Postnatal differentiation of rabbit collecting duct intercalated cells

Lengths of nephron tubule segments and collecting ducts in the CD-1 mouse kidney: an ontogeny study

The kidney continues to mature postnatally, with significant elongation of nephron tubules and collecting ducts to maintain fluid/electrolyte homeostasis. The aim of this project was to develop methodology to estimate lengths of specific segments of nephron tubules and collecting ducts in the CD-1 mouse kidney using a combination of immunohistochemistry and design-based stereology (vertical uniform random sections with cycloid arc test system). Lengths of tubules were determined at postnatal day 21 (P21) and 2 and 12 mo of age and also in mice fed a high-salt diet throughout adulthood. Immunohistochemistry was performed to identify individual tubule segments [aquaporin-1, proximal tubules (PT) and thin descending limbs of Henle (TDLH); uromodulin, distal tubules (DT); aquaporin-2, collecting ducts (CD)]. All tubular segments increased significantly in length between P21 and 2 mo of age (PT, 602% increase; DT, 200% increase; TDLH, 35% increase; CD, 53% increase). However, between 2 and 12 mo, a significant increase in length was only observed for PT (76% increase in length). At 12 mo of age, kidneys of mice on a high-salt diet demonstrated a 27% greater length of the TDLH, but no significant change in length was detected for PT, DT, and CD compared with the normal-salt group. Our study demonstrates an efficient method of estimating lengths of specific segments of the renal tubular system. This technique can be applied to examine structure of the renal tubules in combination with the number of glomeruli in the kidney in models of altered renal phenotype.

Adaptation to metabolic acidosis and its recovery are associated with changes in anion exchanger distribution and expression in the cortical collecting duct

It is well known that acid/base disturbances modulate proton/bicarbonate transport in the cortical collecting duct. To study the adaptation further we measured the effect of three days of acidosis followed by the rapid recovery from this acidosis on the number and type of intercalated cells in the rabbit cortical collecting duct. Immunofluorescence was used to determine the expression of apical pendrin in β-intercalated cells and the basolateral anion exchanger (AE1) in α-intercalated cells. Acidosis resulted in decreased bicarbonate and increased proton secretion, which correlated with reduced pendrin expression and the number of pendrin-positive cells, as well as decreased pendrin mRNA and protein abundance in this nephron segment. There was a concomitant increase of basolateral AE1 and α-cell number. Intercalated cell proliferation did not seem to play a role in the adaptation to acidosis. Alkali loading for 6-20 h after acidosis doubled the bicarbonate secretory flux and reduced proton secretion. Pendrin and AE1 expression patterns returned to control levels, demonstrating that adaptive changes by intercalated cells are rapidly reversible. Thus, regulation of intercalated cell anion exchanger expression and distribution plays a key role in adaptation of the cortical collecting duct to perturbations of acid/base.

Replication of segment-specific and intercalated cells in the mouse renal collecting system

The renal collecting system (CS) is composed of segment-specific (SS) and intercalated (IC) cells. The latter comprise at least two subtypes (type A and non-type A IC). The origin and maintenance of cellular heterogeneity in the CS is unclear. Among other hypotheses, it was proposed that one subtype of IC cells represents a stem cell population from which all cell types in the CS may arise. In the present study, we tested this stem cell hypothesis for the adult kidney by assessing DNA synthesis as a marker for cell replication. SS and IC cells were identified by their characteristic expressions of sodium- (epithelial sodium channel, Na-K-ATPase), water- (aquaporin-2) and acid/base- (H+ -ATPase, anion exchanger AE1) transporting proteins. Immunostaining for bromodeoxyuridine (BrdU) and for the proliferating cell nuclear antigen (PCNA) was used to reveal DNA synthesis in CS epithelium. BrdU- and PCNA-immunostaining as well as mitotic figures were seen in all subtypes of CS cells. Dividing cells retained the cell-type specific expression of marker molecules. Treatment of mice with bumetanide combined with a high oral salt intake, which increases the tubular salt load in the CS, profoundly increased the DNA-synthesis rate in SS and non-type A IC cells, but reduced it in type A IC cells. Thus, our data show that DNA synthesis and cell replication occur in each cell lineage of the CS and in differentiated cells. The replication rate in each cell type can be differently modulated by functional stimulation. Independent proliferation of each cell lineage might contribute to maintain the cellular heterogeneity of the CS of the adult kidney and may also add to the adaptation of the CS to altered functional requirements.

