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REVIEW
published: 25 August 2016
doi: 10.3389/fphys.2016.00361
Frontiers in Physiology | www.frontiersin.org 1August 2016 | Volume 7 | Article 361
Edited by:
Li Zuo,
Ohio State University, USA
Reviewed by:
Han-Zhong Feng,
Wayne State University School of
Medicine, USA
Fan Ye,
University of Florida, USA
Feng He,
California State University, Chico, USA
*Correspondence:
Iskandar Idris
iskandar.idris@nottingham.ac.uk
Philip J. Atherton
philip.atherton@nottingham.ac.uk
†Equal last authors.
Specialty section:
This article was submitted to
Striated Muscle Physiology,
a section of the journal
Frontiers in Physiology
Received: 24 May 2016
Accepted: 08 August 2016
Published: 25 August 2016
Citation:
Rudrappa SS, Wilkinson DJ,
Greenhaff PL, Smith K, Idris I and
Atherton PJ (2016) Human Skeletal
Muscle Disuse Atrophy: Effects on
Muscle Protein Synthesis, Breakdown,
and Insulin Resistance—A Qualitative
Review. Front. Physiol. 7:361.
doi: 10.3389/fphys.2016.00361
Human Skeletal Muscle Disuse
Atrophy: Effects on Muscle Protein
Synthesis, Breakdown, and Insulin
Resistance—A Qualitative Review
Supreeth S. Rudrappa, Daniel J. Wilkinson, Paul L. Greenhaff, Kenneth Smith,
Iskandar Idris *†and Philip J. Atherton *†
Division of Medical Sciences and Graduate Entry Medicine, School of Medicine, MRC-Arthritis Research UK Centre for
Musculoskeletal Ageing Research, Royal Derby Hospital, University of Nottingham, Derby, UK
The ever increasing burden of an aging population and pandemic of metabolic syndrome
worldwide demands further understanding of the modifiable risk factors in reducing
disability and morbidity associated with these conditions. Disuse skeletal muscle atrophy
(sometimes referred to as “simple” atrophy) and insulin resistance are “non-pathological”
events resulting from sedentary behavior and periods of enforced immobilization e.g.,
due to fractures or elective orthopedic surgery. Yet, the processes and drivers regulating
disuse atrophy and insulin resistance and the associated molecular events remain
unclear—especially in humans. The aim of this review is to present current knowledge
of relationships between muscle protein turnover, insulin resistance and muscle atrophy
during disuse, principally in humans. Immobilization lowers fasted state muscle protein
synthesis (MPS) and induces fed-state “anabolic resistance.” While a lack of dynamic
measurements of muscle protein breakdown (MPB) precludes defining a definitive role
for MPB in disuse atrophy, some proteolytic “marker” studies (e.g., MPB genes) suggest
a potential early elevation. Immobilization also induces muscle insulin resistance (IR).
Moreover, the trajectory of muscle atrophy appears to be accelerated in persistent IR
states (e.g., Type II diabetes), suggesting IR may contribute to muscle disuse atrophy
under these conditions. Nonetheless, the role of differences in insulin sensitivity across
distinct muscle groups and its effects on rates of atrophy remains unclear. Multifaceted
time-course studies into the collective role of insulin resistance and muscle protein
turnover in the setting of disuse muscle atrophy, in humans, are needed to facilitate the
development of appropriate countermeasures and efficacious rehabilitation protocols.
Keywords: skeletal muscle, disuse, immobilization, protein metabolism, diabetes
INTRODUCTION
Skeletal muscle tissue represents the largest protein/amino acid (AA) reservoir in the human
body (Bonaldo and Sandri, 2013). Skeletal muscles are not only crucial for locomotion but also
represent the body’s largest metabolically active tissue, glucose disposal site, and fuel reservoir
for other organs in fasting and pathological conditions (i.e., hepatic supply of amino acids for
gluconeogenesis). Loss of muscle mass occurs with many common illnesses (Evans, 2010) including
Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
cancers (Stephens et al., 2010), renal/heart failure (Gordon et al.,
2013), sepsis (Gordon et al., 2013), muscle genetic diseases
(Sandri, 2010), and neurodegenerative disorders (Verdijk et al.,
2012). Muscle atrophy also occurs in otherwise healthy
individuals under situations of reduced neural input—such as
during immobilization, e.g., due to leg casting for fractures
(Phillips et al., 2009), bed-rest, spinal cord injury (Castro et al.,
1999) and during “aging” (i.e., sarcopenia; Evans and Lexell,
1995). The main environmental determinants of muscle mass in
adulthood are exogenous essential amino acids (AA) (needing
to be acquired through dietary protein intake), Newton’s gravity
and locomotion (DeFronzo and Tripathy, 2009). Indicative of
this, lack of energy intake during starvation (Rennie et al., 2010),
inactivity (Wall et al., 2013), spaceflight (Vandenburgh et al.,
1999), or limb immobilization (Phillips et al., 2009) all lead to
a reduction in muscle cross sectional area (CSA), an associated
loss of function, and muscle insulin resistance. Crucially, loss of
muscle mass is associated with greater morbidity and mortality
(Sasaki et al., 2007), reduced independence, especially in older
populations (Leenders et al., 2013) and this is accelerated in type
2 diabetes (Leenders et al., 2013).
Overview of Disuse Atrophy,
Countermeasures, and Muscle Metabolism
Disuse atrophy is often referred to as “simple atrophy” in
that atrophy is intrinsic to the muscle(s) specifically exposed
to disuse; that is, disuse atrophy is not a systemic condition.
Countermeasures for disuse atrophy are limited but include
forms of mechanical loading (Wilkinson et al., 2008) such
as exercise/electrical stimulation (Wall et al., 2012), passive
physiotherapy (Fowles et al., 2000) and harnessing the adjunct
anabolic effects of protein nutrition (Churchward-Venne et al.,
2012b). Ascertaining an understanding of the mechanisms
of disuse atrophy—particularly in relation to the regulation
of muscle protein synthesis (MPS) and breakdown (MPB)—
is important for designing countermeasures or rehabilitation
protocols (Reggiani, 2015). Furthermore, despite accumulating
evidence that physical inactivity plays a causative role in
development of non-communicable diseases such as obesity,
insulin resistance, type 2 diabetes and dyslipidemia (Atherton
et al., 2016), the mechanistic role of muscle insulin resistance
(IR) in driving muscle atrophy in the context of “simple disuse”
remains unclear (Atherton et al., 2016). This review will focus on
identifying different models of human disuse atrophy, the degree
of muscle and strength loss and the regulation of muscle protein
turnover and muscle IR. Future translational studies to mitigate
disuse atrophy will rely upon robust evidence being present in
humans of active mechanisms (often of putative mechanisms that
have been pre-identified in animals). As such, this review will
focus mainly on current evidence from clinical studies.
The Impacts of Experimental Disuse on
Muscle Mass and Strength Loss in Humans
A plethora of clinical studies have investigated the degree of
muscle loss in humans exposed to disuse. The most frequent
employed models to study disuse atrophy in humans are
unilateral limb suspension (ULLS) using a knee brace or cast,
and bed rest; other scenarios include spinal cord injury and
spaceflight. In terms of muscle mass, the observed rate of decline
in muscle size (CSA) for each day of ULLS in knee extensors
was ∼0.40% and ∼0.36% for plantar flexors following 42 days
of unloading (Hackney and Ploutz-Snyder, 2012). Other studies
have demonstrated losses of muscle strength and mass early
on in disuse, i.e., 5 days of cast immobilization lead to ∼3.5%
reductions in quadriceps CSA and ∼9% in strength (Dirks et al.,
2014). This had progressed to ∼8% reductions in CSA and ∼23%
reductions in strength by 14 days (Wall et al., 2013). Additionally,
Suetta et al. reported ∼10% reductions in myofibre area and
∼13% decreases in strength after just 4 days progressing to ∼20%
reductions in myofibre area and strength after 14 days of ULLS
(Suetta et al., 2012, 2013). A further study reported decreases in
mid-thigh CSA of 11% following 28 days of bed rest (Brooks
et al., 2008). Lastly, a study by Castro et al. showed muscle
CSA to be ∼45% less compared to able-bodied controls 6-weeks
after complete spinal cord injury (Castro et al., 1999). Adding
to the above constellation, Gibson et al. studied men who were
immobilized following tibial fracture (thus having 6-weeks of
casting) and reported reductions in quadriceps CSA of ∼17%.
