Characterization of alpha-phosphoglucomutase isozymes from Toxoplasma gondii

Phosphoglucomutase 1 contributes to optimal cyst development in Toxoplasma gondii

OBJECTIVE

Toxoplasma gondii is a ubiquitous parasite of medical and veterinary importance; however, there exists no cure for chronic toxoplasmosis. Metabolic enzymes required for the production and maintenance of tissue cysts represent promising targets for novel therapies. Here, we use reverse genetics to investigate the role of Toxoplasma phosphoglucomutase 1, PGM1, in Toxoplasma growth and cystogenesis.

RESULTS

We found that disruption of pgm1 did not significantly affect Toxoplasma intracellular growth and the lytic cycle. pgm1-defective parasites could differentiate into bradyzoites and produced cysts containing amylopectin in vitro. However, cysts produced in the absence of pgm1 were significantly smaller than wildtype. Together, our findings suggest that PGM1 is dispensable for in vitro growth but contributes to optimal Toxoplasma cyst development in vitro, thereby necessitating further investigation into the function of this enzyme in Toxoplasma persistence in its host.

Enzymatic and structural characterization of HAD5, an essential phosphomannomutase of malaria-causing parasites

The malaria-causing parasite Plasmodium falciparum is responsible for over 200 million infections and 400,000 deaths per year. At multiple stages during its complex life cycle, P. falciparum expresses several essential proteins tethered to its surface by glycosylphosphatidylinositol (GPI) anchors, which are critical for biological processes such as parasite egress and reinvasion of host red blood cells. Targeting this pathway therapeutically has the potential to broadly impact parasite development across several life stages. Here, we characterize an upstream component of parasite GPI anchor biosynthesis, the putative phosphomannomutase (PMM) (EC 5.4.2.8), HAD5 (PF3D7_1017400). We confirmed the PMM and phosphoglucomutase activities of purified recombinant HAD5 by developing novel linked enzyme biochemical assays. By regulating the expression of HAD5 in transgenic parasites with a TetR-DOZI-inducible knockdown system, we demonstrated that HAD5 is required for malaria parasite egress and erythrocyte reinvasion, and we assessed the role of HAD5 in GPI anchor synthesis by autoradiography of radiolabeled glucosamine and thin layer chromatography. Finally, we determined the three-dimensional X-ray crystal structure of HAD5 and identified a substrate analog that specifically inhibits HAD5 compared to orthologous human PMMs in a time-dependent manner. These findings demonstrate that the GPI anchor biosynthesis pathway is exceptionally sensitive to inhibition in parasites and that HAD5 has potential as a specific, multistage antimalarial target.

Implication of Potential Differential Roles of the Two Phosphoglucomutase Isoforms in the Protozoan Parasite Cryptosporidium parvum

Phosphoglucomutase 1 (PGM1) catalyzes the conversion between glucose-1-phosphate and glucose-6-phosphate in the glycolysis/glucogenesis pathway. PGM1s are typically cytosolic enzymes in organisms lacking chloroplasts. However, the protozoan Cryptosporidium parasites possess two tandemly duplicated PGM1 genes evolved by a gene duplication after their split from other apicomplexans. Moreover, the downstream PGM1 isoform contains an N-terminal signal peptide, predicting a non-cytosolic location. Here we expressed recombinant proteins of the two PGM1 isoforms from the zoonotic Cryptosporidium parvum, namely CpPGM1A and CpPGM1B, and confirmed their enzyme activity. Both isoforms followed Michaelis–Menten kinetics towards glucose-1-phosphate (K_m = 0.17 and 0.13 mM, V_max = 7.30 and 2.76 μmol/min/mg, respectively). CpPGM1A and CpPGM1B genes were expressed in oocysts, sporozoites and intracellular parasites at a similar pattern of expression, however CpPGM1A was expressed at much higher levels than CpPGM1B. Immunofluorescence assay showed that CpPGM1A was present in the cytosol of sporozoites, however this was enriched towards the plasma membranes in the intracellular parasites; whereas CpPGM1B was mainly present under sporozoite pellicle, although relocated to the parasitophorous vacuole membrane in the intracellular development. These observations indicated that CpPGM1A played a house-keeping function, while CpPGM1B played a different biological role that remains to be defined by future investigations.

