Oxidative stress and autophagy in the regulation of lysosome-dependent neuron death.
ABSTRACT Lysosomes critically regulate the pH-dependent catabolism of extracellular and intracellular macromolecules delivered from the endocytic/heterophagy and autophagy pathways, respectively. The importance of lysosomes to cell survival is underscored not only by their unique ability effectively to degrade metalloproteins and oxidatively damaged macromolecules, but also by the distinct potential for induction of both caspase-dependent and -independent cell death with a compromise in the integrity of lysosome function. Oxidative stress and free radical damage play a principal role in cell death induced by lysosome dysfunction and may be linked to several upstream and downstream stimuli, including alterations in the autophagy degradation pathway, inhibition of lysosome enzyme function, and lysosome membrane damage. Neurons are sensitive to lysosome dysfunction, and the contribution of oxidative stress and free radical damage to lysosome dysfunction may contribute to the etiology of neurodegenerative disease. This review provides a broad overview of lysosome function and explores the contribution of oxidative stress and autophagy to lysosome dysfunction-induced neuron death. Putative signaling pathways that either induce lysosome dysfunction or result from lysosome dysfunction or both, and the role of oxidative stress, free radical damage, and lysosome dysfunction in pediatric lysosomal storage disorders (neuronal ceroid lipofuscinoses or NCL/Batten disease) and in Alzheimer's disease are emphasized.
- [Show abstract] [Hide abstract]
ABSTRACT: Two decades of the amyloid-β (Aβ) hypothesis in Alzheimer's disease (AD) and the prominence of Aβ-targeting strategies have yet to meet the levels of original expectation. Disappointing results in numerous Phase II/III studies have called for a re-examination of the validity of the Aβ-targeting approaches as an intervention strategy in AD. The mid-life onset of chronic conditions (e.g., hypertension, diabetes, insulin intolerance, and depression nominated as risk factors for the later development of AD) points to the possibility that each condition could involve mechanisms, which while relatively modest over a short-term, could have significant accumulative effects. What may also not be fully appreciated is that a number of these conditions involve potential disturbances to multivalent cations (MC) levels through various mechanisms such as autophagy, oxidative stress, and apoptosis. Furthermore, some MCs have intimate associations with the mechanisms by which Aβ pathology manifests. Considering various lines of evidence and incorporating statistical analysis on Disability-Adjusted Life Years (DALYs) data of both causes of and prevalence of multifactorial risk factors in different world regions, we propose an MC hypothesis for AD. More specifically, we suggest that MC imbalance marks many chronic conditions and because of their involvement with Aβ pathology, could reflect that Aβ may be a vital manifestation and marker of underlying MC imbalance. Thus, careful targeting of MC imbalance may provide an alternative or complementary interventional approach to current Aβ treatment strategies.Journal of Alzheimer's disease: JAD 04/2014; · 3.61 Impact Factor
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ABSTRACT: Lysosomes contribute to a multitude of cellular processes, and the pH of the lysosomal lumen plays a central mechanistic role in many of these functions. In addition to controlling the rate of enzymatic degradation for material delivered through autophagic or phagocytotic pathways, lysosomal pH regulates events such as lysosomal fusion with autophagosomes and the release of lysosomal calcium into the cytoplasm. Disruption of either the steady state lysosomal pH or of the regulated manipulations to lysosomal pH may be pathological. For example, chloroquine elevates the lysosomal pH of retinal pigmented epithelial (RPE) cells and triggers a retinopathy characterized by the accumulation of lipofuscin-like material in both humans and animals. Compensatory responses to restore lysosomal pH are observed; new data illustrate that chronic chloroquine treatment increases mRNA expression of the lysosomal/autophagy master transcription factor TcFEB and of the vesicular proton pump vHATPase in the RPE/choroid of mice. An elevated lysosomal pH with upregulation of TcFEB and vHATPase resembles the pathology in fibroblasts of patients with mutant presenilin 1 (PS1), suggesting a common link between age-related macular degeneration (AMD) and Alzheimer's disease. While the absolute rise in pH is often small in these disorders, elevations of only a few tenths of a pH unit can have a major impact on both lysosomal function and the accumulation of waste over decades. Accurate measurement of lysosomal pH can be complex, and imprecise measurements have clouded the field. Protocols to optimize pH measurement from fresh and cultured cells are discussed, and indirect measurements to confirm changes in lysosomal pH and degradative capacity are addressed. The ability of reacidifying treatments to restore degradative function confirms the central role of lysosomal pH in these disorders and identifies potential approaches to treat diseases of lysosomal accumulation like AMD and Alzheimer's disease. In summary, various approaches to determine lysosomal pH in fresh and cultured cells, as well as the potential to restore pH levels to an optimal range, can help identify and repair pathologies associated with lysosomal defects in RPE cells and perhaps also suggest new approaches to treat lysosomal storage diseases throughout the body.Experimental Eye Research 09/2014; 126:68-76. · 3.02 Impact Factor
- Aktuelle Rheumatologie 07/2013; 38(3):177-179. · 0.06 Impact Factor
ANTIOXIDANTS & REDOX SIGNALING
Volume 11, Number 3, 2009
© Mary Ann Liebert, Inc.
Forum Review Article
Oxidative Stress and Autophagy in the Regulation
of Lysosome-Dependent Neuron Death
Violetta N. Pivtoraiko,1Sara L. Stone,1Kevin A. Roth,1and John J. Shacka1,2
Lysosomes critically regulate the pH-dependent catabolism of extracellular and intracellular macromolecules
delivered from the endocytic/heterophagy and autophagy pathways, respectively. The importance of lyso-
somes to cell survival is underscored not only by their unique ability effectively to degrade metalloproteins
and oxidatively damaged macromolecules, but also by the distinct potential for induction of both caspase-de-
pendent and -independent cell death with a compromise in the integrity of lysosome function. Oxidative stress
and free radical damage play a principal role in cell death induced by lysosome dysfunction and may be linked
to several upstream and downstream stimuli, including alterations in the autophagy degradation pathway, in-
hibition of lysosome enzyme function, and lysosome membrane damage. Neurons are sensitive to lysosome
dysfunction, and the contribution of oxidative stress and free radical damage to lysosome dysfunction may
contribute to the etiology of neurodegenerative disease. This review provides a broad overview of lysosome
function and explores the contribution of oxidative stress and autophagy to lysosome dysfunction–induced
neuron death. Putative signaling pathways that either induce lysosome dysfunction or result from lysosome
dysfunction or both, and the role of oxidative stress, free radical damage, and lysosome dysfunction in pedi-
atric lysosomal storage disorders (neuronal ceroid lipofuscinoses or NCL/Batten disease) and in Alzheimer’s
disease are emphasized. Antioxid. Redox Signal. 11, 481–496.
originally at characterizing liver glucose 6-phosphatase. De
Duve discovered the association of glucose 6-phosphatase
with a labile enzyme called acid phosphatase, which frac-
tionated with populations of mitochondria and microsomes.
