Myogenic progenitor cells express filamin C in developing and regenerating skeletal muscle

Defective Biomechanics and Pharmacological Rescue of Human Cardiomyocytes with Filamin C Truncations

Actin-binding filamin C (FLNC) is expressed in cardiomyocytes, where it localizes to Z-discs, sarcolemma, and intercalated discs. Although FLNC truncation variants (FLNCtv) are an established cause of arrhythmias and heart failure, changes in biomechanical properties of cardiomyocytes are mostly unknown. Thus, we investigated the mechanical properties of human-induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) carrying FLNCtv. CRISPR/Cas9 genome-edited homozygous FLNCKO-/- hiPSC-CMs and heterozygous knock-out FLNCKO+/- hiPSC-CMs were analyzed and compared to wild-type FLNC (FLNCWT) hiPSC-CMs. Atomic force microscopy (AFM) was used to perform micro-indentation to evaluate passive and dynamic mechanical properties. A qualitative analysis of the beating traces showed gene dosage-dependent-manner "irregular" peak profiles in FLNCKO+/- and FLNCKO-/- hiPSC-CMs. Two Young's moduli were calculated: E1, reflecting the compression of the plasma membrane and actin cortex, and E2, including the whole cell with a cytoskeleton and nucleus. Both E1 and E2 showed decreased stiffness in mutant FLNCKO+/- and FLNCKO-/- iPSC-CMs compared to that in FLNCWT. The cell adhesion force and work of adhesion were assessed using the retraction curve of the SCFS. Mutant FLNC iPSC-CMs showed gene dosage-dependent decreases in the work of adhesion and adhesion forces from the heterozygous FLNCKO+/- to the FLNCKO-/- model compared to FLNCWT, suggesting damaged cytoskeleton and membrane structures. Finally, we investigated the effect of crenolanib on the mechanical properties of hiPSC-CMs. Crenolanib is an inhibitor of the Platelet-Derived Growth Factor Receptor α (PDGFRA) pathway which is upregulated in FLNCtv hiPSC-CMs. Crenolanib was able to partially rescue the stiffness of FLNCKO-/- hiPSC-CMs compared to control, supporting its potential therapeutic role.

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

Adaptations to a cold climate promoted social evolution in Asian colobine primates

The biological mechanisms that underpin primate social evolution remain poorly understood. Asian colobines display a range of social organizations, which makes them good models for investigating social evolution. By integrating ecological, geological, fossil, behavioral, and genomic analyses, we found that colobine primates that inhabit colder environments tend to live in larger, more complex groups. Specifically, glacial periods during the past 6 million years promoted the selection of genes involved in cold-related energy metabolism and neurohormonal regulation. More-efficient dopamine and oxytocin pathways developed in odd-nosed monkeys, which may have favored the prolongation of maternal care and lactation, increasing infant survival in cold environments. These adaptive changes appear to have strengthened interindividual affiliation, increased male-male tolerance, and facilitated the stepwise aggregation from independent one-male groups to large multilevel societies.

Skeletal Muscle Regeneration in Cardiotoxin-Induced Muscle Injury Models

Skeletal muscle injuries occur frequently in daily life and exercise. Understanding the mechanisms of regeneration is critical for accelerating the repair and regeneration of muscle. Therefore, this article reviews knowledge on the mechanisms of skeletal muscle regeneration after cardiotoxin-induced injury. The process of regeneration is similar in different mouse strains and is inhibited by aging, obesity, and diabetes. Exercise, microcurrent electrical neuromuscular stimulation, and mechanical loading improve regeneration. The mechanisms of regeneration are complex and strain-dependent, and changes in functional proteins involved in the processes of necrotic fiber debris clearance, M1 to M2 macrophage conversion, SC activation, myoblast proliferation, differentiation and fusion, and fibrosis and calcification influence the final outcome of the regenerative activity.

A gain-of-function filamin A mutation in mouse platelets induces thrombus instability

BACKGROUND

Filaminopathies A are rare disorders affecting the brain, intestine, or skeleton, characterized by dominant X-linked filamin A (FLNA) gene mutations. Macrothrombocytopenia with functionally defective platelets is frequent. We have described a filaminopathy A male patient, exhibiting a C-terminal frame-shift FLNa mutation (Berrou et al., Arterioscler Thromb Vasc Biol. 2017;37:1087-1097). Contrasting with female patients, this male patient exhibited gain of platelet functions, including increased platelet aggregation, integrin αIIbβ3 activation, and secretion at low agonist concentration, raising the issue of thrombosis risk.

