Selective and programmed cleavage of GPI-anchored proteins from the surface membrane by phospholipase C

Cell surface CD55 traffics to the nucleus leading to cisplatin resistance and stemness by inducing PRC2 and H3K27 trimethylation on chromatin in ovarian cancer

BACKGROUND

Platinum resistance is the primary cause of poor survival in ovarian cancer (OC) patients. Targeted therapies and biomarkers of chemoresistance are critical for the treatment of OC patients. Our previous studies identified cell surface CD55, a member of the complement regulatory proteins, drives chemoresistance and maintenance of cancer stem cells (CSCs). CSCs are implicated in tumor recurrence and metastasis in multiple cancers.

METHODS

Protein localization assays including immunofluorescence and subcellular fractionation were used to identify CD55 at the cell surface and nucleus of cancer cells. Protein half-life determinations were used to compare cell surface and nuclear CD55 stability. CD55 deletion mutants were generated and introduced into cancer cells to identify the nuclear trafficking code, cisplatin sensitivity, and stem cell frequency that were assayed using in vitro and in vivo models. Detection of CD55 binding proteins was analyzed by immunoprecipitation followed by mass spectrometry. Target pathways activated by CD55 were identified by RNA sequencing.

RESULTS

CD55 localizes to the nucleus of a subset of OC specimens, ascites from chemoresistant patients, and enriched in chemoresistant OC cells. We determined that nuclear CD55 is glycosylated and derived from the cell surface pool of CD55. Nuclear localization is driven by a trafficking code containing the serine/threonine (S/T) domain of CD55. Nuclear CD55 is necessary for cisplatin resistance, stemness, and cell proliferation in OC cells. CD55 S/T domain is necessary for nuclear entry and inducing chemoresistance to cisplatin in both in vitro and in vivo models. Deletion of the CD55 S/T domain is sufficient to sensitize chemoresistant OC cells to cisplatin. In the nucleus, CD55 binds and attenuates the epigenetic regulator and tumor suppressor ZMYND8 with a parallel increase in H3K27 trimethylation and members of the Polycomb Repressive Complex 2.

CONCLUSIONS

For the first time, we show CD55 localizes to the nucleus in OC and promotes CSC and chemoresistance. Our studies identify a therapeutic mechanism for treating platinum resistant ovarian cancer by blocking CD55 nuclear entry.

SEMA7a primes integrin α5β1 engagement instructing fibroblast mechanotransduction, phenotype and transcriptional programming

Integrins are cellular receptors that bind the extracellular matrix (ECM) and facilitate the transduction of biochemical and biophysical microenvironment cues into cellular responses. Upon engaging the ECM, integrin heterodimers must rapidly strengthen their binding with the ECM, resulting in the assembly of force-resistant and force-sensitive integrin associated complexes (IACs). The IACs constitute an essential apparatus for downstream signaling and fibroblast phenotypes. During wound healing, integrin signaling is essential for fibroblast motility, proliferation, ECM reorganization and, ultimately, restoration of tissue homeostasis. Semaphorin 7A (SEMA7a) has been previously implicated in post-injury inflammation and tissue fibrosis, yet little is known about SEMA7a's role in directing stromal cell, particularly fibroblast, behaviors. We demonstrate that SEMA7a regulates integrin signaling through cis-coupling with active integrin α5β1 on the plasma membrane, enabling rapid integrin adhesion strengthening to fibronectin (Fn) and normal downstream mechanotransduction. This molecular function of SEMA7a potently regulates fibroblast adhesive, cytoskeletal, and migratory phenotype with strong evidence of downstream alterations in chromatin structure resulting in global transcriptomic reprogramming such that loss of SEMA7a expression is sufficient to impair the normal migratory and ECM assembly phenotype of fibroblasts resulting in significantly delayed tissue repair in vivo.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

A Review for the Special Issue on Paramecium as a Modern Model Organism

This review provides background and perspective for the articles contributing to the Special Issue of MDPI Micro-organisms on Paramecium as a Modern Model Organism. The six articles cover a variety of topics, each taking advantage of an important aspect of Paramecium biology: peripheral surface proteins that are developmentally regulated, endosymbiont algae and bacteria, ion channel regulation by calmodulin, regulation of cell mating reactivity and senescence, and the introns that dwell in the large genome. Each article highlights a significant aspect of Paramecium and its versatility.

