Pappa2 is linked to salt-sensitive hypertension in Dahl S rats

Spontaneous vascular dysfunction in Dahl salt-sensitive male rats raised without a high-salt diet

Dahl salt-sensitive (SS) rats fed a high-salt diet, but not low-salt, exhibit vascular dysfunction. Several substrains of SS rats exist that differ in their blood pressure phenotypes and salt sensitivity. The goal of this study was to investigate whether the John-Rapp-derived SS rat (SS/Jr), which exhibits spontaneous hypertension on a low-salt diet, presents with hallmarks of vascular dysfunction observed in another experimental model of hypertension independent of dietary salt, the spontaneously hypertensive rat (SHR). Endothelium-intact aortic rings and mesenteric resistance arteries were isolated from low-salt fed adult male SS/Jr rats and SHRs, or their respective controls, for isometric wire myography. Vessels were challenged with cumulative concentrations of various vasoactive substances, in the absence or presence of nitric oxide synthase or cyclooxygenase inhibitors. Despite showing some differences in their responses to various vasoactive substances, both SS/Jr rats and SHRs exhibited key features of vascular dysfunction, including endothelial dysfunction and hyperresponsiveness to vasocontractile agonists. In conclusion, this study provides evidence to support the utility of the SS/Jr rat strain maintained on a low-salt diet as a valid experimental model for vascular dysfunction, a key feature of human hypertension.

Transcriptomic, epigenomic, and spatial metabolomic cell profiling redefines regional human kidney anatomy

A large-scale multimodal atlas that includes major kidney regions is lacking. Here, we employed simultaneous high-throughput single-cell ATAC/RNA sequencing (SHARE-seq) and spatially resolved metabolomics to profile 54 human samples from distinct kidney anatomical regions. We generated transcriptomes of 446,267 cells and chromatin accessibility profiles of 401,875 cells and developed a package to analyze 408,218 spatially resolved metabolomes. We find that the same cell type, including thin limb, thick ascending limb loop of Henle and principal cells, display distinct transcriptomic, chromatin accessibility, and metabolomic signatures, depending on anatomic location. Surveying metabolism-associated gene profiles revealed non-overlapping metabolic signatures between nephron segments and dysregulated lipid metabolism in diseased proximal tubule (PT) cells. Integrating multimodal omics with clinical data identified PLEKHA1 as a disease marker, and its in vitro knockdown increased gene expression in PT differentiation, suggesting possible pathogenic roles. This study highlights previously underrepresented cellular heterogeneity underlying the human kidney anatomy.

Physiological Mechanisms of Dietary Salt Sensing in the Brain, Kidney, and Gastrointestinal Tract

Excess dietary salt (NaCl) intake is strongly correlated with cardiovascular disease and is a major contributing factor to the pathogenesis of hypertension. NaCl-sensitive hypertension is a multisystem disorder that involves renal dysfunction, vascular abnormalities, and neurogenically-mediated increases in peripheral resistance. Despite a major research focus on organ systems and these effector mechanisms causing NaCl-induced increases in arterial blood pressure, relatively less research has been directed at elucidating how NaCl is sensed by various tissues to elicit these downstream effects. The purpose of this review is to discuss how the brain, kidney, and gastrointestinal tract sense NaCl including key cell types, the role of NaCl versus osmolality, and the underlying molecular and electrochemical mechanisms.

Multi-Omic Approaches to Identify Genetic Factors in Metabolic Syndrome

Metabolic syndrome (MetS) is a highly heritable disease and a major public health burden worldwide. MetS diagnosis criteria are met by the simultaneous presence of any three of the following: high triglycerides, low HDL/high LDL cholesterol, insulin resistance, hypertension, and central obesity. These diseases act synergistically in people suffering from MetS and dramatically increase risk of morbidity and mortality due to stroke and cardiovascular disease, as well as certain cancers. Each of these component features is itself a complex disease, as is MetS. As a genetically complex disease, genetic risk factors for MetS are numerous, but not very powerful individually, often requiring specific environmental stressors for the disease to manifest. When taken together, all sequence variants that contribute to MetS disease risk explain only a fraction of the heritable variance, suggesting additional, novel loci have yet to be discovered. In this article, we will give a brief overview on the genetic concepts needed to interpret genome-wide association studies (GWAS) and quantitative trait locus (QTL) data, summarize the state of the field of MetS physiological genomics, and to introduce tools and resources that can be used by the physiologist to integrate genomics into their own research on MetS and any of its component features. There is a wealth of phenotypic and molecular data in animal models and humans that can be leveraged as outlined in this article. Integrating these multi-omic QTL data for complex diseases such as MetS provides a means to unravel the pathways and mechanisms leading to complex disease and promise for novel treatments. © 2022 American Physiological Society. Compr Physiol 12:1-40, 2022.

