Intracellular retention of newly synthesized insulin in yeast is caused by endoproteolytic processing in the Golgi complex

Molecular engineering of insulin for recombinant expression in yeast

Since the first administration of insulin to a person with diabetes in 1922, scientific contributions from academia and industry have improved insulin therapy and access. The pharmaceutical need for insulin is now more than 40 tons annually, half of which is produced by recombinant secretory expression in Saccharomyces cerevisiae. We discuss how, in this yeast species, adaptation of insulin precursors by removable structural elements is pivotal for efficient secretory expression. The technologies reviewed have been implemented at industrial scale and are seminal for the supply of human insulin and insulin analogues to people with diabetes now and in the future. Engineering of a target protein with removable structural elements may provide a general approach to yield optimisation.

Genome-wide screen identifies new set of genes for improved heterologous laccase expression in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is widely used as a host cell for recombinant protein production due to its fast growth, cost-effective culturing, and ability to secrete large and complex proteins. However, one major drawback is the relatively low yield of produced proteins compared to other host systems. To address this issue, we developed an overlay assay to screen the yeast knockout collection and identify mutants that enhance recombinant protein production, specifically focusing on the secretion of the Trametes trogii fungal laccase enzyme. Gene ontology analysis of these mutants revealed an enrichment of processes including vacuolar targeting, vesicle trafficking, proteolysis, and glycolipid metabolism. We confirmed that a significant portion of these mutants also showed increased activity of the secreted laccase when grown in liquid culture. Notably, we found that the combination of deletions of OCA6, a tyrosine phosphatase gene, along with PMT1 or PMT2, two genes encoding ER membrane protein-O-mannosyltransferases involved in ER quality control, and SKI3, which encode for a component of the SKI complex responsible for mRNA degradation, further increased secreted laccase activity. Conversely, we also identified over 200 gene deletions that resulted in decreased secreted laccase activity, including many genes that encode for mitochondrial proteins and components of the ER-associated degradation pathway. Intriguingly, the deletion of the ER DNAJ co-chaperone gene SCJ1 led to almost no secreted laccase activity. When we expressed SCJ1 from a low-copy plasmid, laccase secretion was restored. However, overexpression of SCJ1 had a detrimental effect, indicating that precise dosing of key chaperone proteins is crucial for optimal recombinant protein expression. This study offers potential strategies for enhancing the overall yield of recombinant proteins and provides new avenues for further research in optimizing protein production systems.

Engineering strategies for enhanced heterologous protein production by Saccharomyces cerevisiae

Microbial proteins are promising substitutes for animal- and plant-based proteins. S. cerevisiae, a generally recognized as safe (GRAS) microorganism, has been frequently employed to generate heterologous proteins. However, constructing a universal yeast chassis for efficient protein production is still a challenge due to the varying properties of different proteins. With progress in synthetic biology, a multitude of molecular biology tools and metabolic engineering strategies have been employed to alleviate these issues. This review first analyses the advantages of protein production by S. cerevisiae. The most recent advances in improving heterologous protein yield are summarized and discussed in terms of protein hyperexpression systems, protein secretion engineering, glycosylation pathway engineering and systems metabolic engineering. Furthermore, the prospects for efficient and sustainable heterologous protein production by S. cerevisiae are also provided.

Improving therapeutic protein secretion in the probiotic yeast Saccharomyces boulardii using a multifactorial engineering approach

The probiotic yeast Saccharomyces boulardii (Sb) is a promising chassis to deliver therapeutic proteins to the gut due to Sb's innate therapeutic properties, resistance to phage and antibiotics, and high protein secretion capacity. To maintain therapeutic efficacy in the context of challenges such as washout, low rates of diffusion, weak target binding, and/or high rates of proteolysis, it is desirable to engineer Sb strains with enhanced levels of protein secretion. In this work, we explored genetic modifications in both cis- (i.e. to the expression cassette of the secreted protein) and trans- (i.e. to the Sb genome) that enhance Sb's ability to secrete proteins, taking a Clostridioides difficile Toxin A neutralizing peptide (NPA) as our model therapeutic. First, by modulating the copy number of the NPA expression cassette, we found NPA concentrations in the supernatant could be varied by sixfold (76-458 mg/L) in microbioreactor fermentations. In the context of high NPA copy number, we found a previously-developed collection of native and synthetic secretion signals could further tune NPA secretion between 121 and 463 mg/L. Then, guided by prior knowledge of S. cerevisiae's secretion mechanisms, we generated a library of homozygous single gene deletion strains, the most productive of which achieved 2297 mg/L secretory production of NPA. We then expanded on this library by performing combinatorial gene deletions, supplemented by proteomics experiments. We ultimately constructed a quadruple protease-deficient Sb strain that produces 5045 mg/L secretory NPA, an improvement of > tenfold over wild-type Sb. Overall, this work systematically explores a broad collection of engineering strategies to improve protein secretion in Sb and highlights the ability of proteomics to highlight under-explored mediators of this process. In doing so, we created a set of probiotic strains that are capable of delivering a wide range of protein titers and therefore furthers the ability of Sb to deliver therapeutics to the gut and other settings to which it is adapted.

The Regulated Secretion and Models of Intracellular Transport: The Goblet Cell as an Example

Transport models are extremely important to map thousands of proteins and their interactions inside a cell. The transport pathways of luminal and at least initially soluble secretory proteins synthesized in the endoplasmic reticulum can be divided into two groups: the so-called constitutive secretory pathway and regulated secretion (RS) pathway, in which the RS proteins pass through the Golgi complex and are accumulated into storage/secretion granules (SGs). Their contents are released when stimuli trigger the fusion of SGs with the plasma membrane (PM). In specialized exocrine, endocrine, and nerve cells, the RS proteins pass through the baso-lateral plasmalemma. In polarized cells, the RS proteins secrete through the apical PM. This exocytosis of the RS proteins increases in response to external stimuli. Here, we analyze RS in goblet cells to try to understand the transport model that can be used for the explanation of the literature data related to the intracellular transport of their mucins.