Furthermore, Alkner et al. reported that 90 days bed rest led
to ∼10 and ∼16% reductions in quadriceps and triceps surae
mass after 29 days, with rates of weekly loss slowing during
the last 2 months to roughly half that observed during the first
month (Alkner and Tesch, 2004). Finally, muscle CSA decreased
by ∼5% (de Boer et al., 2007; Glover et al., 2010) at 14 days
and 10%, at 23 days, i.e., 0.5% day following ULLS (de Boer
et al., 2007). Collectively, these studies indicate a varying degree
of rates of disuse muscle atrophy, depending on the duration
and nature of immobilization but also measurement techniques,
i.e., MRI/DXA/ultrasound/ myofibre CSA; however, it appears
atrophy occurs more rapidly in first 3–14 days of unloading and
eventually reaching a nadir where further loss of muscle occurs
at a slower rate despite continued unloading of muscle (Bodine,
2013).
Differences in the rate of muscle atrophy have also been
observed according to different muscle and fiber types as well
as the mode of immobilization. For example, after prolonged
disuse (∼180 days of space flight), loss of fiber size and force
was reported in the soleus and gastrocnemius muscles with the
order of atrophy (greatest-least) being: soleus type I >soleus type
II >gastrocnemius type I >gastrocnemius type II (Fitts et al.,
2010). Similar effects of disuse on fiber type following 35 days of
bed rest was reported in the vastus lateralis (VL) muscle, i.e., the
loss of fiber CSA was greater in type 1 than type II fibers (Brocca
et al., 2012). Conversely, muscle fiber type specificity has not been
observed in other studies (Bamman et al., 1998; Trappe et al.,
2008; Hvid et al., 2010) where duration of immobilization was
shorter (<14 days). It is notable that, these studies have mainly
focused on a single muscle with muscle biopsy taken from a
single site in the periphery of the muscle. This is relevant because
muscles do not atrophy uniformly across the entire length of a
single muscle (Miokovic et al., 2012), with differential atrophy
across different muscles being observed following 27–60 days
head-down-tilt bed rest. The investigators also reported that the
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Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
posterior calf muscles atrophied faster than the knee extensor
muscles (Vastus Lateralis) and ankle flexors (Tibialis anterior).
In another study where multiple muscles were examined using
MRI over 43 days disuse in the form of ankle immobilization,
the greatest rate of muscle loss was observed in soleus and
medial gastrocnemius muscle followed by lateral gastrocnemius
and tibialis anterior (Psatha et al., 2012). The aetiology driving
distinct fiber type, and individual muscle atrophy susceptibility is
poorly defined.
Regulation of Skeletal Muscle Mass in
Ambulated and Unloaded Human Muscle
Ambulated Regulation of MPS and MPB
Skeletal muscle mass is regulated by the balance between MPS
and MPB. Nutrients (i.e., AA) and nutrient derived hormones
(i.e., insulin) play a crucial role in regulating the balance
between MPS and MPB. In ambulated humans, intake of
dietary protein stimulates MPS due to the essential amino acids
(EAA) components of proteins (Atherton and Smith, 2012).
These anabolic responses are dose-dependent and saturable; at
a maximal stimulus, rates of MPS increase ∼200–300% for a
period of ∼2 h following ∼20 g protein (Cuthbertson et al.,
2005; Atherton and Smith, 2012). In contrast, insulin released
following intake of dietary protein and/or CHO, is neither
necessary nor sufficient to stimulate MPS (Greenhaff et al., 2008).
Reflecting this, the anti-catabolic effects of insulin upon MPB was
not recapitulated via AA infusions when insulin concentrations
were clamped at 5 µU.ml−1(post absorptive; Greenhaff et al.,
2008). Instead, insulin concentrations of just 15 IU/ml (3×post
absorptive) are sufficient to maximally suppress MPB (Wilkes
et al., 2009). This anti-catabolic effect of insulin acting on MPB
was confirmed in a recent systematic review and meta-analysis of
44 human studies, which concluded insulin did not significantly
affect MPS but has a crucial role in reducing MPB (Abdulla et al.,
2016). Therefore, while EAA’s stimulate MPS, insulin suppresses
MPB (and stimulates muscle glucose uptake). On the basis,
EAA and insulin are so vital in maintaining muscle metabolic
homeostasis, failure of these mechanisms inevitably leads to
skeletal muscle atrophy and IR.
Impact of Disuse on MPS and Anabolic Pathways
Disuse of human skeletal muscle alters muscle metabolism
dynamics (Mallinson and Murton, 2013). For instance, early
work by Gibson et al. showed that young men exposed to ULLS
exhibited ∼30% slower rates of fasted state MPS compared
to the contralateral non-immobilized limb (Gibson et al.,
1987). Subsequent studies confirmed reductions in MPS; for
instance, ∼50% reductions in MPS following 2-weeks of bed
rest (Ferrando et al., 1996; Paddon-Jones et al., 2004) and ULLS
with braces/casting (Glover et al., 2008). In further agreement
with this, Kortebein et al. reported ∼30% reductions in post-
absorptive MPS during a 24 h period in older adults after 10
days of bed rest (Kortebein et al., 2007). Crucially, blunting
of MPS in response to muscle unloading is not restricted to
fasted periods. Glover et al. reported that ULLS led to ∼27%
reductions in postprandial rates of MPS at both low and high
doses of AA infusions (Glover et al., 2008). Similarly, Drummond
et al. reported that 7 days of bed rest blunted fed state MPS
following EAA ingestion (Drummond et al., 2012). Similarly,
14 days of ULLS led to a ∼30% reduction in MPS after
ingestion of 20 g dietary protein (Wall et al., 2013). On this
basis, available evidence strongly supports the notion that skeletal
muscle atrophy in humans during a period of disuse is driven by
blunting of both postabsorptive and postprandial MPS (Rennie
et al., 2010; Wall et al., 2013).
The mTOR (mechanistic target of rapamycin) pathway is
the major signal transduction network “hub” involved in the
regulation of mRNA translation. Cell and rodent based research
suggests this system senses important stimuli responsible
for the regulation of MPS, i.e., (1) insulin and insulin-like
growth factor-1 (IGF-1) through IRS (insulin receptor substrate)
and PI3K (Phosphatidylinositol-3) pathways (Wackerhage and
Ratkevicius, 2008); (2) AA through leucyl-tRNA/Rag-mTORc1
pathways (Wackerhage and Ratkevicius, 2008), (3) Energy stress
through AMPK-eukaryotic elongation factor (AMP activated
protein kinase/ eEF2) pathway (Wackerhage and Ratkevicius,
2008), and (4) mechanical stress, e.g., through mechano-sensory
pathways (Wackerhage and Ratkevicius, 2008). However, the
impact of these factors in the regulation of MPS in human
remains unclear. For example, under ambulated conditions, one
of the mTOR upstream effector, the class III PI3K hVps34
(human vacuolar protein sorting-34) was shown to be inhibited
in amino acid starved state (basal condition) and increased
activity of this protein appears to be concomitant with increased
S6K1, suggesting it to be key player in mTOR signaling pathway
(Dickinson and Rasmussen, 2011). Furthermore, intake of
dietary protein typically leads to increase in phosphorylation
status of class III PI3K hVps34 (Dickinson and Rasmussen,
2011), mTOR and its downstream substrates regulating mRNA
translation, such as p70S6K (Drummond et al., 2012). However,
in response to disuse, blunted phosphorylation of mTOR and
p70S6K was shown after 2-weeks of immobilization while, no
significant increases in phosphorylation of mTOR and p70S6K
were noted after immobilization (Wall et al., 2013). In contrast,
others have reported no decreases in Akt/mTOR signaling despite
reductions in MPS (de Boer et al., 2007; Wackerhage and
Ratkevicius, 2008). It is worthwhile noting that peak stimulation
of signaling proteins (mTOR and p70S6K) occurs 1–2 h after
protein intake while many studies have muscle biopsies taken
3–6 h after protein intake when the response would be attenuated
or perhaps absent (Glover et al., 2010; West et al., 2011;
Churchward-Venne et al., 2012a; Drummond et al., 2012).
