Growth plate cartilage formation and resorption are differentially depressed in growth retarded uremic rats

Chronic Kidney Disease and Growth Failure in Children

Chronic kidney disease (CKD) is a significant challenge for pediatric endocrinologists, as children with CKD may present a variety of endocrine complications. Growth failure is common in CKD, and its severity is correlated with the degree of renal insufficiency. Management strategies include addressing reversible comorbidities, optimizing nutrition, and ensuring metabolic control. Kidney replacement therapy, including transplantation, determines a significant improvement in growth. According to a recent Consensus Statement, children with CKD stage 3-or on dialysis older >6 months-are eligible for treatment with recombinant growth hormone (rGH) in the case of persistent growth failure. Treatment with rGH may be considered for those with height between the 3rd and 10th percentile and persistent growth deceleration. In children who received kidney transplantation but continue to experience growth failure, initiation of GH therapy is recommended one year post-transplantation if spontaneous catch-up growth does not occur and steroid-free immunosuppression is not an option. In children with CKD, due to nephropathic cystinosis and persistent growth failure, GH therapy should be considered at all stages of CKD. Potential adverse effects and benefits must be regularly assessed during therapy. Treatment with GH is safe in children with CKD. However, its general efficacy is still controversial. All possible problems with a negative impact on growth should be timely addressed and resolved, whenever possible with a personalized approach to the patient. GH therapy may be useful in promoting catch-up growth in children with residual growth potential. Future research should focus on refining effective therapeutic strategies and establishing consensus guidelines to optimize growth outcomes in this population.

Associations between collagen X biomarker and linear growth velocity in a pediatric chronic kidney disease cohort

BACKGROUND

Collagen X biomarker (CXM) is a novel biomarker of linear growth velocity. We investigated whether CXM correlated with measured growth velocity in children with impaired kidney function.

METHODS

We used data from children aged 2 through 16 years old enrolled in the Chronic Kidney Disease in Children (CKiD) study. We assessed the association between CXM level and growth velocity based on height measurements obtained at study visits using linear regression models constructed separately by sex, with and without adjustment for CKD covariates. Linear mixed-effects models were used to capture the between-individual and within-individual CXM changes over time associated with concomitant changes in growth velocity from baseline through follow-up.

RESULTS

A total of 967 serum samples from 209 participants were assayed for CXM. CXM correlated more strongly in females compared to male participants. After adjustment for growth velocity and CKD covariates, only proteinuria in male participants affected CXM levels. Finally, we quantified the between- and within-participant associations between CXM level and growth velocity. A between-participant increase of 24% and 15% in CXM level in females and males, respectively, correlated with a 1 cm/year higher growth velocity. Within an individual participant, on average, 28% and 13% increases in CXM values in females and males, respectively, correlated with a 1 cm/year change in measured growth.

CONCLUSIONS

CXM measurement is potentially a valuable aid for monitoring growth in pediatric CKD. However, future research, including studies of CXM metabolism, is needed to clarify whether CXM can be a surrogate of growth in children with CKD. A higher resolution version of the Graphical abstract is available as Supplementary information.

Innovative Three-Dimensional Microscopic Analysis of Uremic Growth Plate Discloses Alterations in the Process of Chondrocyte Hypertrophy: Effects of Growth Hormone Treatment

Chronic kidney disease (CKD) alters the morphology and function of the growth plate (GP) of long bones by disturbing chondrocyte maturation. GP chondrocytes were analyzed in growth-retarded young rats with CKD induced by adenine intake (AD), control rats fed ad libitum (C) or pair-fed with the AD group (PF), and CKD rats treated with growth hormone (ADGH). In order to study the alterations in the process of GP maturation, we applied a procedure recently described by our group to obtain high-quality three-dimensional images of whole chondrocytes that can be used to analyze quantitative parameters like cytoplasm density, cell volume, and shape. The final chondrocyte volume was found to be decreased in AD rats, but GH treatment was able to normalize it. The pattern of variation in the cell cytoplasm density suggests that uremia could be causing a delay to the beginning of the chondrocyte hypertrophy process. Growth hormone treatment appears to be able to compensate for this disturbance by triggering an early chondrocyte enlargement that may be mediated by Nkcc1 action, an important membrane cotransporter in the GP chondrocyte enlargement.

