We have seen that Hydra possesses an insulin-receptor and that insulin obtained from cows has the ability to induce a cellular response in this simple animal. We have also seen that bovine insulin likewise can influence the development of plants.

This all suggests that the last common ancestor of both plants and animals could have responded to mammalian insulin. Yet front-loading would lead us to predict that the same theme would hold true among single-celled protozoa.

About two years ago, I suggested that Tetrahymena might represent a useful model for exploring front-loading. And sure enough, an insulin-like receptor was indeed identified in this organism in 2003 (Søren T. Christensen, Charles F. Guerra, Aashir Awan, Denys N. Wheatley and Peter Satir 2003. Insulin receptorlike proteins in Tetrahymena thermophila ciliary membranes Curr Biol. 13:R50-2).

The researchers used antibodies raised against the phosphotyrosine kinase domain of a human insulin receptor and identified a protein that reacted with the antibody. It is most interesting to me that the receptor localizes to the cilia of Tetrahymena, as this is an angle I will be exploring in a month or so.

They were then able to use PCR to identify the gene, which they named TtPTK1. It was determined that “the TtPTK1 kinase domain has ~30% identity and 65% homology to human, mouse and various invertebrate insulin receptor beta–subunit kinase domains.” In conclusion, the authors note:

Our observations suggest that cilia in Tetrahymena have signal transduction components resembling those of insulin receptor-like systems based on phosphotyrosine signaling. Our results agree with the notion that signaling through phosphorylation via PTKs is not a defining character of metazoan cells, as was previously proposed [13], but is present in unicellular eukaryotes.

This finding clearly shows that the RTKs are not restricted to animals and choanoflagellates and are likely to be found in many other protozoa. But an insulin receptor in a single-celled organism? Like Hydra and plants, might mammalian insulin influence the behavior of this humble protozoan?

We’ll consider this in the next installment.