Overexpression of human insulin-like growth factor binding protein-1 in the mouse leads to nephron deficit

IGF Signaling in the Heart in Health and Disease

One of the most vital processes of the body is the cardiovascular system's proper operation. Physiological processes in the heart are regulated by the balance of cardioprotective and pathological mechanisms. The insulin-like growth factor system (IGF system, IGF signaling pathway) plays a pivotal role in regulating growth and development of various cells and tissues. In myocardium, the IGF system provides cardioprotective effects as well as participates in pathological processes. This review summarizes recent data on the role of IGF signaling in cardioprotection and pathogenesis of various cardiovascular diseases, as well as analyzes severity of these effects in various scenarios.

Growth Hormone and IGF1 Actions in Kidney Development and Function

Growth hormone (GH) exerts multiple effects on different organs including the kidneys, either directly or via its main mediator, insulin-like-growth factor-1 (IGF-1). The GH/IGF1 system plays a key role in normal kidney development, glomerular hemodynamic regulation, as well as tubular water, sodium, phosphate, and calcium handling. Transgenic animal models demonstrated that GH excess (and not IGF1) may lead to hyperfiltration, albuminuria, and glomerulosclerosis. GH and IGF-1 play a significant role in the early development of diabetic nephropathy, as well as in compensatory kidney hypertrophy after unilateral nephrectomy. Chronic kidney disease (CKD) and its complications in children are associated with alterations in the GH/IGF1 axis, including growth retardation, related to a GH-resistant state, attributed to impaired kidney postreceptor GH-signaling and chronic inflammation. This may explain the safety of prolonged rhGH-treatment of short stature in CKD.

Insulin-like growth factors and kidney disease

Insulin-like growth factors (IGF-1 and IGF-2) are necessary for normal growth and development. They are related structurally to proinsulin and promote cell proliferation, differentiation, and survival, as well as insulin-like metabolic effects, in most cell types and tissues. In particular, IGFs are important for normal pre- and postnatal kidney development. IGF-1 mediates many growth hormone actions, and both growth hormone excess and deficiency are associated with perturbed kidney function. IGFs affect renal hemodynamics both directly and indirectly by interacting with the renin-angiotensin system. In addition to the IGF ligands, the IGF system includes receptors for IGF-1, IGF-2/mannose-6-phosphate, and insulin, and a family of 6 high-affinity IGF-binding proteins that modulate IGF action. Disordered regulation of the IGF system has been implicated in a number of kidney diseases. IGF activity is enhanced in early diabetic nephropathy and polycystic kidneys, whereas IGF resistance is found in chronic kidney failure. IGFs have a potential role in enhancing stem cell repair of kidney injury. Most IGF actions are mediated by the tyrosine kinase IGF-1 receptor, and inhibitors recently have been developed. Further studies are needed to determine the optimal role of IGF-based therapies in kidney disease.

Evolution of the urinary proteome during human renal development and maturation: variations with gestational and postnatal age

BACKGROUND

Low birth weight is associated with deficits in nephron number in the infant kidney and increased risk of adulthood hypertension and renal dysfunction. Urinary biomarkers may be potential indicators of renal reserve, but little is known about the influence of gestational and postnatal age on the expression of urinary proteins. The aims of this study were to determine the relationships between selected urinary proteins and renal maturation. We hypothesized that urinary protein patterns would change over time during late nephrogenesis and renal maturation.

METHODS

Urine samples were collected at birth and over 12 mo from preterm (33-35 wk) and term (38-40 wk) infants. Candidate urinary proteins were identified by antibody array and quantified with enzyme-linked immunosorbent assay.

RESULTS

Preterm infants at birth were found to have relatively elevated levels of insulin-like growth factor binding protein-1, -2, and -6, monocyte chemotactic protein-1, CD14, and sialic acid-binding Ig-like lectin 5. These markers gradually decline to levels similar to those of full-term infants by 2-6 mo of life. In contrast, many urinary markers in healthy full-term infants remain stable over the first year of life.

CONCLUSION

Gestational and postnatal age must be considered when evaluating the utility of urinary biomarkers.

