Communication between cells takes many forms. There could be communication by direct contact, by sending out free molecules (like hormones) or by special structures (e.g. synapses).
But how can parasites, that dwell inside their host cell, communicate with one another?
A very elegant mechanism used by Malaria parasites was found, and is described in a recent Cell paper (actually, it was published in the same issue as my paper).
Malaria parasites (Plasmodium falciparum) are transferred from mosquitoes to humans, where they infect red blood cells (RBCs). Once inside the RBCs, the parasite need to sexually differentiate into sexual forms that are competent for transmission by the next mosquito.
When trying to understand which signals are transferred between the parasites that dwell in different RBCs, they mixed two cultures, each expresses a different drug resistant gene and a different fluorescent protein. Surprisingly, the mixed cultures survived when both drugs were added, and parasite cells exhibited both colors.
Further analysis showed that the parasites send out tiny vesicles (their size is ~70 nm). These vesicles are similar to endosome-derived “exosomes” ,and therefor are referred in the paper as “exosome-like” vesicles. In most papers that study exosomes, they are visualized by electron microscopy. However, in this paper, Atomic Force Microscopy (AFM) was employed. AFM resolution is ~1000-fold better than light microscopy (yet lower than electron microscopy). In essence, AFM uses a tiny cantilever with a very sharp tip that travels over the sample. The tip is deflected from the sample based on forces exerted from the surface (e.g. mechanical contact force, electrostatic forces, magnetic forces, Van der Waals force). The deflection is registered by a laser light.
Principle of AFM. Source: Wikipedia
These vesicles supposedly contain the plasmid DNA that enables the lateral inheritance of the new characteristic (drug resistance & fluorescent protein).
Malaria parasites-derived “exosome-like” vesicles as seen by Atomic Force Microscopy. Upper row: fraction that does not contain the vesicles. Lower row: fraction that does contain vesicles. Source: Regev-Rudzki et al. Cell 153(5): 1120-1133
The authors put a lot of effort in proving that the information – DNA plasmid – is transferred via these vesicles. They perform DNA FISH and PCR to show that the “acceptor” parasites contain these genes. Alas, they never show that the vesicles that they isolated also contain this DNA. This should have been simple to do: they already have the isolated exosomes and just need to do PCR on them. I do not know why this was not requested by the reviewers.
Their last figure, which is intended to give a broader biological meaning to their findings, suggests that this form of communication is required for production of gametocytes (a sexual differentiation step required for intake by the mosquito). They show that the gametocytes contain both fluorescent markers, and are produced at greater numbers when parasites are co-cultured.
Malaria gametocytes from co-cultures express both fluorescent proteins. Source: Regev-Rudzki et al. Cell 153(5): 1120-1133.
However, they do not show that application of the isolated vesicles can induce this sexual differentiation. More so, the exosomes may contain other factors (proteins, RNAs) that can induce sexual differentiation, and be unrelated to the DNA transfer observed.
In any case, an interesting paper, that may have major clinical applications in the future.
Regev-Rudzki N, Wilson DW, Carvalho TG, Sisquella X, Coleman BM, Rug M, Bursac D, Angrisano F, Gee M, Hill AF, Baum J, & Cowman AF (2013). Cell-Cell Communication between Malaria-Infected Red Blood Cells via Exosome-like Vesicles. Cell, 153 (5), 1120-33 PMID: 23683579