The molecular basis for sarcomere organization in vertebrate skeletal muscle.

Sarcomeres are force-generating and load-bearing devices of muscles. A precise molecular picture of how sarcomeres are built underpins understanding their role in health and disease. Here, we determine the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography. Our reconstruction reveals molecular details of the three-dimensional organization and interaction of actin and myosin in the A-band, I-band, and Z-disc and demonstrates that α-actinin cross-links antiparallel actin filaments by forming doublets with 6-nm spacing. Structures of myosin, tropomyosin, and actin at ~10 Å further reveal two conformations of the "double-head" myosin, where the flexible orientation of the lever arm and light chains enable myosin not only to interact with the same actin filament, but also to split between two actin filaments. Our results provide unexpected insights into the fundamental organization of vertebrate skeletal muscle and serve as a strong foundation for future investigations of muscle diseases.

The structure and regulation of human muscle α-actinin.

The spectrin superfamily of proteins plays key roles in assembling the actin cytoskeleton in various cell types, crosslinks actin filaments, and acts as scaffolds for the assembly of large protein complexes involved in structural integrity and mechanosensation, as well as cell signaling. α-actinins in particular are the major actin crosslinkers in muscle Z-disks, focal adhesions, and actin stress fibers. We report a complete high-resolution structure of the 200 kDa α-actinin-2 dimer from striated muscle and explore its functional implications on the biochemical and cellular level. The structure provides insight into the phosphoinositide-based mechanism controlling its interaction with sarcomeric proteins such as titin, lays a foundation for studying the impact of pathogenic mutations at molecular resolution, and is likely to be broadly relevant for the regulation of spectrin-like proteins.

DNA of neutrophil extracellular traps promotes cancer metastasis via CCDC25.

Neutrophil extracellular traps (NETs), which consist of chromatin DNA filaments coated with granule proteins, are released by neutrophils to trap microorganisms1-3. Recent studies have suggested that the DNA component of NETs (NET-DNA) is associated with cancer metastasis in mouse models4-6. However, the functional role and clinical importance of NET-DNA in metastasis in patients with cancer remain unclear. Here we show that NETs are abundant in the liver metastases of patients with breast and colon cancers, and that serum NETs can predict the occurrence of liver metastases in patients with early-stage breast cancer. NET-DNA acts as a chemotactic factor to attract cancer cells, rather than merely acting as a 'trap' for them; in several mouse models, NETs in the liver or lungs were found to attract cancer cells to form distant metastases. We identify the transmembrane protein CCDC25 as a NET-DNA receptor on cancer cells that senses extracellular DNA and subsequently activates the ILK-ß-parvin pathway to enhance cell motility. NET-mediated metastasis is abrogated in CCDC25-knockout cells. Clinically, we show that the expression of CCDC25 on primary cancer cells is closely associated with a poor prognosis for patients. Overall, we describe a transmembrane DNA receptor that mediates NET-dependent metastasis, and suggest that targeting CCDC25 could be an appealing therapeutic strategy for the prevention of cancer metastasis.

Response to Wyckelsma et al.: Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation.

The common loss-of-function mutation R577X in the structural muscle protein ACTN3 emerged as a potential target of positive selection from early studies and has been the focus of insightful physiological work suggesting a significant impact on muscle metabolism. Adaptation to cold climates has been proposed as a key adaptive mechanism explaining its global allele frequency patterns. Here, we re-examine this hypothesis analyzing modern (n = 3,626) and ancient (n = 1,651) genomic data by using allele-frequency as well as haplotype homozygosity-based methods. The presented results are more consistent with genetic drift rather than selection in cold climates as the main driver of the ACTN3 R577X frequency distribution in human populations across the world. This Matters Arising paper is in response to Wyckelsma et al. (2021),1 published in The American Journal of Human Genetics. See also the response by Wyckelsma et al. (2022),2 published in this issue.

Actinin-4 controls survival signaling in B cells by limiting the lateral mobility of B-cell antigen receptors.

