Caspase activity is required for nephrogenesis in the developing mouse metanephros

Low glomerular number at birth can lead to the development of chronic kidney disease

Chronic kidney disease (CKD) prevalence is increasing worldwide, and reducing the number of patients with CKD is of utmost importance. The environment during the fetal, perinatal, and early childhood stages may influence CKD development (developmental origins of health and disease). Under conditions of maternal malnutrition, the glomerular number of infants reduces, and the risk of developing CKD may increase. Nephron progenitor cells and ureteric buds interact with each other to form glomeruli at the tip of the ureteric bud. Thus, the number of glomeruli is determined by the number of ureteric bud branches, which are reportedly decreased due to maternal malnutrition, in turn reducing the glomerular number. Four possible mechanisms can explain the low glomerular number resulting from maternal malnutrition: 1) suppression of c-Ret expression, 2) suppression of nephron formation by renin-angiotensin-aldosterone system inhibition, 3) exposure to excess glucocorticoids, and 4) promotion of apoptosis. Additionally, nephron formation does not continue after birth in humans. Therefore, a low glomerular number at birth is a lifelong burden on the glomeruli and increases the risk of developing CKD. Therefore, it is important to maintain the glomerular number at birth. Accurate glomerular counts are essential for conducting studies on the glomerular number. The dissector/fractionator method is the gold standard; however, it can only be performed at some institutions. Recently, methods have been developed to measure the glomerular number by combining computed tomography and pathological examination and measure the glomerular count using magnetic resonance imaging. Models of decreased and increased glomerular numbers have been developed. Moreover, research regarding the causes of decreased glomerular number and its relationship with development of lifestyle-related diseases and renal dysfunction has significantly progressed, furthering our understanding of the importance of glomerular number.

Structural and functional changes in the kidney caused by adverse fetal and neonatal environments

Health and disease risk in the adulthood are known to be affected by the early developmental environment. Kidney diseases are one of these diseases, and kidneys are altered both structurally and functionally by adverse pre- and perinatal events. The most known structural change is low nephron number seen in subjects born low birth weight and/or preterm. In various animal models of intrauterine growth restriction (IUGR), one of the causes of low birth weight, the mechanism of low nephron number was investigated. While apoptosis of metanephric mesenchyme has been suggested to be the cause, I showed that suppression of ureteric branching, global DNA methylation, and caspase-3 activity also contributes to the mechanism. Other structural changes caused by adverse fetal and neonatal environments include peritubular and glomerular capillary rarefaction and low podocyte endowment. These are aggravated by postnatal development of focal glomerulosclerosis and tubulointerstitial fibrosis that result from low nephron number. Functional changes can be seen in tubules, endothelium, renin-angiotensin system, sympathetic nervous system, oxidative stress, and others. As an example, I reported that aggravated nitrosative stress in a rat IUGR model resulted in more severe tubular necrosis and tubulointerstitial fibrosis after unilateral ureteral obstruction. The mechanism of various functional changes needs to be clarified but may be explained by epigenetic modifications.

CRKL, AIFM3, AIF, BCL2, and UBASH3A during Human Kidney Development

We aimed to investigate the spatio-temporal expression of possible CAKUT candidate genes CRKL, AIFM3, and UBASH3A, as well as AIF and BCL2 during human kidney development. Human fetal kidney tissue was stained with antibodies and analyzed by fluorescence microscopy and RT-PCR. Quantification of positive cells was assessed by calculation of area percentage and counting cells in nephron structures. Results showed statistically significant differences in the temporal expression patterns of the examined markers, depending on the investigated developmental stage. Limited but strong expression of CRKL was seen in developing kidneys, with increasing expression up to the period where the majority of nephrons are formed. Results also lead us to conclude that AIFM3 and AIF are important for promoting cell survival, but only AIFM3 is considered a CAKUT candidate gene due to the lack of AIF in nephron developmental structures. Our findings imply great importance of AIFM3 in energy production in nephrogenesis and tubular maturation. UBASH3A raw scores showed greater immunoreactivity in developing structures than mature ones which would point to a meaningful role in nephrogenesis. The fact that mRNA and proteins of CRKL, UBASH3A, and AIFM3 were detected in all phases of kidney development implies their role as renal development control genes.