Cellular Mn/Zn Ratio Influences Phosphoglucomutase Activity and Capsule Production in Streptococcus pneumoniae D39

Capsular polysaccharide (CPS) is a major virulence determinant for many human-pathogenic bacteria. Although the essential functional roles for CPS in bacterial virulence have been established, knowledge of how CPS production is regulated remains limited. Streptococcus pneumoniae (pneumococcus) CPS expression levels and overall thickness change in response to available oxygen and carbohydrate. These nutrients in addition to transition metal ions can vary significantly between host environmental niches and infection stage. Since the pneumococcus must modulate CPS expression among various host niches during disease progression, we examined the impact of the nutritional transition metal availability of manganese (Mn) and zinc (Zn) on CPS production. We demonstrate that increased Mn/Zn ratios increase CPS production via Mn-dependent activation of the phosphoglucomutase Pgm, an enzyme that functions at the branch point between glycolysis and the CPS biosynthetic pathway in a transcription-independent manner. Furthermore, we find that the downstream CPS protein CpsB, an Mn-dependent phosphatase, does not promote aberrant dephosphorylation of its target capsule-tyrosine kinase CpsD during Mn stress. Together, these data reveal a direct role for cellular Mn/Zn ratios in the regulation of CPS biosynthesis via the direct activation of Pgm. We propose a multilayer mechanism used by the pneumococcus in regulating CPS levels across various host niches. IMPORTANCE Evolving evidence strongly indicates that maintenance of metal homeostasis is essential for establishing colonization and continued growth of bacterial pathogens in the vertebrate host. In this study, we demonstrate the impact of cellular manganese/zinc (Mn/Zn) ratios on bacterial capsular polysaccharide (CPS) production, an important virulence determinant of many human-pathogenic bacteria, including Streptococcus pneumoniae. We show that higher Mn/Zn ratios increase CPS production via the Mn-dependent activation of the phosphoglucomutase Pgm, an enzyme that functions at the branch point between glycolysis and the CPS biosynthetic pathway. The findings provide a direct role for Mn/Zn homeostasis in the regulation of CPS expression levels and further support the ability of metal cations to act as important cellular signaling mediators in bacteria.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.

Two Phosphoglucomutase Paralogs Facilitate Ionophore-Triggered Secretion of the Toxoplasma Micronemes

Ca²⁺-dependent exocytosis is essential for the life cycle of apicomplexan parasites. Toxoplasma gondii harbors a phosphoglucomutase (PGM) ortholog, PRP1, previously associated with Ca²⁺-dependent microneme secretion. Here it is shown that genetic deletion of either PRP1, its PGM2 ortholog, or both genes is dispensable for the parasite’s lytic cycle, including host cell egress and invasion. Depletion of the proteins abrogated high Ca²⁺-mediated microneme secretion induced by the ionophore A23187; however, the constitutive and phosphatidic acid-mediated release remained unaffected. Secretion mediated by the former pathway is not essential for tachyzoite survival or acute in vivo infection in the mice.

Paralogs of the widely prevalent phosphoglucomutase (PGM) protein called parafusin function in calcium (Ca²⁺)-mediated exocytosis across eukaryotes. In Toxoplasma gondii, the parafusin-related protein 1 (PRP1) has been associated with Ca²⁺-dependent microneme organelle secretion required for essential processes like host cell invasion and egress. Using reverse genetics, we observed PRP1 to be dispensable for completion of the lytic cycle, including host cell invasion and egress by the parasite. However, the absence of the gene affected increased microneme release triggered by A23187, a Ca²⁺ ionophore used to raise the cytoplasmic Ca²⁺ concentration mimicking the physiological role of Ca²⁺ during invasion and egress. The basal levels of constitutive microneme release in extracellular parasites and phosphatidic acid-triggered microneme secretion were unaffected in the mutant. The phenotype of the deletion mutant of the second PGM-encoding gene in Toxoplasma, PGM2, was similar to the phenotype of the PRP1 deletion mutant. Furthermore, the ability of the tachyzoites to induce acute infection in the mice remained normal in the absence of both PGM paralogs. Our data thus reveal that the microneme secretion upon high Ca²⁺ flux is facilitated by the Toxoplasma PGM paralogs, PRP1 and PGM2. However, this protein-mediated release is neither essential for lytic cycle completion nor for acute virulence of the parasite.

