Autocrine regulation of apoptosis and bcl-2 expression by nerve growth factor in early differentiating cerebellar granule neurons involves low affinity neurotrophin receptor

Differential effects of ethanol on bid, tBid, and Bax:tBid interactions in postnatal day 4 and postnatal day 7 rat cerebellum

BACKGROUND

Exposure to ethanol (EtOH) during central nervous system (CNS) development can lead to a wide array of neuroanatomical, behavioral, and cognitive abnormalities, broadly subsumed under the fetal alcohol spectrum disorder classification. One mode of EtOH-induced interference in the normal developmental program appears to be through induction of apoptotic processes mediated by the Bcl-2 family of survival-regulatory proteins. The present series of studies investigated the role of the Bcl-2-related, pro-apoptotic Bid protein, and its truncated, apoptotically active fragment, tBid, in developmental EtOH neurotoxicity.

METHODS

Protein analyses were made via enzyme-linked immunosorbent assays (ELISA) in neonatal rat cerebellum, of basal Bid, and of Bid and tBid, following EtOH exposure via vapor inhalation, at an age of peak EtOH sensitivity in this region (postnatal day 4 [P4]) and a later age of relative resistance (P7). ELISA analyses were also made of Bax:tBid heterodimers, a process which activates Bax, essential for its apoptotic functioning. Finally, in vitro assessments of the importance of tBid to EtOH neurotoxicity were made in cultured cerebellar granule cells, using a specific tBid inhibitor.

RESULTS

Basal levels of Bid were higher at P4 compared to P7, possibly contributing to the differential sensitivity. EtOH exposure elicited further increases in cytosolic Bid and mitochondrial tBid when administration was at P4, but not at P7. Bax:tBid heterodimers were markedly increased by EtOH exposure on P4, an increase which persisted even 2 hours after termination of treatment. Similar effects were not seen at P7. The in vitro analyses revealed that tBid inhibition provided complete protection against EtOH-induced cell death and depressed EtOH-mediated cytochrome-c release.

CONCLUSIONS

These results suggest that Bid/tBid may be important elements in EtOH-mediated neurotoxicity during CNS development. The molecular processes and interactions revealed may represent critical points which can be targeted in studies concerned with designing possible therapeutic strategies for minimizing these devastative effects.

Ethanol influences on Bax associations with mitochondrial membrane proteins in neonatal rat cerebellum

These studies investigated interactions taking place at the mitochondrial membrane in neonatal rat cerebellum following ethanol exposure and focused on interactions between proapoptotic Bax and proteins of the permeability transition pore (PTP), voltage-dependent anion channel (VDAC) and adenine nucleotide translocator (ANT) of the outer and inner mitochondrial membranes, respectively. Cultured cerebellar granule cells were used to assess the role of these interactions in ethanol neurotoxicity. Analyses were made at the age of maximal cerebellar ethanol vulnerability (P4), compared to the later age of relative resistance (P7), to determine whether differential ethanol sensitivity was mirrored by differences in these molecular interactions. We found that, following ethanol exposure, Bax proapoptotic associations with both VDAC and ANT were increased, particularly at the age of greater ethanol sensitivity, and these interactions were sustained at this age for at least 2 h postexposure. Since Bax:VDAC interactions disrupt protective VDAC interactions with mitochondrial hexokinase (HXK), we also assessed VDAC:HXK associations following ethanol treatment and found such interactions were altered by ethanol treatment, but only at 2 h postexposure and only in the P4, ethanol-sensitive cerebellum. Ethanol neurotoxicity in cultured neuronal preparations was abolished by pharmacological inhibition of both VDAC and ANT interactions with Bax but not by a Bax channel blocker. Therefore, we conclude that, at this age, within the constraints of our experimental model, a primary mode of Bax-induced initiation of the apoptosis cascade following ethanol insult involves interactions with proteins of the PTP complex and not channel formation independent of PTP constituents.