Growth plate alterations in chronic kidney disease

Growth retardation is a major feature of chronic kidney disease (CKD) of onset in infants or children and is associated with increased morbidity and mortality. Several factors have been shown to play a causal role in the growth impairment of CKD. All these factors interfere with growth by disturbing the normal physiology of the growth plate of long bones. To facilitate the understanding of the pathogenesis of growth impairment in CKD, this review discusses cellular and molecular alterations of the growth plate during uremia, including structural and dynamic changes of chondrocytes, alterations in their process of maturation and hypertrophy, and disturbances in the growth hormone signaling pathway.

Marked alterations in the structure, dynamics and maturation of growth plate likely explain growth retardation and bone deformities of young Hyp mice

Mechanisms underlying growth impairment and bone deformities in X-linked hypophosphatemia are not fully understood. We here describe marked alterations in the structure, dynamics and maturation of growth plate in growth-retarded young Hyp mice, in comparison with wild type mice. Hyp mice exhibited reduced proliferation and apoptosis rates of chondrocytes as well as severe disturbance in the process of chondrocyte hypertrophy disclosed by abnormal expression of proteins likely involved in cell enlargement, irregular chondro-osseous junction and disordered bone trabecular pattern and vascular invasion in the primary spongiosa. (Hyp mice had elevated circulating FGF23 levels and over activation of ERK in the growth plate.) All these findings provide a basis to explain growth impairment and metaphyseal deformities in XLH. Hyp mice were compared with wild type mice serum parameters, nutritional status and growth impairment by evaluation of growth cartilage and bone structures. Hyp mice presented hyphosphatemia with high FGF23 levels. Weight gain and longitudinal growth resulted reduced in them with numerous skeletal abnormalities at cortical bone. It was also observed aberrant trabecular organization at primary spongiosa and atypical growth plate organization with abnormal proliferation and hypertrophy of chondrocytes and diminished apoptosis and vascular invasion processes. The present results show for the first time the abnormalities present in the growth plate of young Hyp mice and suggest that both cartilage and bone alterations may be involved in the growth impairment and the long bone deformities of XLH.

Growth hormone improves growth retardation induced by rapamycin without blocking its antiproliferative and antiangiogenic effects on rat growth plate

Rapamycin, an immunosuppressant agent used in renal transplantation with antitumoral properties, has been reported to impair longitudinal growth in young individuals. As growth hormone (GH) can be used to treat growth retardation in transplanted children, we aimed this study to find out the effect of GH therapy in a model of young rat with growth retardation induced by rapamycin administration. Three groups of 4-week-old rats treated with vehicle (C), daily injections of rapamycin alone (RAPA) or in combination with GH (RGH) at pharmacological doses for 1 week were compared. GH treatment caused a 20% increase in both growth velocity and body length in RGH animals when compared with RAPA group. GH treatment did not increase circulating levels of insulin-like growth factor I, a systemic mediator of GH actions. Instead, GH promoted the maturation and hypertrophy of growth plate chondrocytes, an effect likely related to AKT and ERK1/2 mediated inactivation of GSK3β, increase of glycogen deposits and stabilization of β-catenin. Interestingly, GH did not interfere with the antiproliferative and antiangiogenic activities of rapamycin in the growth plate and did not cause changes in chondrocyte autophagy markers. In summary, these findings indicate that GH administration improves longitudinal growth in rapamycin-treated rats by specifically acting on the process of growth plate chondrocyte hypertrophy but not by counteracting the effects of rapamycin on proliferation and angiogenesis.

Parathyroid hormone and growth in chronic kidney disease

Growth failure is common in children with chronic kidney disease, and successful treatment is a major challenge in the management of these children. The aetiology is multi-factorial with "chronic kidney disease-metabolic bone disorder" being a key component that is particularly difficult to manage. Parathyroid hormone is at the centre of this mineral imbalance, consequent skeletal disease and, ultimately, growth failure. When other aetiologies are treated, good growth can be achieved throughout the course of the disease when parathyroid hormone (PTH) levels are in the normal range or slightly elevated. A direct correlation between PTH levels and growth has not been convincingly established, and the direct effect of PTH on growth has not been adequately described; furthermore, direct actions of PTH on the growth plate are unproven. The effects of PTH on growth stem from the pivotal role that PTH plays in the development of renal osteodystrophy. In severe secondary hyperparathyroidism, the growth plate is altered and growth is affected. At the other end of the spectrum, with an over-suppressed parathyroid gland, the rate of bone turnover and remodelling is markedly diminished, and some data suggest this is associated with poor growth. Most of the data available suggests that avoiding the development of significant bone disease through the strict control of PTH levels permits good growth. Absolute optimal ranges for PTH that maximise growth or minimise growth failure are not yet established.

