In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily. Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location. Furthermore, we show that the localization of Caspr2 and clustering of K+ channels at the juxtaparanodal region depends on the presence of TAG-1, an immunoglobulin-like cell adhesion molecule that binds Caspr2. These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon–glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.

Autophagy, an evolutionarily conserved process, has functions both in cytoprotective and programmed cell death mechanisms. Beclin 1, an essential autophagic protein, was recently identified as a BH3-domain-only protein that binds to Bcl-2 anti-apoptotic family members. The dissociation of beclin 1 from its Bcl-2 inhibitors is essential for its autophagic activity, and therefore should be tightly controlled. Here, we show that death-associated protein kinase (DAPK) regulates this process. The activated form of DAPK triggers autophagy in a beclin-1-dependent manner. DAPK phosphorylates beclin 1 on Thr 119 located at a crucial position within its BH3 domain, and thus promotes the dissociation of beclin 1 from Bcl-X L and the induction of autophagy. These results reveal a substrate for DAPK that acts as one of the core proteins of the autophagic machinery, and they provide a new phosphorylation-based mechanism that reduces the interaction of beclin 1 with its inhibitors to activate the autophagic machinery.

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.

Myelination in the peripheral nervous system requires close contact between Schwann cells and the axon, but the underlying molecular basis remains largely unknown. Here we show that cell adhesion molecules (CAMs) of the Nectin-like (Necl, also known as SynCAM or Cadm) family mediate Schwann cell-axon interaction during myelination. Necl4 is the main Necl expressed by myelinating Schwann cells and is located along the internodes in direct apposition to Necl1, which is localized on axons. Necl4 serves as the glial binding partner for axonal Necl1, and the interaction between these two CAMs mediates Schwann cell adhesion. Furthermore, the disruption of the interaction between Necl1 and Necl4 by their soluble extracellular domains, as well as the expression of a dominant negative Necl4 in Schwann cells, inhibits myelination. These results suggest a critical role for Necl proteins in mediating axon-glia contact during myelination in peripheral nerves.

The tumor suppressor functions of p19(ARF) have been attributed to its ability to induce cell cycle arrest or apoptosis by activating p53 and regulating ribosome biogenesis. Here we describe another cellular function of p19(ARF), involving a short isoform (smARF, short mitochondrial ARF) that localizes to a Proteinase K-resistant compartment of the mitochondria. smARF is a product of internal initiation of translation at Met45, which lacks the nucleolar functional domains. The human p14(ARF) mRNA likewise produces a shorter isoform. smARF is maintained at low levels via proteasome-mediated degradation, but it increases in response to viral and cellular oncogenes. Ectopic expression of smARF reduces mitochondrial membrane potential (DeltaPsim) without causing cytochrome c release or caspase activation. The dissipation of DeltaPsim does not depend on p53 or Bcl-2 family members. smARF induces massive autophagy and caspase-independent cell death that can be partially rescued by knocking down ATG5 or Beclin-1, suggesting a different prodeath function for this short isoform.