P863Morphometric analysis of the dynamic changes of the interstitium after reperfused myocardial infarction
Authors: Rios Navarro C , Ruiz-Sauri A , Ortega M , Gavara J , Marcos-Garces V , Minana G , Chorro F J , Bodi V 
Published: October 2019

Abstract

Background
The interstitial space is mainly composed by cells, fibers and gels of polysaccharides, which act as a compression buffer against the stress placed on the extracellular matrix (ECM). After myocardial infarction (MI), heart has to withstand higher mechanical stress due to injured cardiomyocytes. ECM composition notably influences the mechanical properties of the myocardium and participates in left ventricular remodeling.

Purpose
To characterize the myocardial ECM changes from ischemia onset until late phases after coronary reperfusion in a swine model of reperfused MI.

Methods
MI was induced in swine by transient 90-min coronary occlusion using angioplasty balloons. One control group and three MI groups were defined: 1) without reperfusion, 2) 1-week, and 3) 1-month reperfusion (n=5, each). Myocardial samples from the infarcted area were isolated and histologically staining to evaluate the presence of collagen type I, collagen type III, elastic fibers, and proteinglycans. Moreover, the presence of laminin, fibronectin, and secreted protein, acidic and rich in cysteine (SPARC) was determined by immunohistochemistry. Five independent photographs for each group and each stain were taken and the presence of the different components of the interstitium was morphometrically quantified. Indeed, the mRNA levels of metalloproteinases (MMP), tissue inhibitors of metalloproteinases (TIMP), and transcription factors implicated in ECM remodeling were quantified.

Results
In the no-reperfusion group, an increase in fibronectin, laminin, and elastic fibers were observed, whereas no changes neither in the quantity nor in the organization of collagen-I and collagen-III fibers were detected. In the 1-week and 1-month reperfusion groups, an augmentation in the content of collagen-I, collagen-III, elastic fibers were found. These fibers displayed a more organized pattern compared to control tissue. The quantity of proteinglycans, laminin, fibronectin, and SPARC were also increased. These changes could be explained by a heightening in the mRNA expression of TIMP1, TIMP2, and TIMP3 from the ischemic period until chronic phases, whereas no changes were observed for TIMP4. MMP2 and MMP9 mRNA levels were boosted in late phases post-MI. When exploring the mRNA levels of transcription factors implicated in ECM remodeling, CTGF increased during the ischemic period, whereas TNF-α and TGF-β augmented after 1-week and 1-month post-reperfusion, respectively.

Conclusions
ECM remodeling starts after ischemia onset, probably aiming to protect cardiomyocytes from the ischemic damage. After reperfusion, the ECM evolves to constitute a fibrotic scar to maintain a proper cardiac function. These changes may be orchestrated by the fluctuation in genes specially implicated in ECM distribution. ECM changes might be accurately regulated since it could provoke an adverse left ventricular remodelling, consequently impairing patient prognosis.

Acknowledgement/Funding
This study was funded by “Instituto de Salud Carlos III” and “Fondos Europeos de Desarrollo Regional FEDER” (PIE15/00013, PI17/01836, and CIBERCV16/11