Regulation of inhibitory synaptic connections and distal neuronal complexity by the Ste20-like kinase Slk

Abstract

Neurons in the brain are typically defined by a characteristic morphology of the dendritic tree. Thereby, they can contact neighboring cells and communicate via excitatory and inhibitory synapses. To allow the establishment of neuronal circuits that ensure normal functions of the brain, the processes of dendrite and synapse development need to be tightly controlled. This is done via a plethora of interacting proteins within and between the developing cells. A protein that has been shown to critically influence neuronal development is the Ste20-like serine/threonine-protein kinase (Slk). Loss of Slk resulted in a simplified dendritic tree and reduced stability of specifically inhibitory synapses, which manifested in a decreased density of inhibitory synapses during postnatal development. The imbalance of excitatory and inhibitory synapses was also reflected on a functional level and impaired the electrophysiological excitation/inhibition balance. <br /> In this study, the role and mechanism of Slk in neurons were further characterized. Slk seemed to be specifically important during development for establishing the dendritic tree while not contributing to maintaining the dendritic tree structure during adulthood. <br /> Although a loss of Slk disturbed the excitation/inhibition balance of an individual neuron, a focal knockdown of Slk in selected neurons of cortical layers II/III did not lead to epileptic seizures or seizure-like activity. However, Slk knockdown did decrease the ability of neurons to adapt their firing behavior to external stimuli such as changed ion concentrations. <br /> Mechanistically, Slk knockdown reduced filamentous actin in the dendrites of neurons across the whole dendritic tree. To understand how Slk influences the actin cytoskeleton, we conducted proteomics studies to identify protein interaction partners of Slk. In these studies, proteins that play a role at postsynaptic sites or are involved in cytoskeletal dynamics were detected with above-average frequency. Novel proteins that were identified to bind to Slk included Crmp1 and Prkcg, which are both involved in cytoskeletal remodeling, the synaptic vesicle protein Sv2a, and the transmembrane protein Tmeff1. Furthermore, we identified new substrates of Slk, which are the closely related kinases Mink1 and Tnik. Both kinases are known to influence dendritic outgrowth and cytoskeletal dynamics. By identifying new interaction partners of Slk in neurons, pathways underlying Slk function in neurons can be proposed. Having obtained the first data set of potential Slk downstream effectors in neurons, this provides the basis to resolve the mechanism of how Slk regulates dendritic growth and inhibitory synapse stabilization in the brain.