INVESTIGATION OF DEFECTIVE CELL SIGNALING CASCADE INVOLVED IN THE OSTEOGENESIS IN HUTCHINSON-GILFORD PROGERIA SYNDROME
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Human bone homeostasis is maintained through constant bone remodeling, which balances bone formation by osteoblasts and bone resorption by osteoclasts. Patients with Hutchinson-Gilford progeria syndrome (HGPS) have low bone mass that manifests in a high risk of fractures and an atypical skeletal geometry, suggesting impaired bone remodeling. HGPS is a premature aging disease caused by truncated lamin A that is permanently farnesylated. The mutant lamin A is referred as progerin. Several previous clinical reports discussed abnormal skeletal development of the children with HGPS, but the molecular mechanistic study on defective osteogenesis of HGPS stem cells need to be further elucidated. The major aim of my dissertation research is to investigate dysfunction in stem cell differentiation due to aberrant cell signaling in osteoprogenitor cells that express progerin. To achieve this aim, the study demonstrates both in vitro and in vivo models of HGPS to support defective mechanism of the canonical WNT/β-catenin pathway, seemingly at the level of efficiency of nuclear import of β-catenin and impaired osteoblast differentiation. Restoring β-catenin activity rescues osteoblast differentiation and significantly improves bone mass. In particular, HGPS patient-derived induced pluripotent stem cells (iPSCs)-osteoprogenitors and primary mesenchymal stem cells (MSCs) expressing the HGPS mutant progerin display defects in osteoblast differentiation, characterized by deficits in alkaline phosphatase activity and mineralizing capacity. Mechanistic investigation reveals that canonical WNT/β-catenin pathway, a major signaling cascade involved in skeletal homeostasis, is impaired by progerin, causing a reduction in nuclear active β-catenin protein levels and reciprocal aberrant cytoplasmic accumulation which causes reduced transcriptional activity for osteogenesis. Non-farnesylation of progerin in MSCs attains higher level of active β-catenin protein expression and consequently increasing the signaling, enhancing mineralization capacity and ameliorating the defective osteogenesis. Moreover, in vivo analysis of the Zmpste24-/- HGPS mouse model demonstrates that treatment with a sclerostin-neutralizing antibody (SclAb), which targets an antagonist of canonical WNT/β-catenin signaling pathway, fully rescues the low bone mass phenotype to wild-type levels. This study implicates β-catenin signaling cascade as a therapeutic target for restoring defective skeletal microarchitecture in HGPS. Given the fundamental nature of WNT/β-catenin signaling to stem cell renewal and lineage allocation, the findings from this dissertation may provide broader inferences for the treatment options in HGPS.