Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms

Impaired mitochondrial morphological plasticity and failure of mitophagy associated with the G11778A mutation of LHON

Mitochondrial dynamics were examined in human dermal fibroblasts biopsied from a confirmed Leber's Hereditary Optic Neuropathy (LHON) patient with a homoplasmic G11778A mutation of the mitochondrial genome. Expression of the G11778A mutation did not impart any discernible difference in mitochondrial network morphology using widefield fluorescence microscopy. However, at the ultrastructural level, cells expressing this mutation exhibited an impairment of mitochondrial morphological plasticity when forced to utilize oxidative phosphorylation (OXPHOS) by transition to glucose-free, galactose-containing media. LHON fibroblasts also displayed a transient increase in mitophagy upon transition to galactose media. These results provide new insights into the consequences of the G11778A mutation of LHON and the pathological mechanisms underlying this disease.

Impairments in contractility and cytoskeletal organisation cause nuclear defects in nemaline myopathy

Nemaline myopathy (NM) is a skeletal muscle disorder caused by mutations in genes that are generally involved in muscle contraction, in particular those related to the structure and/or regulation of the thin filament. Many pathogenic aspects of this disease remain largely unclear. Here, we report novel pathological defects in skeletal muscle fibres of mouse models and patients with NM: irregular spacing and morphology of nuclei; disrupted nuclear envelope; altered chromatin arrangement; and disorganisation of the cortical cytoskeleton. Impairments in contractility are the primary cause of these nuclear defects. We also establish the role of microtubule organisation in determining nuclear morphology, a phenomenon which is likely to contribute to nuclear alterations in this disease. Our results overlap with findings in diseases caused directly by mutations in nuclear envelope or cytoskeletal proteins. Given the important role of nuclear shape and envelope in regulating gene expression, and the cytoskeleton in maintaining muscle fibre integrity, our findings are likely to explain some of the hallmarks of NM, including contractile filament disarray, altered mechanical properties and broad transcriptional alterations.

Induced in vivo knockdown of the Brca1 gene in skeletal muscle results in skeletal muscle weakness

KEY POINTS

Breast cancer 1 early onset gene codes for the DNA repair enzyme, breast cancer type 1 susceptibility protein (BRCA1). The gene is prone to mutations that cause a loss of protein function. BRCA1/Brca1 has recently been found to regulate several cellular pathways beyond DNA repair and is expressed in skeletal muscle. Skeletal muscle specific knockout of Brca1 in mice caused a loss of muscle quality, identifiable by reductions in muscle force production and mitochondrial respiratory capacity. Loss of muscle quality was associated with a shift in muscle phenotype and an accumulation of mitochondrial DNA mutations. These results demonstrate that BRCA1 is necessary for skeletal muscle function and that increased mitochondrial DNA mutations may represent a potential underlying mechanism.

ABSTRACT

Recent evidence suggests that the breast cancer 1 early onset gene (BRCA1) influences numerous peripheral tissues, including skeletal muscle. The present study aimed to determine whether induced-loss of the breast cancer type 1 susceptibility protein (Brca1) alters skeletal muscle function. We induced genetic ablation of exon 11 in the Brca1 gene specifically in the skeletal muscle of adult mice to generate skeletal muscle-specific Brca1 homozygote knockout (Brca1KOsmi ) mice. Brca1KOsmi exhibited kyphosis and decreased maximal isometric force in limb muscles compared to age-matched wild-type mice. Brca1KOsmi skeletal muscle shifted toward an oxidative muscle fibre type and, in parallel, increased myofibre size and reduced capillary numbers. Unexpectedly, myofibre bundle mitochondrial respiration was reduced, whereas contraction-induced lactate production was elevated in Brca1KOsmi muscle. Brca1KOsmi mice accumulated mitochondrial DNA mutations and exhibited an altered mitochondrial morphology characterized by distorted and enlarged mitochondria, and these were more susceptible to swelling. In summary, skeletal muscle-specific loss of Brca1 leads to a myopathy and mitochondriopathy characterized by reductions in skeletal muscle quality and a consequent kyphosis. Given the substantial impact of BRCA1 mutations on cancer development risk in humans, a parallel loss of BRCA1 function in patient skeletal muscle cells would potentially result in implications for human health.

Essential nucleotide- and protein-dependent functions of Actb/β-actin

The highly similar cytoplasmic β- and γ-actins differ by only four functionally similar amino acids, yet previous in vitro and in vivo data suggest that they support unique functions due to striking phenotypic differences between Actb and Actg1 null mouse and cell models. To determine whether the four amino acid variances were responsible for the functional differences between cytoplasmic actins, we gene edited the endogenous mouse Actb locus to translate γ-actin protein. The resulting mice and primary embryonic fibroblasts completely lacked β-actin protein, but were viable and did not present with the most overt and severe cell and organismal phenotypes observed with gene knockout. Nonetheless, the edited mice exhibited progressive high-frequency hearing loss and degeneration of actin-based stereocilia as previously reported for hair cell-specific Actb knockout mice. Thus, β-actin protein is not required for general cellular functions, but is necessary to maintain auditory stereocilia.

β-Actin shows limited mobility and is required only for supraphysiological insulin-stimulated glucose transport in young adult soleus muscle

Studies in skeletal muscle cell cultures suggest that the cortical actin cytoskeleton is a major requirement for insulin-stimulated glucose transport, implicating the β-actin isoform, which in many cell types is the main actin isoform. However, it is not clear that β-actin plays such a role in mature skeletal muscle. Neither dependency of glucose transport on β-actin nor actin reorganization upon glucose transport have been tested in mature muscle. To investigate the role of β-actin in fully differentiated muscle, we performed a detailed characterization of wild type and muscle-specific β-actin knockout (KO) mice. The effects of the β-actin KO were subtle; however, we confirmed the previously reported decline in running performance of β-actin KO mice compared with wild type during repeated maximal running tests. We also found insulin-stimulated glucose transport into incubated muscles reduced in soleus but not in extensor digitorum longus muscle of young adult mice. Contraction-stimulated glucose transport trended toward the same pattern, but the glucose transport phenotype disappeared in soleus muscles from mature adult mice. No genotype-related differences were found in body composition or glucose tolerance or by indirect calorimetry measurements. To evaluate β-actin mobility in mature muscle, we electroporated green fluorescent protein (GFP)-β-actin into flexor digitorum brevis muscle fibers and measured fluorescence recovery after photobleaching. GFP-β-actin showed limited unstimulated mobility and no changes after insulin stimulation. In conclusion, β-actin is not required for glucose transport regulation in mature mouse muscle under the majority of the tested conditions. Thus, our work reveals fundamental differences in the role of the cortical β-actin cytoskeleton in mature muscle compared with cell culture.

The makings of the 'actin code': regulation of actin's biological function at the amino acid and nucleotide level

The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles in vivo We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.