Cortical degeneration in the absence of neurotrophin signaling: dendritic retraction and neuronal loss after removal of the receptor TrkB

PKA Inhibitor H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulfonamide) Attenuates Synaptic Dysfunction and Neuronal Cell Death following Ischemic Injury

The cyclic AMP-dependent protein kinase (PKA), which activates prosurvival signaling proteins, has been implicated in the expression of long-term potentiation and hippocampal long-term memory. It has come to light that H89 commonly known as the PKA inhibitor have diverse roles in the nervous system that are unrelated to its role as a PKA inhibitor. We have investigated the role of H89 in ischemic and reperfusion injury. First, we examined the expression of postsynaptic density protein 95 (PSD95), microtubule-associated protein 2 (MAP2), and synaptophysin in mouse brain after middle cerebral artery occlusion injury. Next, we examined the role of H89 pretreatment on the expression of brain-derived neurotrophic factor (BDNF), PSD95, MAP2, and the apoptosis regulators Bcl2 and cleaved caspase-3 in cultured neuroblastoma cells exposed to hypoxia and reperfusion injury. In addition, we investigated the alteration of AKT activation in H89 pretreated neuroblastoma cells under hypoxia and reperfusion injury. The data suggest that H89 may contribute to brain recovery after ischemic stroke by regulating neuronal death and proteins related to synaptic plasticity.

Discrete BDNF Neurons in the Paraventricular Hypothalamus Control Feeding and Energy Expenditure

Brain-derived neurotrophic factor (BDNF) is a key regulator of energy balance; however, its underlying mechanism remains unknown. By analyzing BDNF-expressing neurons in paraventricular hypothalamus (PVH), we have uncovered neural circuits that control energy balance. The Bdnf gene in the PVH was mostly expressed in previously undefined neurons, and its deletion caused hyperphagia, reduced locomotor activity, impaired thermogenesis, and severe obesity. Hyperphagia and reduced locomotor activity were associated with Bdnf deletion in anterior PVH, whereas BDNF neurons in medial and posterior PVH drive thermogenesis by projecting to spinal cord and forming polysynaptic connections to brown adipose tissues. Furthermore, BDNF expression in the PVH was increased in response to cold exposure, and its ablation caused atrophy of sympathetic preganglionic neurons. Thus, BDNF neurons in anterior PVH control energy intake and locomotor activity, whereas those in medial and posterior PVH promote thermogenesis by releasing BDNF into spinal cord to boost sympathetic outflow.

Regulation of dendrite morphogenesis by extrinsic cues

Dendrites play a central role in the integration and flow of information in the nervous system. The morphogenesis and maturation of dendrites is hence an essential step in the establishment of neuronal connectivity. Recent studies have uncovered crucial functions for extrinsic cues in the development of dendrites. We review the contribution of secreted polypeptide growth factors, contact-mediated proteins, and neuronal activity in distinct phases of dendrite development. We also highlight how extrinsic cues influence local and global intracellular mechanisms of dendrite morphogenesis. Finally, we discuss how these studies have advanced our understanding of neuronal connectivity and have shed light on the pathogenesis of neurodevelopmental disorders.

Brain-derived neurotrophic factor inhibits calcium channel activation, exocytosis, and endocytosis at a central nerve terminal

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that regulates synaptic function and plasticity and plays important roles in neuronal development, survival, and brain disorders. Despite such diverse and important roles, how BDNF, or more generally speaking, neurotrophins affect synapses, particularly nerve terminals, remains unclear. By measuring calcium currents and membrane capacitance during depolarization at a large mammalian central nerve terminal, the rat calyx of Held, we report for the first time that BDNF slows down calcium channel activation, including P/Q-type channels, and inhibits exocytosis induced by brief depolarization or single action potentials, inhibits slow and rapid endocytosis, and inhibits vesicle mobilization to the readily releasable pool. These presynaptic mechanisms may contribute to the important roles of BDNF in regulating synapses and neuronal circuits and suggest that regulation of presynaptic calcium channels, exocytosis, and endocytosis are potential mechanisms by which neurotrophins achieve diverse neuronal functions.

Brain-derived neurotrophic factor is required for axonal growth of selective groups of neurons in the arcuate nucleus

Objective

Brain-derived neurotrophic factor (BDNF) is a potent regulator of neuronal development, and the Bdnf gene produces two populations of transcripts with either a short or long 3′ untranslated region (3′ UTR). Deficiencies in BDNF signaling have been shown to cause severe obesity in humans; however, it remains unknown how BDNF signaling impacts the organization of neuronal circuits that control energy balance.

Methods

We examined the role of BDNF on survival, axonal projections, and synaptic inputs of neurons in the arcuate nucleus (ARH), a structure critical for the control of energy balance, using Bdnf^klox/klox mice, which lack long 3′ UTR Bdnf mRNA and develop severe hyperphagic obesity.

Results

We found that a small fraction of neurons that express the receptor for BDNF, TrkB, also expressed proopiomelanocortin (POMC) or neuropeptide Y (NPY)/agouti-related protein (AgRP) in the ARH. Bdnf^klox/klox mice had normal numbers of POMC, NPY, and TrkB neurons in the ARH; however, retrograde labeling revealed a drastic reduction in the number of ARH axons that project to the paraventricular hypothalamus (PVH) in these mice. In addition, fewer POMC and AgRP axons were found in the dorsomedial hypothalamic nucleus (DMH) and the lateral part of PVH, respectively, in Bdnf^klox/klox mice. Using immunohistochemistry, we examined the impact of BDNF deficiency on inputs to ARH neurons. We found that excitatory inputs onto POMC and NPY neurons were increased and decreased, respectively, in Bdnf^klox/klox mice, likely due to a compensatory response to marked hyperphagia displayed by the mutant mice.

Conclusion

This study shows that the majority of TrkB neurons in the ARH are distinct from known neuronal populations and that BDNF plays a critical role in directing projections from these neurons to the DMH and PVH. We propose that hyperphagic obesity due to BDNF deficiency is in part attributable to impaired axonal growth of TrkB-expressing ARH neurons.

Increased cortical synaptic activation of TrkB and downstream signaling markers in a mouse model of Down Syndrome

Down Syndrome (DS), trisomy 21, is characterized by synaptic abnormalities and cognitive deficits throughout the lifespan and with development of Alzheimer's disease (AD) neuropathology and progressive cognitive decline in adults. Synaptic abnormalities are also present in the Ts65Dn mouse model of DS, but which synapses are affected and the mechanisms underlying synaptic dysfunction are unknown. Here we show marked increases in the levels and activation status of TrkB and associated signaling proteins in cortical synapses in Ts65Dn mice. Proteomic analysis at the single synapse level of resolution using array tomography (AT) uncovered increased colocalization of activated TrkB with signaling endosome related proteins, and demonstrated increased TrkB signaling. The extent of increases in TrkB signaling differed in each of the cortical layers examined and with respect to the type of synapse, with the most marked increases seen in inhibitory synapses. These findings are evidence of markedly abnormal TrkB-mediated signaling in synapses. They raise the possibility that dysregulated TrkB signaling contributes to synaptic dysfunction and cognitive deficits in DS.