Differential effects of ethanol on c-jun N-terminal kinase, 14-3-3 proteins, and Bax in postnatal day 4 and postnatal day 7 rat cerebellum

These studies investigated ethanol effects on upstream cellular elements and interactions which contribute to Bax-related apoptosis in neonatal rat cerebellum at ages of peak ethanol sensitivity (postnatal day 4 [P4]), compared to later ages of relative resistance (P7). Analyses were made of basal levels of the pro-apoptotic c-jun N-terminal kinase (JNK), Bax, and the 14-3-3 anchoring proteins, as well as the responsiveness of these substances to ethanol at P4 versus P7. Dimerization of Bax with 14-3-3 was also investigated at the two ages following ethanol treatment, a process which sequesters Bax in the cytosol, thus inhibiting its mitochondrial translocation and disruption of the mitochondrial membrane potential. Cultured cerebellar granule cells were used to examine the protective potential of JNK inhibition on ethanol-mediated cell death. Basal levels of JNK were significantly higher at P4 than P7, but no differences in the other proteins were found. Activated JNK, and cytosolic and mitochondrially-translocated Bax were increased in P4 but not P7 animals following ethanol exposure, while protective 14-3-3 proteins were increased only at P7. Ethanol treatment resulted in decreases in Bax:14-3-3 heterodimers at P4, but not at P7. Inhibition of JNK activity in vitro provided partial protection against ethanol neurotoxicity. Thus, differential temporal vulnerability to ethanol in this CNS region correlates with differences in both levels of apoptosis-related substances (e.g., JNK), and differential cellular responsiveness, favoring apoptosis at the most sensitive age and survival at the resistant age. The upstream elements contributing to this vulnerability can be targets for future therapeutic strategies.

Ethanol influences on Bax translocation, mitochondrial membrane potential, and reactive oxygen species generation are modulated by vitamin E and brain-derived neurotrophic factor

BACKGROUND

This study investigated ethanol influences on intracellular events that predispose developing neurons toward apoptosis and the capacity of the antioxidant α-tocopherol (vitamin E) and the neurotrophin brain-derived neurotrophic factor (BDNF) to modulate these effects. Assessments were made of the following: (i) ethanol-induced translocation of the pro-apoptotic Bax protein to the mitochondrial membrane, a key upstream event in the initiation of apoptotic cell death; (ii) disruption of the mitochondrial membrane potential (MMP) as a result of ethanol exposure, an important process in triggering the apoptotic cascade; and (iii) generation of damaging reactive oxygen species (ROS) as a function of ethanol exposure.

METHODS

These interactions were investigated in cultured postnatal day 8 neonatal rat cerebellar granule cells, a population vulnerable to developmental ethanol exposure in vivo and in vitro. Bax mitochondrial translocation was analyzed via subcellular fractionation followed by Western blot, and mitochondrial membrane integrity was determined using the lipophilic dye, JC-1, that exhibits potential-dependent accumulation in the mitochondrial membrane as a function of the MMP.

RESULTS

Brief ethanol exposure in these preparations precipitated Bax translocation, but both vitamin E and BDNF reduced this effect to control levels. Ethanol treatment also resulted in a disturbance of the MMP, and this effect was blunted by the antioxidant and the neurotrophin. ROS generation was enhanced by a short ethanol exposure in these cells, but the production of these harmful free radicals was diminished to control levels by cotreatment with either vitamin E or BDNF.

CONCLUSIONS

These results indicate that both antioxidants and neurotrophic factors have the potential to ameliorate ethanol neurotoxicity and suggest possible interventions that could be implemented in preventing or lessening the severity of the damaging effects of ethanol in the developing central nervous system seen in the fetal alcohol syndrome (FAS).

Synergistic effects of osteonectin and NGF in promoting survival and neurite outgrowth of superior cervical ganglion neurons

Schwann cells (SCs) play a major role in the successful regeneration of peripheral nerves regeneration. Here we examined the effects of osteonectin (ON), a major factor secreted by SCs, on survival and neuritogenesis of mouse superior cervical ganglion (SCG) neurons. SC conditioned medium (SCCM) not only promoted the survival and neuritogenesis of SCG neurons at a level comparable to nerve growth factor (NGF) but also doubled the neurite length of NGF-treated SCG neurons. SCCM neuritogenic effects were not blocked by the tyrosine kinase receptor (Trk) inhibitor K252a demonstrating that these are not due solely to classical neurotrophic factors. Anti-ON neutralizing antibody diminished the SCCM-induced survival and neuritogenesis significantly. In the presence of K252a, the SCCM neuritogenic effects were blocked completely by anti-ON which suggests synergistic effects of ON with Trk-mediated growth factors. ON alone increased the survival and neurite outgrowth of SCG neurons significantly at high density cultures. ON at low concentration acts synergistically with NGF which induced maximum survival and neurite outgrowth (>50% increase). However, ON at high concentration was detrimental to survival (64% decrease) and neurite outgrowth (87% decrease) even in the presence of NGF. The well documented counter-adhesive effect of ON may account for this observation. Nevertheless, the growth promoting effects of ON became more pronounced as the cell density increased which suggests a possible interaction of ON with growth factors secreted by SCG neurons (autocrine or paracrine effects). Taken together, our study indicates that ON plays important roles in nervous system repair through its synergistic effects with growth factors.

