Extracellular vesicle transfer of miR-1 to adipose tissue modifies lipolytic pathways following resistance exercise.

Extracellular vesicles (EVs) have emerged as important mediators of inter-tissue signaling and exercise adaptations. In this human study (n = 32), we provide evidence that muscle-specific microRNA-1 (miR-1) was transferred to adipose tissue via EVs following an acute bout of resistance exercise. Using a multi-model machine learning automation tool, we discovered muscle primary miR-1 transcript and CD63+ EV count in circulation as top explanatory features for changes in adipose miR-1 levels in response to resistance exercise. RNA-sequencing (RNA-seq) and in-silico prediction of miR-1 target genes identified caveolin 2 (CAV2) and tripartite motif containing 6 (TRIM6) as miR-1 target genes downregulated in the adipose tissue of a subset of participants with the highest increases in miR-1 levels following resistance exercise (n = 6). Overexpression of miR-1 in differentiated human adipocyte-derived stem cells downregulated these miR-1 targets and enhanced catecholamine-induced lipolysis. These data identify a potential EV-mediated mechanism by which skeletal muscle communicates to adipose tissue and modulates lipolysis via miR-1.

Satellite cell dynamics during skeletal muscle hypertrophy.

Skeletal muscle stem cells (MuSCs) display distinct behavior crucial for tissue maintenance and repair. Upon activation, MuSCs exhibit distinct modes of division: symmetric division, facilitating either self-renewal or differentiation, and asymmetric division, which dictates divergent cellular fates. This review explores the nuanced dynamics of MuSC division and the molecular mechanisms governing this behavior. Furthermore, it introduces a novel phenomenon observed in a subset of MuSCs under hypertrophic stimuli termed division-independent differentiation. Insights into the underlying mechanisms driving this process are discussed, alongside its broader implications for muscle physiology.

microRNA-1 Regulates Metabolic Flexibility in Skeletal Muscle via Pyruvate Metabolism.

MicroRNA-1 (miR-1) is the most abundant miRNA in adult skeletal muscle. To determine the function of miR-1 in adult skeletal muscle, we generated an inducible, skeletal muscle-specific miR-1 knockout (KO) mouse. Integration of RNA-sequencing (RNA-seq) data from miR-1 KO muscle with Argonaute 2 enhanced crosslinking and immunoprecipitation sequencing (AGO2 eCLIP-seq) from human skeletal muscle identified miR-1 target genes involved with glycolysis and pyruvate metabolism. The loss of miR-1 in skeletal muscle induced cancer-like metabolic reprogramming, as shown by higher pyruvate kinase muscle isozyme M2 (PKM2) protein levels, which promoted glycolysis. Comprehensive bioenergetic and metabolic phenotyping combined with skeletal muscle proteomics and metabolomics further demonstrated that miR-1 KO induced metabolic inflexibility as a result of pyruvate oxidation resistance. While the genetic loss of miR-1 reduced endurance exercise performance in mice and in C. elegans, the physiological down-regulation of miR-1 expression in response to a hypertrophic stimulus in both humans and mice causes a similar metabolic reprogramming that supports muscle cell growth. Taken together, these data identify a novel post-translational mechanism of adult skeletal muscle metabolism regulation mediated by miR-1.

Skeletal muscle hypertrophy: cell growth is cell growth.

Roberts et al. have provided an insightful counterpoint to our review article on the utility of the synergist ablation model. The purpose of this review is to provide some further dialogue regarding the strengths and weaknesses of the synergist ablation model. Specifically, we highlight that the robustness of the model overshadows surgical limitations. We also compare the transcriptomic responses to synergist ablation in mice and resistance exercise in humans to identify common pathways. We conclude that "cell growth is cell growth" and that the mechanisms available to cells to accumulate biomass and increase in size are similar across cell types and independent of the rate of growth.

The utility of the rodent synergist ablation model in identifying molecular and cellular mechanisms of skeletal muscle hypertrophy.

Skeletal muscle exhibits remarkable plasticity to adapt to stimuli such as mechanical loading. The mechanisms that regulate skeletal muscle hypertrophy due to mechanical overload have been thoroughly studied. Remarkably, our understanding of many of the molecular and cellular mechanisms that regulate hypertrophic growth were first identified using the rodent synergist ablation (SA) model and subsequently corroborated in human resistance exercise training studies. To demonstrate the utility of the SA model, we briefly summarize the hypertrophic mechanisms identified using the model and the following translation of these mechanism to human skeletal muscle hypertrophy induced by resistance exercise training.

