Paul T Brinkkötter - C 5

Mitochondrial energetics and its contribution to podocyte function

Metabolic signalling pathways orchestrate the dynamic turnover between catabolic and anabolic processes. Glomerular podocytes, which are part of the kidney filtration barrier, are thought to be highly energy demanding cells maintaining a highly dynamic actin cytoskeleton. Here, we investigate the role of mitochondria in podocytes and (i) characterize how mitochondria regulate insulin signalling and (ii) identify the role of the respiratory chain for podocyte function in states of health and glomerular disease. 

Introduction 

The majority of kidney diseases affect the renal filtration unit, the glomerulus, as a consequence of a very limited capacity for regeneration and self-renewal of terminally differentiated glomerular podocytes. Podocytes are epithelial cells that have interdigitating foot processes to cover the outer surface of glomerular capillaries and to form filtration slits. They are essential components of the kidney filtration unit and responsible for ultrafiltration of protein-free urine from plasma. There are several lines of evidence proposing an important role of mitochondria to maintain podocyte function comprising respiratory chain activity and ATP production, inhibition of intrinsic apoptotic pathways and, potentially, buffering cytosolic Ca2+ release.

Anaerobic glycolysis maintains the glomerular filtration barrier independent of mitochondrial metabolism and dynamics

Intrigued by the lack of almost any glomerular phenotype in patients with profound renal ischemia, we comprehensively investigated the primary sources of energy of glomerular podocytes. Combining functional measurements of oxygen consumption rates, glomerular metabolite analysis and determination of mitochondrial density of podocytes in vivo, we demonstrate that anaerobic glycolysis and fermentation of glucose to lactate represent the key energy source of podocytes. Under physiological conditions, we could neither detect a developmental nor late onset pathological phenotype in podocytes with impaired mitochondrial biogenesis machinery, defective mitochondrial fusion-fission apparatus or reduced mtDNA stability and transcription caused by podocyte-specific deletion of Pgc‑1aDrp1 or Tfam, respectively. In conclusion, this study provides insights into podocyte metabolism as podocytes primarily rely on anaerobic glycolysis and only to a minor extent on ß-oxidation of lipids. Overall, mitochondrial oxidative phosphorylation has only very limited impact on the overall podocyte ATP synthesis. As such, anaerobic glycolysis and the fermentation of glucose to lactate is the predominant metabolic pathway of podocytes and, therefore represents a key target for therapeutic interventions in glomerular disease with enhanced metabolism such as diabetic nephropathy (Figure 1). 

Linking mitochondria to insulin signalling

While the mitochondrial contribution to the cellular ATP levels in podocytes are of minor importance, we could recently establish a link between mitochondrial function and insulin signalling pathways in podocytes (Ising et al. EMBO Mol Med 2015). Deletion of the mitochondrial membrane scaffolds prohibtin 1/2 in murine podocytes caused progressive proteinuria, kidney failure and premature death of the animals. Inhibition of the insulin/IGF-1 signalling system through genetic deletion of the insulin receptor alone or in combination with the IGF-1 receptor or treatment with the mTOR inhibitor rapamycin prevented hyperphosphorylation of S6RP without affecting the mitochondrial structural defect. Moreover, renal disease was found to be alleviated and the onset of kidney failure was delayed in PHB2 deficient animals.  
We now identified OMA1 as a critical regulator of podocyte metabolism in vitro and in vivo and demonstrated that a stress-induced OPA1 processing by OMA1 promotes a metabolic switch in glomerular podocytes. This metabolic switch alone was not sufficient to cause podocyte injury. However, when using mice lacking prohibitin membrane scaffolds as a model of mitochondrial dysfunction, we could demonstrate that additional ablation of OMA1 protects mitochondrial cristae formation from degradation leading to a significantly prolonged survival as compared to prohibitin deficient mice. Moreover, single OMA1 knockout animals also showed increased insulin signalling and activation of mTOR in the absence of any disease phenotype further highlighting the importance of mitochondria and the control of insulin signalling (Figure 2).  