Caspase-3 regulates ureteric branching in mice via cell migration

Inhibition of caspase-3 (Casp3) reduces ureteric branching in organ culture but the mechanism remains unclear. Since Casp3 has non-apoptotic functions, we examined whether Casp3 regulates ureteric branching by promoting cell migration, using a ureteric bud (UB) cell line and Casp3-deficient (Casp3-/-) mice. Also, we examined whether Casp3 plays a role in the reduced ureteric branching of metanephroi from nutrient restricted mothers, in which Casp3 activity is suppressed. A Casp3 inhibitor Ac-DNLD-CHO reduced FGF2-induced cord formation of UB cells in 3D culture. UB cell migration assessed by Boyden chamber and wound healing assays was inhibited by Ac-DNLD-CHO. Glomerular number was reduced by ≈ 30%, and ureteric tip number was lower in Casp3-/- mice compared with controls. Maternal nutrient restriction decreased ureteric tip number in controls but not in Casp3-/-. In conclusion, Casp3 regulates ureteric branching by promoting UB cell migration. Inhibited ureteric branching by maternal nutrient restriction may be mediated by Casp3.

The Origins and Functions of Tissue-Resident Macrophages in Kidney Development

The adult kidney hosts tissue-resident macrophages that can cause, prevent, and/or repair renal damage. Most of these macrophages derive from embryonic progenitors that colonize the kidney during its development and proliferate in situ throughout adulthood. Although the precise origins of kidney macrophages remain controversial, recent studies have revealed that embryonic macrophage progenitors initially migrate from the yolk sac, and later from the fetal liver, into the developing kidney. Once in the kidney, tissue-specific transcriptional regulators specify macrophage progenitors into dedicated kidney macrophages. Studies suggest that kidney macrophages facilitate many processes during renal organogenesis, such as branching morphogenesis and the clearance of cellular debris; however, little is known about how the origins and specification of kidney macrophages dictate their function. Here, we review significant new findings about the origins, specification, and developmental functions of kidney macrophages.

Deletion of β1-integrin in collecting duct principal cells leads to tubular injury and renal medullary fibrosis

The renal collecting duct (CD) contains two major cell types, intercalated (ICs) and principal cells (PCs). A previous report showed that deletion of β1-integrin in the entire renal CD causes defective CD morphogenesis resulting in kidney dysfunction. However, subsequent deletion of β1-integrin specifically in ICs and PCs, respectively, did not cause any morphological defects in the CDs. The discrepancy between these studies prompts us to reinvestigate the role of β1-integrin in CD cells, specifically in the PCs. We conditionally deleted β1-integrin in mouse CD PCs using a specific aquaporin-2 (AQP2) promoter Cre-LoxP system. The resulting mutant mice, β-1^f/fAQP2-Cre+, had lower body weight, failed to thrive, and died around 8-12 wk. Their CD tubules were dilated, and some of them contained cellular debris. Increased apoptosis and proliferation of PCs were observed in the dilated CDs. Trichrome staining and electron microscopy revealed the presence of peritubular and interstitial fibrosis that is associated with increased production of extracellular matrix proteins including collagen type IV and fibronectin, as detected by immunoblotting. Further analysis revealed a significantly increased expression of transforming growth factor-β (TGF-β)-induced protein, fibronectin, and TGF-β receptor-1 mRNAs and concomitantly increased phosphorylation of SMAD-2 that indicates the activation of the TGF-β signaling pathway. Therefore, our data reveal that normal expression of β1-integrin in PCs is a critical determinant of CD structural and functional integrity and further support the previously reported critical role of β1-integrin in the development and/or maintenance of the CD structure and function.

