Baoji Xu - US grants

DESCRIPTION (provided by applicant): Neurotrophins regulate function and survival of many populations of neurons in the central nervous system and their deficiencies may play a pivotal role in the progression of neurodegenerative diseases. This application aims to examine the role of brain-derived neurotrophic factor (BDNF) in the pathogenesis of Huntington's disease (HD) in which striatal medium spiny neurons are selectively lost. HD is caused by expansion of CAG repeats in the HD gene that encodes huntingtin. Striatal neurons express the BDNF receptor, TrkB, but do not express BDNF which mainly arrives at the striatum by anterograde transport from cortical neurons. It has been shown that the HD mutation reduces the BDNF level in the striatum by inhibiting BDNF gene transcription and anterograde transport. This project will test the hypothesis that the reduced striatal BDNF supply is relevant to the pathogenesis of HD by examining the role of TrkB signaling in the normal function and survival of striatal neurons, by determining whether a reduction in TrkB signaling sensitizes striatal neurons to the toxic effect of mutant huntingtin, and by testing whether increasing the striatal BDNF level delays the onset of HD. Mouse mutants in which the trkB gene is specifically deleted in the striatum will be used to examine whether TrkB signaling is required for the normal function and survival of striatal neurons. One trkB hypomorphic allele will be used to produce HD transgenic mice that express TrkB at 100% or 25% of the normal amount. These mutant mice will then be used to determine whether striatal neurons expressing mutant huntingtin are more dependent on neurotrophic protection. Finally, the possibility that BDNF overexpression in cortical neurons delays the onset of HD will be investigated in HD and BDNF double transgenic mice. This research may lead to discovery of the BDNF-TrkB pathway as a promising target for designing effective treatments of Huntington's disease.

DESCRIPTION (provided by applicant): Brain-derived neurotrophic factor (BDNF) and its TrkB receptor have been implicated in the control of energy balance in both humans and mice. Previous studies suggest that BDNF expressed in the ventromedial hypothalamus (VMH) plays a critical role in regulating energy balance. BDNF protein is synthesized in both neuronal somas and dendrites. In this application, we will examine the role of dendritic local BDNF synthesis in the control of energy balance. Aim 1 is to determine if leptin regulates local synthesis of BDNF in dendrites. We will employ green fluorescence protein reporter constructs and synaptoneurosome preparations to examine the effect of leptin on dendritic transport and translation of BDNF mRNA. Aim 2 is to determine if dendritically synthesized BDNF in the VMH regulates food intake and body weight. We will attempt to rescue the obese phenotype in a Bdnf mouse mutant which lacks dendritic local BDNF synthesis through viral expression of dendritically localized BDNF mRNA or somatically localized BDNF mRNA in the VMH. Aim 3 is to examine if lack of local BDNF synthesis alters morphologies of dendritic spines of TrkB-expressing hypothalamic neurons. There are TrkB-expressing neurons in the hypothalamic nuclei important for the control of energy balance, such as the arcuate nucleus (ARC) and dorsomedial hypothalamus (DMH). We will examine spine morphologies of TrkB-expressing neurons in the ARC and DMH in the mouse mutant deficient in local BDNF synthesis. PUBLIC HEALTH RELEVANCE Identification and characterization of new molecules and signaling pathways that control energy balance will offer opportunities for designing improved obesity treatments. If successful, findings from this proposed project will suggest that mutations in proteins involved in transport and translation of dendritic mRNA will increase susceptibility to obesity. Therefore, these proteins could be potential drug targets for obesity treatment.

