Question:

SPRED (Sprouty-related proteins with an EVH1domain) proteins are inhibitors of the pro-hypertrophic Ras/ERK-MAPK signaling pathway and are expressed in the heart. To investigate physiological SPRED functions, we generated SPRED2 KO mice. Initial characterizations revealed manifest hyperaldosteronism, an increased heart weight/body weight ratio, and a reduced survival probability. Based on these first results, we investigated cardiac performance of SPRED2 KO mice in a systematic manner.

Methods & Results:

Echocardiographic measurements revealed increased wall thickness, both in systolic (WT: 1.19 vs. KO: 1.47 mm) and diastolic (WT: 0.89 vs. KO: 1.20 mm) dimensions, and, accordingly, decreased inner endsystolic (WT: 2.27 vs. 1.64 mm) and endiastolic (WT: 3.52 vs. 3.07 mm) diameters of the left ventricle of SPRED2 KO mice (n(WT/KO)=15; p<0.05). Hemodynamic studies demonstrated an increased stroke volume (WT: 8.7 vs. KO: 10.2 µl), an elevated ejection fraction (WT: 51.2 vs. KO: 61.1 %), and an augmented peak rate of pressure rise (WT: 8848 vs. KO: 9774 mmHg/s) and, therefore, also an increase in cardiac output (WT: 4.2 vs. KO: 4.8 ml/min) and index (WT: 143 vs. 229 ml/(min*kg))in SPRED2 KOs (n(WT/KO)=16; p<0.5). Accordingly, high resolution ECGs showed an accelerated basal heart rate characterized by a shortened TP segment (WT: 49 vs. 35 ms; n(WT/KO)=9; p<0.01) in the SPRED2 KO mice, which is likely caused by premature depolarization of pacemaker cells. Furthermore, ECG recordings demonstrated various forms of spontaneous arrhythmias in SPRED2 KOs, e.g. different forms of AV blocks and extrasystoles. Stress conditions also provoked arrhythmias in SPRED2 KO mice, because electrophysiological overdrive pacing of atriae at decreasing pulse pause frequencies and increasing pacing rates elicited atrial fibrillation in KOs starting at a pacing rate of 1800 bpm. Atrial pacing at a rate of 700 bpm furthermore demonstrated impaired sinus node function by prolonged sinus node recovery times (WT: 31 vs. KO: 89 ms; n(WT/KO)=8; p>0.05) in the SPRED2 KOs. Western blot analysis of heart lysates revealed an up-regulated Ras/ERK-MAPK signaling due to the loss of SPRED-mediated pathway inhibition in SPRED2 KO hearts and excluded a compensatory up-regulation of the related SPRED1.

Conclusion:

SPRED2-deficiency in mice provokes cardiac hypertrophy and an increased cardiac output and index, presumably caused by facilitated or premature excitation of pacemaker cells. The resulting positive chronotropic and inotropic effect is not beneficial for the SPRED2 KO since it is accompanied by arrhythmias both under resting and stress conditions. The causative mechanisms, e.g. if the cardiac phenotype of the SPRED2 KO mice is either a direct consequence of SPRED2-deficiency and Ras/ERK-MAPK up-regulation or an indirect effect of SPPRED2-deficiency-mediated hyperaldosteronism will be investigated in a rescue model with aldosterone antagonist treatment.