Impact of obesity as an independent risk factor for the development of renal injury: implications from rat models of obesity

Abstract

Diabetes and hypertension are the major causes of chronic kidney disease (CKD). Epidemiological studies within the last few decades have revealed that obesity-associated renal disease is an emerging epidemic and that the increasing prevalence of obesity parallels the increased rate of CKD. This has led to the inclusion of obesity as an independent risk factor for CKD. A major complication when studying the relationship between obesity and renal injury is that cardiovascular and metabolic disorders that may result from obesity including hyperglycemia, hypertension, and dyslipidemia, or the cluster of these disorders [defined as the metabolic syndrome, (MetS)] also contribute to the development and progression of renal disease. The associations between hyperglycemia and hypertension with renal disease have been reported extensively in patients suffering from obesity. Currently, there are several obese rodent models (high-fat diet-induced obesity and leptin signaling dysfunction) that exhibit characteristics of MetS. However, the available obese rodent models currently have not been used to investigate the impact of obesity alone on the development of renal injury before hypertension and/or hyperglycemia. Therefore, the aim of this review is to describe the incidence and severity of renal disease in these rodent models of obesity and determine which models are suitable to study the independent effects obesity on the development and progression of renal disease.

INTRODUCTION

Obesity is a constantly rising global epidemic and has become a public health crisis in the United States (129). Obesity is often associated with metabolic syndrome (MetS), which is not a disease but an assortment of other cardiovascular and metabolic disorders (i.e., insulin resistance, hypertension, hyperglycemia, and dyslipidemia). MetS is commonly defined as the existence of insulin resistance under either a prediabetic or type 2 diabetic state with at least two of the following 1) hypertension with an arterial pressure of ≥140/90 mmHg; 2) dyslipidemia with triglyceride levels of ≥150 mg/dl and high-density lipoprotein (HDL) cholesterol levels < 40 mg/dl; 3) central/abdominal obesity reflected by a waist circumference of >35–40 in.; and 4) hyperglycemia with fasting blood glucose levels of ≥100 mg/dl (61, 97).

Obesity is associated with the two most common causes of renal disease: hypertension and diabetes. Patients suffering from obesity have a greater than twofold risk of developing albuminuria (5, 18, 69, 80, 100, 141). The prevalence of obesity has increased dramatically within the last decade and is now considered an independent risk factor for chronic kidney disease (CKD) (16, 38). Investigation into the relationship between obesity and renal injury is complicated by the presence of the characteristics of MetS (18, 80). The associations between hyperglycemia and hypertension with renal disease have been extensively reported in individuals suffering from obesity. Over the last decade, the prevalence of renal disease has also increased in obese patients without hyperglycemia (17, 112). Kramer et al. (78) observed that the mean body mass index was increased in the end-stage renal disease population and was higher compared with the United States population as a whole. Additionally, nearly 33% of the obese population with end-stage renal disease was nondiabetic. Similarly, Praga et al. (113) reported that 90% of patients, who underwent a unilateral nephrectomy and developed progressive proteinuria and CKD, were obese. Moreover, many of these obese patients were not hypertensive or diabetic. In support of these two studies, Wesson et al. (144) observed that severe obesity independent of diabetes lead to the development of proteinuria and glomerular injury. Overall, these studies provide strong evidence that renal dysfunction in obese patients may occur before the development of hypertension or diabetes.

As mentioned earlier, one of the major difficulties when studying the early development of renal disease associated with obesity is separating the direct effects of obesity from the insulin resistance-mediated effects. Interestingly, insulin resistance rather than the typical risk factors (hypertension, hyperglycemia, and dyslipidemia) may be the most vital characteristic that initiates renal disease. Insulin resistance is able to stimulate two risk factors of obesity: hypertension and hyperglycemia in which both have the ability to raise arterial pressure by increasing sodium tubular reabsorption. Insulin resistance has been observed to activate the renin-angiotensin-aldosterone system (53, 57, 85, 94, 101), and its effect to promote hyperglycemia causes increased hyperglycemia-mediated coreabsorption of sodium in the renal tubules (47, 136, 137). The increase in sodium reabsorption in the proximal tubule and loop of Henle leads to an impairment of tubular glomerular feedback. This stimulates a reduction in afferent arteriole resistance and causes elevations in renal blood flow and glomerular filtration rate (GFR), often referred to as renal hyperfiltration. While obesity alone causes renal hyperfiltration, hyperglycemia and hypertension during obesity augment these elevations in GFR (53, 57, 85). At the same time, lipid accumulation in the kidney in response to dyslipidemia contributes to renal disease associated with obesity (32, 33, 91). Therefore, we hypothesize that the obesity- and/or insulin resistance-mediated renal hyperfiltration in combination with dyslipidemia synergistically interacts to cause proteinuria and renal injury (Fig. 1). However, the mechanisms involved in the development of renal disease associated with obesity, alone, still remain elusive.

Fig. 1.

Potential mechanisms by which obesity and other features of metabolic syndrome (MetS; insulin resistance, hypertension, hyperglycemia, and dyslipidemia) may cause or augment the development of renal disease associated with obesity. RAAS, renin-angiotensin-aldosterone system; GFR, glomerular filtration rate.

