Cardiac Lineage Protein 1 (CLP1) Plays a Distinct Role in Myocardial Infarction

Abstract

Introduction: Cardiovascular disease is one of the leading causes of death worldwide. Atherosclerosis in the vessels in the heart, for example, can contribute to an acute loss of blood supply to the heart, leading to damage or destruction of cardiomyocytes. A sufficient loss of these cardiac myocytes can lead to heart failure. The loss of cardiomyocytes results in a process of remodeling, which includes inflammatory responses that contribute to debris removal and proliferation of fibroblasts to replace muscle tissue with fibrous tissue. The Siddiqui laboratory had identified a protein called cardiac lineage protein 1 (CLP1), also known as HEXIM1, that is involved in directing cardiac lineage and developed a mouse strain that is haplodeficient for the gene. I investigated whether CLP1 plays a role in remodeling the heart following myocardial infarction in mice. In particular, I investigated the role of CLP1 in the development of fibrosis in cardiac tissue using a mouse line that is haplodeficient for the CLP1 protein. More specifically, I studied Smad3 and NFkB activity, each of which are known to play a role in the progression of myocardial infarction, and may contribute to remodeling. Methods: CLP1 wildtype and CLP1 haplodeficicent mice were subjected to permanent occlusion of the left anterior descending artery (LAD) for 1 week or for 4 weeks postoperatively. Mice were evaluated preoperatively and postoperatively for cardiac function via echocardiography. Harvested hearts were evaluated for collagen deposition via Masson’s Trichrome stain. Hearts were, also, evaluated for localization, activation, and transactivation of NFkB and Smad3 via immunofluorescence, Western blot, and qRT-PCR, respectively. Results: The data show that both wildtype and CLP1 haplodeficient hearts exhibit cardiac remodeling, as measured by percent of collagen deposition, 1 week and 4 weeks following ischemic injury both above and below the ligation. CLP haplodeficient hearts had a lower amount of remodeling below the ligation following ischemic injury compared to wildtype hearts 1 week postoperatively. And, this phenotype is switched 4 weeks postoperatively, at which time there is a greater amount of fibrosis in the CLP1 haplodeficient mice compared to the CLP1 wildtype mice observed. Above the ligation, a similar pattern is observed in that CLP1 haplodeficient hearts had a trend of a lower amount of remodeling compared to wildtype 1 week postoperatively and there is a greater amount of remodeling in the CLP1 haplodeficient hearts compared to wildtype hearts 4 weeks postoperatively. Additionally, one can observe increased pSmad3 and pNFkB activation and translocation into the nucleus in the infarcted zone following ischemic damage in both wildtype and in CLP1 haplodeficient hearts. Inflammatory gene expression, specifically IL1b and IL6, is higher in the CLP1 haplodeficient hearts compared to wildtype hearts both 1 week and 4 weeks postoperatively. There was, also, a greater amount of collagen type I and type III transcripts in the haplodeficient mice, compared to wildtype both 1 week and 4 weeks postoperatively, which may suggest other methods of regulation in fibrosis following myocardial infarction. The cardiac function in CLP1 haplodeficient mice was worse in comparison to the wildtype mice 4 weeks postoperatively, as demonstrated by lower fractional shortening and by lower fractional area change. There was not an appreciable difference in LVmass between CLP1 haplodeficient mice and wildtype mice. Conclusion: The data are suggestive that CLP1 has an impact on inflammation following myocardial infarction that yields poorer functional recovery after ischemic damage. Further studies need to be done in order to better elucidate the role of CLP1 in regulating inflammation and in regulating extracellular matrix turnover following myocardial infarction.