NEUROTROPHIC REGULATION OF SYNAPSE DEVELOPMENT AND PLASTICITY
, Head, Section on Neural Development and Plasticity
Feng Yang, MD, PhD, Staff Scientist
Kazuko Sakata, PhD, Research Fellow
Sundar Ganeson, PhD, Visiting Fellow
Yuanyuan Ji, PhD, Visiting Fellow
Guhan Nagappan, PhD, Visiting Fellow
Newton Woo, PhD, Visiting Fellow
Evan Galloway, BS, Predoctoral Fellow
We investigate the role of neurotrophins, a class of secretory proteins critical for synapse development and plasticity. We were among the first to elucidate the synaptic functions of neurotrophins and discovered that (1) brain-derived neurotrophic factor (BDNF) promotes the development of early-phase long-term potentiation (E-LTP) in the hippocampus, which is mediated by an enhanced response to tetanic stimulation through vesicle docking; (2) BDNF elicits synapse-specific modulation by regulating TrkB receptor trafficking through activity-dependent insertion, endocytosis, synaptic localization, and so forth; (3) the two modes of neurotrophic regulation are acute modulation of synaptic transmission and plasticity and long-term alteration of the structure and function of synapses; (4) a single nucleotide polymorphism in the prodomain of BDNF affects activity-dependent BDNF secretion, resulting in impairment in hippocampal function and short-term memory in human; (5) extracellular conversion of proBDNF to mature BDNF by protease tPA/plasmin is essential for late-phase LTP (L-LTP), a cellular model for long-term memory; and (6) proBDNF, if uncleaved, facilitates hippocampal long-term depression (LTD) by activating the p75NTR receptor. This year, we revealed several new aspects of neurotrophin function.
Regulation of the development of the neuromuscular junction
Neurotrophins elicit both structural and functional changes in synapses. It is unknown whether these changes are mediated by the same or different mechanisms. Using the neuromuscular junction (NMJ) as a model, we reported on the mechanistic separation of functional and structural synaptic regulation by neurotrophin 3 (NT-3). Inhibition of cAMP response element (CRE) binding protein (CREB)–mediated transcription blocked the enhancement of transmitter release elicited by NT-3 without affecting the synaptic varicosity of the presynaptic terminals. Further analysis indicated that CREB is activated through the Ca2+/calmodulin-dependent kinase IV (CaMKIV) pathway rather than through the mitogen-activated protein kinase (MAPK) or cAMP pathway. In contrast, inhibition of MAPK prevented NT-3–induced structural, but not functional, changes. Genetic and imaging experiments indicated that the small GTPase Rap1, but not Ras, acts upstream of MAPK activation by NT-3. Thus, NT-3 initiates parallel structural and functional modifications of synapses through the Rap1-MAPK and CaMKIV-CREB pathways, respectively. Our findings may have implications for the general mechanisms of long-term synaptic modulation by neurotrophins.
Long-term synaptic modulation by neurotrophins requires protein synthesis, but the role of protein degradation has not been studied. We investigated whether the ubiquitin-proteasome pathway is involved in the development of the NMJ. We identified a PDZ domain containing RING finger 3 (PDZRN3) as a synapse-associated E3 ubiquitin ligase and demonstrated that it regulates the surface expression of the muscle-specific receptor tyrosine kinase (MuSK), the key organizer of postsynaptic development at the NMJ. PDZRN3 binds to MuSK and promotes its ubiquitination. Regulation of cell-surface levels of MuSK by PDZRN3 requires the ubiquitin ligase domain and is mediated by accelerated endocytosis. Gain- and loss-of-function studies in cultured myotubes showed that regulation of MuSK by PDZRN3 plays an important role in MuSK-mediated ACh receptor clustering. Furthermore, overexpression of PDZRN3 in skeletal muscle of transgenic mice perturbs the growth and maturation of the NMJ. These results identify a synapse-associated E3 ubiquitin ligase as an important regulator of synapse development.