Currently, there are many obese animal models that develop characteristics of MetS that develop varying degrees of renal disease (3, 7, 14, 25, 34–36, 41, 44, 45, 51, 58, 66, 67, 75, 103, 105, 110, 132, 149). The use of these models have provided valuable insights on notable mechanisms involved in the development and progression of renal disease associated with obesity. Thus, due to the exhaustive nature of a comprehensive list of both genetic and nongenetic obese rodent models and limited space, this review will focus on obese rat models resulting from a high-fat (HF) diet and models derived from mutations in leptin or leptin receptor genes. Additionally, we will describe the pathogenesis of renal disease associated with obesity including insulin resistance, hypertension, hyperglycemia, and dyslipidemia. While sex differences have been observed in obesity, diabetes, hypertension, and kidney disease (8, 37, 65, 77, 88, 108, 120, 121), the majority of the studies discussed in the current review were performed in male rats.

HIGH-FAT DIET-INDUCED OBESITY

While leptin resistance has been observed in obese patients, true leptin signaling deficiency is rare and is not the primary cause of clinical obesity. Other factors such as environmental factors (i.e., dietary) play an important role in the development of obesity in humans. Feeding rodents a HF diet to induce obesity causes similar patterns of metabolic and cardiovascular abnormalities observed in patients suffering from obesity and/or MetS. Moreover, these HF diet-induced models have proven to be very useful in studying various mechanisms involved in the development of renal disease associated with obesity. For these reasons, HF diet-induced obesity models are more advantageous than the monogenetic obesity (i.e., leptin or leptin receptor deficiency) strains. However, similar to obese humans and monogenetic obesity strains, the development of metabolic and cardiovascular disorders and renal disease induced by a HF diet vary with different rat strains. Therefore, the following section will describe the development of metabolic and cardiovascular abnormalities associated with obesity along with renal disease in these strains fed various formulas of a HF diet in Wistar, Sprague-Dawley (SD), spontaneously hypertensive rat (SHR), and Dahl salt-sensitive (SS) rats. However, the temporal changes in the metabolic parameters and the development of renal injury of these strains on a HF diet have not been thoroughly investigated.

Wistar Rats

There are quite a few Wistar strains that were created in the mid-late 1940s and late 1990s, but the Wistar rat originated from the Wistar Institute in 1906 (24). When placed on a HF diet between 5 and 7 wk of age, increased weight gain is observed as early as 8 wk of administration (3, 74, 84, 86). During this time frame, insulin resistance and hyperinsulinemia are also noticed in the Wistar strain when fed a HF diet (3, 22, 74, 84, 86, 95). However, the development of hyperglycemia in HF diet-induced obesity in Wistar rats is conflicting. Some studies have demonstrated that fasting blood glucose levels are in the prediabetic to diabetic range (115–170 mg/dl) after being placed on a HF diet for 8 wk (3, 74, 84, 86). Yet, others have reported blood glucose levels within the normal range after 20–40 wk on a HF diet (21, 84, 127, 146). Similar to the other metabolic parameters mentioned above, dyslipidemia is evident as early as 8 wk in Wistar rats fed a HF diet (3, 95). Elevations in arterial pressure are observed in HF diet-fed Wistar rats compared with normal diet-fed rats between 8 and 35 wk that is considered mild to moderate (mean arterial pressure: 125–150 mmHg) (21, 74, 84). Interestingly, renal histological abnormalities are detected as early as 8–10 wk on a HF diet, which include narrowing of Bowman’s space and glomerular and tubular injury (74, 95). These abnormalities become worse when placed on a HF diet longer (>20 wk) in which renal injury consist of hypertrophy, glomerulosclerosis, protein casts, renal fibrosis, and mild proteinuria (21, 127, 146). However, the development of renal injury observed in HF diet-induced obesity in Wistar rats does not progress to reduced renal function. Taken together, these data suggest that Wistar rats fed a HF diet develop renal injury in the presence of insulin resistance, dyslipidemia, and mild hyperglycemia and hypertension.

Sprague-Dawley Rat

The Sprague-Dawley (SD) strain was created from a male hooded hybrid and a female Wistar rats by R. W. Dawley in 1925 and is one of the highly resistant rat strains to the development of renal injury. When administered a HF diet as early as 4 wk of age, these rats gain a significant amount of body weight that can be detected within 8–14 wk of administration (30, 36, 39, 56, 72, 73, 86, 106). During this time frame, SD rats fed a HF diet display insulin resistance, hyperinsulinemia, and mild dyslipidemia (36, 39, 72, 86, 106). Despite having these metabolic disorders, SD rats fed a HF diet do not develop hyperglycemia (34–36, 39, 72, 86, 106, 110). In most studies, these rats develop moderate hypertension (130–160 mmHg) (30, 34, 35, 39, 72, 110). SD rats with HF diet-induced obesity exhibit renal glomerular hypertrophy, mesangial expansion, and podocyte foot process fusion and effacement with mild proteinuria between 14 and 22 wk after administration of a HF diet (30, 56, 106, 110, 153). However, SD rats fed a HF diet do not experience elevations in GFR nor progress to CKD with a decline in renal function (110). HF diet-induced obesity in the SD strain is an appropriate model to study the mechanisms that contribute to early glomerular injury and proteinuria associated with obesity and moderate hypertension but independent of hyperglycemia. However, the glomerular and renal injury observed in this model diet-induced obesity is considered mild.

