The intermediate filament synemin exists as two alternate splice isoforms, and ,

Question

The intermediate filament synemin exists as two alternate splice isoforms, and , and is enriched in skeletal myocytes, but is also found in other tissues, and appears to be represented by at least one isoform in many different eukaryotes. You have been contacted by Dr. Xavier, who wants you to find out if his ghost crab skeletal muscle tissue contains synemin, and if so, whether it is enriched for the or isoform. How will you accomplish this? The intermediate filament synemin exists as two alternate splice isoforms, and , and is enriched in skeletal myocytes, but is also found in other tissues, and appears to be represented by at least one isoform in many different eukaryotes. You have been contacted by Dr. Xavier, who wants you to find out if his ghost crab skeletal muscle tissue contains synemin, and if so, whether it is enriched for the or isoform. How will you accomplish this?

Explanation / Answer

Answer)Ghost crabs possess rapid running capabilities, which make them good candidates for comparing invertebrate exercise physiology with that of more extensively studied vertebrates. While a number of studies have examined various aspects of running physiology and biomechanics in terrestrial crabs, none to date have defined the basic skeletal muscle fiber types that power locomotion. In the current study, we investigated skeletal muscle fiber types comprising the extensor and flexor carpopodite muscles in relation to running performance in the ghost crab. We used kinematic analyses to determine stride frequency and muscle shortening velocity and found that both parameters are similar to those of comparably sized mammals but slower than those observed in running lizards. Using several complementary methods, we found that the muscles are divided into two primary fiber types: those of the proximal and distal regions possess long sarcomeres (6.2+/-2.3 microm) observed in crustacean slow fibers and have characteristics of aerobic fibers whereas those of the muscle mid-region have short sarcomeres (3.5+/-0.4 microm) characteristic of fast fibers and appear to be glycolytic. Each fiber type is characterized by several different myofibrillar protein isoforms including multiple isoforms of myosin heavy chain (MHC), troponin I (TnI), troponin T (TnT) and a crustacean fast muscle protein, P75. Three different isoforms of MHC are differentially expressed in the muscles, with fibers of the mid-region always co-expressing two isoforms at a 1:1 ratio within single fibers. Based on our analyses, we propose that these muscles are functionally divided into a two-geared system, with the aerobic fibers used for slow sustained activities and the glycolytic mid-region fibers being reserved for explosive sprints. Finally, we identified subtle differences in myofibrillar isoform expression correlated with crab body size, which changes by several orders of magnitude during an animal's lifetime.

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