Background Human heart failure (HF) is associated with decreased cardiac voltage-gated Na+ channel current (encoded by SCN5A), and the changes have been implicated in the increased risk of sudden death in HF. Nevertheless, the mechanism of SCN5A downregulation is unclear. A number of human diseases are associated with alternative mRNA splicing, which has received comparatively little attention in the study of cardiac disease. Splicing factor expression profiles during human HF and a specific splicing pathway for SCN5A regulation were explored in this paper. Methods and Results Gene array comparisons between normal human and heart failure tissues demonstrated that 17 splicing factors, associated with all major spliceosome components, were upregulated. Two of these splicing factors, RBM25 and LUC7L3, were elevated in human heart failure tissue and mediated truncation of SCN5A mRNA in both Jurkat cells and human embryonic stem cell-derived cardiomyocytes (hESC-CMs). RBM25/LUC7L3-mediated abnormal SCN5A mRNA splicing reduced Na+ channel current 91.1 ± 9.3% to a range known to cause sudden death. Overexpression of either splicing factor resulted in an increase in truncated mRNA and a concomitant decrease in the full-length SCN5A transcript. Conclusions Of the 17 mRNA splicing factors upregulated in HF, RBM25 and LUC7L3 were sufficient to explain the increase in truncated forms and the reduction in full length Na+ channel transcript. Since the reduction in channels was in the range known to be associated with sudden death, interruption of this abnormal mRNA processing may reduce arrhythmic risk in heart failure.

The node of Ranvier is a site for ionic conductances along myelinated nerves and governs the saltatory transmission of action potentials. Defects in the cross-bridging and spacing of the cytoskeleton is a prominent pathologic feature in diseases of the peripheral nerve. Electron tomography was used to examine cytoskeletal-cytoskeletal, membrane-cytoskeletal, and heterologous cell connections in the paranodal region of the node of Ranvier in peripheral nerves. Focal attachment of cytoskeletal filaments to each other and to the axolemma and paranodal membranes of the Schwann cell via narrow cross-bridges was visualized in both neuronal and glial cytoplasms. A subset of intermediate filaments associates with the cytoplasmic surfaces of supramolecular complexes of transmembrane structures that are presumed to include known and unknown junctional proteins. Mitochondria were linked to both microtubules and neurofilaments in the axoplasm and to neighboring smooth endoplasmic reticulum by narrow cross-bridges. Tubular cisternae in the glial cytoplasm were also linked to the paranodal glial cytoplasmic loop juxtanodal membrane by short cross-bridges. In the extracellular matrix between axon and Schwann cell, junctional bridges formed long cylinders inking the two membranes. Interactions between cytoskeleton, membranes, and extracellular matrix associations in the paranodal region is likely critical not only for scaffolding, but also for intracellular and extracellular communication.

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