Functional role of glycosylation on the recombinant CD38/ADP-ribosyl cyclase in CHO cells

Cancer stem cell marker glycosylation: Nature, function and significance

Glycans are essential for the maintenance of normal biological function, with alterations in glycan expression being a hallmark of cancer. Cancer stem cells (CSCs) are a subset of cells within a tumour capable of self-renewal, cellular differentiation and resistances to conventional therapies. As is the case with stem cells, marker proteins present on the cell surface are frequently used to identify and enrich CSCs, with the expression of these markers statistical correlating with the likelihood of cancer recurrence and overall patient survival. As such CSC markers are of high clinical relevance. The majority of markers currently used to identify CSC populations are glycoproteins, and although the diverse biological roles for many of these markers are known, the nature and function of the glycan moiety on these glycoproteins remains to be fully elucidated. This mini-review summarises our current knowledge regarding the types and extent of CSC marker glycosylation, and the various roles that these glycans play in CSC biology, including in mediating cell adhesion, metastasis, evading apoptosis, tear shear resistance, tumour growth, maintaining pluripotency, self-renewal, trafficking, maintaining stability, maintaining enzymatic activity and aiding epithelial mesenchymal transitioning. Given that CSCs markers have multiple diverse biological functions, and are potentially of significant diagnostic and therapeutic benefit the search for new markers that are uniquely expressed on CSCs is vital to selectively target/identify this subset of cancer cells. As such we have also outlined how high-throughput lectin microarrays can be used to successfully profile the glycosylation status of CSC and to identify glyco-markers unique to CSCs.

NAD and ADP-ribose metabolism in mitochondria

Mitochondrial metabolism is intimately connected to the universal coenzyme NAD. In addition to its role in redox reactions of energy transduction, NAD serves as substrate in regulatory reactions that lead to its degradation. Importantly, all types of the known NAD-consuming signalling reactions have been reported to take place in mitochondria. These reactions include the generation of second messengers, as well as post-translational protein modifications such as ADP-ribosylation and protein deacetylation. Therefore, the availability and redox state of NAD emerged as important factors in the regulation of mitochondrial metabolism. Molecular mechanisms and targets of mitochondrial NAD-dependent protein deacetylation and mono-ADP-ribosylation have been established, whereas poly-ADP-ribosylation and NAD-derived messenger generation in the organelles await in-depth characterization. In this review, we highlight the major NAD-dependent reactions occurring within mitochondria and describe their metabolic and regulatory functions. We also discuss the metabolic fates of the NAD-degradation products, nicotinamide and ADP-ribose, and how the mitochondrial NAD pool is restored.

Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology

The membrane proteins CD38 and CD157 belong to an evolutionarily conserved family of enzymes that play crucial roles in human physiology. Expressed in distinct patterns in most tissues, CD38 (and CD157) cleaves NAD(+) and NADP(+), generating cyclic ADP ribose (cADPR), NAADP, and ADPR. These reaction products are essential for the regulation of intracellular Ca(2+), the most ancient and universal cell signaling system. The entire family of enzymes controls complex processes, including egg fertilization, cell activation and proliferation, muscle contraction, hormone secretion, and immune responses. Over the course of evolution, the molecules have developed the ability to interact laterally and frontally with other surface proteins and have acquired receptor-like features. As detailed in this review, the loss of CD38 function is associated with impaired immune responses, metabolic disturbances, and behavioral modifications in mice. CD38 is a powerful disease marker for human leukemias and myelomas, is directly involved in the pathogenesis and outcome of human immunodeficiency virus infection and chronic lymphocytic leukemia, and controls insulin release and the development of diabetes. Here, the data concerning diseases are examined in view of potential clinical applications in diagnosis, prognosis, and therapy. The concluding remarks try to frame all of the currently available information within a unified working model that takes into account both the enzymatic and receptorial functions of the molecules.

