Juan Ji An

Brain‐derived neurotrophic factor over‐expression in the forebrain ameliorates Huntington’s disease phenotypes in mice

Huntington’s disease (HD), a dominantly inherited neurodegenerative disorder characterized by relatively selective degeneration of striatal neurons, is caused by an expanded polyglutamine tract of the huntingtin (htt) protein. The htt mutation reduces levels of brain‐derived neurotrophic factor (BDNF) in the striatum, likely by inhibiting cortical BDNF gene expression and anterograde transport of BDNF from cortex to striatum. However, roles of the BDNF reduction in HD pathogenesis have not been established conclusively. We reasoned that increasing striatal BDNF through over‐expression would slow progression of the disease if BDNF reduction plays a pivotal role in HD pathogenesis. We employed a Bdnf transgene driven by the promoter for the alpha subunit of Ca2+/calmodulin‐dependent kinase II to over‐express BDNF in the forebrain of R6/1 mice which express a fragment of mutant htt with a 116‐glutamine tract. The Bdnf transgene increased BDNF levels and TrkB signaling activity in the striatum, ameliorated motor dysfunction, and reversed brain weight loss in R6/1 mice. Furthermore, it normalized DARPP‐32 expression of the 32 kDa dopamine and cAMP‐regulated phosphoprotein, increased the number of enkephalin‐containing boutons, and reduced formation of neuronal intranuclear inclusions in the striatum of R6/1 mice. These results demonstrate crucial roles of reduced striatal BDNF in HD pathogenesis and suggest potential therapeutic values of BDNF to HD.

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 Bdnfklox/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. Bdnfklox/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 GABAA 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.

Dendritically targeted Bdnf mRNA is essential for energy balance and response to leptin

Mutations in the Bdnf gene, which produces transcripts with either short or long 3′ untranslated regions (3′ UTRs), cause human obesity; however, the precise role of brain-derived neurotrophic factor (BDNF) in the regulation of energy balance is unknown. Here we show the relationship between Bdnf mRNA with a long 3′ UTR (long 3′ UTR Bdnf mRNA), leptin, neuronal activation and body weight. We found that long 3′ UTR Bdnf mRNA was enriched in the dendrites of hypothalamic neurons and that insulin and leptin could stimulate its translation in dendrites. Furthermore, mice harboring a truncated long Bdnf 3′ UTR developed severe hyperphagic obesity, which was completely reversed by viral expression of long 3′ UTR Bdnf mRNA in the hypothalamus. In these mice, the ability of leptin to activate hypothalamic neurons and inhibit food intake was compromised despite normal activation of leptin receptors. These results reveal a novel mechanism linking leptin action to BDNF expression during hypothalamic-mediated regulation of body weight, while also implicating dendritic protein synthesis in this process.

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.

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.

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.

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.

Distinct cellular toxicity of two mutant huntingtin mRNA variants due to translation regulation

Huntington’s disease (HD) is a neurodegenerative disorder caused by CAG repeat expansion within exon1 of the HTT gene. The gene generates two mRNA variants that carry either a short or long 3′ untranslated region (3′UTR) while encoding the same protein. It remains unknown whether the two mRNA variants play distinct roles in HD pathogenesis. We found that the long HTT 3′UTR was capable of guiding mRNA to neuronal dendrites, suggesting that some long-form HTT mRNA is transported to dendrites for local protein synthesis. To assay roles of two HTT mRNA variants in cell bodies, we expressed mRNA harboring HTT exon1 containing 23x or 145x CAGs with the short or long 3′UTR. We found that mutant mRNA containing the short 3′UTR produced more protein aggregates and caused more apoptosis in both cultured neurons and HEK293 cells, compared with mutant mRNA containing the long 3′UTR. Although the two 3′UTRs did not affect mRNA stability, we detected higher levels of protein synthesis from mRNA containing the short 3′UTR than from mRNA containing the long 3′UTR. These results indicate that the long HTT 3′UTR suppresses translation. Thus, short-form mutant HTT mRNA will be more efficient in producing toxic protein than its long-form counterpart.

Regulation of Energy Balance via BDNF Expressed in Nonparaventricular Hypothalamic Neurons

Brain-derived neurotrophic factor (BDNF) expressed in the paraventricular hypothalamus (PVH) has been shown to play a key role in regulating energy intake and energy expenditure. BDNF is also expressed in other hypothalamic nuclei; however, the role in the control of energy balance for BDNF produced in these structures remains largely unknown. We found that deleting the Bdnf gene in the ventromedial hypothalamus (VMH) during embryogenesis using the Sf1-Cre transgene had no effect on body weight in mice. In contrast, deleting the Bdnf gene in the adult VMH using Cre-expressing virus led to significant hyperphagia and obesity. These observations indicate that the lack of a hyperphagia phenotype in the Sf1-Cre/Bdnf mutant mice is likely due to developmental compensation. To investigate the role of BDNF expressed in other hypothalamic areas, we employed the hypothalamus-specific Nkx2.1-Cre transgene to delete the Bdnf gene. We found that the Nkx2.1-Cre transgene could abolish BDNF expression in many hypothalamic nuclei, but not in the PVH, and that the resulting mutant mice developed modest obesity due to reduced energy expenditure. Thus, BDNF produced in the VMH plays a role in regulating energy intake. Furthermore, BDNF expressed in hypothalamic areas other than PVH and VMH is also involved in the control of energy expenditure.

Anxiety induced by extra-hypothalamic BDNF deficiency instigates resistance to diet-induced obesity

Anxiety disorders are associated with body weight changes in humans. However, mechanisms underlying anxiety-related weight changes remain poorly understood. Using Emx1Cre/+ mice, we deleted the gene for brain-derived neurotrophic factor (BDNF) in the cortex, hippocampus, and some parts of the amygdala. The resulting mutant mice displayed elevated anxiety levels and were markedly lean when fed either chow diet or high-fat diet (HFD). The mice showed higher levels of sympathetic activity, thermogenesis and lipolysis in both brown and white adipose tissues, and higher oxygen consumption and body temperature, compared with control mice. They were still lean at thermoneurality when fed HFD, indicating elevated basal metabolism in addition to activated thermogenesis. Anxiety induced by site-specific Bdnf deletion similarly increased energy expenditure and minimized HFD-induced weight gain. These results reveal that anxiety can stimulate adaptive thermogenesis and basal metabolism by activating sympathetic nervous system, which enhances lipolysis and limits weight gain.