Transgenic mice overexpressing truncated trkB neurotrophin receptors in neurons show increased susceptibility to cortical injury after focal cerebral ischemia

Intranasal insulin intake and exercise improve memory function in amyloid-β induced Alzheimer's-like disease in rats: Involvement of hippocampal BDNF-TrkB receptor

The most prevalent type of dementia, Alzheimer's disease (AD), is a compelling illustration of the link between cognitive deficits and neurophysiological anomalies. We investigated the possible protective effect of intranasal insulin intake with exercise on amyloid-β (Aβ)-induced neuronal damage. The level of hippocampal brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) were analyzed to understand the involvement of BDNF-TrkB pathway in this modulation. In this study, we induced AD-like pathology by amyloid-β (Aβ) administration. Then, we examined the impact of a 4-week pretreatment of moderate treadmill exercise and intranasal intake of insulin on working and spatial memory in male Wistar rats. We also analyzed the mechanisms of improved memory and anxiety through changes in the protein level of BDNF and TrkB. Results showed that animals received Aβ had impaired working memory, increased anxiety which were accompanied by lower protein levels of BDNF and TrkB in the hippocampus. The exercise training and intranasal insulin improved working memory deficits, decreased anxiety, and increased BDNF, and TrkB levels in the hippocampus of animals received Aβ. Our finding of improved memory performance after intranasal intake of insulin and exercise may be of significance for the treatment of memory impairments and anxiety-like behavior in AD.

Brain-derived neurotrophic factor/tropomyosin receptor kinase B signaling in spinal muscular atrophy and amyotrophic lateral sclerosis

Tropomyosin receptor kinase B (TrkB) and its primary ligand brain-derived neurotrophic factor (BDNF) are expressed in the neuromuscular system, where they affect neuronal survival, differentiation, and functions. Changes in BDNF levels and full-length TrkB (TrkB-FL) signaling have been revealed in spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), two common forms of motor neuron diseases that are characterized by defective neuromuscular junctions in early disease stages and subsequently progressive muscle weakness. This review summarizes the current understanding of BDNF/TrkB-FL-related research in SMA and ALS, with an emphasis on their alterations in the neuromuscular system and possible BDNF/TrkB-FL-targeting therapeutic strategies. The limitations of current studies and future directions are also discussed, giving the hope of discovering novel and effective treatments.

TrkB/BDNF signaling pathway and its small molecular agonists in CNS injury

As one of the most prevalent neurotrophic factors in the central nervous system (CNS), brain-derived neurotrophic factor (BDNF) plays a significant role in CNS injury by binding to its specific receptor Tropomyosin-related kinase receptor B (TrkB). The BDNF/TrkB signaling pathway is crucial for neuronal survival, structural changes, and plasticity. BDNF acts as an axonal growth and extension factor, a pro-survival factor, and a synaptic modulator in the CNS. BDNF also plays an important role in the maintenance and plasticity of neuronal circuits. Several studies have demonstrated the importance of BDNF in the treatment and recovery of neurodegenerative and neurotraumatic disorders. By undertaking in-depth study on the mechanism of BDNF/TrkB function, important novel therapeutic strategies for treating neuropsychiatric disorders have been discovered. In this review, we discuss the expression patterns and mechanisms of the TrkB/BDNF signaling pathway in CNS damage and introduce several intriguing small molecule TrkB receptor agonists produced over the previous several decades.

Overexpression of the X-Linked Inhibitor of Apoptosis Protein (XIAP) in Neurons Improves Cell Survival and the Functional Outcome after Traumatic Spinal Cord Injury

Mechanical trauma to the spinal cord causes extensive neuronal death, contributing to the loss of sensory-motor and autonomic functions below the injury location. Apoptosis affects neurons after spinal cord injury (SCI) and is associated with increased caspase activity. Cleavage of X-linked inhibitor of apoptosis protein (XIAP) after SCI may contribute to this rise in caspase activity. Accordingly, we have shown that the elevation of XIAP resulted in increased neuronal survival after SCI and improved functional recovery. Therefore, we hypothesise that neuronal overexpression of XIAP can be neuroprotective after SCI with improved functional recovery. In line with this, studies of a transgenic mice with overexpression of XIAP in neurons revealed that higher levels of XIAP after spinal cord trauma favours neuronal survival, tissue preservation, and motor recovery after the spinal cord trauma. Using human SH-SY5Y cells overexpressing XIAP, we further showed that XIAP reduced caspase activity and apoptotic cell death after pro-apoptotic stimuli. In conclusion, this study shows that the levels of XIAP expression are an important factor for the outcome of spinal cord trauma and identifies XIAP as an important therapeutic target for alleviating the deleterious effects of SCI.

TrkB Truncated Isoform Receptors as Transducers and Determinants of BDNF Functions

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of secreted growth factors and binds with high affinity to the TrkB tyrosine kinase receptors. BDNF is a critical player in the development of the central (CNS) and peripheral (PNS) nervous system of vertebrates and its strong pro-survival function on neurons has attracted great interest as a potential therapeutic target for the management of neurodegenerative disorders such as Amyotrophic Lateral Sclerosis (ALS), Huntington, Parkinson’s and Alzheimer’s disease. The TrkB gene, in addition to the full-length receptor, encodes a number of isoforms, including some lacking the catalytic tyrosine kinase domain. Importantly, one of these truncated isoforms, namely TrkB.T1, is the most widely expressed TrkB receptor in the adult suggesting an important role in the regulation of BDNF signaling. Although some progress has been made, the mechanism of TrkB.T1 function is still largely unknown. Here we critically review the current knowledge on TrkB.T1 distribution and functions that may be helpful to our understanding of how it regulates and participates in BDNF signaling in normal physiological and pathological conditions.

Function and Mechanisms of Truncated BDNF Receptor TrkB.T1 in Neuropathic Pain

Brain-derived neurotrophic factor (BDNF), a major focus for regenerative therapeutics, has been lauded for its pro-survival characteristics and involvement in both development and recovery of function within the central nervous system (CNS). However, studies of tyrosine receptor kinase B (TrkB), a major receptor for BDNF, indicate that certain effects of the TrkB receptor in response to disease or injury may be maladaptive. More specifically, imbalance among TrkB receptor isoforms appears to contribute to aberrant signaling and hyperpathic pain. A truncated isoform of the receptor, TrkB.T1, lacks the intracellular kinase domain of the full length receptor and is up-regulated in multiple CNS injury models. Such up-regulation is associated with hyperpathic pain, and TrkB.T1 inhibition reduces neuropathic pain in various experimental paradigms. Deletion of TrkB.T1 also limits astrocyte changes in vitro, including proliferation, migration, and activation. Mechanistically, TrkB.T1 is believed to act through release of intracellular calcium in astrocytes, as well as through interactions with neurotrophins, leading to cell cycle activation. Together, these studies support a potential role for astrocytic TrkB.T1 in hyperpathic pain and suggest that targeted strategies directed at this receptor may have therapeutic potential.

