MARK1 in dendritic spine neoteny

Project Summary A key cellular substrate underlying learning and memory is the plasticity of dendritic spines, which are sites of excitatory synaptic inputs in the mammalian brain. Dendritic spines show protracted synaptic maturation during human brain development, leading to longer spines with higher density in adulthood as compared with rodents. This extraordinary neoteny of spines is believed to underlie the high cognitive performance in humans. However, there remains a dearth of knowledge regarding the molecular mechanisms that contribute to dendritic spine neoteny unique to humans. One way to approach this issue is to examine genes that show adaptive evolution along the human lineage. These genes are under positive selection pressure and are likely to play a role in human speciation. In particular, genes that show adaptive evolution and genetically linked to neurodevelopmental disorders are strong candidates that may contribute to the high cognitive capacity of the human brain. To date, there are only a handful of genes reported to fit both criteria. One of the genes encodes the Microtubule Affinity Regulating Kinase 1 (MARK1), a Ser/Thr kinase highly expressed in the brain. MARK1 displays strong evidence of adaptive evolution in the lineage leading to humans, and single nucleotide polymorphisms (SNPs) of MARK1 are associated with autism spectrum disorders (ASD) and bipolar disorder. We previously found MARK-mediated phosphorylation of the synaptic scaffolding protein PSD-95 is important for bidirectional dendritic spine plasticity. Moreover, our preliminary studies show loss of MARK1 in forebrain pyramidal neurons leads to reduced spine formation and delayed spatial learning. In addition, we observed a significant increase in the synaptic levels of the AMPA receptor subunit GluR2 in the MARK1 conditional KO (cKO) hippocampus. By contrast, in neurons where rodent MARK1 was replaced with human MARK1, we observed increased spine density and immature morphology reminiscent of human neurons. These exciting data led us to hypothesize that loss of MARK1 leads to premature dendritic spine stabilization, which limits spine density and impairs learning. Conversely, human MARK1 shows enhanced kinase activity leading to dendritic spine neoteny and increased spine density, which contributes to high cognitive functions in humans. Aim 1 will test the hypothesis that human MARK1 contributes to spine neoteny through regulating the PSD scaffold and GluR2 trafficking. Aim 2 will test the hypothesis that altered spatiotemporal dynamics of human MARK1 activity is responsible for its effects on spine neoteny. We will utilize FRET, FRAP, super resolution imaging, and two- photon glutamate uncaging. We will complement these imaging approaches with biochemical analyses and optogenetic manipulation of MARK1 activity. Completion of the proposed experiments will establish a role for MARK1 in dendritic spine neoteny observed in human neurons. The results can shed light on the molecular mechanisms underlying high level cognitive functions of the human brain.