scRNA Transcription Profile of Adult Zebrafish Podocytes Using a Novel Reporter Strain

BACKGROUND/AIMS

The role of podocytes is well conserved across species from drosophila to teleosts, and mammals. Identifying the molecular markers that actively maintain the integrity of the podocyte will enable a greater understanding of the changes that lead to damage.

METHODS

We generated transgenic zebrafish, expressing fluorescent reporters driven by the podocin promoter, for the visualization and isolation of podocytes. We have conducted single cell RNA sequencing (scRNA-seq) on isolated podocytes from a zebrafish reporter line.

RESULTS

We demonstrated that the LifeAct-TagRFP-T fluorescent reporter faithfully replicated podocin expression in vivo. We were also able to show spontaneous GCaMP6s fluorescence using light sheet (single plane illumination) microscopy. We identified many podocyte transcripts, encoding proteins related to calcium-binding and actin filament assembly, in common with those expressed in human and mouse mature podocytes.

CONCLUSION

We describe the establishment of novel transgenic zebrafish and their use to identify and isolate podocyte cells for the preparation of a scRNA-seq library from normal podocytes. The scRNA-seq data identifies distinct populations of cells and potential gene switching between clusters. These data provide a foundation for future comparative studies and for exploiting the zebrafish as a model for kidney development, disease, injury and repair.

Cellular plasticity: A mechanism for homeostasis in the kidney

Cellular plasticity is a topical subject with interest spanning a wide range of fields from developmental biology to regenerative medicine. Even the nomenclature is a subject of debate, and the underlying mechanisms are still under investigation. On top of injury repair, cell plasticity is a constant physiological process in adult organisms and tissues, in response to homeostatic challenges. In this review we discuss two examples of plasticity for the maintenance of homeostasis in the renal system-namely the renin-producing juxtaglomerular cells (JG cells) and cortical collecting duct (CCD) cells. JG cells show plasticity through recruitment mechanisms, answering the demand for an increase in renin production. In the CCD, cells appear to have the ability to transdifferentiate between principal and intercalated cells to help maintain the highly regulated solute transport levels of that segment. These two cases highlight the complexity of plasticity processes and the role they can play in the kidney.

mCCDcl1 cells show plasticity consistent with the ability to transition between principal and intercalated cells

The cortical collecting duct of the mammalian kidney plays a critical role in the regulation of body volume, sodium pH, and osmolarity and is composed of two distinct cells types, principal cells and intercalated cells. Each cell type is detectable in the kidney by the localization of specific transport proteins such as aquaporin 2 (Aqp2) and epithelial sodium channel (ENaC) in principal cells and V-ATPase B1 and connexin 30 (Cx30) in intercalated cells. mCCD_cl1 cells have been widely used as a mouse principal cell line on the basis of their physiological characteristics. In this study, the mCCD_cl1 parental cell line and three sublines cloned from isolated single cells (Ed1, Ed2, and Ed3) were grown on filters to assess their transepithelial resistance, transepithelial voltage, equivalent short circuit current and expression of the cell-specific markers Aqp2, ENaC, V-ATPaseB1, and Cx30. The parental mCCD_cl1 cell line presented amiloride-sensitive electrogenic sodium transport indicative of principal cell function; however, immunocytochemistry and RT-PCR showed that some cells expressed the intercalated cell-specific markers V-ATPase B1 and Cx30, including a subset of cells also positive for Aqp2 and ENaC. The three subclonal lines contained cells that were positive for both intercalated and principal cell-specific markers. The vertical transmission of both principal and intercalated cell characteristics via single cell cloning reveals the plasticity of mCCD_cl1 cells and a direct lineage relationship between these two physiologically important cell types and is consistent with mCCD_cl1 cells being precursor cells.

Renal disease pathophysiology and treatment: contributions from the rat

The rat has classically been the species of choice for pharmacological studies and disease modeling, providing a source of high-quality physiological data on cardiovascular and renal pathophysiology over many decades. Recent developments in genome engineering now allow us to capitalize on the wealth of knowledge acquired over the last century. Here, we review rat models of hypertension, diabetic nephropathy, and acute and chronic kidney disease. These models have made important contributions to our understanding of renal diseases and have revealed key genes, such as Ace and P2rx7, involved in renal pathogenic processes. By targeting these genes of interest, researchers are gaining a better understanding of the etiology of renal pathologies, with the promised potential of slowing disease progression or even reversing the damage caused. Some, but not all, of these target genes have proved to be of clinical relevance. However, it is now possible to generate more sophisticated and appropriate disease models in the rat, which can recapitulate key aspects of human renal pathology. These advances will ultimately be used to identify new treatments and therapeutic targets of much greater clinical relevance.

Summary: This Review highlights the key role that the rat continues to play in improving our understanding of the etiologies of renal pathologies, and how these insights have opened up new therapeutic avenues.

Phenotypic dissection of the mouse Ren1d knockout by complementation with human renin

Normal renin synthesis and secretion is important for the maintenance of juxtaglomerular apparatus architecture. Mice lacking a functional Ren1d gene are devoid of renal juxtaglomerular cell granules and exhibit an altered macula densa morphology. Due to the species-specificity of renin activity, transgenic mice are ideal models for experimentally investigating and manipulating expression patterns of the human renin gene in a native cellular environment without confounding renin–angiotensin system interactions. A 55-kb transgene encompassing the human renin locus was crossed onto the mouse Ren1d-null background, restoring granulation in juxtaglomerular cells. Correct processing of human renin in dense core granules was confirmed by immunogold labeling. After stimulation of the renin–angiotensin system, juxtaglomerular cells contained rhomboid protogranules with paracrystalline contents, dilated rough endoplasmic reticulum, and electron-lucent granular structures. However, complementation of Ren1d^−/− mice with human renin was unable to rescue the abnormality seen in macula densa structure. The juxtaglomerular apparatus was still able to respond to tubuloglomerular feedback in isolated perfused juxtaglomerular apparatus preparations, although minor differences in glomerular tuft contractility and macula densa cell calcium handling were observed. This study reveals that the human renin protein is able to complement the mouse Ren1d^−/− non-granulated defect and suggests that granulopoiesis requires a structural motif that is conserved between the mouse Ren1d and human renin proteins. It also suggests that the altered macula densa phenotype is related to the activity of the renin-1d enzyme in a local juxtaglomerular renin–angiotensin system.