On further optimization of their fractionation protocols, a
“light mitochondrial” fraction was discovered that was in-
termediate in sedimentation to that of mitochondria and mi-
crosomes. Subsequent analysis of this purified fraction de-
lineated several more enzymes, one of which was cathepsin
D (CD), which had acid pH optima. Today the scientific
community appreciates the lysosome as an organelle with
the critical function of regulating the pH-dependent degra-
YSOSOMES were discovered ?50 years ago by Christian de
Duve (41) in a series of serendipitous experiments aimed
dation of intracellular macromolecules. The ability of lyso-
somes to compartmentalize degradation within their lumen
protects the rest of the cell from the transient induction of
oxidative stress and cytoplasmic degradation. Under condi-
tions of cell stress, however, lysosome function and integrity
may become compromised and can trigger regulated cell
death. Instrumental in this cell-death induction are alter-
ations in the vesicular recycling pathway autophagy, which
can induce lysosomal dysfunction or become compromised
as a result of lysosomal dysfunction or both. In addition, ox-
idative stress may cause direct, intralysosomal damage or
cause secondary lysosomal damage through the increased
production of damaged macromolecules or organelles. This
review provides an overview of lysosome function and the
role that oxidative stress and autophagy play in lysosomal
damage. Lysosomal death pathways are explored in great
1Department of Pathology, Neuropathology Division, University of Alabama at Birmingham, Birmingham, Alabama.
2Birmingham VA Medical Center, Birmingham, Alabama.
somal–lysosomal and autophagy–
lysosomal degradation pathways.
Lysosomal hydrolases are produced
in the endoplasmic reticulum (ER)
and, on delivery to the trans-Golgi
network (TGN), are transported in
vesicles by recognition of mannose-6-
phosphate receptors (M6PRs) to the
late endosome (or to the early endo-
some, which then matures to form the
late endosome). The late endosome is
then thought to deliver lysosomal hy-
drolases via a type of fusion event to
their terminal location, the lysosome,
which is M6PR negative. Damaged
organelles and macromolecules are
surrounded by a limiting membrane
from the ER to form a preautophago-
somal structure (PAS), which ma-
tures to form the double membraned
autophagosome. The pH of au-
tophagosomes is not sufficient to de-
grade their intraluminal contents,
and fusion with lysosomes (forming
the autophagolysosome) or with en-
dosomes (forming an amphisome),
Convergence of the endo-
which both contain pH-dependent acid hydrolases, must take place for autophagosomal contents to be effectively de-
graded. Please refer to text for further details.
detail, with particular focus to their role in age-related neu-
rodegenerative diseases including Alzheimer’s disease and
the pediatric neurodegenerative disease neuronal ceroid
lipofuscinoses (NCL)/Batten disease.
Lysosome Structure, Function, and Assembly
Lysosomes serve an important intracellular role as the site
for the terminal proteolytic degradation of damaged proteins
and organelles, which is accomplished in the range of pH 4.5
to 5 via ?50 lysosomal hydrolases with acidic pH optima
(113). Morphologically, lysosomes are cytoplasmic dense
bodies that are either spheroid, ovoid or occasionally tubu-
lar in appearance (113). Neuron lysosomes are typically ?1
?m in size and are often situated in a perinuclear position
(113). Lysosomal hydrolases are surrounded by a limiting
membrane containing an abundance of glycosylated proteins
(117). An intact lysosomal membrane provides the barrier
necessary to maintain such a low pH compared with the neu-
tral pH of the surrounding cytosol. Upward of two dozen
cathepsins are known, with specificities for different peptide
bonds, including the cysteine proteases cathepsins B (CB),
H, and L or the aspartic acid protease CD. Lysosomal hy-
drolases catalyze the pH-dependent degradation of proteins
into amino acid pools for intracellular recycling. As is dis-
cussed in subsequent sections, the increase in posttransla-
tional oxidative modifications has been shown to decrease
the effective degradation of proteins by lysosomal hydro-
lases and may lead to an increase in protein accumulation,
which may contribute to the increase in autofluorescent
lipopigment in postmitotic neurons (149).
Although lysosomal hydrolases reside at their terminal lo-
cation in lysosomes, their synthesis and transport to lyso-
somes requires a complex series of events that carries them
through many different organelles and vesicles (Fig. 1). Their
localization to lysosomes must be confirmed either by colo-
calization with lysosomal membrane proteins such as
LAMP-1 or LAMP-2 (49) or by subcellular fractionation.
Lysosome synthesis begins initially in the endoplasmic retic-
ulum (ER) (142), where newly synthesized hydrolases con-
tain an N-terminal, 20- to 25-amino acid signal peptide,
which allows their translocation into the ER lumen. On cleav-
age of the signal peptide, oligosaccharides are added onto
the hydrolases, which allows the enzymes to be equipped
with mannose-6-phosphate (M6P) recognition markers in the
trans Golgi network (TGN). This M6P tag allows lysosomal
hydrolases to recognize and bind to M6P receptors (M6PRs),
and the receptor–ligand complex subsequently exits from the
TGN in clathrin-coated vesicles as they deliver their contents
directly to late endosomes or indirectly via delivery to early
endosomes, which are thought to mature into late endo-
somes. Endosomes exhibit an acidic pH, as do lysosomes,
but can be distinguished from lysosomes in that lysosomes
are M6PR negative. The low pH of endosomes facilitates dis-
sociation of lysosomal hydrolases from M6PRs, which allows
the vesicle-mediated recycling of M6PRs back to the TGN.
Concomitant with further maturation steps, including de-
phosphorylation, oligosaccharide trimming, and proteolytic
activation, lysosomal hydrolases arrive at the lysosomes,
events that are mediated most likely by a type of fusion event
between the late endosome and lysosome (86).
Intracellular macromolecules and organelles are delivered
to lysosomes for degradation and recycling by autophagy
PIVTORAIKO ET AL. 482
(Greek for “eat oneself”), and several types of autophagy dic-
tate the manner in which macromolecules and organelles ar-
rive at the lysosome (77). Arguably the best-studied type of
autophagy is macroautophagy (Fig. 1), which involves the
generation of a double-membraned autophagosome that
forms nonselectively around bulk cytoplasm, and the shut-
tling of these contents through a series of vesicular fusion
events to the lysosomes for pH-dependent degradation by
lysosomal hydrolases (for review, see ref. 131). Autophago-
somes may fuse with either late endosomes or lysosomes
(131), which both contain lysosomal hydrolases in an acidic
environment that facilitates degradation. The fusion of au-
tophagosomes with endosomes forms single-membraned
amphisomes (59, 85), which fuse ultimately with lysosomes
for terminal degradation. Macroautophagy is induced by in-
tracellular nutrient stress or energy depletion or both and is
regulated at multiple levels by upward of 30 known au-
tophagy-related gene (Atg) proteins, including signals that
stimulate autophagy induction, the initiation and comple-
tion of autophagic vacuole formation, and the recycling of
autophagic vacuoles (for review, see ref. 131). Chaperone-
mediated autophagy (CMA) is a more-selective form of au-
tophagy in which specific cytosolic proteins with KFERQ se-
quences are targeted by chaperone proteins such as hsc70 to
the lysosome, followed by internalization in lysosomes by
the membrane-bound, Lamp2a receptor (45). Microau-
tophagy is a less well-defined type of autophagy in which
lysosomes directly ingest cytosolic nutrients by membrane
involution (158). Although microautophagy has been iden-
tified and studied in simple organisms such as yeast, its
occurrence and significance in mammalian cells is unclear.
