Doctor of Philosophy, Case Western Reserve University, 2024, Biology

The calvaria is composed of interlocking plates of bone with sutures that encase the brain. Calvarial development is an error-prone process potentially leading to persistent gaps between bones or premature suture fusion (craniosynostosis, CS), impacting the brain and sensory organs. The mechanism of directional growth of the calvarial bones and impact on suture patency remains unknown. Here, we identify that graded expression of fibronectin (Fn1) in the mouse embryonic cranial mesenchyme (CM) precedes the apical expansion of calvarial osteoblasts. In humans, syndromic forms of CS exhibit dysregulated fibronectin expression. We find Fn1 expression altered in several mouse CS models as well, including Apert, Crouzon and Twist1+/- mouse embryos, accompanied by abnormal apical expansion of the frontal bone primordia. We hypothesize that fibronectin expression in CM is required for both the apical expansion of calvaria and maintenance of suture patency. Conditional deletion of Fn1 in CM causes diminished apical frontal bone expansion by altering cell shape and actin enrichment without perturbing other cell behaviors. To address how osteoprogenitors use FN1, we conditionally ablate Wasl/N-Wasp in cranial neural crest cells to disrupt F-actin nucleation in lamellipodial protrusions. Dramatic reduction in apical expansion occurs in Wasl mutants, phenocopying Fn1 mutants without a significant change in proliferation, survival, or osteogenesis. Additionally, we find that CM-restricted Fn1 deletion leads to premature fusion of coronal sutures at 81% penetrance. The Fn1 mutants show decreased levels of suture mesenchyme markers, Six2 and Erg, in coronal suture space. The coronal suture mesenchyme has diminished proliferation index and ectopic expression of osteogenic cell markers suggesting premature differentiation. These data support a model of FN1 as a directional substrate for calvarial osteoblast migration that may be a common mechanism underlying many cranial disorders of (open full item for complete abstract)

Committee: Radhika Atit (Advisor); Robert Ward (Committee Member); Matthew Harris (Committee Member); Timothy Mead (Committee Member); Martin Basch (Committee Member) Subjects: Biology; Cellular Biology; Developmental Biology; Genetics