常陸 圭介

Inducing fast myofiber programs offers therapeutic potential for skeletal muscle disorders such as sarcopenia, where fast myofibers are preferentially lost. Engineered muscle-specific AAV (MyoAAV) vectors enable efficient transduction of skeletal muscles after systemic administration; however, cardiac transgene expression limits applications requiring skeletal muscle-selective delivery. We generated modified MyoAAV vectors by incorporating cardiac-specific miR-208a target sequences into the transgene 3′UTR. This design markedly suppressed cardiac expression while preserving skeletal muscle output, with target-site variation enabling tunable trade-offs between cardiac detargeting and skeletal muscle expression levels. We validated this platform using neural retina leucine zipper (Nrl), a large Maf transcription factor regulating type IIb myofiber identity. Systemic delivery of conventional MyoAAV-Nrl caused severe cardiac hypertrophy and uniform lethality within one month. Conversely, incorporating miR-208a target sequences prevented detectable hypertrophy and eliminated mortality during the experimental observation period. This modification significantly reduced cardiac Nrl expression while maintaining skeletal muscle levels, successfully promoting type IIb myofiber formation and hypertrophy across multiple skeletal muscles. These findings demonstrate that miR-208a-mediated cardiac detargeting combined with MyoAAV-Nrl enables safe systemic induction of fast myofiber remodeling and hypertrophy, establishing a platform for gene therapies targeting skeletal muscle disorders associated with fast myofiber loss.

Mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) is an intractable form of epilepsy involving the hippocampus, and temporal lobectomy remains an effective treatment. Human herpesvirus 6B (HHV-6B) establishes latency in the hippocampus and may contribute to MTLE-HS pathogenesis by altering host gene expression; however, transcriptomic data from healthy controls remain limited. This study investigated the role of HHV-6B to MTLE-HS pathogenesis by analyzing gene expression in resected hippocampal tissues. Samples were collected from 12 to 43 HHV-6 DNA-positive and -negative patients, respectively, and three controls. RNA sequencing was performed on eight representative samples, followed by RT-qPCR validation of nine selected genes in 58 samples. RNA sequencing identified 600 differentially expressed genes (210 upregulated, 390 downregulated) between HHV-6B-positive MTLE-HS and controls. Pathway enrichment analysis revealed involvement of synaptic signaling and inflammatory responses, with prostaglandin biosynthesis specifically upregulated in HHV-6B-positive tissues. Two genes were significantly upregulated in HHV-6B-positive compared with HHV-6B-negative samples. RT-qPCR confirmed elevated cholesterol 25-hydroxylase and interleukin 1 beta expression in HHV-6 DNA-positive samples (both p = 0.031). These findings suggest that HHV-6B may contribute to MTLE-HS pathogenesis by modulating the expression of host inflammatory genes, supporting a role for neuroinflammation and the potential benefits of anti-inflammatory therapies.

Transforming growth factorβ (TGFβ) signaling regulates diverse aspects of vertebrate skeletal muscle tissue including differentiation, homeostasis, regeneration and pathogenic degeneration. Ubiquitination of SMAD2, an intracellular transducer of TGFβ signaling, is a well-studied negative feedback regulation of the signaling pathway in the field of cell biology, but it's relevance in skeletal muscle tissue has been elusive. In this study, to elucidate the in vivo role of SMAD2 ubiquitination, we generated Smad2dPY mutant mice in which a 15 bp sequence encoding the PY motif of SMAD2 protein is deleted from Smad2 gene. By removing this motif, the SMAD2 protein escapes from protein-protein interaction with NEDD4 family E3 ligases and thus is devoid of ubiquitination-dependent negative regulation. Smad2dPY mice showed no obvious abnormality in development, growth or fertility, indicating that SMAD2 ubiquitination through PY motif is dispensable for these processes. The skeletal muscle of Smad2dPY mice demonstrated reduced weight and myofiber size reduction at 12 months old. SMAD2 protein level was increased in the skeletal muscle of Smad2dPY mice while SMAD2 ubiquitination was reduced. Primary myoblasts of Smad2dPY mice displayed higher TGFβ responsiveness and suppressed terminal differentiation, which may explain the reduced muscle mass. The TGFβ responsiveness of the interstitial fibroblast population was also increased. Fibrotic tissue remodeling triggered by cardiotoxin injection was exacerbated in Smad2dPY mice. Altogether, our study identified SMAD2 ubiquitination through PY motif as an important regulatory mechanism operating in skeletal muscle tissue to maintain the TGFβ signaling pathway at the desired level in homeostasis and tissue remodeling.