Feasibility of a cinematic-virtual reality training program about opioid use disorder for osteopathic medical students: a single-arm pre-post study.

CONTEXT: Opioid use disorder (OUD) has a considerable morbidity and mortality in the United States. Healthcare providers are key points of contact for those with OUD; however, some providers may hold stigma toward OUD. Stigma toward OUD can lead to lower quality of care and more negative health outcomes. Thus, new trainings designed to reduce stigma toward OUD while increasing empathy are critical. We created a web-based cinematic virtual reality (cine-VR) training program on OUD for osteopathic medical students. OBJECTIVES: The aim of this pilot study was to assess changes in stigma toward OUD and empathy before and after the online cine-VR training program on OUD. METHODS: We employed a single-arm, pre- and posttest pilot study to assess changes in stigma toward OUD and empathy. Osteopathic medical students from one large medical school in the Midwest with three campuses were invited to participate in the online cine-VR training. Participants completed two surveys before and after the cine-VR training. We performed paired t tests to examine changes in stigma toward OUD and empathy scores before and after the cine-VR OUD training program. RESULTS: A total of 48 participants completed the training. We observed a decrease in stigma toward OUD posttraining (t=4.402, p<0.001); this change had a Cohen's d of 0.64, indicating a medium effect. We also observed an increase in participants' empathy scores posttraining (t=-2.376, p=0.023), with a Cohen's d of 0.40 signifying a small effect. CONCLUSIONS: Findings from this pilot study suggest that the online cine-VR training may reduce stigma toward OUD while increasing empathy. Future research employing a randomized controlled trial design with a larger, more diverse sample and a proper attention control condition is needed to confirm the effectiveness of the online cine-VR training. If confirmed, this cine-VR training may be an accessible approach to educating osteopathic medical students about OUD.

The rRNA epitranscriptome and myonuclear SNORD landscape in skeletal muscle fibers contributes to ribosome heterogeneity and is altered by a hypertrophic stimulus.

In cell biology, ribosomal RNA (rRNA) 2'O-methyl (2'-O-Me) is the most prevalent posttranscriptional chemical modification contributing to ribosome heterogeneity. The modification involves a family of small nucleolar RNAs (snoRNAs) and is specified by box C/D snoRNAs (SNORDs). Given the importance of ribosome biogenesis for skeletal muscle growth, we asked if rRNA 2'-O-Me in nascent ribosomes synthesized in response to a growth stimulus is an unrecognized mode of ribosome heterogeneity in muscle. To determine the pattern and dynamics of 2'-O-Me rRNA, we used a sequencing-based profiling method called RiboMeth-seq (RMS). We applied this method to tissue-derived rRNA of skeletal muscle and rRNA specifically from the muscle fiber using an inducible myofiber-specific RiboTag mouse in sedentary and mechanically overloaded conditions. These analyses were complemented by myonuclear-specific small RNA sequencing to profile SNORDs and link the rRNA epitranscriptome to known regulatory elements generated within the muscle fiber. We demonstrate for the first time that mechanical overload of skeletal muscle 1) induces decreased 2'-O-Me at a subset of skeletal muscle rRNA and 2) alters the SNORD profile in isolated myonuclei. These findings point to a transient diversification of the ribosome pool via 2'-O-Me during growth and adaptation in skeletal muscle. These findings suggest changes in ribosome heterogeneity at the 2'-O-Me level during muscle hypertrophy and lay the foundation for studies investigating the functional implications of these newly identified "growth-induced" ribosomes.NEW & NOTEWORTHY Ribosomal RNAs (rRNAs) are posttranscriptionally modified by 2'O-methyl (2'-O-Me). This study applied RiboMeth-seq (RMS) to detect changes in 2'-O-Me levels during skeletal muscle hypertrophy, uncovering transient diversification of the ribosome pool in skeletal muscle fibers. This work implies a role for ribosome heterogeneity in skeletal muscle growth and adaptation.

Microbial-Derived Exerkines Prevent Skeletal Muscle Atrophy.

Regular exercise yields a multitude of systemic benefits, many of which may be mediated through the gut microbiome. Here, we report that cecal microbial transplants (CMTs) from exercise-trained vs. sedentary mice have modest benefits in reducing skeletal muscle atrophy using a mouse model of unilaterally hindlimb-immobilization. Direct administration of top microbial-derived exerkines from an exercise-trained gut microbiome preserved muscle function and prevented skeletal muscle atrophy.