Thyroid hormone and cerebellar development

Thyroid hormone (TH) plays a key role in mammalian brain development. The developing brain is sensitive to both TH deficiency and excess. Brain development in the absence of TH results in motor skill deficiencies and reduced intellectual development. These functional abnormalities can be attributed to maldevelopment of specific cell types and regions of the brain including the cerebellum. TH functions at the molecular level by regulating gene transcription. Therefore, understanding how TH regulates cerebellar development requires identification of TH-regulated gene targets and the cells expressing these genes. Additionally, the process of TH-dependent regulation of gene expression is tightly controlled by mechanisms including regulation of TH transport, TH metabolism, toxicologic inhibition of TH signaling, and control of the nuclear TH response apparatus. This review will describe the functional, cellular, and molecular effects of TH deficit in the developing cerebellum and emphasize the most recent findings regarding TH action in this important brain region.

Enhanced neuronal loss under perinatal hypothyroidism involves impaired neurotrophic signaling and increased proteolysis of p75(NTR)

Recognition of the molecular events that lead to enhanced cell death is vital to understand the developmental cerebellar defects under hypothyroidism. Though neurotrophins promote the survival and development of neurons in the cerebellum, but the mechanism of their insufficiency mediated cell loss under hypothyroidism is unknown. Here in developmental hypothyroid rat model we report that hypothyroidism induced neuronal loss involve down regulation of neurotrophic survival signaling and increased truncation of the receptor p75(NTR). Results showed that perinatal hypothyroidism besides repressing the expression of BDNF also impairs the maturation of NGF which results in decreased activation of ERK, CREB, NF-kappaB and AKT. Furthermore hypothyroidism caused an enhanced expression and proteolysis of p75(NTR). The increased proteolysis of p75(NTR)in vivo and its association with death of granule neurons brings forward hitherto a p75(NTR) dependence signaling which along with compromised survival signaling could provide a neurotrophic basis of understanding the cause of enhanced cell death in developing cerebellum under hypothyroidism.

Bcl-2 and Bax proteins are increased in neocortical but not in thalamic apoptosis following devascularizing lesion of the cerebral cortex in the rat: an immunohistochemical study

The hypothesis that devascularization of somatosensory and motor cortex causes apoptosis in infarcted regions and in the linked thalamic nuclei was evaluated. To unravel whether Bcl-related proteins, known to regulate apoptosis, participate in neuronal and glial responses to devascularization, we analyzed immunohistochemically the distribution and intensity of staining of Bcl-2 and Bax proteins at different time points after lesion. Both early (up to 6 h) and late (1-7 days) responses were studied. Devascularization led to rapid (within hours) apoptosis in the cortex and to a delayed (within 3-7 days) apoptosis in thalamic nuclei. In control groups, Bcl-2 and Bax immunoreactivity (IR) was detected in neurons and oligodendrocytes but not in astrocytes or microglia. Following devascularization, Bcl-2 IR and Bax IR increased in neurons before the onset of the apoptosis. In the ischemic focus, the increase reached maximal values 3 h after the lesion. The increase was of slower onset in the penumbra zone (24 h and after), a region in which both proteins were induced in astrocytes also. The change of Bax IR intensity exceeded four times that of Bcl-2 at all time points investigated, indicating a diminution of Bcl-2/Bax ratio that may direct neurons to apoptotic pathway. In numerous neurons, an increase of IR in the cytoplasm was accompanied by induction of nuclear staining. No changes of Bcl-2 and Bax IR were found in thalamic nuclei. Our results point to different mechanisms underlying apoptosis of cortical and thalamic neurons. Nuclear appearance of Bcl-2 and Bax suggests they possess regulatory role of gene expression changes triggered by cortical infarct.