Labeling cell surface glycosylphosphatidylinositol-anchored proteins through metabolic engineering using an azide-modified phosphatidylinositol

Glycosylphosphatidylinositol (GPI) anchorage is one of the most common mechanisms to attach proteins to the plasma membrane of eukaryotic cells. GPI-anchored proteins (GPI-APs) play a critical role in many biological processes but are difficult to study. Here, a new method was developed for the effective and selective metabolic engineering and labeling of cell surface GPI-APs with an azide-modified phosphatidylinositol (PI) as the biosynthetic precursor of GPIs. It was demonstrated that this azido-PI derivative was taken up by HeLa cells and incorporated into the biosynthetic pathway of GPIs to present azide-labeled GPI-APs on the live cell surface. The azido group was used as a molecular handle to install other labels through a biocompatible click reaction to enable various biological studies, e.g., fluorescent imaging and protein pull-down, which can help explore the functions of GPI-APs and discover new GPI-APs.

Nebulized mRNA-Encoded Antibodies Protect Hamsters from SARS-CoV-2 Infection

Despite the success of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) vaccines, there remains a clear need for new classes of preventatives for respiratory viral infections due to vaccine hesitancy, lack of sterilizing immunity, and for at-risk patient populations, including the immunocompromised. While many neutralizing antibodies have been identified, and several approved, to treat COVID-19, systemic delivery, large doses, and high costs have the potential to limit their widespread use, especially in low- and middle-income countries. To use these antibodies more efficiently, an inhalable formulation is developed that allows for the expression of mRNA-encoded, membrane-anchored neutralizing antibodies in the lung to mitigate SARS-CoV-2 infections. First, the ability of mRNA-encoded, membrane-anchored, anti-SARS-CoV-2 antibodies to prevent infections in vitro is demonstrated. Next, it is demonstrated that nebulizer-based delivery of these mRNA-expressed neutralizing antibodies potently abrogates disease in the hamster model. Overall, these results support the use of nebulizer-based mRNA expression of neutralizing antibodies as a new paradigm for mitigating respiratory virus infections.

Enzymatic Nanosphere-to-Nanofiber Transition and Autophagy Inducer Release Promote Tumor Chemotherapy

Chemotherapy has remained an effective and predominant cancer treatment for the past decades, but is hampered by its low response rate and severe systemic toxicity. Combination chemotherapies are proposed to address these issues, yet their therapeutic outcomes are still far from satisfactory. Thus, it is urgent to develop novel strategies to promote tumor chemosensitivity while reducing toxic side effects of chemotherapeutics. Herein, employing a rationally designed peptide conjugate Nap-Phe-Phe-Lys(SA-AZD8055)-Tyr(H₂ PO₃ )-OH (Nap-AZD-Yp), a novel approach of simultaneous intracellular nanofiber formation and autophagy inducer release is proposed for selectively sensitizing tumor to chemotherapy. Upon sequential catalyses of alkaline phosphatase and carboxylesterase, Nap-AZD-Yp undergoes nanosphere-to-nanofiber transition accompanied by autophagy inducer AZD8055 release in cancer cells. Cell experiments show enhanced endocytosis of anticancer drug doxorubicin and inhibition of cell migration due to the intracellular nanofiber formation. The released AZD8055 further activates excessive autophagy of cancer cells, sensitizing them to chemotherapy. Animal experiment results suggest Nap-AZD-Yp can significantly enhance the therapeutic effects of doxorubicin on tumors while mitigate its toxic adverse effects on normal tissues. It is anticipated that the "smart" concept in this work c be widely employed to develop novel combinational therapies for the treatment of cancers and other diseases in near future.

