Intrarenal insulin-like growth factor-1 axis after unilateral nephrectomy in rat

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.

A marked deficiency in circulating and renal IGF-I peptide does not inhibit compensatory renal enlargement in uninephrectomized mice

OBJECTIVE

Increase in kidney IGF-I levels due to its increased trapping from the circulation was hypothesized to be a key mediator of compensatory renal enlargement. We tested this hypothesis using genetically engineered mice with extremely low circulating IGF-I levels.

DESIGN

Both IGF-I deficient (ID) and normal (N) mice underwent a uninephrectomy (UNx) and sacrificed 2 or 9days later.

RESULTS

Initial body weight (BW) and kidney weight (KW) were significantly reduced in ID vs. N mice, while KW/BW ratios were similar. KW increased post-UNx to a comparable extent in ID and N mice (125±4 and 118±6% of pre-UNx KW, p<0.05 vs. C). Kidney IGF-I mRNA levels were similar in the ID and N mice and did not change post-UNx. Kidney IGF-I peptide levels pre-UNx were significantly lower in ID vs. N mice (25±5 vs. 305±39ng/g) and increased in both groups after UNx, remaining low in ID mice (45±4 in ID vs 561±64ng/g in N). IGF type 1 receptor phosphorylation was unchanged.

CONCLUSION

While a severe deficiency of circulating IGF-I impairs body growth, UNx induces a significant and proportional increase in renal mass in ID mice despite markedly decreased kidney IGF-I levels (>90% reduction) and no significant change in receptor phosphorylation. This all suggests that factors other than circulating and locally produced IGF-I are responsible for compensatory renal enlargement.

Role of sustained overexpression of central nervous system IGF-I in the age-dependent decline of mouse excitation-contraction coupling

We investigated the effects of exclusive and sustained transgenic overexpression of insulin-like growth factor (IGF)-I in the central nervous system (CNS) on the age-dependent decline in muscle strength, excitation-contraction coupling, muscle innervation and neuromuscular junction postterminal architecture. We found that (1) transgenic IGF-I overexpression in the CNS does not modify the decline in extensor digitorum longus (EDL) and soleus muscle weight with aging and (2) strength significantly decreases in transgenic (Tg) compared to wild-type mice. The latter finding is consistent with (3) the decreased absolute and specific force measured in the EDL muscle in vitro and (4) the decreased charge movement and peak intracellular Ca(2+) mobilization in individual muscle fibers from old IGF-I Tg mice compared to young wild-type mice, which also is associated with (5) decreased dihydropyridine receptor alpha(1)-subunit expression in old compared to young IGF-I Tg mice. (6) Tg IGF-I prevents a change in muscle fiber type that is associated with (7) improved muscle innervation and postterminal neuromuscular structure. (8) IGF-I is expressed extensively across the spinal cord gray matter and the lateral motor column. Our results raise questions about the timing and cell location of CNS IGF-I overexpression necessary to prevent or to ameliorate age-dependent alterations in the structure and function of skeletal muscle.

An IGF-I antagonist does not inhibit renal fibrosis in the rat following subtotal nephrectomy

Insulin-like growth factor I (IGF-I) has been proposed as a mediator of kidney scarring, although no interventional studies on the role of IGF-I in models of chronic kidney disease have been reported. The effect of a peptide IGF-I receptor antagonist (JB3) has been examined on kidney fibrosis and function in the rat following 5/6 subtotal nephrectomy (SNx). Male Wistar rats were anesthetized with halothane and subjected to SNx. JB3 was delivered by subcutaneous infusion using Alzet osmotic minipumps. In vitro studies showed JB3 to displace125I-IGF-I binding to isolated rat glomeruli and to inhibit IGF-I-induced receptor phosphorylation in renal tubular cells in culture. In the 7-day SNx rats, IGF-I immunostain was present in collecting tubules and JB3 inhibited compensatory renal growth, the maximum effect occuring at 10 μg·kg−1·day−1. After 90 days, the SNx rats developed proteinuria, hypertension, and a fall in glomerular filtration rate. IGF-I immunostain was present in the tubulointerstitial space of the remnant kidney together with marked tubulointerstitial fibrosis. Treatment with JB3 at a dose of 10 μg·kg−1·day−1had no effect on the renal fibrosis measured by Masson's trichrome staining or immunostain for collagen III and collagen IV. The proteinuria, hypertension, and lower creatinine clearance all remained unchanged. The remnant kidney was associated with a 50% decrease in renal IGF-I mRNA, which was partially restored by treatment with JB3. Thus an interventional study with an IGF-I receptor antagonist does not support a role for IGF-I in the development of renal fibrosis in the SNx rat, although IGF-I does make an important contribution to compensatory kidney growth.