Further studies are needed with frequent biopsy sampling to fully
determine a role for deactivation in mTORC1-related signaling
networks (or indeed other putative mechanisms) regulating
depressions in post-absorptive and post-prandial MPS.
Impact of Disuse on MPB and Catabolic Pathways
In contrast to the recognized deficits in MPS during
immobilization, the role of MPB in disuse atrophy is less
clear. This is partly confounded by the technical and clinical
challenges in measuring in vivo rates of MPB (e.g., arterio-venous
balance methods to measure rate of appearance i.e., breakdown
and pulse chase stable isotope approaches to measure fractional
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Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
breakdown rates; Kumar et al., 2009; Atherton and Smith,
2012; Atherton et al., 2016). In the only study to our knowledge
to measure this, Symons et al. demonstrated no increase in
MPB in his study of healthy young volunteers exposed to 21
days of microgravity setting using a bed rest model (Symons
et al., 2009). Although not directly quantifying MPB, Wall
et al. reported that muscle free tracer enrichment over 4-h
post prandial period was >3 fold higher after immobilization
(Wall et al., 2013). Since MPS has consistently been shown to
be reduced with immobilization, a likely explanation is that
MPB is actually reduced (rather than increased) and hence
the less unlabeled phenylalanine efflux was diluting the muscle
free labeled tracer (L-(ring-2H5) phenylalanine pool (Wall
et al., 2013). Alternatively, an accumulation of free tracer could
simply be explained by established “anabolic resistance,” i.e.,
where a failure of AA incorporation into the muscle lead to its
accumulation (Glover et al., 2008; Phillips et al., 2009; Rennie,
2009; Wall et al., 2013). Nevertheless, regardless of the driving
force behind muscle atrophy (i.e., disuse, aging, cancer, organ
failure), blunted postabsorptive and postprandial MPS (Figure 1;
anabolic resistance) seem to be the major drivers of disuse
atrophy—rather than increases in MPB. Nonetheless, more work
is needed across the time-course of unloading to verify this.
In terms of the molecular regulation of MPB, the ubiquitin
proteasome system (UPS; Lecker et al., 2006) supplemented
by lysosomal and calcium activated calpain (ATP–independent)
and caspase dependent cleavage of actinomyosin complexes
(Glover et al., 2010) are the major catabolic pathways in
muscle. The identification of the “atrogenes” as genes that are
uniformly upregulated irrespective of the atrophy stimulus (e.g.,
denervation, disuse, thermal injury) has received much attention
as key drivers of atrophy programming (Bodine et al., 2001a;
Jones et al., 2004; Milan et al., 2015). This led to members
of the Forkhead Box (Fox) O family (Fox1, 3, and 4) being
identified as downstream targets of Akt pathway (Figure 2) and
as the main transcription factors regulating MAFbx/atrogin-
1expression (Sandri et al., 2004). In terms of disuse atrophy,
mRNA expression of two E3 ubiquitin ligases was initially found
to be crucial in immobilization, unloading and denervation
induced muscle atrophy (Bodine et al., 2001b). These genes,
MuRF-1 (Muscle Ring Finger-1), and MAFbx/atrogin-1 (Muscle
Atrophy F-box), are expressed in skeletal muscle at low levels
FIGURE 1 | Diagrammatic representation of the main mechanisms involved in disuse skeletal muscle atrophy in humans: Immobilization/disuse reduces
both postabsorptive and post prandial muscle protein synthesis (MPS) via the mammalian target of rapamycin (mTORC1) and Akt signaling. The role of MPS, muscle
protein breakdown (MPB) and insulin resistance (IR) in simple disuse atrophy remain poorly defined in humans. So the role of insulin resistance and MPB in the setting
of disuse atrophy needs further evaluation. Inflammation probably leads to IR. Recently, reactive oxygen species (ROS) has been implicated in development of muscle
atrophy in disuse setting, but the mechanism in human remains putative. Solid arrow shows positive association and broken arrow shows putative association. See
text for more details.
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Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
FIGURE 2 | Diagrammatic representation of the overlap between insulin signaling pathway, reactive oxygen species (ROS), inflammatory cytokine
such as NF-κB and ubiquitin-proteasome system in insulin resistant (IR) states particularly diabetes: In IR state, PI3K activity is decreased, leading to
decreased activity of Akt, which in turn release the inhibition of FOXO and caspase-3 resulting in elevation of muscle ring finger-1 (MuRF-1) and muscle atrophy F-box
(MAFbx/atrogin-1) finally leading to increased proteolytic activity. Also, ROS and low grade inflammation via NF-κB pathway lead to muscle atrophy. See text for more
details.
but rapidly induced in response to unloading (Bodine et al.,
2001b). In humans, after 5 days (Dirks et al., 2014) and 2
weeks (Jones et al., 2004) of immobilization, MAFbx and MuRF1
mRNA were reported to be elevated. Nonetheless, while their
expression is thought to be regulated by transcription factors
such as FOXO1, FOXO3a, (Sandri et al., 2004) and NFκβ (p50
and Bcl-3)(Wu et al., 2011), no increase in mRNA expression
in FOXO’s were noted after 4 or 14 days of immobilization
(Suetta et al., 2012). De Boer et al reported the expression of
MuRF-1 but not MAFbx mRNA was increased after 10 days
of ULLS (Jones et al., 2004; de Boer et al., 2007), while both
had decreased by 21 days (de Boer et al., 2007). Furthermore,
increases in UPP components (particularly UBE2E) were up-
regulated 48 h following instigation of ULLS (Urso et al., 2006).
In contrast, a recent study by Brocca et al. found that muscle
atrophy following ULLS found no change in mRNA expression
of ubiquitin-proteasome and autophagy systems (Brocca et al.,
2015). Some work has been done in relation to autophagy
(and calpain-signaling) in relation to human disuse (Jones
et al., 2004). Autophagy is responsible for removing unfolded,
damaged and dysfunctional proteins and organelles via the
formation of autophagosomes for degradation by lysosomes
(Sandri, 2010). Interestingly, up-regulation of autophagy markers
such as Beclin-1 suggested increased autophagosome formation
and hence a higher activity of the macro-autophagy by 24 days
of bed rest; nonetheless, other autophagy markers such as P62,
LC3II/I ratio, and cathepsin-L were not up-regulated (Brocca
et al., 2012). While details of the pathways discussed above
are outside the scope of this review, the readers are referred
to reviews by Bonaldo and Sandri (2013) and Sandri (2010).
What is clear however is that without more clinical studies with
time-course acquisition, including dynamic measures of MPB in
tandem with molecular markers spanning different proteolytic
systems, no firm conclusions can be made surrounding the mixed
results regarding whether existing molecular data suggest MPB
is increased, decreased or unchanged in response to disuse in
humans.
In addition to the anabolic and catabolic pathways described
above, emerging evidence indicates that disturbed redox
signaling may also be an important regulator of MPS and MPB
in muscle disuse atrophy (Powers et al., 2012; Zuo and Pannell,
2015). Oxidative injury has been shown to occur in muscle fibers
during periods of disuse in locomotor skeletal muscles (Min et al.,
2011) and in non-load bearing muscle such as the diaphragm
during prolonged mechanical ventilation (Kavazis et al., 2009).