OBJECTIVES

Our goal is to assess the thrombotic potential of the patient FLNa mutation in an in vivo model.

METHODS

We have established a mutant FlnA knock-in mouse model.

RESULTS

The mutant FlnA mouse platelets phenocopied patient platelets, showing normal platelet count, lower expression level of mutant FlnA, and gain of platelet functions: increased platelet aggregation, secretion, and αIIbβ3 activation, as well as increased spreading and clot retraction. Surprisingly, mutant FlnA mice exhibited a normal bleeding time, but with increased re-bleeding (77%) compared to wild type (WT) FlnA mice (27%), reflecting hemostatic plug instability. Again, in an in vivo thrombosis model, the occlusion time was not altered by the FlnA mutation, but arteriolar embolies were increased (7-fold more frequent in mutant FlnA mice versus WT mice), confirming thrombus instability.

CONCLUSIONS

This study shows that the FlnA mutation found in the male patient induced gain of platelet functions in vitro, but thrombus instability in vivo. Implications for the role of FLNa in physiology of thrombus formation are discussed.

Molecular Tuning of Filamin A Activities in the Context of Adhesion and Migration

The dynamic organization of actin cytoskeleton meshworks relies on multiple actin-binding proteins endowed with distinct actin-remodeling activities. Filamin A is a large multi-domain scaffolding protein that cross-links actin filaments with orthogonal orientation in response to various stimuli. As such it plays key roles in the modulation of cell shape, cell motility, and differentiation throughout development and adult life. The essentiality and complexity of Filamin A is highlighted by mutations that lead to a variety of severe human disorders affecting multiple organs. One of the most conserved activity of Filamin A is to bridge the actin cytoskeleton to integrins, thereby maintaining the later in an inactive state. We here review the numerous mechanisms cells have developed to adjust Filamin A content and activity and focus on the function of Filamin A as a gatekeeper to integrin activation and associated adhesion and motility.

Unraveling local tissue changes within severely injured skeletal muscles in response to MSC-based intervention using MALDI Imaging mass spectrometry

Pre-clinical and clinical studies are now beginning to demonstrate the high potential of cell therapies in enhancing muscle regeneration. We previously demonstrated functional benefit after the transplantation of autologous bone marrow mesenchymal stromal cells (MSC-TX) into a severe muscle crush trauma model. Despite our increasing understanding of the molecular and cellular mechanisms underlying MSC's regenerative function, little is known about the local molecular alterations and their spatial distribution within the tissue after MSC-TX. Here, we used MALDI imaging mass spectrometry (MALDI-IMS) in combination with multivariate statistical strategies to uncover previously unknown peptide alterations within severely injured skeletal muscles. Our analysis revealed that very early molecular alterations in response to MSC-TX occur largely in the region adjacent to the trauma and only to a small extent in the actual trauma region. Using "bottom up" mass spectrometry, we subsequently identified the proteins corresponding to the differentially expressed peptide intensity distributions in the specific muscle regions and used immunohistochemistry to validate our results. These findings extend our current understanding about the early molecular processes of muscle healing and highlights the critical role of trauma adjacent tissue during the early therapeutic response upon treatment with MSC.

Common and Distinctive Functions of the Hippo Effectors Taz and Yap in Skeletal Muscle Stem Cell Function

