Sodium-calcium exchange in renal epithelial cells: Dependence on cell sodium and competitive inhibition by magnesium

Summary Kinetic properties of Na⁺–Ca²⁺ exchange in a renal epithelial cell line (LLC-MK₂) were assessed by measuring cytosolic free Ca²⁺ with fura-2 and⁴⁵Ca²⁺ influx. Replacing external Na⁺ with K⁺ produced relatively small increases in free Ca²⁺ and⁴⁵Ca²⁺ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na⁺ and strongly potentiated the effect of replacing external Na⁺ with K⁺ on free Ca²⁺ and⁴⁵Ca²⁺ uptake.⁴⁵Ca²⁺ influx in 140mm K⁺ or N-methyl-d-glucamine minus influx in 140mm Na⁺ was used to quantify Na⁺–Ca²⁺ exchange activity of Na⁺-loaded cells. The dependence of exchange on cell Na⁺ was sigmoidal; theK _0.5 was 26±3 mmol/liter cell water space, and the Hill coefficient was 3.1±0.2. The kinetic features of the dependence of exchange on cell Na⁺ partly account for the small increase in Ca²⁺ influx when all external Na⁺ is replaced by K⁺. Besides raising cell Na⁺ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg²⁺ was 8.2-fold lower with potassium instead of N-methyl-d-glucamine or choline as the replacement for external Na⁺. Potassium also increased theV _max of exchange by 86% and had no effect on theK _m for Ca²⁺. The exchanger does not cause detectable²²Na⁺–Mg²⁺ exchange and does not appear to require K⁺ or transport⁸⁶Rb⁺. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na⁺–Ca²⁺ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.     

A petite mitochondrial DNA segment arising in exceptionally high frequency in a mit- mutant of Saccharomyces cerevisiae

In cultures of the mit- mutant strain Mb12 of Saccharomyces cerevisiae (carrying a mutation in the oli2 gene), 70% of the cells are petite mutants. More than 80% of the petites from Mb12 contain a particular mtDNA segment, denoted BB5, that is 880 bp long and carries a single MboI site. Thus, in cultures of Mb12, about 56% of the cells are petites containing the defective BB5 mtDNA genome, and only 30% are mit- cells containing parental Mb12 mtDNA. The BB5 mtDNA segment is also found in petites arising from the wild-type strain J69-1B (from which Mb12 was derived), but in this case mtDNA from only five out of 24 petites produced an 880 bp band after MboI digestion. Since J69-1B cultures carry a petite frequency of about 5%, approximately 1% of cells in J69-1B cultures contain the BB5 mtDNA segment. The difference between Mb12 and J69-1B cultures is reflected in the MboI digestion patterns of the respective mtDNAs. While Mb12 mtDNA contains a grossly superstoicheiometric 880 bp MboI fragment, the corresponding fragment in J69-1B mtDNA cannot be seen on stained gels, but can be readily visualized in Southern blots hybridized to a 32P-labelled DNA probe obtained from the 880 bp MboI fragment. The BB5 mtDNA segment was shown to contain the ori1 sequence (one of several very similar sequences in wild-type mtDNA thought to act as origins of replication of mtDNA) which confers the genetic property of very high suppressiveness on petites carrying this mtDNA. The efficient replication of BB5 mtDNA may contribute to its abundance in Mb12 cultures. Nevertheless, other factors must operate to influence the abundance of the BB5 mtDNA segment in cultures of different strains, the most important of which is likely to be the rate of excision of this mtDNA segment from the parental mtDNA genome.     

A copy of the copia transposable element is very tightly linked to the Wa allele at the white locus of D. melanogaster

Results are described demonstrating that several X chromosomes of Drosophila melanogaster carrying the Wa (white-apricot) mutant allele also carry homology to the copia transposable element in distal 3C of the polytene chromosome map as assessed by situ hybridization. The locus of the Wa mutation, white, resides in distal 3C. We further show, using fine scale genetic mapping techniques, that the copia homology in distal 3C in Wa-bearing chromosomes is very tightly linked to the Wa mutation. Both the Wa mutation and the copia homology associated with it map to the central portion of the white locus.     

Design and development of a series of borocycles as selective,covalent kallikrein 5 inhibitors

The connection between Netherton syndrome and overactivation of epidermal/dermal proteases, particularly Kallikrein 5 (KLK5) has been well established and it is expected that a KLK5 inhibitor would improve the dermal barrier and also reduce the pain and itch that afflict Netherton syndrome patients. One of the challenges of covalent protease inhibitors has been achieving selectivity over closely related targets. In this paper we describe the use of structural insight to design and develop a selective and highly potent reversibly covalent KLK5 inhibitor from an initial weakly binding fragment.     

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na?/K?-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.