Myofibroblast and Endothelial Cell Proliferation during Murine Myocardial Infarct Repair

Abstract

Granulation tissue formation is a critical step in infarct repair, however, the kinetics of cell replication and the molecules that regulate this process are poorly understood. In uninjured mouse hearts and at 2 days post-infarction, very little DNA synthesis (measured by incorporation of a BrdU pulse) was detected in any cell type. Four days after permanent coronary occlusion, the rates of myofibroblast (smooth muscle ␣-actin and BrdU double-positive) and endothelial cell (CD31 and BrdU double-positive) proliferation were 15.4 ؎ 1.1% and 2.9 ؎ 0.5%, respectively. Most proliferating cells were located at the interface of the infarct and viable tissue. By 1 week, fibroblast and endothelial cell proliferation declined to 4.1 ؎ 0.6% and 0.7 ؎ 0.1%, respectively. In the 2-week infarct, the remaining necrosis had been phagocytosed, and fibroblast and endothelial cell proliferation were <0.5%. Although leukocytes were abundant throughout infarct repair, no significant proliferation was detected at any time in cells expressing CD45 or mac-3. Infarct size at 4 days was 38 ؎ 5% of the left ventricle and contracted to 20 ؎ 4% by 4 weeks. After 4 days, the chamber dilated to four times that of the control hearts and remained so for the duration of the time course. The vascular density (per mm 2 ) declined from 3643 ؎ 82 in control hearts to 2716 ؎ 197 at 1 week and 1010 ؎ 47 at 4 weeks post-myocardial infarction (MI). The average percent area occupied by vessels did not change significantly between the groups but the area/vessel (m 2 ) increased from 14.1 ؎ 0.3 in control hearts to 16.9 ؎ 1.9 at 1 week and 38.7 ؎ 7.9 at 4 weeks post-MI. These data indicate that mitogens for fibroblasts and endothelial cells peak within 4 days of infarction in the mouse heart. This provides the basis for identifying the responsible molecules and developing strategies to alter wound repair and improve cardiac function. Following coronary artery occlusion, extensive cardiomyocyte necrosis and remodeling of the ventricular wall occurs. Macrophages phagocytose the necrotic myocardium, and, concomitantly, myofibroblasts and endothelial cells proliferate and migrate into the infarct zone.

The immune system and cardiac repair

“…The origin of fibroblasts in healing infarcts remains unclear. Although numerous studies have documented active proliferation of resident fibroblasts [255], [219], [256] the possibility that at least a subset of infarct fibroblasts may be derived from blood-borne precursors should be considered.…”

“…They are predominantly localized in the border zone area, and exhibit intense proliferative activity [257], [255]. The mechanisms responsible for myofibroblast differentiation in the infarcted heart remain poorly understood; however, the modulation of a fibroblast toward myofibroblast likely results from the combined action of several distinct factors:…”

Systems approaches to preventing transplanted cell death in cardiac repair

“…They use different cell types (C2C12 vs. hESC-derived cardiomyocytes), different host animals (C3H vs. Rag2/γ c double knockout mice) and different assay systems (β-gal vs. Alu PCR). Treated and control cells were delivered into mice following thoracotomy and direct left anterior descending coronary artery occlusion [90]. In brief, Avertin-anesthetized mice were ventilated with room air supplemented with oxygen and underwent left-sided thoracotomy under aseptic conditions.…”

“…This led us to use an ischemia-reperfusion (IR) model of cardiac injury for our long term engraftment studies. This procedure followed established surgical techniques [90,99,100], but myocardial infarction was induced by ligating the left anterior descending coronary artery (LAD) with an 8-0 suture tied around a small piece of silicone tubing placed between the coronary artery and the suture to compress the LAD. After 45 minutes, the suture and tubing were removed to initiate reperfusion.…”

Bcl-2 Is a Key Factor for Cardiac Fibroblast Resistance to Programmed Cell Death

“…In addition to a drop in oxygen tension, which inactivates the respiratory chain and, therefore, impairs the efficient production of energy, ischemia also leads to a local deficiency in trophic factors, precursors, and energy-providing metabolites such as glucose. We focused on comparing the response of cardiac and dermal fibroblasts to such an adverse situation because it has been described that cardiac fibroblasts have a certain capacity to overcome heart infarction, at least for a certain time period (35), and die, in part, by apoptosis (36). In our experimental model of serum and glucose deprivation plus hypoxia, dermal fibroblasts readily activated apoptosis (Fig.…”