Insulin-like growth factors in normal and diseased kidney

This article reviews the physiology of the insulin-like growth factor (IGF) system in the kidney and the changes and potential role of this system in selected renal diseases. The potential therapeutic uses of recombinant human IGF-I for the treatment of acute and chronic kidney failure are briefly discussed.

The insulin-like growth factor system in chronic kidney disease: Pathophysiology and therapeutic opportunities

The growth hormone-insulin-like growth factor-insulin-like growth factor binding protein (GH-IGF-IGFBP) axis plays a critical role in the maintenance of normal renal function and the pathogenesis and progression of chronic kidney disease (CKD). Serum IGF-I and IGFBPs are altered with different stages of CKD, the speed of onset, the amount of proteinuria, and the potential of remission. Recent studies demonstrate that growth failure in children with CKD is due to a relative GH insensitivity and functional IGF deficiency. The functional IGF deficiency in CKD results from either IGF resistance due to increased circulating levels of IGFBPs or IGF deficiency due to increased urinary excretion of serum IGF-IGFBP complexes. In addition, not only GH and IGFs in circulation, but locally produced IGFs, the high-affinity IGFBPs, and low-affinity insulin-like growth factor binding protein-related proteins (IGFBP-rPs) may also affect the kidney. With respect to diabetic kidney disease, there is growing evidence suggesting that GH, IGF-I, and IGFBPs are involved in the pathogenesis of diabetic nephropathy (DN). Thus, prevention of GH action by blockade either at the receptor level or along its signal transduction pathway offers the potential for effective therapeutic opportunities. Similarly, interrupting IGF-I and IGFBP actions also may offer a way to inhibit the development or progression of DN. Furthermore, it is well accepted that the systemic inflammatory response is a key player for progression of CKD, and how to prevent and treat this response is currently of great interest. Recent studies demonstrate existence of IGF-independent actions of high-affinity and low-affinity-IGFBPs, in particular, antiinflammatory action of IGFBP-3 and profibrotic action of IGFBP-rP2/CTGF. These findings reinforce the concept in support of the clinical significance of the IGF-independent action of IGFBPs in the assessment of pathophysiology of kidney disease and its therapeutic potential for CKD. Further understanding of GH-IGF-IGFBP etiopathophysiology in CKD may lead to the development of therapeutic strategies for this devastating disease. It would hold promise to use of GH, somatostatin analogs, IGFs, IGF agonists, GHR and insulin-like growth factor-I receptor (IGF-IR) antagonists, IGFBP displacer, and IGFBP antagonists as well as a combination treatment as therapeutic agents for CKD.

Prenatal programming-effects on blood pressure and renal function

Impaired intrauterine nephrogenesis-most clearly illustrated by low nephron number-is frequently associated with low birthweight and has been recognized as a powerful risk factor for renal disease; it increases the risks of low glomerular filtration rate, of more rapid progression of primary kidney disease, and of increased incidence of chronic kidney disease or end-stage renal disease. Another important consequence of impaired nephrogenesis is hypertension, which further amplifies the risk of onset and progression of kidney disease. Hypertension is associated with low nephron numbers in white individuals, but the association is not universal and is not seen in individuals of African origin. The derangement of intrauterine kidney development is an example of a more general principle that illustrates the paradigm of plasticity during development-that is, that transcription of the genetic code is modified by epigenetic factors (as has increasingly been documented). This Review outlines the concept of prenatal programming and, in particular, describes its role in kidney disease and hypertension.

The effects of gestational age and growth restriction on compensatory kidney growth

BACKGROUND

Low birth weight is associated with altered renal development, adult onset hypertension and renal disease. The aim of this prospective longitudinal study was to estimate the renal growth during the first 2 years of life in small-for-gestational age (SGA) infants of varied gestational age (GA) and with differing degrees of growth retardation (GR) at birth. Material and methods. The study included 466 children: SGA, n = 243, and appropriate-for-gestational age (AGA), n = 223, classified according to GA into three groups (28-34, 34-36 and >36 weeks, respectively). The SGA children were also classified according to the degree of GR: birth weight <3rd percentile, and birth weight 3-10th percentiles. Serial renal ultrasonography (US) for kidney length (KL) measurement was performed at the ages of 36 and 40 weeks corrected age and 3, 6, 12 and 24 months of chronological age. The ratios of KL(3) to crown to heel length (CHL), body weight (BW) and body surface area (BSA) were used as estimators of relative kidney length (RKL).