A new view of macula densa cell protein synthesis

Macula densa (MD) cells, a chief sensory cell type in the nephron, are endowed with unique microanatomic features including a high density of protein synthetic organelles and secretory vesicles in basal cell processes ("maculapodia") that suggest a so far unknown high rate of MD protein synthesis. This study aimed to explore the rate and regulation of MD protein synthesis and their effects on glomerular function using novel transgenic mouse models, newly established fluorescence cell biology techniques, and intravital microscopy. Sox2-tdTomato kidney tissue sections and an O-propargyl puromycin incorporation-based fluorescence imaging assay showed that MD cells have the highest level of protein synthesis within the kidney cortex followed by intercalated cells and podocytes. Genetic gain of function of mammalian target of rapamycin (mTOR) signaling specifically in MD cells (in MD-mTORgof mice) or their physiological activation by low-salt diet resulted in further significant increases in the synthesis of MD proteins. Specifically, these included both classic and recently identified MD-specific proteins such as cyclooxygenase 2, microsomal prostaglandin E2 synthase 1, and pappalysin 2. Intravital imaging of the kidney using multiphoton microscopy showed significant increases in afferent and efferent arteriole and glomerular capillary diameters and blood flow in MD-mTORgof mice coupled with an elevated glomerular filtration rate. The presently identified high rate of MD protein synthesis that is regulated by mTOR signaling is a novel component of the physiological activation and glomerular hemodynamic regulatory functions of MD cells that remains to be fully characterized.NEW & NOTEWORTHY This study discovered the high rate of protein synthesis in macula densa (MD) cells by applying direct imaging techniques with single cell resolution. Physiological activation and mammalian target of rapamycin signaling played important regulatory roles in this process. This new feature is a novel component of the tubuloglomerular cross talk and glomerular hemodynamic regulatory functions of MD cells. Future work is needed to elucidate the nature and (patho)physiological role of the specific proteins synthesized by MD cells.

Spatial transcriptional mapping of the human nephrogenic program

Congenital abnormalities of the kidney and urinary tract are among the most common birth defects, affecting 3% of newborns. The human kidney forms around a million nephrons from a pool of nephron progenitors over a 30-week period of development. To establish a framework for human nephrogenesis, we spatially resolved a stereotypical process by which equipotent nephron progenitors generate a nephron anlage, then applied data-driven approaches to construct three-dimensional protein maps on anatomical models of the nephrogenic program. Single-cell RNA sequencing identified progenitor states, which were spatially mapped to the nephron anatomy, enabling the generation of functional gene networks predicting interactions within and between nephron cell types. Network mining identified known developmental disease genes and predicted targets of interest. The spatially resolved nephrogenic program made available through the Human Nephrogenesis Atlas (https://sckidney.flatironinstitute.org/) will facilitate an understanding of kidney development and disease and enhance efforts to generate new kidney structures.

Abnormal neonatal sodium handling in skin precedes hypertension in the SAME rat

We discovered high Na⁺ and water content in the skin of newborn Sprague–Dawley rats, which reduced ~ 2.5-fold by 7 days of age, indicating rapid changes in extracellular volume (ECV). Equivalent changes in ECV post birth were also observed in C57Bl/6 J mice, with a fourfold reduction over 7 days, to approximately adult levels. This established the generality of increased ECV at birth. We investigated early sodium and water handling in neonates from a second rat strain, Fischer, and an Hsd11b2-knockout rat modelling the syndrome of apparent mineralocorticoid excess (SAME). Despite Hsd11b2^−/− animals exhibiting lower skin Na⁺ and water levels than controls at birth, they retained ~ 30% higher Na⁺ content in their pelts at the expense of K⁺ thereafter. Hsd11b2^−/− neonates exhibited incipient hypokalaemia from 15 days of age and became increasingly polydipsic and polyuric from weaning. As with adults, they excreted a high proportion of ingested Na⁺ through the kidney, (56.15 ± 8.21% versus control 34.15 ± 8.23%; n = 4; P < 0.0001), suggesting that changes in nephron electrolyte transporters identified in adults, by RNA-seq analysis, occur by 4 weeks of age. Our data reveal that Na⁺ imbalance in the Hsd11b2^−/− neonate leads to excess Na⁺ storage in skin and incipient hypokalaemia, which, together with increased, glucocorticoid-induced Na⁺ uptake in the kidney, then contribute to progressive, volume contracted, salt-sensitive hypertension. Skin Na⁺ plays an important role in the development of SAME but, equally, may play a key physiological role at birth, supporting post-natal growth, as an innate barrier to infection or as a rudimentary kidney.

