The genetic heterogeneity of autism spectrum disorders (ASDs) has hindered the development of targeted therapies. Recently, genomic studies have revealed that many gene products that confer ASD risk converge on a surprisingly limited number of biological networks, including those controlling synaptic function. Such findings are consistent with the synaptic and behavioral hyperexcitability observed in individuals with ASD and mouse models of ASDs with impaired GABAergic inhibition. These studies suggest that targeting GABA neurotransmission could be an effective ‘network strategy’ of treatment applicable to ASDs of multiple etiologies. However, current GABA agonists are often ineffective and have considerable side effects. Novel drugs that safely restore GABA inhibition are therefore an urgent and unmet clinical need.
GABAA receptors are ligand-gated Cl- channels. GABAA receptor activation can elicit excitatory or inhibitory responses, depending on the intraneuronal Cl- concentration levels. Such levels are largely established by the Cl- co-transporters NKCC1 and KCC2. A progressive postnatal increase in KCC2 over NKCC1 activity drives the emergence of GABAA receptor-mediated synaptic inhibition, and is critical for functional brain maturation. A delay in this NKCC1/KCC2 ‘switch’ contributes to the impairment of GABAergic inhibition observed in Rett syndrome, fragile X syndrome, and other neurodevelopmental conditions, such as epilepsy.
Kristopher Kahle and his colleagues aim to understand the mechanisms that govern these developmental changes in NKCC1/KCC2 activity. They hypothesize that an improved knowledge of these mechanisms will lead to the development of novel strategies for restoring GABAergic inhibition. The researchers propose to exploit their recent discovery of a ‘rheostat’ of Cl- homeostasis, comprising the Cl-sensitive WNK-SPAK kinases and the NKCC1/KCC2 cotransporters1-3. This rheostat provides a phosphorylation-dependent way to reversibly tune the strength of synaptic inhibition in neurons.
The team will create genetic mouse models with inducible expression of phospho-mimetic or constitutively dephosphorylated WNK-SPAK-KCC2 pathway components. They will also develop novel WNK-SPAK kinase inhibitors that function as simultaneous NKCC1 inhibitors and KCC2 activators. These mouse models and compounds will be used to therapeutically restore GABA inhibition in the Rett syndrome MeCP2(R308/Y) mouse model. The researchers will use a combination of two-photon microscopy coupled with improved fluorescent optogenetic Cl- sensing, quantitative phosphoproteomics and patch-clamp electrophysiology to assess cellular and physiological changes in these mice.
Findings from this study are expected to highlight the utility of targeting the NKCC1/KCC2 system, and this unique phosphorylation motif in particular, in ASDs.