The structure and dynamics of F-actin networks in the cortical area of B cells control the signal efficiency of B-cell antigen receptors (BCRs). Although antigen-induced signaling has been studied extensively, the role of cortical F-actin in antigen-independent tonic BCR signaling is less well understood. Because these signals are essential for the survival of B cells and are consequently exploited by several B-cell lymphomas, we assessed how the cortical F-actin structure influences tonic BCR signal transduction. We employed genetic variants of a primary cell-like B-cell line that can be rendered quiescent to show that cross-linking of actin filaments by α-actinin-4 (ACTN4), but not ACTN1, is required to preserve the dense architecture of F-actin in the cortical area of B cells. The reduced cortical F-actin density in the absence of ACTN4 resulted in increased lateral BCR diffusion. Surprisingly, this was associated with reduced tonic activation of BCR-proximal effector proteins, extracellular signal-regulated kinase, and pro-survival pathways. Accordingly, ACTN4-deficient B-cell lines and primary human B cells exhibit augmented apoptosis. Hence, our findings reveal that cortical F-actin architecture regulates antigen-independent tonic BCR survival signals in human B cells.

Meta-analysis of genomic variants in power and endurance sports to decode the impact of genomics on athletic performance and success.

Association between genomic variants and athletic performance has seen a high degree of controversy, as there is often conflicting data as far as the association of genomic variants with endurance, speed and strength is concerned. Here, findings from a thorough meta-analysis from 4228 articles exploring the association of genomic variants with athletic performance in power and endurance sports are summarized, aiming to confirm or overrule the association of genetic variants with athletic performance of all types. From the 4228 articles, only 107 were eligible for further analysis, including 37 different genes. From these, there were 21 articles for the ACE gene, 29 articles for the ACTN3 gene and 8 articles for both the ACE and ACTN3 genes, including 54,382 subjects in total, from which 11,501 were endurance and power athletes and 42,881 control subjects. These data show that there is no statistically significant association between genomic variants and athletic performance either for endurance or power sports, underlying the fact that it is highly risky and even unethical to make such genetic testing services for athletic performance available to the general public. Overall, a strict regulatory monitoring should be exercised by health and other legislative authorities to protect the public from such services from an emerging discipline that still lacks the necessary scientific evidence and subsequent regulatory approval.

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged mdx dystrophic mice by reducing pathological fibre branching.

The common null polymorphism (R577X) in the ACTN3 gene is present in over 1.5 billion people worldwide and results in the absence of the protein α-actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin-deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this, we use a double knockout (dk)Actn3KO/mdx (dKO) mouse model, which lacks both dystrophin and sarcomere α-actinin-3. We used dKO mice and mdx dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an in vitro eccentric contraction protocol. In the aged dKO mouse, we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged mdx EDL fibres (68%), and this correlates with a graded force loss over three eccentric contractions for dKO muscles (~36% after first contraction, ~66% overall) compared to an abrupt drop in mdx upon the first eccentric contraction (~75% after first contraction, ~89% after three contractions). In dKO, protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca2+ pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.

Increased prevalence of the null allele of the p.Arg577Ter variant in the ACTN3 gene in Brazilian long-distance athletes: A retrospective study.

INTRODUCTION: The phenotypic consequences of the p.Arg577Ter variant in the α-actinin-3 (ACTN3) gene are suggestive of a trade-off between performance traits for speed and endurance sports. Although there is a consistent association of the c.1729C allele (aka R allele) with strength/power traits, there is still a debate on whether the null allele (c.1729T allele; aka X allele) influences endurance performance. The present study aimed to test the association of the ACTN3 p.Arg577Ter variant with long-distance endurance athlete status, using previously published data with the Brazilian population. METHODS: Genotypic data from 203 long-distance athletes and 1724 controls were analysed in a case-control approach. RESULTS: The frequency of the X allele was significantly higher in long-distance athletes than in the control group (51.5% vs. 41.4%; p = 0.000095). The R/X and X/X genotypes were overrepresented in the athlete group. Individuals with the R/X genotype instead of the R/R genotype had a 1.6 increase in the odds of being a long-distance athlete (p = 0.012), whereas individuals with the X/X genotype instead of the R/R genotype had a 2.2 increase in the odds of being a long-distance athlete (p = 0.00017). CONCLUSION: The X allele, mainly the X/X genotype, was associated with long-distance athlete status in Brazilians.