Organelle-specific macroautophagy (e.g., mitophagy, reticu-
lophagy) also has been identified and may selectively target
damaged organelles for lysosomal degradation (10, 76, 146).
Heterophagy, by definition, is distinct from autophagy be-
cause it involves the intracellular degradation of extracellu-
lar material, which is mediated by endocytosis and the de-
livery of material to lysosomes from endosomes (120).
Redox-Reactive Iron and Intralysosomal Damage
Lysosomes play a critical role in the breakdown of iron-
containing macromolecules on their delivery to lysosomes
by autophagy, and as such, the lysosome contains high lev-
els of iron (15, 111, 115, 121). Metalloproteins such as ferritin
have been shown to rely on intact lysosome function for their
effective degradation and removal of iron, which is thought
to provide an important source of free iron for essential in-
tracellular functions (75, 82, 111, 115). Although the com-
partmentalization of high concentrations of potentially re-
dox-active iron within lysosomes is in theory a protective
measure for the rest of the cell, it may also increase the sus-
ceptibility for intralysosomal damage and the induction of
cell death (167). The brain and neurons, in particular, con-
tain relatively high levels of iron, and iron has been shown
to accumulate in neurons with aging (125), which further im-
plicates the potential for iron-mediated damage in age-re-
lated neurodegenerative disease. Ferric iron (containing at
least one uncoordinated ligand) may react with hydrogen
peroxide in forming ferrous iron, along with the deleterious
hydroxyl radical, by the Fenton reaction (81). The acidic pH
of lysosomes in addition to the presence of reducing equiv-
alents such as cysteine provides a hospitable environment
for Fenton chemistry (7), and hydrogen peroxide may read-
ily diffuse into the lysosomal lumen from the cytoplasm, es-
pecially under conditions of oxidative stress. In addition,
lysosomes do not ordinarily contain reducing enzymes such
as catalase or glutathione peroxidase unless they are being
degraded by autophagy, which exacerbates the potential for
reactive iron-induced damage in lysosomes (81). Hydroxyl
radical can oxidize a host of macromolecules, including
lipids and proteins, which may not only inhibit their degra-
dation and contribute to the accumulation of intralysosomal
lipofuscin as discussed below, but also may inhibit the func-
tion of lysosomal hydrolases, further decreasing the
degradative capacity of lysosomes (65, 133). In addition, the
accumulation of oxidized lipoproteins within lysosomes may
negatively affect the integrity of lysosomal membranes and
provide a stimulus for the induction of lysosomal membrane
permeabilization (LMP), as discussed later.
Conversely, the autophagy of thiol-rich proteins, includ-
ing metallothioneins, has been proposed to counteract lyso-
somal damage by binding redox-active iron and other tran-
sition metals such as zinc within lysosomes, thus decreasing
the probability of Fenton chemistry occurring (7, 32). In ad-
dition, under some experimental conditions, the iron chela-
tor desferrioxamine has been shown to attenuate cell dam-
age and cell death through its ability to localize within
lysosomes and bind intralysosomal free iron (80, 109, 139,
Lipofuscin and Oxidative Stress
Lipofuscin is an intralysosomal waste material that accu-
mulates in postmitotic cells such as neurons as a function of
aging, or in dividing cells whose rate of proliferation has
been compromised (reviewed in ref. 17). The makeup of lipo-
fuscin is chemically and morphologically amorphous, con-
sisting of protein and lipid, carbohydrates, transition metals,
and autofluorescent pigment (17). The accumulation of lipo-
fuscin in postmitotic cells is closely related to a compromise
in its effective degradation, combined with a lack of effec-
tive exocytosis (148). Lipofuscin accumulation is associated
with age-related neurodegenerative diseases such as Alz-
heimer (25–27) and in lysosomal storage disorders including
NCL/Batten disease (47), which may be related in part to
known alterations in the macroautophagy–lysosomal degra-
dation pathway that exist in these diseases. Whereas it is
clear that lipofuscin accumulation correlates with lysosome
dysfunction, it is not clear the extent to which its accumula-
tion directly contributes to the induction of neuron death, al-
though adverse effects on cell function have been reported
(96), with an increased susceptibility of lipofuscin-loaded fi-
broblasts to apoptosis (147). Regardless, the finding that up
to 75% of a neuron’s perikarya may contain lipofuscin [re-
viewed in (149)] suggests that altered lysosome function may
exacerbate the sensitivity of neurons to lysosomal death sig-
The inhibition of lipofuscin degradation may result from
either the inhibition of lysosomal hydrolases or an increase
in oxidative stress or both. Lipofuscin accumulation has been
described experimentally by the chemical inhibition of lyso-
somal hydrolases, either from treatment with protease in-
hibitors or from the lysosomotropic agent chloroquine (69,
AUTOPHAGY, ROS, AND LYSOSOMAL CELL DEATH 483
70, 148). Age-related decreases in the activity of lysosomal
hydrolases have also been documented, which may contrib-
ute to the age-related increase in lipofuscin with normal
brain aging (3, 70). Conversely, the overloading of cells with
lipofuscin has been shown to cause a decrease in the activ-
ity of lysosomal hydrolases (133), suggesting that lipofuscin
accumulation per se may also initiate a compromise in lyso-
some function. The ability of oxidative stress to enhance lipo-
fuscinogenesis has been documented in several cell types
(137, 151, 159). Lipofuscinogenesis may be caused by pro-
teins that are oxidatively modified outside the lysosome and
subsequently delivered to lysosomes for degradation, or may
be caused by the intralysosomal formation of reactive oxy-
gen species (ROS), as suggested by the potential for lysoso-
mal lipoproteins to acquire oxidative cross-links (16). The ef-
fect of either route would be a net increase in oxidatively
modified lipofuscin, with an inherent compromise in its
degradative capacity. The importance of oxidative stress in
lipofuscin accumulation is further emphasized by its de-
crease on experimental treatment with antioxidants or the
iron chelator desferrioxamine (151). In addition, the inhibi-
tion of lysosomal hydrolases may exacerbate the oxidative
stress–induced accumulation of lipofuscin, because a com-
promise in intralysosomal enzymatic protein degradation
would provide greater opportunities for such proteins to ac-
quire oxidative modifications that contribute to lipofuscin
accumulation. In support of this argument, the accumulation
of lipofuscin induced by combined oxidative stress and pro-
tease inhibition was shown to be three times greater than
that observed by either condition alone (148).
Lipofuscin is formed from a variety of intracellular sources
that are delivered to lysosomes by the autophagy degrada-
tion pathway (for review, see ref. 131). The induction of
macroautophagy may provide a potent stimulus for lipo-
fuscin accumulation (Fig. 2). Nutrient deprivation and re-
sultant oxidative stress are natural stimuli for macroau-
tophagy induction, and as such, may result in the increased
delivery of undegradable, oxidatively modified proteins to
lysosomes that accumulate as part of lipofuscin. Along these
lines, ROS induced by starvation were found recently to reg-
ulate macroautophagy induction critically through the cys-
teine-dependent activity of Atg4, an autophagy-specific pro-
tein that regulates autophagosome formation (124). The
induction of mitophagy may also increase the lysosomal de-
livery of oxidatively damaged mitochondrial membranes
and proteins, in addition to superoxide anion, which is gen-
erated normally in mitochondria by the electron-transport
chain (37). In further support of mitophagy contributing to
lipofuscin accumulation, subunit c of mitochondrial ATP
synthetase has been shown to be a major component of lipo-
fuscin, in particular in aged neurons (47). An increase in in-
tralysosomal redox-active iron also may result from the au-
tophagy-mediated degradation of ferritin (75, 111, 115, 121).
