A Glanzmann's mutation in beta 3 integrin specifically impairs osteoclast function.

Osteoclastic bone resorption requires cell-matrix contact, an event mediated by the alpha v beta 3 integrin. The structural components of the integrin that mediate osteoclast function are, however, not in hand. To address this issue, we generated mice lacking the beta 3 integrin gene, which have dysfunctional osteoclasts. Here, we show the full rescue of beta 3(-/-) osteoclast function following expression of a full-length beta 3 integrin. In contrast, truncated beta 3, lacking a cytoplasmic domain (h beta 3c), is completely ineffective in restoring function to beta 3(-/-) osteoclasts. To identify the components of the beta 3 cytoplasmic domain regulating osteoclast function, we generated six point mutants known, in other circumstances, to mediate beta integrin signaling. Of the six, only the S(752)P substitution, which also characterizes a form of the human bleeding disorder Glanzmann's thrombasthenia, fails to rescue beta 3(-/-) osteoclasts or restore ligand-activated signaling in the form of c-src activation. Interestingly, the double mutation Y(747)F/Y(759)F, which disrupts platelet function, does not affect the osteoclast. Thus similarities and distinctions exist in the mechanisms by which the beta 3 integrin regulates platelets and osteoclasts.

Bcl-2 protein expression during murine development.

Bcl-2 functions as a death repressor molecule in an evolutionarily conserved cell death pathway. To further explore the role of Bcl-2 in development, we assessed its pattern of expression during murine embryogenesis. Immunohistochemical analysis demonstrates that Bcl-2 is widely expressed early in mouse fetal development in tissues derived from all three germ layers and that this expression becomes restricted with maturation. Within epithelium, the E12.5 lung bud demonstrates a proximal to distal gradient of Bcl-2 expression which is enhanced by E18.5. Bcl-2 is expressed throughout the intestinal epithelium through E14.5, but by E18.5 only cells in the crypts and lower villi express Bcl-2. In the mesoderm-derived kidney, Bcl-2 is expressed in both the ureteric bud and metanephric cap tissue at E12.5. Tubular structures also express Bcl-2, although overall levels drop as the kidney matures. Retinal neuroepithelial cells uniformly express Bcl-2 until cells begin to differentiate and then display the topographic distribution maintained into adulthood. The developing limb provides a clear example where Bcl-2 is restricted to zones of cell survival; Bcl-2 is expressed in the digital zones but not in the interdigital zones of cell death. The wide distribution of Bcl-2 in the developing mouse suggests that many immature cells require a death repressor molecule or that Bcl-2 may have roles beyond regulating developmental cell death.