An Intact Dilysine-like Motif in the Carboxyl Terminus of MAL Is Required for Normal Apical Transport of the Influenza Virus Hemagglutinin Cargo Protein in Epithelial Madin-Darby Canine Kidney Cells

The MAL proteolipid, a component of the integral protein sorting machinery, has been demonstrated as being necessary for normal apical transport of the influenza virus hemagglutinin (HA) and the overall apical membrane proteins in Madin-Darby canine kidney (MDCK) cells. The MAL carboxy terminus ends with the sequence Arg-Trp-Lys-Ser-Ser (RWKSS), which resembles dilysine-based motifs involved in protein sorting. To investigate whether the RWKSS pentapeptide plays a role in modulating the distribution of MAL and/or its function in apical transport, we have expressed MAL proteins with distinct carboxy terminus in MDCK cells whose apical transport was impaired by depletion of endogenous MAL. Apical transport of HA was restored to normal levels by expression of MAL with an intact but not with modified carboxyl terminal sequences bearing mutations that impair the functioning of dilysine-based sorting signals, although all the MAL proteins analyzed incorporated efficiently into lipid rafts. Ultrastructural analysis indicated that compared with MAL bearing an intact RWKSS sequence, a mutant with lysine −3 substituted by serine showed a twofold increased presence in clathrin-coated cytoplasmic structures and a reduced expression on the plasma membrane. These results indicate that the carboxyl-terminal RWKSS sequence modulates the distribution of MAL in clathrin-coated elements and is necessary for HA transport to the apical surface.

Glial cell line-derived neurotrophic factor activates the receptor tyrosine kinase RET and promotes kidney morphogenesis.

The receptor tyrosine kinase RET functions during the development of the kidney and the enteric nervous system, yet no ligand has been identified to date. This report demonstrates that the glial cell line-derived neurotrophic factor (GDNF) activates RET, as measured by tyrosine phosphorylation of the intracellular catalytic domain. GDNF also binds RET with a dissociation constant of 8 nM, and 125I-labeled GDNF can be coimmunoprecipitated with anti-RET antibodies. In addition, exogenous GDNF stimulates both branching and proliferation of embryonic kidneys in organ culture, whereas neutralizing antibodies against GDNF inhibit branching morphogenesis. These data indicate that RET and GDNF are components of a common signaling pathway and point to a role for GDNF in kidney development.

Immunolocalization of ion transport proteins in human autosomal dominant polycystic kidney epithelial cells.

The kidneys of patients with autosomal dominant polycystic kidney disease become massively enlarged due to the progressive expansion of myriad fluid-filled cysts. The epithelial cells that line the cyst walls are responsible for secreting the cyst fluid, but the mechanism through which this secretion occurs is not well established. Recent studies suggest that renal cyst epithelial cells actively secrete Cl across their apical membranes, which in turn drives the transepithelial movement of Na and water. The characteristics of this secretory flux suggest that it is dependent upon the participation of an apical cystic fibrosis transmembrane conductance regulator (CFTR)-like Cl channel and basolateral Na,K-ATPase. To test this hypothesis, we have immunolocalized the CFTR and Na,K-ATPase proteins in intact cysts and in cyst epithelial cells cultured in vitro on permeable filter supports. In both settings, cyst epithelial cells were found to possess Na,K-ATPase exclusively at their basolateral surfaces; apical labeling was not detected. The CFTR protein was present at the apical surfaces of cyst epithelial cells that had been stimulated to secrete through incubation in forskolin. CFTR was detected in intracellular structures in cultured cyst epithelial cells that had not received the forskolin treatment. These results demonstrate that the renal epithelial cells that line cysts in autosomal dominant polycystic kidney disease express transport systems with the appropriate polarity to mediate active Cl and fluid secretion.

The heart communicates with the kidney exclusively through the guanylyl cyclase-A receptor: acute handling of sodium and water in response to volume expansion.

Disruption of guanylyl cyclase-A (GC-A) results in mice displaying an elevated blood pressure, which is not altered by high or low dietary salt. However, atrial natriuretic peptide (ANP), a proposed ligand for GC-A, has been suggested as critical for the maintenance of normal blood pressure during high salt intake. In this report, we show that infusion of ANP results in substantial natriuresis and diuresis in wild-type mice but fails to cause significant changes in sodium excretion or urine output in GC-A-deficient mice. ANP, therefore, appears to signal through GC-A in the kidney. Other natriuretic/diuretic factors could be released from the heart. Therefore, acute volume expansion was used as a means to cause release of granules from the atrium of the heart. That granule release occurred was confirmed by measurements of plasma ANP concentrations, which were markedly elevated in both wild-type and GC-A-null mice. After volume expansion, urine output as well as urinary sodium and cyclic GMP excretion increased rapidly and markedly in wild-type mice, but the rapid increases were abolished in GC-A-deficient animals. These results strongly suggest that natriuretic/diuretic factors released from the heart function exclusively through GC-A.

