Sci. Aging Knowl. Environ., 23 July 2003
Vol. 2003, Issue 29, p. pe20
[DOI: 10.1126/sageke.2003.29.pe20]

PERSPECTIVES

The Molecular Basis of Age-Related Kidney Disease

Feng Zheng, Anna Rita Plati, Anita Banerjee, Sharon Elliot, Liliane J. Striker, and Gary E. Striker

The authors are in the Departments of Medicine and Surgery, Vascular Biology Institute, University of Miami, Miami, FL 33136, USA. E-mail: gstriker{at}med.miami.edu

http://sageke.sciencemag.org/cgi/content/full/sageke;2003/29/pe20

Key Words: glomerular disease • hypertrophy • scarring • renal • bone marrow • cell cycle

Incidence of Renal Disease in the Aged

Until recently, renal disease, as evidenced by an elevated level of serum creatinine, was believed to be relatively rare. Creatinine is a waste product that is normally excreted in the urine, and an elevated level in the serum is indicative of kidney malfunction. However, because the serum creatinine value is a relatively crude measure of renal function and rises only when kidney function has decreased to less than one-half of the normal capacity, the incidence of kidney disease has been underestimated. Recently, the National Health and Nutrition Examination Survey (NHANES), which compiles information about the health and diet of people in the United States, provided data showing that nearly 3% of the U.S. population has an elevated serum creatinine level. This suggests that renal disease is a much more common health problem than previously appreciated (1). These new data also revealed that the incidence of renal disease increases with age. Even after removing subjects with obvious causes of renal impairment, such as diabetes mellitus and hypertension, 11% of people over the age of 65 had renal function that was less than 60% of that seen in a normal individual. Thus, age-related renal disease appears to represent a newly recognized syndrome separate from other known types of renal disease.

Causes of Renal Disease in the Aged

Decrease in the number of functioning nephrons

The underlying cause of age-related renal disease is unknown. However, it has been suggested that the development and progression of renal disease are associated with a decrease in the number of functioning nephrons, structures in the kidney that filter the blood (2). Studies of hospital patients have shown that the number of functioning nephrons decreases with age (3). In addition, the development of glomerulosclerosis--scarring of a group of blood vessels in the kidneys that form structures called glomeruli, which assist the kidneys in filtering the blood--has been correlated with a decrease in the number of functioning nephrons that occurs in the postmenopausal period in humans. These results have led many to conclude that age-related renal disease is likely the result of a loss of functional nephrons, which is an irreversible process.

Causes of renal disease unrelated to the number of functioning nephrons

Three observations made in our laboratory led us to question the hypothesis that a decrease in the number of functional nephrons is the principal cause of postmenopausal glomerulosclerosis. First, the development of renal disease as a result of decreased nephron number depends on the genetic background of the individual in premenopausal mice (4). Second, mice that are resistant to renal disease during the premenopausal period do develop renal disease characterized by glomerular enlargement and scarring in the postmenopausal period (5). In addition, renal disease in these postmenopausal mice does not appear to require, and is not associated with, a decrease in the number of functioning nephrons. Third, renal disease in mice can be transferred to a new host by transplantation of bone marrow-derived mesangial cell precursors (cells destined to become the mesangial, or smooth muscle cells, of the glomeruli in the kidney) and is independent of nephron number. These observations left open the question of whether estrogen status (or the status of other ovarian hormones) mediates renal disease in the postmenopausal period. In addition, these data suggest that glomerulosclerosis and increased glomerular size (hypertrophy) in the postmenopausal period may be related.

Genetic background

Using mice as a model, we have shown a close link between glomerular hypertrophy and increased glomerular cell turnover with the development of glomerulosclerosis, but only in the presence of a glomerulosclerosis-prone genetic background (4). We also showed that, in sclerosis-prone mice, various physiological and disease conditions result in the same pathophysiological outcome (6). Specifically, diabetes and a reduction in functional nephron number both lead to glomerular hypertrophy and glomerulosclerosis. In contrast, these same conditions in mice with a sclerosis-resistant genetic background resulted in little, or no, glomerular hypertrophy or glomerulosclerosis. Thus, in the resistant mice strains, increased cell turnover and glomerular hypertrophy could be dissociated from the development of glomerulosclerosis.

