Cell signalling cascades regulating neuronal growth-promoting and inhibitory cues

During development of the nervous system, neurons extend axons over considerable distances in a highly stereospecific fashion in order to innervate their targets in an appropriate manner. This involves the recognition, by the axonal growth cone, of guidance cues that determine the pathway taken by the axons. These guidance cues can act to promote and/or repel growth cone advance, and they can act either locally or at a distance from their place of synthesis. The directed growth of axons is partly governed by cell adhesion molecules (CAMs) on the neuronal growth cone that bind to CAMs on the surface of other axons or non-neuronal cells. In vitro assays have established the importance of the CAMs (N-CAM, N-cadherin and the L1 glycoprotein) in promoting axonal growth over cells, such as Schwann cells, astrocytes and muscle cells. Strong evidence now exists implicating the fibroblast growth factor receptor tyrosine kinase as the primary signal transduction molecule in the CAM pathway. Cell adhesion molecules are important constituents of synapses, and CAMs appear to play important and diverse roles in regulating synaptic plasticity associated with learning and memory. Negative extracellular signals which physically direct neurite growth have also been described. The latter include the neuronal growth inhibitory proteins Nogo and myelin-associated glycoprotein, as well as the growth cone collapsing Semaphorins/neuropilins. Although less well characterised, evidence is now beginning to emerge describing a role for Rho kinase-mediated signalling in inhibition of neurite outgrowth. This review focuses on some of the major themes and ideas associated with this fast-moving field of neuroscience.

CAMs and axonal growth: a critical evaluation of the role of calcium and the MAPK cascade

Calcium has long been recognized as a key player in the control of axonal growth and guidance. Recent studies lend support to this pivotal role by showing that local changes in calcium can directly induce the formation of filopodia in vivo and turn a growth cone in vitro. Under normal growth conditions, the L1 adhesion molecule has now been shown to induce local rather than global changes in calcium in growth cones, and this suggests that cell adhesion molecules (CAMs) use localized calcium transients to stimulate axonal growth and guidance. A number of recent reports have demonstrated that the neurite outgrowth response stimulated by L1 and other adhesion molecules (NCAM, N-cadherin, laminin) also depends in part upon the integrity of the MAPK cascade in cells. In this review we consider the recent data and suggest that calcium and the MAPK cascade might be required for very distinct growth cone functions. Finally, we will consider the contentious issue of how the above CAMs activate signaling cascades in growth cones and review the recently available data that support the hypothesis that at least one of these CAMs (N-cadherin) might promote growth cone motility by directly interacting with the FGFR in growth cones.

Calphostin C induces AP1 synthesis and AP1-dependent c-jun transactivation in normal human chondrocytes independent of protein kinase C-alpha inhibition: possible role for c-jun N-terminal kinase

Activator protein-1 (AP1) regulates the promoter activity of a large number of genes associated with developmental, proliferative, inflammatory, and homeostatic processes in human connective tissue cells. Some of these genes (e.g., cyclooxygenase-2) are regulated by the protein kinase C (PKC) inhibitor, calphostin C (CalC). We examined whether CalC could indeed induce AP1 and AP1 gene transactivation (c-jun) in human chondrocytes. Exploratory studies confirmed the anti-PKC effects of CalC, as equal molar concentrations of CalC blocked the PMA-induced translocation of PKC-alpha from the cytosolic to the membrane fraction. CalC induction of AP1, as judged by gel-shift analysis, using a consensus AP1 oligonucleotide, was biphasic with an initial increase (maximum 4 h), followed by a decline, reaching its nadir after 16 h, and finally a major upregulation phase at 24 h. Maximum induction of AP-1 was reached at a concentration of 250 nmol/L of CalC. CalC did not block PMA-induced AP1 synthesis. Gel-shift analysis in the presence of specific antibodies to c-Jun, JunB, JunD, c-Fos, and CREB/ATF showed that the AP1 complexes were probably c-Jun/c-Jun, c-Fos/c-Jun, c-Fos/JunB, or c-Jun/JunB dimers. Northern blot analysis confirmed that c-jun, junB, and c-Fos were the principal proto-oncogenes induced by CalC. To confirm that c-jun induction occurs at the transcriptional level and to examine the role of the AP1 site present in the c-jun promoter in the induction of c-jun by CalC, we performed transient transfections of c-jun promoter-CAT constructs harboring either wild-type (WT) AP1 regulatory element sites or mutant AP1 sites. CalC (250 nmol/L) induced a marked increase in CAT activity (i.e., promoter activation) with WT AP1 c-jun promoter-CAT plasmids, but the response was completely abrogated when using constructs where the AP1 site was mutated. PMA produced similar results, but the induction of the WT AP1 c-jun promoter-CAT plasmid was smaller. CalC (250 nmol/L) inhibited MAPK (p42/44) activity while stimulating c-Jun N-terminal kinase activity in a time-frame coincident with the activation of AP1. We conclude that CalC induces signaling pathways that activate AP1 and transactivate genes harboring AP1 enhancer sites independent of PKC-alpha.

Quantitative high-performance liquid chromatography-based detection method for calphostin C, a naturally occurring perylenequinone with potent antileukemic activity

Calphostin C is a potent inhibitor of protein kinase C and can induce Ca2+-dependent apoptosis in human ALL cells. Further development of calphostin C will require detailed pharmacodynamic studies in preclinical animal models. Therefore, we established a sensitive and accurate high-performance liquid chromatography (HPLC)-based quantitative detection method for the measurement of calphostin C levels in plasma. Extraction of calphostin C from plasma was performed by precipitation of plasma protein using acetonitrile and an aliquot of extracted supernatant was injected onto a Hewlett-Packard HPLC system constituting a 250x4 mm LiChrospher 100, RP-18 (5 microm) in conjunction with a 4x4 mm LiChrospher 100, RP-18 guard column (5 microm). The eluted compounds were detected by diode array detection set at a wavelength of 479 nm. Acetonitrile-water containing 0.1% trifluoroacetic acid and 0.1% triethylamine (70:30, v/v) was used as the mobile phase. The average extraction recovery from plasma was 97.3%. Good linearity (r>0.999) was observed throughout the concentration range of 0.05-40 microM for calphostin C in 50 microl of plasma. Intra- and inter-assay variabilities were less than 6% in plasma. The lowest detection limit of calphostin C in 50 microl plasma was 0.02 microM at a signal-to-noise ratio of approximately 3. The availability of this assay will now permit detailed pharmacodynamic and pharmacokinetic studies of calphostin C in vivo.