N-linked glycosylation of CD38 is required for its structure stabilization but not for membrane localization

CD38 is a type II transmembrane protein with 25% of its molecular mass consisting of glycosyl moieties. It has long been predicted that the carbohydrate moieties of glycoproteins play important roles in the physical function and structural stability of the proteins on cell surfaces. To determine the structural/functional significance of glycosylation of the human CD38, the four potential N-linked glycosylation sites asparagine residues, N100, N164, N209 and N219 were mutated. The mutant (CD38mu) and wild-type (CD38wt) were expressed separately in Escherichia coli, HeLa, and MCF-7 cells. SDS-polyacrylamide gel electrophoresis under reducing conditions and western blotting indicated that the molecular mass of the CD38wt is 45 kDa, and that of the CD38mu is 34 kDa in HeLa cells. Importantly, the CD38mu protein expressed in HeLa cells, showed the high molecular weight oligomers in addition to the 34 kDa monomeric form. Similarly, in E. coli, the CD38wt formed dimers and other oligomers besides the monomeric form. Moreover, MCF-7 cells stably transfected with CD38wt cDNA, also revealed the presence of cross-linked oligomers when treated with a N-linked glycosylation inhibitor tunicamycin (TM). These results suggested that the N-linked glycosylation of CD38 plays a crucial role in the structure stability by preventing the formation inter-molecular cross-links. In addition, immunostaining, enzyme activity (cyclase), and western blotting data revealed that the glycosylation of human CD38 protein is not required for its localization to the cell membrane.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

CD157, the Janus of CD38 but with a unique personality

CD157 is a pleiotropic ectoenzyme which belongs to the CD38 family and to the growing number of leukocyte surface molecules known to act independently as both receptors and enzymes. A 45-kDa surface structure with a GPI anchor, the CD157 molecule displays two distinct domains in its extracellular component. The first is implicated in the enzymic activities of the molecule and the second features adhesion/signalling properties. CD157 shares several characteristics with CD38, including a similar amino acid sequence and enzymic functions. Both molecules are involved in the metabolism of NAD(+), and the CD157 gene is synthenic on 4p15 with CD38, with which it also shares a unique genomic organization. Their conservation in phylogeny is striking evidence for their relevance in the life and death cycle of the cell.

Site-directed removal of N-glycosylation sites in BST-1/CD157: effects on molecular and functional heterogeneity

Cyclic ADP ribose (cADPR) is a novel second messenger that releases calcium from intracellular calcium stores, but works independently of inositol 1,4,5-trisphosphate. In mammals ADP-ribosyl cyclase function is found in two membrane proteins, CD38 and bone marrow stromal cell antigen 1 (BST-1)/CD157. These enzymes are exposed extracellularly and also possess cADPR hydrolase activity, but an intracellular soluble ADP-ribosyl cyclase has been reported in human T-cells. Previously, a soluble form of BST-1/CD157 (sBST-1), which lacked the glycosylphosphatidylinositol-anchored portion, was expressed by a baculovirus-insect-cell system. In this study, we have purified the sBST-1, and it migrated as two major bands by SDS/PAGE, suggesting that it is post-translationally modified. BST-1 contains four putative N-glycosylation sites. Tunicamycin treatment reduced sBST-1 expression in the culture medium, indicating that N-glycosylation is essential for secretion. Site-directed mutagenesis was performed to generate sBST-1 mutants (N1-N4), each preserving a single N-glycosylation site. N1, N3 and N4 were well secreted into the medium, and were each detected as a single band. Although N3 and N4 retained the ADP-ribosyl cyclase activity, the cADPR-hydrolase activity was retained only in N4. We conclude that N-glycosylation of sBST-1 facilitates the folding of the nascent polypeptide chain into a conformation that is conductive for intracellular transport and enzymic activity. Furthermore a crystal has been obtained using the N4 mutant, but not the wild-type sBST-1. Thus the artificial engineering of N-glycosylation sites could be an effective method to generate homogeneous material for structural studies.