The role of microRNAs in muscle wasting and recovery during critical illness: a systematic review

Critical illness associated with intensive care unit (ICU) admission often results in persistent skeletal muscle wasting and may lead to frailty in older and patients with multi‐morbidity. Early recognition of patients at high‐risk of long‐term complications could provide opportunities to minimize the impact of critical illness and improve health and quality of life. MicroRNAs (miRs) are short, non‐coding RNAs that regulate approximately two‐thirds of the human genome and are involved in most biological processes. Multiple studies have demonstrated their role in muscle development and disease and their potential as biomarkers of muscle wasting.


Introduction
The COVID-19 pandemic has highlighted the burden of critical illness on society.2][3][4] These patients often experience muscle weakness contributing to ICU-acquired weakness (ICU-AW), which is associated with functional limitations, decreased independence and reduced quality of life. 4lthough there is no consensus on how to quantify muscle changes in critically ill patients, 5 ICU-AW is present in up to 75% of mechanically ventilated and critically ill patients, [6][7][8][9][10][11][12] and is more pronounced in older adults, owing to baseline weaker muscle including sarcopenia, 8,11,[13][14][15] frailty 16 and age-associated risk of adverse health outcomes. 179][20][21][22] In addition, drugs used in critically ill patients, such as muscle relaxants, antibiotics and steroids, can contribute to muscle wasting during critical illness, 23 which can be considered extreme, with up to 75% decrease in cross-sectional area (CSA) in both type I and II fibres 7 and similar to muscle wasting during late stages of cancer. 24n a mechanistic level, disrupted proteostasis in the early phases of critical illness is a hallmark of critical illness-related muscle wasting. 15Loss of myosin relative to actin, sarcomere disorganization, reduction in specific force and changes in Ca 2+ sensitivity are also prominent in the muscle of ICU patients. 7,8Moreover, lipid infiltration and necrosis, in addition to muscle injury during mechanical ventilation, as well as fibre atrophy, have been suggested drivers of diaphragm wasting during ICU stay. 8,9,25Defective muscle regeneration and inflammation are considered another hallmark of critical illness 8,25,26 and key drivers of early muscle atrophy via the ubiquitin-proteasome system (UPS) 8,9,[27][28][29][30] and reduced protein synthesis. 9,25,31]32 Considering the rate and damage of muscle atrophy induced in the early stages of critical illness, interventions usually focus on the metabolic mechanisms of protein synthesis, although direct evidence supporting the use of early nutritional interventions is missing 15,33 and long-term growth hormone use has negative consequences. 34The limited success of these therapies may be related to an incomplete understanding of the molecular factors underlying muscle wasting during ICU-AW.Among genes regulated between early and persistent ICU-AW were those associated with mitochondrial homeostasis, lipid oxidation, 31,35,36 E3 ubiquitin ligases, autophagy and calpain systems, apoptosis and chaperones. 37YOD1, p38 MAPK and GDF-15 have also been proposed to regulate muscle homeostasis in critically ill patients by regulating TGF-β-mediated expression of microRNAs (miRs), 31 which themselves are disrupted in the muscle of critically ill patients. 38iRs are short, non-coding RNAs that bind to the 3′UTR region, in most cases, of their target mRNA, resulting in inhibition of translation and/or mRNA degradation and ultimately reduced levels of target protein(s). 10][41][42] miR levels change in muscle and plasma/serum during ageing and disease; hence, miRs represent both potential therapeutic targets for preventing muscle loss and novel, non-invasive biomarkers of muscle loss. 10,43,44Given that specific gene 'modules' linked to physiological processes are dysregulated in the muscle of critically ill patients, changes in the expression of miRs may mediate aberrant expression of genes associated with loss of muscle mass and strength in persons recovering from critical illness.However, the role of miRs in muscle wasting and recovery from critical illness is largely unknown.
The purpose of this systematic review was to identify changes in miR levels in skeletal muscle and/or blood and their association with muscle mass, strength and/or function and frailty during and post critical illness to determine their diagnostic/prognostic potential of ICU-AW and their mechanistic involvement in muscle wasting post-critical illness.As this is an area with limited research, we aimed to explore the potential role of miRs as therapeutics and/or biomarkers of muscle wasting in critically ill patients based on known miR function and their target genes.