Species-Specific Duplication of Surface Antigen Genes in Paramecium

Paramecium is a free-living ciliate that undergoes antigenic variation and still the functions of these variable surface antigen coats in this non-pathogenic ciliate remain elusive. Only a few surface antigen genes have been described, mainly in the two model species P. tetraurelia strain 51 and P. primaurelia strain 156. Given the lack of suitable sequence data to allow for phylogenetics and deeper sequence comparisons, we screened the genomes of six different Paramecium species for serotype genes and isolated 548 candidates. Our approach identified the subfamilies of the isogenes of individual serotypes that were mostly represented by intrachromosomal gene duplicates. These showed different duplication levels, and chromosome synteny suggested rather young duplication events after the emergence of the P. aurelia species complex, indicating a rapid evolution of surface antigen genes. We were able to identify the different subfamilies of the surface antigen genes with internal tandem repeats, which showed consensus motifs across species. The individual isogene families showed additional consensus motifs, indicating that the selection pressure holds individual amino acids constant in these repeats. This may be a hint of the receptor function of these antigens rather than a presentation of random epitopes, generating the variability of these surface molecules.

Dynamic Continuum of Nanoscale Peptide Assemblies Facilitates Endocytosis and Endosomal Escape

Alkaline phosphatase (ALP) enables intracellular targeting by peptide assemblies, but how the ALP substrates enter cells remains elusive. Here we show that nanoscale phosphopeptide assemblies cluster ALP to enable caveolae-mediated endocytosis (CME) and endosomal escape. Specifically, fluorescent phosphopeptides undergo enzyme-catalyzed self-assembly to form nanofibers. Live cell imaging unveils that phosphopeptides nanoparticles, coincubated with HEK293 cells overexpressing red fluorescent protein-tagged tissue-nonspecific ALP (TNAP-RFP), cluster TNAP-RFP in lipid rafts to enable CME. Further dephosphorylation of the phosphopeptides produces peptidic nanofibers for endosomal escape. Inhibiting TNAP, cleaving the membrane anchored TNAP, or disrupting lipid rafts abolishes the endocytosis. Decreasing the transformation to nanofibers prevents the endosomal escape. As the first study establishing a dynamic continuum of nanoscale assemblies for cellular uptake, this work illustrates an effective design for enzyme-responsive supramolecular therapeutics and provides mechanism insights for understanding the dynamics of cellular uptake of proteins or exogenous peptide aggregates.

CD52 Is Elevated on B cells of SLE Patients and Regulates B Cell Function

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by B cell dysregulation and breaks in tolerance that lead to the production of pathogenic autoantibodies. We performed single-cell RNA sequencing of B cells from healthy donors and individuals with SLE which revealed upregulated CD52 expression in SLE patients. We further demonstrate that SLE patients exhibit significantly increased levels of B cell surface CD52 expression and plasma soluble CD52, and levels of soluble CD52 positively correlate with measures of lupus disease activity. Using CD52-deficient JeKo-1 cells, we show that cells lacking surface CD52 expression are hyperresponsive to B cell receptor (BCR) signaling, suggesting an inhibitory role for the surface-bound protein. In healthy donor B cells, antigen-specific BCR-activation initiated CD52 cleavage in a phospholipase C dependent manner, significantly reducing cell surface levels. Experiments with recombinant CD52-Fc showed that soluble CD52 inhibits BCR signaling in a manner partially-dependent on Siglec-10. Moreover, incubation of unstimulated B cells with CD52-Fc resulted in the reduction of surface immunoglobulin and CXCR5. Prolonged incubation of B cells with CD52 resulted in the expansion of IgD+IgMlo anergic B cells. In summary, our findings suggest that CD52 functions as a homeostatic protein on B cells, by inhibiting responses to BCR signaling. Further, our data demonstrate that CD52 is cleaved from the B cell surface upon antigen engagement, and can suppress B cell function in an autocrine and paracrine manner. We propose that increased expression of CD52 by B cells in SLE represents a homeostatic mechanism to suppress B cell hyperactivity.

Amyloid binding and beyond: a new approach for Alzheimer's disease drug discovery targeting Aβo-PrPC binding and downstream pathways

Amyloid β oligomers (Aβo) are the main toxic species in Alzheimer's disease, which have been targeted for single drug treatment with very little success. In this work we report a new approach for identifying functional Aβo binding compounds. A tailored library of 971 fluorine containing compounds was selected by a computational method, developed to generate molecular diversity. These compounds were screened for Aβo binding by a combined 19F and STD NMR technique. Six hits were evaluated in three parallel biochemical and functional assays. Two compounds disrupted Aβo binding to its receptor PrPC in HEK293 cells. They reduced the pFyn levels triggered by Aβo treatment in neuroprogenitor cells derived from human induced pluripotent stem cells (hiPSC). Inhibitory effects on pTau production in cortical neurons derived from hiPSC were also observed. These drug-like compounds connect three of the pillars in Alzheimer's disease pathology, i.e. prion, Aβ and Tau, affecting three different pathways through specific binding to Aβo and are, indeed, promising candidates for further development.

