Microvillar membrane microdomains exist at physiological temperature. Role of galectin-4 as lipid raft stabilizer revealed by "superrafts"

Lipid rafts (glycosphingolipid/cholesterol-enriched membrane microdomains) have been isolated as low temperature, detergent-resistant membranes from many cell types, but despite their presumed importance as lateral sorting and signaling platforms, fundamental questions persist concerning raft function and even existence in vivo. The nonionic detergent Brij 98 was used to isolate lipid rafts from microvillar membrane vesicles of intestinal brush borders at physiological temperature to compare with rafts, obtained by "conventional" extraction using Triton X-100 at low temperature. Microvillar rafts prepared by the two protocols were morphologically different but had essentially similar profiles of protein- and lipid components, showing that raft microdomains do exist at 37 degrees C and are not "low temperature artifacts." We also employed a novel method of sequential detergent extraction at increasing temperature to define a fraction of highly detergent-resistant "superrafts." These were enriched in galectin-4, a beta-galactoside-recognizing lectin residing on the extracellular side of the membrane. Superrafts also harbored the glycosylphosphatidylinositol-linked alkaline phosphatase and the transmembrane aminopeptidase N, whereas the peripheral lipid raft protein annexin 2 was essentially absent. In conclusion, in the microvillar membrane, galectin-4, functions as a core raft stabilizer/organizer for other, more loosely raft-associated proteins. The superraft analysis might be applicable to other membrane microdomain systems.     

Bone regenerates via dedifferentiation of osteoblasts in the zebrafish fin

While mammals have a limited capacity to repair bone defects, zebrafish can completely regenerate amputated bony structures of their fins. Fin regeneration is dependent on formation of a blastema, a progenitor cell pool accumulating at the amputation plane. It is unclear which cells the blastema is derived from, whether it forms by dedifferentiation of mature cells, and whether blastema cells are multipotent. We show that mature osteoblasts dedifferentiate and form part of the blastema. Osteoblasts downregulate expression of intermediate and late bone differentiation markers and induce genes expressed by bone progenitors. Dedifferentiated osteoblasts proliferate in a FGF-dependent manner and migrate to form part of the blastema. Genetic fate mapping shows that osteoblasts only give rise to osteoblasts in the regenerate, indicating that dedifferentiation is not associated with the attainment of multipotency. Thus, bone can regenerate from mature osteoblasts via dedifferentiation, a finding with potential implications for human bone repair.