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.

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction.

to the excess in reactive oxygen species (ROS) and Ca 2þ signals responsible for contraction injury in diseased muscle. In the present study, we hypothesized that a reduction in muscle redox buffering, as a result of chronic oxidative stress, is central to the MT changes that impair muscle function. Evidence suggest that oxidative stress is central to MT alterations in disease. Specifically, Nox2 expression precedes MT alterations (Khairallah et al, 2011), and its signaling regulates MT structure (Loehr et al, 2016) in dystrophic muscle. Furthermore, evidence shows oxidative regulation of the tubulin carboxypeptidase responsible for deTyr-tub (Nieuwenhuis et al, 2017). In order to test the contribution of altered redox capacity on MTs and muscle function, we treated adult C57BL/6J mice, with saline, butathione sulfoxamine (BSO; to deplete glutathione), taxol (MT polymerizer, promotes deTyr), or BSOþTaxol for 4 days. We show redox buffer capacity (GSH:GSSG) was significantly reduced with BSO and BSOþTaxol. However, only BSOþTaxol increased MT density and its level of deTyr-tub; a result consistent with BSO promoting Taxol's chemotherapeutic efficacy (Liebmann et al, 1993). With these MT alterations, we show that in vivo gastrocnemius function was unchanged with BSO or Taxol alone, while BSOþTaxol reduced muscle contractility and enhanced muscle contraction injury. Lastly, our examination of gastrocnemius muscle 1 day post contraction injury revealed that BSO treatment yielded a significant increase in MT density and deTyr-tub vs control despite injuring to the same extent. Taken together, we conclude that while decreased redox buffer capacity is insufficient to elicit MT alterations that impact function, it establishes a permissive environment for Taxol or injury to induce these changes. Huntington's disease (HD) is an inherited disorder with debilitating motor, cognitive, and psychiatric dysfunction. Motor symptoms, including chorea, rigidity, dystonia, and weakness, have principally been attributed to central neurodegeneration, but recent work suggests that primary skeletal muscle defects may also contribute to the motor symptoms. Electrophysiology studies in our lab have revealed a marked decrease in specific capacitance in HD muscle fibers from R6/2 transgenic mice compared with controls. This reduced capacitance is greater than expected when accounting for the smaller size of HD fibers. Because the transverse tubular system (TTS) in skeletal muscle makes up a significant proportion of total cell membrane, we hypothesized that structural changes in the TTS were responsible for reduced capacitance in the disease state. A loss or disruption of membrane in the TTS could directly contribute to weakness in HD. Decreased t-tubule diameters in HD fibers without changes in overall cellular TTS density were confirmed using quantitative electron and confocal microscopy. Here, we present results of calculations based on a cylindrical model of skeletal muscle incorporating these changes in muscle fiber and TTS architectu...