Hippo pathway downstream effectors Yap and Taz play key roles in cell proliferation and regeneration, regulating gene expression especially via Tead transcription factors. To investigate their role in skeletal muscle stem cells, we analyzed Taz in vivo and ex vivo in comparison with Yap. Small interfering RNA knockdown or retroviral‐mediated expression of wild‐type human or constitutively active TAZ mutants in satellite cells showed that TAZ promoted proliferation, a function shared with YAP. However, at later stages of myogenesis, TAZ also enhanced myogenic differentiation of myoblasts, whereas YAP inhibits such differentiation. Functionally, while muscle growth was mildly affected in Taz (gene Wwtr1^–/–) knockout mice, there were no overt effects on regeneration. Conversely, conditional knockout of Yap in satellite cells of Pax7^Cre‐ERT2/+: Yap^fl°^x/fl°^x:Rosa26^Lacz mice produced a regeneration deficit. To identify potential mechanisms, microarray analysis showed many common TAZ/YAP target genes, but TAZ also regulates some genes independently of YAP, including myogenic genes such as Pax7, Myf5, and Myod1 (ArrayExpress–E‐MTAB‐5395). Proteomic analysis revealed many novel binding partners of TAZ/YAP in myogenic cells, but TAZ also interacts with proteins distinct from YAP that are often involved in myogenesis and aspects of cytoskeleton organization (ProteomeXchange–PXD005751). Neither TAZ nor YAP bind members of the Wnt destruction complex but both regulated expression of Wnt and Wnt‐cross talking genes with known roles in myogenesis. Finally, TAZ operates through Tead4 to enhance myogenic differentiation. In summary, Taz and Yap have overlapping functions in promoting myoblast proliferation but Taz then switches to enhance myogenic differentiation. Stem Cells2017;35:1958–1972

Slow recovery of the impaired fatigue resistance in postunloading mouse soleus muscle corresponding to decreased mitochondrial function and a compensatory increase in type I slow fibers

Unloading or disuse rapidly results in skeletal muscle atrophy, switching to fast-type fibers, and decreased resistance to fatigue. The recovery process is of major importance in rehabilitation for various clinical conditions. Here we studied mouse soleus muscle during 60 days of reloading after 4 wk of hindlimb suspension. Unloading produced significant atrophy of soleus muscle with decreased contractile force and fatigue resistance, accompanied by switches of myosin isoforms from IIa to IIx and IIb and fast troponin T to more low-molecular-weight splice forms. The total mass, fiber size, and contractile force of soleus muscle recovered to control levels after 15 days of reloading. However, the fatigue resistance showed a trend of worsening during this period with significant infiltration of inflammatory cells at days 3 and 7, indicating reloading injuries that were accompanied by active regeneration with upregulations of filamin-C, αB-crystallin, and desmin. The fatigue resistance partially recovered after 30-60 days of reloading. The expression of peroxisome proliferator-activated receptor γ coactivator 1α and mitofusin-2 showed changes parallel to that of fatigue resistance after unloading and during reloading, suggesting a causal role of decreased mitochondrial function. Slow fiber contents in the soleus muscle were increased after 30-60 days of reloading to become significantly higher than the normal level, indicating a secondary adaption to compensate for the slow recovery of fatigue resistance.

The NAD(+)-dependent SIRT1 deacetylase translates a metabolic switch into regulatory epigenetics in skeletal muscle stem cells

Stem cells undergo a shift in metabolic substrate utilization during specification and/or differentiation, a process that has been termed metabolic reprogramming. Here, we report that during the transition from quiescence to proliferation, skeletal muscle stem cells experience a metabolic switch from fatty acid oxidation to glycolysis. This reprogramming of cellular metabolism decreases intracellular NAD(+) levels and the activity of the histone deacetylase SIRT1, leading to elevated H4K16 acetylation and activation of muscle gene transcription. Selective genetic ablation of the SIRT1 deacetylase domain in skeletal muscle results in increased H4K16 acetylation and deregulated activation of the myogenic program in SCs. Moreover, mice with muscle-specific inactivation of the SIRT1 deacetylase domain display reduced myofiber size, impaired muscle regeneration, and derepression of muscle developmental genes. Overall, these findings reveal how metabolic cues can be mechanistically translated into epigenetic modifications that regulate skeletal muscle stem cell biology.

MALDI imaging mass spectrometry: discrimination of pathophysiological regions in traumatized skeletal muscle by characteristic peptide signatures