Under conditions of oxidative stress, the diffusion of read-
ily available hydrogen peroxide into the lysosomal lumen
may drive Fenton chemistry to form the highly reactive hy-
droxyl radical that would promote oxidative cross-links that
enhance lipofuscin accumulation, a hypothesis that has been
previously proposed and is further supported by the increase
in lipofuscin accumulation on inhibition of lysosomal pro-
PIVTORAIKO ET AL.484
lysosomal membrane damage, LMP, and cell death may be directly influenced by both the aberrant induction and inhibi-
tion of macroautophagy, which can lead to the induction of intralysosomal oxidative stress. It has also been proposed that
an initial overinduction of macroautophagy induction may lead to an eventual inhibition of macroautophagy, which also
may be related in part to the induction of oxidative stress. Please see text for further details.
Macroautophagy induction versus inhibition in oxidative stress–induced lysosome damage. The induction of
teases (148). The generation of intralysosomal free radicals
may cause peroxidation of membrane polyunsaturated fatty
acids to form relatively stable and cytotoxic aldehydes, alke-
nals, or hydroxyalkenals, including malondialdehyde or 4-
hydroxy-nonenal (4-HNE) (50). Treatment of purified pro-
tein with 4-HNE, for instance, has been shown not only to
form protein cross-links (36, 54, 156) and generate protein-
associated fluorescence similar to that found in the autoflu-
orescent lipofuscin (55, 63, 152), but also to cause enzyme in-
activation (30, 38, 50, 134, 144, 156) that may further enhance
Inhibition of macroautophagy completion may also con-
tribute to the accumulation of lipofuscin (Fig. 2), as was
shown previously by treatment with the lysosomotropic
agent chloroquine or with protease inhibitors (69, 70, 148).
Treatment with lysosomotropic agents and protease inhibi-
tors has been shown to increase intralysosomal ferritin sta-
bility and decrease the available pools of redox-active iron
(75, 82), which, in contrast to macroautophagy induction,
may suggest a limited role for redox-active iron and Fenton
chemistry in the intralysosomal production of ROS after
macroautophagy inhibition. Rather, the inhibition of lysoso-
mal hydrolases may initially play a more direct role in lipo-
fuscin accumulation after macroautophagy inhibition, be-
cause in this setting, it would be logical to predict a
more-direct compromise in lysosome function as the initial
stimulus for altered macroautophagy. Because oxidized
lipoproteins or lipofuscin accumulation has been shown ex-
perimentally to decrease the activity of lysosomal hydrolases
(65, 133), it is possible that lipofuscin accumulation per se
may also initiate a compromise in lysosome function that
would lead to macroautophagy inhibition, perhaps as a re-
sponse to initial macroautophagy induction. This explana-
tion is attractive for the etiology of Alzheimer disease
neuropathology, as it was hypothesized previously that
macroautophagy is induced early in the course of AD onset,
which is followed in later stages by macroautophagy inhi-
bition (103, 105).
Lysosomotropic Agents Generate Oxidative Stress
Christian De Duve (42) coined the term “lysosomotropic”
in 1974 to delineate a group of uncharged compounds, typ-
ically amphiphilic weak bases, that are attracted to acidic
compartments within cells, or are, in other words, “aci-
dotropic.” Such uncharged molecules diffuse passively
through the membranes of acidic organelles, including lyso-
somes, which have a typical pH range of 4.5 to 5 (106). Once
inside lysosomes, these agents become protonated, and their
charge effectively precludes their transport across lysosomal
membranes, resulting ultimately in an effective increase in
intralysosomal pH and the impairment of lysosome-medi-
ated degradation (42, 126). Accumulation of such agents in
lysosomes depends initially on the pH gradient between the
intra- and extralysosomal compartments and can be pre-
vented by the prior increase in intralysosomal pH.
lamino)quinoline; see structure, Fig. 3] is a well-known an-
timalarial agent that has been used for many years to inves-
tigate lysosome function. Chloroquine exerts its antimalarial
effects by concentrating in the acidic digestive vacuole of
Plasmodium parasites, where it is hypothesized to complex
with ferric heme (ferriprotoporphyrin IX, FPIX) monomer
(51), which is produced on parasitic degradation of host he-
moglobin. By complexing with FPIX, chloroquine promotes
accumulation of the toxic, undimerized form of FPIX, which
increases susceptibility to iron-dependent peroxidation of
lipid membranes (51), an effect that has been observed with
treatment of liposomes with the chloroquine–FPIX complex
(143). It is thus reasonable to predict that chloroquine also
forms a similar type of lipid peroxidation–generating com-
plex with iron-containing proteins in the lysosomes of mam-
malian cells. In support of this argument, chloroquine has
been shown effectively to inhibit the intralysosomal release
of free iron from ferritin, which is known to require intact
lysosome function (75, 82). Regardless, chloroquine does in-
duce lipid peroxidation in mammalian cells (11, 68, 112), and
future studies are needed to delineate whether this occur-
rence is specific for lysosomal membranes. Because chloro-
quine effectively inhibits the intralysosomal release of free
iron from ferritin, Fenton chemistry may not play a princi-
pal role in the induction of lysosomal damage mediated by
chloroquine and subsequent macroautophagy inhibition.
Alternatively, chloroquine-induced oxidative damage to
lysosomal membranes and the accumulation of oxidatively
modified lipoproteins may result from macroautophagy in-
hibition combined with its inhibition of lysosomal proteases
(4, 43, 53, 62, 163) mechanisms that may be responsible for
its induction of lipofuscin, as previously described (70).
Chloroquine also was shown recently to reduce intracellular
levels of glutathione (110), which could lead to an increased
production of cytosolic hydrogen peroxide and concomitant
extralysosomal damage of macromolecules and organelle
The intralysosomal accumulation of chloroquine has been
shown to induce profound alterations in lysosome function,
including inhibition of both the proteolytic maturation and
enzyme activities of CB and CD (4, 43, 53, 62, 101, 163), which
may be secondary to chloroquine-induced increase in in-
tralysosomal pH and disruption of pH optima for these en-
zymes. In our laboratory, we observed similar results in SH-
SY5Y cells, such that a death-inducing concentration of
chloroquine markedly decreases maturation of CD, as mea-
sured by Western blot (Fig. 4). However, recent reports also
indicate that chloroquine increased CD levels, as measured
by Western blot, but it is unclear from these studies which
form of CD (pro versusmature, “active” forms) was increased
(9, 18). Earlier studies reported an increase in lysosome size
or swelling by chloroquine and other lysosomotropic agents
AUTOPHAGY, ROS, AND LYSOSOMAL CELL DEATH 485
line] represents the class of fluoroquinolones.