Targeted Single-Cell RNA-seq Identifies Minority Cell Types of Kidney Distal Nephron

BACKGROUND

Proximal tubule cells dominate the kidney parenchyma numerically, although less abundant cell types of the distal nephron have disproportionate roles in water and electrolyte balance.

METHODS

Coupling of a FACS-based enrichment protocol with single-cell RNA-seq profiled the transcriptomes of 9099 cells from the thick ascending limb (CTAL)/distal convoluted tubule (DCT) region of the mouse nephron.

RESULTS

Unsupervised clustering revealed Slc12a3 ⁺/Pvalb ⁺ and Slc12a3 ⁺/Pvalb ^- cells, identified as DCT1 and DCT2 cells, respectively. DCT1 cells appear to be heterogeneous, with orthogonally variable expression of Slc8a1, Calb1, and Ckb. An additional DCT1 subcluster showed marked enrichment of cell cycle-/cell proliferation-associated mRNAs (e.g., Mki67, Stmn1, and Top2a), which fit with the known plasticity of DCT cells. No DCT2-specific transcripts were found. DCT2 cells contrast with DCT1 cells by expression of epithelial sodium channel β- and γ-subunits and much stronger expression of transcripts associated with calcium transport (Trpv5, Calb1, S100g, and Slc8a1). Additionally, scRNA-seq identified three distinct CTAL (Slc12a1 ⁺) cell subtypes. One of these expressed Nos1 and Avpr1a, consistent with macula densa cells. The other two CTAL clusters were distinguished by Cldn10 and Ptger3 in one and Cldn16 and Foxq1 in the other. These two CTAL cell types were also distinguished by expression of alternative Iroquois homeobox transcription factors, with Irx1 and Irx2 in the Cldn10 ⁺ CTAL cells and Irx3 in the Cldn16 ⁺ CTAL cells.

CONCLUSIONS

Single-cell transcriptomics revealed unexpected diversity among the cells of the distal nephron in mouse. Web-based data resources are provided for the single-cell data.

A new view of macula densa cell microanatomy

Although macula densa (MD) cells are chief regulatory cells in the nephron with unique microanatomical features, they have been difficult to study in full detail due to their inaccessibility and limitations in earlier microscopy techniques. The present study used a new mouse model with a comprehensive imaging approach to visualize so far unexplored microanatomical features of MD cells, their regulation, and functional relevance. MD-GFP mice with conditional and partial induction of green fluorescent protein (GFP) expression, which specifically and intensely illuminated only single MD cells, were used with fluorescence microscopy of fixed tissue and live MD cells in vitro and in vivo with complementary electron microscopy of the rat, rabbit, and human kidney. An elaborate network of major and minor cell processes, here named maculapodia, were found at the cell base, projecting toward other MD cells and the glomerular vascular pole. The extent of maculapodia showed upregulation by low dietary salt intake and the female sex. Time-lapse imaging of maculapodia revealed highly dynamic features including rapid outgrowth and an extensive vesicular transport system. Electron microscopy of rat, rabbit, and human kidneys and three-dimensional volume reconstruction in optically cleared whole-mount MD-GFP mouse kidneys further confirmed the presence and projections of maculapodia into the extraglomerular mesangium and afferent and efferent arterioles. The newly identified dynamic and secretory features of MD cells suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD cells and between MD and other target cells.NEW & NOTEWORTHY This study illuminated a physiologically regulated dense network of basal cell major and minor processes (maculapodia) in macula densa (MD) cells. The newly identified dynamic and secretory features of these microanatomical structures suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD and other target cells. Detailed characterization of the function and molecular details of MD cell intercellular communications and their role in physiology and disease warrant further studies.