α-Actinin-1 promotes activity of the L-type Ca2+ channel Cav 1.2.

The L-type Ca2+ channel CaV 1.2 governs gene expression, cardiac contraction, and neuronal activity. Binding of α-actinin to the IQ motif of CaV 1.2 supports its surface localization and postsynaptic targeting in neurons. We report a bi-functional mechanism that restricts CaV 1.2 activity to its target sites. We solved separate NMR structures of the IQ motif (residues 1,646-1,664) bound to α-actinin-1 and to apo-calmodulin (apoCaM). The CaV 1.2 K1647A and Y1649A mutations, which impair α-actinin-1 but not apoCaM binding, but not the F1658A and K1662E mutations, which impair apoCaM but not α-actinin-1 binding, decreased single-channel open probability, gating charge movement, and its coupling to channel opening. Thus, α-actinin recruits CaV 1.2 to defined surface regions and simultaneously boosts its open probability so that CaV 1.2 is mostly active when appropriately localized.

Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation.

The protein α-actinin-3 expressed in fast-twitch skeletal muscle fiber is absent in 1.5 billion people worldwide due to homozygosity for a nonsense polymorphism in ACTN3 (R577X). The prevalence of the 577X allele increased as modern humans moved to colder climates, suggesting a link between α-actinin-3 deficiency and improved cold tolerance. Here, we show that humans lacking α-actinin-3 (XX) are superior in maintaining core body temperature during cold-water immersion due to changes in skeletal muscle thermogenesis. Muscles of XX individuals displayed a shift toward more slow-twitch isoforms of myosin heavy chain (MyHC) and sarcoplasmic reticulum (SR) proteins, accompanied by altered neuronal muscle activation resulting in increased tone rather than overt shivering. Experiments on Actn3 knockout mice showed no alterations in brown adipose tissue (BAT) properties that could explain the improved cold tolerance in XX individuals. Thus, this study provides a mechanism for the positive selection of the ACTN3 X-allele in cold climates and supports a key thermogenic role of skeletal muscle during cold exposure in humans.

ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype.

ACTN1-related thrombocytopenia is a rare disorder caused by heterozygous variants in the ACTN1 gene characterized by macrothrombocytopenia and mild bleeding tendency. We describe for the first time two patients affected with ACTN1-RT caused by a homozygous variant in ACTN1 (c.982G>A) with mild heart valve defects unexplained by any other genetic variants investigated by WES. Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size. Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities.

miR-129-5p inhibits anchorage-independent growth through silencing of ACTN1 and the ELK4/c-FOS axis in HPV-transformed keratinocytes.

