A Quest for Small Molecule Inhibitors of the Cell Cycle Regulator, P27

Abstract

Hearing loss is primarily caused by alteration of inner ear hair cells (HC). Humans and other mammals cannot spontaneously regenerate damaged hair cells. However, chicken, fish and amphibians have the ability to regenerate the damaged sensory HCs and recover their hearing.In mice, HC and SC progenitors exit the cell cycle between embryonic days 12 and 14 and do not proliferate thereafter. This stage of development correlates very well with the expression of the cell cycle inhibitor, p27Kip1 (p27), in the organ of Corti, suggesting that p27 plays a role in terminal differentiation in this tissue. This hypothesis is supported by studies in which inactivation of p27 in mouse postnatal SCs or HCs has led to cell proliferation and HC regeneration, revealing the potential of p27 small molecule inhibitors (therapeutic drugs) for regeneration of functional HCs in mammalian vertebrates.Protein p27 is an intrinsically disordered protein (IDP) that inhibits the enzymatic catalytic activity of nuclear Cdk2/cyclin E (and A) complexes, blocking progression from G1 to S phase of the cell cycle and accounting for the ability of p27 to enforce cell cycle arrest.We hypothesize that small molecules which bind specifically and tightly to p27 will compete for its binding and thus activate Cdk/cyclin complexes. Such small molecules have potential as transient inhibitors of p27-mediated cell cycle arrest and terminal differentiation, and may serve to initiate HC regeneration in the hearing system. We are using fragment-based drug discovery methods, utilizing NMR spectroscopy as the primary screening technique. After the validation of initial fragment hits we used cheminformatics analysis to identify 2nd generation inhibitors. We seek to demonstrate that p27, a prototype IDP, is a “druggable” target and that its function can be modulated by small molecules in vivo.