Genetic susceptibility of hypertension-induced kidney disease

Hypertension is the second leading cause of end-stage renal disease (ESRD) after diabetes mellitus. The significant differences in the incidence of hypertensive ESRD between different patient populations worldwide and patients with and without family history indicate that genetic determinants play an important role in the onset and progression of this disease. Recent studies have identified genetic variants and pathways that may contribute to the alteration of renal function. Mechanisms involved include affecting renal hemodynamics (the myogenic and tubuloglomerular feedback responses); increasing the production of reactive oxygen species in the tubules; altering immune cell function; changing the number, structure, and function of podocytes that directly cause glomerular damage. Studies with hypertensive animal models using substitution mapping and gene knockout strategies have identified multiple candidate genes associated with the development of hypertension and subsequent renal injury. Genome-wide association studies have implicated genetic variants in UMOD, MYH9, APOL-1, SHROOM3, RAB38, and DAB2 have a higher risk for ESRD in hypertensive patients. These findings provide genetic evidence of potential novel targets for drug development and gene therapy to design individualized treatment of hypertension and related renal injury.

Rat models of human diseases and related phenotypes: a systematic inventory of the causative genes

The laboratory rat has been used for a long time as the model of choice in several biomedical disciplines. Numerous inbred strains have been isolated, displaying a wide range of phenotypes and providing many models of human traits and diseases. Rat genome mapping and genomics was considerably developed in the last decades. The availability of these resources has stimulated numerous studies aimed at discovering causal disease genes by positional identification. Numerous rat genes have now been identified that underlie monogenic or complex diseases and remarkably, these results have been translated to the human in a significant proportion of cases, leading to the identification of novel human disease susceptibility genes, helping in studying the mechanisms underlying the pathological abnormalities and also suggesting new therapeutic approaches. In addition, reverse genetic tools have been developed. Several genome-editing methods were introduced to generate targeted mutations in genes the function of which could be clarified in this manner [generally these are knockout mutations]. Furthermore, even when the human gene causing a disease had been identified without resorting to a rat model, mutated rat strains (in particular KO strains) were created to analyze the gene function and the disease pathogenesis. Today, over 350 rat genes have been identified as underlying diseases or playing a key role in critical biological processes that are altered in diseases, thereby providing a rich resource of disease models. This article is an update of the progress made in this research and provides the reader with an inventory of these disease genes, a significant number of which have similar effects in rat and humans.

NOX4/H2O2/mTORC1 Pathway in Salt-Induced Hypertension and Kidney Injury

We have reported that a high-salt (4.0% NaCl) dietary intake activates mTORC1 and inhibition of this pathway with rapamycin blunts the chronic phase of salt-induced hypertension and renal injury in Dahl salt-sensitive (SS) rats. In SS rats, high-salt intake is known to increase the renal production of H2O2 by NOX4, the most abundant NOX isoform in the kidney, and the global knockout of NOX4 blunts salt-sensitivity in these rats. Here, we explored the hypothesis that elevations of H2O2 by NOX4 in high-salt fed SS rat stimulate mTORC1 for the full development of salt-induced hypertension and renal injury. Our in vitro studies found that H2O2 activates mTORC1 independent of PI3K/AKT and AMPK pathways. To determine the in vivo relevance of NOX4/H2O2/mTORC1 in the salt-induced hypertension, SS-Nox4 knockout (SSNox4-/-) rats were daily administrated with vehicle/rapamycin fed a high-salt diet for 21 days. Rapamycin treatment of SSNox4-/- rats had shown no augmented effect on the salt-induced hypertension nor upon indices of renal injury. Significant reductions of renal T lymphocyte and macrophage together with inhibition of cell proliferation were observed in rapamycin treated rats suggesting a role of mTORC1 independent of NOX4 in the proliferation of immune cell. Given the direct activation of mTORC1 by H2O2 and absence of any further protection from salt-induced hypertension in rapamycin-treated SSNox4-/- rats, we conclude that NOX4-H2O2 is a major upstream activator of mTORC1 that contributes importantly to salt-induced hypertension and renal injury in the SS rat model.

Chromosomal Substitution Strategies to Localize Genomic Regions Related to Complex Traits

