Susan C. Roberts

Recent advances towards development and commercialization of plant cell culture processes for the synthesis of biomolecules

Plant cell culture systems were initially explored for use in commercial synthesis of several high-value secondary metabolites, allowing for sustainable production that was not limited by the low yields associated with natural harvest or the high cost associated with complex chemical synthesis. Although there have been some commercial successes, most notably paclitaxel production from Taxus sp., process limitations exist with regards to low product yields and inherent production variability. A variety of strategies are being developed to overcome these limitations including elicitation, in situ product removal and metabolic engineering with single genes and transcription factors. Recently, the plant cell culture production platform has been extended to pharmaceutically active heterologous proteins. Plant systems are beneficial because they are able to produce complex proteins that are properly glycosylated, folded and assembled without the risk of contamination by toxins that are associated with mammalian or microbial production systems. Additionally, plant cell culture isolates transgenic material from the environment, allows for more controllable conditions over field-grown crops and promotes secretion of proteins to the medium, reducing downstream purification costs. Despite these benefits, the increase in cost of heterologous protein synthesis in plant cell culture as opposed to field-grown crops is significant and therefore processes must be optimized with regard to maximizing secretion and enhancing protein stability in the cell culture media. This review discusses recent advancements in plant cell culture processing technology, focusing on progress towards overcoming the problems associated with commercialization of these production systems and highlighting recent commercial successes.

Effect of mammalian cell culture medium on the gelation properties of Pluronic F127.

We investigate the gelation of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer, Pluronic F127, in mammalian cell culture medium for applications in tissue engineering and cell encapsulation. In both minimum essential medium (MEM) and MEM with added fetal bovine serum (MEM-FBS), the gel-phase boundary shifts to lower temperatures and concentrations as compared to pure water. The thermodynamics of gel formation are similar in MEM, MEM-FBS, and pure water, suggesting that the mechanism of gelation is similar in all three solvents. The shift of the sol-gel boundary to lower concentrations is particularly significant for development of cell encapsulation protocols using Pluronics and applications where copolymer concentration must be minimized due to toxicity concerns.

Hydrogel‐Perfluorocarbon Composite Scaffold Promotes Oxygen Transport to Immobilized Cells

Cell encapsulation provides cells a three‐dimensional structure to mimic physiological conditions and improve cell signaling, proliferation, and tissue organization as compared to monolayer culture. Encapsulation devices often encounter poor mass transport, especially for oxygen, where critical dissolved levels must be met to ensure both cell survival and functionality. To enhance oxygen transport, we utilized perfluorocarbon (PFC) oxygen vectors, specifically perfluorooctyl bromide (PFOB) immobilized in an alginate matrix. Metabolic activity of HepG2 liver cells encapsulated in 1% alginate/10% PFOB composite system was 47–104% higher than alginate systems lacking PFOB. A cubic model was developed to understand the oxygen transport mechanism in the alginate/PFOB composite system. The theoretical flux enhancement in alginate systems containing 10% PFOB was 18% higher than in alginate‐only systems. Oxygen uptake rates (OURs) of HepG2 cells were enhanced with 10% PFOB addition under both 20% and 5% O2 boundary conditions, by 8% and 15%, respectively. Model predictions were qualitatively and quantitatively verified with direct experimental OUR measurements using both a perfusion reactor and oxygen sensing plate, demonstrating a greater OUR enhancement under physiological O2 boundary conditions (i.e., 5% O2). Inclusion of PFCs in an encapsulation matrix is a useful strategy for overcoming oxygen limitations and ensuring cell viability and functionality both for large devices (>1 mm) and over extended time periods. Although our results specifically indicate positive enhancements in metabolic activity using the model HepG2 liver system encapsulated in alginate, PFCs could be useful for improving/stabilizing oxygen supply in a wide range of cell types and hydrogels.

Advancements in the understanding of Paclitaxel metabolism in tissue culture.

Paclitaxel is a potent chemotherapeutic agent approved in the treatment of a variety of cancers, and under evaluation for the treatment of Alzheimers and heart disease. Originally isolated from Taxus brevifolia, this highly substituted ring diterpenoid belongs to a family of plant secondary metabolites known as taxoids. Paclitaxel is currently supplied through both a semi-synthetic process and plant cell culture. Taxus spp. cell culture offers the potential to produce large amounts of paclitaxel and related taxoids, although variability in accumulation and low yields represent key limitations. Thus, intense efforts have been put forth towards understanding Taxus spp. metabolism to increase paclitaxel accumulation in cell culture. While elicitation and environmental optimization have provided some success in increasing paclitaxel accumulation in vitro, understanding metabolism of paclitaxel on the molecular level is essential for process optimization. Utilizing direct and indirect molecular techniques, a further understanding of paclitaxel biosynthesis has been gained, though knowledge into other aspects of paclitaxel global metabolism, such as regulation, transport, and degradation is lacking. Taxus spp. cell cultures are highly heterogeneous, displaying significant cell-cell variability in growth and paclitaxel accumulation. Information gathered on culture subpopulations as well as putative transcriptional bottlenecks in paclitaxel biosynthesis, coupled with successful transformation of Taxus spp. will allow for the targeted metabolic engineering of Taxus spp. or other model organisms for paclitaxel accumulation to ensure future supply of this important pharmaceutical.