After 35 days of bed rest, vastus lateralis muscle showed ∼18%
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Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
muscle fiber atrophy and increased protein carbonylation (Dalla
Libera et al., 2009). Furthermore, an inverse linear relationship
was observed between normalized levels of protein oxidation and
muscle fiber CSA (Dalla Libera et al., 2009). An analysis of gene
expression showed up-regulation of pathways involved in the
oxidative stress response (increase in mRNA for stress response
gene heme oxygenase-1) following 48 h of unilateral lower leg
suspension (ULLS; Reich et al., 2010). Conversely, in a limb
immobilization human model with ∼5.7% muscle and 11.8%
muscle fiber loss after 14 days of immobilization, no increase in
lipid peroxidation and protein oxidation in vastus lateralis was
observed (Glover et al., 2010). Although information on oxidative
stress and potential mechanisms explaining proteolysis in disuse
human muscle is still sparse (Pellegrino et al., 2011), these
findings support the extensive evidence available from animal
studies that oxidative stress inhibits MPS (Powers et al., 2011a)
and increases muscle MPB (via increased gene expression of key
proteins involved in the proteolytic pathways such as autophagy,
calpains and proteosomes; activation of both calpain and caspase-
3 and possibly by modification of myofibrillar proteins which
enhances their susceptibility to proteolytic processing; Powers
et al., 2011a). Interaction between ROS and insulin signaling
pathway has also been described, i.e., ROS may regulate Insulin
growth factor-1 (IGF-1) signaling either positively or negatively
depending on the amount of ROS produced (Papaconstantinou,
2010). Low levels of endogenous ROS due to their reversible
oxidative inhibition of protein tyrosine phosphatases induces
phosphorylation of tyrosine residue on the insulin receptor and
its substrates triggering IGF-1 signaling (Bashan et al., 2009).
In contrast, the IGF-1 signaling pathway is inhibited by higher
levels of ROS and recent evidence suggests ROS down regulates
the IGF-1 cascade and induces insulin resistance (Bashan et al.,
2009;Figure 2). For detailed discussion of the signaling pathways
linking ROS and muscle atrophy, the interested reader is referred
to recent reviews on oxidative stress and disuse muscle atrophy
(Pellegrino et al., 2011; Powers et al., 2011b, 2014; Zuo and
Pannell, 2015).
Impact of Disuse on Muscle IR and Links to Muscle
Mass in Persistent IR States
Insulin-mediated glucose uptake is also blunted with muscle
disuse (Mikines et al., 1991; Biensø et al., 2012); that is, unloaded
muscle becomes IR. This IR can be observed at a whole-body
level following bed-rest, but is most apparent at the muscle level
across the physiological range of insulin concentrations under
clamp conditions (Mikines et al., 1991). Recently, a 1 week bed-
rest study in young males by Dirks et al. revealed reduced muscle
mass (∼1.4 kg lean tissue and ∼3% quadriceps CSA) and whole-
body insulin sensitivity (∼29%)(Dirks et al., 2016). Thus, disuse
lowers MPS, induces anabolic resistance to nutrients and impairs
insulin-mediated muscle glucose uptake—even in healthy adults
(Fink et al., 1983).
The role of IR in driving muscle atrophy however is poorly
defined. Evidence from large cross-sectional and longitudinal
studies reports accelerated loss of muscle mass in individuals
with persistent IR (i.e., people with Type 2 Diabetes), perhaps
pointing to mechanistic links. For instance, declines in muscle
mass were inversely related to duration of diabetes or HbA1c
(Park et al., 2007, 2009; Kalyani et al., 2013) and attenuated
with insulin sensitizers (Kuo et al., 2009). Human muscle
tissue accounts for 80% of glucose uptake after food ingestion
and insulin resistance (HOMA-IR) is associated with reduced
quadriceps muscle strength (Kalyani et al., 2013; Leenders
et al., 2013), power (Kalyani et al., 2013) and muscle mass
(Leenders et al., 2013) in humans. Approximately a 50% more
rapid decline in knee extensor strength has been observed in
older patients with type 2 diabetes compared with patients
without diabetes over a 3 year period, suggesting that decreased
muscle strength may be accelerated in type 2 diabetes (Park
et al., 2007). In a further study, Volpato et al. reported
differences in walking speed, muscle strength, power and muscle
quality between individuals with and without diabetes were
independent of co-existing peripheral motor neuropathy or
peripheral vascular disease, suggesting a direct effect of diabetes
per se on muscle performance (Volpato et al., 2012). These
findings are important because in catabolic conditions such as
diabetes, atrophy in combination with reduced activity decrease
quality of life and increase mortality (Zinna and Yarasheski,
2003). Yet despite clear evidence linking accelerated muscle loss
in diabetes compared to non-diabetes, studies investigating the
direct effect of immobilization on muscle protein turnover in
patients with diabetes compared to those without diabetes are
scant. Furthermore, clear distinction between Type 1 and Type
2 diabetes needs to be made when investigating patients with
diabetes. This is because Type 1 diabetes is a condition with
severe depletion of energy stores and reduced mitochondrial
function resulting in accelerated muscle protein loss (Hebert
and Nair, 2010), which can be reversed by insulin replacement
(Workeneh and Bajaj, 2013). In contrast, muscle loss, whilst
accelerated in type 2 diabetes, is unaffected by insulin treatment
(Workeneh and Bajaj, 2013), possibly due to IR. Hence,
skeletal muscle mass loss whilst common, appears to occur less
predictably and to varying degree in Type 2 diabetes compared
with Type 1 diabetes (Workeneh and Bajaj, 2013). Collectively,
these data are consistent with the notion that diabetes causes
muscle mass loss possibly due to mechanisms driving muscle
IR, however there is lack of data regarding the effects of
immobilization or disuse on muscle mass in individuals with
diabetes.
The mechanistic regulation of muscle IR in driving muscle
atrophy in the setting of “simple disuse” remains vague (Atherton
et al., 2016). Early human studies by Shulman et al. showed
that, under steady state plasma concentration of both glucose
and insulin mimicking postprandial conditions, the mean rate
of muscle glycogen synthesis accounted for most of the whole
body glucose uptake and virtually all of non-oxidative glucose
metabolism in both healthy and diabetic subjects (Shulman,
2000), with defects in muscle glycogen synthesis playing a
major role in causing insulin resistance in type 2 diabetes
(Shulman, 2000). This may be explained by defects in the
insulin receptor substrate (IRS)-1/phosphatidylinositol (PI) 3-
kinase pathway, leading to reduced glucose uptake and utilization
in insulin target tissues (Draznin, 2006). Free fatty acids induce
muscle IR by inhibiting glucose transport/phosphorylation and
Frontiers in Physiology | www.frontiersin.org 6August 2016 | Volume 7 | Article 361
Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
reductions in both the rate of muscle glycogen synthesis and
glucose oxidation (Roden et al., 1996). Additionally, many
other mechanisms have been postulated to explain free fatty
acid-induced muscle IR, including the Randle cycle, oxidative
stress, inflammation and mitochondrial dysfunction (Martins
et al., 2012). Full details regarding above mechanisms escape
the scope of this article and readers are referred to a review
by Martins et al. (2012). With regard to disuse induced muscle
atrophy, following (7 days) bed-rest healthy volunteers showed
reduced glucose infusion rate and leg glucose extraction (after
bed rest) along with reduced muscle GLUT4, hexokinase II,
protein kinase B/Akt1, and Akt2 protein levels, and a tendency
for reduced 3-hydroxyacyl-CoA dehydrogenase activity (Biensø
et al., 2012). Further in the same study, the ability of insulin to
phosphorylate Akt and activate glycogen synthase was reduced
post bed-rest (Biensø et al., 2012); but whether this observation
is causative or a consequence of immobilization is not clear.