The physiological and pathophysiological roles of the GH/IGF-axis in the kidney: lessons from experimental rodent models

The growth hormone (GH)/insulin-like growth factor (IGF) system plays an important role in renal development, growth, function and pathophysiology. IGF-I has been associated with renal/glomerular hypertrophy and compensatory renal growth. Potential effects on glomerular size are of interest, since an increase in glomerular size may be permissive for the development of glomerulosclerosis. In an effort to abolish the decline of renal function and possibly to restore the renal structure, different approaches have been tested in experimental models of nephropathy, focusing mainly on early renal changes. The involvement of the GH/IGF system in renal pathophysiology has been studied in much detail in the rat. In view of the growing interest in murine physiology, occurring in large part by genetically modified animals, this review examines those aspects of GH, IGFs, their receptors and binding proteins that relate both to mouse kidney physiology and to a number of conditions characterized by pathophysiological renal changes. A deeper understanding of the role of the GH/IGF system in renal dysfunction may stimulate the development of novel therapeutic approaches aiming at preventing or retarding various kidney diseases.

Successful six-day kidney preservation using trophic factor supplemented media and simple cold storage

This study examined the effect of trophic factor supplementation [TFS; bovine neutrophil peptide-1 (bactenecin), 1 mg/L; substance P, 2.5 mg/L; nerve growth factor, 20 microg/L; epidermal growth factor, 10 microg/L; insulin-like growth factor-1, 10 microg/L] during cold storage with UW lactobionate solution. Dogs transplanted with kidneys stored for 4days in TFS-UW had significantly lower peak serum creatinine values (mean 2.9 +/- 0.2mg/dL) and returned to normal values faster (6 days) than kidneys stored for 3days in unmodified UW solution (4.2 +/- 0.3 mg/dL and 14 days, respectively). Kidneys stored for 5days in TFS-UW (mean peak creatinine 3.7 +/- 0.3) functioned equivalently to kidneys stored for 3days and better than kidneys stored for 4 days in UW alone. Dogs with kidneys stored for 6days in TFS-UW had mean peak creatinines of 5.7 +/- 0.4 mg/dL. These returned to normal creatinine values in 14 days, equal to 3-day stored and significantly better than kidneys stored for 4days in UW alone (20days recovery time). This study shows trophic factor deprivation appears to be a critical mechanism of injury in organ preservation with current synthetic storage media, and marks the initial development of a synthetic biologically active preservation solution, the next generation of preservation media.

JB3, an IGF-I receptor antagonist, inhibits early renal growth in diabetic and uninephrectomized rats