Alleviating vacuolar transport improves cellulase production in Trichoderma reesei

Increasing cellulase production in cellulolytic fungus Trichoderma reesei is of interest for biofuels and biorefineries. Previous studies indicated that secreted protein was occasionally accumulated in vacuoles; this phenomenon has also been reported in T. reesei. Therefore, alleviating vacuolar transport seems to be a promising strategy for improving cellulase production in T. reesei. Herein, we found that knockout of vps10, vps13, and vps21, among 11 vacuolar protein sorting factors, improved cellulase production in T. reesei. The filter paper activity in Δvps10, Δvps13, and Δvps21 increased by 1.28-, 2.45-, and 2.11-fold than that of the parent strain. Moreover, the β-glucosidase activity in Δvps13 and Δvps21 increased by 3.22- and 3.56-fold after 6 days of fermentation. Furthermore, we also found that the vacuolar trafficking towards vacuoles was partially impaired in three knockout mutants, and disruption of vps13 alleviated the autophagy process. These results indicated that alleviated transport and degradation towards vacuole in Δvps10, Δvps13, and Δvps21 might improve cellulase production. Of note, the expression of cellulase genes in Δvps13 and Δvps21 was dramatically increased in the late induction phase compared to the parent. These results suggested that Vps13 and Vps21 might influence the cellulase production at transcription level. And further transcriptome analysis indicated that increased cellulase gene expression might be attributed to the differential expression of sugar transporters. Our study unravels the effect of alleviating vacuolar transport through knockout vps10, vps13, and vps21 for efficient cellulase secretion, providing new clues for higher cellulase production in T. reesei. KEY POINTS: • Disruption of vps10, vps13 or vps21 improves cellulase production • Vacuolar transport is impaired in three vps KO mutants • Deletion of vps13 or vps21 increases the transcript of cellulase genes in late stage.

Customized yeast cell factories for biopharmaceuticals: from cell engineering to process scale up

The manufacture of recombinant therapeutics is a fastest-developing section of therapeutic pharmaceuticals and presently plays a significant role in disease management. Yeasts are established eukaryotic host for heterologous protein production and offer distinctive benefits in synthesising pharmaceutical recombinants. Yeasts are proficient of vigorous growth on inexpensive media, easy for gene manipulations, and are capable of adding post translational changes of eukaryotes. Saccharomyces cerevisiae is model yeast that has been applied as a main host for the manufacture of pharmaceuticals and is the major tool box for genetic studies; nevertheless, numerous other yeasts comprising Pichia pastoris, Kluyveromyces lactis, Hansenula polymorpha, and Yarrowia lipolytica have attained huge attention as non-conventional partners intended for the industrial manufacture of heterologous proteins. Here we review the advances in yeast gene manipulation tools and techniques for heterologous pharmaceutical protein synthesis. Application of secretory pathway engineering, glycosylation engineering strategies and fermentation scale-up strategies in customizing yeast cells for the synthesis of therapeutic proteins has been meticulously described.

VPS4B deficiency causes early embryonic lethality and induces signal transduction disorders of cell endocytosis

VPS4B (vacuolar protein sorting 4B), a member of the ATPase associated with diverse cellular activities (AAA) protein family, is a component of the endosomal sorting complexes required for transport machinery which regulates the internalization and lysosomal degradation of membrane proteins. We previously reported that VPS4B is one of the pathogenic genes related to dentin dysplasia type I, although its function was largely unknown. To investigate the role of VPS4B in tooth development, we deleted the Vps4b gene in mice. We found that heterozygous knockout mice (Vps4b^+/- ) developed normally and were fertile. However, homozygous deletion of the Vps4b gene resulted in early embryonic lethality of Vps4b^-/- mice at approximately embryonic day 9.5 (E9.5). To investigate the underlying molecular mechanisms, we examined the molecular functions of VPS4B in vivo and in vitro. Cell experiments showed that VPS4B influenced the proliferation, apoptosis, and cell cycle of transfected human neuroblastoma cells (IMR-32 cells) with over-expression or knockdown of VPS4B. Moreover, qRT-PCR detection showed that the mRNA expression levels of apoptosis-, cell cycle-, and endocytosis-related genes was significantly down or up-regulated in RNA interference-mediated knockdown of VPS4B in IMR-32 cells and Vps4b^+/- E12.5 embryos. We accordingly speculated that signal transduction disorders of cell endocytosis are a contributing factor to the prenatal lethality of Vps4b^-/- mice.

Exploiting strain diversity and rational engineering strategies to enhance recombinant cellulase secretion by Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulosic material into bioethanol has progressed in the past decades; however, several challenges still exist which impede the industrial application of this technology. Identifying the challenges that exist in all unit operations is crucial and needs to be optimised, but only the barriers related to the secretion of recombinant cellulolytic enzymes in Saccharomyces cerevisiae will be addressed in this review. Fundamental principles surrounding CBP as a biomass conversion platform have been established through the successful expression of core cellulolytic enzymes, namely β-glucosidases, endoglucanases, and exoglucanases (cellobiohydrolases) in S. cerevisiae. This review will briefly address the challenges involved in the construction of an efficient cellulolytic yeast, with particular focus on the secretion efficiency of cellulases from this host. Additionally, strategies for studying enhanced cellulolytic enzyme secretion, which include both rational and reverse engineering approaches, will be discussed. One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development. Furthermore, with the advancement in next-generation sequencing, studies that utilise this method of exploiting intra-strain diversity for industrially relevant traits will be reviewed. Finally, future prospects are discussed for the creation of ideal CBP strains with high enzyme production levels.Key Points• Several challenges are involved in the construction of efficient cellulolytic yeast, in particular, the secretion efficiency of cellulases from the hosts.• Strategies for enhancing cellulolytic enzyme secretion, a core requirement for CBP host microorganism development, include both rational and reverse engineering approaches.• One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development.

Mining Data From Plasma Cell Differentiation Identified Novel Genes for Engineering of a Yeast Antibody Factory

Saccharomyces cerevisiae is a common platform for production of therapeutic proteins, but it is not intrinsically suited for the manufacturing of antibodies. Antibodies are naturally produced by plasma cells (PCs) and studies conducted on PC differentiation provide a comprehensive blueprint for the cellular transformations needed to create an antibody factory. In this study we mined transcriptomics data from PC differentiation to improve antibody secretion by S. cerevisiae. Through data exploration, we identified several new target genes. We tested the effects of 14 genetic modifications belonging to different cellular processes on protein production. Four of the tested genes resulted in improved antibody expression. The ER stress sensor IRE1 increased the final titer by 1.8-fold and smaller effects were observed with PSA1, GOT1, and HUT1 increasing antibody titers by 1. 6-, 1. 4-, and 1.4-fold. When testing combinations of these genes, the highest increases were observed when co-expressing IRE1 with PSA1, or IRE1 with PSA1 and HUT1, resulting in 3.8- and 3.1-fold higher antibody titers. In contrast, strains expressing IRE1 alone or in combination with the other genes produced similar or lower levels of recombinantly expressed endogenous yeast acid phosphatase compared to the controls. Using a genetic UPR responsive GFP reporter construct, we show that IRE1 acts through constitutive activation of the unfolded protein response. Moreover, the positive effect of IRE1 expression was transferable to other antibody molecules. We demonstrate how data exploration from an evolutionary distant, but highly specialized cell type can pinpoint new genetic targets and provide a novel concept for rationalized cell engineering.