Maternal nutrient restriction inhibits ureteric bud branching but does not affect the duration of nephrogenesis in rats

BACKGROUND

Maternal nutrient restriction produces offspring with fewer nephrons. We studied whether the reduced nephron number is due to the inhibition of ureteric branching or early cessation of nephrogenesis in rats. Signaling pathways involved in kidney development were also examined.

METHODS

The offspring of dams given food ad libitum (control (CON)) and those subjected to 50% food restriction (nutrient restriceted (NR)) were examined.

RESULTS

At embryonic day 13 (E13), there was no difference between NR and CON in body weight or kidney size. Ureteric buds branched once in both NR and CON. At E14 and E15, body and kidney size were significantly reduced in NR. Ureteric bud tip numbers were also reduced to 50% of CON. On the other hand, the disappearance of nephrogenic zone and a nephron progenitor marker Cited1 was not different between CON and NR. The final glomerular number of NR was 80% of CON. Activated extracellular signal-regulated kinase (ERK), p38, PI3K, Akt, and mammallian target of rapamycin (mTOR), and protein expression of β-catenin were downregulated at E15.

CONCLUSION

Ureteric branching is inhibited and developmentally regulated signaling pathways are downregulated at an early stage by maternal nutrient restriction. These changes, not early cessation of nephrogenesis, may be a mechanism for the inhibited kidney growth and nephrogenesis.

The regulation of apoptosis in kidney development: implications for nephron number and pattern?

Apoptosis is essential to remodel developing structures and eliminate superfluous cells in a controlled manner during normal development, and continues to be an important component of tissue remodeling and regeneration during an organism's lifespan, or as a response to injury. This mini review will discuss recent studies that have provided insights into the roles of apoptosis in the determination of nephron number and pattern, during normal and abnormal kidney development. The regulation of congenital nephron endowment has implications for risk of chronic kidney disease in later life, whereas abnormalities in nephron pattern are associated with congenital anomalies of the kidney and urinary tract (the leading cause of renal disease in children). Tight regulation of apoptosis is required in normal renal morphogenesis, although many questions remain regarding the regulation of apoptosis by genetic, epigenetic, and environmental factors, in addition to the functional requirement of different components of the apoptotic pathway.

Kidney and urinary tract development: an apoptotic balancing act

Development of the mammalian urogenital system requires a balance between survival and programmed cell death. Pro-survival molecules are crucial in preserving metanephric mesenchyme viability, and thus allowing nephrogenesis to proceed. At the same time, localized areas of apoptosis mediated by effector caspases are required for the appropriate morphogenesis of the kidney and urinary tract. Activation of the intrinsic pathway of apoptosis seems to be fundamental to the progression of cell death necessary to aid ureteric bud branching, nephrogenesis, and ureter-bladder connection. Here, we review what is currently known about survival and apoptosis in building functional kidneys and urinary tracts.

Primary enamel knot cell death in Apaf-1 and caspase-9 deficient mice

During molar development, apoptosis occurs in a well-characterised pattern suggesting several roles for cell death in odontogenesis. However, molecular mechanisms of dental apoptosis are only poorly understood. In this study, Apaf-1 and caspase-9 knockouts were used to uncover the engagement of these members of the apoptotic machinery during early tooth development, concentrating primarily on their function in the apoptotic elimination of primary enamel knot cells. Molar tooth germ morphology, proliferation and apoptosis were investigated on frontal histological sections of murine heads at embryonic days (ED) 15.5, the stage when the primary enamel knot is eliminated apoptotically. In molar tooth germs of both knockouts, no apoptosis was observed according to morphological (haematoxylin-eosin) as well as biochemical criteria (TUNEL). Morphology of the mutant tooth germs, however, was not changed. Additionally, knockout mice showed no changes in proliferation compared to wild type mice. According to our findings on knockout embryos, Apaf-1 and caspase-9 are involved in apoptosis during tooth development; however, they seem dispensable and not necessary for proper tooth shaping. Compensatory or other mechanisms of cell death may act to eliminate the primary enamel knot cells in the absence of Apaf-1 and caspase-9.

