Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4−CD8− (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56lck. At the CD4+CD8+ (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR–major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4+ or CD8+ (single positive, SP) thymocytes is induced by engagement of the α/β-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR–MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.

Expression of the transcription factor GATA-3 is strongly associated with T helper type 2 (T(H)2) differentiation, but genetic evidence for its involvement in this process has been lacking. Here, we generated a conditional GATA-3-deficient mouse line. In vitro deletion of Gata3 diminished both interleukin 4 (IL-4)-dependent and IL-4-independent T(H)2 cell differentiation; without GATA-3, T(H)1 differentiation occurred in the absence of IL-12 and interferon-gamma. Gata3 deletion limited the growth of T(H)2 cells but not T(H)1 cells. Deletion of Gata3 from established T(H)2 cells abolished IL-5 and IL-13 but not IL-4 production. In vivo deletion of Gata3 using OX40-Cre eliminated T(H)2 responses and allowed the development of interferon-gamma-producing cells in mice infected with Nippostrongylus brasiliensis. Thus, GATA-3 serves as a principal switch in determining T(H)1-T(H)2 responses.

Dopamine (DA) neurotransmission has been implicated in several neurological and psychiatric disorders. The dopamine transporter (DAT) is highly expressed in dopaminergic neurons of the ventral mesencephalon and regulates neurotransmission by transporting DA back into the presynaptic terminals. To mediate restricted DNA recombination events into DA neurons using the Cre/loxP technology, we have generated a knockin mouse expressing Cre recombinase under the transcriptional control of the endogenous DAT promoter. To minimize interference with DAT function by preservation of both DAT alleles, Cre recombinase expression was driven from the 3' untranslated region (3'UTR) of the endogenous DAT gene by means of an internal ribosomal entry sequence. Crossing this murine line with a LacZ reporter showed colocalization of DAT immunocytochemistry and beta-galactosidase staining in all regions analyzed. This knockin mouse can be used for generating tissue specific knockouts in mice carrying genes flanked by loxP sites, and will facilitate the analysis of gene function in dopaminergic neurons.

The two iron regulatory proteins IRP1 and IRP2 bind to transcripts of ferritin, transferrin receptor and other target genes to control the expression of iron metabolism proteins at the post-transcriptional level. Here we compare the effects of genetic ablation of IRP1 to IRP2 in mice. IRP1À/À mice misregulate iron metabolism only in the kidney and brown fat, two tissues in which the endogenous expression level of IRP1 greatly exceeds that of IRP2, whereas IRP2À/À mice misregulate the expression of target proteins in all tissues. Surprisingly, the RNA-binding activity of IRP1 does not increase in animals on a lowiron diet that is sufficient to activate IRP2. In animal tissues, most of the bifunctional IRP1 is in the form of cytosolic aconitase rather than an RNA-binding protein.Our findings indicate that the small RNA-binding fraction of IRP1, which is insensitive to cellular iron status, contributes to basal mammalian iron homeostasis, whereas IRP2 is sensitive to iron status and can compensate for the loss of IRP1 by increasing its binding activity. Thus, IRP2 dominates post-transcriptional regulation of iron metabolism in mammals.

p52 is a subunit of nuclear factor (NF)-κB transcription factors, most closely related to p50. Previously, we have shown that p52, but not p50 homodimers can form transactivating complexes when associated with Bcl-3, an unusual member of the IκB family. To determine nonredundant physiologic roles of p52, we generated mice deficient in p52. Null mutant mice were impaired in their ability to generate antibodies to T-dependent antigens, consistent with an absence of B cell follicles and follicular dendritic cell networks in secondary lymphoid organs, and an inability to form germinal centers. Furthermore, the splenic marginal zone was disrupted. These phenotypes are largely overlapping with those observed in Bcl-3 knockout animals, but distinct from those of p50 knockouts, supporting the notion of a physiologically relevant complex of p52 homodimers and Bcl-3. Adoptive transfer experiments further suggest that such a complex may be critical in accessory cell functions during antigen-specific immune reactions. Possible roles of p52 and Bcl-3 are discussed that may underlie the oncogenic potential of these proteins, as evidenced by recurrent chromosomal translocations of their genes in lymphoid tumors.