However, a substantial decline in glucose uptake within 24
h of immobilization would argue against a causative effect
(Atherton et al., 2016). Recently, Vigelso et al. showed an
inverse association between the increase in muscle pyruvate
dehydrogenase complex (PDC) activation and leg lactate
release during contraction after 2 weeks unilateral lower limb
immobilization, suggesting PDC as a potential key regulator
of immobilization induced muscle IR (Vigelsø et al., 2016).
Overall the above data suggests that muscle disuse results in
development of whole body and muscle specific IR, fuelling
the argument that lack of muscle contraction per se may be
the main physiological driver for this dysregulation, however a
mechanistic explanation for this still remains unclear (Atherton
et al., 2016).
CONCLUSIONS
Disuse muscle atrophy causes many undesirable complications.
There seems to be complex interplay of numerous mechanisms
contributing to the aetiology of disuse muscle atrophy. During
muscle disuse, both post-absorptive and post-prandial MPS is
suppressed, with little evidence to support there being an increase
in “bulk” protein MPB. Moreover, animal models show increased
(2.5 times) rate of muscle protein turnover and are also very
sensitive to disuse, while exhibiting marked fiber-type-dependant
differences in rates of muscle protein turnover (type I fibers being
twice as great as type II fibers) when compared to humans. Due
to these inherent species-specific differences, pre-clinical findings
cannot easily be reconciled with nor extrapolated to humans.
So, further research quantifying MPS and MPB and their
temporal relationship during disuse in humans is warranted.
There is strong evidence that type 2 diabetes accelerates muscle
loss, possibly due to mechanisms innate to diabetes. Crucially,
muscle IR secondary to disuse appears to drive the procession
of disuse muscle atrophy independent of other mechanisms
known to cause muscle IR. Nonetheless, the mechanistic role
of muscle IR driving this atrophic response is poorly defined.
Because, common proteolytic mechanisms may exist across
“simple muscle atrophy” and other catabolic conditions (e.g.,
type 2 diabetes, inflammation, cachexia etc.), these two process
can rarely be seen as being mutually exclusive (Atherton et al.,
2016). Further, many questions remain unanswered especially
the molecular regulation of MPS and MPB and muscle insulin
resistance. This whole area of research has potential implications
for the wider clinical community as similar metabolic processes
occur during cancer cachexia, metabolic syndromes including
type 2 diabetes, aging (i.e., sarcopenia), sepsis and many
neurodegenerative disorders. Henceforth, further translational
research is necessary before this knowledge can be effectively
applied in developing targeted strategies to prevent this in the
setting of disuse muscle atrophy.
AUTHOR CONTRIBUTIONS
All authors listed, have made substantial, direct and intellectual
contribution to the work, and approved it for publication.
ACKNOWLEDGMENTS
The first author is a doctoral research student funded through
the University of Nottingham within the MRC-ARUK Centre
for Musculoskeletal Ageing Research. The MRC-ARUK Centre
for Musculoskeletal Ageing Research was funded through
grants from the Medical Research Council [grant number
MR/K00414X/1] and Arthritis Research UK [grant number
19891] awarded to the Universities of Nottingham and
Birmingham.
REFERENCES
Abdulla, H., Smith, K., Atherton, P. J., and Idris, I. (2016). Role of insulin in
the regulation of human skeletal muscle protein synthesis and breakdown:
a systematic review and meta-analysis. Diabetologia 59, 44–55. doi: 10.1007/
s00125-015-3751-0
Alkner, B. A., and Tesch, P. A. (2004). Knee extensor and plantar flexor
muscle size and function following 90 days of bed rest with or without
resistance exercise. Eur. J. Appl. Physiol. 93, 294–305. doi: 10.1007/s00421-004-
1172-8
Atherton, P. J., Paul, L. G., Stuart, M. P., Sue, C. B., Christopher, M. A., and
Charles, H. L. (2016). Control of skeletal muscle atrophy in response to
disuse: clinical/preclinical contentions and fallacies of evidence. Am. J. Physiol.
Endocrinol. Metab. doi: 10.1152/ajpendo.00257.2016. [Epub ahead of print].
Atherton, P. J., and Smith, K. (2012). Muscle protein synthesis in response to
nutrition and exercise. J. Physiol. 590, 1049–1057. doi: 10.1113/jphysiol.2011.
225003
Bamman, M. M., Clarke, M. S., Feeback, D. L., Talmadge, R. J., Stevens, B. R.,
Lieberman, S. A., et al. (1998). Impact of resistance exercise during bed rest
on skeletal muscle sarcopenia and myosin isoform distribution. J. Appl. Physiol.
(1985) 84, 157–163.
Bashan, N., Kovsan, J., Kachko, I., Ovadia, H., and Rudich, A. (2009). Positive
and Negative regulation of insulin signaling by reactive oxygen and nitrogen
species. Physiol. Rev. 89, 27–71. doi: 10.1152/physrev.00014.2008
Biensø, R. S., Ringholm, S., Kiilerich, K., Aachmann-Andersen, N. J., Krogh-
Madsen, R., Guerra, B., et al. (2012). GLUT4 and Glycogen Synthase are key
players in bed rest-induced insulin resistance. Diabetes 61, 1090–1099. doi:
10.2337/db11-0884
Frontiers in Physiology | www.frontiersin.org 7August 2016 | Volume 7 | Article 361
Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
Bodine, S. C., Latres, E., Baumhueter, S., Lai, V. K., Nunez, L., Clarke, B. A., et al.
(2001a). Identification of ubiquitin ligases required for skeletal muscle atrophy.
Science 294, 1704–1708. doi: 10.1126/science.1065874
Bodine, S. C., Stitt, T. N., Gonzalez, M., Kline, W. O., Stover, G. L., Bauerlein,
R., et al. (2001b). Akt/mTOR pathway is a crucial regulator of skeletal muscle
hypertrophy and can prevent muscle atrophy in vivo.Nat. Cell Biol. 3,
1014–1019. doi: 10.1038/ncb1101-1014
Bodine, S. C. (2013). Hibernation: the search for treatments to prevent
disuse-induced skeletal muscle atrophy. Exp. Neurol. 248, 129–135. doi:
10.1016/j.expneurol.2013.06.003
Bonaldo, P., and Sandri, M. (2013). Cellular and molecular mechanisms of muscle
atrophy. Dis. Model. Mech. 6, 25–39. doi: 10.1242/dmm.010389
Brocca, L., Cannavino, J., Coletto, L., Biolo, G., Sandri, M., Bottinelli, R., et al.
(2012). The time course of the adaptations of human muscle proteome to
bed rest and the underlying mechanisms. J. Physiol. 590, 5211–5230. doi:
10.1113/jphysiol.2012.240267
Brocca, L., Longa, E., Cannavino, J., Seynnes, O., de Vito, G., McPhee, J., et al.
(2015). Human skeletal muscle fibre contractile properties and proteomic
profile: adaptations to 3 weeks unilateral lower limb suspension and active
recovery. J. Physiol. 24, 5361–5385. doi: 10.1113/jp271188
Brooks, N., Cloutier, G. J., Cadena, S. M., Layne, J. E., Nelsen, C. A., Freed, A.
M., et al. (2008). Resistance training and timed essential amino acids protect
against the loss of muscle mass and strength during 28 days of bed rest and
energy deficit. J. Appl. Physiol. (1985) 105, 241–248. doi: 10.1152/japplphysiol.
01346.2007
Castro, M. J., Apple, D. F. Jr., Staron, R. S., Campos, G. E., and Dudley, G. A.
(1999). Influence of complete spinal cord injury on skeletal muscle within 6 mo
of injury. J. Appl. Physiol. 86, 350–358.
Churchward-Venne, T. A., Burd, N. A., Mitchell, C. J., West, D. W., Philp, A.,
Marcotte, G. R., et al. (2012a). Supplementation of a suboptimal protein dose
with leucine or essential amino acids: effects on myofibrillar protein synthesis
at rest and following resistance exercise in men. J. Physiol. 590, 2751–2765. doi:
10.1113/jphysiol.2012.228833
Churchward-Venne, T. A., Burd, N. A., and Phillips, S. M. (2012b). Nutritional
regulation of muscle protein synthesis with resistance exercise: strategies to
enhance anabolism. Nutr. Metab. 9:40. doi: 10.1186/1743-7075-9-40
Cuthbertson, D., Smith, K., Babraj, J., Leese, G., Waddell, T., Atherton, P., et al.
