Overexpression of SMPX in adult skeletal muscle does not change skeletal muscle fiber type or size

White-throated sparrow (Zonotrichia albicollis) liver and pectoralis flight muscle transcriptomic changes in preparation for migration

Migratory songbirds undertake challenging journeys to reach their breeding grounds each spring. They accomplish these nonstop flapping feats of endurance through a suite of physiological changes, including the development of substantial fat stores and flight muscle hypertrophy and an increased capacity for fat catabolism. In addition, migratory birds may show large reductions in organ masses during flight, including the flight muscle and liver, which they must rapidly rebuild during their migratory stopover before replenishing their fat stores. However, the molecular basis of this capacity for rapid tissue remodeling and energetic output has not been thoroughly investigated. We performed RNA-sequencing analysis of the liver and pectoralis flight muscle of captive white-throated sparrows in experimentally photostimulated migratory and nonmigratory condition to explore the mechanisms of seasonal change to metabolism and tissue mass regulation that may facilitate these migratory journeys. Based on transcriptional changes, we propose that tissue-specific adjustments in preparation for migration may alleviate the damaging effects of long-duration activity, including a potential increase to the inflammatory response in the muscle. Furthermore, we hypothesize that seasonal hypertrophy balances satellite cell recruitment and apoptosis, while little evidence appeared in the transcriptome to support myostatin-, insulin-like growth factor 1-, and mammalian target of rapamycin-mediated pathways for muscle growth. These findings can encourage more targeted molecular studies on the unique integration of pathways that we find in the development of the migratory endurance phenotype in songbirds.NEW & NOTEWORTHY Migratory songbirds undergo significant physiological changes to accomplish their impressive migratory journeys. However, we have a limited understanding of the regulatory mechanisms underlying these changes. Here, we explore the transcriptomic changes to the flight muscle and liver of white-throated sparrows as they develop the migratory condition. We use these patterns to develop hypotheses about metabolic flexibility and tissue restructuring in preparation for migration.

SMPX Deficiency Causes Stereocilia Degeneration and Progressive Hearing Loss in CBA/CaJ Mice

The small muscle protein, x-linked (SMPX) encodes a small protein containing 88 amino acids. Malfunction of this protein can cause a sex-linked non-syndromic hearing loss, named X-linked deafness 4 (DFNX4). Herein, we reported a point mutation and a frameshift mutation in two Chinese families who developed gradual hearing loss with age. To explore the impaired sites in the hearing system and the mechanism of DFNX4, we established and validated an Smpx null mouse model using CRISPR-Cas9. By analyzing auditory brainstem response (ABR), male Smpx null mice showed a progressive hearing loss starting from high frequency at the 3rd month. Hearing loss in female mice was milder and occurred later compared to male mice, which was very similar to human beings. Through morphological analyses of mice cochleas, we found the hair cell bundles progressively degenerated from the shortest row. Cellular edema occurred at the end phase of stereocilia degeneration, followed by cell death. By transfecting exogenous fluorescent Smpx into living hair cells, Smpx was observed to be expressed in stereocilia. Through noise exposure, it was shown that Smpx might participate in maintaining hair cell bundles. This Smpx knock-out mouse might be used as a suitable model to explore the pathology of DFNX4.

Mammalian and Invertebrate Models as Complementary Tools for Gaining Mechanistic Insight on Muscle Responses to Spaceflight

Bioinformatics approaches have proven useful in understanding biological responses to spaceflight. Spaceflight experiments remain resource intensive and rare. One outstanding issue is how to maximize scientific output from a limited number of omics datasets from traditional animal models including nematodes, fruitfly, and rodents. The utility of omics data from invertebrate models in anticipating mammalian responses to spaceflight has not been fully explored. Hence, we performed comparative analyses of transcriptomes of soleus and extensor digitorum longus (EDL) in mice that underwent 37 days of spaceflight. Results indicate shared stress responses and altered circadian rhythm. EDL showed more robust growth signals and Pde2a downregulation, possibly underlying its resistance to atrophy versus soleus. Spaceflight and hindlimb unloading mice shared differential regulation of proliferation, circadian, and neuronal signaling. Shared gene regulation in muscles of humans on bedrest and space flown rodents suggest targets for mitigating muscle atrophy in space and on Earth. Spaceflight responses of C. elegans were more similar to EDL. Discrete life stages of D. melanogaster have distinct utility in anticipating EDL and soleus responses. In summary, spaceflight leads to shared and discrete molecular responses between muscle types and invertebrate models may augment mechanistic knowledge gained from rodent spaceflight and ground-based studies.