Decreased cortical thickness in drug naïve first episode schizophrenia: in relation to serum levels of BDNF

This study was to examine cortical thickness in drug naïve, first episode schizophrenia patients, and to explore its relationship with serum levels of brain-derived neurotrophic factor (BDNF). Forty-five drug naive schizophrenia patients and 28 healthy controls were enrolled in the study. Freesurfer was used to parcellate cortical regions, and vertex-wise group analysis was used for whole brain cortical thickness. The clusters for the brain regions that demonstrated group differences were extracted, and the mean values of thickness were calculated. Serum levels of BDNF were measured using enzyme-linked immunosorbent assay (ELISA). After controlling for age and gender, significantly thinner cortical thickness was found in left insula and superior temporal gyrus in the patient group compared with the healthy control group (HC group) (p's < 0.001). Lower serum levels of BDNF were also found in the patient group compared with the HC group (p = 0.001). Correlation analysis showed a significant positive relationship between thickness of left insula and serum levels of BDNF within the HC group (r = 0.396, p = 0.037) but there was no such relationship within the patient group (r = 0.035, p = 0.819). Cortical thinning is present in drug naïve, first episode schizophrenia patients, indicating neurodevelopmental abnormalities at the onset of schizophrenia. Left insula might be an imaging biomarker in detecting the impaired protective role of neurotrophic factor for the brain development in schizophrenia.

BDNF signaling and survival of striatal neurons

The striatum, a major component of the basal ganglia, performs multiple functions including control of movement, reward, and addiction. Dysfunction and death of striatal neurons are the main causes for the motor disorders associated with Huntington’s disease (HD). Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is among factors that promote survival and proper function of this neuronal population. Here, we review recent studies showing that BDNF determines the size of the striatum by supporting survival of the immature striatal neurons at their origin, promotes maturation of striatal neurons, and facilitates establishment of striatal connections during brain development. We also examine the role of BDNF in maintaining proper function of the striatum during adulthood, summarize the mechanisms that lead to a deficiency in BDNF signaling and subsequently striatal degeneration in HD, and highlight a potential role of BDNF as a therapeutic target for HD treatment.

Molecular mechanisms of activity-dependent changes in dendritic morphology: role of RGK proteins

The nervous system has the amazing capacity to transform sensory experience from the environment into changes in neuronal activity that, in turn, cause long-lasting alterations in neuronal morphology. Recent findings indicate that, surprisingly, sensory experience concurrently activates molecular signaling pathways that both promote and inhibit dendritic complexity. Historically, a number of positive regulators of activity-dependent dendritic complexity have been described, whereas the list of identified negative regulators of this process is much shorter. In recent years, there has been an emerging appreciation of the importance of the Rad/Rem/Rem2/Gem/Kir (RGK) GTPases as mediators of activity-dependent structural plasticity. In the following review, we discuss the traditional view of RGK proteins, as well as our evolving understanding of the role of these proteins in instructing structural plasticity.

Serotonin-prefrontal cortical circuitry in anxiety and depression phenotypes: pivotal role of pre- and post-synaptic 5-HT1A receptor expression

Decreased serotonergic activity has been implicated in anxiety and major depression, and antidepressants directly or indirectly increase the long-term activity of the serotonin system. A key component of serotonin circuitry is the 5-HT1A autoreceptor, which functions as the major somatodendritic autoreceptor to negatively regulate the "gain" of the serotonin system. In addition, 5-HT1A heteroreceptors are abundantly expressed post-synaptically in the prefrontal cortex (PFC), amygdala, and hippocampus to mediate serotonin actions on fear, anxiety, stress, and cognition. Importantly, in the PFC 5-HT1A heteroreceptors are expressed on at least two antagonist neuronal populations: excitatory pyramidal neurons and inhibitory interneurons. Rodent models implicate the 5-HT1A receptor in anxiety- and depression-like phenotypes with distinct roles for pre- and post-synaptic 5-HT1A receptors. In this review, we present a model of serotonin-PFC circuitry that integrates evidence from mouse genetic models of anxiety and depression involving knockout, suppression, over-expression, or mutation of genes of the serotonin system including 5-HT1A receptors. The model postulates that behavioral phenotype shifts as serotonin activity increases from none (depressed/aggressive not anxious) to low (anxious/depressed) to high (anxious, not depressed). We identify a set of conserved transcription factors including Deaf1, Freud-1/CC2D1A, Freud-2/CC2D1B and glucocorticoid receptors that may confer deleterious regional changes in 5-HT1A receptors in depression, and how future treatments could target these mechanisms. Further studies to specifically test the roles and regulation of pyramidal vs. interneuronal populations of 5-HT receptors are needed better understand the role of serotonin in anxiety and depression and to devise more effective targeted therapeutic approaches.

Postsynaptic localization of PSD-95 is regulated by all three pathways downstream of TrkB signaling

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB regulate synaptic plasticity. TrkB triggers three downstream signaling pathways; Phosphatidylinositol 3-kinase (PI3K), Phospholipase Cγ (PLCγ) and Mitogen activated protein kinases/Extracellular signal-regulated kinases (MAPK/ERK). We previously showed two distinct mechanisms whereby BDNF-TrkB pathway controls trafficking of PSD-95, which is the major scaffold at excitatory synapses and is critical for synapse maturation. BDNF activates the PI3K-Akt pathway and regulates synaptic delivery of PSD-95 via vesicular transport (Yoshii and Constantine-Paton, 2007). BDNF-TrkB signaling also triggers PSD-95 palmitoylation and its transport to synapses through the phosphorylation of the palmitoylation enzyme ZDHHC8 by a protein kinase C (PKC; Yoshii etal., 2011). The second study used PKC inhibitors chelerythrine as well as a synthetic zeta inhibitory peptide (ZIP) which was originally designed to block the brain-specific PKC isoform protein kinase Mϖ (PKMϖ). However, recent studies raise concerns about specificity of ZIP. Here, we assessed the contribution of TrkB and its three downstream pathways to the synaptic distribution of endogenous PSD-95 in cultured neurons using chemical and genetic interventions. We confirmed that TrkB, PLC, and PI3K were critical for the postsynaptic distribution of PSD-95. Furthermore, suppression of MAPK/ERK also disrupted PSD-95 expression. Next, we examined the contribution of PKC. While both chelerythrine and ZIP suppressed the postsynaptic localization of PSD-95, RNA interference for PKMϖ did not have a significant effect. This result suggests that the ZIP peptide, widely used as the "specific" PKMϖ antagonist by many investigators may block a PKC variant other than PKMϖ such as PKCλ/ι. Our results indicate that TrkB regulates postsynaptic localization of PSD-95 through all three downstream pathways, but also recommend further work to identify other PKC variants that regulate palmitoylation and synaptic localization of PSD-95.