Inheritance of an inactivated form of the VHL tumor suppressor gene predisposes patients to develop von Hippel-Lindau disease, and somatic VHL inactivation is an early genetic event leading to the development of sporadic renal cell carcinoma. The VHL gene was disrupted by targeted homologous recombination in murine embryonic stem cells, and a mouse line containing an inactivated VHL allele was generated. While heterozygous VHL (؉͞؊) mice appeared phenotypically normal, VHL ؊͞؊ mice died in utero at 10.5 to 12.5 days of gestation (E10.5 to E12.5). Homozygous VHL ؊͞؊ embryos appeared to develop normally until E9.5 to E10.5, when placental dysgenesis developed. Embryonic vasculogenesis of the placenta failed to occur in VHL ؊͞؊ mice, and hemorrhagic lesions developed in the placenta. Subsequent hemorrhage in VHL ؊͞؊ embryos caused necrosis and death. These results indicate that VHL expression is critical for normal extraembryonic vascular development.Germ-line mutations in the VHL tumor suppressor gene predispose von Hippel-Lindau disease patients to develop tumors at multiple sites, including retinal angiomas, hemangioblastomas of the central nervous system, pheochromocytomas, renal cell carcinomas, and pancreatic cancers (reviewed in refs. 1 and 2). In addition, somatic VHL inactivation through deletion of one allele coupled with either mutation or hypermethylation of the remaining allele has been detected in approximately 80% of sporadic clear cell renal carcinomas examined (2), supporting a tumor suppressor function for VHL.The deduced amino acid sequence of the VHL protein gives no indication of the protein's function. Studies designed to assign VHL function through the identification of physically associating proteins found that VHL stably binds the B and C subunits of the elongin complex (3-5). Elongin, an RNA polymerase II transcription elongation factor, contains a catalytic A subunit that is stabilized and activated by the B and C regulatory subunits. In addition, the heterotrimeric VHLelongin B͞C complex (VBC) stably interacts with the human CUL-2 homolog, a member of a conserved gene family involved in cell cycle and growth control in lower eukaryotes (6). The cellular function(s) of the VBC and͞or VBC-CUL-2 complexes have yet to be elucidated.To analyze the role of VHL in normal cell growth and differentiation, we used targeted homologous recombination to develop a mouse line that is deleted for one VHL allele. Heterozygous VHL mice (ϩ͞Ϫ) have survived thus far to beyond 15 months without evidence of spontaneous disease.However, VHL-deficient mice (Ϫ͞Ϫ) develop placental lesions at 9.5 to 10.5 days of gestation (E9.5 to E10.5) and die in utero between E10.5 to E12.5 due to the absence of placental embryonal vasculogenesis and subsequent hemorrhage and necrosis. MATERIALS AND METHODSConstruction of a Murine VHL-Targeting Vector. Two genomic DNA clones corresponding to the VHL gene were isolated from a 129 mouse-derived P1 library (Genome Systems, St. Louis). Physical mapping showed that three Ba...

To evaluate the role of hypoxia-inducible factor 1α (HIF-1α) and its TCR activation-inducible short isoform I.1 in T cell functions, we genetically engineered unique mice with: 1) knockout of I.1 isoform of HIF-1α; 2) T cell-targeted HIF-1α knockdown; and 3) chimeric mice with HIF-1α gene deletion in T and B lymphocytes. In all three types of mice, the HIF-1α-deficient T lymphocytes, which were TCR-activated in vitro, produced more proinflammatory cytokines compared with HIF-1α-expressing control T cells. Surprisingly, deletion of the I.1 isoform, which represents <30% of total HIF-1α mRNA in activated T cells, was sufficient to markedly enhance TCR-triggered cytokine secretion. These data suggest that HIF-1α not only plays a critical role in oxygen homeostasis but also may serve as a negative regulator of T cells.

Multidrug resistance protein 3 (MRP3) is an ATP-binding cassette transporter that is able to confer resistance to anticancer agents such as etoposide and to transport lipophilic anions such as bile acids and glucuronides. These capabilities, along with the induction of the MRP3 protein on hepatocyte sinusoidal membranes in cholestasis and the expression of MRP3 in enterocytes, have led to the hypotheses that MRP3 may function in the body to protect normal tissues from etoposide, to protect cholestatic hepatocytes from endobiotics, and to facilitate bile-acid reclamation from the gut. To elucidate the role of Mrp3 in these processes, the Mrp3 gene (Abcc3) was disrupted by homologous recombination. Homozygous null animals were healthy and physically indistinguishable from wild-type mice. Mrp3Ϫ/Ϫ mice did not exhibit enhanced lethality to etoposide phosphate, although an analysis of transfected human embryonic kidney 293 cells indicated that the potency of murine Mrp3 toward etoposide (ϳ2.0-to 2.5-fold) is comparable with that of human MRP3. After induction of cholestasis by bile duct ligation, Mrp3 Ϫ/Ϫ mice had 1.5-fold higher levels of liver bile acids and 3.1-fold lower levels of serum bilirubin glucuronide compared with ligated wild-type mice, whereas significant differences were not observed between the respective sham-operated mice. Bile acid excretion, pool size, and fractional turnover rates were similar in Mrp3 Ϫ/Ϫ and wild-type mice. We conclude that Mrp3 functions as an alternative route for the export of bile acids and glucuronides from cholestatic hepatocytes, that the pump does not play a major role in the enterohepatic circulation of bile acids and that the lack of chemosensitivity is probably attributable to functional redundancy with other pumps.