Role of mitotic, pro-apoptotic and anti-apoptotic factors in human kidney development

The expression pattern of mitotic Ki-67 and anti-apoptotic bcl-2 proteins, as well as apoptotic caspase-3 and p53 proteins, were investigated in the human mesonephros and metanephros of 5-9 week-old human conceptuses. Apoptotic cells were additionally detected using the terminal deoxynucleotidyl transferase (TdT) nick-end labelling (TUNEL) method. Between the 5th and 7th developmental weeks Ki-67, caspase-3 and TUNEL-positive cells characterized all mesonephric structures, indicating importance of cell proliferation in the growth of the mesonephros and role of apoptosis in nephrogenesis. From the 7th week on, p53 and bcl-2 positive cells appeared in the mesonephros as well. Regressive changes in the mesonephros could be regulated by activation of p53, while bcl-2 could contribute to selective survival of some tubules giving rise to adult structures. In the early human metanephros (5-7 weeks), Ki-67 positive cells characterized all metanephric structures, indicating a role of cell proliferation in branching of the ureteric bud and in nephron formation. During the same period bcl-2, caspase-3 and TUNEL-positive cells were found only in the metanephric mesenchyme and nephrons. Bcl-2 protein probably protected nephrons from apoptosis, while caspase-3 protein controlled cell death in the mesenchyme. At later stages (7-9-weeks), appearance of p53-expressing cells could participate in further morphogenesis of the metanephric collecting system. The factors investigated had a spatially and temporally restricted pattern of appearance in developing kidneys. Changes in that pattern might lead to serious disturbances of kidney formation and function in early childhood.

Spatiotemporal regulation of morphogenetic molecules during in vitro branching of the isolated ureteric bud: toward a model of branching through budding in the developing kidney

In search of guiding principles involved in the branching of epithelial tubes in the developing kidney, we analyzed branching of the ureteric bud (UB) in whole kidney culture as well as in isolated UB culture independent of mesenchyme but in the presence of mesenchymally derived soluble factors. Microinjection of the UB lumen (both in the isolated UB and in the whole kidney) with fluorescently labeled dextran sulfate demonstrated that branching occurred via smooth tubular epithelial outpouches with a lumen continuous with that of the original structure. Epithelial cells within these outpouches cells were wedge-shaped with actin, myosin-2 and ezrin localized to the luminal side, raising the possibility of a "purse-string" mechanism. Electron microscopy and decoration of heparan sulfates with biotinylated FGF2 revealed that the basolateral surface of the cells remained intact, without the type of cytoplasmic extensions (invadopodia) that are seen in three-dimensional MDCK, mIMCD, and UB cell culture models of branching tubulogenesis. Several growth factor receptors (i.e., FGFR1, FGFR2, c-Ret) and metalloproteases (i.e., MT1-MMP) were localized toward branching UB tips. A large survey of markers revealed the ER chaperone BiP to be highly expressed at UB tips, which, by electron microscopy, are enriched in rough endoplasmic reticulum and Golgi, supporting high activity in the synthesis of transmembrane and secretory proteins at UB tips. After early diffuse proliferation, proliferating and mitotic cells were mostly found within the branching ampullae, whereas apoptotic cells were mostly found in stalks. Gene array experiments, together with protein expression analysis by immunoblotting, revealed a differential spatiotemporal distribution of several proteins associated with epithelial maturation and polarization, including intercellular junctional proteins (e.g., ZO-1, claudin-3, E-cadherin) and the subapical cytoskeletal/microvillar protein ezrin. In addition, Ksp-cadherin was found at UB ampullary cells next to developing outpouches, suggesting a role in epithelial-mesenchymal interactions. These data from the isolated UB culture system support a model where UB branching occurs through outpouching possibly mediated by wedge-shaped cells created through an apical cytoskeletal purse-string mechanism. Additional potential mechanisms include (1) differential localization of growth factor receptors and metalloproteases at tips relative to stalks; (2) creation of a secretory epithelium, in part manifested by increased expression of the ER chaperone BiP, at tips relative to stalks; (3) after initial diffuse proliferation, coexistence of a balance of proliferation vs. apoptosis favoring tip growth with a very different balance in elongating stalks; and (4) differential maturation of the tight and adherens junctions as the structures develop. Because, without mesenchyme, both lateral and bifid branching occurs (including the ureter), the mesenchyme probably restricts lateral branching and provides guidance cues in vivo for directional branching and elongation as well as functioning to modulate tubular caliber and induce differentiation. Selective cadherin, claudin, and microvillar protein expression as the UB matures likely enables the formation of a tight, polarized differentiated epithelium. Although, in vivo, metanephric mesenchyme development occurs simultaneously with UB branching, these studies shed light on how (mesenchymally derived) soluble factors alone regulate spatial and temporal expression of morphogenetic molecules and processes (proliferation, apoptosis, etc.) postulated to be essential to the UB branching program as it forms an arborized structure with a continuous lumen.