RNAi expression tuning, microfluidic screening, and genome recombineering for improved protein production in Saccharomyces cerevisiae

The cellular machinery that supports protein synthesis and secretion lies at the foundation of cell factory-centered protein production. Due to the complexity of such cellular machinery, the challenge in generating a superior cell factory is to fully exploit the production potential by finding beneficial targets for optimized strains, which ideally could be used for improved secretion of other proteins. We focused on an approach in the yeast Saccharomyces cerevisiae that allows for attenuation of gene expression, using RNAi combined with high-throughput microfluidic single-cell screening for cells with improved protein secretion. Using direct experimental validation or enrichment analysis-assisted characterization of systematically introduced RNAi perturbations, we could identify targets that improve protein secretion. We found that genes with functions in cellular metabolism (YDC1, AAD4, ADE8, and SDH1), protein modification and degradation (VPS73, KTR2, CNL1, and SSA1), and cell cycle (CDC39), can all impact recombinant protein production when expressed at differentially down-regulated levels. By establishing a workflow that incorporates Cas9-mediated recombineering, we demonstrated how we could tune the expression of the identified gene targets for further improved protein production for specific proteins. Our findings offer a high throughput and semirational platform design, which will improve not only the production of a desired protein but even more importantly, shed additional light on connections between protein production and other cellular processes.

Systematic Optimization of Protein Secretory Pathways in Saccharomyces cerevisiae to Increase Expression of Hepatitis B Small Antigen

Hepatitis B is a major disease that chronically infects millions of people in the world, especially in developing countries. Currently, one of the effective vaccines to prevent Hepatitis B is the Hepatitis B Small Antigen (HBsAg), which is mainly produced by the recombinant yeast Saccharomyces cerevisiae. In order to bring down the price, which is still too high for people in developing countries to afford, it is important to understand key cellular processes that limit protein expression. In this study, we took advantage of yeast knockout collection (YKO) and screened 194 S. cerevisiae strains with single gene knocked out in four major steps of the protein secretory pathway, i.e., endoplasmic-reticulum (ER)-associated protein degradation, protein folding, unfolded protein response (UPR), and translocation and exocytosis. The screening showed that the single deletion of YPT32, SBH1, and HSP42 led to the most significant increase of HBsAg expression over the wild type while the deletion of IRE1 led to a profound decrease of HBsAg expression. The synergistic effects of gene knockout and gene overexpression were next tested. We found that simultaneously deleting YPT32 and overexpressing IRE1 led to a 2.12-fold increase in HBsAg expression over the wild type strain. The results of this study revealed novel genetic targets of protein secretory pathways that could potentially improve the manufacturing of broad scope vaccines in a cost-effective way using recombinant S. cerevisiae.

Disruption of genes involved in CORVET complex leads to enhanced secretion of heterologous carboxylesterase only in protease deficient Pichia pastoris

The methylotrophic yeast Pichia pastoris (Komagataella spp.) is a popular microbial host for the production of recombinant proteins. Previous studies have shown that mis-sorting to the vacuole can be a bottleneck during production of recombinant secretory proteins in yeast, however, no information was available for P. pastoris. In this work the authors have therefore generated vps (vacuolar protein sorting) mutant strains disrupted in genes involved in the CORVET (class C core vacuole/endosome tethering) complex at the early stages of endosomal sorting. Both Δvps8 and Δvps21 strains contained lower extracellular amounts of heterologous carboxylesterase (CES) compared to the control strain, which could be attributed to a high proteolytic activity present in the supernatants of CORVET engineered strains due to rerouting of vacuolar proteases. Serine proteases were identified to be responsible for this proteolytic degradation by liquid chromatography-mass spectrometry and protease inhibitor assays. Deletion of the major cellular serine protease Prb1 in Δvps8 and Δvps21 strains did not only rescue the extracellular CES levels, but even outperformed the parental CES strain (56 and 80% higher yields, respectively). Further deletion of Ybr139W, another serine protease, did not show a further increase in secretion levels. Higher extracellular CES activity and low proteolytic activity were detected also in fed batch cultivation of Δvps21Δprb1 strains, thus confirming that modifying early steps in the vacuolar pathway has a positive impact on heterologous protein secretion.

Changes in the expression of the type 2 diabetes-associated gene VPS13C in the β-cell are associated with glucose intolerance in humans and mice

Single nucleotide polymorphisms (SNPs) close to the VPS13C, C2CD4A and C2CD4B genes on chromosome 15q are associated with impaired fasting glucose and increased risk of type 2 diabetes. eQTL analysis revealed an association between possession of risk (C) alleles at a previously implicated causal SNP, rs7163757, and lowered VPS13C and C2CD4A levels in islets from female (n = 40, P < 0.041) but not from male subjects. Explored using promoter-reporter assays in β-cells and other cell lines, the risk variant at rs7163757 lowered enhancer activity. Mice deleted for Vps13c selectively in the β-cell were generated by crossing animals bearing a floxed allele at exon 1 to mice expressing Cre recombinase under Ins1 promoter control (Ins1Cre). Whereas Vps13c(fl/fl):Ins1Cre (βVps13cKO) mice displayed normal weight gain compared with control littermates, deletion of Vps13c had little effect on glucose tolerance. Pancreatic histology revealed no significant change in β-cell mass in KO mice vs. controls, and glucose-stimulated insulin secretion from isolated islets was not altered in vitro between control and βVps13cKO mice. However, a tendency was observed in female null mice for lower insulin levels and β-cell function (HOMA-B) in vivo. Furthermore, glucose-stimulated increases in intracellular free Ca(2+) were significantly increased in islets from female KO mice, suggesting impaired Ca(2+) sensitivity of the secretory machinery. The present data thus provide evidence for a limited role for changes in VPS13C expression in conferring altered disease risk at this locus, particularly in females, and suggest that C2CD4A may also be involved.

