Unit 1 - Axonal Growth, Synaptogenesis, and Tropism Flashcards
neuronal polarization
first step in neuronal identity and the formation of connections (primary property of all cells)
- neuron starts round with no obvious processes (neurites)
- over time it begins to extend multiple processes and becomes multipolar
- one of these processes become the axon, which then defines the polarity of the neuron
- at the tip of the axon is/are specialized structure(s) called growth cones
axonal growth cone structure, function, and morphological characteristics
specialized motile structure at tip of extending axon
- explores extracellular environment, determines direction of growth, and guides extension of axon
- key decision-making structure in axon pathfinding
- lamellapodium - fan-shaped sheet at tip of axon; contains actin filaments and microtubules
- filopodia - fine processes extending out from lamellapodium; contain actin filaments; form and disappear rapidly
cytoskeletal elements in growth cones
globular actin can be incorporated into filamentous actin at leading edge of filopodium in response to environmental cues
- key to growth cone turning is binding of F-actin binding PRO to F-actin to regulate retrograde flow
- when encountering attractive cue, assembly is increased and retrograde flow slowed, causing turning towards attractive cue (opposite if repulsion)
- microtubules make core of cytoskeleton in axon, and are stable and strong
- modulation of interactions with microtubules modulate stability and turning of axons
what are microtubules and F-actin primarily responsible for?
microtubules: axon elongation
F-actin: direction
cytoskeleton components in growth cones
- F-actin
- tyrosinated microtubles
- acetylated microtubules
discrete regions of growth cones can be detected by different types of actin and tubulin
- F-actin is in lamellipodium and filopodia
- tyrosinated microtubules are in lamellipodia, and around for a shorter amount of time than acetylated
- acetylated microtubules are only in axons
growth cones and “decision points”
growth cone changes shape when they encounter specific environmental cues
4 types of axon guidance molecules
non-diffusible (short-range) 1. contact attraction 2. contract repulsion diffusible (long-range) 3. chemoattraction 4. chemorepulsion
all act in concert to guide axon to appropriate target to ensure accurate guidance
what do attractive and repellent interactions promote in terms of growth cone movement?
attractive: promote actin polymerization in direction of attractive molecules
repellent: promote actin depolymerization and growth cone collapse
are ECM molecules attractive or repellent? what kinds are there?
both PNS (attractive) and CNS (repellent) non-diffusible molecules
- PNS: laminins, collagens, and fibronectin; bind to integrin growth cone receptors to trigger signaling cascades for axon growth and elongation
- CNS: hyaluronan, proteoglycans, and glycoproteins; lacks “typical” matrix molecules of PNS
are cell adhesion molecules attractive or repellent? where are they? how do they work? are they Ca++ independent or dependent? what is the L1 CAM associated with?
attractive non-diffusible molecule; located on surface of growing axons, growth cones, and surrounding cells or targets
- act as ligands and receptors via homophilic binding
- Ca++ independent
- ligand/receptor interaction induces interaction with cytoplasmic kinases in growth cone
- L1 CAM has been associated with fasciculation (bundling) of groups of axons as they grow
are cadherins attractive or repellent? where are they? what do they do? are they Ca++ independent or dependent?
attractive non-diffusible molecule; located on surface of growing axons, growth cones, and surrounding cells or targets
- act as ligands and receptors via homophilic binding
- Ca++ dependent
- ligand/receptor interaction triggers intracellular signaling pathways that lead to actin binding and organization
are semaphorins attractive or repellent? where are they? what do they do? what family are the growth cone receptors?
mostly repellent non-diffusible molecules; can be attractive in some situations
- can be secreted or anchored to the cell surface; secreted forms are attached to cell surface or ECM, so not really diffusible
- receptors on growth cones are plexins; cell surface forms bind directly to plexins, while secreted forms bind neuropilins then complex with plexins
- ligand/receptor interaction results in growth cone collapse and inhibition of axon extension via receptor interaction with intracellular signaling molecules
are ephrins attractive or repellent? what are the classes? what do they do?
