Cellular targets and trophic functions of neurotrophin-3 in the developing rat hippocampus

Overexpression of NT-3 in the hippocampus suppresses the early phase of the adult neurogenic process

The dentate gyrus (DG) of the hippocampus regulates stress-related emotional behaviors and ensures neurogenesis throughout life. Neurotrophin-3 (NT-3) is a neurotrophic factor that regulates neuronal differentiation, survival, and synaptic formation in both the peripheral and central nervous systems. NT-3 is expressed in the adult DG of the hippocampus; several chronic stress conditions enhance NT-3 expression in rodents. However, functional modulation of the adult DG by NT-3 signaling remains unclear. To directly investigate the impact of NT-3 on DG function, NT-3 was overexpressed in the hippocampal ventral DG by an adeno-associated virus carrying NT-3 (AAV-NT-3). Four weeks following the AAV-NT-3 injection, high NT-3 expression was observed in the ventral DG. We examined the influence of NT-3 overexpression on the neuronal responses and neurogenic processes in the ventral DG. NT-3 overexpression significantly increased the expression of the mature DG neuronal marker calbindin and immediate early genes, such as Fos and Fosb, thereby suggesting DG neuronal activation. During neurogenesis, the number of proliferating cells and immature neurons in the subgranular zone of the DG significantly decreased in the AAV-NT-3 group. Among the neurogenesis-related factors, Vegfd, Lgr6, Bmp7, and Drd1 expression significantly decreased. These results demonstrated that high NT-3 levels in the hippocampus regulate the activation of mature DG neurons and suppress the early phase of neurogenic processes, suggesting a possible role of NT-3 in the regulation of adult hippocampal function under stress conditions.

BDNF guides neural stem cell-derived axons to ventral interneurons and motor neurons after spinal cord injury

Neural stem cells (NSCs) implanted into sites of spinal cord injury (SCI) extend very large numbers of new axons over very long distances caudal to the lesion site, and support partial functional recovery. Newly extending graft axons distribute throughout host gray and white matter caudal to the injury. We hypothesized that provision of trophic gradients caudal to the injury would provide neurotrophic guidance to newly extending graft-derived axons to specific intermediate and ventral host gray matter regions, thereby potentially further improving neural relay formation. Immunodeficient rats underwent C5 lateral hemisection lesions, following by implants of human NSC grafts two weeks later. After an additional two weeks, animals received injections of AAV2-BDNF expressing vectors three spinal segments (9 mm) caudal to the lesion in host ventral and intermediate gray matter. After 2 months additional survival, we found a striking, 5.5-fold increase in the density of human axons innervating host ventral gray matter (P < 0.05) and 2.7-fold increase in intermediate gray matter (P < 0.01). Moreover, stem cell-derived axons formed a substantially greater number of putative synaptic connections with host motor neurons (P < 0.01). Thus, trophic guidance is an effective means of enhancing and guiding neural stem cell axon growth after SCI and will be used in future experiments to determine whether neural relay formation and functional outcomes can be improved.

Review : Gene Expression after Cerebral Ischemia

Global and focal ischemias induce a variety of gene families, including immediate early genes, cytokines, neurotransmitter receptors, and heat-shock proteins. The Janus-like effects of several of these gene prod ucts promote neuronal survival and degeneration. Therefore, determining the molecular pathways respon sible for the differential regulation of these genes is of paramount importance. The discovery of apoptosis as a mediator of delayed neuronal death has led to the identification of a number of other genes involved in postischemic brain damage. Future neuroprotective therapies for cerebral ischemia may be directed at preventing alterations in gene expression. NEUROSCIENTIST 5:238-253, 1999

Review : Stress, Antidepressant Treatments, and Neurotrophic Factors: Molecular and Cellular Mechanisms

Repeated stress or an excess of glucocorticoids can exacerbate neuronal damage in response to insults and, in severe cases, can lead to neuronal atrophy and death. These effects are thought to be related to the actions of stress and glucocorticoids on glutamate function, neuronal metabolism, and the generation of cytotoxic free radicals. Recent studies demonstrate that the regulation of neurotrophic factors may contribute to the actions of stress on neuronal function. Acute or chronic stress decreases the expression of brain derived neurotrophic factor, the most abundant neurotrophin in the brain, in specific regions of the hippocampus, and other forebrain regions. In addition, chronic stress increases the expression of neurotrophin-3 in certain regions of the hippocampus and may, thereby, help to protect these regions from the neurotoxic effects of chronic stress. The deleterious effects of stress may contribute to psy chiatric illnesses, such as depression, that can be precipitated or worsened by stress and that are often characterized by hypercortisolism. Electroconvulsive seizure therapy, as well as antidepressant drugs, increase the expression of brain derived neurotrophic factor and its receptor, trkB, in the brain, demon strating that neurotrophins are a target of antidepressant treatments. These findings outline a role of neurotrophic factors in the etiology and treatment of certain psychiatric illnesses. The Neuroscientist 1:351-360, 1995

Calbindin-D28K is increased in the ventral horn of spinal cord by neuroprotective factors for motor neurons

Slow glutamate-mediated neuronal degeneration is implicated in the pathophysiology of motor neuron diseases such as amyotrophic lateral sclerosis (ALS). The calcium-binding proteins calbindin-D28K and parvalbumin have been reported to protect neurons against excitotoxic insults. Expression of calbindin-D28K is low in adult human motor neurons, and vulnerable motor neurons additionally may lack parvalbumin. Thus, it has been speculated that the lack of calcium-binding proteins may, in part, be responsible for early degeneration of the population of motor neurons most vulnerable in ALS. Using a rat organotypic spinal cord slice system, we examined whether the most potent neuroprotective factors for motor neurons can increase the expression of calbindin-D28K or parvalbumin proteins in the postnatal spinal cord. After 4 weeks of incubation of spinal cord slices with 1) glial cell line-derived neurotrophic factor (GDNF), 2) neurturin, 3) insulin-like growth factor I (IGF-I), or 4) pigment epithelium-derived factor (PEDF), the number of calbindin-D28K -immunopositive large neurons (>20 μm) in the ventral horn was higher under the first three conditions, but not after PEDF, compared with untreated controls. Under the same conditions, parvalbumin was not upregulated by any neuroprotective factor. The same calbindin increase was true of IGF-I and GDNF in a parallel glutamate toxicity model of motor neuron degeneration. Taken together with our previous reports from the same model, which showed that all these neurotrophic factors can potently protect motor neurons from slow glutamate injury, the data here suggest that upregulation of calbindin-D28K by some of these factors may be one mechanism by which motor neurons can be protected from glutamate-induced, calcium-mediated excitotoxicity.

Basic fibroblast growth factor attenuates bisphosphonate-induced oxidative injury but decreases zinc and copper levels in oral epithelium of rat

Recent studies have reported oxidative damage due to bisphosphonate (BP) in various cancer tissues and neurons, although basic fibroblast growth factor (bFGF) induced antioxidant effects in the cells. The bFGF may modulate the BP-induced oxidative stress in oral epithelium of rats. This study was undertaken to explore possible beneficial antioxidant effects of bFGF on oxidative stress induced by BP in oral epithelium of rats. Twenty-eight rats were equally divided into four groups. The first group was used as control. The second, third and fourth groups intraperitoneally received BP (zoledronic acid), bFGF and BP + bFGF. At the end of 10 weeks, the rats were sacrificed, and oral epithelium samples were taken for analyses. In BP group, the lipid peroxidation levels were increased in the oral epithelium, while the activities of glutathione peroxidase (GSH-Px) and the concentrations of total antioxidant status (TAS) were decreased. In rats treated with bFGF, lipid peroxidation levels decreased, and the activities of GSH-Px and concentrations of TAS improved in the oral epithelium. However, zinc and copper levels were decreased in the oral epithelium by BP and bFGF treatments. Concentrations of vitamin E and reduced glutathione in the samples did not change in the groups. In conclusion, treatment with bFGF modulated the antioxidant redox system and reduced the oral epithelium oxidative stress induced by BP. However, zinc and copper levels were decreased by BP and bFGF treatments.

