Construction of Neural Circuits Flashcards
What are polarized epithelial cells?
cells that will develop into neurons
These all have an apical side specialized for secretion of chemical signals and a basal side involved in receiving intercellular signals
The distribution of cytoskeletal proteins creates this apical/basal polarity
What do fully committed polarized epithelial cells extend
Once a cell has fully committed to the neural fate, it begins to extend numerous, temporary outgrowths called neurites
Eventually, actin and microtubule elements will redistribute and concentrate in a single neurite that will be the axon. The remaining neurites will become the dendrites.
Two components of the axon growth cone
A structure at the tip of the growing/developing axon that guides it to its’ appropriate target.
Lamellipodium – the larger, sheet-like component of the growth cone (the “cone” part)
Filopodia – the smaller, finger-like projections that extend out from the lamellipodium
What is the growth cone for?
The growth cone is capable of detecting signals that guide the axon to its appropriate target. This includes changes in direction when necessary (think about how the axons of the retinal ganglion cells are distributed in the optic chiasm/optic tract).
Once the appropriate target is reached the growth cone converts into a presynaptic terminal.
What is the movement of the growth cone controlled by? How does the axon grow to follow the growth cone?
Movement of the growth cone and the axon shaft is controlled by the assembly and disassembly of the cytoskeletal proteins.
Actin forms the filaments that create the structure of the lamellipodium and filopodia.
Tubulin forms the microtubules that create the structure/shape of the axon. Microtubules are also responsible for the trafficking of proteins, mRNAs, vesicles and other cellular components up and down the axon.
What happens in the growth cone during projection toward a cue?
When the growth cone is projecting towards a cue, globular or monomeric actin (g-actin) is polymerized to form fibrillar actin (f-actin) that forms the actual actin filaments. Happening at the tip or leading edge.
This process pushes the filopodium forward (the actin filaments are connected to membrane-bound proteins to provide the necessary mechanical stability).
Other regions of the growth cone are undergoing actin fibrile depolymerization.
A similar process is occurring with tubulin; tubulin monomers are polymerized to form microtubules at the end closest to the growth cone, extending the axon.
What is the polymerization of actin/ tubulin in the growth cone controlled by?
The process of polymerization of actin and tubulin is controlled by actin-binding and tubulin-binding proteins.
The activity of these assembly proteins is regulated by intracellular Ca2+. The sources for this Ca2+ signal include:
Voltage-gated Ca2+ channels
TRP channels (why?)
Ca2+ release from intracellular stores
Thus, Ca2+ signals are a critical component for determining the direction/path of the growth cone.
What does the axon cone eventually become?
the presynaptic terminal
What are the types of non-diffusing axon guidance cues?
extracellular matrix adhesion molecules (ECMs)
cell adhesion molecules (CAMs) and Cadherins
ephrin-eph complexes
Describe how the extracellular matrix adhesion (ECMs) molecules guide axonal growth
These are proteins such as laminin, collagen & fibronectin that are secreted by the cell and then polymerize to create a complex that remains in close proximity to its source.
The receptor for these ECM molecules are integrins.
When a cell/axon expressing integrins make contact with ECM molecules, the integrins initiate an intracellular signaling cascade that regulates the path of axon growth.
Usually used for growth along tissue borders, e.g. the neural tube and the surrounding mesenchyme.
describe how the cell adhesion molecules (CAMs) and cadherins guide axonal growth.
CAMs and Cadherins:
Expressed on both the growing axon and the target tissue.
These undergo homophilic binding, so that whenever two of the same protein encounter each other, they bind and initiate an intracellular signaling cascade.
CAMs are associated with fasiculation, the groups of axons as part of a nerve.
Cadherins play a role in axonal target selection and the conversion from growth cone to functioning synapse.
Describe how the Ephrin-Eph complex guides axonal growth.
Ephrin is technically the ligand in this type of signaling, despite the fact that it is a membrane-bound protein.
Eph is the receptor. It is a tyrosine kinase receptor (see Chapter 7)
The signaling is bi-directional. Ephrin-Eph binding can initiate signaling cascade within the ephrin-containing cell via activation of protein kinases. A signaling cascade can also be initiated in the eph-containing cell via the tyrosine kinase mediated process.
This pathway can also be used to limit axon growth by the cleavage of either ephrin or eph.
Produce a repulsive signal. Both cells have signalling cascades
What are axon guidance diffusible signals?
These are secreted molecules that diffuse to form a signaling gradient that can either be attractive or repulsive.
