Regulation of CNS synapses by neuronal MHC class I
C. Alex Goddard*, Daniel A. Butts†, and Carla J. Shatz‡
Department of Neurobiology, Harvard Medical School, Boston, MA 02115
Contributed by Carla J. Shatz, March 6, 2007 (sent for review January 9, 2007)
Until recently, neurons in the healthy brain were considered
immune-privileged because they did not appear to express MHC
class I (MHCI). However, MHCI mRNA was found to be regulated by
neural activity in the developing visual system and has been
detected in other regions of the uninjured brain. Here we show
that MHCI regulates aspects of synaptic function in response to
activity. MHCI protein is colocalized postsynaptically with PSD-95
in dendrites of hippocampal neurons. In vitro, whole-cell record-
ings of hippocampal neurons from ?2m/TAP1 knockout (KO) mice,
which have reduced MHCI surface levels, indicate a 40% increase
in mini-EPSC (mEPSC) frequency. mEPSC frequency is also increased
100% in layer 4 cortical neurons. Similarly, in KO hippocampal
cultures, there is a modest increase in the size of presynaptic
boutons relative to WT, whereas postsynaptic parameters (PSD-95
puncta size and mEPSC amplitude) are normal. In EM of intact
hippocampus, KO synapses show a corresponding increase in
vesicles number. Finally, KO neurons in vitro fail to respond
normally to TTX treatment by scaling up synaptic parameters.
Together, these results suggest that postsynaptically localized
MHCl acts in homeostatic regulation of synaptic function and
morphology during development and in response to activity block-
ade. The results also imply that MHCI acts retrogradely across the
synapse to translate activity into lasting change in structure.
homeostatic ? neuron ? plasticity ? synapsin ? PSD-95
activity remodels synaptic connections during development is
termed ‘‘activity-dependent plasticity,’’ in which electrical sig-
nals induce specific patterns of gene transcription to alter
synaptic properties and structural connectivity. Genes including
BDNF and CamKII are known to be critical for this plasticity
(2–4); however, many other molecules are likely involved as well.
MHC class I (MHCI) family members are well known for their
roles in cellular immunity, but a neuronal function has not been
generally appreciated. In the immune system, MHCI genes act
in concert with T cell receptors to discriminate self- versus
non-self-proteins. The CNS was considered ‘‘immune privi-
leged,’’ in part, because it was thought that healthy neurons do
not express MHCI protein (5, 6). Recently, MHCI gene family
members have been found at low levels in CNS neurons (7–11).
MHCI mRNA is expressed and regulated in cortical and tha-
lamic neurons during development and is down-regulated by
chronic activity blockade with Tetrodotoxin (TTX) in vivo (7).
MHCI is also a downstream target of the transcription factor
CREB, required for Hebbian synaptic plasticity (8, 12, 13).
MHCI is thus implicated in several forms of activity-dependent
The proteins encoded are heavy chains comprising the largest
portion of the MHCI protein complex. Functional MHCI is
usually a trimer consisting of the heavy chain, ?-2-microglobulin
(?2m), and a 9–11 aa peptide generated from proteosomal
degradation (15). The transporter associated with antigen pro-
cessing [(TAP) a heterodimer of TAP1 and TAP2] is required
for transport of peptide fragments from cytoplasm into the
lumen of the endoplasmic reticulum for assembly (16). For most
MHCI proteins, cell surface expression of heavy chain only
xperience transduced into neural activity is required for
proper brain development (1). The process by which neural
occurs if ?2m and peptide are present (16, 17). In their absence,
both surface and intracellular levels of MHCI are down-
regulated (18). Therefore, brains of mice deficient in ?2m and
TAP1 were studied here as MHCI ‘‘loss of function.’’ These mice
have altered Hebbian synaptic plasticity in the hippocampus and
abnormal patterning of visual system connections (19), reminis-
cent of animals that have undergone blockade of neural activity
(20–22). Despite this new appreciation of MHCI function in
neuronal plasticity and the discovery of a candidate neuronal
receptor (23, 24), however, it is not known whether MHCI
protein is present at CNS synapses or whether it is part of
molecular machinery regulating synaptic function and structure.
Here we investigate the subcellular localization of MHCI and
show that neurons with low levels of MHCI have altered synaptic
function and structure. Moreover, MHCI appears to play a role
in homeostatically regulating aspects of synaptic structure and
function in response to low levels of neural activity.
The subcellular localization of MHCI protein was examined by
immunostaining cultures of hippocampal neurons with a pan-
specific MHCI antibody, Ox18 (7, 25). Punctate immunostaining
is present in soma and dendrites (Fig. 1a); at higher magnifica-
tion, MHCI immunostaining is in spine-like dendritic protuber-
ances (Fig. 1b). To determine prior postsynaptic location for
MHCI, PSD-95 [present at postsynaptic densities of excitatory
synapses (26)] or synapsin [associated with presynaptic vesicles
(27)] was also detected immunofluorescently (Fig. 1b). Signal for
MHCI protein overlaps extensively with PSD-95 signal; 57% of
Ox18 immunoreactive pixels overlap with PSD-95 pixels (Fig.
1c). In contrast, the distribution of synapsin immunoreactivity is
one of close apposition and minimal overlap with MHCI (Fig. 1
b and c), suggesting that the two proteins are in separate,
adjacent pre- and postsynaptic compartments. Thus, MHCI
appears to be located postsynaptically at excitatory synapses,
consistent with a recent report of dendritic localization of MHCI
mRNAs in hippocampal neurons (28).
Given the presence of MHCI at synapses, as well as previously
reported alterations in activity-dependent plasticity in ?2m/
TAP1 knockout (KO) mice (19), it is possible that cultured KO
neurons have altered basal synaptic transmission. Spontaneous
mini-EPSCs (mEPSCs) from WT or KO hippocampal cultures
were recorded by using whole-cell patch-clamp (Fig. 2a). mEP-
SCs from WT neurons have a median instantaneous frequency
of 1.8 Hz and a median amplitude of 7.1 pA. In contrast, the
Author contributions: C.A.G., D.A.B., and C.J.S. designed research; C.A.G. performed re-
search; C.A.G. and D.A.B. contributed new reagents/analytic tools; C.A.G. analyzed data;
and C.A.G. and C.J.S. wrote the paper.
The authors declare no conflict of interest.
Abbreviations: KO, knockout; MHCI, MHC class I; PSD, postsynaptic densities.
*Present address: Department of Neurobiology, Stanford University, Stanford, CA 94305.
†Present address: Department of Physiology and Biophysics, Institute of Computational
Biomedicine, Weill Medical College of Cornell University, New York, NY 10021.
‡To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2007 by The National Academy of Sciences of the USA
April 17, 2007 ?
vol. 104 ?
We thank the Harvard Center for Neurodegeneration and Repair for
access to the Confocal Microscopy Core facility. This work was sup-
ported by National Institutes of Health Grant R01 MH071666 and the
Dana Foundation (to C.J.S.), a Goldenson Research Fellowship (to
D.A.B.), and National Institutes of Health Grant T32 MH20017 and a
Victoria and Stuart Quan fellowship (to C.A.G.).
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