Long-lasting effects of chronic rTMS to treat chronic rodent model of depression

Repetitive transcranial magnetic stimulation (rTMS) has been demonstrated in the pre-clinical and clinical settings to have an antidepressant effect. However, studies on the long-lasting effect of rTMS, especially when the effect is measured after treatment has ceased for a few weeks is lacking. We examined this question in a chronic unpredicted mild stress (CUMS) rat model of depression. We gave 3 weeks of high frequency (15 Hz) rTMS, venlafaxine, or these two treatments combined to a modified CUMS paradigm, and then investigated the prolonged effect of treatments. Behavioral testing (sucrose preference test, open field test, forced swimming test, novelty suppressed feeding test), plasma hormone level, hippocampal BrdU labeling, and amount of related neurotropic factors were used to assess the effects of stress and treatments. Long-term chronic rTMS significantly reversed andehonic-like behavior, increased hippocampus cell proliferation, BDNF protein level, phosphorylation of ERK1/2 compared with CUMS rats two weeks after the cessation of rTMS treatment. However, the changes in plasma hormone level were not sustained for that amount of time. Venlafaxine had no interaction with the physical stimulation. Our results suggest that high frequency rTMS has long-lasting effects, which may have some relationship with neuroplasticity.

Induction of the GABA cell phenotype: an in vitro model for studying neurodevelopmental disorders

Recent studies of the hippocampus have suggested that a network of genes is associated with the regulation of the GAD₆₇ (GAD1) expression and may play a role in γ-amino butyric acid (GABA) dysfunction in schizophrenia (SZ) and bipolar disorder (BD). To obtain a more detailed understanding of how GAD₆₇ regulation may result in GABAergic dysfunction, we have developed an in vitro model in which GABA cells are differentiated from the hippocampal precursor cell line, HiB5. Growth factors, such as PDGF, and BDNF, regulate the GABA phenotype by inducing the expression of GAD₆₇ and stimulating the growth of cellular processes, many with growth cones that form appositions with the cell bodies and processes of other GAD₆₇-positive cells. These changes are associated with increased expression of acetylated tubulin, microtubule-associated protein 2 (MAP2) and the post-synaptic density protein 95 (PSD95). The addition of BDNF, together with PDGF, increases the levels of mRNA and protein for GAD₆₇, as well as the high affinity GABA uptake protein, GAT1. These changes are associated with increased concentrations of GABA in the cytoplasm of "differentiated" HiB5 neurons. In the presence of Ca²⁺ and K⁺, newly synthesized GABA is released extracellularly. When the HiB5 cells appear to be fully differentiated, they also express GAD₆₅, parvalbumin and calbindin, and GluR subtypes as well as HDAC1, DAXX, PAX5, Runx2, associated with GAD₆₇ regulation. Overall, these results suggest that the HiB5 cells can differentiate into functionally mature GABA neurons in the presence of gene products that are associated with GAD₆₇ regulation in the adult hippocampus.

Neural Differentiation of Brain-Derived Neurotrophic Factor-Expressing Human Umbilical Cord Blood-Derived Mesenchymal Stem Cells in Culture via TrkB-Mediated ERK and β-Catenin Phosphorylation and following Transplantation into the Developing Brain

The ability of mesenchymal stem cells (MSCs) to differentiate into neural cells makes them potential replacement therapeutic candidates in neurological diseases. Presently, overexpression of brain-derived neurotrophic factor (BDNF), which is crucial in the regulation of neural progenitor cell differentiation and maturation during development, was sufficient to convert the mesodermal cell fate of human umbilical cord blood-derived MSCs (hUCB-MSCs) into a neuronal fate in culture, in the absence of specialized induction chemicals. BDNF overexpressing hUCB-MSCs (MSCs-BDNF) yielded an increased number of neuron-like cells and, surprisingly, increased the expression of neuronal phenotype markers in a time-dependent manner compared with control hUCB-MSCs. In addition, MSCs-BDNF exhibited a decreased labeling for MSCs-related antigens such as CD44, CD73, and CD90, and decreased potential to differentiate into mesodermal lineages. Phosphorylation of the receptor tyrosine kinase B (TrkB), which is a receptor of BDNF, was increased significantly in MSC-BDNF. BDNF overexpression also increased the phosphorylation of β-catenin and extracellular signal-regulated kinases (ERKs). Inhibition of TrkB availability by treatment with the TrkB-specific inhibitor K252a blocked the BDNF-stimulated phosphorylation of β-catenin and ERKs, indicating the involvement of both the β-catenin and ERKs signals in the BDNF-stimulated and TrkB-mediated neural differentiation of hUCB-MSCs. Reduction of β-catenin availability using small interfering RNA-mediated gene silencing inhibited ERKs phosphorylation. However, β-catenin activation was maintained. In addition, inhibition of β-catenin and ERKs expression levels abrogated the BDNF-stimulated upregulation of neuronal phenotype markers. Furthermore, MSC-BDNF survived and migrated more extensively when grafted into the lateral ventricles of neonatal mouse brain, and differentiated significantly into neurons in the olfactory bulb and periventricular astrocytes. These results indicate that BDNF induces the neural differentiation of hUCB-MSCs in culture via the TrkB-mediated phosphorylation of ERKs and β-catenin and following transplantation into the developing brain.

Conditional knockout of brain-derived neurotrophic factor in the hippocampus increases death of adult-born immature neurons following traumatic brain injury

It has been reported that the hippocampus is particularly vulnerable to traumatic brain injury (TBI), the consequence of which results in hippocampal-dependent cognitive impairment. In the previous study we found that adult-born immature neurons in the hippocampal dentate gyrus are the most vulnerable cell type to moderate TBI insult. However, the molecular mechanisms that regulate the survival of adult-born immature neurons in the hippocampus following TBI are still not well understood. Here, we conditionally knocked out brain-derived neurotrophic factor (BDNF) in the hippocampal dentate gyrus and examined the death of adult-born immature neurons following moderate TBI. The results showed that the amount of adult-born immature neuron death in the hippocampal dentate gyrus significantly increased in the BDNF conditional knockout mice. This result suggests that BDNF is involved in regulating the survival of adult-born immature neurons in the hippocampus following TBI, and potentially might be a useful target for preventing the adult-born immature neurons from death following TBI.

