Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physiology

Supervisor

Dr Frank Beier

Abstract

Glycogen Synthase Kinase 3 beta (GSK-3b) is an important node of regulation in many cellular signaling pathways including insulin and Wnt. Previous work has shown that ubiquitous deletion of GSK-3b is lethal and causes various skeletal abnormalities, but it is not known whether these defects are skeleton-intrinsic. A second GSK-3 protein which shares almost complete homology, GSK-3a, has been shown to have completely overlapping roles with the b form in some cases and independent functions in other cases. Therefore examination of the role of GSK-3b in skeletal development is very poignant.

To examine the role of GSK-3b in skeletal development we used three models: 1. Ex vivo tibia organ cultures treated with pharmacological inhibitors, 2. In vivo cartilage-specific GSK-3b deletion, 3. In vivo bone-specific GSK-3b deletion. The tibia organ culture experiment revealed that inhibiting both GSK-3 forms caused increased bone growth. We also demonstrated that GSK-3 effects on chondrocyte proliferation were PI3K-dependent and regulated by GSK-3a whereas effects on hypertrophy were PI3K-independent and GSK-3b regulated. However when GSK-3b was deleted specifically in cartilage, there was no change in growth or growth plate morphology. One unique finding was that GSK-3a was upregulated in the pre-hypertrophic/hypertrophic region of mutant mice, presumably through a compensating mechanism.

Since the global knockout (KO) showed a skeletal phenotype and our cartilage-specific KO did not, we postulated that GSK-3b function in osteoblast was important for skeletal development. To test this we generated osteoblast-specific GSK-3b KO mice. These mice displayed skeletal phenotypes seen in the global deletion study plus several more since we were able to study postnatal development. These mice also displayed a metabolic phenotype. The mice had lower blood glucose and insulin but their pancreases were unchanged. Most interestingly, almost 100% of the mutant males died with high blood glucose and damage to their urogenital track, all reminiscent of type II diabetes development.

In conclusion we showed that cartilage GSK-3b is not important for skeletal development but both GSK-3s are required to regulate growth. We showed that osteoblast GSK-3b is important for skeletal development and whole body metabolic regulation.


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