The effects of rotenone on TH, BDNF and BDNF-related proteins in the brain and periphery: Relevance to early Parkinson's disease

Loss of dopaminergic neurons in the substantia nigra (SN) is one of the pathological hallmarks in Parkinson's disease (PD). This neuron loss is accompanied by reduced protein and activity levels of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. Reduced nigral brain-derived neurotrophic factor (BDNF) has been postulated to contribute to the loss of nigral dopaminergic neurons in PD by causing a lack of trophic support. Prior to this nigral cell loss many patients develop non-motor symptoms such as hyposmia, constipation and orthostatic hypotension. We investigated how TH, BDNF and BDNF related receptors are altered in the SN, olfactory bulb, adrenal glands and colon (which are known to be affected in PD) using rotenone-treated rats. Rotenone was administered to Sprague-Dawley rats at a dose of 2.75 mg/kg, 5 days/week for 4 weeks, via intraperitoneal injections. Rats underwent behavioural testing, and tissues were collected for western blot and ELISA analysis. This rotenone treatment induced reduced rears and distance travelled in the rearing and open field test, respectively but caused no impairments in forced movement (rotarod test). The SN had changes consistent with a pro-apoptotic state, such as increased proBDNF but no change in TH; whereas, the colon had significantly reduced TH and increased sortilin. Thus, our results indicate further investigation is warranted for this rotenone-dosing paradigm's capacity for reproducing the early stage of PD, as we observed impairments in voluntary movement and pathology in the colon without overt motor symptoms or nigral dopaminergic loss.

Inhibition of BDNF signaling in the paraventricular nucleus of the hypothalamus lowers acute stress-induced pressor responses

Brain-derived neurotrophic factor (BDNF) expression increases in the paraventricular nucleus of the hypothalamus (PVN) during stress, and our recent studies indicate that BDNF induces sympathoexcitatory and hypertensive responses when injected acutely or overexpressed chronically in the PVN. However, it remained to be investigated whether BDNF is involved in the mediation of stress-induced cardiovascular responses. Here we tested the hypothesis that inhibition of the high-affinity BDNF receptor TrkB in the PVN diminishes acute stress-induced cardiovascular responses. Male Sprague-Dawley rats were equipped with radiotelemetric transmitters for blood pressure measurement. BDNF-TrkB signaling was selectively inhibited by viral vector-mediated bilateral PVN overexpression of a dominant-negative truncated TrkB receptor (TrkB.T1, n = 7), while control animals ( n = 7) received green fluorescent protein (GFP)-expressing vector injections. Rats were subjected to acute water and restraint stress 3-4 wk after vector injections. We found that body weight, food intake, baseline mean arterial pressure (MAP), and heart rate were unaffected by TrkB.T1 overexpression. However, peak MAP increases were significantly reduced in the TrkB.T1 group compared with GFP both during water stress (GFP: 39 ± 2 mmHg, TrkB.T1: 27 ± 4 mmHg; P < 0.05) and restraint stress (GFP: 41 ± 3 mmHg, TrkB.T1: 34 ± 2 mmHg; P < 0.05). Average MAP elevations during the poststress period were also significantly reduced after both water and restraint stress in the TrkB.T1 group compared with GFP. In contrast, heart rate elevations to both stressors remained unaffected by TrkB.T1 overexpression. Our results demonstrate that activation of BDNF high-affinity TrkB receptors within the PVN is a major contributor to acute stress-induced blood pressure elevations. NEW & NOTEWORTHY We have shown that inhibition of the high-affinity brain-derived neurotrophic factor receptor TrkB in the paraventricular nucleus of the hypothalamus significantly reduces blood pressure elevations to acute stress without having a significant impact on resting blood pressure, body weight, and food intake.

Bidirectional coordination of actions and habits by TrkB in mice

Specific corticostriatal structures and circuits are important for flexibly shifting between goal-oriented versus habitual behaviors. For example, the orbitofrontal cortex and dorsomedial striatum are critical for goal-directed action, while the dorsolateral striatum supports habits. To determine the role of neurotrophin signaling, we overexpressed a truncated, inactive form of tropomyosin receptor kinase B [also called tyrosine receptor kinase B (TrkB)], the high-affinity receptor for Brain-derived Neurotrophic Factor, in the orbitofrontal cortex, dorsomedial striatum and dorsolateral striatum. Overexpression of truncated TrkB interfered with phosphorylation of full-length TrkB and ERK42/44, as expected. In the orbitofrontal cortex and dorsomedial striatum, truncated trkB overexpression also occluded the ability of mice to select actions based on the likelihood that they would be reinforced. Meanwhile, in the dorsolateral striatum, truncated trkB blocked the development of habits. Thus, corticostriatal TrkB-mediated plasticity appears necessary for balancing actions and habits.

Social Learning Requires Plasticity Enhanced by Fluoxetine Through Prefrontal Bdnf-TrkB Signaling to Limit Aggression Induced by Post-Weaning Social Isolation

Escalated or abnormal aggression induced by early adverse experiences is a growing issue of social concern and urges the development of effective treatment strategies. Here we report that synergistic interactions between psychosocial and biological factors specifically ameliorate escalated aggression induced by early adverse experiences. Rats reared in isolation from weaning until early adulthood showed abnormal forms of aggression and social deficits that were temporarily ameliorated by re-socialization, but aggression again escalated in a novel environment. We demonstrate that when re-socialization was combined with the antidepressant fluoxetine, which has been shown to reactivate juvenile-like state of plasticity, escalated aggression was greatly attenuated, while neither treatment alone was effective. Early isolation induced a permanent, re-socialization-resistant reduction in Bdnf expression in the amygdala and the infralimbic cortex. Only the combined treatment of fluoxetine and re-socialization was able to recover Bdnf expression via epigenetic regulation. Moreover, the behavior improvement after the combined treatment was dependent on TrkB activity. Combined treatment specifically strengthened the input from the ventral hippocampus to the mPFC, suggesting that this pathway is an important mediator of the beneficial behavioral effects of the combined psychosocial and pharmacological treatment of abnormal aggression. Our findings suggest that synergy between pharmacological induction of plasticity and psychosocial rehabilitation could enhance the efficacy of therapies for pathological aggression.

Tetramethylpyrazine Protects Neurons from Oxygen-Glucose Deprivation-Induced Death

Background

To explore the theoretical basis for protecting the brain from ischemic stroke with tetramethylpyrazine, we studied whether and how tetramethylpyrazine could protect neurons against the oxygen-glucose deprivation (OGD)-induced death and whether transient receptor potential cation channel, subfamily C, member 6 (TRPC6) was involved.

Material/Methods

Primary rat cortical neurons were cultured and an OGD model was established in the presence or absence of tetramethylpyrazine. Neuronal death was assessed by measuring the uptake of membrane-impermeable PI. Western blot analysis was used to determine the protein expressions of TRPC6 and caspase-3. The involvement of TRPC6 was tested via RNAi against TRPC6.

Results

OGD-induced neuronal death was decreased by tetramethylpyrazine in a concentration-dependent manner. The expression of TRPC6 protein was decreased by OGD. Furthermore, downregulating TRPC6 by RNA interfering mimicked the effect of OGD in neuronal death. Tetramethylpyrazine attenuated OGD-induced TRPC6 downregulation in a tetramethylpyrazine concentration-dependent manner. However, these effects of tetramethylpyrazine on attenuating OGD-induced neuronal death were abolished by TRPC6 RNAi.

Conclusions

Tetramethylpyrazine can protect neurons from oxygen-glucose deprivation-induced death, possibly via TRPC6.

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3β (GSK3β). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.