Sex-different effect of angiotensin II type 2 receptor on ischemic brain injury and cognitive function

We previously reported that angiotensin II type 2 (AT(2)) receptor signaling prevents neural damage and cognitive impairment after focal cerebral ischemia. We investigated the possible roles of the AT(2) receptor in the sex difference, focusing on cognitive function and ischemic brain damage using AT(2) receptor-deficient mice (Agtr2(-)). In Agtr2(-), spatial memory evaluated by the Morris water maze test was impaired in female compared with that in male Agtr2(-) and female wild-type (Agtr2(+)), while no significant sex-different change was observed in Agtr2(+). Interestingly, bromodeoxyuridine incorporation assay showed a significant decrease of hippocampal neurogenesis in female Agtr2(-) compared with that in female Agtr2(+). In contrast, ischemic area after middle cerebral artery (MCA) occlusion was significantly increased in male compared with female mice in Agtr2(-), while no significant sex-different change was observed in Agtr2(+). Male Agtr2(-) mice showed higher AT(1) receptor expression and significantly impaired cerebral blood flow (CBF) in the ipsilateral side 24 hours after MCA occlusion compared with female Agtr2(-) mice. In conclusion, deletion of the AT(2) receptor showed a sex-different effect such as a severe cognitive impairment with a decrease of hippocampal neurogenesis in females and a larger ischemic brain damage with a decrease of CBF in males.

Signalling by neurotrophins and hepatocyte growth factor regulates axon morphogenesis by differential beta-catenin phosphorylation

Tyrosine phosphorylation of beta-catenin, a component of adhesion complexes and of the Wnt pathway, affects cell adhesion, migration and gene transcription. By reducing beta-catenin availability using shRNA-mediated gene silencing or expression of intracellular N-cadherin, we show that beta-catenin is required for axon growth downstream of brain-derived neurotrophic factor (BDNF) signalling and hepatocyte growth factor (HGF) signalling. We demonstrate that the receptor tyrosine kinases (RTKs) Trk and Met interact with and phosphorylate beta-catenin. Stimulation of Trk receptors by neurotrophins (NTs) results in phosphorylation of beta-catenin at residue Y654, and increased axon growth and branching. Conversely, pharmacological inhibition of Trk or expression of a Y654F mutant blocks these effects. beta-catenin phosphorylated at Y654 colocalizes with the cytoskeleton at growth cones. However, HGF, which also increases axon growth and branching, induces beta-catenin phosphorylation at Y142 and a nuclear localization. Interestingly, dominant-negative DeltaN-TCF4 abolishes the effects of HGF in axon growth and branching, but not that of NTs. We conclude that NT- and HGF-signalling differentially phosphorylate beta-catenin, targeting this protein to distinct compartments to regulate axon morphogenesis by TCF4-transcription-dependent and -independent mechanisms. These results place beta-catenin downstream of growth-factor-RTK signalling in axon differentiation.

Cigarette smoking decreases neurotrophin-3 expression in rat hippocampus after transient forebrain ischemia

Stroke is a common cause of death and severe disability among adults in developed countries. Cigarette smoking adversely affects human health in many ways and is considered to be a risk factor for a stroke. However, the mechanism that determines the relative importance of neurotrophins in this process remains unclear. To study the effect of chronic cigarette smoking on ischemic stroke, in situ hybridization and immunohistochemistry were employed to detect the mRNA and protein expression of neurotrophin-3 (NT-3), respectively, which is thought to play a critical role in protection against neuronal death in brain ischemia. Rats, with or without chronic cigarette smoking, were subjected to 20 min of transient forebrain ischemia. Distribution and quantification of mRNA and protein of NT-3 in the whole hippocampus and the cell death in the hippocampal CA1-CA3 regions were determined in these rats. Experimental results show that chronic cigarette smoking produces a significantly delay and persistent down-regulation of ischemia-induced NT-3 mRNA and protein changes at 6-24h post-ischemia, and seemingly increases neuron death 7 days after reperfusion. These experimental results indicate that by influencing NT-3 expression, directly or indirectly, chronic cigarette smoking has a potentially harmful effect when acute brain ischemia attacks.

Cellular mechanisms underlying the effect of a single exposure to neonatal handling on neurotrophin-3 in the brain of 1-day-old rats

Neurotrophin-3 (NT-3) has an important role in brain development and is thus a good candidate molecule to be involved in the cellular mechanisms mediating the effects of early experiences on the brain. In the present work we employed the model of neonatal handling, which is known to affect the ability of the adult organism to respond to stressful stimuli, and determined its effects on NT-3 levels in the rat hippocampus and cortex 2, 4 and 8 h after handling on postnatal day 1. We also recorded maternal behavior during the 8 h following handling. At both the 4 and 8 h time-points there was an increase in NT-3 positive cells in field 1 of Ammon's horn (CA1 area of the hippocampus) and parietal cortex of the handled animals. In the parietal cortex NT-3 levels increased with time following handling: at 8 h there were more NT-3 positive cells than at 4 h. During the 4 h following the end of handling, handled pups were subject to more maternal licking, indicating that the more intense maternal care could underlie the handling-induced increase in NT-3. In the hippocampus, the handling induced increase in NT-3 was cancelled by inhibition of N-methyl-D-aspartate (NMDA), AMPA/kainate, or GABA-A receptors, as well as L-type voltage-gated Ca(2+) channels. It thus appears that neonatal handling activates these neurotransmitter receptors and channels, leading to increased intracellular Ca(2+) and increased NT-3 expression. NT-3 can then activate downstream effectors and exert its morphogenetic actions and thus imprint the effects of handling on the brain.

Spontaneous glutamate release controls NT-3-dependent development of hippocampal calbindin-D28k phenotype through activation of sodium channels ex vivo

Functional NMDA and AMPA ionotropic glutamate receptors are expressed in embryonic hippocampal glutamatergic pyramidal neurons prior to synapse formation but their function and mechanisms of action are still unclear. At the same time, these neurons develop their calbindin-D(28k) phenotype through an activity-dependent NT-3 autocrine loop. Using single-neuron microcultures, we show here that immature pyramidal neurons spontaneously secreted glutamate and that chronic blockade of either NMDA or AMPA receptors down-regulated the number of calbindin-D(28k)-positive pyramidal neurons without affecting neuronal survival. This antagonistic effect of glutamate ionotropic receptors was mimicked by anti-TrkC antibodies and reversed by the application of NT-3. Similar results were obtained in ex vivo embryonic hippocampal slice cultures. Moreover, glutamate receptor blockade inhibited the generation of spontaneous sodium-driven action potentials which, in turn, regulate both the endogenous secretion of NT-3 and the calbindin-D(28k) phenotype acquisition. Altogether, these results suggest an unexpected role for glutamate in the development of the physiological and biochemical properties of hippocampal pyramidal neurons and support the idea that glutamate may underlie an activity-dependent mode of differentiation prior to synapse formation.

Early pharmacological treatment of autism: a rationale for developmental treatment

Autism is a dynamic neurodevelopmental syndrome in which disabilities emerge during the first three postnatal years and continue to evolve with ongoing development. We briefly review research in autism describing subtle changes in molecules important in brain development and neurotransmission, in morphology of specific neurons, brain connections, and in brain size. We then provide a general schema of how these processes may interact with particular emphasis on neurotransmission. In this context, we present a rationale for utilizing pharmacologic treatments aimed at modifying key neurodevelopmental processes in young children with autism. Early treatment with selective serotonin reuptake inhibitors (SSRIs) is presented as a model for pharmacologic interventions because there is evidence in autistic children for reduced brain serotonin synthesis during periods of peak synaptogenesis; serotonin is known to enhance synapse refinement; and exploratory studies with these agents in autistic children exist. Additional hypothetical developmental interventions and relevant published clinical data are described. Finally, we discuss the importance of exploring early pharmacologic interventions within multiple experimental settings in order to develop effective treatments as quickly as possible while minimizing risks.