Protocol registration
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement was followed as a reference protocol standard. 45The protocol was registered at the International Prospective Register of Systematic Reviews PROSPERO (Registration Number CRD42022339531).

Search and eligibility criteria
A systematic review of journal articles was conducted within the PubMed and Scopus databases (from inception to June 2022).Articles were searched through the selection of terms associated with muscle wasting and/or critical illness and microRNAs as follows: ( Article titles, keywords and abstracts were reviewed according to the inclusion criteria.The collated articles were searched for by two independent researchers (MBG, SC) and reviewed by a third independent researcher (JCM).Articles were limited to the English language.Included articles were (1) primary journal publications, (2) described human research, (3) included adults aged ≥18 years and (4) patients with a critical illness.Articles were excluded if they were (1) review or correspondence articles, (2) non-human subject research and (3) studying persons aged <18 years.The following biological samples were considered for inclusion: skeletal muscle, diaphragm, blood, serum and plasma.

Data extraction
A standardized form was used to extract trial features (authors, published year), study and patient characteristics (study design, methods, outcomes, healthy/disease, age) and the subset of differentially expressed miRs.Extracted information was verified by a third reviewer.The risk of bias in individual studies was assessed using either two measurement methods or two study groups.All studies used at least two study groups, and no studies validated the results in an independent cohort of patients.No studies validated the results using an additional miR level profiling tool.

Outcome measure
The outcome of interest was differential expression of miRs in human skeletal muscle, diaphragm, blood, serum or plasma with at least 1.5-fold changes and/or statistically significant change (p < 0.05) using RT-qPCR, microarray or RNA-sequencing.Circulating miRs may be useful as non-invasive biomarkers, whereas miRs in muscle or diaphragm may provide mechanistic insight into muscle wasting during critical illness.

Synthesis of results and additional analyses
miRs reported as differentially expressed in muscle, blood, serum or plasma were considered in association with critical illness and muscle wasting.An independent literature search was performed to identify miR function and target genes.A narrative synthesis of findings from these studies was also provided.

Study characteristics
A total of 537 citations were identified through the search (Figure 1).After duplicates were removed (n = 260), 228 articles were excluded, leaving 49 suitable for full-text review.From these, seven met the inclusion criteria.

MicroRNAs reported as dysregulated in muscle during critical illness
Three miRs were dysregulated in plasma, 27 in muscle and one in serum and blood in the context of muscle wasting in patients during or following an ICU stay (Tables 1-3).MyomiRs miR-1 and miR-133, as well as miR-181a, were dysregulated in both skeletal muscle and plasma of critically ill patients.Among these miRs, no miRs were reported to change expression in the same direction in muscle in more than one article (Table 1).However, as noted in Table 2, the timing of the biopsy was different in the two studies: Biopsies were either taken during or several days after an ICU stay, which may indicate different stages of muscle loss and consecutive repair (Table 1).miR-133a was reported as upregulated in the serum of critically ill patients and was proposed to be associated with muscle weakness.miR-133a was also downregulated in skeletal muscle of critically ill patients (Tables 2 and 3).MiRs regulated during critical illness or COVID-19, but not investigated in the context of muscle wasting, are included in Tables S2 and S3.

microRNAs dysregulated in muscle
Twenty-seven miRs were changed in the muscle of critically ill patients (Table 2).Of these, miR-424-5p and miR-542-5p were reported in two articles. 38,41However, the direction of expression changes of these miRs was opposite in the two articles, likely due to difference in the time of the analysis: miR-424-5p and miR-542-5p were quantified either during surgery in patients requiring an aortic surgery or with established ICU-AW 41 or 7 days post ICU discharge 38 (Table 2).The expression levels of several microRNAs, including muscle-enriched myomiRs, with roles previously demonstrated in muscle atrophy, myopathies or muscle ageing (reviewed in Sannicandro et al. 42 ), were also reported to change during/post ICU stay in patients who developed ICU-AW: miR-1, miR-133, miR-181, miR-29, miR-499, miR-422 (Table 2).