Opposing effects of APP/PS1 and TrkB.T1 genotypes on midbrain dopamine neurons and stimulated dopamine release in vivo

Brain derived neurotrophic factor (BDNF) signaling disturbances in Alzheimer׳s disease (AD) have been demonstrated. BDNF levels fall in AD, but the ratio between truncated and full-length BDNF receptors TrkB.T1 and TrkB.TK, respectively, increases in brains of AD patients and APPswe/PS1dE9 (APP/PS1) AD model mice. Dopaminergic (DAergic) system disturbances in AD and detrimental effects of BDNF signaling deficits on DAergic system functions have also been indicated. Against this, we investigated changes in nigrostriatal dopamine (DA) system in mice carrying APP/PS1 and/or TrkB.T1 transgenes, the latter line modeling the TrkB.T1/TK ratio change in AD. Employing in vivo voltammetry, we found normal short-term DA release in caudate-putamen of mice carrying APP/PS1 or TrkB.T1 transgenes but impaired capacity to recruit more DA upon prolonged stimulation. However, mice carrying both transgenes did not differ from wild-type controls. Immunohistochemistry revealed normal density of tyrosine hydroxylase positive axon terminals in caudate-putamen in all genotypes and intact presynaptic machinery for DA release and reuptake, as shown by unchanged levels of SNAP-25, α-synuclein and DA transporter. However, we observed increased DAergic neurons in substantia nigra of TrkB.T1 mice resulting in decreased tyrosine hydroxylase per neuron in TrkB.T1 mice. The finding of unchanged nigral DAergic neurons in APP/PS1 mice largely confirms earlier reports, but the unexpected increase in midbrain DA neurons in TrkB.T1 mice is a novel finding. We suggest that both APP/PS1 and TrkB.T1 genotypes disrupt DAergic signaling, but via separate mechanisms.

The Physiology of BDNF and Its Relationship with ADHD

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin in the central nervous system that plays a critical role in the physiological brain functions via its two independent receptors: tropomyosin-related kinase B (TrkB) and p75, especially in the neurodevelopment. Disrupting of BDNF and its downstream signals has been found in many neuropsychological diseases, including attention-deficit hyperactivity disorder (ADHD), a common mental disorder which is prevalent in childhood. Understanding the physiological functions of BDNF during neural development and its potential relationship with ADHD will help us to elucidate the possible mechanisms of ADHD and to develop therapeutic approaches for this disease. In this review, we summarized the important literatures for the physiological functions of BDNF in the neurodevelopment. We also performed an association study on the functional genetic variation of BDNF and ADHD by a case-control study in the Chinese mainland population and revealed the potential correlation between BDNF and ADHD which needs further research to confirm.

Potential therapeutic effects of neurotrophins for acute and chronic neurological diseases

The neurotrophins (NTs) nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), NT-3, and NT-4/5 are proteins that regulate cell proliferation, differentiation, and survival in both the developing and mature central nervous system (CNS) by binding to two receptor classes, Trk receptors and p75 NTR. Motivated by the broad growth- and survival-promoting effects of these proteins, numerous studies have attempted to use exogenous NTs to prevent the death of cells that are associated with neurological disease or promote the regeneration of severed axons caused by mechanical injury. Indeed, such neurotrophic effects have been repeatedly demonstrated in animal models of stroke, nerve injury, and neurodegenerative disease. However, limitations, including the short biological half-lives and poor blood-brain permeability of these proteins, prevent routine application from treating human disease. In this report, we reviewed evidence for the neuroprotective efficacy of NTs in animal models, highlighting outstanding technical challenges and discussing more recent attempts to harness the neuroprotective capacity of endogenous NTs using small molecule inducers and cell transplantation.

Expression of full-length and truncated trkB in human striatum and substantia nigra neurons: implications for Parkinson's disease

Brain derived neurotrophic factor (BDNF) is a potent mediator of cell survival and differentiation and can reverse neuronal injury associated with Parkinson's disease (PD). Tropomyosin receptor kinase B (trkB) is the high affinity receptor for BDNF. There are two major trkB isoforms, the full-length receptor (trkB.tk(+)) and the truncated receptor (trkB.t1), that mediate the diverse, region specific functions of BDNF. Both trkB isoforms are widely distributed throughout the brain, but the isoform specific distribution of trkB.t1 and trkB.tk(+) to human neurons is not well characterized. Therefore, we report the regional and neuronal distribution of trkB.tk(+) and trkB.t1 in the striatum and substantia nigra pars compacta (SNpc) of human autopsy tissues from control and PD cases. In both PD and control tissues, we found abundant, punctate distribution of trkB.tk(+) and trkB.t1 proteins in striatum and SNpc neurons. In PD, trkB.tk(+) is decreased in striatal neurites, increased in striatal somata, decreased in SNpc somata and dendrites, and increased in SNpc axons. TrkB.t1 is increased in striatal somata, decreased in striatal axons, and increased in SNpc distal dendrites. We believe changes in trkB isoform distribution and expression levels may be markers of pathology and affect the neuronal response to BDNF.

Differential regulation of BDNF, synaptic plasticity and sprouting in the hippocampal mossy fiber pathway of male and female rats

Many studies have described potent effects of BDNF, 17β-estradiol or androgen on hippocampal synapses and their plasticity. Far less information is available about the interactions between 17β-estradiol and BDNF in hippocampus, or interactions between androgen and BDNF in hippocampus. Here we review the regulation of BDNF in the mossy fiber pathway, a critical part of hippocampal circuitry. We discuss the emerging view that 17β-estradiol upregulates mossy fiber BDNF synthesis in the adult female rat, while testosterone exerts a tonic suppression of mossy fiber BDNF levels in the adult male rat. The consequences are interesting to consider: in females, increased excitability associated with high levels of BDNF in mossy fibers could improve normal functions of area CA3, such as the ability to perform pattern completion. However, memory retrieval may lead to anxiety if stressful events are recalled. Therefore, the actions of 17β-estradiol on the mossy fiber pathway in females may provide a potential explanation for the greater incidence of anxiety-related disorders and post-traumatic stress syndrome (PTSD) in women relative to men. In males, suppression of BDNF-dependent plasticity in the mossy fibers may be protective, but at the 'price' of reduced synaptic plasticity in CA3. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Brain-derived neurotrophic factor promotes vasculature-associated migration of neuronal precursors toward the ischemic striatum

Stroke induces the recruitment of neuronal precursors from the subventricular zone (SVZ) into the ischemic striatum. In injured areas, de-routed neuroblasts use blood vessels as a physical scaffold to their migration, in a process that resembles the constitutive migration seen in the rostral migratory stream (RMS). The molecular mechanism underlying injury-induced vasculature-mediated migration of neuroblasts in the post-stroke striatum remains, however, elusive. Using adult mice we now demonstrate that endothelial cells in the ischemic striatum produce brain-derived neurotrophic factor (BDNF), a neurotrophin that promotes the vasculature-mediated migration of neuronal precursors in the RMS, and that recruited neuroblasts maintain expression of p75NTR, a low-affinity receptor for BDNF. Reactive astrocytes, which are widespread throughout the damaged area, ensheath blood vessels and express TrkB, a high-affinity receptor for BDNF. Despite the absence of BDNF mRNA, we observed strong BDNF immunolabeling in astrocytes, suggesting that these glial cells trap extracellular BDNF. Importantly, this pattern of expression is reminiscent of the adult RMS, where TrkB-expressing astrocytes bind and sequester vasculature-derived BDNF, leading to the entry of migrating cells into the stationary phase. Real-time imaging of cell migration in acute brain slices revealed a direct role for BDNF in promoting the migration of neuroblasts to ischemic areas. We also demonstrated that cells migrating in the ischemic striatum display higher exploratory behavior and longer stationary periods than cells migrating in the RMS. Our findings suggest that the mechanisms involved in the injury-induced vasculature-mediated migration of neuroblasts recapitulate, at least partially, those observed during constitutive migration in the RMS.

