Calcineurin primes immature gonadotropin-releasing hormone-secreting neuroendocrine cells for migration.

During development, many neurons display calcium-dependent migration, but the role of this messenger in regulating gene expression leading to this event has not yet been elucidated. Among the decoders of calcium signals is calcineurin, a Ca(2+)/calmodulin serine/threonine phosphatase that has been involved in both short-term and long-term cellular changes. By using immortalized GnRH-secreting neurons, we now show that, in vitro, Ca(2+)-dependent gene expression, proceeding via calcineurin and the transcription factor nuclear factor of activated T cells, is a key player controlling the chemomigratory potential of developing GnRH-secreting neurons. Furthermore, our data highlight the switch nature of this phosphatase, whose activation or inactivation guides cells to proceed from one genetic program to the next.

Kallmann's syndrome, a neuronal migration defect.

Infertility and inability to smell are the phenotypical features of Kallmann's syndrome (KS), a genetic disease which affects 1 in 10,000 males and 1 in 50,000 females, the majority of the cases being sporadic. The molecular pathogenesis of KS is complex but mainly referable to the impairment of olfactory axon development and of the migration of gonadotropin-releasing hormone (GnRH) neurons. Only two different genes have been identified so far as responsible for the disease: KAL1 and KAL2, encoding anosmin-1 and fibroblast growth factor receptor 1 (FGFR1), respectively. In this review we focus our attention on insights evoked by recent studies, which propose a new direct role for anosmin-1 in the migration GnRH neurons, and a fascinating hypothesis of interactions between anosmin-1 and FGFR1 systems.

Factors involved in the migration of neuroendocrine hypothalamic neurons.

Neuroendocrine control of physiological functions needs a complex developmental organisation of the hypothalamic parvicellular neurons, which synthesise and release hypophysiotropic hormones. Among the hypothalamic neuroendocrine cells, Gonadotropin-releasing hormone (GnRH) neurons represent a unique class; they are generated in the olfactory placode and, during embryonic life, migrate to the septo/hypothalamic region along terminal and vomeronasal nerves. At this level GnRH neurons undergo terminal differentiation and start to release GnRH to modulate the secretion of pituitary gonadotropins. All these steps are under the strict control of several developmental cues and their defect might represent a cause of clinical disorders. A number of factors have been proposed to be involved in the migration of GnRH neurons, but their role is still unclear. By using gene knockout techniques it has been found that mice carrying a targeted deletion of Ebf2 gene, a component of Olf/Ebf bHLH transcription factors, show a defective migration of GnRH neurons, providing the first evidence of a mouse model of such defect. Since the investigation of GnRH neurons is hindered by their peculiar anatomical distribution, other studies has been forwarded by the availability of immortalized GnRH-expressing neurons (GN11 cells) that retain a strong chemomigratory response "in vitro". Among the factors analysed, we found that hepatocyte growth factor/scatter factor (HGF/SF) and vascular endothelial growth factor (VEGF) induce specific chemotaxis of GN 11 neurons, suggesting that migratory signals can arise from nasal mesenchyme and from the concomitant vasculogenesis. Finally, anosmin-1 (the product of the gene responsible of the X-linked form of Kallmann's disease) was found to induce a significant chemotactic response of GN11 cells, confirming a permissive/instructive role of KAL1 gene product in the migratory behaviour of GnRH neurons. In conclusion, the migration of the GnRH neurons appears to be a complex process, which involves the interplay of multiple molecular cues. These studies may provide new insights on the etiopathogenesis of the large proportion of reproductive dysfunctions that affect humans and could provide novel insights on common biochemical events controlling neuronal development and migration.

Hepatocyte growth factor/scatter factor facilitates migration of GN-11 immortalized LHRH neurons.

The molecular cues regulating the migratory process of LHRH neurons from the olfactory placode into the brain are not well known, but gradients of chemotropic and chemorepellent factors secreted by the targets are likely to play a key role in guidance mechanisms. Hepatocyte growth factor/scatter factor (HGF/SF) is a pleiotropic cytokine inducing cell migration. It is involved in a variety of developmental processes through interaction with its receptor c-Met. Here we show that c-Met-antibody labels LHRH migrating neurons in the olfactory mesenchyme of E12 mouse and analyze the potential chemotropic effect of HGF/SF on two immortalized LHRH cell lines, GT1-7 and GN11, isolated from tumors developed in the hypothalamus and in the olfactory bulb, respectively. By RT-PCR analysis, Western blotting, and immunocytochemistry, we provide evidence for a high level of c-Met expression in GN11, but not in GT1-7, cells. In addition, HGF/SF treatment promotes specific migratory activity of GN11 cells, as demonstrated by collagen gel assay, time-lapse video microscopy, and Boyden's chamber experiments. Such promotion is inhibited by the neutralizing antibody. The data reported here represent the first direct evidence of a chemotactic effect of HGF/SF on immortalized LHRH neurons.