IMPORTANCE Ca²⁺-dependent exocytosis is essential for the life cycle of apicomplexan parasites. Toxoplasma gondii harbors a phosphoglucomutase (PGM) ortholog, PRP1, previously associated with Ca²⁺-dependent microneme secretion. Here it is shown that genetic deletion of either PRP1, its PGM2 ortholog, or both genes is dispensable for the parasite’s lytic cycle, including host cell egress and invasion. Depletion of the proteins abrogated high Ca²⁺-mediated microneme secretion induced by the ionophore A23187; however, the constitutive and phosphatidic acid-mediated release remained unaffected. Secretion mediated by the former pathway is not essential for tachyzoite survival or acute in vivo infection in the mice.

Structural and functional characterization of the phosphoglucomutase from Xanthomonas citri subsp. citri

Citrus canker, caused by bacteria Xanthomonas citri subsp. citri, can affect all economically important varieties of citrus. Studying Xanthomonas genes related to the invasive capacity may improve the knowledge on how this works and ultimately use the information to avoid the disease. Some annotated genes from Xanthomonas citri subsp. citri published genome are addressed to an interesting class of genes named "pathogenicity, virulence and adaptation". One of them is xanA, which encodes a predicted phosphoglucomutase. Phosphoglucomutases are ubiquitous enzymes among the living kingdoms that play roles in carbohydrate metabolism, catalyzing the reversible conversion of 1- to 6-phosphoglucose. In Xanthomonas, phosphoglucomutase activity is required to synthesize precursors of the pathogenesis-related polysaccharide xanthan. In this work, a characterization of this gene product is presented by structural and functional studies. Molecular cloning was used for heterologous expression and deletion of xanA. A Michaelis-Menten kinetics model was obtained using the recombinant protein. The protein structure was also determined by X-ray diffraction on the recombinant enzyme substrate-free, bound to glucose-1,6-biphosphate and to glucose-1-phosphate. Deletion of xanA was done with a suicide plasmid construct and the obtained mutant was tested for pathogenic capacity. This study is the first describing the properties of the Xanthomonas citri subsp. citri phosphoglucomutase.

Crystal structure of phosphoglucomutase from Leishmania major at 3.5 Å resolution

The crystal structure of phosphoglucomutase (LmPGM) from the parasite Leishmania major has been solved at 3.5 Å resolution. Although the active form of the enzyme is monomeric in solution, four molecules (A, B, C, D) were found in the asymmetric unit, of which the pairs (A,D) and (B,C) were of identical inter-subunit geometry. The parasitic enzyme constituted of four domains exhibited the canonical 'heart' shape of the protein, with domains I to III having a conserved α|β core, while the fourth (IV) domain being structurally distinct from the rest. Conformational variability of the IVth domain, postulated to be responsible for the catalytic function of the enzyme has been studied by normal mode analysis (NMA) and the conformational features responsible for domain movement in the 'hinge region' analyzed in detail. Although the active site of phosphoglucomutase is highly conserved from parasite to human, initial calculations show that a ligand binding pocket could exist near the hinge region, which is unique to the parasite. The enzymatic parameters of LmPGM have been determined and compared with other PGMs from orthologous species in addition to elucidating its mechanism of action by docking the substrate, intermediate onto the active site.

Heterologous expression and biochemical characterization of recombinant alpha phosphoglucomutase from Mycobacterium tuberculosis H37Rv

Phosphoglucomutase (PGM) plays an important role in polysaccharide capsule formation and virulence in a number of bacterial pathogens. However, the enzyme has not yet been characterized from Mycobacterium tuberculosis (Mtb). Here, we report the biochemical properties of recombinant Mtb-PGM as well as the in silico structural analysis from Mtb H37Rv. The purified recombinant enzyme was enzymatically active with a specific activity of 67.5 U/mg and experimental k(cat) of 70.31 s(-1) for the substrate glucose-1-phosphate. The enzyme was stable in pH range 6.5-7.4 and exhibited temperature optima range between 30 and 40°C. Various kinetic parameters and constants of the rPGM were determined. A structural comparison of Modeller generated 3D Mtb-PGM structure with rabbit muscle PGM revealed that the two enzymes share the same overall heart shape and four-domain architecture, despite having only 17% sequence identity. However, certain interesting differences between the two have been identified, which provide an opportunity for designing new drugs to specifically target the Mtb-PGM. Also, in the absence of the crystal structure of the Mtb-PGM, the modeled structure could be further explored for in silico docking studies with suitable inhibitors.