Desmin Intermediate Filaments Form a Stress-Transmitting and Stress-Signaling Network In Muscle

Abstract

INTRODUCTION: Muscle cells respond to stimuli such as stretch, exercise and electrical activation. Structurally, the cell’s desmin intermediate filament (IF) is strategically located between Z-disks to provide support and connections for intracellular organelles such as nuclei and mitochondria. However, the biological and/or mechanical significance of these interconnections are not clear. PURPOSE: The purpose of this study was to define the role of the desmin IF network in regulating nuclear strain, force transmission and mechanical stress-induced JNK signaling in muscle. METHODS: Plasmid DNA encoding a GFP-desmin (GFPDS) chimeric protein was introduced into desmin null (DSN) mouse muscle by electroporation. Muscles from wild type (WT, n=6) and either DSN (n=7), or transfected DSN muscles (n=9) were examined. Desmin’s biological role was probed by stretching isolated muscles and measuring Jun N-terminal kinase (JNK) phosphorylation. Force transmission was quantified by measuring isometric force generation after an eccentric contraction bout. Single fibers were imaged during deformation to quantify nuclear shape and strain. RESULTS: Nuclei in WT muscle subjected to stretch increased length/width ratio (i.e., aspect ratio with increased sarcomere length (1.25±.04 μm-1) while nuclei in DSN were significantly less deformed (0.268±.07 μm-1). Nuclear deformation pattern in DSN muscles transfected with GFPDS showed restored shape and deformation (1.22±.06 μm-1). WT muscle subjected to an eccentric contraction protocol demonstrated significantly more stress decrease (69±10%Po) compared to DSN muscles (37±4%Po) but this change within DSN muscles transfected with GFPDS was rescued to near WT levels (57±9 %Po). Finally, while stretch of WT muscle strongly phosphorylated both the 54 kDa and 46 kDa isoforms of JNK, DSN muscle showed a near absence of stretch-induced signaling to JNK. This signaling was partially rescued in DSN muscles transfected with GFPDS. DISCUSSION: These experiments clearly demonstrate that desmin IFs are involved in mechanical stress transmission, nuclear deformation and signaling. This demonstration of biomechanical integration by the desmin IF system suggests that it plays an active biological role in muscle in addition to its accepted structural role.