Je H-S, Yang F, Zhou J, Lu B. Neurotrophin 3 induces structural and functional modification of synapses through distinct molecular mechanisms. J Cell Biol 2006;18:1029-42.
Lu Z, Je H-S, Young P, Gross J, Lu B, Feng G. Regulation of synaptic growth and maturation by a synapse-associated E3 ubiquitin ligase at the neuromuscular junction. J Cell Biol 2007;18:1077-89.
Neurotrophic regulation of adult neurogenesis, memory, and mood disorders
While adult neurogenesis is thought to play a role in learning and memory as well as in depression, how newly generated neurons contribute to the cognitive process remains unknown. Fibroblast growth factor 2 (FGF-2) is known to stimulate the proliferation of neuronal progenitor cells (NPCs) in adult brain. Using conditional knockout mice that lacked brain expression of FGFR1, a major receptor for FGF-2, we investigated the role of adult neurogenesis in hippocampal synaptic plasticity and in learning and memory. Bromodeoxyuridine labeling experiments demonstrated that FGFR1 is required for the proliferation of NPCs as well as for the generation of new neurons in the adult dentate gyrus (DG). Moreover, deficits in neurogenesis in Fgfr1 mutant mice are accompanied by a severe impairment of LTP at the medial perforant path (MPP)–granule neuron synapses in the hippocampal dentate. Finally, Fgfr1 mutant mice exhibit significant deficits in memory consolidation but not in spatial learning. Our study suggests a critical role of FGFR1 in adult neurogenesis in vivo, provides a potential link between proliferative neurogenesis and dentate LTP, and raises the possibility that adult neurogenesis might contribute to memory consolidation. The role of FGFR1 in depression remains to be investigated further.
The “neurotrophin hypothesis of depression” is based largely on correlations between stress or antidepressant treatment and down- or upregulation, respectively, of BDNF. The current status of research on BDNF and depression suggests that genetic disruption of the signaling pathways involving BDNF and its TrkB receptor does not cause depressive behaviors but does hamper the effect of antidepressant drugs. Thus, BDNF may be a target of antidepressants, but not the sole mediator of depression or anxiety. Advances in BDNF cell biology, including an understanding of BDNF’s transcription through several promoters, its trafficking and secretion, may provide new insights into the role of the neurotrophin in mood disorders. Moreover, given that the precursor proBDNF and the mature protein mBDNF can elicit opposite effects on cellular functions, the impact of proBDNF and its cleavage on mood should be considered. Opposing influences of mBDNF and proBDNF on LTP and LTD might contribute to the dichotomy of BDNF actions on behaviors mediated by the brain’s stress and reward systems.
BDNF and serotonin (5-hydroxytryptamine, or 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions, including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. The data on the relationship between BDNF and 5-HT in the context of depression and anxiety indicate that BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant SSRIs (selective serotonin reuptake inhibitors) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.
Martinowich K, Lu B. Interaction between BDNF and serotonin: role in mood disorders. Neuropsychopharmacology Rev 2007 [E-pub ahead of print].
Martinowich K, Manji H, Lu B. New perspective of BDNF cell biology and its implications in depression. Nat Neurosci 2007;10:1089-93.
Zhao M, Li D, Shimazu K, Zhou YX, Deng C-X, Lu B. Fibroblast growth factor receptor-1 is required for long-term potentiation, memory consolidation, and neurogenesis. Biol Psychiatry 2007;62:381-90.
