Neurotrophic factors and Their receptors Huang Shu-HongInstitute of neurobiology1nFor decades, scientists believed that brain cells of the central nervous system could not regrow following damage due to trauma such as head injury or disorders such as Alzheimers disease. Treatments did not exist. The outlook was bleak. 2nBut now that thinking has been turned upside down. Scientists recently have discovered a whole family of proteins called neurotrophic factors. These proteins play a crucial role in the development and survival of nerve cells, or neurons, and in supporting adult neurons to keep them healthy throughout life. 31934: Victor Hamburger discovered that removal of a limb bud resulted in reduced numbers of sensory and motor neurons in the spinal cord.The neurotrophic factor hypothesis4Effect of Removing or Augmenting Neural Targets on the Survival of Related NeuronsnPN23092.JPG 1939: Victor Hamburger showed that transplantation of a supernumerary limb resulted in increased numbers of sensory and motor neurons in the spinal cord.5Based on his limb-bud experiments, V. Hamburger hypothesized that the targets of innervating neurons provide signals that recruit undifferentiated cells to develop into sensory or motor neurons.(he was wrong)In 1942, Levi-Montalcini and Levi proposed that target derived signals maintain survival of differentiating neurons. In 1949, Hamburger and Levi-Montalcini repeated the limb bud experiments and found that their results supported the neurotrophic hypothesis.Hamburger, V. and Levi-Montalcini, R. (1949) J. Exp. Zool. 111: 457-502.The neurotrophic factor hypothesis671954: neurite outgrowth assay extract+ extract1969: NGF purified to homogeneityStanley CohenRita Levi-Montalcini1986: Levi-Montalcini and Cohen split the Nobel prize for Physiology or Medicine “for their discovery of growth factors”1960: NGF purified8NGF: sympathetic neurons and some sensory neurons(CNS neurons do not require NGF for survival)BDNF: NGF-related factor purified in 1982 from pig brain (shares 50% homolog with NGF)NT-3 and NT-4/5: were obtained by PCR cloningAll these factors are synthesized as 250 aa precursors that are processed into 120 aa proteinsThe main List of Neurotrophic Factors9Neurotrophins BDNF (brain derived neurotrophic factor) on cortical neurons10111213Trk receptor signalingWhen a neurotrophin binds to a trk receptor, the kinase domain is activated resulting in autophosphorylation.Autophoshorylation results in further activation of the kinase domain, leading to activation of three potential signaling cascades:MAPKPI3KPLC-g g1415Other neurotrophic Factors1617Structure of neurotrophic Factor receptors18Alternative splicing generates many Trk receptor isoformsRoux and Barker (2002) Prog Neurobiol 67:203-233NGF KD for TrkA=10-11M19p75NTR structureNGF KD=10-9M(all neurotrophinscan bind p75NTR)Roux and Barker (2002) Prog Neurobiol 67:203-23320Models for Trk and p75NTR interactionChao and Bothwell (2002) Neuron 33:9-1221The effect of NT/NTR knockouts on neurons in the DRG Farias et al. (2002) Brain Res Bull 57:809-81622Our axons can be 1 m in length-how does the neurotrophin/receptor complex signal to the neuronal cell body?Miller and Kaplan (2001) Neuron 32:767-77023Campenot, RB (1977) Local control of neurite development by nerve growth factor. Proc Natl Acad Sci U S A. 74(10):4516-9. (A method that can be used to study how NTs added to distal axons signal retrogradely)Miller and Kaplan (2001) Neuron 32:767-770NGF+K252a2425Activated Trk can signal locally and retrogradely using different signalling pathwaysMiller and Kaplan (2001) Neuron 32:767-770Slow (2-20 mm/hr)26Differential control of TrkA trafficking and signaling may also be the basis for the different functions of NGF and NT-3Kuruvilla et al. (2004) Cell 118: 243-25527In vitro assays have shown that NTs enhance both axonal and dendritic growthIn vivo, the situation is more difficult to studyWhy? In standard knockouts, it is difficult to separate the survival effects of NTs from their effects on the morphology of neurons.This problem has begun to be addressed by using conditional knockouts, or by crossing NT knockouts with mouse mutants lacking pro-apoptotic genes.Recent evidence from these kinds of experiments suggests that long distance peripheral sensory axon growth in vivo is NT-dependent.28NTs roles in neuronal development and function NTs are expressed in regions of the developing embryo that are traversed by sensory axons en route to their targets. NTs affect the proliferation and differentiation of CNS neuroepithelial progenitors, neural crest cells, and progenitors of enteric neurons in vitro (and in some cases also confirmed in vivo).In the CNS, BDNF/TrkB signaling is implicated in the development and maintenance of cortical circuits.29NTs in the CNSThe highest levels of NTs are found in the hippocampusLindsay et al. (1995) Trends Neurosci 17:182-190.30Cytokines and Growth Factors in the Nervous SystemCNTF, LIF (oncostatin M, cardiotrophin-1): neuropoietic cytokines. These factors may be important in neuronal response to injury.GDNF, neurturin, artemin, persephin: exhibit distant homology with the TGF-b family. They signal through a receptor complex composed of the Ret tyrosine kinase and a GPI-linked binding subunit (GFRa family; GFRa1, a2, a3, and a4). These factors are potent axon-promoting growth factors in vivo for developing sympathetic and parasympathetic neurons.31SummarynNeurotrophic factors play essential roles in the developing and mature nervous system. nNow more than 50 years old, this field of study continues to contribute breakthrough discoveries regarding the fundamental biology of cell-cell communication and the development and maintenance of the most complex and elegant biological machine - the brain. Neurotrophic factors influence cell proliferation, survival, differentiation, migration, axon and dendrite growth, synaptic plasticity and the interactions of neuronal and glial cells. 32What is a growth factor ?What is a trophic factor ?nDEFINITIONnGrowth factors include substances that stimulate cells to dividen(hyperplasia) or increase in size (hypertrophy). Many growthnfactors are now known to existnTrophic factors include those substances that have effects onncell differentiation, cell survival, expression of a specific cellularnphenotype (e.g. a cell becomes an inhibitory or an excitatorynneuron), cellular morphological plasticity, as well as cellnhypertrophy including, for example, the induction of neuritenextention (also considered a trophic action). Importantly, somengrowth factors may also act as trophic factors and viceversa andneach growth/trophic factor may have a specific combination ofncellular effects.33Neurotrophic factorsnNeurotrophic factors are endogenous soluble proteins regulating survival, growth, morphological and synaptic* plasticity, or synthesis of proteins for differentiated functions of neurons. (* synaptic plasticity = regulation of the transmission activity at the level of the synapse)34Which cell sythesizes the neurotrophic factors ?nNeurotrophic factors are produced by all cells of the nervous system, including neurons, glial cells, ependimal cells, blood vessels endothelial cells and cells from innervated tissues such as muscles, epidermis, etc35