A Case of Metastatic Chromophobe Renal Cell Carcinoma Masked as Suspected Hepatic Abscesses.

Background: Characterizing multiple hepatic lesions on cross-sectional imaging, particularly differentiating abscesses from metastatic lesions, can be challenging. Case Presentation: A male aged 53 years with a history of chromophobe renal cell carcinoma presented with fevers and abdominal pain and was found to have multiple hepatic lesions concerning for hepatic abscesses. The lesions initially evaded diagnosis on imaging, laboratory tests, and biopsy, but ultimately were determined to be a rare case of metastatic chromophobe renal cell carcinoma of the liver. Conclusions: The finding of multiple new liver lesions on imaging during a febrile illness is concerning for hepatic abscess or malignancy, which can be difficult to diagnose with imaging alone. Differentiation between infectious and neoplastic etiologies may require additional imaging and/or tissue sampling.

Associations of Mortality Outcomes With Employment Status at Discharge From VA Vocational Rehabilitation Service Programs.

OBJECTIVE: The authors evaluated associations between employment at discharge from Veterans Health Administration Vocational Rehabilitation Service (VR) programs and suicide and other causes of death. METHODS: For veterans receiving VR between October 1, 2005, and September 30, 2014 (N=78,293), proportional hazards analyses were used to test associations of employment with suicide, drug overdose, and external and natural cause mortality rates over 1 and 5 years postdischarge and through December 31, 2019. The analyses were adjusted for clinical and sociodemographic characteristics and propensity for employment. RESULTS: Of the veterans, 94.1% had a psychiatric diagnosis, and 35.5% were employed at VR discharge. In proportional hazards analyses, employment was associated with lower mortality rates through 1 year (suicide, hazard ratio [HR]=0.54; overdose, HR=0.70; external causes, HR=0.62; and natural causes, HR=0.51) and 5 years postdischarge (overdose, HR=0.72; external causes, HR=0.81; and natural causes, HR=0.72). Through December 31, 2019, employment was associated with lower risks for overdose (HR=0.80) and death by external (HR=0.81) and natural (HR=0.80) causes. CONCLUSIONS: Employment at VR discharge was associated with lower mortality risk among veterans with psychiatric diagnoses.

Combination therapy of itraconazole and an acylhydrazone derivative (D13) for the treatment of sporotrichosis in cats.

Acylhydrazone (AH) derivatives represent a novel category of anti-fungal medications that exhibit potent activity against Sporothrix sp., both in vitro and in a murine model of sporotrichosis. In this study, we demonstrated the anti-fungal efficacy of the AH derivative D13 [4-bromo-N'-(3,5-dibromo-2-hydroxybenzylidene)-benzohydrazide] against both planktonic cells and biofilms formed by Sporothrix brasiliensis. In a clinical study, the effect of D13 was then tested in combination with itraconazole (ITC), with or without potassium iodide, in 10 cats with sporotrichosis refractory to the treatment of standard of care with ITC. Improvement or total clinical cure was achieved in five cases after 12 weeks of treatment. Minimal abnormal laboratory findings, e.g., elevation of alanine aminotransferase, were observed in four cats during the combination treatment and returned to normal level within a week after the treatment was ended. Although highly encouraging, a larger and randomized controlled study is required to evaluate the effectiveness and the safety of this new and exciting drug combination using ITC and D13 for the treatment of feline sporotrichosis. IMPORTANCE: This paper reports the first veterinary clinical study of an acylhydrazone anti-fungal (D13) combined with itraconazole against a dimorphic fungal infection, sporotrichosis, which is highly endemic in South America in animals and humans. Overall, the results show that the combination treatment was efficacious in ~50% of the infected animals. In addition, D13 was well tolerated during the course of the study. Thus, these results warrant the continuation of the research and development of this new class of anti-fungals.

Walking net V ˙ O2 rises with advancing age in older women: where to go from here?