Chemical structure of chloroquine. Chloroquine
(97, 107, 138), which results from intralysosomal chloroquine
reaching isotonicity with levels in the cytosol and the sub-
sequent increase in water flow into the lysosome. Such
“swollen” lysosomes may exhibit increased membrane
fragility, as indicated in isolated preparations by their in-
creased latency to release lysosomal enzymes (97, 138) by an
increase in lysosomal enzymes in purified cytosolic prepa-
rations (89). These findings clearly suggest the induction of
LMP and may play a significant role in the induction of cell
death after chloroquine treatment, as described later.
ROS, Autophagy, and Lysosomal Membrane
Permeabilizations Death Stimuli
The susceptibility of lysosomes to oxidative stress or mem-
brane destabilization or both is thought to play a major role
in the induction of LMP, which results in the release of lyso-
somal enzymes into the cytosol and the potent induction of
cell death. Both macroautophagy induction (14, 19, 157) and
inhibition (14) have been shown to regulate cell death po-
tently through the induction of LMP, which may involve the
generation of reactive oxygen species (Fig. 5). For many
years, it was believed that LMP-induced cell death was un-
regulated and necrotic (40). Today, it is well established that
LMP may induce both apoptosis and necrosis, which seems
to depend in part on the magnitude of LMP and the amount
of proteolytic enzymes released into the cytosol. Many stud-
ies have indicated that stimuli that produce LMP tend to in-
duce apoptosis at lower concentrations and necrosis at
higher concentrations (84). Because multiple types of cell
death can be induced after LMP, it is not surprising that the
inhibition of apoptosis after LMP has been shown to shunt
the type of death to a more-necrotic nature (57). To this end,
we also showed that the inhibition of Bax-dependent neu-
ron death after lysosome dysfunction does not attenuate the
degree of neuron loss or neurodegeneration (130).
The cysteine protease CB and the aspartic acid protease
CD are two of the most ubiquitous lysosomal enzymes (61),
and as such, they have been shown to play a major role in
the stimulus-specific induction of cell death after LMP. Be-
cause lysosomal hydrolases possess optimal activation at
acidic pH, it is fair to question their ability to function once
released into the cytosol. However, in vitro studies have
shown that lysosomal proteases can function for several min-
utes to more than an hour at neutral pH (154), confirming
their potential for activation outside of lysosomes. In addi-
tion, recent studies indicated that the cytoplasmic pH is re-
duced in the course of cell death (98, 99), which increases the
potential for lysosomal proteases directly to influence cell
death after LMP.
Many studies have used hydrogen peroxide to generate
oxidative stress–induced LMP and apoptosis, in both neural
(21, 67) and nonneural cell types (5, 33). In addition, studies
have indicated the induction of LMP by other stimuli that
indirectly induce hydrogen peroxide, including TNF-? (60)
and lipopolysaccharide (161). The induction of LMP by hy-
drogen peroxide is believed to occur through its ability to
diffuse freely from the cytosol into iron-rich lysosomes,
where it uses Fenton chemistry to induce the production of
the highly reactive hydroxyl radical (150). In addition, both
hydrogen peroxide and stimuli known to produce hydrogen
peroxide indirectly (such as TNF-?) have been shown to in-
duce activation of phospholipase A2 (PLA2), which in the-
ory stimulates the degradation of membrane lipids that
could potentially increase lysosome destabilization and LMP
(71, 172). LMP-induced apoptosis has also been evidenced
after treatment with other oxidative stress–inducing com-
pounds, including naphthazarin (73), which generates ROS
through redox cycling, and hypochlorous acid, shown re-
cently to induce lysosome destabilization in cultured corti-
cal neurons (165).
ROS-induced LMP is a potent stimulus that has been
shown in many studies to precede the induction of mito-
chondrial-dependent apoptosis (21), which has also been in-
dicated by treatment with lysosomotropic agents or other
agents that mediate indirect production of ROS (14). In ad-
dition, several studies have shown that CB (153) and CD (74)
mediate mitochondrial apoptosis, findings that strongly im-
plicate LMP in the “lysosomal–mitochondrial axis” theory of
cell death, as previously described (150). Further proof of
this paradigm came from an elegant study whereby the cy-
PIVTORAIKO ET AL. 486
cells follows alterations in the processing of CD. (A) Treat-
ment of human SH-SY5Y cells with chloroquine (50 ?M) sig-
nificantly attenuates cell viability at 48 h vs. vehicle control
but not at 24 h. *p ? 0.05 vs. vehicle control (Student’s un-
paired t test). (B) By 24 h, chloroquine induces a modest de-
crease in the mature “active” form of CD, migrating at ?30
kDa, along with a marked increase in the inactive, “pre-pro”
fragment migrating at ?50 kDa, in comparison to vehicle
control. After 48 h of chloroquine treatment, levels of the ma-
ture active form of CD appear to be further reduced in com-
parison to 24 h. Levels of ?-tubulin (migrating at ?50 kDa)
serve as the loading control.
Chloroquine-induced death of human SH-SY5Y
tosolic microinjection of CD induced caspase-dependent
death, an effect that was inhibited by combined microinjec-
tion of CD with its inhibitor pepstatin A (114). Conversely,
lysosomal enzymes have been shown to increase production
of mitochondrial ROS, which may result in further lysoso-
mal destabilization as part of a deleterious feedback loop
Recent studies have shown that one mechanism by which
cytosolic cathepsins induce mitochondrial apoptosis is
through direct effects on Bcl-2 family members (Fig. 5). This
concept was first suggested by Stoka et al. (141) in 2001,
which reported cleavage of the proapoptotic Bcl-2 family
member Bid by lysosomal extracts, and the ability of this
cleavage product to induce cytochrome c release from mi-
tochondria. Bid cleavage along with induction of mito-
chondrial apoptosis was first shown to be mediated via the
cysteine protease caspase-8 (83) . A follow-up study con-
firmed that CB is directly responsible, at least in part, for
Bid cleavage and induction of mitochondrial apoptosis (35).
Another study suggested that CD plays a role in apoptosis
mediated by Bid cleavage after treatment with ceramide
(64). In addition, recent evidence has shown that after the
induction of LMP, cytosolic CD interacts directly with
proapoptotic Bax in the promotion of mitochondrial apop-
tosis by a variety of stimuli, including treatment with hy-
drogen peroxide (21). This CD–Bax–mitochondrial death
pathway has also been shown to stimulate downstream mi-
tochondrial release of apoptosis-inducing factor (AIF) (12),
a mitochondrial flavoprotein that, on release from mito-
chondria, is implicated in caspase-independent apoptosis
and necrosis (39). Thus, the interaction of cytosolic cathep-
sins with Bcl-2 family members has the potential to induce
multiple types of cell death, and future studies are war-
ranted to determine whether this pathway also plays an im-
portant role in the induction of neuron death in acute in-
jury or neurodegenerative disease.