A persistent infection with human papillomavirus (HPV) can induce precancerous lesions of the cervix that may ultimately develop into cancer. Cervical cancer development has been linked to altered microRNA (miRNA) expression, with miRNAs regulating anchorage-independent growth being particularly important for the progression of precancerous lesions to cancer. In this study, we set out to identify and validate targets of miR-129-5p, a previously identified tumor suppressive miRNA involved in anchorage-independent growth and HPV-induced carcinogenesis. We predicted 26 potential miR-129-5p targets using online databases, followed by KEGG pathway enrichment analysis. RT-qPCR and luciferase assays confirmed that 3'UTR regions of six genes (ACTN1, BMPR2, CAMK4, ELK4, EP300, and GNAQ) were targeted by miR-129-5p. Expressions of ACTN1, CAMK4, and ELK4 were inversely correlated to miR-129-5p expression in HPV-transformed keratinocytes, and their silencing reduced anchorage-independent growth. Concordantly, miR-129-5p overexpression decreased protein levels of ACTN1, BMPR2, CAMK4 and ELK4 in anchorage-independent conditions. Additionally, c-FOS, a downstream target of ELK4, was downregulated upon miR-129-5p overexpression, suggesting regulation through the ELK4/c-FOS axis. ACTN1 and ELK4 expression was also upregulated in high-grade precancerous lesions and cervical cancers, supporting their clinical relevance. In conclusion, we identified six targets of miR-129-5p involved in the regulation of anchorage-independent growth, with ACTN1, BMPR2, ELK4, EP300, and GNAQ representing novel targets for miR-129-5p. For both ACTN1 and ELK4 functional and clinical relevance was confirmed, indicating that miR-129-5p-regulated ACTN1 and ELK4 expression contributes to HPV-induced carcinogenesis.

The Immunosuppressive Drug LF15-0195 Acts Also on Glomerular Lesions, by a Change in Cytoskeleton Distribution in Podocyte.

INTRODUCTION: Buffalo/Mna rats spontaneously develop nephrotic syndrome (NS) which recurs after renal transplantation. The immunosuppressive drug LF15-0195 can promote regression of the initial and post-transplantation nephropathy via induction of regulatory T cells. We investigate if this drug has an additional effect on the expression and localization of podocyte specific proteins. METHODS: Buffalo/Mna kidney samples were collected before and after the occurrence of proteinuria, and after the remission of proteinuria induced by LF15-0195 treatment and compared by quantitative RT-PCR, Western blot, electron, and confocal microscopy to kidney samples of age-matched healthy rats. Cytoskeleton changes were assessed in culture by stress fibers induction by TNFα. RESULTS: We observed, by electron microscopy, a restoration of foot process architecture in the LF15-0195-treated Buff/Mna kidneys, consistent with proteinuria remission. Nephrin, podocin, CD2AP, and α-actinin-4 mRNA levels remained low during the active disease in the Buff/Mna, in comparison with healthy rats which increase, while podocalyxin and synaptopodin transcripts were elevated before the occurrence of the disease but did not differ from healthy animals after. No difference in the mRNA and protein expression between the untreated and the LF15-0195-treated proteinuric Buff/Mna were seen for these 6 proteins. No changes were observed by confocal microscopy in the protein distribution at a cellular level, but a more homogenous distribution similar to healthy rats, was observed within the glomeruli of LF15-0195-treated rats. In addition, LF15-0195 could partially restore actin cytoskeleton of endothelial cells in TNFα-induced-cell stress experiment. CONCLUSION: The effect of LF15-0195 treatment appears to be mediated by 2 mechanisms: an immunomodulatory effect via regulatory T cells induction, described in our previous work and which can act on immune cell involved in the disease pathogenesis, and an effect on the restoration of podocyte cytoskeleton, independent of expression levels of the proteins involved in the slit diaphragm and podocyte function, showed in this article.

Alternative Splicing Mediated by RNA-Binding Protein RBM24 Facilitates Cardiac Myofibrillogenesis in a Differentiation Stage-Specific Manner.

