Reactive astrocytes adjacent to a forebrain stab injury were selectively ablated in adult mice expressing HSV-TK from the Gfap promoter by treatment with ganciclovir. Injured tissue that was depleted of GFAP-positive astrocytes exhibited (1) a prolonged 25-fold increase in infiltration of CD45-positive leukocytes, including ultrastructurally identified monocytes, macrophages, neutrophils, and lymphocytes, (2) failure of blood-brain barrier (BBB) repair, (3) substantial neuronal degeneration that could be attenuated by chronic glutamate receptor blockade, and (4) a pronounced increase in local neurite outgrowth. These findings show that genetic targeting can be used to ablate scar-forming astrocytes and demonstrate roles for astrocytes in regulating leukocyte trafficking, repairing the BBB, protecting neurons, and restricting nerve fiber growth after injury in the adult central nervous system.

To investigate the roles of astroglial cells, we targeted their ablation genetically. Transgenic mice were generated expressing herpes simplex virus thymidine kinase from the mouse glial fibrillary acidic protein (GFAP) promoter. In adult transgenic mice, 2 weeks of subcutaneous treatment with the antiviral agent ganciclovir preferentially ablated transgene-expressing, GFAP-positive glia from the jejunum and ileum, causing a fulminating and fatal jejuno-ileitis. This pathology was independent of bacterial overgrowth and was characterized by increased myeloperoxidase activity, moderate degeneration of myenteric neurons, and intraluminal hemorrhage. These findings demonstrate that enteric glia play an essential role in maintaining the integrity of the bowel and suggest that their loss or dysfunction may contribute to the cellular mechanisms of inflammatory bowel disease.

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Genesis of the trophectoderm and inner cell mass (ICM) lineages occurs in two stages. It is initiated via asymmetric divisions of eight-and 16-cell blastomeres that allocate cells to inner and outer positions, each with different developmental fates. Outside cells become committed to the trophectoderm at the blastocyst stage through Cdx2 activity, but here we show that Cdx2 can also act earlier to influence cell allocation. Increasing Cdx2 levels in individual blastomeres promotes symmetric divisions, thereby allocating more cells to the trophectoderm, whereas reducing Cdx2 promotes asymmetric divisions and consequently contribution to the ICM. Furthermore, both Cdx2 mRNA and protein levels are heterogeneous at the eight-cell stage. This heterogeneity depends on cell origin and has developmental consequences. Cdx2 expression is minimal in cells with unrestricted developmental potential that contribute preferentially to the ICM and is maximal in cells with reduced potential that contribute more to the trophectoderm. Finally, we describe a mutually reinforcing relationship between cellular polarity and Cdx2: Cdx2 influences cell polarity by up-regulating aPKC, but cell polarity also influences Cdx2 through asymmetric distribution of Cdx2 mRNA in polarized blastomeres. Thus, divisions generating inside and outside cells are truly asymmetric with respect to cell fate instructions. These two interacting effects ensure the generation of a stable outer epithelium by the blastocyst stage.[Keywords: Cdx2; mouse embryo; polarization; ICM; blastocyst; trophectoderm] Supplemental material is available at http://www.genesdev.org.

Regulation of erythropoietin production by the kidneys is central to the control of erythropoiesis. Uncertainty about the identity of the renal cells involved has been a major obstacle to understanding this mechanism. We have used sequence from the mouse erythropoietin locus to direct expression of a marker gene, SV40 T antigen, to these cells in transgenic mice. The transgenic constructs contained an oligonucleotide marker (Epo-M) or SV40 sequence (Epo-TAg) in the 5' untranslated region of the mouse erythropoietin gene, flanked on each side by 9 and 7.5 kb of DNA from the mouse erythropoietin locus. Anemia-inducible expression of Epo-M and Epo-TAg was observed in the kidney. In one of thirteen lines, homologous integration of Epo-TAg into the mouse erythropoietin locus occurred. In transgenic mice bearing Epo-TAg at homologous and heterologous insertion sites, renal expression was restricted to a population of cells in the interstitium of the cortex and outer medulla. Immunohistochemical characterization by light and electron microscopy shows that these are the fibroblast-like type I interstitial cells.