by Dr. Stephen Mayfield
ver the past ten years, work from our lab has identified mechanisms of chloroplast gene expression that have allowed for development of recombinant protein expression and metabolic engineering in the algal chloroplast.
Transformation of algae is relatively easy. You can transform either the nuclear or the chloroplast genome. If you get DNA in, and you have a good selectable marker and a good selection system, you can get transformation.
There are rather complex structures that fold into three-dimensional RNA elements that are bound by protein factors, and that is a requirement for translation. We still haven’t sorted this all out, but we’ve identified a number of elements that are required, and a number of proteins that interact in order to get translation.
Having accumulated these proteins and showing that they were bioactive, we went back to ask, “What’s the advantage of expressing something inside of a chloroplast, or inside of an algae? What biological advantages does that give you over expressing the protein in a bacterial or a mammalian cell culture?”
So one of the things that the lab came up with was to try and express malarial proteins. And the reason we wanted to express these was because malarial proteins have many different domains inside of them, folded in very complex proteins. Malaria is a euchariotic parasite, and their proteins form complex structures that have many disulfite bonds, but the proteins are not glycosolated.
That’s important because when you try to express these proteins in bacterial systems, they are incapable of doing the complex fold and they won’t form disulfate bonds. If you’re trying to express these in mammalian systems, they’ll form the disulfite bonds and correctly fold them, but then they decorate the proteins with sugar—they glycosolate them—so that if you use these as a vaccine, you end up getting antibodies to the sugars rather than the proteins.
So, we knew that inside of chloroplasts—inside of all plastids—we could fold complex proteins. We could make disulfate bonds. But we also knew there was no mechanism to glycosolate these. So we expressed three different surface antigens, PFS-25, 28 and 45. All of those proteins accumulate very well inside the chloroplast. Importantly, they all fold correctly. So then, antibodies directed against native proteins, which only recognize the correctly folded native proteins, also recognized the algal-expressed proteins.
Most importantly, when we injected these proteins into mice, the mice-generated antibodies recognized the correctly folded proteins, and we had an immune response. Those antibodies blocked malaria transmission within the mice.
It’s important to understand, for something like malaria, that most recombinant vaccines today cost about a hundred dollars a dose, and generally you need two or three injections. So clearly for the people in the malaria belt—and there are about two billion people on this planet in the malaria belt—they simply do not have the resources to even think about spending two or three hundred dollars for a vaccine.
I think these developments using algal proteins are beginning to give us the opportunity to make those vaccines cheap enough that we can think about the real possibility of inoculating two billion people.
A co-founder of the San Diego Center for Algae Biotechnology (SD-CAB), and Sapphire Energy, Dr. Mayfield is Professor of Molecular Biology, and the John Dove Isaacs Chair of Natural Philosophy, at UC San Diego.