The tubular hypothesis of nephron filtration and diabetic kidney disease

Abstract

Kidney size and glomerular filtration rate (GFR) often increase with the onset of diabetes, and elevated GFR is a risk factor for the development of diabetic kidney disease. Hyperfiltration mainly occurs in response to signals passed from the tubule to the glomerulus: high levels of glucose in the glomerular filtrate drive increased reabsorption of glucose and sodium by the sodium-glucose cotransporters SGLT2 and SGLT1 in the proximal tubule. Passive reabsorption of chloride and water also increases. The overall capacity for proximal reabsorption is augmented by growth of the proximal tubule, which (alongside sodium-glucose cotransport) further limits urinary glucose loss. Hyperreabsorption of sodium and chloride induces tubuloglomerular feedback from the macula densa to increase GFR. In addition, sodium-glucose cotransport by SGLT1 on macula densa cells triggers the production of nitric oxide, which also contributes to glomerular hyperfiltration. Although hyperfiltration restores sodium and chloride excretion it imposes added physical stress on the filtration barrier and increases the oxygen demand to drive reabsorption. Tubular growth is associated with the development of a senescence-like molecular signature that sets the stage for inflammation and fibrosis. SGLT2 inhibitors attenuate the proximal reabsorption of sodium and glucose, normalize tubuloglomerular feedback signals and mitigate hyperfiltration. This tubule-centred model of diabetic kidney physiology predicts the salutary effect of SGLT2 inhibitors on hard renal outcomes, as shown in large-scale clinical trials.

Fig. 1 |. Tubular hypothesis of glomerular filtration and nephropathy in diabetes mellitus.

Hyperglycaemia activates renal mechanisms that act to retain the increased amounts of filtered glucose. Tubular growth and the activity of sodium–glucose cotransporters 2 and 1 (SGLT2 and SGLT1) in the proximal tubule increase glucose reabsorption. These adaptations also induce reabsorption of sodium; as a result, the glomerular filtration rate (GFR) also increases, to restore sodium excretion. However, the onset of tubular growth, hyperreabsorption and glomerular hyperfiltration promotes the development of diabetic kidney disease. The increase in GFR increases renal transport and oxygen requirements, which (together with enhanced glomerular filtration of albumin and other tubulotoxic factors) promotes hypoxia, inflammation, fibrosis and tubulointerstitial damage. The molecular signature of tubular growth in the context of diabetes is characterized by a senescence-like cellular phenotype, which is associated with the release of pro-inflammatory and pro-fibrotic factors, and probably also explains the ‘salt paradox’ of the diabetic kidney, whereby renal vasodilation unexpectedly occurs in response to a low dietary NaCl intake. Together, these changes promote hypoxia and apoptosis, facilitate episodes of acute kidney injury (AKI) and eventually lead to kidney failure. We propose that the tubular growth response to hyperglycaemia differs from patient to patient, in part because of unidentified genetic and environmental influences, which may determine not only the extent of tubular sodium and glucose hyperreabsorption and glomerular hyperfiltration in early diabetes mellitus but also the subsequent progression of renal disease. Adapted with permission from REF.

Fig. 2 |. Primary tubular hyperreabsorption drives hyperfiltration in diabetes.

Diabetes induces hyperreabsorption of glucose in the proximal tubule resulting from enhanced glucose reabsorption via sodium–glucose cotransporters 2 and 1 (SGLT2 and SGLT1) and growth of the proximal tubule, which also leads to enhanced reabsorption of sodium, chloride and fluid in the proximal tubule. The reduced levels of sodium, chloride and potassium delivered to the macula densa ([Na⁺/Cl⁻K⁺]_MD) induce glomerular hyperfiltration via tubuloglomerular feedback. The reduced fluid delivery to the distal nephron increases hyperfiltration via lowering hydrostatic back pressure in the Bowman’s space (P_BOW). SGLT inhibition mitigates diabetic proximal tubular hyperreabsorption and thereby attenuates glomerular hyperfiltration. The graph illustrates how tubuloglomerular feedback results in an inverse relationship between [Na⁺/Cl⁻/K⁺]_MD and the single nephron glomerular filtration rate (SNGFR). A red dot indicates the natural operating point of tubuloglomerular feedback, which lies in the steepest part of the curve. NaCl, sodium chloride (table salt).

Fig. 3 |. Renal glucose reabsorption in the proximal tubule.

