Possible functions of p94 in connectin-mediated signaling pathways in skeletal muscle cells

Abstract

Calpains are intracellular Ca2+ -requiring 'modulator proteases', which modulate cellular functions by limited and specific proteolysis. p94/calpain3, a skeletal-muscle specific calpain, has been one of the representative calpain species which indicates physiological importance of calpain proteolytic system; a defect of proteolytic activity of p94 causes limb girdle muscular dystrophy type2A (LGMD2A, also called 'calpainopathy'). Immunohistochemical studies on myofibrils showed that p94 localizes at the Z- and N2-line regions of sarcomeres. It was also identified by the yeast two hybrid studies that p94 binds to the N2A and M-line regions of connectin. Furthermore, genetic studies indicate that p94 is indispensable for skeletal muscles, although its precise functions are still unclear. Interestingly, connectin provides sarcomere not only with elasticity but also with binding sites to various multi-functional proteins such as muscle ankyrin repeat proteins (MARPs), muscle RING finger proteins (MURFs), titin-capping protein (T-cap/telethonin), sarcomeric-alpha-actinin, p94 etc. Binding sites for these proteins are not randomly placed along connectin but rather accumulated in the Z-, N2-, and/or M-line regions, indicating the existence of 'signal complexes' unique to each regions. The concept of these complexes are strongly supported by the facts that mutations of connectin or its binding proteins in these regions severely perturb muscle functions, as in the case of LGMD2A caused by mutations in the p94 gene. Therefore, it is hypothesized that the 'signal complexes' in the Z-, N2-, and M-lines modulate muscle cell homeostasis by transducing signals of external stimulations/stresses to trigger appropriate response at various different cellular events such as protein modification and gene expressions. In this article, we performed detailed immunohistochemical analyses of p94 on isolated single myofibers. Together with recent findings about p94, it is suggested that sarcomeric localization of p94, especially its M-line localization, is affected by the combination of cellular contexts such as contractile status of myofibrils, fiber type compositions, sarcomeric maturation, and the composition of the 'signal complexes' in each region.

Gigantic Business

“…Moreover, elastic titin contains a binding site for calpain-1 within the proximal-Ig region (Ig13), thus affording a reservoir to the myofibrils, from which the peptidase can be released Figures 2B and 3). 51 Titin also binds skeletal muscle-specific calpain-3 at the N2-A element, 52,53 as well as matrix metalloproteinase-2 at the Z-/I-band region 54 ; both proteinases cleave titin at various sites. Relatively high basal proteolytic activity toward I-band titin could be the reason why protein gel electrophoresis of heart tissue usually reveals, next to intact titin (T1, ≥3000 kDa), ≥1 proteolytic titin fragments (T2, 2000-2500 kDa) containing the molecule's A-band portion but lacking most of the I-band region.…”

Endogenous Calpain-3 Activation Is Primarily Governed by Small Increases in Resting Cytoplasmic [Ca2+] and Is Not Dependent on Stretch

“…Although it is now recognized that autolysis is actually the process endowing proteolytic activity, it is still often said to occur in a Ca 2ϩ -independent manner (11,12). However, this does not seem an appropriate description.…”

“…Calpain-3 has been shown to bind to titin at both the N2A line and the M-line (19), although the latter binding site is not present in adult fast twitch muscle (12). The N terminus of calpain-3 also binds at the Z-band to ␣-actinin (20).…”

Inherited progressive limb-girdle muscular dystrophy type 2A (calpainopathy): a review of literature

“…Субстратами, расщепляемыми кальпаинами, в частности, являются белки, ответст-венные за передачу сигналов (родопсин, протеинки-назы А и С), а также белки цитоскелета (миофибрил-лярные белки -актин, миозин, парамиозин, тропомиозин и др., и белки нейрофиламентов). Сле-довательно, можно предположить, что кальпаин-3 непосредственно принимает участие в этих процессах, регулирует механизмы саркомер-ремоделирования и обеспечивает стабильность цитоскелета [21,[25][26][27]. Многочисленные исследования с использованием кле-точных культур, животных моделей и биопсии мышц пациентов свидетельствуют об участии продукта гена CAPN3 в дифференцировке мышечных волокон и их регенерации, а также в развитии апоптоза [28,29].…”