| Sumari: | <p>Synapse formation, differentiation and plasticity involve modulation of pre- and post-synaptic
elements. During synaptogenesis synaptic compartments differentiate in response to molecular cues. Once
contact is made, both dendritic and axonal cells undergo further morphological changes resulting in the
formation of a synapse which then matures in response to activity. The α-amino-3-hydroxy-5-methyl-4-
isoxazole-propionic acid (AMPA) receptors are responsible for fast excitatory neurotransmission in the central
nervous system. Here the AMPA receptor GluA2 amino terminal domain (ATD) structure was solved to 1.8Å
resolution and revealed important insights into the domain’s involvement in subunit specific assembly and the
presence of a putative ligand binding site.</p>
<p>In a physiological context, the AMPA receptor spends little time alone. Shuttle proteins control its
transport to the cell surface, intracellular scaffolding molecules synaptically anchor the receptor and auxiliary
proteins modulate channel conductance but all interactions are structurally uncharacterised. This thesis aimed
to give structural and biophysical insights into protein-protein interactions involving the AMPA receptor
ectodomain. Biophysical analysis confirmed that neuronal pentraxins (NPs) interact with AMPA receptor
ATDs (all GluA2, 3 &4 ATDs and NP1, NP2 and NP receptor-pentraxin (PTX) domain combinations tested),
where a preferential interaction occurred between GluA4_ATD and NPI_PTX. Stable homogenous oligomeric
neuronal pentraxins were developed to investigate their physiological effects on AMPA receptors where
receptor clustering had previously been observed. Cellular analysis indicated that AMPA receptors are
internalised when stimulated with trimeric NP1. Additional AMPA receptor-interacting proteins were also
investigated where biophysical analysis found that Brorin, a BMP antagonist, interacts with the LBC of GluA2.
Attempts were also made to optimise the expression of the AMPA receptor auxiliary protein cystine-knot
AMPA receptor modulating protein-44 (CKAMP44) and transmembrane AMPA regulatory protein-2 (TARP2)
that are also postulated to interact with the AMPA receptor ectodomain but remain structurally uncharacterised.</p>
<p>Regulation of synapse formation and function can also be achieved through adhesive mechanisms.
The recent identification of meprin/A5 protein/ receptor protein tyrosine phosphatase mu (MAM)-domain
containing glycosylphosphatidylinositol (GPI)- anchor protein 1 (MDGA1) as a key modulator of the neurexinneuroligin interaction in inhibitory synapse formation suggested its potential role as the first identified negative
regulator of inhibition. MDGA1 was structurally uncharacterised but here the structure of the secreted MDGA1
ectodomain was solved to 3.4Å and presented a highly unusual interlocked dimer in a ‘tubular bells’ formation.
Biophysical analysis determined that MDGA1 interacts with both neuroligin1, usually post-synaptically
expressed at glutamatergic synapses, and neuroligin-2, expressed at γ-aminobutyric acid (GABA)-ergic
synapses. MDGA1 may therefore be a key regulator of both excitatory and inhibitory synapse formation and
function. </p>
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