Biochemical and immunological characterization of renal protein kinase C

Role of protein kinase C-α in hypertonicity-stimulated urea permeability in mouse inner medullary collecting ducts

The kidney's ability to concentrate urine is vitally important to our quality of life. In the hypertonic environment of the kidney, urea transporters must be regulated to optimize function. We previously showed that hypertonicity increases urea permeability and that the protein kinase C (PKC) blockers chelerythrine and rottlerin decreased hypertonicity-stimulated urea permeability in rat inner medullary collecting ducts (IMCDs). Because PKCα knockout (PKCα(-/-)) mice have a urine-concentrating defect, we tested the effect of hypertonicity on urea permeability in isolated perfused mouse IMCDs. Increasing the osmolality of perfusate and bath from 290 to 690 mosmol/kgH(2)O did not change urea permeability in PKCα(-/-) mice but significantly increased urea permeability in wild-type mice. To determine whether the response to protein kinase A was also missing in IMCDs of PKCα(-/-) mice, tubules were treated with vasopressin and subsequently with the PKC stimulator phorbol dibutyrate (PDBu). Vasopressin stimulated urea permeability in PKCα(-/-) mice. Like vasopressin, forskolin stimulated urea permeability in PKCα(-/-) mice. We previously showed that, in rats, vasopressin and PDBu have additive stimulatory effects on urea permeability. In contrast, in PKCα(-/-) mice, PDBu did not further increase vasopressin-stimulated urea permeability. Western blot analysis showed that expression of the UT-A1 urea transporter in IMCDs was increased in response to vasopressin in wild-type mice as well as PKCα(-/-) mice. Hypertonicity increased UT-A1 phosphorylation in wild-type mice but not in PKCα(-/-) mice. We conclude that PKCα mediates hypertonicity-stimulated urea transport but is not necessary for vasopressin stimulation of urea permeability in mouse IMCDs.

Immunolocalization of Protein Kinase C Isoenzymes α, βI, βII and γ in Adult and Developing Rat Kidney

Protein kinase C (PKC) plays an important role not only in signal transduction mechanisms in various biological processes, but also in the regulation of growth and differentiation during development. We studied the classical PKCα, βI, βII and γ, with regard to their expression in adult and developing rat kidney. PKCα appeared in the ureteric bud at embryonic day (E) 16, and the proximal and distal anlage at E18. After birth, the immunoreactivity of PKCα gradually decreased. In adult, PKCα was expressed intensely in the connecting tubule (CNT), the collecting ducts (CD) and the renal corpuscle, and weakly in the proximal and distal tubules. PKCβI appeared in the ureteric bud at E16, and the proximal anlage at E18. After birth, the immunoreactivity of PKCβI gradually disappeared from the CD and proximal tubule. In adult, PKCβI was expressed in the intercalated cells of the CNT and cortical CD, the proximal straight tubule, and the renal corpuscle. PKCβII appeared in distal anlage at E18, and increased markedly after birth. In the CD, PKCβII immunoreactivity appeared after birth. In adult, PKCβII was expressed in the distal tubule, the CNT and the CD. The immunoreactivity for PKCγ appeared only in the proximal anlage at E18, and increased temporally around the time of birth. However, no immunoreactivity for PKCγ was observed in adult rat kidney. These results indicate that classical PKC isoforms appear to play a role in the regulation of various renal functions and differentiation within specific functional units of the uriniferous tubule in rat kidney.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-betal and plasma glucose concentration

AIMS/HYPOTHESIS

GLUT2 is the main renal glucose transporter upregulated by hyperglycaemia, when it becomes detectable at the brush border membrane (BBM). Since glucose-induced protein kinase C (PKC) activation in the kidney is linked to diabetic nephropathy, we investigated the effect of glycaemic status on the protein levels of PKC isoforms alpha, betaI, betaII, delta and epsilon in the proximal tubule, as well as the relationship between them and changes in GLUT2 production at the BBM.

METHODS

Plasma glucose concentrations were modulated in rats by treatment with nicotinamide 15 min prior to induction of diabetes with streptozotocin. Levels of GLUT2 protein and PKC isoforms in BBM were measured by western blotting. Additionally, the role of calcium signalling and PKC activation on facilitative glucose transport was examined by measuring glucose uptake in BBM vesicles prepared from proximal tubules that had been incubated either with thapsigargin, which increases cytosolic calcium, or with the PKC activator phorbol 12-myristate,13-acetate (PMA).

RESULTS

Thapsigargin and PMA enhanced GLUT-mediated glucose uptake, but had no effect on sodium-dependent glucose transport. Diabetes significantly increased the protein levels of GLUT2 and PKC-betaI at the BBM. Levels of GLUT2 and PKC-betaI correlated positively with plasma glucose concentration. Diabetes had no effect on BBM levels of alpha, betaII, delta or epsilon isoforms of PKC.

CONCLUSIONS/INTERPRETATION

Enhanced GLUT2-mediated glucose transport across the proximal tubule BBM during diabetic hyperglycaemia is closely associated with increased PKC-betaI. Thus, altered levels of GLUT2 and PKC-betaI proteins in the BBM may be important factors in the pathogenic processes underlying diabetic renal injury.