Circulating microRNAs in critical illness
miR levels have also been investigated in the blood, plasma and serum of patients admitted to ICU, as potential non-invasive biomarkers of ICU-AW/muscle wasting (Table 3).Although multiple studies investigated the levels of circulating miRs in the context of critical illness, only two articles examined these in the context of muscle wasting/ICU-AW.Circulating miRs levels, which were altered during critical illness or COVID-19 that were identified in critically ill patients but not investigated in the context of muscle wasting, are included in Table S3.The approach to determine the levels of miRs used in the reported studies was qPCR.Hence, no study to date examined changes in circulating microRNAs in ICU pa-tients using a high-throughput approach.Not surprisingly, myomiRs miR-1, miR-133a, as well as miR-181a, with known roles in skeletal muscle, were selected for quantification in these studies (Table 3).mir-181a and miR-1 were transiently increased in ICU patients with muscle wasting as compared to patients without muscle wasting (Days 1 and 2 only).miR-16 was significantly decreased at Day 1 compared to baseline in muscle wasting and non-wasting ICU patients. 20iR-16 is ubiquitously expressed and serum miR-16 has been shown to be upregulated in patients with sepsis as compared to healthy controls.46 The levels of circulating miR-1 and miR-133 were reported to be increased in other conditions associated with muscle wasting, for example, chronic obstructive pulmonary disease (COPD) and following exercise.39,40,47 Figure 1 PRISMA flow chart of the systematic review of articles, based on.45 Note no automation tools were used.Exclusion criteria: (1) article type; (2) language; (3) organism; (4)

Discussion
This systematic review summarizes the available literature on miRs in the of muscle wasting and ICU-AW following critical illness.As several miR-based therapies are currently undergoing clinical trials, this review provides a theoretical underpinning for their potential in patients with critical illnesses.Furthermore, with evidence emerging of the role of miR as biomarkers, monitoring miR levels can potentially be utilized to identify those at high risk of poor outcomes following critical illness to target personalized, adaptive rehabilitation pathways.

