Contribution of glutamate receptors to brain-derived neurotrophic factor-induced elevation of intracellular Ca2+levels

Stamina-Enhancing Effects of Human Adipose-Derived Stem Cells

Stamina-enhancing effects of human adipose derived stem cells (hADSCs) were investigated in young Sprague-Dawley rats. Ten-day-old male rats were transplanted intravenously (IV) or intracerebroventricularly (ICV) with hADSCs (1 × 10⁶ cells/rat), and physical activity was measured by locomotor activity and rota-rod performance at post-natal day (PND) 14, 20, 30, and 40, as well as a forced swimming test at PND 41. hADSCs injection increased the moving time in locomotor activity, the latency in rota-rod performance, and the maximum swimming time. For the improvement of physical activity, ICV transplantation was superior to IV injection. In biochemical analyses, ICV transplantation of hADSCs markedly reduced serum creatine phosphokinase, lactate dehydrogenase, alanine transaminase, and muscular lipid peroxidation, the markers for muscular and hepatic injuries, despite the reduction in muscular glycogen and serum triglycerides as energy sources. Notably, hADSCs secreted brain-derived neurotrophic factor (BDNF) and nerve growth factor in vitro, and increased the level of BDNF in the brain and muscles in vivo. The results indicate that hADSCs enhance physical activity including stamina not only by attenuating tissue injury, but also by strengthening the muscles via production of BDNF.

Activity-dependent alterations in the sensitivity to BDNF-TrkB signaling may promote excessive dendritic arborization and spinogenesis in fragile X syndrome in order to compensate for compromised postsynaptic activity

Fragile X syndrome (FXS), the most common cause of inherited human mental retardation, results from the loss of function of fragile X mental retardation protein (FMRP). To date, most researchers have thought that FXS neural pathologies are primarily caused by extreme dendritic branching and spine formation. With this rationale, several researchers attempted to prune dendritic branches and reduce the number of spines in FXS animal models. We propose that increased dendritic arborization and spinogenesis in FXS are developed rather as secondary compensatory responses to counteract the compromised postsynaptic activity during uncontrollable metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD). When postsynaptic and electrical activities become dampened in FXS, dendritic trees can increase their sensitivity to brain-derived neurotrophic factor (BDNF) by using the molecular sensor called eukaryotic elongation factor 2 (eEF2) and taking advantage of the tight coupling of mGluR and BDNF-TrkB signaling pathways. Then, this activity-dependent elevation of the BDNF signaling can strategically alter dendritic morphologies to foster branching and develop spine structures in order to improve the postsynaptic response in FXS. Our model suggests a new therapeutic rationale for FXS: correcting the postsynaptic and electrical activity first, and then repairing structural abnormalities of dendrites. Then, it may be possible to successfully fix the dendritic morphologies without affecting the survival of neurons. Our theory may also be generalized to explain aberrant dendritic structures observed in other neurobehavioral diseases, such as tuberous sclerosis, Rett syndrome, schizophrenia, and channelopathies, which accompany high postsynaptic and electrical activity.

Intratympanic administration of methylprednisolone reduces impact of experimental intensive impulse noise trauma on hearing

CONCLUSION

The present findings demonstrated that intratympanic methylprednisolone (MP) reduces the impact of impulse noise trauma on hearing and in part preserves the hair cells from death 1 h after exposure to intensive impulse noise.

OBJECTIVE

To examine the treatment efficiency of intratympanic MP (IT-MP) with different methods of administration on cochlear injury induced by exposure to intensive impulse noise.

MATERIALS AND METHODS

Fifty-five guinea pigs were assigned into six groups and exposed to intensive impulse noise, i.e. 60 impulses at 165 dB SPL peak pressure with 0.5 ms duration and 2 s intervals. The auditory brainstem response (ABR) was used to examine the hearing thresholds. Cochlear morphology was examined to estimate the inner and outer hair cell loss induced by impulse noise exposure. MP was applied as a rescue agent via different modalities of administration.

RESULTS

The ABR threshold value of IT-MP1 or IT-MP4 groups significantly decreased at 4 weeks as compared with the IT-NS (IT 0.9% physiological saline) group. The ABR threshold value of the group that received intramuscular administration of MP (IM-MP) also decreased at 4 weeks as compared with the IT-NS group. Significant hair cell loss was observed at the region 40-50% from the apex in the present model. Within this region, the residual hair cell number in the IT-MP1 or IT-MP4 groups was significantly greater than that in the IT-NS group.

The basal level of intracellular calcium gates the activation of phosphoinositide 3-kinase-Akt signaling by brain-derived neurotrophic factor in cortical neurons

Brain-derived neurotrophic factor (BDNF) mediates survival and neuroplasticity through the activation of phosphoinositide 3-kinase-Akt pathway. Although previous studies suggested the roles of mitogen-activated protein kinase, phospholipase C-gamma-mediated intracellular calcium ([Ca2+]i) increase, and extracellular calcium influx in regulating Akt activation, the cellular mechanisms are largely unknown. We demonstrated that sub-nanomolar BDNF significantly induced Akt activation in developing cortical neurons. The TrkB-dependent Akt phosphorylation at S473 and T308 required only phosphoinositide 3-kinase, but not phospholipase C and mitogen-activated protein kinase activity. Blocking NMDA receptors, L-type voltage-gated calcium channels, and chelating extracellular calcium by EGTA failed to block BDNF-induced Akt phosphorylation. In contrast, chelating [Ca2+]i by 1,2-bis(o-aminophenoxy)ethane-N,N,N ',N '-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) abolished Akt phosphorylation. Interestingly, sub-nanomolar BDNF did not stimulate [Ca2+]i increase under our culture conditions. Together with that NMDA- and membrane depolarization-induced [Ca2+]i increase did not activate Akt, we conclude that the basal level of [Ca2+]i gates BDNF function. Furthermore, inhibiting calmodulin by W13 suppressed Akt phosphorylation. On the other hand, inhibition of protein phosphatase 1 by okadaic acid and tautomycin rescued Akt phosphorylation in BAPTA-AM and W13-treated neurons. We further demonstrated that the phosphorylation of phosphoinositide-dependent kinase-1 did not correlate with Akt phosphorylation at T308. Our results suggested novel roles of basal [Ca2+]i, rather than activity-induced calcium elevation, in BDNF-Akt signaling.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

