Human sialidase NEU4 long and short are extrinsic proteins bound to outer mitochondrial membrane and the endoplasmic reticulum, respectively

NEU4-mediated desialylation enhances the activation of the oncogenic receptors for the dissemination of ovarian carcinoma

Glycosylation profoundly influences the interactions between cancer cells and microenvironmental stromal cells during the peritoneal disseminated metastasis of ovarian carcinoma (OC), which is the major cause of cancer-related death. Although the characteristic cancer glycoconjugates are widely used as biomarkers for cancer diagnosis, our knowledge about cancer glycome remains quite fragmented due to the technique limitations in analyzing glycan chains with tremendous structural and functional heterogeneity. Given the dysregulated cancer glycome is defined by the altered glycosylation machinery, here we performed a systematic loss-of-function screen on 498 genes involved in glycosylation for key regulators of OC dissemination. We identified neuraminidase 4 (NEU4), an enzyme capable of hydrolyzing terminal sialic acid from glycoconjugates, as a vital peritoneal dissemination-promoting modifier of OC glycome. In human patients with high-grade serous OC (HGSOC), increased NEU4 was detected in the disseminated OC cells when compared with that in the primary tumor cells, which significantly correlated with the worse survival. Among three alternative splice-generated isoforms of human NEU4, we revealed that only the plasma membrane-localized NEU4 isoform 2 (NEU4-iso2) and intracellular isoform 3 promoted the peritoneal dissemination of OC by enhancing the cell motility and epithelial-mesenchymal transition. We also identified NEU4-iso2-regulated cell surface glycoproteome and found that NEU4-iso2 desialylated the epithelial growth factor receptor (EGFR), in particular at N196 residue, for the hyperactivation of EGFR and its downstream tumor-promoting signaling cascades. Our results provide new insights into how the OC glycome is dysregulated during OC progression and reveal a functionally important glycosite on EGFR for its abnormal activation in cancer.

Targeting Neuraminidase 4 Attenuates Kidney Fibrosis in Mice

Despite significant progress in therapy, there remains a lack of substantial evidence regarding the molecular factors that lead to renal fibrosis. Neuraminidase 4 (NEU4), an enzyme that removes sialic acids from glycoconjugates, has an unclear role in chronic progressive fibrosis. Here, this study finds that NEU4 expression is markedly upregulated in mouse fibrotic kidneys induced by folic acid or unilateral ureter obstruction, and this elevation is observed in patients with renal fibrosis. NEU4 knockdown specifically in the kidney attenuates the epithelial-to-mesenchymal transition, reduces the production of pro-fibrotic cytokines, and decreases cellular senescence in male mice. Conversely, NEU4 overexpression exacerbates the progression of renal fibrosis. Mechanistically, NEU4254-388aa interacts with Yes-associated protein (YAP) at WW2 domain (231-263aa), promoting its nucleus translocation and activation of target genes, thereby contributing to renal fibrosis. 3,5,6,7,8,3',4'-Heptamethoxyflavone, a natural compound, is identified as a novel NEU4 inhibitor, effectively protecting mice from renal fibrosis in a NEU4-dependent manner. Collectively, the findings suggest that NEU4 may represent a promising therapeutic target for kidney fibrosis.

Cellular translocation and secretion of sialidases

Sialidases (or neuraminidases) catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly the removal of the terminal Sia on glycans (desialylation) of either glycoproteins or glycolipids. Therefore, sialidases can modulate the functionality of the target glycoprotein or glycolipid and are involved in various biological pathways in health and disease. In mammalian cells, there are four kinds of sialidase, which are Neu1, Neu2, Neu3, and Neu4, based on their subcellular locations and substrate specificities. Neu1 is the lysosomal sialidase, Neu2 is the cytosolic sialidase, Neu3 is the plasma membrane-associated sialidase, and Neu4 is found in the lysosome, mitochondria, and endoplasmic reticulum. In addition to specific subcellular locations, sialidases can translocate to different subcellular localizations within particular cell conditions and stimuli, thereby participating in different cellular functions depending on their loci. Lysosomal sialidase Neu1 can translocate to the cell surface upon cell activation in several cell types, including immune cells, platelets, endothelial cells, and epithelial cells, where it desialylates receptors and thus impacts receptor activation and signaling. On the other hand, cells secrete sialidases upon activation. Secreted sialidases can serve as extracellular sialidases and cause the desialylation of both extracellular glycoproteins or glycolipids and cell surface glycoproteins or glycolipids on their own and other cells, thus playing roles in various biological pathways as well. This review discusses the recent advances and understanding of sialidase translocation in different cells and secretion from different cells under different conditions and their involvement in physiological and pathological pathways.

