PDZRN3 regulates differentiation of myoblasts into myotubes through transcriptional and posttranslational control of Id2

PDZRN3 regulates adipogenesis of mesenchymal progenitors in muscle

Introduction

Intramuscular adipose tissue (IMAT) is frequently formed in certain pathological conditions, such as biological aging, and ectopic fat accumulation leads to muscle weakness and a subsequent decline in physical function. Although mesenchymal progenitors (MPs) are present in postnatal skeletal muscle and are the cells from which IMAT originates, the molecular mechanism by which MPs contribute to IMAT formation has not been completely elucidated. Recently, we found that PDZ domain-containing ring finger 3 (PDZRN3), an E3-ubiquitin ligase, was highly expressed in MPs. In this study, we aimed to clarify the functions of PDZRN3 in MPs and the roles of PDZRN3 in IMAT formation using in vitro and in vivo experiments.

Methods

Primary mouse MPs isolated from hindlimb muscles were applied to adipogenic differentiation conditions, and expression fluctuation of PDZRN3 was verified with adipogenic differentiation and Wnt signaling markers. The role of PDZRN3 on MP's adipogenesis was evaluated in vitro by gene knock-down experiments. To evaluate the contribution of PDZRN3 to IMAT formation in vivo, tamoxifen-inducible MP-specific Pdzrn3 knockout (Pdzrn3 MPcKO) mice were developed.

Results

PDZRN3 was more expressed in MPs than in muscle stem cells, and its expression profile of PDZRN3 fluctuated with the adipogenic differentiation of MPs. Our results revealed that PDZRN3 suppressed the adipogenesis of MPs in vitro through the activation of Wnt signaling and that a decrease in PDZRN3 accelerated adipogenesis. Indeed, IMAT significantly increased in the denervated muscles of Pdzrn3 MPcKO mice.

Conclusions

Our findings suggest that PDZRN3 is a key molecule in regulating IMAT formation. Since ectopic fat accumulation is frequently found in the skeletal muscles of older adults and also muscular dystrophy patients, PDZRN3 and its related pathways may represent a novel therapeutic target for these muscle pathologies.

Intracellular delivery of a phospholamban-targeting aptamer using cardiomyocyte-internalizing aptamers

The sarco (endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a)-phospholamban (PLN) system within the sarcoplasmic reticulum is crucial for regulating intracellular Ca2+ cycling in ventricular cardiomyocytes. Given that impaired Ca2+ cycling is associated with heart failure, modulating SERCA2a activity represents a promising therapeutic strategy. Previously, we engineered an RNA aptamer (Apt30) that binds to PLN, thereby activating SERCA2a by alleviating PLN's inhibitory effect. However, Apt30 alone cannot reach intracellular PLN, necessitating the development of a mechanism for its specific internalization into cardiomyocytes. Using the systematic evolution of ligands by exponential enrichment (SELEX) method, we isolated RNA aptamers capable of internalizing into cardiomyocytes. These aptamers demonstrated sub-micromolar EC50 values for cardiomyocyte internalization and exhibited significantly reduced activity against various non-myocardial cells, highlighting their specificity for cardiomyocytes. Moreover, some of these cardiomyocyte-internalizing aptamers could be linked to Apt30 as a single RNA strand without compromising their internalization efficacy. Supplementing the culture medium with these hybrid aptamers enhanced Ca2+ transients and contractile function in rat cardiomyocytes. These findings provide critical insights for developing novel therapeutics directly acting on PLN in cardiomyocytes, potentially compensating for the disadvantages of conventional methods that involve viral vector-mediated intracellular transduction or alterations in endogenous protein expression.

Postnatal Pdzrn3 deficiency causes acute muscle atrophy without alterations in endplate morphology

PDZ domain-containing RING finger family protein 3 (PDZRN3) is expressed in various tissues, including the skeletal muscle. Although PDZRN3 plays a crucial role in the terminal differentiation of myoblasts and synaptic growth/maturation in myogenesis, the role of this molecule in postnatal muscles is completely unknown despite its lifelong expression in myofibers. In this study, we aimed to elucidate the function of PDZRN3 in mature myofibers using myofiber-specific conditional knockout mice. After tamoxifen injection, PDZRN3 deficiency was confirmed in both fast and slow myofibers of Myf6-CreERT2; Pdzrn3flox/flox (Pdzrn3mcKO) mice. Transcriptome analysis of the skeletal muscles of Pdzrn3mcKO mice identified differentially expressed genes, including muscle atrophy-related genes such as Smox, Amd1/2, and Mt1/2, suggesting that PDZRN3 is involved in the homeostatic maintenance of postnatal muscles. PDZRN3 deficiency caused muscle atrophy, predominantly in fast-twitch (type II) myofibers, and reduced muscle strength. While myofiber-specific PDZRN3 deficiency did not influence endplate morphology or expression of neuromuscular synaptic formation-related genes in postnatal muscles, indicating that the relationship between PDZRN3 and neuromuscular junctions might be limited during muscle development. Considering that the expression of Pdzrn3 in skeletal muscles was significantly lower in aged mice than in mature adult mice, we speculated that the PDZRN3-mediated muscle maintenance system might be associated with the pathophysiology of age-related muscle decline, such as sarcopenia.