repellent non-diffusible molecules, similar to CAMs
- ephrin-A: GPI_linked to cell surface
- ephrin-B: single pass transmembrane PRO
- each class has its own set of receptors on growth cones that are tyrosine kinases
- ephrin interaction with its receptor on the growth cone results in a repellent interaction that collapses the growth cone
ephrins in the optic tectum (superior colliculus)
-differences in anterior and posterior tectum
- in developing brain, axons from the retina make precise connections with optic tectum
- -temporal/lateral retina to anterior tectum
- -nasal/medial retina to posterior tectum
- ephrins A2 and A5 are expressed in anterior-to-posterior gradient with highest concentration at posterior side
- axons from temporal retina are repulsed by ephrin in posterior tectum b/c they express high levels of EphA3 receptor
- axons from nasal retina are blind to ephrin b/c they lack the eph receptor
are netrins attractive or repellent? what secretes them? what happens if it’s knocked out?
attractive or repellent diffusible molecules, depending on receptors expressed on growth cone
- secreted by target cells in midline of embryo
- attractive receptors are members of DCC family
- repellent receptors are members of UNC5 family
- KO eliminates crossing of commisural axons
are slits attractive or repellent?
repellent diffusible secreted PRO
- receptors on growth cones are members of Robo family
- slit interaction with its receptor on growth cone results in repellent interaction
what do commisural axons do in the spinal cord and why is this a problem?
axons cross in spinal cord, and make contact with cells on opposite side
- this is a problem b/c:
1. cell has to avoid its target cell on the same side of the spinal cord
2. has to cross at a very specific site in the cord
3. once crossed, cannot re-cross
4. has to find its target cell on the other side of the cord
how to netrin and slit act in concert at midline?
comissural axons express DCC (attractive netrin) and are originally attracted to midline (which produces high levels of netrin)
- as they cross the midline, the upregulate Robo (repellent slit) which keeps them from recrossing b/c of the high level of slit at the midline
- there is also cross-talk between Robo and DCC, where Robo signaling inhibits DCC signaling
steps to axon guidance cues
- ECM adhesion (attractive, non-diffusible collagen)
- cell surface adhesion (attractive, non-diffusible NCAM)
- fasciculation (attractive, non-diffusible L1CAM)
- chemoattraction (attractive, diffusible netrins)
- contact inhibition (repellent, non-diffusible ephrins)
- chemorepulsion (repellent, diffusible slits)
selective synapse formation
after reaching correct, final target region, axons must decide which cells to innervate
-synaptogenesis is best understood in PNS system (neuromuscular junction, especially) although it is different from CNS
synaptogenesis at the NMJ
- motor axon approaches and makes contact with myotubule, seemingly at random
- both nerve terminal and myotubule differentiate after contact has been made (with nerve becoming motor terminal and muscle forming postsynaptic apparatus)
- differentiation of muscle is induced by agrin, which activates MuSK (muscle-specific kinase), causing clustering and increased local expression of AchR through rapsyn
- both motor nerve and muscle make ECM components to form basal lamina, which stabilizes synaptic structure
synaptogenesis in superior cervical ganglion; T1 and T4 axons
axons arriving from T1 and T4 form synapses on cell bodies of neurons that project to targets in eye and ear, respectively
- use same guidance cues to reach ganglion via cervical sympathetic trunk, so differential innervation of ganglionic neurons must occur at level of synapse formation
- very selective; correct pre/postsynaptic neurons have higher affinity for one another, so incoming axons preferentially form synapses on correct targets
- system of bias in synapse formation guides innervation throughout nervous system during development without limiting it
synaptogenesis in CNS
- nascent presynaptic process, derived from growth cone, recognizes appropriate site on target cell via cadherin/protocadherin family of adhesion molecule
- synaptic vesicles and active zone components begin to accumulate
- additional adhesion molecules, including neurexin in presynaptic membrane and neuroligin in postsynaptic membrane, are recruited to developing synapse