Treatment of cerebellar granule cell neurons with the neurotrophic factor pigment epithelium-derived factor in vitro enhances expression of other neurotrophic factors as well as cytokines and chemokines

Microarray analyses demonstrated that a variety of genes was affected by treatment of cerebellar granule cell neurons with the neurotrophic factor pigment epithelium-derived factor (PEDF). The genes for neurotrophins, glial cell-derived neurotrophic factor (GDNF), and their receptors were regulated differentially in immature versus mature neurons; however, nerve growth factor (NGF), neurotrophin (NT)-3, and GDNF did not contribute to the protective effect of PEDF. Brain-derived neurotrophic factor (BDNF) seemed capable of inducing apoptosis, because a blocking antibody enhanced the protective effect of PEDF. In addition, PEDF exposure also stimulated expression of several cytokine and chemokine genes. Removal of the less than 1% of microglia in the cultures by treatment with L-leucine methyl ester, combined with enzyme-linked immunosorbent assays (ELISAs), demonstrated that the cerebellar granule cells constitutively produce three chemokines, macrophage inflammatory protein (MIP)-1alpha, MIP-2, and MIP-3alpha, whose production is enhanced further by treatment with PEDF. Blocking antibodies to each of the chemokines was protective under control conditions, suggesting that they may contribute to the "natural" apoptosis occurring in the cultures, and enhanced the effects of PEDF. Although PEDF enhanced production of all three chemokines, the blocking antibodies did not increase its protective effect against induced apoptosis. These results suggest that although PEDF enhances expression of other neurotrophic factors or chemokines, it does not exert its neuroprotective effect on cerebellar granule cells through their production.

Ethanol effects on neonatal rat cortex: comparative analyses of neurotrophic factors, apoptosis-related proteins, and oxidative processes during vulnerable and resistant periods

The developing central nervous system (CNS) is highly susceptible to ethanol, with acute or chronic exposure producing an array of anomalies and cell loss. Certain periods of vulnerability have been defined for various CNS regions, and are often followed by periods of relative ethanol resistance. In the present study, neonatal rats were acutely exposed to ethanol during a time when peak cell death is found in developing cerebral cortex (postnatal day 7; P7), and during a later neonatal period of ethanol resistance (P21). Comparisons at the two ages were made of basal levels of neurotrophic factors (NTFs), and in addition, ethanol-mediated changes in NTFs, apoptosis-related proteins, antioxidant activities, and generation of reactive oxygen species (ROS) were quantified at 0, 2, and 12 h following termination of exposure. It was found that at P21, basal levels of NTF nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) were considerably higher than at P7, possibly affording protection against ethanol neurotoxicity at this age. Following ethanol treatment at P7, approximately equal numbers of pro-apoptotic and pro-survival changes were produced, although most of the pro-apoptotic alterations occurred rapidly following termination of treatment, a critical period for initiation of apoptosis. At P21, however, the large majority of ethanol-mediated changes were adaptive, favoring survival. We speculate that the capacity of the older CNS to upregulate a number of protective elements within the cellular milieu serves to greatly mitigate ethanol neurotoxicity, while in younger animals, such adjustments are minimal, thus enhancing ethanol vulnerability within this developing region.

Sympathetic neurons synthesize and secrete pro-nerve growth factor protein

Postmitotic sympathetic neuronal survival is dependent upon nerve growth factor (NGF) provided by peripheral targets, and this dependency serves as a central tenet of the neurotrophic hypothesis. In some other systems, NGF has been shown to play an autocrine role, although the pervasiveness and significance of this phenomenon within the nervous system remain unclear. We show here that rat sympathetic neurons synthesize and secrete NGF. NGF mRNA is expressed in nearly half of superior cervical ganglion sympathetic neurons at embryonic day 17, rising to over 90% in the early postnatal period, and declining in the adult. Neuronal immunoreactivity is reduced when retrograde transport is interrupted by axotomy, but persists in a subpopulation of neurons despite diminished mRNA expression, suggesting that intrinsic protein synthesis occurs. Cultured neonatal neurons express NGF mRNA, which is maintained even when they are undergoing apoptosis. To determine which NGF isoforms are secreted, we performed metabolic labeling and immunoprecipitation of NGF-immunoreactive proteins synthesized by cultured NGF-dependent and -independent neurons. Conditioned medium contained high molecular weight NGF precursor proteins, which varied depending upon the state of NGF dependence. Mature NGF was undetectable by these methods. High molecular weight NGF isoforms were also detected in ganglion homogenates, and persisted at diminished levels following axotomy. We conclude that sympathetic neurons express NGF mRNA, and synthesize and secrete pro-NGF protein. These findings suggest that a potential NGF-sympathetic neuron autocrine loop may exist in this prototypic target-dependent system, but that the secreted forms of this neurotrophin apparently do not support neuronal survival.

