By: Susana Cohen-Cory, Adhanet H. Kidane, Nicole J. Shirkey, Sonya Marshak
Department of Neurobiology and Behavior, University of California Irvine, Irvine, California 92697
Received 15 August 2009; revised 17 November 2009; accepted 19 November 2009
ABSTRACT: During development, neural networks are established in a highly organized manner, which persists throughout life. Neurotrophins play crucial roles in the developing nervous system. Among the neurotrophins, brain-derived neurotrophic factor (BDNF) is highly conserved in gene structure and function during vertebrate evolution, and serves an important role during brain development and in synaptic plasticity. BDNF participates in the formation of appropriate synaptic connections in the brain, and disruptions in this process contribute to disorders of cognitive function.
In this review, we first briefly highlight current knowledge on the expression, regulation, and secretion of BDNF. Further, we provide an overview of the possible actions of BDNF in the development of neural circuits, with an emphasis on presynaptic actions of BDNF during the structural development of central neurons.
Keywords:
Neurotrophin; central nervous system; axon;dendrite; synapse.
INTRODUCTION
Neurotrophins are growth factors with crucial roles in the developing and mature nervous system.
They are initially synthesized as precursor proteins (proneurotrophins), which are processed intracellularly to be secreted mostly in a mature, biologically active form (Mowla et al., 1999, 2001; Matsumoto et al., 2008; for review see Lu et al., 2005). Proneurotrophins can also influence developing and mature neural circuits, and may be released in a developmentally regulated manner (Lee et al., 2001; Lu et al., 2005; Teng et al., 2005; Yang et al., 2009). Neurotrophins bind two classes of membrane receptors, the tropomyosin receptor kinase (Trk) family of receptors and the p75 neurotrophin receptor (p75NTR) (for a review see Chao, 2003). The actions of mature neurotrophins are mediated by the high affinity full-length Trk receptors, which signal through their intrinsic tyrosine kinase activity to promote growth. Trk receptors signal by dimerization of receptor molecules, leading to intracellular phosphorylation and activation of intracellular signaling cascades (Ullrich and Schlessinger, 1990; Jing et al., 1992). Truncated Trk receptors (Trk.T) are splice variants of full-length Trks, which lack the intracellular tyrosine kinase domain, and are thought to act as negative effectors of full-length receptors (Luikart et al., 2003), although they may also have their own signaling properties (Rose et al., 2003; Ohira et al., 2006). The neurotrophins show binding specificity for particular Trk receptors: nerve growth factor (NGF) binds to TrkA, brain-derived neurotrophic factor (BDNF) and neurotrophin 4 (NT4) to TrkB, and neurotrophin 3 (NT3) to TrkC(Chao, 2003). The p75NTR has low affinity for the mature neurotrophins, but can form a complex with Trk receptors to form high affinity binding sites for neurotrophins, enabling the receptor to participate in the stimulation of growth processes (Esposito et al., 2001). Moreover, p75NTR displays high affinity binding with proneurotrophins, and induces apoptosis by interacting with sortilin (Lee et al., 2001; Nykjaer et al., 2004). Thus, proneurotrophins and mature neurotrophins may utilize distinct receptors to mediate divergent neuronal actions. This review focuses on the actions of mature BDNF, highlighting the role that BDNF plays in the development of synaptic connectivity in the central nervous system (CNS). Evidence supporting presynaptic actions by target-released BDNF, and the influence that BDNF exerts during the structural development of neurons are reviewed here.
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