Bioluminescence imaging: a shining future for cardiac regeneration

Abstract

Advances in bioanalytical techniques have become crucial for both basic research and medical practice. One example, bioluminescence imaging (BLI), is based on the application of natural reactants with light-emitting capabilities (photoproteins and luciferases) isolated from a widespread group of organisms. The main challenges in cardiac regeneration remain unresolved, but a vast number of studies have harnessed BLI with the discovery of aequorin and green fluorescent proteins. First described in the luminous hydromedusan Aequorea victoria in the early 1960s, bioluminescent proteins have greatly contributed to the design and initiation of ongoing cell-based clinical trials on cardiovascular diseases. In conjunction with advances in reporter gene technology, BLI provides valuable information about the location and functional status of regenerative cells implanted into numerous animal models of disease. The purpose of this review was to present the great potential of BLI, among other existing imaging modalities, to refine effectiveness and underlying mechanisms of cardiac cell therapy. We recount the first discovery of natural primary compounds with light-emitting capabilities, and follow their applications to bioanalysis. We also illustrate insights and perspectives on BLI to illuminate current efforts in cardiac regeneration, where the future is bright.

Fig. 1

Global ‘bioluminescent’ biomes. Rarely on land, but widely common in deep-seas and oceans, a vast number of creatures has been discovered as visible light-emitting organisms.

Fig. 2

Aequorea victoria bioluminescence. Ca²⁺-activated aequorin and accessory GFP naturally coexist in the luminous organs of the crystal jellyfish. When stimulated, photocytes distributed along the edge of the umbrella give off green light due to a radiationless energy transfer (black arrow) from aequorin to GFP. This phenomenon has allowed the development of the novel hybrid bioluminescence FRET techniques, based on the combination (protein fusion) of photoproteins or luciferases and fluorescent proteins, to visualize protein–protein interactions inside single living cells and animals. Abbreviations: Ca²⁺, calcium; GFP, green fluorescent protein.

Fig. 3

Schematic representations of major BLI applications. Bioluminescence-based assays are being currently applied for (A) detection of biological molecules, (B) gene/protein expression and intracellular trafficking, and (C) implanted cell distribution and function.

Fig. 4

Harnessing of BLI in cardiac regeneration. BLI is supported by advanced gene technology that allows the generation of efficient reporters comprising genes encoding for distinct luciferases under the control of constitutive and inducible, lineage-specific promoters. This approach is first useful in vitro to modify therapeutic cells, to validate, e.g. for duplicate in multi-well culture plates, the increase in photonic detection levels with increasing cell number, and, lately in vivo, to track at any given time implanted cell survival and differentiation. As a result, researchers can finely quantify cardiac regeneration degree relative to the number of surviving cells under ischemic conditions.

Fig. 5

Example of non-invasive BLI monitoring of cardiac differentiation in the experimental model of acute myocardial infarction in mice. Representative BLI images showing decrease in the activity of RLuc driven by the constitutive CMV promoter, but increase in that from PLuc under the control of the cardiac-specific cTnI promoter within adipose tissue–derived progenitor cells at indicated times after myocardial implantation.