Transcription controls growth, cell kinetics and cholesterol supply to sustain ACTH responses

Chronic ACTH exposure is associated with adrenal hypertrophy and steroidogenesis. The underlying molecular processes in mice have been analysed by microarray, histological and immunohistochemical techniques. Synacthen infused for 2 weeks markedly increased adrenal mass and plasma corticosterone levels. Microarray analysis found greater than 2-fold changes in expression of 928 genes (P < 0.001; 397 up, 531 down). These clustered in pathways involved in signalling, sterol/lipid metabolism, cell proliferation/hypertrophy and apoptosis. Signalling genes included some implicated in adrenal adenomas but also upregulated genes associated with cyclic AMP and downregulated genes associated with aldosterone synthesis. Sterol metabolism genes were those promoting cholesterol supply (Scarb1, Sqle, Apoa1) and disposal (Cyp27a1, Cyp7b1). Oil red O staining showed lipid depletion consistent with reduced expression of genes involved in lipid synthesis. Genes involved in steroidogenesis (Star, Cyp11a1, Cyp11b1) were modestly affected (P < 0.05; <1.3-fold). Increased Ki67, Ccna2, Ccnb2 and Tk1 expression complemented immunohistochemical evidence of a 3-fold change in cell proliferation. Growth arrest genes, Cdkn1a and Cdkn1c, which are known to be active in hypertrophied cells, were increased >4-fold and cross-sectional area of fasciculata cells was 2-fold greater. In contrast, genes associated with apoptosis (eg Casp12, Clu,) were downregulated and apoptotic cells (Tunel staining) were fewer (P < 0.001) and more widely distributed throughout the cortex. In summary, long-term steroidogenesis with ACTH excess is sustained by genes controlling cholesterol supply and adrenal mass. ACTH effects on adrenal morphology and genes controlling cell hypertrophy, proliferation and apoptosis suggest the involvement of different cell types and separate molecular pathways.

Zebrafish mesonephric renin cells are functionally conserved and comprise two distinct morphological populations

Zebrafish provide an excellent model in which to assess the role of the renin-angiotensin system in renal development, injury, and repair. In contrast to mammals, zebrafish kidney organogenesis terminates with the mesonephros. Despite this, the basic functional structure of the nephron is conserved across vertebrates. The relevance of teleosts for studies relating to the regulation of the renin-angiotensin system was established by assessing the phenotype and functional regulation of renin-expressing cells in zebrafish. Transgenic fluorescent reporters for renin (ren), smooth muscle actin (acta2), and platelet-derived growth factor receptor-beta (pdgfrb) were studied to determine the phenotype and secretory ultrastructure of perivascular renin-expressing cells. Whole kidney ren transcription responded to altered salinity, pharmacological renin-angiotensin system inhibition, and renal injury. Mesonephric ren-expressing cells occupied niches at the preglomerular arteries and afferent arterioles, forming intermittent epithelioid-like multicellular clusters exhibiting a granular secretory ultrastructure. In contrast, renin cells of the efferent arterioles were thin bodied and lacked secretory granules. Renin cells expressed the perivascular cell markers acta2 and pdgfrb Transcriptional responses of ren to physiological challenge support the presence of a functional renin-angiotensin system and are consistent with the production of active renin. The reparative capability of the zebrafish kidney was harnessed to demonstrate that ren transcription is a marker for renal injury and repair. Our studies demonstrate substantive conservation of renin regulation across vertebrates, and ultrastructural studies of renin cells reveal at least two distinct morphologies of mesonephric perivascular ren-expressing cells.

Renin expression in developing zebrafish is associated with angiogenesis and requires the Notch pathway and endothelium

Although renin is a critical regulatory enzyme of the cardiovascular system, its roles in organogenesis and the establishment of cardiovascular homeostasis remain unclear. Mammalian renin-expressing cells are widespread in embryonic kidneys but are highly restricted, specialized endocrine cells in adults. With a functional pronephros, embryonic zebrafish are ideal for delineating the developmental functions of renin-expressing cells and the mechanisms governing renin transcription. Larval zebrafish renin expression originates in the mural cells of the juxtaglomerular anterior mesenteric artery and subsequently at extrarenal sites. The role of renin was determined by assessing responses to renin-angiotensin system blockade, salinity variation, and renal perfusion ablation. Renin expression did not respond to renal flow ablation but was modulated by inhibition of angiotensin-converting enzyme and altered salinity. Our data in larval fish are consistent with conservation of renin's physiological functions. Using transgenic renin reporter fish, with mindbomb and cloche mutants, we show that Notch signaling and the endothelium are essential for developmental renin expression. After inhibition of angiogenesis, renin-expressing cells precede angiogenic sprouts. Arising from separate lineages, but relying on mutual interplay with endothelial cells, renin-expressing cells are among the earliest mural cells observed in larval fish, performing both endocrine and paracrine functions.

