#3828 KINETICS OF THE DE NOVO NAD/NADH PATHWAY FROM AKI TO CKD
Authors: Serre Justine , Bras Alexandre , Merlier Franck , Samson Chloé , Placier Sandrine , Louedec Liliane , Haupt Karsten , Galichon Pierre , Hadchouel Juliette 
Published: June 2023

Abstract

Background and Aims
Mitochondrial dysfunction and energy metabolism deficiency are part of the acute kidney injury (AKI) pathophysiology. In particular, recent studies highlighted the role of nicotinamide adenine dinucleotide (NAD) production in the pathogenesis of AKI. NAD is an electron carrier for mitochondria and a cofactor for cytoplasmic redox reactions. Its production is decreased during AKI following a decreased of the expression of PGC1α (Peroxisome proliferator activated receptor gamma co-activator-1-α). PGC1α inactivation worsened and its overexpression prevented renal injury in a mouse model of renal ischemia. It was also shown that the consequences of PGC1α decreased expression included the alteration of the de novo NAD+ synthesis pathway. This was due to a reduction in the expression of the Quinolinate PhosphoRibosylTransferase (QPRT), which led to an accumulation of quinolinate. Consequently, the quinolinate/tryptophane (uQ/T) ratio increased in the urine. The modulation of this de novo has been studied mainly in the acute phase immediately following the injury. However its evolution with degree of severity and during the transition from AKI to CKD has not been described yet. In order to answer these questions, we designed the following studies: (1) A “severity” study in which we provoked ischemic AKI of increasing intensity in mice and quantified kidney PGC1alpha and QPRT mRNA expression. (2) An “AKI to CKD” study in which we followed kidney PGC1alpha and QPRT mRNA expression and uQ/T.

Method
Renal ischemia-reperfusion was performed in C57Bl6/J male mice using the new vascular occluder we recently developed (RIRI clamp), after nephrectomy of the contralateral kidney. For the ‘severity’ study, a 5–30 minutes ischemia was performed. Renal function (measurement of plasma creatinine and urea) and structure (Periodic Acid Schiff staining) were assessed 24h after ischemia. For the ‘AKI to CKD’ study, renal ischemia was performed during 10 minutes. Mice were sacrificed 1, 2, 3, 6 and 28 days after ischemia. For both studies, PGC1α and QPRT mRNA expression was quantified by qPCR in the kidneys. Urinary Q/T was determined by mass spectrometry.

Results
PGC1α and QPRT mRNA expression is inversely corrected to the AKI severity
We induced AKI in mice by unilateral ischemia reperfusion injury of increasing time to induce several degrees of AKI severity. The measurement of plasma urea and creatinine concentration confirmed that the severity increased with the duration of ischemia. PGC1α and QPRT mRNA expression decreased progressively with ischemia severity until reaching a plateau at 15 minutes of ischemia for PGC1α (Fig. 1A), like plasma creatinine and urea, whereas QPRT (Fig. 1B) decreased is linear until 30 min of ischemia. PGC1α mRNA decrease is also inversely correlated to kidney dysfunction (p&lt;0.001) (Figue 1C).
PGC1α and QPRT expression recovery after AKI
A 10-minute ischemia led to the development of Chronic Kidney Disease (CKD), as evidenced by an incomplete recuperation of the kidney function at 28 days. QPRT and PGC1α mRNA expression showed a progressive but incomplete recuperation during the transition from AKI to CKD compared to sham mice (Fig. 1D, E). uQ/T increases after ischemia and is restored during transition from AKI to CKD (Fig. 1F).

Conclusion
In addition to confirming the decrease of PGCα and QPRT mRNA expression during AKI, we show that it is correlated to the severity of the ischemic AKI. Furthermore, we describe the recovery during renal repair and transition to CKD.