Cloning and characterization of a novel gene promoting ureteric bud branching in the metanephros

BACKGROUND

The ureteric buds and metanephric mesenchymal cells reciprocally induce each other's maturation during kidney development, and implicated transcription factors, secreted growth factors, and cell surface signaling peptides are critical regulators of renal branching morphogenesis. Protein kinase C (PKC) is a key enzyme in the signal transduction mechanisms in various biologic processes, including development, because it regulates growth and differentiation. Inhibition of PKC by the sphingolipid product ceramide interferes with nephron formation in the developing kidney, but the molecule that controls ureteric bud branching downstream of PKC is still unknown.

METHODS

Differential display polymerase chain reaction (PCR) of metanephroi cultured with a PKC activator and inhibitor was performed. We also examined the role of a novel gene in kidney development with organ culture system.

RESULTS

A novel gene encoding a 759 bp mRNA was identified, and we named it metanephros-derived tubulogenic factor (MTF)/L47. Inhibition of MTF with antisense oligonucleotide impaired ureteric bud branching by cultured metanephroi, and addition of recombinant MTF protein promoted ureteric bud branching in cultured metanephroi and increased cell proliferation.

CONCLUSION

We identified a novel molecule in developing kidney that is capable of modulating ureteric bud branching and kidney differentiation.

Caspase-9 takes part in programmed cell death in developing mouse kidney

Programmed cell death is a mechanism by which organisms dispose of unwanted cells, and it is thought to be an important process in organogenesis. We have already reported the role of caspase-3 in the developing metanephros. While caspase-3 is thought to be positioned downstream of the caspase-activating cascade, the upstream caspase for programmed cell death in the developing kidney is still unknown. In an attempt to identify it, we blocked caspase activity in metanephric explants with caspase inhibitors. Administration of a caspase-9 inhibitor (Ac-IETD-CHO) effectively prevented both ureteric bud branching and nephrogenesis, the same as a caspase-3 inhibitor (Ac-DEVD-CHO). On the other hand, administration of a caspase-8 inhibitor (Ac-LETD-CHO) did not inhibit ureteric bud branching or nephrogenesis. Apaf-1, which executes programmed cell death in the caspase-9-related pathway, was detected in the cells exhibiting caspase-9 activity, and our results suggest that Apaaf-1/caspase-9 activates caspase-3 in kidney organogenesis.

Protein restriction in pregnancy is associated with increased apoptosis of mesenchymal cells at the start of rat metanephrogenesis

BACKGROUND

In rats, offspring born to mothers supplied low protein diets during pregnancy have fewer glomeruli than normal. We hypothesized that such nephron deficits are associated with altered cell turnover in the metanephros, the embryonic precursor of the adult kidney.

METHODS

Wistar rats were supplied with one of three isocaloric diets from day 0 of pregnancy: control (18% protein) or low protein (9% or 6%) diets. All had a normal chow after birth. Groups were compared by multilevel statistical modeling.

