Because stem cells are often found to improve repair tissue including heart without evidence of engraftment or differentiation, mechanisms underlying wound healing are still elusive. Several studies have reported that stem cells can fuse with cardiomyocytes either by permanent or partial cell fusion processes. However, the respective physiological impact of these two processes remains unknown in part because of the lack of knowledge of the resulting hybrid cells. To further characterize cell fusion, we cocultured mouse fully differentiated cardiomyocytes with human multipotent adipose-derived stem (hMADS) cells as a model of adult stem cells. We found that heterologous cell fusion promoted cardiomyocyte reprogramming back to a progenitor-like state. The resulting hybrid cells expressed early cardiac commitment and proliferation markers such as GATA-4, myocyte enhancer factor 2C, Nkx2.5, and Ki67 and exhibited a mouse genotype. Interestingly, human bone marrow-derived stem cells shared similar reprogramming properties than hMADS cells but not human fibroblasts, which suggests that these features might be common to multipotent cells. Furthermore, cardiac hybrid cells were preferentially generated by partial rather than permanent cell fusion and that intercellular structures composed of f-actin and microtubule filaments were involved in the process. Finally, we showed that stem cell mitochondria were transferred into cardiomyocytes, persisted in hybrids and were required for somatic cell reprogramming. In conclusion, by providing new insights into previously reported cell fusion processes, our data might contribute to a better understanding of stem cell-mediated regenerative mechanisms and thus, the development of more efficient stem cell-based heart therapies. STEM CELLS 2011; 29:812-824 Disclosure of potential conflicts of interest is found at the end of this article.

The role of Ca(2+) in insect olfactory transduction was studied in the moth Spodoptera littoralis. Single sensillum recordings were made to investigate in vivo the role of sensillar Ca(2+) on the electrophysiological properties of sex pheromone responsive olfactory receptor neurons (ORNs). Lowering the sensillar Ca(2+) concentration to 2 x 10(-8) M increased ORN spontaneous firing activity and induced long bursts of action potentials (APs) superimposed on spontaneous negative deflections of the transepithelial potential. We inferred that Ca(2+) stabilizes the membrane potential of ORNs, keeping the spontaneous firing activity at a low and regular level. Neither the amplitude and kinetics of the rising phase of sensillar potentials (SPs) recorded in response to pheromone stimuli nor the AP generation during stimulation depended on the extracellular Ca(2+) concentration. Thus, extracellular Ca(2+) is not absolutely necessary for ORN response. Partial inhibition of responses with a calmodulin antagonist, W-7, also indicates that intracellular Ca(2+) contributes to the ORN response and suggests that Ca(2+) release from internal stores is involved. In 2 x 10(-8) M Ca(2+), the repolarization of the SP was delayed when compared with higher Ca(2+) concentrations. Therefore, in contrast to depolarization, ORN repolarization depends on extracellular Ca(2+). Ca(2+)-gated K(+) channels identified from cultured ORNs with whole-cell recordings are good candidates to mediate ORN repolarization.

Olfactory receptors are G protein-coupled receptors. Some of them are expressed in tumor cells, such as the OR51E2 receptor overexpressed in LNCaP prostate cancer cells. It is considered a prostate tumor marker. We previously demonstrated that this receptor is able to promote LNCaP cell invasiveness in vitro upon stimulation with its odorant agonist β-ionone, leading to increased generation of metastases in vivo. In the present study, we show that even a relatively short exposure to β-ionone is sufficient to promote metastasis emergence. Moreover, α-ionone, considered an OR51E2 antagonist, in fact promotes prostate tumor growth in vivo. The combination of α-ionone with β-ionone triggers a higher increase in the total tumor burden than each molecule alone. To support the in vivo results, we demonstrate in vitro that α-ionone is a real agonist of OR51E2, mainly sustaining LNCaP cell growth, while β-ionone mainly promotes cell invasiveness. So, while structurally close, α-ionone and β-ionone appear to induce different cellular effects, both leading to increased tumor aggressiveness. This behaviour could be explained by a different coupling to downstream effectors, as it has been reported for the so-called biased ligands of other G protein-coupled receptors.

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