Lecture 24 -Construction_of_Neural_Circuits_Part_1_and_2

Importance of Cytoskeleton in Growth Cone Behavior

• The cytoskeleton is critical for the development and function of growth cones, which guide axons during neural circuit formation.

Molecular Mechanisms of Axon Guidance

• Extracellular matrix (ECM) provides physical and biochemical cues for axon guidance, utilizing various signaling molecules.
• ECM ligands, such as fibronectin and laminin, interact with integrin receptors, modulating growth cone activity.
• Guidance molecules can function as chemoattractants (promoting growth) or repellents (inhibiting growth).

Trophic vs. Tropic Molecules

• Trophic molecules: Support cell survival and growth.
• Tropic molecules: Directly influence the direction of axonal growth.

Mechanisms Enabling Synaptogenesis

• Synaptogenesis is regulated by adhesion molecules.
• Activities of molecules like cadherins and protocadherins facilitate synapse formation via initial membrane recognition and recruitment of additional adhesion molecules.

Neuronal Polarization

• Neurons are highly polarized cells with distinct axonal and dendritic structures.
• The differentiation of neurites involves the partition defective protein (Par-3).
• Axons are characterized by secretion, while dendrites are involved in signal transduction.
• Microtubules and actin filaments are distributed to favor axon development.

Growth Cone Dynamics

• Growth cones are specialized structures at the ends of growing axons, critical for navigating the extracellular environment during development.
• The morphology of growth cones changes depending on their location, becoming more complex at decision points and reverting to simpler forms afterward.

Types of Cytoskeletal Elements

• Growth cones possess distinct types of actin and microtubules, coordinating growth and stability.
• Filamentous actin (F-actin) and globular actin (G-actin) arrangements dictate growth cone motility.

Calcium Ion (Ca2+) Signaling

• Changes in intracellular calcium concentrations are crucial for growth cone dynamics.
• Actin-binding proteins respond to Ca2+ to facilitate the assembly and disassembly of filopodia.

Cell Adhesion Molecules (CAMs)

• Non-diffusible signals such as CAMs play dual roles as ligands and receptors and participate in axon guidance and fasciculation (axon bundling).
• Cadherins, primarily calcium-dependent, activate signaling pathways critical for growth guidance.

Ephrins and Their Role

• Ephrins serve as cell-recognition molecules, signaling through Eph receptors to guide growth and support synaptic formation.

Visualizing Axon Guidance Disorders

• Diffusion Tensor Imaging (DTI) can reveal axon guidance disorders by highlighting structural changes in the brain, such as partial agenesis of the corpus callosum.

Disorders Arising from Defects

• Disorders like L1 syndrome and horizontal gaze palsy are linked to defects in axon pathfinding and are associated with specific genetic mutations affecting neuronal pathways.

Axon Guidance Signals

• Chemoattractants guide growth cones toward targets while chemorepellents prevent inappropriate trajectory changes.

Netrin and Slit Family Signals

• Netrins and Slits play significant roles in axonal navigation by providing attractive and repulsive cues through specific receptors (e.g., DCC for netrins, Robo for Slit).

Semaphorins as Repulsive Cues

• Semaphorins are primarily repulsive, signaling through plexin and neuropilin receptors to retract growth cones during development.

Topographic Mapping of Axons

• Axons exhibit specific targeting to structures in the brain based on spatial orientation, exemplified in visual systems of frogs and other vertebrates.

Synapse Formation and Specialization

• Initial synapse formation relies on specific adhesion interactions, followed by recruitment of additional proteins to establish synaptic structures.

Neurotrophic Factors in Development

• Neurotrophic factors like NGF stimulate neurite outgrowth and are essential for neuronal survival and differentiation.
• Different neurotrophic factors have tissue-specific influences on growth, assisting in developing neuronal circuits.

Local Control of Neuritic Growth

• Neurotrophins can locally regulate growth cone activity by modulating intracellular calcium levels and guiding growth towards target tissues.

Neurotrophin Receptor Specificity

• Trk receptors bind specific neurotrophins (e.g., TrkA for NGF) and activate distinct intracellular signaling pathways, leading to varied cellular outcomes based on the neurotrophin present.