The small membrane protein MgrB regulates PhoQ bifunctionality to control PhoP target gene expression dynamics.

In Escherichia coli and other γ-proteobacteria, the PhoQ-PhoP two-component signaling system responds to low extracellular Mg++ and cationic antimicrobial peptides. On transition to inducing conditions, the expression of PhoP-dependent genes increases rapidly, but then decays to a new, intermediate steady-state level, a phenomenon often referred to as partial adaptation. The molecular basis for this partial adaptation has been unclear. Here, using time-lapse fluorescence microscopy to examine PhoP-dependent gene expression in individual E. coli cells we show that partial adaptation arises through a negative feedback loop involving the small protein MgrB. When E. coli cells are shifted to low Mg++ , PhoQ engages in multiple rounds of autophosphorylation and phosphotransfer to PhoP, which, in turn, drives the expression of mgrB. MgrB then feeds back to inhibit the kinase activity of PhoQ. PhoQ is bifunctional such that, when not active as a kinase, it can stimulate the dephosphorylation of PhoP. Thus, MgrB drives the inactivation of PhoP and the observed adaptation in PhoP-dependent gene expression. Our results clarify the source of feedback inhibition in the E. coli PhoQ-PhoP system and reveal how exogenous factors, such as MgrB, can combine with a canonical two-component signaling pathway to produce complex temporal dynamics in target gene expression.

Identification and experimental validation of splicing regulatory elements in Drosophila melanogaster reveals functionally conserved splicing enhancers in metazoans.

RNA sequence elements involved in the regulation of pre-mRNA splicing have previously been identified in vertebrate genomes by computational methods. Here, we apply such approaches to predict splicing regulatory elements in Drosophila melanogaster and compare them with elements previously found in the human, mouse, and pufferfish genomes. We identified 99 putative exonic splicing enhancers (ESEs) and 231 putative intronic splicing enhancers (ISEs) enriched near weak 5' and 3' splice sites of constitutively spliced introns, distinguishing between those found near short and long introns. We found that a significant proportion (58%) of fly enhancer sequences were previously reported in at least one of the vertebrates. Furthermore, 20% of putative fly ESEs were previously identified as ESEs in human, mouse, and pufferfish; while only two fly ISEs, CTCTCT and TTATAA, were identified as ISEs in all three vertebrate species. Several putative enhancer sequences are similar to characterized binding-site motifs for Drosophila and mammalian splicing regulators. To provide additional evidence for the function of putative ISEs, we separately identified 298 intronic hexamers significantly enriched within sequences phylogenetically conserved among 15 insect species. We found that 73 putative ISEs were among those enriched in conserved regions of the D. melanogaster genome. The functions of nine enhancer sequences were verified in a heterologous splicing reporter, demonstrating that these sequences are sufficient to enhance splicing in vivo. Taken together, these data identify a set of predicted positive-acting splicing regulatory motifs in the Drosophila genome and reveal regulatory sequences that are present in distant metazoan genomes.

Differential and shared genetic effects on kidney function between diabetic and non-diabetic individuals.

Reduced glomerular filtration rate (GFR) can progress to kidney failure. Risk factors include genetics and diabetes mellitus (DM), but little is known about their interaction. We conducted genome-wide association meta-analyses for estimated GFR based on serum creatinine (eGFR), separately for individuals with or without DM (nDM = 178,691, nnoDM = 1,296,113). Our genome-wide searches identified (i) seven eGFR loci with significant DM/noDM-difference, (ii) four additional novel loci with suggestive difference and (iii) 28 further novel loci (including CUBN) by allowing for potential difference. GWAS on eGFR among DM individuals identified 2 known and 27 potentially responsible loci for diabetic kidney disease. Gene prioritization highlighted 18 genes that may inform reno-protective drug development. We highlight the existence of DM-only and noDM-only effects, which can inform about the target group, if respective genes are advanced as drug targets. Largely shared effects suggest that most drug interventions to alter eGFR should be effective in DM and noDM.

Genome-Wide Association for HbA1c in Malay Identified Deletion on SLC4A1 that Influences HbA1c Independent of Glycemia.