Spontaneous Hypertensive Rat

The spontaneous hypertensive rat (SHR) is a rodent model of hypertension that was created during the 1960s by Okamoto and Aoki (104) by specifically breeding Wistar-Kyoto rats with hypertension. When fed a HF diet for 10–15 wk at 4–8 wk of age, body weight significantly increases by 10–30% compared with SHRs fed a normal fat diet (14, 22, 71, 75, 147). During this same time period, elevated plasma insulin levels and insulin resistance are observed along with very mild hyperglycemia (130–170 mg/dl) in HF diet-fed SHRs compared with SHRs on normal fat diet (14, 22, 71, 147). Similarly, plasma lipid levels are markedly increased during HF diet-induced obesity after 10–15 wk in SHRs (14, 22, 71, 75, 147). However, when the effect of a HF diet on arterial pressure was examined, there have been conflicting reports. Feeding SHRs a HF diet for 10 wk does not augment the level of hypertension compared with their normal fat diet counterparts (71, 75, 147). In contrast, arterial pressure increases by >30 mmHg in SHRs fed a HF diet for 12–15 wk (14, 22). HF diet-induced obesity in SHRs induces renal disease, which includes increased proteinuria, glomerular injury with mesangial expansion, and lipid accumulation that progress to a decline in renal function compared with SHRs on a normal fat diet (14, 22, 71, 75, 147). These results provide evidence that HF diet-induced obesity in SHRs causes renal injury associated with many of pathophysiological disorders similar to those observed in MetS, which include severe hypertension and mild hyperglycemia. However, the HF diet-induced obesity in SHRs represents an appropriate model to study mechanisms that are involved in the development of renal injury that progresses to impaired renal function related to obesity.

Dahl Salt-Sensitive Rat

One of the most commonly used rodent models to study mechanisms involved in the development and progression hypertension-induced renal disease is the Dahl salt-sensitive (SS) rat. In 1962, Dahl et al. (29) performed selective breeding from SD rats that resulted in a population of rats that were very sensitive to the development of hypertension from a high-salt diet. When placed on a HF diet between 3 and 5 wk of age, SS rats gain a significant amount weight compared with rats fed a normal fat diet within 8 wk (6, 96). Plasma insulin and blood glucose levels are markedly elevated in SS rats fed a HF diet for 8–18 wk in comparison to rats fed a control diet (96). However, no differences in cholesterol and triglyceride levels were observed during this time frame. Arterial pressure was significantly elevated in SS rats fed a HF diet in comparison to normal fat fed rats (6, 96). Additionally, SS rats fed a HF diet under these conditions exhibit no to very small differences in proteinuria (6, 96). Interestingly, most studies characterizing SS rats on a HF diet usually begin after 10–12 wk of age (66, 67, 124, 125). When these SS rats are placed on a HF diet for 4 wk, increased weight gain is detected along with hypertension and glomerular injury (124). However, when the salt content of the HF diet was increased, SS rats become hyperglycemic and severely hypertensive and develop progressive renal disease, which includes proteinuria, glomerulosclerosis, and vascular and tubulointerstitial injury after 4 wk (66, 67). These data provide evidence that SS rats fed a HF diet gain weight and develop hypertension and mild hyperglycemia with proteinuria, and administration of high salt augments the hypertension and renal damage in SS rats fed a HF diet.

OBESE LEPTIN AND LEPTIN RECEPTOR DYSFUNCTIONAL MUTATION MODELS

The majority of obese rodent models used to study mechanisms associated with renal disease are the result of genetic mutations in the leptin signaling pathway. While these obese models have been utilized to study the relationship between obesity and renal disease, the development and progression of renal disease vary among these animal models due to differences in genetic backgrounds and the incidence of the various characteristics of MetS. These genetic mutations occurred spontaneously or were introduced via genetic engineering, leading to dysfunctional proteins that inhibit leptin signaling. Disruption of the leptin signaling pathway in the hypothalamus of the brain, which is responsible for appetite suppression and energy expenditure, results in obesity (11). In the following section, we discuss the development of renal injury in various obese models of leptin signaling dysfunction and determine if these renal abnormalities occur before the presence of hypertension and hyperglycemia.