PURPOSE: Walking net V ˙ O2 tends to increase with advancing age; however, factors contributing to this relationship have not been widely described. The implications of such findings could inform targeted strategies to promote independent mobility in older adults. Herein, we evaluated the relationship between net V ˙ O2 and age at two submaximal workloads while exploring potential moderators of this relationship. METHODS: Secondary analyses were performed on 35 older (65 ± 3 years) women who completed a battery of physical assessments including fixed-speed, non-graded and graded (+ 2.5%) treadmill walking with indirect calorimetry to determine net V ˙ O2. Maximal oxygen uptake ( V ˙ O2max), knee extensor maximal isometric voluntary contraction (MVC), peak rate of torque development (RTD), and plantar flexor range-of-motion (PFROM) were also measured. RESULTS: Bivariate correlations showed non-graded (r = 0.403, p = 0.017) and graded (r = 0.413, p = 0.014) net V ˙ O2 were positively related to age. Notably, these relationships strengthened after adjusting for V ˙ O2max. Regression modeling showed age, RTD:MVC ratio (composite of muscle performance), and PFROM together explained 49% and 34% of the variance in non-graded and graded net V ˙ O2, respectively. Further analyses suggested knee extensor MVC moderates the relationship between non-graded net V ˙ O2 and age, accounting for 9% of the variance [ΔR2 = 0.090, F (1,31) = 4.13, p = 0.05]. CONCLUSION: These data support the premise that, in older women, walking net V ˙ O2 rises with advancing age, and additionally, the RTD:MVC ratio and PFROM are independent correlates of non-graded net V ˙ O2. Exercise interventions with a high degree of training specificity including explosive, velocity-based elements may promote independent mobility in older women.

Implementing and evaluating a veterans crisis line quality improvement initiative: The safety planning pilot program.

The Veterans Crisis Line (VCL) is part of the U.S. Department of Veterans Affairs' suicide prevention mission. In 2021, VCL assessed the impact of a pilot implementation project of conducting six-part safety plans (SPs) instead of VCL's usual risk mitigation plan. VCL responders offered to complete six-part SPs with eligible callers. Parametric and nonparametric methods compared call characteristics and Veteran Health Administration (VHA) utilization of eligible callers, by SP completion. We forecasted the operational impact of VCL-wide implementation. 27.37% (N = 448/1,637) of calls to designated responders were eligible for SPs. Of those, 27.23% (N = 122/448) completed SPs. Common barriers were call interruptions and the veteran declining. Among veteran callers who use VHA, SP completers were more likely to accept clinical referrals and had more outpatient mental health appointments before and after their VCL call. Calls involving SPs had a call plus documentation time 175% longer than eligible calls without SPs (87.78 vs. 49.66 min). If SPs were implemented VCL-wide, this would require 3-5(4.12%) more responders per hour to maintain current VCL call answer speed. SPs are adaptable to VCL; however, implementation presents logistical barriers. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Impact of Implementation Facilitation on the REACH VET Clinical Program for Veterans at Risk for Suicide.

OBJECTIVE: In 2017, the Veterans Health Administration (VHA) implemented a national suicide prevention program, called Recovery Engagement and Coordination for Health-Veterans Enhanced Treatment (REACH VET), that uses a predictive algorithm to identify, attempt to reach, assess, and care for patients at the highest risk for suicide. The authors aimed to evaluate whether facilitation enhanced implementation of REACH VET at VHA facilities not meeting target completion rates. METHODS: In this hybrid effectiveness-implementation type 2 program evaluation, a quasi-experimental pre-post design was used to assess changes in implementation outcome measures evaluated 6 months before and 6 months after onset of facilitation of REACH VET implementation at 23 VHA facilities. Measures included percentages of patients with documented coordinator and provider acknowledgment of receipt, care evaluation, and outreach attempt. Generalized estimating equations were used to compare differences in REACH VET outcome measures before and after facilitation. Qualitative interviews were conducted with personnel and were explored via template analysis. RESULTS: Time had a significant effect in all outcomes models (p<0.001). An effect of facilitation was significant only for the outcome of attempted outreach. Patients identified by REACH VET had significantly higher odds of having a documented outreach attempt after facilitation of REACH VET implementation, compared with before facilitation. Site personnel felt supported and reported that the external facilitators were helpful and responsive. CONCLUSIONS: Facilitation of REACH VET implementation was associated with an improvement in outreach attempts to veterans identified as being at increased risk for suicide. Outreach is critical for engaging veterans in care.

Impact of the Communities That HEAL Intervention on Buprenorphine-Waivered Practitioners and Buprenorphine Prescribing: A Prespecified Secondary Analysis of the HCS Randomized Clinical Trial.