RESULTS

A total of 1898 measurements were performed. In the full-term and near-term SGA infants (GA >36 weeks), RKL was similar to or even higher than that in AGA controls (P < 0.05 at 12 and 24 months). In two groups of preterm infants (GA 34-36, 28-34 weeks), RKL was lower than in AGA controls either after the first 6 months (GA 34-36 group, P < 0.05) or throughout the study period (GA 28-34 group, P < 0.05). The absolute KL was more severely affected in the preterm babies (GA <36 weeks) with BW <3rd percentile than in those of GA 3rd-10th percentile.

CONCLUSION

While in full-term and near-term SGA infants RKL is similar to or even higher than that of AGA infants, in smaller preterm babies (<36 weeks of GA) the RKL is impaired up to the second year of life.

Genetic low nephron number hypertension is associated with dysregulation of the hepatic and renal insulin-like growth factor system during nephrogenesis

OBJECTIVE

Low nephron number may represent a major determinant of human primary hypertension in adult life. This hypothesis is supported by a genetic rat model, namely the Munich-Wistar-Frömter (MWF) rat, which demonstrates an inherited deficit in nephron number and the development of spontaneous hypertension. Insulin-like growth factor (IGF) I and II exert endocrine and paracrine effects that are required for normal growth and nephron development. We tested the hypothesis that low nephron number is already present during fetal development, and the expression pattern of important molecules of the IGF system is altered in MWF rat during the critical period of kidney development.

METHODS

We compared MWF and normal Wistar rats during nephrogenesis at day 19 of fetal development (E19) and adult rats at postnatal day 100 (D100). Histomorphometric analysis was performed by stereological methods. Quantitative messenger RNA and protein expression was determined by real-time polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

At E19, glomerular density (-32%) and hepatic mRNA (-48%) and protein (-18%) expression of IGF-I were decreased (P < 0.05, respectively), whereas renal mRNA expression of IGF-II receptor (+52%) and IGF binding protein 3 (+113%) were increased in MWF compared with Wistar rats (P < 0.05, respectively). Systolic blood pressure, urinary albumin excretion, and mean glomerular area were significantly elevated in MWF compared with Wistar rats at D100 (P < 0.05, respectively).

CONCLUSIONS

The fetal expression of IGF system molecules in the MWF rat model points towards a link between the decreased availability of active IGF-I and IGF-II and the fetal development of low nephron number, with manifestation of genetic hypertension in adult life.

Consequences of Intrauterine Growth Restriction for the Kidney

Low birth weight due to intrauterine growth restriction is associated with various diseases in adulthood, such as hypertension, cardiovascular disease, insulin resistance and end-stage renal disease. The purpose of this review is to describe the effects of intrauterine growth restriction on the kidney. Nephrogenesis requires a fine balance of many factors that can be disturbed by intrauterine growth restriction, leading to a low nephron endowment. The compensatory hyperfiltration in the remaining nephrons results in glomerular and systemic hypertension. Hyperfiltration is attributed to several factors, including the renin-angiotensin system (RAS), insulin-like growth factor (IGF-I) and nitric oxide. Data from human and animal studies are presented, and suggest a faltering IGF-I and an inhibited RAS in intrauterine growth restriction. Hyperfiltration makes the kidney more vulnerable during additional kidney disease, and is associated with glomerular damage and kidney failure in the long run. Animal studies have provided a possible therapy with blockage of the RAS at an early stage in order to prevent the compensatory glomerular hyperfiltration, but this is far from being applicable to humans. Research is needed to further unravel the effect of intrauterine growth restriction on the kidney.

Oligonephropathy, developmental programming and nutritional management of low-gestation newborns

Intrauterine growth retardation is associated with oligonephropathy and the risk of hypertension and renal disease in adulthood. Animal study data show that nephron density is directly related to protein nutrition status during nephrogenesis. Among infants born at low gestation, postnatal growth retardation and major deficits in energy and protein intake have been reported.

