Gradual Formation and Accumulation of Tubular Aggregates in Fast Twitch Muscle Fibers: SERCA and Calsequestrin Involvement

Abstract

Tubular aggregates (TAs), ordered arrays of elongated sarcoplasmic reticulum (SR) membranes, are present in skeletal muscle fibers from patients with various myopathies. TAs have been also described in ageing wild type (WT) mice, where they display a dependence on sex (male), and fiber type (fast twitch). The mechanism(s) leading to TAs formation are, though, not yet clear. Here, we investigated the sequential stages leading to maturation of TAs in extensor digitorum longus (EDL) from male WT and calsequestrin knockout (CASQ-null) mice. A crucial step in the formation of TAs, seems to be the swelling of free SR cisternae at the sarcomere I band. This dilated SR, which is abundant at 1 year of age contains electron-dense material, which likely indicates abnormal accumulation of CASQ. Lately, the enlarged SR sacs mature into multiple and longitudinally oriented tubules containing CASQ, which first elongates into the A band, and then gradually acquire cylindrical shape and uniform size. Apparently the latter changes occur in concert with partial crystallization of sarco(endo)plasmic reticulum Ca2+ ATPases (SERCA) on its surface, as suggested by freeze-fracture (FF) evidence indicative of an inactive ATPase. Finally, multiple small TAs associate to form fewer mature aggregates of very large size. Interestingly, in fibers from CASQ1-knockout mice the initial swelling of the SR does not occur, possibly due to lack of CASQ accumulation, and aggregates of SR tubules remain small/wavy and never develop into ordered aggregates of straight cylinders. Based on these results, we propose the hypothesis that TAs do not participate actively in the fibers’ Ca2+ homeostasis, but may simply act as deposit sites for accumulated proteins.