[Analysis of differentially expressed proteome in urine from non-small cell lung cancer patients]

背景与目的

筛查非小细胞肺癌（non-small cell lung cancer, NSCLC）患者尿液中差异表达蛋白，确定可用于NSCLC早期诊断、监测预后和治疗评估的生物标记物。

方法

分别收集40例已病理证实初诊NSCLC患者、8例肺部良性疾病患者和22例健康志愿者的尿液样本。利用0.9%一维十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（sodium dode-cyl sulfate polyacrylamide gel electrophoresis, 1D SDS-PAGE）技术和MS-Thermo-Orbitrap-Velos质谱分析仪对NSCLC组和非肿瘤组尿液中蛋白质进行分离、提取及识别，鉴定出NSCLC患者尿液中的差异表达蛋白。应用SPSS 20.0软件中受试者工作特征曲线（receiver operating characteristic curve, ROC）分别对其敏感性、特异性进行分析，并进行实验验证，从而确定出与NSCLC相关的生物标记物。

结果

NSCLC患者组和非肿瘤组尿液差异性表达蛋白质集中表现在90 kDa、60 kDa和20 kDa-30 kDa凝胶条带中。在NSCLC患者尿液蛋白分析中发现了4种与NSCLC相关的差异表达蛋白，包括上调蛋白LRG1、CA1和下调蛋白VPS4B、YWHAZ。这4种差异表达蛋白作为独立的NSCLC生物标记物其敏感性较低：LRG1蛋白敏感性83.0%（25/30）、特异性90.0%（18/20）；CA1蛋白敏感性60.0%（18/30）、特异性90.0%（18/20)；VPS4B蛋白敏感性73.3%（22/30）、特异性90.0%（18/20）；YWHAZ蛋白敏感性60.0%（18/30）、特异性95.0%（19/20）。而采用蛋白质组合模式对NSCLC进行筛查、诊断，则其敏感性和特异性分别可高达96.7%（29/30）和85%（17/20）。

结论

LRG1、CA1蛋白在NSCLC患者尿液中高表达，而VPS4B、YWHAZ蛋白呈低表达，差异表达蛋白均提示有可能成为用于NSCLC早期筛查、监测预后和治疗评估的生物标记物。LRG1、CA1、VPS4B和YWHAZ尿液蛋白作为单一生物标记物应用于NSCLC筛查和诊断的敏感性较低，而采用蛋白质组合模式明显优于独立模式对NSCLC的筛查和诊断，故蛋白质组合模式在临床诊疗中将更具有良好应用价值和前景。

Gene expression profiling for human iPS-derived motor neurons from sporadic ALS patients reveals a strong association between mitochondrial functions and neurodegeneration

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease that leads to widespread motor neuron death, general palsy and respiratory failure. The most prevalent sporadic ALS form is not genetically inherited. Attempts to translate therapeutic strategies have failed because the described mechanisms of disease are based on animal models carrying specific gene mutations and thus do not address sporadic ALS. In order to achieve a better approach to study the human disease, human induced pluripotent stem cell (hiPSC)-differentiated motor neurons were obtained from motor nerve fibroblasts of sporadic ALS and non-ALS subjects using the STEMCCA Cre-Excisable Constitutive Polycistronic Lentivirus system and submitted to microarray analyses using a whole human genome platform. DAVID analyses of differentially expressed genes identified molecular function and biological process-related genes through Gene Ontology. REVIGO highlighted the related functions mRNA and DNA binding, GTP binding, transcription (co)-repressor activity, lipoprotein receptor binding, synapse organization, intracellular transport, mitotic cell cycle and cell death. KEGG showed pathways associated with Parkinson's disease and oxidative phosphorylation, highlighting iron homeostasis, neurotrophic functions, endosomal trafficking and ERK signaling. The analysis of most dysregulated genes and those representative of the majority of categorized genes indicates a strong association between mitochondrial function and cellular processes possibly related to motor neuron degeneration. In conclusion, iPSC-derived motor neurons from motor nerve fibroblasts of sporadic ALS patients may recapitulate key mechanisms of neurodegeneration and may offer an opportunity for translational investigation of sporadic ALS. Large gene profiling of differentiated motor neurons from sporadic ALS patients highlights mitochondrial participation in the establishment of autonomous mechanisms associated with sporadic ALS.

VPS13D Gene Variant Is Associated with Altered IL-6 Production and Mortality in Septic Shock

BACKGROUND

Genetic variations contribute to septic shock mortality. To discover a novel locus, we performed in vitro genome-wide association studies (GWAS) and further tested the result in a cohort of septic shock patients.

METHODS

Two in vitro GWAS using a quantitative trait locus analysis of stimulated IL-6 production in lymphoblastoid cells from 60 individuals of European ancestry were performed. VPS13D rs6685273 was genotyped in European ancestry patients (n = 498). The VPS13D gene was silenced in vitro.

RESULTS

Two GWAS using lymphoblastoid cells identified the locus of VPS13D rs6685273 that was significant in the same direction in both GWAS. The VPS13D rs6685273 C allele was associated with increased IL-6 production. Patients with septic shock who had the VPS13D rs6685273 CC genotype had an increased 28-day mortality (p = 0.023) and more organ failure (p < 0.05) compared to the CT/TT genotypes. VPS13D in vitro gene silencing in the HeLa cell line increased IL-6 production. Furthermore, the rs6685273 genotype was associated with differential VPS13D splice variant expression.

CONCLUSIONS

The VPS13D rs6685273 C allele was associated with increased IL-6 production in vitro. The patients with the VPS13D rs6685273 CC genotype had increased 28-day mortality and increased organ failure. VPS13D appears to regulate IL-6 production.

High CHMP4B expression is associated with accelerated cell proliferation and resistance to doxorubicin in hepatocellular carcinoma

Charged multivesicular body protein 4B (CHMP4B), a subunit of the endosomal sorting complex required for transport (ESCRT)-III complex, plays an important part in cytokinetic membrane abscission and the late stage of mitotic cell division. In this study, we explored the prognostic significance of CHMP4B in human hepatocellular carcinoma (HCC) and its impact on the physiology of HCC cells. Western blot and immunohistochemistrical analyses showed that CHMP4B was significantly upregulated in HCC tissues, compared with adjacent non-tumorous tissues. Meanwhile, clinicopathological analysis revealed that high CHMP4B expression was correlated with multiple clinicopathological variables, including AFP, cirrhosis, AJCC stage, Ki-67 expression, and poor prognosis. More importantly, univariate and multivariate survival analyses demonstrated that CHMP4B served as an independent prognostic factor for survival of HCC patients. Using HCC cell cultures, we found that the expression of CHMP4B was progressively upregulated after the release from serum starvation. To verify whether CHMP4B could regulate the proliferation of HCC cells, CHMP4B was knocked down through the transfection of CHMP4B-siRNA oligos. Flow cytometry and CCK-8 assays indicated that interference of CHMP4B led to cell cycle arrest and proliferative impairment of HCC cells. Additionally, depletion of CHMP4B expression could increase the sensitivity to doxorubicin in HepG2 and Huh7 cells. Taken together, our results implied that CHMP4B could be a promising prognostic biomarker as well as a potential therapeutic target of HCC.