Ibandronate affects bone growth and mineralization in rats with normal and reduced renal function

Bisphosphonates have been shown to attenuate ectopic calcification in experimental uremia. While they are known to reduce bone turnover, the effects on endochondral bone formation have not yet been addressed. To address this issue, we administered male Sprague-Dawley rats weekly subcutaneous injections of either vehicle or ibandronate (1.25 μg/kg body weight) for a total of 10 weeks. The rats were randomly allocated into one of four groups: (1) vehicle-treated, sham-operated rats; (2) ibandronate-treated, sham-operated rats; (3) vehicle-treated, 5/6 nephrectomized rats; (4) ibandronate-treated, 5/6 nephrectomized rats. Bones were double labeled with tetracycline and demeclocycline in vivo, and tibiae were removed for analysis. Weight gain was similar in all groups. Ibandronate reduced body length gain and tibial growth rate in the sham-operated animals but not in the rats showing chronic renal failure (CRF). The height of the proliferative zone of the epiphyseal growth plate was reduced in the ibandronate-treated controls and tended to be reduced in CRF rats. A significant correlation between tibial growth rate and height of the proliferative zone was observed. Mineral apposition rates were significantly reduced in ibandronate-treated, sham-operated rats and tended to be reduced in CRF rats. In conclusion, ibandronate interferes with tibial growth and bone mineralization in young rats with normal and reduced renal function.

Effects of hyperhomocysteinemia during the gestational period on ossification in rat embryo

Severe hyperhomocysteinemia, as seen in classic homocystinuria, is associated with several skeletal malformations and osteopenia. Moreover, hyperhomocysteinemia during pregnancy has been associated with multiple developmental defects in the fetus. This study was undertaken to determine whether offspring of hyperhomocysteinemic mothers have demonstrable changes in bone volume and the epiphyseal growth plate. Ten adult female Sprague-Dawley rats were randomly assigned to the control or experimental group. The experimental group received 100 mg/kg/day of homocysteine in their drinking water for 3 weeks before mating and for the total duration of pregnancy. In each group, three pups per mother were randomly selected. The histomorphometric properties of tibial, radial and vertebral growth plates of newborn rats and the volume fraction of bone were compared between groups. The plasma homocysteine concentration at the end of study was significantly higher in dams in the experimental group (16.42+/-1.5 vs. 4.7+/-1.7 mumol/L, P<0.05). In offspring born to dams given the homocysteine supplement, the volume fraction of bone in the tibia (30.7+/-1.5% vs. 36.8+/-1.9%, P<0.05), radius (29.6+/-1.1% vs. 37.4+/-2%, P<0.05) and vertebra (34.4+/-1.8% vs. 41+/-1.9%, P<0.05) were significantly decreased whereas vertical heights of proliferative (423+/-25.1 vs. 301.8+/-28.1 microm for radius and 131.9+/-5.9 vs. 107.8+/-3.5 microm for vertebra) and hypertrophic zones (213.1+/-12 vs. 163.3+/-7.5 microm for tibia, 153.2+/-7.7 vs. 121.1+/-7.9 microm for radius and 112+/-9.9 vs. 88.4+/-10.1 microm for the vertebra) were increased (P<0.05). The results showed that the administration of homocysteine caused osteopenia in newborn rats. In addition, these data suggest that hyperhomocysteinemia may induce disruption of normal development of epiphyseal cartilage in the rat embryo.