Chromosomal substitution strategies provide a powerful tool to anonymously reveal the relationship between DNA sequence variants and a normal or disease phenotype of interest. Even in this age of CRISPR-Cas9 genome engineering, the knockdown or overexpression of a gene provides relevant information to our understanding of complex disease only when a close association of an allelic variant with the phenotype has first been established. Limitations of genetic linkage approaches led to the development of more efficient breeding strategies to substitute chromosomal segments from one animal strain into the genetic background of a different strain, enabling a direct comparison of the phenotypes of the strains with variant(s) that differ only at a defined locus. This substitution can be a whole chromosome (consomic), a part of a chromosome (congenic), or as small as only a single or several alleles (subcongenics). In contrast to complete knockout of a specific candidate gene of interest, which simply studies the effects of complete elimination of the gene, the substitution of naturally occurring variants can provide special insights into the functional actions of wild-type alleles. Strategies for production of these inbred strains are reviewed, and a number of examples are used to illustrate the utility of these model systems. Consomic/congenic strains provide a number of experimental advantages in the study of functions of genes and their variants, which are emphasized in this article, such as replication of experimental studies; determination of temporal relationships throughout a life; rigorously controlled experiments in which relations between genotype and phenotype can be tested with the confounding effects of heterogeneous genetic backgrounds, both targeted and multilayered; and "omic" studies performed at many levels of functionality, from molecules to organelles, cells to organs, and organs to organismal behavior across the life span. The application of chromosomal substitution strategies and development of consomic/congenic rat and mouse strains have greatly expanded our knowledge of genomic variants and their phenotypic relationship to physiological functions and to complex diseases such as hypertension and cancer. © 2020 American Physiological Society. Compr Physiol 10:365-388, 2020.

Temporal Expression and Cellular Localization of PAPPA2 in the Developing Kidney of Rat

PAPPA2 is a metalloproteinase which cleaves insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5, and its role in pregnancy and postnatal growth is primarily studied. Using exclusion mapping, we reported a subcongenic (26-P) rat where a 0.71-Mbp region containing the pregnancy-associated plasma protein a2 (Pappa2) allele of salt-insensitive Brown Norway (BN) was introgressed into Dahl saltsensitive (SS) genetic background, resulting in the reduction of salt sensitivity. Pappa2 was differentially expressed in the adult kidney of 26-P and SS rats. Here, the expression and cellular localization of Pappa2 in embryonic and postnatal kidneys of 26-P and SS rats were examined. Pappa2 mRNA expression was 5-fold higher in the embryonic kidney (day 20.5) of the 26-P rat compared with the SS rat. Pappa2 mRNA expression progressively increased with the development of kidney, reaching a peak at postnatal day 5 before trending downward in subsequent stages of development in both strains. At all tested time points, Pappa2 remained higher in the 26-P compared with the SS rat kidney. Immunohistochemistry studies localized PAPPA2 in the ureteric bud (UB) and distal part of S-shaped body. PAPPA2 was colocalized with IGFBP-5 in the UB and Na⁺/K⁺/2Cl^- cotransporter-stained tubules, respectively. Future studies are needed to determine the role of Pappa2 in kidney development and mechanistic pathways involved in this process.

Dissecting Epistatic QTL for Blood Pressure in Rats: Congenic Strains versus Heterogeneous Stocks, a Reality Check

Advances in molecular genetics have provided well-defined physical genetic maps and large numbers of genetic markers for both model organisms and humans. It is now possible to gain a fundamental understanding of the genetic architecture underlying quantitative traits, of which blood pressure (BP) is an important example. This review emphasizes analytical techniques and results obtained using the Dahl salt-sensitive (S) rat as a model of hypertension by presenting results in detail for three specific chromosomal regions harboring genetic elements of increasing complexity controlling BP. These results highlight the critical importance of genetic interactions (epistasis) on BP at all levels of structure, intragenic, intergenic, intrachromosomal, interchromosomal, and across whole genomes. In two of the three examples presented, specific DNA structural variations leading to biochemical, physiological, and pathological mechanisms are well defined. This proves the usefulness of the techniques involving interval mapping followed by substitution mapping using congenic strains. These classic techniques are compared to newer approaches using sophisticated statistical analysis on various segregating or outbred model-organism populations, which in some cases are uniquely useful in demonstrating the existence of higher-order interactions. It is speculated that hypertension as an outlier quantitative phenotype is dependent on higher-order genetic interactions. The obstacle to the identification of genetic elements and the biochemical/physiological mechanisms involved in higher-order interactions is not theoretical or technical but the lack of future resources to finish the job of identifying the individual genetic elements underlying the quantitative trait loci for BP and ascertaining their molecular functions. © 2019 American Physiological Society. Compr Physiol 9:1305-1337, 2019.