Human stem/progenitor cells from bone marrow promote neurogenesis of endogenous neural stem cells in the hippocampus of mice

Stem/progenitor cells from bone marrow and other sources have been shown to repair injured tissues by differentiating into tissue-specific phenotypes, by secreting chemokines, and, in part, by cell fusion. Here we prepared the stem/progenitor cells from human bone marrow (MSCs) and implanted athem into the dentate gyrus of the hippocampus of immunodeficient mice. The implanted human MSCs markedly increased the proliferation of endogenous neural stem cells that expressed the stem cell marker Sox2. Labeling of the mice with BrdUrd demonstrated that, 7 days after implantation of the human MSCs, BrdUrd-labeled endogenous cells migrated throughout the dorsal hippocampus (positive for doublecortin) and expressed markers for astrocytes and for neural or oligodendrocyte progenitors. Subpopulations of BrdUrd-labeled cells exhibited short cytoplasmic processes immunoreactive for nerve growth factor and VEGF. By 30 days after implantation, the newly generated cells expressed markers for more mature neurons and astrocytes. Also, subpopulations of BrdUrd-labeled cells exhibited elaborate processes immunoreactive for ciliary neurotrophic factor, neurotrophin-4/5, nerve growth factor, or VEGF. Therefore, implantation of human MSCs stimulated proliferation, migration, and differentiation of the endogenous neural stem cells that survived as differentiated neural cells. The results provide a paradigm to explain recent observations in which MSCs or related stem/progenitor cells were found to produce improvements in disease models even though a limited number of the cells engrafted.

Overexpression of calbindin‐D 28K in hippocampal progenitor cells increases neuronal differentiation and neurite outgrowth

Excitatory stimuli are known to be a potent regulator for induction of neuronal differentiation. Calbindin-D28K buffers intracellular Ca2+ and modifies synaptic functions in neurons. However, the effects of calbindin-D28K on the regulation of activity-induced neuronal differentiation and related biochemical modifications remain unsolved. In the present study, by a gain-of-function study with retroviral vector system and dicer-generated small interfering RNA (d-siRNA) to effectively knock down the expression of calbindin-D28K, we demonstrated that calbindin-D28K at a physiologically relevant level promoted neuronal differentiation and neurite outgrowth. Increase of neuronal differentiation by calbindin-D28K overexpression was concurrent with the expression of basic helix-loop-helix (bHLH) transcriptional factors, phosphorylation of calcium and calmodulin-dependent protein kinase II (CaMKII) and NeuroD at Ser(336). KN-62, a highly specific CaMKII inhibitor, blocked the up-regulation of proneural bHLH genes, p-CaMKII, and pSer(336)NeuroD. Calbindin-D28K appeared to facilitate neuronal differentiation of both fetal and adult hippocampal progenitor cells. Together, these findings establish the novel calbindin-regulated function of CaMKII and NeuroD in control of neuronal differentiation and neurite outgrowth.

Tolloid-like 1 is negatively regulated by stress and glucocorticoids

Glucocorticoids affect a variety of tissues to enable the organism to adapt to the stress. Hippocampal neurons contain glucocorticoid receptors and respond to elevated glucocorticoid levels by down-regulating the HPA axis. Chronically, however, stress is deleterious to hippocampal neurons. Chronically elevated levels of glucocorticoids result in a decrease in the number of dendritic spines, reduced axonal growth and synaptogenesis, and decreased neurogenesis in the hippocampus. Tolloid-like 1 (Tll-1) is a metalloprotease that potentiates the activity of the bone morphogenetic proteins (BMPs). Neurogenesis in the hippocampus of both developing and adult mammals requires BMPs. In this study, we demonstrate that Tll-1 expression is increased in mice that have increased neurogenesis. The Tll-1 promoter contains glucocorticoid response elements which are capable of binding to purified glucocorticoid receptor. Glucocorticoids decrease Tll-1 expression in vitro. Finally, prenatal stress leads to a decrease in Tll-1 mRNA expression in the hippocampus of adult female mice that is not observed in adult male mice indicating that Tll-1 expression is differentially regulated in males and females. The results of this study indicate that Tll-1 is responsive to glucocorticoids and this mechanism might influence neurogenesis in the hippocampus.

Effect of GDNF on differentiation of cultured ventral mesencephalic dopaminergic and non-dopaminergic calretinin-expressing neurons

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for ventral mesencephalic (VM) dopaminergic neurons. Subpopulations of dopaminergic and non-dopaminergic VM neurons express the calcium-binding proteins calbindin (CB) and calretinin (CR). Characterization of the actions of GDNF on distinct subpopulations of VM cells is of great importance for its potential use as a therapeutic molecule and for understanding its role in neuronal development. The present study investigated the effects of GDNF on the survival and morphological differentiation of dopaminergic and non-dopaminergic neurons in primary cultures of embryonic day (E) 18 rat VM. As expected from our results obtained using E14 VM cells, GDNF significantly increased the morphological complexity of E18 CB-immunoreractive (CB-ir), tyrosine hydroxylase (TH)-ir, and CR-ir neurons and also the densities of CB-ir and TH-ir neurons. Interestingly, densities of E18 CR-ir neurons, contrarily to our previous observations on E14 CR-ir neurons, were significantly higher after GDNF treatment (by 1.5-fold). Colocalization analyses demonstrated that GDNF increased the densitiy of dopaminergic neurons expressing CR (TH+/CR+/CB-), while no significant effects were observed for TH-/CR+/CB- cell densities. In contrast, we found that GDNF significantly increased the total fiber length (2-fold), number of primary neurites (1.4-fold), number of branching points (2.5-fold), and the size of neurite field per neuron (1.8-fold) of the non-dopaminergic CR-expressing neurons (TH-/CR+/CB-). These cells were identified as GABA-expressing neurons. In conclusion, our findings recognize GDNF as a potent differentiation factor for the development of VM dopaminergic and non-dopaminergic CR-expressing neurons.

Dexamethasone enhances NT-3 expression in rat hippocampus after traumatic brain injury

The cellular events in traumatic brain injury (TBI) are complicated, and the factors mediating neurotrophins to protect and repair the injured brain cells are only beginning to be identified. This study examined the effect of dexamethasone (DEX) on neurotrophin-3 (NT-3) expression following TBI. Levels of NT-3 mRNA and protein in rat hippocampus were measured using in situ hybridization and immunohistochemistry, respectively. After TBI, the NT-3 mRNA expression was down-regulated during the first 24 h. DEX reversed the post-traumatic reduction of NT-3 mRNA expression at 2, 4, 6, and 12 h in the hippocampus, and also decreased the cell death in hippocampal hilum and supraventricular cerebral cortex after 7 days. The NT-3 protein levels generally corresponded to the mRNA levels in the hippocampal region. DEX enhanced the NT-3 expression after TBI, indicating that post-traumatic neuroprotection in the hippocampus is at least partially mediated by NT-3 and thus can be modulated by DEX treatment.