Growth-plate cartilage in chronic renal failure

Bone growth occurs in the growth-plate cartilage located at the ends of long bones. Changes in the architecture, abnormalities in matrix organization, reduction in protein staining and RNA expression of factors involved in cell signaling have been described in the growth-plate cartilage of nephrectomized animals. These changes can lead to a smaller growth plate associated with decrease in chondrocyte proliferation, delayed hypertrophy, and prolonged initiation of mineralization and vascular invasion. As a result, chronic renal failure can result in stunted body growth and skeletal deformities. Multiple etiologic factors can contribute to impaired bone growth in renal failure, including suboptimal nutrition, metabolic acidosis, and secondary hyperparathyroidism. Recent findings have also shown the tight connection between chondro/osteogenesis, hematopoiesis, and immunogenesis.

Alterations of growth plate and abnormal insulin-like growth factor I metabolism in growth-retarded hypokalemic rats: effect of growth hormone treatment

Hypokalemic tubular disorders may lead to growth retardation which is resistant to growth hormone (GH) treatment. The mechanism of these alterations is unknown. Weaning female rats were grouped ( n = 10) in control, potassium-depleted (KD), KD treated with intraperitoneal GH at 3.3 mg·kg−1·day−1 during the last week (KDGH), and control pair-fed with KD (CPF). After 2 wk, KD rats were growth retarded compared with CPF rats, the osseous front advance (±SD) being 67.07 ± 10.44 and 81.56 ± 12.70 μm/day, respectively. GH treatment did not accelerate growth rate. The tibial growth plate of KD rats had marked morphological alterations: lower heights of growth cartilage (228.26 ± 23.58 μm), hypertrophic zone (123.68 ± 13.49 μm), and terminal chondrocytes (20.8 ± 2.39 μm) than normokalemic CPF (264.21 ± 21.77, 153.18 ± 15.80, and 24.21 ± 5.86 μm). GH administration normalized these changes except for the distal chondrocyte height. Quantitative PCR of insulin-like growth factor I (IGF-I), IGF-I receptor, and GH receptor genes in KD growth plates showed downregulation of IGF-I and upregulation of IGF-I receptor mRNAs, without changes in their distribution as analyzed by immunohistochemistry and in situ hybridization. GH did not further modify IGF-I mRNA expression. KD rats had normal hepatic IGF-I mRNA levels and low serum IGF-I values. GH increased liver IGF-I mRNA, but circulating IGF-I levels remained reduced. This study discloses the structural and molecular alterations induced by potassium depletion on the growth plate and shows that the lack of response to GH administration is associated with persistence of the disturbed process of chondrocyte hypertrophy and depressed mRNA expression of local IGF-I in the growth plate.

Differential gene expression induced by growth hormone treatment in the uremic rat growth plate

OBJECTIVES

Treatment with growth hormone (GH) improves growth retardation of chronic renal failure. cDNA microarrays were used to investigate GH-induced modifications in gene expression in the tibial growth plate of young rats.

DESIGN

RNA was extracted from the tibial growth plate from two groups, untreated and treated with GH, of young rats made uremic by subtotal nephrectomy (n=10). To validate changes shown by the Agilent oligo microarrays, some modulated genes known to play a physiological role in growth plate metabolism were analyzed by real-time quantitative polymerase chain reaction (qPCR).

RESULTS

The microarrays showed that GH modified the expression of 224 genes, 195 being upregulated and 29 downregulated. qPCR results confirmed the sense of expression change found in the arrays for insulin-like growth factor I, insulin-like growth factor II, collagen V alpha 1, bone morphogenetic protein 3 and proteoglycan type II.

CONCLUSIONS

This study shows for the first time the profile of growth plate gene expression modifications caused by GH treatment in experimental uremia and provides a basis to further investigate selected individual genes with potential implication in the stimulating effect on the growth of GH treatment in chronic renal failure.

Growth of prepubertal children on dialysis

Growth failure is a common and significant clinical problem for children on dialysis and often remains a major impediment to their rehabilitation. Early referral to a paediatric nephrology centre and appropriate management before the initiation of dialysis may significantly prevent growth deterioration. Growth in children on dialysis can be affected by nutritional, metabolic, and hormonal changes. Early diagnosis of malnutrition and aggressive management should be a priority. Gastrostomy feeding should be used when adequate oral intake to maintain normal height and weight velocity cannot be achieved. Active vitamin D metabolites should be used carefully, to prevent low-turnover bone disease. All children should have an adequate regimen of dialysis and an appropriate management of malnutrition, renal osteodystrophy, metabolic acidosis, salt wasting and anaemia, before recombinant human growth hormone (rhGH) administration is considered. The current challenge of reversing growth impairment in children on dialysis can only be achieved by optimization of their care.

