J. Lu1, M. Griffin1, J. L. Guo1, J. A. Norton1, M. T. Longaker1 1Stanford University, Department Of Surgery, Stanford, CA, USA
Introduction:
Cardiac fibrosis is the key hallmark of nearly all cardiac diseases, which collectively are the leading cause of mortality in the US. Cardiac fibrosis can reduce tissue compliance to cause systolic/diastolic dysfunction and heart failure, interrupt electrical excitation pathways to cause cardiac arrythmias, and impair regeneration of cardiomyocytes after acute myocardial infarction.
Fibrosis is characterized by excessive extracellular matrix deposition after injury. The accumulation of extracellular matrix is a normal response to injury: after cardiac injury such as myocardial infarction (MI), cardiac fibroblasts create a dense collagen scar that replaces the dead cardiac muscle to prevent acute cardiac rupture. Over time, however, the fibrotic scar is maladaptive and triggers hypertrophy and fibrosis in the remote myocardium, leading to ischemic cardiomyopathy and congestive heart failure. Large fibrotic scars are a poor prognostic predictor in post-MI patients.
Despite the central role the MI collagen scar in the pathogenesis of ischemic cardiomyopathy, the formation and remodeling of the MI scar over time are poorly understood. To study MI scar formation, we sought to quantitatively analyze changes in the ultrastructure of the extracellular matrix after MI.
Methods:
We subjected adult C57BL/6 mice to ischemic cardiac injury by surgically ligating the left anterior descending artery using a rapid non-ventilated thoracotomy. Sham surgeries were performed using the same procedure without artery occlusion. We harvested hearts from ligated and sham-operated mice at 4, 7, and 14 days after ischemic injury, and stained short-axis tissue sections with Picrosirius Red. To examine the ultrastructure of the extracellular matrix, we developed an image-processing algorithm to profile 294 ultrastructural features (fiber length, width, etc.) from Picrosirius Red–stained tissue sections viewed under polarized light.
Results:
We demonstrate that the extracellular matrix progressively expands after acute MI (Fig 1a). Computational analysis of the matrix ultrastructure reveals that the matrix ultrastructure follows a pro-fibrotic trajectory after MI (Fig 1b). We propose a model by which cardiac fibroblasts clonally proliferate to generate the MI scar. To test this model, we are actively inducing ischemic cardiac injury in Col1a1CreERT/+; Rainbow mice, which will allow for clonal lineage tracing of Col1a1+ cardiac fibroblasts.
Conclusion:
The extracellular matrix ultrastructure follows a pro-fibrotic trajectory after MI. In the future, our extracellular matrix ultrastructure algorithm can be used quantify the effects of new anti-fibrotic therapies.
