The Link Between Diabetes and Kidney Disease
Learn more about the link between diabetes and kidney disease, including three main ways that hyperglycemia negatively impacts renal health, and how diabetic kidney disease can be modeled preclinically.
What is Diabetic Kidney Disease?
Diabetic kidney disease (DKD) covers chronic kidney disease that is specifically attributable to type 1 and 2 diabetes. Within the US, DKD accounts for 24% of chronic kidney disease, and strongly associates with cardiovascular disease progression and mortality.
Diabetic kidney disease progresses in stages. Clinical presentation usually starts with hypertrophy, urinary protein excretion (microalbuminuria), and glomerular hyperfiltration. DKD can progress to higher levels of urinary protein excretion (macroalbuminuria) and diminished glomerular filtration rate (GFR), but both of these conditions are dynamic and can improve or deteriorate between follow-up studies.
DKD transitions to overt nephropathy when:
- Urinary albumin-to-creatinine ratio exceeds 30 mg/g
- With GFR below 60 mL/min per 1.73 m2
- And both are sustained over three months.
More categorical indicators of diabetic kidney disease are histopathological changes. Profound morphological changes to the renal structure are caused by increases in extracellular matrix (ECM) deposition. This starts with thickening of glomerular and tubular basement membranes, and kidney size and weight may increase by 15% following the onset of diabetes.
Next, the mesangial matrix within the glomerulus expands, which occludes its capillaries. End stage renal disease (ESRD) is signaled by an accumulation of nodular scleroses and patients can experience a 10% annual decline in filtering capacity rates. Approximately 2/3 of ESRD patients die within 5 years of starting dialysis treatment.
How Does Diabetes Cause Kidney Disease?
Not all diabetic patients develop DKD. Polygenic factors play a major role in deciding which patients will be affected, though these are not yet fully understood.
Glycemic control is the first line treatment for DKD. It is mostly effective if initiated early in diabetes and before the onset of DKD complications. In susceptible individuals, the metabolites and inflammatory stimuli arising from hyperglycemia have direct effects on kidneys, and drive a progressively irreversible pathophysiology. Other strategies to slow down DKD involve reducing blood pressure and cholesterol.
There are numerous mechanisms involved in hyperglycemia affecting renal function, and three main axes are examined here:
- Advanced glycation end products.
- Transforming growth factor β.
- The inflammasome.
These metabolites and inflammatory stimuli have direct effects on the kidneys, resulting in a progressively irreversible pathophysiology, and are reviewed in more detail below.
Advanced Glycation End Products
Advanced glycation end products (AGE) build up naturally and progressively with metabolism and aging. They occur when reducing sugars react with free amino groups, lipoproteins, or nucleic acids to irreversibly modify the proteins and generate derivatives with a modified structure and function. The accumulation of AGE is accelerated in diabetic patients. AGE are slowly degraded and persist in diabetic blood vessels long after glycemic control is established.
One glycation target which has been extensively studied in diabetic kidney disease is collagen. AGE formation affects collagen cross-linking, altering its packing density and surface charge within the kidney basement membrane. This disrupts its form, cellular interactions, and maintenance by matrix metalloproteinases, leading to mesangial expansion and thickening of the basement membrane.
AGE engagement with their receptor (RAGE) activates signal transduction pathways triggering proliferative, inflammatory, and fibrotic processes. RAGE are expressed on tubular and glomerular epithelial cells, as well as on mesangial cells. In the kidney, RAGE signaling activates proinflammatory regulator NFκβ. This in turn leads to macrophage attraction and activation, with generation of proteases and reactive oxygen species contributing to DKD pathogenesis. RAGE knockout mice are protected against many diabetic nephropathy symptoms including reduced filtration rate, albuminuria, nephromegaly, basement membrane thickening, and sclerosis.
The AGE/RAGE axis also suppresses the nitric oxide (NO) synthase activity of endothelial cells. NO is a vasodilatory molecule with a crucial regulatory role in renal hemodynamic and tubular functioning, helping maintain normally low renal vascular resistance and increasing tubular sodium excretion. Interruption of NO synthesis by the AGE/RAGE axis leads to hypertension and renal vasoconstriction in advanced diabetic kidney disease. Interestingly, NO production is enhanced at the start of DKD and may contribute to the early symptom of hyperfiltration.
The use of cholesterol-lowering atorvastatin has been associated with improved kidney function in chronic kidney disease, including DKD. Statins induce RAGE shedding into soluble forms which act as AGE decoys and can slow DKD progression.
Transforming Growth Factor Beta Activation
Transforming growth factor beta (TGFβ) isoforms are a group of multifunctional cytokines regulating a wide variety of basic biological processes in different cell types, including ECM building and stabilization. They are secreted as latent molecules and deposited into the matrix. The TGFβ1 isoform in particular is increased in the glomeruli of diabetic patients. A number of signals free TGFβ molecules from their latency complexes, activating signal transduction pathways that trigger the transcription of key fibrotic ECM components such as fibronectin and collagen genes. TGFβ1 is a key driver of glomerulosclerosis and interstitial fibrosis in kidney disease.
Hyperglycemia generates numerous molecular stimulators for TGFβ secretion and activation. In mesangial cells, these include AGE, which activate the NFκβ signaling pathway. TGFβ-mediated gene expression changes are blunted in the kidneys of RAGE knockout mice, and their mesangial cells are protected against the apoptosis often seen in diabetic nephropathy.
