The mechanisms that regulate the activity of the neuron specific K+Cl- cotransporter (KCC2) remain poorly understood, despite the critical importance of this transporter in inhibitory synaptic transmission and plasticity. In this thesis I made three novel discoveries which reveal the cellular and molecular mechanisms of KCC2 regulation. First, I assayed the K+Cl- cotransport function of KCC2 under isotonic conditions and determined the molecular domain of the cotransporter required for constitutive Cl- transport in hippocampal neurons (Acton et al 2012). Specifically, I identified the 15 amino acid domain of the C-terminus in neurons that is responsible for the ability of KCC2 to cotransport K+Cl- under basal isotonic conditions, allowing it to remain constitutively active to create the steep Cl- gradient across the neuronal membrane required for synaptic inhibition. Secondly, I investigated a novel KCC2-interacting protein named Neto2 and determined its effect on the postsynaptic action of GABA (Ivakine et al 2013). I have found that Neto2, which is also an auxiliary protein of kainate-type ionotropic receptors, can also regulate the activity of the KCC2. Neto2 is required for neurons to maintain low [Cl-]i and strong synaptic inhibition. Third, I examined the functional relevance of the KCC2:Neto2:KAR multiprotein complex and found that this complex regulates the surface level membrane expression pattern of KCC2 and the stability of the cotransporter in the membrane. Moreover, I have provided the first evidence that the interactions of KCC2:Neto2:GluK2 regulate KCC2 via a PKC-mediated phosphorylation of the cotransporter. Taken together, these results resolve three novel mechanisms of KCC2 regulation: the identity of the key C-terminal domain of KCC2 required for isotonic transport, the functional significance of the KCC2:Neto2 interaction, and the mechanism by which the KCC2:Neto2:KAR complex regulates KCC2 expression and mobility in the neuronal membrane.

The mechanisms that regulate the activity of the neuron specific K+Cl- cotransporter (KCC2) remain poorly understood, despite the critical importance of this transporter in inhibitory synaptic transmission and plasticity. In this thesis I made three novel discoveries which reveal the cellular and molecular mechanisms of KCC2 regulation. First, I assayed the K+Cl- cotransport function of KCC2 under isotonic conditions and determined the molecular domain of the cotransporter required for constitutive Cl- transport in hippocampal neurons (Acton et al 2012). Specifically, I identified the 15 amino acid domain of the C-terminus in neurons that is responsible for the ability of KCC2 to cotransport K+Cl- under basal isotonic conditions, allowing it to remain constitutively active to create the steep Cl- gradient across the neuronal membrane required for synaptic inhibition. Secondly, I investigated a novel KCC2-interacting protein named Neto2 and determined its effect on the postsynaptic action of GABA (Ivakine et al 2013). I have found that Neto2, which is also an auxiliary protein of kainate-type ionotropic receptors, can also regulate the activity of the KCC2. Neto2 is required for neurons to maintain low [Cl-]i and strong synaptic inhibition. Third, I examined the functional relevance of the KCC2:Neto2:KAR multiprotein complex and found that this complex regulates the surface level membrane expression pattern of KCC2 and the stability of the cotransporter in the membrane. Moreover, I have provided the first evidence that the interactions of KCC2:Neto2:GluK2 regulate KCC2 via a PKC-mediated phosphorylation of the cotransporter. Taken together, these results resolve three novel mechanisms of KCC2 regulation: the identity of the key C-terminal domain of KCC2 required for isotonic transport, the functional significance of the KCC2:Neto2 interaction, and the mechanism by which the KCC2:Neto2:KAR complex regulates KCC2 expression and mobility in the neuronal membrane.

Fast postsynaptic inhibition in the brain is mediated by ionotropic GABA_A receptors (GABA_ARs), which are activated by the release of the neurotransmitter GABA from presynaptic interneurons. The GABA_AR is primarily permeable to chloride ions (Cl-) and therefore the transmembrane gradient for Cl- sets the reversal potential of the receptor (E_GABA-A). When intracellular Cl^- concentrations are relatively low, E_GABA-A is more negative than the membrane potential and GABA_AR responses will have a hyperpolarising and inhibitory effect upon the postsynaptic cell. In contrast, when intracellular Cl^- concentrations are relatively high, E_GABA-A will be more positive and GABA_AR activation will have a depolarising effect. How a neuron controls its intracellular Cl^- concentrations is a fundamental question that has direct relevance to hyperexcitability conditions such as epilepsy. Recently, it has become clear that Cl^- homeostasis is altered in epileptic tissue such that postsynaptic inhibition via the GABA_AR is reduced and, under some conditions, GABA_AR signalling may even be excitatory. In my thesis I explore some of the mechanisms and factors that are responsible for regulating postsynaptic GABA_AR signalling in the context of epileptic seizure activity in the rat hippocampus. In the first series of experiments I combined pharmacological approaches with electrophysiological recordings from pyramidal neurons in the CA3 region of the hippocampus to trigger seizure activity. My results show that intense neuronal activity during a seizure leads to a transient accumulation of intracellular Cl^-, which generates a pronounced depolarising shift in E_GABA-A. Under these conditions, GABAergic synapses become excitatory and contribute to ongoing neuronal activity rather than exerting their normal inhibitory role. I found that the same seizure activity also induces the release of a neuromodulator called adenosine, which serves to limit the deleterious effects of excitatory GABA_AR responses. Adenosine exerts these effects by activating downstream potassium channels, which increase the postsynaptic cell’s membrane conductance and, in doing so, ‘shunt’ incoming GABA_AR responses. In the second series of experiments I examined Cl^- homeostasis and E_GABA-A in the context of neonatal seizures. One of the main mechanisms by which neurons maintain their intracellular Cl^- levels is through the activity of ion transporter proteins that reside in the membrane and move Cl^- either into, or out of, the cell. I discovered that the intracellular trafficking of an important Cl^- transporter protein, NKCC1, correlates with changes in Cl^- homeostasis. Using a combination of biochemical and molecular techniques, I then identified a novel molecular association between NKCC1 and a motor protein, Myosin Va, which has been implicated in the intracellular trafficking of membrane proteins. Using electrophysiological recordings I found that Myosin Va is required for NKCC1’s contribution to Cl^- homeostasis, which may be important for E_GABA-A changes in epilepsy. In the final series of experiments I developed methods to study the temporal dynamics in E_GABA-A during a single seizure. These revealed a Cl^- unloading mechanism that emerges at the end of a seizure and which depends upon hyperpolarisation of the postsynaptic membrane potential. This mechanism aids E_GABA-A recovery after the seizure and moves E_GABA-A to more hyperpolarised values. This mechanism could boost postsynaptic inhibition after a seizure and thereby help to protect against further seizure episodes. In conclusion, this work extends our understanding of postsynaptic GABAergic transmission in the context of epileptic seizure activity and suggests new mechanisms that could be relevant for the development of rational anti-epileptic treatments.