P2Y1 Gene Deficiency Protects from Renal Disease Progression and Capillary Rarefaction during Passive Crescentic Glomerulonephritis Bernd Hohenstein,* Sindy Renk,* Kathrin Lang, † Christoph Daniel,* Monique Freund, ‡§ Catherine Le ´on, ‡§ Kerstin U. Amann, † Christian Gachet, ‡§ and Christian P.M. Hugo* Divisions of *Nephrology and † Pathology, University Erlangen-Nuremberg, Erlangen, Germany; and ‡ INSERM U.311 and § Etablissement Francais du Sang-Alsace, Strasbourg, France The metabotropic receptor P2Y1 is necessary for full ADP-induced platelet activation and is localized on various intrinsic renal cells, including mesangial cells, podocytes, and endothelial cells. To date, nothing is known about the role of the P2Y1 receptor during inflammatory renal disease. The role of the P2Y1 receptor was investigated using 22 P2Y1 gene– deficient (/) and 27 wild-type (wt) mice during the time course of passive crescentic nephrotoxic glomerulonephritis. Six P2Y1 / and six wt mice served as undiseased controls. Renal tissues were harvested on days 1, 10, and 28 after disease induction. No renal phenotype was found in P2Y1 / versus wt mice. In contrast, during crescentic glomerulonephritis, approximately 50% of all wt mice died, whereas all P2Y1 / mice survived. Renal function as assessed by creatinine clearance measurements, glomeruloscle- rosis, and tubulointerstitial injury indices as well as glomerular and interstitial matrix expansion were improved significantly in P2Y1 / compared with wt mice. These changes were preceded by reduced glomerular and peritubular capillary rarefaction indices in P2Y1 / compared with wt mice. The alteration of the rates of both peritubular apoptosis and endothelial cell proliferation suggests improved capillary preservation in P2Y1 / mice early in disease (day 10) and an additional enhanced repair reaction in P2Y1 / mice at the late time point (day 28), whereas injury on day 1 seemed to be equivalent in both groups. It is concluded that loss of P2Y1 receptor function safeguards against capillary loss, fibrosis, and death by renal failure during experimental crescentic glomerulonephritis. J Am Soc Nephrol 18: 494 –505, 2007. doi: 10.1681/ASN.2006050439 N ucleotides (e.g., ADP, ATP, UTP) are secreted under physiologic and pathophysiologic conditions and specifically activate P2 membrane nucleotide recep- tors. They can be found in all tissues, including the kidney, and are subdivided into metabotropic, G protein– coupled P2Y, and ionotropic and ligand-gated ion channels, the P2X receptors (1). The metabotropic P2Y1 receptor is of high importance for ADP-induced platelet activation (2). Besides its presence on megakaryocytes and platelets, there is evidence for P2Y1 recep- tors on many other cell types, including lymphocytes, mono- cytes (3), endothelial cells (4) and intrinsic renal cells such as mesangial cells, podocytes, and tubular cells (5). P2Y1 gene– deficient (-/-) mice have no apparent pheno- type and demonstrate normal development and survival (2). Platelets from P2Y1 -/- mice are unable to aggregate in response to usual concentrations of ADP and display mildly impaired aggregation to collagen, whereas high concentrations of ADP induce platelet aggregation without shape change (2). P2Y1 -/- mice have no spontaneous bleeding tendency but are resistant to experimentally induced thrombosis as evi- denced in various models of systemic thromboembolism or localized arterial thrombosis (6). In contrast to the well-established role of the P2Y1 receptor in platelets, little is known about its importance regarding other cell types. The high relevance of P2 receptors as modulators of leukocyte, smooth muscle cell, platelet, and endothelial cell function has been demonstrated clearly for inflammatory pro- cesses, chemotaxis, cytokine generation, and the innate host defense (7–10). Specifically, there is evidence for a role of the P2Y1 receptor in proliferation and apoptosis of endothelial cells as well as for regulatory processes via the nitric oxide (NO) system in endothelial and smooth muscle cells (4). Glomerulonephritis as an inflammatory renal disease that is mediated via humoral or cellular immune processes leads to acute damage of intrinsic glomerular cells that may be followed by chronic progression or resolution of disease, depending on the response to injury. Several studies have demonstrated a role for platelets as a source of inflammatory molecules during glomerulonephritis (11–13). In parallel, the balance of endothe- lial cell injury and repair in determining the degree of capillary rarefaction has been established as an important mechanism for chronic renal disease progression versus healing (14 –18). Con- sidering the wide distribution of the P2Y1 receptor on various renal cell types, experimental glomerulonephritis seemed to be Received May 6, 2006. Accepted November 20, 2006. Published online ahead of print. Publication date available at www.jasn.org. Address correspondence to: Dr. Christian Hugo, Department of Nephrology and Hypertension, University Erlangen-Nuremberg, Loschgestrasse 8, 91054 Erlan- gen, Germany. Phone: +49-9131-8539002; Fax: +49-9131-8539209; E-mail: christian.hugo@rzmail.uni-erlangen.de Copyright © 2007 by the American Society of Nephrology ISSN: 1046-6673/1802-0494