Polo-like kinase 1 controls activation of adult neural stem cells and represents a druggable entry-point to boost adult neurogenesis

Neural stem cells (NSCs) persist in the mammalian brain after development, but their potential for replacing neural populations lost in brain damage is dramatically restricted. Quiescence, or the ability to transition in and out of the cell cycle is a hallmark of adult NSCs. It deepens as the organism ages and is regarded as a strategy to protect the reservoirs of NSC for the long term. Deciphering the molecular code of dormancy is a major interest in cellular and molecular biology of ageing and in regenerative medicine since it has important implications for potentially manipulating NSCs to improve tissular homeostasis. In this work we have identified the mitotic kinase Polo like kinase 1 (Plk1) as a novel intrinsic regulator of the dynamic transition between NSC quiescence and activation in adult neurogenic niches. Using quantitative phosphoproteomics to pinpoint mechanistical mediators of the role of Plk1, we have identified the quiescence/activation molecular switch formed by E3 ubiquitin ligase Huwe1 and its target, the transcription factor Ascl1/Mash1, as novel effectors of Plk1 in NSCs. We discover that Plk1 restrains the accumulation of Ascl1/Mash1 by acting upstream of Huwe1, providing a direct molecular connection between completion of the cell cycle and the re-establishment of a low activation state following NSC division, which supports the concept that quiescence is an actively maintained state. Moreover we demonstrate that pharmacological inhibition of Plk1 is sufficient to enhance adult neurogenesis in vivo, identifying Plk1 as a viable target to acutely boost endogenous stem cell activation.