Exploring novel pathways and potential therapeutic targets for diabetic nephropathy: The interplay of podocytes and proximal tubular epithelial cells

Abstract

This article discusses a recent article by Zha et al, which explores new pathogenic pathways contributing to diabetic nephropathy in a study conducted on male mice. Diabetic kidney disease outcomes remain suboptimal; it is the leading cause of end-stage kidney disease in the developed world. Previous knowledge about diabetic nephropathy focused on glomerular pathology. Advancing knowledge led to the introduction of new pathogenic concepts beyond glomerulopathy. This work by Zha et al explored an important podocyte pathway and its link to the proximal tubular cells. Podocytes are essential for maintaining glomerular health and preserving body proteins. In a state of hyperglycaemic stress, podocytes show features of internalisation of nephrin, an integral surface protein of the podocytes. The novel pathway uncovered in this experimental study involved crosstalk between the podocytes and the proximal tubular cells, more precisely, the secretion of interleukin 6 (IL-6) and Rab5 by the proximal tubular cells. When the podocytes were cultured in the conditioned medium, this resulted in podocyte dysfunction. IL-6 neutralising antibodies ameliorated this effect. Nicotinamide mononucleotide is essential for the integrity of the proximal tubular cells. Interestingly, it has been found that nicotinamide mononucleotide treatment can disrupt the IL-6-Rab signalling between the proximal tubules and podocytes, leading to improved podocyte morphology and function. The clinical applicability of this novel pathway is yet to be explored; however, it is one of the key pathways mediating inflammation and dysfunction in diabetic nephropathy.

Key Words: Diabetic nephropathy; Diabetic kidney disease; Nicotinamide mononucleotide; Podocyte; Proximal tubular epithelial cells

Core Tip: This article sheds light on the interesting study by Zha et al. In this experimental study, a novel pathogenic pathway has been described. A novel concept was also demonstrated, namely the crosstalk between the podocytes and the proximal tubular epithelial cells. The crosstalk involves the secretion of interleukin 6 by the proximal tubular epithelial cells, which aggravates nephrin endocytosis in the podocytes using the Rab5 activation pathway. The application of nicotinamide mononucleotide nicotinamide mononucleotide intercepted the pathway. A new horizon for therapeutics could be further explored using the nicotinamide mononucleotide.

INTRODUCTION

In 2021, more than 500 million people (10.5% of the population) were estimated to be affected by diabetes mellitus. Diabetic kidney disease remains the most common cause of end-stage kidney disease in the Western world[1,2]. Diabetic nephropathy, or the more recent nomenclature diabetic kidney disease, is the leading cause of end-stage kidney disease in the Western world. Therapeutic options for diabetic nephropathy are still limited.

Podocytes are highly specialized cells that surround the capillary basement membrane of the glomeruli. Podocyte integrity is key to the function of glomeruli. Podocyte injury is now recognized as one of the main pathogenic pathways in diabetic kidney disease[3,4]. What is even more recently revealed is that there is crosstalk between the proximal tubular cells and podocytes. In this way, remote cells (the proximal tubular cells) affect the podocytes’ morphology and function. This recently discovered connection, which is called proximal tubule-podocyte communication, adds to the complexity of diabetic nephropathy pathogenesis. This pathogenesis involves a milieu of immunopathological pathways.

THE NOVEL PATHWAY OF INTERLEUKIN 6 RAB5 ACTIVATION, THE CROSSTALK BETWEEN THE PROXIMAL TUBULAR EPITHELIAL CELLS AND PODOCYTES

Zha et al[5] have discovered this novel pathway. Recently, interest has been shown in the interaction between podocytes and proximal tubular epithelial cells (PTECs). Although these cells are not adjacent, they affect each other. When podocytes are injured, they leak protein. The leaked protein induces oxidative stress in the PTECs, causing the release of inflammatory chemokines and triggering the signalling pathway Figure 1. This leads to decreased sirtuin 1 and increased expression of claudin on the podocytes, followed by endocytosis of the nephrin into the podocytes. This gives rise to dysmorphology of the podocytes and functional damage and contributes to increased albuminuria. The pathogenic immunoinflammatory pathway is mediated through the interleukin (IL) 6/Rab5 pathway. Rab5 is a small GTPase that significantly controls the process of internalisation of nephrin. This key finding is the first demonstration that under hyperglycaemic conditions, nephrin internalisation into podocytes is mediated through Rab5. In this study, it was possible to block nephrin endocytosis in podocytes, which mitigated the pathologic and morphologic changes in the podocytes.