Rapamycin retards growth and causes marked alterations in the growth plate of young rats

Rapamycin is a potent immunosuppressant with antitumoral properties widely used in the field of renal transplantation. To test the hypothesis that the antiproliferative and antiangiogenic activity of rapamycin interferes with the normal structure and function of growth plate and impairs longitudinal growth, 4-week-old male rats (n = 10/group) receiving 2 mg/kg per day of intraperitoneal rapamycin (RAPA) or vehicle (C) for 14 days were compared. Rapamycin markedly decreased bone longitudinal growth rate (94 +/- 3 vs. 182 +/- 3 microm/day), body weight gain (60.2 +/- 1.4 vs. 113.6 +/- 1.9 g), food intake (227.8 +/- 2.6 vs. 287.5 +/- 3.4 g), and food efficiency (0.26 +/- 0.00 vs. 0.40 +/- 0.01 g/g). Signs of altered cartilage formation such as reduced chondrocyte proliferation (bromodeoxiuridine-labeled cells 32.9 +/- 1.4 vs. 45.2 +/- 1.1%), disturbed maturation and hypertrophy (height of terminal chondrocytes 26 +/- 0 vs. 29 +/- 0 microm), and decreased cartilage resorption (18.7 +/- 0.5 vs. 31.0 +/- 0.8 tartrate-resistant phosphatase alkaline reactive cells per 100 terminal chondrocytes), together with morphological evidence of altered vascular invasion, were seen in the growth plate of RAPA animals. This study indicates that rapamycin can severely impair body growth in fast-growing rats and distort growth-plate structure and dynamics. These undesirable effects must be kept in mind when rapamycin is administered to children.

Transient disturbance in physeal morphology is associated with long-term effects of nitrogen-containing bisphosphonates in growing rabbits

Bisphosphonates have clinical benefit in children with severe osteogenesis imperfecta or osteoporosis and potential benefit in children with Perthes disease or undergoing distraction osteogenesis. However, there is concern about the effects of bisphosphonates on the physis and bone length. In 44 growing rabbits, zoledronic acid caused a transient disruption of physeal morphology, retention of cartilaginous matrix in trabeculae and cortical bone of the metaphysis, and a minor decrement in tibial bone length at maturity.

INTRODUCTION

Data from growing animal models suggest that bisphosphonates cause retention of longitudinal cartilaginous septa at the chondro-osseous junction, extension of trabeculae to the metaphyseal-diaphyseal junction, and varying dose-dependent effects on longitudinal growth. However, there is a lack of data regarding effects of intermittent use of nitrogen-containing bisphosphonates on the physis and on tibial length in models reaching maturity.

MATERIALS AND METHODS

Contralateral tibias of juvenile rabbits were examined after right tibial distraction osteogenesis from two previous studies. Animals were randomized to receive 0.1 mg/kg zoledronic acid (ZA) IV at 8 weeks of age (ZA*1) or 8 and 10 weeks of age (ZA*2) or saline. Body mass was analyzed from 5 to 44 weeks of age; tibial length and proximal physeal-metaphyseal histology and histomorphometry were analyzed at 8-52 weeks of age.

RESULTS

Tibial length was 3% less at 14 weeks of age in the ZA*2-treated versus saline group (p<0.05) in both studies, and this difference persisted at maturity in the long-term study group (26 weeks of age, p<0.05). Total body mass gain from 5 to 26 weeks of age was 14% less in ZA*2-treated than saline animals (p<0.05). Rate of weight gain from 8 to 10 weeks of age was 76% less in ZA*2 compared with saline animals (p<0.05). Radiographs showed radiodense lines in the metaphyses of ZA-treated bones, corresponding to the number of doses. Histologically, lines resulting from the first dose of ZA contained longitudinal cartilaginous matrix cores surrounded by bone, whereas those from the second dose contained spherical cores of matrix caused by transient disruption of physeal morphology after the first dose of ZA. Resorption of these lines at later times was radiographically and histologically evident, but remnants of cartilaginous matrix remained in the cortical bone of ZA-treated animals.