Rem2 is an activity-dependent negative regulator of dendritic complexity in vivo

A key feature of the CNS is structural plasticity, the ability of neurons to alter their morphology and connectivity in response to sensory experience and other changes in the environment. How this structural plasticity is achieved at the molecular level is not well understood. We provide evidence that changes in sensory experience simultaneously trigger multiple signaling pathways that either promote or restrict growth of the dendritic arbor; structural plasticity is achieved through a balance of these opposing signals. Specifically, we have uncovered a novel, activity-dependent signaling pathway that restricts dendritic arborization. We demonstrate that the GTPase Rem2 is regulated at the transcriptional level by calcium influx through L-VGCCs and inhibits dendritic arborization in cultured rat cortical neurons and in the Xenopus laevis tadpole visual system. Thus, our results demonstrate that changes in neuronal activity initiate competing signaling pathways that positively and negatively regulate the growth of the dendritic arbor. It is the balance of these opposing signals that leads to proper dendritic morphology.

Distribution and function of TrkB receptors in the developing brain of the opossum Monodelphis domestica

The expression, development pattern, spatiotemporal distribution, and function of TrkB receptors were investigated during the postnatal brain development of the opossum. Full-length TrkB receptor expression was detectable in the newborn opossum, whereas three different short forms that are expressed in the adult brain were almost undetectable in the newborn opossum brain. The highest level of full-length TrkB receptor expression was observed at P35, which corresponds to the time of eye opening. We found that in different brain structures, TrkB receptors were localized in various compartments of cells. The hypothalamus was distinguished by the presence of TrkB receptors not only in cell bodies but also in the neuropil. Double immunofluroscent staining for TrkB and a marker for the identification of the cell phenotype in several brain regions such as the olfactory bulb, hippocampus, thalamus, and cerebellum showed that unlike in eutherians, in the opossum, TrkB receptors were predominantly expressed in neurons. A lack of TrkB receptors in glial cells, particularly astrocytes and oligodendrocytes, provides evidence that TrkB receptors can play a functionally different role in marsupials than in eutherians. The effects of TrkB signaling on the development of cortical progenitor cells were examined in vitro using shRNAs. Blockade of the endogenous TrkB receptor expression induced a decrease in the number of progenitor cells proliferation, whereas the number of apoptotic progenitor cells increased. These changes were statistically significant but relatively small. In contrast, TrkB signaling was strongly involved in regulation of the cortical progenitor cell differentiation process.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

Ablation of TrkB expression in RGS9-2 cells leads to hyperphagic obesity

Brain-derived neurotrophic factor (BDNF) and its cognate receptor, TrkB (tropomyosin receptor kinase B), are widely expressed in the brain where they regulate a wide variety of biological processes, including energy homeostasis. However, the specific population(s) of TrkB-expressing neurons through which BDNF governs energy homeostasis remain(s) to be determined. Using the Cre-loxP recombination system, we deleted the mouse TrkB gene in RGS9-2-expressing cells. In this mouse mutant, TrkB expression was abolished in several hypothalamic nuclei, including arcuate nucleus, dorsomedial hypothalamus, and lateral hypothalamus. TrkB expression was also abolished in a small number of cells in other brain regions, including the cerebral cortex and striatum. The mutant animals developed hyperphagic obesity with normal energy expenditure. Despite hyperglycemia under fed conditions, these animals exhibited normal fasting blood glucose levels and normal glucose tolerance. These results suggest that BDNF regulates energy homeostasis in part through TrkB-expressing neurons in the hypothalamus.

Molecular mechanisms of dendrite stability

In the developing brain, dendrite branches and dendritic spines form and turn over dynamically. By contrast, most dendrite arbors and dendritic spines in the adult brain are stable for months, years and possibly even decades. Emerging evidence reveals that dendritic spine and dendrite arbor stability have crucial roles in the correct functioning of the adult brain and that loss of stability is associated with psychiatric disorders and neurodegenerative diseases. Recent findings have provided insights into the molecular mechanisms that underlie long-term dendrite stabilization, how these mechanisms differ from those used to mediate structural plasticity and how they are disrupted in disease.

BDNF regulates Rab11-mediated recycling endosome dynamics to induce dendritic branching

Dendritic arborization of neurons is regulated by brain-derived neurotrophic factor (BDNF) together with its receptor, TrkB. Endocytosis is required for dendritic branching and regulates TrkB signaling, but how postendocytic trafficking determines the neuronal response to BDNF is not well understood. The monomeric GTPase Rab11 regulates the dynamics of recycling endosomes and local delivery of receptors to specific dendritic compartments. We investigated whether Rab11-dependent trafficking of TrkB in dendrites regulates BDNF-induced dendritic branching in rat hippocampal neurons. We report that TrkB in dendrites is a cargo for Rab11 endosomes and that both Rab11 and its effector, MyoVb, are required for BDNF/TrkB-induced dendritic branching. In addition, BDNF induces the accumulation of Rab11-positive endosomes and GTP-bound Rab11 in dendrites and the expression of a constitutively active mutant of Rab11 is sufficient to increase dendritic branching by increasing TrkB localization in dendrites and enhancing sensitization to endogenous BDNF. We propose that Rab11-dependent dendritic recycling provides a mechanism to retain TrkB in dendrites and to increase local signaling to regulate arborization.

Neurotrophin and Wnt signaling cooperatively regulate dendritic spine formation

Dendritic spines are major sites of excitatory synaptic transmission and changes in their numbers and morphology have been associated with neurodevelopmental and neurodegenerative disorders. Brain-derived Neurotrophic Factor (BDNF) is a secreted growth factor that influences hippocampal, striatal and neocortical pyramidal neuron dendritic spine density. However, the mechanisms by which BDNF regulates dendritic spines and how BDNF interacts with other regulators of spines remain unclear. We propose that one mechanism by which BDNF promotes dendritic spine formation is through an interaction with Wnt signaling. Here, we show that Wnt signaling inhibition in cultured cortical neurons disrupts dendritic spine development, reduces dendritic arbor size and complexity, and blocks BDNF-induced dendritic spine formation and maturation. Additionally, we show that BDNF regulates expression of Wnt2, and that Wnt2 is sufficient to promote cortical dendrite growth and dendritic spine formation. Together, these data suggest that BDNF and Wnt signaling cooperatively regulate dendritic spine formation.