Mineralocorticoid Excess or Glucocorticoid Insufficiency: Renal and Metabolic Phenotypes in a Rat Hsd11b2 Knockout Model

Obesity and hypertension are 2 major health issues of the 21st century. The syndrome of apparent mineralocorticoid excess is caused by deficiency of 11β-hydroxysteroid dehydrogenase type 2 (Hsd11b2), which normally inactivates glucocorticoids, rendering the mineralocorticoid receptor aldosterone–specific. The metabolic consequences of Hsd11b2 knockout in the rat are investigated in parallel with electrolyte homeostasis. Hsd11b2 was knocked out, by pronuclear microinjection of targeted zinc-finger nuclease mRNAs, and 1 line was characterized for its response to renal and metabolic challenges. Plasma 11-dehydrocorticosterone was below detection thresholds, and Hsd11b2 protein was undetected by Western blot, indicating complete ablation. Homozygotes were 13% smaller than wild-type littermates, and were polydipsic and polyuric. Their kidneys, adrenals, and hearts were significantly enlarged, but mesenteric fat pads and liver were significantly smaller. On a 0.3% Na diet, mean arterial blood pressure was ≈65 mm Hg higher than controls but only 25 mm Hg higher on a 0.03% Na⁺ diet. Urinary Na/K ratio of homozygotes was similar to controls on 0.3% Na⁺ diet but urinary albumin and calcium were elevated. Corticosterone and aldosterone levels showed normal circadian variation on both a 0.3% and 0.03% Na⁺ diet, but plasma renin was suppressed in homozygotes on both diets. Plasma glucose responses to an oral glucose challenge were reduced despite low circulating insulin, indicating much greater sensitivity to insulin in homozygotes. The rat model reveals mechanisms linking electrolyte homeostasis and metabolic control through the restriction of Hsd11b1 substrate availability.

Hypertrophy in the Distal Convoluted Tubule of an 11β-Hydroxysteroid Dehydrogenase Type 2 Knockout Model

Na(+) transport in the renal distal convoluted tubule (DCT) by the thiazide-sensitive NaCl cotransporter (NCC) is a major determinant of total body Na(+) and BP. NCC-mediated transport is stimulated by aldosterone, the dominant regulator of chronic Na(+) homeostasis, but the mechanism is controversial. Transport may also be affected by epithelial remodeling, which occurs in the DCT in response to chronic perturbations in electrolyte homeostasis. Hsd11b2(-/-) mice, which lack the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) and thus exhibit the syndrome of apparent mineralocorticoid excess, provided an ideal model in which to investigate the potential for DCT hypertrophy to contribute to Na(+) retention in a hypertensive condition. The DCTs of Hsd11b2(-/-) mice exhibited hypertrophy and hyperplasia and the kidneys expressed higher levels of total and phosphorylated NCC compared with those of wild-type mice. However, the striking structural and molecular phenotypes were not associated with an increase in the natriuretic effect of thiazide. In wild-type mice, Hsd11b2 mRNA was detected in some tubule segments expressing Slc12a3, but 11βHSD2 and NCC did not colocalize at the protein level. Thus, the phosphorylation status of NCC may not necessarily equate to its activity in vivo, and the structural remodeling of the DCT in the knockout mouse may not be a direct consequence of aberrant corticosteroid signaling in DCT cells. These observations suggest that the conventional concept of mineralocorticoid signaling in the DCT should be revised to recognize the complexity of NCC regulation by corticosteroids.

Activation of thiazide-sensitive co-transport by angiotensin II in the cyp1a1-Ren2 hypertensive rat

Transgenic rats with inducible expression of the mouse Ren2 gene were used to elucidate mechanisms leading to the development of hypertension and renal injury. Ren2 transgene activation was induced by administration of a naturally occurring aryl hydrocarbon, indole-3-carbinol (100 mg/kg/day by gastric gavage). Blood pressure and renal parameters were recorded in both conscious and anesthetized (butabarbital sodium; 120 mg/kg IP) rats at selected time-points during the development of hypertension. Hypertension was evident by the second day of treatment, being preceded by reduced renal sodium excretion due to activation of the thiazide-sensitive sodium-chloride co-transporter. Renal injury was evident after the first day of transgene induction, being initially limited to the pre-glomerular vasculature. Mircoalbuminuria and tubuloinsterstitial injury developed once hypertension was established. Chronic treatment with either hydrochlorothiazide or an AT1 receptor antagonist normalized sodium reabsorption, significantly blunted hypertension and prevented renal injury. Urinary aldosterone excretion was increased ∼20 fold, but chronic mineralocorticoid receptor antagonism with spironolactone neither restored natriuretic capacity nor prevented hypertension. Spironolactone nevertheless ameliorated vascular damage and prevented albuminuria. This study finds activation of sodium-chloride co-transport to be a key mechanism in angiotensin II-dependent hypertension. Furthermore, renal vascular injury in this setting reflects both barotrauma and pressure-independent pathways associated with direct detrimental effects of angiotensin II and aldosterone.

THE RADIOACTIVITY OF POTASSIUM FROM HUMAN SOURCES

A comparison of the radioactivity of potassium from human and commercial sources indicates that the radioactive isotope K(40) is probably 1 or 2 per cent less abundant in human potassium.

Transcriptional and physiological responses to chronic ACTH treatment by the mouse kidney

We investigated the effects on urinary steroid and electrolyte excretion and renal gene expression of chronic infusions of ACTH in the mouse. ACTH caused a sustained increase in corticosteroid excretion; aldosterone excretion was only transiently elevated. There was an increase in the excretion of deoxycorticosterone, a weak mineralocorticoid, to levels of physiological significance. Nevertheless, we observed neither antinatriuresis nor kaliuresis in ACTH-treated mice, and plasma renin activity was not suppressed. We identified no changes in expression of mineralocorticoid target genes. Water turnover was increased in chronic ACTH-treated mice, as were hematocrit and hypertonicity: volume contraction is consistent with high levels of glucocorticoid. ACTH-treated mice exhibited other signs of glucocorticoid excess, such as enhanced weight gain and involution of the thymus. We identified novel ACTH-induced changes in 1) genes involved in vitamin D (Cyp27b1, Cyp24a1, Gc) and calcium (Sgk, Calb1, Trpv5) metabolism associated with calciuria and phosphaturia; 2) genes that would be predicted to desensitize the kidney to glucocorticoid action (Nr3c1, Hsd11b1, Fkbp5); and 3) genes encoding transporters of enzyme systems associated with xenobiotic metabolism and oxidative stress. Although there is evidence that ACTH-induced hypertension is a function of physiological cross talk between glucocorticoids and mineralocorticoids, the present study suggests that the major changes in electrolyte and fluid homeostasis and renal function are attributable to glucocorticoids. The calcium and organic anion metabolism pathways that are affected by ACTH may explain some of the known adverse effects associated with glucocorticoid excess.

