Localization of osteopontin during mouse development

Rat osteopontin antibody is cross-reactive to a novel myelin-associated protein in chick

Osteopontin (OPN) was initially identified as glycosylated phosphoprotein in bones of vertebrates. Recently, OPN is reported to express in the primitive neuroepithelia of early chick embryonic hindbrain. We have demonstrated that rat OPN is immunohistochemically localized in the white matter of chick CNS. We have further confirmed the specificity of OPN cross-immunoreaction in myelin using demyelinated optic nerve induced by lysophosphatidylcholine (LPC), where the intensity of immunoreaction was closely related to the degree of demyelination. Immunoblot analyses showed that rat OPN antibody recognized a protein with molecular weights of approximately 47 kDa from chick CNS. Our data suggest that the antigen recognized by rat OPN is a previously undescribed myelin-associated protein in the chick CNS.

Identification of osteopontin as a novel ligand for the integrin alpha8 beta1 and potential roles for this integrin-ligand interaction in kidney morphogenesis

Epithelio-mesenchymal interactions during kidney organogenesis are disrupted in integrin alpha8 beta1-deficient mice. However, the known ligands for integrin alpha8 beta1-fibronectin, vitronectin, and tenascin-C-are not appropriately localized to mediate all alpha8 beta1 functions in the kidney. Using a method of general utility for determining the distribution of unknown integrin ligands in situ and biochemical characterization of these ligands, we identified osteopontin (OPN) as a ligand for alpha8 beta1. We have coexpressed the extracellular domains of the mouse alpha8 and beta1 integrin subunits as a soluble heterodimer with one subunit fused to alkaline phosphatase (AP) and have used the alpha8 beta1-AP chimera as a histochemical reagent on sections of mouse embryos. Ligand localization with alpha8 beta1-AP in developing bone and kidney was observed to be overlapping with the distribution of OPN. In "far Western" blots of mouse embryonic protein extracts, bands were detected with sizes corresponding to fibronectin, vitronectin, and unknown proteins, one of which was identical to the size of OPN. In a solid-phase binding assay we demonstrated that purified OPN binds specifically to alpha8 beta1-AP. Cell adhesion assays using K562 cells expressing alpha8 beta1 were used to confirm this result. Together with a recent report that anti-OPN antibodies disrupt kidney morphogenesis, our results suggest that interactions between OPN and integrin alpha8 beta1 may help regulate kidney development and other morphogenetic processes.

Early expression of Osteopontin in the chick is restricted to rhombomeres 5 and 6 and to a subpopulation of neural crest cells that arise from these segments

BACKGROUND

Osteopontin previously has been shown to be expressed in the early notochord and rostral hindbrain during formation of the rat neuraxis. Both of these sites are known to have important organizing capacities associated with the expression of a number of transcription factors and secreted signaling molecules. Thus multiple types of molecules are involved in neuraxis patterning. Osteopontin is an extracellular matrix molecule, whose expression within the early hindbrain and notochord suggests that it too could play a role in patterning of the neuraxis.

METHODS

By wholemount in situ hybridization, we analyzed osteopontin expression in the chick embryo at Hamburger and Hamilton (HH) stages 5-15, and patterns of expression were compared to those of other hindbrain and neural crest cell markers. Early developmental expression was further examined in the rat by wholemount in situ hybridization and immunohistochemistry.

RESULTS

Our results have demonstrated that whereas notochord expression of osteopontin is not conserved between rat and chick, expression was clearly present in the chick caudal hindbrain between HH stages 9 and 11. In the chick, osteopontin expression was restricted to rhombomeres 5 and 6 and to a subpopulation of neural crest cells arising from these rhombomeres, up to HH stage 13. Osteopontin expression was present in a cranial-to-caudal gradient, and in the dorsoventral plane, expression was absent from both the floor plate and roof plate, with the exception of occasional cells within the roof plate.

CONCLUSIONS

Localized expression in the 5th and 6th rhombomeres suggests that osteopontin may have a role in patterning of postotic neural crest cells that arise from these segments of the hindbrain. In addition, comparison of our current data with those obtained earlier in the rat suggests that there are significant differences either between the developmental role(s) of osteopontin in the chick and in the rat, or between osteopontin gene products in the two species.