β-Amyloid (Aβ) oligomers impair brain-derived neurotrophic factor retrograde trafficking by down-regulating ubiquitin C-terminal hydrolase, UCH-L1

We previously found that BDNF-dependent retrograde trafficking is impaired in AD transgenic mouse neurons. Utilizing a novel microfluidic culture chamber, we demonstrate that Aβ oligomers compromise BDNF-mediated retrograde transport by impairing endosomal vesicle velocities, resulting in impaired downstream signaling driven by BDNF/TrkB, including ERK5 activation, and CREB-dependent gene regulation. Our data suggest that a key mechanism mediating the deficit involves ubiquitin C-terminal hydrolase L1 (UCH-L1), a deubiquitinating enzyme that functions to regulate cellular ubiquitin. Aβ-induced deficits in BDNF trafficking and signaling are mimicked by LDN (an inhibitor of UCH-L1) and can be reversed by increasing cellular UCH-L1 levels, demonstrated here using a transducible TAT-UCH-L1 strategy. Finally, our data reveal that UCH-L1 mRNA levels are decreased in the hippocampi of AD brains. Taken together, our data implicate that UCH-L1 is important for regulating neurotrophin receptor sorting to signaling endosomes and supporting retrograde transport. Further, our results support the idea that in AD, Aβ may down-regulate UCH-L1 in the AD brain, which in turn impairs BDNF/TrkB-mediated retrograde signaling, compromising synaptic plasticity and neuronal survival.

Molecular and neural bases underlying roles of BDNF in the control of body weight

Brain-derived neurotrophic factor (BDNF) is a potent regulator of neuronal development and synaptic plasticity that is fundamental to neural circuit formation and cognition. It is also involved in the control of appetite and body weight, with mutations in the genes for BDNF and its receptor, TrkB, resulting in remarkable hyperphagia and severe obesity in humans and mice. Recent studies have made significant progress in elucidating the source, action sites, and regulatory pathways of BDNF with regard to its role in the control of energy homeostasis, and have shed light on the relationships between BDNF and other molecules involved in the control of body weight. Here we provide a comprehensive review of evidence from pharmacological, genetic, and mechanistic studies, linking BDNF to the control of body weight. This review also aims to organize the main findings on this subject into a more refined framework and to discuss the future research directions necessary to advance the field.

EGF transactivation of Trk receptors regulates the migration of newborn cortical neurons

The development of neuronal networks in the neocortex depends on control mechanisms for mitosis and migration that allow newborn neurons to find their accurate position. Multiple mitogens, neurotrophic factors, guidance molecules and their corresponding receptors are involved in this process, but the mechanisms by which these signals are integrated are only poorly understood. We found that TrkB and TrkC, the receptors for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are activated by epidermal growth factor receptor (EGFR) signaling rather than by BDNF or NT-3 in embryonic mouse cortical precursor cells. This transactivation event regulated migration of early neuronal cells to their final position in the developing cortex. Transactivation by EGF led to membrane translocation of TrkB, promoting its signaling responsiveness. Our results provide genetic evidence that TrkB and TrkC activation in early cortical neurons do not depend on BDNF and NT-3, but instead on transactivation by EGFR signaling.

TrkB downregulation is required for dendrite retraction in developing neurons of chicken nucleus magnocellularis

The chick embryo (Gallus domesticus) is one of the most important model systems in vertebrate developmental biology. The development and function of its auditory brainstem circuitry is exceptionally well studied. These circuits represent an excellent system for genetic manipulation to investigate mechanisms controlling neural circuit formation, synaptogenesis, neuronal polarity, and dendritic arborization. The present study investigates the auditory nucleus, nucleus magnocellularis (NM). The neurotrophin receptor TrkB regulates dendritic structure in CNS neurons. TrkB is expressed in NM neurons at E7-E8 when these neurons have dendritic arbors. Downregulation of TrkB occurs after E8 followed by retraction of dendrites and by E18 most NM cells are adendritic. Is cessation of TrkB expression in NM necessary for dendritic retraction? To answer this question we combined focal in ovo electroporation with transposon mediated gene transfer to obtain stable expression of Doxycycline (Dox) regulated transgenes, specifically TrkB coexpressed with EGFP in a temporally controlled manner. Electroporation was performed at E2 and Dox added onto the chorioallointoic membrane from E7.5 to E16. Expression of EGFP had no effect on development of the embryo, or cell morphology and organization of auditory brainstem nuclei. NM cells expressing EGFP and TrkB at E17-E18 had dendrites and biophysical properties uncharacteristic for normal NM cells, indicating that cessation of TrkB expression is essential for dendrite retraction and functional maturation of these neurons. These studies indicate that expression of transposon based plasmids is an effective method to genetically manipulate events in mid to late embryonic brain development in chick.

BDNF promotes differentiation and maturation of adult-born neurons through GABAergic transmission

Brain-derived neurotrophic factor (BDNF) has been implicated in regulating adult neurogenesis in the subgranular zone (SGZ) of the dentate gyrus; however, the mechanism underlying this regulation remains unclear. In this study, we found that Bdnf mRNA localized to distal dendrites of dentate gyrus granule cells isolated from wild-type (WT) mice, but not from Bdnf(klox/klox) mice where the long 3' untranslated region (UTR) of Bdnf mRNA is truncated. KCl-induced membrane depolarization stimulated release of dendritic BDNF translated from long 3' UTR Bdnf mRNA in cultured hippocampal neurons, but not from short 3' UTR Bdnf mRNA. Bdnf(klox/klox) mice exhibited reduced expression of glutamic acid decarboxylase 65 (a GABA synthase), increased proliferation of progenitor cells, and impaired differentiation and maturation of newborn neurons in the SGZ. These deficits in adult neurogenesis were rescued with administration of phenobarbital, an enhancer of GABA(A) receptor activity. Furthermore, we observed similar neurogenesis deficits in mice where the receptor for BDNF, TrkB, was selectively abolished in parvalbumin (PV)-expressing GABAergic interneurons. Thus, our data suggest that locally synthesized BDNF in dendrites of granule cells promotes differentiation and maturation of progenitor cells in the SGZ by enhancing GABA release, at least in part, from PV-expressing GABAergic interneurons.