Physiological and pathophysiological applications of sensitive ELISA methods for urinary deoxycorticosterone and corticosterone in rodents

Deoxycorticosterone (DOC: a weak mineralocorticoid) is the precursor to corticosterone (B: the major glucocorticoid in rodents) and aldosterone (the major mineralocorticoid). The genes Cyp11b1 and Cyp11b2 that encode the enzymes responsible for DOC to B (11beta-hydroxylase) and DOC to aldosterone (aldosterone synthase) conversions are located on the same chromosome. The aim of this study was to develop sensitive and specific ELISA methods to quantify urinary DOC and B concentrations to assess the physiological and genetic control of the Cyp11b1/b2 locus. Antibodies raised in rabbits against DOC and B and horse radish peroxidase-goat anti-rabbit IgG enzyme tracer were used to develop the assays. Urine samples collected from mice held in metabolic cages were extracted with dichloromethane and reconstituted in assay buffer. The assays were validated for specificity, sensitivity, parallelism, accuracy and imprecision. Cross-reactivities with major interfering steroids were minimal: DOC assay (progesterone=0.735% and corticosterone=0.045%), and for B assay (aldosterone=0.14%, 11-dehydro-B=0.006%, cortisol=0.016% and DOC=0.04%) and minimum detection limit for DOC ELISA was 2.2 pg/mL (6.6 pmol/L), and for B ELISA was 6.2 pg/mL (17.9 pmol/L). The validity of urinary DOC and B ELISAs was confirmed by the excellent correlation between the results obtained before and after solvent extraction and HPLC (DOC ELISA: Y=1.092X-0.054, R(2)=0.988; B ELISA: Y=1.047X-0.226, R(2)=0.996). Accuracy studies, parallelism and imprecision data were determined and all found to be satisfactory. The methods were used in a series of metabolic cage studies which demonstrated that (i) females produce more DOC and corticosterone than males; (ii) DOC and corticosterone respond to ACTH treatment but not dietary sodium restriction; (iii) DOC:B ratios in Cyp11b1 null mice were >200-fold greater than wild type.

Development of a highly sensitive ELISA for aldosterone in mouse urine: validation in physiological and pathophysiological states of aldosterone excess and depletion

BACKGROUND

Clinical studies have established aldosterone as a critical physiological and pathophysiological factor in salt and water homeostasis, blood pressure control and in heart failure. Genetic and physiological studies of mice are used to model these processes. A sensitive and specific assay for aldosterone is therefore needed to monitor adrenocortical activity in murine studies of renal function and cardiovascular diseases.

METHODS

Antibodies against aldosterone were raised in sheep as previously described. HRP-Donkey-anti-sheep IgG enzyme tracer was produced in our laboratory using the Lightning-Link HRP technique. Aldosterone ELISA protocol was validated and optimised to achieve the best sensitivity. The assay was validated by analysing the urine of mice collected under various experimental conditions designed to stimulate or suppress aldosterone in the presence of other potentially interfering steroid hormones.

RESULTS

Cross-reactivity with the steroids most likely to interfere was minimal: corticosterone=0.0028%, cortisol=0.0006%, DOC=0.0048% except for 5alpha-dihydro-aldosterone=1.65%. Minimum detection limit of this ELISA was 5.2 pmole/L (1.5 pg/mL). The validity of urinary aldosterone ELISA was confirmed by the excellent correlation between results obtained before and after solvent extraction and HPLC separation step (Y=1.092X+0.03, R(2)=0.995, n=54). Accuracy studies, parallelism and imprecision data were determined and all found to be satisfactory. Using this assay, mean urinary aldosterone levels were (i) approximately 60-fold higher in females than males mice; (ii) increased 6-fold by dietary sodium restriction; (iii) increased 10-fold by ACTH infusion and (iv) reduced by >60% in Cyp11b1 null mice.

CONCLUSION

We describe an ELISA for urinary aldosterone that is suitable for repeated non-invasive measurements in mice. Female aldosterone levels are higher than males. Unlike humans, most aldosterone in mouse urine is not conjugated. Increased levels were noted in response to dietary sodium restriction and ACTH treatment. The sensitivity of the assay is sufficient to detect suppressed levels in mouse models of congenital adrenal hyperplasia.

Angiotensin-converting enzyme is a modifier of hypertensive end organ damage

Severe forms of hypertension are characterized by high blood pressure combined with end organ damage. Through the development and refinement of a transgenic rat model of malignant hypertension incorporating the mouse renin gene, we previously identified a quantitative trait locus on chromosome 10, which affects malignant hypertension severity and morbidity. We next generated an inducible malignant hypertensive model where the timing, severity, and duration of hypertension was placed under the control of the researcher, allowing development of and recovery from end organ damage to be investigated. We have now generated novel consomic Lewis and Fischer rat strains with inducible hypertension and additional strains that are reciprocally congenic for the refined chromosome 10 quantitative trait locus. We have captured a modifier of end organ damage within the congenic region and, using a range of bioinformatic, biochemical and molecular biological techniques, have identified angiotensin-converting enzyme as the modifier of hypertension-induced tissue microvascular injury. Reciprocal differences between angiotensin-converting enzyme and the anti-inflammatory tetrapeptide, N-acetyl-Ser-Asp-Lys-Pro in the kidney, a tissue susceptible to end organ damage, suggest a mechanism for the amelioration of hypertension-dependent damage.