CONCLUSIONS

ZA treatment within the final 13.5% of the rabbit tibial growth period caused a transient disruption in physeal morphology and resorption associated with retention of cartilaginous matrix and coinciding with a persistent 3% decrement in tibial length. Disruption of physeal morphology and potential loss of bone length should be considered when administering nitrogen-containing bisphosphonates to children before closure of the major physes.

Experimental renal failure and iron overload: a histomorphometric study in rat tibia

Renal failure (RF) is a serious disease of relatively high incidence, known to cause bone alterations. RF patients frequently suffer anemia, which is usually treated with iron. Given that iron overload inhibits bone formation, the aim of the present study was to evaluate the effect of iron on the subchondral bone of rat tibiae, using a model of renal failure. Male Wistar rats were subjected to experimental nephrectomy in order to induce renal failure and to iron overload by daily intraperitoneal injections of 88 mg/kg body weight of iron-dextran for 16 days. Tetracyclines were injected intraperitoneally to evaluate dynamic parameters of bone. Undecalcified histological sections of the tibiae were obtained. Serum urea, creatinine, and paratohormone (PTH) levels were evaluated 30 days after the onset of the experiment. Static and dynamic histomorphometric measurements were performed. Iron overload modified the response of the animals with renal failure: a reduction in bone forming activity compatible with adynamic bone disease and a decrease in peritrabecular fibrosis were observed. Our results suggest that iron is yet one more factor involved in the imbalance in bone metabolism typically found in renal failure patients treated with iron, rendering diagnosis and treatment of bone disease in these patients more complex.

Alterations of the growth plate in chronic renal failure

Chronic renal failure modifies the morphology and dynamics of the growth plate (GP) of long bones. In young uremic rats, the height of cartilage columns of GP may vary markedly. The reasons for this variation are unknown, although the severity and duration of renal failure and the type of renal osteodystrophy have been shown to influence the height of GP cartilage. Expansion of GP cartilage is associated with that of the hypertrophic stratum. The interference of uremia with the process of chondrocyte differentiation is suggested by some morphological features. However, analysis by immunohistochemistry and/or in situ hybridization of markers of chondrocyte maturation in the GP of uremic rats has yielded conflicting results. Thus, there have been reported normal and reduced mRNA levels for collagen X, parathyroid hormone/parathyroid hormone-related peptide receptor, and matrix metalloproteinase 9, as well as normal mRNA and protein expression for vascular endothelial growth factor and chondromodulin I, peptides related to the control of angiogenesis. In addition, a decreased immunohistochemical signal for growth hormone receptor and low insulin-like growth factor I mRNA in the proliferative zone of uremic GP are supportive of reduced chondrocyte proliferation. Growth hormone treatment improves chondrocyte maturation and activates bone metabolism in the primary spongiosa.

Bone elongation in rats with renal failure and mild or advanced secondary hyperparathyroidism

BACKGROUND

Impairment of growth in children with chronic renal failure may be due, in part to the insensitivity to the actions of growth hormone by insulin-like growth factor-I (IGF-I) because of accumulations of IGF binding proteins. There are a few studies describing the changes that occur in the growth plate in renal failure. None of these studies has simultaneously compared the modifications in the expression of selected markers of endochondral bone formation in renal failure with mild or advanced secondary hyperparathyroidism.

METHODS

Forty-six rats that underwent 5/6 nephrectomy (Nx) were fed either standard rodent diet (Nx-control) or high phosphorus diet to induce advanced secondary hyperparathyroidism (Nx-phosphorus) for 4 weeks. Sections of the tibia were obtained for growth plate histomorphometry, immunohistochemistry studies, and in situ hybridization experiments for selected markers of endochondral bone formation.

RESULTS

Weight gain, gain in length, and tibial length were less in Nx animals. Serum parathyroid hormone (PTH) and phosphorus levels were higher and serum calcium levels were lower in the Nx-phosphorus group. The width of the growth plate was much shorter in the Nx-phosphorus group due to a decrease in both proliferative and hypertrophic zones. IGF-I protein and IGF binding protein-3 staining were diminished in both Nx groups without changes in the IGF-I receptor expression; the decline in IGF-I protein expression was much lower in the Nx-phosphorus group. PTH/PTH receptor protein (PTHrP) receptor mRNA transcripts decline and tartrate-resistant acid phosphastase (TRAP) staining increased only in the Nx-phosphorus group.