Increases in two truncated TrkB isoforms in the prefrontal cortex of people with schizophrenia

The truncated brain-derived neurotrophic factor (BDNF) receptors (truncated TrkB [TrkB-TK-] and sarc homology containing TrkB [TrkB-Shc]) are alternative transcripts of the full-length TrkB receptor (TrkB-TK+) that produce isoforms capable of binding to BDNF but not being able to mediate the classic neurotrophic response via tyrosine kinase signaling. We hypothesized that in the dorsolateral prefrontal cortex (DLPFC) of people with schizophrenia, truncated TrkB receptors (TK- and Shc) would be altered and may contribute to deficits in BDNF function. Using a large cohort of controls and schizophrenics (n = 72/72), we measured mRNA expression of the full-length TrkB receptor, TrkB-TK+ and the truncated TrkB receptors, TrkB-TK- and TrkB-Shc, by quantitative real-time polymerase chain reaction and protein expression by western blotting. We found highly significant increases in mRNA expression of both truncated TrkB receptor isoforms in people with schizophrenia. When we examined the full-length TrkB-TK+:truncated TrkB ratios, we observed significant decreases in schizophrenia both on the mRNA and protein level. We found a slight reduction in TrkB-TK+ mRNA and a significant reduction in TrkB-TK+ protein expression in schizophrenia, which was evident in females. No gender-specific changes were found for the truncated TrkB receptors. Diagnostic changes in TrkB-TK+ mRNA and protein may be subtle and/or gender-specific, whereas changes in TrkB-TK- and TrkB-Shc expression are robust and may generalize to both males and females with schizophrenia. Increased truncated TrkB receptors may contribute to reduced overall BDNF/tyrosine receptor kinase B (TrkB) signaling and lead to reduced neuronal plasticity in the DLPFC in schizophrenia suggesting that therapies aimed at ameliorating neurotrophin deficits may need to consider blocking excessive truncated TrkB function.

Human immunodeficiency virus type 1 alters brain-derived neurotrophic factor processing in neurons

The molecular mechanisms leading to synaptic simplification and neuronal apoptosis in human immunodeficiency virus type 1 (HIV-1)-positive subjects are unknown. The HIV protein gp120 reduced the length of neuronal processes similarly to the proneurotrophin pro-brain-derived neurotrophic factor (proBDNF). Intriguingly, the effects of both proBDNF and gp120 were blocked by inhibitors of the p75 neurotrophin receptor, suggesting that proBDNF and gp120 share a similar mechanism of neurotoxicity. Therefore, we tested the hypothesis that gp120 affects the release of proBDNF. Using rat primary neurons, we observed that gp120 promotes a time-dependent intracellular and extracellular accumulation of proBDNF concomitantly with a decrease in mature BDNF. A similar imbalance in the ratio proBDNF/mature BDNF was confirmed in postmortem brains of HIV-positive subjects cognitively impaired and motor impaired. Therefore, it is conceivable to formulate the hypothesis that HIV neurotoxicity includes a gp120-mediated alteration of BDNF processing. To determine the cellular mechanism whereby gp120 produces an accumulation of proBDNF, we examined the levels of intracellular and extracellular enzymes that proteolytically cleave proBDNF furin and tissue plasminogen, respectively. In rat neurons exposed to gp120, intracellular furin levels decreased before cell death, whereas tissue plasminogen changed only during apoptosis. Our data suggest that HIV, through gp120, reduces proBDNF processing by affecting furin levels, and therefore causes an altered balance between antiapoptotic and proapoptotic neurotrophins. Our studies identify a new mechanism that may explain how HIV promotes neuronal injury.

Dendritic BDNF synthesis is required for late-phase spine maturation and recovery of cortical responses following sensory deprivation

Sensory experience in early postnatal life shapes neuronal connections in the brain. Here we report that the local synthesis of brain-derived neurotrophic factor (BDNF) in dendrites plays an important role in this process. We found that dendritic spines of layer 2/3 pyramidal neurons of the visual cortex in mutant mice lacking dendritic Bdnf mRNA and thus local BDNF synthesis were normal at 3 weeks of age, but thinner, longer, and more closely spaced (morphological features of immaturity) at 4 months of age than in wild-type (WT) littermates. Layer 2/3 of the visual cortex in these mutant animals also had fewer GABAergic presynaptic terminals at both ages. The overall size and shape of dendritic arbors were, however, similar in mutant and WT mice at both ages. By using optical imaging of intrinsic signals and single-unit recordings, we found that mutant animals failed to recover cortical responsiveness following monocular deprivation (MD) during the critical period, although they displayed normally the competitive loss of responsiveness to an eye briefly deprived of vision. Furthermore, MD still induced a loss of responsiveness to the closed eye in adult mutant mice, but not in adult WT mice. These results indicate that dendritic BDNF synthesis is required for spine pruning, late-phase spine maturation, and recovery of cortical responsiveness following sensory deprivation. They also suggest that maturation of dendritic spines is required for the maintenance of cortical responsiveness following sensory deprivation in adulthood.

Signaling mechanisms that coordinate the development and maintenance of dendritic fields

The establishment of a dendritic tree is a highly dynamic process characterized by extension and retraction of branches, followed by stabilization of existing dendrites and synaptic connections. To properly cover the receptive fields, all of these processes are tightly coordinated at all time points. Recent in vivo studies suggest that several signaling pathways, including Hippo pathway and epigenetic mechanisms, play important roles in maintenance of matured dendrites. This review focuses on the current molecular understanding of how established dendritic fields in functional neuronal circuits are maintained in the brain. The relevance of this knowledge to the pathological mechanisms underlying some neurodegenerative disorders is also discussed.

Sustained expression of brain-derived neurotrophic factor is required for maintenance of dendritic spines and normal behavior

Brain-derived neurotrophic factor (BDNF) plays important roles in the development, maintenance, and plasticity of the mammalian forebrain. These functions include regulation of neuronal maturation and survival, axonal and dendritic arborization, synaptic efficacy, and modulation of complex behaviors including depression and spatial learning. Although analysis of mutant mice has helped establish essential developmental functions for BDNF, its requirement in the adult is less well documented. We have studied late-onset forebrain-specific BDNF knockout (CaMK-BDNF(KO)) mice, in which BDNF is lost primarily from the cortex and hippocampus in early adulthood, well after BDNF expression has begun in these structures. We found that although CaMK-BDNF(KO) mice grew at a normal rate and can survive more than a year, they had smaller brains than wild-type siblings. The CaMK-BDNF(KO) mice had generally normal behavior in tests for ataxia and anxiety, but displayed reduced spatial learning ability in the Morris water task and increased depression in the Porsolt swim test. These behavioral deficits were very similar to those we previously described in an early-onset forebrain-specific BDNF knockout. To identify an anatomical correlate of the abnormal behavior, we quantified dendritic spines in cortical neurons. The spine density of CaMK-BDNF(KO) mice was normal at P35, but by P84, there was a 30% reduction in spine density. The strong similarities we find between early- and late-onset BDNF knockouts suggest that BDNF signaling is required continuously in the CNS for the maintenance of some forebrain circuitry also affected by developmental BDNF depletion.

Phosphoinositide 3-kinase enhancer (PIKE) in the brain: is it simply a phosphoinositide 3-kinase/Akt enhancer?

Since its discovery in 2000, phosphoinositide 3-kinase enhancer (PIKE) has been recognized as a class of GTPase that controls the enzymatic activities of phosphoinositide 3-kinase (PI3K) and Akt in the central nervous system (CNS). However, recent studies suggest that PIKEs are not only enhancers to PI3K/Akt but also modulators to other kinases including insulin receptor tyrosine kinase and focal adhesion kinases. Moreover, they regulate transcription factors such as signal transducer and activator of transcription and nuclear factor κB. Indeed, PIKE proteins participate in multiple cellular processes including control of cell survival, brain development, memory formation, gene transcription, and metabolism. In this review, we have summarized the functions of PIKE proteins in CNS and discussed their potential implications in various neurological disorders.

