Pathophysiological changes of muscle after ischemic stroke: a secondary consequence of stroke injury

Hu Qi ,Dan Tian ,Fei Luan Ruocong Yang,Nan Zeng

Abstract Sufficient clinical evidence suggests that the damage caused by ischemic stroke to the body occurs not only in the acute phase but also during the recovery period,and that the latter has a greater impact on the long-term prognosis of the patient.However,current stroke studies have typically focused only on lesions in the central nervous system,ignoring secondary damage caused by this disease.Such a phenomenon arises from the slow progress of pathophysiological studies examining the central nervous system.Further,the appropriate therapeutic time window and benefits of thrombolytic therapy are still controversial,leading scholars to explore more pragmatic intervention strategies.As treatment measures targeting limb symptoms can greatly improve a patient’s quality of life,they have become a critical intervention strategy.As the most vital component of the limbs,skeletal muscles have become potential points of concern.Despite this,to the best of our knowledge,there are no comprehensive reviews of pathophysiological changes and potential treatments for post-stroke skeletal muscle.The current review seeks to fill a gap in the current understanding of the pathological processes and mechanisms of muscle wasting atrophy,inflammation,neuroregeneration,mitochondrial changes,and nutritional dysregulation in stroke survivors.In addition,the challenges,as well as the optional solutions for individualized rehabilitation programs for stroke patients based on motor function are discussed.

Key Words: inflammation;ischemic stroke;mitochondria;muscle atrophy;muscle fiber;muscle nutrition;quality of life;rehabilitation;ubiquitin

Introduction

Ischemic stroke is caused by an interruption to cerebral blood flow due to blood clots (Scherbakov et al.,2013).Stroke is characterized by high morbidity,mortality,and disability;has imposed a huge economic and psychological burden on countless patients;and has impeded the positive development of society (Kamalian and Lev,2019).Presently,there are several misconceptions about stroke.Most studies on the subject have only focused on the solutions to central problems in the field,which are to reduce neuroinflammation and enhance neuroprotection of stroke patients in the acute pathological phase,but ignore the impact of stroke on pathological changes in the periphery (Ferriero et al.,2019).Post-stroke pathological symptoms can be observed not only in the acute phase but also in the recovery phase.

Stroke is a complex systemic lesion that not only affects the brain but can also affect multiple other organs and tissues.As such,changes in limb muscle function after stroke can strongly impact prognosis and quality of life in patients (Scherbakov et al.,2013;Shao et al.,2022;Liu et al.,2023).Even with timely acute treatment after stroke,two-thirds of patients do not fully recover (Kamalian and Lev,2019).Previous stroke studies have largely ignored secondary damage in the periphery of the body.Following stroke,muscle issues exert the greatest influence on post-stroke limb symptoms.In fact,muscle is one of the most crucial effectors supporting motor function.To the best of our knowledge,pathophysiological studies of skeletal muscle after stroke are still lacking (Haruyama et al.,2017).The specific mechanisms underlying muscle changes after stroke remain to be elucidated,which could limit the development of rehabilitation treatment and disease prognosis for limb symptoms after stroke.Therefore,the current review examines recent research progress on disuse atrophy of muscles,reinnervation,inflammation,changes in muscle fiber type,mitochondrial changes,and nutritional supply after stroke.This review also discusses the current limitations of rehabilitation for stroke patients and how the development of individualized rehabilitation in a multidimensional manner,which considers the patient’s unique circumstances,is vital for effective treatment.

Search Strategy and Selection Criteria

PubMed (https://www.ncbi.nlm.nih.gov/pubmed) and Web of Science (https://www.webofscience.com/wos) were used to collect relevant papers published from 2010 to 2022.The search strategy was set as follows: “(((rehabilitation) OR (skeletal muscle)) OR (muscle atrophy)) AND (ischemic stroke)” or “(TS=(ischemic stroke)) AND (TS=(skeletal muscle) OR TS=(muscle atrophy) OR TS=(rehabilitation))”.The collected literature was sorted and filtered,and articles that were highly consistent with the described topic were selected.

Muscle Changes after Stroke

Skeletal muscle is the key effector of limb movement (Konopka and Harber,2014);however,European guidelines (Kernan et al.,2014) do not provide guidance related to muscle atrophy,muscle loss,and decreased muscle function (Hafer-Macko et al.,2008) following stroke.At present,substantial progress has been made in the pathological study of limb muscles after stroke (Okada and Suzuki,2020).Clinically,muscle loss and atrophy begin to occur in older patients on the 10thday after stroke (Kortebein et al.,2007).These studies suggest that,during paralytic exercise,intact brain regions in the damaged and/or nondamaged hemispheres are recruited and innervate limb musculature through the recruitment of adjacent neural pathways to promote recovery from hemiplegia after stroke (Luft et al.,2005).The incidence of dysphagia in stroke patients is 24.3-52.6%,resulting in malnutrition rates of 8.2-49.0% due to severe dysphagia.Patient undernutrition seriously affects limb muscle mass (Liu et al.,2022).The results of a study involving 9 older adults and 11 healthy adults,who underwent 2 weeks of muscle fixation followed by 4 weeks of training,showed that muscle mass decreased,and muscle fiber type (I,IIA,and IIx) changed after the 2-week fixation period (English et al.,2010).Due to the specific course of the disease,two-thirds of stroke patients are only able to stay in bed or sit still due to activity limitations (Powers et al.,2015),further aggravating the problem of muscle disuse after stroke.Despite the occurrence of varied degrees of muscle atrophy and reduction,weakness,and other functional changes among stroke patients,the precise underlying mechanisms of limb muscle alterations remain elusive.Consequently,it is imperative to conduct further investigations into the pathophysiology of limb muscle changes following a stroke.

