Analysis of CRE-mediated recombination driven by myosin light chain 1/3 regulatory elements in embryonic and adult skeletal muscle: a tool to study fiber specification

Skeletal Muscle-Specific Bis Depletion Leads to Muscle Dysfunction and Early Death Accompanied by Impairment in Protein Quality Control

Bcl-2-interacting cell death suppressor (BIS), also called BAG3, plays a role in physiological functions such as anti-apoptosis, cell proliferation, autophagy, and senescence. Whole-body Bis-knockout (KO) mice exhibit early lethality accompanied by abnormalities in cardiac and skeletal muscles, suggesting the critical role of BIS in these muscles. In this study, we generated skeletal muscle-specific Bis-knockout (Bis-SMKO) mice for the first time. Bis-SMKO mice exhibit growth retardation, kyphosis, a lack of peripheral fat, and respiratory failure, ultimately leading to early death. Regenerating fibers and increased intensity in cleaved PARP1 immunostaining were observed in the diaphragm of Bis-SMKO mice, indicating considerable muscle degeneration. Through electron microscopy analysis, we observed myofibrillar disruption, degenerated mitochondria, and autophagic vacuoles in the Bis-SMKO diaphragm. Specifically, autophagy was impaired, and heat shock proteins (HSPs), such as HSPB5 and HSP70, and z-disk proteins, including filamin C and desmin, accumulated in Bis-SMKO skeletal muscles. We also found metabolic impairments, including decreased ATP levels and lactate dehydrogenase (LDH) and creatine kinase (CK) activities in the diaphragm of Bis-SMKO mice. Our findings highlight that BIS is critical for protein homeostasis and energy metabolism in skeletal muscles, suggesting that Bis-SMKO mice could be used as a therapeutic strategy for myopathies and to elucidate the molecular function of BIS in skeletal muscle physiology.

Sirt6 reprograms myofibers to oxidative type through CREB-dependent Sox6 suppression

Expanding the exercise capacity of skeletal muscle is an emerging strategy to combat obesity-related metabolic diseases and this can be achieved by shifting skeletal muscle fibers toward slow-twitch oxidative type. Here, we report that Sirt6, an anti-aging histone deacetylase, is critical in regulating myofiber configuration toward oxidative type and that Sirt6 activator can be an exercise mimetic. Genetic inactivation of Sirt6 in skeletal muscle reduced while its transgenic overexpression increased mitochondrial oxidative capacity and exercise performance in mice. Mechanistically, we show that Sirt6 downregulated Sox6, a key repressor of slow fiber specific gene, by increasing the transcription of CREB. Sirt6 expression is elevated in chronically exercised humans, and mice treated with an activator of Sirt6 showed an increase in exercise endurance as compared to exercise-trained controls. Thus, the current study identifies Sirt6 as a molecular target for reprogramming myofiber composition toward the oxidative type and for improving muscle performance.

Inducible Rpt3, a Proteasome Component, Knockout in Adult Skeletal Muscle Results in Muscle Atrophy

The ubiquitin–proteasome system has the capacity to degrade polyubiquitinated proteins and plays an important role in many cellular processes. However, the role of Rpt3, a crucial proteasomal gene, has not been investigated in adult muscles in vivo. Herein, we generated skeletal-muscle-specific Rpt3 knockout mice, in which genetic inactivation of Rpt3 could be induced by doxycycline administration. The Rpt3-knockout mice showed a significant reduction by more than 90% in the expression of Rpt3 in adult muscles. Using this model, we found that proteasome dysfunction in adult muscles resulted in muscle wasting and a decrease in the myofiber size. Immunoblotting analysis showed that the amounts of ubiquitinated proteins were markedly higher in muscles of Rpt3-deficient mice than in those of the control mice. Analysis of the autophagy pathway in the Rpt3-deficient mice showed that the upregulation of LC3II, p62, Atg5, Atg7, and Beclin-1 in protein levels, which supposed to be compensatory proteolysis activation. Our results suggest that the proteasome inhibition in adult muscle severely deteriorates myofiber integrity and results in muscle atrophy.

CORP: Using transgenic mice to study skeletal muscle physiology

The development of tissue-specific inducible transgenic mice has provided a powerful tool to study gene function and cell biology in almost any tissue of interest at any given time within the animal's life. The purpose of this review is to describe how to use two different inducible transgenic systems, the Cre-loxP system and the Tet-ON/OFF system, that can be used to study skeletal muscle physiology. Myofiber- and satellite cell-specific Cre-loxP transgenic mice are described as is how these mice can be used to knockout a gene of interest or to deplete satellite cells in adult skeletal muscle, respectively. A myofiber-specific Tet-ON system is described as is how such mice can be used to overexpress a gene of interest or to label myonuclei. How to effectively breed and genotype the transgenic mice are also described in detail. The hope is this review will provide the basic information necessary to facilitate the incorporation of tissue-specific inducible transgenic mice into a skeletal muscle research program.

Loss of RNA-Binding Protein Sfpq Causes Long-Gene Transcriptopathy in Skeletal Muscle and Severe Muscle Mass Reduction with Metabolic Myopathy

Growing evidences are suggesting that extra-long genes in mammals are vulnerable for full-gene length transcription and dysregulation of long genes is a mechanism underlying human genetic disorders. How long-distance transcription is achieved is a fundamental question to be elucidated. In previous study, we had discovered that RNA-binding protein SFPQ preferentially binds to long pre-mRNAs and specifically regulates the cluster of neuronal genes >100 kbp. Here we investigated the roles of SFPQ for long gene expression, target specificities, and also physiological functions in skeletal muscle. Loss of Sfpq selectively downregulated genes >100 kbp including Dystrophin, which is 2.26 Mbp in length. Sfpq knockout (KO) mice showed progressive muscle mass reduction and metabolic myopathy characterized by glycogen accumulation and decreased abundance of mitochondrial oxidative phosphorylation complexes. Functional clustering analysis identified energy metabolism pathway genes as SFPQ's targets. These findings indicate target gene specificities and tissue-specific physiological functions of SFPQ in skeletal muscle.