TrkB and protein kinase Mζ regulate synaptic localization of PSD-95 in developing cortex

Postsynaptic density 95 (PSD-95), the major scaffold at excitatory synapses, is critical for synapse maturation and learning. In rodents, eye opening, the onset of pattern vision, triggers a rapid movement of PSD-95 from visual neuron somata to synapses. We showed previously that the PI3 kinase-Akt pathway downstream of BDNF/TrkB signaling stimulates synaptic delivery of PSD-95 via vesicular transport. However, vesicular transport requires PSD-95 palmitoylation to attach it to a lipid membrane. Also, PSD-95 insertion at synapses is known to require this lipid modification. Here, we show that BDNF/TrkB signaling is also necessary for PSD-95 palmitoylation and its transport to synapses in mouse visual cortical layer 2/3 neurons. However, palmitoylation of PSD-95 requires the activation of another pathway downstream of BDNF/TrkB, namely, signaling through phospholipase Cγ and the brain-specific PKC variant protein kinase M ζ (PKMζ). We find that PKMζ selectively regulates phosphorylation of the palmitoylation enzyme ZDHHC8. Inhibition of PKMζ results in a reduction of synaptic PSD-95 accumulation in vivo, which can be rescued by overexpressing ZDHHC8. Therefore, TrkB and PKMζ, two critical regulators of synaptic plasticity, facilitate PSD-95 targeting to synapses. These results also indicate that palmitoylation can be regulated by a trophic factor. Our findings have implications for neurodevelopmental disorders as well as aging brains.

Neurotoxicity of human immunodeficiency virus-1: viral proteins and axonal transport

Human immunodeficiency virus-1 (HIV) infection of the central nervous system may cause a neurological syndrome termed HIV-associated neurocognitive disorder (HAND) which includes minor neurocognitive disorders or a more severe form of motor and cognitive impairments. Although treatment with highly active antiretroviral agents decreases the load of HIV in the brain, the prevalence of mild forms of HAND is actually increased due to longer life. Therefore, adjunctive and combined therapies must be developed to prevent and perhaps reverse the neurologic deficits observed in individuals with HAND. Key to developing effective therapies is a better understanding of the molecular and cellular mechanisms by which the virus causes this disorder. A number of HIV proteins has been shown to be released from HIV-infected cells. Moreover, these proteins have been shown to possess neurotoxic properties. This review describes new evidence of a direct interaction of the HIV protein gp120 with neurons, which might play a role in the etiopathology of HAND.

TrkB signaling in parvalbumin-positive interneurons is critical for gamma-band network synchronization in hippocampus

Although brain-derived neurotrophic factor (BDNF) is known to regulate circuit development and synaptic plasticity, its exact role in neuronal network activity remains elusive. Using mutant mice (TrkB-PV(-/-)) in which the gene for the BDNF receptor, tyrosine kinase B receptor (trkB), has been specifically deleted in parvalbumin-expressing, fast-spiking GABAergic (PV+) interneurons, we show that TrkB is structurally and functionally important for the integrity of the hippocampal network. The amplitude of glutamatergic inputs to PV+ interneurons and the frequency of GABAergic inputs to excitatory pyramidal cells were reduced in the TrkB-PV(-/-) mice. Functionally, rhythmic network activity in the gamma-frequency band (30-80 Hz) was significantly decreased in hippocampal area CA1. This decrease was caused by a desynchronization and overall reduction in frequency of action potentials generated in PV+ interneurons of TrkB-PV(-/-) mice. Our results show that the integration of PV+ interneurons into the hippocampal microcircuit is impaired in TrkB-PV(-/-) mice, resulting in decreased rhythmic network activity in the gamma-frequency band.

Phosphoinositide 3-kinase enhancer regulates neuronal dendritogenesis and survival in neocortex

Phosphoinositide 3-kinase enhancer (PIKE) binds and enhances phosphatidylinositol 3-kinase (PI3K)/Akt activities. However, its physiological functions in brain have never been explored. Here we show that PIKE is important in regulating the neuronal survival and development of neocortex. During development, enhanced apoptosis is observed in the ventricular zone of PIKE knock-out (PIKE(-/-)) cortex. Moreover, PIKE(-/-) neurons show reduced dendritic complexity, dendritic branch length, and soma size. These defects are due to the reduced PI3K/Akt activities in PIKE(-/-) neurons, as the impaired dendritic arborization can be rescued when PI3K/Akt cascade is augmented in vitro or in PIKE(-/-)PTEN(-/-) double-knock-out mice. Interestingly, PIKE(-/-) mice display behavioral abnormality in locomotion and spatial navigation. Because of the diminished PI3K/Akt activities, PIKE(-/-) neurons are more vulnerable to glutamate- or stroke-induced neuronal cell death. Together, our data established the critical role of PIKE in regulating neuronal survival and development by substantiating the PI3K/Akt pathway.

Retrolinkin cooperates with endophilin A1 to mediate BDNF-TrkB early endocytic trafficking and signaling from early endosomes

Both retrolinkin and its interaction partner endophilin A1 are required for BDNF-induced dendrite outgrowth of cultured hippocampal neurons. They function sequentially in an early endocytic trafficking pathway for BDNF-activated TrkB, which provides spatiotemporal control of downstream ERK signaling from endosomes.

Brain-derived neurotrophic factor (BDNF) binds to its cell surface receptor TrkB to regulate differentiation, development, synaptic plasticity, and functional maintenance of neuronal cells. Binding of BDNF triggers TrkB dimerization and autophosphorylation, which provides docking sites for adaptor proteins to recruit and activate downstream signaling molecules. The molecular mechanisms underlying BDNF–TrkB endocytic trafficking crucial for spatiotemporal control of signaling pathways remain to be elucidated. Here we show that retrolinkin, a transmembrane protein, interacts with endophilin A1 and mediates BDNF-activated TrkB (pTrk) trafficking and signaling in CNS neurons. We find that activated TrkB colocalizes and interacts with the early endosome marker APPL1. Both retrolinkin and endophilin A1 are required for BDNF-induced dendrite development and acute extracellular signal-regulated kinase activation from early endosomes. Suppression of retrolinkin expression not only blocks BDNF-triggered TrkB internalization, but also prevents recruitment of endophilin A1 to pTrk vesicles trafficking through APPL1-positive endosomes. These findings reveal a novel mechanism for BDNF–TrkB to regulate signaling both in time and space through a specific membrane trafficking pathway.