Obese Zucker Rat

The obese Zucker (OZ) rat strain was discovered in the early 1960s at the Harriet G. Bird Memorial Laboratory in Stow, MA, by Louis M. Zucker and Theodore F. Zucker (154). The OZ strain emerged from a colony created by crossing Sherman and Merck Stock M rats (M13 strain) that spontaneously developed a mutation in the fa/fa (fatty) gene, also known as the leptin receptor gene. Three decades later, it was revealed that this mutation was due to a single amino acid substitution of glutamine to proline at position 269 caused by a single nucleotide substitution at position 880 of the leptin receptor gene (20). Consequently, this missense mutation resulted in a decreased binding affinity of leptin to the leptin receptor (145). Obesity is visually evident as early as 3–6 wk of age and progresses very rapidly during the first 16 wk of life (25, 103, 128). The OZ strain develops hyperinsulinemia and dyslipidemia by 6 wk of age that worsens with age (25, 28, 45, 46, 103). However, the early development of obesity and insulin resistance does not lead to the development of overt diabetes. OZ rats exhibit either normoglycemia or mild hyperglycemia (<120 mg/dl) (12, 25, 64, 103). In contrast, few studies have provided evidence that OZ rats display hyperglycemia after 20 wk of age (≥170 mg/dl) (25, 45). Despite developing many of the characteristics of MetS, the OZ strain exhibits mild hypertension (<150 mmHg) after 15 wk of age (45). Renal injury manifests and progresses very slowly over the life span of the OZ strain. Elevations in renal blood flow and GFR (renal hyperfiltration) have been identified at 13 wk of age in these obese rats compared with their lean littermates (2). Evidence of mild progressive proteinuria/albuminuria is detectable after 20 wk of age but may not be related to insulin resistance and hyperglycemia (25, 103). McCaleb and Sredy (89) concluded from his studies that albuminuria was dependent on age and independent of hyperglycemia in both normoglycemic and hyperglycemic OZ rats. Focal glomerularsclerosis, glomerular hypertrophy, and tubular injury first become detectable at 30–40 wk of age (2, 25, 103, 105) followed by a decline in renal function (elevated plasma creatinine) in the OZ strain at 60 wk of age (25). The OZ strain displays a small number of early changes in renal structure (6–10 wk of age) including elevated macrophage infiltration, podocyte damage, and renal lipid accumulation (25, 46, 103). However, the majority of evidence of renal disease (mild proteinuria, renal structural damage, and decline in renal function) occurs after 20 wk of age despite the presence of the development of MetS. Overall, these data suggest that the development of renal injury in the OZ strain strongly correlates with aging rather than suffering from obesity, insulin resistance, and hypertension. However, the OZ strain may be appropriate to study renal injury associated with mild hyperglycemia, moderate hypertension, and aging.

Zucker Diabetic Fatty Rat

Once the OZ strain was established, this model was widely distributed to several facilities. At the Indiana University School of Medicine in 1985, Dr. Richard Peterson created a subsequent strain of the OZ rats called the Zucker diabetic fatty (ZDF) strain by breeding OZ rats that developed severe hyperglycemia (≥300 mg/dl) (81). One possible explanation responsible for the development of severe hyperglycemia in the ZDF strain is an autosomal recessive defect in the islet cells of the pancreas independent of the leptin receptor mutation in which there is a significant reduction in insulin levels at 12 wk of age (23, 26, 148). Similar to its parental OZ strain, the ZDF rat develops obesity and insulin resistance but also mild hyperglycemia early (<8 wk of age) (26, 27, 79, 140). Unlike the OZ strain, these rats become very hyperglycemic due to the loss of insulin production by 12 wk of age (31, 60, 107). This progressively worsens that ultimately leads to weight loss by 30 wk of age (13). Hypertriglyceridemia also develops before 10 wk of age (26). Since the onset of severe hyperlipidemia also coincides with a huge impairment in glucose tolerance, it has been proposed that the decreased pancreatic β-cell mass and thus loss of insulin production observed in this strain may also be due to both lipo- and gluco-toxicity (107). Obese ZDF rats also develop a nonprogressive mild hypertension (<140 mmHg) as they age (27, 60, 123, 140). Albuminuria is slightly elevated in the ZDF strain, compared with control lean rats at 6 wk of age but increases progressively with age after 30 wk in the absence of hypertension (60, 123, 140). By 12 wk of age, these obese ZDF rats experience a twofold increase in GFR that leads to renal hypertrophy. Eventually, GFR decreases in the ZDF strain compared with that of their lean controls by 30 wk of age (60). Despite the early onset of renal hyperfiltration, renal pathological changes including glomerulosclerosis, tubulointerstitial fibrosis, and inflammation do not occur until after 20 wk of age (60, 123, 140). Additionally, the ZDF rats, like the OZ strain, develop podocyte injury and characteristics of renal injury similar to other rodent models of diabetic nephropathy (60). Collectively, these data indicate that the ZDF strain develops mild renal disease that does not progress to CKD and is the most appropriate to explore the development of renal injury associated with hyperglycemia and age related mechanisms in the absence of hypertension.

Spontaneously Hypertensive Obese Rat

In 1969, an obese model of the spontaneously hypertensive rat (SHR) called the Koletsky rat or the spontaneously hypertensive obese rat (SHROB) was developed in Dr. Richard Koletsky laboratory at Case Western Reserve University School of Medicine in Cleveland, OH, as a result of a spontaneous mutation in the leptin receptor now known as the cp mutation (76, 131). This mutation evolved after many generations of inbreeding animals derived from a male normotensive SD rat and a female SHR, a strain that develops hypertension (76) and, consequently, results in the development of obesity. However, the rapid progression of weight gain in the SHROB strain does not begin until 8 wk of age (41, 71a, 152). Insulin resistance in SHROBs becomes evident by 12 wk of age and worsens without developing hyperglycemia (40, 44, 102). Signs of severe dyslipidemia occur as early as 5 wk of age before the onset of obesity (41, 44, 152). Although systolic arterial pressures (150–170 mmHg) are elevated between 6 and 8 wk of age, hypertension in the SHROB is much less severe compared with their lean counterparts until after 30 wk of age (40, 41, 71a, 83, 152). The delay in the severity of the hypertension in the SHROB may be due to the lack of leptin signaling mediated activation of the sympathetic nervous system (10). The first indication of renal injury in the SHROBs occurs between 16 and 18 wk of age and includes microalbuminuria (30–300 mg/day), mild glomerular injury, and elevated nephrin excretion (600 µg/day), a marker of podocyte injury, and worsens with age (71a, 152). Additionally, markers of tubular injury such as kidney injury marker-1 are elevated in the SHROB strain at 18 wk of age but remain unchanged with aging (152). Furthermore, GFR decreases by 50% between the ages of 30 and 40 wk in SHROBs (40, 152). Therefore, in the SHROB strain, the compounding effects of severe weight gain, hyperinsulinemia, and dyslipidemia along with aging may play a greater role than hypertension in the development of CKD. The SHROB strain may be best suitable to examine renal injury associated with obesity and hypertension in the absence of hyperglycemia.