RESULTS

At two weeks postnatally, when nephrogenesis has finished, controls had 16.8 x 103 +/- 0.7 x 10(3) (mean +/- SEM) glomeruli/kidney, whereas offspring exposed to 9% diet had 5.1 x 10(3) +/- 1.2 x 10(3) fewer and those exposed to 6% diet had 6.9 x 10(3) +/- 1.7 x 10(3) fewer glomeruli/kidney (P < 0.001, both diets). At embryonic day 13 (E13), when the metanephros has just formed, control metanephroi contained 2.35 x 10(4) +/- 0.15 x 10(4) cells, with no significant differences in low protein groups. At E15, when mesenchyme begins forming primitive nephrons but glomeruli are still absent, controls had 2.00 x 10(6) +/- 0.13 x 10(6) cells. E15 embryos exposed to 9% protein had 1.09 x 10(6) +/- 0.36 x 10(6) fewer cells/metanephros than controls, while those exposed to 6% diet had 1.45 x 10(6) +/- 0.37 x 10(6) fewer (P < 0.01, both diets). Apoptotic cells were detected by molecular (in-situ end-labeling) and morphological (propidium iodide staining) techniques. In all diets, apoptosis was noted in condensing mesenchyme (nephron precursors) and loose mesenchyme (interstitial precursors). Control E13 metanephroi had 63 +/- 7 apoptotic cells/mm2, whereas those exposed to 9% diet had an increase of 77 +/- 26 cells/mm2 (P < 0.01) and those exposed to 6% diet had an increase of 55 +/- 26 cells/mm2 (P < 0.05). By E15, apoptosis was similar in all groups but metanephric mitosis was significantly increased in the 6% protein diet group. No change was found in the level of apoptosis in E13 mesonephroi.

CONCLUSIONS

Maternal low protein diets reduce final numbers of glomeruli in association with enhanced deletion of mesenchymal cells at the start of kidney development. Whether aberrant nephrogenesis is a direct effect from deletion of nephron precursors, or an indirect effect from loss of supportive interstitial precursors, requires further investigation.

Apoptosis – a death-inducing mechanism tightly linked with morphogenesis in Hydractina echinata (Cnidaria, Hydrozoa)

Programmed cell death is not only known as a mechanism mediating tissue destruction, but also as an organismic tool for body shaping and regulation of morphological events during development. Here we report the tight and vital link of the most prominent form of programmed cell death, apoptosis, to one of the oldest, most basic, and most radical developmental processes, the metamorphosis of the marine hydrozoon Hydractinia echinata. Apoptosis, represented by DNA fragmentation, appears very early during metamorphosis, approximately 20 minutes post induction. It is then executed in a very distinct spatial and temporal pattern, including the removal or phagocytosis of a large number of larval cells prior to the appearance of stolons and tentacles. Our data indicate a developmental program striving to reduce all body parts that are no longer necessary, before reaching a distinct turning point, when the development of adult features is initiated. During these events, morphogenesis of basal and apical structures correlates with recycling of that particular larval region, indicated by the presence of apoptosis. Based on these data, the necessity of apoptosis for normal development of adult patterns is inferred and a fundamental association of apoptosis with developmental processes can be stated.

Intracellular and extracellular regulation of ureteric bud morphogenesis

The urinary collecting duct system of the permanent kidney develops by growth and branching of an initially unbranched epithelial tubule, the ureteric bud. Formation of the ureteric bud as an outgrowth of the wolffian duct is induced by signalling molecules (such as GDNF) that emanate from the adjacent metanephrogenic mesenchyme. Once it has invaded the mesenchyme, growth and branching of the bud is controlled by a variety of molecules, such as the growth factors GDNF, HGF, TGFbeta, activin, BMP-2, BMP-7, and matrix molecules such as heparan sulphate proteoglycans and laminins. These various influences are integrated by signal transduction systems inside ureteric bud cells, with the MAP kinase, protein kinase A and protein kinase C pathways appearing to play major roles. The mechanisms of morphogenetic change that produce branching remain largely obscure, but matrix metalloproteinases are known to be necessary for the process, and there is preliminary evidence for the involvement of the actin/myosin contractile cytoskeleton in creating branch points.