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The cerebral cortex is organized into six cell layers, each of which contains neurons with similar morphology, functions, gene-expression profiles, and projection patterns. These layer-specific neuronal phenotypes are sequentially generated from common cortical progenitor cells in the ventricular zone of dorsal telencephalon. Although recent investigations have clarified important roles of intrinsic factors such as transcription factors and regulators of the cell cycle for the maturation of cortical progenitors, growth factors and neurotrophic factors environmentally supplied by the cerebral cortex are thought to regulate proliferation and neural development and determine neuronal differentiation in the cerebral cortex. In this review, I focus on the function of neurotrophin-family neurotrophic factor, including nerve growth factor, brain-derived neurotrophic factor (BDNF), neurotropin-3 (NT-3) and neurotrophin-4 in the formation of the neuronal layer of the cerebral cortex. Especially, BDNF and NT-3 are expressed in the proliferating cortical progenitors and influence the biological properties of cortical progenitors prior to neurogenesis and play distinct roles in generation of cortical neuronal subtypes.

The effects of neurotrophin-3 (NT3) administered into the ventricular space of 13.5-day-old mouse embryos on neurogenesis in the developing cerebral cortex were examined. 5-Bromo-2'-deoxyuridine (BrdU) was injected into pregnant mice 3 hr after the NT3 administration to label the neural progenitor cells. NT3 increased the number of BrdU-positive cells without altering their distribution. The increment in BrdU-positive cells 24 hr after the BrdU injection was attributed to Pax6-/BrdU-positive cells (neural stem cells), which was followed by a significant elevation of the number of Tuj1-/BrdU-positive cells (neurons) 36 or 48 hr after the BrdU injection, suggesting that NT3 facilitated neurogenesis by acting in two sequential steps, i.e., causing proliferation of neural stem cells and generation of neurons from these progenitors. NT3 stimulated phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and ERK5 in the cortical progenitors, and the effects of NT3 on the increase in total BrdU-positive cells and Pax6-/BrdU-positive cells were diminished by an MEK inhibitor, suggesting the involvement of MEK-mediated ERK signal transduction in the NT3 actions., (2008 Wiley-Liss, Inc.)

To understand the molecular basis of inflammation-induced neurotrophic influences, we investigated the effects of lipopolysaccharide (LPS) on production of glial cell line-derived neurotrophic factor (GDNF) in the injured rat spinal cord or in cultured rat macrophages in comparison with the effects on synthesis/secretion of inducible nitric oxide synthase (iNOS) and nitric oxide (NO). We found that GDNF mRNA expression lasted longer than that of iNOS mRNA in the injured spinal cord after injection of the high-dose LPS that had improved locomotor function, suggesting that the GDNF expression and its balance with NO generation were critical for injury regeneration. Therefore, we next investigated the effects of LPS on cultured macrophages. Levels of iNOS mRNA and secreted NO were enhanced by LPS at lower concentrations (10 ng/mL and above), whereas mRNA expression and secretion of GDNF were elevated only at higher concentrations (100 ng/mL and above). The culture medium of macrophages treated with 10 ng/mL of LPS was actually neurotoxic against cultured cortical neurons, whereas that conditioned at 1000 ng/mL was not. These observations suggest that neurotoxicity partly based on NO is induced by a lower degree of inflammation, whereas neurotrophic effects based on GDNF are manifested at a higher degree of inflammatory activity.

Royal jelly (RJ) is known to have a variety of biological activities toward various types of cells and tissues of animal models, but nothing is known about its effect on brain functions. Hence, we examined the effect of oral administration of RJ on the mRNA expression of various neurotrophic factors, their receptors, and neural cell markers in the mouse brain. Our results revealed that RJ selectively facilitates the mRNA expression of glial cell line-derived neurotrophic factor (GDNF), a potent neurotrophic factor acting in the brain, and neurofilament H, a specific marker predominantly found in neuronal axons, in the adult mouse hippocampus. These observations suggest that RJ shows neurotrophic effects on the mature brain via stimulation of GDNF production, and that enhanced expression of neurofilament H mRNA is involved in events subsequently caused by GDNF. RJ may play neurotrophic and/or neuroprotective roles in the adult brain through GDNF.

