Mesenchymal stem cells (MSCs) in bone marrow provide progenitors for both adipocyte and osteoblast cells and the output of the MSC pool reflects a reciprocal relationship between these two lineages. The ability of mechanical signals to promote osteogenic lineage has raised the exciting possibility that exercise might be able to regulate MSC lineage. Our work indicates that mechanical input can inhibit adipogenesis, exerting a significant control over MSC reciprocity through control of ?-catenin signaling. Signals which promote MSC adipogenesis involve diminution in ?-catenin signaling, followed by a rise in PPAR?, adiponectin and lipid content. We have compelling data showing that mechanical strain induces persistent ?-catenin activation in MSC through alteration of GSK3? phosphorylation via AKT in MSCs. Repetitive loading bouts increase the ?-catenin signal duration such that downstream events such as the rise in adiponectin and lipid droplets are inhibited. Our results suggest that even a strongly adipogenic microenvironment can be counteracted in this way by repetitive bouts of mechanical input. This allows us to hypothesize that """"""""mechanical stimulation represses adipogenic conversion through ?-catenin inhibition of PPAR? action"""""""". With this grant proposal we propose to test this hypothesis, fully characterizing the mechanisms by which mechanical input prevents adipogenesis and controls MSC lineage selection. We will investigate the temporal nature of the signal: how much and how many repetitions are required to regulate adipogenesis, and we will ask if mechanical input induces an alternate lineage selection, e.g., osteoprogenitor or myocyte with the help of unique reporter mice from which we make MSC clones for study. Interactions between local cells will be probed asking whether soluble factors secreted from strained cells can act on unstrained cells (SA1). We will ascertain the mechanisms by which mechanical strain activates ?-catenin (via AKT and GSK32), as well consider other mechanical targets (Wnts and BMPs) that could exert local control. We will consider alternative targets of GSK3? such as NFATc1 and mTOR (SA2). We will define how mechanical activation perturbs PPAR? promotion of adipogenesis directly and indirectly in SA3. Our proposal has significance for understanding the fate of MSC in a sedentary and aging population. It will be critically important to characterize the cascade of signals involved in mechanical regulation of MSC lineage selection, as this should identify modifiable steps in pathways that suppress adipogenesis and stimulate osteogenesis.
Evolution has led to interrelationships between bone and fat together to allow individuals to move to food sources (skeleton necessary for locomotion), and store energy (as fat). Clearly our nation's health is impacted by counterproductive """"""""activities"""""""" -lack of exercise and caloric excess. Indeed, as the presence of mechanical information prevents emergence of adipocytes from the marrow, exercise may be considered a way to prevent bone marrow senescence as """"""""old"""""""" bone marrow mimics unloading with increased fat. As such, understanding the mechanisms by which mechanical input controls lineage selection is highly relevant to an aging population, and should have high importance for understanding the pathophysiology behind the decreased osteoprogenitor pool and how to reverse it. Consensus building now needs to be a targeted goal, examining the basics of how exercise impacts these relationships by defining the fate of mesenchymal stem cells. We propose here a focused investigation of how loading cells can affect lineage selection when that lineage is already directed toward fat (as would be seen in non-exercising individuals, or aged individuals). We seek insights into the type of mechanical input (how much and for how long), and the targets, beginning with ?-catenin, but considering other factors which determine lineage selection, and what strained cells become. We will examine proximal nodes in strain activation of ?-catenin and investigate alternative effectors of mechanical strain. We will look at direct effects to prevent adipogenesis (inhibition of PPAR? expression) and indirect ?- catenin inhibition of PPAR? responses.
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