Production of cell wall polypeptides by different cell wall mutants of the unicellular green alga Chlamydomonas reinhardtii

Based on Transcriptome Sequencing of Cell Wall Deficient Strain, Research on Arabinosyltransferase Inhibition's Effect on the Synthesis of Cell Wall in Chlamydomonas reinhardtii

To explore the key genes involved in cell wall synthesis and understand the molecular mechanism of cell wall assembly in the model alga-Chlamydomonas reinhardtii, transcriptome sequencing was used to discover the differentially expressed genes in the cell wall defective strain. In the glucose metabolism, lipid metabolism, and amino acid metabolism pathways, the gene expressions involved in the synthesis of cell wall functional components were analyzed. The results showed that in the cell wall defective strain, arabinosyltransferase gene (XEG113, RRA) related to synthesis of plant extensin and some cell wall structural protein genes (hyp, PHC19, PHC15, PHC4, PHC3) were up-regulated, 1,3-β-glucan synthase gene (Gls2) and endoglucanase gene (EG2) about synthesis and degradation of glycoskeleton were both mainly up-regulated. Then, ethambutol dihydrochloride, an arabinosyltransferase inhibitor, was found to affect the permeability of the cell wall of the normal strain, while the cell wall deficient strain was not affected. To further research the function of arabinosyltransferase, the RRA gene was inactivated by knockout in the normal cell wall algal strain. Through a combination of microscope observation and physiological index detection, it was found that the cell wall of the mutant strains showed reduced structure levels, suggesting that the structure and function of the cell wall glycoprotein were weakened. Therefore, arabinosyltransferase may affect the glycosylation modification of cell wall glycoprotein, further affecting the structure assembly of cell wall glycoprotein.

Unassembled cell wall proteins form aggregates in the extracellular space of Chlamydomonas reinhardtii strain UVM4

Abstract

The green microalga Chlamydomonas reinhardtii is emerging as a promising cell biofactory for secreted recombinant protein (RP) production. In recent years, the generation of the broadly used cell wall–deficient mutant strain UVM4 has allowed for a drastic increase in secreted RP yields. However, purification of secreted RPs from the extracellular space of C. reinhardtii strain UVM4 is challenging. Previous studies suggest that secreted RPs are trapped in a matrix of cell wall protein aggregates populating the secretome of strain UVM4, making it difficult to isolate and purify the RPs. To better understand the nature and behaviour of these extracellular protein aggregates, we analysed and compared the extracellular proteome of the strain UVM4 to its cell-walled ancestor, C. reinhardtii strain 137c. When grown under the same conditions, strain UVM4 produced a unique extracellular proteomic profile, including a higher abundance of secreted cell wall glycoproteins. Further characterization of high molecular weight extracellular protein aggregates in strain UVM4 revealed that they are largely comprised of pherophorins, a specific class of cell wall glycoproteins. Our results offer important new insights into the extracellular space of strain UVM4, including strain-specific secreted cell wall proteins and the composition of the aggregates possibly related to impaired RP purification. The discovery of pherophorins as a major component of extracellular protein aggregates will inform future strategies to remove or prevent aggregate formation, enhance purification of secreted RPs, and improve yields of recombinant biopharmaceuticals in this emerging cell biofactory.

Key points

• Extracellular protein aggregates hinder purification of recombinant proteins in C. reinhardtii

• Unassembled cell wall pherophorins are major components of extracellular protein aggregates

• Known aggregate composition informs future strategies for recombinant protein purification

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11960-9.

Nutritional influences on biomass behaviour and metabolic products by Chlamydomonas reinhardtii

The recent works have shown the unicellular green alga Chlamydomonas reinhardtii is a relevant model for investigations of algal bioprocesses. In the current work, several media were evaluated in batch mode for a better understanding of C. reinhardtii metabolism. Nutrient-suppression using heterotrophic and mixotrophic conditions were performed. The findings showed C. reinhardtii metabolized lactose (from milk whey permeate) resulting in high biomass density (2.08 g/L) and total chlorophyll content (86.74 mg/m³). It was observed a specific growth rate of 0.023 h and 29 h for the doubling time. In sulfur-suppression, the algal growth (1.17 g/L) was reduced even though a carbon source (glucose) has been supplemented. Also, the specific growth rate (0.022 h) and the doubling time (31 h) was verified. The production of ethanol was slight and the acetic acid-suppression affected the C. reinhardtii performance providing slow cell growth (0.004 h) and high doubling time (154 h).

Mapping Single Walled Carbon Nanotubes in Photosynthetic Algae by Single-Cell Confocal Raman Microscopy

Carbon nanotubes (CNTs) are among the most exploited carbon allotropes in the emerging technologies of molecular sensing and bioengineering. However, the advancement of algal nanobiotechnology and nanobionics is hindered by the lack of methods for the straightforward visualization of the CNTs inside the cell. Herein, we present a handy and label-free experimental strategy based on visible Raman microscopy to assess the internalization of single-walled carbon nanotubes (SWCNTs) using the model photosynthetic alga Chlamydomonas reinhardtii as a recipient. The relationship between the properties of SWCNTs and their biological behavior was demonstrated, along with the occurrence of excitation energy transfer from the excited chlorophyll molecules to the SWCNTs. The non-radiative deactivation of the chlorophyll excitation promoted by the SWCNTs enables the recording of Raman signals originating from cellular compounds located near the nanotubes, such as carotenoids, polyphosphates, and starch. Furthermore, the outcome of this study unveils the possibility to exploit SWCNTs as spectroscopic probes in photosynthetic and non-photosynthetic systems where the fluorescence background hinders the acquisition of Raman scattering signals.