Pathophysiology

During the acute phase following a stroke,cell metabolism disturbances can lead to cytotoxic oedema and result in extensive necrosis of neurons and glial cells (Peisker et al.,2017).Compared to the non-acute phase after stroke,the primary focus during the acute phase is on reperfusion of the blocked area and neuroprotection (Morettiet al.,2015),in an effort to limit more severe neurological functional impairment (Smith et al.,2015).In the non-acute phase of stroke patients,the focus is on reducing the central nervous system’s impact on peripheral tissue function,and the most easily affected,as well as the most intuitive effector,is muscle tissue (McColl et al.,2008).

The term “sarcopenia” was originally used to describe the age-related loss of muscle mass and muscle strength,mainly observed in older adult individuals (Larsson et al.,1979).More recently sarcopenia has been redefined as a syndrome of widespread and progressive loss of skeletal muscle mass and strength at any age (Morley et al.,2011),with a risk of adverse outcomes,such as impaired physical activity and reduced quality of life (Severinsen et al.,2016).The main clinical manifestation of sarcopenia is a reduction in muscle strength and muscle mass,which is difficult to accurately assess due to the lack of specific indicators for muscle system evaluation and individual patient differences.The main feature that distinguishes ischemic stroke-induced muscle changes from those occurring with normal aging are (1) muscle reinnervation,(2) muscle inflammation,(3) disuse muscle atrophy,(4) protein synthesis and catabolism,(5) muscle fiber type transformation,(6) changes in muscle mitochondrial function,and (7) nutrient supply.The main causes of muscle atrophy after stroke are summarized inFigure 1.The muscle changes that occur after stroke are attributed to the brain damage produced by ischemic stroke,and most of the current indicators used for the evaluation of sarcopenia are only applicable to muscle loss due to aging.Proper evaluation protocols are necessary for more accurate disease diagnosis,likely improving overall prognosis.Understanding the specific mechanisms of muscle cell autophagy and apoptosis after ischemic stroke,protein synthesis (Sacheck et al.,2007),muscle fiber type transformation,muscle reinnervation,and altered mitochondrial function.Further,raising awareness of the physical symptoms associated with ischemic stroke are a critical key to improving patient quality of life and reducing social burdens in the future.

Figure 1｜The causes of muscle atrophy after stroke.

Reinnervation

Disruption of the lower motor neuron pathway after stroke typically leads to hemiplegia of the contralateral limb (Hidler et al.,2007),with 35% of stroke survivors not regaining mobility despite active rehabilitation,and 25% unable to walk without assistance (Langhorne et al.,2009).Within 4 hours after a stroke,a reduction in motor units has been observed in the affected muscle tissue of patients (Li et al.,2020).An additional study found a quarter of 149 patients exhibited motor neuron loss (Drey et al.,2014).Neurological deficits also contribute to structural changes in skeletal muscle and the accompanying atrophy of skeletal muscle that produces mobility problems.In humans,the descending motor pathways include the reticulospinal,vestibulospinal,rubrospinal,and corticospinal tract (Arasaki et al.,2006).Normal movement of the body is mediated primarily by the brainstem and spinal cord (Li et al.,2018).The central nervous system determines whether a limb needs to respond by processing excitatory and inhibitory signals (Babyar et al.,2008),and modulating neuromuscular junctions to achieve force output (Taylor et al.,2016).After the onset of stroke,the nervous system activates paretic motor neurons to limit muscle force production (Murphy et al.,2018),and reduces neural coupling during bilateral limb movements (Charalambous et al.,2016).The entire neuromuscular pathway is altered throughout the movement process.A human walking on a horizontal plane at a comfortable speed is primarily mediated by brainstem and spinal cord mechanisms (Lin et al.,2014).Spinal control adds complexity,flexibility,and multifunctionality to gait control to meet the demands of a dynamic environment (Owen et al.,2017).The neural system after stroke shows plasticity,but damage to the motor cortex and its descending corticospinal tract directly results in muscle weakness (Li et al.,2018).Loss of spinal inhibition leads to hyperexcitability of the spinal cord,known as the unmasking effect (Li et al.,2018).The absence of neural excitation can cause a lack of appropriate inhibition of motor neurons,resulting in spasms,muscle weakness,and other muscle function abnormalities.

Apoptotic damage to neurons can be induced through two mechanisms: (1) by reducing the expression of apoptotic genes and (2) by promoting the expression of repair genes.As some neurons are damaged,the remaining neurons are not able to innervate muscle fibers at normal levels,resulting in an “undersupply” phenomenon,which also decreases muscle function (Larsson et al.,2019).The mammalian target of rapamycin (mTOR),a key kinase in the regulation of neural function,is a crucial factor in cell proliferation and growth,as well as an important regulator of muscle size;phosphatidylinositol 3-kinase (PI3K) has been documented to reduce ischemic neuronal injury after the onset of cerebral ischemia by inhibiting neuronal apoptosis through phosphorylation of activated protein kinase B (AKT) and prompting the expression of the downstream targets of mTOR;and p70 S6 kinase (P70S6K) and eukaryotic translation initiation factor (eIF) 4E-binding protein (4E-BP1;Chong et al.,2012),as well as the Pl3K/AKT/mTOR/P70S6K signaling pathway stimulates the activation of anabolic molecules in muscles (Bodine et al.,2001a).After stroke,neuronal cells and microglia rapidly undergo apoptosis due to ischemia and hypoxia,while increased mTOR activity can help reduce the neurological damage caused by apoptosis (Liu et al.,2012).The combined activation of transcription activator and mTORoffsignaling pathways also helps maintain the regeneration of distant axons in the central nervous system (Sun et al.,2011).In contrast,F-box protein 32 (Atrogin-1) plays a role in denervation-induced muscle atrophy (Bonaldo and Sandri,2013).The N-methyl-D-aspartate receptor,located upstream of the PI3K/AKT kinase and mitogen-activated protein kinase (MAPK) pathways,is an important regulator of neuronal injury and ischemic stroke,activating neural stem cells at low N-methyl-D-aspartate concentrations to play a neuroprotective role (Chen et al.,2008).

