Neural Development - Extension of axons to targets, formation and modification of synaptic connection

Question 1

What is axon guidance?

Answer 1

After neuron has migrated to final position
= must create neurites (axons and dendrites)
= to connect it to other neurons
= neurites often travel long distances to reach final targets

Growth cone
= at leading edge of each growing neurite
= it extends amoeboid- like fingers = Filopodia
= Filopodia interact with immediate environment

Question 2

What does Axonal Transport do?

Answer 2

= provides supplies to growth cone

Cell body
= only site of protein synthesis

Membrane components for growth cone extension
= made in cell body as vesicles
= transported along microtubules in the growing neurite
= proteins for neuronal extension transported inside these vesicles to growth cone

= by anterograde axonal transport

Question 3

What is the role of Actin and Tubulin? What is growth influenced by?

Answer 3

Movement of filopodia
= driven by filaments of actin connected to plasma membrane
= in association with myosin (connected to cytoskeleton)

As growth cone moves forwards
= backbone is continually reinforced with microtubules (tubulin)
= if it keeps moving forwards it must adhere to adjacent cells and ECM

Growth is influenced by
= cell adhesion molecules (CAMs)
= substrate adhesion molecules (SAMs)
= diffusible guidance molecules

Question 4

What is the structure of the growth cone?

Answer 4

Central Domain
= microtuvules, organneles, vesicles

Transitional Zone

Peripheral domain
= actin, lamellipodia, filopodia

= lamellipodia found in between the filopodia
(flat regions of dense actin meshwork instead of the bundled F-actin found in filopodia)

Question 5

How does the Axon know where to go?

Answer 5

Highly motile growth cone
= very sensitive to attractive and repellent cues
= they act as molecular guideposts for the developing neuron

= combination of CAMs/SAMs and diffusible chemotropic molecules gives guidance

e.g. if Neuron A can only attach to SAM = X
neuron B to SAM = Y
(resulting in correct orientation to path A or B)

Question 6

What is N-CAM (cell adhesion molecule)?

Answer 6

= found on surface of neurons and glial cells

= belongs to immunoglobulin superfamily

= shows homophilic adhesion via immunoglobulin domains

= >27 different isoforms

= N-CAM KO mice show axon migration defects

Question 7

What is N-Cadherin (cell adhesion molecule)?

Answer 7

= contain 5 cadherin repeats
(each comprising of a sandwhich of β sheets)

= show homophilic Ca2+-dependent adhesion at each end

= functional unit is a dimer

= anti-N-cadherin antibodies prevent outgrowth of Xenopus retinal axons

Question 8

What are SAMs (substrate adhesion molecule)?

Answer 8

= substrate adhesion molecules

= important ECM adhesion molecules: laminin, fibronectin, tenascin

= can inhibit or promote neurite attachment and growth

= laminin / fibronectin in ECM interact with integrin on cell surface

= integrin then interacts with actin to influence growth cone motility

= laminins = trimeric glycoproteins (alpha, beta and gamma chains) with many isoforms to give specificity

= have cross-like structure with multiple binding properties

Question 9

What are diffusible Chemotropic Molecules?

Answer 9

= can act over longer distances
(substrate-associated cues only work over short distances)

= can act as chemoattractants or chemorepellents

e.g. Netrins - chemoattractant

Question 10

What are Netrins?

Answer 10

= netrin 1 = made by floor plate of developing spinal cord
= guides development of commisural neurons in spinal cord

= has high homology to ECM molecules like laminin

= ventral floor plate produces netrin-1, netrin-2 and Shh (as chemoattractants)

AND

BMPs in dorsal spinal cord = repel commissural neurons
(drive connections to the ventral spinal cord)

Question 11

What are Semaphorins?

Answer 11

= some secreted / some membrane-bound

= often guide growth cones by repulsion (short-range inhibitory signals)

= their receptors are neuropilins (for class 3) and plexins (class 1 and others)

Neuropilins
(NRP-1, NRP-2)
= single pass transmembrane receptors that work as dimers
= have no signal transduction system of their own
= so co-receptor: plexin is needed

Plexins
= receptors either alone or in combination with neuropilins
= trigger a novel signal trandsduction pathway controlling cell repulsion
= binding of semaphorin leads to microtubule dissassembly and actin depolymerisation
= leads to growth cone collapse

Semaphorin 3
= secreted by developing spinal cord cells in ventral part
= repels incoming thermoreceptor and pain receptor neurites
(role in axonal guidance)

= group 1a sensory neurons are not affected
(muscle proprioceptor input)

Question 12

What are Ephrins and EPH?