Novel functional interactions between Trk kinase and p75 neurotrophin receptor in neuroblastoma cells

To understand the functional interactions between the TrkA and p75 nerve growth factor (NGF) receptors, we stably transfected LAN5 neuroblastoma cells with an expression vector for ET-R, a chimeric receptor with the extracellular domain of the epidermal growth factor receptor (EGFR), and the TrkA transmembrane and intracellular domains. EGF activated the ET-R kinase and induced partial differentiation. NGF, which can bind to endogenous p75, did not induce differentiation but enhanced the EGF-induced response, leading to differentiation of almost all cells. A mutated NGF, 3T-NGF, that binds to TrkA but not to p75 did not synergize with EGF. Enhancement of EGF-induced differentiation required at least nanomolar concentrations of NGF, consistent with the low-affinity p75 binding site. EGF may induce a limited number of neuronal cells because it also enhanced apoptosis. Both NGF and a caspase inhibitor reduced apoptosis and, thereby, enhanced differentiation. NGF seems to enhance survival through the phosphatidylinositol-3 kinase (PI3K) pathway. Consistent with this hypothesis, Akt, a downstream effector of the PI3K pathway, was hyperphosphorylated in the presence of EGF+NGF. These results demonstrate that TrkA kinase initiates differentiation, and p75 enhances differentiation by rescuing differentiating cells from apoptosis via the PI3K pathway. Even though both EGF and NGF are required for differentiation of LAN5/ET-R cells, only NGF is required for survival of the differentiated cells. In the absence of NGF, the cells die by an apoptotic mechanism, involving caspase-3. An anti-p75 antibody blocked the survival effect of NGF. Brain-derived neurotrophic factor also enhanced cell survival, indicating that in differentiated cells, NGF acts through the p75 receptor to prevent apoptosis.

Influence of ethanol on neonatal cerebellum of BDNF gene-deleted animals: analyses of effects on Purkinje cells, apoptosis-related proteins, and endogenous antioxidants

The sensitivity of the developing central nervous system (CNS) to the deleterious effects of ethanol has been well documented, with exposure leading to a wide array of CNS abnormalities. Certain CNS regions are susceptible to ethanol during well-defined critical periods. In the neonatal rodent cerebellum, a profound loss of Purkinje cells is found when ethanol is administered early in the postnatal period [on postnatal days 4 or 5 (P4-5)], while this neuronal population is much less vulnerable to similar ethanol insult slightly later in the postnatal period (P7-9). Prior studies have shown that neurotrophic factors (NTFs) can be altered by ethanol exposure, and both in vitro and in vivo studies have provided evidence that such substances have the potential to protect against ethanol neurotoxicity. In the present study, it was hypothesized that depletion of an NTF shown to be important to cerebellar development would exacerbate ethanol-related effects within this region, when administration was confined to a normally ethanol-resistant ontogenetic period. For this study, brain-derived neurotrophic factor (BDNF) gene-deleted ("knockout") and wild-type mice were exposed to ethanol via vapor inhalation or to control conditions during the normally ethanol-resistant period (P7 and P8). Two hours after termination of exposure on P8, analyses were made of body weight, crown-rump length, and brain weight. In subsequent investigations, the number and density of Purkinje cells and the volume of cerebellar lobule I were determined, and the expression of anti- and pro-apoptotic proteins and the activities of endogenous antioxidants were assessed. It was found that the BDNF knockouts were significantly smaller than the wild-type animals, with smaller brain weights. Purkinje cell number and density was reduced in ethanol-treated knockout, but not wild-type animals, and the volume of lobule I was significantly decreased in the gene-deleted animals compared to wild-types, but was not further affected by ethanol treatment. The loss of Purkinje cells in the BDNF knockouts was accompanied by decreases in anti-apoptotic Bcl-xl and in phosphorylated (and hence inactivated) pro-apoptotic Bad, and reduced activity of the antioxidant glutathione reductase, while the antioxidant catalase was increased by ethanol treatment in this genotype. In the wild-type animals, anti-apoptotic Bcl-2 was decreased by ethanol treatment, but the pro-apoptotic c-Jun N-terminal kinase (JNK) was markedly diminished by ethanol exposure, while the activity of the protective antioxidant superoxide dismutase (SOD) was significantly enhanced. These results suggest that neurotrophic factors have the capacity to protect against ethanol neurotoxicity, perhaps by regulation of expression of molecules critical to neuronal survival such as elements of the apoptosis cascade and protective antioxidants.

