120324 lecture

Axon Outgrowth Signals

  • Permissive Signals

    • Sources:

      • Extracellular matrix

      • Other cells located far away

      • Other axons

    • Types:

      • Chemoattractants (guide axon growth toward them)

      • Chemorepellents (deter axon growth)

    • Reference: Lowery and Van Vactor 2009

The Axon Growth Cone: Structure and Function

  • Growth Cone Function

    • Facilitated by: Fasciculation

  • What is Fasciculation?

    • Adhesion of axons together

    • Caused by surface cell adhesion molecules (CAMs)

    • Examples:

      • Cadherins

      • DSCAMs

Pioneer Axons

  • Function:

    • Serve as a scaffold for follower axon extension

    • Important for establishing neural pathways

Regulation of Axon Fasciculation

  • Involves Cell Adhesion Molecules (CAMs)

    • Fasciculation leads to tight interactions between axons

    • Types of CAM interactions:

      • Homotypic: Same CAMs

      • Heterotypic: Different CAMs

Cadherins and Axon Fasciculation

  • Cadherins:

    • Regulation of axon fasciculation

    • Protocadherin-17 (pcdh17):

      • Important for homotypic fasciculation of amygdala axons extending to the hippocampus and ventral striatum

      • Loss of Pcdh17 impacts axon growth

    • Reference: Hayashi et al. 2014

Synapse Formation in the CNS

  • Pre- and Post-synaptic Specializations:

    • Formation at sites of active contact

    • Steps in the process:

      • Dendritic filopodia contact axons

      • Recruitment of synaptic vesicles and active zone proteins to the presynaptic membrane

      • Accumulation of receptors at the postsynaptic membrane

Neuromuscular Junction Formation

  • Steps involved:

    1. Motor neuron axon secretes agrin into the basal lamina.

    2. Agrin binds to the MuSK (muscle-specific kinase) receptor on muscle fibers.

    3. MuSK activates Rapsyn, clustering acetylcholine receptors into plaques.

Neurexin and Neuroligin in Synapse Formation

  • Neurexin:

    • Typically found on the pre-synaptic side

    • Consists of three genes with various isoforms in the brain

  • Neuroligin:

    • Typically found on the post-synaptic side

    • Consists of four genes with multiple isoforms

    • Reference: Sudhof 2008

Types of Synaptic Refinement

  1. Changes in synaptic capacity

  2. Synaptic rearrangement

  3. Synaptic segregation

  4. Programmed cell death

Apoptosis: Elimination of Neurons

  • Apoptosis = programmed cell death

    • Occurs after axons reach their targets, leading to a decline in neuron and axon numbers

Apoptosis During Brain Development

  • Approximately 1/3 of differentiated neurons die before adulthood; estimates range from 20%-50%.

  • Even a 20% loss signifies a major reduction in neuron number.

Role of Apoptosis in Development

  • Apoptosis occurs across all assessed parts of the nervous system.

    • Example: Frog hindlimb development shows decline from 4000 to ~1200 motor neurons (60% eliminated).

    • ~50% of rat retinal ganglion cells also undergo apoptosis during development.

Essentiality of Apoptosis for Neural Development

  • Stages of mouse development show that blocking apoptosis leads to lethal defects.

    • Emphasized by research (Kuida et al. 1998).

Regulation of Apoptosis by Trophic Factors

  • Trophic factors play a role in regulating neuronal survival and selective elimination of neurons based on competition for them.

Overview of Trophic Factors

  • Definition and Importance:

    • Survival factors provided in limited quantities, necessary for maintenance of neuronal connections and neuronal survival.

Sources of Trophic Factors

  • Neurons receive them from:

  1. Target tissues they innervate (retrograde signaling)

  2. Synaptic inputs (anterograde signaling)

  3. Neighboring neurons (paracrine signaling)

  4. Distant cells (through the circulatory system)

  5. Glial cells

Diversity of Trophic Factors

  • Families of structurally related neurotrophic factors include neurotrophins and others, each with its specific receptors.

  • Examples of neurotrophic factors and preferred receptors listed.

Discovery of Nerve Growth Factor (NGF)

  • Identified by Rita Levi-Montalcini in the 1940s.

    • Produced by target tissues of sympathetic neurons

    • Key roles in regulating body functions (e.g., gut motility, pupil dilation, etc.).

Influence of Target Tissue on Neuronal Population

  • Target tissue dictating numbers of innervating neurons illustrated with limb manipulation results and their impact on dorsal root ganglia sensory neurons.

Levi-Montalcini’s Hypothesis and Experiments

  • Hypothesis: Target tissue secretes factors stimulating neuron proliferation.

  • Initial experiments indicated no change in differentiated neuron counts, leading to the new hypothesis that these factors are essential for neuronal survival.

Key Observations in NGF Discovery

  • Notable observations: Tumors and snake venom promoted neuronal survival, leading to isolation of proteins for identifying critical factors.

Machinery Behind Neuronal Survival from NGF

  • Experimental approaches to test the necessity of the factor involved antibody blocking and resulting neuron survival assessments.

Nobel Prize Achievements

  • Nobel Prize in Physiology or Medicine awarded in 1986 to Rita Levi-Montalcini and Stanley Cohen for their discoveries on trophic factors.

Timeline of Neuronal Cell Death due to NGF Withdrawal

  • Insights into sympathetic neurons undergoing programmed cell death following NGF withdrawal.

Dynamics of the Internal Cell Death Program

  • Following NGF withdrawal, the mitochondrial cell death program is activated, leading to apoptotic pathways.

Critical Role of Trophic Factors

  • Trophic factors are crucial for proper development of sympathetic ganglia.

True or False Questions

  • Evaluate understanding of NGF's role in neuronal proliferation and its importance in neuronal survival.

Neurotrophins & Their Characteristics

  • Neurotrophins as a subset of neurotrophic factors with structural similarities and critical functions in neuronal health.

Neurotrophin Structure and Function

  • Overview of neurotrophin maturation, with emphasis on peptide structure and receptor binding capabilities.

Neurotrophin Receptor Affinities

  • Neurotrophin receptors exhibit high homology and splicing variations influencing numerous receptor sequence variations.

Neurotrophin Signaling Importance

  • Signaling impacts neuronal survival and cellular processes in the nucleus essential for survival.

Total Overview of Neurotrophin Sources

  • Comprehensive summary emphasizing that trophic factors originate from various local and distant sources and their effects on neuronal health.

Retrograde Transport Mechanism

  • Neurotrophin signaling reveals the importance of retrograde transport in maintaining neuronal transcriptional changes in response to signaling.

Signaling Cascade Implications

  • Mechanisms of how neurotrophin signaling lead to transcriptional responses vital for cell survival identified.

Activation Pathways

  • Discusses various roles and effects of neurotrophins once they bind to their receptors, contributing to neuron maintenance and survival.

Internalization Dynamics of Neurotrophin Receptors

  • Understanding the internalization processes of neurotrophin receptors plays a crucial role in signaling and cellular responses.

Signaling Endosomes and Their Functions

  • The role and importance of signaling endosomes in carrying activated neurotrophin receptors from axon terminals to cell bodies discussed.