Role of collapsin response mediator protein-2 in neurite outgrowth of PC12 cells

Effects of geldanamycin on neurite outgrowth-related proteins and kinases in nerve growth factor-differentiated pheochromocytoma 12 cells

Heat shock protein 90 (HSP90) antagonists are currently being evaluated as potential anticancer drugs. However, adverse effects related to these drugs, such as fatigue and pain, suggest that they affect neurons. Therefore, to understand the influence of HSP90 inhibitors on neurons, we investigated the effects of geldanamycin, an HSP90 antagonist, on nerve growth factor (NGF)-differentiated pheochromocytoma 12 (PC12) cells, particularly, on the expression and phosphorylation of proteins and kinases in the NGF pathway. Geldanamycin significantly inhibited NGF-induced neurite outgrowth and phosphorylation of Akt and extracellular signal-related kinase 1/2 in PC12 cells. Furthermore, geldanamycin inhibited the phosphorylation of collapsin response mediator protein 2 and the expression of cyclin-dependent kinase 5 in the presence of NGF, but did not significantly affect the expression of glycogen synthase kinase 3β. These results suggest that geldanamycin influences microtubule-binding proteins and kinases relating to neurite outgrowth, thereby inducing neuronal impairment.

Phosphorylation of serine 774 of the neural cell adhesion molecule (NCAM) is involved in the interaction with collapsin response mediator protein-2

The neural cell adhesion molecule NCAM is a major adhesion receptor involved in the development and regeneration of the nervous system. It is expressed in three major isoforms of which two have large intracellular domains of different lengths (NCAM140 and NCAM180). Several intracellular ligands of NCAM have been described. One of them is the collapsin response mediator protein-2 (CRMP-2), which is known to be involved in cell differentiation and axonal growth. The cytoplasmic domains of NCAM contain up to 49 phosphorylation sites and it has been demonstrated recently that the phosphorylation of serine 774 is crucial for NCAM-mediated signal transduction and neurite outgrowth. Here we analyzed the interaction of NCAM with CRMP-2 in more detail using a biochemical approach. We found that CRMP-2 binds specifically to NCAM180 in a sequence between amino acid 788 and 819. In addition we could demonstrate that serine 774, which has been shown previously to be phosphorylated and involved in neurite outgrowth, is also important for the interaction of CRMP-2 with NCAM.

Targeting of CRMP-2 to the primary cilium is modulated by GSK-3β

CRMP-2 plays a pivotal role in promoting axon formation, neurite outgrowth and elongation in neuronal cells. CRMP-2′s role in other cells is unknown. Our preliminary results showed CRMP-2 expression in cilia of fibroblasts. To localize CRMP-2, define its role and study the regulation of CRMP-2′s expression in cilia we carried out the following experiments. We find that in fibroblasts CRMP-2 localizes to the centrosome and is associated with the basal body and -at a low level- is present in primary cilia. Phosphorylated pCRMP-2 can only be detected at the basal body. RNAi knockdown of CRMP-2 interfered with primary cilium assembly demonstrating a critical requirement for CRMP-2. Deletion analysis of CRMP-2 identified a 51 amino acid sequence in the C-terminus that is required for targeting to the basal body and primary cilium. This domain contains GSK-3β phosphorylation sites as well as two repeats of the VxPx motif, previously identified as a cilium targeting signal in other primary cilium proteins. To our surprise, mutation of the CRMP-2 VxPx motifs did not eliminate primary cilium targeting. Instead, mutation of the GSK-3β phosphorylation sites abolished CRMP-2 targeting to the primary cilium without affecting basal body localization. Treatment of cells with lithium, a potent GSK-3β inhibitor, or with two specific GSK-3β inhibitors (the L803-mts peptide inhibitor and CHIR99021) resulted in cilium elongation and decreased basal body levels of pCRMP-2 as well as increased levels of total CRMP-2 at the primary cilium. In summary, we identified CRMP-2 as a protein critically involved in primary cilia formation. To our knowledge this is the first demonstration of modulation of primary cilium targeting by GSK-3β.

Activation of CRHR2 exerts an inhibitory effect on the expression of collapsin response mediator protein 3 in hippocampal neurons

Corticotropin-releasing hormone (CRH) family peptides as well as their receptors have been shown to exhibit various functions in hippocampus. However, effects of CRH receptors activation on collapsin response mediator protein 3 (CRMP3), the key protein for dendrite outgrowth and cell apoptosis, remain unclear. In the present study, we determined the effects of CRHR1 and CRHR2 on CRMP3 expression in cultured hippocampal neurons. CRH and urocortin II (UCNII) dose-dependently suppressed CRMP3 mRNA and protein expression. The inhibitory effect on CRMP3 expression was completely reversed by CRHR2 antagonist but not by CRHR1 antagonist. Investigations on the signaling pathways of UCNII showed that CRHR2 mediated UCNII-induced increase in phosphorylated phospholipase C (PLC)-β3 expression. Blocking PLC activity with U73122 and PKC with Gö6976 completely prevented UCNII-inhibited CRMP3 expression. Our results suggest that CRHR2 activation decrease CRMP3 expression in hippocampal neurons via a mechanism that is dependent on PLC/PKC signaling pathways.