Cellular and subcellular localization of the vasopressin- regulated urea transporter in rat kidney.

The renal urea transporter (RUT) is responsible for urea accumulation in the renal medulla, and consequently plays a central role in the urinary concentrating mechanism. To study its cellular and subcellular localization, we prepared affinity-purified, peptide-derived polyclonal antibodies against rat RUT based on the cloned cDNA sequence. Immunoblots using membrane fractions from rat renal inner medulla revealed a solitary 97-kDa band. Immunocytochemistry demonstrated RUT labeling of the apical and subapical regions of inner medullary collecting duct (IMCD) cells, with no labeling of outer medullary or cortical collecting ducts. Immunoelectron microscopy directly demonstrated labeling of the apical plasma membrane and of subapical intracellular vesicles of IMCD cells, but no labeling of the basolateral plasma membrane. Immunoblots demonstrated RUT labeling in both plasma membrane and intracellular vesicle-enriched membrane fractions from inner medulla, a subcellular distribution similar to that of the vasopressin-regulated water channel, aquaporin-2. In the outer medulla, RUT labeling was seen in terminal portions of short-loop descending thin limbs. Aside from IMCD and descending thin limbs, no other structures were labeled in the kidney. These results suggest that: (i) the RUT provides the apical pathway for rapid, vasopressin-regulated urea transport in the IMCD, (ii) collecting duct urea transport may be increased by vasopressin by stimulation of trafficking of RUT-containing vesicles to the apical plasma membrane, and (iii) the rat urea transporter may provide a pathway for urea entry into the descending limbs of short-loop nephrons.

Fumonisins and Alternaria alternata lycopersici toxins: sphinganine analog mycotoxins induce apoptosis in monkey kidney cells.

Fusarium moniliforme toxins (fumonisins) and Alternaria alternata lycopersici (AAL) toxins are members of a new class of sphinganine analog mycotoxins that occur widely in the food chain. These mycotoxins represent a serious threat to human and animal health, inducing both cell death and neoplastic events in mammals. The mechanisms by which this family of chemical congeners induce changes in cell homeostasis were investigated in African green monkey kidney cells (CV-1) by assessing the appearance of apoptosis, cell cycle regulation, and putative components of signal transduction pathways involved in apoptosis. Structurally, these mycotoxins resemble the sphingoid bases, sphingosine and sphinganine, that are reported to play critical roles in cell communication and signal transduction. The addition of fumonisin B1 or AAL toxin, TA, to CV-1 cells induced the stereotypical hallmarks of apoptosis, including the formation of DNA ladders, compaction of nuclear DNA, and the subsequent appearance of apoptotic bodies. Neither mycotoxin induced cell death, DNA ladders, or apoptotic bodies in CV-1 cells expressing simian virus 40 large T antigen (COS-7) at toxin concentrations that readily killed CV-1 cells. Fumonisin B1 induced cell cycle arrest in the G1 phase in CV-1 cells but not in COS-7 cells. AAL toxin TA did not arrest cell cycle progression in either cell line. The induction of apoptosis combined with the widespread presence of these compounds in food crops and animal feed identifies a previously unrecognized health risk to humans and livestock. These molecules also represent a new class of natural toxicants that can be used as model compounds to further characterize the molecular and biochemical pathways leading to apoptosis.

Polycystin, the polycystic kidney disease 1 protein, is expressed by epithelial cells in fetal, adult, and polycystic kidney.

Polycystic kidney disease 1 (PKD1) is the major locus of the common genetic disorder autosomal dominant polycystic kidney disease. We have studied PKD1 mRNA, with an RNase protection assay, and found widespread expression in adult tissue, with high levels in brain and moderate signal in kidney. Expression of the PKD1 protein, polycystin, was assessed in kidney using monoclonal antibodies to a recombinant protein containing the C terminus of the molecule. In fetal and adult kidney, staining is restricted to epithelial cells. Expression in the developing nephron is most prominent in mature tubules, with lesser staining in Bowman's capsule and the proximal ureteric bud. In the nephrogenic zone, detectable signal was observed in comma- and S-shaped bodies as well as the distal branches of the ureteric bud. By contrast, uninduced mesenchyme and glomerular tufts showed no staining. In later fetal (>20 weeks) and adult kidney, strong staining persists in cortical tubules with moderate staining detected in the loops of Henle and collecting ducts. These results suggest that polycystin's major role is in the maintenance of renal epithelial differentiation and organization from early fetal life. Interestingly, polycystin expression, monitored at the mRNA level and by immunohistochemistry, appears higher in cystic epithelia, indicating that the disease does not result from complete loss of the protein.