(2005). Anabolic signaling deficits underlie amino acid resistance of wasting,
aging muscle. FASEB J. 19, 422–424. doi: 10.1096/fj.04-2640fje
Dalla Libera, L., Ravara, B., Gobbo, V., Tarricone, E., Vitadello, M., Biolo, G., et al.
(2009). A transient antioxidant stress response accompanies the onset of disuse
atrophy in human skeletal muscle. J. Appl. Physiol. (1985) 107, 549–557. doi:
10.1152/japplphysiol.00280.2009
de Boer, M. D., Selby, A., Atherton, P., Smith, K., Seynnes, O. R., Maganaris,
C. N., et al. (2007). The temporal responses of protein synthesis, gene
expression and cell signalling in human quadriceps muscle and patellar
tendon to disuse. J. Physiol. 585, 241–251. doi: 10.1113/jphysiol.2007.
142828
DeFronzo, R. A., and Tripathy, D. (2009). Skeletal muscle insulin resistance is the
primary defect in type 2 diabetes. Diabetes Care 32(Suppl. 2), 157–163. doi:
10.2337/dc09-S302
Dickinson, J. M., and Rasmussen, B. B. (2011). Essential amino acid sensing,
signaling, and transport in the regulation of human muscle protein metabolism.
Curr. Opin. Clin. Nutr. Metab. Care 14, 83–88. doi: 10.1097/MCO.0b013e3
283406f3e
Dirks, M. L., Wall, B. T., Snijders, T., Ottenbros, C. L., Verdijk, L. B., and van
Loon, L. J. (2014). Neuromuscular electrical stimulation prevents muscle disuse
atrophy during leg immobilization in humans. Acta Physiol. 210, 628–641. doi:
10.1111/apha.12200
Dirks, M. L., Wall, B. T., van de Valk, B., Holloway, T. M., Holloway, G. P.,
Chabowski, A., et al. (2016). One week of bed rest leads to substantial muscle
atrophy and induces whole-body insulin resistance in the absence of skeletal
muscle lipid accumulation. Diabetes. doi: 10.2337/db15-1661. [Epub ahead of
print].
Draznin, B. (2006). Molecular mechanisms of insulin resistance: serine
phosphorylation of insulin receptor substrate-1 and increased expression of
p85alpha: the two sides of a coin. Diabetes 55, 2392–2397. doi: 10.2337/db06-
0391
Drummond, M. J., Dickinson, J. M., Fry, C. S., Walker, D. K., Gundermann, D.
M., Reidy, P. T., et al. (2012). Bed rest impairs skeletal muscle amino acid
transporter expression, mtorc1 signaling, and protein synthesis in response to
essential amino acids in older adults. Am. J. Physiol. Endocrinol. Metab. 302,
E1113–E1122. doi: 10.1152/ajpendo.00603.2011
Evans, W. J. (2010). Skeletal muscle loss: cachexia, sarcopenia, and inactivity. Am.
J. Clin. Nutr. 91, 1123S–1127S. doi: 10.3945/ajcn.2010.28608A
Evans, W. J., and Lexell, J. (1995). Human aging, muscle mass, and fiber
type composition. J. Gerontol. Series A Biol. Sci. Med. Sci. 50, 11–16. doi:
10.1093/gerona/50A.Special_Issue.11
Ferrando, A. A., Lane, H. W., Stuart, C. A., Davis-Street, J., and Wolfe, R. R. (1996).
Prolonged bed rest decreases skeletal muscle and whole body protein synthesis.
Am. J. Physiol. Endocrinol. Metab. 270, E627–E633.
Fink, R. I., Kolterman, O. G., Griffin, J., and Olefsky, J. M. (1983). Mechanisms of
insulin resistance in ageing. J. Clin. Invest. 71, 1523–1535.
Fitts, R. H., Trappe, S. W., Costill, D. L., Gallagher, P. M., Creer, A. C., Colloton,
P. A., et al. (2010). Prolonged space flight-induced alterations in the structure
and function of human skeletal muscle fibres. J. Physiol. 588, 3567–3592. doi:
10.1113/jphysiol.2010.188508
Fowles, J. R., MacDougall, J. D., Tarnopolsky, M. A., Sale, D. G., Roy, B. D., and
Yarasheski, K. E. (2000). The effects of acute passive stretch on muscle protein
synthesis in humans. Can. J. Appl. Physiol. 25, 165–180. doi: 10.1139/h00-012
Gibson, J. N., Halliday, D., Morrison, W. L., Stoward, P. J., Hornsby, G. A.,
Watt, P. W., et al. (1987). Decrease in human quadriceps muscle protein
turnover consequent upon leg immobilization. Clin. Sci. 72, 503–509. doi:
10.1042/cs0720503
Glover, E. I., Phillips, S. M., Oates, B. R., Tang, J. E., Tarnopolsky, M. A., Selby, A.,
et al. (2008). Immobilization induces anabolic resistance in human myofibrillar
protein synthesis with low and high dose amino acid infusion. J. Physiol. 586,
6049–6061. doi: 10.1113/jphysiol.2008.160333
Glover, E. I., Yasuda, N., Tarnopolsky, M. A., Abadi, A., and Phillips, S. M. (2010).
Little change in markers of protein breakdown and oxidative stress in humans
in immobilization-induced skeletal muscle atrophy. Appl. Physiol. Nutr. Metab.
35, 125–133. doi: 10.1139/H09-137
Gordon, B. S., Kelleher, A. R., and Kimball, S. R. (2013). Regulation of muscle
protein synthesis and the effects of catabolic states. Int. J. Biochem. Cell Biol.
45, 2147–2157. doi: 10.1016/j.biocel.2013.05.039
Greenhaff, P. L., Karagounis, L. G., Peirce, N., Simpson, E. J., Hazell, M., Layfield,
R., et al. (2008). Disassociation between the effects of amino acids and
insulin on signaling, ubiquitin ligases, and protein turnover in human muscle.
Am. J. Physiol. Endocrinol. Metab. 295, E595–E604. doi: 10.1152/ajpendo.
90411.2008
Hackney, K. J., and Ploutz-Snyder, L. L. (2012). Unilateral lower limb suspension:
integrative physiological knowledge from the Past 20 Years (1991-2011). Eur. J.
Appl. Physiol. 112, 9–22. doi: 10.1007/s00421-011-1971-7
Hebert, S. L., and Nair, K. S. (2010). Protein and energy metabolism in type 1
diabetes. Clin. Nutr. 29, 1–11. doi: 10.1016/j.clnu.2009.09.001
Hvid, L., Aagaard, P., Justesen, L., Bayer, M. L., Andersen, J. L., Ørtenblad,
N., et al. (2010). Effects of aging on muscle mechanical function and
muscle fiber morphology during short-term immobilization and subsequent
retraining. J. Appl. Physiol. 109, 1628–1634. doi: 10.1152/japplphysiol.
00637.2010
Jones, S. W., Hill, R. J., Krasney, P. A., O’Conner, B., Peirce, N., and Greenhaff,
P. L. (2004). Disuse atrophy and exercise rehabilitation in humans profoundly
affects the expression of genes associated with the regulation of skeletal muscle
mass. FASEB J. 18, 1025–1027. doi: 10.1096/fj.03-1228fje
Kalyani, R. R., Tra, Y., Yeh, H. C., Egan, J. M., Ferrucci, L., and Brancati,
F. L. (2013). Quadriceps strength, quadriceps power, and gait speed in
older, u.s. adults with diabetes mellitus: results from the national health and
nutrition examination survey, 1999-2002. J. Am. Geriatr. Soc. 61, 769–775. doi:
10.1111/jgs.12204
Kavazis, A. N., Talbert, E. E., Smuder, A. J., Hudson, M. B., Nelson, W. B.,
and Powers, S. K. (2009). Mechanical ventilation induces diaphragmatic
mitochondrial dysfunction and increased oxidant production. Free Radic. Biol.