Biochemical evidence suggests that insulin-like growth factor I (IGF-I) may play an important role as a mediator of kidney growth. In the present study, an IGF-I receptor antagonist (JB3) was synthesized, and its effect on the renal growth that follows the induction of diabetes or unilateral nephrectomy (UNx) was examined. JB3 was generated by solid phase peptide synthesis. Its activity as an IGF-I antagonist was confirmed in an opossum kidney cell line from its inhibitory effect on the increase in thymidine incorporation into DNA induced by recombinant human IGF-I. Male Wistar rats were anesthetized with halothane and subjected to either the induction of diabetes by streptozotocin (intravenous 60 mg/kg) for 4 d (control animals received citrate buffer) or UNx for 11 d (control animals were sham operated). JB3 was delivered by subcutaneous infusion using an osmotic minipump implanted 3 d before the induction of diabetes or UNx. Kidney wet weight, DNA, and protein all were significantly higher 4 d after the induction of diabetes (24%) or 11 d after UNx (55%). Dose-response studies (1 to 30 microg/kg per day) showed JB3 administration to inhibit the increase in kidney growth in both diabetic and UNx rats. The increase in kidney wet weight, DNA, and protein was significantly lower in UNx rats that were treated with JB3 10 microg/kg per day (P: < 0.05) than in saline vehicle controls but was abolished in diabetic rats that were treated with JB3 3 microg/kg per day (P: < 0. 01). Increasing the dose of JB3 to 30 microg/kg per day was associated with a decrease in its inhibitory effect, resulting in bell-shaped dose-response curves. JB3 administration had no effect on the blood glucose concentration or food consumption by either diabetic or nondiabetic animals. The results support the concept of IGF-I as an important mediator of the early renal growth that follows the induction of diabetes or UNx in the rat.

Characterization of the selectivity filter of the epithelial sodium channel

The epithelial sodium channel (ENaC) is composed of three homologous subunits termed alpha, beta, and gamma. Previous studies suggest that selected residues within a hydrophobic region immediately preceding the second membrane-spanning domain of each subunit contribute to the conducting pore of ENaC. We probed the pore of mouse ENaC by systematically mutating all 24 amino acids within this putative pore region of the alpha-subunit to cysteine and co-expressing these mutants with wild type beta- and gamma-subunits of mouse ENaC in Xenopus laevis oocytes. Functional characteristics of these mutants were examined by two-electrode voltage clamp and single channel recording techniques. Two distinct domains were identified based on the functional changes associated with point mutations. An amino-terminal domain (alpha-Val(569)-alpha-Gly(579)) showed minimal changes in cation selectivity or amiloride sensitivity following cysteine substitution. In contrast, cysteine substitutions within the carboxyl-terminal domain (alpha-Ser(580)-alpha-Ser(592)) resulted in significant changes in cation selectivity and moderately altered amiloride sensitivity. The mutant channels containing alphaG587C or alphaS589C were permeable to K(+), and mutation of a GSS tract (positions alpha587-alpha589) to GYG resulted in a moderately K(+)-selective channel. Our results suggest that the C-terminal portion of the pore region within the alpha-subunit contributes to the selectivity filter of ENaC.

Compensatory renal growth in uninephrectomized adult mice is growth hormone dependent

Compensatory renal growth in uninephrectomized adult mice is growth hormone dependent.

BACKGROUND

Growth hormone (GH) and insulin-like growth factors (IGFs) have been implicated as pathogenic factors in compensatory renal growth (CRG) following unilateral nephrectomy in rodents. CRG in adult rats has been suggested to be GH dependent and GH independent in immature rats. However, the exact role of GH as a regulating or permissive factor in CRG in adult rodents has not been fully resolved to date.

METHODS

To elucidate a possible direct, permissive role of GH in CRG, we examined the effect of a newly developed specific GH receptor (GHR) antagonist (G120K-PEG) on kidney IGF-I accumulation and renal/glomerular hypertrophy over seven days after uninephrectomy in adult mice.

RESULTS

Placebo-treated uninephrectomized mice were characterized by a transient increase in kidney IGF-I concentration preceding CRG and an increase in glomerular volume. In G120K-PEG-treated uninephrectomized animals, increased kidney IGF-I levels, kidney weight, and glomerular volume were fully abolished. No differences were seen between the two uninephrectomized groups with respect to body weight, food intake, blood glucose, serum GH, IGF-I, or IGFBP-3 levels.

CONCLUSIONS

The administration of a GHR antagonist in uninephrectomized adult mice has renal effects without affecting circulating levels of GH/IGFs, indicating that the effect of G120K-PEG may be mediated through a direct inhibitory effect on renal IGF-I accumulation through the renal GHR. This study shows, to our knowledge for the first time, that CRG in adult mice is strictly GH dependent.