BACKGROUND: Mutations in genes encoding sarcomeric proteins lead to failures in sarcomere assembly, the building blocks of contracting muscles, resulting in cardiomyopathies that are a leading cause of morbidity and mortality worldwide. Splicing variants of sarcomeric proteins are crucial at different stages of myofibrillogenesis, accounting for sarcomeric structural integrity. RBM24 (RNA-binding motif protein 24) is known as a tissue-specific splicing regulator that plays an essential role in cardiogenesis. However, it had been unclear if the developmental stage-specific alternative splicing facilitated by RBM24 contributes to sarcomere assembly and cardiogenesis. Our aim is to study the molecular mechanism by which RBM24 regulates cardiogenesis and sarcomere assembly in a temporal-dependent manner. METHODS: We ablated RBM24 from human embryonic stem cells (hESCs) using CRISPR/Cas9 techniques. RESULTS: Although RBM24-/- hESCs still differentiated into sarcomere-hosting cardiomyocytes, they exhibited disrupted sarcomeric structures with punctate Z-lines due to impaired myosin replacement during early myofibrillogenesis. Transcriptomics revealed >4000 genes regulated by RBM24. Among them, core myofibrillogenesis proteins (eg, ACTN2 [α-actinin 2], TTN [titin], and MYH10 [non-muscle myosin IIB]) were misspliced. Consequently, MYH6 (muscle myosin II) cannot replace nonmuscle myosin MYH10, leading to myofibrillogenesis arrest at the early premyofibril stage and causing disrupted sarcomeres. Intriguingly, we found that the ABD (actin-binding domain; encoded by exon 6) of the Z-line anchor protein ACTN2 is predominantly excluded from early cardiac differentiation, whereas it is consistently included in human adult heart. CRISPR/Cas9-mediated deletion of exon 6 from ACTN2 in hESCs, as well as forced expression of full-length ACTN2 in RBM24-/- hESCs, further corroborated that inclusion of exon 6 is critical for sarcomere assembly. Overall, we have demonstrated that RBM24-facilitated inclusion of exon 6 in ACTN2 at distinct stages of cardiac differentiation is evolutionarily conserved and crucial to sarcomere assembly and integrity. CONCLUSIONS: RBM24 acts as a master regulator to modulate the temporal dynamics of core myofibrillogenesis genes and thereby orchestrates sarcomere organization.

Revealing the progression and pathologic features of intraperitoneal infection of Trichomonas vaginalis in mice via parasite α-actinin-based immunological detection.

BACKGROUND: Trichomoniasis caused by Trichomonas vaginalis is the most prevalent nonviral sexually transmitted disease in women and has frequently damaged public health. To better use the animal model and take a step forward fully elucidating this pathogen, intraperitoneal infection of T. vaginalis in mice, one of the most common mouse models, was highly concerned. METHODS: By adjusting the number of parasites inoculated, acute and chronic infection models were established. Pathological changes and the presence of T. vaginalis in organs were observed at different timepoints post inoculation using histological and TV-α-actinin-based immunological detection. RESULTS: The results reconfirmed the correlation between inoculum size of parasites and infection duration, as well as the multiplication capacity of T. vaginalis in mouse enterocoelia or invaded organs. The progression and pathologic features of vital organs (e.g., liver and spleen) from mice intraperitoneally infected with T. vaginalis in both the acute and chronic groups were also revealed. In particular, a reliable immunological method based on TV-α-actinin was first verified to clearly present the invasion of T. vaginalis into infected mouse organs. CONCLUSIONS: In brief, this study presented a clearer and more detailed pathologic characteristic of the intraperitoneal infection model, which probably provides more basic information for the use of this model in future studies. Especially, expanding on specific research applications of this model would be valuable.

Exploring the nexus between sports performance and genetics: a comprehensive literature review.

Sport is a multifactorial phenomenon that is influenced by many factors. Although many factors affect sports performance, genetic factors may be important issues that need to be examined. In addition, the relationship between sports performance and genes is still unclear. Due to the developments in omics technologies, approximately 185 genetic markers have been identified for the relationship between sports performance and genes. These genes are expressed differently in metabolism according to the characteristics of sports performance. The aim of this study was to investigate the relationship between sports and genetics. Pubmed, Pubmed Central and Google Scholar internet search engines were used in current study. Additionally, the PRISMA technique was used in the study design. For this purpose, COL1A1, COL5A1, ACTN3 and ELN genes may be important regulators on soft tissues. For endurance sports, genes like ACE, ACTN3, ADRB2, HFE, COL5A1, BDKRB2, NOS3, HIF, VEGF, AMPD and PPARGC1A significantly may influence performance limits. ACE and ACTN3 genes, on the other hand, may determine power/strength and speed skills in athletes. As a result, knowing the athlete's genetic predisposition to sports can be effective in achieving success.