Expression cloning and functional characterization of the kidney cortex high-affinity proton-coupled peptide transporter.

The presence of a proton-coupled electrogenic high-affinity peptide transporter in the apical membrane of tubular cells has been demonstrated by microperfusion studies and by use of brush border membrane vesicles. The transporter mediates tubular uptake of filtered di- and tripeptides and aminocephalosporin antibiotics. We have used expression cloning in Xenopus laevis oocytes for identification and characterization of the renal high-affinity peptide transporter. Injection of poly(A)+ RNA isolated from rabbit kidney cortex into oocytes resulted in expression of a pH-dependent transport activity for the aminocephalosporin antibiotic cefadroxil. After size fractionation of poly(A)+ RNA the transport activity was identified in the 3.0- to 5.0-kb fractions, which were used for construction of a cDNA library. The library was screened for expression of cefadroxil transport after injection of complementary RNA synthesized in vitro from different pools of clones. A single clone (rPepT2) was isolated that stimulated cefadroxil uptake into oocytes approximately 70-fold at a pH of 6.0. Kinetic analysis of cefadroxil uptake expressed by the transporter's complementary RNA showed a single saturable high-affinity transport system shared by dipeptides, tripeptides, and selected amino-beta-lactam antibiotics. Electrophysiological studies established that the transport activity is electrogenic and affected by membrane potential. Sequencing of the cDNA predicts a protein of 729 amino acids with 12 membrane-spanning domains. Although there is a significant amino acid sequence identity (47%) to the recently cloned peptide transporters from rabbit and human small intestine, the renal transporter shows distinct structural and functional differences.

Neurotrophin 3 rescues neuronal precursors from apoptosis and promotes neuronal differentiation in the embryonic metanephric kidney.

We analyzed the developmental regulation and role of the neurotrophins during metanephric kidney morphogenesis. RNase protection assay revealed the presence of nerve growth factor, neurotrophin 3 (NT-3), and brain-derived neurotrophic factor mRNAs and the regulation of their expression during embryonic development of rat metanephros. NT-3 induced differentiation (neurite outgrowth) and survival (inhibition of apoptosis) of the neuronal precursors in cultured nephrogenic mesenchymes and neuronal differentiation in cultured whole kidneys, whereas NT-4/5, brain-derived neurotrophic factor, and nerve growth factor were without effect. The neurotrophins did not trigger tubular differentiation of isolated nephrogenic cells, which underwent apoptosis when cultured with or without the neurotrophins. NT-3 is thus an inducer of differentiation and a survival factor for renal neuronal cells, but none of the neurotrophins is a morphogen in kidney tubule induction.

A possible role of vitamin D receptors in regulating vitamin D activation in the kidney.

The vitamin D endocrine system is regulated reciprocally by renal 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylases. Previously, we reported that renal proximal convoluted tubules, the major site of 1 alpha, 25-dihydroxyvitamin D3 production, have vitamin D receptors. In the presence of vitamin D receptors, renal proximal convoluted tubules cannot maintain the state of enhanced production of 1 alpha, 25-dihydroxyvitamin D3. To clarify this discrepancy, we proposed a working hypothesis for the reciprocal control of renal 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylase activities. In rat models of enhanced renal production of 1 alpha, 25-dihydroxyvitamin D3, expression of vitamin D receptors and 25-hydroxyvitamin D3 24-hydroxylase mRNAs was strikingly suppressed in renal proximal convoluted tubules but not in the cortical collecting ducts. In vitamin D-deficient rats with up-regulated renal 25-hydroxyvitamin D3 1 alpha-hydroxylase activity, expression of vitamin D receptor mRNA in renal proximal convoluted tubules was also down-regulated, indicating that the down-regulation of vitamin D receptor mRNA is not the result of the enhanced production of 1 alpha, 25-dihydroxyvitamin D3. In Japanese quail models with up-regulated renal 25-hydroxyvitamin D3 1 alpha-hydroxylase activity by sex steroids, expression of vitamin D receptor mRNA was also down-regulated in the kidney but not in the duodenum. These results suggest that the down-regulation of vitamin D receptors plays a critical role in production of 1 alpha, 25-dihydroxyvitamin D3 in renal proximal convoluted tubules.