Loss of Allometric Control of Glomerular Size

In mice, rats, and humans in whom renal disease progresses rapidly, allometric control of glomerular size is lost, particularly in mice with a glomerulosclerosis-prone genetic background (4, 6). In other words, the glomerular size continues to increase, whereas other aspects of the kidney architecture remain appropriate relative to both the overall body and kidney size. This stands in sharp contrast to mice, rats, and human patients who are resistant to glomerulosclerosis. In these resistant subjects, a decrease in functional nephron number is associated with an appropriate and limited increase in glomerular size. For example, if one kidney is removed, the remaining kidney approximately doubles in size, and all kidney elements remain in allometric balance with one another. Renal function also remains normal.

Increased glomerular size has been shown to be a predictor of genetic susceptibility to the development of renal disease before other morphological changes are visible (7, 8). This genetic predisposition is relatively common, as shown by the NHANES data and by the observation that in about one-third of patients with renal carcinomas, the glomeruli appear enlarged and sclerotic, and these changes are independent of functional nephron number (9).

Before menopause, C57B6 mice are resistant to the development of glomerulosclerosis and glomerular hypertrophy, but both of these conditions develop and progress during the postmenopausal period (9). In the early postmenopausal period, the glomerular phenotype consists of hypertrophy and minimal sclerosis. As a direct function of time after menopause, the hypertrophy increases and the glomerulosclerosis becomes more severe (Fig. 1). Thus, there appears to be a progressive loss of allometric control of glomerular size, associated with a progressive increase in the amount of glomerulosclerosis in the postmenopausal period (Fig. 1). Importantly, nephron number remains normal during this period.



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Fig. 1. Progressive increase in glomerular size and sclerosis with age. Glomeruli are normal in 5-month-old C57B6 mice (A), but develop progressive sclerosis from 22 months (B) to 30 months (C) [periodic acid-Schiff staining (PAS); magnification, x200]. There is also an increase in cell number in 30-month-old mice, as determined by cell count (data not shown). (D) Morphometric analysis shows a progressive increase in glomerular volume as the mice age. (**) P < 0.01 versus 5-month-old mice; (&&) P < 0.01 versus 22-month-old mice (E). The reduction of nephron number by uninephrectomy (+Nx) is associated with an increase in glomerular volume in 2.5-month-old mice, but the values are still lower than those for 20-month-old mice with intact kidneys. A trend toward a further increase in glomerular volume is found in 20-month-old uninephrectomized mice. (##) P < 0.01 versus 2.5-month-old mice; (**) P < 0.01 versus 2.5-month-old mice +Nx. (F) The allometry curve of body weight (filled circles), kidney weight (lightly shaded circles), and glomerular volume (open circles) (all on the y axis) shows that the increase in glomerular volume with age in mice is not proportionate to the increase in body and kidney weight.

 
Molecular Changes Associated with Increased Glomerular Size

Increased glomerular size could result either from an increase in the number of glomerular cells or an increase in the size of the individual cells that constitute the glomerulus. In fact, we found that glomerular size is increased soon after menopause, at a time when glomerular cell number does not differ from that in the premenopausal period, suggesting that individual cells are becoming enlarged.

In the cell division cycle, cell growth occurs during the G1 to S phase. After mitosis, the sizes of the two daughter cells are returned to what is normal for the specific cell type. We found that mesangial cells isolated from the kidneys of postmenopausal mice are enlarged and have elevated amounts of the p27 protein, as assessed by immunochemistry (Fig. 2). In addition, mesangial cells isolated from these mice contain increased concentrations of p27. p27 is a member of the Kip/Cip family of kinase inhibitors. It binds to cyclin-cyclin-dependent kinase (cyclin-CDK) complexes and inhibits their kinase activity, which in turn blocks the cell cycle at the point where cells enter the S, or DNA synthesis, phase. The observation that mesangial cells from postmenopausal mice contain high amounts of p27 suggests that the cell cycle may be partially blocked in these cells, which could result in an increase in cell size. It is also possible, however, that p27 affects cell size by a mechanism that is independent of its ability to regulate CDKs.