Oxidative stress and programmed cell death in diabetic neuropathy

Recent evidence in both animal models and human sural nerve biopsies indicates an association with oxidative stress, mitochondrial (Mt) membrane depolarization (MMD), and induction of programmed cell death (PCD). In streptozotocin (STZ)-treated diabetic rats, hyperglycemia induces typical apoptotic changes as well as swelling and disruption of the Mt cristae in diabetic dorsal root ganglion neurons (DRG) and Schwann cells (SC), but these changes are only rarely observed in control neurons. In human sural nerve biopsies, from patients with diabetic sensory neuropathy, there is transmission electromicrograph evidence of swelling and disruption of the Mt and cristae compared to patients without peripheral neuropathy. In human SH-SY5Y neurons, rat sensory neurons, and SC, in vivo, there is an increase in reactive oxygen species (ROS) after exposure to 20 mM added glucose. In parallel, there is an initial Mt membrane hyperpolarization followed by depolarization (MMD). In turn, MMD is coupled with cleavage of caspases. Various strategies aimed at inhibiting the oxidative burst, or stabilizing the DeltaPsi(M), block induction of PCD. First, growth factors such as NGF can block induction of ROS and/or stabilize the DeltaPsi(M). This, in turn, is associated with inhibition of PCD. Second, reduction of ROS generation in neuronal Mt prevents neuronal PCD. Third, up-regulation of uncoupling proteins (UCPs), which stabilize the DeltaPsi(M), blocks induction of caspase cleavage. Collectively, these findings indicate that hyperglycemic conditions observed in diabetes mellitus are associated with oxidative stress-induced neuronal and SC death, and targeted therapies aimed at regulating ROS may prove effective in therapy of diabetic neuropathy.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

Skin-derived nerve growth factor blocks programmed cell death in the trigeminal ganglia but does not enhance neuron proliferation

Development of the cutaneous sensory nervous system is dependent on the production of neurotrophic factors, such as nerve growth factor (NGF), by the skin. Limited synthesis of NGF in developing skin is thought to underlie programmed cell death and cause a 50% neuronal loss. This loss does not occur in transgenic mice that overexpress NGF in the skin, which have double the number of neurons (J. Neurosci. 14 (1994) 1422). To determine whether increased NGF blocks neuronal death and/or increases neuronal precursor replication, we analyzed the trigeminal ganglia at embryonic days E12.5, E14.5 and E16.5 using transferase-mediated dUTP nick-end labeling (TUNEL) and bromodeoxyuridine labeling. Results show that excess target-derived NGF causes a major decrease in the percent of TUNEL-labeled neurons without affecting the percent of replicating neurons. Analysis of RNA and protein expression suggests this block in cell death is mediated via the anti-apoptotic protein bcl-2.

Neurotrophic factor protection against ethanol toxicity in rat cerebellar granule cell cultures requires phosphatidylinositol 3-kinase activation

Neonatal rat cerebellar granule cells were used to assess the possible role of the phosphatidylinositol 3-kinase (PI3-K) signaling pathway in the neuroprotective effects of neurotrophic factors against ethanol toxicity. Culture conditions included medium with ethanol (400 and 600 mg/dl), nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), ethanol+NGF or BDNF, the PI3-K inhibitor wortmannin (10 or 100 microM), and wortmannin+ethanol+NGF or BDNF. Neuronal survival was determined via the MTT assay. The results indicated that both NGF and BDNF ameliorate ethanol neurotoxicity, and wortmannin abolished this effect, except at the higher ethanol concentration combined with the lower wortmannin level. These data strongly implicate the PI3-K pathway in growth factor protection against ethanol neurotoxicity.