Identification of a human ortholog of the mouseDcppgene locus, encoding a novel member of the CSP-1/Dcpp salivary protein family

Salivary fluid, the collective product of numerous major and minor salivary glands, contains a range of secretory proteins that play key defensive, digestive, and gustatory roles in the oral cavity. To understand the distinct protein “signature” contributed by individual salivary glands to salivary secretions, we studied a family of proteins shown by in vitro mRNA translation to be abundantly expressed in mouse sublingual glands. Molecular cloning, Southern blotting, and restriction fragment length polymorphism analyses showed these to represent one known and two novel members of the common salivary protein (CSP-1)/Demilune cell and parotid protein (Dcpp) salivary protein family, the genes for which are closely linked in the T-complex region of mouse chromosome 17. Bioinformatic analysis identified a putative human CSP-1/Dcpp ortholog, HRPE773, expressed predominantly in human salivary tissue, that shows 31% amino acid identity and 45% amino acid similarity to the mouse Dcpp query sequence. The corresponding human gene displays a similar structure to the mouse Dcpp genes and is located on human chromosome 16 in a region known to be syntenic with the T-complex region of mouse chromosome 17. The predicted mouse and human proteins both display classical NH2-terminal signal sequences, putative jacalin-related lectin domains, and potential N-linked glycosylation sites, suggesting secretion via sublingual saliva into the oral cavity where they may display antimicrobial activity or provide a defensive coating to enamel. Identification of a human CSP-1/Dcpp ortholog therefore provides a key tool for investigation of salivary protein function in human oral health and disease.

Hypertension, kidney, and transgenics: a fresh perspective

In this review, we outline the application and contribution of transgenic technology to establishing the genetic basis of blood pressure regulation and its dysfunction. Apart from a small number of examples where high blood pressure is the result of single gene mutation, essential hypertension is the sum of interactions between multiple environmental and genetic factors. Candidate genes can be identified by a variety of means including linkage analysis, quantitative trait locus analysis, association studies, and genome-wide scans. To test the validity of candidate genes, it is valuable to model hypertension in laboratory animals. Animal models generated through selective breeding strategies are often complex, and the underlying mechanism of hypertension is not clear. A complementary strategy has been the use of transgenic technology. Here one gene can be selectively, tissue specifically, or developmentally overexpressed, knocked down, or knocked out. Although resulting phenotypes may still be complicated, the underlying genetic perturbation is a starting point for identifying interactions that lead to hypertension. We recognize that the development and maintenance of hypertension may involve many systems including the vascular, cardiac, and central nervous systems. However, given the central role of the kidney in normal and abnormal blood pressure regulation, we intend to limit our review to models with a broadly renal perspective.

Insights from the rat genome sequence

Detailed three-way comparison of the rat, mouse and human genomes, is revealing a great deal about mammalian genome evolution and together with recent developments in cloning technologies, this heralds an important phase in rat research.

The availability of the rat genome sequence, and detailed three-way comparison of the rat, mouse and human genomes, is revealing a great deal about mammalian genome evolution. Together with recent developments in cloning technologies, this heralds an important phase in rat research.

Nuclear transfer in rodents

Cloning is the asexual reproduction of an individual, such that the offspring have an essentially identical nuclear genome. Nuclear transfer and cloning have been achieved in a number of species, namely sheep, cows, goats, rabbits, cats and mice, but have been largely unsuccessful, so far, in dogs, primates and rats. Clearly, contributory factors which affect the outcome of successful cloning experiments are not universally applicable to all species. One theme common to all cloning experiments, however, is the overall inefficiency of the process, typically 0-4%. A number of factors contribute to nuclear transfer inefficiency, and we will review mouse cloning experiments, which address these problems, highlighting the importance of donor nucleus choice (somatic or ES cell, fetal or adult, quiescent or actively dividing). Finally, we will summarize the emerging principles which appear to govern nuclear reprogramming and production of clones, and will consider the application of nuclear transfer to the rat.

Granulation rescue and developmental marking of juxtaglomerular cells using "piggy-BAC" recombination of the mouse ren locus

Mice lacking a functional Ren-1(d) gene exhibit a complete lack of renal juxtaglomerular cell granulation and atypical macula densa morphology. Transgenic mice carrying a 145-kilobase BAC clone encompassing the Ren-1(d) and Ren-2 loci were generated, characterized, and backcrossed with Ren-1(d-/-) mice. Homozygous Ren-1(d)-null mice expressing the BAC clone exhibited complete restoration of normal renal structure. Homologous recombination in Escherichia coli was used to generate a modified version of the BAC clone, in which an IRESbeta-geo cassette was inserted specifically into the Ren-1(d) gene. When introduced into the germline, the modified clone provided a marker for juxtaglomerular cell differentiation and beta-geo was expressed appropriately in juxtaglomerular cells throughout development. Parallel backcross experiments onto the Ren-1(d)-null background demonstrated that the juxtaglomerular cells expressed the modified Ren-1(d) locus in the absence of regranulation. These data demonstrate that the nongranulated cells constitute bona fide juxtaglomerular cells despite their altered morphology, that overexpression of renin-2 cannot compensate for the loss of renin-1(d), and that primary structural differences between the two isoforms are responsible for the differences in granulation. The use of BAC modification as part of functional complementation studies illustrates the potential for in vivo molecular dissection of key physiological mechanisms.

