Capturing the Cardiac Injury Response of Targeted Cell Populations via Cleared Heart Three-Dimensional Imaging

Summary

Cardiomyocyte proliferation following injury is a dynamic process that requires a symphony of extracellular cues from non-myocyte cell populations. Utilizing lineage tracing, passive CLARITY, and three-dimensional whole-mount confocal microscopy techniques, we can analyze the influence of a variety of cell types on cardiac repair and regeneration.

Abstract

Cardiovascular disease outranks all other causes of death and is responsible for a staggering 31% of mortalities worldwide. This disease manifests in cardiac injury, primarily in the form of an acute myocardial infarction. With little resilience following injury, the once healthy cardiac tissue will be replaced by fibrous, non-contractile scar tissue and often be a prelude to heart failure. To identify novel treatment options in regenerative medicine, research has focused on vertebrates with innate regenerative capabilities. One such model organism is the neonatal mouse, which responds to cardiac injury with robust myocardial regeneration. In order to induce an injury in the neonatal mouse that is clinically relevant, we have developed a surgery to occlude the left anterior descending artery (LAD), mirroring a myocardial infarction triggered by atherosclerosis in the human heart. When matched with the technology to track changes both within cardiomyocytes and non-myocyte populations, this model provides us with a platform to identify the mechanisms that guide heart regeneration. Gaining insight into changes in cardiac cell populations following injury once relied heavily on methods such as tissue sectioning and histological examination, which are limited to two-dimensional analysis and often damage the tissue in the process. Moreover, these methods lack the ability to trace changes in cell lineages, instead providing merely a snapshot of the injury response. Here, we describe how technologically advanced methods in lineage tracing models, whole organ clearing, and three-dimensional (3D) whole-mount microscopy can be used to elucidate mechanisms of cardiac repair. With our protocol for neonatal mouse myocardial infarction surgery, tissue clearing, and 3D whole organ imaging, the complex pathways that induce cardiomyocyte proliferation can be unraveled, revealing novel therapeutic targets for cardiac regeneration.

Introduction

The heart has long been considered to be a post-mitotic organ, yet recent evidence demonstrates that cardiomyocyte renewal occurs in the adult human heart at about 1% per year¹. However, these low rates of cardiomyocyte turnover are insufficient to replenish the massive loss of tissue that occurs following injury. A heart that has suffered a myocardial infarction will lose around one billion cardiomyocytes, often serving as a prelude to heart failure and sudden cardiac death^2,3. With over 26 million people affected by heart failure worldwide, there is an unmet need for therapeutics that....

Protocol

All experiments were conducted in accordance with the Guide for the Use and Care of Laboratory Animals and in compliance with the Institutional Animal Care and Use Committee in the School of Medicine and Public Health at the University of Wisconsin–Madison. All methods were performed on wild type C57BL/6J (B6) and transgenic mouse lines obtained from Jackson Laboratories.

1. Coronary Artery Occlusion (Myocardial Infarction) Induced via Ligation of the Left Anterior Descending Artery (LAD) in 1-Day-Old Neonatal Mice⁶

1. Separate the 1-day-old neonatal pups from the mother by placing them into a clean ca....

Results

Often the two most challenging steps are guiding the heart out of the chest cavity and ligating the LAD. To troubleshoot these steps, adjustments may be made in the placement of the initial puncture between the fourth intercostal muscles; if the puncture and blunt dissection are too close in proximity to the sternum, the heart may not be able to exit the chest cavity (Figure 1A).

Additionally, increased pressure on the left abdomen may be needed t.......

Discussion

Cell-cell interactions between cardiomyocytes and non-myocyte populations are a determining factor of whether the heart will undergo fibrosis or repair following injury. Discoveries have been made demonstrating that a variety of cell types, including nerves¹⁴, epicardial cells²⁴, peritoneal macrophages²⁵, arterioles^12,13, and lymphatic endothelial cells²⁶, all play an .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1-thioglycerol
6-0 Prolene Sutures	Ethicon	8889H	Polypropylene Sutures
Acrylamide
Boric acid
Curved Forceps	Excelta	16-050-146	Half Curved, Serrated, 4 in
Dressing Forceps	Fisherbrand	13-812-39	Dissecting, 4.5 in
Glass Vial	Fisherbrand	03-339-26A	12 x 35 mm Vial with Cap
Histodenz	Sigma-Aldrich		Density gradient medium
Iridectomy Scissors	Fine Science Tools	15000-03	2 mm Cutting Edge
Large Dissecting Scissors	Fisherbrand	08-951-20	Straight, 6 in
Needle Holder	Fisherbrand	08-966	Mayo-Hegar, 6 in
Paraformaldehyde
Phosphate Buffer
Sharp Forceps	Sigma-Adrich	Z168777	Fine Tip, Straight, 4.25 in
Small Dissecting Scissor	Walter Stern Inc	25870-002	30 mm Cutting Edge
Sodium Azide
Sodium Dodecyl Sulfate (SDS)
Tissue Forceps	Excelta	16050133	Medium Tissue, 1X2 Teeth
VA-044	Wako Chemicals		Water-soluble azo initiator