Deletion of the BDNF truncated receptor TrkB.T1 delays disease onset in a mouse model of amyotrophic lateral sclerosis

Brain Derived Neurotrophic Factor (BDNF) exerts strong pro-survival effects on developing and injured motoneurons. However, in clinical trials, BDNF has failed to benefit patients with amyotrophic lateral sclerosis (ALS). To date, the cause of this failure remains unclear. Motoneurons express the TrkB kinase receptor but also high levels of the truncated TrkB.T1 receptor isoform. Thus, we investigated whether the presence of this receptor may affect the response of diseased motoneurons to endogenous BDNF. We deleted TrkB.T1 in the hSOD1^G93A ALS mouse model and evaluated the impact of this mutation on motoneuron death, muscle weakness and disease progression. We found that TrkB.T1 deletion significantly slowed the onset of motor neuron degeneration. Moreover, it delayed the development of muscle weakness by 33 days. Although the life span of the animals was not affected we observed an overall improvement in the neurological score at the late stage of the disease. To investigate the effectiveness of strategies aimed at bypassing the TrkB.T1 limit to BDNF signaling we treated SOD1 mutant mice with the adenosine A2A receptor agonist CGS21680, which can activate motoneuron TrkB receptor signaling independent of neurotrophins. We found that CGS21680 treatment slowed the onset of motor neuron degeneration and muscle weakness similarly to TrkB.T1 removal. Together, our data provide evidence that endogenous TrkB.T1 limits motoneuron responsiveness to BDNF in vivo and suggest that new strategies such as Trk receptor transactivation may be used for therapeutic intervention in ALS or other neurodegenerative disorders.

Excitotoxicity downregulates TrkB.FL signaling and upregulates the neuroprotective truncated TrkB receptors in cultured hippocampal and striatal neurons

Brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival through activation of TrkB receptors. The trkB gene encodes a full-length receptor tyrosine kinase (TrkB.FL) and its truncated (T1/T2) isoforms. We investigated the changes in TrkB protein levels and signaling activity under excitotoxic conditions, which are characteristic of brain ischemia, traumatic brain injury, and neurodegenerative disorders. Excitotoxic stimulation of cultured rat hippocampal or striatal neurons downregulated TrkB.FL and upregulated a truncated form of the receptor (TrkB.T). Downregulation of TrkB.FL was mediated by calpains, whereas the increase in TrkB.T protein levels required transcription and translation activities. Downregulation of TrkB.FL receptors in hippocampal neurons correlated with a decrease in BDNF-induced activation of the Ras/ERK and PLCγ pathways. However, calpain inhibition, which prevents TrkB.FL degradation, did not preclude the decrease in signaling activity of these receptors. On the other hand, incubation with anisomycin, to prevent the upregulation of TrkB.T, protected to a large extent the TrkB.FL signaling activity, suggesting that truncated receptors may act as dominant-negatives. The upregulation of TrkB.T under excitotoxic conditions was correlated with an increase in BDNF-induced inhibition of RhoA, a mediator of excitotoxic neuronal death. BDNF fully protected hippocampal neurons transduced with TrkB.T when present during excitotoxic stimulation with glutamate, in contrast with the partial protection observed in cells overexpressing TrkB.FL or expressing GFP. These results indicate that BDNF protects hippocampal neurons by two distinct mechanisms: through the neurotrophic effects of TrkB.FL receptors and by activation of TrkB.T receptors coupled to inhibition of the excitotoxic signaling.

Truncated TrkB: beyond a dominant negative receptor

BDNF activates trkB receptors to regulate neuronal survival, differentiation, and proliferation. Mutations in the BDNF gene, altered BDNF expression, and altered trkB expression are associated with degenerative and psychiatric disorders. The full-length trkB receptor (trkB.tk(+)) undergoes autophosphorylation to activate intracellular signaling pathways. The truncated trkB receptor (trkB.t1) is abundantly expressed in the brain but lacks the catalytic tyrosine kinase domain. TrkB.t1 is a dominant-negative receptor that inhibits trkB.tk(+) signaling. While this is an important function of trkB.t1, it is only one of its many functions. TrkB.t1 sequesters and translocate BDNF, induces filopodia and neurite outgrowth, stimulates intracellular signaling cascades, regulates Rho GTPase signaling, and modifies cytoskeletal structures. TrkB.t1 is an active signaling molecule with regulatory effects on neurons and astrocytes.

Impaired TrkB receptor signaling contributes to memory impairment in APP/PS1 mice

Brain-derived neurotrophic factor (BDNF) plays an important role in neuronal plasticity, learning, and memory. Levels of BDNF and its main receptor TrkB (TrkB.TK) have been reported to be decreased while the levels of the truncated TrkB (TrkB.T1) are increased in Alzheimer's disease. We show here that incubation with amyloid-β increased TrkB.T1 receptor levels and decreased TrkB.TK levels in primary neurons. In vivo, APPswe/PS1dE9 transgenic mice (APdE9) showed an age-dependent relative increase in cortical but not hippocampal TrkB.T1 receptor levels compared with TrkB.TK. To investigate the role of TrkB isoforms in Alzheimer's disease, we crossed AP mice with mice overexpressing the truncated TrkB.T1 receptor (T1) or the full-length TrkB.TK isoform. Overexpression of TrkB.T1 in APdE9 mice exacerbated their spatial memory impairment while the overexpression of TrkB.TK alleviated it. These data suggest that amyloid-β changes the ratio between TrkB isoforms in favor of the dominant-negative TrkB.T1 isoform both in vitro and in vivo and supports the role of BDNF signaling through TrkB in the pathophysiology and cognitive deficits of Alzheimer's disease.

Alternative splicing in oncogenic kinases: from physiological functions to cancer

Among the 518 protein kinases encoded by the human kinome, several of them act as oncoproteins in human cancers. Like other eukaryotic genes, oncogenes encoding protein kinases are frequently subjected to alternative splicing in coding as well as noncoding sequences. In the present paper, we will illustrate how alternative splicing can significantly impact on the physiological functions of oncogenic protein kinases, as demonstrated by mouse genetic model studies. This includes examples of membrane-bound tyrosine kinases receptors (FGFR2, Ret, TrkB, ErbB4, and VEGFR) as well as cytosolic protein kinases (B-Raf). We will further discuss how regular alternative splicing events of these kinases are in some instances implicated in oncogenic processes during tumor progression (FGFR, TrkB, ErbB2, Abl, and AuroraA). Finally, we will present typical examples of aberrant splicing responsible for the deregulation of oncogenic kinases activity in cancers (AuroraB, Jak2, Kit, Met, and Ron).