Na⁺/K⁺-ATPase in the basolateral membrane lowers intracellular Na⁺ concentrations and thereby provides the driving force for sodium uptake across the apical membrane. Sodium–glucose cotransporters 2 and 1 (SGLT2 and SGLT1) are expressed in the apical membrane of the early and late proximal tubule, respectively. SGLT2 reabsorbs one glucose molecule along with one Na⁺ ion, whereas SGLT1 reabsorbs one glucose molecule along with two Na⁺ ions. In individuals with euglycaemia, SGLT2 and SGLT1 reabsorb ~97% and ~3%, respectively, of filtered glucose. Glucose leaves the cells via basolateral facilitated glucose transporter 2 (GLUT2), and potentially in part by GLUT1 in the late proximal tubule. Inhibition of SGLT2 increases the delivery of glucose to the late proximal tubule and unmasks the high capacity of SGLT1 for glucose reabsorption (which rises from 3% to 40–50% of filtered glucose); thus, only ~50–60% of filtered glucose is excreted. Na⁺/H⁺-exchanger 3 (NHE3) in the apical membrane is important for sodium and bicarbonate (HCO₃⁻) reabsorption in the proximal tubule. SGLT2 might be positively linked to NHE3 and therefore to sodium and bicarbonate reabsorption in the proximal tubule. However, the implications of this link (in terms of the effects of SGLT2 inhibition on tubular reabsorption, glomerular filtration rate (GFR) and blood pressure) remain to be determined. NBC1, electrogenic sodium bicarbonate cotransporter 1. Adapted with permission from Annual Review of Medicine volume 66 ©2015 (REF.).

Fig. 4 |. Diabetes induces the hyperreabsorption of glucose and sodium in the proximal tubule.

(1) Hyperglycaemia enhances filtered glucose and, via sodium–glucose cotransporter 2 (SGLT2; and also SGLT1, not shown), increases the reabsorption of glucose and sodium in the proximal tubule. (2) Diabetes can increase renal membrane expression of SGLT2; proposed mechanisms include activation of angiotensin II (AngII), hepatocyte nuclear factor 1α (HNF1α) and tubular growth. (3) Hyperinsulinaemia and tubular growth might induce the coordinated upregulation of proximal tubular transporters, including SGLT2, Na⁺/H⁺-exchanger 3 (NHE3), solute carrier family 22 member 12 (URAT1) and Na⁺/K⁺-ATPase. The resulting increase in proximal tubular Na⁺ retention enhances the glomerular filtration rate (GFR) via tubuloglomerular feedback, which, by increasing flow rate and thereby torque, may further increase luminal membrane transporter density in the early proximal tubule. (4) Diabetes, in part because of its associated acidosis, can enhance gluconeogenesis in the early proximal tubule. The resulting increase in intracellular glucose may inhibit SGLT2 expression through negative feedback mechanisms. (5) HNF1α and HNF3β have been implicated in stimulating the basolateral exit of glucose through upregulation of basolateral facilitated glucose transporter 2 (GLUT2). (6) The relevance of the apical translocation of GLUT2 in diabetes remains to be determined, but may be secondary to enhanced SGLT2-mediated glucose uptake. (7) Increased glucose reabsorption maintains hyperglycaemia. The induction of transforming growth factor (β1 (TGFβ1) and tubular growth might be particularly sensitive to the basolateral uptake of glucose via GLUT1.(8) Hyperreabsorption of sodium might induce hypoxia-inducible factor 1α (HIF1α), which inhibits apical transporters, including SGLT2 and NHE3, and facilitates basolateral glucose uptake and a metabolic shift to glycolysis.

Fig. 5 |. Mechanisms of kidney protection in response to SGLT2 inhibition.

Sodium–glucose cotransporter 2 (SGLT2) inhibition reduces the diabetes-induced hyperreabsorption of glucose and sodium in the early proximal tubule, lowering hyperglycaemia and increasing delivery of sodium chloride (NaCl) and fluid to the macula densa, resulting in higher macula densa concentrations of sodium, chloride and potassium ([Na⁺/Cl⁻/K⁺]_MD). The increase in [Na⁺/Cl⁻/K⁺]_MD reduces the glomerular filtration rate (GFR) through tubuloglomerular feedback by inducing afferent arteriole constriction and potentially also efferent arteriole dilation, which both reduce glomerular capillary pressure (P_GC) (1). Increased delivery of fluid to the distal nephron reduces GFR by increasing hydrostatic back pressure in the Bowman’s space (P_BOW) (2). The reduction in GFR is the primary mechanism for reducing tubular transport work (3), particularly in the proximal convoluted tubule, which thereby lowers cortical oxygen demand (Q_O2) (4) and increases cortical oxygen tension (P_O2) (5). Lowering of GFR attenuates tubular growth and albuminuria and consequently kidney inflammation (6). Tubular transport work is further reduced by lowering blood glucose levels and by cellular SGLT2 blockade itself (7). The reduction in hyperglycaemia attenuates tubular growth, albuminuria and inflammation (8). SGLT2 inhibition also shifts glucose reabsorption downstream, particularly to the S3 segment of the proximal tubule, where glucose uptake by SGLT1 compensates for the inhibition of SGLT2 and reduces the risk of hypoglycaemia. Shifting glucose and sodium reabsorption to the S3 segment and medullary thick ascending limb (mTAL) raises oxygen demand (9) and lowers P_O2 in the outer medulla (OM) (5). Conversely, lower medullary P_O2 might stimulate pathways induced by hypoxia-inducible factors (HIFs), including erythropoietin (EPO) production (10), thereby increasing the haematocrit levels (11), which improves O₂ delivery to the kidney medulla and cortex (12) and increases O₂ supply to the heart (13). The diuretic and natriuretic effects of SGLT2 inhibition further increase the haematocrit (14) and reduce circulating volume, blood pressure and body weight (15), which might protect the failing heart. The overall reduction in and increased distribution of renal transport activity increases cortical oxygen availability, which improves the cortical energy balance and tubular integrity, thereby enabling the kidney to maintain tubular transport capacity and GFR in the long term (16). UNaV, urinary sodium excretion; UV, urinary flow rate. Adapted with permission from REF., Portland Press on behalf of The Biochemical Society.