Nutrients as trophic factors in neurons and the central nervous system: role of retinoic acid

In multicellular organisms, death, survival, proliferation, and differentiation of a given cell depend on signals produced by neighboring and/or distant cells, resulting in the coordinated development and function of the various tissues. In the nervous system, control of cell survival and differentiation is achieved through the action of a distinct group of polypeptides collectively known as neurotrophic factors. Recent findings support the view that trophic factors also are involved in the response of the nervous system to acute injury. By contrast, nutrients are not traditionally viewed as potential trophic factors; however, there is increasing evidence that at least some influence neuronal differentiation. During development the brain is responsive to variations in nutrient supply, and this increased sensitivity or vulnerability of the brain to nutrient supply may reappear during neuronal repair, a period during which a rapid membrane resynthesis and reestablishment of synthetic pathways occur. To further evaluate the potential of specific nutrients to act as pharmacologic agents in the repair of injured neurons, the effects of retinoic acid, an active metabolite of vitamin A, and its role as a trophic factor are discussed. This literature review is intended to provide background information regarding the effect of retinoic acid on the cholinergic phenotype and the differentiation of these neurons and to explain how it may promote neuronal repair and survival following injury.

Mood stabilizer valproate promotes ERK pathway-dependent cortical neuronal growth and neurogenesis

Manic-depressive illness has been conceptualized as a neurochemical illness. However, brain imaging and postmortem studies reveal gray-matter reductions, as well as neuronal and glial atrophy and loss in discrete brain regions of manic-depressive patients. The roles of such cerebral morphological deficits in the neuropathophysiology and therapeutic mechanisms of manic-depressive illness are unknown. Valproate (2-propylpentanoate) is a commonly used mood stabilizer. The ERK (extracellular signal-regulated kinase) pathway is used by neurotrophic factors to regulate neurogenesis, neurite outgrowth, and neuronal survival. We found that chronic treatment of rats with valproate increased levels of activated phospho-ERK44/42 in neurons of the anterior cingulate, a region in which we found valproate-induced increases in expression of an ERK pathway-regulated gene, bcl-2. Valproate time and concentration dependently increased activated phospho-ERK44/42 and phospho-RSK1 (ribosomal S6 kinase 1) levels in cultured cortical cells. These increases were attenuated by Raf and MEK (mitogen-activated protein kinase/ERK kinase) inhibitors. Although valproate affects the functions of GSK-3 (glycogen synthase kinase-3) and histone deacetylase (HDAC), its effects on the ERK pathway were not fully mimicked by selective inhibitors of GSK-3 or HDAC. Similar to neurotrophic factors, valproate enhanced ERK pathway-dependent cortical neuronal growth. Valproate also promoted neural stem cell proliferation-maturation (neurogenesis), demonstrated by bromodeoxyuridine (BrdU) incorporation and double staining of BrdU with nestin, Tuj1, or the neuronal nuclei marker NeuN (neuronal-specific nuclear protein). Chronic treatment with valproate enhanced neurogenesis in the dentate gyrus of the hippocampus. Together, these data demonstrate that valproate activates the ERK pathway and induces ERK pathway-mediated neurotrophic actions. This cascade of events provides a potential mechanism whereby mood stabilizers alleviate cerebral morphometric deficits associated with manic-depressive illness.

Morphometric analysis of embryonic rat trigeminal neurons treated with different neurotrophins

In whole-mount explant cultures of the trigeminal ganglion (TG) with intact peripheral and brainstem targets, exogenous application of nerve growth factor (NGF) and neurotrophin-3 (NT-3) leads to elongation and precocious arborization of embryonic trigeminal axons, respectively. In addition, neurotrophins play a major role in survival and differentiation of distinct classes of TG neurons. In the present study, we conducted morphometric analyses of trigeminal neurons exposed to exogenous NGF or NT-3 in whole-mount explant cultures. Explants dissected from embryonic day (E) 13 and E15 rats were cultured in the presence of serum-free medium (SFM) or in SFM supplemented with NGF or NT-3 for 3 days. TG neurons were then retrogradely labeled with lipophilic tracer DiI and their soma size distributions were compared following different treatments. The mean diameters of E13 and E15 trigeminal neurons grown in the presence of NT-3 were similar to those grown in SFM. On the other hand, in cultures supplemented with NGF, the mean diameters of neurons were larger at E13, but smaller at E15. Double immunolabeling with TrkA and TrkC antibodies confirmed the presence of large-diameter TrkA-positive neurons in E13 TG, but not in E15 TG. At both ages, other large-diameter neurons expressed only TrkC. These results show that exposure to NGF leads to phenotypic changes in TrkA-expressing trigeminal neurons at early embryonic development, but selective survival of small diameter neurons at later ages.

Fibroblast growth factors and neuroprotection

Several members of the FGF family, in particular FGF2, are intimately involved in neuronal protection and repair after ischemic, metabolic or traumatic brain injury. Expression of Fgf2 mRNA and protein is strongly upregulated after neuronal damage, with glial cells as the predominant source. Given its survival-promoting effects on cultured neurons, exogenous FGF2 was tested in several animal models of stroke and excitotoxic damage, in which it consistently proved protective against neuronal loss. FGF2 affords neuroprotection by interfering with a number of signaling pathways, including expression and gating of NMDA receptors, maintenance of Ca2+ homeostasis and regulation of ROS detoxifying enzymes. FGF2 prevents apoptosis by strengthening anti-apoptotic pathways and promotes neurogenesis in adult hippocampus after injury. The protective action of FGF2 has been linked to its augmenting effect on the lesion-induced upregulation of activin A, a member of the TGF-beta superfamily. Despite the well-documented benefits of FGF2 in animal models of stroke, there is currently no clinical development in stroke, after a phase II/III trial with FGF2 in acute stroke patients was discontinued because of an unfavorable risk-to-benefit ratio. As the molecular targets of FGF2 are going to be unraveled over the next years, new therapeutic strategies will hopefully emerge that enable us to influence the various protective mechanisms of FGF2 in a more specific fashion.

Insulin or bFGF and C2 ceramide increase newborn rat retinal ganglion cell survival rate

Treatment of RGCs with insulin or C2 ceramide alone increased survival rate by 30%. Adding both insulin and C2 ceramide increased survival rate by 80%. Protein phosphatase 2A (PP2A) inhibitor okadaic acid (OA) eliminated the effect of C2 ceramide, but not that of insulin. Protein kinase inhibitor K252a decreased the effect of C2 ceramide in a dose-dependent manner, but the effect of insulin was not changed. Treatment of RGCs with bFGF increased survival rate by 36%. Adding both bFGF and C2 ceramide increased survival rate by 102%. OA did not alter the effect of bFGF, whereas K252a increased survival rate in a dose-dependent manner. Inhibition of C2 ceramide by OA suggests that PP2A activation is involved in its pathway, whereas PP2A is not involved in the insulin- and bFGF-activated pathway. Elimination of the effect of C2 ceramide by K252a suggests that sphingomyelin cycle activation is mediated by a protein kinase not important in the insulin-activated pathway. Moreover, the increased effect of bFGF and dose-dependently decreased effect of C2 ceramide by K252a suggest that different protein kinases are important in bFGF- and ceramide-mediated enhancement of RGC survival rate.

Signal transduction and neurosurvival in experimental models of brain injury

Brain injury and neurodegenerative disease are linked by their primary pathological consequence-death of neurons. Current approaches for the treatment of neurodegeneration are limited. In this review, we discuss animal models of human brain injury and molecular biological data that have been obtained from their analysis. In particular, signal transduction pathways that are associated with neurosurvival following injury to the brain are presented and discussed.