Due to formation of fibrosis and the loss of contractile muscle tissue, severe muscle injuries often result in insufficient healing marked by a significant reduction of muscle force and motor activity. Our previous studies demonstrated that the local transplantation of mesenchymal stromal cells into an injured skeletal muscle of the rat improves the functional outcome of the healing process. Since, due to the lack of sufficient markers, the accurate discrimination of pathophysiological regions in injured skeletal muscle is inadequate, underlying mechanisms of the beneficial effects of mesenchymal stromal cell transplantation on primary trauma and trauma adjacent muscle area remain elusive. For discrimination of these pathophysiological regions, formalin-fixed injured skeletal muscle tissue was analyzed by MALDI imaging MS. By using two computational evaluation strategies, a supervised approach (ClinProTools) and unsupervised segmentation (SCiLS Lab), characteristic m/z species could be assigned to primary trauma and trauma adjacent muscle regions. Using "bottom-up" MS for protein identification and validation of results by immunohistochemistry, we could identify two proteins, skeletal muscle alpha actin and carbonic anhydrase III, which discriminate between the secondary damage on adjacent tissue and the primary traumatized muscle area. Our results underscore the high potential of MALDI imaging MS to describe the spatial characteristics of pathophysiological changes in muscle.

Impaired macrophage and satellite cell infiltration occurs in a muscle-specific fashion following injury in diabetic skeletal muscle

BACKGROUND

Systemic elevations in PAI-1 suppress the fibrinolytic pathway leading to poor collagen remodelling and delayed regeneration of tibialis anterior (TA) muscles in type-1 diabetic Akita mice. However, how impaired collagen remodelling was specifically attenuating regeneration in Akita mice remained unknown. Furthermore, given intrinsic differences between muscle groups, it was unclear if the reparative responses between muscle groups were different.

PRINCIPAL FINDINGS

Here we reveal that diabetic Akita muscles display differential regenerative responses with the TA and gastrocnemius muscles exhibiting reduced regenerating myofiber area compared to wild-type mice, while soleus muscles displayed no difference between animal groups following injury. Collagen levels in TA and gastrocnemius, but not soleus, were significantly increased post-injury versus controls. At 5 days post-injury, when degenerating/necrotic regions were present in both animal groups, Akita TA and gastrocnemius muscles displayed reduced macrophage and satellite cell infiltration and poor myofiber formation. By 10 days post-injury, necrotic regions were absent in wild-type TA but persisted in Akita TA. In contrast, Akita soleus exhibited no impairment in any of these measures compared to wild-type soleus. In an effort to define how impaired collagen turnover was attenuating regeneration in Akita TA, a PAI-1 inhibitor (PAI-039) was orally administered to Akita mice following cardiotoxin injury. PAI-039 administration promoted macrophage and satellite cell infiltration into necrotic areas of the TA and gastrocnemius. Importantly, soleus muscles exhibit the highest inducible expression of MMP-9 following injury, providing a mechanism for normative collagen degradation and injury recovery in this muscle despite systemically elevated PAI-1.

CONCLUSIONS

Our findings suggest the mechanism underlying how impaired collagen remodelling in type-1 diabetes results in delayed regeneration is an impairment in macrophage infiltration and satellite cell recruitment to degenerating areas; a phenomena that occurs differentially between muscle groups.

A novel GFP reporter mouse reveals Mustn1 expression in adult regenerating skeletal muscle, activated satellite cells and differentiating myoblasts

AIM

Mustn1 has been implicated in myofusion as well as skeletal muscle growth and repair; however, the exact role and spatio-temporal expression of Mustn1 have yet to be fully defined.

METHODS

Transgenic mice were generated with a 1512-bp sequence of the Mustn1 promoter directing the expression of GFP (Mustn1(PRO) -GFP). These mice were used to investigate the spatio-temporal expression of Mustn1(PRO) -GFP during skeletal muscle development and adult skeletal muscle repair, as well as various phases of the satellite cell lifespan (i.e. quiescence, activation, proliferation, differentiation).

RESULTS

Mustn1(PRO) -GFP expression was observed within somites at embryonic day 12 and developing skeletal muscles at embryonic day 15 and 18. While uninjured adult tibialis anterior muscle displayed no detectable Mustn1(PRO) -GFP expression, cardiotoxin injury robustly elevated Mustn1(PRO) -GFP expression at 3 days post-injury with decreasing levels observed at 5 days and minimal, focal expression seen at 10 days. The expression of Mustn1(PRO) -GFP at 3 days post-injury consistently overlaid with MyoD although the strongest expression of Mustn1(PRO) -GFP was noted in newly formed myotubes that were expressing minimal levels of MyoD. By 5 days post-injury, Mustn1(PRO) -GFP overlaid in all myotubes expressing myogenin although cells were present expressing Mustn1(PRO) -GFP alone. The expression patterns of Mustn1(PRO) -GFP in regenerating muscle preceded the expression of desmin throughout the regenerative time course consistent with Mustn1 being upstream of this myogenic protein. Further, quiescent satellite cells located on freshly isolated, single myofibers rarely expressed Mustn1(PRO) -GFP, but within 24 h of isolation, all activated satellite cells expressed Mustn1(PRO) -GFP. Expression of Mustn1(PRO) -GFP in primary myoblasts diminished with prolonged time in proliferation media. However, in response to serum withdrawal, the expression of Mustn1(PRO) -GFP increased during myofusion (day 2) followed by declining expression thereafter.