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Fig. 2. p27 concentrations are increased in the mesangial cells within the glomeruli of 22-month-old C57B6 mice. Increased p27 immunostaining in the glomeruli of 22-month-old (B) compared with 8-month-old (A) C57B6 mice. Staining with p27 monoclonal antibody was performed in paraffin sections of kidneys from mice and revealed with alkaline phosphatase (x400). Arrows point to the positive p27 staining in glomerui. (C) Western blot analysis shows that mesangial cells isolated from 22-month-old C57B6 mice (22m MC) exhibit higher amounts of p27 (two lanes on left) compared with cells isolated from 5-month-old C57B6 mice (5m MC) (two lanes on right). Cells were isolated, placed in culture, and serum-starved for 48 hours before analysis. (D) Increased size of mesangial cells isolated from 22-month-old compared with 5-month-old mice. Mesangial cell size was analyzed by flow cytometry. The size of the cells from the 5-month-old mice was arbitrarily defined as 100%. *P < 0.05.

 
Disease-Related Phenotypic Changes in Mesangial Cells

We had previously found that mesangial cells isolated from mice that developed various forms of progressive renal disease during the premenopausal period retain phenotypic changes when grown in vitro, such as increased size and deposition of connective tissue (4-6, 10). The fact that all glomeruli are affected in mice with progressive renal disease and that similar phenotypic changes are present in mesangial cells isolated from glomeruli of mice with various types of renal disease suggests that the origin and some mechanisms may be common among the various forms of progressive renal diseases. Thus, we postulated that the ultimate phenotypic changes originate in a population of mesangial cell progenitors.

We demonstrated that a population of mesangial cell progenitors exists in the bone marrow of ROP Os/+ mice by showing that transplantation of bone marrow from these mice with established glomerular disease into the bloodstream of syngeneic hosts (ROP +/+) that have normal glomeruli transferred this disease to the glomeruli of the hosts. The mesangial cell progenitors contained in the donor bone marrow localize to the recipient's glomeruli (Fig. 3). Furthermore, we were able to transfer the phenotypic changes typical of the glomerular lesions associated with diabetes mellitus to glomeruli of nondiabetic syngeneic mice that initially had normal glomeruli. In this experiment, bone marrow from diabetic mice, which have diabetic nephropathy (diabetic glomerular disease), is transplanted into nondiabetic syngeneic mice. The bone marrow contains mesangial cell progenitors that deliver the diabetic nephropathy phenotype to nondiabetic recipient glomeruli. The glomerulosclerosis in the recipients cannot be controlled by the diabetes, because the recipients did not have diabetes. Rather, the recipients developed lesions in the kidney that were identical to those in their diabetic donors. Thus, the lesions in diabetic glomerular disease do not depend on the continued presence of hyperglycemia for the persistence or progression of the disease. In both of these cases, the genotype and the phenotype (glomerular hypertrophy and glomerulosclerosis) were transmitted by the bone marrow-derived progenitors (10). Glomerular hypertrophy and glomerulosclerosis occurred independently of functional nephron number in normal syngeneic bone marrow recipients, and this number closely resembled that of the donor. It is interesting that glomerular hypertrophy and glomerulosclerosis can be associated with a chromosomal defect (the Os mutation) and with a metabolic disease (diabetes mellitus). At any rate, the information needed to produce these stable phenotypic changes in the kidney is present in mesangial cell progenitors during the premenopausal period. The causes of these phenotypic changes are the subject of future investigations.