Manipulating large genomic clones via in vivo recombination in bacteria

Transgenesis is proving to be a powerful technique in studying the molecular genetics of hypertension. The ability to target specific mutations resulting in either loss of function, by gene deletion, the insertion of reporter sequences, or the subtle change of function by nucleotide replacement, can facilitate the understanding of gene function and its role in the manifestation of diseases. However an inherent problem associated with transgenic studies is the lack of consistent expression observed between independent lines of animals which have integrated the same transgene, a phenomenon known as 'position effect'. Small transgenes are almost invariably subject to position effect due to the absence of essential regulatory elements required to maintain an open chromatin structure. This phenomenon may be overcome if larger transgenes, isolated using vectors such as yeast artifical chromosomes (YACs), bacterial artificial chromosomes (BACs) or P1-based vectors, are used. Studies using such transgenes have reported levels of expression which are consistent between lines and dependent upon the number of copies integrated. The introduction of modifications into these large genomic clones is not practical by traditional restriction endonuclease strategies and so is dependent upon in vivo recombination to maintain structural integrity. Here we demonstrate the modification of a 100 Kb P1 clone spanning the renin locus using the BAC targeting strategy described by Yang et al (Nat Biotechnol 1997; 15: 859-865).

Efficient Cre-lox linearisation of BACs: applications to physical mapping and generation of transgenic animals

Due to the size of BAC, PAC and P1 clones, it is often difficult to construct detailed restriction maps, with large number of restriction fragments leading to ambiguity of mapping data. We report the use of Cre recombinase to linearise and asymmetrically introduce label at the unique loxP site of large loxP-containing clones. Subsequent partial digestion allows the direct ordering of restriction fragments. Additionally, BAC DNA linearised using the Cre-lox system has been used successfully to generate transgenic animals.

Transgenics and essential hypertension

Primary or "essential' hypertension is generally perceived to be a multifactorial or complex genetic trait. An individual's susceptibility to high blood pressure (BP) is influenced not only by the many genetic factors, which effect control through biochemical and physiological mechanisms, but also by environmental determinants. In a small proportion of human hypertensives the cause is a single genetic defect, exhibiting Mendelian characteristics. The vast heterogeneous majority, however, result from a multitude of contributing factors, making identification of the underlying etiology very difficult. We will briefly review a number of strategies which have helped to identify genetic factors involved in hypertension. These include the search for genetic defects in Mendelian forms of hypertension, intensive study of classical animal models such as the spontaneously hypertensive rat, and linkage analyses in animal models and hypertensive patients. We will then discuss the role which transgenesis can play in complementing and extending such analyses.

Transgenesis in nonmurine species

Although the mouse remains the species of choice for most transgenic experimentation, it may be preferable or even necessary to use alternative species for certain applications. We review the strategies by which transgenic technology has been applied to other animals, specifically, the rat, rabbit, pig, sheep, goat, and cow. Additionally, we outline the potential applications of alternative transgenic species with reference to the field of hypertension and cardiovascular research.

Intracellular ionized calcium changes in squid giant axons monitored by Fura-2 and aequorin

Squid giant axons were injected simultaneously with Ca indicators Fura-2 and aequorin. Fura-2 was calibrated in situ by measuring fluorescence at 510 nm upon UV excitation at 340 nm, 360 nm, and 380 nm with a time-sharing multiple wavelength spectrofluorimeter. Limiting values for dye fluorescence were obtained by allowing a massive load of Ca to enter the axon with the aid of procedures such as prolonged depolarization in the presence of CN (for saturation) and by sequestration of all Ca present in the axoplasm accomplished with injection of EGTA into the axon (for a zero-Ca signal). The average intracellular Ca concentration obtained with Fura-2 was 184 nM. The sensitivity of Fura-2 to intracellular Ca is at least as great as that of aequorin, thus permitting its use in the characterization of Ca homeostasis mechanisms such as Na-Ca exchange. It was found, however, that for voltage-clamp experiments requiring an internal current electrode, Fura-2 is not a convenient Ca probe because electrode reactions in the axoplasm denature the dye, thereby restricting its use in characterization of Ca movements associated with electrically induced changes in membrane potential. A comparison of aequorin luminescence with Fura-2 fluorescence demonstrated that light output by aequorin is linear with intracellular Ca concentrations up to values of 750 nM, changing to a square law relationship from 750 nM up to 10 microM Ca.

Linkage of the erythroid transcription factor gene (Gf-1) to the proximal region of the X chromosome of mice

We have used a cDNA probe for mouse Gf-1 gene that encodes the erythroid cell transcription factor to identify genetic variation in genomic DNA between Mus species. The segregation of Gf-1 DNA variation was analyzed in Mus species crosses that have been previously typed for the segregation of more than 30 genes spanning 80 cM of the mouse X chromosome from the centromere to the border of the X-Y pairing region. We identified a single X chromosome locus in the mouse, Gf-1, and an analysis of recombinants from 203 backcross progeny mapped Gf-1 to the proximal portion of the chromosome, coincident with the Cybb locus and proximal to Otc gene locus. A gene order of centromere, DXWas70, Cybb/Gf-1, Otc, Timp was established for the mouse X chromosome, which is in agreement with the map position observed on the human X chromosome.

Linkage of a gene for neural cell adhesion molecule, L1 (CamL1) to the Rsvp region of the mouse X chromosome

L1 is a glycoprotein with an apparent molecular weight of 200 kDa in the developing fetus and adult central nervous system. In the peripheral nervous system, it has a molecular weight of 230 kDa. The L1 protein appears to be encoded by a single gene that has been located on the human X chromosome by in situ hybridization. In this paper we describe restriction variation in genomic DNA Southern analysis between Mus species for the K13 cDNA probe for the L1 neural cell adhesion molecule. We have designated the locus described by this variation as cell adhesion molecule L1, CamL1. The X chromosome linkage and the relative position on the X chromosome coincident with the genes Rsvp/G6pd/Cf-8 were defined in backcross matings involving M. spretus and M. musculus.