Perspectives 

Accumulating evidence emphasizes the mitochondrial impact on podocyte function and the regulation of metabolic pathways. Within this project, we will provide further mechanistic insights into the role of mitochondria with respect to oxidative phosphorylation (OXPHOS) and generation of ATP as well as mitochondrial signalling events in states of health and disease. The long-term perspective of this project is to determine the mitochondrial contribution to podocyte-driven glomerular disease and potentially characterize novel, drugable signalling pathways. We expect that our results will not only lead to a better understanding of glomerular diseases but also to important insights into mitochondrial physiology in general.

Selected publications (CMMC-project related)

1. Brinkkoetter PT, Bork T, Salou S, Liang W, Mizi A, Özel C, Koehler S, Hagmann  HH, Ising C, Kuczkowski A, Schnyder S, Abed A, Schermer B, Benzing T, Kretz O, Puelles VG, Lagies S, Schlimpert M, Kammerer B, Handschin C, Schell C, Huber TB. Anaerobic Glycolysis Maintains the Glomerular Filtration Barrier Independent of Mitochondrial Metabolism and Dynamics. Cell Rep. 2019 Apr 30;27(5):1551-1566.e5.

2. Schroeter CB, Koehler S, Kann M, Schermer B, Benzing T, Brinkkoetter PT, Rinschen MM. Protein half-life determines expression of proteostatic networks in Podocyte differentiation. FASEB J. 2018 Apr 25

3. Hagmann H, Mangold N, Rinschen MM, Koenig T, Kunzelmann K, Schermer B, BenzingT, Brinkkoetter PT. Proline-dependent and basophilic kinases phos-phorylate human TRPC6 at serine 14 to control channel activity through increased membrane expression. FASEB J. 2018 32(1):208-219.

4. Koehler S, Brähler S, Braun F, Hagmann H, Rinschen MM, Späth MR, Höhne M, Wunderlich FT, Schermer B, Benzing T, Brinkkoetter PT.Construction of a viral T2A-peptide based knock-in mouse model for enhanced Cre recombinase activity and fluorescent labeling of podocytes. Kidney Int. 2017 91(6):1510-1517.


Prof. Dr. Paul T Brinkkötter

Dept. II of Internal Medicine / RG location - CECAD Building

Prof. Dr. Paul T Brinkkötter

Principal Investigator C 5 / CAP 1

paul.brinkkoetter@uk-koeln.de

Work +49 221 478 89593

Dept. II of Internal Medicine Nephrology, Rheumatology, Diabetology and general Internal Medicine
Kerpener Str. 62
50937 Cologne

http://www.kidneyresearchcenter.org/43/Research/Podocyte-Biology-and-Glomerular-Diseases/Inflammatory-and-metabolic-signaling-and-proteinuria-%E2%80%93-Paul-Brinkkoetter.htm

Publications - Paul T Brinkkötter

Link to PubMed

Group Members

Sybille Köhler (PostDoc)
Sebastian Brähler 
(Medical doctor)
Henning Hagmann (Medical doctor)
Lucas Kühne (Medical doctor)
Cem Özel (Medical doctor)
Mahsa Matin (PhD student)
Johanna Odenthal (PhD student)
Thomas Schömig (Medical student)
Anja Klein (Medical student)
Bastian Trinsch (Medical student)
Angelika Köser (technician)
Vivian Ludwig (technician)

Figure 1

CMMC Research Odenthal

Figure 1. Podocyte function is independent  of the mitochondrial OXPHOS machinery. (Brinkkoetter et al. Cell Reports 2019).

Figure 2

CMMC Research Odenthal

Figure 2. Loss of Phb2 and/or Oma1 both lead to mTOR activation in glomerular podocytes asassessed by pS6 immunohistochemistry.