Increase in BDNF-mediated TrkB signaling promotes epileptogenesis in a mouse model of mesial temporal lobe epilepsy

Mesio-temporal lobe epilepsy (MTLE), the most common drug-resistant epilepsy syndrome, is characterized by the recurrence of spontaneous focal seizures after a latent period that follows, in most patients, an initial insult during early childhood. Many of the mechanisms that have been associated with the pathophysiology of MTLE are known to be regulated by brain-derived neurotrophic factor (BDNF) in the healthy brain and an excess of this neurotrophin could therefore play a critical role in MTLE development. However, such a function remains controversial as other studies revealed that BDNF could, on the contrary, exert protective effects regarding epilepsy development. In the present study, we further addressed the role of increased BDNF/TrkB signaling on the progressive development of hippocampal seizures in the mouse model of MTLE obtained by intrahippocampal injection of kainate. We show that hippocampal seizures progressively developed in the injected hippocampus during the first two weeks following kainate treatment, within the same time-frame as a long-lasting and significant increase of BDNF expression in dentate granule cells. To determine whether such a BDNF increase could influence hippocampal epileptogenesis via its TrkB receptors, we examined the consequences of (i) increased or (ii) decreased TrkB signaling on epileptogenesis, in transgenic mice overexpressing the (i) TrkB full-length or (ii) truncated TrkB-T1 receptors of BDNF. Epileptogenesis was significantly facilitated in mice with increased TrkB signaling but delayed in mutants with reduced TrkB signaling. In contrast, TrkB signaling did not influence granule cell dispersion, an important feature of this mouse model which is also observed in most MTLE patients. These results suggest that an increase in TrkB signaling, mediated by a long-lasting BDNF overexpression in the hippocampus, promotes epileptogenesis in MTLE.

Neurotrophin receptors TrkB.T1 and p75NTR cooperate in modulating both functional and structural plasticity in mature hippocampal neurons

Tropomyosin-related kinase (Trk) receptors modulate neuronal structure and function both during development and in the mature nervous system. Interestingly, TrkB and TrkC are expressed as full-length and as truncated splice variants. The cellular function of the kinase-lacking isoforms remains so far unclear. We investigated the role of the truncated receptor TrkB.T1 in the hippocampus of transgenic mice overexpressing this splice variant by analyzing both neuronal structure and function. We observed an impairment in activity-dependent synaptic plasticity as indicated by deficits in long-term potentiation and long-term depression in acute hippocampal slices of transgenic TrkB.T1 mice. In addition, dendritic complexity and spine density were significantly altered in TrkB.T1-overexpressing CA1 neurons. We found that the effect of TrkB.T1 overexpression differs between subgroups of CA1 neurons. Remarkably, overexpression of p75(NTR) and its activation by chemical induction of long-term depression in slice cultures rescued the TrkB.T1-dependent morphological alterations specifically in one of the two subgroups observed. These findings suggest that the TrkB.T1 and p75(NTR) receptor signaling systems might be cross-linked. Our findings demonstrate that TrkB.T1 regulates the function and the structure of mature pyramidal neurons. In addition, we showed that the ratio of expression levels of p75(NTR) and TrkB.T1 plays an important role in modulating dendritic architecture and synaptic plasticity in the adult rodent hippocampus, and, indeed, that the endogenous expression patterns of both receptors change reciprocally over time. We therefore propose a new function of TrkB.T1 as being dominant-negative to p75(NTR).

Inhibition of TRPC6 degradation suppresses ischemic brain damage in rats

Brain injury after focal cerebral ischemia, the most common cause of stroke, develops from a series of pathological processes, including excitotoxicity, inflammation, and apoptosis. While NMDA receptors have been implicated in excitotoxicity, attempts to prevent ischemic brain damage by blocking NMDA receptors have been disappointing. Disruption of neuroprotective pathways may be another avenue responsible for ischemic damage, and thus preservation of neuronal survival may be important for prevention of ischemic brain injury. Here, we report that suppression of proteolytic degradation of transient receptor potential canonical 6 (TRPC6) prevented ischemic neuronal cell death in a rat model of stroke. The TRPC6 protein level in neurons was greatly reduced in ischemia via NMDA receptor-dependent calpain proteolysis of the N-terminal domain of TRPC6 at Lys¹⁶. This downregulation was specific for TRPC6 and preceded neuronal death. In a rat model of ischemia, activating TRPC6 prevented neuronal death, while blocking TRPC6 increased sensitivity to ischemia. A fusion peptide derived from the calpain cleavage site in TRPC6 inhibited degradation of TRPC6, reduced infarct size, and improved behavioral performance measures via the cAMP response element-binding protein (CREB) signaling pathway. Thus, TRPC6 proteolysis contributed to ischemic neuronal cell death, and suppression of its degradation preserved neuronal survival and prevented ischemic brain damage.

Dual response of BDNF to sublethal concentrations of beta-amyloid peptides in cultured cortical neurons

Beta-amyloid (Abeta) deposition is one important pathological hallmark in Alzheimer's disease (AD). However, low levels of Abeta may modify critical endogenous protection systems before neurodegeneration occurs. We examined the time-course effect of sublethal concentrations of Abeta on total BDNF (panBDNF), BDNF transcripts (I, II, IV and VI), trkB.FL, trkB.T1 and p75(NGFR) mRNA expression in cultured cortical neurons. We have shown that Abeta exhibited a dual response on BDNF mRNA, i.e. an increase at short times (3-5 h) and a dramatic decrease at longer times (24 or 48 h). The early increase in BDNF expression seems to be driven by increased expression of transcripts I and IV. The BDNF drop was specific since did not occur for other mRNAs examined. The BDNF protein content showed a similar profile but did not follow the dramatic reduction as its encoding mRNA. These observations may help to explain cognitive deficits observed at initial stages of AD.

Endogenous truncated TrkB.T1 receptor regulates neuronal complexity and TrkB kinase receptor function in vivo

Pathological or in vitro overexpression of the truncated TrkB (TrkB.T1) receptor inhibits signaling through the full-length TrkB (TrkB.FL) tyrosine kinase receptor. However, to date, the role of endogenous TrkB.T1 is still unknown. By studying mice lacking the truncated TrkB.T1 isoform but retaining normal spatiotemporal expression of TrkB.FL, we have analyzed TrkB.T1-specific physiological functions and its effect on endogenous TrkB kinase signaling in vivo. We found that TrkB.T1-deficient mice develop normally but show increased anxiety in association with morphological abnormalities in the length and complexity of neurites of neurons in the basolateral amygdala. However, no behavioral abnormalities were detected in hippocampal-dependent memory tasks, which correlated with lack of any obvious hippocampal morphological deficits or alterations in basal synaptic transmission and long-term potentiation. In vivo reduction of TrkB signaling by removal of one BDNF allele could be partially rescued by TrkB.T1 deletion, which was revealed by an amelioration of the enhanced aggression and weight gain associated with BDNF haploinsufficiency. Our results suggest that, at the physiological level, TrkB.T1 receptors are important regulators of TrkB.FL signaling in vivo. Moreover, TrkB.T1 selectively affects dendrite complexity of certain neuronal populations.

Pharmacologically diverse antidepressants rapidly activate brain-derived neurotrophic factor receptor TrkB and induce phospholipase-Cgamma signaling pathways in mouse brain

Previous studies suggest that brain-derived neurotrophic factor and its receptor TrkB are critically involved in the therapeutic actions of antidepressant drugs. We have previously shown that the antidepressants imipramine and fluoxetine produce a rapid autophosphorylation of TrkB in the rodent brain. In the present study, we have further examined the biochemical and functional characteristics of antidepressant-induced TrkB activation in vivo. We show that all the antidepressants examined, including inhibitors of monoamine transporters and metabolism, activate TrkB rapidly in the rodent anterior cingulate cortex and hippocampus. Furthermore, the results indicate that acute and long-term antidepressant treatments induce TrkB-mediated activation of phospholipase-Cgamma1 (PLCgamma1) and increase the phosphorylation of cAMP-related element binding protein, a major transcription factor mediating neuronal plasticity. In contrast, we have not observed any modulation of the phosphorylation of TrkB Shc binding site, phosphorylation of mitogen-activated protein kinase or AKT by antidepressants. We also show that in the forced swim test, the behavioral effects of specific serotonergic antidepressant citalopram, but not those of the specific noradrenergic antidepressant reboxetine, are crucially dependent on TrkB signaling. Finally, brain monoamines seem to be critical mediators of antidepressant-induced TrkB activation, as antidepressants reboxetine and citalopram do not produce TrkB activation in the brains of serotonin- or norepinephrine-depleted mice. In conclusion, our data suggest that rapid activation of the TrkB neurotrophin receptor and PLCgamma1 signaling is a common mechanism for all antidepressant drugs.

