Penumbra encodes a novel tetraspanin that is highly expressed in erythroid progenitors and promotes effective erythropoiesis.

In a search for new genes involved in the regulation of erythropoiesis, we identified murine Penumbra cDNA from a multipotent hematopoietic cell line based on its predominant expression in erythroblasts. Subsequently, we identified the human PENUMBRA from a bone marrow cDNA library. Penumbra is a new member of the tetraspanin superfamily of membrane proteins, many of which are thought to function as organizers of supramolecular signaling complexes. Human and murine Penumbras contain 283 amino acids and are 97% identical. The human PENUMBRA gene is mapped to chromosome 7q32, a hot spot for deletions in myelodysplastic syndromes and acute myelogenous leukemias. Penumbra is targeted to the cell surface and forms disulfide-bonded homodimers. To study the effects of Penumbra deletions, we created a knockout mouse model by gene targeting. Penumbra(-/-) mice develop massive splenomegaly, basophilic macrocytic red blood cells, and anemia as they age. A multipotent hematopoietic cell line, EMX, was established from the bone marrow of a Penumbra(-/-) mouse. EMX exhibits ineffective erythropoiesis in the presence of erythropoietin, a defect that is reversed by reexpression of Penumbra. These findings indicate that Penumbra has a positive function in erythropoiesis and its deletion or mutation may result in anemia.

Jagged2 promotes the development of natural killer cells and the establishment of functional natural killer cell lines.

Emerging evidence indicates that Notch receptors and their ligands play important roles in the development of T cells and B cells. However, little is known about their possible roles in the development of other lymphoid cells. Here we demonstrate that Jagged2, a Notch ligand, stimulates the development of natural killer (NK) cells from Lin(-) Sca-1(+) c-kit(+) hematopoietic stem cells. Our culture system supports NK cell development for 2 to 3 months, often leading to the establishment of continuous NK cell lines. The prototype of such cell lines is designated as KIL. KIL depends on interleukin-7 for survival and proliferation and is NK1.1(+) CD3(-) TCRalphabeta(-) TCRdeltagamma(-) CD4(-) CD8(-) CD19(-) CD25(+) CD43(+) CD45(+) CD49b(-) CD51(+) CD94(+) NKG2D(+) Mac-1(-/low) B220(-) c-kit(+) perforin I(+) granzyme B(+) Notch-1(+), and cytotoxic. Like normal natural killer cells, the T-cell receptor-beta loci of KIL remain in the germ-line configuration. In response to interleukin-2, KIL proliferates extensively (increasing cell number by approximately 10(10)-fold) and terminally differentiates into adherent, hypergranular NK cells. Our findings indicate that Jagged2 stimulates the development of natural killer cells and the KIL cell line preserves most properties of the normal NK precursors. As such, KIL provides a valuable model system for NK cell research.