CONCLUSION

The growth impairment in renal failure may be worsened by the severity of secondary hyperparathyroidism.

Chondromodulin-I expression in the growth plate of young uremic rats

BACKGROUND

Growth retardation of chronic renal failure is associated with alterations in the growth plate suggestive of a disturbed chondrocyte maturation process and abnormal vascular invasion at the chondro-osseous interphase. Chondromodulin I (ChM-I) is a potent cartilage-specific angiostatic factor. Its pattern of expression in the uremic rat growth plate is unknown. Persistence of ChM-I synthesis and/or imbalance between ChM-I and vascular endothelial growth factor (VEGF) expressions might play a role in the alterations of uremic growth plate.

METHODS

Growth cartilage ChM-I expression was investigated by immunohistochemistry, in situ hybridization, and reverse transcription-polymerase chain reaction (RT-PCR) in growth-retarded young uremic rats (UREM), control rats, fed ad libitum (SAL) or pair-fed with the UREM group (SPF), and uremic rats treated with growth hormone (UREM-GH). VEGF expression was analyzed by immunohistochemistry.

RESULTS

ChM-I and ChM-I mRNA were confined to the proliferative and early hypertrophic zones of growth cartilage. A similar number of chondrocytes per column was positive for ChM-I in the 4 groups. In accordance with the elongation of the hypertrophic stratum in uremia, the distance (X+/-SEM, microm) between the extracellular ChM-I signal and the metaphyseal end of growth cartilage was higher (P < 0.003) in UREM (236 +/- 40) and UREM-GH (297 +/- 17) than in SAL (92 +/- 7) and SPF (113 +/- 6). No differences in ChM-I expression were appreciated by RT-PCR. Similar VEGF positivity was observed in the hypertrophic chondrocytes of all groups.

CONCLUSION

In experimental uremia, expansion of growth cartilage does not result from increased or persistent expression of ChM-I or from reduced VEGF expression at the cartilage-metaphyseal bone interphase. GH treatment does not modify ChM-I and VEGF expressions.

Special aspects of renal osteodystrophy in children

Renal osteodystrophy represents a spectrum of skeletal lesions that range from high-turnover to low-turnover bone disease. Similar factors are involved in the pathogenesis of renal osteodystrophy in adult and pediatric patients with chronic kidney disease (CKD). However, growth retardation and the development of bone deformities are specific complications that occurred in pediatric patients with CKD. Metabolic acidosis, renal osteodystrophy, malnutrition, and disturbances in the insulin growth factor (IGF)/growth hormone (GH) are among the main factors involved and they are discussed briefly in this article. In addition to disturbances in bone remodeling, longitudinal bone growth occurs at the growth plate cartilage by endochondral ossification. Although young rats with experimental CKD have growth retardation, the characteristics of the growth plate are markedly different between animals with severe secondary hyperparathyroidism and those with calcium-induced adynamic osteodystrophy. These disturbances may suggest potential molecular mechanisms by which endochondral bone formation may be altered in renal failure, consequently leading to growth retardation.

Histologic and dynamic changes induced by chronic metabolic acidosis in the rat growth plate