For many years, it was widely believed that the regulation
of cell death by Bcl-2 family members was due solely to their
manipulation of mitochondrial membrane integrity. How-
ever, several intriguing studies over the last few years sug-
gested that other organelles, including the ER and lysosomes,
may also be regulated by Bcl-2 family members in the in-
duction of cell death (61). The first reports of Bcl-2 fam-
ily–mediated regulation of lysosome function were from the
laboratory of Ulf Brunk (173), which suggested that lyso-
some-localized Bcl-2 attenuated hydrogen peroxide–induced
apoptosis, at least in part, by promoting lysosome stabiliza-
tion. Subsequent studies have shown that proapoptotic Bax
not only localizes to lysosomal membranes after stressful
stimuli but also regulates the induction of LMP (162). The
BH3 domain–only molecules Bim and Bad were also shown
to localize to lysosomes after a death stimulus and regulate
the induction of LMP, although their induction of LMP re-
quired the presence of Bax (162). Together these findings
support the potential for the additional “upstream” influ-
ence of Bcl-2 family members in the regulation of lysosome-
AUTOPHAGY, ROS, AND LYSOSOMAL CELL DEATH 487
meabilization, and the induction of necrotic versus
apoptotic death. Agents that promote the direct or in-
direct production of oxidative stress may lead to lyso-
some membrane permeabilization (LMP) and cell
death. It is thought that the induction of total LMP fa-
vors the onset of necrosis, whereas partial LMP favors
the onset of apoptosis. LMP is associated with the re-
lease of lysosomal cathepsins into the cytosol and the
interaction with pro-apoptotic Bcl-2 family members,
which leads to the induction of mitochondrial apop-
tosis. Proapoptotic Bcl-2 family members may also act
directly at the lysosomal membrane as a stimulus for
LMP. For further details, please see the text.
Oxidative stress, lysosomal membrane per-
dependent neuron death (Fig. 5), and as a result, the poten-
tial for their regulation of multiple types of neuron death.
Chloroquine-induced Neuron Death
One of the most striking observations after treatment of
cells or tissues with chloroquine is the massive accumulation
of autophagic vacuoles that results from the inhibition in
completion of the macroautophagy–lysosomal degradation
pathway. We and others have shown that sustained incuba-
tion with chloroquine potently induces cell death that is char-
acterized by morphologic and biochemical markers of apop-
tosis and is preceded by autophagic vacuole accumulation
(14, 129, 168). In our laboratory, chloroquine-induced cell
death has been evidenced in a variety of cell types, includ-
ing immature and fully differentiated primary neurons,
neural precursor cells, and a variety of neural cell lines (Fig.
4). At present, whether the accumulation of AVs directly me-
diates chloroquine-induced neuron death has not been thor-
oughly investigated, although the ability of macroautophagy
inhibition to induce cell death was clearly indicated previ-
ously in the literature (14, 129). Death induced by macroau-
tophagy inhibition may result from a compromise in home-
ostatic organelle turnover, thus increasing the accumulation
of damaged organelles with compromised function, which
could trigger the initiation or completion of death-pathway
signaling. Certainly the accumulation of undegradeable ox-
idized lipoproteins may cause associated damage to lysoso-
Mitochondrial dysfunction appears to play a major role in
chloroquine-induced cell death, as indicated previously by
a decrease in mitochondrial membrane potential and an at-
tenuation of cell death by the targeted disruption of proapop-
totic bax or bcl-2 overexpression, and the exacerbation of cell
death after the targeted disruption of antiapoptotic bcl-x (14,
129, 168). Although chloroquine induces robust activation of
caspase-3, the targeted genetic disruption of caspase-3 or
treatment with general caspase inhibitors does not attenuate
chloroquine-induced neuron death (129, 168). Together,
these findings suggest either that the commitment point for
chloroquine-induced neuron death lies upstream of caspase
activation, or indicates that the potential for both caspase-
dependent and -independent death pathways triggered by
disruption of the macroautophagy–lysosomal degradation
pathway. We have also shown that chloroquine-induced
death of immature neurons is attenuated by the protoonco-
gene p53, an effect that was not observed in cultures of post-
mitotic neurons (68, 70), which suggests that p53-dependent
autophagic cell death may be cell-type or differentiation de-
pendent or both. As such, chloroquine-induced, p53-depen-
dent autophagic death is being actively investigated as a
potential therapeutic target in several types of cancers, in-
cluding glioblastomas (110).
We showed recently that the plecomacrolide antibiotic
bafilomycin A1 (BafA1) and other structurally similar com-
pounds significantly attenuate chloroquine-induced neuron
death (128, 129), at concentrations (?1 nM) shown previ-
ously not to inhibit vacuolar-type ATPase (13). Although a
previous study suggested that a high dose of 100 nM BafA1
attenuated cell death induced by hydroxychloroquine by at-
tenuating the pH-dependent fusion of chloroquine into the
lysosome (14), our results suggest that “neuroprotective”
concentrations of BafA1 (?1 nM) do not alter the ability of
chloroquine to inhibit macroautophagy, because AVs still ac-
cumulate in chloroquine?BafA1–treated cells, concomitant
with an absence of apoptotic morphology, and that the
chloroquine-induced inhibition of long-lived protein degra-
dation was not affected by 1 nM BafA1 (129). Ongoing stud-
ies in our laboratory are delineating the potential mecha-
nisms by which plecomacrolide antibiotics attenuate neuron
death induced by lysosomotropic agents and whether cell
death induced by other disruptions in lysosome function can
also be attenuated by plecomacrolides.
Chloroquine-induced cell death was shown previously to
involve LMP, as indicated immunocytochemically by the dif-
fuse cytosolic immunoreactivity of the lysosomal protease
CB in chloroquine-treated cells (14). LMP was suggested as
an upstream mediator of mitochondrial cell death, because
selective inhibition of CB significantly attenuated chloro-
quine-induced mitochondrial dysfunction concomitant with
an increase in viability (14). Interestingly, chloroquine also
enhances the extracellular secretion of many lysosomal en-
zymes, including ?-hexaminodase, CB, and CD (58, 88, 100,
101, 122), which effectively blocks the delivery of newly syn-
thesized lysosomal hydrolases to lysosomes. It will be im-
portant in future studies of chloroquine-induced LMP to
confirm results of immunocytochemistry with rigorous bio-
chemical analyses indicating an increased appearance of
lysosomal enzymes in purified cytosolic fractions via west-
Chloroquine has also been shown to inhibit the activities
of sphingolipid-metabolizing enzymes, including sphin-
gomyelinase and acid ceramidase (48, 72), which are local-
ized to lysosomes and most likely reflect the deleterious
alterations in lysosome function that are induced on chloro-
quine treatment. Inhibition of these lipid-metabolizing en-
zymes causes the accumulation of ceramide and sphingo-
sine, two highly reactive lipid mediators that have been
shown to mediate oxidative stress–induced apoptosis (123,
170). Sphingosine has been shown to exhibit detergent-like
properties toward lysosome membranes, which may con-
tribute to chloroquine-induced LMP and subsequent apop-
tosis (74). Together, these results suggest that the aberrant
production of reactive lipid metabolites not only may medi-
ate cell death induced as a result of lysosome dysfunction
mediated during macroautophagy inhibition but also may
further exacerbate lysosome dysfunction and stimulate LMP-
induced cell death. It should be noted, however, that inhi-
bition of sphingolipid-metabolizing enzymes also increases
levels of the antiapoptotic sphingolipid sphingosine-1-phos-
phate concomitant with proapoptotic sphingolipids (48),
which suggests a potential balance of pro- versus antiapop-
totic lipid mediators that must be addressed appropriately
to understand the net contribution of lipid mediators in neu-
ron death regulation.