Chronic psychosocial stress and concomitant repetitive transcranial magnetic stimulation: effects on stress hormone levels and adult hippocampal neurogenesis

BACKGROUND

Repetitive transcranial magnetic stimulation is increasingly used as a therapeutic tool in psychiatry and has been demonstrated to attenuate the activity of the stress hormone system. Stress-induced structural remodeling in the adult hippocampus may provide a cellular basis for understanding the impairment of neural plasticity in depressive illness. Accordingly, reversal of structural remodeling might be a desirable goal for antidepressant therapy. The present study investigated the effect of chronic psychosocial stress and concomitant repetitive transcranial magnetic stimulation treatment on stress hormone regulation and hippocampal neurogenesis.

METHODS

Adult male rats were submitted to daily psychosocial stress and repetitive transcranial magnetic stimulation (20 Hz) for 18 days. Cell proliferation in the dentate gyrus was quantified by using BrdU immunohistochemistry, and both the proliferation rate of progenitors and the survival rate of BrdU-labeled cells were evaluated. To characterize the activity of the hypothalamic-pituitary-adrenocortical system, plasma corticotropin and corticosterone concentrations were measured.

RESULTS

Chronic psychosocial stress resulted in a significant increase of stress hormone levels and potently suppressed the proliferation rate and survival of the newly generated hippocampal granule cells. Concomitant repetitive transcranial magnetic stimulation treatment normalized the stress-induced elevation of stress hormones; however, despite the normalized activity of the hypothalamic-pituitary-adrenocortical system, the decrement of hippocampal cell proliferation was only mildly attenuated by repetitive transcranial magnetic stimulation, while the survival rate of BrdU-labeled cells was further suppressed by the treatment.

CONCLUSIONS

These results support the notion that attenuation of the hypothalamic-pituitary-adrenocortical system is an important mechanism underlying the clinically observed antidepressant effect of repetitive transcranial magnetic stimulation, whereas this experimental design did not reveal beneficial effects of repetitive transcranial magnetic stimulation on adult hippocampal neurogenesis.

Overexpression of the signaling adapter FRS2 reconstitutes the cell cycle deficit of a nerve growth factor non-responsive TrkA receptor mutant

We have characterized the cell cycle deficit of a novel TrkA receptor mutant (TrkAS3) that fails to support nerve growth factor (NGF)-dependent cell cycle arrest and neurite outgrowth. TrkAS3 receptors fail to support an NGF-dependent increase in the expression of cyclin D1 and the cell cycle inhibitor, p21(Waf1/Cip1), two important regulators of G(1) /S transition, and do not down-regulate expression of the G(2) /M phase marker, cdc2/cdk1, or the S phase marker, proliferating cell nuclear antigen. Moreover, NGF-activated TrkAS3 receptors do not down-regulate cyclin-dependent kinase 4 phosphorylation of the retinoblastoma protein, essential for G(1) arrest, in comparison to NGF-activated wild-type TrkA. Collectively these data indicate that TrkAS3 receptors fail to support NGF-dependent G(1) arrest. Interestingly, ectopic expression of regulators of G(1) /S arrest, such as cyclin D1 or inhibitors of cell cycle (p21(Waf1/Cip1), p16(INK4A) ), or the fibroblast growth factor (FGF) receptor substrate-2 (FRS2) in cells expressing TrkAS3 reconstitutes NGF-dependent neurite outgrowth. Collectively, these data suggest a model in which NGF-stimulated TrkA-dependent activation of FRS2 supports neurite outgrowth through a mechanism that likely involves the induction of p21(Waf1/Cip1) expression and the arrest of cells at G(1) /S.

Effect of dexamethasone on the expression of brain-derived neurotrophic factor and neurotrophin-3 messenger ribonucleic acids after forebrain ischemia in the rat

OBJECTIVE

To determine whether a large dose of dexamethasone affected brain damage induced by concurrent cerebral ischemia, we used in situ hybridization to examine the expression of brain-derived neurotrophic factor and neurotrophin-3 messenger ribonucleic acids (mRNAs) in rats with and without dexamethasone administration after transient forebrain ischemia.

DESIGN

Prospective experimental study in rats.

SETTING

Experimental laboratory in a teaching hospital and university.

SUBJECTS

Eighty adult rats.

INTERVENTIONS

Twenty minutes of transient forebrain ischemia was induced by occlusion of four vessels in lightly anesthetized rats. Thirty-six animals received dexamethasone (15 mg/kg, intraperitoneally) after initial reperfusion. Thirty-six dexamethasone-control rats were injected with saline, and the remaining animals underwent sham surgery but no ischemia or dexamethasone.

MEASUREMENTS AND MAIN RESULTS

Using in situ hybridization, we determined hippocampal brain-derived neurotrophic factor and neurotrophin-3 mRNA expression 2, 4, 6, 12, and 24 hrs and 2, 3, 4, and 7 days after brain ischemia. Additionally, hippocampal CA1 region cell death was measured with Nissl stains. Both brain-derived neurotrophic factor and neurotrophin-3 mRNA exhibited a biphasic response after ischemia. Brain-derived neurotrophic factor mRNA showed two peaks of 4.07-fold and 2.84-fold increases relative to sham operation at 6 hrs and 2 days, respectively. Neurotrophin-3 mRNA initially decreased to 59% of sham levels at 4 hrs and then increased to 146% at 3 days before it returned to basal levels. When the ischemic rats were treated with dexamethasone, the elevation of brain-derived neurotrophic factor mRNA and the reduction of neurotrophin-3 mRNA level were prevented within the first 24 hrs, and hippocampal CA1 neurons were protected from ischemia-induced cell loss 7 days after brain ischemia. The protein levels of both brain-derived neurotrophic factor and neurotrophin-3 in general correspond to the mRNA levels in the hippocampal region.

CONCLUSIONS

Dexamethasone modulates the intriguing temporal and spatial expression of brain-derived neurotrophic factor and neurotrophin-3 that predominantly supports neuronal innervation at different times after brain ischemia and also may provide specific trophic support for various neurons in the central nervous system.

An activity-dependent neurotrophin-3 autocrine loop regulates the phenotype of developing hippocampal pyramidal neurons before target contact

Neurotrophin-3 (NT-3), its cognate receptor trkC, and voltage-gated calcium channels are coexpressed by embryonic pyramidal neurons before target contact, but their functions at this stage of development are still unclear. We show here that, in vitro, anti-NT-3 and anti-trkC antibodies blocked the increase, and NT-3 reversed the decrease in the number of calbindin-D(28k)-positive pyramidal neurons induced by, respectively, calcium channel activations and blockades. Similar results were obtained with single-neuron microcultures. In addition, voltage-gated calcium channel inhibition downregulates the extracellular levels of NT-3 in high-density cultures. Moreover, electrophysiological experiments in single-cell cultures reveal a tetrodotoxin-sensitive spontaneous electrical activity allowing voltage-gated calcium channel activation. The mouse NT-3 (-/-) mutation decreases by 40% the number of developing calbindin-D(28k)-positive pyramidal neurons, without affecting neuronal survival, both in vitro and in vivo. Thus, present results strongly support that an activity-dependent autocrine NT-3 loop provides a local, intrinsic mechanism by which, before target contact, hippocampal pyramidal-like neurons may regulate their own differentiation, a role that may be important during early CNS differentiation or after adult target disruption.