Spontaneously Hypertensive Heart Failure Rat

The obese spontaneously hypertensive and heart failure (SHHF) rat is a genetic rodent model developed for the study of obesity and diabetes in the late 1970s by Dr. Carl T. Hasen at the National Institute of Health as a result of crossing heterozygote SHROB (Kolestzky) rats with the SHR/N strain (117). To eliminate non cp Koletsky genes, 12 or more backcrosses were conducted to establish a colony now known as SHR/N-cp since the cp gene is the only genetic difference between SHR/N-cp and the SHR/N strain (92). A set of breeder pairs were obtained from Dr. Hasen by Dr. McCune after only seven backcross generations, and this substrain was designated as the SHHF/Mccp-cp strain (90). They become visibly obese by 10 wk of age (92, 149). In contrast to their parental strains, SHHF/Mccp-cp rats develop hyperglycemia and hyperinsulinemia at 6 wk of age before the onset of obesity. After the development of obesity, the hyperglycemia is worse (glucose range 130–200 mg/dl, insulin >30 µU/ml) (58, 92, 149). Severe dyslipidemia occurs later in life after the establishment of severe obesity (92, 149). Elevations in arterial pressure are detected by 6 wk of age in SHHF/Mccp-cp before obesity (150 mmHg) and progressively increases with age (92, 149). However, similar to the SHROB strain, the hypertension is less severe in the obese SHHF/Mccp-cp strain when compared with their lean counterparts, which may be due to the lack of leptin signaling (10). After 50 wk of age, systolic arterial pressure in the obese SHHF/Mccp-cp rats peaks at ~200 mmHg, which is comparable to their lean littermates (149). This would suggest that the late onset of severe hypertension in these obese rats may be attributed to age-associated mechanisms. Despite having hypertension, obese SHHF/Mccp-cp rats do not develop progressive proteinuria or changes in renal structure until 14 wk of age (59, 149). Kidneys from the SHHF/Mccp-cp strain display increased glomerulus surface area, protein casts, and renal hypertrophy, consistent with changes seen in diabetic nephropathy and inflammation (149). Furthermore, by 50 wk of age, GFR decreases by more than half corresponding with an elevation in plasma creatinine (81, 149). Moreover, renal injury and a decline in renal function are present in the lean littermates but are less severe despite elevations in arterial pressure at an earlier age (149). These results indicate that the development of renal disease occurs after the presence of hyperglycemia and hypertension in the obese SHHF/Mccp-cp strain, and the progression of renal disease may be essentially due to the combination of effects from the other characteristics of MetS and age-associated mechanisms. However, the renal disease exhibited in the SHHF/Mccp-cp strain recapitulates human diabetic nephropathy.

Obese Zucker Spontaneously Hypertensive Fatty Rat

The Zucker spontaneously hypertensive fatty (ZSF1) rat was created at Genetic Models in Indianapolis, IN, in the early 2000s, by breeding a lean female heterozygote Zucker fatty rat (ZDF), a strain of the fa mutation, and a lean male heterozygote spontaneously hypertensive heart failure rat model (SHHR), a strain of the fa^cp mutation (51). Since its creation, the obese ZSF1 strain has been considered as an acceptable model of human diabetic nephropathy and cardiac disease (81). Obesity in the ZSF1 strain can be observed as early as 8 wk of age (7, 81, 111, 132) with the development of dyslipidemia by 12 wk of age (37, 115, 150). Like both parent strains, the ZSF1 strain displays hyperglycemia (>140 mg/dl) and hyperinsulinemia (>10 ng/ml) at 6 wk of age with severe hyperglycemia present after 12 wk of age (7, 81, 150). However, after 30 wk of age, plasma insulin levels eventually decline by 70% due to β-cell insufficiency (7). Both lean and obese ZSF1 rats are equally hypertensive (140–150 mmHg) by 8 wk of age (7, 111), but by 20 wk of age, the hypertension worsens in ZSF1 compared with their age-matched lean counterparts (81, 111, 115). The onset of progressive proteinuria is detected by 16 wk of age (7, 9, 81, 93, 115, 133, 151). By 20 wk of age, kidneys from ZSF1 rats display glomerular sclerosis, mesangial expansion, and tubular injury/atrophy (7, 70, 111, 115, 138). Moreover, a decline in renal function is observed in the ZSF1 after 40 wk of age (37, 70, 111, 133, 138). These results suggest the development of renal injury occurs in the ZSF1 strain after the presence of some features of MetS (hyperglycemia, hypertension, and dyslipidemia) by 12 wk of age that later progresses to CKD. Similar to the SHHF/Mccp-cp strain, ZSF1 rats are an applicable model to study mechanisms involved in the development of diabetic nephropathy.