Etiology of a genetically complex seizure disorder in Celf4 mutant mice

Mice deficient for the gene encoding the RNA-binding protein CELF4 (CUGBP, ELAV-like family member 4) have a complex seizure phenotype that includes both convulsive and non-convulsive seizures, depending upon gene dosage and strain background, modeling genetically complex epilepsy. Invertebrate CELF is associated with translational control in fruit fly ovary epithelium and with neurogenesis and neuronal function in the nematode. Mammalian CELF4 is expressed widely during early development, but is restricted to the central nervous system in adults. To better understand the etiology of the seizure disorder of Celf4 deficient mice, we studied seizure incidence with spatial and temporal conditional knockout Celf4 alleles. For convulsive seizure phenotypes, it is sufficient to delete Celf4 in adulthood at the age of 7 weeks. This timing is in contrast to absence-like non-convulsive seizures, which require deletion before the end of the first postnatal week. Interestingly, selective deletion of Celf4 from cerebral cortex and hippocampus excitatory neurons, but not from inhibitory neurons, is sufficient to lower seizure threshold and to promote spontaneous convulsions. Correspondingly, Celf4 deficient mice have altered excitatory, but not inhibitory, neurotransmission as measured by patch-clamp recordings of cortical layer V pyramidal neurons. Finally, immunostaining in conjunction with an inhibitory neuron-specific reporter shows that CELF4 is expressed predominantly in excitatory neurons. Our results suggest that CELF4 plays a specific role in regulating excitatory neurotransmission. We posit that altered excitatory neurotransmission resulting from Celf4 deficiency underlies the complex seizure disorder in Celf4 mutant mice.

Cell adhesion signaling pathways: First responders to cocaine exposure?

The 100 billion neurons comprising the human brain are wired together using structural extensions termed axons, dendrites and dendritic spines. Addictive drugs remodel dendritic spine structure in certain brain regions and with repeated exposure, induce psychomotor sensitization and impair behavioral flexibility. We recently reported that low-dose cocaine exposure, in combination with knockout of Arg-an adhesion-regulated nonreceptor tyrosine kinase that stabilizes neuronal shape starting in adolescence-recapitulates both features of chronic drug exposure in rodents. In light of these and other recent findings in the field, we suggest that cell adhesion receptors and their downstream cytoskeletal effectors act as "first responders" to psychostimulant exposure. In this model, cell adhesion factors act to stabilize existing dendritic spines in response to cocaine, and reduced expression/function is expected to increase vulnerability. Moreover, this model anticipates that increased sensitivity to psychostimulants in adolescence relates to neuronal pruning processes that occur during this developmental period.

Regulation of protein prenyltransferase in central neurons

Geranylgeranyltransferase I (GGT) is a protein prenyltransferase that mediates lipid modification of some proteins such as Rho family small GTPases. Since the activation of Rho GTPases mediates tumorgenesis and metastasis, GGT has become an attractive target for anti-tumor drug design. Although GGT is extensively expressed in the brain, the function of GGT in central nervous system (CNS) is totally unknown. We have previously shown that GGT was involved in neuromuscular synaptogenesis. In this study, we report that neuronal activity- and brain-derived neurotropic factor (BDNF)-dependent dendritic morphogenesis requires activation of GGT. Furthermore, GGT was activated by depolarization or BDNF in cultured neurons or in hippocampus of the mice under novelty exploration test, suggesting that neuronal activity activates GGT in vitro and in vivo. In this addendum, we further discuss the significance of this study and the possible implication to the field.

Loss of nuclear factor E2-related factor 1 in the brain leads to dysregulation of proteasome gene expression and neurodegeneration

The ubiquitin-proteasome pathway plays an important role in the pathogenesis of neurodegeneration, but mechanisms controlling expression of components in this pathway remain poorly understood. Nuclear factor E2-related factor 1 (Nrf1) transcription factor has been shown to regulate expression of antioxidant and cytoprotective genes. To determine the function of Nrf1 in the brain, mice with a late-stage deletion of Nrf1 in neuronal cells were generated. Loss of Nrf1 leads to impaired proteasome function and neurodegeneration. Gene expression profiling and RT-PCR analysis revealed a coordinate down-regulation of various proteasomal genes including PsmB6, which encodes a catalytic subunit of the proteasome. Transcriptional analysis and chromatin immunoprecipitation experiments demonstrated that PsmB6 is an Nrf1 target gene. These findings reveal Nrf1 as a key transcriptional regulator required for the expression of proteasomal genes in neurons and suggest that perturbations of Nrf1 function may contribute to the pathogenesis of neurodegenerative diseases.

Venlafaxine modulates depression-induced behaviour and the expression of Bax mRNA and Bcl-xl mRNA in both hippocampus and myocardium

OBJECTIVES

Major depressive disorder is associated with progressive brain changes and is frequently comorbid with cardiovascular disease. There may be shared pathophysiological pathways between cerebral and myocardial dysfunction that impact on apoptosis related proteins. Our aim was to examine behaviour changes of rats with chronic mild stress (CMS), explore the expression of Bax and Bcl-xl in the hippocampus and myocardium, and additionally evaluate the effects of venlafaxine on these molecular mechanisms.

METHODS

Rats were randomly divided into three groups. The behaviour was assessed using the open field and sucrose consumption tests. Gene expression was measured by RT-PCR.

RESULTS

In CMS, there was a significant reduction of movements and sucrose consumption, an increased Bax level and a decreased Bcl-xl level in both the hippocampus and myocardium. The venlafaxine group showed an increase in movements and sucrose consumption, as well as upregulated expression of Bcl-xl and downregulated expression of Bax in both the hippocampus and myocardium.

CONCLUSIONS

These results demonstrate that in CMS, there is an increase in pro-apoptotic pathways that is reversed by venlafaxine. This suggests that there are shared active biochemical pathways that may play a role in the process of neuroprogression that is seen in depression and cardiovascular disorders.

Mechanisms of synapse and dendrite maintenance and their disruption in psychiatric and neurodegenerative disorders

Emerging evidence indicates that once established, synapses and dendrites can be maintained for long periods, if not for the organism's entire lifetime. In contrast to the wealth of knowledge regarding axon, dendrite, and synapse development, we understand comparatively little about the cellular and molecular mechanisms that enable long-term synapse and dendrite maintenance. Here, we review how the actin cytoskeleton and its regulators, adhesion receptors, and scaffolding proteins mediate synapse and dendrite maintenance. We examine how these mechanisms are reinforced by trophic signals passed between the pre- and postsynaptic compartments. We also discuss how synapse and dendrite maintenance mechanisms are compromised in psychiatric and neurodegenerative disorders.

