The retinoblastoma gene pathway regulates the postmitotic state of hair cells of the mouse inner ear

Abstract

Precursors of cochlear and vestibular hair cells of the inner ear exit the cell cycle at midgestation. Hair cells are mitotically quiescent during late-embryonic differentiation stages and postnatally. We show here that the retinoblastoma gene Rb and the encoded protein pRb are expressed in differentiating and mature hair cells. In addition to Rb, the cyclin dependent kinase inhibitor (CKI) p21 is expressed in developing hair cells, suggesting that p21 is an upstream effector of pRb activity. p21 apparently cooperates with other CKIs, as p21-null mice exhibited an unaltered inner ear phenotype. By contrast, Rb inactivation led to aberrant hair cell proliferation, as analysed at birth in a loss-of-function/transgenic mouse model. Supernumerary hair cells expressed various cell typespecific differentiation markers, including components of stereocilia. The extent of alterations in stereociliary bundle morphology ranged from near-normal to severe disorganization. Apoptosis contributed to the mutant phenotype, but did not compensate for the production of supernumerary hair cells, resulting in hyperplastic sensory epithelia. The Rb-null-mediated proliferation led to a distinct pathological phenotype, including multinucleated and enlarged hair cells, and infiltration of hair cells into the mesenchyme. Our findings demonstrate that the pRb pathway is required for hair cell quiescence and that manipulation of the cell cycle machinery disrupts the coordinated development within the inner ear sensory epithelia.

Sox2 and Jagged1 Expression in Normal and Drug-Damaged Adult Mouse Inner Ear

“…Deletion of the cyclin-dependent kinase inhibitor, p27 Kip1 , in mice leads to production of supernumerary cells in the organ of Corti through excessive mitoses that extend significantly past the normal developmental period (Chen and Segil 1999;Löwenheim et al 1999). Two other cell cycle regulatory genes, p19 Ink4d and retinoblastoma, are also necessary for normal developmental cell cycle exit in the organ of Corti (Chen et al 2003;Sage et al 2005;Mantela et al 2005). Deletion of either gene leads to highly disregulated DNA synthesis, including DNA synthesis in mature HCs.…”

“…Non-sensory SCs serve as progenitors to regenerated HCs in nonmammalian ears and in the adult mammalian vestibular sensory epithelia (SE; reviewed in Matsui and Cotanche 2004;Matsui et al 2005;Oesterle and Stone 2008). Under certain experimental circumstances, SCs in neonatal organ of Corti can give rise to new HCs via mitotic pathways (Chen and Segil 1999;Löwenheim et al 1999;Mantela et al 2005;Sage et al 2005;White et al 2006) or by the non-mitotic conversion of SCs into HCs upon altering Notch signaling with N-S-phenyl-glycine-t-butyl ester (DAPT: an inhibitor of gamma secretase: Woods et al 2004;Yamamoto et al 2006). SCs can be converted into HCs upon transfection of the developmental gene Atoh1 (Math1, Zheng and Gao 2000;Shou et al 2003;Woods et al 2004;Izumikawa et al 2005).…”

p19Ink4dand p21Cip1Collaborate to Maintain the Postmitotic State of Auditory Hair Cells, Their Codeletion Leading to DNA Damage and p53-Mediated Apoptosis

“…In contrast, inactivation of the retinoblastoma gene (Rb) in the inner ear leads to an apparent complete loss of control of the postmitotic state of HCs (Mantela et al, 2005;Sage et al, 2005Sage et al, , 2006. The retinoblastoma protein (pRb) functions as a master switch for the G 1 /S-phase transition of the cell cycle and is regulated primarily through modulation of its phosphorylation state (Weinberg, 1995).…”

“…This leads to the working hypothesis that active inhibition of CDK activity, through the action of CKIs, is required to keep pRb in an inactive state to maintain the postmitotic state of numerous cell types, including HCs. The more extreme loss of control of the postmitotic state observed in the Rb mutant mice (Mantela et al, 2005;Sage et al, 2005Sage et al, , 2006 …”

Essential role of retinoblastoma protein in mammalian hair cell development and hearing

“…Some negative regulators of cell proliferation have been identified, notably p27kip1 (8,9), p19ink4d (10), and pRb (11,12), which control cell-cycle exit in the inner ear. p27kip1 is primarily involved in cell-cycle arrest of inner ear sensory progenitor cells and cochlear supporting cells, whereas p19ink4d plays a role in hair cell survival.…”