Bridging the GAPs in plant reproduction: a comparison of plant and animal GPI-anchored proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins (GAPs) are a unique type of membrane-associated proteins in eukaryotes. GPI and GAP biogenesis and function have been well studied in non-plant models and play an important role in the fertility of mouse sperm and egg. Although GPI and GAP biogenesis and function in plants are less known, they are critical for flowering plant reproduction because of their essential roles in the fertility of the male and female gametophytes. In Eukaryotes, GPI, a glycolipid molecule, can be post-translationally attached to proteins to serve as an anchor in the plasma membrane. GPI-anchoring, compared to other modes of membrane attachment and lipidation processes, localizes proteins to the extracellular portion of the plasma membrane and confers several unique attributes including specialized sorting during secretion, molecular painting onto membranes, and enzyme-mediated release of protein through anchor cleavage. While the biosynthesis, structure, and role of GPI are mostly studied in mammals, yeast and protists, the function of GPI and GAPs in plants is being discovered, particularly in gametophyte development and function. Here, we review GPI biosynthesis, protein attachment, and remodeling in plants with insights about this process in mammals. Additionally, we summarize the reproductive phenotypes of all loss of function mutations in Arabidopsis GPI biosynthesis and GAP genes and compare these to the reproductive phenotypes seen in mice to serve as a framework to identify gaps in our understanding of plant GPI and GAPs. In addition, we present an analysis on the gametophyte expression of all Arabidopsis GAPs to assist in further research on the role of GPI and GAPs in all aspects of the gametophyte generation in the life cycle of a plant.

Enzymatically Formed Peptide Assemblies Sequestrate Proteins and Relocate Inhibitors to Selectively Kill Cancer Cells

Herein, we show that an enzymatic reaction can generate peptide assemblies that sequestrate proteins to selectively kill cancer cells. A phosphopeptide bearing the antagonistic motif (AVPI) to the inhibitors of apoptotic proteins (IAPs) enters cancer cells and normal cells by caveolin-dependent endocytosis and macropinocytosis, respectively. The AVPI-bearing peptide assemblies sequestrates IAPs and releases bortezomib (BTZ), a proteasome inhibitor, in the cytosol of cancer cells, but rescues the normal cells (namely, HS-5 cells) by trafficking the BTZ into lysosomes. Alkaline phosphatase (ALP) acts as a context-dependent signal for trafficking the peptide/BTZ assemblies and selectively induces the death of the cancer cells. The assemblies of AVPI exhibit enhanced proteolytic resistance. This work, which utilizes the difference in endocytic uptake of enzymatically formed peptide assemblies to selectively kill cancer cells, promises a new way to develop selective cancer therapeutics.

Complex regulation of ecto-5'-nucleotidase/CD73 and A2AR-mediated adenosine signaling at neurovascular unit: A link between acute and chronic neuroinflammation

The review summarizes available data regarding the complex regulation of CD73 at the neurovascular unit (NVU) during neuroinflammation. Based on available data we propose the biphasic pattern of CD73 regulation at NVU, with an early attenuation and a postponed up-regulation of CD73 activity. Transient attenuation of CD73 activity on leukocyte/vascular endothelium and leukocyte/astrocyte surface, required for the initiation of a neuroinflammatory response, may be effectuated either by catalytic inhibition of CD73 and/or by shedding of the CD73 molecule from the cell surface, while postponed induction of CD73 is effectuated by transcriptional up-regulation of Nt5e and posttranslational modifications. Neuroinflammatory conditions are also associated with significant enhancement and gain-of-function of A_2AR-mediated adenosine signaling. However, in contrast to the temporary prevalence of A_2AR over A₁R signaling during an acute inflammatory response, prolonged induction of A_2AR and resulting perpetual CD73/A_2AR coupling may be a contributing factors in the transition between acute and chronic neuroinflammation. Thus, pharmacological targeting of the CD73/A_2AR axis may attenuate inflammatory response and ameliorate neurological deficits in chronic neuroinflammatory conditions.