Med. 46, 842–850. doi: 10.1016/j.freeradbiomed.2009.01.002
Kortebein, P., Ferrando, A., Lombeida, J., Wolfe, R., and Evans, W. J. (2007). Effect
of 10 days of bed rest on skeletal muscle in healthy older adults. JAMA 297,
1772–1774. doi: 10.1001/jama.297.16.1772-b
Frontiers in Physiology | www.frontiersin.org 8August 2016 | Volume 7 | Article 361
Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
Kumar, V., Atherton, P., Smith, K., and Rennie, M. J. (2009). Human muscle
protein synthesis and breakdown during and after exercise. J. Appl. Physiol. 106,
2026–2039. doi: 10.1152/japplphysiol.91481.2008
Kuo, C. K., Lin, L. Y., Yu, Y. H., Wu, K. H., and Kuo, H. K. (2009). Inverse
association between insulin resistance and gait speed in nondiabetic older
men: results from the U.S. National Health and Nutrition Examination Survey
(NHANES) 1999-2002. BMC Geriatrics 9:49. doi:10.1186/1471-2318-9-49
Lecker, S. H., Goldberg, A. L., and Mitch, W. E. (2006). Protein degradation by
the ubiquitin-proteasome pathway in normal and disease states. J. Am. Soc.
Nephrol. 17, 1807–1819. doi: 10.1681/ASN.2006010083
Leenders, M., Verdijk, L. B., van der Hoeven, L., Adam, J. J., van Kranenburg, J.,
Nilwik, R., et al. (2013). Patients with type 2 diabetes show a greater decline in
muscle mass, muscle strength, and functional capacity with aging. J. Am. Med.
Dir. Assoc. 14, 585–592. doi: 10.1016/j.jamda.2013.02.006
Mallinson, J. E., and Murton, A. J. (2013). Mechanisms responsible for
disuse muscle atrophy: potential role of protein provision and exercise as
countermeasures. Nutrition 29, 22–28. doi: 10.1016/j.nut.2012.04.012
Martins, A. R., Nachbar, R. T., Gorjao, R., Vinolo, M. A., Festuccia, W. T.,
Lambertucci, R. H., et al. (2012). Mechanisms underlying skeletal muscle
insulin resistance induced by fatty acids: importance of the mitochondrial
function. Lipids Health Dis. 11:30. doi: 10.1186/1476-511X-11-30
Mikines, K. J., Richter, E. A., Dela, F., and Galbo, H. (1991). Seven days of bed
rest decrease insulin action on glucose uptake in leg and whole body. J. Appl.
Physiol. 70, 1245–1254.
Milan, G., Romanello, V., Pescatore, F., Armani, A., Paik, J. H., Frasson, L., et al.
(2015). Regulation of autophagy and the ubiquitin-proteasome system by the
foxo transcriptional network during muscle atrophy. Nat. Commun. 6:6670.
doi: 10.1038/ncomms7670
Min, K., Smuder, A. J., Kwon, O. S., Kavazis, A. N., Szeto, H. H., and Powers,
S. K. (2011). Mitochondrial-Targeted antioxidants protect skeletal muscle
against immobilization-induced muscle atrophy. J. Appl. Physiol. (1985) 111,
1459–1466. doi: 10.1152/japplphysiol.00591.2011
Miokovic, T., Armbrecht, G., Felsenberg, D., and Belavý, D. L. (2012).
Heterogeneous atrophy occurs within individual lower limb muscles during
60 days of bed rest. J. Appl. Physiol. 113, 1545–1559. doi: 10.1152/japplphysiol.
00611.2012
Paddon-Jones, D., Sheffield-Moore, M., Zhang, X. J., Volpi, E., Wolf, S. E.,
Aarsland, A., et al. (2004). Amino acid ingestion improves muscle protein
synthesis in the young and elderly. Am. J. Physiol. Endocrinol. Metab. 286,
E321–E328. doi: 10.1152/ajpendo.00368.2003
Papaconstantinou, J. (2010). Insulin/IGF-1 and ROS signaling pathway cross-talk
in aging and longevity determination. Mol. Cell Endocrinol. 299, 89–100. doi:
10.1016/j.mce.2008.11.025
Park, S. W., Goodpaster, B. H., Lee, J. S., Kuller, L. H., Boudreau, R., de Rekeneire,
N., et al. (2009). Excessive loss of skeletal muscle mass in older adults with type
2 diabetes. Diabetes Care 32, 1993–1997. doi: 10.2337/dc09-0264
Park, S. W., Goodpaster, B. H., Strotmeyer, E. S., Kuller, L. H., Broudeau, R.,
Kammerer, C., et al. (2007). Accelerated loss of skeletal muscle strength in older
adults with type 2 diabetes: the health, aging, and body composition study.
Diabetes Care 30, 1507–1512. doi: 10.2337/dc06-2537
Pellegrino, M. A., Desaphy, J. F., Brocca, L., Pierno, S., Camerino, D. C., and
Bottinelli, R. (2011). Redox homeostasis, oxidative stress and disuse muscle
atrophy. J. Physiol. 589, 2147–2160. doi: 10.1113/jphysiol.2010.203232
Phillips, S. M., Glover, E. I., and Rennie, M. J. (2009). Alterations of protein
turnover underlying disuse atrophy in human skeletal muscle. J. Appl. Physiol.
(1985) 107, 645–654. doi: 10.1152/japplphysiol.00452.2009
Powers, S. K., Smuder, A. J., and Criswell, D. S. (2011a). Mechanistic links
between oxidative stress and disuse muscle atrophy. Antioxid. Redox Signal. 15,
2519–2528. doi: 10.1089/ars.2011.3973
Powers, S. K., Smuder, A. J., and Judge, A. R. (2012). Oxidative stress and disuse
muscle atrophy: cause or consequence? Curr. Opin. Clin. Nutr. Metab. Care 15,
240–245. doi: 10.1097/MCO.0b013e328352b4c2
Powers, S. K., Talbert, E. E., and Adhihetty, P. J. (2011b). Reactive oxygen and
nitrogen species as intracellular signals in skeletal muscle. J. Physiol. 589,
2129–2138. doi: 10.1113/jphysiol.2010.201327
Powers, S. K., Kavazi, A. N., and McClung, J. M. (2014). Oxidative stress and disuse
muscle atrophy. J. Appl. Physiol. 102, 2389–2397. doi: 10.1152/japplphysiol.
01202.2006
Psatha, M., Wu, Z., Gammie, F. M., Ratkevicius, A., Wackerhage, H., Lee, J. H.,
et al. (2012). A longitudinal mri study of muscle atrophy during lower leg
immobilization following ankle fracture. J. Magn. Reson. Imaging 35, 686–695.
doi: 10.1002/jmri.22864
Reggiani, C. (2015). Not all disuse protocols are equal: new insight into the
signalling pathways to muscle atrophy. J. Physiol. 593, 5227–5228. doi:
10.1113/JP271613
Reich, K. A., Chen, Y. W., Thompson, P. D., Hoffman, E. P., and Clarkson, P. M.
(2010). Forty-Eight hours of unloading and 24 h of reloading lead to changes
in global gene expression patterns related to ubiquitination and oxidative
stress in humans. J. Appl. Physiol. 109, 1404–1415. doi: 10.1152/japplphysiol.
00444.2010
Rennie, M. J. (2009). Anabolic resistance: the effects of aging, sexual dimorphism,
and immobilization on human muscle protein turnover. Appl. Physiol. Nutr.