TrkB receptor controls striatal formation by regulating the number of newborn striatal neurons

In the peripheral nervous system, target tissues control the final size of innervating neuronal populations by producing limited amounts of survival-promoting neurotrophic factors during development. However, it remains largely unknown if the same principle works to regulate the size of neuronal populations in the developing brain. Here we show that neurotrophin signaling mediated by the TrkB receptor controls striatal size by promoting the survival of developing medium-sized spiny neurons (MSNs). Selective deletion of the gene for the TrkB receptor in striatal progenitors, using the Dlx5/6-Cre transgene, led to a hindpaw-clasping phenotype and a 50% loss of MSNs without affecting striatal interneurons. This loss resulted mainly from increased apoptosis of newborn MSNs within their birthplace, the lateral ganglionic eminence. Among MSNs, those expressing the dopamine receptor D2 (DRD2) were most affected, as indicated by a drastic loss of these neurons and specific down-regulation of the DRD2 and enkephalin. This specific phenotype of mutant animals is likely due to preferential TrkB expression in DRD2 MSNs. These findings suggest that neurotrophins can control the size of neuronal populations in the brain by promoting the survival of newborn neurons before they migrate to their final destinations.

Chronic deprivation of TrkB signaling leads to selective late-onset nigrostriatal dopaminergic degeneration

The pathological hallmark of Parkinson's disease (PD) is a selective and progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc). In the vast majority of cases the appearance of PD is sporadic, and its etiology remains unknown. Several postmortem studies demonstrate reduced levels of brain-derived neurotrophic factor (BDNF) in the SNc of PD patients. Application of BDNF promotes the survival of DA neurons in PD animal models. Here we show that BDNF signaling via its TrkB receptor tyrosine kinase is important for survival of nigrostriatal DA neurons in aging brains. Immunohistochemistry revealed that the TrkB receptor was expressed in DA neurons located in the SNc and ventral tegmental area (VTA). However, a significant loss of DA neurons occurred at 12-24 months of age only in the SNc but not in the VTA of TrkB hypomorphic mice in which the TrkB receptor was expressed at a quarter to a third of the normal amount. The neuronal loss was accompanied by a decrease in dopaminergic axonal terminals in the striatum and by gliosis in both the SNc and striatum. Furthermore, nigrostriatal DA neurons in the TrkB mutant mice were hypersensitive to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a mitochondrial complex I inhibitor that selectively kills DA neurons. These results suggest that BDNF-to-TrkB signaling plays an important role in the long-term maintenance of the nigrostriatal system and that its deficiency may contribute to the progression of PD.

Characterisation of a new regulator of BDNF signalling, Sprouty3, involved in axonal morphogenesis in vivo

During development, many organs, including the kidney, lung and mammary gland, need to branch in a regulated manner to be functional. Multicellular branching involves changes in cell shape, proliferation and migration. Axonal branching, however, is a unicellular process that is mediated by changes in cell shape alone and as such appears very different to multicellular branching. Sprouty (Spry) family members are well-characterised negative regulators of Receptor tyrosine kinase (RTK) signalling. Knockout of Spry1, 2 and 4 in mouse result in branching defects in different organs, indicating an important role of RTK signalling in controlling branching pattern. We report here that Spry3, a previously uncharacterised member of the Spry family plays a role in axonal branching. We found that spry3 is expressed specifically in the trigeminal nerve and in spinal motor and sensory neurons in a Brain-derived neurotrophin factor (BDNF)-dependent manner. Knockdown of Spry3 expression causes an excess of axonal branching in spinal cord motoneurons in vivo. Furthermore, Spry3 inhibits the ability of BDNF to induce filopodia in Xenopus spinal cord neurons. Biochemically, we show that Spry3 represses calcium release downstream of BDNF signalling. Altogether, we have found that Spry3 plays an important role in the regulation of axonal branching of motoneurons in vivo, raising the possibility of unexpected conservation in the involvement of intracellular regulators of RTK signalling in multicellular and unicellular branching.

BDNF overexpression in the forebrain rescues Huntington's disease phenotypes in YAC128 mice

Huntington's disease (HD) is caused by an expansion of the polyglutamine tract at the N terminus of huntingtin. This mutation reduces levels of BDNF in the striatum, likely by inhibiting cortical Bdnf gene expression and anterograde transport of BDNF from the cerebral cortex to the striatum. Substantial evidence suggests that this reduction of striatal BDNF plays a crucial role in HD pathogenesis. Here we report that overexpression of BDNF in the forebrain rescues many disease phenotypes in YAC128 mice that express a full-length human huntingtin mutant with a 128-glutamine tract. The Bdnf transgene, under the control of the promoter for α subunit of Ca(2+)/calmodulin-dependent protein kinase II, greatly increased BDNF levels in the cerebral cortex and striatum. BDNF overexpression in YAC128 mice prevented loss and atrophy of striatal neurons and motor dysfunction, normalized expression of the striatal dopamine receptor D2 and enkephalin, and improved procedural learning. Furthermore, quantitative analyses of Golgi-impregnated neurons revealed a decreased spine density and abnormal spine morphology in striatal neurons of YAC128 mice, which was also reversed by increasing BDNF levels in the striatum. These results demonstrate that reduced striatal BDNF plays a crucial role in the HD pathogenesis and suggest that attempts to restore striatal BDNF level may have therapeutic effects to the disease.

Pik3c3 deletion in pyramidal neurons results in loss of synapses, extensive gliosis and progressive neurodegeneration

The lipid kinase PIK3C3 (also known as VPS34) regulates multiple aspects of endo-membrane trafficking processes. PIK3C3 is widely expressed by neurons in the CNS, and its catalytic product PI3P is enriched in dendritic spines. Here we generated a line of conditional mutant mouse in which Pik3c3 is specifically deleted in hippocampal and in small subsets of cortical pyramidal neurons using the CaMKII-Cre transgene. We found that Pik3c3-deficiency initially causes loss of dendritic spines accompanied with reactive gliosis, which is followed by progressive neuronal degeneration over a period of several months. Layers III and IV cortical neurons are more susceptible to Pik3c3-deletion than hippocampal neurons. Furthermore, in aged conditional Pik3c3 mutant animals, there are extensive gliosis and severe secondary loss of wild type neurons. Our analyses show that Pik3c3 is essential for CNS neuronal homeostasis and Pik3c3flox/flox; CaMKII-Cre mouse is a useful model for studying pathological changes in progressive forebrain neurodegeneration.

Therapy of neurodegenerative diseases using neurotrophic factors: cell biological perspective

Neurotrophic growth factors are a structurally diverse group of endogenous proteins, which profoundly regulate survival and multiple other cellular functions of healthy and diseased neurons. Their physiological targets include all neuronal cell types affected by neurodegenerative diseases. Initial clinical trials, which were based on strong and convincing neuroprotective effects in animal models of neurodegenerative diseases, have failed so far. In this review, we adopt a cell biological perspective to outline both the potential and problems of the therapeutic use of trophic factors in the nervous system. Emphasis is given to the functional interplay of neurotrophic factors in the regulation of cell death, as well as to the effects these proteins exert on neuronal morphology and synaptic transmission.