Efficient linkage of 10 loci in the proximal region of the mouse X chromosome

Interspecific Mus species crosses were used to construct a multilocus genetic map of the mouse X chromosome that extends for more than 50 cM. In these studies, we established the segregation of eight loci in more than 200 backcross progeny from crosses of M. musculus and M. spretus with a common inbred strain (C57BL/6JRos). Genetic divergence at the level of the nucleotide sequences makes these crosses a useful cumulative genetic resource for mapping additional genes defined by genomic or cDNA probes in a highly efficient manner. We have therefore devised a mapping strategy that uses a subset of these backcrosses that are recombinant between successive anchor loci to both localize and order an additional set of six genes without necessarily resorting to an analysis of the entire backcross series. Using this approach, we have defined the linkage of cytochrome b245 beta-chain (Cybb), synapsin (Syn-1), and two members of the X-linked lymphocyte-regulated gene family (Xlr-1, Xlr-2), as well as DXSmh141 and DXSmh172, two loci defined by random genomic probes. All six loci have been localized to the proximal portion of the mouse X chromosome and their order has been defined as Cybb, Otc, Syn-1/Timp, DXSmh141/Xlr-1, DXSmh172, Hprt, Xlr-2, Cf-9. Gene order was established by minimizing multiple recombination events across the region spanning an estimated 20 cM of the proximal X chromosome. The possible significance of the Xlr loci is discussed with respect to other X-chromosome loci that regulate the immune response.

The nucleotide sequence of a mouse renin-encoding gene, Ren-1d, and its upstream region

The renin-encoding genes have been cloned from high (Ren-1d, Ren-2d)- and low (Ren-1c)-renin-producing strains of mice (DBA/2J and C57BL/10). Each of the genes is approx. 9.6 kb in length and consists of nine exons and eight introns. The entire nucleotide sequence of the Ren-1d gene has been determined and the 5'-flanking regions of the three genes, Ren-1c, Ren-1d and Ren-2d, have been compared. The significance of several potential regulatory signals found in the DNA is discussed.

Calcium entry in squid axons during voltage clamp pulses

Squid giant axons were injected with aequorin and tetraethylammonium and were impaled with sodium ion sensitive, current and voltage electrodes. The axons were usually bathed in a solution of varying Ca2+ concentration ([Ca2+]o) containing 150mM each of Na+, K+ and an inert cation such as Li+, Tris or N-methylglucamine and had ionic currents pharmacologically blocked. Voltage clamp pulses were repeatedly delivered to the extent necessary to induce a change in the aequorin light emission, a measure of axoplasmic Ca2+ level, [Ca2+]i. The effect of membrane voltage on [Ca2+]i was found to depend on the concentration of internal Na+ ([Na+]i). Voltage clamp hyperpolarizing pulses were found to cause a reduction of [Ca2+]i. For depolarizing pulses a relationship between [Ca2+]i gain and [Na+]i indicates that Ca2+ entry is sigmoid with a half maximal response at 22 mM Na+. This Ca2+ entry is a steep function of [Na+]i suggesting that 4 Na+ ions are required to promote the influx of 1 Ca2+. There was little change in Ca2+ entry with depolarizing pulses when [Ca2+]o is varied from 1 to 10mM, while at 50mM [Ca2+]o calcium entry clearly increases suggesting an alternate pathway from that of Na+/Ca2+ exchange. This entry of Ca2+ at high [Ca2+]o, however, was not blocked by Cs+o. The results obtained lend further support to the notion that Na+/Ca2+ exchange in squid giant axon is sensitive to membrane voltage no matter whether this is applied as a constant change in membrane potential or as an intermittent one.

Changes in internal ionized Ca2+ and H+ in voltage clamped squid axons

Squid giant axons were injected with aequorin and tetraethylammonium and were impaled with hydrogen ion sensitive, current and voltage electrodes. A newly designed horizontal microinjector was used to introduce the aequorin. It also served, simultaneously, as the current and voltage electrode for voltage clamping and as the reference for ion-sensitive microelectrode measurements. The axons were usually bathed in a solution containing 150 mM each of Na+, K+, and some inert cation, at either physiological or zero bath Ca2+ concentration [( Ca2+]o), and had ionic currents pharmacologically blocked. Voltage clamp pulses were repeatedly delivered to the extent necessary to induce a change in the aequorin light emission, a measure of axoplasmic ionized Ca2+ level, [( Ca2+]i). Alternatively, membrane potential was steadily held at values that represented deviations from the resting membrane potential observed at 150 mM [K+]o (i.e. approximately -15 mV). In the absence of [Ca2+]o a significant steady depolarization brought about by current flow increased [Ca2+]i (and acidified the axoplasm). Changes in internal hydrogen activity, [H+]i, induced by current flow from the internal Pt wire limited the extent to which valid measurements of [Ca2+]i could be made. However, there are effects on [Ca2+]i that can be ascribed to membrane potential. Thus, in the absence of [Ca2+]o, hyperpolarization can reduce [Ca2+]i, implying that a Ca2+ efflux mechanism is enhanced. It is also observed that [Ca2+]i is increased by depolarization. These results are consistent with the operation of an electrogenic mechanism that exchanges Na+ for Ca2+ in squid giant axon.

Multilocus molecular mapping of the mouse X chromosome

Using restriction fragment length polymorphisms (RFLPs) and enzymatic variants between distantly related mouse species, we have assigned three genes to the mouse X chromosome and concurrently mapped a total of eight genes spanning an estimated 50 cM of the chromosome. Segregation of RFLPs in over 200 male progeny from interspecies backcrosses between the inbred strain C57BL/6JRos and either wild-derived Mus musculus or Mus spretus was followed for the murine genes Timp (tissue inhibitor of metalloproteinases), Cf-8 (coagulation factor VIII), and Rsvp (red-sensitive visual pigment) and the known X-linked markers Otc, Hprt, Cf-9, G6pd, and Ags. From the centromere, the gene order was defined as Otc, Timp, Hprt, Cf-9, (Cf-8/Rsvp/G6pd), Ags, by minimizing the number of multiple recombinational events. No significant differences in map order or frequency of recombination were observed between the two backcross series studied. The use of Southern analysis has allowed us to add new genes to the map in a cumulative manner, and as probes become available, additional markers can be mapped, using the same set of mice, by utilizing existing blots or resampling the DNAs. The use of probes for functional genes has allowed us to directly compare the X chromosomes of mouse and man and has provided insight into chromosomal rearrangements which have occurred during the evolutionary divergence of these species, as well as to define the extent of linkage homologies.