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Fig. 3. Genotype and phenotype of bone marrow-derived mesangial cells. (A) Repopulation of the glomeruli of ROP+/+ recipients with mesangial cells derived from ROP Os/+ bone marrow. Microsatellite analysis using polymerase chain reaction (PCR) with primers that span part of the Os mutation shows a double band for ROP Os/+ mice (lane 3) and a single band for ROP+/+ mice (lane 2). Glomeruli (lanes 4 and 5, which show results for two mice from the same group) and mesangial cells (not shown) of ROP+/+ recipients of ROP Os/+ bone marrow have the genotype ROP Os/+. Lane 1, negative control for the microsatellite analysis. (B) Decreased matrix metalloproteinase 2 (MMP-2) mRNA expression in mesangial cells from both ROP Os/+ and ROP+/+ recipients of ROP Os/+ bone marrow (BM). MMP-2 mRNA levels were determined by competitive PCR. The levels in mesangial cells isolated from ROP+/+ mice were used as controls and arbitrarily defined as 100%. (*) P < 0.05 and (**) P < 0.01 versus cells isolated from ROP+/+ mice. (C) Decreased MMP-2 activity as determined by gelatin zymography in mesangial cells of ROP Os/+ and ROP+/+ recipients of ROP Os/+ bone marrow. MMP-2 activity in mesangial cells isolated from ROP+/+ mice was used as a control. (**) P < 0.01 versus cells isolated from ROP +/+ mice. (D) Representative zymography of MMP-2. Lanes 1 and 2 show the activities of mesangial cells from ROP+/+ mice. Lanes 3 and 4 show the activities of mesangial cells from ROP Os/+ mice. Lanes 5 and 6 show the activities of mesangial cells from ROP+/+ recipients of ROP Os/+ bone marrow. [Modified from (10)]

 
Modulation of Glomerular Disease in the Postmenopausal Period

Because glomerulosclerosis and glomerular hypertrophy can be independent of functional nephron number, but dependent on the phenotype of mesangial cells derived from precursors in the bone marrow, we exchanged bone marrow between normal premenopausal C57B6 mice and postmenopausal C57B6 mice with glomerulosclerosis. Bone marrow-derived progenitors from the postmenopausal mice with glomerulosclerosis transferred the lesions of the donor to the recipient; that is, the premenopausal recipients developed lesions in their glomeruli consisting of both glomerulosclerosis and glomerular hypertrophy. Conversely, the administration of bone marrow-derived progenitors from premenopausal B6 mice with normal glomeruli to postmenopausal B6 mice with glomerulosclerosis resulted in a significant reduction in both glomerulosclerosis and glomerular hypertrophy in the postmenopausal mice. Examination of the whole glomeruli and isolated mesangial cells showed that the cell cycle phenotype of the donors with regard to p27 concentrations was delivered to the recipient glomeruli. Also delivered to the recipients were genes associated with increased deposition of extracellular matrix in the glomeruli.

Summary

Aging-related renal disease appears to be much more common than was previously thought. Our recent data obtained with female C57B6 mice show that the renal disease associated with the postmenopausal period consists of phenotypic changes in mesangial cell progenitors that induce both glomerulosclerosis and glomerular hypertrophy. These changes are independent of the number of functional nephrons. Furthermore, the glomerular changes can be reduced if mesangial progenitor cells from premenopausal mice are transplanted into postmenopausal mice with glomerulosclerosis. Thus, the development, regression, and progression of postmenopausal changes in the kidney are dependent on the molecular phenotype of the progenitor cells. The functional nephron number does not appear to be a primary factor in the development of glomerulosclerosis in the postmenopausal period. In addition, our recent data showing that glomerulosclerosis in the postmenopausal period can be substantially reduced raises the possibility that progressive, aging-related renal disease can be reduced.

A more complete delineation of the molecular phenotypes of postmenopausal glomerulosclerosis awaits further study. Similarly, the events that initiate these phenotypic changes also remain undetermined. However, the fact that the changes reside in extraglomerular progenitors, which are available in the circulation, makes it possible to envision efforts to modify their phenotype by pharmacological or gene therapy manipulations.


July 23, 2003
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Citation: F. Zheng, A. R. Plati, A. Banerjee, S. Elliot, L. J. Striker, G. E. Striker, The Molecular Basis of Age-Related Kidney Disease. Sci. SAGE KE 2003, pe20 (23 July 2003)
http://sageke.sciencemag.org/cgi/content/full/sageke;2003/29/pe20








Science of Aging Knowledge Environment. ISSN 1539-6150