UC Merced

The need to treat bone loss disorders, such as osteoporosis, continually compounds as societie’s age advances with over 200 million postmenopausal women and elderly men are affected annually. Additionally, the limitions associated with existing therapeautics underscore the need to developed improved osteoanabolic drugs.

The research herein focuses on transcription factor mediated gene regulatory networks (GRN) that govern key bone physiology. Mef2c is a transcription factor that is emerging as a key regulator of bone and endochondral ossification. Given the wide expression profile of Mef2c amongst bone cell populations characterizing the Mef2c GRN could provide incites into novel drug targets or suggest improved strategies in reprogramming MSCs to treat osteoporosis.

Accessility to the underlying bone cell populations is a widely excepted practical limitation associated with bone research. Current bone cell isolation methods are limited and have led to a lack of robust osteocyte-specific data. The development of a improved protocol, adaption of current practices, was used to isolate primary bone cell populations for downstream single cell RNA sequencing (scRNAseq). Additionally, flow cytometry was used to capture floursescent labeling bone cell populations. Transcriptomic analysis revealed that the isolation method was capapble of capturing rare populations such as osteocytes.

The isolation method made it suitable to characterize the Mef2c function in bone. Previous work has shown evidence that Mef2c mediates key cellular behaviors that promote endochondral ossification and bone formation involved in maintaining bone strength and homeostasis. Such findings underscore the need to uncover the underlying mechaninsm by which Mef2c regulates bone cell activity. Here, Mef2c-deficient bone cell populations from Mef2cfl/fl; Bglap-Cre and Mef2cfl/fl; Dmp1-Cre mutant mice were isolated to profile the gene expression changes scRNAseq. The work revealed a population level shift towards Lum+ mesenchymal progenitor subpopulations. Whereas osteoblasts and osteocytes exhibited defective energy and bone metabolism genes in the absence of Mef2c. Taken together, the wide expression amongst bone cell populations exhibits a cell types specific Mef2c function.

Recently, Mef2c function was also found amongst bone resorbing osteoclast populations and was found mediating bone erosion associated with inflammatory arthiritis. Here, we isolated Mef2c-defecient CD11b+ osteoclast progenitors from Mef2cfl/fl; Ctsk-Cre and characterized their ability to differentiate into mature osteoclasts. While in vitro studies demonstrated the negative impact the absence of Mef2c has on osteoclast differentiation, transcriptional analysis did not highlight a similar impact. Future studies will have to focus on targeting Mef2c at early or the initial stages of osteoclast cell states.

In summary, the current findings illustrate the partial view of how osteogenic cell populations are programmed in order to maintain bone tissues throughout adulthood. By continually characterizing Mef2c will lead to a greater understanding of how cells are programmed and the pathophysiology associated with osteoporosis.