(IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle. cytoskeleton; costameres; elastimetry; biomechanical properties; desmin A LARGE NUMBER OF HUMAN DISEASES, ranging from skin disorders (51), to premature aging (34, 64), to skeletal and cardiac myopathies (19,37), are linked to mutations in genes encoding intermediate filament (IF) proteins (25). Since the discovery of desmin, the major IF protein of striated muscle (49), its role and that of other IF proteins in muscle have been of keen interest to many laboratories (reviewed in 16). Our research on muscle IF proteins has addressed their role in force transmission and in organizing the sarcoplasm and stabilizing it against injury during force generation.Force generated in muscle can be transmitted radially, from the myofibrils, across the sarcolemma to the extracellular matrix and neighboring cells (11, 67). As a result, a significant amount of force is exerted on the sarcolemma and the structures that link it to the underlying contractile apparatus and the extracellular matrix. The ability of the sarcolemma with the underlying contractile apparatus to withstand the distortions caused during isotonic contractions (2, 11, 12) depends on specialized structures, termed "costameres" (66), that mediate the radial transmission of force across the sarcolemma (11). Costameres are structures at the sarcolemma of striated muscle fibers that align circumferentially around the Z disks and the M bands of the nearest myofibrils, and longitudinally, to form a rectilinear sarcolemmal network comprised of integral membrane proteins (such as dystroglycan, the sarcoglycans, and Na-K-ATPase), proteins of the extracellular matrix (such as laminin and collagen IV), and proteins of t...

Biological roles of obscurin and its close homolog Obsl1 (obscurin-like 1) have been enigmatic. While obscurin is highly expressed in striated muscles, Obsl1 is found ubiquitously. Accordingly, obscurin mutations have been linked to myopathies, whereas mutations in Obsl1 result in 3M-growth syndrome. To further study unique and redundant functions of these closely related proteins, we generated and characterized Obsl1 knockouts. Global Obsl1 knockouts are embryonically lethal. In contrast, skeletal muscle-specific Obsl1 knockouts show a benign phenotype similar to obscurin knockouts. Only deletion of both proteins and removal of their functional redundancy revealed their roles for sarcolemmal stability and sarcoplasmic reticulum organization. To gain unbiased insights into changes to the muscle proteome, we analyzed tibialis anterior and soleus muscles by mass spectrometry, uncovering additional changes to the muscle metabolism. Our analyses suggest that all obscurin protein family members play functions for muscle membrane systems.

Background: Small ankyrin 1 (sAnk1) is required for stability of the network sarcoplasmic reticulum and shares transmembrane similarity with sarcolipin. Results: sAnk1-SERCA interactions and their effects on SERCA were demonstrated by co-immunoprecipitation, FRET, blot overlay, and Ca 2ϩ -ATPase assays. Conclusion: sAnk1 binds SERCA and reduces the affinity of SERCA for Ca 2ϩ . Significance: sAnk1 may play a role in maintaining Ca 2ϩ homeostasis in skeletal muscle.

SERCA1, the sarco(endo)plasmic reticulum Ca-ATPase of skeletal muscle, is essential for muscle relaxation and maintenance of low resting Ca levels in the myoplasm. We recently reported that small ankyrin 1 (sAnk1) interacts with the sarco(endo)plasmic reticulum Ca-ATPase in skeletal muscle (SERCA1) to inhibit its activity. We also showed that this interaction is mediated at least in part through sAnk1's transmembrane domain in a manner similar to that of sarcolipin (SLN). Earlier studies have shown that SLN and phospholamban, the other well studied small SERCA-regulatory proteins, oligomerize either alone or together. As sAnk1 is coexpressed with SLN in muscle, we sought to determine whether these two proteins interact with one another when coexpressed exogenously in COS7 cells. Coimmunoprecipitation (coIP) and anisotropy-based FRET (AFRET) assays confirmed this interaction. Our results indicated that sAnk1 and SLN can associate in the sarcoplasmic reticulum membrane and after exogenous expression in COS7 cells but that their association did not require endogenous SERCA2. Significantly, SLN promoted the interaction between sAnk1 and SERCA1 when the three proteins were coexpressed, and both coIP and AFRET experiments suggested the formation of a complex consisting of all three proteins. Ca-ATPase assays showed that sAnk1 ablated SLN's inhibition of SERCA1 activity. These results suggest that sAnk1 interacts with SLN both directly and in complex with SERCA1 and reduces SLN's inhibitory effect on SERCA1 activity.

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