Target specific differentiation of peripheral trigeminal axons in rat-chick chimeric explant cocultures

Avian and rodent trigeminal ganglion (TG) neurons share common features in their neurotrophin requirements and axonal projections between the sensory periphery and the brainstem. In rodents, the whisker pad (WP) is a major peripheral target of the infraorbital (IO) nerve component of the TG. The chick IO nerve is much smaller and innervates the maxillary process (MP). In the embryonic WP, IO axons course in fascicles from a caudal to rostral direction and form terminal plexuses around follicles. In the chick, IO axons travel as a thin bundle to the MP and branch out with no specific patterning. We cocultured E15 rat TG with E5-6 chick MP or chick TG with rat WP explants to examine target influences on trigeminal axon growth patterns as visualized with DiI labeling or neurofilament immunohistochemistry. Chick TG axons showed robust growth into WP explants, and the ganglion increased in size. Thick bundles of axons traveled between rows of follicles and formed a distinct pattern as they developed terminal arbors around individual follicles. In contrast, rat TG axon growth was sparse in chick MP explants and the ganglion size reduced over time. Furthermore, rat TG axons did not show any patterning in the chick MP. Similar target-specific growth patterns were observed when TG explants were given a choice between chick MP and rat WP explants. Collectively these results indicate that both the chick and rat TG cells respond to similar target-specific peripheral cues in the establishment of innervation density and patterning in peripheral orofacial targets.

Hyperbaric oxygen treatment decreases post-ischemic neurotrophin-3 mRNA down-regulation in the rat hippocampus

The therapeutic effect of hyperbaric oxygen (HBO) on ischemic injury was investigated using in situ hybridization to detect the mRNA expression of neurotrophin-3 (NT-3), which is thought to play a crucial role in protecting against neuronal death induced by brain ischemia. The rats under investigation were subjected to 10 min transient forebrain ischemia, and subsequently exposed to HBO (100% oxygen, 2.5 atm absolute) for 2 h. Levels of NT-3 mRNA in the CA1, CA2 and CA3 regions, and the dentate gyrus of the hippocampus were measured after various reperfusion periods. Neuronal death in the hippocampal CA1 region was also measured by Nissl staining, seven days post ischemia. The results demonstrated that HBO treatment significantly reduced the ischemia-induced down-regulation of the NT-3 mRNA level at 4 h post ischemia, and significantly increased cell survival 7 days after reperfusion. The findings suggest that an HBO treatment maintaining the NT-3 mRNA level in the hippocampus can be beneficial to the ischemic brain within a certain time frame.

Overexpression of calbindin-D28K induces neurite outgrowth in dopaminergic neuronal cells via activation of p38 MAPK

An MN9D dopaminergic neuronal cell line overexpressing calbindin-D28K (MN9D/Calbindin) was established in order to investigate directly the potential role of calcium-binding protein in neuronal differentiation. Overexpression of calbindin-D28K in MN9D cells resulted in significant increases in the number of neurites, the length of primary neurites, and the total extent of neurites. This robust neurite outgrowth occurred without cessation of cell division. Analysis of immunoblots revealed that this morphological differentiation was accompanied by increased expression of such markers of maturation as the synaptosomal protein SNAP-25. During calbindin-D28K-evoked neurite outgrowth in MN9D cells, phosphorylation of p38 mitogen-activated protein kinase (MAPK) dramatically increased while the levels and extent of phosphorylation of such other MAPKs as c-Jun N-terminal kinase (JNK) or extracellular response kinase (ERK) were not altered. Consequently, calbindin-D28K-induced neurite outgrowth was largely abolished by treatment with a p38 inhibitor, PD 169316, while the level of SNAP-25 in MN9D/Calbindin cells was not altered by this treatment. These data support an idea that calbindin-D28K and its associated p38 signaling pathway play a role in dopaminergic neuronal differentiation.

Serotonergic cells of the rat raphe nuclei express mRNA of tyrosine kinase B (trkB), the high-affinity receptor for brain derived neurotrophic factor (BDNF)

Here we have studied the distribution of mRNA for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF) amongst serotonergic cell bodies of the raphe nuclei and their ascending projections into the dorsal hippocampus in the rat brain. Previous studies have shown that BDNF has got trophic action on serotonergic neurons. In the present study, we provide evidence that serotonergic neurons express mRNA for the functional receptor of BDNF, trkB. Intracerebro-ventricular (i.c.v.) injection of the 5-HT-specific neurotoxin, 5,7-dihydroxytryptamine, which lesions serotonergic cell bodies in the raphe nuclei as well as their ascending projections into the dorsal hippocampus, caused a dramatic loss of trkB mRNA from serotonergic cell bodies of the dorsal raphe nucleus. In contrast, there was no change in the abundance of trkB mRNA within the dorsal hippocampus. These findings provide direct evidence for the expression of trkB mRNA by serotonergic neurons and suggest distinct mechanisms of action of BDNF upon serotonergic neurons at the levels of their cell bodies and terminal projection sites.

Locally reduced levels of acidic FGF lead to decreased expression of 28-kda calbindin and contribute to the selective vulnerability of the neurons in the entorhinal cortex in Alzheimer's disease

Recent studies demonstrate that a disturbed calcium-homeostasis leading to increased susceptibility to excitotoxic triggers plays a major role in the neurodegenerative process initiating in layer 2 of the entorhinal cortex (EC2) during Alzheimer's disease (AD). Thus, proteins binding free Ca++ (i.e. calbindin) and factors regulating these proteins are of great importance for the neuroprotective-neurotoxic balance in the affected brain regions. In the present combined human and in vitro study evidence is provided that altered levels of the acidic fibroblast growth factor (aFGF) and calbindin expression are concomitantly present in EC2 neurons and have interactive effects. A dramatic loss of aFGF- and calbindin-labeled EC2 neurons was found. Further analysis of the surviving EC2 neurons revealed a strong immunoreactivity to calbindin and aFGF. In vitro experiments show that aFGF regulates calbindin expression, because treatment of differentiating neurons with recombinant aFGF increases calbindin expression in a time-dependent fashion. The data imply that a reduced expression of aFGF in EC2 neurons of AD brains leads to lower levels of calbindin resulting in decreased neuroprotective capacity.

Neurotrophin responsiveness is differentially regulated in neurons and precursors isolated from the developing striatum

Sequential exposure to members of the neurotrophin family, nerve growth factor (NGF), neurotrophin-type 3 (NT3), brain-derived neurotrophic factor (BDNF), and neurotrophin-type 4 (NT4), determines the generation, survival, and maturation of developing neurons. The effects of neurotrophins depend on the stage of development and the target cell population. However, the nature of the responding cells is often unclear. In this study neurotrophin responsiveness was analyzed in murine embryonic striatal precursors and neurons. Individual neurotrophin-responsive cells were identified based on activation of intracellular signaling pathways to the transcription factor CREB and were further characterized using differentiation-stage specific markers. A dramatic developmentally regulated decrease in BDNF responsiveness was observed: BDNF targeted more than 40% of striatal neurons at E14 but only 12% at E18. The percentage of NT3-responsive neurons also moderately decreased during development while no change was observed in the fraction of neuronal cells targeted by NT4 and NGF. A different type of developmental change was found in striatal precursors. BDNF, NT3, and NT4 each targeted about 15% of striatal precursors at E14 but no NGF responsive-precursors were detected at this age. In contrast, only NT3 and NGF could induce a response in precursor cells at E18. NGF-responsive precursors shared a distinct morphology with a large cell body and high levels of nestin expression. These results indicate that during striatal development, the regulation of neurotrophin responsiveness is different in neurons and precursor cells.