Regional localization of the murine Duchenne muscular dystrophy gene on the mouse X chromosome

The murine locus corresponding to the human Duchenne/Becker muscular dystrophy (DMD) gene has been regionally mapped on the mouse X chromosome by hybridizing DNA from interspecies mouse crosses with a cDNA clone for the mouse Dmd gene. The results demonstrate that the relative organization of genes on the murine and human X chromosomes is more divergent than has previously been postulated. Furthermore, the mouse Dmd gene maps to a similar region of the X chromosome as does the mouse muscular dystrophy mutation mdx, providing further evidence that the mdx mutant may be a murine equivalent of human DMD. However, Southern analysis of portions of the mouse Dmd gene has not yet revealed any differences between mdx and wild-type mice.

Differential methylation of the ornithine carbamoyl transferase gene on active and inactive mouse X chromosomes

Ornithine carbamoyl transferase (Oct) is an X-linked gene which exhibits tissue-specific expression. To determine whether methylation of specific CpG sequences plays a role in dosage compensation or tissue-specific expression of the gene, 13 potentially methylatable sites were identified over a 30-kilobase (kb) region spanning from approximately 15 kb upstream to beyond exon II. Fragments of the Mus hortulanus Oct gene were used as probes to establish the degree of methylation at each site. By considering the methylation status in liver (expressing tissue) versus kidney (nonexpressing tissue) from male and female mice, the active and inactive genes could be investigated on active and inactive X-chromosome backgrounds. One MspI site, 12 kb 5' of the Oct-coding region, was cleaved by HpaII in liver DNA from males but not in kidney DNA from males and thus exhibited complete correlation with tissue-specific expression of the gene. Six other sites showed partial methylation, reflecting incomplete correlation with tissue-specific expression.

Increases in internal Ca2+ and decreases in internal H+ are induced by general anesthetics in squid axons

Squid axons were injected with arsenazo III and treated with sea water containing compounds usually classified as general anesthetics, (pentanol-decanol and a variety of hydrocarbons and their derivatives). Such treatment led to an increase in absorbance by arsenazo III at wavelengths sensitive to [Ca]i. The effect was independent of the presence or absence of Ca++ in sea water and it was not modified by substances that release Ca from internal stores. The effect was easily reversible. In axons injected with phenol red or impaled with a glass electrode sensitive to H+, a similar treatment led to an alkalinization that was also readily reversible. Both Ca release and the change to an alkaline pH had identical time courses. The dose required for action by all of the chemical agents studied could be predicted from a knowledge of their fractional saturation in sea water, i.e. from their thermodynamic activity. For compounds with 8-10 carbon atoms, Ca-release effects can occur at concentration less than those necessary to block either conduction or Na/Ca exchange. A special chemical agent was octylamine, which induced a marked rise in pHi and in addition its nonionic form produced the typical Ca release associated with general anesthetics.

Dependence of ionized and total Ca in squid axons on Nao-free or high-Ko conditions

The level of intracellular Ca in squid axons (both ionized and total Ca) was studied as a function of the experimental variables [Na]i, [Na]o, pHi, cyanide, and depolarization. Ionized Ca was measured by following the light emission of aequorin while total Ca was measured by the atomic absorption analysis of samples of axoplasm. Aequorin glow is known to be increased either by the application of Nao-free solutions or by depolarization produced by external solutions containing greater than normal K concentrations. The present results show that if [Na]i is low, the depolarization that is brought about by solutions with elevated [K] leads to a resting light emission that is decreased rather than increased, as is the case when [Na]i is high. In axons where [Na]i is varied, a comparison of the increments in light emission produced by the application first of Na-free and then of high-K solutions shows that they have an identical dependence on [Na]i, with a half-activation of Ca entry produced by an [Na]i of 25-30 mM. Changes in pHi affect the aequorin signal produced by depolarization, with acidification reducing and alkanization increasing the response. Cyanide did not greatly affect the size of the signal resulting from either Nao removal or that from depolarization.

The influence of chemical agents on the level of ionized [Ca2+] in squid axons

Squid giant axons injected with either aequorin or arsenazo III and bathed in 3 mM Ca (Na) seawater were transferred to 3 mM Ca (K) seawater and the response of the aequorin light or the change in the absorbance of arsenazo III was followed. These experimental conditions were chosen because they measure the change in the rate of Na/Ca exchange in introducing Ca into the axon upon depolarization; [Ca]o is too low to effect a channel-based system of Ca entry. This procedure was applied to axons treated with a variety of compounds that have been implicated as inhibitors of Na/Ca exchange. The result obtained was that the substances tested could be placed in three groups. (a) Substances that were without effect on Ca entry effected by Na/Ca exchange were: D600 at 10-100 microM, nitrendipine at 1-5 microM, Ba2+ and Mg2+ at concentrations of 10-50 mM, lidocaine at 0.1-10 mM, cyanide at 2 mM, adriamycin at a concentration of 3 microM, chloradenosine at 35 microM, 2,4-diaminopyridine at 1 mM, Cs+ at 45-90 mM, and tetrodotoxin at 10(-7). (b) Substances that had a significant inhibitory effect on Na/Ca exchange were: Mn2+, Cd2+, and La3+ at 1-50 mM, and quinidine at 50 microM. (c) There were also blocking agents and biochemical inhibitors whose action appeared to be the inhibition of nonmitochondrial Ca buffering in axoplasm rather than an inhibition of Na/Ca exchange. These were the general anesthetic l-octanol at 0.1 mM and 1 mM orthovanadate plus apyrase.