Cross-hybridizing snake satellite, Drosophila, and mouse DNA sequences may have arisen independently

Previous reports have interpreted hybridization between snake satellite DNA and DNA clones from a variety of distant taxonomic groups as evidence for evolutionary conservation, which implies common ancestry (homology) and/or convergence (analogy) to produce the cross- hybridizing sequences. We have isolated 11 clones from a genomic library of Drosophila melanogaster, using a cloned 2.5-kb snake satellite probe of known nucleotide sequence. We have also analysed published sequence data from snakes, mice, and Drosophila. These data show that (1) all of the cross-hybridization between the snake, fly, and mouse clones can be accounted for by the presence of either of two tandem repeats, [GATA]n and [GACA]n and (2) these tandem repeats are organized differently among the different species. We find no evidence that these sequences are homologous apart from the existence of the simple repeat itself, although their divergence from a common ancestral sequence cannot be ruled out. The sequences contain a variety of homogeneous clusters of tandem repeats of CATA, GA, TA, and CA, as well as GATA and GACA. We suggest that these motifs may have arisen by a self-accelerating process involving slipped-strand mispairing of DNA. Homogeneity of the clusters might simply be the result of a rate of accumulation of tandem repeats that exceeds that of other mutations.      

Structural comparison of fibroblast growth factor-specific heparan sulfates derived from a growing or differentiating neuroepithelial cell line

Heparan sulfate (HS) glycosaminoglycans are essential modulators of fibroblast growth factor (FGF) activity both in vivo and in vitro, and appear to act by cross-linking particular forms of FGF to appropriate FGF receptors. We have recently isolated and characterized two separate HS pools derived from immortalized embryonic day 10 mouse neuroepithelial 2.3D cells: one from cells in log growth phase, which greatly potentiates the activity of FGF-2, and the other from cells undergoing contact-inhibition and differentiation, which preferentially activates FGF-1. These two pools of HS have very similar functional activities to those species isolated from primary neuroepithelial cells at corresponding stages of active proliferation or differentiation. We present here a structural comparison between these cell line HS species to establish the nature of the changes that occur in the biosynthesis of HS. A combination of chemical and enzymatic cleavage, low pressure chromatography and strong anion-exchange HPLC were used to generate full chain models of each species. Overall, the HS pools synthesized in the dividing cell line pools possessed less complex sulfation than those derived from more differentiated, growth arrested cells.      

Carbohydrate inhibitors of concanavalin A that inhibit binding of insulin-sepharose to fat cells and antagonize and mimic insulin's bioactivity. A possible role for membrane carbohydrate in insulin's action.

A consistent pattern of insulin-like properties is expressed by a variety of glycoside inhibitors of concanavalin A (Con A), and is suggestive of a common mechanism of action to explain these effects. Various exogenously added glycoside derivatives inhibit the binding of insulin-Sepharose beads to insulin receptors on isolated intact rat fat cells with a specificity resembling that for Con A-Sepharose binding to these cells. A more limited number of glycosides tested were also found to inhibit the binding of 125I-insulin, although some enhancement of binding that preceded the inhibition was observed for some of these saccharides. The glycosides also antagonize insulin-stimulated glucose utilization by the cells, but in some cases also mimic the hormone by stimulating glucose utilization. A few glycosides mimic insulin without appearing to antagonize its bioactivity. Radiolabeled glycoside inhibitors fail to bind to insulin in equilibrium dialysis experiments although they readily bind to Con A, indicating that the glycosides act directly on the cell rather than on the insulin molecule. The latter observation is consistent with the ability of those glycosides that act like insulin to do so independent of the hormone. In view of the known insulin-like properties of Con A, the effects of the glycosides seen in the present study suggest roles for a membrane carbohydrate and a carbohydrate binding site in the mechanisms of action of both insulin and Con A. In addition to various alternative explanations, a working hypothesis is presented to rationalize the present observations. It proposes that the effects of the exogenously added glycosides (and Con A) may reflect the presence on the membrane of a native carbohydrate moiety by either mimicking or competitively inhibiting its ability to interact reversibly with a lectin-like carbohydrate binding site associated with the function of the insulin receptor.20     

Complex phylogeographic history of central African forest elephants and its implications for taxonomy

Background

The mitochondrial genomes of snakes are characterized by an overall evolutionary rate that appears to be one of the most accelerated among vertebrates. They also possess other unusual features, including short tRNAs and other genes, and a duplicated control region that has been stably maintained since it originated more than 70 million years ago. Here, we provide a detailed analysis of evolutionary dynamics in snake mitochondrial genomes to better understand the basis of these extreme characteristics, and to explore the relationship between mitochondrial genome molecular evolution, genome architecture, and molecular function. We sequenced complete mitochondrial genomes from Slowinski's corn snake (Pantherophis slowinskii) and two cottonmouths (Agkistrodon piscivorus) to complement previously existing mitochondrial genomes, and to provide an improved comparative view of how genome architecture affects molecular evolution at contrasting levels of divergence.

