Abstract Body (Do not enter title and authors here): Background: We have previously reported that Sorbs2 (sorbin and SH3 domain 2) knockout mice have significant cardiac structural and electrical remodeling, including atrial enlargement and fibrosis. The mechanisms of such changes were unknown.
Hypothesis: We hypothesized that loss of Sorbs2 in cardiac fibroblasts promotes profibrotic signaling pathways and alters the balance between matrix metalloproteinases (MMPs), and their inhibitors (TIMPs) resulting in excessive collagen deposition in the atria.
Methods: We used Sorbs2 global knockout (Sorbs2 KO) mice and age-matched wild-type (WT) controls at 4 to 5 months of age. Left atrial fibrosis was quantitated by Masson's trichrome staining in serial sections. Scratch wound healing assays and transwell migration assays were performed in isolated Sorbs2 KO and WT atrial fibroblasts. Protein expression was determined by Western blots. Interleukin-6 (IL-6) production in cultured fibroblast supernatants was measured by ELISA.
Results: Sorbs2 KO mice showed significant increase in atrial diameter (16.6 ± 1.5 mm vs 12.0 ± 1.1 mm in WT; n=3, p < 0.05) and atrial fibrosis (fraction of fibrotic areas: 18.6± 2.9 % vs. 7.6 ± 0.9 % in WT; p < 0.01). Collagen 1 and collagen 3 protein levels in Sorbs2 KO mouse atria were 1.8-fold and 1.7-fold higher than those of WT respectively. Sorbs2 KO atrial fibroblasts showed impaired function with significantly reduced migration and slower scratch-wound closure (migration area after 24h: 18.5 ± 3.8 % vs 70.1 ± 6.7 % in WT; n=6, p < 0.01).Moreover, transforming growth factor β1 (TGF-β1), α-smooth muscle actin, and periostin expression levels were significantly reduced in isolated fibroblasts and atrial myocardia from Sorbs2 KO mice, suggesting that fibrosis is not caused by TGF-b1-dependent mechanisms or from myofibroblast transformation. Analysis of ECM proteins showed significant downregulation of MMP1 and MMP3 but upregulation of TIMP1, while TIMP3 was unchanged. Consistent with a pro-inflammatory phenotype, Sorbs2 KO atrial fibroblasts secreted higher IL-6 levels at baseline (3303± 463 pg/mL vs. 2075 ± 731 pg/mL in WT; n=6, p < 0.01) and with LPS stimulation (7791 ± 604 pg/mL vs. 5588 ± 763 pg/mL in WT; n=6, p < 0.01).
Conclusion: Sorbs2 deficiency is associated with abnormal atrial fibroblast function showing impaired migration and scratch wound healing, as well as increased IL-6 production and imbalanced MMP/TIMP. These changes result in collagen accumulation and enhanced fibrosis.