Skeletal regenerative medicine promises to mitigate age-related bone loss and support fracture healing but requires manipulation of biological properties of osteogenic progenitor cells. Osteogenic cells must sustain their phenotype during lineage-expansion yet undergo a number of mitotic cell cycles to generate the requisite number of cells for proper tissue-organization. The proposed study will examine fundamental mechanisms that ensure cells retain a post-mitotic molecular memory of the bone phenotype. The central hypothesis of our study is that key mRNAs for osteogenic factors are passed on to daughter cells during mitosis as a component of a non-genomic epigenetic mechanism and that selected microRNAs (miRs) attenuate the translation of these transmitted mRNAs. Our hypothesis is based on a robust set of preliminary data showing that the osteogenic master regulator Runx2 is controlled by mitotic miRs in osteoblastic cell lines. Based on these and other preliminary data, we will (i) characterize the full complement of miRs that suppress expression of Runx2 in proliferating osteogenic cells, as well as begin characterization of mRNAs and cognate miRs during mitosis;(ii) examine miR dependent changes in fidelity of cell growth, survival and lineage-direction in proliferating osteoblasts, and (iii) characterize the physiological role of selected mitosis-related miRs during skeletal development in vivo. Validation of this concept would (i) establish a major new dimension in cell cycle regulation, (ii) reveal a previously unrecognized function for miRs during mitosis, (iii) define a novel molecular mechanism for biological control of gene expression in lineage-committed cells, and (iv) identify specific miRs that are transmitted to osteogenic progeny cells upon mitosis ('mito-miRs'). From a molecular therapeutic perspective, these miRs permit generation of epigenomic agents that control cellular inheritance by targeting cell fate determining factors which mediate mesenchymal stem cell expansion and differentiation.
Bone formation requires expansion and differentiation of osteoprogenitor cells. Osteogenic cells pass on genetic (chromosomes) and epigenetic information (e.g., CpG methylation, histone codes and bookmarking by transcription factors) to progeny cells to maintain growth potential and osteogenic phenotype identity. We will characterize a fundamentally novel RNA based epigenetic mechanism involving mitotic co-transmission of mRNAs and cognate microRNAs during mitosis.
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