Mature Mice Lacking Rbl2/p130 Gene Have Supernumerary Inner Ear Hair Cells and Supporting Cells

Abstract

Adult mammalian auditory hair cells (HCs) and their associated supporting cells (SCs) do not proliferate, and HC death leads to irreversible neurosensory hearing loss and balance impairment. In non-mammalian vertebrates, loss of HCs induces mitotic proliferation of adjacent non-sensory SCs and/or direct SC transdifferentiation to generate replacement cells. This results in the structural and functional recovery of the non-mammalian sensory systems. Potential replacement of mammalian auditory HCs, either by transplanting cells or by transforming existing cells through molecular therapy, has long been proposed. However, HC replacement strategies with clear therapeutic potential remain elusive. The retinoblastoma (pRB) family of cell cycle regulators, Rb1, Rbl1 (p107), and Rbl2 (p130), regulate the G1 to S phase transition in proliferating cells. In the inner ear, the biochemical and molecular pathways involving pRBs, particularly p107 and p130 are relatively unexplored and their therapeutic suitability is yet to be determined. In this study, we analyzed the cochleae of adult p130 knockout (p130−/−) mice and showed that lack of the p130 gene results in extra rows of HCs and SCs in the more apical regions of the cochlea. No evidence of transdifferentiation of these supernumerary SCs into HCs was observed in the p130−/− mouse. Nevertheless, unscheduled proliferation of SCs in the adult p130−/− cochlea coupled to downregulation of bona fide cell cycle inhibitors provides a mechanistic basis for p130’s role as a regulator of SC and HC mitotic quiescence in the more apical regions of the cochlea. Interestingly, p130−/− mice exhibited nearly normal peripheral auditory sensitivity.

Inner ear supporting cells: Rethinking the silent majority

“…Adult supporting cells express cyclin-dependent kinase inhibitors and pocket proteins that block their transition from G1 to S phase (e.g. [15, 16, 164, 165]). Deletion of p27 kip1 or Rbl2 in adult mice enables some supporting cells to enter the cell cycle [15, 16, 165].…”

“…[15, 16, 164, 165]). Deletion of p27 kip1 or Rbl2 in adult mice enables some supporting cells to enter the cell cycle [15, 16, 165]. Forced misexpression of cyclinD1 [77] or c-Myc [166] promotes adult mammalian vestibular supporting cells to re-enter the cell cycle.…”

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

“…As another example, the neonatal ablation of the retinoblastoma protein (pRb) from HCs (Weber et al, 2008) or SCs (Yu et al, 2010) results in the proliferation of these cells between P3 and P6, yet ablation of pRb at 3 months of age does not appear to elicit any signs of proliferation (Huang et al, 2011). Similarly, germline deletion of the related retinoblastoma-like 2 protein (p130) results in supernumerary HCs and SCs that incorporate EdU when the mice are injected with the thymidine analog at P21 but not when they are injected at P30 (Rocha-Sanchez et al, 2011). Taken together, these reports strongly suggest an age-related decline in proliferative potential of cells in the organ of Corti as well as the combined role of several different factors to promote quiescence in the postnatal mouse cochlea.…”

Understanding the evolution and development of neurosensory transcription factors of the ear to enhance therapeutic translation

“…No matter the starting point, in forcing cell cycle re-entry in differentiated cells, not only must the correct number of cells be formed by a controlled balance between proliferation and differentiation as in avenue “A” but this re-entry into the cell cycle must produce the normal cytoarchitecture of the OC. Nonetheless, re-entry through inhibition of cell cycle inhibitors (Laine et al 2007; Liu and Zuo 2008; Minoda et al 2007; Oesterle et al 2011; Ono et al 2009; Rocha-Sanchez et al 2011; Sulg et al 2010; Weber et al 2008; Yu et al 2010) or over-expression of proto-oncogenes (Loponen et al 2011; Ozeki et al 2007) have been successful for at least a short period of time but all newly formed cells eventually undergo apoptosis as there appears to be a more complicated intrinsic regulation than previously thought (Huang et al 2011). Because of the possibility of cross-talk between bHLH TFs and the intrinsic interplay evident with the Mycs, IDs and differentiation cues such as Atoh1 and Neurod1, the understanding of both the positive (proto-oncogenes such as N-Myc ) and negative (tumor suppressors such as p21/27Kip1 and pRb) regulation of the cell cycle has never been more important to hair cell restoration.…”