Cell fate decisions require appropriate regulation of key genes. , a direct target of SRY, is pivotal in mammalian sex determination. In vivo high-throughput chromatin accessibility techniques, transgenic assays, and genome editing revealed several novel gonadal regulatory elements in the 2-megabase gene desert upstream of Although others are redundant, enhancer 13 (Enh13), a 557-base pair element located 565 kilobases 5' from the transcriptional start site, is essential to initiate mouse testis development; its deletion results in XY females with transcript levels equivalent to those in XX gonads. Our data are consistent with the time-sensitive activity of SRY and indicate a strict order of enhancer usage. Enh13 is conserved and embedded within a 32.5-kilobase region whose deletion in humans is associated with XY sex reversal, suggesting that it is also critical in humans.

SUMMARYTbx4 and Tbx5 are two closely related T-box genes that encode transcription factors expressed in the prospective hindlimb and forelimb territories, respectively, of all jawed vertebrates. Despite their striking limb type-restricted expression pattern, we have shown that these genes do not participate in the acquisition of limb type-specific morphologies. Instead, Tbx4 and Tbx5 play similar roles in the initiation of hindlimb and forelimb outgrowth, respectively. We hypothesized that different combinations of Hox proteins expressed in different rostral and caudal domains of the lateral plate mesoderm, where limb induction occurs, might be involved in regulating the limb type-restricted expression of Tbx4 and Tbx5 and in the later determination of limb type-specific morphologies. Here, we identify the minimal regulatory element sufficient for the earliest forelimb-restricted expression of the mouse Tbx5 gene and show that this sequence is Hox responsive. Our results support a mechanism in which Hox genes act upstream of Tbx5 to control the axial position of forelimb formation.

SummaryThe retinoic acid (RA)- and β-catenin-signaling pathways regulate limb bud induction and initiation; however, their mechanisms of action are not understood and have been disputed. We demonstrate that both pathways are essential and that RA and β-catenin/TCF/LEF signaling act cooperatively with Hox gene inputs to directly regulate Tbx5 expression. Furthermore, in contrast to previous models, we show that Tbx5 and Tbx4 expression in forelimb and hindlimb, respectively, are not sufficient for limb outgrowth and that input from RA is required. Collectively, our data indicate that RA signaling and Tbx genes act in a coherent feed-forward loop to regulate Fgf10 expression and, as a result, establish a positive feedback loop of FGF signaling between the limb mesenchyme and ectoderm. Our results incorporate RA-, β-catenin/TCF/LEF-, and FGF-signaling pathways into a regulatory network acting to recruit cells of the embryo flank to become limb precursors.

Page 2, text line 9 should read ' … for the record, this M r for type B is 20 200 (p800), for type A 24 000 (p1300) and for type C 22 700 (p1000). The glycoform ratios for each type were as follows : type B, %H 66, %L 27 ; type A, %H 15-50, %L 22-46 ; … ' IFB

Biochemical typing of scrapie strainsDifferences in the molecular heterogeneity of the abnormal isoforms of the prion protein, PrP 5 c, due to differential glycosylation and partial endogenous or exogenous proteo-lysis1'2 have been found in different scrapie strains\ sources of Creutzfeldt-Jakob disease (CJD) and fatal familial insomnia (FFI) 4 ';, Recently Collinge et a/. 6 extended this analysis to include sporadic CJD and the new variant of Creutzfeldt-Jakob disease ( vCJD) and found that the vCJD 'type 4' pattern of PrPSc was common to mice, cats and monkeys infected with bovine spongiform encephalopathy (BSE). We have extended this analysis to include scrapie strains passaged in mice and originally derived from sheep or goats.Collinge et a/. 6 suggested that the prion molecular phenotype 'type 4' was distinct and characteristic ofBSE, and that its presence in vCJD was further evidence of a direct link between vCJD and BSE. Furthermore, they suggested that this pattern, though also seen in FPC, might be used to identify a BSE origin of the disease-in particular that it would differentiate BSE and natural scrapie in sheep and goats. Unfortunately, no control data of scrapie in mice or the other species were given to support this idea.We present a glycoform ratio analysis of mouse-passaged scrapie strains from sheep or goats and compare them with a BSE-60 ~ I 20 40 60 80 100 PrPSetn high Mr glyco!Ofm (%)