Importance: Buprenorphine significantly reduces opioid-related overdose mortality. From 2002 to 2022, the Drug Addiction Treatment Act of 2000 (DATA 2000) required qualified practitioners to receive a waiver from the Drug Enforcement Agency to prescribe buprenorphine for treatment of opioid use disorder. During this period, waiver uptake among practitioners was modest; subsequent changes need to be examined. Objective: To determine whether the Communities That HEAL (CTH) intervention increased the rate of practitioners with DATA 2000 waivers and buprenorphine prescribing. Design, Setting, and Participants: This prespecified secondary analysis of the HEALing Communities Study, a multisite, 2-arm, parallel, community-level, cluster randomized, open, wait-list-controlled comparison clinical trial was designed to assess the effectiveness of the CTH intervention and was conducted between January 1, 2020, to December 31, 2023, in 67 communities in Kentucky, Massachusetts, New York, and Ohio, accounting for approximately 8.2 million adults. The participants in this trial were communities consisting of counties (n = 48) and municipalities (n = 19). Trial arm randomization was conducted using a covariate constrained randomization procedure stratified by state. Each state was balanced by community characteristics including urban/rural classification, fatal opioid overdose rate, and community population. Thirty-four communities were randomized to the intervention and 33 to wait-list control arms. Data analysis was conducted between March 20 and September 29, 2023, with a focus on the comparison period from July 1, 2021, to June 30, 2022. Intervention: Waiver trainings and other educational trainings were offered or supported by the HEALing Communities Study research sites in each state to help build practitioner capacity. Main Outcomes and Measures: The rate of practitioners with a DATA 2000 waiver (overall, and stratified by 30-, 100-, and 275-patient limits) per 100 000 adult residents aged 18 years or older during July 1, 2021, to June 30, 2022, were compared between the intervention and wait-list control communities. The rate of buprenorphine prescribing among those waivered practitioners was also compared between the intervention and wait-list control communities. Intention-to-treat and per-protocol analyses were performed. Results: A total of 8 166 963 individuals aged 18 years or older were residents of the 67 communities studied. There was no evidence of an effect of the CTH intervention on the adjusted rate of practitioners with a DATA 2000 waiver (adjusted relative rate [ARR], 1.04; 95% CI, 0.94-1.14) or the adjusted rate of practitioners with a DATA 2000 waiver who actively prescribed buprenorphine (ARR, 0.97; 95% CI, 0.86-1.10). Conclusions and Relevance: In this randomized clinical trial, the CTH intervention was not associated with increases in the rate of practitioners with a DATA 2000 waiver or buprenorphine prescribing among those waivered practitioners. Supporting practitioners to prescribe buprenorphine remains a critical yet challenging step in the continuum of care to treat opioid use disorder. Trial Registration: ClinicalTrials.gov Identifier: NCT04111939.

Impact of Primary Care-Mental Health Care Integration on Mental Health Care Engagement Across Racial and Ethnic Groups.

OBJECTIVE: Receiving mental health services as part of primary care in the Veterans Health Administration (VHA) might increase engagement in specialty mental health care. The authors reexamined the association between primary care-mental health integration (PCMHI) and continued engagement in specialty mental health care for VHA patients and assessed differences by race and ethnicity. METHODS: The study included 437,051 primary care patients with a first in-person specialty mental health encounter in 2015-2016 (no specialty mental health encounters in prior 12 months), including 46,417 patients with new PCMHI encounters in the year before the first specialty mental health encounter. Multivariable logistic regression assessed odds of follow-up specialty mental health care within 3 months of the first specialty mental health encounter. The dependent variable was care engagement (attending a second specialty mental health appointment); independent variables were whether patients were seen by PCMHI on the same day as the primary care appointment ("same-day access"), the time between PCMHI and first specialty mental health appointments, and race and ethnicity. RESULTS: PCMHI was associated with increased engagement in specialty mental health care for all patients, with a greater likelihood of engagement among non-Hispanic White patients. Same-day access to PCMHI was positively associated with care engagement, with no significant differences by race or ethnicity. PCMHI care within 3 months before a first specialty mental health encounter was associated with greater care engagement. CONCLUSIONS: PCMHI, especially same-day access to PCMHI care, may boost engagement in mental health care, although the study design precluded conclusions regarding causal relationships.

Design of aided augmentative and alternative communication systems for children with vision impairment: psychoacoustic perspectives.

Children with complex communication needs often have multiple disabilities including visual impairments that impact their ability to interact with aided augmentative and alternative communication (AAC) systems. Just as the field benefited from a consideration of visual cognitive neuroscience in construction of visual displays, an exploration of psychoacoustics can potentially assist in maximizing the possibilities within AAC systems when the visual channel is either (a) not the primary sensory mode, or (b) is one that can be augmented to ultimately benefit AAC outcomes. The purpose of this paper is to highlight background information about psychoacoustics and present possible future directions for the design of aided AAC system technologies for children with visual impairments who rely on auditory information to learn and utilize AAC.

