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Summary Adaptive brain chapter 23
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Summary of Purves Chapter 22: Early Brain Development
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Neuronal networks chapter 23Learning objectives: The Molecular Basis of Growth Cone Motility
• You can explain how neurons find their target cell. Growth cone motility is arranged by the rapid movement of the
• You can differentiate between classes of signalling molecules cytoskeleton, the energy for this is generated by ATP-dependent
that mediate network formation. modification of the actin cytoskeleton and microtubules. The actin
• You can describe the steps in synapse formation. cytoskeleton regulates changes in the lamellipodia and filopodia
• You can explain how synaptic contacts in a network are shape for directed growth, while the microtubules are responsible
maintained. for the elongation of the axon itself. Actin filaments are found in
the lamellipodia and filopodia of growth cones and tubulin is
Neurons are polarised epithelial cells that assemble into sheets found in microtubules that run parallel to the axis of the axon
that absorb molecules from the environment in one domain and and give it both structural integrity and means for transporting
secrete proteins and other cell products in the other. The apical proteins from the nerve cell body to the axon terminal. Actin and
surface of epithelium cells on the gut, lung, kidney, and pancreas tubulin are found in two forms in the growth cone and axon:
have specialisations such as cilia or microvilli that increase the freely soluble monomers (cytoplasm) and polymers that form
surface area. The basolateral surface is specialised for filaments in organised bundles.
intercellular communication and is being held together by
junctions.
Steps of neuron development
Because neurons are highly specialised the first step in their
development is distinguishing the polarity between the dendrites
and the axon. Once the neurogenesis is complete, the outgrowth
of neuronal processes begins. Initially, several small extensions of Polymerisation and depolymerisation of actin at the membrane of
neurites protrude from the neuron, microtubules and action the lamellipodium and filopodium sets the direction of growth
components of the cytoskeleton and other proteins are cone movement by generating local forces that orient the growth
redistributed among the neurites so that a single process is cone toward or away from substrates. Polymerisation and
identified as the axon. The remaining processes become dendrites. depolymerisation of tubulin into microtubules determine the
direction of movement of the growth cone by stabilizing the axon
The Axon Growth Cone shaft. The interaction of motor proteins with tubulin allows the
Once the axon has been specified, it navigates to find appropriate axon to remain stable during the expansion of the growth cone
synaptic partners. There are two features of axonal growth, the and extension of the axon.
first one is the growth cone, which is a specialised structure at
the tip of the extending axon. Growth cones are highly motile
and explore the extracellular environment to determine the
direction of growth, and then guide the extension of the axon in
that direction. The morphological characteristic of a growth cone
is an expansion of the growing axon at its tip called a
lamellipodium. The numerous extensions from the lamellipodium
are called filopodia.
Transient receptor potential (TRP)
TRP channels are channels that are activated by second
messengers or second-messenger pathways that mobilise
intracellular calcium stores and are thought to be a mediator of
Lamellipodia can concentrate molecular receptors on their cell actin and microtubule dynamics in the growing axon. The
surfaces based on their cytoskeleton. ATP-dependent interactions fluctuations of calcium are localised in the growth cone and
between cytoskeletal proteins provide energy and power to influence the decisions on the direction of growth. Thus, changes
propel the growth cone and its axon to its target. The in the cytoskeleton and local composition of membrane proteins of
lamellipodium of the growth cone is highly dynamic, once it the growth cone and axon, are mediated by intracellular signal
encounters a potential target it makes a loop to stabilise the transduction in response to adhesion molecules and diffusible
lamellipodia from which the filopodia grow out if. This requires signals in the embryonic terrain through which the growth cone
special cues from the target cell. extends.
, Growth cone steering There are two ways of signalling, inside out and inside in, where
When guidance cue molecules, such as BDNF are in the the growth cone binds to the ECM. It is also possible that
surroundings of the growth cone, this gets picked up by one side instead of a molecule that binds on the outside of the cell, the
of the cone. Depending on whether this is a repellent or molecule binds to the tails of
attractant cue, the growth cone moves in a certain direction. The the integrin within the cytoplasm:
cues get picked up by receptors and trigger all sorts of cascades this provides cell adhesion, cell
to move the cone in a certain direction. To move the cone, there migration, ECM assembly and
is a balance in the endo- and exocytosis of the growth cone. If recognition. This makes the
you want the growth cone to move to the right side, there is growth cones more sensitive.
more exocytosis/more insertion of the membrane that is coming
from the other side of the cell. On the other side is more CAMS and Cadherins
endocytosis, so from this side, the excess membrane is removed. CAMs and cadherins have dual functions (hand-shake molecules),
This allows the filopodia to extend. the same molecule can function as ligands and receptors, usually
via homophilic binding, which is important for recognition between
axons and targets. This means that cadherin can bind to cadherin
and CAM can bind to CAM. The calcium-independent CAMs
interact with cytoplasmic kinases that initiate cellular responses,
whereas the calcium-dependent cadherins engage in the β-
catenin pathway
Non-Diffusible Signals for Axon Guidance
For the growth cones to extend there are certain cues needed
that cause the growth cone to move in a particular direction.
There are also receptors and transduction mechanisms that
respond to these cues. The cues are part of a group of proteins
that are associated with cell adhesion and cell-cell recognition
and initiate intracellular signalling cascades that alter the actin Ephrins and tyrosine kinase receptors
or microtubule cytoskeleton, the complement of receptors and The last class of non-diffusable axon guidance molecules are
channels on the cell surface, or gene expression. The association ephrins and their Eph receptors, which are specialised tyrosine
of cell adhesion molecules with axon growth is based on kinase receptors (TKR). These are present in the plasma
experiments where the addition or removal of a molecule results membrane and are involved in cell-cell recognition and recognise
in modifying the behaviour of axons or where deletion/ pathways for growth as well as sites for synaptogenesis. The
manipulation disrupts the growth of an axon pathway. binding of ephrins with Eph receptors activates the receptor by
cross-phosphorylation, which can interact with cytoplasmic protein
kinases. Ephrins and Ephs activate a variety of signalling
pathways and can promote or limit growth. To modulate the
signal, the extracellular and intracellular domains of the Eph
receptor can be cleaved.
Molecules that are involved in axon growth
Non-diffusable cell-cell contact molecules:
• ECM adhesion molecules: integrins
• Ephrins
• Cadherins
• Calcium-independent adhesion molecules (CAMs)
Diffusable:
• Semaphorins The dependence on axon growth and guidance that is activated
• Netrin/slits by these interactions is involved in several developmental
• Neurotrophic factors (NGF, BDNF) neurological disorders such as mental retardation. Mutations in
genes can also lead to a partial absence of the corpus callosum
Integrins that connects the two cerebral hemispheres or the corticospinal
Dimers that stretch out upon activation that allow interaction tract that carries information to the spinal cord.
with the ECM. The integrin receptors bind to ECM molecules,
which triggers a cascade via interactions between the
cytoplasmic domains of integrins with non-
receptor cytoplasmic kinases and other
signalling molecules, as well as calcium
channels that stimulate axon growth
and elongation.
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