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SFPQ is essential for long gene expression, including Dystrophin, in skeletal muscle

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Disruption of Sfpq caused severe muscle mass reduction and premature death

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SFPQ is required for metabolic pathway gene expression in skeletal muscle

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Loss of Sfpq decreased OXPHOS complexes and caused glycogen accumulation

Mouse models of peripheral metabolic disease

Metabolic disease risk is driven by defects in the function of cells that regulate energy homeostasis, as well as altered communication between the different tissues or organs that these cells occupy. Thus, it is desirable to use model organisms to understand the contribution of different cells, tissues and organs to metabolism. Mice are widely used for metabolic research, since well-characterised mouse strains (in terms of their genotype and phenotype) allow comparative studies and human disease modelling. Such research involves strains containing spontaneous mutations that lead to obesity and diabetes, surgically- and chemically-induced models, those that are secondary to caloric excess, genetic mutants created by transgenesis and gene knockout technologies, and peripheral models generated by Cre-Lox or CRISPR/Cas9 approaches. Focussing on obesity and type 2 diabetes as relevant metabolic diseases, we systematically review each of these models, discussing their use, limitations, and future potential.

Tdp-43 cryptic exons are highly variable between cell types

BACKGROUND

TDP-43 proteinopathy is a prominent pathological feature that occurs in a number of human diseases including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and inclusion body myositis (IBM). Our recent finding that TDP-43 represses nonconserved cryptic exons led us to ask whether cell type-specific cryptic exons could exist to impact unique molecular pathways in brain or muscle.

METHODS

In the present work, we investigated TDP-43's function in various mouse tissues to model disease pathogenesis. We generated mice to conditionally delete TDP-43 in excitatory neurons or skeletal myocytes and identified the cell type-specific cryptic exons associated with TDP-43 loss of function.

RESULTS

Comparative analysis of nonconserved cryptic exons in various mouse cell types revealed that only some cryptic exons were common amongst stem cells, neurons, and myocytes; the majority of these nonconserved cryptic exons were cell type-specific.

CONCLUSIONS

Our results suggest that in human disease, TDP-43 loss of function may impair cell type-specific pathways.

Mitochondrial Dynamics is a Distinguishing Feature of Skeletal Muscle Fiber Types and Regulates Organellar Compartmentalization

Skeletal muscle fibers differentiate into specific fiber types with distinct metabolic properties determined by their reliance on oxidative phosphorylation (OXPHOS). Using in vivo approaches, we find that OXPHOS-dependent fibers, compared to glycolytic fibers, contain elongated mitochondrial networks with higher fusion rates that are dependent on the mitofusins Mfn1 and Mfn2. Switching of a glycolytic fiber to an oxidative IIA type is associated with elongation of mitochondria, suggesting that mitochondrial fusion is linked to metabolic state. Furthermore, we reveal that mitochondrial proteins are compartmentalized to discrete domains centered around their nuclei of origin. The domain dimensions are dependent on fiber type and are regulated by the mitochondrial dynamics proteins Mfn1, Mfn2, and Mff. Our results indicate that mitochondrial dynamics is tailored to fiber type physiology and provides a rationale for the segmental defects characteristic of aged and diseased muscle fibers.

Proteasome dysfunction induces muscle growth defects and protein aggregation

The ubiquitin–proteasome and autophagy–lysosome pathways are the two major routes of protein and organelle clearance. The role of the proteasome pathway in mammalian muscle has not been examined in vivo. In this study, we report that the muscle-specific deletion of a crucial proteasomal gene, Rpt3 (also known as Psmc4), resulted in profound muscle growth defects and a decrease in force production in mice. Specifically, developing muscles in conditional Rpt3-knockout animals showed dysregulated proteasomal activity. The autophagy pathway was upregulated, but the process of autophagosome formation was impaired. A microscopic analysis revealed the accumulation of basophilic inclusions and disorganization of the sarcomeres in young adult mice. Our results suggest that appropriate proteasomal activity is important for muscle growth and for maintaining myofiber integrity in collaboration with autophagy pathways. The deletion of a component of the proteasome complex contributed to myofiber degeneration and weakness in muscle disorders that are characterized by the accumulation of abnormal inclusions.

Inducible Cre transgenic mouse strain for skeletal muscle-specific gene targeting

Background

The use of the Cre/loxP system for gene targeting has been proven to be a powerful tool for understanding gene function. The purpose of this study was to create and characterize an inducible, skeletal muscle-specific Cre transgenic mouse strain.

Methods

To achieve skeletal muscle-specific expression, the human α-skeletal actin promoter was used to drive expression of a chimeric Cre recombinase containing two mutated estrogen receptor ligand-binding domains.

Results

Western blot analysis, PCR and β-galactosidase staining confirmed that Cre-mediated recombination was restricted to limb and craniofacial skeletal muscles only after tamoxifen administration.

Conclusions

A transgenic mouse was created that allows inducible, gene targeting of floxed genes in adult skeletal muscle of different developmental origins. This new mouse will be of great utility to the skeletal muscle community.