Kai Martin Schmidt-Ott

Introduction: Arginine vasopressin receptor 2 (AVPR2) signaling regulates kidney collecting duct signaling and urinary concentration, but the kidney’s cell-type-specific responses to its pharmacological inhibition and their relevance to non-cystic kidney injury remain incompletely characterized. Methods: We examined physiological and transcriptional responses to the AVPR2 antagonist Tolvaptan in uninjured male mice and assessed tissue outcomes after unilateral ischemia-reperfusion injury (IRI) and repeated cisplatin administration. Analyses included renal tissue osmolality measurements, single nucleus and bulk RNA sequencing of kidney tissues, quantitative PCR, in situ hybridization, and histology. Results: In uninjured kidneys, Tolvaptan reduced urine and medullary tissue osmolality and altered transcription predominantly in medullary collecting-duct populations, including lower expression of selected stress-associated and proinflammatory genes. Following IRI, Tolvaptan-treated mice exhibited attenuated expression of inflammatory and monocyte recruitment genes, reduced macrophage accumulation in the renal medulla, and diminished fibrotic remodeling. In cisplatin-induced kidney injury Tolvaptan reduced tubular injury markers and inflammatory activation in the outer medulla. Conclusions: Tolvaptan attenuates medullary tubular stress responses and post-injury inflammation and fibrosis. These findings extend the effects of pharmacological AVPR2 antagonism beyond water homeostasis to tissue-level responses in non-cystic kidney injury. TRANSLATIONAL STATEMENT: Acute kidney injury can lead to persistent inflammation and fibrotic remodeling, yet the contribution of medullary Avpr2 signaling to these processes remains poorly understood. In mice, treatment with the clinically used Avpr2 antagonist Tolvaptan reduced medullary osmolality and modulated stress-associated tubular transcriptomes. Tolvaptan treatment initiated before injury reduced selected inflammatory and tissue remodeling endpoints in experimental kidney injury. These findings support further investigation of pharmacological AVPR2 antagonism in non-cystic kidney disease.

Red pulp macrophages (RPM) and bone marrow macrophages (BMM) are iron-recycling cells, involved in iron homeostasis and erythropoiesis. Here we show, by conditional deletion strategies using Cx3cr1Cre-mediated targeting to inactivate Notch signalling components in mice, that canonical Notch2 signalling regulates the development of RPM and BMM. Loss of functional Notch2, or its nuclear mediator Rbpj, impair RPM and BMM development and cause iron overload in the spleen and bone marrow. In the absence of Notch2, prototypic RPM genes are downregulated, which is accompanied by splenic extramedullary haematopoiesis and changes in splenic microarchitecture. Treatment of mice with an anti-Notch2 blocking antibody recapitulates the impaired erythrophagocyte and de novo extramedullary hematopoiesis phenotypes characteristic to the genetic model. Furthermore, early postnatal transfer of bone marrow and fetal liver progenitors rescues the defects in RPM and BMM in a Notch2-dependent manner, demonstrating the potential to restore defective tissue resident macrophage niches by Notch-competent progenitors. Thus, our study demonstrates that canonical Notch2 signalling is required for the development, maintenance, and function of iron-recycling macrophages.

This study reveals ferroptosis, a pharmacologically inducible form of cell death, as a novel mechanism to eliminate activated fibroblasts, the main drivers of kidney fibrosis. Due to their high ferroptosis sensitivity, these cells are selectively depleted by RSL3 in vitro, in kidney tissue slice cultures, and in fibrotic kidneys in vivo. These findings highlight ferroptosis induction as a promising antifibrotic strategy in kidney disease.