Spatial memory training modifies the expression of brain-derived neurotrophic factor tyrosine kinase receptors in young and aged rats

Aging leads to alterations in the function of the hippocampus, a brain structure largely involved in learning processes. This study aimed at examining the basal levels and the impact of a learning-associated task on brain-derived neurotrophic factor (BDNF), on BDNF full-length catalytic receptor (TrkB.FL) and on the truncated forms (TrkB.T1 and TrkB.T2) receptor expression (mRNA and protein) in the hippocampus of young (2-month-old) and aged (24-month-old) Wistar rats. Spatial memory was evaluated using a water-maze procedure involving visible and invisible platform location learning. Aged rats showed higher latencies during the first two training days but rapidly exhibited learning performances similar to patterns observed with young rats. Real-time PCR measurements showed that aged rats had significantly higher levels of trkB.FL mRNAs than young rats under basal conditions. In situ hybridization analysis indicated that the highest level of trkB.FL mRNA (mRNA encoding for TrkB.FL receptor) was noted in the dentate gyrus, and in the CA2 and CA3 hippocampal layers. In contrast, there was no marked difference in trkB.T1 signal in any hippocampal region. Training induced a significant reduction in trkB.FL mRNA levels solely in aged rats. In contrast, in young and aged rats, trkB.T2 mRNA levels were significantly increased after training. Measurements of proteins revealed that learning significantly increased TrkB.FL content in aged rats. Untrained aged rats presented higher levels of BDNF and brain-derived neurotrophic factor precursor (proBDNF) proteins than young rats. Training strongly increased precursor BDNF metabolism in young and aged rats, resulting in increased levels of proBDNF in the two groups but in old rats the mature BDNF level did not change. This study shows that Wistar rats present age-related differences in the levels of BDNF and TrkB isoforms and that spatial learning differentially modifies some of these parameters in the hippocampus.

Neurotrophins in the dentate gyrus

Since the discovery of nerve growth factor (NGF) in the 1950s and brain-derived neurotrophic factor (BDNF) in the 1980s, a great deal of evidence has mounted for the roles of neurotrophins (NGF; BDNF; neurotrophin-3, NT-3; and neurotrophin-4/5, NT-4/5) in development, physiology, and pathology. BDNF in particular has important roles in neural development and cell survival, as well as appearing essential to molecular mechanisms of synaptic plasticity and larger scale structural rearrangements of axons and dendrites. Basic activity-related changes in the central nervous system (CNS) are thought to depend on BDNF modulation of synaptic transmission. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders. In this chapter, I review neurotrophin structure, signal transduction mechanisms, localization and regulation within the nervous system, and various potential roles in disease. Modulation of neurotrophin action holds significant potential for novel therapies for a variety of neurological and psychiatric disorders.

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: complexity of steroid hormone-growth factor interactions in the adult CNS

In the CNS, there are widespread and diverse interactions between growth factors and estrogen. Here we examine the interactions of estrogen and brain-derived neurotrophic factor (BDNF), two molecules that have historically been studied separately, despite the fact that they seem to share common targets, effects, and mechanisms of action. The demonstration of an estrogen-sensitive response element on the BDNF gene provided an impetus to explore a direct relationship between estrogen and BDNF, and predicted that the effects of estrogen, at least in part, might be due to the induction of BDNF. This hypothesis is discussed with respect to the hippocampus, where substantial evidence has accumulated in favor of it, but alternate hypotheses are also raised. It is suggested that some of the interactions between estrogen and BDNF, as well as the controversies and implications associated with their respective actions, may be best appreciated in light of the ability of BDNF to induce neuropeptide Y (NPY) synthesis in hippocampal neurons. Taken together, this tri-molecular cascade, estrogen-BDNF-NPY, may be important in understanding the hormonal regulation of hippocampal function. It may also be relevant to other regions of the CNS where estrogen is known to exert profound effects, such as amygdala and hypothalamus; and may provide greater insight into neurological disorders and psychiatric illness, including Alzheimer's disease, depression and epilepsy.

Overexpression of a truncated TrkB isoform increases the proliferation of neural progenitors

The truncated isoform of TrkB, TrkB.T1, has been shown to be expressed in the neurogenic region of rodent brain. TrkB.T1 lacks tyrosine kinase activity and it may modify the action of the full-length TrkB. We show here that the full-length TrkB and TrkB.T1 are expressed at the same relative expression levels in mouse neural progenitor cell cultures. The number of neurosphere-forming progenitors was reduced and apoptosis increased in neurospheres generated from mice overexpressing TrkB.T1 when compared with wild-type neurospheres. The proliferation of the transgenic neural progenitors was increased, as indicated by the larger average diameter of spheres (140% of control), the increased cell growth in an MTT assay (137% of control) and the faster rate of 3H-thymidine incorporation (128% of control) in the transgenic cell cultures than in controls. The proliferation of neural progenitors was also increased after lentivirus-mediated TrkB.T1 overexpression. A significant increase in 3H-thymidine incorporation (119% of control) and the average diameter of spheres (112% of control) in the TrkB.T1-transduced neurospheres compared with neurospheres transduced with the control vectors confirmed the role of TrkB.T1 in proliferation of neural progenitor. When induced to differentiate, progenitors overexpressing TrkB.T1 generated two to three times more neurons than did wild-type cells. The increase in the number of neurons correlated with an increase in the number of apoptotic cells (two-fold) at these time points. The data indicate that changes in the relative expression levels of different TrkB isoforms influence the replicative capacity of neural progenitors.

Identification of Brain Cell Death Associated Proteins in Human Post-mortem Cerebrospinal Fluid

Following any form of brain insult, proteins are released from damaged tissues into the cerebrospinal fluid (CSF). This body fluid is therefore an ideal sample to use in the search for biomarkers of neurodegenerative disorders and brain damage. In this study, we used human post-mortem CSF as a model of massive brain injury and cell death for the identification of such protein markers. Pooled post-mortem CSF samples were analyzed using a protocol that combined immunoaffinity depletion of abundant CSF proteins, off-gel electrophoresis, SDS-PAGE and protein identification by LC-MS/MS. A total of 299 proteins were identified, of which 172 proteins were not previously described to be present in CSF. Of these 172 proteins, more than 75% have been described as intracellular proteins suggesting that they were released from damaged cells. Immunoblots of a number of proteins were performed on individual post-mortem CSF samples and confirmed elevated concentrations in post-mortem CSF compared to ante-mortem CSF. Interestingly, among the proteins specifically identified in the post-mortem CSF, several have been previously described as biochemical markers of brain damage.