MicroRNAs are highly correlated with muscle strength in ICU patients
miR-424-5p was increased in the muscle of patients with established ICU-AW, as compared to age-matched healthy controls. 13In a cohort of older patients, miR-424-5p was also higher in sarcopenic individuals; however, the correlation be-tween miR levels and reduced physical performance measured by 6 m timed up and go or 3-min gait speed was not statistically significant (r = 0.23, p = 0.066 and r = 0.23, p = 0.069). 13miR-424 levels, however, were higher in patients who lost >10% cross-sectional area of rectus femoris than in those who lost <10% and was correlated with the amount of muscle loss that the patients would lose over the subsequent 7 days. 13miR-424-5p expression levels positively correlated with the length of ICU stay. 13Walsh et al. have found miR-424-5p levels to positively correlate with quadriceps size and strength and demonstrated a decrease in miR-424-3p/ 5p microRNAs in muscle from patients 7 days post-ICU discharge who developed ICU-AW. 38The complex nature of the association of miR-424-5p with muscle loss and regeneration requires further investigation.However, miRs-424-3p/ 5p were not differentially regulated in 'improvers' versus 'non-improvers' 6 months post discharge.miR-424-5p was shown to target several components of the protein synthesis pathways, such as UBTF, PolR1A and RRN3, known to be affected in muscle of ICU patients. 13miR-424-5p increases the activity of TGF-β pathway, likely regulating muscle  regeneration, 13 and its other targets are associated rRNA synthesis.miR-424-5p has also been shown to target HSPA90AA 48 and inhibit myogenesis, and miR-424-5p levels were also altered in muscle of cachexic patients. 49In addition, data from animal studies support the role of miR-424 as a negative regulator of muscle growth; specifically, ectopic expression of miR-322(494)/miR-503, driven by muscle-specific creatine kinase promoter, has been shown to inhibit muscle growth through decreased levels of translation initiation factors, but not the IGF1/AKT/mTOR signalling pathway. 50Conversely, lower levels of these miRs have been shown to facilitate muscle growth: Supporting these data, Geng et al. demonstrated that miR-322 (494) aggravated dexamethasone-induced atrophy of C2C12 myotubes and in vivo, through regulation of Igf1r and Insr expression. 51ogether, these data suggest that increases in miR-424-5p may be associated with suppression of anabolic processes in muscle during ICU admissions and its subsequent downregulation may be a part of the regenerative process.
Other miRs belonging to miR-424 cluster, including miR-542-3p/miR-542-5p, were upregulated in rectus femoris from patients with ICU-AW, as compared to age-matched healthy controls. 41Levels of miR-542-3p were significantly increased in patients with COPD and correlated with the length of hospital stay; however, this increase was more marked in patients with ICU-AW. 41This suggests that increased levels of miR-542-3p may indicate muscle atrophy in ICU patients.Consistently, the levels of miR-542-3p were higher in the muscle of patients with SOFA (sepsis-related organ failure assessment) scores >10, indicating higher likelihood of death, as compared to those with SOFA scores <10.These miRs have also been altered in other conditions associated with muscle wasting, for example, ALS, 52 suggesting that potentially a common set of miRs may regulate muscle wasting irrespective of underlying pathophysiology.Both miR-424-5p and miR-524-3p/5p were proposed to promote muscle wasting by promoting mitochondrial and cytoplasmic ribosomal stress. 13,41miR-542-3p/5p, similar to miR-424-5p, increases TGF-β signalling by targeting inhibitory components of the TGF-β pathway (SMAD7 and PP2a), consistent with suggested mechanisms underlying muscle wasting in patients with ICU-AW. 41miR-542-3p/5p were downregulated at Day 7 post ICU, 38,41 suggesting that downregulation of these miRs may be needed for muscle repair following critical illness. 38owever, they were not differentially regulated in muscle of those labelled as 'improvers' compared to 'non-improvers' 6 months after ICU. 38iR-422a levels in the quadriceps of male patients with COPD and those undergoing aortic surgery and in ICU were positively associated with muscle strength (maximal voluntary contraction), whereas pre-surgery levels of miR-422a were inversely associated with the amount of muscle lost in the first post-operative week.53 This suggests that miR-422a expression in muscle may have the potential to predict mus-cle wasting in chronic and acute disease.53 Paul et al. 53 further demonstrated that individuals with higher levels of miR-422a lost less muscle than those with low levels of miR-422a and suggested that miR-422a, through regulation of Smad2, Smad3 and Smad4, may confer the resistance to catabolism by reducing the activity of the TGF-ß signalling in skeletal muscle, which is a known mediator of muscle wasting.
miR-29b-3p was downregulated in the muscle of ICU patients at Day 7 post discharge from ICU, whereas its target genes were upregulated at this time point. 38However, there was no difference detected in miR-29b-3p levels between 'improvers' and 'non-improvers' 6 months after ICU discharge.Downregulation of miR-29b may enable muscle hypertrophy at day seven post ICU, as miR-29b has been shown to promote muscle atrophy, whereas its inhibition attenuated muscle atrophy following immobilization through regulation of IGF-1 and PI3K. 54YY1 (myogenesis suppressing transcription factor) has been shown to increase the levels of miR-29b, 54 whereas Wang et al. have shown that miR-29 is downregulated in muscle of mice with chronic kidney disease and suggested that concomitant upregulation of YY1 may be associated with miR-29 downregulation, as YY1 is a predicted target of this miR. 55miR-29b-3p has also been shown to be increased in the muscle tissue and plasma of mice during ageing and increased in the plasma of older people. 56Circulating levels of miR-29b were also negatively correlated with relative appendicular skeletal muscle. 56Together, these data support the negative regulation of muscle mass by miR-29b.