CONCLUSION

Since nephrogenesis occurs mainly in late gestation, increasing protein intake during nephrogenesis could minimize the risk of long-term morbidities associated with oligonephropathy.

The role of the kidney in cardiovascular medicine

The relationship between the kidney and hypertension is complex. The kidney is both culprit and victim. Renal disease and even renal structural abnormalities (nephron underdosing) lead to hypertension. On the other hand, blood pressure (even blood pressure values in the range of normotension) accelerates a progressive loss of renal function in patients with primary renal disease. This review discusses some recent work in this field, emphasizing that multiple mechanisms are operative in renal hypertension, particularly a shift in the natriuresis-blood pressure relationship (blood pressure natriuresis), inappropriate activation of the renin-angiotensin system, sympathetic overactivity, and impaired endothelial cell-dependent vasodilatation. It also emphasizes the substantial revision of past recommendations concerning blood pressure targets. In renal patients, blood pressure should be reduced to levels below 125/75 mm Hg. In addition to blood pressure, a reduction in proteinuria is widely considered an additional treatment target.

Glomerular Number and Function Are Influenced by Spontaneous and Induced Low Birth Weight in Rats

A link exists between low birth weight and diseases in adulthood, such as hypertension, cardiovascular disease, and insulin resistance. Intrauterine growth restriction (IUGR) has been used to explain this association and has been shown to lead to a nephron endowment in humans. A reduction in glomerular number has been described in animal models with induced low birth weight as well but not in animals with spontaneous low birth weight. It therefore is debatable whether the models are suitable. The effect on glomerular number and size was studied in rats with naturally occurring IUGR and experimental IUGR, induced by bilateral uterine artery ligation. Design-based stereologic methods were used. Urinary protein excretion was determined as a measure of renal damage. Results showed a decrease of approximately 20% in glomerular number in both groups of IUGR (control 35,400, naturally occurring IUGR 30,900, and experimental IUGR 28,000 glomeruli per kidney). Mean glomerular volume was increased in both IUGR groups, which was associated with an increased proteinuria. It is concluded that IUGR leads to a nephron endowment with a compensatory glomerular enlargement. This compensation is associated with more proteinuria in the long run. Uterine artery ligation in the pregnant rat is a suitable model to study the effects of IUGR on the kidney.

Insulin-Like Growth Factor-1 and Its Binding Proteins, IGFBP-1 and IGFBP-3, in Adolescents with Type-1 Diabetes mellitus and Microalbuminuria

BACKGROUND/AIMS

Numerous clinical and experimental studies suggest that growth factors may contribute to the development of diabetic microvascular complications. The aim of the study was to test the hypothesis that in adolescents with type-1 diabetes mellitus and microalbuminuria (MA) there are specific disorders of serum insulin-like growth factor-1 (IGF-1) and concentrations of its binding proteins, IGFBP-1 and IGFBP-3, that could be of importance in the pathogenesis of microvascular diabetic complications.

METHODS

25 adolescents with MA, 24 adolescents with diabetes without complications, and 17 controls were examined. There were no differences with regard to age, puberty stage, HbA1c and body mass index between the groups examined. Two of the patients in the first group also had diabetic retinopathy. Serum fasting concentrations of IGF-1 and overnight urine albumin concentrations were measured by radioimmunoassay, IGFBP-1 and IGFBP-3 concentrations by immunoradiometric assay and HbA1c by high-performance liquid chromatography methods. Diabetic patients were examined by an experienced ophthalmologist and neurologist. The data were analyzed using Kruskal-Wallis ANOVA and multiple regression analysis.

RESULTS

Significantly lower IGF-1 concentrations were found in adolescents with diabetes and MA compared to diabetic patients without complications and healthy contemporaries. IGFBP-1 concentrations were significantly higher and IGFBP-3 concentrations were statistically lower in diabetic patients with MA than in patients without complications.

CONCLUSIONS

The IGF-IGFBP system is deranged in adolescents with type-1 diabetes mellitus and MA. Our results suggest the participation of circulating IGFBP-1 in the origin of diabetic complications. It could be also possible that IGFBP-3 takes part in the protection from them.