QTL mapping of rat blood pressure loci on RNO1 within a homologous region linked to human hypertension on HSA15

Fine-mapping of regions linked to the inheritance of hypertension is accomplished by genetic dissection of blood pressure quantitative trait loci (BP QTLs) in rats. The goal of the current study was to further fine-map two genomic regions on rat chromosome 1 with opposing blood pressure effects (BP QTL1b1 and BP QTL1b1a), the homologous region of which on human chromosome 15 harbors BP QTLs. Two new substrains were constructed and studied from the previously reported BP QTL1b1, one having significantly lower systolic BP by 17 mmHg than that of the salt-sensitive (S) rat (P = 0.007). The new limits of BP QTL1b1 were between 134.09 Mb and 135.40 Mb with a 43% improvement from the previous 2.31 Mb to the current 1.31 Mb interval containing 4 protein-coding genes (Rgma, Chd2, Fam174b, and St8sia2), 2 predicted miRNAs, and 4 lncRNAs. One new substrain was constructed and studied from the previously reported BPQTL1b1a having a significantly higher systolic BP by 22 mmHg (P = 0.006) than that of the S rat. The new limits of BPQTL1b1a were between 133.53 Mb and 134.52 Mb with a 32% improvement from the previous1.45 Mb to the current 990.21 Kb interval containing 1 protein-coding gene, Mctp2, and a lncRNA. The congenic segments of these two BP QTLs overlapped between 134.09 Mb and 134.52 Mb. No exonic variants were detected in any of the genes. These findings reiterate complexity of genetic regulation of BP within QTL regions, where elements beyond protein-coding sequences could be factors in controlling BP.

Combined linkage and association analysis identifies rare and low frequency variants for blood pressure at 1q31

High blood pressure (BP) is a major risk factor for cardiovascular disease (CVD) and is more prevalent in African Americans as compared to other US groups. Although large, population-based genome-wide association studies (GWAS) have identified over 300 common polymorphisms modulating inter-individual BP variation, largely in European ancestry subjects, most of them do not localize to regions previously identified through family-based linkage studies. This discrepancy has remained unexplained despite the statistical power differences between current GWAS and prior linkage studies. To address this issue, we performed genome-wide linkage analysis of BP traits in African-American families from the Family Blood Pressure Program (FBPP) and genotyped on the Illumina Human Exome BeadChip v1.1. We identified a genomic region on chromosome 1q31 with LOD score 3.8 for pulse pressure (PP), a region we previously implicated in DBP studies of European ancestry families. Although no reported GWAS variants map to this region, combined linkage and association analysis of PP identified 81 rare and low frequency exonic variants accounting for the linkage evidence. Replication analysis in eight independent African ancestry cohorts (N = 16,968) supports this specific association with PP (P = 0.0509). Additional association and network analyses identified multiple potential candidate genes in this region expressed in multiple tissues and with a strong biological support for a role in BP. In conclusion, multiple genes and rare variants on 1q31 contribute to PP variation. Beyond producing new insights into PP, we demonstrate how family-based linkage and association studies can implicate specific rare and low frequency variants for complex traits.

Brief Overview of a Decade of Genome-Wide Association Studies on Primary Hypertension

Primary hypertension is widely believed to be a complex polygenic disorder with the manifestation influenced by the interactions of genomic and environmental factors making identification of susceptibility genes a major challenge. With major advancement in high-throughput genotyping technology, genome-wide association study (GWAS) has become a powerful tool for researchers studying genetically complex diseases. GWASs work through revealing links between DNA sequence variation and a disease or trait with biomedical importance. The human genome is a very long DNA sequence which consists of billions of nucleotides arranged in a unique way. A single base-pair change in the DNA sequence is known as a single nucleotide polymorphism (SNP). With the help of modern genotyping techniques such as chip-based genotyping arrays, thousands of SNPs can be genotyped easily. Large-scale GWASs, in which more than half a million of common SNPs are genotyped and analyzed for disease association in hundreds of thousands of cases and controls, have been broadly successful in identifying SNPs associated with heart diseases, diabetes, autoimmune diseases, and psychiatric disorders. It is however still debatable whether GWAS is the best approach for hypertension. The following is a brief overview on the outcomes of a decade of GWASs on primary hypertension.

Chronic cathepsin inhibition by E-64 in Dahl salt-sensitive rats

Cysteine cathepsins are lysosomal enzymes expressed in the kidneys and other tissues, and are involved in the maturation and breakdown of cellular proteins. They have been shown to be integrally involved in the progression of many cardiovascular and renal diseases. The goal of this study was to determine the involvement of cysteine cathepsins in the development of salt‐sensitive hypertension and associated kidney damage. In our experiments, Dahl salt‐sensitive (SS) rats were fed an 8% high salt NaCl diet and intravenously infused with the irreversible cysteine cathepsin inhibitor E‐64 (1 mg/day) or the vehicle (control). Both the control and E‐64 infused groups developed significant hypertension and kidney damage, and no difference of the mean arterial pressure and the hypertension‐associated albuminuria was observed between the groups. We next tested basal calcium levels in the podocytes of both control and infused groups using confocal calcium imaging. Basal calcium did not differ between the groups, indicative of the lack of a protective or aggravating influence by the cathepsin inhibition. The efficacy of E‐64 was tested in Western blotting. Our findings corresponded to the previously reported, E‐64 induced increase in cathepsin B and L abundance. We conclude that the inhibition of cysteine cathepsins by E‐64 does not have any effects on the blood pressure development and kidney damage, at least under the studied conditions of this model of SS hypertension.

