Differing Requirements for RAD51 and DMC1 in Meiotic Pairing of Centromeres and Chromosome Arms in Arabidopsis thaliana

During meiosis homologous chromosomes pair, recombine, and synapse, thus ensuring accurate chromosome segregation and the halving of ploidy necessary for gametogenesis. The processes permitting a chromosome to pair only with its homologue are not fully understood, but successful pairing of homologous chromosomes is tightly linked to recombination. In Arabidopsis thaliana, meiotic prophase of rad51, xrcc3, and rad51C mutants appears normal up to the zygotene/pachytene stage, after which the genome fragments, leading to sterility. To better understand the relationship between recombination and chromosome pairing, we have analysed meiotic chromosome pairing in these and in dmc1 mutant lines. Our data show a differing requirement for these proteins in pairing of centromeric regions and chromosome arms. No homologous pairing of mid-arm or distal regions was observed in rad51, xrcc3, and rad51C mutants. However, homologous centromeres do pair in these mutants and we show that this does depend upon recombination, principally on DMC1. This centromere pairing extends well beyond the heterochromatic centromere region and, surprisingly, does not require XRCC3 and RAD51C. In addition to clarifying and bringing the roles of centromeres in meiotic synapsis to the fore, this analysis thus separates the roles in meiotic synapsis of DMC1 and RAD51 and the meiotic RAD51 paralogs, XRCC3 and RAD51C, with respect to different chromosome domains

The orphan nuclear receptor, shp, mediates bile acid-induced inhibition of the rat bile acid transporter, ntcp

Background & Aims: Hepatic bile acid homeostasis is regulated by negative feedback inhibition of genes involved in the uptake and synthesis of bile acids. Bile acids down-regulate the rate-limiting gene for bile acid synthesis, cholesterol 7 alpha -hydroxylase (cyp7a), via bile acid receptor (fxr) activation of an inhibitory nuclear receptor, shp. We hypothesized that shp would also mediate negative feedback regulation of ntcp, the principal hepatic bile acid transporter. Methods: Primary rat hepatocytes or transfected HepG2 and Cos cells were treated with retinoids with or without bile acids, and effects on bile acid transport and ntcp and shp gene expression and promoter activity were determined. Gel shift assays were performed using synthetic fry, rxr, and rar proteins. Results: Bile acid treatment of primary rat hepatocytes prevented retinoid activation of ntcp gene expression and function; this corresponded temporally with shp gene activation. Bile acid-mediated down-regulation occurred via fxr-dependent suppression of the ntcp RXR:RAR response element. Moreover, cotransfected shp directly inhibited retinoid activation of the ntcp promoter. Conclusions: These studies show negative feedback regulation of ntcp by bile acid-activated fxr via induction of shp. This novel regulatory pathway provides a means for coordinated down-regulation of bile acid import and synthesis, thereby protecting the hepatocyte from bile acid-mediated damage in cholestatic conditions

Kupffer cell depletion with liposomal clodronate prevents suppression of Ntcp expression in endotoxin-treated rats

Background/Aims: In sepsis-associated cholestasis, expression of many genes involved in bile acid transport, including Ntcp, is suppressed by cytokines. Kupffer cells (KC) are an important source of cytokines in sepsis. To assess the consequences of KC depletion on hepatic Ntcp expression in endotoxemic rats. Methods: Sprague-Dawley rats received liposomal clodronate (CLO) or vehicle (PBS) to deplete KC prior to lipopolysaccharide (LPS) exposure. Plasma and liver samples were taken 1 and 16 h after LPS exposure. Results: Complete CLO-depletion of KC by was demonstrated by inmumohistochemistry. Hepatic gene expression of IL-1beta and TNFalpha as well as TNFalpha plasma levels in CLO/LPS-injected animals were significantly reduced to a mean of 41, 36 and 23% of controls injected with LPS only. Ntcp RNA- and protein expression was significantly higher whereas plasma bile salt concentration was lower in CLO/LPS animals vs. animals injected with LPS only. Binding activity of transcription factors RXR:RAR and HNF1alpha was decreased in LPS only controls but preserved in CLO/LPS treated animals. Conclusions: Clodronate-depletion of KC blocks cytokine-mediated Ntcp suppression upon endotoxin exposure. KC may represent pharmacological targets for treatment of sepsis-associated cholestasis. (C) 2004 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved

Linear assembly of a human centromere on the Y chromosome

The human genome reference sequence remains incomplete owing to the challenge of assembling long tracts of near-identical tandem repeats in centromeres. We implemented a nanopore sequencing strategy to generate high-quality reads that span hundreds of kilobases of highly repetitive DNA in a human Y chromosome centromere. Combining these data with short-read variant validation, we assembled and characterized the centromeric region of a human Y chromosome