From farm to pharm: taking plant pharmaceuticals from research to production

Abstract

Affordable production of pharmaceuticals with high potency and low side effects is a major challenge of the 21st century. Peptides are an emerging class of therapeutics that have the potential to marry the specificity and efficacy of protein drugs with the stability and membrane permeability of small molecule drugs. Although many peptides are amenable to chemical synthesis, their cost of production is high, as is the generation of waste products. Peptide production in plants has the potential to be a scalable, cost effective, and a less environmentally taxing alternative. Cyclotides, first discovered in Oldenlandia affinis, are a unique class of backbone cyclic peptides containing three stabilising disulfide bridges that form a knot-like structure. Their stability, and for some variants, the ability to traverse cellular membranes make them ideal candidates for pharmaceutical and agricultural applications. Even though highly constrained sterically, cyclotides are amenable to engineering by replacing native sequences with bioactive epitopes. In this thesis, cyclotide production strategies in plant cell suspensions are examined with a special focus placed on O. affinis as an archetypical cyclotide producer. Additional species investigated are Clitoria ternatea, Hybanthus enneaspermus, Nicotiana benthamiana, and Petunia hybrida. Insights into how cyclotides and their biosynthetic processing machinery are regulated in suspension plant cells are reported and provide first steps towards an affordable and environmentally friendly peptide production system in plants.Chapter 1 introduces the relevant scientific knowledge. The journey starts with a description of proteins and peptides, emphasising cyclotides and their synthesis. Then, current production strategies are compared with plant-based systems. The advantages and disadvantages of bioreactor setups suitable for medium to large scale production of cyclotides in plant cell suspensions are subsequently outlined. Finally, the importance of downstream processing is discussed. At the end of Chapter 1, the scope of this thesis is outlined to give an overview of the scientific questions addressed.Chapter 2 lays the experimental groundwork upon which the rest of this thesis is built. Protocols to establish C. ternatea, H. enneaspermus, N. benthamiana, O. affinis, and P. hybrida suspension cultures were developed and their cyclotide accumulation profiles presented. Cycloviolacin O2, a cyclotide previously reported only in the Violaceae, was discovered in O. affinis suspension cells and corroborated in the O. affinis leaf transcriptome. Two new cyclotides, kalata B22 and kalata B23, were characterized from O. affinis suspensions and were shown to have high sequence similarity to a group of cyclotides found in suspension cells of multiple plant families. Additionally, a strong time dependent effect was observed for cyclotide production in O. affinis suspension cells and was connected to reduced cyclotide mRNA expression. Cyclotide production was tissue specific in all tested species, an outcome with potential consequences for large scale production.Chapter 3 presents the scale-up of plant cell suspension cultures for cyclotide production. C. ternatea, H. enneaspermus, and O. affinis suspensions were cultivated in a rocking motion bioreactor with culture volumes up to 5 L. Growth characteristics and biomass yields of all species were promising; however, contamination, sampling and consequential downregulation of cyclotides in suspension cultures presented both challenges and opportunities which are discussed in depth.Chapter 4 investigates several elicitors for boosting cyclotide production in plant cell suspensions. O. affinis, C. ternatea, and H. enneaspermus cultures were either immobilized or treated with methyl jasmonate or sodium chloride. Cyclotide production in C. ternatea and H. enneaspermus could not be enhanced, but their base level was already high. Cyclotide production in O. affinis suspension cells was low but was enhanced by treatment with 3 mM sodium chloride and by cell immobilization.Chapter 5 explores Agrobacterium-mediated transformation of plant suspension cells for production of natural and engineered cyclotides. The first reported transformation of O. affinis was achieved in plant cell packs, which is a stacked filter cake of suspension cells devoid of liquid medium. Several parameters necessary for transformation of plant cell packs were identified, but the observed transformation rates varied greatly. Still, the insights gained will prove useful for research of cyclotide biosynthesis and production in O. affinis.Chapter 6 presents four protocols tested to enable cell cryo-banking of O. affinis, a feat necessary for large scale production in engineered cell lines. No positive results were observed, but future attempts might find this work a useful staring point.Chapter 7 summarizes the results, focused on plant cell suspensions and cyclotide production respectively, whilst they are put in context and possible research paths going forward are discussed. In conclusion, new species were brought into suspension, new cyclotides were discovered, new cyclotide production methods were explored and the first transformation of O. affinis is reported. The application of knowledge generated in this thesis by analysing cell suspension of cyclotide producing plants could lead to affordable and flexible production of pharmaceutical peptides in plants.