Yeast synthetic biology for the production of recombinant therapeutic proteins

The production of recombinant therapeutic proteins is one of the fast-growing areas of molecular medicine and currently plays an important role in treatment of several diseases. Yeasts are unicellular eukaryotic microbial host cells that offer unique advantages in producing biopharmaceutical proteins. Yeasts are capable of robust growth on simple media, readily accommodate genetic modifications, and incorporate typical eukaryotic post-translational modifications. Saccharomyces cerevisiae is a traditional baker's yeast that has been used as a major host for the production of biopharmaceuticals; however, several nonconventional yeast species including Hansenula polymorpha, Pichia pastoris, and Yarrowia lipolytica have gained increasing attention as alternative hosts for the industrial production of recombinant proteins. In this review, we address the established and emerging genetic tools and host strains suitable for recombinant protein production in various yeast expression systems, particularly focusing on current efforts toward synthetic biology approaches in developing yeast cell factories for the production of therapeutic recombinant proteins.

Blocking endocytotic mechanisms to improve heterologous protein titers in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a useful platform for protein production of biopharmaceuticals and industrial enzymes. To date, substantial effort has focused on alleviating several bottlenecks in expression and the secretory pathway. Recently, it has been shown that highly active endocytosis could decrease the overall protein titer in the supernatant. In this study, we block endocytosis and trafficking to the vacuole using a modified TEV Protease-Mediated Induction of Protein Instability (mTIPI) system to disrupt the endocytotic and vacuolar complexes. We report that conditional knock-down of endocytosis gene Rvs161 improved the concentration of α-amylase in supernatant of S. cerevisiae cultures by 63.7% compared to controls. By adaptive evolution, we obtained knock-down mutants in Rvs161 and End3 genes with 2-fold and 3-fold α-amylase concentrations compared to controls that were not evolved. Our study demonstrates that genetic blocking of endocytotic mechanisms can improve heterologous protein production in S. cerevisiae. This result is likely generalizable to other eukaryotic secretion hosts.

Current state and recent advances in biopharmaceutical production in Escherichia coli, yeasts and mammalian cells

Almost all of the 200 or so approved biopharmaceuticals have been produced in one of three host systems: the bacterium Escherichia coli, yeasts (Saccharomyces cerevisiae, Pichia pastoris) and mammalian cells. We describe the most widely used methods for the expression of recombinant proteins in the cytoplasm or periplasm of E. coli, as well as strategies for secreting the product to the growth medium. Recombinant expression in E. coli influences the cell physiology and triggers a stress response, which has to be considered in process development. Increased expression of a functional protein can be achieved by optimizing the gene, plasmid, host cell, and fermentation process. Relevant properties of two yeast expression systems, S. cerevisiae and P. pastoris, are summarized. Optimization of expression in S. cerevisiae has focused mainly on increasing the secretion, which is otherwise limiting. P. pastoris was recently approved as a host for biopharmaceutical production for the first time. It enables high-level protein production and secretion. Additionally, genetic engineering has resulted in its ability to produce recombinant proteins with humanized glycosylation patterns. Several mammalian cell lines of either rodent or human origin are also used in biopharmaceutical production. Optimization of their expression has focused on clonal selection, interference with epigenetic factors and genetic engineering. Systemic optimization approaches are applied to all cell expression systems. They feature parallel high-throughput techniques, such as DNA microarray, next-generation sequencing and proteomics, and enable simultaneous monitoring of multiple parameters. Systemic approaches, together with technological advances such as disposable bioreactors and microbioreactors, are expected to lead to increased quality and quantity of biopharmaceuticals, as well as to reduced product development times.

Imbalance of heterologous protein folding and disulfide bond formation rates yields runaway oxidative stress

Background

The protein secretory pathway must process a wide assortment of native proteins for eukaryotic cells to function. As well, recombinant protein secretion is used extensively to produce many biologics and industrial enzymes. Therefore, secretory pathway dysfunction can be highly detrimental to the cell and can drastically inhibit product titers in biochemical production. Because the secretory pathway is a highly-integrated, multi-organelle system, dysfunction can happen at many levels and dissecting the root cause can be challenging. In this study, we apply a systems biology approach to analyze secretory pathway dysfunctions resulting from heterologous production of a small protein (insulin precursor) or a larger protein (α-amylase).

Results

HAC1-dependent and independent dysfunctions and cellular responses were apparent across multiple datasets. In particular, processes involving (a) degradation of protein/recycling amino acids, (b) overall transcription/translation repression, and (c) oxidative stress were broadly associated with secretory stress.

Conclusions

Apparent runaway oxidative stress due to radical production observed here and elsewhere can be explained by a futile cycle of disulfide formation and breaking that consumes reduced glutathione and produces reactive oxygen species. The futile cycle is dominating when protein folding rates are low relative to disulfide bond formation rates. While not strictly conclusive with the present data, this insight does provide a molecular interpretation to an, until now, largely empirical understanding of optimizing heterologous protein secretion. This molecular insight has direct implications on engineering a broad range of recombinant proteins for secretion and provides potential hypotheses for the root causes of several secretory-associated diseases.

