Analysis of cell cycle activity and population dynamics in heterogeneous plant cell suspensions using flow cytometry

Two parametric cell cycle analyses of plant cell suspension cultures with fragile, isolated nuclei to investigate heterogeneity in growth of batch cultivations

Plant cell suspensions are frequently considered to be heterogeneous with respect to growth in terms of progression of the cells through the cell cycle and biomass accumulation. Thus, segregated data of fractions in different cycle phases during cultivation is needed to develop robust production processes. Bromodeoxyuridine (BrdU) incorporation and BrdU-antibodies or 5-ethynyl-2'-deoxyuridine (EdU) click-it chemistry are frequently used to acquire such information. However, their use requires centrifugation steps that cannot be readily applied to sensitive cells, particularly if nuclei have to be extracted from the protective cellular milieu and envelopes for DNA analysis. Therefore, we have established a BrdU-Hoechst stain quenching protocol for analyzing nuclei directly isolated from delicate plant cell suspension cultures. After adding BrdU to test Harpagophytum procumbens cell suspension cultures the cell cycle distribution could be adequately resolved using its incorporation for the following 72 h (after which BrdU slowed biomass accumulation). Despite this limitation, the protocol allows resolution of the cell cycle distribution of cultures that cannot be analyzed using commonly applied methods due to the cells' fragility. The presented protocol enabled analysis of cycling heterogeneities in H. procumbens batch cultivations, and thus should facilitate process control of secondary metabolite production from fragile plant in vitro cultures. Biotechnol. Bioeng. 2016;113: 1244-1250. © 2015 Wiley Periodicals, Inc.

Methyl jasmonate represses growth and affects cell cycle progression in cultured Taxus cells

Methyl jasmonate elicitation of Taxus cultures enhances paclitaxel accumulation, but represses growth by inhibition of cell cycle progression. Growth repression is evident both at the culture level and transcriptional level. Methyl jasmonate (MeJA) elicitation is an effective strategy to induce and enhance synthesis of the anticancer agent paclitaxel (Taxol(®)) in Taxus cell suspension cultures; however, concurrent decreases in growth are often observed, which is problematic for large-scale bioprocessing. Here, increased accumulation of paclitaxel in Taxus cuspidata suspension cultures with MeJA elicitation was accompanied by a concomitant decrease in cell growth, evident within the first 3 days post-elicitation. Both MeJA-elicited and mock-elicited cultures exhibited similar viability with no apoptosis up to day 16 and day 24 of the cell culture period, respectively, suggesting that growth repression is not attributable to cell death. Flow cytometric analyses demonstrated that MeJA perturbed cell cycle progression of asynchronously dividing Taxus cells. MeJA slowed down cell cycle progression, impaired the G1/S transition as observed by an increase in G0/G1 phase cells, and decreased the number of actively dividing cells. Through a combination of deep sequencing and gene expression analyses, the expression status of Taxus cell cycle-associated genes correlated with observations at the culture level. Results from this study provide valuable insight into the mechanisms governing MeJA perception and subsequent events leading to repression of Taxus cell growth.

Cellular aggregation is a key parameter associated with long term variability in paclitaxel accumulation in Taxus suspension cultures

Plant cell cultures provide a renewable source for synthesis and supply of commercially valuable plant-derived products, particularly for secondary metabolites. However, instability in product yields over multiple passages has hampered the efficient and sustainable use of this technology. Paclitaxel accumulation in Taxus cell suspension culture was quantified over multiple passages and correlated to mean aggregate size, extracellular sugar level, ploidy, and cell cycle distribution. Paclitaxel levels varied approximately 6.9-fold over the six-month timeframe investigated. Of all of the parameters examined, only mean aggregate size correlated with paclitaxel accumulation, where a significant negative correlation (r = - 0.75, p < 0.01) was observed. These results demonstrate the relevance of measuring, and potentially controlling, aggregate size during long term culture passages, particularly for plant suspensions where industrially relevant secondary metabolites are not pigmented to enable rapid culture selection.

Analysis of aggregate size as a process variable affecting paclitaxel accumulation in Taxus suspension cultures

Plant cell aggregates have long been implicated in affecting cellular metabolism in suspension culture, yet the rigorous characterization of aggregate size as a process variable and its effect on bioprocess performance has not been demonstrated. Aggregate fractionation and analysis of biomass-associated product is commonly used to assess the effect of aggregation, but we establish that this method is flawed under certain conditions and does not necessarily agree with comprehensive studies of total culture performance. Leveraging recent advances to routinely measure aggregate size distributions, we developed a simple method to manipulate aggregate size and evaluate its effect on the culture as a whole, and found that Taxus suspension cultures with smaller aggregates produced significantly more paclitaxel than cultures with larger aggregates in two cell lines over a range of aggregate sizes, and where biomass accumulation was equivalent before elicitation with methyl jasmonate. Taxus cuspidata (T. cuspidata) P93AF cultures with mean aggregate sizes of 690 and 1,100 μm produced 22 and 11 mg/L paclitaxel, respectively, a twofold increase for smaller aggregates, and T. cuspidata P991 cultures with mean aggregate sizes of 400 and 840 μm produced 6 and 0.3 mg/L paclitaxel, respectively, an increase of 20-fold for smaller aggregates. These results demonstrate the importance of validating experiments aimed at a specific phenomenon with total process studies, and provide a basis for treating aggregate size as a targeted process variable for rational control strategies.