AGR2-mediated unconventional secretion of 14-3-3ε and α-actinin-4, responsive to ER stress and autophagy, drives chemotaxis in canine mammary tumor cells.

BACKGROUND: Canine mammary tumors (CMTs) in intact female dogs provide a natural model for investigating metastatic human cancers. Our prior research identified elevated expression of Anterior Gradient 2 (AGR2), a protein disulfide isomerase (PDI) primarily found in the endoplasmic reticulum (ER), in CMT tissues, highly associated with CMT progression. We further demonstrated that increased AGR2 expression actively influences the extracellular microenvironment, promoting chemotaxis in CMT cells. Unraveling the underlying mechanisms is crucial for assessing the potential of therapeutically targeting AGR2 as a strategy to inhibit a pro-metastatic microenvironment and impede tumor metastasis. METHODS: To identify the AGR2-modulated secretome, we employed proteomics analysis of the conditioned media (CM) from two CMT cell lines ectopically expressing AGR2, compared with corresponding vector-expressing controls. AGR2-regulated release of 14-3-3ε (gene: YWHAE) and α-actinin 4 (gene: ACTN4) was validated through ectopic expression, knockdown, and knockout of the AGR2 gene in CMT cells. Extracellular vesicles derived from CMT cells were isolated using either differential ultracentrifugation or size exclusion chromatography. The roles of 14-3-3ε and α-actinin 4 in the chemotaxis driven by the AGR2-modulated CM were investigated through gene knockdown, antibody-mediated interference, and recombinant protein supplement. Furthermore, the clinical relevance of the release of 14-3-3ε and α-actinin 4 was assessed using CMT tissue-immersed saline and sera from CMT-afflicted dogs. RESULTS: Proteomics analysis of the AGR2-modulated secretome revealed increased abundance in 14-3-3ε and α-actinin 4. Ectopic expression of AGR2 significantly increased the release of 14-3-3ε and α-actinin 4 in the CM. Conversely, knockdown or knockout of AGR2 expression remarkably reduced their release. Silencing 14-3-3ε or α-actinin 4 expression diminished the chemotaxis driven by AGR2-modulated CM. Furthermore, AGR2 controls the release of 14-3-3ε and α-actinin 4 primarily via non-vesicular routes, responding to the endoplasmic reticulum (ER) stress and autophagy activation. Knockout of AGR2 resulted in increased α-actinin 4 accumulation and impaired 14-3-3ε translocation in autophagosomes. Depletion of extracellular 14-3-3ε or α-actinin 4 reduced the chemotaxis driven by AGR2-modulated CM, whereas supplement with recombinant 14-3-3ε in the CM enhanced the CM-driven chemotaxis. Notably, elevated levels of 14-3-3ε or α-actinin 4 were observed in CMT tissue-immersed saline compared with paired non-tumor samples and in the sera of CMT dogs compared with healthy dogs. CONCLUSION: This study elucidates AGR2's pivotal role in orchestrating unconventional secretion of 14-3-3ε and α-actinin 4 from CMT cells, thereby contributing to paracrine-mediated chemotaxis. The insight into the intricate interplay between AGR2-involved ER stress, autophagy, and unconventional secretion provides a foundation for refining strategies aimed at impeding metastasis in both canine mammary tumors and potentially human cancers.

Sarcomeres regulate murine cardiomyocyte maturation through MRTF-SRF signaling.