Growth patterns of the heart and kidney suggest inter-organ collaboration in facultative fetal growth

Maternal smoking during pregnancy has been associated with a number of negative sequelae among offspring, including elevated postnatal blood pressure. While animal studies have described organ level alterations with smoke exposure, human data have been more limited. Thirty-four healthy maternal/fetal pairs (24 nonsmokers, 10 smokers) participated in a longitudinal growth study from the thirteenth week of pregnancy to document fetal kidney and heart growth trajectories and morphology. Curve fitting followed by a mixed model for repeated measures identified significantly different growth patterns in kidney width, thickness, length, and volume growth with exposure: the smoke-exposed fetal kidney was wide and thick compared to the unexposed kidney during the second and early third trimester, declining to proportionately thin kidneys for length and width subsequently. Cardiac growth in width and volume followed a reverse pattern: a surge in cardiac volume occurred after 30 weeks with acceleration in cardiac width, resulting in a heart that was wide for length and for fetal weight. Smoke exposure altered fetal growth in size and timing of the heart and kidneys during midgestation, with changes in organ morphology suggesting compensatory growth. These are the first data providing anatomical evidence of altered renal/cardiac volume relationships that may provide a mechanism to previously reported sequelae of in utero smoke exposure. They suggest that cell-level adaptive responses to hypoxia and/or chemical insults are operative and illustrate the importance of longitudinal ultrasound to directly assess the organ-level growth response of the human fetus to a prenatal stress, in lieu of relying on birth outcome measures.

Diabetic nephropathy: recent insights into the pathophysiology and the progression of diabetic nephropathy

Diabetes has become the single most frequent comorbid condition in patients admitted for renal replacement therapy. This is the result of a greater prevalence of type 2 diabetes and better survival of diabetic patients. Progress has been made in pinpointing the predisposition to diabetes on metabolic abnormalities of muscle mitochondrial metabolism, but the long sought genes predisposing to diabetes and to diabetic nephropathy have not yet been identified. Of great concern are experimental studies documenting that maternal hyperglycemia causes nephron underdosing in the offspring. Relevant to pathogenesis and treatment of diabetic nephropathy are, among others, recent insights that hyperglycemia sensitizes target organs to blood pressure-induced damage, and that local renin-angiotensin systems play an important role in genesis and progression of diabetic nephropathy.

Functional consequences of IGFBP excess-lessons from transgenic mice

The functions of insulin-like growth factor-binding proteins (IGFBPs) have been studied extensively in vitro, revealing IGF-dependent and also IGF-independent effects on cell growth, differentiation, and survival. In contrast, the biological relevance of IGFBPs in vivo is only partially understood. In the past decade, mouse models lacking or overexpressing specific IGFBPs have been generated by transgenic technology. Phenotypic analysis revealed features that are common for most IGFBPs (growth inhibition), but also effects that appear to be specific for some but not all IGFBPs, such as disturbed glucose homeostasis (IGFBP-1 and -3) or impaired fertility (IGFBP-1, -5, and -6). Future systematic comparison of IGFBP functions in transgenic mice will be facilitated by targeted insertion of IGFBP expression vectors and by standardized phenotype assessment. Furthermore, analysis of IGFBP expression in growth-selected mouse lines or pedigrees segregating for growth phenotypes will be important to understand the roles of IGFBPs in multigenic growth regulation.

Nephron endowment and blood pressure: what do we really know?

It has been hypothesized that a reduced number of nephrons at birth contributes to the development of essential hypertension. Nephron number in normal human kidneys has been shown to vary up to eightfold. Therefore, a significant proportion of the population appears to be at risk for developing hypertension. Furthermore, nephron deficits might explain why some racial groups have a higher incidence of hypertension and end-stage renal disease than others. Animal studies have demonstrated that maternal limitations in nutrient supply, both gross and nutrient-specific; exposure to elevated levels of hormones or toxins; and genetic factors can lead to permanent deficits in nephron number and, when examined, elevated blood pressure. In this review, maternal and genetic factors influencing nephron endowment and the implications of nephron deficit for hypertension and renal disease in humans are discussed.