Inner Ear and Muscle Developmental Defects in Smpx-Deficient Zebrafish Embryos

The last decade has witnessed the identification of several families affected by hereditary non-syndromic hearing loss (NSHL) caused by mutations in the SMPX gene and the loss of function has been suggested as the underlying mechanism. In the attempt to confirm this hypothesis we generated an Smpx-deficient zebrafish model, pointing out its crucial role in proper inner ear development. Indeed, a marked decrease in the number of kinocilia together with structural alterations of the stereocilia and the kinocilium itself in the hair cells of the inner ear were observed. We also report the impairment of the mechanotransduction by the hair cells, making SMPX a potential key player in the construction of the machinery necessary for sound detection. This wealth of evidence provides the first possible explanation for hearing loss in SMPX-mutated patients. Additionally, we observed a clear muscular phenotype consisting of the defective organization and functioning of muscle fibers, strongly suggesting a potential role for the protein in the development of muscle fibers. This piece of evidence highlights the need for more in-depth analyses in search for possible correlations between SMPX mutations and muscular disorders in humans, thus potentially turning this non-syndromic hearing loss-associated gene into the genetic cause of dysfunctions characterized by more than one symptom, making SMPX a novel syndromic gene.

Expression pattern of the small muscle protein, X-linked (smpx) gene during zebrafish embryonic and larval developmental stages

The small muscle protein, X-linked (SMPX) gene encodes a cytoskeleton-associated protein, highly expressed in both cardiac and skeletal muscles, as well as in fetal inner ears, with suggested roles as mechanotransductor. Recently, several mutations in the SMPX gene have been associated with X-chromosomal progressive deafness in human. However, very little information is known concerning the roles of SMPX, and no in-vivo models are currently available. Therefore, we characterized the zebrafish ortholog of SMPX to pave the way towards the establishment of a biotool for future functional studies. Despite the genome duplication occurred in the ancestry of teleosts, zebrafish retain only one copy of smpx which shares a high degree of similarity with the mammalian counterpart in terms of genomic organization, syntenic map, and encoded protein. RT-PCR, as well as whole-mount in-situ hybridization and immunofluorescence analyses, revealed that smpx is expressed in several embryonic areas starting from the 4-somite stage. Specifically, smpx mRNA marked the Kupffer's vesicle (KV), the somites, the myocardium, the hair cells of the anterior and the posterior macula of the inner ear, the pronephric ducts, and the muscles of the branchial arches, eyes and pectoral fins. According to our data, zebrafish smpx expression pattern closely resembles that observed in mouse and human, supporting the notion that zebrafish might represent a suitable in-vivo model to disclose the cellular and molecular mechanisms underlying the involvement of SMPX in development and disease.

Increased Expression of Lipid Metabolism Genes in Early Stages of Wooden Breast Links Myopathy of Broilers to Metabolic Syndrome in Humans

Wooden breast is a muscle disorder affecting modern commercial broiler chickens that causes a palpably firm pectoralis major muscle and severe reduction in meat quality. Most studies have focused on advanced stages of wooden breast apparent at market age, resulting in limited insights into the etiology and early pathogenesis of the myopathy. Therefore, the objective of this study was to identify early molecular signals in the wooden breast transcriptional cascade by performing gene expression analysis on the pectoralis major muscle of two-week-old birds that may later exhibit the wooden breast phenotype by market age at 7 weeks. Biopsy samples of the left pectoralis major muscle were collected from 101 birds at 14 days of age. Birds were subsequently raised to 7 weeks of age to allow sample selection based on the wooden breast phenotype at market age. RNA-sequencing was performed on 5 unaffected and 8 affected female chicken samples, selected based on wooden breast scores (0 to 4) assigned at necropsy where affected birds had scores of 2 or 3 (mildly or moderately affected) while unaffected birds had scores of 0 (no apparent gross lesions). Differential expression analysis identified 60 genes found to be significant at an FDR-adjusted p-value of 0.05. Of these, 26 were previously demonstrated to exhibit altered expression or genetic polymorphisms related to glucose tolerance or diabetes mellitus in mammals. Additionally, 9 genes have functions directly related to lipid metabolism and 11 genes are associated with adiposity traits such as intramuscular fat and body mass index. This study suggests that wooden breast disease is first and foremost a metabolic disorder characterized primarily by ectopic lipid accumulation in the pectoralis major.

