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.

BACKGROUND:Orofacial clefts are common worldwide and result from insufficient growth and/or fusion during the genesis of the derivatives of the first pharyngeal arch and the frontonasal prominence. Recent studies in mice carrying conditional and tissue-specific deletions of the human ortholog Dicer1, an RNAse III family member, have highlighted its importance in cell survival, differentiation, proliferation, and morphogenesis. Nevertheless, information regarding Dicer1 and its dependent microRNAs (miRNAs) in mammalian palatogenesis and orofacial development is limited.AIMS:To describe the craniofacial phenotype, gain insight into potential mechanisms underlying the orofacial defects in the Pax2-Cre/Dicer1 CKO mouse, and shed light on the role of Dicer1 in mammalian palatogenesis.MATERIALS AND METHODS:Histological and molecular assays of wild type (WT) and Pax2-Cre/Dicer1loxP/loxP (Dicer1 CKO) mice dissected tissues have been performed to characterize and analyze the orofacial dysmorphism in Pax2-Cre/Dicer1loxP/loxP mouse.RESULTS:Dicer1 CKO mice exhibit late embryonic lethality and severe craniofacial dysmorphism, including a secondary palatal cleft. Further analysis suggest that Dicer1 deletion neither impacts primary palatal development nor the initial stages of secondary palatal formation. Instead, Dicer1 is implicated in growth, differentiation, mineralization, and survival of cells in the lateral palatal shelves. Histological and molecular analysis demonstrates that secondary palatal development becomes morphologically arrested prior to mineralization around E13.5 with a significant increase in the expression levels of apoptotic markers (P < 0.01).CONCLUSIONS:Pax2-Cre-mediated Dicer1 deletion disrupts lateral palatal outgrowth and bone mineralization during palatal shelf development, therefore providing a mammalian model for investigating the role of miRNA-mediated signaling pathways during palatogenesis.

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