Complexation of Eu(III), Pb(II), and U(VI) with a Paramecium glycoprotein: Microbial transformation of heavy elements in the aquatic environment

This study investigated the interaction of inorganic aqueous Eu(III), Pb(II), and U(VI) with Paramecium sp., a representative single-celled protozoan that lives in freshwater. Living and prekilled Paramecium cells were tested. The prekilled cells were killed with a fixative. After 24 h exposure of the cells to inorganic aqueous solutions containing Eu(III) or U(VI), analyses by microparticle-induced X-ray emission with a focused beam (<1 μm) did not detect Eu and U in the living cells, whereas Eu and U were detected in the prekilled cells. Size exclusion chromatography coupled with on-line ultraviolet-visible detection and elemental detection by inductively coupled plasma mass spectrometry of the aqueous phases collected after the living cell experiments revealed that a fraction of the Eu, Pb, and U in the aqueous phase bound to a large (ca. 250 kDa) Paramecium biomolecule and formed a metal-organic complex. The characteristics of the biomolecule were consistent with those of the soluble glycoproteins covering the surfaces of Paramecium cells. These results show that Paramecium cells transform inorganic aqueous Eu, Pb, and U to organic complexes. This paper discusses the relation between this novel complexation and the sorption of these heavy elements on Paramecium cells.

Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

During evolution, the cell as a fine-tuned machine had to undergo permanent adjustments to match changes in its environment, while "closed for repair work" was not possible. Evolution from protists (protozoa and unicellular algae) to multicellular organisms may have occurred in basically two lineages, Unikonta and Bikonta, culminating in mammals and angiosperms (flowering plants), respectively. Unicellular models for unikont evolution are myxamoebae (Dictyostelium) and increasingly also choanoflagellates, whereas for bikonts, ciliates are preferred models. Information accumulating from combined molecular database search and experimental verification allows new insights into evolutionary diversification and maintenance of genes/proteins from protozoa on, eventually with orthologs in bacteria. However, proteins have rarely been followed up systematically for maintenance or change of function or intracellular localization, acquirement of new domains, partial deletion (e.g. of subunits), and refunctionalization, etc. These aspects are discussed in this review, envisaging "evolutionary cell biology." Protozoan heritage is found for most important cellular structures and functions up to humans and flowering plants. Examples discussed include refunctionalization of voltage-dependent Ca²⁺ channels in cilia and replacement by other types during evolution. Altogether components serving Ca²⁺ signaling are very flexible throughout evolution, calmodulin being a most conservative example, in contrast to calcineurin whose catalytic subunit is lost in plants, whereas both subunits are maintained up to mammals for complex functions (immune defense and learning). Domain structure of R-type SNAREs differs in mono- and bikonta, as do Ca²⁺ -dependent protein kinases. Unprecedented selective expansion of the subunit a which connects multimeric base piece and head parts (V0, V1) of H⁺ -ATPase/pump may well reflect the intriguing vesicle trafficking system in ciliates, specifically in Paramecium. One of the most flexible proteins is centrin when its intracellular localization and function throughout evolution is traced. There are many more examples documenting evolutionary flexibility of translation products depending on requirements and potential for implantation within the actual cellular context at different levels of evolution. From estimates of gene and protein numbers per organism, it appears that much of the basic inventory of protozoan precursors could be transmitted to highest eukaryotic levels, with some losses and also with important additional "inventions."

Proteolytic cleavage of the hydrophobic domain in the CaVα2δ1 subunit improves assembly and activity of cardiac CaV1.2 channels