DahlS.Z-Lepr^fa/Slc Rat

At the Hamamatsu University School of Medicine in 2011, a new obese rodent model called Dahl SS.Z-Lepr^fa/Slc (DS/obese) rat was generated from crossing a male Zucker rat heterozygous for the fa allele of the leptin receptor gene with a female SS/Jr rat (54). The final congenic strain was established by several rounds of backcrossing male rats with the fa allele from the F1 prodigy to the SS strain to eliminate undesired genes from the Zucker strain (54). Obesity is apparent as early as 5–6 wk of age in DS/obese rats (54, 55). By 13 wk of age, plasma cholesterol and triglycerides in the DS/obese strain are abnormally high and continue to increase by 18 wk of age (54, 55, 130). Furthermore, DS/obese and DS/lean rats develop similar levels of mild hyperglycemia (54, 55, 87, 130). Development of hyperinsulinemia in the DS/obese strain occurs as early as 9 wk (87, 130). While arterial pressure is similar in both the DS/obese and DS/lean rats at 9 wk of age (140 mmHg), the DS/obese strain exhibits a significant rise in arterial pressure and develops severe hypertension by 13 wk of age (190–200 mmHg) (55, 87, 130). Evaluation of renal disease in the DS/obese strain has been very limited with most studies focusing on mechanisms involved in cardiac dysfunction (54, 55, 87, 130). However, the DS/obese rats display progressive proteinuria and a decline in renal function by 18 wk of age when compared with their age-matched DS/lean rats (54, 130). The DS/obese strain develops renal disease in the presence of insulin resistance, hyperinsulinemia, dyslipidemia, as well as hypertension that leads to CKD by 18 wk of age. Therefore, the DS/obese strain is suitable to investigate the effects of hypertension and very mild hyperglycemia on the development of progressive proteinuria and renal injury.

Obese SS^LepRMutant Rat

Recently, another obese rat model was created on the Dahl SS genetic background using Zinc-finger technology with the PhysGen Knockout Program at the Medical College of Wisconsin in 2012 (49, 50). Zinc-finger constructs targeting a specific region on chromosome 5 of exon 11 were used to cause a 16 basepair frame-shift deletion in the leptin receptor gene (SS^LepRmutant strain). The SS^LepRmutant strain is visually obese and develops dyslipidemia as early as 6 wk of age (91). Although insulin resistance and hyperinsulinemia were observed in the SS^LepRmutant strain by 6 wk of age, they do develop hyperglycemia by 18 wk of age (91). Arterial pressure in SS^LepRmutant rats is similar to their lean control counterparts (<150 mmHg) between 6 and 14 wk of age (91). However, the SS^LepRmutant strain develops severe hypertension after 14 wk of age while their lean littermates did not (91). Interestingly, progressive proteinuria is observed early at 6 wk of age before elevations in arterial pressure and independent of hyperglycemia (91). By 18 wk of age, these obese rats exhibit a reduction in urinary markers of renal injury, which is primarily due to a decline in renal function (91). The kidneys from the SS^LepRmutant strain display glomerulosclerosis and mesangial expansion as early as 6 wk of age that worsen with age (91). Collectively, these data indicate the SS^LepRmutant strain develops glomerular injury early independent of hyperglycemia and elevations in arterial pressure that ultimately progresses to CKD.

SUMMARY

Investigations to identify the independent, direct role of obesity in renal disease are often challenged by the need to separate the direct effects of obesity alone versus the effects of obesity-mediated features of MetS on the development of renal injury. With the rate of renal disease in nondiabetic obese patients on the rise, an appropriate animal model of obesity that mimics the progression of renal disease in this population is needed for the investigation into the underlying mechanisms responsible for obesity-associated renal injury. A number of animal models of obesity are currently available for investigating the relationship between obesity and renal disease. However, these available obese models have limited use in understanding the direct effect of obesity alone on the development and progression of renal injury due to the concurrent presence of other cardiovascular and metabolic disorders, including hypertension, insulin resistance, hyperglycemia, and dyslipidemia, which have independent effects on renal disease. We have summarized below and highlighted advantages and disadvantages for each obese model resulting from a HF diet in Table 1 and derived from leptin signaling mutations in Table 2.

Table 1.