Early fish domestication affects methylation of key genes involved in the rapid onset of the farmed phenotype

Animal domestication is a process of environmental modulation and artificial selection leading to permanent phenotypic modifications. Recent studies showed that phenotypic changes occur very early in domestication, i.e., within the first generation in captivity, which raises the hypothesis that epigenetic mechanisms may play a critical role on the early onset of the domestic phenotype. In this context, we applied reduced representation bisulphite sequencing to compare methylation profiles between wild Nile tilapia females and their offspring reared under farmed conditions. Approximately 700 differentially methylated CpG sites were found, many of them associated not only with genes involved in muscle growth, immunity, autophagy and diet response but also related to epigenetic mechanisms, such as RNA methylation and histone modifications. This bottom-up approach showed that the phenotypic traits often related to domestic animals (e.g., higher growth rate and different immune status) may be regulated epigenetically and prior to artificial selection on gene sequences. Moreover, it revealed the importance of diet in this process, as reflected by differential methylation patterns in genes critical to fat metabolism. Finally, our study highlighted that the TGF-β1 signalling pathway may regulate and be regulated by several differentially methylated CpG-associated genes. This could be an important and multifunctional component in promoting adaptation of fish to a domestic environment while modulating growth and immunity-related traits.

EZH2 modulates retinoic acid signaling to ensure myotube formation during development

Sequential differentiation of pre-somitic progenitors into myocytes and subsequently into myotubes and myofibers is essential for the myogenic differentiation program (MDP) crucial for muscle development. Signaling factors involved in MDP are Polycomb Repressive Complex 2 (PRC2) targets in various developmental contexts. PRC2 is active in the developing myotomes during MDP, but how it regulates MDP is unclear. Here, we found that myocyte differentiation to myotubes requires Enhancer of Zeste 2 (EZH2), the catalytic component of PRC2. We observed elevated retinoic-acid (RA) signaling in the prospective myocytes in the Ezh2 mutants (E8.5-Mus^Ezh2 ), and its inhibition can partially rescue the myocyte differentiation defect. Together, our data demonstrate a new role for PRC2-EZH2 during myocyte differentiation into myotubes by modulating RA signaling.

MethylationToActivity: a deep-learning framework that reveals promoter activity landscapes from DNA methylomes in individual tumors

Although genome-wide DNA methylomes have demonstrated their clinical value as reliable biomarkers for tumor detection, subtyping, and classification, their direct biological impacts at the individual gene level remain elusive. Here we present MethylationToActivity (M2A), a machine learning framework that uses convolutional neural networks to infer promoter activities based on H3K4me3 and H3K27ac enrichment, from DNA methylation patterns for individual genes. Using publicly available datasets in real-world test scenarios, we demonstrate that M2A is highly accurate and robust in revealing promoter activity landscapes in various pediatric and adult cancers, including both solid and hematologic malignant neoplasms.

The Molecular and Pathophysiological Functions of Members of the LNX/PDZRN E3 Ubiquitin Ligase Family

The ligand of Numb protein-X (LNX) family, also known as the PDZRN family, is composed of four discrete RING-type E3 ubiquitin ligases (LNX1, LNX2, LNX3, and LNX4), and LNX5 which may not act as an E3 ubiquitin ligase owing to the lack of the RING domain. As the name implies, LNX1 and LNX2 were initially studied for exerting E3 ubiquitin ligase activity on their substrate Numb protein, whose stability was negatively regulated by LNX1 and LNX2 via the ubiquitin-proteasome pathway. LNX proteins may have versatile molecular, cellular, and developmental functions, considering the fact that besides these proteins, none of the E3 ubiquitin ligases have multiple PDZ (PSD95, DLGA, ZO-1) domains, which are regarded as important protein-interacting modules. Thus far, various proteins have been isolated as LNX-interacting proteins. Evidence from studies performed over the last two decades have suggested that members of the LNX family play various pathophysiological roles primarily by modulating the function of substrate proteins involved in several different intracellular or intercellular signaling cascades. As the binding partners of RING-type E3s, a large number of substrates of LNX proteins undergo degradation through ubiquitin-proteasome system (UPS) dependent or lysosomal pathways, potentially altering key signaling pathways. In this review, we highlight recent and relevant findings on the molecular and cellular functions of the members of the LNX family and discuss the role of the erroneous regulation of these proteins in disease progression.

PDZRN3 protects against apoptosis in myoblasts by maintaining cyclin A2 expression

PDZRN3 is a PDZ domain-containing RING-finger family protein that functions in various developmental processes. We previously showed that expression of PDZRN3 is induced together with that of MyoD during the early phase of skeletal muscle regeneration in vivo. We here show that PDZRN3 suppresses apoptosis and promotes proliferation in myoblasts in a manner dependent on cyclin A2. Depletion of PDZRN3 in mouse C2C12 myoblasts by RNA interference reduced the proportion of Ki-67-positive cells and the level of Akt phosphorylation, implicating PDZRN3 in regulation of both cell proliferation and apoptosis. Exposure of C2C12 cells as well as of C3H10T1/2 mesenchymal stem cells and NIH-3T3 fibroblasts to various inducers of apoptosis including serum deprivation resulted in a greater increase in the amount of cleaved caspase-3 in PDZRN3-depleted cells than in control cells. The abundance of cyclin A2 was reduced in PDZRN3-depleted C2C12 myoblasts, as was that of Mre11, which contributes to the repair of DNA damage. Overexpression of cyclin A2 restored the expression of Mre11 and Ki-67 as well as attenuated caspase-3 cleavage in PDZRN3-depleted cells deprived of serum. These results indicate that PDZRN3 suppresses apoptosis and promotes proliferation in myoblasts and other cell types by maintaining cyclin A2 expression.