New concepts: growth hormone, insulin-like growth factor-I and the kidney

Both growth hormone (GH) and IGF-1 have major effects on normal kidney growth, structure and function and participate in the pathogenesis of certain kidney diseases. Furthermore when the kidneys fail there are profound changes in the circulating GH-IGF-1 system and the renal and systemic responses to these hormones. In this brief review we address the advances that have been made in our understanding of the relationship between growth hormone GH and IGF-1 and the kidney in health and the systemic and local perturbations that occur in kidney disease and identify key unanswered questions.

Leucine-rich repeat-containing, G protein-coupled receptor 4 null mice exhibit intrauterine growth retardation associated with embryonic and perinatal lethality

Leucine-rich repeat-containing, G protein-coupled receptors (LGRs) belong to the largest mammalian superfamily of proteins with seven-transmembrane domains. LGRs can be divided into three subgroups based on their unique domain arrangement. Although two subgroups have been found to be receptors for glycoprotein hormones and relaxin-related ligands, respectively, the third LGR subgroup, consisting of LGR4-6, are orphan receptors with unknown physiological roles. To elucidate the functions of this subgroup of LGRs, LGR4 null mice were generated using a secretory trap approach to delete the majority of the LGR4 gene after the insertion of a beta-galactosidase reporter gene immediately after exon 1. Tissues expressing LGR4 were analyzed based on histochemical staining of the transgene driven by the endogenous LGR4 promoter. LGR4 was widely expressed in kidney, adrenal gland, stomach, intestine, heart, bone/cartilage, and other tissues. The expression of LGR4 in these tissues was further confirmed by immunohistochemical studies in wild-type animals. Analysis of the viability of 250 newborn animals suggested a skewed inheritance pattern, indicating that only 40% of the expected LGR4 null mice were born. For the LGR4 null mice viable at birth, most of them died within 2 d. Furthermore, the LGR4 null mice showed intrauterine growth retardation as reflected by a 14% decrease in body weight at birth, together with 30% and 40% decreases in kidney and liver weights, respectively. The present findings demonstrate the widespread expression of LGR4, and an essential role of LGR4 for embryonic growth, as well as kidney and liver development. The observed pre- and postnatal lethality of LGR4 null mice illustrates the importance of the LGR4 signaling system for the survival and growth of animals during the perinatal stage.

Cyclosporin A Administration during Pregnancy Induces a Permanent Nephron Deficit in Young Rabbits

ABSTRACT. Cyclosporin A (CsA) is an immunosuppressive agent used to prevent graft rejection and to treat autoimmune disorders. Successful pregnancies can be achieved among CsA-treated women, although it is known that CsA is nephrotoxic and crosses the human placenta. The aim of this study was to evaluate the harmlessness of CsA toward the embryonic kidney. Twenty-one pregnant rabbits were divided into four groups. Groups of six and four female animals were subjected to daily injections of 10 mg/kg per d CsA (administered subcutaneously) for 5 d, from day 14 to day 18 of gestation or from day 20 to day 24 of gestation, respectively. In the third group, five female animals received the CsA diluent (Cremophor) from day 14 to day 18 of gestation. The fourth group consisted of six untreated female animals. Pregnancy outcomes among CsA-treated does demonstrated a reduced number of living pups, which were also growth-retarded, with exposure to CsA from day 20 to day 24 of gestation. However, pups exposed to CsA from day 14 to day 18 of gestation exhibited normal fetal growth, and blood concentrations of CsA matched human data. Examinations of kidneys at birth demonstrated vacuolation of proximal and collecting tubules and ureteric bud ends. Increased glomerular volumes and decreased nephron densities suggested nephron mass reduction, which was quantitatively evaluated in 1-mo-old animals. The nephron numbers were reduced by 25 and 33% in day 14 to 18 CsA-treated and day 20 to 24 CsA-treated animals, respectively, which displayed compensatory adaptation of the existing nephrons. However, foci of segmental glomerular sclerosis were already present, which would possibly jeopardize renal function later in life.

