The neurotrophic hypothesis: Targets of innervation secrete limiting amounts of survival factors to generate a balance between the size of the target organ and the number of innervating neurons. Note: This hypothesis is mainly concerned with peripheral neurons and peripheral target tissues.

Effect of Removing or Augmenting Neural Targets on the Survival of Related Neurons PN23091.JPG 1934: Victor Hamburger discovered that removal of a limb bud resulted in reduced numbers of sensory and motor neurons in the spinal cord.

Effect of Removing or Augmenting Neural Targets on the Survival of Related Neurons PN23092.JPG 1939: Victor Hamburger showed that transplantation of a supernumerary limb resulted in increased numbers of sensory and motor neurons in the spinal cord.

Based on his limb-bud experiments, V Based 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.

1954: neurite outgrowth assay 1960: NGF purified 1969: NGF purified to homogeneity – extract Stanley Cohen Rita Levi-Montalcini 1986: Levi-Montalcini and Cohen win the Nobel prize for Physiology or Medicine “for their discovery of growth factors” + extract

NGF: 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 cloning All these factors are synthesized as ~250 aa precursors that are processed into 120 aa proteins

Neurotrophin Evolution Neurotrophins have only been isolated from chordates Hallbook (1999) Curr Opin Neurobiol 9: 616-21

Structure of NGF bound to its receptor

The Trk Family of Receptor Tyrosine Kinases for the Neurotrophins PN23160.JPG p75NTR: purified and cloned 1st, homology to TNFR Trk: tropomyosin-related kinase, originally known as orphan receptors

(Found only in fish)

The effect of NT/NTR knockouts on neurons in the DRG Fariñas et al. (2002) Brain Res Bull 57:809-816

Trk receptor signaling When 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: MAPK PI3K PLC-g

Our axons can be >1 m in length---how does the neurotrophin/receptor complex signal to the neuronal cell body? Miller and Kaplan (2001) Neuron 32:767-770

Campenot, 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) NGF +K252a Miller and Kaplan (2001) Neuron 32:767-770

In vitro assays have shown that NTs enhance both axonal and dendritic growth In vivo, the situation is more difficult to study Why? 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.

NT’s 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.

What happens to neurons in the absence of neurotrophic factors? ap·o·pto·sis Pronunciation: "a-p&p-'tO-s&s, -p&-'tO- Function: noun Inflected Form(s): plural ap·o·pto·ses /-"sEz /Etymology: New Latin, from Greek apoptOsis a falling off first use in Kerr et al. (1972) “Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics.” Br. J. Cancer 26: 239-257 Apoptosis is a mechanism contributing to programmed cell death (PCD)

Stages during neuronal development where PCD occurs

Morphological types of cell death Apoptosis: originally defined according to a set of characteristic ultrastructural features that include nuclear and cytoplasmic condensation, cell fragmentation and phagocytosis. Necrosis: cell death as the result of injury, disease, or pathological state (usually involves large numbers of cells and is associated with inflammation). Chromatin condenses in multiple small clumps and at later stages, cell membranes and organelles disintegrate. Autophagy: (from the Greek, self-eating) Cytoplasm is destroyed by lysosomal enzymes before any nuclear changes become visible. A characteristic feature is the appearance of large autophagic vacuoles in the cytoplasm. At later stages, chromatin condenses, DNA laddering is evident and phagocytosis occurs.

Necrosis PCD apoptotic autophagic Morphology of cell death

Ultrastructure of cell death apoptosis necrosis

PCD also occurs in the glial lineage