Skeletal Muscle Atrophy in R6/2 Mice – Altered Circulating Skeletal Muscle Markers and Gene Expression Profile Changes

Abstract

Background: In addition to classical neurological symptoms, Huntington's disease (HD) is complicated by peripheral pathology, including progressive skeletal muscle wasting, and common skeletal muscle gene expression changes have been shown in HD mice and human HD. Objective: To highlight possible mechanisms underlying muscle wasting in HD, we examined gene expression in pathways governing skeletal muscle contractility, skeletal myogenesis, skeletal muscle wasting, apoptosis and the NFκB signaling pathway in two HD mouse models (the transgenic R6/2 and full-length knock-in Q175). In addition, we assessed circulating markers that increase in response to skeletal muscle injury, skeletal Troponin I (sTnI), fatty acid binding protein 3 (FABP3), and Myosin light chain 3 (Myl3). Methods: We measured gene expression in muscle tissue as well as in cultured primary myocytes using qPCR. Concentrations of cytokines and muscle proteins were obtained using multiplex ELISA. Results: Circulating markers of muscle injury (sTnI, FABP3, and Myl3) were significantly increased in mouse serum. In skeletal muscle, we observed reduced gene expression of components involved in muscle contractility, with pronounced downregulation of Acta1, Myh2 and Tnni2, among others. Alongside, we found increased expression of caspases (3 and 8) and key elements of the NFκB signaling pathway, p65/RelA, Tradd, and TRAF5. We also found similar gene expression alterations in cultured primary myocytes from R6/2 mice stimulated with TNF-␣. Conclusions: Our results indicate that activation of apoptotic and NFκB pathways occur alongside down-regulation of key compartments of the muscle contractility unit in skeletal muscle of HD mice, and muscle atrophy could possibly be a source of circulating disease progression markers.

A shared mechanism of muscle wasting in cancer and Huntington's disease

“…In general, it is believed that mitochondrial dysfunction and energy deficits underline HD pathology; for a review see [14]. Our results were in line with a previous study in the R6/2 mouse model that showed increased levels of circulating markers of muscle injury in the serum and a reduction of contractile transcripts [15]. It is interesting to compare these results to those found in cancer cachexia; animal models of urothelial carcinoma or Lewis lung carcinoma showed decreased respiratory chain activity [16] and a lower level of ATP [17], respectively, similarly to HD mouse models [11].…”

“…However, it is also possible that an intrinsic component of the HTT mutant gene product, expressed within muscle cells, might directly lead to pathogenic consequences. In addition, it has been shown that R6/2 mice had elevated levels of NFκB pathways that may be involved in muscle atrophy [15]. Similarly, increased levels of pro‐inflammatory cytokines like tumor necrosis factors (TNF) and interleukin 1 (IL‐1), caused by dysfunction of hypothalamic serotonergic neurons, have been implicated in cancer cachexia [26].…”

Ghrelin rescues skeletal muscle catabolic profile in the R6/2 mouse model of Huntington’s disease

“…Altered gene expression in skeletal muscle has been shown to reflect HD progression 8 , including activation of apoptotic and NFκ-B pathway transcripts in R6/2 mice 26 . We therefore evaluated gene expression changes related to muscle damage and cachexia in gastrocnemius skeletal muscle, and the effects of 2 and 4 weeks of ghrelin treatment on these alterations.…”

“…HD myocytes have been shown to exhibit increased expression of genes encoding, for example, chaperones and heat shock proteins, and muscle-specific mRNAs have been shown to be altered 8 , 48 . The increased expression of genes involved in apoptosis and autophagy 25 , 26 suggests that these mechanisms may contribute to a catabolic phenotype and muscle wasting. In line with this, we here confirmed that R6/2 mouse skeletal muscle exhibits a catabolic gene expression profile.…”

Early Neurodegeneration in R6/2 Mice Carrying the Huntington’s Disease Mutation with a Super-Expanded CAG Repeat, Despite Normal Lifespan

“…The reason for the disparity between brain pathology and phenotype are likely to be complex. For example, there is considerable peripheral pathology in these mice [60][61][62][63][64][65], so the rapid deterioration in phenotype is unlikely to be due solely to brain dysfunction.…”