Role of hensin in mediating the adaptation of the cortical collecting duct to metabolic acidosis

PURPOSE OF REVIEW

The cortical collecting duct is able to secrete HCO3-, a state that can be converted to acid secretion during metabolic acidosis. Bicarbonate secretion in this segment is mediated by beta-intercalated cells whereas alpha-intercalated cells perform acid secretion. During metabolic acidosis, the number of beta-intercalated cells is reduced while that of alpha-intercalated cells increases without a change in the total number of intercalated cells, suggesting conversion of one cell type to another. Using an immortalized intercalated cell line we found that this adaptation is mediated by an extracellular protein named hensin. Hensin is secreted as a monomer which is then polymerized in the extracellular environment by a complex process requiring at least three other proteins.

RECENT FINDINGS

We describe that a cyclophilin, via its cis/trans prolyl isomerase activity, is required for this polymerization. This may explain the distal renal tubular acidosis observed with cyclosporin A therapy. In addition, galectin-3 is needed to aggregate the protein. Finally, we recently found that activation of integrins is also necessary for the development of the hensin fiber. Hensin is expressed in all epithelia and deletion of its gene is embryonic lethal at an early stage when the first columnar epithelia develop.

SUMMARY

These studies suggest that the response of intercalated cells to metabolic acidosis uses a pathway that is involved in terminal differentiation of columnar epithelia.

Terminal differentiation of intercalated cells: the role of hensin

During the response to metabolic acidosis, the intercalated cell of the collecting tubule converts from one that secretes HCO3(-) to one that absorbs HCO3(-) by H(+) secretion. The molecular basis of this complex change in phenotype was studied in an immortalized intercalated cell line. We found that it was induced by secretion, polymerization, and deposition of a protein, which we termed hensin, into the extracellular matrix. Surprisingly, this change in phenotype is identical to terminal differentiation of epithelial cells in that it recapitulated all the characteristics of terminal differentiation, including a change in cell shape, acquisition of specialized apical structures (microvilli and ruffles), and the ability to secrete and endocytose materials in a regulated manner from the apical membrane. Hensin is expressed in most epithelia, and others have discovered that it is deleted in a large number of epithelial tumors. These results suggest that conversion of polarity of the intercalated cells represents a process of terminal differentiation.

Hypoxic-ischemic injury induces macrophage inflammatory protein-1alpha expression in immature rat brain

BACKGROUND AND PURPOSE

Macrophage inflammatory protein (MIP)-1alpha is a well-characterized monocyte chemoattractant; its role in regulating monocyte and microglial recruitment and activation in the injured neonatal brain is unknown. We evaluated the impact of acute hypoxic-ischemic (HI) brain injury on the expression of MIP-1alpha in neonatal rat brain.

METHODS

To elicit forebrain ischemic injury, 7-day-old (P7) rats underwent right carotid ligation, followed by 2.5 hours of 8% oxygen exposure. We used an enzyme-linked immunosorbent assay and immunohistochemistry to detect MIP-1alpha; double-labeling immunofluorescence assays were analyzed with confocal microscopy to identify cellular sources of MIP-1alpha. Immunocytochemistry assays were also used to detect 2 MIP-1alpha receptors, CCR1 and CCR5.

RESULTS

We found marked increases in tissue concentrations of MIP-1alpha in the HI cerebral hemisphere, peaking from 8 to 72 hours after lesioning. Immunocytochemistry assays revealed that MIP-1alpha was constitutively expressed in physiologically activated microglia; from 8 to 120 hours after lesioning, MIP-1alpha immunoreactive monocytes and microglia accumulated in the lesion territory. In immunoreactive cells, MIP-1alpha was diffusely distributed throughout the cytoplasm at early post-HI time intervals; by 72 hours, MIP-1alpha immunoreactivity was typically concentrated adjacent to the nucleus, a pattern indicative of active MIP-1alpha production. In P7 to P12 brain, many cells expressed MIP-1alpha receptors; both CCR1 and CCR5 immunoreactivity were localized to endothelium and ependyma; CCR1-immunoreactive astrocytes and neurons and CCR5-immunoreactive microglia were also identified.

CONCLUSIONS

These data implicate MIP-1alpha as a mediator of the complex and sustained inflammatory response initiated by perinatal HI braininjury.

