The potential of biophotonic techniques in stem cell tracking and monitoring of tissue regeneration applied to cardiac stem cell therapy

Melatonin enhances survival and preserves functional integrity of stem cells: A review

Despite state-of-the-art pharmaceutical regimens, continuous improvements in diagnostic techniques as well as refinements in equipment and interventional procedures, many diseases remain refractory to conventional therapies. Recent advances in stem cell (SC) biology have opened an avenue to exploring its therapeutic potential in various disease entities, especially those that are ischemia-related and refractory to conventional treatment. A number of experimental studies and clinical trials have already demonstrated promising outcomes. On the other hand, SC therapy is associated with major problems. For instance, ischemia, inflammation, and oxidative stress are some of the factors unfavorable for SC survival once SCs are implanted into the ischemic area in an attempt to enhance tissue regeneration and restore organ function. Melatonin, which is originally derived from pineal gland in the regulation of human circadian rhythms and sleep, is a potent free radical scavenger and metal chelator with the capacity to alleviate oxidative stress and inflammatory reactions as well as stabilizing cell membranes. Accumulating data have demonstrated that melatonin-supported SC therapy is superior to SC alone for improving ischemia-related organ dysfunction. In this review, we describe and interpret the potential role of melatonin in sustaining the survival and preserving the functional integrity of SC.

Non-invasive monitoring of cell metabolism and lipid production in 3D engineered human adipose tissues using label-free multiphoton microscopy

Non-linear optical microscopy methods can characterize over time multiple functional properties of engineered tissues during development. Here, we demonstrate how the combined use of third-harmonic generation (THG) and two-photon excited fluorescence (2PEF) imaging can provide direct quantitative biomarkers of adipogenic stem cell differentiation and metabolic state, respectively. Specifically, we imaged over nine weeks silk scaffolds embedded with human mesenchymal stem cells and exposed to either propagation (PM) or adipogenic differentiation media (AM). THG was employed to visualize the formation of lipid droplets. 2PEF was used to assess the metabolic state of the cells through the redox ratio defined based on the endogenous FAD and NADH fluorescence. The redox ratio of cells in the AM scaffold was significantly lower than that in the PM scaffold during week 5 and 9, and correlated with significant increases in lipid-to-cell volume ratio, and number and size of lipid droplets in the AM scaffold. These findings indicate that the decrease in redox ratio during adipogenic differentiation is associated with fatty acid synthesis and lipid accumulation. Our methods therefore enabled us to identify and measure dynamic correlations between lipid droplet formation and cell metabolic state, while providing insight on the spatial heterogeneity of the observed signals.

Elastin overexpression by cell-based gene therapy preserves matrix and prevents cardiac dilation

After a myocardial infarction, thinning and expansion of the fibrotic scar contribute to progressive heart failure. The loss of elastin is a major contributor to adverse extracellular matrix remodelling of the infarcted heart, and restoration of the elastic properties of the infarct region can prevent ventricular dysfunction. We implanted cells genetically modified to overexpress elastin to re-establish the elastic properties of the infarcted myocardium and prevent cardiac failure. A full-length human elastin cDNA was cloned, subcloned into an adenoviral vector and then transduced into rat bone marrow stromal cells (BMSCs). In vitro studies showed that BMSCs expressed the elastin protein, which was deposited into the extracellular matrix. Transduced BMSCs were injected into the infarcted myocardium of adult rats. Control groups received either BMSCs transduced with the green fluorescent protein gene or medium alone. Elastin deposition in the infarcted myocardium was associated with preservation of myocardial tissue structural integrity (by birefringence of polarized light; P < 0.05 versus controls). As a result, infarct scar thickness and diastolic compliance were maintained and infarct expansion was prevented (P < 0.05 versus controls). Over a 9-week period, rats implanted with BMSCs demonstrated better cardiac function than medium controls; however, rats receiving BMSCs overexpressing elastin showed the greatest functional improvement (P < 0.01). Overexpression of elastin in the infarcted heart preserved the elastic structure of the extracellular matrix, which, in turn, preserved diastolic function, prevented ventricular dilation and preserved cardiac function. This cell-based gene therapy provides a new approach to cardiac regeneration.

Polarization birefringence measurements for characterizing the myocardium, including healthy, infarcted, and stem-cell-regenerated tissues

Myocardial infarction leads to structural remodeling of the myocardium, in particular to the loss of cardiomyocytes due to necrosis and an increase in collagen with scar formation. Stem cell regenerative treatments have been shown to alter this remodeling process, resulting in improved cardiac function. As healthy myocardial tissue is highly fibrous and anisotropic, it exhibits optical linear birefringence due to the different refractive indices parallel and perpendicular to the fibers. Accordingly, changes in myocardial structure associated with infarction and treatment-induced remodeling will alter the anisotropy exhibited by the tissue. Polarization-based linear birefringence is measured on the myocardium of adult rat hearts after myocardial infarction and compared with hearts that had received mesenchymal stem cell treatment. Both point measurement and imaging data show a decrease in birefringence in the region of infarction, with a partial rebound back toward the healthy values following regenerative treatment with stem cells. These results demonstrate the ability of optical polarimetry to characterize the micro-organizational state of the myocardium via its measured anisotropy, and the potential of this approach for monitoring regenerative treatments of myocardial infarction.

"Microenvironmental contaminations" induced by fluorescent lipophilic dyes used for noninvasive in vitro and in vivo cell tracking

Determining how normal and leukemic stem cells behave in vivo, in a dynamic and noninvasive way, remains a major challenge. Most optical tracking technologies rely on the use of fluorescent or bioluminescent reporter genes, which need to be stably expressed in the cells of interest. Because gene transfer in primary leukemia samples represents a major risk to impair their capability to engraft in a xenogenic context, we evaluated the possibility to use gene transfer-free labeling technologies. The lipophilic dye 3,3,3',3' tetramethylindotricarbocyanine iodide (DiR) was selected among 4 near-infrared (NIR) staining technologies. Unfortunately we report here a massive transfer of the dye occurring toward the neighbor cells both in vivo and in vitro. We further demonstrate that all lipophilic dyes tested in this study (1,1'-dioctadecyl-3,3,3',3'-tetramethylindotricarbocyanine perchlorate [DiI], DiD, DiR, and PKH26) can give rise to microenvironmental contamination, including when used in suboptimal concentration, after extensive washing procedures and in the absence of phagocytosis or marked cell death. This was observed from all cell types tested. Eventually, we show that this microenvironmental contamination is mediated by both direct cell-cell contacts and diffusible microparticles. We conclude that tracking of labeled cells using non-genetically encoded markers should always be accompanied by drastic cross validation using multimodality approaches.