19 Ischemia reperfusion (I/R) injury triggers the activation of coagulation and inflammation processes 20 involved in the pathophysiology of acute kidney injury (AKI). Coagulation proteases upregulated 21 upon renal I/R injury activate protease activated receptors (PARs), which form an important 22 molecular link between inflammation and coagulation. PAR4 is the major thrombin receptor on 23 mouse platelets, and the only PAR that is expressed on both human and murine platelets. In 24 addition, PAR4 is expressed on other cells including podocytes. We here sought to determine the 25 contribution of PAR4 in the host response to renal I/R injury. Hence, we subjected PAR4 knockout 26 and wild-type mice to renal I/R injury. PAR4 knockout mice exhibited an increased tolerance to 27 renal tubular necrosis and showed a decreased neutrophil influx in response to renal I/R, 28 independent from platelet PAR4. On the other hand, PAR4 deficiency resulted in albumin cast 29 formation in peritubular capillaries and showed a tendency towards albuminuria. Transmission 30 Electron Microscopy revealed an increase in podocyte foot process effacement. Our findings 31 suggest that PAR4 contributes to renal injury likely through facilitating neutrophil migration, 32 independent from platelet PAR4. In addition, PAR4 fulfils an important function in the 33 maintenance of podocyte integrity following renal I/R insult. Subsequently, loss of PAR4 results 34 in albuminuria. 35 36 37 38 3 39 Introduction40 Acute kidney injury (AKI) is a common and costly complication in hospitalized patients, and is 41 independently associated with increased risk of death(1). AKI severity is directly related to patient 42 outcome; even minor changes in serum creatinine predict prognosis in AKI patients after major 43 surgery.(2) Despite the progress in knowledge of the underlying pathophysiology, a recent 44 epidemiological study demonstrated that the incidence of AKI continues to grow.(3) AKI is often 45 triggered by ischemia reperfusion (I/R), which leads to a complex interplay between inflammatory 46 and coagulation processes and renal tissue remodelling(4, 5). More comprehensive understanding 47 of the mechanism underlying these processes is needed to devise strategies to control AKI or 48 accelerate renal recovery. Host-derived coagulation proteases, such as the serine proteases 49 thrombin and factor Xa have been implicated in renal I/R injury (5, 6). Serine proteases regulate 50 haemostasis as well as inflammation and tissue remodelling, thus linking the underlying processes 51 involved in the pathophysiology of AKI (7, 8). Serine proteases elicit cellular effects through 52 cleavage of Protease Activated Receptors (PARs). PARs are a family of seven transmembrane 53 spanning G-protein-coupled receptors that are broadly expressed on immune cells, platelets, and 54 also on renal cells.(9) Upon cleavage, a previously cryptic sequence becomes exposed and acts as 55 a receptor-activated tethered ligand(10), hereby initiating downstream signalling. The PAR fa...