Inflammation

After the onset of ischemic stroke,tissue damage is further exacerbated over time (Dirnagl et al.,1999).During the acute phase of ischemia (within minutes to hours;Amantea et al.,2009),vascular endothelial cells and immune cells are activated by regulating the release of a series of danger-associated molecular pattern molecules that recognize toll-like receptors (Broughton et al.,2012) and trigger proinflammatory intracellular signaling switches,nuclear factor kappa-light-chain-enhancer of activated B-cell (NF-κB) expression,reactive oxygen species (ROS) generation (Yilmaz and Granger,2010),and the release of proinflammatory cytokines,such as matrix metalloproteinases (Amantea et al.,2009),interleukin (IL)-1β,and tumor necrosis factor-α (TNF-α).All of these factors further increase ischemic tissue damage (Valko et al.,2007),leading to more dramatic cell death.

Under normal physiological conditions,the blood-brain barrier allows the passage of selected substances into the brain,maintaining the stability of the internal environment (Ju et al.,2018).With injuries,such as stroke,acute traumatic stroke,and Alzheimer’s disease,the structure and function of the blood-brain barrier are altered (Gao et al.,2019),and microglia,as sensitive receivers of brain injury signals,migrate to the site of brain tissue damage within minutes (Zhang et al.,2017).This migration leads to significant leakage of the blood-brain barrier and accelerates the entry and release of inflammatory cytokines,such as IL-6,IL-1β,and TNF-α (Ceulemans et al.,2010),into and out of the injured brain tissue area,creating a positive feedback loop.Increased blood-brain barrier permeability is considered a significant pathological feature of ischemic stroke (Chen et al.,2018a).IL-1β also activates cyclooxygenase-2 in skeletal muscle cells (Yuan et al.,2015) and inducible nitric oxide (NO) synthase expression in rat chondrocytes (Fei et al.,2019),triggering the release of prostaglandin E2 and NO,which results in chronic inflammation and sarcopenia (Cruz-Jentoft and Sayer,2019).TNF-α and other proinflammatory cytokines up-regulate the expression of muscle atrophy F-box (Atrogin-1/MAFbx) and muscle ring finger 1 (MuRF1;Fang et al.,2021),which are two specific E3 ubiquitin ligases that are highly expressed in muscle and target myogenic fibronectin (Bodine and Baehr,2014).Myogenic fibronectin is an important protein that comprises muscle.Some studies have reported that inflammatory factors,such as TNF-α,IL-1β,and IL-6 affect muscle production and differentiation by inhibiting myogenic differentiation antigen (MyoD) activity through the NF-κB pathway.Increased inducible NO synthase expression under the stimulation of inflammatory factors reduces the binding ability of hu antigen R (HuR),a protein that maintains the stability of MyoD mRNA,leading to the inhibition of muscle differentiation (Di Marco et al.,2005).Inhibition of MuRF1 and MAFbx expression is effective in reducing the occurrence of muscle atrophy (Nguyen et al.,2020).MuRF1 and MAFbx,which are two key indicators of muscle atrophy (Larsson et al.,2019) and inflammation (one of the most common complications of ischemic stroke) can induce muscle atrophy by activating ubiquitination pathways through multiple routes (Bodine and Baehr,2014).While suppression of inflammation in the acute phase of stroke can effectively reduce damage to brain tissue and the central system at the ischemic site by inflammatory factors,also focusing on chronic inflammation during the rehabilitation period may be an effective method to reduce muscle loss and improve rehabilitation efficiency.

Disuse atrophy of muscles

Muscle unloading

Skeletal muscle atrophy reduces muscle cell cross-sectional area and reduced muscle volume/strength are closely related to “disuse atrophy” in stroke survivors (Lang et al.,2010).The lack of exercise stimulation of skeletal muscles further aggravates the degeneration of the locomotor system,creating a positive feedback loop due with disease progression and rehabilitation requirements that force the deloading of muscles,resulting in a significant reduction in exercise (Knops et al.,2013).One of the most obvious characteristics of decreased limb motor function is inconsistent gait and hemiparesis (Sauder et al.,2019).Decreased muscle tissue function on one side of the body can produce uncoordinated strides,inconsistent spans,and movement deformities.These mobility difficulties are due to inconsistent force application on the left and right limbs,which is known as a slow and asymmetrical gait pattern (Verma et al.,2012).Abnormal gait patterns can also lead to changes in neural control and muscle compensation,making the limb more susceptible to fatigue,given the increased intensity and duration of force applied to some muscles,compared to muscles that are typically used (Taylor et al.,2016).This fatigue further reduces the urge to exercise,as stroke patients prefer a quiet and comfortable resting position to exercise (Sandri,2008).

An extensive study on older adults stroke patients found that 14.86% experienced fractures,with the primary causes of fractures including muscle atrophy,muscle weakness,osteoporosis,and decreased bone density;features that cause patients to be unstable while standing and thus,more prone to falls (Kristensen et al.,2020).The selection of lower limb muscles in post-stroke patients as a research subject is more appropriate (Liu et al.,2021),given that lower limb strength reflects the health of this skeletal muscle group (Mentiplay et al.,2015).Two important manifestations of muscle function include the peak force produced by the muscle group (muscle power) and the speed at which that force is produced (Mentiplay et al.,2015).A small but growing body of data suggests that regular weight training and increased muscle movement can help hemiplegic rehabilitation after stroke (Minn and Suk,2017).One study found that people who exercised in conjunction with rehabilitation after stroke had less skeletal muscle loss than those who did not exercise (English et al.,2010).Conversely,muscle strength was shown to be reduced by 16% in healthy older individuals who were bedridden for only 10 days,due to “wasting atrophy” caused by a lack of exercise (Kortebein et al.,2007).Appropriate aerobic exercise after stroke helps the body recover motor capacity (English et al.,2012),while also improving spinal cord plasticity by increasing mTOR expression and ribosome S6 protein kinase (p70S6K) activity.Maintaining an appropriate exercise routine is also a key inducer of protein synthesis,which is necessary to promote long-distance axon growth in the motor spinal cord to better control muscles and maintain normal muscle function (Liu et al.,2012).

