Wiring of the Nervous System

Question 1

Schematic of neuronal differentiation

Answer 1

Question 2

During CNS development, new post-mitotic neuron bodies migrate. . .

Answer 2

. . .past their older cousins to the pial surface.

Question 3

Migration of GABAergic neurons

Answer 3

Neurons become post-mitotic in the medial and caudal ganglionic eminences and then migrate. The MGE and CGE together supply the cerebral cortex and the GABAergic interneurons in the amygdala and basal ganglia. Some MGE cells also become Martinotti cells, expressing serotonin.

Question 4

Lateral inhibition

Answer 4

Frequently utilized by the nervous system to produce a variety of neurons during development.

The Delta-Notch-Numb pathway is one such example. If Notch signaling is less in one cell, that cell will upregulate Delta and Numb. Meanwhile, its upregulation of Delta suppresses Numb and activates Notch in an adjacent cell. Thus, you end up with one strongly Delta+ cell and one strongly Notch+ cell.

Question 5

Role of Sonic Hedge Hog (SHH) in spinal cord development

Answer 5

Relies on creating a concentration gradient that cells can respond to by up-regulating specific transcription factors at specific concentrations.

Question 6

Ephrins regulate the direction of axonal growth in the spinal nerve

Answer 6

Question 7

Axon guiding/repressing transcription factors in the CNS

Answer 7

Satb2 - promotes crossing of the corpus callosum
Fezf2 - promotes subcerebellar projection
Tbr1 - promotes corticothalamic projection

These transcription factors all cross-repress one another.

Question 8

Regulation of midline crossing

Answer 8

Slit - secreted by midline glia. Binds to Robo to repulse axon growth.
Robo - receptor for Slit. Binding repels the axon from this direction.
Comm - secretory pathway protein which down-regulates Robo, enabling midline crossing.
Netrin - chemo attractant that attracts cells to the midline. However, the receptor becomes suppressed in the presence of Robo/Slit signaling.

Once axons have crossed, Comm is typically deactivated, preventing axons from re-crossing the midline.

This system not only regulates midline crossing, but also determines how lateral an axon sits relative to the midline. Different Robo receptors with different affinities for Slit result in closer or further distances from the midline.

Question 9

After midline crossing, commissural axons turn anteriorly. What regulates this anterior turn?

Answer 9

A gradient of Wnt4 strongest on the anterior side of the spinal cord.

Question 10

Process of axonal selection in a new neuron

Answer 10

Question 11

Factors that determine which process becomes the axon

Answer 11

1. Actin destabilization
2. Microtubule stabilization and orientation (+ end faces axon terminus)
3. LKB1 signaling (PKA -> LKB1 -> SAD kinases)

Question 12

“Golgi outposts”

Answer 12

Names for the bodies of Golgi apparatus in the dendrites and axon of a neuron, separate from the full Golgi apparatus body in the cell soma. Strips of smooth and rough ER may also be found near these outposts.

In neurons, Golgi outposts may also serve as microtubule nucleation sites.

Question 13

While axons have +-end-out microtubules, dendrites have. . .

Answer 13

. . . both (+)-end-out and (-)-end-out microtubules.

Question 14

Mechanisms of dendritic and axonal branching

Answer 14

1. Growth cone splitting (splitting at the growing end of the process)
2. Interstitial branching (splitting/branching off in the middle of a process as the growth cone continues on)

Question 15

Role of DSCAM proteins

Answer 15

Found in Drosophila

Homophilic cell adhesion molecules which exist in multiple isoforms. When different isoforms bind, they repel one another. Each branch of a neuron expresses one isoform.

Different neurons also express different sets of DSCAM isoforms, to prevent cross-neuron connections.

Question 16

Role of protocadherins

Answer 16

Found in humans and other mammals

Protocadherins serve effectively the role that DSACMs do in Drosophila, and display a similar level of intramolecular diversity.

Question 17

Neurofascin

Answer 17

One of many linkers used by neurons.

Localized to the axon base on Purkinje cells via its association with Ankyrin G on the intracytoplasmic surface. Used to target the exact subcellular site of synapse by Basket cells.

Loss of Ankyrin G expression results in diffuse neurofascin expression and resultant loss of synapse site specificity.

Question 18

Post-synaptic infrastructure for receptor clustering

Answer 18

At the motor endplate, you will find a protein called aggrin expressed by the terminal axon. This is secreted and binds on the muscle cell surface, where it activates LDL-receptor-related 4 and Muscle-specific Receptor Tyrosine Kinase (MuSK).

Together, these components control signaling that targets a high density of AChR to the membrane nearby.

Question 19

Neurexin and neuroligin

Answer 19

Neurexin is enriched pre-synaptically while neuroligin is enriched post-synaptically.

Like aggrin, neurexin can induce the complexing of post-synaptic structures on the opposing neuron, such as GABA receptors. Meanwhile, neuroligin can also induce the complexing of pre-synaptic structures on the opposing neuron, such as synaptic vesicle clusters. Thus, their signaling is bi-directional.

The neuroligin involved appears to determine the type of synapse formed: Neuroligin-1 results in the recruitment of glutaminergic proteins while neuroligin-2 results in the recruitment of GABAergic proteins.

Question 20

Glia-derived factors in synapse development and regulation

Answer 20

Thrombospondins - Factors from glia which support structural synapse development, but not synapse function. Loss of TSPs reduces synapse number.
Glypicans - Glycosylated, extracellular, GPI-anchored proteins on glia which recruit post-synaptic GluA1 (an AMPA glutamate receptor subunit). Loss of glypicans also reduces synapse number.

Question 21

Synapse elimination

Answer 21

Principle that most neural networks develop an excess of synapses initially and subsequently undergo pruning. For example, innervation of muscle fibers, shown.

Question 22

Stereotyped axon pruning

Answer 22

Refers to the principle of axon pruning in developed, long neurons in maturing animals. This is used to select against certain neuron types or groups during development.

These degeneration systems rely upon the ubiquitin-proteasome pathway and upon absorption of axon fragments by surrounding glia.

Question 23

Nerve growth factor (NGF)

Answer 23

Major growth factor and inhibitor of apoptosis for neurons, aka neurotrophins.

Question 24

Neurotrophins

Answer 24

Nerve growth factor (NGF)
Brain-derived neurotrophic factor (BDNF)
Neurotrophin-3
Neurotrophin-4

All are cleavage products of “pro-neurotrophins.” All neurotrophins bind to the p75NTR (neurotrophin receptor) and they have varying affinity for the three Trk receptor kinases. Trks are classical receptor tyrosine kinases which must dimerize in order to signal and activate the RAS, MAPK, and PI3K pathways.

Question 25

Continuous vs Discrete Mapping

Answer 25

Question 26

Possible mechanisms of establishing wiring specificity

Answer 26

Question 27

Axon attraction and axon separation

Answer 27