Intravenous brain-derived neurotrophic factor reduces infarct size and counterregulates Bax and Bcl-2 expression after temporary focal cerebral ischemia

BACKGROUND AND PURPOSE

Pretreatment with intraventricular brain-derived neurotrophic factor (BDNF) reduces ischemic damage after focal cerebral ischemia. In this experiment we studied the effect of intravenous BDNF delivered after focal cerebral ischemia on neurological outcome, infarct size, and expression of proapoptotic and antiapoptotic proteins Bax and Bcl-2, respectively.

METHODS

With the use of the suture occlusion technique, the right middle cerebral artery in rats was temporarily occluded for 2 hours. Thirty minutes after vessel occlusion, BDNF (300 microg/kg per hour in vehicle; n=12) or vehicle alone (n=13) was continuously infused intravenously for 3 hours. After 24 hours the animals were weighed and neurologically assessed on a 5-point scale. The animals were then killed, and brains underwent either 2,3,5-triphenyltetrazolium chloride staining for assessment of infarct volume or paraffin embedding for morphology and immunohistochemistry (Bax, Bcl-2).

RESULTS

Physiological parameters (mean arterial blood pressure, PO(2), PCO(2), pH, body temperature, glucose) and weight revealed no difference between groups. Neurological deficit was improved in BDNF-treated animals versus controls (P:<0.05, unpaired, 2-tailed t test). Mean+/-SD infarct volume was 229.7+/-97.7 mm(3) in controls and 121.3+/-80.2 mm(3) in BDNF-treated animals (P:<0.05, unpaired, 2-tailed t test). Cortical infarct volume was 155.5+/-78.5 mm(3) in the placebo group and 69.9+/-50.2 mm(3) in the BDNF-treated group (P:<0.05, unpaired, 2-tailed t test). Subcortical infarct volume was 74.1+/-30.6 mm(3) in the placebo group and 51.1+/-26.8 mm(3) in the BDNF-treated group (P:=NS). Bax-positive neurons were significantly reduced in the ischemic penumbra in BDNF-treated animals (P:<0.05, unpaired, 2-tailed t test), whereas Bcl-2-positive neurons were significantly increased in this area (P:<0.001, unpaired, 2-tailed t test).

CONCLUSIONS

This study demonstrates a neuroprotective effect of BDNF when delivered intravenously after onset of focal cerebral ischemia. As shown here, one possible mechanism of action of neuroprotection of BDNF after focal ischemia appears to be counterregulation of Bax/Bcl-2 proteins within the ischemic penumbra.

Effect of neonatal ethanol exposure on parvalbumin-expressing GABAergic neurons of the rat medial septum and cingulate cortex

This study was performed to determine the long-term effects of ethanol exposure during the brain growth spurt (postnatal days 4-10) on the number of parvalbumin-immunoreactive (PA+) GABAergic neurons in the adult (P60) rat medial septum and anterior cingulate cortex. Significant loss of neurons within each of these populations has previously been demonstrated following prenatal ethanol exposure. In the present study, no significant differences in the number of PA+ neurons were found in either the medial septum or the cingulate cortex when control and ethanol-exposed animals were compared. The cellular densities and volumetric measures in both brain regions were also similar in the two groups. We speculate that compensatory up-regulative mechanisms may have accounted for the protection of the PA neuronal populations in these two areas following the early neonatal exposure.

