PDGFRbeta-mediated skeletal-vascular crosstalk in calvarial bone marrow development

About This Grant

PROJECT SUMMARY/ABSTRACT The calvaria bones of the skull begin to form bone marrow (BM) after birth, and this BM continues to expand with age in both mice and humans. Recent studies show that calvaria BM is not static but dynamically remodels in responses to local and systemic conditions, including pregnancy and disorders of skeletal, hematological, and neurological origin. BM niche formation is thought to be regulated by the intercommunication of skeletal cells (SKCs), vascular endothelial cells (ECs), and hematopoietic cells (HCs), but crosstalk mechanisms between them are complex and largely unknown. BM residing SKCs express platelet-derived growth factor (PDGF) receptors, PDGFR and PDGFRβ. The PDGFRs regulate the proliferation, migration, and differentiation of various mesenchyme-derived cell types in organ development, homeostasis, and tissue repair. Increased receptor signaling has been implicated in diseases such as fibrosis, cardiovascular disease, and cancer. Therefore, PDGF receptor activation and its downstream signaling have been proposed as potential therapeutic targets. Recent work in mice suggests a novel role for platelet-derived growth factor receptor beta (PDGFR) signaling in calvaria BM formation. Mice with a gain-of-function (GOF) mutation in PDGFR exhibit dramatically increased calvaria bone and BM size by 2-3 weeks of age, compared to wild type mice that do not generate significant calvaria BM until full adulthood. This precocious BM growth is accompanied by an expanded vascular network. Because PDGFR is expressed in SKCs but not in ECs or HCs, these results point to SKCs as the cellular origin of signals that increase calvaria vasculature and BM. The central hypothesis is that skeletal PDGFR promotes BM formation through an angiogenesis-directed mechanism. This project will address the central hypothesis with the following aims: 1) Characterize how PDGFR GOF causes dramatic expansion of calvaria vasculature and BM in juvenile mice, and 2) Determine the extent to which angiogenesis mediates BM formation in wild type and PDGFR mutants. In Aim 1, a skeletal-specific Cre recombinase mouse line will be used to conditionally induce a GOF Pdgfrb knock-in allele. The developmental processes of BM niche formation in wild type and PDGFRβ mutants will be defined using multiphoton light sheet imaging at 3 dimensions and transcriptomics. In Aim 2, experiments will focus on evaluating the contribution of ECs to BM formation with pharmacological angiogenic inhibition. The importance of skeletal and vascular coordination is evident from a close relationship between skeletal disorders, vascular malformation, and hematopoietic defects. The results of this project will generate a new mouse model to investigate calvaria bone/BM regeneration in future R01s and provide novel insights into the cell-cell interaction mechanisms that rebuild calvaria bone and BM.