One issue in identifying these molecules was distinguishing between those responsible for guidance (tropic) vs. those responsible support/survivial/growth (trophic).
Many of these molecules or signaling complexes were initially identified in C. elegans or Drosophila
*May help thing to pass through the substance. *
Netrins
secreted protein that is chemoattractive when it binds to the DCC receptor and chemorepellant when it binds to the Unc5 receptor.
Both receptors interact with Rho/Gap signaling cascades that act on axonal cytoskeletal elements.
Slit/Robo
slit is the secreted protein and robo is its’ receptor. Are chemorepulsive
and also work through the Rho/Gap pathway
Semaphorins
chemorepulsive factor (can be an attractant too); is actually non-diffusible (bound to cell membranes or ECM). Its receptors are plexin and neuropilin (which interacts with the Rho/Gap pathway).
Describe the example of axonal guidance involving the floor plate.
As the neural tube develops into the spinal cord, axon that will form the spinothalamic tract project to the floorplate that express high levels of netrins.
Netrin attracts the axons until they cross the ventral midline (axons express DCC receptors)
Once the axons cross the midline, 2 factors prevent them from re-crossing
- Expression of the robo receptor which makes the axons now sensitive to the chemorepulsive signal from slit.
- Axon’s lose their sensitivity to netrins (DCC receptors not-expressed)
Mutations of the netrin gene disrupt the formation of this midline-crossing axonal pathway.
What is the initial stage of synaptogenesis
Initial stages mediated by the same cell-adhesion molecules used for axon-guidance: cadherins, CAMs & ephrins.
What is the second stage of synaptogenesis (general, including protein names)
, the two cells undergo specialization to establish their presynaptic & postsynaptic identities.
Neuregulin
neurexin/neuroligin
What does neuregulin do in synaptogenesis?
transmembrane protein expressed on the presynaptic neuron; extracellular region is cleaved and it diffuses across the cleft to bind to the ErB receptor (a receptor kinase); this interaction contributes to the synthesis and insertion of NT receptors
- expressed presynaptically, works postsynaptically.*
- leads to addition of post-syn receptors*
What does neurexin/neuroligin do in synaptogenesis
Neurexin/Neuroligin – adhesion molecules; neurexin is expressed presynaptically and selectively binds to neuroligin expressed in the postsynaptic cell
Neurexin interacts with cytoskeletal elements that localize NT-containing vesicles, vesicle release machinery and voltage-gated Ca2+ channels
Neuroligins interact with postsynaptic density proteins that concentrate NT receptors to the postsynaptic side.
How are the correct connections made in the developing nervous system? (role of eph/ephrin, DSCAM, and protocadherins)
Multiple signaling mechanisms involved, including…
Eph/Ephrin – there is considerable variability in both the receptor & ligand; they appear to be distributed in a manner indicates a role in labeling synapses specificity
DSCAM – a cell adhesion molecule, so proteins expressed on the pre- and postsynaptic cell are binding with each other (see slide 7)
In Drosophila it has over 38,000 isoforms due to alternative splicing and which isoform is expressed determines which cells synapse with each other. It is actually heterophillic binding that leads to synaptic connectivity; homophillic binding actually leads to repulsion (studies in the Drosophila retina).
Protocadherins – again multiple splice variants lead to a large diversity of isoforms (in the extracellular regions of the proteins)
Protocadherins expressed in the pre- and postsynaptic cells bind to each other (different from cadherins) based on how similar the isoforms are to each other (the more similar the higher affinity for binding)
Not universally expressed at all synaptic sites within a single neuron so may be important for giving each synapse a distinct identity; this would be important for distinguishing on a single neuron which synapses will be GABAergic vs. glutamatergic
What is synapse elimination?
refers to the reduction in the number of inputs to a target cell, not a reduction in the overall number of synapses.
Early in development target cells are innervated by multiple inputs and there is a competitive interaction for which synapse “wins” (remains connected) and which synapse “loses”
Where are the signals for synapse elimination from?
This competitive interaction involves bot signals generated from both the pre- and postsynaptic cell
In the developing NMJ, blocking either postsynaptic activity (curare) or presynaptic activity (TTX) will result in polyneuronal innervation persisting.
The “losing” axon begins to withdraw presynaptic terminals; this is accompanied by receptor loss on the postsynaptic side (again, an interactive process)
Eventually the losing axon completely withdraws and the remaining axon takes over the abandoned region (ultimately an increase in gross number of synapses)