Metab. 34, 377–381. doi: 10.1139/H.09-012
Rennie, M. J., Selby, A., Atherton, P., Smith, K., Kumar, V., Glover, E. L., et al.
(2010). Facts, noise and wishful thinking: muscle protein turnover in aging and
human disuse atrophy. Scand. J. Med. Sci. Sports 20, 5–9. doi: 10.1111/j.1600-
0838.2009.00967.x
Roden, M., Price, T. B., Perseghin, G., Petersen, K. F., Rothman, D. L., Cline, G. W.
et al. (1996). Mechanism of free fatty acid-induced insulin resistance in humans.
J.Clin.Invest. 97, 2859–65. doi: 10.1172/JCI118742
Sandri, M. (2010). Autophagy in skeletal muscle. FEBS Lett. 584, 1411–1416. doi:
10.1016/j.febslet.2010.01.056
Sandri, M., Sandri, C., Gilbert, A., Skurk, C., Calabria, E., Picard, A., et al.
(2004). Foxo transcription factors induce the atrophy- related ubiquitin
ligase atrogin-1 and cause skeletal muscle atrophy. Cell 117, 1–2. doi:
10.1016/j.jsbmb.2011.07.002.Identification
Sasaki, H., Kasagi, F., Yamada, M., and Fujita, S. (2007). Grip strength predicts
cause-specific mortality in middle-aged and elderly persons. Am. J. Med. 120,
337–342. doi: 10.1016/j.amjmed.2006.04.018
Shulman, G. I. (2000). Cellular mechanisms of insulin resistance. J. Clin. Invest.
106, 171–176. doi: 10.1172/JCI10583
Stephens, N. A., Gallagher, I. J., Rooyackers, O., Skipworth, R. J., Tan, B. H.,
Marstrand, T., et al. (2010). Using transcriptomics to identify and validate novel
biomarkers of human skeletal muscle cancer cachexia. Genome Med. 2:122. doi:
10.1186/gm122
Suetta, C., Frandsen, U., Jensen, L., Jensen, M. M., Jespersen, J. G., Hvid,
L. G., et al. (2012). Aging affects the transcriptional regulation of human
skeletal muscle disuse atrophy. PLoS ONE 7:e51238. doi: 10.1371/journal.pone.
0051238
Suetta, C., Frandsen, U., Mackey, A. L., Jensen, L., Hvid, L. G., Bayer, M. L.,
et al. (2013). Ageing is associated with diminished muscle re-growth and
myogenic precursor cell expansion early after immobility-induced atrophy in
human skeletal muscle. J. Physiol. 591, 3789–3804. doi: 10.1113/jphysiol.2013.
257121
Symons, T. B., Sheffield-Moore, M., Chinkes, D. L., Ferrando, A. A., and Paddon-
Jones, D. (2009). Artificial gravity maintains skeletal muscle protein synthesis
during 21 days of simulated microgravity. J. Appl. Physiol. (1985) 107, 34–38.
doi: 10.1152/japplphysiol.91137.2008
Trappe, S., Creer, A., Minchev, K., Slivka, D., Louis, E., Luden, N., et al.
(2008). Human soleus single muscle fiber function with exercise or nutrition
countermeasures during 60 days of bed rest. Am. J. Physiol. 294, R939–R947.
doi: 10.1152/ajpregu.00761.2007
Urso, M. L., Scrimgeour, A. G., Chen, Y. W., Thompson, P. D., and Clarkson, P.
M. (2006). Analysis of human skeletal muscle after 48h immobilization reveals
alterations in mrna and protein for extracellular matrix components. J. Appl.
Physiol. 101, 1136–1148. doi: 10.1152/japplphysiol.00180.2006
Vandenburgh, H., Chromiak, J., Shansky, J., Del Tatto, M., and Lemaire, J. (1999).
Space travel directly induces skeletal muscle atrophy. FASEB J. 13, 1031–1038.
Verdijk, L. B., Dirks, M. L., Snijders, T., Prompers, J. J., Beelen, M., Jonkers,
R. A., et al. (2012). Reduced satellite cell numbers with spinal cord
injury and aging in humans. Med. Sci. Sports Exerc. 44, 2322–2330. doi:
10.1249/MSS.0b013e3182667c2e
Vigelsø, A., Gram, M., Dybboe, R., Kuhlman, A. B., Prats, C., Greenhaff, P. L.,
et al. (2016). The effect of age and unilateral leg immobilization for 2 weeks
on substrate utilization during moderate-intensity exercise in human skeletal
muscle. J. Physiol. 594, 2339–2358. doi: 10.1113/JP271712
Frontiers in Physiology | www.frontiersin.org 9August 2016 | Volume 7 | Article 361
Rudrappa et al. Mechanisms of Disuse Atrophy in Humans
Volpato, S., Bianchi, L., Lauretani, F., Lauretani, F., Bandinelli, S., Guralnik, J. M.,
et al. (2012). Role of muscle mass and muscle quality in the association between
diabetes and gait speed. Diabetes Care 35, 1672–1679. doi: 10.2337/dc11-2202
Wackerhage, H., and Ratkevicius, A. (2008). Signal transduction pathways
that regulate muscle growth. Essays Biochem. 44, 99–108. doi: 10.1042/BS
E0440099
Wall, B. T., Dirks, M. L., Verdijk, L. B., Snijders, T., Hansen, D., Vranckx, P.,
et al. (2012). Neuromuscular electrical stimulation increases muscle protein
synthesis in elderly type 2 diabetic men. Am. J. Physiol. 303, E614–E623. doi:
10.1152/ajpendo.00138.2012
Wall, B. T., Snijders, T., Senden, J. M., Ottenbros, C. L., Gijsen, A. P., Verdijk,
L. B., et al. (2013). Disuse impairs the muscle protein synthetic response to
protein ingestion in healthy men. J. Clin. Endocrinol. Metab. 98, 4872–4881.
doi: 10.1210/jc.2013-2098
West, D. W., Burd, N. A., Coffey, V. G., Baker, S. K., Burke, L. M., Hawley, J.
A., et al. (2011). Rapid aminoacidemia enhances myofibrillar protein synthesis
and anabolic intramuscular signaling responses after resistance exercise. Am. J.
Clin.Nutr. 94, 795–803. doi: 10.3945/ajcn.111.013722
Wilkes, E. A., Selby, A. L., Atherton, P. J., Patel, R., Rankin, D., Smith, K., et al.
(2009). Blunting of insulin inhibition of proteolysis in legs of older subjects
may contribute to age-related sarcopenia. Am. J. Clin. Nutr. 90, 1343–1350. doi:
10.3945/ajcn.2009.27543
Wilkinson, S. B., Phillips, S. M., Atherton, P. J., Patel, R., Yarasheski, K.
E., Tarnopolsky, M. A., et al. (2008). Differential effects of resistance and
endurance exercise in the fed state on signalling molecule phosphorylation
and protein synthesis in human muscle. J. Physiol. 586, 3701–3717. doi:
10.1113/jphysiol.2008.153916
Workeneh, B., and Bajaj, M. (2013). The regulation of muscle protein turnover
in diabetes. Int. J. Biochem. Cell Biol. 45, 2239–2244. doi: 10.1016/j.biocel.
2013.06.028
Wu, C. L., Kandarian, S. C., and Jackman, R. W. (2011). Identification of genes
that elicit disuse muscle atrophy via the transcription factors p50 and bcl-3.
PloS ONE 6:e16171. doi: 10.1371/journal.pone.0016171
Zinna, E. M., and Yarasheski, K. E. (2003). Exercise treatment to counteract protein
wasting of chronic diseases. Curr. Opin. Clin. Nutr. Metab. Care 6, 87–93. doi:
10.1097/00075197-200301000-00013
Zuo, L., and Pannell, B. K. (2015). Redox characterization of functioning skeletal
muscle. Front. Physiol. 6:338. doi: 10.3389/fphys.2015.00338
Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2016 Rudrappa, Wilkinson, Greenhaff, Smith, Idris and Atherton. This
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