RNA-Seq Analysis Reveals Spatial and Sex Differences in Pectoralis Major Muscle of Broiler Chickens Contributing to Difference in Susceptibility to Wooden Breast Disease

Wooden Breast Disease (WBD) is a novel myopathy affecting the pectoralis major muscle of modern broiler chickens. The etiology of WBD is not currently known, but has been linked to increased feed efficiency, growth rate, and muscle yield in broiler chickens. Differential effect of WBD has been detected between regions of the P. major and between sexes of broilers-male birds and the cranial aspect of the muscle tend to be more severely affected by the disease than females and the caudal aspect. This study aimed to characterize biological differences in the P. major between regions of the muscle and sexes of birds. Samples were taken from the cranial and caudal aspects of P. major muscles of 3-week-old, unaffected male and female birds for RNA sequencing. RNA was extracted and used for preparation of cDNA libraries, which were sequenced by the Delaware Biotechnology Institute (DBI) using HiSeq2500. Sequence reads were aligned to the chicken reference genome with HISAT, and genes were analyzed for differential expression between regions of the breast muscle and sexes of birds using CuffDiff. Functional analysis was performed on differentially expressed genes (DEGs) between sex groups using DAVID and Ingenuity Pathway Analysis (IPA). There were 12 DEGs between cranial and caudal samples, and 260 between male and female birds. Out of the 260 genes differentially expressed between sexes, 189 were upregulated in males. Of this subset, 103 genes (55%) were located on the Z-chromosome. There was increased expression of genes involved in fat metabolism and oxidative stress responses in the cranial region of the P. major muscle, as well as increased expression of fat metabolism, oxidative stress response, antiangiogenesis, and connective tissue proliferation genes in male broilers. These results support the hypothesis that there are biological characteristics in male broilers and the cranial region of the breast muscle that may make them more susceptible to WBD, as well as raising the possibility of a metabolic switch in modern broiler chickens that may be more prominent in males.

Proteomics analysis indicated the protein expression pattern related to the development of fetal conotruncal defects

Abnormal development of embryonic conus arteriosus could lead to conotruncal defects in fetal heart, and increase the incidence of fetal congenital heart disease. Tetralogy of Fallot (TOF) is one of the most common forms of congenital heart disease. It may be helpful for us to solve this clinical problem through exploring the molecular mechanisms of development in embryonic congenital heart disease. Proteomics has attracted much attention in understanding the development of human diseases during the past decades. However, there is still little information about the relationship between protein expression pattern and TOF. In this study, we aimed to explore the potential linkage of proteomics and TOF development. Briefly, 121 differentially expressed proteins were identified from a TOF group, compared with a control group. The expression levels of 34 of these proteins were significantly different (>1.5 absolute fold change, p < 0.05) between the two groups. Gene ontology (GO) and pathway analysis showed that these proteins were mainly associated with carbon metabolism, biosynthesis of antibodies, positive regulation of transcription from RNA polymerase II promoter, nucleus, ATP binding, and so on. The ingenuity pathway analysis (IPA) results indicated that 435 of upstream regulators were identified of these differentially expressed proteins, which might be involved in the development of TOF. Data of string analysis showed the protein-protein interaction network among the differentially expressed proteins and regulators, which are related to TOF. In conclusion, our study explored the protein expression pattern of TOF, which might provide new insights into understanding the mechanism of TOF development and afford potential targets for TOF diagnosis and therapy.

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.