Studies on genes involved in schizophrenia
We have initiated a new line of research aimed at studying genes involved in cognition and schizophrenia. We recently collaborated with Hong-Jun Song’s group to investigate the role of Disrupted-In-Schizophrenia 1 (disc1), a susceptibility gene for schizophrenia. DISC1 expression is broad in many brain regions during embryonic development and fairly restricted in the adult brain, with particularly high expression in dentate granule cells of the hippocampus and interneurons of the olfactory bulb, two neuronal types that are continuously generated through adult neurogenesis. Biochemical identification of interacting proteins first suggested a role for DISC1 in neuronal development. For example, DISC1 binds to Ndel1 (NUDEL), a molecule involved in embryonic neuronal development, including migration. In vitro studies with PC12 cells and primary neurons showed that blocked DISC1 function impairs neurite outgrowth while in-utero, electroporation-mediated expression of DISC1 shRNAs in embryos leads to retarded migration. The finding that DISC1 promotes migration in the embryonic cortex and neurite outgrowth in vitro, coupled with restricted expression of DISC1 in neurons produced during adult neurogenesis, raises a tantalizing possibility that DISC1 may play an important role in regulating the process of adult neurogenesis.
To ascertain the in vivo function of DISC1 in adult neurogenesis, we used an oncoretrovirus-mediated RNA interference approach for genetically manipulating DISC1 expression within individual cells in specific brain regions. Such an in vivo “single-cell genetic” approach permits characterization of cell-autonomous roles of DISC1 specifically in adult neurogenesis, without the complication of potential developmental defects and/or compensations in traditional germ-line knockout animals. We demonstrated that DISC1 regulates almost all essential steps of neuronal integration in adult neurogenesis. In contrast to what has been found in embryonic cortical development and cultured neuronal cells, DISC1 knockdown in newborn dentate granule cells of the adult hippocampus leads to soma hypertrophy, accelerated dendritic outgrowth with the appearance of ectopic dendrites, mis-positioning from overextended migration, enhanced intrinsic excitability, and accelerated synapse formation of new neurons. These findings indicate that DISC1 is a major regulator that controls the tempo of neuronal development and therefore keeps in check the progress of new neuron integration in the adult brain.
Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu X, Yang CH, Jordan DJ, Ma DK, Liu CY, Ganesan S, Cheng H-J, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell 2007;130:1146-58.
Galloway E, Woo N, Lu B. Functional role of BDNF in prefrontal cortex. Progr in Brain Res 2007, in press.
Lu B, Martinowich K. Cell biology of BDNF and its relevance to schizophrenia. Novartis Foundation Symposium 2007;296, in press.
Lu Y, Christian K, Lu B. BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory? Neurobiol Learn Mem 2007 [E-pub ahead of print].
1 Hyon-Soo Je, BS, former Predoctoral Fellow
2 Mingrui Zhao, PhD, former Visiting Fellow
3 Kazuhiro Shimazu, MD, PhD, former Visiting Fellow
COLLABORATOR
Hwai-Jong Cheng, PhD, University of California Davis, Davis, CA
Chuxia Deng, PhD, Genetics of Development and Disease Branch, NIDDK, Bethesda, MD
Xin Duan, BS, The Johns Hopkins University School of Medicine, Baltimore, MD
Regina L. Faulkner, University of California Davis, Davis, CA
Guoping Feng, PhD, Duke University Medical Center, Durham, NC
Shaoyu Ge, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Jimmy Gross, Duke University Medical Center, Durham, NC
J. Dedrick Jordan, MD, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Ju Young Kim, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Yasuji Kitabatake, MD, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Dan Li, PhD, Genetics of Development and Disease Branch, NIDDK, Bethesda, MD
Cindy Y. Liu, The Johns Hopkins University School of Medicine, Baltimore, MD
Zhonghua Lu, Duke University Medical Center, Durham, NC
Dengke K. Ma, BS, The Johns Hopkins University School of Medicine, Baltimore, MD
Husseini Manji, MD, FRCPC, Laboratory of Molecular Pathophysiology, NIMH, Bethesda, MD
Guo-li Ming, MD, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Hongjun Song, PhD, The Johns Hopkins University School of Medicine, Baltimore, MD
Chih-Hao Yang, Graduate Student, The Johns Hopkins University School of Medicine, Baltimore, MD
Paul Young, PhD, Duke University Medical Center, Durham, NC
Zhuan Zhou, PhD, Institute of Neuroscience, Shanghai, China
For further information, contact lub@mail.nih.gov.