Effect of aging on the expression of BDNF and TrkB isoforms in rat pituitary

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity in adult rat brain and its effects are mediated through TrkB receptors. BDNF and its receptors are also localized in the pituitary, but their expressions throughout the rat lifespan are poorly known. Here we analyzed levels of BDNF and the different subtypes of TrkB receptors (mRNA and proteins) in the rat pituitary at different stages of life. BDNF immunoreactivity was expressed in folliculo-stellate cells from the anterior pituitary and in the intermediate lobe. TrkB.FL and TrkB.T1 receptors were strongly and essentially expressed in the intermediate lobe similar to the alpha-MSH localization pattern. These receptors begun decreasing at middle-age but TrkB.T2 was not detected in the pituitary at any age. Finally, in vitro alpha-MSH release from the intermediate lobe was correlated with the receptor content throughout the lifespan. The present results demonstrate the presence of BDNF in folliculo-stellate cells and indicated that receptors, rather than BDNF itself, are impaired with aging. These changes can contribute to explain age-dependent endocrine changes.

Secretion of brain-derived neurotrophic factor from brain microvascular endothelial cells

The cerebral microvasculature has recently been identified as a source of factors that can influence the generation and survival of neurons, including brain-derived neurotrophic factor (BDNF). However, relatively little is known about signals that regulate secretion of endothelial cell derived BDNF. To approach this issue the present study examined BDNF secretion from brain endothelial cells in response to reduced oxygen availability (hypoxia), using the mouse brain microvascular endothelial cell line, bEnd.3. We found that exposure of bEnd.3 cells to either sustained or intermittent hypoxia (IH) stimulates BDNF expression and release and that IH is the more potent stimulus. IH-induced BDNF release can be partially inhibited by either N-acetyl-L-cysteine, a scavenger of reactive oxygen species, or by the stable superoxide dismutase mimetic manganese(III)tetrakis1-methyl-4-pyridylporphyrin, indicating that oxyradical formation contributes to enhanced secretion of BDNF. In addition, we found that IH-induced BDNF release requires Ca2+ mobilization from internal stores through ryanodine- and inositol (1,4,5-triphosphate) IP3 receptors and is completely blocked by SKF 96365, a nonselective inhibitor of transient receptor potential (TRP) channels. These data demonstrate that bEnd.3 cells respond to oxidative stress by increasing BDNF secretion and, in addition, highlight TRP channels as potential therapeutic targets for enhancing BDNF availability from the cerebral microvasculature.

Secretion of brain-derived neurotrophic factor from PC12 cells in response to oxidative stress requires autocrine dopamine signaling

Expression of brain-derived neurotrophic factor (BDNF) is sensitive to changes in oxygen availability, suggesting that BDNF may be involved in adaptive responses to oxidative stress. However, it is unknown whether or not oxidative stress actually increases availability of BDNF by stimulating BDNF secretion. To approach this issue we examined BDNF release from PC12 cells, a well-established model of neurosecretion, in response to hypoxic stimuli. BDNF secretion from neuronally differentiated PC12 cells was strongly stimulated by exposure to intermittent hypoxia (IH). This response was inhibited by N-acetyl-l-cysteine, a potent scavenger of reactive oxygen species (ROS) and mimicked by exogenous ROS. IH-induced BDNF release requires activation of tetrodotoxin sensitive Na+ channels and Ca2+ influx through N- and L-type channels, as well as mobilization of internal Ca2+ stores. These results demonstrate that oxidative stress can stimulate BDNF release and that underlying mechanisms are similar to those previously described for activity-dependent BDNF secretion from neurons. Surprisingly, we also found that IH-induced secretion of BDNF was blocked by dopamine D2 receptor antagonists or by inhibition of dopamine synthesis with alpha-methyl-p-tyrosine. These data indicate that oxidative stress can stimulate BDNF release through an autocrine or paracrine loop that requires dopamine receptor activation.

Brain-derived neurotrophic factor and antidepressant drugs have different but coordinated effects on neuronal turnover, proliferation, and survival in the adult dentate gyrus

Antidepressants increase proliferation of neuronal progenitor cells and expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. We investigated the role of BDNF signaling in antidepressant-induced neurogenesis by using transgenic mice with either reduced BDNF levels (BDNF+/-) or impaired trkB activation (trkB.T1-overexpressing mice). In both transgenic strains, chronic (21 d) imipramine treatment increased the number of bromodeoxyuridine (BrdU)-positive cells to degree similar to that seen in wild-type mice 24 h after BrdU administration, although the basal proliferation rate was increased in both transgenic strains. Three weeks after BrdU administration and the last antidepressant injection, the amount of newborn (BrdU- or TUC-4-positive) cells was significantly reduced in both BDNF+/- and trkB.T1-overexpressing mice, which suggests that normal BDNF signaling is required for the long-term survival of newborn hippocampal neurons. Moreover, the antidepressant-induced increase in the surviving BrdU-positive neurons seen in wild-type mice 3 weeks after treatment was essentially lost in mice with reduced BDNF signaling. Furthermore, we observed that chronic treatment with imipramine or fluoxetine produced a temporally similar increase in both BrdU-positive and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end-labeled neurons in the dentate gyrus, indicating that these drugs simultaneously increase both neurogenesis and neuronal elimination. These data suggest that antidepressants increase turnover of hippocampal neurons rather than neurogenesis per se and that BDNF signaling is required for the long-term survival of newborn neurons in mouse hippocampus.

Age-related changes in brain-derived neurotrophic factor and tyrosine kinase receptor isoforms in the hippocampus and hypothalamus in male rats

A large amount of aging individuals show diminished cognitive and endocrine capabilities. The main brain areas involved in these changes are the hippocampus and hypothalamus, two regions possessing high plasticity and implicated in cognitive and endocrine functions, respectively. Among neurotrophins (considered as genuine molecular mediators of synaptic plasticity), brain-derived neurotrophic factor (BDNF) exhibits in adult rats, the highest concentrations in the hippocampus and hypothalamus. Most of neuronal effects of BDNF are mediated through high-affinity cell surface BDNF tyrosine kinase receptors (TrkB). Different TrkB isoforms are issued by alternative splicing of mRNA encoding for TrkB (trkB mRNA) generating at least three different TrkB receptors with different signaling capabilities. The goal of this study was to examine simultaneously the expression (mRNAs and proteins) of BDNF and its three specific receptors, in the hippocampus and hypothalamus throughout lifespan in rats. We observed that BDNF essentially increased during the first 2 postnatal weeks in the hippocampus and hypothalamus, with no close correlation to its mRNA levels. In these regions, mRNA encoding for BDNF full-length catalytic receptor (trkB.FL mRNA) showed no important changes throughout life but of the mRNA truncated forms of TrkB receptors (trkB.T1 mRNA and trkB.T2 mRNA) trkB.T1 mRNA strongly increased after birth, then remaining stable during aging. trkB.T2 mRNA gradually decreased from 1 postnatal week becoming undetectable in the hippocampus in old-rats. Proteins issued from these mRNAs showed substantial quantitative modifications with aging. From 2 months old, the BDNF full-length catalytic receptor (TrkB.FL) gradually and significantly decreased in the hippocampus and the hypothalamus. Of the truncated forms of TrkB receptors (TrkB.T1 and TrkB.T2) TrkB.T1, which is essentially localized in glial cells, significantly increased from the first postnatal week in the hippocampus and in the hypothalamus, remaining stable during aging but reduced in old rats. TrkB.T2 which similarly to TrkB.FL has a neuronal localization also gradually decreased in the hippocampus and in the hypothalamus throughout lifespan. These reductions were significant at 21 and 30 days old, respectively. All the changes reported here could contribute to the reduced plasticity of these regions observed in old rats.

