Biotechnology has dramatically increased the supply of many pharmaceuticals and other products for human consumption, especially thanks to the technique of heterologous expression, which seeks to express genes in a “heterologous system”, a different organism from the one that originally produced the compound.

Perhaps the greatest example is human insulin, a polypeptide hormone that is industrially produced by a variety of different microbes. Modified E. coli is typically used as the expression system, although fungi and other bacteria are also widely used. This has been a safe and globally adapted practice for years, although it is perhaps not too well known among the general public.

Of course, it is also used to manufacture animal proteins and enzymes for human consumption. Bovine chymosin – the enzyme that coagulates cheese – is just one of the products best implemented in this production model. In the past, cheese was traditionally made by slaughtering a suckling calf and treating one of its stomachs, the abomasum, to extract these enzymes from the chyme. This was, frankly, a reaction as inefficient as it is unnecessary today, as there are cheaper, cleaner, and more ethical alternatives using synthetic biology.

Why microorganisms?

It is possible to think of heterologous expression systems different from bacteria and filamentous fungi. Indeed, it is common practice in research laboratories to use murine systems to test drugs. However, when it comes to production yield, heterologous systems in microorganisms have many advantages at the moment.

One notable advantage is the growth capacity of a microbial culture versus a eukaryotic culture. Transformation is also easier in bacteria/fungi. This is because they are simpler organisms (simplicity being understood as more easily achievable homeostasis since they are composed of fewer elements). This, in turn, means that the cultures are also more stable and controllable because they have a large number of individuals in them, unlike other expression systems.

Other expression systems

After listing all the advantages of a microbial system, one must also understand its limitations. The truth is that each microorganism is different, and both its production rate and tolerance to the protein produced vary greatly from one to another; even from one environmental circumstance to another. To this must be added that they are expressing not native proteins to their genome, so they can interact with a multitude of metabolic pathways that are even unknown. Finally, the greatest disadvantages are usually in extracting and isolating the gene product. Bacteria often make intracytoplasmic aggregates, which are useless because the catalytic sites are lost in the case of enzymes. Filamentous fungi are good secreters, but they also make secondary metabolites called mycotoxins that can contaminate the medium, and extra purification is necessary. There are many similar cases depending on the species and the type of protein to be extracted.

Thus, this industry has room for competition to emerge in very original ways. Animal proteins can be produced in living multicellular organisms, as mentioned above, but in an industrial way and without affecting their life cycle. It is a matter of designing a sequence that codes for our protein and adding accessory elements to it to ensure its functioning and safety. These can be of various types, such as an inducible promoter that ensures gene expression when required, a “recombination sign” so that the sequence enters a specific site of the heterologous genome and does not interrupt any vital function, or even unnatural domains; regions added to the protein that is recognized by endogenous transporters and thus accumulate the protein in a specific compartment. Of course, this must also be done in microbial systems, but being unicellular entails to do not have structures designed to accumulate substances.

For example, cows can accumulate desired proteins in the mammary gland, where they dissolve with the milk. On the other hand, we can use the immobilization of a gene product in vegetable fruits, taking advantage of the willingness of these organisms to accumulate reserve substances in them. As we can see, these heterologous expression systems also have many advantages, since they can combine the normal production of fruits or milk with the extraction of the second protein of interest for human consumption.

Current Research & Start-ups

In terms of recombinant DNA, we have developed very efficient ways to maintain the desired heterologous expression with a variety of techniques using plasmids, or yeast artificial chromosomes, among many other ways. We also understand the differences between different expression systems quite well and have been able to develop very safe for consumption. However, the options for this technology are almost endless, as we require protein products with ever greater specificity and abundance, so there are always new and interesting initiatives on the market.

A good catalog is what Shiru is doing, as it uses artificial intelligence to develop new non-natural proteins that have interesting or positive properties for food. Another way to use microorganisms to produce a protein product without requiring heterologous expression is The Protein Brewery, which uses the fermentation of a fungus.

The Future

Although this technology has dominated the world of isolated protein production for years, it will continue to evolve. New advances in proteomics will allow higher production yields and gradually this type of biotechnological application should colonize new market niches that remain unexplored. Certainly, the sustainability of our civilization will be closer if we cede a good part of food production to microbes since their growth is ubiquitous, inexpensive, and allows the renewal of natural cycles.