Japanese researchers have managed to incorporate red algae chloroplasts into mammalian cells and observe photosynthetic activity in them for at least two days. A report on the work was published in the journal Proceedings of the Japan Academy, Series B.
The first indications of photosynthesis date back about 3.4 billion years; direct evidence in the form of thylakoid membranes of cyanobacteria dates back about 1.8 billion years. Between 1.2 and 1.6 billion years ago, cyanobacteria were captured by eukaryotic cells, leading to successful endosymbiosis that produced plant chloroplasts. In the mid-20th century, isolated chloroplasts were shown to retain photosynthetic activity for some time, although the mechanisms by which this occurs are not well understood. Since then, various scientific groups have attempted to incorporate these organelles into cells that are not typical for them, but they retained their morphology for only a few hours, and their photosynthetic activity in the new environment has not been confirmed.
Sachihiro Matsunaga of the University of Tokyo and colleagues isolated chloroplasts from the unicellular red alga Cyanidioschyzon merolae, which retains primitive features and lives in volcanic hot springs with highly acidic water. Its chloroplasts are active at temperatures below 37 degrees Celsius, retain 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 phonosynthetic activity and morphology even after six days of storage at four degrees Celsius.
The resulting chloroplasts were co-cultured with Chinese hamster ovary cells (Cricetulus griseus) of the immortalized CHO-K1 cell line, widely used in biotechnology, at a ratio of 100 to 1. On the same day, confocal microscopy showed that about 20 percent of the cells had captured 1–3 chloroplasts, and about one percent contained a large number (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 co-cultivation, the number of captured organelles in the cells decreased, probably due to either their intracellular digestion or random distribution between daughter cells during division.
Chloroplasts were located in intracellular vesicles circularly near the nucleus without penetrating it and were surrounded by mitochondria; native DNA was preserved in them. The authors of the work used fluorescence microscopy to identify these organelles by their chlorophyll content and scanning electron microscopy to study their membranes in detail. 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 of them, while in others it was partially deformed. After two days, the distance between the thylakoid membranes and the size of plastoglobuli (lipoprotein particles with plastoquinone 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 captured 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 right selection of chloroplasts and recipient cells, these organelles can maintain their structure and photosynthetic activity for at least two days after capture. Such a top-down approach by synthetic biology can serve as a basis for obtaining artificially photosynthetic animal cells, the authors of the work conclude.
Previously, Chinese researchers created nanostructures with chloroplast thylakoids and introduced 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 main transmembrane protein of the reaction center of purple bacteria.