Hyperglycemia also leads to elevated diacylglycerol, an activator of protein kinase C enzymes, which are key signal transducers for TGFβ expression in mesangial cells. Excess glucose also leads to generation of reactive oxygen species (ROS) by mitochondria and NADPH oxidase. ROS activate both the PKC and NFκβ signaling pathways contributing to inflammation and renal pathologies. Antioxidants inhibit TGF-β1 and ECM expression in glomerular mesangial and tubular epithelial cells and ameliorate diabetic nephropathy.
Pirfenidone is a standard of care drug for idiopathic pulmonary fibrosis that reduces TGFβ expression and activity, and has shown promise in clinical trials for DKD.
The NLRP3 Inflammasome
Inflammatory cell recruitment into the kidney is associated with a decline in renal function. The persistent high glucose of diabetic patients leads to increased release of inflammatory interleukins (IL) by renal cells. This pathogen-independent phenomenon is termed “sterile inflammation”, and is increasingly described as a leading mechanism of diabetic kidney disease.
Inflammasomes are innate immune cytosolic complexes that respond to molecular “danger” patterns, activating caspase enzymes enabling maturation and secretion of inflammatory cytokines, and triggering pyroptotic cell death pathways. The NLRP3 subset inflammasome mediates release of IL1β and IL18. Its activation is causatively linked with DKD, and has been shown to be active in nonimmune cells of the kidney such as podocytes. The full spectrum of mechanisms which activate the inflammasome in DKD is not well documented, although mitochondrial ROS are proposed as one key stimulus.
Glomerular apoptosis triggered by caspase-1-dependent inflammasome activation contributes to nephropathy pathophysiology. Recruited immune cells, notably macrophages, produce substances causing renal injury, including excess ROS and NO, and profibrotic factors. Macrophage-generated ROS and tumor necrosis factor also initiate apoptosis in podocytes. Animal models have shown that neutralization of IL1β and antagonism of the IL1 receptor prevents, and even reverses, nephropathy.
The small molecule MCC950 is a potent and specific inhibitor of the NLRP3 pathway, and has shown efficacy in a wide spectrum of NLRP3-mediated inflammatory diseases. Recent studies using obese and hypertensive rodents have revealed its potential for treating associated chronic kidney disease symptoms.
Preclinical Modeling of Diabetic Kidney Disease
Animal models of diabetic nephropathy should feature progressively increasing albuminuria with:
- Declining renal function, including a period of hyperfiltration followed by reduced GFR.
- Histopathological changes in the glomeruli.
- Tubulointerstitial lesions.
There is no ideal rodent model featuring all of the hallmarks for the human condition, but many models are available for studying particular disease endpoints.
Owing to its widespread use in type 2 diabetes and obesity research, the db/db mouse is a popular choice for DKD studies. The db/db mouse is a homozygous loss of function mutant for the leptin receptor. In the C57BLKS/J background, early onset hyperglycemia is followed by albuminuria (10-12 weeks), nonprogressive renal function decline (15-18 weeks), mesangial expansion (5-6 months), and thickening of the basement membrane (18-20 months).
While the db/db mouse is useful for studying early DKD features, severe disease with histopathological hallmarks is slow to emerge, and glomerulosclerosis does not present. Histopathology can be accelerated through a uninephrectomy procedure, and recapitulates glomerulosclerosis in about one third of individuals within 6 months.
The Akita mouse is a popular model for studies on DKD with type 1 diabetes. This model harbors a dominant mutation in the ins2 insulin gene. Heterozygotes produce a misfolded protein that is toxic to pancreatic βcells. Nephropathy is evident as albuminuria, mesangial expansion, and renal injury. Phenotype severity depends on the genetic background, with the FVB/NJ strain reported as the most prominently affected.
ZDF and ZSF Rats
The Zucker diabetic fatty rat (ZDF) is another leptin receptor mutant rodent with rapidly progressive type 2 diabetes and many of its associated conditions. Overt diabetes occurs starting at 8 weeks of age accompanied by steadily increasing albuminuria. ZDF rats develop a progressive nephropathy with glomerular, vascular, and tubulointerstitial pathologies, including mesangial expansion and tubular fibrosis. Early onset hyperfiltration is apparent, but GFR eventually returns to levels comparable to controls.
The leptin impaired ZSF rat was generated by crossing ZDF “lean” leptin mutant heterozygotes with hypertensive SHHF rats, also heterozygous for leptin signaling. GFR decline has been observed in ZSF obese animals relative to lean controls. This strain also presents the advantage of more rapid renal fibrosis with a relatively quick onset of 20 weeks, providing a potentially valuable endpoint for trials targeting the TGFβ and NLRP3 axes.
A new diabetic rat with a polygenic mechanism of diabetes onset is also being described for diabetic nephropathy research and drug development. This provides a more translational model for diabetic complications than leptin signaling-impaired animals. The ZDSD rat develops type 2 diabetes accompanied by albuminuria, urinary markers of renal injury, mesangial expansion, and basement membrane thickening. Studies are currently ongoing to characterize effects on GFR.
This article has introduced three main axes by which hyperglycemia contributes to the decline in renal function of diabetic kidney disease, but numerous other pathways interplay or present divergent mechanisms to further contribute to pathophysiology. Furthermore, these pathways form multiple feedback loops to amplify each other.
The diabetic kidney disease paradigm is convoluted and incompletely described, but suggests multiple targets may need to be impaired or engaged for effective therapy. The consensus that can be elucidated from the literature is that early and effective glycemic management is the best way to prevent ESRD in the diabetic population.