Hormesis: a promising strategy to sustain endogenous neuronal survival pathways against neurodegenerative disorders

The brain developed adaptive mechanisms in the face of changing environments and stresses imposed on the nervous system. The addition of glutamate as the major excitatory amino acid neurotransmitter to the brain's complement of amino acids and peptides dictated a coordinated transcriptional and translational program to meet the demands of excitatory neurotransmission. One such program is the ability of neurons to sustain and maintain their survival given the nature of glutamate-mediated receptor activation. The unique development of endogenous neuronal pathways activated by glutamate receptors transformed neurons and allowed them to survive under conditions of high energy demands. These same endogenous survival pathways also mediate plastic responses to meet another demand of the brain, adaptation. An endogenous protein that plays a central role in glutamate receptor-mediated survival pathways is brain-derived neurotrophic factor (BDNF). Intermittent but frequent synaptic ionotropic glutamate receptor activation ensures neuronal survival through a BDNF autocrine loop. In sharp contrast, overactivation of ionotropic glutamate receptors leads to neuronal cell death. Thus, innovative strategies that induce endogenous neuronal survival pathways through low-level activation of ionotropic glutamate receptors or those that bypass receptor activation but upregulate endogenous survival pathways may not only prevent neurodegenerative disorders that involve glutamate as a final common pathway that kills neurons, but may also provide treatment alternatives critical for neurons to survive stressful conditions such as stroke, status epilepticus and hypoglycemic-induced neuronal cell death.

Hepatocyte growth factor-induced enhancement of dendritic branching is blocked by inhibitors of N-methyl-D-aspartate receptors and calcium/calmodulin-dependent kinases

Hepatocyte growth factor (HGF) and its receptor, Met, are clustered at excitatory synapses and can enhance N-methyl-D-aspartate (NMDA) receptor current and promote formation of neurites and dendrites. In this study, we examine the effects of HGF on dendritic arborization in mature cultures of dissociated hippocampal neurons. Exogenous HGF treatment caused a dose-dependent increase in total dendritic branch tip number, total dendritic branch length, and dendritic complexity in these neurons. NMDA receptor activity has been linked to changes in dendritic structure, so we tested the effects of HGF on the dendritic arbor in the presence of DL-2-amino-5-phosphonopentanoic acid (APV), an NMDA receptor inhibitor. APV blocked the HGF-induced enhancement of the dendritic arbor in a dose-dependent manner. Similarly, pretreatment of neurons with KN62, an inhibitor of calcium-dependent kinases, suppressed changes in dendritic branching induced by HGF. These results suggest that HGF initiates Ca2+-dependent processes, so we examined the effect of HGF on intracellular calcium levels and autophosphorylation of the calcium/calmodulin-dependent protein kinase II (CaMKII). HGF caused a persistent increase in fluorescence in clusters along dendrites of neurons preloaded with the Ca2+ indicator Fluo-4. HGF treatment also enhanced autophosphorylation of CaMKII. The increases in Fluo-4 fluorescence and autophosphorylation of CaMKII were blocked by pretreatment of neurons with APV. These results indicate that HGF stimulates Ca2+ influx into dendrites through the NMDA receptor and that this effect is necessary for the changes in dendritic morphology induced by HGF.

BDNF induces calcium elevations associated with IBDNF, a nonselective cationic current mediated by TRPC channels

Brain-derived neurotrophic factor (BDNF) has potent actions on hippocampal neurons, but the mechanisms that initiate its effects are poorly understood. We report here that localized BDNF application to apical dendrites of CA1 pyramidal neurons evoked transient elevations in intracellular Ca(2+) concentration, which are independent of membrane depolarization and activation of N-methyl-d-aspartate receptors (NMDAR). These Ca(2+) signals were always associated with I(BDNF), a slow and sustained nonselective cationic current mediated by transient receptor potential canonical (TRPC3) channels. BDNF-induced Ca(2+) elevations required functional Trk and inositol-tris-phosphate (IP(3)) receptors, full intracellular Ca(2+) stores as well as extracellular Ca(2+), suggesting the involvement of TRPC channels. Indeed, the TRPC channel inhibitor SKF-96365 prevented BDNF-induced Ca(2+) elevations and the associated I(BDNF). Thus TRPC channels emerge as novel mediators of BDNF-induced intracellular Ca(2+) elevations associated with sustained cationic membrane currents in hippocampal pyramidal neurons.

Insulin-like growth factor-I enhances the biological activity of brain-derived neurotrophic factor on cerebrocortical neurons

Insulin-like growth factor (IGF)-I and brain-derived neurotrophic factor (BDNF) act within the brain to enhance neuronal survival and plasticity. We extend these findings by showing that the presence of both neurotrophins is required to depress the rise in intracellular Ca2+ caused by glutamate in primary cultures of cerebrocortical neurons. IGF-I enhanced expression of BDNF receptors (Trk-B) and increased the ability of BDNF to induce ERK1/2 phosphorylation. This IGF-I-induced increase in BDNF responsiveness describes a new interaction between these peptides in the brain.