Lysosome Dispersion in Osteoblasts Accommodates Enhanced Collagen Production during Differentiation

Abstract

Lysosomes are essential organelles for intracellular degradation and are generally sequestered near the cell center to receive vesicles with contents targeted for destruction. During ascorbic acid (AA)-induced differentiation of osteogenic cells ( Beck, G. R., Jr., Zerler, B., and Moran, E. (2001) Cell Growth Differ. 12, 61-83 ), we saw a marked increase in total lysosome organelles in osteoblastic cells, in addition to an enhanced endocytic rate. Interestingly, lysosomes were dispersed toward the cell periphery in differentiating osteoblasts. We determined that lysosome dispersion in differentiated osteoblasts required intact microtubules for long range transport and was dependent on kinesin motors but did not involve cytosolic acidification. Impairment of lysosome dispersion markedly reduced AA-induced osteoblast differentiation. Lysosomes were not secreted in differentiated osteoblasts, implicating them instead in intracellular degradation. We assayed the degradative capacity and saw a significant increase in DQ-ovalbumin fluorescence in differentiated osteogenic cells compared with undifferentiated control cells. Osteogenic cells are specialized for type I collagen production, and we noted enhanced secreted and intracellular collagen in AA-differentiated osteoblasts versus control cells. Importantly, osteoblasts displayed procollagen-containing vesicles that were distributed throughout the cytoplasm, a portion of which colocalized with lysosomes. Treatment of cells with 2,2'-dipyridyl to inhibit procollagen trimerization enhanced colocalization of lysosomes with procollagen-containing organelles, implicating dispersed lysosomes in collagen processing in osteogenic cells.