Maturation of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV facilitates renal acidification by catalyzing the dehydration of luminal H(2)CO(3). CA IV is expressed in proximal tubules, medullary collecting ducts, and A-intercalated cells of the mature rabbit kidney (Schwartz GJ, Kittelberger AM, Barnhart DA, and Vijayakumar S. Am J Physiol 278: F894-F904, 2000). In view of the maturation of HCO transport in the proximal tubule and collecting duct, the ontogeny of CA IV expression was examined. During the first 2 wk, CA IV mRNA was expressed in maturing cortex and medulla at ~20% of adult levels. The maturational increase was gradual in cortex over 3-5 wk of age but surged in the medulla, so that mRNA levels appeared higher than those in the adult medulla. In situ hybridization showed very little CA IV mRNA at 5 days, with increases in deep cortex and medullary collecting ducts by 21 days. Expression of CA IV protein in the cortex and medulla was minimal at 3 days of age but then apparent in the juxtamedullary region, A-intercalated cells and medullary collecting ducts by 18 days; there was little labeling of the proximal straight tubules of the medullary rays. Thus CA IV expression may be regulated to accommodate the maturational increase in HCO absorption in the proximal tubule. In the medullary collecting duct, there is a more robust maturation of CA IV mRNA and protein, commensurate with the high rate of HCO absorption in the neonatal segment.

Carbonic anhydrase IV is expressed in H(+)-secreting cells of rabbit kidney

Carbonic anhydrase (CA) IV is a membrane-bound enzyme that catalyzes the dehydration of carbonic acid to CO(2) and water. Using peptides from each end of the deduced rabbit CA IV amino acid sequence, we generated a goat anti-rabbit CA IV antibody, which was used for immunoblotting and immunohistochemical analysis. CA IV was expressed in a variety of organs including spleen, heart, lung, skeletal muscle, colon, and kidney. Rabbit kidney CA IV had two N-glycosylation sites and was sialated, the apparent molecular mass increasing by at least 11 to approximately 45 kDa in the cortex. Medullary CA IV was much more heavily glycosylated than CA IV from cortex or any other organ, such modifications increasing the molecular mass by at least 20 kDa. CA IV was expressed on the apical and basolateral membranes of proximal tubules with expression levels on the order of S2 > S1 > S3 = 0. Because CA IV is believed to be anchored to the apical membrane by glycosylphosphatidylinositol, the presence of basolateral CA IV suggests an alternative mechanism. CA IV was localized on the apical membranes of outer medullary collecting duct cells of the inner stripe and inner medullary collecting duct cells, as well as on alpha-intercalated cells. However, CA IV was not expressed by beta-intercalated cells, glomeruli, distal tubule, or Henle's loop cells. Thus CA IV was expressed by H(+)-secreting cells of the rabbit kidney, suggesting an important role for CA IV in urinary acidification.

Postnatal development of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV activity facilitates renal acidification by catalyzing the dehydration of luminal carbonic acid. CA IV has been localized to the proximal tubules and medullary collecting ducts. Maturation of CA IV expression has been considered to be important in the development of renal acid excretion. The purpose of the present study was to determine the maturational expression of CA IV in rabbit kidney. A guinea pig polyclonal antibody to purified rabbit lung microsomal membrane CA IV was generated. Immunoblotting of membrane proteins after peptide-N-glycosidase F treatment revealed two N-glycosylation sites and reduction in size from approximately 52 to 35 kDa; there appeared to be heavier glycosylation in the medulla. In membrane and total proteins from the kidney cortex, CA IV was 15-30% of the adult level during the first 2 wk of life but increased to mature levels by 5 wk of age. The maturational pattern in the cortex was confirmed by measuring SDS-resistant CA hydratase activity. In the medulla, both membrane and total proteins were generally less than one-fourth of the adult level of CA IV during the first 2 wk of life before reaching mature levels by 5 wk of age. Immunohistochemistry showed staining in proximal tubules (apical > basolateral), with maximal label in the S2 segment. CA IV also appeared on the apical membranes of a minority cell type of the cortical collecting duct, presumably the alpha-intercalated cell. Several labeled cells also appeared to be the process of being extruded from medullary collecting ducts of 1- to 2-wk rabbits. The antibody did not reliably detect medullary CA IV expression in sections from mature rabbits. These studies indicate that there is a substantial postnatal increase in expression of CA IV in the maturing kidney in both the cortex and medulla. The disappearance of intercalated cells in the maturing rabbit medullary collecting duct may be part of a normal renal developmental program as previously reported [J. Kim, J.-H. Cha, C. C. Tisher, and K. M. Madsen. Am. J. Physiol. 270 (Renal Fluid Electrolyte Physiol. 39): F575-F592, 1996]. It is likely that the maturation of CA IV expression contributes to the increase in renal acidification observed early in postnatal life.