Translational downregulation of the noncatalytic growth factor receptor TrkB.T1 by ischemic preconditioning of primary neurons

Short episodes of ischemia can protect neuronal cells and tissue against a subsequent lethal ischemia by a phenomenon called ischemic preconditioning. The development of this tolerance depends on protein synthesis and takes at least 1 day. It therefore seems reasonable that preconditioning leads to upregulation and translation of protective genes or posttranslational modification of pro- or antiapoptotic proteins. We recently used suppression subtractive hybridization to identify transcripts upregulated in rat primary neuronal cultures preconditioned by oxygen glucose deprivation. In this contribution, we describe the previously unknown 7-kb full-length sequence of an upregulated expressed sequence tag and show that it constitutes the 3' end of the large untranslated region of the noncatalytic "truncated" growth factor receptor TrkB.T1. TrkB.T1 is expressed most prominently in the adult brain and its mRNA was found to be 2.1-fold upregulated by ischemic preconditioning. At the protein level, however, TrkB.T1 was clearly downregulated, possibly by increased degradation in preconditioned cultures. TrKB.T1 can act as a dominant-negative inhibitor of its catalytic counterpart TrkB, which is the receptor for brain-derived neurotrophic factor (BDNF), a factor induced by ischemia that can protect from ischemia-induced neuron loss. We hypothesize that the downregulation of TrkB.T1 at the protein level can prolong BDNF-mediated protective signaling via the catalytic receptor and thus participates in the development of ischemic preconditioning.

Brain-derived neurotrophic factor

Since the purification of BDNF in 1982, a great deal of evidence has mounted for its central roles in brain development, physiology, and pathology. Aside from its importance in neural development and cell survival, BDNF appears essential to molecular mechanisms of synaptic plasticity. Basic activity-related changes in the central nervous system are thought to depend on BDNF modification of synaptic transmission, especially in the hippocampus and neocortex. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders.

Transgenic mice overexpressing the full-length neurotrophin receptor trkB exhibit increased activation of the trkB-PLCgamma pathway, reduced anxiety, and facilitated learning

We have investigated the biochemical, physiological, and behavioral properties of transgenic mice overexpressing the full-length neurotrophin receptor trkB (trkB.TK+). The highest trkB.TK+ mRNA overexpression was achieved in the cerebral cortex and hippocampal subfields, both areas also showing strongly increased trkB.TK+ receptor protein expression and phosphorylation. Furthermore, as a result of trkB.TK+ overexpression, partial activation of trkB downstream signaling was observed. Phosphorylation of phospholipaseCgamma-1 was increased but unexpectedly, the expression and phosphorylation levels of signaling molecules Shc and mitogen-activated protein kinase (MAPK) were unaltered. Behavioral studies revealed improved learning and memory in the water maze, contextual fear conditioning, and conditioned taste aversion tests, and reduced anxiety in the elevated plus maze (EPM) and light-dark exploration tests in trkB.TK+ transgenic mice. Electrophysiological studies revealed a reduced long-term potentiation (LTP) at the Schaffer collateral-CA1 synapse in trkB.TK+ mice. Altogether, overexpression of the trkB.TK+ receptor postnatally leads to selective activation of trkB signaling pathways and enhanced learning and memory.

Alterations of cerebellar mRNA specific for BDNF, p75NTR, and TrkB receptor isoforms occur within hours of ethanol administration to 4-day-old rat pups

Developing cerebellar Purkinje cells of the rat are extremely sensitive to ethanol during postnatal days (PN) 4-6, but not at later times during development. Ethanol exposure during this vulnerable window induces rapid apoptotic Purkinje cell death that is hypothesized to result from ethanol inhibition in brain-derived nerve growth factor (BDNF)-TrkB neurotrophic signaling that results in loss of apoptotic suppression. In this study, the effect that different concentrations of ethanol (1.5, 3.0, 4.5 and 6.0 g/kg) have on steady-state mRNA expression of BDNF and different TrkB receptor isoforms in the cerebellum on PN4 was determined at 1, 4, 6, and 8 h after treatment. Significant decreases in mRNA specific for BDNF and TrkB isoforms were detected within 1 h after ethanol administration. No significant alterations in expression of mRNA specific to the low affinity p75(NTR) receptor were identified. These alterations are concurrent with the PN4 vulnerable period for Purkinje cells since equivalent treatment of PN9 rat pups does not produce significant alterations in mRNA specific to BDNF or TrkB at 4 h after exposure. These results support the hypothesis that ethanol induces a disruption of BDNF-TrkB signaling that results in loss of apoptotic suppression in vulnerable Purkinje cells by growth factor withdrawal.

Intracerebral transplantation of carotid body in rats with transient middle cerebral artery occlusion

Recent laboratory and clinical studies demonstrate therapeutic efficacy of intracerebral transplantation of carotid body (CB) in Parkinson's disease, possibly through secretion of neurotrophic factors. Here, we examined the role of CB in experimental stroke. In the first experiment, we hypothesized that removal of CB would exacerbate cerebral infarction and stroke-related behavioral deficits. Eight-week-old, male Sprague-Dawley rats were randomly divided into two groups: stroke with intact CB and stroke with surgically removed CB. We used the stroke model of temporary middle cerebral artery occlusion. The ipsilateral CB was removed in animals assigned to treatment group exposed to stroke with surgically removed CB. Behavioral tests, using the elevated body swing test, were conducted at days 1-3 after surgery. Cerebral infarction was visualized by TTC staining on day 3 post-surgery. The data revealed no significant differences in behavioral deficits and infarct volumes between the two groups. In the second experiment, CB cell suspension grafts or control adult tissue grafts were intracerebally transplanted into the ischemic penumbra immediately (within 1 h) after stroke surgery. The results revealed significant reduction of behavioral deficits and infarct volumes, accompanied by increased levels of neurotrophic factors, as detected by ELISA, in transplanted ischemic striatum collected from CB-grafted stroke animals. These observations suggest that surgical resection of CB in the periphery did not alter stroke pathology; however, CB when made available in the CNS, via intracerebral transplantation, could protect against stroke possibly through the synergistic release of neurotrophic factors. The present study extends the use of CB as efficacious graft source for transplantation.

X-linked inhibitor of apoptosis (XIAP) protein protects against caspase activation and tissue loss after neonatal hypoxia–ischemia

Nine-day-old transgenic XIAP overexpressing (TG-XIAP) and wild-type mice were subjected to left carotid artery ligation and 10% O(2) for 60 min, leading to widespread infarctions in the ipsilateral hemisphere during reperfusion. The activation of caspase-3 and -9 seen in wild-type animals was virtually abolished in TG-XIAP mice. Tissue loss was significantly reduced from 54.4 +/- 4.1 mm(3) (mean +/- SEM) in wild-type mice to 33.1 +/- 2.1 mm(3) in the TG-XIAP mice. Injured neurons displayed stronger XIAP staining during reperfusion, particularly in the nuclei. XIAP was colocalized with XAF-1, Smac, and HtrA2 in injured neurons after hypoxia-ischemia (HI). XIAP was cleaved after HI, and Smac immunoprecipitation co-precipitated a 25-kDa C-terminal fragment of XIAP, indicating that Smac preferentially bound to cleaved XIAP. These findings provide the first evidence that increased XIAP levels protect the neonatal brain against HI.