Oxidative Stress, Autophagy, and Lysosome
Dysfunction in CNS Aging and Alzheimer’s Disease
Several properties of the aging brain make it uniquely sus-
ceptible to age-related oxidative damage. First, neurons are
postmitotic; thus, over their life span, age-related macro-
molecular damage accumulates and compromises their func-
tion. This is evidenced by the age-related increase in lipo-
PIVTORAIKO ET AL. 488
fuscin, which may provide both cause and effect for age-re-
lated declines in lysosome function and autophagy signal-
ing (69, 70, 87, 149). Second, neurons have high energy de-
mands compared with other cell types, and they may be
more vulnerable to the deleterious effects of mitochondrial
dysfunction, combined with the fact that the electron-trans-
port chain of oxidative phosphorylation generates ROS (1).
Third, the brain is composed of large amounts of lipids and
transition metals including iron (1, 52, 169), which increases
the probability of age-related lipid peroxidation. Last, the ag-
ing brain contains fewer reducing equivalents that in theory
would contribute to an increase in oxidative stress (136).
Age-related oxidative stress in the cytoplasm may cause
macroautophagy induction, in particular as cytoplasmic
macromolecules or organelles become damaged and are de-
livered to lysosomes for degradation. Conversely, age-re-
lated oxidative stress in the lysosome may lead to macroau-
tophagy inhibition if the result of sustained oxidative stress
is a net compromise in lysosome function.
Many studies have indicated pronounced alterations in
the endosomal–lysosomal pathway in human AD brain,
which are some of the earliest reported abnormalities in AD
brain neurons and precede the onset of both A?-containing
plaque and tangle neuropathology (28). Enlarged, A?-im-
munoreactive endosomes have been reported in brains of AD
patients (102) before A? deposition, which suggests a po-
tential for endosome-mediated A? secretion and deposition.
Increased levels of CD have also been localized to endosomes
of AD patients (23, 24). Endosomal CD may be linked to A?
formation, in that CD possesses inherent ?- and -secretase
activity, enzymes that are responsible for the cleavage of
amyloid precursor protein into A? (31), although APP pro-
cessing was shown previously to be unaffected by CD defi-
ciency in mice (119). Increased levels and activity of CB and
CD in AD brain have been shown to occur concomitant with
lysosome proliferation (2, 22, 24, 27, 29) and may reflect a
compensatory response to altered macroautophagy, but it is
not clear whether such alterations serve a beneficial role to
promote protein degradation or death signaling. In addition,
both CB and CD have been localized extracellularly to amy-
loid plaque, which may indicate their potential to regulate
plaque deposition (27, 29). In support of this argument, CB
has been shown effectively to decrease levels of the more
amyloidogenic A?1-42, and CB deficiency in mice was shown
to cause an increase in extracellular A? deposition (94).
Alterations in the autophagy–lysosomal degradation path-
way have been indicated in AD by pathologic increases in
autophagic vacuoles (AVs) observed in cortical biopsies ob-
tained from AD brain (104, 166). Accumulating AVs in AD
brain have been found to localize in large part to dystrophic
neurites, which may be related to alterations in intracellular
trafficking that either cause AV accumulation or result from
AV accumulation. Both immature, double-membraned au-
tophagic vacuoles and mature, single-membraned au-
tophagolysosomes have been shown to accumulate in AD
dystrophic neurites, which implicates both macroautophagy
induction and inhibition in AD and may reflect both an early
and late autophagic response of individual neurons to AD-
associated stress. Recent evidence suggests the localization
of A? in AVs both in human AD brain and in experimental
models of AD, and that the processing of APP into A? may
even occur within AVs (166). The intracellular accumulation
of A? has also been reported after the chemical inhibition of
macroautophagy completion mediated by in vivo and in vitro
treatment with chloroquine (34, 90), providing further evi-
dence that macroautophagy plays a vital role in A? pro-
cessing and degradation.
Taken together, evidence suggests that alterations in both
the endosomal–lysosomal and autophagy–lysosomal degra-
dation pathways play an intimate role in the generation of
AD neuropathology, and oxidative stress may play a major
role in inducing alterations in intracellular recycling path-
ways. Oxidative stress has been proposed to play a major
role in the onset and progression of AD. Many reports of in-
creased oxidative damage have been reported in AD brain
(reviewed in refs. 20 and 92), which may have important
ramifications in the macroautophagy-lysosome degradation
pathway. An increase in mitophagy has been reported in AD
brain (93), which is likely a response of autophagy to clear
oxidatively damaged mitochondria in postmitotic neurons.
Treatment of neuronal cells under conditions of oxidative
stress was shown recently to induce macroautophagy of A?
and promote its localization in lysosomes (174), which may
reflect a stimulus early in the progression of AD to clear in-
tracellular levels of A?. The effects of oxidative stress on
lysosomal function in experimental models of AD have not
been directly tested, although as described later, treatment
with A? produces profound effects on lysosomal function
that may be related in part to the generation of oxidative
A?-Induced Neuron Death
Although it is obvious that a definite progression of neu-
ron loss occurs in AD, studies of AD brain have in large part
shown inconsistent findings regarding a role for apoptosis
as an important mechanism of neuron death (reviewed in
refs. 108 and 116). This variability of results may be explained
by the inherent heterogeneity of the human AD population
at the time of tissue biopsy and by differences in the pro-
cessing of postmortem tissue. In addition, an inherent chal-
lenge exists in proving with great confidence the relevance
of neuron death mechanisms in age-related neurodegenera-
tive disease, because only a small number of neurons suc-
cumb to cell death at any one time. Nevertheless, countless
studies focused on delineating the mechanisms of A?-medi-
ated neuron death as a contributing factor to neuron loss in
human AD brain.
Results of in vitro studies indicate a clear link between A?,
oxidative stress, and cell death (reviewed in ref. 108). Many
studies have shown that A?-induced cell death and apopto-
sis is mediated by oxidative stress (145), effects that in many
cases were inhibited on treatment with antioxidants (8).
Treatment with A? has been shown to increase free radical
production and markers of oxidative stress (66), and the in-
duction of oxidative stress has been shown to induce the in-
tracellular accumulation of A? (173). As discussed earlier,
the autophagy–lysosomal degradation pathway is a sensi-
tive target for oxidative stress–induced damage, and treat-
ment with soluble forms of A? at death-inducing concen-
trations has been shown to result in its intralysosomal
localization (46) and induction of intralysosomal oxidative
stress (46) and the induction of LMP (46). Other alterations
in lysosome function, including alterations in levels of CD,
AUTOPHAGY, ROS, AND LYSOSOMAL CELL DEATH 489
have also been reported after A? treatment (18). Although
many studies of A?-induced cell death suggest a role for
apoptosis (44), our laboratory previously showed that A?-
induced neuron death is Bax dependent but caspase inde-
pendent (127). Even though we observed activation of
caspase 3 after treatment with A?, neither inhibition of cas-
pase-3 nor the targeted genetic disruption of caspase-3 at-
tenuated A?-induced neuron death (127). Together, these
findings suggest that A? may play a significant role in ox-
idative stress–induced neuron death in AD brain, although
the role of caspase-dependent apoptosis is still controversial.
