Periodic Reporting for period 4 - RENOIR (RENal prOgenItoRs as tools to understand kidney pathophysiology and treat kidney disorders) Reporting period: 2020-01-01 to 2020-06-30 Summary of the context and overall objectives of the project Renal diseases represent one of the major global health burden of the 21st century. Acute kidney injury (AKI) affects one in five people admitted to hospital via emergency departments. In addition, more than 10% of people have chronic kidney disease (CKD), with high morbidity, and social impact. In other organs, discovery of endogenous stem/progenitor systems fueled understanding of disease mechanisms, establishment of novel disease models and identification of innovative treatment strategies. In the kidney, identification of an endogenous progenitor system remained a challenge until very recently. Conclusive evidence for the existence of renal progenitors (RPC) in human kidney has been reported only in the most recent years and this discovery opens a wide range of possibilities to support progress in several fields of nephrology. Indeed, we recently demonstrated that RPC are key player in the pathogenesis of kidney disorders, and their study is increasing knowledge about the mechanisms of kidney response to injury. Thus, RPC may represent potential tools and/or targets for therapeutic purposes and to promote innovative renal replacement strategies for kidney disorders. In this project, by using a series of innovative experimental models, we proposed to address a series of problems that are central to better understanding kidney pathophysiology. Specific topics are represented by acute kidney injury, chronic kidney disease, the development and progression of renal cell carcinoma, as well the use of patients-specific human RPC cultures derived from urine for personalized medicine. One essential feature of this project is the translation of this knowledge to new promising therapeutic and diagnostic approaches to kidney diseases. Consequently, these efforts will have a potential positive impact for patients and public health. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far 1. We stablished the role of RPC in glomerular disease. During the first period of the project, we established and validated new transgenic models for tracing the RPC system in the mouse. Using these models, we demonstrated that RPC localized within the glomerulus behave as podocyte progenitors during childhood and adolescence. Moreover, we established the role of RPC in progression and resolution of glomerular injury demostrating that remission or progression of disease was determined by the amount of regenerated podocytes. Indeed, the disease remitted if renal progenitors successfully differentiated into podocytes, while it persisted if differentiation was ineffective, resulting in glomerulosclerosis. Moreover, pharmacological treatment with a GSK3s inhibitor significantly increased disease remission enhancing RPC differentiation towards podocytes. Thus, we demonstrated that RPC represent an attractive therapeutic strategy to promote remission of glomerular disorders.Moreover, we demonstrated that CXCL12 blockade increased podocyte numbers and attenuated proteinuria enhancing de novo podocyte formation from RPC. 2. We established the role of RPC in tubular injury. We developed inducible transgenic mouse models to track RPC and all tubular cells and analyze their proliferative capacity. Using these models we demonstrated that following AKI tubular regeneration is only provided by RPC, while differentiated tubular cells can only undergo hypertrophy, which would explain why functional recovery after AKI could be fast, but the risk for progressive CKD is high if many nephrons are lost.3. We established the role of abnormal RPC function in the pathogenesis of renal cell carcinomas (RCC). We developed new transgenic mouse models to evaluate the contribution of differentiated tubular cells and of tubular progenitor cells with abnormal activation of Notch pathway to the development of RCC. Our data support the notion that hyperactivation of Notch pathway in differentiated tubular cells led to the formation of a broad spectrum of lesions of proliferative and not-proliferative nature and highlight the role of tubular progenitor cells in the formation of RCC.4. We provided the proof of concept for the use of urine-derived RPC for personalized kidney medicine. We have started to apply exome sequencing and filtering for all known genes causative of steroid-resistant nephrotic syndrome together with personalized disease modeling using cultures of human renal progenitors derived from the patient following the protocol previously patented and published. During this period, we provided evidence that this method helps clinical diagnosis of kidney disorders and proof of concept was provided in a young girl affected by Lupus Nephritis refractory to every immunosuppressive treatment. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) 1. The discovery that the RPC response during glomerular disorders has the potential to determine the outcome and can be pharmacologically modulated, changes the idea that podocyte loss cannot be rescued and, more importantly, suggest that drugs can be used to promote podocyte regeneration and treat even severe glomerular disorders, with important clinical implications.2. The discovery that RPC are the drivers of repair also after tubular regeneration challenges the current knowledge about acute kidney injury (AKI). Indeed, currently AKI is considered largely reversible based on the capacity of surviving tubular cells to dedifferentiate and to replace lost cells via cell division. However, we showed that AKI causes substantial tubular cell loss despite renal function recovery, that tubular cell proliferation is erroneously indicated by proliferation markers unable to ultimately identify cell division and that endocycle-mediated hypertrophy of tubular cells is the main cellular response upon AKI. By contrast, tubular progenitors got enriched via higher stress resistance and clonal expansion, regenerating tubule segments. Treatment with drugs that enhance tubular regeneration confirmed that this occurred only through progenitor mitosis. These results disprove current paradigms and demonstrate that, although a limited regeneration occurs through tubular progenitor proliferation, endocycle-mediated hypertrophy and persistent tubular cell loss are the dominant features after AKI. This change of paradigm also opens to the possibility of new strategies of treatment for AKI.3. The observation that Notch overexpression causes different types of renal cell carcinoma in addition to chronic kidney disease (CKD), provides an explanation of the link among CKD and cancer and suggests the innovative idea that tubular progenitors are the cells of origin of at least some types of kidney cancers.4. The successful use of urinary RPC cultures to support diagnosis of of a genetic podocytopathy in a patient affected by Lupus Nephritis resistant to immunosuppressive treatment provides the proof of concept that this technique can be successfully used in clinical practice and extremely useful to establish the correct diagnosis in difficult cases. Science Translational Medicine, Cover of Issue 536, 25 march 2020 related to Peired et al.