WT1 and glomerular function

The Wilms' Tumour 1 (WT1) gene plays an important role at three different stages of kidney development. The onset of kidney formation, the progression of kidney formation and the maintenance of normal kidney function. Disruption of WT1 may lead to a whole spectrum of kidney diseases ranging from tumour development to mild forms of renal failure. However, the underlying mechanisms are largely unknown. The WT1 proteins have been implicated in various cellular processes like proliferation, differentiation and apoptosis and in agreement with these diverse functions, the number of target genes is still mounting. The development of mouse models in recent years has contributed considerably to a better understanding of the biological activities of WT1, and in this article we will discuss the role of WT1 during kidney formation and kidney function.

Effect of maternal iron restriction during pregnancy on renal morphology in the adult rat offspring

In rats, maternal anaemia during pregnancy causes hypertension in the adult offspring, although the mechanism is unknown. The present study investigated the renal morphology of adult rats born to mothers who were Fe-deficient during pregnancy. Rats were fed either a control (153 mg Fe/kg diet, n 7) or low-Fe (3 mg/kg diet, n 6) diet from 1 week before mating and throughout gestation. At delivery, the Fe-restricted (IR) mothers were anaemic; the IR pups were also anaemic and growth-retarded at 2 d of age. At 3 and 16 months, systolic blood pressure in the IR offspring (163 (sem 4) and 151 (sem 4) mmHg respectively, n 13) was greater than in control animals (145 (sem 3) and 119 (sem 4) mmHg respectively, n 15, P<0.05). At post mortem at 18 months, there was no difference in kidney weight between treatment groups, although relative kidney weight as a fraction of body weight in the IR offspring was greater than in control animals (P<0.05). Glomerular number was lower in the IR offspring (11.4 (sem 1.1) per 4 mm(2), n 13) compared with control rats (14.8 (sem 0.7), n 15, P<0.05). Maternal treatment had no effect on glomerular size, but overall, female rats had smaller and more numerous glomeruli per unit area than male rats. When all animals were considered, inverse relationships were observed between glomerular number and glomerular size (r-0.73, n 28, P<0.05), and glomerular number and systolic blood pressure at both 3 months (r-0.42, n 28, P<0.05) and 16 months of age (r-0.64, n 28, P<0.05). Therefore, in rats, maternal Fe restriction causes hypertension in the adult offspring that may be due, in part, to a deficit in nephron number.

Critical periods in human growth and their relationship to diseases of aging

It has long been recognized that there are "critical periods" during mammalian development when exposure to specific environmental stimuli are required in order to elicit the normal development of particular anatomical structures or their normal functioning. The responses of the organism to these stimuli depend on a specific level of anatomical maturation and a state of rapid anatomical and/or functional change. This discussion of critical periods in growth is not confined to the classic definition of a narrow time frame of development during which a particular environmental threshold or limit must exist for normal growth and function to ensue. Using both auxological and epidemiological approaches, we suggest a lifespan perspective which encompasses accumulating and interacting risks that are manifest from prenatal life onward. By understanding the process of growth development, and by scrutinizing the growth process, early variations that lead to later disease can be identified. Here we review a significant amount of the evidence that links exposure during growth to later morbidity and mortality. The fetus appears to respond to insults during the prenatal period through the process of "programming," which has short-term survival advantages but may have a long-term disadvantage in that it is associated with cardiovascular disease, hypertension, type II diabetes, and later obesity. Low birth weight combined with rapid postnatal growth during infancy also appears to be associated, for instance, with later childhood and adult sequelae in terms of glucose tolerance and obesity. Independent of birth weight, the timing of adiposity rebound during mid-childhood also predicts later obesity. The timing, magnitude, and duration of adolescent growth and maturationare associated with critical body composition changes, including the normal acquisition of body fat and bone mineralization. In particular, the acquisition of appropriate peak bone mass is critical in determining the later risk of osteoporosis. A putative causal mechanism linking early growth variation to later chronic disease risk through telomeric attrition is discussed. The obligatory loss of telomeric DNA with each cell division serves as a mitotic clock and marks the rate of growth and repair processes in the cell. Although much more work is required, existing studies support the notion that telomere shortening is not only a clock of cellular division, but also marks relative growth rate, as well as contributing to common degenerative processes of aging through its impact on cellular senescence.