Figure 1 Interaction between injured podocytes and proximal tubular epithelial cells. NMN: Nicotinamide mononucleotide; PCT: Proximal convoluted tubule; PTEC: Proximal tubular epithelial cells; IL-6: Interleukin 6.

Another key finding of the study was that increased IL-6 secretion by the stressed PTECs augmented nephrin endocytosis via Rab5. The study then examined the pharmacological effects of blocking IL-6 using neutralising antibodies, which eliminated nephrin endocytosis. Also, nicotinamide mononucleotide (NMN) treatment helped downregulate Rab-mediated nephrin endocytosis.

CURRENT AND EMERGING THERAPY FOR DIABETIC KIDNEY DISEASE

The gap between the current and the desired treatment for diabetic kidney disease is significant. Currently, the hallmark of treatment for diabetic kidney disease consists of inhibition of the renin angiotensin axis, angiotensin converting enzyme inhibitors, alongside tightening glycaemic control and other risk factors like hypertension and dyslipidaemias[6].

In the contemporary era, sodium-glucose cotransporter 2 inhibitors revolutionised the treatment of diabetic kidney disease for both the albuminuria and non-albuminuria[7]. Despite the above, a mismatch remains between advances in science concerning the pathogenesis of diabetic nephropathy and the desired outcome and treatment.

CLINICAL AND THERAPEUTIC IMPLICATIONS

Nicotinamide mononucleotide as an emerging therapeutic target in diseases

NMN is a precursor of the nicotinamide adenine dinucleotide (NAD+), which is a vital coenzyme found in eukaryotic cells[8]. Recently, interest has been growing in NAD+ and its association with sirtuins, proteins that affect a variety of metabolic and biochemical processes. Deficient levels of NAD+ have been extensively studied and implicated as a key contributor to the pathophysiological determinants of major diseases, specifically age-related disease and diabetes mellitus[9].

Nicotinamide has shown favourable effects in previous experimental studies, exerting a beneficial effect on glucose uptake in patients with type 2 diabetes mellitus through a favourable effect on the adipose tissues[10]. However, NMN in human studies have failed to show beneficial effects. Currently, a few clinical trials have obtained approval for the use of nicotinamide mononucleotide in the treatment of various diseases.

Although the newly discovered pathogenic pathway is interesting, its blockage by NMN may not provide tangible positive effects, as was the case with previous human clinical studies. This would require more studies in human participants. Numerous studies in human participants have yielded mixed results. Some studies revealed subtle improvements in physical performance, muscle function, and muscle insulin sensitivity while other studies failed to find clear benefits[11,12].

The explanation of the mismatch between the preclinical studies and studies with human participants may be: (1) Most of the animal studies use higher weight-based doses than studies performed on human subjects (the doses used in mice were about 300 mg/kg while the dose used in humans was 500 mg in total); (2) The difference in metabolism pharmacokinetics between lab animals and humans; (3) The limited number, sample size and short duration of human studies; and (4) The fact that, after oral ingestion, NMN breaks down to two metabolites: Nicotinamide and nicotinamide riboside, which can have unpredictable effects compared to the precursor[13,14].

The above limitations have resulted in the Food and Drug Administration prohibiting the pharmacologic use of NMN in human subjects except for investigative research purposes.