Voltage-gated L-type Ca_V1.2 channels in cardiomyocytes exist as heteromeric complexes with the pore-forming Ca_Vα1, Ca_Vβ, and Ca_Vα2δ1 subunits. The full complement of subunits is required to reconstitute the native-like properties of L-type Ca²⁺ currents, but the molecular determinants responsible for the formation of the heteromeric complex are still being studied. Enzymatic treatment with phosphatidylinositol-specific phospholipase C, a phospholipase C specific for the cleavage of glycosylphosphatidylinositol (GPI)-anchored proteins, disrupted plasma membrane localization of the cardiac Ca_Vα2δ1 prompting us to investigate deletions of its hydrophobic transmembrane domain. Patch-clamp experiments indicated that the C-terminally cleaved Ca_Vα2δ1 proteins up-regulate Ca_V1.2 channels. In contrast, deleting the residues before the single hydrophobic segment (Ca_Vα2δ1 Δ1059-1063) impaired current up-regulation. Ca_Vα2δ1 mutants G1060I and G1061I nearly eliminated the cell-surface fluorescence of Ca_Vα2δ1, indicated by two-color flow cytometry assays and confocal imaging, and prevented Ca_Vα2δ1-mediated increase in peak current density and modulation of the voltage-dependent gating of Ca_V1.2. These impacts were specific to substitutions with isoleucine residues because functional modulation was partially preserved in Ca_Vα2δ1 G1060A and G1061A proteins. Moreover, C-terminal fragments exhibited significantly altered mobility in denatured immunoblots of Ca_Vα2δ1 G1060I and Ca_Vα2δ1 G1061I, suggesting that these mutant proteins were impaired in proteolytic processing. Finally, Ca_Vα2δ1 Δ1059-1063, but not Ca_Vα2δ1 G1060A, failed to co-immunoprecipitate with Ca_V1.2. Altogether, our data support a model in which small neutral hydrophobic residues facilitate the post-translational cleavage of the Ca_Vα2δ1 subunit at the predicted membrane interface and further suggest that preventing GPI anchoring of Ca_Vα2δ1 averts its cell-surface expression, its interaction with Ca_Vα1, and modulation of Ca_V1.2 currents.

Differential subcellular distribution of four phospholipase C isoforms and secretion of GPI-PLC activity

Phospholipase C (PLC) is an important enzyme of signal transduction pathways by generation of second messengers from membrane lipids. PLCs are also indicated to cleave glycosylphosphatidylinositol (GPI)-anchors of surface proteins thus releasing these into the environment. However, it remains unknown whether this enzymatic activity on the surface is due to distinct PLC isoforms in higher eukaryotes. Ciliates have, in contrast to other unicellular eukaryotes, multiple PLC isoforms as mammals do. Thus, Paramecium represents a perfect model to study subcellular distribution and potential surface activity of PLC isoforms. We have identified distinct subcellular localizations of four PLC isoforms indicating functional specialization. The association with different calcium release channels (CRCs) argues for distinct subcellular functions. They may serve as PI-PLCs in microdomains for local second messenger responses rather than free floating IP₃. In addition, all isoforms can be found on the cell surface and they are found together with GPI-cleaved surface proteins in salt/ethanol washes of cells. We can moreover show them in medium supernatants of living cells where they have access to GPI-anchored surface proteins. Among the isoforms we cannot assign GPI-PLC activity to specific PLC isoforms; rather each PLC is potentially responsible for the release of GPI-anchored proteins from the surface.

Pharmacological Inhibition of Protein Lipidation

Lipid modifications of mammalian proteins are widespread, modifying thousands of targets involved in all aspects of cellular physiology cellular physiology. Broadly, lipidations serve to increase protein hydrophobicity and association with cellular membranes. Often, these modifications are absolutely essential for protein stability and localization, and serve critical roles in dynamic regulation of protein function. A number of lipidated proteins are associated with diseases, including parasite infections, neurological diseases, diabetes, and cancer, suggesting that lipid modifications represent potentially attractive targets for pharmacological intervention. This review briefly describes the various types of posttranslational protein lipid modifications, proteins modified by them, and the enzymatic machinery associated with these. We then discuss several case studies demonstrating successful development of lipidation inhibitors of potential (and more rarely, realized) clinical value. Although this field remains in its infancy, we believe these examples demonstrate the potential utility of targeting protein lipidation as a viable strategy for inhibiting the function of pathogenic proteins.