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 microM) on day 7 post-transfer as compared to unelicited cultures. Maximum accumulation was observed by day 12 post-transfer in both total paclitaxel (approximately 0.25 mg/L) and the percentage of paclitaxel-accumulating cells (approximately 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.

Cell cycle analysis ofTaxus suspension cultures at the single cell level as an indicator of culture heterogeneity

Single cell growth and division was measured via flow cytometry in order to characterize the metabolic variability of Taxus cuspidata suspension cultures, which produce the valuable secondary metabolite Taxol. Good agreement was observed between the cell cycle distribution and biomass accumulation over the batch culture period. Specific growth rates of 0.13 days(-1) by fresh weight and 0.15 days(-1) by dry weight were measured. Elicitation with methyl jasmonate (MJ) significantly decreased both cell cycle progression and biomass accumulation, as the specific growth rate decreased to 0.027 days(-1) by fresh and dry weight. Despite the decrease in biomass accumulation for MJ elicited cultures, sucrose utilization was not significantly different from control cultures. MJ elicitation also increased the accumulation of paclitaxel and other taxanes. The accumulation of upstream taxanes (baccatin III and 10-deactylbaccatin III) increased during exponential growth, reached a maximum around day 12, and then declined throughout the stationary phase. The paclitaxel concentration increased during both exponential growth and stationary phase, reaching a maximum around days 20-25. Throughout the culture period, greater than 70% of the cells were in G(0)/G(1) phase of the cell cycle. Studies using bromodeoxyuridine (BrdU) incorporation showed that approximately 65% of the Taxus cells are noncycling, even during exponential growth. Although the role of these cells is currently unknown, the presence of a large, noncycling subpopulation can have a significant impact on the utilization of plant cell culture technology for the large-scale production of paclitaxel. These results demonstrate that there is a high degree of metabolic heterogeneity in Taxus cuspidata suspension cultures. Understanding this heterogeneity is important for the optimization of plant cell cultures, particularly the reduction of production variability.

Biochemical engineering of the production of plant-specific secondary metabolites by cell suspension cultures

Plant cell culture has recently received much attention as a useful technology for the production of valuable plant-derived secondary metabolites such as paclitaxel and ginseng saponin. The numerous problems that yet bewilder the optimization and scale-up of this process have not been over emphasized. In spite of the great progress recorded in recent years towards the selection, design and optimization of bioreactor hardware, manipulation of environmental factors such as medium components, light irradiation, shear stress and O2 supply needs detailed investigations for each case. Recent advances in plant cell processes, including high-density suspension cultivation, continuous culture, process monitoring, modeling and control and scale-up, are also reviewed in this chapter. Further developments in bioreactor cultivation processes and in metabolic engineering of plant cells for metabolite production are expected in the near future.

Cleavage of lamin-like proteins in in vivo and in vitro apoptosis of tobacco protoplasts induced by heat shock

Apoptosis in heat shock-treated tobacco protoplasts was evidenced by DNA fragmentation, flow cytometric analysis and activation of caspase 3-like protease. Furthermore, an in vitro apoptosis system was established which reproduced the apoptotic events. Western blotting analysis using an antibody against lamin A and C showed that in both in vivo and in vitro systems lamin-like proteins were cleaved into a 35-kDa fragment, and that lamin-like protein degradation precedes DNA fragmentation. Moreover, we found a 22.8-fold increase in caspase 6-like activity in cytosol of heat-treated protoplasts as compared with the control.

Effects of hydrodynamic and interfacial forces on plant cell suspension systems

Plant cells are perceived to be sensitive to the hydrodynamic environment in conventional bioreactors. Heightened sensitivity, relative to most bacterial cultures, is frequently attributed to larger plant cell sizes, extensive vacuolization and aggregation patterns. Early studies of shear sensitivity focused on cell lysis and/or loss of viability. More recently, an extensive array of sub-lethal responses has been identified. A fuller understanding of these sub-lytic effects may assist in the optimization of large-scale cultivation conditions. This paper reviews recent work on the hydrodynamic shear sensitivity of plant cell systems, under cultivation conditions and in purpose-built shearing devices. The relevance of different approaches to the characterization of the intensity of a given hydrodynamic environment is discussed. Indicators of cell response to hydrodynamic stress are evaluated. The potential significance of cellular defense mechanisms, observed in response to mechanical stimulants, is identified.

Flow cytometry of plant cells with applications in large-scale bioprocessing

In recent years, there has been a significant upsurge in the application of flow cytometry to plant cells and plant cell cultures. As well as a range of uses in plant biology, flow cytometry offers many advantages for monitoring plant cell cultures used in large-scale bioprocessing operations. This review summarizes the current status of the field, concentrating on methods for DNA measurement and multiparameter cell cycle analysis. Techniques for screening and selection of elite cell lines with high productivity of secondary metabolites are also addressed.