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making neural connections - Coggle Diagram
making neural connections
key points
how correct target innervation is important for neuronal survival?
only neurons that are able to make appropriate connections with target cells are able to survive. those neurons that make no contact will eventually die
what is the structure and function of the growth cone?
the growth cone is responsible for guiding the axon by sensing the environment for cues. this process is dependent on dynamic polymerisation and depolymerisation of actin filaments
how does cell adhesion influence axon pathfinding?
cell adhesion links the extracellular environment to the actin cytoskeleton of the growth cone, either directly or indirectly through messenger signalling
how many different types of axon guidance mechanisms exist?
there are four main types of axon guidance mechanisms: contact attraction, contact repulsion, chemoattraction and chemorepulsion
what are the effects of each major class of guidance cue family?
each major class of guidance cue family can exert repulsion or attraction to the growth cone, which is largely context/tissue dependent and relies on specific ligand-receptor combination and other proteins in the environment
axonal guidance and synapse formation
connecting to targets
growth cone
adhesion molecules
guidance cues
connecting to targets
synapse formation allows neuronal survival
junctions between nerve cells and muscle cells in vertebrates are neuromuscular junctions
neurons that do not connect with target undergo apoptosis
20,000 motor neurons are formed in the spinal cord of the chick but approximately half die
survival may depend upon establishing a functional synapse with a muscle cell
even after neuromuscular connections are made, some are eliminated until each muscle fibre is innervated by only one motor neuron
neurons project axons towards target
axons require growth (trophic) factors from target to survive when they reach it
connections form
- selection of correct connections
programmed cell death eliminated inappropriate connections (cells in the projecting neuronal group and the target begin apoptosis)
programmed cell death
- elimination of excess cells by selecting those that make the correct connection followed by pruning of redundant connections
those neurons that survive are able to take up growth factors from the target
neuronal death occurs at least in part, because neurons are produced in excess during development and compete with each other for the limited amounts of the survival-promoting trophic factors secreted by target tissues
when axons reach their targets they form synapses
topographic mapping
even within the target, there is a graded innervation for finer control or to provide more information
mapping of retinal axis across the axis of a target nucleus in the brain, tectum (fish, frogs, birds_, geniculate nucleus or superior colliculus (mammals)
mapping of the medial-lateral axis of motor neurons from the spinal cord to a single muscle across its axis (dorsal ventral) eg. gluteal muscle
timing in dependence
neurons are not initially dependent on target cells
switch in dependence occurs - axons must reach target and form connection before this
factors in pathway also important - often depend on factors from glia or other cells in the pathway (adhesion factors and guidance cues)
target switching
during development of the brain some neurons must innervate multiple targets or they must innervate a secondary target first before reaching the primary target
there is therefore a transitory dependence on a secondary target
final survival then depends on innervation of the primary target
there is then pruning or complete loss of innervation of the secondary target
how does the axon navigate complex environments to find its target?
once the axons reach the target the neurons become dependent on the target for survival
ie. cell death occurs if the growth factor does not bind to the receptor
growth cone
the motile axonal growth cone
ramón y cajal, a famous neuroanatomist, was the first person who observed growth cones (in fixed brain tissues)
in the early 20th century ross Harrison cultured pieces of embryonic neural tube tissue and observed growth cones moving in real time
in the 1940s Speidel was the first one to observe dynamic movements of growth cones in vivo
the driver and engine of axon pathfinding
th growth cone
contains many lamellipodia (membrane protrusions) and filopodia (motile fine processes extending outwards from the lamellipodia)
if we want to know how a growth cone navigates, we must understand how it moves first, and this means looking into its dynamic cytoskeleton
the cytoskeleton of a growth cone is filled with molecules that are involved in cell movements
growth cones have the ability to sense the environment and translate the extracellular signals into a directed movement
the locomotory apparatus of an axon
neuronal growth cones repond to both contact mediated and chemotropic guidance cues that are either attractive or repulsive
the physical movement of the growth cone requires dynamic polymerisation and breakdown of actin filaments
growth cone is the only motile part of the axon
two agents inhibit the normal activity of the growth cone due to their ability to bind actin
pharmacology of actin filaments
drugs such as the actin depolymerising agent
cytochalasin
and the filament stabilising agent
phalloidin
have been used to investigate the functions of actin in the growth cone
treatments of growth cones with such drugs prevents filopodia formation and such growth cones slow down dramatically, showing that actin-rich fibres are important for the forward progress of a growth cone
cytochalasin
binds to monomeric actin
blocks actin polymerisation
induces depolymerisation of filaments
causes axons to lose directionality and grow randomly
cytochalasin-treated growth cones do not steer properly and usually lose their way in the developing organism
phalloidin
binds to polymeric actin
stabilises actin filaments and prevents filament depolymerisation
the process of axon growth
penetration of tissue is caused by the secretion of proteases that allow the passage of the growth cone
the extension of the axon requires microtubules
when microtubule polymerisation is inhibited (by addition of colchicine) axons retract
adhesion molecules
guidance by adhesion gradients - haptotaxis
haptotaxis
is a directional cell movement in response to adhesive substrates such as ECM
growth cone expresses
cell adhesion molecules
(CAMs), enable it to move by modulating adhesion to the extracellular matrix (ECM) and other cells
CAMs
on growth cone recognise proteins found in certain basal laminae
Cell Adhesion Molecules (CAMs)
sit around cell membrane and bind molecules in membranes of neighbouring cells, eg.
Ig-CAMs, NCAM
immunoglobulin family CAMs (Ig-CAMs)
structurally similar to IgGs
IgG domains bind similar domains sticking out from surface of other cells (same Ig-CAM or different) eg.
NCAM
with
NCAM
.