Circulating microRNAs dysregulated in critical illness
Serum biomarkers have been suggested as promising non-invasive tools for the diagnosis of muscle atrophy. 57hanges in the levels of circulating miRs have been examined as biomarkers for the early detection of muscle weakness during an ICU stay with critical illness and of poor outcomes following hospital discharge. 46,57,588][59][60][61][62][63][64] Wang et al. reported that serum levels of several myomiRs included (see Table 3) may have a role as biomarkers for muscle atrophy diagnosis. 57iR-1, miR-133, miR-206 and miR-499 are called myomiRs due to their high expression in muscle and important function in muscle differentiation, regeneration, development and fibre type remodelling.39,57,58 In addition, they have also been proposed as biomarkers in DMD and myotonic dystrophies.57,58 These miRs detected in plasma of stable COPD patients and were suggested to be derived from atrophying muscle or, more muscle with elevated turnover.40 The authors also suggested that combining plasma levels of miRs with physiological parameters improves the identification of type I fibre shift in some patients.
miR-1, increased in plasma of ICU patients, 20,40 has also been reported to be increased in plasma from patients suffering from rare muscular dystrophy, LGMD2A. 58Circulating miR-133, increased in ICU patients, has also been proposed as a biomarker for type 2 diabetes. 58miR-206 has been broadly studied in the context of muscle wasting due to its role in muscle differentiation, regeneration and development. 58It was suggested as a biomarker of muscle wasting, 58 since it was increased in the blood of patients with various forms of muscle atrophy and was associated with disease severity. 58Circulating miR-181a levels were reported to be higher on the second post-operative day in patients who underwent high-risk cardiothoracic surgery, particularly in those who developed muscle wasting at 1 week compared to those who did not develop muscle wasting. 20miR-181a, therefore, has been proposed as a strong candidate biomarker of muscle wasting.Together, several miRs have been reported to be increased in the circulation during conditions associated with muscle wasting.However, care needs to be taken in the interpretation of these results as these miRs are also expressed/enriched in other tissues, for example, miR-1 is highly expressed in cardiac muscle.Therefore, the increase of miR-1 levels in circulation could be due to cardiac pathology in addition/instead of muscle wasting.Further research is needed to fully establish the diagnostic/ prognostic potential of miRs as biomarkers of muscle wasting.