Three-finger snake neurotoxins and Ly6 proteins targeting nicotinic acetylcholine receptors: pharmacological tools and endogenous modulators

Snake venom neurotoxins and lymphocyte antigen 6 (Ly6) proteins, most of the latter being membrane tethered by a glycosylphosphatidylinositol (GPI) anchor, have a variety of biological activities, but their three-finger (3F) folding combines them in one Ly6/neurotoxin family. Subsets of two groups, represented by α-neurotoxins and Lynx1, respectively, interact with nicotinic acetylcholine receptors (nAChR) and, hence, are of therapeutic interest for the treatment of neurodegenerative diseases, pain, and cancer. Information on the mechanisms of action and 3D structure of the binding sites, which is required for drug design, is available from the 3D structure of α-neurotoxin complexes with nAChR models. Here, I compare the structural and functional features of α-neurotoxins versus Lynx1 and its homologs to get a clearer picture of Lynx1-nAChR interactions that is necessary for fundamental science and practical applications.

Characterization of the GPI-anchored lipid transfer proteins in the moss Physcomitrella patens

The non-specific lipid transfer proteins (nsLTPs) are characterized by a compact structure with a central hydrophobic cavity very suitable for binding hydrophobic ligands, such as lipids. The nsLTPs are encoded by large gene families in all land plant lineages, but seem to be absent from green algae. The nsLTPs are classified to different types based on molecular weight, sequence similarity, intron position or spacing between the cysteine residues. The Type G nsLTPs (LTPGs) have a GPI-anchor in the C-terminal region which may attach the protein to the exterior side of the plasma membrane. Here, we present the first characterization of nsLTPs from an early diverged plant, the moss Physcomitrella patens. Moss LTPGs were heterologously produced and purified from Pichia pastoris. The purified moss LTPGs were found to be extremely heat stable and showed a binding preference for unsaturated fatty acids. Structural modeling implied that high alanine content could be important for the heat stability. Lipid profiling revealed that cutin monomers, such as C16 and C18 mono- and di-hydroxylated fatty acids, could be identified in P. patens. Expression of a moss LTPG-YFP fusion revealed localization to the plasma membrane. The expressions of many of the moss LTPGs were found to be upregulated during drought and cold treatments.

Comparative transcriptome of wild type and selected strains of the microalgae Tisochrysis lutea provides insights into the genetic basis, lipid metabolism and the life cycle

The applied exploitation of microalgae cultures has to date almost exclusively involved the use of wild type strains, deposited over decades in dedicated culture collections. Concomitantly, the concept of improving algae with selection programs for particular specific purposes is slowly emerging. Studying since a decade an economically and ecologically important haptophyte Tisochrysis lutea (Tiso), we took advantage of the availability of wild type (Tiso-Wt) and selected (Tiso-S2M2) strains to conduct a molecular variations study. This endeavour presented substantial challenges: the genome assembly was not yet available, the life cycle unknown and genetic diversity of Tiso-Wt poorly documented. This study brings the first molecular data in order to set up a selection strategy for that microalgae. Following high-throughput Illumina sequencing, transcriptomes of Tiso-Wt and Tiso-S2M2 were de novo assembled and annotated. Genetic diversity between both strains was analyzed and revealed a clear conservation, while a comparison of transcriptomes allowed identification of polymorphisms resulting from the selection program. Of 34,374 transcripts, 291 were differentially expressed and 165 contained positional polymorphisms (SNP, Indel). We focused on lipid over-accumulation of the Tiso-S2M2 strain and 8 candidate genes were identified by combining analysis of positional polymorphism, differential expression levels, selection signature and by study of putative gene function. Moreover, genetic analysis also suggests the existence of a sexual cycle and genetic recombination in Tisochrysis lutea.

Dynamic chromatin remodelling of ciliate macronuclear DNA as determined by an optimized chromatin immunoprecipitation (ChIP) method for Paramecium tetraurelia

We report the detailed evaluation of crucial parameters for chromatin immunoprecipitation (ChIP) of macronuclear DNA in the unicellular eukaryote Paramecium tetraurelia. Optimized parameters include crosslinking conditions, chromatin sonication and antibody titration thus providing a detailed protocol for successful ChIP in P. tetraurelia. As this ciliate is bacterivorous and RNAi by feeding represents a powerful tool for analysis of gene function, we moreover determined the effects of ingested nucleic acids by food bacteria. Feasibility of our protocol is demonstrated by characterisation of chromatin remodelling at promoters of cytosolic HSP70 isoforms during transcriptional activation under heat shock conditions by analyzing RNA abundance, nucleosome occupancy and levels of H3 lysine 9 acetylation.