DESCRIPTION (provided by applicant): Loss of fragile X mental retardation protein (FMRP) leads to fragile X syndrome, the most common form of inherited mental retardation, which is associated with cognitive impairment and more numerous and thinner dendritic spines in cortical neurons. FMRP is an mRNA binding protein and has been shown to stabilize select mRNA species and to repress local protein synthesis in dendrites. However, its role in dendritic targeting of mRNAs remains to be determined, although it is associated with transport RNA granules. Mice with deletion of the gene for FMRP, Fmr1, have deficits in hippocampus-dependent learning. In agreement with this phenotype, long-term potentiation (LTP) elicited by threshold levels of theta burst stimulation is impaired in hippocampal slices prepared from Fmr1 knockout (Fmr1 KO) mice, which can be rescued by application of brain-derived neurotrophic factor (BDNF). The Bdnf gene produces two pools of BDNF mRNA species, with either a short 3'untranslated region (3'UTR) or a long 3'UTR. The long 3'UTR has been shown to be necessary and sufficient to target BDNF mRNA to dendrites. Interestingly, a mouse mutant in which dendritic targeting of BDNF mRNA is impaired also exhibits more numerous dendritic spines and deficits in LTP at hippocampal CA1 synapses, as does the Fmr1 knockout. This research project will determine the role of FMRP in trafficking of BDNF mRNA to dendrites. In Aim 1 the association of FMRP with BDNF mRNA will be examined in brain lysates using immunoprecipitation and in cultured neurons using in situ hybridization and immunocytochemistry. Electrophoretic mobility shift assays will be employed to identify FMRP binding sites in the long BDNF 3'UTR. In Aim 2 two studies will be carried out to determine whether FMRP is required for trafficking of BDNF mRNA to dendrites. One is to examine levels of BDNF mRNA in dendrites of Fmr1 KO neurons using in situ hybridization, and the other is to determine the effect of mutations in the FMRP binding sites on dendritic targeting of BDNF mRNA in cultured neurons. In Aim 3 the role of FMRP in the local synthesis of the BDNF protein will be examined in cultured neurons and synaptoneurosome preparations. PUBLIC HEALTH RELEVANCE: Fragile X syndrome is the most common form of inherited mental retardation, which is associated with a wide range of neurological symptoms and signs. This research will make a significant contribution to the understanding of fragile X syndrome.

DESCRIPTION (provided by applicant): Loss of fragile X mental retardation protein (FMRP) leads to fragile X syndrome, the most common form of inherited mental retardation, which is associated with cognitive impairment and more numerous and thinner dendritic spines in cortical neurons. FMRP is an mRNA binding protein and has been shown to stabilize select mRNA species and to repress local protein synthesis in dendrites. However, its role in dendritic targeting of mRNAs remains to be determined, although it is associated with transport RNA granules. Mice with deletion of the gene for FMRP, Fmr1, have deficits in hippocampus-dependent learning. In agreement with this phenotype, long-term potentiation (LTP) elicited by threshold levels of theta burst stimulation is impaired in hippocampal slices prepared from Fmr1 knockout (Fmr1 KO) mice, which can be rescued by application of brain-derived neurotrophic factor (BDNF). The Bdnf gene produces two pools of BDNF mRNA species, with either a short 3'untranslated region (3'UTR) or a long 3'UTR. The long 3'UTR has been shown to be necessary and sufficient to target BDNF mRNA to dendrites. Interestingly, a mouse mutant in which dendritic targeting of BDNF mRNA is impaired also exhibits more numerous dendritic spines and deficits in LTP at hippocampal CA1 synapses, as does the Fmr1 knockout. This research project will determine the role of FMRP in trafficking of BDNF mRNA to dendrites. In Aim 1 the association of FMRP with BDNF mRNA will be examined in brain lysates using immunoprecipitation and in cultured neurons using in situ hybridization and immunocytochemistry. Electrophoretic mobility shift assays will be employed to identify FMRP binding sites in the long BDNF 3'UTR. In Aim 2 two studies will be carried out to determine whether FMRP is required for trafficking of BDNF mRNA to dendrites. One is to examine levels of BDNF mRNA in dendrites of Fmr1 KO neurons using in situ hybridization, and the other is to determine the effect of mutations in the FMRP binding sites on dendritic targeting of BDNF mRNA in cultured neurons. In Aim 3 the role of FMRP in the local synthesis of the BDNF protein will be examined in cultured neurons and synaptoneurosome preparations. PUBLIC HEALTH RELEVANCE: Fragile X syndrome is the most common form of inherited mental retardation, which is associated with a wide range of neurological symptoms and signs. This research will make a significant contribution to the understanding of fragile X syndrome.