Presentation on theme: "Axon Outgrowth and Pathfinding. Wiring of the nervous system Axon guidance is part of a genetic program that controls neuronal connections. Patterning."— Presentation transcript:
Wiring of the nervous system Axon guidance is part of a genetic program that controls neuronal connections. Patterning of the brain Neuronal cell fate determination Neuronal differentiation Axon pathfinding Dendrite development Map formation Layer formation Synaptogenesis Synaptic competition, homeostasis, and plasticity
Axonal growth cone Movement of the growth cone is mediated by a cytoskeletal lattice containing the motor proteins actin and myosin. As the neurite extends behind the moving growth cone, the microtubule backbone of the neurite is constructed from molecules of tubulin. Karl H. Pfenninger
Guidance of Axons by short- and long-range cues: Attractive or repulsive Four types of mechanisms contribute to the guidance of the growth cone: Contact attraction, chemoattraction contact repulsion, chemorepulsion. Individual growth cones might be "pushed" from behind by a chemorepellent, "pulled" from in front by a chemoattractant, and "hemmed in" by attractive and repulsive local cues (cell surface or extracellular matrix molecules). Adapted from Tessier-Lavigne and Goodman (1996).
Molecular guidance molecules Conserved families of guidance molecules (A) and their receptors (B). Examples: ● SLIT secreted proteins, control midline repulsion, dual role, signaling through roundabout receptors (Robo) ● Ephrins (A +B) membrane anchored, repellent and attractive functions, receptors: EphA, EphB ● Netrins and their receptors ● Semaphorins 5 different subfamilies characterized by a 500 aa semaphorin domain, secreted and anchored. Cell Adhesion Molecules ( N-CAM, L1 or Fasciclins
Semaphorins example for dual function:. E) In the presence of NT3, Sema III elicits outgrowth of neurites (NT-3) D) In the presence of NGF Sema III has a repellent effect on neurite growth Dual function: Secreted (subclass 2 + 3) or membrane bound ligands (GPI anchored or transmembrane domain) have Chemorepellent or chemoattractive functions.
Linkage of the actin cytoskeleton to a permissive surface is required for forward advance. Actin is polymerized at the leading edge of the growth cone (right) and is swept toward the rear. If the actin meshwork is not linked to cell surface receptors that bind permissive molecules on adjacent cell surfaces, the actin cycles from front to rear but does not advance the growth cone. If the actin meshwork is attached to these receptors, the meshwork remains in place and newly polymerized actin helps advance the leading edge. Modified from Lin et al.(1994).
Projections from preplate guide thalamocortical fibers Top: Preplate cells send out their axons towards the internal capsule (red). Thalamic axons project through the IC and meet cortical axons. Right: Handshake between thalamic and preplate axons and precise topography of early thalamicortical projections Note: Axons travel together (fasciculation) Axons use preexisting projections Guidepost cells show the way
Growth cones are sensory-motile organelles at the tip of growing axons and dendrites. Golgi-stained section of the spinal cord (specimen prepared by Ramon y Cajal, 1892, photographed 100 years later)
The cytoskeleton of the growth cone continuously changes during outgrowth and navigation.
Growth cones are highly dynamic structures. Mauthner cell axon labeled with DiI in the spinal cord of a zebrafish embryo contacting a motoneuron (left) and forming an en passant synapse (right) Jontes et al., 2000
How does the growth cone get from A to B? Consider Enormous distances. Neuronal diversity.
Growth cones turn in response to gradients of axon guidance molecules Dickson, 2002
Axon guidance cues can be either attractive or repulsive
Four families of axon guidance molecules and their receptors Netrins (DCC, Unc5) Slits (Robo) Semaphorins (plexin, neuropilin) Ephrins/Eph (Eph/ephrin)
Gradient reading requires detection of small concentration changes (a few percent over the length of the growth cone)
Gradient reading can be achieved by two mechanisms n n n n n n n n n n n n n n n n n n n n n n 1) Local autocatalysis (plus lateral inhibition) No gradient Shallow, unreadable gradient 2) Adaptation Intracellularly enhanced gradient Netrin gradient
1) Local autocatalysis amplifies a small concentration difference to generate a larger absolute difference. Lateral inhibition prevents the autocatalysis to spread and suppresses competing activation foci. 2) Adaptation shifts the baseline down to generate a larger relative concentration difference. Growth cone "sensory physiology"
Growth cones are sensitive to external concentration differences of ca. 2% 1) Local autocatalysis (plus lateral inhibition) Shallow gradient 2) Adaptation Enhanced gradient 102:100 300:100 3 : 1
Genetic analysis of the retinotectal projection DiI injection DiO injection Chiasm Retina Tectum Optic tract Optic nerve
A screen for mutations disrupting axon pathfinding and retinotopy in zebrafish
Lipophilic axon tracers DiI (di-C18-...indo-carbocyanine)DiO (di-C18-oxa-carbocyanine) DiD (di-C18...-indo-di-carbocyanine)DiA (di-C16-…amino…styryl…pyrimidinium)
Axon pathfinding phenotypes discovered in the retinotectal screen Baier et al., 1996; Trowe et al., 1996; Karlstrom et al., 1996
Somatotopic mapping: Body surface map in the cortex
The retinotectal projection creates a faithful map of the visual space in the brain DV (L) VD (M) NP (C) TA (R)
Sperry's chemoaffinity theory Connections between retinal and tectal neurons are specified by "key-and-lock" interactions of cell-surface molecules specific to these cells.
Positional information is graded and is being "read" by retinal axons 1. Growth cone guidance 2. Axon branching (not in all systems) 3. Refinement of axonal arbors
Axon guidance by gradients of attractive and repulsive cues in a two-dimensional field Branching Guidance from ectopic position Normal route DVDV APAP Attraction=Repulsion
In vitro retinotectal guidance: The stripe assay Walter et al., 1987 Stripe assay was first carried out with crude membrane preparations from different parts of the tectum. antpost
Stripe assay......was used to test molecules that were differentially expressed between anterior and posterior tectum. In 1995, the Bonhoeffer and Flanagan labs independently discovered the ephrins (under different names). Ephrin-A2 and ephrin-A5 are expressed as gradients in the tectum. Their receptors are expressed as gradients in the retina. ant: low ephrin-Apost: high ephrin-A
assuming crowding results in a countergradient and/or more competition in anterior tectum Ephrin-A2 and A5 both specify A/P position in the tectum Feldheim et al., 2000
Basic model of retinotectal mapping (along the A/P axis)
Axon competition for tectal territory? Evidence from surgical manipulations
Summary 1) Growth cones are sensory and motile organelles at the tip of axons 2) 4 + 4 families of axon guidance molecules are responsible for most of the pathfinding decisions observed so far in the nervous system. 3) Axon guidance depends on gradient sensing by the growth cone (or entire axon). 4) Growth cone responses are not static, but are dynamically regulated by the local environment and the intracellular state. 5) Most sensory projections are topographically organized (neighborhood is preserved). This is achieved by axon guidance (plus other mechanisms).