The number of motor axons that can be stimulated after ischemic stroke is reduced,which in turn decreases the speed of the body’s neuromodulation feedback mechanisms (Uyeda and Muramatsu,2020).One study performed in healthy men found that quadricep mass decreases at a rate of 4.1% per decade,and quadricep volume decreases at a rate of 3.4% per decade (Kostka,2005).In contrast,muscle volume in the thigh on the paralyzed side of stroke patients decreased by 24%,whereas subcutaneous fat volume increased by 5% (Ryan et al.,2011),this was accompanied by a significant decrease in bone mineral content,bone mineral density,and cortical thickness (Pang et al.,2007).Significant plasticity in lower extremity muscle function is noted in both younger and older individuals during short-term discontinuation and reuse (rehabilitation exercise;Kristensen et al.,2020).Individualized and appropriate rehabilitation treatment programs can maximize the rehabilitation efficiency of patients and improve their quality of life.As such,a suitable exercise regimen is needed as a medical intervention to maintain and repair normal muscle function following stroke.

Ubiquitin-proteosome system

Muscle atrophy is a dynamic process controlled by specific signaling pathways and transcriptional programs rather than a short period of acute change (Zhao et al.,2007).Atrogin-1 (MAFbx) and MuRF1 serve as two representative genes that regulate muscle atrophy,and they are commonly induced by two activated transcription factors (Bodine et al.,2001b),NF-κB and forkhead box O1 (FoxO1),in the resting phase after stroke onset (Bonaldo and Sandri,2013).Ubiquitin-protein ligase (E3) determines the overall rate and specificity of the ATP-ubiquitin-proteosome system.The E3 ubiquitin-protein ligase bound to the protein modified by the first transcription (Yang et al.,2009) after phosphorylation promotes muscle protein degradation by MAFbx.Lack of motor stimulation of muscle leads to de-burdening,resulting in the FoxO3 transcription factor being upregulated,promoting expression of atrophy genes Bnip3,autophagy-related 12 homologs (Atg12),and Atg4B,which ultimately induces autophagy in skeletal muscle (Zhao et al.,2007).

Local injection of insulin-like growth factor 1 (IGF-1) blocks the proteolysis that occurs upon the upregulation of MAFBX and MurF1 expression induced by disuse atrophy (Stitt et al.,2004).The PI3K/AKT pathway is located downstream of IGF-1.PI3K/AKT promotes protein synthesis by upregulating the expression of mammalian target of rapamycin complex 1 (mTORC1),a complex of mTOR,which activates p70 and eukaryotic translation initiation factor 4 gamma 1 (elF4G1),and inhibits the expression of 4E-BP1.AMPactivated protein kinase (AMPK) activation and phosphorylation directly inhibit mTOR,thereby reducing protein synthesis and inhibiting FoxO1 transcription factor phosphorylation (Stitt et al.,2004).Nonphosphorylated FoxO1 regulates MuRF1 gene transcription,by promoting increased MurF1 protein expression.Atrogin-1/MAFbx or MuRF1 knockout mice exhibit reduced muscle atrophy caused by denervation (Castillero et al.,2013).The total rate of protein degradation is closely related to Atrogin-1 mRNA levels.Additionally,MuRF1 is directly related to both Atrogin-1 mRNA and myogenic fiber levels (Centner et al.,2001),through maintenance of the stability of the M-line region of sarcoma,myosin heavy chain protein,troponin-T,and troponin I (Clarke et al.,2007).MuRF1 perturbs calpain and other proteolytic pathways,and inhibition of MuRF1 expression preserves the levels of myosin heavy chain protein,which is the primary sarcomere component,to combat disease-induced muscle atrophy (Clarke et al.,2007).

The relationship between the ubiquitin-proteosome pathway and skeletal muscle atrophy is well established and is shown inFigure 2.Previous research has shown that the loss of muscle tissue can be reduced by inhibiting key factors in this pathway;however,this effect has only been demonstrated in animal and cellular experiments.Effective clinical measures to improve disease sequelae are currently unavailable.MuRF1 and MAFb,two important E3 ubiquitin ligases in skeletal muscle,activate the ubiquitin-proteosome system after the onset of ischemic stroke through multiple factors induced by brain injury that promote muscle atrophy and post-stroke hemiplegia,forming a negative closed-loop that reduces the efficiency of rehabilitation (Sandri et al.,2006).Atrogin-1/MAFbx or MuRF1 can be targeted as a clinically relevant and selective inhibitor of certain components of the ubiquitin-proteosome pathway (Zhang and Ding,2015).However,a more important aspect is to further elucidate the upstream activation mechanisms of the ubiquitinproteosome pathway,as well as other key factors that can intervene in the muscle degradation process,providing potential avenues for the treatment of post-stroke muscle wasting and muscle loss.

Figure 2｜ Ischemic stroke changes fiber size of skeletal muscle by regulating the synthesis and degradation system via NF-κB and FoxO signaling pathways.