CONCLUSION

Mustn1(PRO) -GFP is expressed in activated satellite cells and myoblasts but continued time in proliferation media diminished Mustn1(PRO) -GFP expression. However, myoblasts exposed to serum withdrawal increased Mustn1(PRO) -GFP expression consistent with its demonstrated role in myofusion. The in vivo expression pattern of Mustn1 observed in regenerating and developing skeletal muscle is consistent with its presence in satellite cells and its critical role in myofusion.

Filamins in mechanosensing and signaling

Filamins are essential, evolutionarily conserved, modular, multidomain, actin-binding proteins that organize the actin cytoskeleton and maintain extracellular matrix connections by anchoring actin filaments to transmembrane receptors. By cross-linking and anchoring actin filaments, filamins stabilize the plasma membrane, provide cellular cortical rigidity, and contribute to the mechanical stability of the plasma membrane and the cell cortex. In addition to binding actin, filamins interact with more than 90 other binding partners including intracellular signaling molecules, receptors, ion channels, transcription factors, and cytoskeletal and adhesion proteins. Thus, filamins scaffold a wide range of signaling pathways and are implicated in the regulation of a diverse array of cellular functions including motility, maintenance of cell shape, and differentiation. Here, we review emerging structural and functional evidence that filamins are mechanosensors and/or mechanotransducers playing essential roles in helping cells detect and respond to physical forces in their local environment.

Mutations in the N-terminal actin-binding domain of filamin C cause a distal myopathy

Linkage analysis of the dominant distal myopathy we previously identified in a large Australian family demonstrated one significant linkage region located on chromosome 7 and encompassing 18.6 Mbp and 151 genes. The strongest candidate gene was FLNC because filamin C, the encoded protein, is muscle-specific and associated with myofibrillar myopathy. Sequencing of FLNC cDNA identified a c.752T>C (p.Met251Thr) mutation in the N-terminal actin-binding domain (ABD); this mutation segregated with the disease and was absent in 200 controls. We identified an Italian family with the same phenotype and found a c.577G>A (p.Ala193Thr) filamin C ABD mutation that segregated with the disease. Filamin C ABD mutations have not been described, although filamin A and filamin B ABD mutations cause multiple musculoskeletal disorders. The distal myopathy phenotype and muscle pathology in the two families differ from myofibrillar myopathies caused by filamin C rod and dimerization domain mutations because of the distinct involvement of hand muscles and lack of pathological protein aggregation. Thus, like the position of FLNA and B mutations, the position of the FLNC mutation determines disease phenotype. The two filamin C ABD mutations increase actin-binding affinity in a manner similar to filamin A and filamin B ABD mutations. Cell-culture expression of the c.752T>C (p.Met251)Thr mutant filamin C ABD demonstrated reduced nuclear localization as did mutant filamin A and filamin B ABDs. Expression of both filamin C ABD mutants as full-length proteins induced increased aggregation of filamin. We conclude filamin C ABD mutations cause a recognizable distal myopathy, most likely through increased actin affinity, similar to the pathological mechanism of filamin A and filamin B ABD mutations.