The physiological roles of glial cell line-derived neurotrophic factor (GDNF) expressed in the microglia/macrophages of the injured spinal cord have not yet been clarified. mRNA expression of chemokines, including monocyte chemoattractant protein (MCP)-1, was evoked within 1 hr after transection of the spinal cord, and GDNF mRNA expression was similarly up-regulated. Immunohistochemical analysis showed that GDNF was coexpressed with MCP-1 in the CD11b-positive cells. Therefore, we examined further the effects of GDNF on cultured rat peritoneal macrophages. GDNF enhanced the phagocytic activity of the macrophages via GFRalpha-1, glycosylphosphatidylinositol-anchored specific binding site of GDNF, in a c-Ret-independent manner. The influence of autocrine and/or paracrine GDNF synthesis was evaluated by performing activation experiments using macrophages cultured from heterozygous (+/-) GDNF gene-deficient mice or wild-type (+/+) mice. There were no morphological differences dependent on genetic types or stimulators. However, the GDNF mRNA level, but not the MCP-1 or GFRalpha-1 mRNA level, was substantially lower in the mutant macrophages than in the +/+ cells irrespective of stimulation with MCP-1 or lipopolysaccharide (LPS). The phagocytic activity enhanced by MCP-1 or LPS was significantly lower in the mutant cells (+/-) than in the +/+ ones, demonstrating the involvement of endogenous GDNF in the activation processes of macrophages in vitro and suggesting that not only neuroprotective function but also activation of macrophages is effected by the GDNF produced after a spinal cord injury., (Copyright (c) 2005 Wiley-Liss, Inc.)

Activation of microglia/macrophages after injury occurs limitedly in the CNS, which finding may explain unsuccessful axonal regeneration. Therefore, the relationship between lipopolysaccharide (LPS)-induced inflammation and recovery of locomotor function of rats after spinal cord injury was examined. High-dose LPS improved locomotor function greater than low-dose LPS, being consistent with the expression of neurotrophic factor (GDNF) in microglia/macrophages. Experiments using GDNF gene mutant mice confirmed that the increase in the GDNF mRNA level, rather than the reduction in the mRNA level of inducible NO synthase, could be correlated with the restoration activity of locomotor function. These results suggest that a higher degree of inflammation leads to a higher degree of repair of CNS injuries through GDNF produced by activated microglia/macrophages.

We investigated whether certain hydrophobic dipeptides, Leu-Ile, Leu-Pro, and Pro-Ile, which partially resemble the site on FK506 that binds to immunophilin, could stimulate glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) synthesis in cultured neurons and found only Leu-Ile to be an active dipeptide. Leu-Ile protected against the death of mesencephalic neurons from wild-type mice but not from mice lacking the BDNF or GDNF gene. Next, we examined the effects of i.p. or i.c.v. administration of Leu-Ile on BDNF and GDNF contents. Both types of administration increased the contents of BDNF and GDNF in the striatum of mice. Also, peripheral administration of Leu-Ile inhibited dopaminergic (DA) denervation caused by unilateral injection of 6-hydroxydopamine (6-OHDA) into the striatum of mice. The number of rotations following a methamphetamine challenge was lower in the Leu-Ile-treated group than in the nontreated group. Next, we compared the calcineurin activity and immunosuppressant activity of Leu-Ile with those of FK506. Leu-Ile was not inhibitory toward calcineurin cellular activity in cultured neuronal cells. Furthermore, Leu-Ile did not suppress concanavalin A (ConA)-induced synthesis/secretion of interleukin-2 by cultured spleen cells, suggesting that the immunosuppressant activity of Leu-Ile may be negligible when used as a therapeutic tool for neurodegenerative diseases., (Copyright 2004 Wiley-Liss, Inc.)

The effect of axonal regeneration on de novo synthesis of glial cell line-derived neurotrophic factor (GDNF) in rat sciatic nerves was examined. Transection of the sciatic nerve caused a prominent increase in the GDNF content in the distal segments within 1 week. The high level was sustained until 4 weeks in the animal model in which the nerve ends were ligated with thread (non-regeneration group); however, it was reduced to the original level within 2 or 4 weeks after the transection only in the segments invaded by regenerating axons in the models in which the nerve ends were coaptated (regeneration group). Expression of both GDNF protein and mRNA was decreased with a reciprocal increase in the density of neurofilaments, used as a marker of axonal ingrowth in distal segments of the regeneration group, suggesting that axonal contact turned off the GDNF-mediated nerve regeneration activity., (Copyright 2004 Elsevier Ireland Ltd.)