Change in muscle fiber type

During normal aging,a decrease in fast muscle fibers (type IIa oxidative enzymatic fibers,type IIb enzymatic fibers,and type IIX oxidative enzymatic fibers) and an increase in slow muscle [major histocompatibility complex (MHC) I] fibers is observed (Conrad et al.,2017).In contrast,the opposite muscle fiber changes are noted after stroke.These changes include an increase in fast muscle fibers and MHC type II muscle fibers,and a decrease in mitochondria-rich slow muscle (MHC type I) fibers (Snow et al.,2019).These changes make exercise more anaerobically dependent,prompting lactic acid accumulation,which increases soreness and fatigue (Severinsen et al.,2016).A change in muscle fiber type occurs after the onset of ischemic stroke.Skeletal muscle consists of fast and slow muscle fibers,which are subdivided into fast fatigue-prone (2A) and fast fatigue-resistant (2B) types (Li et al.,2017).The ratio of fast to slow muscle fiber content,muscle fiber diameter,and number of adipocytes are all directly related to muscle strength.Normal individuals typically lose a certain number of motor units with age,accompanied by a change in muscle fiber type,and this change is generally most pronounced in the fifth decade of life (Larsson et al.,2019).The ubiquitin ligase MuRF-1 and the “fork-headbox” (FoxO) are highly associated with skeletal muscle atrophy (Kamei et al.,2004).AKT promotes FoxO phosphorylation,increasing the ability of FoxOs to pass through the nuclear pore,and its export from the nucleus to the cytoplasm.The effect of changes in the activity of FoxO family proteins on Atrogin-1/MAFbx and MuRF1 in disease-induced symptoms of muscle atrophy is substantial (Ji and Yeo,2019).MuRF1 and Atrogin-1 expression is significantly increased in aging rats,and FoxO1 and FoxO3a expression levels are reduced by 73% and 50%,respectively,in skeletal muscle tissues of 70-year-old humans.Muscle type differences between older and younger individuals are mainly reflected in the ratio of fast to slow muscle fibers (Hsu and Kao,2018).FoxO overexpression leads to decreased expression of genes related to structural proteins in type I muscles,a shift from type I to type II muscle fibers (Chen et al.,2018c),and a significant reduction in the size of both fiber types (Kamei et al.,2004).AMPK,an AMP-dependent protein kinase,acts as an intracellular energy receptor and is thought to be a potential factor regulating myofiber-type transformation in skeletal muscle (Hvid et al.,2010).The expression of the closely related peroxisome proliferators-activated receptor γ coactivator 1 alpha (PGC-1α) plays a role in promoting slow muscle fiber expression and pro-mitotic mitochondrial biosynthesis to improve motility (Boström et al.,2012),and changes in AMPK levels are positively correlated with PGC-1α expression (Thomson,2018).The AMPK/PGC-1α pathway promotes the conversion of skeletal muscle fibers from type II muscle fibers (involved in generating force) to type I muscle fibers (involved in endurance),which are also known as slowcontracting fibers.Type I muscle fibers are rich in mitochondria and more resistant to fatigue than fast muscle fibers,In contrast,fast muscle fibers provide energy mainly through glycolysis reactions,so these muscles are prone to lactic acid accumulation,causing soreness (Ciciliot et al.,2013).PGC-1α overexpression in mouse muscle reduces MAFbx/Atrogin-1 and MuRF1 expression,and inhibits NF-κB activity,thereby reducing muscle atrophy caused by fiber changes (Ciciliot et al.,2013).It has been shown that there is an increase in the percentage of type I muscle fibers in patients with strokeinduced hemiparesis,possibly due to a compensatory mechanism of lateral limb reinnervation caused by a decrease in the activity of neurons innervating type II muscle fibers due to the disruption of the corticospinal tract caused by ischemic stroke (Dattola et al.,1993).The alteration of skeletal muscle fibers is an extremely complex and sophisticated process that is primarily controlled by various myogenic precursor cells and protein synthesis signaling pathways (Deshmukh et al.,2021).Additionally,the involvement of various cytokines is required for coordinated participation.

Mitochondrial changes

Mitochondria,as organelles widely distributed in the body’s cells,are most abundant in muscle tissue (Avellaneda et al.,2021).Changes in the number and function of mitochondria will have a significant impact on muscle cells.Mitochondria have several functions,such as fuel supply,regulation of cell proliferation,regulation of apoptotic signals,and regulation of cell homeostasis (Picca et al.,2018),while changes in the number,morphology,and function of mitochondria in soleus are thought to be one of the main causes of muscle degeneration in skeletal muscle (Park et al.,2009).

When an ischemic stroke occurs,a series of deleterious changes occur in skeletal muscle mitochondria,which are manifested as increased ROS generation and release (DeGregorio-Rocasolano et al.,2018),increased FoxO family factor activation and NF-κB inflammatory pathway activation,and finally,apoptosis and phagocytosis.Mitochondria are both a source of oxygen radicals and susceptible to oxygen radical-induced damage (Sandri et al.,2006);mitochondrial dysfunction accelerates the production of ROS;and increased ROS also leads to peroxidation of lipids in subsarcolemmal mitochondria,proton leakage,and decreased membrane potential (Zheng et al.,2019);which are factors closely associated with sarcopenia (Crane et al.,2010).Proteins in skeletal muscle are regulated through two systems,ubiquitin-proteosome and autophagic lysosomes,and both pathways are activated by disease,leading to muscle loss and decreased muscle function (Sandri,2008).It has been shown that knocking out autophagy-related gene-7 (Atg7) in mouse muscles leads to muscle atrophy and accelerated muscle loss during fasting (Masiero et al.,2009).

In a stable internal environment,autophagy plays a critical role in degrading damaged or misfolded organelles into amino acids that can be recycled and transported throughout the body,thereby maintaining muscle quality (Vinel et al.,2018).In the context of stroke,autophagy is continuously activated,resulting in accelerated protein degradation.Muscle atrophy can occur if there is a lack of energy and insufficient protein synthesis (Aoyama et al.,2021).Experiments have shown that in a rat model of ischemic stroke,the expression of autophagy protein microtubule-associated protein 1,light chain 3 beta (LC3B),ATG5,and Beclin-1 was upregulated in the skeletal muscle of the rats,leading to excessive autophagy activation and mitochondrial dysfunction,ultimately resulting in muscle atrophy (Qi et al.,2023).Atrophic muscle shows an accumulation of abnormal mitochondria,forming an abnormal sarcoplasmic reticulum swelling,disorganized sarcomeres,and concentric membrane structures (Conrad et al.,2017).Therefore,maintaining mitochondrial autophagic flux (Zhang et al.,2020) in an appropriate range is also an entry point to reduce muscle loss after stroke.

