Abstract 15006: Serotonin Transporter (SERT) Knockout Leads to Increased Myocardial and Mitral Valvular Fibrosis in a Murine Model

Abstract

Introduction: Heart valve disease attributed to serotonin (5HT) has been observed with 5HT-secreting carcinoid tumors and with the diet drug, Dexfenfluoramine, a 5HT transporter (SLC6A4, also known as SERT) inhibitor and 5HT receptor (HTR) 2B agonist. 5HT/SLC6A4 mechanisms had not been investigated in mitral valve (MV) regurgitation. SLC6A4 internalizes 5HT, limiting HTR signaling. Selective 5HT reuptake inhibitor (SSRI) antidepressants target SLC6A4 to increase 5HT. We showed that SLC6A4 reduction in human MV influences MV regurgitation through enhanced HTR signaling. We further investigate the cardiac impact of SLC6A4 deficiency in a SERT -/- mouse reported to have cardiac abnormalities. Hypothesis: Absence of SERT leads to increased myocardial and MV fibrosis. Methods: SERT -/- and wild type (WT) C57BL/6J mice underwent echocardiography at 8 and 16 weeks (w) of age (N=5 / group). Collagen staining was performed with Masson’s trichrome. Markers of myocardial fibrosis α-smooth muscle actin (αSMA) and collagen type1, alpha 1 (COL1A1) were measured by PCR. Results: 8w and 16w SERT -/- mice showed increased epicardial fibrosis and MV sub-valvular fibrosis, particularly at leaflet attachment points, and in 16w SERT -/- mice MV leaflet fibrosis was also present. At 8w, posterior ventricular wall thickness was greater in SERT -/- mice vs. WT, both in systole (1.30 ± 0.09mm vs. 1.61 ± 0.2mm, p=0.003) and diastole (0.78 ± 0.11mm vs. 1.22 ± 0.29mm, p=0.001). At 16 w, ventricular dimensions were larger in SERT -/- mice vs. WT at end-systole (2.29 ± 0.54mm vs. 3.0 ± 0.47mm, p=0.008) and end-diastole (3.48 ± 0.45mm vs. 4.07 ± 0.35, p=0.006). Fractional shortening was reduced in 16w SERT -/- mice vs. WT (35.20 ± 7.66 vs. 26.74 ± 6.14, p=0.017) and vs. 8w SERT -/- mice (38.85 ± 4.57 vs. 26.74 ± 6.14, p=0.002). PCR showed increased fibrosis markers αSMA and COL1A1 in SERT -/- mice (8w and 16w) vs. WT (all p<0.05). Conclusions: SERT -/- mice show pathologic structural changes and fibrosis in both myocardium and MV, corresponding to functional differences that worsen with age. As inhibition of SLC6A4, combined with active HTR2A/B signaling would hypothetically increase MV remodeling and regurgitation, these animals could be ideal to test 5HT pharmacology.