Point Mutation in trkB Causes Loss of NT4-Dependent Neurons without Major Effects on Diverse BDNF Responses

Abstract

Neurotrophins are a family of soluble ligands that promote the survival and differentiation of peripheral and central neurons and regulate synaptic function. The two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), bind and activate a single high-affinity receptor, TrkB. Experiments in cell culture have revealed that an intact Shc adaptor binding site on TrkB and subsequent activation of the Ras/MAPK pathway are important for neuronal survival and neurite outgrowth. To elucidate the intracellular signaling pathways that mediate the diverse effects of BDNF and NT4 in vivo, we have mutated in the mouse germline the Shc binding site in the trkB gene. This trkB(shc) mutation revealed distinctive responses to BDNF and NT4. While nearly all NT4-dependent sensory neurons were lost in trkB(shc/shc) mutant mice, BDNF-dependent neurons were only modestly affected. Activation of MAP kinases and in vitro survival of cultured trkB(shc/shc) neurons were reduced in response to both neurotrophins, with NT4 being less potent than BDNF, suggesting differential activation of TrkB by the two ligands. Moreover, while the Ras/MAPK pathway is required for in vitro differentiation of neuronal cells, trkB(shc/shc) mutant mice do not show any defects in BDNF-dependent differentiation of CNS neurons or in the function of sensory neurons that mediate innocuous touch.

Restoration of synapse formation inMuskmutant mice expressing a Musk/Trk chimeric receptor

“…For example, in TrkA, the receptor for nerve growth factor, phosphorylation of a juxtamembrane tyrosine leads to Ras and PI3-kinase activation, which are important for cell survival and neurite outgrowth (Greene and Kaplan, 1995;Huang and Reichardt, 2001), whereas phosphorylation of a tyrosine in the C-terminal region of TrkA leads to PLCγ activation, which is crucial for NGF-dependent Na + channel and VRI channel regulation (Choi et al, 2001;Chuang et al, 2001). Moreover, in TrkB, phosphorylation of a single juxtamembrane tyrosine is required for nearly all of NT4-dependent signaling whereas phosphorylation of other tyrosine residues are required to mediate BDNF-dependent signaling in vivo (Minichiello et al, 1998). Similarly, in Met, the receptor for hepatocyte growth factor, a binding site for Grb2 is essential for late steps in myogenesis (Maina et al, 1996), a binding-site for PI3-kinase is essential for placental development, hepatocyte survival and myoblast migration, and a Src binding-site is essential for motor axon outgrowth (Maina et al, 2001).…”

Neurotrophin-mediated neuroprotection of hippocampal neurons following traumatic brain injury is not associated with acute recovery of hippocampal function

“…The NT-4/5 gene codes for a 26-kDa homodimeric protein of 123 amino acids, sharing 54% homology with BDNF (Ip et al, 1992, Ibanez, 1996, Salin et al, 1997. Although NT-4/5 and BDNF are both highaffinity ligands for the TrkB receptor, the two neurotrophins have distinct biological activities in the nervous system (Klein et al, 1992, Curtis et al, 1995, McAllister et al, 1995, Ryden et al, 1995, Strohmaier et al, 1996, Minichiello et al, 1998, and some populations of neurons require NT-4/5 but not BDNF for survival (Ardelt et al, 1994, Conover et al, 1995, Liu et al, 1995, Riddle et al, 1995. Furthermore, replacing the spatio-temporal expression pattern of BDNF with NT4/5 by 'knocking-in' NT4/5 to the BDNF locus does not rescue all of the deficits, providing direct evidence that the two neurotrophins have some non-overlapping actions (Fan et al, 2000).…”

“…Interestingly, infusion of NT-4/5 but not BDNF is neuroprotective after LFP-TBI (Blaha et al, 2000, Royo et al, 2006. The mechanisms for this difference may include partially different binding sites on the trkB receptor (Klein et al, 1992), differential activation of a particular shc domain on the cytoplasmic portion of the trkB receptor (Minichiello et al, 1998), and/or differential downregulation of trkB (Frank et al, 1996, Knusel et al, 1997). …”

Recent and Emerging Trends in Pharmacotherapy of Neuropathic Pain

“…[75] NT-4 is synthesized by DRG and expressed in the rat spinal cord. [76] It is a ligand of the TrkB tyrosine kinase receptor, but it mediates to diverse effects in relation to BDNF. [74] It has also been demonstrated that repeated injections of a specific antibody to NT-4 failed to reverse the thermal hyperalgesia caused by sciatic nerve ligation in mice.…”

“…[74] It has also been demonstrated that repeated injections of a specific antibody to NT-4 failed to reverse the thermal hyperalgesia caused by sciatic nerve ligation in mice. [76] Glial cell line-derived neurotrophic factor (GDNF) has been shown to prevent and reverse sensory abnormalities that developed in various NP models, without affecting pain-related behavior. GDNF reduces ectopic discharges within sensory neurons after nerve injury and this effect may involve modulation of sodium channels.…”