Paul T Brinkkötter | Sebastian Brähler - B 01

Inflammatory diseases of the kidney filter are leading causes of chronic renal insufficiency. Despite tremendous advances in our disease understanding, the treatment options remain limited. Targeted strategies to tackle specific intracellular pathways are warranted. Metabolic reprogramming is one the key steps in immune cells activation. Our project will not only advance our knowledge about a fundamental disease mechanism with the perspective of new treatment approaches for renal diseases.

The kidney contains a complex network of immune cells important in the defence against pathogens and also in development and tissue homeostasis in states of health and disease. The prominent role of inflammatory processes is reflected by the wide use of anti-inflammatory drugs in the treatment of patients with glomerular diseases of many different causes. Metabolic reprogramming has recently been recognized as central event in the regulation of T cell and macrophage function. With respect to the kidney, very little is known about the inflammatory signal transduction itself and how cells of the immune system are orchestrated during glomerular disease. Our overarching goal is to gain mechanistic insights into the role of metabolic signalling events and their influence on myeloid cell function in glomerulonephritis. For this purpose, we perform detailed metabolic profiling of FACS sorted immune cells of the kidney in a model of experimental glomerulonephritis. We are particularly interested in the role of insulin-receptor signalling in the activation of various immune cell subsets in inflammatory kidney diseases.

In this project we will also analyse the role of the tricarboxylic acid (TCA) cycle during inflammatory macrophage activation by studying the function of TCA-cycle associated proteins and their metabolites.

Our ultimate goal is to identify new therapeutic targets and transfer these findings in innovative treatment strategies for our patients.

1. Analysis of metabolic properties of key immune cell populations during glomerulonephritis.
2. Characterization of the role of insulin signaling in macrophages, DCs and T-cells.
3. Evaluation of the function of the TCA cycle and associated metabolites in inflammatory kidney disease.

Our lab is interested in the metabolic and inflammatory pathways that lead to kidney failure.

Recently, using innovative imaging techniques like multiphoton-micorscopy were able to redefine the morphology of podocytes, macrophages and two major DC subsets in the mouse kidney: cDC1 and cDC2 (1,2).

We challenged mice with the standard model of immune complex-mediated nephritis, i.e. nephrotoxic nephritis, a disease mechanism that causes the most devastating form of GN: crescentic or rapid-progressive GN. We found that the known extensive network of immune cells in the kidney under healthy conditions is not consisting of dendritic cells, but of macrophages, while cDC1 and cDC2 are smaller migratory populations located in clusters (Figure 1).  We were able to characterize the function of cDC1 and cDC2. cDC2, which represent about 90% of DCs in the kidney, have a strong pro-inflammatory function during GN by recruiting Th17 through IL23 and secreting neutrophil-attracting cytokines. In contrast, the cDC1 subset had a robust anti-inflammatory function by counteracting the activity of the cDC2.

In order to understand the function of cDC1 and cDC2 in detail, we obtained comprehensive gene expression data by RNAseq from sorted mouse kidney cDCs under healthy and inflamed conditions. Our data showed fundamental differences in the gene expression and metabolic profiles of the two DC subsets, supporting the hypothesis that the opposing functions of these immune cell subsets are reflected by their metabolic profile demanding for a comprehensive metabolic analysis of the major players in GN.

Alterations in metabolic signaling have been implicated in a variety of human diseases. Podocytes, which are part of the kidney filtration barrier, do not represent classical insulin-responsive target cells. However, recent fundamental studies highlighted the pivotal role of insulin signaling in podocytes beyond mere glucose uptake comprising activation of AKT and mTOR.

We have shown that insulin signaling in podocytes is tightly controlled by mitochondrial function. An impairment of the mitochondrial fusion and fission machinery caused by loss of the expression of Oma1 or Phb2 is associated with an increased activation of the insulin signal cascade (3). In a recently published comprehensive analysis podocyte metabolism, we found that anaerobic glycolysis represents the predominant metabolic pathway of podocytes (4).

These findings offer a strategy to therapeutically interfere with the enhanced podocyte metabolism in various progressive kidney diseases, such as diabetic nephropathy or focal segmental glomerulosclerosis (FSGS).