Axonal Specificity

Growth cone

o   Only present at tip of developing neuron

o   Later urns into synapse

o   Sensory and motor structure that allows neuron/axon to find it way to its target by responding to cues in their environment.

axon growth cone 2 major cytoskeletal components

1. actin 

2. tubulin

Actin

§  Present in finger like extensions called filopodia

·        Filopodia have receptors for guidance cues to know where to go to get to their targets

Tubulin 

§  Makes up cytoskeleton on axon

§  When tubulin comes together in long chains it gives us tubules

·        Microtubules are present throughout the growth cone

Central domain

o   Central region where a lot of microtubules are coming up

Growth cones can encounter two different types of signals 

attractive or repulsive 

• this initiate signaling causing the growth cone to turn

What needs to happen in order for the growth cone to turn?

1. disassemble the cytoskeleton

2. Axon elongation  

Disassemble the cytoskeleton

·        Remove cytoskeleton on repulsive side and build up cytoskeleton on attractive side

Axon needs to be elongated

·        Growth cone moves forward by building up microtubules behind it allowing it to move forward

Guidance cues

-        Affect cytoskeleton

-        Can be attractive or repulsive

-        Can be

-Contact meditated

• Juxtracrine

o   Diffusible

• Paracrine

Rho GTPases 

-        Interpret and relay external guidance signals to the actin cytoskeleton

-        Intermediate step between guidance cues binding to receptors and cytoskeleton changes-        Interpret and relay external guidance signals to the actin cytoskeleton

•   Guidance cues binds to receptors which either turns on or turns off Rho GTPases which leads to cytoskeleton changes by either assembling or disassembling actin.

Common Guidance Cues

1. Ephrins

2. Netrins

3. Slit

4. Semaphorins

Ephrins

bind to receptor Eph

Netrins 

binds to receptors UNC5/ DCC

Slit

binds to receptor Robo

Semaphorins

bind to receptor Plexin

Local translation

-        Some mRNA present within the growth cone that can be translated locally in the growth cone to gives the growth cone the proteins it needs

-        This allows the growth cone to not have to send a signal all the way back to the cell body and wait for the cell body to send the protein needed

Local translation accomplished by 

-        tubulin mRNA

o   When growth cone receives attractive guidance cue it signals the tubulin mRNA to make proteins

How are Axonal Connections Made?

1.      Axon guidance

2.      Synaptogenesis

3.      Synaptic refinement

Axon guidance

• route to target

• pioneer and follower growth cones 

Synaptogenesis

recognize and bind to target

Synaptic refinement

refinement of target connectivity

Example of Axon Guidance: motor neurons

-        Ephrins important guidance cue

-        Contact-mediated Axon Guidance – Repulsive

• Ephs (receptor) expressed by motor neuron growth cones

• Ephrin’s expressed by cells in posterior part of the sclerotome

Ephrins binding to Ephs is repulsive

axons only grow through anterior part not posterior part because posterior region expresses ephrins

What assay was used to determine attractive or repulsive

-        Used strip assay where + have ephrins and – do not have ephrins

o   Axons only grow in medium without ephrins (-)

Example of Axon Guidance: Commissural axons

-        Sit at top of developing neural tube / spinal cord

-        Cell body at the top of the spinal cord and they put out axons that are guided down to bottom of spinal cord cross the midline and then go through the spinal cord up towards the brain.

-        Netrin 1

Netrin 1 

o   Attractive cue that guides the axon down to the midline

o   DCC present in the growth cone

 Robo/Slit regulation of midline crossing by axons of commissural neurons

-        Repulsive Axon Guidance

o   Slit expressed by midline glial cells

o   Robo (receptor for slit) expressed by commissural neuron growth cone

o   Commissural neurons upregulate Robo after crossing.

Once growth cone reaches midline it activates a

different signaling pathway that removes DCC and puts Robos into its place.