Disadvantages and advantages of rat models of high-fat-induced obesity when studying obesity as an independent risk factor for renal injury

Obese Model	Strain	Disadvantages	Advantages
Renal disease associated with hyperglycemia and mild hypertension	Wistar rat	The development of renal disease is associated with MetS including mild hypertension and hyperglycemia	Represents a good model to study renal injury that is very similar to the early pathogenesis of renal disease observed in obese patients
Renal disease associated with hyperglycemia and severe hypertension	Spontaneous hypertensive rat (SHR)	SHRs develop severe hypertension very early, and it may be the direct cause of the renal injury along with the other features of MetS	Being able to potentially investigate mechanisms that are involved in renal injury linked to obesity that advances to impaired renal function
	Dahl salt-sensitive rat (SS)	Similar to SHRs in which the renal disease is associated with severe hypertension and most of the features of MetS	Represents an appropriate model to study the effects of a HF and HS diet on the development and progression of renal disease; a combined HF and HS diet is considered the standard diet of obese humans
Renal disease associated with mild hypertension in the absence of hyperglycemia	Sprague-Dawley rat (SD)	Slow development of renal injury that is considered mild and is associated with other MetS characteristics	May be a good model to study the early mechanisms of obesity that contribute to the initial development of proteinuria

MetS, metabolic syndrome; HF, high fat; HS, high salt.

Table 2.

Disadvantages and advantages of rat models of obesity with dysfunctional leptin signaling when studying obesity as an independent risk factor for renal injury

Obese Model	Strain	Disadvantages	Advantages
Renal disease associated with hyperglycemia independent of hypertension	Zucker diabetic fatty rat (ZDF)	Slow development of renal disease that primarily may be due to aging	Investigate hyperglycemia-induced renal disease in the absence of hypertension
Renal disease associated with hyperglycemia and hypertension	Obese Zucker rat (OZ)		Examine renal injury associated with hyperglycemia and moderate hypertension
	DahlS.Z-Leprfa/Slc rat (DS/obese)	Studies have only focused on renal injury after the development of hypertension and the presence of other characteristics of MetS	Study the development of severe proteinuria in the presence of hypertension and very mild hyperglycemia
	Spontaneous hypertensive heart failure rat (SHHF/Mcc-cp)	Renal disease occurs after the development of hypertension and hyperglycemia along with other characteristics of MetS	Explore progressive renal disease that recapitulates human diabetic and hypertensive nephropathy
	Obese Zucker spontaneous fatty-1 rat (ZSF1)
Renal disease associated with hypertension in the absence of hyperglycemia	Spontaneous hypertensive obese rat (Kolestzky or SHROB)	Development of renal injury follows hypertension and other MetS characteristics	Examine renal injury associated with obesity and hypertension in the absence of hyperglycemia
	Dahl SS leptin receptor mutant rat (SSLepRmutant)	Displays moderate hypertension along with other characteristics of MetS during the early development of renal disease	Investigate the early development of renal injury independent of hyperglycemia and before elevations in arterial pressure

MetS, metabolic syndrome.

An increased caloric or HF intake has been associated with many diet-induced obesity complications including MetS and cardiometabolic diseases. Various combinations of carbohydrate and fat-rich dietary components have been used in rodents to study various mechanisms involved in the development and progression of renal disease associated with obesity. In the current review, we discussed the development of renal disease related to HF diet-induced obesity in four different rat strains: Wistar, SD, SHR, and SS. When fed a HF diet, Wistar and SD rats develop mild renal injury in the presence of insulin resistance, elevated plasma lipids, and mild to moderate hypertension. However, the renal disease observed in HF diet fed SD rats occurs independent of hyperglycemia. In contrast to Wistar and SD rats, the degree of hypertension is more severe and is enhanced in SHRs fed a HF diet. HF diet-induced obesity in SHRs causes many of the characteristics of MetS along with progressive hypertension and renal disease that advances to reduced renal function. Unlike the other strains, the SS strain is the more susceptible strain due to the accelerated development of renal injury in the presence of insulin resistance, elevated plasma lipid and blood glucose levels, and hypertension within 4 wk after being placed on a HF diet. Moreover, the degree of hyperglycemia and hypertension is exacerbated leading to progressive renal disease in SS rats when the salt content in the HF diet is increased. While HF diet-induced obesity in these strains may be ideal for studying mechanisms involved in the development of renal disease associated with obesity, more studies are needed to determine the temporal changes in metabolic and renal disease parameters in these obese models.

In addition to HF diet-induced obesity, we discussed the development of renal injury in leptin dysfunctional signaling strains. The OZ and ZDF strains develop renal injury gradually, which primarily may be due to aging, but are considered the most appropriate models of obesity to study renal disease associated with hyperglycemia in presence and absence of hypertension, respectively. When exploring renal disease similar to diabetic nephropathy (hyperglycemia and hypertension), the SHHF/Mccp-cp and ZSF1 strains are the most appropriate. However, evaluating the direct effects of obesity on renal disease in the obese SHHF/Mccp-cp and ZSF1 rats is extremely difficult, since they both develop renal injury after the presence of hypertension and hyperglycemia. Similarly, the DS/obese strain is suitable to investigate the development of renal disease associated hypertension but with very mild hyperglycemia. Interestingly, to our knowledge studies examining renal injury before the development of hypertension and hyperglycemia in the DS/obese strain have not been conducted. The SHROB and SS^LepRmutant strains are the most fitting obese models to investigate renal disease associated with hypertension independent of hyperglycemia. While both of these strains develop hypertension before renal injury, the SS^LepRmutant strain develops early progressive proteinuria and renal disease before elevations in arterial pressure occur when compared with their lean littermate counterparts. All of these strains have proven to be useful in studying renal injury associated with obesity, but these obesity-related disorders in humans are not due primarily to leptin signaling dysfunction.