Key Points: Transferrin receptor 1 is critical for perinatal nephron growth and maturation; its deletion results in widespread cystic hypodysplasia. Nontransferrin bound iron traffic is a physiologic source of iron in the midgestation embryo and complements transferrin receptor 1. Cystic hypodysplasia can be rescued by exogenous iron or by the activation of systemic iron traffic with hypoxia inducible factor activators. Background: Periconceptual maternal iron deficiency (FeD) is a worldwide cause of premature births and low birth weights. Yet, it is unknown whether FeD affects all developing tissues equally or rather target- s pecific lineages. In addition, since FeD restricts both transferrin bound and nontransferrin bound iron species, their unique contributions to organogenesis are indeterminant. Methods: To address questions of iron traffic and kidney development, we examined the deletion of the singular transferrin receptor ( TfR1−/− ), created green flourescent protein-labeled TfR1−/− embryonic stem cells for inoculation into wild blastocysts, and created TfR1 -floxed mice to generate cell autonomous deletions of TfR1 in mesenchymal, ureteric, and stromal derivatives. Finally, we created a model of global FeD with iron poor diets, for comparison with cell autonomous TfR1 deletions. Results: Transferrin receptor deletions only modestly suppressed tubulogenesis, had little, if any effect on the growth of the ureteric bud and no gross effects on kidney stroma at mid gestation. By contrast, nutritional FeD nearly abolished kidney development, highlighting the limited phenotypes induced by transferrin receptor deletion. Yet, in the second postnatal week, the critical function of TfR1 became evident by the growth of residual TfR1+ cells that had escaped Cre-mediated deletion and by tubular segment-specific polycystic transformation. Timed treatment with iron or systemic activators of iron trafficking prevented both cystic dysplasia and the terminal loss of kidney function, reversing extensive malformations of the kidney. Conclusions: TfR1 is the critical iron species targeting postnatal tubulogenesis, but in the embryo, TfR1 must be complemented by alternative iron species called nontransferrin bound iron. Iron-deficient kidney disease is reversible postnatally.

T-cell–mediated rejection (TCMR) remains a major cause of kidney transplant failure, despite being considered treatable. Its impact reflects a limited understanding of the underlying molecular mechanisms and their clinical consequences. To address this, we induced acute TCMR in mouse kidney transplants and profiled molecular changes using single-nucleus RNA sequencing (snRNA-seq), spatial transcriptomics and immunofluorescence. Results were compared with human snRNA-seq data from TCMR and stable allografts, as well as single-cell deconvolution analysis of bulk transcriptomic data from kidney transplant biopsies. Here we show that TCMR induces injured epithelial cell states in mouse kidney allografts, particularly in proximal tubules and thick ascending limbs. Spatial transcriptomics of these injured epithelial states demonstrated heterogeneous localization, interactions with immune cells and cellular microenvironments. Cross-species analysis confirmed similar severely injured epithelial states in human samples, whose abundances correlated with transplant survival and persisted despite TCMR resolution. Collectively, our results identify epithelial injury cell states as a determinant of outcome after TCMR.

Mutations in the transcription factor TFAP2A are linked to congenital anomalies of the kidney and urinary tract in humans. While Tfap2a knockout (KO) in mouse collecting ducts leads to tubular epithelial abnormalities, its precise molecular functions in kidney tubules remain unclear. To investigate Tfap2a -dependent gene regulatory networks in the mouse kidney collecting ducts, we employed conditional KO ( Hoxb7-Cre; Tfap2a ^fl/fl ) models combined with transcriptomics. Histomorphological and physiological assessments of Tfap2a -KO mice revealed progressive postnatal dilation of the outer medullary collecting ducts. Integrating bulk and single-nucleus RNA sequencing with in silico motif mapping in ATAC-seq datasets demonstrated that Tfap2a is highly expressed and active in normal collecting duct principal cells. Comparative transcriptomics between 3-month-old Tfap2a -KO and control mice identified dysregulated genes associated with cell adhesion and WNT signaling, including Alcam and Wnt9b . These changes were confirmed by in situ hybridization. Our findings reveal that Tfap2a regulates medullary collecting duct diameter by orchestrating a transcriptional network involving Wnt9b and Alcam , providing insights into its role in kidney structural integrity.