After stroke onset,insufficient blood flow leads to hypoxic damage in infarcted tissues,causing upregulation of Bcl-2 protein family member Bnip3,which results in free Beclin-1 binding to anti-apoptotic proteins Bcl-2 or Bcl-xL as phakellistatin 13 (PK13) complexes (Dobrowolny et al.,2008),and leads to mitochondrial autophagy through regulation of the PK13/AKT pathway.Bcl-2 family proteins regulate apoptosis by altering the permeability of the mitochondrial membrane (Masiero et al.,2009).Bcl-2 and Bcl-xL are usually localized to the outer mitochondrial membrane as anti-apoptotic proteins and inhibit the release of cytochrome c (Umaki et al.,2002),Bax and Bak,members of the pro-apoptotic Bcl-2 family,are typically localized in the cytosol and enter the mitochondria upon stimulation by apoptotic signals to promote the release of cytochrome C (Jeon and Choung,2021).The sarcoplasm cytochrome c level is an indicator of mitochondrial damage or outer membrane rupture,and the high levels of cytochrome c in the cytoplasm is indicative of mitochondrial damage (Ji and Yeo,2019).Caspase-8 can induce apoptosis by directly activating the terminal shear enzyme caspase-3 and cleaving the pro-apoptotic protein Bid (tBid,truncated Bid),as caspase-8 can induce Bax to bind to mitochondria to form a transmembrane channel that allows the cleaved protein to bind to the mitochondria (Clarke et al.,2007).As caspase-8 induces Bax to bind to mitochondria to form transmembrane channels,the cleaved Bid enters mitochondria to promote the activation of cytochrome c (Stitt et al.,2004),while the inflammatory cytokine TNF-α has both pro-apoptotic and apoptosis-inhibiting effects (Sandri,2008).The inflammatory factor TNF-α entering tissues promotes the upregulation of caspase-8 and caspase-9,and then activates the expression of the anti-apoptotic gene Bcl-2 through NF-κB (Sandri et al.,2006).In different muscle types,activation of caspase-3 increases proteolysis of key myofibrillar proteins and autointegration,such as troponin-T,µ-calpain,titin,and nebulin.The mechanisms involved are shown inFigure 3.Although research on muscular atrophy has increased over the years,limb symptoms after stroke have not necessarily been associated with muscle atrophy.In muscle fiber tissue,mitochondria are important in regulating metabolism,and also a potential signal source for regulating partial muscle protein decomposition,providing a basis for the detection of muscle atrophy.

Figure 3｜Mitochondrial related apoptosis leading to post-stroke muscle atrophy.

Nutritional supply

The probability of acute stroke occurring secondary to malnutrition may range from 8% to 34% (Ojo and Brooke,2016).The 2018 clinical practice guidelines for rehabilitation nutrition recommend intensive nutritional therapy for several symptoms,such as hemiplegia,sarcopenia,and muscle weakness caused by cerebrovascular disease (Nakahara et al.,2021).It should also be noted that excessive energy intake can also lead to disruption of insulin signaling in skeletal muscles,resulting in insulin resistance (Caballero-García and Córdova-Martínez,2022),energy absorption,and metabolic problems that can affect muscle function (Kernan et al.,2014).As such,appropriate nutritional supplementation is a key factor in recovery.Proper nutritional supplementation also presents an opportunity for stroke prevention,with studies demonstrating that B vitamins and folic acid supplements can decrease stroke risk (Spence,2019),as B vitamins play a crucial role in energy metabolism and muscle tissue repair (Burgos et al.,2018).Increasing the consumption of vegetables,fruits,legumes,and olive oil,while reducing saturated and trans-fat intake,can reduce stroke incidence in high-risk patients by at least 40% (Spence,2019).This dietary approach,also known as the Mediterranean diet,may also have a positive impact on muscle strength (Spence,2019).

Vitamin E plays a crucial role in oxidative metabolism,oxygen transport,muscle health,and neuronal maintenance (Servais et al.,2007).A study of Finaud et al.(2006) suggested that vitamin E can decrease muscle atrophy by modulating mitochondrial biogenesis and downregulating the expression of Murf-1 and MAFbx,two key factors in the ubiquitin-proteasome system involved in muscle protein degradation induced by inflammatory cytokines,such as IL-1β and TNF-α.Studies suggest that magnolol and epicatechin promote muscle protein synthesis by downregulating myostatin expression,inhibiting the ubiquitin-proteasome system,and ultimately leading to enhanced expression of myosin heavy chain,myogenin,and MyoD (Ge et al.,2020).Vitamin D deficiency is closely associated with sarcopenia,a condition characterized by muscle loss and reduced function (Dawson-Hughes,2017).Vitamin D accelerates muscle function recovery,promotes cell proliferation,reduces apoptosis in muscle cells,increases the expression of vitamin D receptors in muscle,enhances type II muscle fiber content,and improves athletic performance (Garcia et al.,2019).In addition to increasing vitamin intake,the use of nutritional supplements,such as creatine,β-hydroxy-β-methyl butyric acid,and omega-3 fatty acids can help maintain muscle function (Hayes et al.,2020).Leucine supplementation regulates C2C12 myotubes by activating the mTOR downstream of PI3K,as well as the expression of 4E-BP1 and p70 ribosomal protein S6 kinase,thereby regulating the muscle cell cycle,cell migration,and protein synthesis (Schmid et al.,2007;Xiong et al.,2022).The nature of muscle wasting involves an imbalance in protein metabolism,where protein synthesis is less than the net degradation of proteins,due to breakdown.However,it was found that supplementation of soy protein-derived peptides in the diet attenuated burn-induced muscle atrophy in Wistar rats by modulating the ubiquitinproteasome system and autophagy signaling pathway (Garber et al.,2011).

