A mouse model of Huntington’s disease shows altered ultrastructure of transverse tubules in skeletal muscle fibers

Abstract

Huntington’s disease (HD) is a fatal and progressive condition with severe debilitating motor defects and muscle weakness. Although classically recognized as a neurodegenerative disorder, there is increasing evidence of cell autonomous toxicity in skeletal muscle. We recently demonstrated that skeletal muscle fibers from the R6/2 model mouse of HD have a decrease in specific membrane capacitance, suggesting a loss of transverse tubule (t-tubule) membrane in R6/2 muscle. A previous report also indicated that Cav1.1 current was reduced in R6/2 skeletal muscle, suggesting defects in excitation–contraction (EC) coupling. Thus, we hypothesized that a loss and/or disruption of the skeletal muscle t-tubule system contributes to changes in EC coupling in R6/2 skeletal muscle. We used live-cell imaging with multiphoton confocal microscopy and transmission electron microscopy to assess the t-tubule architecture in late-stage R6/2 muscle and found no significant differences in the t-tubule system density, regularity, or integrity. However, electron microscopy images revealed that the cross-sectional area of t-tubules at the triad were 25% smaller in R6/2 compared with age-matched control skeletal muscle. Computer simulation revealed that the resulting decrease in the R6/2 t-tubule luminal conductance contributed to, but did not fully explain, the reduced R6/2 membrane capacitance. Analyses of bridging integrator-1 (Bin1), which plays a primary role in t-tubule formation, revealed decreased Bin1 protein levels and aberrant splicing of Bin1 mRNA in R6/2 muscle. Additionally, the distance between the t-tubule and sarcoplasmic reticulum was wider in R6/2 compared with control muscle, which was associated with a decrease in junctophilin 1 and 2 mRNA levels. Altogether, these findings can help explain dysregulated EC coupling and motor impairment in Huntington’s disease.

Huntington’s disease skeletal muscle has altered T-tubules

“…In a surprise finding, the authors of this follow up study ( Romer et al, 2021 ) did not detect a difference in T-tubule density, spacing, or regularity between R6/2 and control skeletal fibers. However, R6/2 skeletal muscle fibers had smaller cross-sectional areas and openings ( Romer et al, 2021 ). Thus, T-tubules are not disrupted as much as they are smaller.…”

“…In an earlier issue of the Journal of General Physiology , Romer and colleagues used a mouse model of Huntington’s disease (HD) to explore a primary skeletal myocyte origin component of HD. The hypothesis tested in the study is that T-tubules are altered in HD skeletal muscle, leading to EC coupling changes and muscle dysfunction ( Romer et al, 2021 ).…”

“…In the present study, Romer et al (2021) used a R6/2 mouse model of HD to explore primary pathogenesis in skeletal muscle. They previously reported that R6/2 HD skeletal muscle fibers have a decrease in specific membrane capacitance ( Miranda et al, 2017 ).…”

“…A reduction in T-tubule size can at least partially account for reduced total cellular membrane capacitance. Furthermore, R6/2 muscle fibers had increased spacing between SR terminal cisternae and T-tubule membranes ( Romer et al, 2021 ), which will contribute to impaired EC coupling. T-tubule and SR docking may precede the incorporation of the RYRs into SR membrane, and especially that of the newly formed T-tubule–SR junctions ( Takekura et al, 2001 ).…”

Purine Nucleotides Metabolism and Signaling in Huntington’s Disease: Search for a Target for Novel Therapies

“…Moreover, HD mice models were characterized by skeletal muscle atrophy [ 69 ]. It is also noted that R6/2 mice have altered the ultrastructure of transverse tubules in skeletal muscle fibers [ 70 ]. At the cellular level, aggregation of mHTT, the inclusion of poly-ubiquitinated proteins were found in myofibers and myonuclei in R6/2 mice [ 9 , 71 ].…”