Chapter ten - Regulation of Nucleocytoplasmic Transport in Skeletal Muscle
Introduction
Skeletal muscle is a very plastic tissue that readily undergoes changes in mass and function in response to aging, injury, and disease. Such changes in muscle can impact breathing, locomotion, and metabolism and affect motility and lifespan. Proper skeletal muscle function is dependent on spatial and temporal control of gene expression mediated by proteins, such as transcription factors, that require facilitated transport to enter the nuclei. The subcellular localization of these regulatory proteins must be tightly controlled because altered import or export could result in aberrant muscle function.
Proper muscle function is dependent on myofibers, which are multinucleated cells containing many hundreds of nuclei distributed along the length of the cell in a common cytoplasm. Alongside each myofiber are adult muscle stem cells, called satellite cells, that lay beneath the basal lamina surrounding each myofiber. These satellite cells are normally quiescent but in response to muscle damage they are activated to begin proliferating and undergo differentiation and eventual fusion with each other or existing myofibers to repair muscles in a process called myogenesis. Myogenesis can be modeled in vitro by culturing myoblasts, the progeny of satellite cells, and inducing them to differentiate into multinucleated myotubes by changes in culture media. Myogenesis both in vivo and in vitro requires the coordinate activation and repression of many genes. Numerous nuclear proteins are required for proper gene expression and the nuclear repertoire of these proteins is very different along the myogenic continuum of quiescent satellite cells to mature postmitotic myofibers. How satellite cells differentially regulate nucleocytoplasmic transport of these nuclear proteins critical for regulating gene expression during quiescence, activation, and differentiation is unknown. In addition, how a myofiber with hundreds of nuclei coordinates and regulates nucleocytoplasmic transport is not clear.
Section snippets
Nuclear Envelope
The nuclear envelope of eukaryotic cells provides separation of the genetic material and transcriptional machinery within the nucleus from the translational machinery in the cytoplasm enhancing regulation of gene expression. The nuclear envelope is comprised of two lipid membrane bilayers, the outer nuclear membrane which is contiguous with the endoplasmic reticulum and the inner nuclear membrane which faces the nucleoplasm (Hetzer and Wente, 2009). The inner nuclear membrane contains integral
Nuclear Pore Complexes
The nuclear envelope is perforated by nuclear pore complexes (NPCs) which fuse the outer nuclear membrane and inner nuclear membrane together to create channels for nucleocytoplasmic transport (Fig. 10.1; Lim and Fahrenkrog, 2006). NPCs are multiprotein suprastructures (~ 50 MDa) which provide channels for the nucleocytoplasmic exchange of ions and macromolecules (Alber et al., 2007). While smaller ions and molecules can diffuse through the NPC, molecules larger than ~ 40 kDa require a targeting
Nuclear Import Pathways
Nuclear transport is a process whereby proteins or other macromolecules traverse the NPC either by directly interacting with peripheral FG-Nups within the NPC channel or by binding to import or export receptors that mediate transport through the NPC (Walde and Kehlenbach, 2010). The majority of nuclear transport receptors are karyopherin family members termed importins, transportins, or exportins which mediate transport across the NPC in an energy-dependent manner (Tran and Wente, 2006, Weis,
Identifying Classical Nuclear Import-Dependent Cargoes
Identifying the specific cargo proteins that are transported via various nucleocytoplasmic transport pathways is key for understanding the regulatory networks that govern cell function. Here we focus on how cNLS-dependent cargoes are identified since this pathway is the best characterized nuclear transport pathway; however, many of the challenges in cargo identification presented here apply also to other receptor-mediated transport pathways. In Mus musculus, 30–55% of nuclear proteins are
Remodeling of the Nuclear Transport Machinery
Alterations in global nucleocytoplasmic transport provide another layer of control over gene expression. Global changes in the efficiency or rate of nuclear transport can occur through alterations or remodeling of key components of the nuclear transport machinery. For example, altering the expression or localization of karyopherin transport receptors, Ran and/or Ran-associated proteins, or Nups results in changes in transport efficiency (Hodel et al., 2001, Hodel and Harreman, 2006; Riddick and
Challenges in Studying Nucleocytoplasmic Transport in Multinucleated Cells
The basic mechanics for nucleocytoplasmic transport of proteins and RNA identified to date and described in this review have almost exclusively been examined in cells with a single nucleus. Skeletal muscle is the only permanent multinucleated cell type in the body and constitutes ~ 50% of body mass; yet, how these cells spatially and temporally regulate and coordinate nucleocytoplasmic transport among hundreds of nuclei is unknown.
Spatial and temporal regulation of nucleocytoplasmic transport
Summary
Nucleocytoplasmic transport plays a key regulatory role in cellular physiology. While much is known about facilitated nuclear transport in other cell types, the study of nucleocytoplasmic transport in skeletal muscle is still in its infancy. Multinucleated myofibers are faced with unique challenges compared to most other mammalian cell types in controlling the function of hundreds of nuclei in a common cytoplasm. Although a fair bit is known about nuclear envelope proteins in skeletal muscle
Acknowledgments
G. K. P. is supported by National Institute of Health grants AR051372, AR052730, AR047314, and NS059340.
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