Importance of axon-glial interactions for the normal postnatal development of the mouse peripheral nervous system
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Roche2015.docx (89.57Mb)
Date
29/06/2015Author
Roche, Sarah Louise
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Abstract
The mouse nervous system undergoes a vast remodelling of synaptic connections
postnatally, resulting in a reduced number of innervating axons to target cells within
the first few weeks of life. This extensive loss of connections is known as synapse
elimination and it plays a critical role in sculpting and refining neural connectivity
throughout the nervous system, resulting in a finely tuned and well-synchronised
network of innervation. This process has been well characterised at the mouse
neuromuscular junction (NMJ), where synapse elimination takes place postnatally in
all skeletal muscles. It has been well studied for the reasons that it is easily accessible
for live imaging and post-mortem experimental analysis. Studies utilising this
synapse to uncover regulators of synapse elimination have mainly focused on the
importance of glial cell lysosomal activity, nerve conduction and target-derived
growth factor supply. It is clear that non-axonal cell types play key roles in the
success of developmental axon retraction at the NMJ, however the role of glial cells
in the regulation of this process has not been fully explored, as lysosomal activity is
thought of as a consequence of axon pruning rather than a molecular driver.
Previous studies have shown that signals emanating from myelinating glial cells can
modulate neurofilament composition and transport within the underlying axons. We
know that these changes in neurofilament composition and transport are underway
during developmental synapse elimination at the NMJ, so it seems logical to predict
that myelinating glial cells may play a role in the regulation of axonal pruning.
Myelinating glial cells are found along the entire length of lower motor neurons and
form physical interactions with the underlying axons at regions known as paranodes.
At the paranode, Neurofascin155 (Nfasc155: expressed by the myelinating glial cell)
interacts with a Caspr/contactin complex (expressed by the axon). This site has been
proposed as a likely site for axon-glial signalling due to the close apposition of the
cell membranes.
The main focus of this PhD project was to study the potential role of myelinating
glial cells in the success of synapse elimination at the NMJ, using a mouse model of
paranodal disruption (Nfasc155-/-). Chapters 3 and 4 show the results of this work.
This work has revealed a novel role for glia in the modulation of synapse elimination
at the mouse neuromuscular junction, mediated by Nfasc155 in the myelinating
Schwann cell. Synapse elimination was profoundly delayed in Nfasc155-/- mice and
was found to be associated with a non-canonical role for Nfasc155, as synapse
elimination occurred normally in mice lacking the axonal paranodal protein Caspr.
Loss of Nfasc155 was sufficient to disrupt axonal proteins contributing to
cytoskeletal organisation and trafficking pathways in peripheral nerve of Nfasc155-/-
mice and lower levels of neurofilament light (NF-L) protein in maturing motor axon
terminals. Synapse elimination was delayed in mice lacking NF-L, suggesting that
Nfasc155 influences neuronal remodelling, at least in part, by modifying cytoskeletal
dynamics in motor neurons. This work provides the first clear evidence for
myelinating Schwann cells acting as drivers of synapse elimination, with Nfasc155
playing a critical role in glial cell-mediated postnatal sculpting of neuronal
connectivity in the peripheral nervous system. A small section of the results within this thesis are devoted to the study of axon-glial
interactions in a mouse model of childhood motor neuron disease, otherwise known
as spinal muscular atrophy (SMA). In SMA, there are reduced levels of the
ubiquitously expressed survival motor neuron (SMN) protein. The NMJ is a
particularly vulnerable target in SMA, manifesting as a breakdown of neuromuscular
connectivity and progressive motor impairment. Recent studies have begun to shed
light on the role of non-neuronal cell types in the onset and progression of the
disease, presenting SMA as a multi-system disease rather than a purely neuronal
disorder. Recent evidence has highlighted that myelinating glial cells are
significantly affected in a mouse model of SMA, manifesting as an impaired ability
to produce key myelin proteins, resulting in deficient myelination. The final results
chapter of this thesis (Chapter 5) is focussed on further exploring the effects that loss
of SMN has in Schwann cells including their interactions with underlying axons.
This work reveals a disruption to axon-glial interaction, shown by a delay in the
development of paranodes, supporting the idea that non-neuronal cell types are also
affected in SMA.
The results within this thesis reveal a novel role for a glial cell protein, Nfasc155, in
the modulation of synapse elimination at the NMJ. Mechanistic insight in to
Nfasc155’s role in this process is also uncovered and likely involves axonal
cytoskeletal transport systems and the filamentous protein NF-L, which have not
previously been implicated in the process of synapse elimination. This work
highlights an important role for axon-glial interactions during normal postnatal
development of the mouse NMJ. This work also highlights a role for axon-glial
interactions in disease states of the NMJ. Using a mouse model of SMA, axon-glial
interaction was assessed with the finding of a delay in paranodal maturation due to
loss of SMN.