Japanese researchers have successfully incorporated red algae chloroplasts into mammalian cells and observed photosynthetic activity for at least two days. A report on the study was published in the journal Proceedings of the Japan Academy, Series B.
The first indications of photosynthesis date back approximately 3.4 billion years; direct evidence, in the form of the thylakoid membranes of cyanobacteria, dates back approximately 1.8 billion years. Between 1.2 and 1.6 billion years ago, cyanobacteria were hijacked by eukaryotic cells, leading to a successful endosymbiosis that gave rise to plant chloroplasts. In the mid-20th century, it was shown that isolated chloroplasts retain photosynthetic activity for some time, although the mechanisms behind this remain unclear. Subsequently, various scientific groups attempted to incorporate these organelles into alien cells using various methods, but they retained their morphology for only a few hours, and their photosynthetic activity in the new environment was not confirmed.
Sachihiro Matsunaga of the University of Tokyo and colleagues isolated chloroplasts from the unicellular red alga Cyanidioschyzon merolae, which retains primitive features and inhabits highly acidic volcanic hot springs. Its chloroplasts are active at temperatures below 37 degrees Celsius, maintain their structure when isolated, and rarely differentiate into other plastids when environmental conditions change, making them promising candidates for incorporation into other cells. The isolated chloroplasts retained their phonosynthetic activity and morphology even after six days of storage at 4 degrees Celsius.
The resulting chloroplasts were cocultured with Chinese hamster ovary (Cricetulus griseus) cells, a widely used immortalized cell line in biotechnology, CHO-K1, at a ratio of 100 to 1. On the same day, confocal microscopy revealed that approximately 20 percent of the cells had engulfed 1–3 chloroplasts, and about one percent contained large numbers (7–45) of these organelles. On the second day, cells with chloroplasts demonstrated a higher growth rate than controls. After two and four days of coculture, the number of engulfed organelles in the cells decreased—likely due to either intracellular digestion or random distribution among daughter cells during cell division.
Chloroplasts were located in intracellular vesicles circularly near the nucleus without penetrating it and surrounded by mitochondria; native DNA was preserved. The authors of the study used fluorescence microscopy to identify these organelles by their chlorophyll content and scanning electron microscopy to study their membranes in detail. The chloroplasts had a double outer membrane and multiple layers of thylakoid membranes. On the first day of co-cultivation, this layered structure was intact in some chloroplasts, while in others it was partially deformed. After two days, the distance between the thylakoid membranes and the size of plastoglobuli (plastoquinone-containing lipoprotein particles that appear in response to stress) increased in some chloroplasts. After four days, the thylakoid membranes degraded.
To assess photosynthetic activity—electron transport in photosystem II—the researchers used imaging pulse-amplitude modulation (Imaging-PAM) fluorometry. On the first day and after two days of co-cultivation, this activity in the chloroplasts engulfed by the cells did not differ significantly from that in the isolated organelles, but by the fourth day, it had significantly decreased.
Thus, with the correct selection of chloroplasts and recipient cells, these organelles can maintain their structure and photosynthetic activity for at least two days after capture. This top-down approach to synthetic biology could serve as the basis for producing artificially photosynthetic animal cells, the authors conclude.
Previously, Chinese researchers created nanostructures with chloroplast thylakoids and implanted them into mouse chondrocytes, which were then transplanted into the articular cartilage of living animals. Their Italian colleagues were able to assemble a photosynthetic apparatus in an artificial cell using only the major transmembrane protein of the reaction center of purple bacteria.
