Erythroblastic islands: specialized microenvironmental niches for erythropoiesis.
OBJECTIVE
This review focuses on current understanding of molecular mechanisms operating within erythroblastic islands including cell-cell adhesion, regulatory feedback, and central macrophage function.


RESULTS
Erythroblasts express a variety of adhesion molecules and recently two interactions have been identified that appear to be critical for island integrity. Erythroblast macrophage protein, expressed on erythroblasts and macrophages, mediates cell-cell attachments via homophilic binding. Erythroblast intercellular adhesion molecule-4 links erythroblasts to macrophages through interaction with macrophage alphav integrin. In intercellular adhesion molecule-4 knockout mice, erythroblastic islands are markedly reduced, whereas the erythroblast macrophage protein null phenotype is severely anemic and embryonic lethal. Retinoblastoma tumor suppressor (Rb) protein stimulates macrophage differentiation by counteracting inhibition of Id2 on PU.1, a transcription factor that is a crucial regulator of macrophage differentiation. Rb-deficient macrophages do not bind Rb null erythroblasts and the Rb null phenotype is anemic and embryonic lethal. Lastly, extruded nuclei rapidly expose phosphatidylserine on their surface, providing a recognition signal similar to apoptotic cells.


CONCLUSIONS
Although understanding of molecular mechanisms operating within islands is at an early stage, tantalizing evidence suggests that erythroblastic islands are specialized niches where intercellular interactions in concert with cytokines play critical roles in regulating erythropoiesis.
Life Sciences Division, University of California, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. jachasis@lbl.gov
Joel Anne
JA
Chasis
D000818:Animals; D002448:Cell Adhesion / Q000502:physiology; D002450:Cell Communication / Q000502:physiology; D004900:Erythroblasts / Q000378:metabolism / Q000502:physiology*; D004920:Erythropoiesis / Q000502:physiology*; D006801:Humans; D008264:Macrophages / Q000502:physiology; D008954:Models, Biological