Differentiation and proliferation of hematopoietic progenitors take place in the bone marrow and is a tightly controlled process. Cell adhesion molecules of the integrin and immunoglobulin families have been shown to be involved in these processes, but almost nothing was known about the involvement of the cadherin family in the hematopoietic system. A PCR screening of RNA of human bone marrow mononuclear cells with specific primers for classical cadherins revealed that E-cadherin, which is mainly expressed by cells of epithelial origin, is also expressed by bone marrow cells. Western blot analysis and immunofluorescence staining of bone marrow sections confirmed this unexpected finding. A more detailed analysis using immunoaffinity columns and dual color flow cytometry showed that the expression of E-cadherin is restricted to defined maturation stages of the erythropoietic lineage. Erythroblasts and normoblasts express E-cadherin, mature erythrocytes do not. A functional role of E-cadherin in the differentiation process of the erythroid lineage was indicated by antibody-inhibition studies. The addition of anti-E-cadherin antibody to bone marrow mononuclear cultures containing exogeneous erythropoietin drastically diminished the formation of erythropoietic cells. These data suggest a non-anticipated expression and function of E-cadherin in one defined hematopoietic cell lineage.
Tenascin is a large extracellular matrix (ECM) glycoprotein found in restricted tissue locations in the adult organism. It is copiously synthesized in regenerative organs or regenerating tissues and by certain tumors. We have analyzed the expression of tenascin in human long term bone marrow cultures as well as in cryostat sections of native bone marrow and found it strongly expressed by the stromal cells of the microenvironment. Two different protein subunits of 280 and 220 kD were detected by immunoblotting. These two forms are derived most likely from two different mRNA splice variants of 6 and 8 kb detected by Northern blotting. The in vivo analysis of cryostat sections showed a codistribution with other ECM molecules such as fibronectin and collagen type III in the microenvironment surrounding the maturing hematopoietic cells. Using two independent cell adhesion assays tenascin could be shown to function as a cytoadhesive molecule for hematopoietic cells. These data suggest a direct involvement of tenascin in the retention of hematopoietic progenitor cells in the stroma.
To study genes that may be crucial for the male germ cell development of Drosophila we screened a cDNA expression library with a polyclonal antiserum against testis proteins of Drosophila hydei. We identified a cDNA fragment that exhibited a complete sequence similarity with the cDNA of the laminin B2 chain, an important component of the extracellular matrix. Transcripts of laminin B2 were detected in the RNA of male germ cells with the polymerase chain reaction and by in situ hybridization. We studied the reaction of different polyclonal antibodies including those against a Drosophila laminin B2-lac fusion protein, the entire Drosophila laminin complex, or against the mouse laminin complex and against laminin A and B1 chains with specific structures in developing male germ cells of Drosophila. Antigenic sites against laminin B2 were found in the lampbrush loops in primary spermatocyte nuclei, in nuclei of spermatids, and in heads of spermatozoa. The axonemes of elongating spermatids react with antibodies against the Drosophila laminin B1, B2 and laminin A chains. The possible biological functions of the laminin in the male germ cells of Drosophila are discussed.