Expression of protein kinase C isoenzymes alpha, betaI, and delta in subtypes of intercalated cells of mouse kidney

Recent studies of the distribution of PKC isoenzymes in the mouse kidney demonstrated that PKC-alpha, -beta(I), and -delta are expressed in intercalated cells. The purpose of this study was to identify the intercalated cell subtypes that express the different PKC isoenzymes and determine the location of the PKC isoenzymes within these cells. Adult C57BL/6 mice kidney tissues were processed for multiple-labeling immunohistochemistry. Antibodies against the vacuolar H(+)-ATPase and pendrin were used to identify intercalated cell subtypes, whereas antibodies against calbindin D(28K) and aquaporin-2 (AQP2) were used to identify connecting tubule cells and principal cells of the collecting duct, respectively. Within type A intercalated cells, PKC-delta was highly expressed in the apical part of the cells, whereas immunoreactivity for both PKC-alpha and PKC-beta(I) was weak. Type B intercalated cells exhibited strong expression of PKC-alpha, -beta(I), and -delta. PKC-alpha and -beta(I) were localized throughout the cytoplasm, whereas PKC-delta was restricted to the basal domain. Within non-A-non-B cells, immunoreactivity for both PKC-alpha and PKC-beta(I) was high in intensity and localized diffusely in the cytoplasm, whereas PKC-delta was localized in the apical part of the cells. None of the PKC isoenzymes (PKC-alpha, -beta(I), or -delta) were expressed in the calbindin D(28K)-positive connecting tubule cells. Within AQP2-positive principal cells of the collecting duct, PKC-alpha was expressed on the basolateral plasma membrane, but no significant staining was detected for PKC-beta(I) and -delta. In summary, this study demonstrates distinct and differential expression patterns of PKC-alpha, -beta(I), and -delta in the three subtypes of intercalated cells in the mouse kidney.

Immunolocalization of protein kinase C isoenzymes alpha, beta I, beta II, delta, and epsilon in mouse kidney

Localization of protein kinase C (PKC) isoenzymes alpha, beta I, beta II, delta, and epsilon was studied employing Western blot analysis and immunohistochemical methods including confocal laser-scanning microscopy in the kidney of two mice strains, namely, C57BL/6 and 129/Sv, which have recently been used as genetic backgrounds for respective knockout mice. Immunoblot analysis identified immunoreactive bands for each isoenzyme in total kidney cell extracts. Isoenzyme expression sites were identical for both strains. Glomeruli expressed PKC-alpha, -beta I, and -epsilon. The latter isoenzyme was also detected in apical aspects of proximal convoluted but not in proximal straight tubules. In contrast to rats, neither PKC-alpha nor PKC-beta I was detectable in the proximal tubule. Immunofluorescence was observed in luminal membranes of medullary (MTAL) and cortical thick ascending limbs for PKC-beta I and in MTAL for PKC-epsilon. The cortical collecting duct expressed PKC-alpha, -beta I, and -delta in intercalated cells only. In the outer medullary collecting duct, PKC-alpha and -beta I were detectable in principal cells, whereas PKC-delta was found in intercalated cells. In the inner medullary collecting duct, PKC-alpha, -beta I, and -beta II were detected. As described for the rat, the expression of PKC-beta II was otherwise restricted to cortical and medullary interstitial cells. The specificity of all labeling was confirmed in respective PKC isoenzyme knockout mice. In summary, distinct expression patterns were shown for PKC isoenzymes alpha, beta I, beta II, delta, and epsilon in the mouse kidney.

Evidence for a role of protein kinase C-alpha in urine concentration

In mouse kidney, the conventional protein kinase C (PKC) isoenzyme alpha is expressed in glomeruli, the cortical collecting duct (intercalated cells only), and medullary collecting duct. To get insights on its function, PKC-alpha knockout (-/-) and wild-type (+/+) mice were studied. When provided free access to water, PKC-alpha -/- mice showed approximately 50% greater urine flow rate and lower urinary osmolality in 24-h metabolic cage experiments despite a greater urinary vasopressin-to-creatinine ratio vs. PKC-alpha +/+ mice. Renal albumin excretion was not different. Clearance experiments under inactin/ketamine anesthesia revealed a modestly reduced glomerular filtration rate and showed a reduced absolute and fractional renal fluid reabsorption in PKC-alpha -/- mice. The sodium-restricting response to a low-sodium diet was unaffected in PKC-alpha -/- mice. Urinary osmolality was reduced to similar hypotonic levels in PKC-alpha -/- and +/+ mice during acute oral water loading or application of the vasopressin V(2)-receptor antagonist SR-121463. In comparison, the lower urinary osmolality observed in PKC-alpha -/- mice vs. wild-type mice under basal conditions persisted during water restriction for 36 h. In conclusion, PKC-alpha appears not to play a major role in renal sodium reabsorption but, consistent with its expression in the medullary collecting duct, contributes to urinary concentration in mice. Considering that PKC-beta I and -beta II are coexpressed with PKC-alpha in mouse medullary collecting duct, the present results indicate that conventional PKC isoenzymes cannot fully compensate for each other.