A simple method for enhancing paclitaxel release from Taxus canadensis cell suspension cultures utilizing cell wall digesting enzymes

Paclitaxel storage in Taxus suspension cell cultures was studied through the simple use of cell wall digesting enzymes. The application of cellulase (1%) and pectolyase (0.1%) to Taxus canadensis suspension cultures induced a significant increase in the paclitaxel present in the extracellular medium while maintaining membrane integrity, suggesting that paclitaxel is stored in the cell wall. The addition of cell wall digesting enzymes to a cell culture bioprocess may be an effective way of enhancing paclitaxel release to the extracellular medium and hence simplify product recovery.

Flow Cytometric Identification of Paclitaxel-Accumulating Subpopulations

An immunofluorescence procedure was developed for paclitaxel quantification at the single cell level via flow cytometry in Taxus cuspidata suspension cultures. Intracellular staining was validated via fluorescence microscopy. Paclitaxel content of isolated cells and protoplasts was compared to total paclitaxel levels measured via HPLC. Paclitaxel accumulation was significantly increased by elicitation with methyl jasmonate (100 μM) on day 7 post‐transfer as compared to unelicited cultures. Maximum accumulation was observed by day 12 post‐transfer in both total paclitaxel (∼0.25 mg/L) and the percentage of paclitaxel‐accumulating cells (∼95%). A similar trend was observed with isolated protoplasts, although protoplasts accumulated only ca. 40–75% of the paclitaxel present in single cells. In unelicited cell cultures, a small subpopulation (ca. 3–5%) of single cells was shown to accumulate paclitaxel. Although nearly all cells were observed to accumulate paclitaxel in methyl jasmonate‐elicited cell cultures, a high degree of cell‐to‐cell variation was observed in paclitaxel content. The identified subpopulations represent targets for cell sorting, which may be applied to develop higher‐accumulating cell lines. The quantification of single cell paclitaxel content is useful for characterizing production variability in cell cultures and can be utilized to develop rational strategies to increase paclitaxel production.

Application of Colorimetric Assays to Assess Viability, Growth and Metabolism of Hydrogel-Encapsulated Cells

The applicability of the colorimetric 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assays to measure cell growth and viability in hydrogel encapsulation systems was investigated using HepG2 liver cells encapsulated in alginate matrices. The MTT assay was effective in measuring viable cell density in alginate-encapsulated cell systems, demonstrating less variance and higher throughput capability than hemocytometry. The LDH assay was effective in measuring dead cell density in monolayer cultures and in alginate-encapsulated cells simply by measuring the LDH concentration secreted into the medium. Further validation of these assays was shown in two additional cell lines (rat muscle and mouse embryonic fibroblasts). The MTT and LDH assays are particularly significant in the rapid evaluation of in vitro cell encapsulation device design.

Identification and expression analysis of methyl jasmonate responsive ESTs in paclitaxel producing Taxus cuspidata suspension culture cells.

BackgroundTaxol® (paclitaxel) promotes microtubule assembly and stabilization and therefore is a potent chemotherapeutic agent against wide range of cancers. Methyl jasmonate (MJ) elicited Taxus cell cultures provide a sustainable option to meet the growing market demand for paclitaxel. Despite its increasing pharmaceutical importance, the molecular genetics of paclitaxel biosynthesis is not fully elucidated. This study focuses on identification of MJ responsive transcripts in cultured Taxus cells using PCR-based suppression subtractive hybridization (SSH) to identify genes involved in global pathway control.ResultsSix separate SSH cDNA libraries of paclitaxel-accumulating Taxus cuspidata P991 cell lines were constructed at three different post-elicitation time points (6h, 18h and 5 day) to identify genes that are either induced or suppressed in response to MJ. Sequencing of 576 differentially screened clones from the SSH libraries resulted in 331 unigenes. Functional annotation and Gene Ontology (GO) analysis of up-regulated EST libraries showed enrichment of several known paclitaxel biosynthetic genes and novel transcripts that may be involved in MJ-signaling, taxane transport, or taxane degradation. Macroarray analysis of these identified genes unravelled global regulatory expression of these transcripts. Semi-quantitative RT-PCR analysis of a set of 12 candidate genes further confirmed the MJ-induced gene expression in a high paclitaxel accumulating Taxus cuspidata P93AF cell line.ConclusionsThis study elucidates the global temporal expression kinetics of MJ responsive genes in Taxus suspension cell culture. Functional characterization of the novel genes identified in this study will further enhance the understanding of paclitaxel biosynthesis, taxane transport and degradation.

Contribution of taxane biosynthetic pathway gene expression to observed variability in paclitaxel accumulation in Taxus suspension cultures

Variability in product accumulation is one of the major obstacles limiting the widespread commercialization of plant cell culture technology to supply natural product pharmaceuticals. Despite extensive process engineering efforts, which have led to increased yields, plant cells exhibit variability in productivity that is poorly understood. Elicitation of Taxus cultures with methyl jasmonate (MeJA) induces paclitaxel accumulation, but to varying extents in different cultures. In the current study, cultures with different aggregation profiles were established to create predictable differences in paclitaxel accumulation upon MeJA elicitation. Expression of known paclitaxel biosynthetic genes in MeJA‐elicited cultures exhibiting both substantial (15‐fold) and moderate (2‐fold) differences in paclitaxel accumulation was analyzed using quantitative reverse transcriptase PCR. Each population exhibited the characteristic large increase in paclitaxel pathway gene expression following MeJA elicitation; however, differences in expression between populations were minor, and only observed for the cultures with the 15‐fold variation in paclitaxel content. These data suggest that although upregulation of biosynthetic pathway gene expression contributes to observed increases in paclitaxel synthesis upon elicitation with MeJA, there are additional factors that need to be uncovered before paclitaxel productivity can be fully optimized.

Plant natural products from cultured multipotent cells

Cultured cambial meristematic cells could enable large-scale production of certain natural products.