Producing lab-grown meat – made with animal cells grown in bioreactors – is a promising avenue for sustainable meat production. However, scaling up this process to produce tons of meat at a reasonable cost is going to be difficult. One of the hurdles in the scaling process is producing the large quantities of growth factors (which includes cytokines – see previous blog article) required for culturing muscle and other cells.
A 2020 analysis indicated that 55-95% of the cost of cultured meat production could be attributed to the cost of growth media for cells. Drilling down, 99% of that cost was accounted for by growth factors, principally TGF-β. In 2013, when the first lab-grown hamburger was produced, it took nearly two years and cost approximately $375,000. Current cost estimates vary between $10-50 per pound ($11-$55 per kg) or €9 for a burger. A lot more needs to be done to make this competitive with conventional meat.
Broadly speaking, techniques used for the production of growth factors fall into two categories: bioproduction or cell-free synthesis. Bioproduction methods use either bioreactors or in vivo production (e.g. plants, insects) to produce the growth factors. In contrast, synthetic production methods are cell-free and do not require a host – whether a cell or an organism. Large-scale synthetic growth factor production methods are in their infancy and the majority of efforts for producing growth factors use bioproduction.
The expression hosts for the bioproduction of growth factors can include yeast, bacteria, mammalian cells, insect cells, transgenic plants and whole insects. For a growth factor to have bioactivity, the proper folding and post-translational modifications such as glycosylation, phosphorylation, or disulfide bonding are essential to obtain bioactive, functional proteins. The ease with which a bioactive protein can be made is highly dependent on the individual growth factor. This is an important consideration in determining growth factor pricing. Growth factors which are more difficult to make, such as BMP-4 and Wnt-3a, tend to be more expensive.
TGF-β1, widely used in muscle cell culture, requires post-translational disulfide bonding for proper folding. As well as functionality, glycosylation can affect serum half-life and other characteristics of growth factors. Each production cell host has its own pros and cons. Reproducible glycosylation patterns are the most difficult to attain, since glycosylation profiles in plants, yeast, and bacteria differ from mammalian glycosylation patterns, although modifications to expression hosts have helped with this endeavour. CHO (Chinese hamster ovary) cells are the most widely used host for producing TGF-β1 and CHO-derived TGF-β1 is available from a variety of companies, though the cost remains prohibitively high for meat production.
The Good Food Institute released a report in 2020 reporting current costs for producing lab-grown meat and outlining scenarios to lower the cost of growth factors – particularly TGF-β and FGF2 – needed for culture media, which included lowering the required amount, sourcing from different brands, and switching to slightly more affordable E. coli-derived TGF-β3 instead of the CHO-derived TGF-β1. They suggest that, in future, it should be possible to produce all the growth factors for $4 per gram of growth factor compared to the millions per gram it currently costs for TGF-β and FGF2 (based on microgram scale catalog pricing). The $4/gram target was based solely on the cost of other growth factors and it was unclear how they would achieve this price point using standard sourcing and production methods. A more recent report from the Good Food Institute outlined scenarios for reducing the total cost of lab-grown meat products to make them comparable with conventional meat. The absolute lowest cost of producing growth factors may be as little as $0.10 per gram. This is the cost at which industrial-scale enzymes used in washing detergents are produced.
Moving away from cell culture for factor production, there are many unique bioproduction methods using transgenic animals, plants, and insects. Examples include chicken bioreactors to produce growth factors in egg whites, TGF-β produced in silkworm cocoons, transgenic barley plants, and transgenic fruit flies to produce FGF2.
Orf Genetics has generated transgenic barley plants to produce a variety of human and animal cytokines and growth factors. They have a wide variety of cytokines and growth factors including IL-6, FGF, EGF, TNFα, and IFN-γ. Future Fields, a Canadian company, uses transgenic drosophila melanogaster (fruit flies) to produce FGF2 and transferrin growth factors. Though currently only available for research purposes, both companies aim to be a scalable source of growth factors for the lab-grown meat market.
One advantage of cell-free production options is that cell viability does not need to be retained and no transgenic organism needs to be cared for. However, reproducing functional growth factors and cytokines in vitro can prove difficult.
One promising in vitro option is e. coli based open cell-free synthesis (OCFS). This platform mimicks the cytoplasm of E. coli – indeed it relies on extracts isolated from E. coli cultures. Their system builds on the previous Cytomim cell-free protein synthesis platform. Using OCFS, they were able to produce large-scale quantities of the cytokine granulocyte-macrophage colony-stimulating factor (rhGM-CSF). Importantly, like TGF-β, rhGM-CSF has multiple post-translational disulfide bonds. Thus, OCFS may be a great option for producing TGF-β at scale for biomanufacturing lab-grown meat.
While at least one company has announced the large-scale chemical synthesis of cytokines like IL-10, there has been little to no coverage or scientific literature to verify this. In comparison to the cell-free Cytomim and OCFS platforms described above, that still require extracts from cells, reconstituted systems rely on purified components that are truly independent of any cell culture. These systems are more costly, and to date, there has not been any described use of reconstituted systems for cytokine production.
Stabilizing growth factors and reconfiguring culture systems
As well as producing growth factors more cheaply, another way to reduce growth factor costs in meat production is to make better growth factors and use these more efficiently. Growth factors are unstable molecules with half-lives that vary based on the environment. These range from a few minutes (e.g. IL-2 in circulating blood) to days (IL-6). Stabilized versions of growth factors such as IGF-1 and FGF-2 have been produced by modifying their amino acid sequence. Increased stability reduces the need to replenish, thereby reducing costs.
Growth factor concentrations needed in-vitro are much higher than in-vivo levels. Could amounts be reduced? To some extent, the high in-vitro concentrations are required to offset the troughs of growth factor availability that occur as growth factor proteins degrade in in vitro culture systems. Growth factor depot formulations such as Cell Guidance Systems’ PODS address this issue by slowly releasing growth factors. This continuous replenishment from the PODS crystal eliminates the need for high starting levels and leads to more consistent growth factor levels within a culture system. Moreover, PODS can be incorporated into microcarriers to reduce the amount of growth factor needed in a culture system making them available just in the 1% of the culture system where the cells are growing.
The scale of biomanufacturing envisioned for commercial cultured meat production is orders of magnitude above anything that has been contemplated previously in cell culture. Growth factor cost is a major hurdle for this nascent cultured meat industry. The level of innovation seen in this area and previous experience in producing industrial-scale quantities of enzymes for other consumer products suggest that these goals will be met.
IMAGE Sausages - creative commons
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