The total number of neurons and calcium binding protein positive neurons during aging in the cochlear nucleus of CBA/CaJ mice: a quantitative study

The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and the total number of neurons immunostained with parvalbumin, calbindin-D28k (calbindin), and calretinin in the dorsal and posteroventral cochlear nucleus (DCN and PVCN) in CBA/CaJ (CBA) mice during aging (1-39 months old). CBA mice have only a modest sensorineural pathology late in life. An age-related decrease of the total number of neurons was demonstrated in the DCN (r=-0.54, P<0.03), while the total number of neurons in the PVCN did not show any significant age-related differences (r=0.16, P=0.57). In the DCN 5.5% of neurons were parvalbumin positive in the very old (30-39 months) mice, vs. 2.2% in the 1 month old mice. In the DCN 3% of the neurons were calbindin immunopositive in the 30-39 months mice compared to 1.9% in the 1 month old group. In the PVCN, 20% of the neurons in the very old mice were parvalbumin immunopositive, compared to 12% in the young mice. Calbindin did not show any significant age-related differences in the PVCN. The total number of calretinin immunopositive neurons both in the DCN and PVCN did not show any significant change with increasing age. In conclusion, the total neuronal number in the DCN and PVCN was age-related and region-specific. While the neuronal number in the DCN and PVCN was decreased or unchanged, respectively, the calcium binding protein positive neuronal number showed a graded increase during aging in a region-specific and protein-specific manner.

Malonate-induced cortico-motoneuron death is attenuated by NT-4, but not by BDNF or NT-3

Neurotrophins are promising candidates to slow the progression of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease in which spinal and cortical motoneurons selectively degenerate. In a long-term in vitro model, malonate-induced toxicity and cell death of motoneurons have been demonstrated. Here we studied the neuroprotective effect of BDNF, NT-3, and NT-4 on the cell death of cortical motoneurons in an organotypic culture model after chronic mitochondrial inhibition with malonate. Our data show that NT-4 completely prevents malonate-induced toxicity, whereas BDNF or NT-3 had no neuroprotective effect. In clinical trials for ALS, predominantly focussed on the survival of spinal motoneurons, BDNF has already been tested with disappointing results; our results suggest that NT-4 may be a better neurotrophin to prevent motoneuron loss.

Therapeutic implications of fibroblast growth factors in traumatic spinal cord injury

Neurological damage after acute traumatic spinal cord injury (SCI) results from both the primary mechanical injury as well as the subsequent activation of cell death cascades mediating delayed tissue damage. Since secondary injury following traumatic SCI is a tightly regulated process in which several neurotrophic factors seem to be implicated, administration of these proteins has a clinical interest. Fibroblast growth factor may be one of the agents to be used for the treatment of traumatic SCI.

Distinct signaling pathways involved in multiple effects of basic fibroblast growth factor on cultured rat hippocampal neurons

We investigated possible involvement of voltage-dependent Ca(2+) channels (VDCCs) and several intracellular signaling mechanisms in multiple actions of basic fibroblast growth factor (bFGF), such as survival promotion, induction of calbindin D(28k) expression as well as acceleration of neuritic branch formation of cultured rat hippocampal neurons. Immunocytochemical staining with anti-gamma-aminobutyric acid (GABA) antibody showed that the promotion of neuron survival by bFGF in high cell-density cultures were exerted exclusively on GABA-negative neurons. Nicardipine (5 microM) attenuated the effect of bFGF on neuronal survival and formation of neurite branches, suggesting that the activity of L-type VDCCs is required for these effects. In contrast, stimulation of calbindin expression by bFGF was not attenuated by nicardipine. A phospholipase C inhibitor U73122 (1 microM) prevented the effect of bFGF on neurite branch formation, but not on neuronal survival or calbindin expression. On the other hand, chronic application of phorbol-12-myristate-13-acetate (1 microM) inhibited the effect of bFGF on neuronal survival, without inhibiting the other bFGF actions. Forskolin (100 microM) attenuated the effect of bFGF on neuronal survival and neurite branch formation, indicating that cyclic AMP plays negative regulatory roles in these actions of bFGF. Taken together, these results suggest that multiple biological actions of bFGF on hippocampal neurons are exerted through, and modulated by, distinct signaling pathways.

Expression of c-Met in developing rat hippocampus: evidence for HGF as a neurotrophic factor for calbindin D-expressing neurons

Hepatocyte growth factor-scatter factor (HGF) is expressed in different parts of the nervous system, and has been shown to exhibit neurotrophic activity. Here we show that c-Met, the receptor for HGF, is expressed in developing rat hippocampus, with the highest levels during the first postnatal weeks. To study the function of HGF, hippocampal neurons were prepared from embryonic rats and treated with different HGF concentrations. In these cultures, HGF increased the number of neurons expressing the 28-kDa calcium-binding protein (calbindin D) in a dose-dependent manner. The effect of HGF was larger than that observed with either brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), and cotreatment of the cultures with HGF and the neurotrophins was additive with respect to calbindin D neurons. Besides affecting the number of neurons, HGF significantly increased the degree of sprouting of calbindin D-positive neurons, suggesting an influence on neuronal maturation. BDNF and NT-3 stimulated neurite outgrowth of calbindin D neurons to a much smaller degree. In contrast to calbindin D neurons, HGF did not significantly increase the number of neurons immunoreactive with the neurotransmitter gamma-aminobutyric acid (GABA) in the hippocampal cultures. Immunohistochemical studies showed that c-Met-, calbindin D- and HGF-immunoreactive cells are all present in the dentate gyrus and partly colocalize within neurons. These results show that HGF acts on calbindin D-containing hippocampal neurons and increases their neurite outgrowth, suggesting that HGF plays an important role for the maturation and function of these neurons in the hippocampus.

Differential effects of NGF and NT-3 on embryonic trigeminal axon growth patterns

We examined the effects of neurotrophins nerve growth factor (NGF) and neurotrophin-3 (NT-3) on trigeminal axon growth patterns. Embryonic (E13-15) wholemount explants of the rat trigeminal pathway including the whisker pads, trigeminal ganglia, and brainstem were cultured in serum-free medium (SFM) or SFM supplemented with NGF or NT-3 for 3 days. Trigeminal axon growth patterns were analyzed with the use of lipophilic tracer DiI. In wholemount cultures grown in SFM, trigeminal axon projections, growth patterns, and differentiation of peripheral and central targets are similar to in vivo conditions. We show that in the presence of NGF, central trigeminal axons leave the tract and grow into the surrounding brainstem regions in the elongation phase without any branching. On the other hand, NT-3 promotes precocious development of short axon collaterals endowed with focal arbors along the sides of the central trigeminal tract. These neurotrophins also affect trigeminal axon growth within the whisker pad. Additionally, we cultured dissociated trigeminal ganglion cells in the presence of NGF, NT-3, or NGF+NT-3. The number of trigeminal ganglion cells, their size distribution under each condition were charted, and axon growth was analyzed following immunohistochemical labeling with TrkA and parvalbumin antibodies. In these cultures too, NGF led to axon elongation and NT-3 to axon arborization. Our in vitro analyses suggest that aside from their survival promoting effects, NGF and NT-3 can differentially influence axon growth patterns of embryonic trigeminal neurons.