Neurofascin
with
CASPR1
calcium independent homophobic interection
fibronectin like domains
links to actin cytoskeleton
may be transmembrane or surface-bound
sit in cell membrane and bind molecules in the extracellular matrix eg.
Integrins
integrins
transmembrane proteins, bind adhesive glycoproteins eg. fibronectin, in the ECM
calcium-dependent
recognise an arginine-glycine-aspartate sequence in ECM proteins including vitronectin, fibronectin, laminin
several genes
alpha and beta subunits form heterodimeers
localised to specific junctions and bind proteins in similar junctions on neighbouring cells eg.
Cadherins
cadherins
calcium-dependent, transmembrane,
homophilic
, act as dimers, major component of adherents juncitons
links to actin cytoskeleton via
catenins
links to cytoskeleton + secondary messenger signalling pathways affecting cell behaviour
shape
movement
fate
laminin
appears to pave many axonal tracts, even if only transiently
glycosaminoglycans
appear to impede neural outgrowths
because laminin and CAMs are found in several places, they can only provide general cues
guidance cues
four guidance mechanisms in axon pathfinding
chemorepulsion
chemoattraction
contract repulsion
contact attraction
4 types of molecular cues influencing the direction in which growth cones will travel
long range
short range
attractive
repulsive
guidance cue families
genetic studies of neuronal development in simple invertebrate systems including
c elegans
and
d melanogaster
have identified four major pathways of axonal guidance
studies of mammalian systems have confirmed the importance and diversity of these evolutionarily conserved pathways
semaphorins
about 20 different proteins - mostly transmembrane
about 1/3 are secreted - positively charged carboxy terminus that increases their affinity for the extracellular matrix
semaphorin domain is highly conserved - 500 amino acids
most repel the growth cone
plexins are the transmembrane protein receptors for the semaphorins
diversit of semaphoring ligands and plexin receptors
the outcome in terms of attraction or repulsion is also dependent on which receptors are present in the growth cone
also there are different family members for both ligands and receptors and this complexity can influence the outcome in terms of attraction or repulsion depending on which receptors are expressed in the growth cone
transmembrane semaphorins bind directly to plexins but...
secreted semaphorins require an additional protein (neuropilin) to bind to the plexins
dual function
in the presence of NGF Sema III has a repellent effect on neurone growth
in the presence of NT3, Sema III elicits outgrowth of neurites
secreted (class 2 & 3) or membrane bound ligands (GPI anchored or transmembrane domain) can have chemorepellant or chemoattractive functions (dependent on proteins present in the environment)
whether semaphorins mediate attraction or repulsion, this depends on other proteins present in the environment, such as growth factors
netrins
diffusible proteins that can mediate either repulsion or attraction
netrins show strong homology to a region of the ECM molecule laminin-1
attraction is mediated through receptors of the DCC/UNC-40 family of receptors
repulsion is mediated through the UNC-5 family of receptors
it is likely that an unidentified tyrosine kinase is associated with the UNC-40 receptor and that this kinase mediates effects of netrins
netrins in commissural axon guidance
in the absence of netrin-1 abnormal migration of commissural axons is observed
slits
large (~190kDa) extracellular matrix proteins. In vertebrates there are three Slit proteins
Slit proteins contain
leucine-rich
and
epidermal growth factor-like
repeats
slits are predominantly
repellent
ligands - they are well known for axon guidance at the
midline
vertebrates have four Robo receptors
transmembrane proteins
contain
Ig
and
fibronectin
domains in the extracellular portion of the protein
midline crossing in
drosophila melanogaster
Robo is expressed by axons that navigate the midline and prevents them crossing
Robo expressing axons run longitudinally and do not cross the midline
some commissural axons, however, are able to cross the midline because they down regulate Robo
commissural axons up regulate Robo after they have crossed the midline
in Robo and Slit mutant axons freely cross the midline
analogous midline crossing functions of Slits have been identified in vertebrates
optic chiasm
commissural axons in ventral spinal cord
cells along the midline express
high levels of slit
ephrins
CLASS A ephrins are tethered to the cell surface by a GPI linkage
CLASS B ephrins are tethered to the cell surface by transmembrane domains
ephrins must be clustered in order to activate their receptors
Eph receptors are RTKs that bind specifically to either Class A or Class B ephrins
EphA receptors bind and activate Ephexin which is a GEF for rho family GTPases
GPI: glycophosphatidylinositol
RTK: receptor tyrosine kinase
ephrin receptor signalling
ephA receptors bind and activate Ephexin which can then bind Rho family GTPases
the RhoA-GTPase influences the rate of actin polymerisation at the growth cone leading edge
EphB receptors signal through PDZ containing proteins including GRIP-1 and a novel G protein regulator (PDZ-RGS3)
topographical map formation in the retina-tectal projection
ephrinA2 in a smooth gradient across the whole tectum
EphrinA5 in a steeper gradient confined roughly to the posterior half of the tectum
ephrinA2 and ephrinA5 bind EphA receptors on RGC axons (EphA4, EphA5 and EphA6)
EphA5 and A6 is preferred receptor for EphrinA2 and ephrinA5 increasing nasal to temporal gradient
Eph4 is expressed uniformly
location
netrins, slits and some semaphorins are secreted and associate with cells or the ECM
ephrins and some semaphorins are membrane bound
action
netrins can act as
attractants
or
repellents
slits, semaphorins and ephrins act primarily as
repellents
but can be attractant in some cases
receptors
for each cue there is one or more transmembrane receptors
may alter properties (attract v repel
summary of effects
semaphorins - repellent, modified to attract by other proteins
slits - repellent (but some attract
netrins - attract or repel - depend on receptors
ephrin/eph receptor - mostly repel - create boundaries or gradients for topographic mapping