Clear correlation of genotype with disease phenotype in very-long-chain acyl-CoA dehydrogenase deficiency

Very-long-chain acyl-CoA dehydrogenase (VLCAD) catalyzes the initial rate-limiting step in mitochondrial fatty acid beta-oxidation. VLCAD deficiency is clinically heterogenous, with three major phenotypes: a severe childhood form, with early onset, high mortality, and high incidence of cardiomyopathy; a milder childhood form, with later onset, usually with hypoketotic hypoglycemia as the main presenting feature, low mortality, and rare cardiomyopathy; and an adult form, with isolated skeletal muscle involvement, rhabdomyolysis, and myoglobinuria, usually triggered by exercise or fasting. To examine whether these different phenotypes are due to differences in the VLCAD genotype, we investigated 58 different mutations in 55 unrelated patients representing all known clinical phenotypes and correlated the mutation type with the clinical phenotype. Our results show a clear relationship between the nature of the mutation and the severity of disease. Patients with the severe childhood phenotype have mutations that result in no residual enzyme activity, whereas patients with the milder childhood and adult phenotypes have mutations that may result in residual enzyme activity. This clear genotype-phenotype relationship is in sharp contrast to what has been observed in medium-chain acyl-CoA dehydrogenase deficiency, in which no correlation between genotype and phenotype can be established.

Lower incidence of necrotizing enterocolitis in infants fed a preterm formula with egg phospholipids

Necrotizing enterocolitis (NEC) causes approximately 4000 deaths/y and significant morbidity among U.S.-born preterm infants alone. Various combinations of inadequate tissue oxygenation, bacterial overgrowth, and enteral feeding with immaturity may cause the initial damage to intestinal mucosa that culminates in necrosis. Presently, there is not a way to predict the onset of the disease or to prevent its occurrence. As part of risk-benefit assessment, we compared disease in hospitalized preterm infants fed a commercial (control) preterm formula or an experimental formula with egg phospholipids for a randomized, double-masked, clinical study of diet and infant neurodevelopment. Infants fed the experimental formula developed significantly less stage II and III NEC compared with infants fed the control formula (2.9 versus 17.6%, p < 0.05), but had similar rates of bronchopulmonary dysplasia (23.4 versus 23.5%), septicemia (26 versus 31%), and retinopathy of prematurity (38 versus 40%). Compared with the control formula, the experimental formula provided 7-fold more esterified choline, arachidonic acid (AA, 0.4% of total fatty acids), and docosahexaenoic acid (0.13%). Phospholipids are constituents of mucosal membranes and intestinal surfactant, and their components, AA and choline, are substrates for intestinal vasodilatory and cytoprotective eicosanoids (AA) and the vasodilatory neurotransmitter, acetylcholine (choline), respectively. One or more of these components of egg phospholipids may have enhanced one or more immature intestinal functions to lower the incidence of NEC in this study. Regardless of the potential mechanism, a larger randomized trial designed to test the effect of this egg phospholipid-containing formula on NEC seems warranted.

Insulin-like growth factor I receptor gene expression during postnatal development of rabbit kidney

BACKGROUND

Insulin-like growth factor-I (IGF-I) is a peptide growth factor whose biological effects are mediated through a specific receptor (IGF-IR). IGF-I and IGF-IR are detected in both fetal and adult kidneys and have both metabolic and growth effects. IGF-IR expression during postnatal kidney development is not well defined and the biological role of this receptor during the postnatal stage is not clearly established. The purpose of the present study was to analyze IGF-IR gene expression during the postnatal development of rabbit kidney to achieve a better understanding of the correlation between growth and differentiation of kidney tissues and IGF-IR expression.

METHODS

Using in situ hybridization, we studied changes in IGF-IR expression in the kidneys of newborn rabbits and those up to 35 days old. Evaluation of the stage of kidney development and morphological maturation was made on histological sections stained with hematoxylin-eosin.

RESULTS

High levels of IGF-IR gene expression in the rabbit kidney occurred in the last stages of postnatal development and in the adult stages; during the development of the subcapsular metanephrogenic zone, IGF-IR gene expression was not observed. IGF-IR mRNA was expressed by proximal and distal tubules and by collecting ducts after these tissues attained morphological maturation. The appearance of IGF-IR mRNA in these kidney structures followed a precise temporo-spatial sequence. IGF-IR was not expressed by renal corpuscles, Henle's loops, inner medullary collecting ducts, vessels, or interstitial cells at any study stage.

CONCLUSIONS

The temporal and spatial patterns of IGF-IR gene expression during postnatal development of the rabbit kidney suggest that IGF-IR and its ligands are relevant for the acquisition of the function, and not for development events, by proximal and distal tubules and collecting ducts. This study also suggests that IGF-IR mRNA localization constitutes a useful marker to determine the functional maturation of these renal structures.