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Positional cloning of quantitative trait nucleotides for blood pressure and cardiac QT-interval by targeted CRISPR/Cas9 editing of a novel long non-coding RNA

Multiple GWAS studies have reported strong association of cardiac QT-interval to a region on HSA17. Interestingly, a rat locus homologous to this region is also linked to QT-intervals. The high resolution positional mapping study located the rat QT-interval locus to a <42.5kb region on RNO10. This region contained no variants in protein-coding sequences, but a prominent contiguous 19bp indel polymorphism was noted within a novel predicted long non-coding RNA (lncRNA), which we named as Rffl-lnc1. To assess the candidacy of this novel lncRNA on QT-interval, targeted CRISPR/Cas9 based genome-engineering approaches were applied on the rat strains used to map this locus. Targeted disruption of the rat Rffl-lnc1 locus caused aberrant, short QT-intervals and elevated blood pressure. Further, to specifically examine the significance of the 19bp polymorphism within the Rffl-lnc1 locus, a CRISPR/Cas9 based targeted knock-in rescue model was constructed by inserting the 19bp into the strain which contained the deletion polymorphism. The knock-in alleles successfully rescued the aberrant QT-interval and blood pressure phenotypes. Further studies revealed that the 19bp polymorphism was necessary and sufficient to recapitulate the phenotypic effect of the previously mapped <42.5kb rat locus. To our knowledge, this study is the first demonstration of a combination of both CRISPR/Cas9 based targeted disruption as well as CRISPR/Cas9 based targeted knock-in rescue approaches applied for a mammalian positional cloning study, which defines the quantitative trait nucleotides (QTNs) within a rat long non-coding RNA as being important for the pleiotropic regulation of both cardiac QT-intervals and blood pressure.

Diseases of the cardiovascular system such as essential hypertension do not have a clear cause, but are known to run in families. The inheritance patterns of essential hypertension and other cardiac diseases suggest that they are not due to a single defective gene but instead are caused by multiple genetic defects that are inherited together in a patient. This complex inheritance makes it difficult to pinpoint the underlying defects. Here, we describe a panel of genetically-engineered rats, using which we have discovered a novel gene, which does not code for any protein, as a gene required for maintenance of normal blood pressure. Structural defects within this non-coding RNA cause hypertension and cardiac short-QT interval. Further, by performing genome surgery to correct the gene defect, we demonstrate the precise error in nucleotides that was inherited and caused hypertension and cardiac short-QT interval syndrome.

Inhibition of Mammalian Target of Rapamycin Complex 1 Attenuates Salt-Induced Hypertension and Kidney Injury in Dahl Salt-Sensitive Rats

The goal of the present study was to explore the protective effects of mTORC1 (mammalian target of rapamycin complex 1) inhibition by rapamycin on salt-induced hypertension and kidney injury in Dahl salt-sensitive (SS) rats. We have previously demonstrated that H₂O₂ is elevated in the kidneys of SS rats. The present study showed a significant upregulation of renal mTORC1 activity in the SS rats fed a 4.0% NaCl for 3 days. In addition, renal interstitial infusion of H₂O₂ into salt-resistant Sprague Dawley rats for 3 days was also found to stimulate mTORC1 activity independent of a rise of arterial blood pressure. Together, these data indicate that the salt-induced increases of renal H₂O₂ in SS rats activated the mTORC1 pathway. Daily administration of rapamycin (IP, 1.5 mg/kg per day) for 21 days reduced salt-induced hypertension from 176.0±9.0 to 153.0±12.0 mm Hg in SS rats but had no effect on blood pressure salt sensitivity in Sprague Dawley treated rats. Compared with vehicle, rapamycin reduced albumin excretion rate in SS rats from 190.0±35.0 to 37.0±5.0 mg/d and reduced the renal infiltration of T lymphocytes (CD3⁺) and macrophages (ED1⁺) in the cortex and medulla. Renal hypertrophy and cell proliferation were also reduced in rapamycin-treated SS rats. We conclude that enhancement of intrarenal H₂O₂ with a 4.0% NaCl diet stimulates the mTORC1 pathway that is necessary for the full development of the salt-induced hypertension and kidney injury in the SS rat.