Different expression systems for production of recombinant proteins in Saccharomyces cerevisiae

Yeast Saccharomyces cerevisiae has become an attractive cell factory for production of commodity and speciality chemicals and proteins, such as industrial enzymes and pharmaceutical proteins. Here we evaluate most important expression factors for recombinant protein secretion: we chose two different proteins (insulin precursor (IP) and α-amylase), two different expression vectors (POTud plasmid and CPOTud plasmid) and two kinds of leader sequences (the glycosylated alpha factor leader and a synthetic leader with no glycosylation sites). We used IP and α-amylase as representatives of a simple protein and a multi-domain protein, as well as a non-glycosylated protein and a glycosylated protein, respectively. The genes coding for the two recombinant proteins were fused independently with two different leader sequences and were expressed using two different plasmid systems, resulting in eight different strains that were evaluated by batch fermentations. The secretion level (µmol/L) of IP was found to be higher than that of α-amylase for all expression systems and we also found larger variation in IP production for the different vectors. We also found that there is a change in protein production kinetics during the diauxic shift, that is, the IP was produced at higher rate during the glucose uptake phase, whereas amylase was produced at a higher rate in the ethanol uptake phase. For comparison, we also refer to data from another study, (Tyo et al. submitted) in which we used the p426GPD plasmid (standard vector using URA3 as marker gene and pGPD1 as expression promoter). For the IP there is more than 10-fold higher protein production with the CPOTud vector compared with the standard URA3-based vector, and this vector system therefore represent a valuable resource for future studies and optimization of recombinant protein production in yeast.

Decreased secretion and unfolded protein response upregulation

Recombinant antibody fragments, for example, the classic monovalent single-chain antibody (scFv), are emerging as credible alternatives to monoclonal antibody (mAb) products. scFv fragments maintain a diverse range of potential applications in biotechnology and can be implemented as powerful therapeutic and diagnostic agents. As such, a variety of hosts have been used to produce antibody fragments resulting in varying degrees of success. Yeast, Saccharomyces cerevisiae, is an attractive host due to quality control mechanisms of the secretory pathway that ensure secreted proteins are properly folded. However, the expression of a recombinant protein in yeast is not trivial; neither are the quality control mechanisms the cell initiates to respond to overwhelming stress, such as an increased protein load, simplistic. The endoplasmic reticulum (ER) is a dynamic organelle, capable of sensing and adjusting its folding capacity in response to increased demand. When protein abundance or terminally misfolded proteins overwhelm the ER's capacity, the unfolded protein response (UPR) is activated. In the guidelines presented here, we discuss varying aspects of quality control, its modulation, and ways to design appropriate constructs for yeast recombinant protein expression. Furthermore, we have provided protocols and methods to monitor intracellular protein expression and trafficking as well as evaluation of the UPR, with essential controls. The latter part of this chapter will review considerations for the experimental design of microarray and quantitative polymerase chain reaction (q-PCR) techniques while suggesting appropriate means of data analysis.

Enhanced production and secretion of heterologous proteins by the filamentous fungus Aspergillus oryzae via disruption of vacuolar protein sorting receptor gene Aovps10

Filamentous fungi have received attention as hosts for heterologous protein production because of their high secretion capability and eukaryotic posttranslational modifications. However, despite these positive attributes, a bottleneck in posttranscriptional processing limits protein yields. The vacuolar protein sorting gene VPS10 encodes a sorting receptor for the recognition and delivery of several yeast vacuolar proteins. Although it can also target recombinant and aberrant proteins for vacuolar degradation, there is limited knowledge of the effect of its disruption on heterologous protein production. In this study, cDNA encoding AoVps10 from the filamentous fungus Aspergillus oryzae was cloned and sequenced. Microscopic observation of the transformant expressing AoVps10 fused with enhanced green fluorescent protein showed that the fusion protein localized at the Golgi and prevacuolar compartments. Moreover, disruption of the Aovps10 gene resulted in missorting and secretion of vacuolar carboxypeptidase AoCpyA into the medium, indicating that AoVps10 is required for sorting of vacuolar proteins to vacuoles. To investigate the extracellular production levels of heterologous proteins, DeltaAovps10 mutants expressing either bovine chymosin (CHY) or human lysozyme (HLY) were constructed. Interestingly, the DeltaAovps10 mutation increased the maximum extracellular production levels of CHY and HLY by 3- and 2.2-fold, respectively. Western blot analysis of extracellular heterologous proteins also demonstrated an improvement in productivity. These results suggest that AoVps10 plays a role in the regulation of heterologous protein secretion in A. oryzae and may be involved in the vacuolar protein degradation through the Golgi apparatus.

Identification of evidence suggestive of an association with peripheral arterial disease at the OSBPL10 locus by genome-wide investigation in the Japanese population

AIM

Peripheral arterial disease (PAD) is a common cause of cardiovascular morbidity and an independent predictor of cardiovascular mortality. However, little is known about the genetic basis of PAD. To elucidate this, we performed a two-staged genome-wide association study in Japanese individuals.

METHODS

We initially tested 222,285 single-nucleotide polymorphisms (SNPs). After the first screening in a panel of 195 PAD cases and 1,358 controls, 2,696 SNPs (1.2%) were further genotyped in the second screening using another panel of 699 PAD cases and 1540 controls. In both screenings, controls were subjects affected with some diseases other than PAD.

RESULTS

When analyzed in the combined panel, the strongest signal of PAD association was observed at rs1902341 in the intron of OSBPL10 (p=4.7E-7 for trend test; OR=1.31, 95% CI 1.18-1.46). Also, PAD was modestly associated at several other loci such as rs2554503 in CSMD1 (p=5.7E-5; OR=1.32, 95% CI 1.15-1.51) or rs235243 in VSP13D (p=0.04; OR=1.18, 95% CI 1.01-1.37).

CONCLUSION

Our genome-wide exploration identified suggestive evidence of PAD association at the OSBPL10 locus. Because the association has not reached a genome-wide significant level, further replication study is warranted for verification in the Japanese population.

Enhanced protein secretion from multiprotease-deficient fission yeast by modification of its vacuolar protein sorting pathway

Previously, we achieved approximately 30-fold enhanced secretion of the protease-sensitive model protein human growth hormone (hGH) by multiple gene deletion of seven obstructive proteases in the fission yeast Schizosaccharomyces pombe. However, intracellular retention of secretory hGH was found in the resultant multiprotease-deficient strains. As a solution, genetic modification of the intracellular trafficking pathway that is related to intracellular retention of hGH was attempted on a protease octuple deletant strain. Vacuolar accumulation of the intracellularly retained hGH was identified by secretory expression of hGH fused with EGFP, and three vacuolar protein sorting (vps)-deficient strains, vps10Delta, vps22Delta, and vps34Delta, were determined on account of their hGH secretion efficiency. The mutant vps10Delta was found to be effective for hGH secretion, which suggested a role for vps10 in the vacuolar accumulation of the intracellularly retained hGH. Finally, vps10 deletion was performed on the protease octuple deletant strain, which led to an approximately 2-fold increase in hGH secretion. This indicated the possible application of secretory-pathway modification and multiple protease deletion for improving heterologous protein secretion from the fission yeast S. pombe.