Cadmium accumulation and toxicity affect the extracytoplasmic polyphosphate level in Chlamydomonas reinhardtii

Cadmium is a non-essential metal and highly toxic for biological functions. Depending on the dose of Cd²⁺, the biochemical response will differ. In this study, we investigated the level of extracytoplasmic polyphosphate (polyP) when Chlamydomonas reinhardtii was exposed to the effect of CdCl₂. When compared to control cells, Cd²⁺-treated cells to 400-600 µM showed a decrease in the growth rate from 0.78 to 0.38 d^-1 for the strain CC-125, and a decrease from 0.81 to 0.32-0.35 d^-1 for CC-503. This indicated a strong toxicity effect on the population growth of cells during 72 h. In addition, the results demonstrated the decrease in the synthesis and/or the degradation of polyP that was correlated with the accumulation of Cd²⁺ in both algal strains. Furthermore, the level of polyP decreased in relation to the decrease of F_V/F_M value. The toxicity of Cd²⁺ induced a high level of cell necrosis for CC-503, and the level of polyP could not be recovered at 72 h. In response to the toxic effects of Cd²⁺, the observed formation of palmelloid colonies by CC-125 cells was correlated with the recovery of the polyP level. Nevertheless, both algal strains were able to accumulate significant amount of Cd²⁺ in their biomass. Therefore, our study demonstrated a distinct impact of Cd²⁺ on the metabolism of polyP (synthesis and/or degradation), which was dependent on the concentration of CdCl₂ and the Chlamydomonas strain. Based on this study, the level of polyP can be used as a biomarker of Cd²⁺ toxicity at 24-48 h, even with the cell wall-deficient strain CC-503.

Engineered Fusion Proteins for Efficient Protein Secretion and Purification of a Human Growth Factor from the Green Microalga Chlamydomonas reinhardtii

Light-driven recombinant protein (RP) production in eukaryotic microalgae offers a sustainable alternative to other established cell-culture systems. RP production via secretion into the culture medium enables simple product separation from the cells adding a layer of process value in addition to the algal biomass, which can be separately harvested. For the model microalga Chlamydomonas reinhardtii, a broad range of molecular tools have been established to enable heterologous gene expression; however, low RP production levels and unreliable purification from secretion concepts have been reported. Domesticated C. reinhardtii strains used for genetic engineering are often cell-wall deficient. These strains nevertheless secrete cell-wall components such as insoluble (hydroxy)proline-rich glycoproteins into the culture media, which hinder downstream purification processes. Here, we attempted to overcome limitations in secretion titers and improve protein purification by combining fusion partners that enhance RP secretion and enable alternative aqueous two-phase (ATPS) RP extraction from the culture medium. Protein fusions were strategically designed to contain a stably folded peptide, which enhanced secretion capacities and gave insights into (some) regulatory mechanisms responsible for this process in the algal host. The elevated protein titers mediated by this fusion were then successfully applied in combination with a fungal hydrophobin tag, which enabled protein purification from the complex microalgal extracellular environment by ATPS, to yield functional recombinant human epidermal growth factor (hEGF) from the algal host.

The Cytotoxic Effects of Camptothecin and Mastoparan on the Unicellular Green Alga Chlamydomonas reinhardtii

We have recently reported that protease inhibitors affecting the activity of the proteasome cause necrotic cell death in Chlamydomonas reinhardtii instead of inducing apoptosis as shown for some mammalian cell lines. Therefore, we have studied other well-known inducers of apoptosis in mammalian cells for their effects on C. reinhardtii cells. Mastoparan caused rapid cell death without a prominent lag-phase under all growth conditions, whereas the cytotoxic effect of the topoisomerase I inhibitor camptothecin exclusively occurred during the cell-division phase. Essentially no differences between wall-deficient and wild-type cells were observed with respect to dose-response and time-course of camptothecin and mastoparan. In cultures of the wall-deficient strain, cell death was accompanied by swelling and subsequent disruption of the cells, established markers of necrosis. In case of the wild-type strain, camptothecin and mastoparan caused accumulation of apparently intact, but dead cells instead of cell debris due to the presence of the wall. Both in cultures of the wall-deficient and the wild-type strains, cell death was accompanied by an increase of the protein concentration in the culture medium indicating a lytic process like necrosis. Taking together, we have severe doubts on the existence of an apoptotic program in case of C. reinhardtii.

Isolation and proteomic alalysis of cell wall-deficientHaematococcus pluvialis mutants

The green alga Haematococcus pluvialis has a plant-like cell wall consisting of glycoproteins and cellulose that is modified during the cell cycle and under various conditions. These features allow Haematococcus to be used as a model organism for studying cell wall biology. Development of the Haematococcus model is hampered by the absence of mutants that could provide insight into the biosynthesis and assembly of wall components. Haematococcus mutants (WM#537 and WM#2978) (WM--wall mutant) with defective cell walls were obtained by chemical mutagenesis. WM#537 features a secondary wall of considerably reduced thickness, whereas WM#2978 possesses a somewhat reduced secondary wall with little intervening space between the wall and plasmalemma. 2-DE revealed that a majority of the cell wall proteins were present in the wild-type and mutant cell walls throughout the cell cycle. PMF identified 55 wall protein orthologs from these strains, including a subset of induced proteins known to be involved in wall construction, remodeling, and defense. Down-regulation of certain wall proteins in the two mutants was associated with the wall defects, whereas overexpression of other proteins may have compensated for the defective walls in the two mutants.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.