Intracellular CD22 rapidly moves to the cell surface in a tyrosine kinase-dependent manner following antigen receptor stimulation.

CD22 is a key accessory molecule for Ag receptor signaling in B cells that becomes tyrosine phosphorylated in the signaling process. CD22 associates with sIg and strongly amplifies sIg-induced signals. During B cell development, CD22 is initially expressed intracellularly, with surface expression appearing with IgD expression. We used confocal laser-scanning microscopy and flow cytometry to analyze CD22 translocation responses to signaling events. Cross-linking surface IgM (sIgM) induced rapid movement of CD22 to the cell surface in both Ramos and Daudi B cells, with a 50 to 100% increase in surface expression observed within 5 min of stimulation. The increase in CD22 surface expression was specific in that CD19 expression was not affected. Both cell surface and intracellular CD22 were directed toward the site of sIgM stimulation. Treatment with the phosphotyrosine phosphatase inhibitor bis(maltolato)oxovanadium(IV) also increased CD22 surface expression. The tyrosine kinase inhibitor tyrphostin A10 inhibited CD22 movement at concentrations that inhibited tyrosine phosphorylation of CD22 and other cellular proteins. In contrast to the B cell lines, mature peripheral blood B cells contained very little intracellular CD22 and showed no significant increase in surface expression following sIgM stimulation. The rapid directed movement of intracellular CD22 provides a new mechanism to dynamically regulate Ag receptor signaling, as well as CD22-mediated adhesion.

Modulation of thrombospondin gene expression during osteoblast differentiation in MC3T3-E1 cells.

The levels of expression of two related extracellular matrix protein genes, thrombospondins 1 and 2 (TSP1 and TSP2), were analyzed in the mouse osteogenic cell line, MC3T3-E1. To monitor differentiation, we also measured two potential markers of the osteoblastic phenotype, alkaline phosphatase (ALP) activity, and alpha 1(I) collagen mRNA levels. TSP1 mRNA levels increased 10- to 15-fold during the first nine days of osteoblastic conversion, and then dropped to a level still significantly above baseline values. This increase in TSP1 mRNA closely paralleled that observed in ALP activity. In contrast, TSP2 mRNA levels were unchanged throughout the 21-day time course. These findings suggest that TSP1 is a marker for osteoblast differentiation and could play a role in the cellular changes that accompany acquisition of the osteoblastic phenotype in MC3T3-E1 cells.