From development through postnatal life, a subset of mesenchymal stromal/stem cells (MSCs) found in the bone marrow compartment, display an ability to differentiate into osteoblasts or adipocytes. While altering MSC shape, extracellular matrix (ECM) rigidity or ECM dimensionality in vitro have all been shown to modulate differentiation, the existence - or identity - of MSC-intrinsic effectors capable of regulating thee environmental cues has remained unclear. Recently, we identified the membrane- anchored matrix metalloproteinase, MT1-MMP, as an MSC-intrinsic regulator of stem cell lineage commitment and differentiation - in vitro as well as in vivo (Dev Cell, 25:402-416, 2013). These studies demonstrated that MT1-MMP-dependent remodeling of the ECM initiated a ?1 integrin-Rho/ROCK signaling cascade that controlled the activity of the key MSC transcriptional activators, YAP and TAZ. However, the molecular mechanisms underlying the changes in MSC transcriptional programs remained unexplored. In new studies, we find that MT1-MMP plays a heretofore unexpected role in modulating MSC nuclear architecture, chromatin organization and transcriptional competence. We further establish that the MT1-MMP-dependent transduction of ECM-directed signals to the nucleus requires functional interactions between the cytoskeleton, actomyosin-generated mechanical tension and Klarsicht, ANC-1, Syne homology (KASH) domain-containing cytoplasmic proteins that transmit signals to the lamin A-rich nuclear scaffold via members of the SUN protein family (i.e., the so-called linker of nucleoskeleton and cytoskeleton;LINC complex). These preliminary findings outline a novel MSC mechanotransduction program that links the 3-D ECM to the nuclear compartment and governs MSC lineage commitment and differentiation. Thus, we propose to i) characterize the dynamics of cytoskeletal and nuclear architecture reorganization on MSC transcriptional activity during MT1-MMP- dependent lineage commitment and differentiation, ii) define the role of MT1-MMP-dependent remodeling of the 3-D pericellular ECM as the upstream regulator of MSC nuclear architecture and function via the nesprin-SUN signaling axis and iii) identify the LINC complex/lamin A-YAP/TAZ axis as the dominant mechanotransduction system responsible for linking ECM-derived cues to the nuclear compartment during MSC lineage commitment and differentiation. As virtually all stem cells reside within a dynamically remodeled ECM, we posit that the molecular mechanisms outlined in this proposal will have broad implications for understanding the regulation of stem cell function and lineage commitment in growth and development as well as during aging and in genetic disorders affecting the nuclear scaffolding.
Mesenchymal stem/stromal cells play key roles in normal development and disease as a function of their ability to differentiate into osteoblasts, chondrocytes or adipocytes - cell types critical to bone and cartilage formation, hematopoiesis and metabolism. Currently, the molecular mechanisms that control mesenchymal stem cell fate in vivo remain largely undefined. Herein we outline a series of new animal models and experimental approaches that identify a novel pathway wherein the membrane-anchored matrix metalloproteinase, MT1-MMP, regulates nuclear architecture and function to control the transcriptional programs that underlie stem cell lineage commitment and differentiation via a novel nesprin-SUN-lamin A-YAP/TAZ signaling network.