The other potential therapeutic implication is the blockage of IL-6 through neutralising antibodies, as in the current experiment. IL-6 is a key proinflammatory regulatory cytokine with hormone-like action. Diabetes mellitus involves complex immunopathological processes. IL-6 is implicated in the pathogenic process underlying type 1 diabetes and, to a lesser extent, in type 2 diabetes mellitus[15]. Meta-analyses and genome-wide analysis have shown that certain variants of the IL-6 gene are associated with the development of diabetic kidney disease. Interestingly, anecdotal evidence from large cohort studies shows a marked increase in the levels of IL-6 in the serum of patients with diabetic kidney diseases. The levels are proportionate to a more advanced stage, suggesting implication in the pathogenesis.

The effects of IL-6 on the kidneys are complex. IL-6 plays a pivotal role in the initiation phase of inflammation and the resolution phase. Under stressful hyperglycaemic conditions, podocytes secrete IL-6, which leads to hypertrophy and apoptosis. IL-6 promotes mesangial cell proliferation and apoptosis. However, the IL-6 that is produced after the exercise improves glucose uptake, glycaemic control and muscle mass. IL-6 infusion to produce the physiological effects of post-exercise has resulted in the same beneficial effect in healthy volunteers[16].

Interestingly, in patients with type 2 diabetes, IL-6 infusion to produce post-exercise levels has resulted in improved insulin sensitivity, as evidenced by decreased insulin levels[17]. In animal models, blockage of IL-6 has resulted in the mitigation of renal fibrosis. In patients with rheumatoid arthritis and diabetes mellitus, treatment with the drug Tocilizumab, which blocks the effects of IL-6 by neutralising antibodies, has resulted in improvement of glycaemic control, as evidenced by a reduction in the haemoglobin A1c[18].

Although clinical trials in human participants are still sparse, there are encouraging signals. In one clinical trial, the use of ziltivekimab in patients with moderate to severe chronic kidney disease has resulted in amelioration of atherosclerosis markers[19].

Another study has shown that the Janus kinase 1/Janus kinase 2 inhibitor baricitinib reduced albuminuria in patients with diabetic kidney disease[20]. However, blocking IL-6 has not resulted in benefits for the survival of beta islet cells in the pancreas, as shown in the results from the EXTEND study[21].

The study by Zha et al[5] re-demonstrated the crosstalk between the PTECs and the podocytes, both in vivo and in vitro. Crosstalk means that under hyperglycaemic conditions, injured podocytes can provoke an inflammatory state in PTECs through increased proteinuria. The stressed PTECs secrete IL-6, among other chemokines. This leads to decreased NMN, and both adversely affect the podocytes by increasing nephrin endocytosis, resulting in damage to the cytoskeleton and dysfunction.

The study broadens our understanding of the immunoinflammatory pathogenesis of diabetic kidney disease and factors affecting podocytopathy and its crosstalk with PTECs.

The study has several key findings: One of these key findings was demonstrated for the first time: Rab5-mediated nephrin endocytosis by the podocytes. Rab5 is upregulated by the IL-6 secreted by the stressed PTECs. This interaction and its effect on nephrin were first demonstrated in this study. The increased IL-6 decreases NMN. This results in decreased sirtuin 1 expression and increased claudin-1 expression on the podocytes. As a result of the above changes, the podocyte cytoskeleton is distorted, and the function is impaired. A second key finding is that pharmacological intervention by applying NMN and blocking IL-6 by neutralising antibodies can ameliorate the effects mentioned above. Despite the preclinical success of the above agents, the translational benefit is limited for several reasons. However, the study provides a rationale and potential for further study of these two pharmacologic interventions in patients with diabetic nephropathy.

CONCLUSION

This study reveals a novel pathogenic pathway in diabetic nephropathy. This pathway provides new insight into the crosstalk between PTECs and podocytes, where IL-6 secreted by the PTEC leads to Rab5-mediated nephrin endocytosis in the podocytes. Interestingly, the pathway can be blocked by NMN and IL-6 neutralising antibodies. However, despite these experimental beneficial effects, neither therapeutic option has shown tangible benefits in humans, with various reasons for the lack of clinical efficacy of the two novel pharmacological agents despite preclinical success. Numerous recent advances in knowledge about diabetic nephropathy have been made. Transforming this knowledge into meaningful benchmark therapeutic options is challenging, as the results of clinical trials on human subjects remain disappointing.