HYPERACTIVATION OF ERK SIGNALING REGULATES EMBRYONIC CORTICAL GABAERGIC NEURON DEVELOPMENT

The extracellular regulated kinases (ERK1 and ERK2) are key components of the canonical RAS/ERK intracellular signaling cascade. De novo mutations in regulators of this pathway occur in approximately 1:2000 births and are associated with a family of syndromes called RASopathies. Patients typically exhibit craniofacial dysmorphia, cardiac defects, neurological delay, and epilepsy. While previous studies have implicated GABAergic interneuron dysfunction in learning deficits observed in adult models of RASopathies, the developmental mechanisms of disease remain poorly understood. In this study, we have investigated the effects of hyperactive ERK signaling on the early development of mouse cortical GABAergic inhibitory interneurons. Mice with hyperactive ERK signaling in GABAergic circuits display a decreased seizure threshold and phenotypically exhibit absence seizure-like behavior. At the cellular level, there is a significant neuronal migration issue that leads to a reduced number of GABAergic interneurons in the sensory cortex of mutant mice as early as embryonic day 17. To examine whether this was a global effect on the entire population of GABAergic neurons, we immunolabeled specific subpopulations of GABAergic interneurons with antibodies directed toward Parvalbumin and Somatostatin. Surprisingly, we only found significant decreases in Parvalbumin expressing GABAergic interneurons following hyperactivation of the ERK cascade. A two-pronged approach was taken to assess the morphology of the Parvalbumin-expressing interneurons present in mutant and control mice: somal sizes and dendritic arbor complexity were both evaluated. Parvalbumin-expressing interneuron somal sizes did not appear to be altered in the mutants. However, mutants exhibited increased dendritic arbor complexity in comparison to littermate controls. This increase in dendritic complexity suggests that the remaining Parvalbumin-expressing GABAergic interneurons may have a greater extent of innervation. Further analysis of the cellular response to hyperactive ERK signaling may assist in understanding the pathogenesis of neurological defects in RASopathies. We are also exploring pharmacological approaches to reverse GABAergic neuron loss in ERK/MAPK mutant mice in an effort to define possible therapeutic strategies. As a whole, this study revealed a significant embryonic neuronal migration issue in mice with hyperactive ERK signaling that selectively affects Parvalbumin-expressing interneurons.