Inhibition of p53-MDM2 binding reduces senescent cell abundance and improves the adaptive responses of skeletal muscle from aged mice.

Skeletal muscle adaptation to external stimuli, such as regeneration following injury and hypertrophy in response to resistance exercise, are blunted with advanced age. The accumulation of senescent cells, along with defects in myogenic progenitor cell (MPC) proliferation, have been strongly linked as contributing factors to age-associated impairment in muscle adaptation. p53 plays an integral role in all these processes, as upregulation of p53 causes apoptosis in senescent cells and prevents mitotic catastrophe in MPCs from old mice. The goal of this study was to determine if a novel pharmaceutical agent (BI01), which functions by upregulating p53 through inhibition of binding to MDM2, the primary p53 regulatory protein, improves muscle regeneration and hypertrophy in old mice. BI01 effectively reduced the number of senescent cells in vitro but had no effect on MPC survival or proliferation at a comparable dose. Following repeated oral gavage with 2 mg/kg of BI01 (OS) or vehicle (OV), old mice (24 months) underwent unilateral BaCl2 injury in the tibialis anterior (TA) muscle, with PBS injections serving as controls. After 7 days, satellite cell number was higher in the TA of OS compared to OV mice, as was the expression of genes involved in ATP production. By 35 days, old mice treated with BI01 displayed reduced senescent cell burden, enhanced regeneration (higher muscle mass and fiber cross-sectional area) and restoration of muscle function relative to OV mice. To examine the impact of 2 mg/kg BI01 on muscle hypertrophy, the plantaris muscle was subjected to 28 days of mechanical overload (MOV) in OS and OV mice. In response to MOV, OS mice had larger plantaris muscles and muscle fibers than OV mice, particularly type 2b + x fibers, associated with reduced senescent cells. Together our data show that BI01 is an effective senolytic agent that may also augment muscle metabolism to enhance muscle regeneration and hypertrophy in old mice.

Division-Independent Differentiation of Muscle Stem Cells During a Growth Stimulus.

Adult muscle stem cells (MuSCs) are known to replicate upon activation before differentiating and fusing to regenerate myofibers. It is unclear whether MuSC differentiation is intrinsically linked to cell division, which has implications for stem cell population maintenance. We use single-cell RNA-sequencing to identify transcriptionally diverse subpopulations of MuSCs after 5 days of a growth stimulus in adult muscle. Trajectory inference in combination with a novel mouse model for tracking MuSC-derived myonuclei and in vivo labeling of DNA replication revealed an MuSC population that exhibited division-independent differentiation and fusion. These findings demonstrate that in response to a growth stimulus in the presence of intact myofibers, MuSC division is not obligatory.

ApoE isoform does not influence skeletal muscle regeneration in adult mice.

Introduction: Apolipoprotein E (ApoE) has been shown to be necessary for proper skeletal muscle regeneration. Consistent with this finding, single-cell RNA-sequencing analyses of skeletal muscle stem cells (MuSCs) revealed that Apoe is a top marker of quiescent MuSCs that is downregulated upon activation. The purpose of this study was to determine if muscle regeneration is altered in mice which harbor one of the three common human ApoE isoforms, referred to as ApoE2, E3 and E4. Methods: Histomorphometric analyses were employed to assess muscle regeneration in ApoE2, E3, and E4 mice after 14 days of recovery from barium chloride-induced muscle damage in vivo, and primary MuSCs were isolated to assess proliferation and differentiation of ApoE2, E3, and E4 MuSCs in vitro. Results: There was no difference in the basal skeletal muscle phenotype of ApoE isoforms as evaluated by section area, myofiber cross-sectional area (CSA), and myonuclear and MuSC abundance per fiber. Although there were no differences in fiber-type frequency in the soleus, Type IIa relative frequency was significantly lower in plantaris muscles of ApoE4 mice compared to ApoE3. Moreover, ApoE isoform did not influence muscle regeneration as assessed by fiber frequency, fiber CSA, and myonuclear and MuSC abundance. Finally, there were no differences in the proliferative capacity or myogenic differentiation potential of MuSCs between any ApoE isoform. Discussion: Collectively, these data indicate nominal effects of ApoE isoform on the ability of skeletal muscle to regenerate following injury or the in vitro MuSC phenotype.