Isoform divergence of the filamin family of proteins

The vertebrate filamin family (A, B, and C) is part of the spectrin family of actin cross-linking proteins. Family members share high sequence similarity (>64%) and have both common and isoform-distinct functionalities. To identify the basis for isoform-specific functionality, we perform an evolutionary trace of chordate filamin at the granularity of single residues. Our trace methodology is constrained to focus on neofunctionality by requiring that one isoform remain the ancestral type, whereas at least one isoform has an accepted mutation. We call divergence meeting these characteristics "class-distinctive." To obtain a temporal and spatial context for class-distinctive residues, we derive an all-atom model of full-length filamin A by homology modeling and joining individual domains. We map onto our model both conserved and class-distinctive residues along with the period (Teleostei, Amphibian, and Mammalian) in which they diverged. Our phylogenetic analysis suggests that filamins diverged from a common ancestral gene between urochordate and vertebrate lineages. Filamins also diverged the most just after gene duplication, in the Teleostei period, with filamin C remaining closest to ancestral filamin. At the residue level, domains with well-characterized interfaces, IgFLN 17 and IgFLN 21 (immunoglobulin, Ig), have diverged in potentially critical residues in their adhesion protein-binding interfaces, signifying that isoforms may bind or regulate ligand binding differentially. Similarly, isoform divergence in a region associated with F actin-binding regulation suggests that isoforms differentially regulate F-actin binding. In addition, we observe some class-distinctive residues in the vicinity of missense mutations that cause filamin A and B-associated skeletal disorders. Our analysis, utilizing both spatial and temporal granularity, has identified potentially important residues responsible for vertebrate filamin isoform-specific divergence-significantly in regions where few binding partners have been discovered to date- and suggests yet to be discovered filamin-binding partners and isoform-specific differential regulation with these binding partners.

Actin associated proteins function as androgen receptor coregulators: an implication of androgen receptor's roles in skeletal muscle

This review of androgen receptor (AR) coregulators, which also function as actin-binding proteins, intends to establish the connection between actin cytoskeletal components and androgen signaling, especially in skeletal muscle. In cellular and animal models, androgen activated AR modulates myoblasts proliferation, promotes sexual dimorphic muscle development, and alters muscle fiber type. In the clinical setting, administration of anabolic androgens can decrease cachexia and speed wound healing. During myogenesis and regeneration of skeletal muscle in embryo and adult, the membrane of myoblasts fuse and the actin cytoskeleton is rearranged to form an alignment with myosin to form myotubes then ultimately the myofibrils. Contraction of skeletal muscle promotes the growth of myocytes by coordinating signals from the neuromuscular junction to intra-myofibrils through costameres, the functional structure comprised of signal proteins closely associated with actin filaments and involved in muscular dystrophy. Therefore, the discovery of actin-binding proteins functioning as AR coregulators implies that androgen signaling is tightly regulated during the process of the development and regeneration of skeletal muscle. The search for selective androgen receptor modulators (SARM) that act precisely in skeletal muscle instead of other tissues could target the engineering of a SARM-AR complex that selectively recruits these coregulators.

Stem cell antigen-1 regulates the tempo of muscle repair through effects on proliferation of alpha7 integrin-expressing myoblasts

Skeletal muscle repair occurs through a programmed series of events including myogenic precursor activation, myoblast proliferation, and differentiation into new myofibers. We previously identified a role for Stem cell antigen-1 (Sca-1) in myoblast proliferation and differentiation in vitro. We demonstrated that blocking Sca-1 expression resulted in sustained myoblast cell division. Others have since demonstrated that Sca-1-null myoblasts display a similar phenotype when cultured ex vivo. To test the importance of Sca-1 during myogenesis in vivo, we employed a myonecrotic injury model in Sca-1(-/-) and Sca-1(+/+) mice. Our results demonstrate that Sca-1(-/-) myoblasts exhibit a hyperproliferative response consisting of prolonged and accelerated cell division in response to injury. This leads to delayed myogenic differentiation and muscle repair. These data provide the first in vivo evidence for Sca-1 as a regulator of myoblast proliferation during muscle regeneration. These studies also suggest that the balance between myogenic precursor proliferation and differentiation is critical to normal muscle repair.

Identification of CAP as a costameric protein that interacts with filamin C

Cbl-associated protein (CAP) is an adaptor protein that interacts with both signaling and cytoskeletal proteins. Here, we characterize the expression, localization and potential function of CAP in striated muscle. CAP is markedly induced during myoblast differentiation, and colocalizes with vinculin during costamerogenesis. In adult mice, CAP is enriched in oxidative muscle fibers, and it is found in membrane anchorage complexes, including intercalated discs, costameres, and myotendinous junctions. Using both yeast two-hybrid and proteomic approaches, we identified the sarcomeric protein filamin C (FLNc) as a binding partner for CAP. When overexpressed, CAP recruits FLNc to cell-extracellular matrix adhesions, where the two proteins cooperatively regulate actin reorganization. Moreover, overexpression of CAP inhibits FLNc-induced cell spreading on fibronectin. In dystrophin-deficient mdx mice, the expression and membrane localization of CAP is increased, concomitant with the elevated plasma membrane content of FLNc, suggesting that CAP may compensate for the reduced membrane linkage of the myofibrils due to the loss of the dystroglycan-sarcoglycan complex in these mice. Thus, through its interaction with FLNc, CAP provides another link between the myofibril cytoskeleton and the plasma membrane of muscle cells, and it may play a dynamic role in the regulation and maintenance of muscle structural integrity.

