Regulation of somatosensory cortex development downstream of glutamate
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Date
2009Author
Petrie, Anne
Metadata
Abstract
Development of the rodent somatosensory cortex is well characterised and involves
activity-dependent mechanisms that occur during the first postnatal week. Glutamate
is a key neurotransmitter responsible for signalling events that result in formation of
cortical barrels - aggregates of cells in the cortex corresponding to whiskers on the
face pad. The molecular mechanisms that occur downstream of glutamate signalling
are not fully understood and data here contributes to the unveiling of some of these
mechanisms. Transgenic mice with deletions of genes that encode members of the
post-synaptic complex associated with NMDARs were used to understand the role of
individual genes in the formation of barrels. SynGAP, a ras GTPase activating
protein (GAP) that negatively regulates the ERK-MAPK pathway downstream of
NMDARs is required for the formation of barrels and data here agrees with other
findings that the ras GAP NF1 has a similar role. Examination of RICS, a RhoGAP
and Dusp6 - a phosphatase that inactivates ERK reveals that neither are necessary for
the formation of barrels. This finding adds to previous data postulating that barrels
form in an ERK-independent manner (Watson et al., 2006, Barnett et al., 2006).
MAGUKs are important scaffolding molecules in the PSD and bind NMDARs to
downstream signalling molecules such as SynGAP. Two of these MAGUKs SAP102
and PSD-95 have roles in hippocampal plasticity, and learning and memory and
Sap102 mutations result in a form of X-linked human retardation (Tarpey et al.,
2004). Deletion of either gene does not cause defects in the development of barrels,
perhaps due to compensation mechanisms already described in hippocampus
(Vickers et al., 2006 Cuthbert et al., 2007). Double knockout mice die by P3 and
analysis of all other mutants revealed a defect in the formation of barrels and
segregation of TCAs in Sap102-/y Psd-95+/-. Surprisingly this defect was not seen in
Sap102+/- Psd-95-/- mice, agreeing with previous findings that SAP102 is better able
to compensate for loss of PSD-95 (is up-regulated) than PSD-95 is for SAP102. An
explanation for this effect may lie with the fact that Sap102 is X-linked and therefore
females that are heterozygote for Sap102 are mosaic with a population of cells
expressing SAP102 and a population not expressing SAP102. Using β-Galactosidase
antibody to label one population of cells, female mice that had two populations of
cells were examined. In these mice one population of cells were Sap102-Psd-95+/-, and did not previously segregate into normal barrels and the other population were
Sap102+Psd-95+/- and should segregate normally. Both populations of cells
segregated normally, indicating that the cells expressing SAP102 were rescuing the
cells not expressing SAP102 by a cell non-autonomous mechanism.
The final part of this thesis focuses on the role of glutamate-dependent signalling
pathways in the regulation of CSPGs- key extracellular matrix proteoglycans that
regulate the termination of the sensitive period. Analysis of 3 overlapping but distinct
subsets of chondroitin-sulphate proteoglycans (CSPGs) reveals that expression of
each of the three is different throughout development. After 2-3 weeks perineuronal
nets (PNNs) labelled with Cat-315 and Cat-316 are visible and locate to specific
regions within the cortical barrel-field. To determine whether the formation of PNNs
is regulated by proteins involved in glutamate signalling, expression of the three
CSPG subsets was analysed in mice with barrel defects due to mutations of Plcβ1,
Mglur5, Syngap and Prkar2b. Interestingly, Prkar2b mutant adults but no other
mutants have reduced Cat-315-PNNs, indicating that PKARIIβ regulates pathways
that lead to formation of Cat-315-PNNs in adulthood. Cat-315 has previously been
found to be regulated in the cortex of visually deprived cats and the cortex of
whisker-trimmed mice, indicating that specific subsets of CSPGs are regulated by
neuronal activity. Molecular pathways that lead to expression of Cat-315 positive
PNNs involve PKARIIβ and the formation of PNNs may be an important step in the
plasticity of circuits in barrels.
Taken together, these results demonstrate that an important part of molecular
signalling downstream of glutamate enabling barrels to form is played by molecules
that maintain structure inside the synapse and outside the cell.