Directed evolution of a secretory leader for the improved expression of heterologous proteins and full-length antibodies in Saccharomyces cerevisiae

Because of its eukaryotic nature, simple fermentation requirements, and pliable genetics, there have been many attempts at improving recombinant protein production in Saccharomyces cerevisiae. These strategies typically involve altering the expression of a native protein thought to be involved in heterologous protein trafficking. Usually, these approaches yield three- to tenfold improvements over wild-type strains and are almost always specific to one type of protein. In this study, a library of mutant alpha mating factor 1 leader peptides (MFalpha1pp) is screened for the enhanced secretion of a single-chain antibody. One of the isolated mutants is shown to enhance the secretion of the scFv up to 16-fold over wild type. These leaders also confer a secretory improvement to two other scFvs as well as two additional, structurally unrelated proteins. Moreover, the improved leader sequences, combined with strain engineering, allow for a 180-fold improvement over previous reports in the secretion of full-length, functional, glycosylated human IgG(1). The production of full-length IgG(1) at milligram per liter titers in a simple, laboratory-scale system will significantly expedite drug discovery and reagent synthesis while reducing antibody cloning, production, and characterization costs.

Secretory protein trafficking: genetic and biochemical analysis

Acute insulin secretion from stimulated pancreatic beta-cells is derived from the intracellular pool of insulin secretory granules wherein insulin is packaged in a highly concentrated (and in some species, crystalline) state. Here we review experimental work, principally from our laboratory, on the question of biogenesis of mature secretory granules within the broader context of intracellular protein trafficking. Events occurring in the lumen of organelles at various stages of intracellular transport within the secretory pathway and events at the limiting membrane of newly forming secretory granules each contribute to formation of the insulin storage compartment comprising the readily releasable pool.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

Microneme proteins in apicomplexans

Microneme secretion supports several key cellular processes including gliding motility, active cell invasion and migration through cells, biological barriers, and tissues. The modular design of microneme proteins enables these molecules to assist each other in folding and passage through the quality control system, accurately target to the micronemes, oligimerizing with other parasite proteins, and engaging a variety of host receptors for migration and cell invasion. Structural and biochemical analyses of MIC domains is providing new perspectives on how adhesion is regulated and the potentially distinct roles MICs might play in long or short range interactions during parasite attachment and entry. New access to complete genome sequences and ongoing advances in genetic manipulation should provide fertile ground for refining current models and defining exciting new roles for MICs in apicomplexan biology.

Dominant-negative behavior of mammalian Vps35 in yeast requires a conserved PRLYL motif involved in retromer assembly

The retromer protein complex assists in recycling selected integral membrane proteins from endosomes to the trans Golgi network. One protein subcomplex (Vps35p, Vps26p and Vps29p) combines with a second (Vps17p and Vps5p) to form a coat involved in sorting and budding of endosomal vesicles. Yeast Vps35p (yVps35) exhibits similarity to human Vps35 (hVps35), especially in a completely conserved PRLYL motif contained within an amino-terminal domain. Companion studies indicate that an R(98)W mutation in yVps35 causes defective retromer assembly in Saccharomyces cerevisiae. Herein, we find that the expression of hVps35 in yeast confers dominant-negative vacuolar proenzyme secretion and defective secretory proprotein processing. The mutant phenotype appears to be driven by hVps35 competing with endogenous yVps35, becoming incorporated into defective retromer complexes and causing proteasomal degradation of endogenous Vps26 and Vps29. Increased expression of yVps35 displaces some hVps35 to a 100 000 x g supernatant and suppresses the dominant-negative phenotype. Remarkably, mutation of the conserved R(107)W of hVps35 displaces some of the protein to the 100 000 x g supernatant, slows protein turnover and restores stability of Vps26p and Vps29p and completely abrogates dominant-negative trafficking behavior. We show that hVps35 coprecipitates Vps26, whereas the R(107)W mutant does not. In pancreatic beta cells, the R(107)W mutant shifts hVps35 from peripheral endosomes to a juxtanuclear compartment, affecting both mannose phosphate receptors and insulin. These data underscore importance of the Vps35 PRLYL motif in retromer subcomplex interactions and function.

A role for Lte1p (a low temperature essential protein involved in mitosis) in proprotein processing in the yeast secretory pathway

We previously identified six single gene disruptions in Saccharomyces cerevisiae that allow enhanced immunoreactive insulin secretion primarily because of defective Kex2p-mediated endoproteolytic processing. Five eis mutants disrupted established VPS (vacuolar protein sorting) genes, The sixth, LTE1, is a Low Temperature (<15 degrees C) Essential gene encoding a large protein with potential guanine nucleotide exchange (GEF) domains. Lte1p functions as a positive regulator of the mitotic GTPase Tem1p, and overexpression of Tem1p suppresses the low temperature mitotic defect of lte1. By sequence analysis, Tem1p has highest similarity to Vps21p (yeast homolog of mammalian Rab5). Unlike TEM1, LTE1 is not restricted to mitosis but is expressed throughout the cell cycle. Lte1p function in interphase cells is largely unknown. Here we confirm the eis phenotype of lte1 mutant cells and demonstrate a defect in proalpha factor processing that is rescued by expression of full-length Lte1p but not a C-terminally truncated Lte1p lacking its GEF homology domain. Neither overexpression of Tem1p nor 13 other structurally related GTPases can suppress the secretory proprotein processing defect. However, overexpression of Vps21p selectively restores proprotein processing in a manner dependent upon the active GTP-bound form of the GTPase. By contrast, a vps21 mutant produces a synthetic defect with lte1 in proprotein processing, as well as a synthetic growth defect. Together, the data underscore a link between the mitotic regulator, Lte1p, and protein processing and trafficking in the secretory/endosomal system.

