Developmental renal pathology: its past, present, and future

From the radiologic pathology archives: pediatric polycystic kidney disease and other ciliopathies: radiologic-pathologic correlation

Genetic defects of cilia cause a wide range of diseases, collectively known as ciliopathies. Primary, or nonmotile, cilia function as sensory organelles involved in the regulation of cell growth, differentiation, and homeostasis. Cilia are present in nearly every cell in the body and mutations of genes encoding ciliary proteins affect multiple organs, including the kidneys, liver, pancreas, retina, central nervous system (CNS), and skeletal system. Genetic mutations causing ciliary dysfunction result in a large number of heterogeneous phenotypes that can manifest with a variety of overlapping abnormalities in multiple organ systems. Renal manifestations of ciliopathies are the most common abnormalities and include collecting duct dilatation and cyst formation in autosomal recessive polycystic kidney disease (ARPKD), cyst formation anywhere in the nephron in autosomal dominant polycystic kidney disease (ADPKD), and tubulointerstitial fibrosis in nephronophthisis, as well as in several CNS and skeletal malformation syndromes. Hepatic disease is another common manifestation of ciliopathies, ranging from duct dilatation and cyst formation in ARPKD and ADPKD to periportal fibrosis in ARPKD and several malformation syndromes. The unifying molecular pathogenesis of this emerging class of disorders explains the overlap of abnormalities in disparate organ systems and links diseases of widely varied clinical features. It is important for radiologists to be able to recognize the multisystem manifestations of these syndromes, as imaging plays an important role in diagnosis and follow-up of affected patients.

Fetal genitourinary imaging

The fetal urinary tract is routinely evaluated sonographically beginning in the first trimester with documentation of fetal bladder visualization. Fetal MR might be indicated to further clarify abnormalities found sonographically. The primary imaging modality for evaluation of the fetal kidney is US, which plays an important role in the detection of collecting system dilatation and parenchymal diseases that influence counseling and postnatal care. A commonly seen birth defect affecting the fetal kidney is pyelectasis. The significance of this finding has been extensively evaluated by a number of fetal imaging centers and will be presented in this review. Further topics of interest within the fetal genitourinary system include fetal renal parenchymal disorders and fetal bladder abnormalities. Characteristic imaging features, as well as developmental pathology and differential considerations, are discussed here.

Obesity-related nephropathy in children

Concurrent with the global obesity epidemic, there is an increasing number of people of all ages developing chronic kidney disease associated with obesity. In adults, the definition of obesity is a BMI greater than 30 kg/m2. Whereas, in children, a BMI greater than the 85th percentile for age is considered overweight and greater than the 95th percentile is classified as obese. Clinical and pathologic characteristics of a distinct nephropathy have emerged independent of that of diabetic or hypertensive glomerulosclerosis. These include a silent presentation in an obese individual with heavy proteinuria, normal serum albumin and the absence of edema. Renal pathologic findings are notable for mesangial matrix expansion, glomerular hypertrophy and reduced density of podocytes with detachment of foot processes from the glomerular basement membrane. These findings are frequently associated with the development of secondary focal segmental glomerulosclerosis. Obesity alone does not appear to be the sole mediator of this nephropathy. It is most likely the ‘second hit’ for individuals who have congenital or acquired reduced nephron mass as well as an inherited genetic vulnerability to the metabolic consequences imposed by cytokines released by adipose tissue. In children, those born of low birthweight, whether small for gestational age and/or preterm, are likely to have reduced nephron mass as well as an increased tendency for early insulin resistance and the development of obesity and the metabolic syndrome. This in turn is perpetuated by the practice of feeding high-calorie fortified formulas to low-birthweight infants. Rapid catch-up growth, early obesity and insulin resistance are major contributors to the emergence of obesity-related glomerulopathy in children and adolescents. Early detection requires recognizing the demographics of high-risk infants and monitoring them for the development of hypertension, elevated glomerular filtration rate, hyperfiltration and proteinuria. After 6 months of age, angiotensin-blocking agents may be used to control blood pressure, glomerular hyperfiltration and proteinuria. If obesity is present, a comprehensive program of weight loss, including diet and exercise, should be the mainstay of treatment. In older children and adolescents, lipid-lowering medications may be indicated. With morbid obesity, bariatric surgery may be an option.

Reactive oxygen species in the presence of high glucose alter ureteric bud morphogenesis

Renal malformations are a major cause of childhood renal failure. During the development of the kidney, ureteric bud (UB) branching morphogenesis is critical for normal nephrogenesis. These studies investigated whether renal UB branching morphogenesis is altered by a high ambient glucose environment and studied underlying mechanism(s). Kidney explants that were isolated from different periods of gestation (embryonic days 12 to 18) from Hoxb7-green fluorescence protein mice were cultured for 24 h in either normal d-glucose (5 mM) or high d-glucose (25 mM) medium with or without various inhibitors. Alterations in renal morphogenesis were assessed by fluorescence microscopy. Paired-homeobox 2 (Pax-2) gene expression was determined by real-time quantitative PCR, Western blotting, and immunohistology. The results revealed that high d-glucose (25 mM) specifically stimulates UB branching morphogenesis via Pax-2 gene expression, whereas other glucose analogs, such as d-mannitol, l-glucose, and 2-deoxy-d-glucose, had no effect. The stimulatory effect of high glucose on UB branching was blocked in the presence of catalase and inhibitors of NADPH oxidase, mitochondrial electron transport chain complex I, and Akt signaling. Moreover, in in vivo studies, it seems that high glucose induces, via Pax-2 (mainly localized in UB), acceleration of UB branching but not nephron formation. Taken together, these data demonstrate that high glucose alters UB branching morphogenesis. This occurs, at least in part, via reactive oxygen species generation, activation of Akt signaling, and upregulation of Pax-2 gene expression.

Reactive oxygen species and nuclear factor-kappa B pathway mediate high glucose-induced Pax-2 gene expression in mouse embryonic mesenchymal epithelial cells and kidney explants

Diabetic mellitus confers a major risk of congenital malformations, and is associated with diabetic embryopathy, affecting multiple organs including the kidney. The DNA paired box-2 (Pax-2) gene is essential in nephrogenesis. We investigated whether high glucose alters Pax-2 gene expression and aimed to delineate its underlying mechanism(s) of action using both in vitro (mouse embryonic mesenchymal epithelial cells (MK4) and ex vivo (kidney explant from Hoxb7-green florescent protein (GFP) mice) approaches. Pax-2 gene expression was determined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunofluorescent staining. A fusion gene containing the full-length 5'-flanking region of the human Pax-2 promoter linked to a luciferase reporter gene, pGL-2/hPax-2, was transfected into MK4 cells with or without dominant negative IkappaBalpha (DN IkappaBalpha) cotransfection. Fusion gene expression level was quantified by cellular luciferase activity. Reactive oxygen species (ROS) generation was measured by lucigenin assay. Embryonic kidneys from Hoxb7-GFP mice were cultured ex vivo. High D(+) glucose (25 mM), compared to normal glucose (5 mM), specifically induced Pax-2 gene expression in MK4 cells and kidney explants. High glucose-induced Pax-2 gene expression is mediated, at least in part, via ROS generation and activation of the nuclear factor kappa B signaling pathway, but not via protein kinase C, p38 mitogen-activated protein kinase (MAPK), and p44/42 MAPK signaling.