Physiology of erythropoietin during mammalian development

Growth is a fundamental process of mammalian development. Several observations regarding regulation of erythropoiesis during growth are not easily explained by the hypoxia-erythropoietin (Epo) concept. This review focuses primarily on this aspect of the physiology of Epo. The question is raised of whether this regulation during growth is based on the hypoxia-Epo mechanism alone, or whether Epo acts in concert with general growth-promoting factors, particularly growth hormone (GH) and the insulin-like growth factors (IGF-I and -II). Supporting the latter hypothesis is the observation that the Epo and GH/IGF systems are activated by hypoxia and share similar receptors and pathways. Recent studies indicate that human fetal and infant growth is stimulated by GH, IGF-I and IGF-II. Epo, GH and IGFs are expressed early in fetal life. Although the rate of erythropoiesis in the fetus is high, serum Epo levels are low. The Epo response to hypoxia in the fetus and neonate is reduced compared with adults. Following delivery the Epo levels vary between species, probably related to the oxygen transport capacity of the hemoglobin (Hb) mass. IGF-I levels are low in the fetus and increase slowly following birth, except in preterm infants in whom the levels decline. In all mammals Hb declines following birth, giving rise to "early anemia". Except in the human, Epo levels increase proportionally with the fall in Hb, but there is a discrepancy between the curves for serum immunoreactive Epo (siEpo) and for erythropoiesis stimulating factors (ESF): the latter include other stimulatory factors in addition to Epo. Hypertransfusion of mice in the period of "early anemia" suppresses siEpo, but not ESF and erythropoiesis, as it does in adult mice. GH and IGF-I have direct effects on erythropoiesis in vitro and act particularly at the later stages of red cell differentiation. IGF-I acts synergistically with Epo, and its effects are most marked when Epo levels are low. Human recombinant (rhu) IGF-I stimulates erythropoiesis in neonatal rats, but not in newborn mice and lambs. In adult mice, in hypophysectomized rats and in mice with end-stage renal failure, however, a stimulatory effect of this growth factor was found on red cell production. RhuGH stimulates erythropoiesis in GH-deficient short children.

CONCLUSION

Fetal and early postnatal erythropoiesis are dependent on factors in addition to Epo. The likely candidates are GH and IGF-I. The in vitro stimulating effects of these factors on erythropoiesis are convincing, but more data are needed on the in vivo effects.

Nephron number, renal function, and arterial pressure in aged GDNF heterozygous mice

The loss of one allele for glial cell line-derived neurotrophic factor (GDNF) results in approximately 30% fewer but normal sized glomeruli in young mice. Low nephron number, inherited or acquired, has been linked to increased risk of development of hypertension and renal failure. This study examines whether GDNF heterozygous mice, with an inherent reduction in nephron number, demonstrate a deterioration in renal structure and function and rise in arterial pressure in later life. Fourteen-month-old male GDNF heterozygous (n=7) and wild-type (n=6) mice were anesthetized and prepared for measurement of mean arterial pressure, glomerular filtration rate (GFR), and renal blood flow. After measurement of renal function, kidneys were fixed for stereological determination of total glomerular number and mean glomerular volume. Mean arterial pressure was, on average, 18 mm Hg higher in GDNF heterozygous (98+/-4 mm Hg) than wild-type mice (80+/-2 mm Hg; P<0.01). However, GFR (0.656+/-0.054 versus 0.688+/-0.076 mL/min per g kidney wt) and renal blood flow (5.29+/-0.42 versus 4.70+/-0.34 mL/min per g kidney wt) were not different between groups. Fourteen-month-old GDNF heterozygous mice had approximately 30% fewer glomeruli than wild-type mice (9206+/-934 versus 13440+/-1275; P<0.01) and significantly larger glomeruli (4.51+/-0.39 versus 3.72+/-0.63x10(-4)mm(3); P<0.01). Thus, aged GDNF heterozygous mice maintained a normal GFR and renal blood flow despite reduced nephron numbers. The elevated arterial pressure, glomerular hypertrophy, and hyperfiltration demonstrated in the GDNF heterozygous mice at this age may indicate a compensatory mechanism whereby GFR is maintained in the presence of a reduced nephron endowment.