CONTEXT: Glycated hemoglobin A1c (HbA1c) level is used to screen and diagnose diabetes. Genetic determinants of HbA1c can vary across populations and many of the genetic variants influencing HbA1c level were specific to populations. OBJECTIVE: To discover genetic variants associated with HbA1c level in nondiabetic Malay individuals. DESIGN AND PARTICIPANTS: We conducted a genome-wide association study (GWAS) analysis for HbA1c using 2 Malay studies, the Singapore Malay Eye Study (SiMES, N = 1721 on GWAS array) and the Living Biobank study (N = 983 on GWAS array and whole-exome sequenced). We built a Malay-specific reference panel to impute ethnic-specific variants and validate the associations with HbA1c at ethnic-specific variants. RESULTS: Meta-analysis of the 1000 Genomes imputed array data identified 4 loci at genome-wide significance (P < 5 × 10-8). Of the 4 loci, 3 (ADAM15, LINC02226, JUP) were novel for HbA1c associations. At the previously reported HbA1c locus ATXN7L3-G6PC3, association analysis using the exome data fine-mapped the HbA1c associations to a 27-bp deletion (rs769664228) at SLC4A1 that reduced HbA1c by 0.38 ±â€…0.06% (P = 3.5 × 10-10). Further imputation of this variant in SiMES confirmed the association with HbA1c at SLC4A1. We also showed that these genetic variants influence HbA1c level independent of glucose level. CONCLUSION: We identified a deletion at SLC4A1 associated with HbA1c in Malay. The nonglycemic lowering of HbA1c at rs769664228 might cause individuals carrying this variant to be underdiagnosed for diabetes or prediabetes when HbA1c is used as the only diagnostic test for diabetes.

Discovering metabolic disease gene interactions by correlated effects on cellular morphology.

OBJECTIVE: Impaired expansion of peripheral fat contributes to the pathogenesis of insulin resistance and Type 2 Diabetes (T2D). We aimed to identify novel disease-gene interactions during adipocyte differentiation. METHODS: Genes in disease-associated loci for T2D, adiposity and insulin resistance were ranked according to expression in human adipocytes. The top 125 genes were ablated in human pre-adipocytes via CRISPR/CAS9 and the resulting cellular phenotypes quantified during adipocyte differentiation with high-content microscopy and automated image analysis. Morphometric measurements were extracted from all images and used to construct morphologic profiles for each gene. RESULTS: Over 107 morphometric measurements were obtained. Clustering of the morphologic profiles accross all genes revealed a group of 14 genes characterized by decreased lipid accumulation, and enriched for known lipodystrophy genes. For two lipodystrophy genes, BSCL2 and AGPAT2, sub-clusters with PLIN1 and CEBPA identifed by morphological similarity were validated by independent experiments as novel protein-protein and gene regulatory interactions. CONCLUSIONS: A morphometric approach in adipocytes can resolve multiple cellular mechanisms for metabolic disease loci; this approach enables mechanistic interrogation of the hundreds of metabolic disease loci whose function still remains unknown.

Protein evolution. Pervasive degeneracy and epistasis in a protein-protein interface.

Mapping protein sequence space is a difficult problem that necessitates the analysis of 20(N) combinations for sequences of length N. We systematically mapped the sequence space of four key residues in the Escherichia coli protein kinase PhoQ that drive recognition of its substrate PhoP. We generated a library containing all 160,000 variants of PhoQ at these positions and used a two-step selection coupled to next-generation sequencing to identify 1659 functional variants. Our results reveal extensive degeneracy in the PhoQ-PhoP interface and epistasis, with the effect of individual substitutions often highly dependent on context. Together, epistasis and the genetic code create a pattern of connectivity of functional variants in sequence space that likely constrains PhoQ evolution. Consequently, the diversity of PhoQ orthologs is substantially lower than that of functional PhoQ variants.

Determinants of specificity in two-component signal transduction.

Maintaining the faithful flow of information through signal transduction pathways is critical to the survival and proliferation of organisms. This problem is particularly challenging as many signaling proteins are part of large, paralogous families that are highly similar at the sequence and structural levels, increasing the risk of unwanted cross-talk. To detect environmental signals and process information, bacteria rely heavily on two-component signaling systems comprised of sensor histidine kinases and their cognate response regulators. Although most species encode dozens of these signaling pathways, there is relatively little cross-talk, indicating that individual pathways are well insulated and highly specific. Here, we review the molecular mechanisms that enforce this specificity. Further, we highlight recent studies that have revealed how these mechanisms evolve to accommodate the introduction of new pathways by gene duplication.

Structural basis of a rationally rewired protein-protein interface critical to bacterial signaling.

Two-component signal transduction systems typically involve a sensor histidine kinase that specifically phosphorylates a single, cognate response regulator. This protein-protein interaction relies on molecular recognition via a small set of residues in each protein. To better understand how these residues determine the specificity of kinase-substrate interactions, we rationally rewired the interaction interface of a Thermotoga maritima two-component system, HK853-RR468, to match that found in a different two-component system, Escherichia coli PhoR-PhoB. The rewired proteins interacted robustly with each other, but no longer interacted with the parent proteins. Analysis of the crystal structures of the wild-type and mutant protein complexes and a systematic mutagenesis study reveal how individual mutations contribute to the rewiring of interaction specificity. Our approach and conclusions have implications for studies of other protein-protein interactions and protein evolution and for the design of novel protein interfaces.