Overall, the discussion from these obese models in the current review support the scientific view that it is extremely difficult to examine the direct impact of obesity on the development of renal injury due to the associations with insulin resistance, hyperglycemia, hypertension, and other characteristics of MetS having their own influence on renal disease.

FUTURE CONSIDERATIONS

Obesity has reached epidemic proportions and continues to be a growing problem worldwide (129). Excess weight gain appears to be a major risk factor for diabetes, hypertension, and CKD (1, 19, 48, 109, 143). The potential mechanisms involve insulin resistance, inflammation, renal renin-angiotensin-aldosterone system hyperactivity, sympathetic nervous system hyperactivity, and perhaps other unknown mechanisms. To find and generate novel therapeutic targets for renal disease associated with obesity, future preclinical studies need to focus on 1) prepubertal/childhood obesity, 2) the long-term effects of bariatric surgery on the progression of renal injury, and 3) the influence of the gut microbiome on the development of renal disease related to obesity.

Childhood/Prepubertal Obesity

One key area of research that has not been thoroughly explored is investigating the mechanisms involved in the development of renal injury during childhood obesity. Childhood obesity has emerged as an epidemic and major health problem over the last few decades (52, 126, 142). Moreover, obesity in children is a risk factor for the development of proteinuria. However, the mechanisms involved remain unclear due to a lack of an appropriate animal model of childhood obesity that mimics the progression of renal disease in this population. HF diet-induced obesity model would be the best to study renal injury associated with childhood obesity. However, feeding rats a HF diet at wean or during the prepubertal stage may not be enough to cause a significant increase in body weight and cause proteinuria and sufficient renal disease during this time frame. Perhaps the SS^LepRmutant strain is the most optimal obese rat model discussed in the current review that is available to study the impact of obesity on the early development of renal injury independent of hyperglycemia and elevations in arterial pressure before puberty (6 wk of age). With childhood obesity on the rise, there is an urgent need to understand the underlying mechanisms that contribute to the future risk of CKD. By understanding these early changes, novel therapeutic strategies may be developed to improve clinical outcomes for these patients as they reach adulthood.

Bariatric Surgery

Bariatric surgery, which alters the anatomy and physiology of the gut, is considered one of the most efficient intervention to significantly reduce body weight in extreme obese patients. Moreover, these surgeries have been shown to have a major impact on the progression of renal injury by preventing renal hyperfiltration and decreasing proteinuria due to obesity (15, 43, 99, 119, 122). At the same time, bariatric surgery increases GFR in patients with reduced renal function (42, 63, 68, 98, 118). The improvement of renal injury after bariatric surgery may be attributed to weight loss, increased insulin sensitivity, and the decline of blood glucose levels. However, the mechanisms involved in long-term consequences from bariatric surgery on the progression of renal injury and function associated with obesity remain unclear. The obese models derived from impaired leptin signaling discussed in the current review experience hyperphagia and may not be appropriate to study the long-term effects of bariatric surgery on the progression of renal disease. The most appropriate model to study the effects of bariatric surgery on renal function and injury would be Wistar or SD rats fed a HF diet. The development of renal injury in these two obese models occurs slowly and mimics the progression of renal disease observed in the obese population.

Gut Microbiome

Over the last few years the gut microbiome has received a significant amount of attention in cardiovascular disease. In particular, the gut microbiome has been observed to play a role in digestion and metabolism during obesity (116). Furthermore, previous findings suggest that the gut microbiome may play a direct role in the development of obesity and its related disorders including insulin resistance (82, 114, 134, 135). Bäckhead et al. (4) recently observed that germ-free mice fed a HF diet do not gain weight, and rescuing germ-free mice gut with microbiota from either lean or diet-induced obese mice triggers weight gain. Although the gut microbiome is altered in rats experiencing chronic renal failure (139), it remains to be determined if the gut microbiome influences renal disease or if the alterations of the gut microbiome are the result of renal disease. Furthermore, studies examining the role of the gut microbiome on the development of renal injury linked to obesity are limited. HF diet-induced obesity in the Wistar and SD strains would be the most appropriate to study the alterations of the gut microbiome during the development of renal disease associated with obesity versus severe hypertensive SHRs and SS rats.

GRANTS

This work was financially supported by the National Institute of Diabetes and Digestive and Kidney Diseases Grants 1F31-DK-109571 (to K. C. McPherson) and DK-109133 (to J. M. Williams) and National Heart, Lung, and Blood Institute Grant HL-130456 and National Institute of General Medical Sciences (Obesity, Cardiorenal and Metabolic Diseases–COBRE) Grants P20-GM-104357 (to D. C. Cornelius) and GM-115428 supported summer undergraduate students (to A. Pennington and B. Fizer) and P20-GM-104357 (to J. M. Williams).

DISCLAIMERS

The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.M.W. and K.C.M. prepared figure; K.C.M., C.A.S, B.P., B.F., A.P., A.S., W.L.T., D.C.C., and J.M.W. drafted manuscript; K.C.M., C.A.S., B.P., W.L.T., D.C.C., and J.M.W. edited and revised manuscript; and K.C.M., C.A.S., B.P., W.L.T., D.C.C., and J.M.W approved final version of manuscript.