The yeast tumor suppressor homologue Sro7p is required for targeting of the sodium pumping ATPase to the cell surface

The SRO7/SOP1 encoded tumor suppressor homologue of Saccharomyces cerevisiae is required for maintenance of ion homeostasis in cells exposed to NaCl stress. Here we show that the NaCl sensitivity of the sro7Delta mutant is due to defective sorting of Ena1p, the main sodium pump in yeast. On exposure of sro7Delta mutants to NaCl stress, Ena1p fails to be targeted to the cell surface, but is instead routed to the vacuole for degradation via the multivesicular endosome pathway. SRO7-deficient mutants accumulate post-Golgi vesicles at high salinity, in agreement with a previously described role for Sro7p in late exocytosis. However, Ena1p is not sorted into these post-Golgi vesicles, in contrast to what is observed for the vesicles that accumulate when exocytosis is blocked in sec6-4 mutants at high salinity. These observations imply that Sro7p has a previously unrecognized role for sorting of specific proteins into the exocytic pathway. Screening for multicopy suppressors identified RSN1, encoding a transmembrane protein of unknown function. Overexpression of RSN1 restores NaCl tolerance of sro7Delta mutants by retargeting Ena1p to the plasma membrane. We propose a model in which blocked exocytic sorting in sro7Delta mutants, gives rise to quality control-mediated routing of Ena1p to the vacuole.

Chiral mutagenesis of insulin. Contribution of the B20-B23 beta-turn to activity and stability

Insulin contains a beta-turn (residues B20-B23) interposed between two receptor-binding elements, the central alpha-helix of the B chain (B9-B19) and its C-terminal beta-strand (B24-B28). The turn contains conserved glycines at B20 and B23. Although insulin exhibits marked conformational variability among crystal forms, these glycines consistently maintain positive phi dihedral angles within a classic type-I beta-turn. Because the Ramachandran conformations of GlyB20 and GlyB23 are ordinarily forbidden to L-amino acids, turn architecture may contribute to structure or function. Here, we employ "chiral mutagenesis," comparison of corresponding D- and L-Ala substitutions, to investigate this turn. Control substitutions are introduced at GluB21, a neighboring residue exhibiting a conventional (negative) phi angle. The D- and L-Ala substitutions at B23 are associated with a marked stereospecific difference in activity. Whereas the D-AlaB23 analog retains native activity, the L analog exhibits a 20-fold decrease in receptor binding. By contrast, D- and L-AlaB20 analogs each exhibit high activity. Stereospecific differences between the thermodynamic stabilities of the analogs are nonetheless more pronounced at B20 (delta deltaG(u) 2.0 kcal/mole) than at B23 (delta deltaG(u) 0.7 kcal/mole). Control substitutions at B21 are well tolerated without significant stereospecificity. Chiral mutagenesis thus defines the complementary contributions of these conserved glycines to protein stability (GlyB20) or receptor recognition (GlyB23).

Contrasting secretory processing of simultaneously expressed heterologous proteins in Saccharomyces cerevisiae

In this study, secretory processing of cell-surface displayed Aga2p fusions to bovine pancreatic trypsin inhibitor (BPTI) and the single chain Fv (scFv) antibody fragment D1.3 are examined. BPTI is more efficiently processed than D1.3 both when secreted and surface-displayed, and D1.3 expression imparts a greater amount of secretory stress on the cell as assayed by a reporter of the unfolded protein response (UPR). Surprisingly, simultaneous expression of the two proteins in the same cell somewhat improves BPTI surface display while decreasing D1.3 surface display with minimal effect on UPR activation. Furthermore, co-expression leads to the accumulation of punctate vacuolar aggregates of D1.3 and increased secretion of the D1.3-Aga2p fusion into the supernatant. Overexpression of the folding chaperones protein disulfide isomerase (PDI) and BiP largely mitigates the D1.3 surface expression decrease, suggesting that changes in vacuolar and cell surface targeting may be due, in part, to folding inefficiency. Titration of constitutive UPR expression across a broad range progressively decreases surface display of both proteins as UPR increases. D1.3-Aga2p traffic through the late secretory pathway appears to be strongly affected by overall secretory load as well as folding conditions in the ER.

Golgi-mediated vacuolar sorting of the endoplasmic reticulum chaperone BiP may play an active role in quality control within the secretory pathway

Quality control in the endoplasmic reticulum (ER) prevents the arrival of incorrectly or incompletely folded proteins at their final destinations and targets permanently misfolded proteins for degradation. Such proteins have a high affinity for the ER chaperone BiP and are finally degraded via retrograde translocation from the ER lumen back to the cytosol. This ER-associated protein degradation (ERAD) is currently thought to constitute the main disposal route, but there is growing evidence for a vacuolar role in quality control. We show that BiP is transported to the vacuole in a wortmannin-sensitive manner in tobacco (Nicotiana tabacum) and that it could play an active role in this second disposal route. ER export of BiP occurs via COPII-dependent transport to the Golgi apparatus, where it competes with other HDEL receptor ligands. When HDEL-mediated retrieval from the Golgi fails, BiP is transported to the lytic vacuole via multivesicular bodies, which represent the plant prevacuolar compartment. We also demonstrate that a subset of BiP-ligand complexes is destined to the vacuole and differs from those likely to be disposed of via the ERAD pathway. Vacuolar disposal could act in addition to ERAD to maximize the efficiency of quality control in the secretory pathway.

Preparing for an invasion: charting the pathway of adhesion proteins to Toxoplasma micronemes

Toxoplasma gondii is an apicomplexan parasite capable of infecting a broad host range including humans. The tachyzoite lytic cycle begins with active invasion of host cells involving the release of adhesive proteins from apical secretory organelles called micronemes. A protein complex consisting of the transmembrane adhesin MIC2 and a tightly associated partner, M2AP, is abundantly released from the micronemes. Similar to many proteins in a regulated secretory pathway, T. gondii proteins destined for micronemes and rhoptries (another secretory organelle associated with invasion) undergo proteolytic maturation. M2AP contains a propeptide that is removed in a post-Golgi compartment. By expressing an M2AP propeptide deletion mutant in the M2AP knockout background, we show that the propeptide is required for the MIC2-M2AP complex to exit from the early endosome. Although a cleavage-resistant M2AP mutant was able to efficiently reach the micronemes, it was unable to rapidly mobilize from the micronemes to the parasite surface. Strikingly, both mutants were unable to support normal parasite invasion and were partially attenuated in virulence to a degree that is indistinguishable from M2AP knockout parasites. Conditional expression of MIC2 showed that it is also required for correct M2AP sorting to the micronemes. These parasites were severely impaired in invasion efficiency. They switched almost exclusively to a non-productive circular gliding motility and were incapable of establishing an infection in mice when inoculated at a normally lethal dose. These findings underscore the importance of correct trafficking of invasion-related proteins. Our results also serve as a basis for future studies aimed at defining the branch points of protein sorting in T. gondii and at a deeper understanding of the precise roles of M2AP propeptide and MIC2 targeting motifs in MIC protein trafficking.