Calpain inhibition rescues troponin T3 fragmentation, increases Cav1.1 , and enhances skeletal muscle force in aging sedentary mice

Abstract

SummaryLoss of strength in human and animal models of aging can be partially attributed to a well‐recognized decrease in muscle mass; however, starting at middle‐age, the normalized force (force/muscle cross‐sectional area) in the knee extensors and single muscle fibers declines in a curvilinear manner. Strength is lost faster than muscle mass and is a more consistent risk factor for disability and death. Reduced expression of the voltage sensor Ca2+ channel α1 subunit (Cav1.1) with aging leads to excitation–contraction uncoupling, which accounts for a significant fraction of the decrease in skeletal muscle function. We recently reported that in addition to its classical cytoplasmic location, fast skeletal muscle troponin T3 (TnT3) is fragmented in aging mice, and both full‐length TnT3 (FL‐TnT3) and its carboxyl‐terminal (CT‐TnT3) fragment shuttle to the nucleus. Here, we demonstrate that it regulates transcription of Cacna1s, the gene encoding Cav1.1. Knocking down TnT3 in vivo downregulated Cav1.1. TnT3 downregulation or overexpression decreased or increased, respectively, Cacna1s promoter activity, and the effect was ablated by truncating the TnT3 nuclear localization sequence. Further, we mapped the Cacna1s promoter region and established the consensus sequence for TnT3 binding to Cacna1s promoter. Systemic administration of BDA‐410, a specific calpain inhibitor, prevented TnT3 fragmentation, and Cacna1s and Cav1.1 downregulation and improved muscle force generation in sedentary old mice.

Skeletal muscle performance and ageing

“…74 Further, systemic administration of a calpain inhibitor (BDA-410) prevented TnT3 fragmentation and Cav1.1 down-regulation and improved soleus muscle quality bỹ 20-30 + % depending on the stimulation frequency ration in sedentary old mice (23-25 month old C57BL/6 female mice). 74 These findings suggest that excitation-contraction uncoupling contributes significantly to the reduction in muscle quality observed with advancing age and highlights the processes in the E-CC pathway as a potential therapeutic target.…”

“…Elegant work from Delbonno and colleagues has demonstrated reduced expression of the voltage sensor calcium channel α1 subunit (Cav1.1) with ageing in three different groups of muscles (soleus, extensor digitorum longus, and in a pool of several skeletal muscles consisting of a mixture of fast‐twitch and slow‐twitch muscle fibres), which has been shown to lead to excitation–contraction uncoupling . This group recently reported that fast skeletal muscle troponin T3 (TnT3) is fragmented in ageing mice and that the full‐length TnT3 and its carboxyl‐terminal fragment shuttle to the nucleus and regulate the gene encoding Cav1.1 . Further, systemic administration of a calpain inhibitor (BDA‐410) prevented TnT3 fragmentation and Cav1.1 down‐regulation and improved soleus muscle quality by ~20–30 + % depending on the stimulation frequency ration in sedentary old mice (23–25 month old C57BL/6 female mice) .…”

“…This group recently reported that fast skeletal muscle troponin T3 (TnT3) is fragmented in ageing mice and that the full‐length TnT3 and its carboxyl‐terminal fragment shuttle to the nucleus and regulate the gene encoding Cav1.1 . Further, systemic administration of a calpain inhibitor (BDA‐410) prevented TnT3 fragmentation and Cav1.1 down‐regulation and improved soleus muscle quality by ~20–30 + % depending on the stimulation frequency ration in sedentary old mice (23–25 month old C57BL/6 female mice) . These findings suggest that excitation–contraction uncoupling contributes significantly to the reduction in muscle quality observed with advancing age and highlights the processes in the E‐CC pathway as a potential therapeutic target.…”

Cardiac troponin T and fast skeletal muscle denervation in ageing

“…For instance, Tn and Tm are found in the nucleus in various tissues 2, 3. We recently found that fast skeletal muscle TnT3 can function as a transcription regulator that modulates muscle contraction‐related gene expression, including calcium channel beta1a and alpha1s subunits, by nuclear translocation 4, 5…”

“…2,3 We recently found that fast skeletal muscle TnT3 can function as a transcription regulator that modulates muscle contraction-related gene expression, including calcium channel beta1a and alpha1s subunits, by nuclear translocation. 4,5 Unlike cardiac TnI, which is highly specific for cardiac muscle, TnT2, the cardiac isoform of troponin T (cTnT), is also expressed in developing skeletal muscles, after sciatic nerve denervation, and in various neuromuscular diseases. [6][7][8] Whereas injury to myocardium causes the simultaneous release of both cTnI and cTnT into the circulation, denervation and chronic injury of skeletal muscle may induce expression and release of cTnT, but not cTnI, from the skeletal muscle.…”