Utrophin Compensates dystrophin Loss during Mouse Spermatogenesis

Dystrophin- and Utrophin-Based Therapeutic Approaches for Treatment of Duchenne Muscular Dystrophy: A Comparative Review

Duchenne muscular dystrophy is a devastating disease that leads to progressive muscle loss and premature death. While medical management focuses mostly on symptomatic treatment, decades of research have resulted in first therapeutics able to restore the affected reading frame of dystrophin transcripts or induce synthesis of a truncated dystrophin protein from a vector, with other strategies based on gene therapy and cell signaling in preclinical or clinical development. Nevertheless, recent reports show that potentially therapeutic dystrophins can be immunogenic in patients. This raises the question of whether a dystrophin paralog, utrophin, could be a more suitable therapeutic protein. Here, we compare dystrophin and utrophin amino acid sequences and structures, combining published data with our extended in silico analyses. We then discuss these results in the context of therapeutic approaches for Duchenne muscular dystrophy. Specifically, we focus on strategies based on delivery of micro-dystrophin and micro-utrophin genes with recombinant adeno-associated viral vectors, exon skipping of the mutated dystrophin pre-mRNAs, reading through termination codons with small molecules that mask premature stop codons, dystrophin gene repair by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genetic engineering, and increasing utrophin levels. Our analyses highlight the importance of various dystrophin and utrophin domains in Duchenne muscular dystrophy treatment, providing insights into designing novel therapeutic compounds with improved efficacy and decreased immunoreactivity. While the necessary actin and β-dystroglycan binding sites are present in both proteins, important functional distinctions can be identified in these domains and some other parts of truncated dystrophins might need redesigning due to their potentially immunogenic qualities. Alternatively, therapies based on utrophins might provide a safer and more effective approach.

Signaling through the dystrophin glycoprotein complex affects the stress-dependent transcriptome in Drosophila

Deficiencies in the human dystrophin glycoprotein complex (DGC), which links the extracellular matrix with the intracellular cytoskeleton, cause muscular dystrophies, a group of incurable disorders associated with heterogeneous muscle, brain and eye abnormalities. Stresses such as nutrient deprivation and aging cause muscle wasting, which can be exacerbated by reduced levels of the DGC in membranes, the integrity of which is vital for muscle health and function. Moreover, the DGC operates in multiple signaling pathways, demonstrating an important function in gene expression regulation. To advance disease diagnostics and treatment strategies, we strive to understand the genetic pathways that are perturbed by DGC mutations. Here, we utilized a Drosophila model to investigate the transcriptomic changes in mutants of four DGC components under temperature and metabolic stress. We identified DGC-dependent genes, stress-dependent genes and genes dependent on the DGC for a proper stress response, confirming a novel function of the DGC in stress-response signaling. This perspective yields new insights into the etiology of muscular dystrophy symptoms, possible treatment directions and a better understanding of DGC signaling and regulation under normal and stress conditions.

Utrophin modulator drugs as potential therapies for Duchenne and Becker muscular dystrophies

Utrophin is an autosomal paralogue of dystrophin, a protein whose deficit causes Duchenne and Becker muscular dystrophies (DMD/BMD). Utrophin is naturally overexpressed at the sarcolemma of mature dystrophin‐deficient fibres in DMD and BMD patients as well as in the mdx Duchenne mouse model. Dystrophin and utrophin can co‐localise in human foetal muscle, in the dystrophin‐competent fibres from DMD/BMD carriers, and revertant fibre clusters in biopsies from DMD patients. These findings suggest that utrophin overexpression could act as a surrogate, compensating for the lack of dystrophin, and, as such, it could be used in combination with dystrophin restoration therapies. Different strategies to overexpress utrophin are currently under investigation. In recent years, many compounds have been reported to modulate utrophin expression efficiently in preclinical studies and ameliorate the dystrophic phenotype in animal models of the disease. In this manuscript, we discuss the current knowledge on utrophin protein and the different mechanisms that modulate its expression in skeletal muscle. We also include a comprehensive review of compounds proposed as utrophin regulators and, as such, potential therapeutic candidates for these muscular dystrophies.

Tsga8 is required for spermatid morphogenesis and male fertility in mice

During spermatogenesis, intricate gene expression is coordinately regulated by epigenetic modifiers, which are required for differentiation of spermatogonial stem cells (SSCs) contained among undifferentiated spermatogonia. We have previously found that KMT2B conveys H3K4me3 at bivalent and monovalent promoters in undifferentiated spermatogonia. Because these genes are expressed late in spermatogenesis or during embryogenesis, we expect that many of them are potentially programmed by KMT2B for future expression. Here, we show that one of the genes targeted by KMT2B, Tsga8, plays an essential role in spermatid morphogenesis. Loss of Tsga8 in mice leads to male infertility associated with abnormal chromosomal distribution in round spermatids, malformation of elongating spermatid heads and spermiation failure. Tsga8 depletion leads to dysregulation of thousands of genes, including the X-chromosome genes that are reactivated in spermatids, and insufficient nuclear condensation accompanied by reductions of TNP1 and PRM1, key factors for histone-to-protamine transition. Intracytoplasmic sperm injection (ICSI) of spermatids rescued the infertility phenotype, suggesting competency of the spermatid genome for fertilization. Thus, Tsga8 is a KMT2B target that is vitally necessary for spermiogenesis and fertility.