Cytokine receptors: a new superfamily of receptors

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Signal transduction in the erythropoietin receptor system

Developing erythroid cells require the glycoprotein hormone, erythropoietin (EPO) as an activator of the rapid proliferation of early proerythroblasts (colony forming units-erythroid [CFU-e]), and subsequently as an activator of late erythroid gene expression. Activation of these growth and differentiation events proceeds from the binding of EPO at its transmembrane receptor (Class I cytokine receptor), to the engagement of a complex set of signaling pathways. Studies of reconstituted activities of the cloned EPO receptor in transfected hematopoietic cell lines have served well in identifying receptor domains and downstream mediators involved in proliferative signaling. Extracellular domains have been defined which contribute to ligand binding, receptor processing and transport, and possible dimerization. Cytosolic regions have been delineated which mediate induced mitogenesis, early gene transcription, activated protein tyrosine phosphorylation, down modulation of EPO- and granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced proliferation, and direct association with PI3- and JAK-2 kinases. These newly defined properties begin to align the EPO receptor mechanistically with growth factor receptors (GFR) which encode, or likewise associate with, regulated protein tyrosine kinases including the Class II cytokine receptors for interferons alpha/beta and gamma. An improved understanding of factors which mediate EPO-induced late erythroid gene activation also is emerging. These factors and pathways may be distinct from those associated with EPO-induced proliferation and may involve induced increases in cellular Ca++, cAMP and arachidonic acid, as well as the modulation of GATA-1, and/or SCL. Attributes of model systems used in studies of the role of EPO in late erythroid differentiation also are considered.