SerpinC1/Antithrombin III in kidney-related diseases

The gene SerpinC1 encodes a serine protease inhibitor named antithrombin III (ATIII). This protease demonstrates both anticoagulant and anti-inflammatory action. ATIII is the most important coagulation factor inhibitor, and even minor changes in ATIII can significantly alter the risk of thromboembolism. ATIII can also suppress inflammation via a coagulation-dependent or -independent effect. Moreover, apart from ATIII deficiency, ATIII and its gene SerpinC1 may also be related to many diseases (e.g. hypertension, kidney diseases). The present review summarizes how ATIII affects the progress of kidney disease and its mechanism. Further studies are required to investigate how ATIII affects renal function and the treatment.

Cleave but not leave: Astrotactin proteins in development and disease

Over the years, animal studies have identified astrotactins as important membrane proteins for glial-guided neuronal migration during central nervous system development and hair follicle polarity control during skin development. Biochemical studies have revealed intramembrane proteolysis as an important feature of astrotactins. The two fragments of astrotactins remain linked together by a disulfide bond after the proteolytic cleavage. In humans, mutations in astrotactin genes have also been linked to a wide range of diseases, including several developmental brain disorders, neurodegenerative diseases and cancer. In this review, I will summarize the current knowledge of the biological function of astrotactins in development, highlight the linkage between mutations in astrotactin genes and human disease and discuss several outstanding questions that remain unanswered. © 2017 IUBMB Life, 69(8):572-577, 2017.

Unique Transcriptional Programs Identify Subtypes of AKI

Two metrics, a rise in serum creatinine concentration and a decrease in urine output, are considered tantamount to the injury of the kidney tubule and the epithelial cells thereof (AKI). Yet neither criterion emphasizes the etiology or the pathogenetic heterogeneity of acute decreases in kidney excretory function. In fact, whether decreased excretory function due to contraction of the extracellular fluid volume (vAKI) or due to intrinsic kidney injury (iAKI) actually share pathogenesis and should be aggregated in the same diagnostic group remains an open question. To examine this possibility, we created mouse models of iAKI and vAKI that induced a similar increase in serum creatinine concentration. Using laser microdissection to isolate specific domains of the kidney, followed by RNA sequencing, we found that thousands of genes responded specifically to iAKI or to vAKI, but very few responded to both stimuli. In fact, the activated gene sets comprised different, functionally unrelated signal transduction pathways and were expressed in different regions of the kidney. Moreover, we identified distinctive gene expression patterns in human urine as potential biomarkers of either iAKI or vAKI, but not both. Hence, iAKI and vAKI are biologically unrelated, suggesting that molecular analysis should clarify our current definitions of acute changes in kidney excretory function.

High-resolution mapping of a novel rat blood pressure locus on chromosome 9 to a region containing the Spp2 gene and colocalization of a QTL for bone mass

Through linkage analysis of the Dahl salt-sensitive (S) rat and the spontaneously hypertensive rat (SHR), a blood pressure (BP) quantitative trait locus (QTL) was previously located on rat chromosome 9. Subsequent substitution mapping studies of this QTL revealed multiple BP QTLs within the originally identified logarithm of odds plot by linkage analysis. The focus of this study was on a 14.39 Mb region, the distal portion of which remained unmapped in our previous studies. High-resolution substitution mapping for a BP QTL in the setting of a high-salt diet indicated that an SHR-derived congenic segment of 787.9 kb containing the gene secreted phosphoprotein-2 (Spp2) lowered BP and urinary protein excretion. A nonsynonymous G/T polymorphism in the Spp2 gene was detected between the S and S.SHR congenic rats. A survey of 45 strains showed that the T allele was rare, being detected only in some substrains of SHR and WKY. Protein modeling prediction through SWISSPROT indicated that the predicted protein product of this variant was significantly altered. Importantly, in addition to improved cardiovascular and renal function, high salt-fed congenic animals carrying the SHR T variant of Spp2 had significantly lower bone mass and altered bone microarchitecture. Total bone volume and volume of trabecular bone, cortical thickness, and degree of mineralization of cortical bone were all significantly reduced in congenic rats. Our study points to opposing effects of a congenic segment containing the prioritized candidate gene Spp2 on BP and bone mass.