The paucity of knowledge about cardiomyocyte maturation is a major bottleneck in cardiac regenerative medicine. In development, cardiomyocyte maturation is characterized by orchestrated structural, transcriptional, and functional specializations that occur mainly at the perinatal stage. Sarcomeres are the key cytoskeletal structures that regulate the ultrastructural maturation of other organelles, but whether sarcomeres modulate the signal transduction pathways that are essential for cardiomyocyte maturation remains unclear. To address this question, here we generated mice with cardiomyocyte-specific, mosaic, and hypomorphic mutations of α-actinin-2 (Actn2) to study the cell-autonomous roles of sarcomeres in postnatal cardiomyocyte maturation. Actn2 mutation resulted in defective structural maturation of transverse-tubules and mitochondria. In addition, Actn2 mutation triggered transcriptional dysregulation, including abnormal expression of key sarcomeric and mitochondrial genes, and profound impairment of the normal progression of maturational gene expression. Mechanistically, the transcriptional changes in Actn2 mutant cardiomyocytes strongly correlated with those in cardiomyocytes deleted of serum response factor (SRF), a critical transcription factor that regulates cardiomyocyte maturation. Actn2 mutation increased the monomeric form of cardiac α-actin, which interacted with the SRF cofactor MRTFA and perturbed its nuclear localization. Overexpression of a dominant-negative MRTFA mutant was sufficient to recapitulate the morphological and transcriptional defects in Actn2 and Srf mutant cardiomyocytes. Together, these data indicate that Actn2-based sarcomere organization regulates structural and transcriptional maturation of cardiomyocytes through MRTF-SRF signaling.

Compliant 3D frameworks instrumented with strain sensors for characterization of millimeter-scale engineered muscle tissues.

Tissue-on-chip systems represent promising platforms for monitoring and controlling tissue functions in vitro for various purposes in biomedical research. The two-dimensional (2D) layouts of these constructs constrain the types of interactions that can be studied and limit their relevance to three-dimensional (3D) tissues. The development of 3D electronic scaffolds and microphysiological devices with geometries and functions tailored to realistic 3D tissues has the potential to create important possibilities in advanced sensing and control. This study presents classes of compliant 3D frameworks that incorporate microscale strain sensors for high-sensitivity measurements of contractile forces of engineered optogenetic muscle tissue rings, supported by quantitative simulations. Compared with traditional approaches based on optical microscopy, these 3D mechanical frameworks and sensing systems can measure not only motions but also contractile forces with high accuracy and high temporal resolution. Results of active tension force measurements of engineered muscle rings under different stimulation conditions in long-term monitoring settings for over 5 wk and in response to various chemical and drug doses demonstrate the utility of such platforms in sensing and modulation of muscle and other tissues. Possibilities for applications range from drug screening and disease modeling to biohybrid robotic engineering.

Identification of key genes involved in collagen hydrogel-induced chondrogenic differentiation of mesenchymal stem cells through transcriptome analysis: the role of m6A modification.

Collagen hydrogel has been shown promise as an inducer for chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), contributing to the repair of cartilage defects. However, the precise molecular mechanism underlying this phenomenon remains poorly elucidated. Here, we induced chondrogenic differentiation of BMSCs using collagen hydrogel and identified 4451 differentially expressed genes (DEGs) through transcriptomic sequencing. Our analysis revealed that DEGs were enriched in the focal adhesion pathway, with a notable decrease in expression levels in the collagen hydrogel group compared to the control group. Protein-protein interaction network analysis suggested that actinin alpha 1 (ACTN1) and actinin alpha 4 (ACTN4), two proteins also involved in cytoskeletal recombination, may be crucial in collagen hydrogel-induced chondrogenic differentiation of BMSCs. Additionally, we found that N6-methyladenosine RNA methylation (m6A) modification was involved in collagen hydrogel-mediated chondrogenic differentiation, with fat mass and obesity-associated protein (FTO) implicated in regulating the expression of ACTN1 and ACTN4. These findings suggest that collagen hydrogel might regulate focal adhesion and actin cytoskeletal signaling pathways through down-regulation of ACTN1 and ACTN4 mRNA via FTO-mediated m6A modification, ultimately driving chondrogenic differentiation of BMSCs. In conclusion, our study provides valuable insights into the molecular mechanisms of collagen hydrogel-induced chondrogenic differentiation of BMSCs, which may aid in developing more effective strategies for cartilage regeneration.