Xin, an actin binding protein, is expressed within muscle satellite cells and newly regenerated skeletal muscle fibers

Xin is a muscle-specific actin binding protein of which its role and regulation within skeletal muscle is not well understood. Here we demonstrate that Xin mRNA is robustly upregulated (>16-fold) within 12 h of skeletal muscle injury and is localized to the muscle satellite cell population. RT-PCR confirmed the expression pattern of Xin during regeneration, as well as within primary muscle myoblast cultures, but not other known stem cell populations. Immunohistochemical staining of single myofibers demonstrate Xin expression colocalized with the satellite cell marker Syndecan-4 further supporting the mRNA expression of Xin in satellite cells. In situ hybridization of regenerating muscle 5-7 days postinjury illustrates Xin expression within newly regenerated myofibers. Promoter-reporter assays demonstrate that known myogenic transcription factors [myocyte enhancer factor-2 (MEF2), myogenic differentiation-1 (MyoD), and myogenic factor-5 (Myf-5)] transactivate Xin promoter constructs supporting the muscle-specific expression of Xin. To determine the role of Xin within muscle precursor cells, proliferation, migration, and differentiation analysis using Xin, short hairpin RNA (shRNA) were undertaken in C2C12 myoblasts. Reducing endogenous Xin expression resulted in a 26% increase (P < 0.05) in cell proliferation and a 20% increase (P < 0.05) in myoblast migratory capacity. Skeletal muscle myosin heavy chain protein levels were increased (P < 0.05) with Xin shRNA administration; however, this was not accompanied by changes in myoglobin protein (another marker of differentiation) nor overt morphological differences relative to differentiating control cells. Taken together, the present findings support the hypothesis that Xin is expressed within muscle satellite cells during skeletal muscle regeneration and is involved in the regulation of myoblast function.

Molecular signatures define myogenic stem cell populations

Developmental and regenerative mechanisms are directed by stem cell populations. Skeletal muscle is a dynamic tissue that is capable of adapting to stress and severe injury due to a resident somatic stem cell population. In response to a severe injury that destroys upward of 90% of the tissue, skeletal muscle efficiently and reproducibly regenerates damaged tissue and restores the cellular architecture within a 2-wk period. Recent studies have localized and examined the molecular regulation of skeletal muscle stem cell populations using emerging molecular biological technologies. These studies enhance the understanding of the regulatory mechanisms that direct the somatic stem cell populations and the role they play in development and regeneration. Furthermore, these basic science studies will serve as a platform for future therapies directed toward patients with myopathic diseases.

MEKK4 signaling regulates filamin expression and neuronal migration

Periventricular heterotopia (PVH) is a congenital malformation of human cerebral cortex frequently associated with Filamin-A (FLN-A) mutations but the pathogenetic mechanisms remain unclear. Here, we show that the MEKK4 (MAP3K4) pathway is involved in Fln-A regulation and PVH formation. MEKK4(-/-) mice developed PVH associated with breaches in the neuroependymal lining which were largely comprised of neurons that failed to reach the cortical plate. RNA interference (RNAi) targeting MEKK4 also impaired neuronal migration. Expression of Fln was elevated in MEKK4(-/-) forebrain, most notably near sites of failed neuronal migration. Importantly, recombinant MKK4 protein precipitated a complex containing MEKK4 and Fln-A, and MKK4 mediated signaling between MEKK4 and Fln-A, suggesting that MKK4 may bridge these molecules during development. Finally, we showed that wild-type FLN-A overexpression inhibited neuronal migration. Collectively, our results demonstrate a link between MEKK4 and Fln-A that impacts neuronal migration initiation and provides insight into the pathogenesis of human PVH.