Answer 12

Ephrins = ligand

EPH = receptor

Migrating axon is repelled away from site of Eph/Ephrin activation
= growth cone collapse

Ephrin A ligand
= GPI anchor to membrane

Ephrin B ligand
= transmembrane

Eph receptor
= tyrosine kinase receptor

= bi-directional signalling

Question 13

How is Axonal Guidance affected by Ephrins/EPH signalling? What is an example?

Answer 13

Retinotopic map formation
= nerve cells in retina send out projections to the tectum
(superior colliculus in mammals that directs eye movements)

Temporal retinal neurons
= have high sensitivity to Ephrins (repelled)
= break down early on, don’t grow

Nasal retinal neurons
= have low sensitivity to Ephrins
= so extend to a higher concentration of Ephrins
(grow all the way to posterior)

Question 14

What are Pioneer Axons?

Answer 14

Axons migrate to targets in bundles
= called fascicles

Pioneer axons sent first
= heavily dependent on guidance cues

Follower axons
= use existing axons as the pathway to follow

Question 15

What are NGF / other Neurotropic Factors for?

Answer 15

NGF
= Initiate neurite outgrowth and needed for survival

Without NGF
= poor neurite outgrowth
= neurons die

With NGF
= prolific neurote outgrowth
= neurons survive

BDNF - Brain derived neurotrophic factor and neutrophins are others

Receptors = are tyrosine receptor kinases (trk)

Question 16

How are synapses formed? (e.g. neuromuscular junction - model system) How are unwanted synapses eliminated?

Answer 16

When axon has not yet reached target
= immature muscle cell produces Ach receptors but not concentrated at end plate

Axon makes contact
= converts into a synaptic terminal (CAMs involved)
= Ach receptors cluster and are made at contact point

Some synaptic contacts are eliminated
= leaving only one motor neuron and its end plate

Overproduction of synapses and elimination of unwanted
= common in developing nervous system

Formation of the synapse
= involves signals sent and received from both presynaptic and postsynaptic cells

Nerve terminals secrete
= Agrin = involved in clustering of Ach receptors
= ARIA (acetylcholine receptor inducing activity) = stimulates production of new Ach receptors
(without clustering, synapses are lost)

Question 17

How does synpatogenesis continue after birth?

Answer 17

At birth
= have full complement of neurons

After birth
= immense increase in connections of the same neurons
= each neuron makes thousands/millions of connections with others
= needed for many functions (e.g. listening to speech, thinking about words, thinking and speaking, reading)

Question 18

What are neural tube defects?

Answer 18

= give rise to (often lethal) major developmental malformations of brain and spinal cord

= known as dysraphic defects

= occur within first 3 weeks of gestation and are caused by incomplete closure of the neural tube

e.g. 1. Anencephaly
= good part of the brain is absent
= skull is wide open
= affected infants are stillborn or die in a few days
(anterior neuropore not closed)

e.g. 2. Myelomeningocele (spina bifida)
= spinal cord (myelon) and its membrane coverings (meninges) herniate through defect in vertebral column
= early surgical repair improves chances of survival
= affected infants have motor and sensory deficits
(posterior neuropore not closed)

e.g. 3. Craniorachischisis
= due to failure of whole neural tube to close throughout length of body
= generally results in death before of soon after birth
= most severe

Question 19

What are disorders of Neuronal Proliferation + Migration?

Answer 19

= result in variety of brain malformations that are compatible with life but often result in psychomotor (slowing down of thought and reduction of physical movements) and seizures

e.g. 1. Microcephaly
= brain and skull small in size
= due to defect in neuronal proliferation

e.g. 2. Lissencephaly
= brain surface is smooth
= due to failure of neuronal migration
= affected children have small brains and often develop intractable seizures, with severe impairment of neurological development

e.g. 3. Pachygyri
= thick gyri
= convolutions of brain surface are broad, shallow and reduced in number
= due to defect in neuronal migration
= affected infants survive but are severely neurologically impaired with intractable seizures

e.g. 4. Polymicrogyria
= numerous small gyri
= due to interference in neuronal migration
= affected infants are moderately to severely neurologically impaired with intractable seizures

e.g. 5. Cortical Heterotopias
= characterised by islands of neurons in abnormal location
= due to arrest in their migration process
= extent of heterotopia determines degree of neurological impairment and seizures