Axon Guidance and Neuronal Specification 3 lecture

Discussion initiated regarding the key concepts from previous lectures.
- Importance of morphogens in establishing neuronal identity.
- Consensus on the idea that neuronal specification occurs through gradients, involving:
- Mutual repression
- Reciprocal co-expression
Differential Gene Expression:
- Activation of specific gene expression programs contributes to the specification of different neuronal subtypes and the establishment of boundaries.
- Important for the layering of the cortex.

The significance of the cytoskeleton in the growth cone dynamics.
- Interactions between actin and microtubules play a vital role in neuronal guidance.

Guidance Cues:
- Definition: Signals that guide axons through gradients.
- Distinction made between morphogen functions and guidance cues.
- Importance of flexibility and plasticity in axonal guidance:
- Axon growth is not predetermined, allowing adaptability to environmental signals.

Axons grow and respond to various signals that can be:
- Attractive or repulsive
- Long-range or short-range
Statement on how the growth process is characterized by growth, not migration:
- Examining the collective movement of axons through facilitated growth, guided by interactions among axons.
- Pioneer Axons:
- Role of leader axons in guiding follower axons towards targets.

The importance of attractive and repulsive signals in determining the direction of axon growth.
- The axons operate in a multi-step process, guided by:
- Various signaling pathways responding at the right timing and location.

Adhesion Molecules:
- Facilitate axon growth and stabilization during guidance.
- Relevant examples include members of the cadherin superfamily.

Lateral and Medial Tongue Experiments:
- Observation of neuronal attraction to lateral tongue segments, demonstrating attractive vs. repulsive cues.
Macro Fabrics Experiment:
- Utilizing patterned surfaces to control axon growth.
- Demonstrating how neuronal behavior is directed based on previous attractive and repulsive cues in culture.

Focus on commissural neurons - those that cross the midline to interact with motor neurons.
- Contrasting ipsilateral neurons, which remain on the same side of the spinal cord.
Comprehension of how signalling pathways regulate directionality for both populations:
- Examples of BMP (a repulsive signal) and Netrin (an attractive signal).

BMP Usage:
- Released by roof plate, providing localized repulsive signals to the commissural neurons.
Netrin Usage:
- Released by floor plate, assisting in guiding neurons downward to the midline where they can cross.
- Induces expression of receptors necessary for crossing, alongside regulatory proteins (e.g., com).

Importance of timing and spatial regulation:
- Slit, a signaling pathway crucial for repulsion, is noted for ensuring successfully crossing of the axon once BMP is no longer influencing it.
- Components such as Robo receptors must be activated in a timely manner to respond effectively to Slit signals.

The interaction of Netrin shifts its nature post-crossing, becoming a repulsive cue for motor neurons.
- Highlights how guidance cues can serve dual roles at distinct stages of axon targeting.

Overall remarks on the importance of spatial and temporal precision in neuronal guidance:
- Mechanistic support is provided through differential receptor expression among neuronal populations (such as between medial and lateral motor neurons).

Discussion on how scientific experiments validate the role and complexity of neuronal signaling in development, particularly through methods such as in utero electroporation and live imaging.
Phenotypic observations on controlled experimental animals demonstrate physiological impacts of genetic manipulations.