- adhesive signaling initiates differentiation of presynaptic active zone and postsynaptic density
- neurexin helps localize cytoskeletal elements, synaptic vesicles, active zone PRO, and voltage-gated Ca2+ channels to presynaptic membrane
- neuroligin recruits nt receptors and other postsynaptic PRO to postsynaptic membrane
trophic interactions between neurons and their target cells
after synaptogenesis, neurons are dependent on targets for survival and differentiation
- target cells secrete neurotrophic factors
- neurons compete for limited amounts of neurotropic factors
- neurons receiving insufficient neurotrophic support from targets degenerate and die via apoptosis
- thus, target cells play a role in determining the number of cells that innervate them
synaptic rearrangement during postnatal life
after neuronal populations are established, trophic interactions continue to modulate synaptic connections
-trophic interactions ensure that each target cell is innervated by right number of axons, and that each axon innervates the right number of cells
synaptic refinement via competition (synapse elimination)
happens in PNS post-natally
- initially, skeletal muscle fibers and some parasympathetic neurons are innervated by multiple neurons (polyneuronal innervation)
- during postnatal development, synaptic inputs are gradually eliminated until only a single one remains
- “competition” between different neurons for “ownership” of target cell
- -dependent on electrical activity in both pre/postsynaptic cells, since blocking either causes persistence of polyneuronal innervation
- competing axons gradually segregate at synaptic site, and losing axon atrophies and retracts while postsynaptic specializations under losing axon are lost
- the single remaining nerve terminal enlarges as endplate region expands during postnatal muscle growth
functions of neurotrophins
- survival of subset of neurons
- formation and maintenance of appropriate numbers of connections (number of target cells contacted)
- elaboration of axonal and dendritic branches to support connections (number of synapses formed)
neutrophin family
nerve growth factor (NGF) - first characterized and best studied
-acts on a few specific populations of peripheral neurons
also include BDNF, NT-3/4/5, but hard to identify b/c in such small quantities
evidence for trophic function of NGF
- neuronal death in absence of NGF
- survival of excess neurons with increased levels of NGF
- presence and production of NGF in target cells
- presence of NGF receptors in innervating nerve terminals
which growth factors do DRGs, nodose ganglia, and sympathetic ganglia respond to?
DRG respond to all growth factors
NG respond most to NT3
SG respond mostly to NGF, and not at all to BDNF
2 types of neurotrophic receptors
Trk - “short form” family of receptor tyrosine kinases (TrkA/B/C)
- each binds a different subset of neurotrophins
- all 3 have high affinity for processed/cleaved neurotrophins only
p75 - “long form” family activated by all neurotrophins
-high affinity for unprocessed neurotrophins, and low affinity for processed
expression of distinct receptors by subsets of neurons allows them to respond selectively to different neurotrophins being secreted by different target cells
what neurotrophins bind to:
- TrkA
- TrkB
- TrkC
- p75
TrkA - NGF only
TrkB - BDNF, NT-4/5, sometimes NT-3
TrkC - NT-3 (usually goes here)
p75 - all of the above
neutrophin signaling overview
activation of each class of neutrophic receptors by neutrophins triggers distinct intracellular signaling cascades
- a variety of intracellular signaling cascades can be activated by each class of receptors
- -the specific cascade that is activated will determine cellular response
- -cell survival or death
- -cell progress growth or differentiation
- -activity dependent synaptic stabilization or elimination
what do neurotrophic interactions depend on?
- neurotrophins secreted by target cells
- neurotrophin receptors on neuron
- intracellular signaling cascades present in neuron
what do neurotrophic interactions determine?
- numbers of neurons
- shape of neurons (degree of arborization)
- patterns of neuronal connections (number of synapses)
what happens if Trk receptor signals:
- PI3 kinase
- ras
- PLC
- PI3 kinase: cell survival
- ras: neurite outgrowth and neuronal differentiation
- PLC: activity-dependent plasticity
what happens if p75 receptor signals:
- RhoA
- c-Jun
- NF-kappa-beta
- RhoA: neurite growth
- c-Jun” cell death
- NF-kappa-beta: cell survival