To understand better the pathophysiology of growth impairment in persistent metabolic acidosis, the morphology and dynamics of the growth plate were studied in young rats grouped as follows: rats that were made acidotic by oral administration of ammonium chloride for 14 d (AC), nonacidotic rats that were fed ad libitum (control [C]), and nonacidotic rats that were pair-fed with the AC group (PF). AC rats became markedly acidotic and growth retarded. The volume of newly formed bone per day (mean +/- SEM) was significantly lowered (P < 0.05) in AC rats (AC, 3.4 +/- 0.4; C, 8.4 +/- 0.6; PF, 6.4 +/- 0.5 mm(3)/d). Growth plate height was lower in AC rats (303.8 +/- 12.7 microm) than in either C (478.0 +/- 16.0 microm) or PF rats (439.0 +/- 21.4 microm). The processes of chondrocyte proliferation (assessed by bromodeoxyuridine labeling) and maturation (assessed by stereologic estimators of size and shape of chondrocytes and the volume of matrix per cell) were not impaired by acidosis. By contrast, the dynamics of hypertrophic chondrocytes were altered significantly: both cell turnover per column per day (AC, 4.4 +/- 0.4; C, 8.0 +/- 0.8; PF, 6.2 +/- 0.6) and linear velocity of advance of chondrocytes (AC, 5.7 +/- 0.5; C, 11.2 +/- 0.9; PF, 9.4 +/- 0.8 microm/h) were lowered significantly. The study presented here shows the inhibitory effect of metabolic acidosis on cartilage cell progression and endochondral bone formation. Finally, the data show that metabolic acidosis caused a marked shortening of the growth plate because chondrocyte turnover was affected to a greater extent than bone tissue formation.

Collagen metabolism is markedly altered in the hypertrophic cartilage of growth plates from rats with growth impairment secondary to chronic renal failure

Skeletal growth depends on growth plate cartilage activity, in which matrix synthesis by chondrocytes is one of the major processes contributing to the final length of a bone. On this basis, the present work was undertaken to ascertain if growth impairment secondary to chronic renal insufficiency is associated with disturbances of the extracellular matrix (ECM) of the growth plate. By combining stereological and in situ hybridization techniques, we examined the expression patterns of types II and X collagens and collagenase-3 in tibial growth plates of rats made uremic by subtotal nephrectomy (NX) in comparison with those of sham-operated rats fed ad libitum (SAL) and sham-operated rats pair-fed with NX (SPF). NX rats were severely uremic, as shown by markedly elevated serum concentrations of urea nitrogen, and growth retarded, as shown by significantly decreased longitudinal bone growth rates. NX rats showed disturbances in the normal pattern of chondrocyte differentiation and in the rates and degree of substitution of hypertrophic cartilage with bone, which resulted in accumulation of cartilage at the hypertrophic zone. These changes were associated with an overall decrease in the expression of types II and X collagens, which was especially marked in the abnormally extended zone of the hypertrophic cartilage. Unlike collagen, the expression of collagenase-3 was not disturbed severely. Electron microscopic analysis proved that changes in gene expression were coupled to alterations in the mineralization as well as in the collagen fibril architecture at the hypertrophic cartilage. Because the composition and structure of the ECM have a critical role in regulating the behavior of the growth plate chondrocytes, results obtained are consistent with the hypothesis that alteration of collagen metabolism in these cells could be a key process underlying growth retardation in uremia.

Growth hormone receptor abundance in tibial growth plates of uremic rats: GH/IGF-I treatment

BACKGROUND

Children with chronic renal failure (CRF) exhibit growth retardation and a disturbed growth hormone/insulin-like growth factor-I (GH/IGF-I) axis. Treatment of children with CRF with GH or GH/IGF-I can partially restore linear growth. The molecular basis for decreased longitudinal growth is not known but may involve an impaired action of GH.

METHODS

We used the growth-retarded uremic rat model to determine the abundance and distribution of GH receptors (GHRs) in the tibial epiphyseal growth plate and the influence of GH, IGF-I, or combined GH/IGF-I treatment. Pair-fed rats were used as the control.

RESULTS

While all treatment regimes increased body length and weight in both rat groups, only GH/IGF-I treatment increased the total growth plate width. This involved an increase in cell number in the hypertrophic zone, which could also be induced by IGF-I alone. Immunohistochemical analysis showed that uremic rats had decreased abundance of GHRs in the proliferative zone, and only GH/IGF-I therapy could overcome this decrease. These data thus suggest that growth retardation in uremic rats is, at least in part, due to a decrease in GHR abundance in chondrocytes of the proliferative zone of the tibial growth plate. This decreased GHR abundance can be overcome by combined GH/IGF-I therapy, thus enhancing generation and proliferation of hypertrophic zone chondrocytes and increasing growth-plate width.

CONCLUSION

These studies point to a mechanism for the growth retardation seen in children with CRF, and suggest that combined GH/IGF-I treatment may provide more effective therapy for these patients than GH alone.