miRs differentially regulated in both muscle and circulation and their potential function in muscle wasting related to critical illness
microRNAs miR-1, miR-133a and miR-181a, which are highly expressed in muscle, were reported to be downregulated during ICU stay and upregulated in the circulation of ICU patients but not after 7 days post ICU discharge (Tables 2 and 3).The decreased levels of these miRs in muscle may indicate their mechanistic role in muscle wasting, while their increased levels in plasma/serum/blood during critical illness associated with muscle wasting/ICU-AW suggests the potential release of these miRs from damaged muscle and has been demonstrated in other muscle wasting conditions, suggesting that this profile may be characteristic of muscle wasting but perhaps not exclusive to ICU. 58,62,65,66 miR-133 levels were found to be elevated in patients with sepsis and treated in ICU in a study performed by Tacke et al., 47 and miR-133a levels were also high in non-septic patients suffering from cardiopulmonary critical illness. 47In those patients, serum levels of miR-133a were associated with myocardial infarction and muscle injury. 47In addition, Pegoraro et al. compared myomiR levels in the serum of patients with myotonic dystrophy (MD1), a rare muscle disorder, against control subjects. 39miR-133a and miR-1 are enriched in cardiac and skeletal muscle, 67 and both regulate muscle development. 39,40,47,679][70][71] miR-133a has been proposed to regulate insulin-like growth factor-1 receptor (IGF1R) in C2C12 myoblasts, although the phenotypic changes associated with regulation of Igf1r by miR-133 were not reported in this study. 724][75] However, miR-133-deficient mice showed impaired exercise tolerance, mitochondrial biogenesis and muscle fibre maintenance. 76iR-133 has also been shown to regulate energy metabolism through targeting Prdm16. 77,78Given the contradictory findings concerning the role of miR-133 in muscle, possibly suggesting that it is highly context-dependent, downregulation of miR-133a in muscle during critical illness may contribute to muscle wasting and its upregulation in circulation may suggest muscle breakdown in critically ill patients, further research is needed to clarify its role in ICU-AW.
The role of miR-1 has also been extensively studied (summary in Safa et al. 79 ).miR-1 is generally considered a positive regulator of myogenesis. 71Another study suggested that miR-1 mediates dexamethasone-induced atrophy of C2C12 myotubes through targeting Hsp70, therefore promoting upregulation of ubiquitin ligases Murf-1 and atrogin-1 and enhancing the activity of Foxo3 nuclear factor, an important signalling in muscle atrophy. 80myomiRs, including miR-1, have been proposed to mediate muscle adaptation to exercise, 81 whereas lifelong reduction of myomiRs did not adversely affect skeletal muscle morphology. 82miR-206, another canonical myomiR with roles in muscle development and maintenance of muscle homeostasis in adulthood, was also downregulated in muscle of ICU patients. 39,40irculating miR-206 levels have been reported higher in COPD patients. 40miR-206 delivery into injured muscle have been previously shown to contribute to increased regenerative capacity of skeletal muscle. 83,84Furthermore, miR-1/ 133/206 were demonstrated to be critical regulators of the DOK7-CRK-RAC1 signalling cascade, which is critical for the stabilisation and anchoring of post-transcriptional AChRs during NMJ development and maintenance. 85On the other hand, inhibition of miR-206 partially protected from denervation-induced atrophy through regulating YY1 and eIF4E3, translation initiation factor.This may suggest context-specific function of miRs, as suggested by the authors and previously discussed. 84,86It is noteworthy that level of redundancy may exist between miR-1 and miR-206 given the same seed sequences, which direct microRNA-target interactions.miR-1 and miR-206 were upregulated in all patients with MD1 when compared to healthy controls. 39nterestingly, miR-1, miR-206 and myostatin levels were downregulated after the patients completed a rehabilitation protocol, correlating with an increase in muscle strength and resistance. 39These myomiRs were not associated with muscular disorders, such as muscular dystrophy or myositis 63 in a study by Eisenberg et al., while other groups proposed these miRs as biomarkers of muscular dystrophy. 66Trifunov et al. demonstrated changes in the levels of circulating miR-206, as well as miR-181a, in patients with DMD or Becker muscular dystrophy, highlighting the importance of the timing of tissue sampling. 87t could be considered that due to miR-1, miR-206 and miR-133, accounting for over 25% of muscle miR content, 88 as the muscle undergoes atrophy, their relative levels also decline, and this decline may not necessarily contribute to mechanistic changes underlying muscle atrophy.The most consistent role of these myomiRs in the literature to date is their positive regulation of myogenesis, which could support a hypothesis that their role is more critical in muscle regeneration rather than muscle atrophy.This remains to be established.
miR-181a was downregulated in muscle and increased in the blood of patients with ICU-AW. 20miR-181a has been previously reported to regulate muscle homeostasis through the regulation of mitochondrial dynamics 89 and mitochondrial function in neural and immune cells (reviewed in Borja-Gonzalez et al. 90 ).miR-181a may be considered a potential early biomarker for the development of acute muscle wasting during ICU stay, corresponding to a reduction in the cross-sectional area of the rectus femoris. 20miR-181a may regulate mechanistic processes occurring in the muscle of critically ill patients through genes associated with mitochondrial dynamics. 25,43,91,92miR-181a has also been shown to reduce TNF-alpha-induced inflammatory response, 93 and its levels, as well as miR-1 levels, were reported to increase in quadriceps femoris muscle following endurance exercise suggesting a potential role in muscle adaptation to exercise. 94ogether, these miRs are key regulators of muscle development and homeostasis in adulthood by regulating signalling pathways and factors critical to the maintenance of muscle mass and function (Figure 2).Their precise role in muscle wasting during critical illness, however, remains unknown.It, therefore, remains to be established whether their increased levels in the circulation during muscle wasting conditions, such as critical illness, are associated with the damage of muscle tissue and release of its contents into circulation or whether these could indicate a mechanistic insight into muscle wasting during critical illness.