Characterization of the subventricular zone of the adult human brain: evidence for the involvement of Bcl-2

The subventricular zone (SVZ) is an embryonic remnant that persists and remains mitotically active throughout adulthood. The rodent SVZ harbors neuronal precursors, principally in its anterior part, and generates neuroblasts that migrate tangentially into the olfactory bulb, thus forming the so-called rostral migratory stream. This study aimed at characterizing the SVZ in the human brain. Antibodies raised against the widely used SVZ molecular markers nestin, glial fibrillary acidic protein, beta-tubulin-III and polysialylated neural cell adhesion molecule, have allowed us to characterize in detail a zone similar to the rodent SVZ in humans. Virtually all portions of the lateral ventricle, as well as the ventral (hypothalamic) sector of the third ventricle, displayed immunoreactivity for most of the molecular markers. The midline region of the septum (septal recess) and the ventral portion of the SVZ displayed a particularly intense immunostaining for all SVZ markers. These two regions may represent zones of adult neurogenesis that are unique to primates. Furthermore, the anti-apoptotic protein Bcl-2 was found to be actively synthesized and co-expressed with all the other markers throughout the entire SVZ. This study reveals that a well-developed SVZ exists in the adult human brain and suggests that Bcl-2 might play an important role in the functional organization of such a system.

Neonatal ethanol exposure alters bcl-2 family mRNA levels in the rat cerebellar vermis

BACKGROUND

The objective of the present work was to determine whether ethanol-induced cerebellar cell death during development is related to alterations in the expression of bcl-2 family genes.

METHODS

Rats were exposed to ethanol or control conditions during the neonatal period and transcript levels of bcl-2 family members relative to cyclophilin were determined. Pups exposed in parallel were taken for cerebellar cell counts.

RESULTS

Ethanol exposure during the first postnatal week significantly reduced Purkinje and granule cell numbers by postnatal day 21 (P21). Acute first postnatal week ethanol exposure up-regulated mRNA transcripts encoding the cell death-promoting molecules bax and bcl-xs as measured on P4. An additional day of exposure on P5 resulted in no further alterations in bcl-2 family transcripts, likely because Purkinje cell death was detectable as early as P5. To determine whether proapoptotic gene expression changes were specific to first postnatal week ethanol neurotoxicity, we examined bcl-2 family mRNA levels in rats exposed to ethanol during a developmental period of cerebellar insusceptibility, the second postnatal week. Exposure on P7 to P8 produced no change in cerebellar cell number, but also resulted in increased levels of bax, although only after 2-day ethanol exposure and not after acute exposure on P7.

CONCLUSIONS

These data implicate altered expression of proapoptotic members of the bcl-2 gene family in acute ethanol-mediated cerebellar cell death during the first postnatal week. They also suggest that the differential survival of cerebellar neurons after ethanol exposure during more mature developmental stages may be related to more successful suppression of proapoptotic processes.

Activation of the Bcl-2 promoter by nerve growth factor is mediated by the p42/p44 MAPK cascade

The Bcl-2 protein has an anti-apoptotic effect in neuronal and other cell types. We show for the first time that the Bcl-2 promoter is activated by the neuronal survival factor nerve growth factor (NGF) and that this effect is dependent on a region of the promoter from -1472 to -1414. This activation requires the Rap-1 G protein and the MEK-1 and p42/p44 MAPK enzymes but is independent of other NGF-activated signalling pathways involving protein kinase A or protein kinase C.

Bcl-2 protein as a marker of neuronal immaturity in postnatal primate brain

The distribution of neurons expressing immunoreactivity for the protein Bcl-2 was studied in the brain of squirrel monkeys (Saimiri sciureus) of various ages. Several subsets of small and intensely immunoreactive neurons displaying an immature appearance were disclosed in the amygdala and piriform cortex. The piriform cortex exhibited clusters of various forms in which Bcl-2+ neurons appeared linked to one another by their own neurites. The subventricular zone, which is known to harbor the largest population of rapidly and constitutively proliferating cells in the adult rat brain, was intensely stained, particularly at the basis of the lateral ventricle. A long and dorsoventrally oriented Bcl-2+ fiber fascicle was seen to emerge from the subventricular zone, together with numerous Bcl-2+ cells that formed a densely packed column directed at the olfactory tubercle. In adult and aged monkeys, the small and intensely labeled neurons were progressively replaced by larger and more weakly stained neurons in the amygdala and piriform cortex. In contrast, Bcl-2 immunostaining did not change with age in the subventricular zone and olfactory tubercle, the islands of Calleja of which were markedly enriched with Bcl-2. The dentate gyrus contained only a few layers of intensely labeled granule cells in juvenile monkeys, but the number of these layers increased markedly in adult and aged monkeys. These findings suggest that Bcl-2 can serve as a marker of both proliferating and differentiating neurons and indicate that such immature neurons may be much more widespread than previously thought in postnatal primate brain.