The lack of well-targeted nutritional plans for post-stroke patients results in less apparent effects of rehabilitation therapy.A strategy to promote stroke patient recovery and reduce muscle loss is for clinical physicians to develop targeted nutritional supplementation plans by referring to different scales based on various stages of disease.

Potential Therapeutic Strategies

Modulation of the expression of key factors

Although muscle degradation via ubiquitin-proteosome has been extensively studied,muscle function improvements in the context of poststroke hemiplegia may represent a promising entry point for suppressing multifactor-induced muscle atrophy by inhibiting the expression ofMuRF1and muscle atrophy box F (MAFbx) gene expression.Local injection of IGF-1 reduces atrophy caused by muscle disuse (Stitt et al.,2004),decreases FMS-like receptor tyrosine kinase-3 expression,and inhibits the expression of FoxO1 and FoxO3 in the ubiquitination system (Long et al.,2020).Dieckol obtained from Ecklonia cava exhibits antioxidant,anti-inflammatory,and neuroprotective effects,leading to decreased IL-1β/Murf-1/Atrogin-1 expression,and decreased NF-κB expression in dexamethasone-induced muscle atrophy (Oh et al.,2021).Similarly,L-carnitine and trimetazidine attenuate dexamethasone-induced muscle atrophy by inhibiting the expression of Murf-1,Atrogin-1,caspase-1,and gasdermin-D (Keller et al.,2012;Wang et al.,2021).Other studies have shown that triptolide inhibits muscle atrophy by upregulating protein synthesis signals of IGF,phosphorylated IGF-1R,phosphorylated AKT,and phosphorylated mTOR protein,as well as downregulating ubiquitin-proteosome molecules nuclear Forkhead box O3a (n-FoxO3a),Atrogin-1,and MuRF1.Triptolide is also a potential treatment for muscle wasting after stroke (Fang et al.,2021).P70S6K and 4E-BP1,inhibit neuronal apoptosis to reduce muscle reinnervation atrophy caused by ischemic neuronal injury.The targets and potential inhibitors associated with muscle wasting after stroke are listed inTable 1.

Table 1｜Promising key targets to improve symptoms,such as muscle wasting,sarcopenia,and muscle weakness after ischemic stroke

B vitamins and folic acid reduce the risk of stroke occurrence and promote muscle tissue repair and facilitate energy conversion.Patients should increase the intake of vitamin D (Garcia et al.,2019),creatine,beta-hydroxy-betamethyl butyrate,and omega-3 fatty acids to maintain normal muscle function,improve motor performance,and reduce the impact of disease on muscle tissue.Targeted nutrient supplementation reduces damage to the hemipelvic girdle from ischemia and hypoxia and promotes muscle repair to improve limb symptoms.Some common sources of nutrients are listed inTable 2.A proper diet is also a key to post-stroke rehabilitation and should be supplemented with additional nutrients needed to improve post-stroke decline in muscle function,along with appropriate exercise,to aid recovery.

Table 2｜Mechanisms and effects of common nutrients derived from action on muscle

Personalized rehabilitation program

Although there are many scales that can accurately assess a patient’s condition (Harrison et al.,2013),the rating system for post-stroke lower limb muscles is incomplete (Quinn et al.,2011).Specifically,a single scale that can comprehensively guide clinicians in developing a comprehensive rehabilitation plan for stroke patients is unavailable (Bernhardt et al.,2019).In clinical practice,many patient factors,such as age,sex,weight,underlying diseases,and even socioeconomic conditions,play a role in rehabilitation (Yoshimura et al.,2022).Different pathological processes and time points require the use of multiple assessment scales to dynamically monitor patient condition during the rehabilitation process (Yoshimura et al.,2022),necessitating collaboration among clinicians and more comprehensive assessment systems.Gait pattern,speed,and endurance,as well as physical performance,can be used as reference factors to evaluate lower limb muscle function after stroke (Quan et al.,2020).Further,significant differences in gait speed have been noted among stroke patients with different levels of impairment.The normal walking speed of a healthy person is approximately 1.39 m/s (Looney et al.,2019).Patients with post-stroke hemiparesis are classified according to the degree of impairment as having a household ambulation (i.e.,severely impaired) speed of 0.4 m/s (Schmid et al.,2007),limited community ambulation (i.e.,moderately impaired) speed between 0.4 and 0.8 m/s,and full community ambulation (i.e.,mildly impaired) speed of 0.8 m/s.One study reported that increasing walking training for stroke patients reduced muscle loss,and that for 1-km/h increase in walking speed,the risk of stroke was reduced by 13% (Quan et al.,2020).Therefore,a gait evaluation index should be included in guidelines for muscle function evaluation after stroke.

The proper amount of exercise should also be correctly understood.In the literature,“appropriate” typically refers to exercise levels based on a patient’s physical condition,environment,climate,and available auxiliary equipment.Additionally,the level of exertion should be based on an exercise program that the patient is familiar with,and the patient should feel comfortable after exercise,without shortness of breath or excessive fatigue (Levin et al.,2009).For example,patients with post-stroke disuse muscle atrophy are unable to maintain normal muscle mass due to a lack of normal exercise (Sandri,2008).The hemiplegia caused by stroke itself and discomfort experienced during exercise prevents the patient from exercising,thus forming a feedback loop.The role of a professional rehabilitation therapist is to supervise and encourage the patient to perform the “uncomfortable,” but appropriate movements.Clinical physicians and rehabilitation therapists should participate in professional training to apply different scales in a more appropriate manner,and develop personalized treatment plans for patients at different stages,as a key strategy to improve the efficiency of rehabilitation treatment.