The likely disruption of the macroautophagy–lysosomal
degradation pathway in AD brain, however, suggests the po-
tential contribution of multiple types of neuron death, both
caspase dependent and independent, to AD neuropathology.
Regulation of Neuron Death in CD-Deficient Mice as a
Model of NCL/Batten Disease
NCL is a heterogeneous group of pediatric lysosomal stor-
age disorders known collectively as Batten disease. Clinical
features of NCL/Batten disease include seizures and pro-
gressive blindness, with eventual loss of motor control and
ultimate death (56). NCLs were classified originally by their
age at onset and include congenital (at birth), infantile (INCL,
within 1 year of birth); late infantile (LINCL, 2–4 years); ju-
venile (JNCL, 4–7 years); or the very rare adult form (ANCL).
Presently seven gene mutations are known in humans to
cause NCL (CLN1, CLN2, CLN3, CLN5, CLN6, CLN8, and
CD), which produce distinct biochemical alterations in lyso-
some function and are also defined by the type of storage
protein that accumulates as a result of lysosome dysfunction
A major focus of our laboratory is the study of CD defi-
ciency–induced neuron death as a model of lysosome dys-
function in congenital NCL/Batten disease. It was not until
2006 that human CD mutations were first reported in two
separate studies (135, 140). In one study, a complete loss of
CD function was reported in four patients with congenital
NCL (135), and these patients exhibited perinatal seizures
before dying by 2 weeks of age. In the other study, a partial
loss of CD enzymatic activity was observed in an adolescent
patient diagnosed with NCL-like symptoms at early school
age (135). Before the finding of CD mutations in humans,
however, sporadic mutations in CD resulting in a character-
istic NCL-like phenotype were reported in sheep (155) and
more recently in American bulldogs (6). In 1995, the effects
of experimental CD deficiency in mice were initially reported
in an attempt to characterize further the role of CD in lyso-
some function (118). CD deficiency was found to inhibit bulk
proteolysis, and CD-deficient mice died by postnatal day 26
of a plethora of morbidities including intestinal necrosis,
thromboembolia, and seizures (118).
An increase in seizures and blindness in CD-deficient mice
led to the subsequent analyses of brain function in these
mice, when it was determined that CD deficiency resulted
in robust neurodegeneration characterized by the massive
accumulation of lipofuscin-laden AVs (78) and significant
neuron loss (130). AVs in CD-deficient mice were found to
accumulate as early as postnatal day 8 (79), and their accu-
mulation has been shown to precede the induction of apop-
tosis (78), which occurs as early as postnatal day 16 (78). Al-
though a role for oxidative stress has not been directly ver-
ified in the induction of AV accumulation, the accumulation
of lipofuscin and likely inhibition of autophagy are strong
indicators that the overproduction of oxidative stress plays
a prominent role in CD deficiency–induced neuropathology,
and further studies are needed to confirm this. Previous
studies suggested that the induction of nitrosative stress in
CD-deficient brain accelerates CD deficiency–induced neu-
ropathology, arising potentially from an increase in nitric ox-
ide and peroxynitrite from microglial activation (95, 164).
Treatment of CD-deficient mice with inhibitors of nitric ox-
ide synthase attenuated the appearance of apoptotic neurons
but not neurons exhibiting AV accumulation and lacking
apoptotic morphology (95). In particular, such apoptotic neu-
rons were found in many cases to be localized adjacent to
neurons undergoing such “autophagic stress,” which led to
the hypothesis that cells undergoing autophagic neurode-
generation induced microglial activation, which in turn re-
sulted in nitric oxide–dependent apoptotic death of neigh-
boring neurons (95). This hypothesis, if correct, would
explain subsequent findings in our laboratory indicating in-
activation of prosurvival Akt and activation of proapoptotic
GSK-3? in CD-deficient neurons with a time course related
to that of apoptosis induction (160), along with findings in
our laboratory indicating inactivation of Akt and apoptosis
induction in cultured cells on treatment with peroxynitrite
Further to investigate the role of apoptosis in CD defi-
ciency–induced neuron death, we generated mice deficient
in both CD and the proapoptotic molecule Bax (130).
Whereas Bax deficiency clearly reduced the induction of
apoptosis following CD deficiency, no decrease in neuron
loss, neurodegeneration, or autofluorescent storage material
was found (130). Together these results suggest that although
CD deficiency induces apoptosis, the resultant lysosome dys-
function contributes to the induction of multiple types of
neuron death and that apoptosis plays a limited role in the
neurodegenerative phenotype induced by CD deficiency. Al-
though the relative contribution of apoptosis to neuron death
and neurodegeneration is obviously disease specific, it is
clear that in the present study of neuron death, a whole host
of cell-death mechanisms should be considered, including
apoptotic versus nonapoptotic, or with different types of
apoptosis that are either caspase dependent or independent.
Lysosome dysfunction is quickly emerging as a prominent
area of research in which to study potential mechanisms of
neuron death. Multiple types of neuron death delineated in
the literature appear to involve, at some level, the disruption
of lysosome function, and both the induction of oxidative
stress and altered autophagy signaling have the capacity to
regulate neuron death through the lysosome. Potential thera-
pies such as the phosphodiesterase inhibitor zaprinast,
the lysosomal “modulator” Z-Phe-Ala-diazomethylketone
(PADK), plecomacrolide antibiotics such as BafA1, calpain in-
hibitors, cathepsin inhibitors, and metal chelators (9, 18, 33,
67, 128, 129, 165) may act through their direct attenuation of
oxidative stress or indirect attenuation of oxidative stress–in-
PIVTORAIKO ET AL. 490
duced damage. In theory, this would promote the stabiliza-
tion of lysosome membranes and decrease the onset of LMP
in neurons. The use of these agents will undoubtedly receive
greater prominence in the near future as the lysosome in turn
receives greater attention as a therapeutic target in the onset
and progression of neurodegenerative disease.
We thank Angela Schmeckebier and Barry Bailey for expert
technical assistance in preparation of the manuscript. We also
thank the UAB Neuroscience Core Facilities (NS47466 and
NS57098) for technical assistance. This work is supported by
grants from the National Institutes of Health (NS35107 and
NS41962), a pilot grant from the UAB Alzheimer’s Disease Re-
search Center, and a VISN7 Career Development Award from
the Birmingham VA Medical Center.
A?, beta amyloid; AD, Alzheimer disease; AIF, apoptosis-
inducing factor; APP, amyloid precursor protein; Atg, au-
tophagy-related gene; AV, autophagic vacuole; BafA1,
bafilomycin A1; CB, cathepsin B; CD, cathepsin D; CMA,
chaperone-mediated autophagy; ER, endoplasmic reticulum;
FPIX, ferriprotoporphyrin IX; 4-HNE, 4-hydroxy-nonenal;
LMP, lysosomal membrane permeabilization; M6P, man-
nose-6-phosphate; NCLs, neuronal ceroid lipofuscinoses;
PLA2, phospholipase A2; ROS, reactive oxygen species;
TGN, trans-Golgi network.
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