ABSTRACT
Complex intrinsic and extrinsic mechanisms determine neural cell fate during development of the nervous system. Using Id4 deficient mice, we show that Id4 is required for normal development of the central nervous system (CNS), timing neural differentiation in the developing forebrain. In the absence of Id4, the ventricular zone of the neocortex, future hippocampus as well as lateral and medial ganglionic eminences exhibited a 20-30% reduction in mitotic neural precursor cells (NPCs). Although the number of apoptotic cells was significantly increased, the neocortex of Id4(-/-) embryos was consistently thicker due to premature neuronal differentiation, which resulted in an increase in early-born neurons in the adult Id4(-/-) cortex. Late-born cortical neurons and astrocytes in the cortex, septum, hippocampus and caudate putamen of Id4(-/-) adult brains were decreased, however, likely due to the depletion of the NPC pool. Consequently, adult Id4(-/-) brains were smaller and exhibited enlarged ventricles. In vitro analysis of neurosphere cultures revealed that proliferation of Id4-deficient NPCs was impaired and that BMP2-mediated astrocyte differentiation was accelerated in the absence of Id4. Together, these in vivo and in vitro data suggest a crucial role for Id4 in regulating NPC proliferation and differentiation.
Subject(s)
Central Nervous System/embryology , DNA-Binding Proteins/physiology , Gene Expression Regulation, Developmental , Neurons/cytology , Transcription Factors/physiology , Animals , Apoptosis , Astrocytes/metabolism , Bone Morphogenetic Protein 2 , Bone Morphogenetic Proteins/metabolism , Brain/embryology , Brain/metabolism , Cell Differentiation , Cell Proliferation , Central Nervous System/metabolism , DNA-Binding Proteins/metabolism , Embryo, Mammalian/metabolism , Hippocampus/embryology , Immunohistochemistry , Inhibitor of Differentiation Proteins , Mice , Mice, Mutant Strains , Mice, Transgenic , Mutation , Neocortex/metabolism , Neurons/metabolism , Phenotype , Prosencephalon/embryology , Time Factors , Transcription Factors/metabolism , Transforming Growth Factor beta/metabolismABSTRACT
In mammals, the fetal liver is the first site of definitive erythropoiesis-the generation of mature, enucleated red cells. The functional unit for definitive erythropoiesis is the erythroblastic island, a multicellular structure composed of a central macrophage surrounded by erythroblasts at various stages of differentiation. Targeted disruption of the retinoblastoma (Rb) tumour suppressor gene in the mouse leads to embryonic death caused by failure of erythroblasts to enucleate. The erythroid defect has been attributed to loss of Rb in cells that support erythropoiesis, but the identity of these cells is unknown. Here we show that Rb-deficient embryos carry profound abnormalities of fetal liver macrophages that prevent physical interactions with erythroblasts. In contrast, wild-type macrophages bind Rb-deficient erythroblasts and lead them to terminal differentiation and enucleation. Loss of Id2, a helix-loop-helix protein that mediates the lethality of Rb-deficient embryos, rescues the defects of Rb-deficient fetal liver macrophages. Rb promotes differentiation of macrophages by opposing the inhibitory functions of Id2 on the transcription factor PU.1, a master regulator of macrophage differentiation. Thus, Rb has a cell autonomous function in fetal liver macrophages, and restrains Id2 in these cells in order to implement definitive erythropoiesis.