Advances and Challenges in Stroke Rehabilitation

At present,the focus of attention on ischemic stroke patients primarily involves blockage recanalization and acute neuroprotection;however,post-rehabilitation care is lacking.The quality of life of patients after a stroke depends on muscle integrity;yet,a systematic understanding of the pathological changes in the muscles after stroke is lacking and not addressed in treatment guidelines.Correctly assessing the degree of limb symptom impairment in stroke patients is the premise of administering correct clinical treatment and rehabilitation advice.Although the professional skills of caregivers to enhance stroke patient rehabilitation have improved,knowledge about complications and the overall progression of the disease remains lacking (Kumar et al.,2010).A professional and personalized rehabilitation treatment plan and a comprehensive training program will give patients a better prognosis and a greater chance of recovery (Kernan et al.,2014).Current rehabilitation treatment emphasizes the completion of tasks,such as walking up and down steps ten times and walking back and forth five times.These tasks are not at the “training” level of rehabilitation because they emphasize the outcome and do not focus on the quality of the movement process (Levin et al.,2009).Significant differences in disease severity are noted among people of different sexes,ages,and weights,and there is no clear standard for what constitutes “recovery” for different individuals.Existing evaluation methods,such as gait speed,stride length,functional walking,and SPPBT,are not suitable for stroke survivors.These tests are often associated with low-limit effects (Verma et al.,2012).How to define “moderate exercise” is a key part of the problem.Different patients need different amounts of exercise for different degrees of disease.Individualized rehabilitation programs are particularly important to avoid inappropriate amounts of exercise and low-quality training.

Numerous stroke scoring scales,such as National Institutes of Health Stroke Scale (Kwah and Diong,2014),the modified Rankin Scale,the Barthel index,Glasgow outcome scale,and the Stroke Impact Scale (Kasner,2006),are currently available.However,none of the scales can describe all the components of disease development after stroke when used alone.In addition,some of the scales are difficult to use.A high level of clinician competence is required to maximize the use of each scale,and most patients are not provided with the most appropriate treatment plan.Most of these existing scales are used for early prediction and systematic assessment of stroke and do not assess muscle health,which is the key to mobility and quality of life in patients with hemiplegia.

Despite the pathological and physiological basis for changes in limb muscles after stroke being hypothesized,no multicenter randomized controlled trial has demonstrated the efficacy of non-invasive stimulation in improving patient motor performance (Stinear et al.,2020).The failure of the bidirectional interaction between preclinical research and clinical reality also hinders the development of effective rehabilitative treatments for stroke patients (Bernhardt et al.,2019).

Conclusion and Perspectives

At present,research on muscle atrophy following stroke encounters two critical limitations.First,the mechanisms underlying post-stroke limb atrophy remain incompletely understood.Second,existing research and clinical guidelines may not fully address the needs of stroke patients suffering from muscle atrophy,leading to suboptimal treatment outcomes.

We have identified that inflammation pathways,the ubiquitin-proteasome system,and oxidative stress-related pathways are the major contributing factors related to limb atrophy after stroke.Proteins including mTOR,PI3KAKT,MAPK,MuRF-1/MAFbx,MyoD,and NF-κB have been recognized as key targets for muscle changes after stroke.Downstream inflammatory factors,such as IL-6,IL-1β,and TNF-α,and various oxidative stress products have been implicated in the process of muscle atrophy in muscle tissue of stroke patients.Moreover,mitochondrial damage has played a crucial role in the pathological process,and mitochondrial dysfunction is considered as one of the key triggers for various types of muscle atrophy after stroke.

Existing stroke patient rating scales are complex and diverse,but the scales for evaluating limb muscle changes in stroke patients are inadequate.This forces clinical physicians to spend more time using different scales to assess the motor ability of stroke patients.Furthermore,individual differences such as age,gender,disease progression,and socioeconomic situation make it difficult for existing treatment plans to achieve appropriate and efficient therapeutic effects for every patient.The lack of effective interaction between preclinical research and clinical treatment has hindered the development of stroke rehabilitation treatment.Therefore,streamlining and integrating the existing complex and diverse stroke patient rating scales is one of the necessary factors for clinical physicians to develop personalized and effective rehabilitation treatment plans for different patients.

The easily implemented nutritional intake program for post-illness care of patients is summarized inTable 2,which can provide references for clinicians and rehabilitation therapists to develop personalized nutrition supplement plans.In addition,through the induction and summary of key targets and related pathways involved in muscle damage in stroke patients,potential intervention targets for innovative strategies are provided.This also offers a theoretical framework and research ideas for addressing muscle damage in stroke patients from a mechanistic perspective.In summary,this review introduces current research on the mechanisms of muscle atrophy following stroke,analyses and discusses treatment strategies based on these mechanisms and current guidelines,proposes potential intervention strategies for limb atrophy in stroke survivors,advocates for simplifying and integrating complex and diverse scoring systems,and provides a possibility for clinicians to develop personalized and effective treatment plans for patients with different disease courses.

This review provides an examination of the advances in the pathophysiological study of post-stroke limb injuries and suggests promising treatment strategies.However,there are still limitations.First,the implementation of methods that are able to be translated between preclinical research and clinical treatment have not been adequately elaborated.Next,the solution of how to execute the transformation of fundamental research and clinical application for post-stroke physician injuries needs to be further summarized.Finally,the potential use of existing muscle damage intervention strategies in post-stroke rehabilitation needs to be further evaluated and debated.

Acknowledgments:We are very grateful to Dr.Jiuseng Zeng from Chengdu University of Traditional Chinese Medicine for his insightful advices on this manuscript.

Author contributions:Conceptualization and manuscript draft: HQ,DT;manuscript revision: FL,RY,NZ.All authors have read and approved the final version of the manuscript.

Conflicts of interest:The authors declare no competing interests.

Data availability statement:The data are available from the corresponding author on reasonable request.

Open access statement:This is an open access journal,and articles are distributed under the terms of the Creative Commons AttributionNonCommercial-ShareAlike 4.0 License,which allows others to remix,tweak,and build upon the work non-commercially,as long as appropriate credit is given and the new creations are licensed under the identical terms.

Open peer reviewer:Md Imamul Islam,University of Manitoba,Canada.

Additional file:Open peer review report 1.