Results

We present a Bayesian genetic approach that suggests that the duplicated control region can function as an additional origin of heavy strand replication. The two control regions also appear to have different intra-specific versus inter-specific evolutionary dynamics that may be associated with complex modes of concerted evolution. We find that different genomic regions have experienced substantial accelerated evolution along early branches in snakes, with different genes having experienced dramatic accelerations along specific branches. Some of these accelerations appear to coincide with, or subsequent to, the shortening of various mitochondrial genes and the duplication of the control region and flanking tRNAs.

Conclusion

Fluctuations in the strength and pattern of selection during snake evolution have had widely varying gene-specific effects on substitution rates, and these rate accelerations may have been functionally related to unusual changes in genomic architecture. The among-lineage and among-gene variation in rate dynamics observed in snakes is the most extreme thus far observed in animal genomes, and provides an important study system for further evaluating the biochemical and physiological basis of evolutionary pressures in vertebrate mitochondria.     

CXCR7 expression disrupts endothelial cell homeostasis and causes ligand-dependent invasion

The homeostatic function of endothelial cells (EC) is critical for a number of physiological processes including vascular integrity, immunity, and wound healing. Indeed, vascular abnormalities resulting from EC dysfunction contribute to the development and spread of malignancies. The alternative SDF-1/CXCL12 receptor CXCR7 is frequently and specifically highly expressed in tumor-associated vessels. In this study, we investigate whether CXCR7 contributes to vascular dysfunction by specifically examining the effect of CXCR7 expression on EC barrier function and motility. We demonstrate that CXCR7 expression in EC results in redistribution of CD31/PECAM-1 and loss of contact inhibition. Moreover, CXCR7+ EC are deficient in barrier formation. We show that CXCR7-mediated motility has no influence on angiogenesis but contributes to another motile process, the invasion of CXCR7+ EC into ligand-rich niches. These results identify CXCR7 as a novel manipulator of EC barrier function via alteration of PECAM-1 homophilic junctions. As such, aberrant expression of CXCR7 in the vasculature has the potential to disrupt vascular homeostasis and could contribute to vascular dysfunction in cancer systems.     

A rare genetically determined electrophoretic variant of human leucocyte type III hexokinase

Various lines of evidence from starch gel electrophoretic experiments demonstrate the existence of a genetically determined rare variant form of the type III isozyme of hexokinase (HK) in the leucocytes of a small percentage of the general human population. This enzymatically active variant (designated IIIS) migrates slightly, but significantly, slower than the common form (designated IIIF). In addition to finding various individuals with a two-banded pattern (heterozygotes containing both IIIS and IIIF), a finding reported previously by S. Povey, G. Corney, and H. Harris ((1975) Ann. Hum. Genet. 38, 407-415), we discovered one person homozygous for the variant phenotype. In close agreement with Povey et al., screening of 59 individuals at random indicated a gene frequency of about 0.017 for the IIIS allele, corresponding to a homozygous genotype for this allele that would be expected in about one of every 3500 individuals. Experiments involving the mixing of blood samples from the individual homozygous for IIIS with those homozygous for IIIF indicate that secondary in vitro changes, a possibility suggested by Povey et al., are not responsible for the appearance of the variant. This conclusion was supported by a demonstration of the specificity of the alteration in type III's mobility in comparison with the lack of alterations in any of the LDH isozymes, glucose-6-phosphate dehydrogenase, and various amido black-stainable proteins. These studies confirm the proposal for a genetically determined polymorphism of type III HK. No differences could be found between the total HK activity (according to spectrophotometric assays) of extracts from the subject homozygous for the variant and the activity from the homozygote for the common form, in terms of either their Km values for glucose or their heat stability properties. The similarity of Km values was supported by kinetic assays performed during staining of the individual forms on electrophoretic gels. Previous findings, reported elsewhere, of type III HK in RBC extracts were shown here to be attributable to contamination, by leucocytes, of the extracts. As a consequence of these studies, slight, but significant, amounts of type II-like HK were also discovered in leucocytes. Because our studies described above were completed in 1969, advantage was taken of the opportunity to test the HK pattern 17 years later from some of the same subjects. The patterns of the homozygotes for IIIS and for IIIF and the heterozygotes were found to be identical to the original ones, indicating no age-, environmental-, or other time-related changes that could explain the variation in type III HK.