SPRED (Sprouty-related EVH-1 domain containing) proteins are inhibitors of the mitogen-activated protein kinase (MAPK) signaling pathway controlling cell proliferation and differentiation. To investigate the physiological impact of SPREDs in vivo we generated SPRED2 knockout mice using a gene trap approach. The lack of functional SPRED2 resulted in dysregulation of hormones secreted along the hypothalamic-pituitary-adrenal (HPA) axis. Because the SPRED2^-/- mice showed a bad general state of health, we determined all common serum parameters revealing significantly increased Na⁺ (152.1 vs. 146.3 mmol/l) and Cl^- levels (120.7 vs. 114.4 mmol/l) accompanied by hyperosmolality (332.2 vs. 322.2 mosmol/kg; *p<0.01; n(wt/ko)=12). Associated with the higher salt load in blood, SPRED2^-/- mice showed a nearly doubled daily water uptake (200.0 vs. 114.7 ml*gBW^-1*24h^-1; *p<0.01; n(wt/ko) =12). Polydipsia induced kidney damage in knockout mice, resulting in pathologically altered kidneys with extremely dilated renal pelves and Bowman's capsules, massive lymphocytic infiltration and apoptotic processes in renal tubules confirmed by TUNEL assays. In order to investigate salt and water imbalances, we systematically analyzed hormones involved in the renin-angiotensin-system and the HPA axis. These hormone tests demonstrated significantly elevated serum aldosterone (573.6 vs. 365.2 pg/ml; *p<0.05) and corticosterone levels (578.2 vs. 321.2 ng/ml; *p<0.01; n(wt/ko)=14) together with significantly higher corticotropin releasing hormone (CRH) in brain lysates (652.6 vs 499.0 pg/mg protein; *p<0.01, n(wt/ko) =20). Serum adrenocorticotropic hormone (ACTH) and angiotensinII levels were unaltered in younger mice aged 6 months, whereas in one year old mice both hormone levels were significantly decreased, most likely due to negative feedback mechanisms (ACTH: 2.45 vs. 5.0 ng/ml; *p<0,01; n(wt/ko)=10; angiotensinII: 0.8 vs. 2.6 ng/ml; *p<0.05; n(wt/ko)=8). Western blots with tissue lysates from wild-type and knockout mice revealed an increased ERK phosphorylation in organs of the HPA axis confirming the inhibitory effect of SPREDs in vivo. Immunohistochemistry and spred2 expression profiling underlined the importance of SPRED2 in tissues of the HPA axis. Kaplan-Meier survival analysis showed a significantly reduced life expectancy in SPRED2^-/- mice (25 vs. 80%; *p<0.01, n(wt/ko)=20).

We conclude that the SPRED2 deficiency leads to an upregulated MAPK signaling, causing an elevated hormone secretion in the HPA axis, which further results in salt and water imbalances and is associated with kidney damage.