Limitations of the study
While the levels of multiple miRs were reported to change in serum/plasma and/or muscle during critical illness, this review found few articles that reported consistent changes in the expression levels of specific miRs.This highlights the need for more research in this area.It is noteworthy that several studies investigated the levels of miRs in muscle during ICU stay, while Walsh et al. investigated miR levels after ICU discharge. 38Moreover, between different articles, different population characteristics, disorders, disease severity and methodologies were used to detect miRs (Table 1).The levels of miR can change rapidly 87 ; hence, the timing of the sample collection would also influence the levels of miRs investigated.In this regard, the study by Walsh et al. 38 could indicate miRs with a potential role in muscle after discharge from ICU, while other studies reported in Table 2 may indicate changes in miRs mechanistically involved in muscle wasting in the early stages of critical illness or miRs persistently dysregulated in patients with established ICU-AW.It is therefore not surprising that changes in miR levels reported in different studies demonstrated different direction of these changes.This also supports the need for further investigation of the role of miRs in the development of ICU-AW and their potential as biomarkers.Another limitation of studies reported here is that no study validated the findings in an independent cohort of patients; hence, the data reported here in our review relate to only a small number of patients.However, some of the findings were consistent with changes in miR levels observed in other disorders associated with muscle wasting, which further supports the findings reported here.
Together, while the data presented here indicate the potential role of selected miRs as candidate biomarkers of muscle wasting in ICU patients, their role in pathogenesis is not clear and requires further in-depth study.Circulating miRs reported here are enriched in skeletal muscle; however, they are also expressed in other tissues, for example, cardiac muscle.Hence, miRs reported in the circulation of critically ill patients may not be specifically associated with muscle wasting but rather indicate organ failure, 20 suggesting that careful miR-level data analysis is required taking into account individual patient data on organ function, co-morbidities and other relevant clinical information.Future studies are required to validate these data in a larger cohort of patients to determine whether miRs truly are potential biomarkers with the potential to assess muscle wasting post-critical illness.This review also emphasizes the need for further studies into mechanisms of muscle wasting and ICU-AW following critical illness.

Conclusions
The pathogenesis underlying muscle wasting during critical illness, as well as the molecular basis of recovery from critical illness and interpersonal variability, are not fully understood.This systematic review examined the potential of miRs in the context of muscle wasting during critical illness.Based on available published data, we have identified a panel of miRs, levels of which are altered in the muscle and blood, serum or plasma, of critically ill patients, either during ICU stay or post ICU discharge, indicating their role in either muscle wasting or the recovery phase of critical illness.We have also described circulating miRs, which may have the potential to lead to effective screening, monitoring or treatment strategies for muscle wasting during critical illness.However, further research is warranted to validate the findings presented here.
: patients stayed in ICU a minimum of 24 h and developed ICU-AW/muscle wasting.ICU-AW*: prolonged ICU stay and (>1 week) mechanical ventilation with ICU-AW/muscle wasting.&: measured during ICU stay.*: measured 7 days post-ICU.microRNAs in critical illness muscle wasting

Figure 2
Figure 2 Summary of the potential role of dysregulated microRNAs during muscle loss and consecutive recovery from critical illness in the development of muscle wasting and intensive care unit-acquired weakness (ICU-AW).microRNA function is presented in relation to the mechanistic changes underlying muscle wasting during critical illness.Created with BioRender.com.

Table 1
study no related to skeletal muscle or ICU stay; (5) age of participants.microRNAs in critical illness muscle wasting Summary of patient cohorts from articles included in Tables2 and 3 6MWD, 6-min walk distance; ARDS, adult respiratory distress syndrome; COPD, chronic obstructive pulmonary disease; CSA, cross-sectional area; ECMO, extracorporeal membrane oxygenation; ICU-AW, intensive care unit acquired weakness; IHD, ischaemic heart disease; MVC, maximum voluntary contraction.

Table 2
MicroRNAs dysregulated in muscle during or post ICU

Table 3
Circulating microRNAs levels of which were reported to change during critical illness with accompanied muscle wasting