Axon Guidance
This lecture focuses on the early steps of axon growth and guidance, examining how neurons develop distinct axons and dendrites, and how axons navigate through a complex environment to reach their targets.
Axon-Dendrite Specification
The lecture begins by addressing a fundamental question: how does a neuron determine which neurite will become the axon and which will become dendrites?
Observations from in vitro experiments, where neurons are grown in culture, reveal that:
Initially, young neurons extend multiple processes called neurites.
One of these neurites eventually differentiates into the axon, characterized by its unique morphology and molecular composition.
If the developing axon is cut, one of the remaining dendrites can transform into a new axon, suggesting that neurons possess an intrinsic ability to re-establish polarity.
These findings suggest the existence of an inhibitory signal that prevents multiple neurites from becoming axons, ensuring that each neuron develops a single axon.
Growth Cone Dynamics
To understand how axons navigate, the lecture then introduces the growth cone, a specialized structure found at the leading edge of a growing axon.
Growth cones exhibit remarkable motility, constantly extending and retracting finger-like projections called filopodia and sheet-like projections called lamellipodia.
These dynamic extensions explore the surrounding environment, sensing and responding to guidance cues that direct axon growth.
The lecture emphasizes that growth cone motility is driven by the underlying cytoskeleton, which is composed of microtubules that provide structural support and actin filaments that generate force for movement.
By regulating the assembly and disassembly of these cytoskeletal components, guidance cues can steer the growth cone in a specific direction.
Axon Guidance Cues and Receptors
The lecture then explores the nature of axon guidance cues, emphasizing that they can be either attractive, pulling the growth cone towards them, or repulsive, pushing it away.
These guidance cues can act over short or long distances and exert their effects through specific receptors located on the growth cone surface.
Binding of guidance cues to their receptors triggers intracellular signaling cascades that ultimately modulate cytoskeletal dynamics, thereby directing axon growth.
Examples of Axon Guidance Cues
The lecture provides specific examples of repulsive and attractive axon guidance cues:
Repulsive Cues:
Class 3 Semaphorins: These secreted molecules act as long-range repellents for specific axon populations.
Experiments show that Semaphorin 3A prevents sensory axons from entering certain regions of the spinal cord.
Neuropilins, transmembrane receptors, mediate the repulsive effects of Class 3 Semaphorins.
Attractive Cues:
Netrin: This secreted molecule acts as a chemoattractant for commissural axons, which are axons that cross the midline of the nervous system.
Experiments show that Netrin attracts commissural axons towards the floor plate, a specialized structure located at the ventral midline of the developing spinal cord.
Axon Growth Substrates
In addition to diffusible guidance cues, the lecture also highlights the importance of the extracellular matrix as a substrate for axon growth.
Adhesive molecules present in the extracellular matrix provide traction for the growing axon, allowing it to extend.
Experiments show that neurons cultured on a permissive substrate, which contains adhesive molecules, can extend axons, whereas neurons cultured on a non-permissive substrate cannot.
This lecture continues the discussion of axon growth and guidance, focusing on how axons navigate long distances to reach their specific targets.
Long-Range Guidance Cues
The lecture emphasizes that long-range guidance cues, such as Netrin and Semaphorins, can establish initial trajectories for growing axons, but these cues alone are not sufficient to ensure precise target innervation.
Intermediate Targets and Local Guidance Cues
Instead, axons rely on a series of intermediate targets and local guidance cues to refine their paths and reach their final destinations.
Intermediate targets are cells or structures located along the axon's path that release specific guidance cues, acting as "stepping stones" for the growing axon.
These local guidance cues can be either attractive or repulsive, fine-tuning the axon's trajectory as it approaches its target.
Midline Crossing
The lecture provides an example of how the interplay between attractive and repulsive cues guides axons across the midline of the nervous system:
Netrin, released from the floor plate, initially attracts commissural axons towards the midline.
However, once axons cross the midline, they need to be repelled from the floor plate to prevent them from stalling or turning back.
This repulsion is mediated by Slits, a family of secreted molecules that act as long-range repellents.
Roundabout (Robo) receptors, present on the surface of commissural axons, mediate the repulsive effects of Slits.
To ensure that commissural axons are initially attracted to the midline, mechanisms exist to temporarily silence Robo receptor activity before crossing.
For example, in Drosophila, the protein Commissureless (Comm) regulates the trafficking and degradation of Robo receptors, preventing them from responding to Slits before midline crossing.
Once axons cross the midline, Robo expression or activity is upregulated, rendering them sensitive to Slit repulsion and ensuring that they move away from the midline towards their targets.
Topographic Mapping
The lecture then introduces the concept of topographic mapping, where axons from neighboring neurons in a source structure project to neighboring targets in a target structure, preserving spatial relationships.
For example, retinal ganglion cells (RGCs), located in the retina of the eye, project their axons to the superior colliculus (SC) in the brain, forming a topographic map of visual space.
The establishment of such precise topographic maps requires a combination of guidance cues that act at different stages of axon growth.
Establishing Topographic Maps in the Retina
The lecture outlines how topographic maps are established in the retinotectal projection, where RGC axons from different positions in the retina map to corresponding positions in the SC:
Initially, RGC axons are guided out of the eye by repulsive cues.
As axons reach the SC, they encounter gradients of guidance cues that provide positional information along the anterior-posterior and medial-lateral axes.
One well-studied guidance cue system involved in retinotectal mapping is the Ephrin-Eph system.
Ephrins are cell surface molecules that act as repulsive cues, and Ephs are their corresponding receptors.
Gradients of Ephrins and Ephs in the retina and SC contribute to the establishment of a coarse topographic map by ensuring that axons from neighboring RGCs project to neighboring regions in the SC.
Refining Topographic Maps
While long-range and gradient cues establish an initial topographic map, further refinement is often achieved through activity-dependent mechanisms that occur after axons reach their target regions.
This refinement involves the formation and elimination of synapses based on neuronal activity, ensuring that connections between neurons representing similar features in the sensory world are strengthened, while those representing disparate features are weakened or eliminated.
This summary provides an overview of the key concepts and processes covered in lectures 9 and 10, highlighting the complex interplay of intrinsic neuronal properties, extrinsic guidance cues, and cellular interactions in guiding axons to their targets during nervous system development. It also lays the foundation for understanding how topographic maps, crucial for sensory processing and motor control, are established in the brain. Please note that further details and specific examples are provided in the original lecture notes.