Fig. 6 |. The role of SGLT1 in the diabetic kidney.

a | Hyperglycaemia enhances filtered glucose, induces tubular growth and increases sodium–glucose cotransport in the proximal tubule, thereby reducing urinary glucose, sodium (Na) and fluid excretion, and maintaining hyperglycaemia. The contribution of sodium–glucose cotransporter 2 (SGLT2) to this process is greater than that of SGLT1. A reduction in urinary sodium and fluid excretion would retain sodium and fluid, increase the effective circulating volume (ECV) and induce a compensating natriuresis by increasing blood pressure (BP). However, tubular hyperreabsorption of glucose also lowers tubular backpressure in the Bowman’s space (P_BOW, not shown) and the concentration of sodium chloride (NaCl) at the macula densa, which increases the glomerular filtration rate (GFR) to restore urinary sodium and fluid excretion. An increase in glucose delivery to the macula densa indicates that upstream sodium–glucose cotransport is saturated. This saturation is sensed by SGLT1 in the macula densa (MD) and, by stimulating macula densa nitric oxide synthase 1 (NOS1) to produce nitric oxide (NO), further increases GFR to compensate for maximized sodium–glucose cotransport.As a consequence of hyperfiltration, ECV and BP remain near normal. SGLT1-mediated glucose sensing might also trigger glomerular and tubular growth, and the latter enhances tubular glucose transport capacity. Inhibition of SGLT1 has a smaller effect than SGLT2 inhibition on tubular hyperreabsorption and thus induces little natriuresis and diuresis (asterisks indicate small changes). By inhibiting macula densa glucose-sensing and upregulation of macula densa NOS1 and lowering of hyperfiltration, however, the inhibition of SGLT1 induces a relatively larger net antinatriuretic and antidiuretic effect in individuals with diabetes. As a consequence, and to limit the increase in ECV, SGLT1 inhibition suppresses renin expression and increases BP, in an attempt to restore renal sodium and fluid excretion. b | Sensing of proximal tubular hyperreabsorption, via the sensing of changes in concentrations of NaCl and glucose at the macula densa, enables the kidney to induce adaptive increases in GFR over a wide range of amounts of filtered glucose. An increase in filtered glucose up to the glucose transport maximum (TM glucose) of the proximal tubule is accompanied by an increase in sodium and glucose reabsorption, which lowers the NaCl concentration at the macula densa. When filtered glucose levels exceed TM glucose, glucose concentrations start to rise at the macula densa. The osmotic effect of non-reabsorbed glucose also enhances NaCl delivery to the macula densa.

Fig. 7 |. Mechanisms and consequences of tubular growth in diabetes.

Growth of the proximal tubule in response to diabetes involves an initial phase of tubular cell proliferation, followed by G1 cell cycle arrest, hypertrophy and finally the development of a senescence-like cellular phenotype. The stimuli for the initial proliferation phase include filtered and locally produced growth factors, angiotensin II (AngII)-induced activation of protein kinase C β1 (PKCβ1), and inhibition of 5′-AMP-activated protein kinase (AMPK) owing to excess energy substrate. Hyperglycaemia also enhances reactive oxygen species (ROS), and ROS-induced hypoxia and activation of mechanistic target of rapamycin complex 1 (mTORCi) can promote apoptosis. Activation of mTORC1 also impairs autophagy and promotes cell proliferation. Induction of transforming growth factor β (TGFβ) drives the expression of cyclin-dependent kinase inhibitors that induce G1 cell cycle arrest and the transition from proliferation to hypertrophy and a senescence-like phenotype associated with the secretion of pro-inflammatory and pro-fibrotic factors. Thus, the molecular pathways involved in and activated by tubular growth in the diabetic kidney are linked to impaired autophagy, inflammation and tubulointerstitial fibrosis and might set the stage for the later development of diabetic kidney disease. AGE, advanced glycation end product; AKT, protein kinase B; bFGF, basic fibroblast growth factor; CTGF, connective tissue growth factor; CycD1, cyclin D1; EGF, epidermal growth factor; HGF, hepatocyte growth factor; HIF1α, hypoxia-inducible factor 1α; IGF1, insulin-like growth factor 1; JAK–STAT, Janus kinase–signal transducer and activator of transcription; MCP1, monocyte chemoattractant protein 1; p16, p16^INK4a; ODC, ornithine decarboxylase; p21, p21^Cip1; p27, p27^Kip1; PDGF, platelet-derived growth factor ; PI3K , phosphoinositide 3-kinase; RAS, renin–angiotensin system; RHEB, Ras homologue enriched in brain; SOCS, suppressor of cytokine signalling protein; TSC1, tuberous sclerosis complex 1; VEGF, vascular endothelial growth factor. Adapted with permission from REF., Wiley-VCH.