Accumulation of Na(+) channels at the nodes of Ranvier is a prerequisite for saltatory conduction. In peripheral nerves, clustering of these channels along the axolemma is regulated by myelinating Schwann cells through a yet unknown mechanism. We report the identification of gliomedin, a glial ligand for neurofascin and NrCAM, two axonal immunoglobulin cell adhesion molecules that are associated with Na+ channels at the nodes of Ranvier. Gliomedin is expressed by myelinating Schwann cells and accumulates at the edges of each myelin segment during development, where it aligns with the forming nodes. Eliminating the expression of gliomedin by RNAi, or the addition of a soluble extracellular domain of neurofascin to myelinating cultures, which caused the redistribution of gliomedin along the internodes, abolished node formation. Furthermore, a soluble gliomedin induced nodal-like clusters of Na+ channels in the absence of Schwann cells. We propose that gliomedin provides a glial cue for the formation of peripheral nodes of Ranvier.

The transition from juvenile to adult life is accompanied by programmed remodeling in many tissues and organs, which is key for organisms to adapt to the demand of the environment. Here we report a novel regulated alternative splicing program that is crucial for postnatnal heart remodeling in the mouse. We identify the essential splicing factor ASF/SF2 as a key component of the program, regulating a restricted set of tissue-specific alternative splicing events during heart remodeling. Cardiomyocytes deficient in ASF/SF2 display an unexpected hypercontraction phenotype due to a defect in postnatal splicing switch of the Ca(2+)/calmodulin-dependent kinase IIdelta (CaMKIIdelta) transcript. This failure results in mistargeting of the kinase to sarcolemmal membranes, causing severe excitation-contraction coupling defects. Our results validate ASF/SF2 as a fundamental splicing regulator in the reprogramming pathway and reveal the central contribution of ASF/SF2-regulated CaMKIIdelta alternative splicing to functional remodeling in developing heart.

The terminal differentiation of myelinating glia involves complex interactions that culminate in the formation of myelin. The POU domain transcription factor Tst-l/Oct-6/SCIP is expressed transiently during myelination, and we report here that it has a critical role in this developmental process. Deletion of the Tst-l/Oct-6/SCIP gene produces a severe defect in peripheral myelination by arresting Schwann cell maturation before axonal wrapping. Unexpectedly, the activation of major myelin-specific genes appears to be unaffected by the Tst-l/Oct-6/SCIP mutation, demonstrating that multiple, independently regulated events are required for terminal differentiation of Schwann cells. In addition, aberrant differentiation and migration of specific neurons in Tst-l/Oct-6/SCIP mutant homozygotes is associated with a fatal breathing defect, providing a model for investigating the regulation of pulmonary homeostasis.

Fibroblasts and smooth muscle cells (FSMCs) are principal cell types of connective and adventitial tissues that participate in the development, physiology and pathology of internal organs, with incompletely defined cellular origins. Here, we identify and prospectively isolate from mesothelium a mouse cell lineage that is committed to FSMCs. Mesothelium is an epithelial monolayer covering the vertebrate thoracic and abdominal cavities and internal organs. Time-lapse imaging and transplantation experiments reveal robust generation of FSMCs from the mesothelium. By targeting Mesothelin (MSLN), a surface marker expressed on mesothelial cells, we identify and isolate precursors capable of clonally generating FSMCs. Using a genetic lineage tracing approach, we show that embryonic and adult mesothelium represents a common lineage to trunk FSMCs, and trunk vasculature, with minimal contributions from neural crest, or circulating cells. The isolation of FSMC precursors enables examination of multiple aspects of smooth muscle and fibroblast biology as well as the prospective isolation of these precursors for potential regenerative medicine purposes.