No abstract

The heparan sulfate proteoglycan (HSPGs) is a components of the extracellular matrix of skeletal muscle that is concentrated at the neuromuscular junction (NMJ). Recent studies have suggested that HSPG, together with its bound peptide growth factors, plays important roles in autocrine or paracrine types of regulation of cell growth and differentiation. Heparin-binding growth-associated molecule (HB-GAM; also known as pleiotrophin, or p18) is a newly discovered HSPG-bound factor that is expressed at high levels in the developing CNS and PNS. In this study, we examined the role of this factor in NMJ development by examining its relationship to the formation of ACh receptor (AChR) clusters. Using an antibody against recombinant rat brain HB-GAM, we found that this protein is present prominently on the surface of cultured Xenopus myotomal muscle cells by immunocytochemistry. It is associated with HSPGs as evidenced by the fact that heparin and heparinase treatment greatly diminished the antibody labeling. HB-GAM is concentrated at preexisting AChR hot spots as well as at those induced by polystyrene beads. In addition, this molecule is also concentrated at AChR clusters induced by spinal cord neurons in nerve-muscle cocultures. To assess its function in synaptic induction, we applied recombinant HB-GAM-coated beads to cultured muscle cells to effect its focal presentation. Over 70% of these beads induced the formation of AChR clusters as shown by fluorescent alpha-bungarotoxin labeling. Furthermore, bath application of HB-GAM inhibited the nerve-induced formation of AChR clusters. Thus, HB-GAM is an endogenous muscle-derived factor that may be a component of the molecular mechanism in postsynaptic induction.

No abstract

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR). In mammalian cells, UPR signals generated by several ER membrane resident proteins, including the bifunctional protein kinase endoribonuclease IRE1α, control cell survival and the decision to execute apoptosis. Processing of XBP1 mRNA by the RNase domain of IRE1α promotes survival of ER stress, while activation of the mitogen-activated protein kinase JNK by IRE1α late in the ER stress response promotes apoptosis. Here we show that activation of JNK in the ER stress response precedes activation of XBP1. This activation of JNK is dependent on IRE1α and TRAF2 and coincides with JNK-dependent induction of expression of several antiapoptotic genes, including cIAP1, cIAP2, XIAP, and BIRC6. ER-stressed jnk1-/- jnk2-/- mouse embryonic fibroblasts (MEFs) display more pronounced mitochondrial permeability transition and increased caspase 3/7 activity compared to wild type MEFs. Caspase 3/7 activity is also elevated in ER-stressed ciap1-/- ciap2-/-, and xiap-/- MEFs. These observations suggest that JNK-dependent transcriptional induction of several inhibitors of apoptosis contributes to inhibiting apoptosis early in the ER stress response.