Topographic map

o   There is a spatial arrangement of neurons where adjacent areas of the body or sensory surface are represented by adjacent areas in the brain.

o   Ex

§  Receptors in finger

Retinotopic map

o   Type of topographic map

o   Neighboring points on the retina connect to retinal connect to neighboring neurons in the brain

-        Neighboring areas of the visual field connect to neighboring neurons in the brain

Retinal Ganglion Cells

-        Multiple guidance cues direct the movement of retinal ganglion cell axon to the optic tectum

-        Projections from left eye go to

right tectum and projections from right eye go to left tectum

-        When image gets projected to the retina it is flipped

-        Ventral RGC send axons to the

-        Dorsal RGC send axons and connect to the

• medial part of the tectum

• lateral part of the tectum

Temporal RGC send to

Nasal RGC send to

caudal/posterior part of tectum

rostral/anterior part of tectum

-        Eph receptors produced by

RGCs region of retina and have Eph receptors

• in gradient

• High temporal and low nasal

-        Eph ligands are in the

tecta

o   Low rostral and high caudal

-        RGC in the nasal regions has

-        RGC in the temporal region

low levels of Eph receptors

high levels of Eph receptors

Ephrin ligands are

low anterior and high posterior

-        Less eph receptors means it can

grow further to more posterior region of tectum where ephrin ligands are high.

o   Low levels of receptor is not enough to

stop / repel them so they have to go to region where there are more ligands

-        More Eph receptors stop at

o   They stop immediately

more anterior region of tectum because they have so many receptors, they do not need a lot of ligands to cause them to stop

• they stop immediately 

this gradient results in 

orderly connections 

Chemo affinity hypothesis

-        Complicated nerve fiber circuits of the brain grow, assemble, and organize themselves through the use of intricate chemical codes under genetic control

Synaptogenesis

1.      Growth cone comes into contact with myotube (muscle)

2.      Growth cone then releases factor Agrin

3.      Get formation of synaptic basal lamina

4.      Transient polyneuraonal innervation

5. Axon that stays branches and makes multiple synapses that cluster tougher on fiber

6. Sheathing by schwann cells

2.      Growth cone then releases factor Agrin

a.      Activates signaling pathway within the muscle that tells Ach receptors to cluster

• Ach NT that makes our muscles contract

3.      Get formation of synaptic basal lamina

a.      Synaptic basal lamia: Dense ECM matrix between growth cone and muscle

4. Transient polyneuraonal innervation

a.      Temporally multiple motor neurons will innervate the muscle 

b.      In maturity only one motor neuron innervates one muscle fiber

• One wins and one prunes itself

6. Sheathing by Schwann cells

a.      Myelinates the axon

The growth cone is the

sensory and motor structure of the developing neuron and rearranges its
cytoskeletal architecture (actin and tubulin) in response to environmental guidance cues.

Guidance cues can be

contact-mediated or diffusible.

• Each of those categories can be further
  divided into attractive or repulsive to the growth cone

Guidance cues bind to

receptors in the growth cone, which activates an intracellular pathway that
changes the cytoskeleton and guides the axon to its target.

_____ and signaling through _____ are two important intracellular mechanisms that are activated by

Local translation Rho GTPases

guidance cues and regulate axon guidance.

Motor neurons respond to

ephrins during axon guidance

Changes in growth cone responsiveness to the secreted attractive and repulsive cues secreted from the midline enable

commissural axons to cross the midline and connect the two sides of the central nervous system.

Retinal ganglion cells in frogs and chicks send axons that synapse in specific regions of the optic tectum. This process is mediated by

numerous guidance cues, and target selection appears to be mediated through ephrins

The chemoaffinity hypothesis explains how

neurons can make appropriate connections even
when moved from their original locations.

9. Synaptogenesis requires a series of steps including

contact

receptor clustering

formation of the basal lamina

transient polyneuronal innervation

sheathing by Schwann cells