Lysosomal sialidase NEU1, its intracellular properties, deficiency, and use as a therapeutic agent

Neuraminidase 1 (NEU1) is a lysosomal sialidase that cleaves terminal α-linked sialic acid residues from sialylglycans. NEU1 is biosynthesized in the rough endoplasmic reticulum (RER) lumen as an N-glycosylated protein to associate with its protective protein/cathepsin A (CTSA) and then form a lysosomal multienzyme complex (LMC) also containing β-galactosidase 1 (GLB1). Unlike other mammalian sialidases, including NEU2 to NEU4, NEU1 transport to lysosomes requires association of NEU1 with CTSA, binding of the CTSA carrying terminal mannose 6-phosphate (M6P)-type N-glycan with M6P receptor (M6PR), and intralysosomal NEU1 activation at acidic pH. In contrast, overexpression of the single NEU1 gene in mammalian cells causes intracellular NEU1 protein crystallization in the RER due to self-aggregation when intracellular CTSA is reduced to a relatively low level. Sialidosis (SiD) and galactosialidosis (GS) are autosomal recessive lysosomal storage diseases caused by the gene mutations of NEU1 and CTSA, respectively. These incurable diseases associate with the NEU1 deficiency, excessive accumulation of sialylglycans in neurovisceral organs, and systemic manifestations. We established a novel GS model mouse carrying homozygotic Ctsa IVS6 + 1 g/a mutation causing partial exon 6 skipping with simultaneous deficiency of Ctsa and Neu1. Symptoms developed in the GS mice like those in juvenile/adult GS patients, such as myoclonic seizures, suppressed behavior, gargoyle-like face, edema, proctoptosis due to Neu1 deficiency, and sialylglycan accumulation associated with neurovisceral inflammation. We developed a modified NEU1 (modNEU1), which does not form protein crystals but is transported to lysosomes by co-expressed CTSA. In vivo gene therapy for GS and SiD utilizing a single adeno-associated virus (AAV) carrying modNEU1 and CTSA genes under dual promoter control will be created.

Elucidation of the mechanism of nuclear localization of Mexican tetra Neu4 via bipartite nuclear localization signal and less conserved regions

Nuclear sialoglycans are minor components in the nucleus, and their biological significance was not well understood. Recently, Nile tilapia Neu4 sialidase (OnNeu4) was identified and reported as the first nuclear sialidase in vertebrates. Although OnNeu4 possesses the nuclear localization signal (NLS) required for nuclear localization, other fish Neu4 sialidases, such as zebrafish and Japanese medaka, also possess NLS, but their subcellular localizations are not nucleus. To understand the nuclear localization mechanism of fish Neu4, we focused on Mexican tetra Neu4 (AmNeu4), which, unlike Neu4 in other fishes, has a bipartite NLS. AmNeu4 exhibited a wide range of optimal pH and substrate specificity, and its gene expression was specifically detected in the liver, spleen, and gut in adult fish. AmNeu4, like OnNeu4, exhibited nuclear localization, which was attenuated by importin inhibitor, and deletion of the bipartite NLS completely reduced the nuclear localization. In addition, the conjugation of the bipartite NLS of AmNeu4 made GFP show nuclear localization. To understand the mechanism of nuclear localization of AmNeu4 and OnNeu4, we compared fish Neu4 amino acid sequences and focused on the less conserved region of Neu4 sialidase (LCR). LCR-deletion mutants of AmNeu4 and OnNeu4 showed significantly reduced the nuclear localization. The LCR region in AmNeu4 and OnNeu4 possessed consecutive Ser/Thr. The Neu4 mutants in which consecutive Ser/Thr in LCR were changed to Ala or deleted significantly suppressed the nuclear localization. These results suggest that the nuclear localization of Neu4 in Nile tilapia and Mexican tetra may be regulated by NLS and LCR.

Mass Spectrometry Detects Sphingolipid Metabolites for Discovery of New Strategy for Cancer Therapy from the Aspect of Programmed Cell Death

Sphingolipids, a type of bioactive lipid, play crucial roles within cells, serving as integral components of membranes and exhibiting strong signaling properties that have potential therapeutic implications in anti-cancer treatments. However, due to the diverse group of lipids and intricate mechanisms, sphingolipids still face challenges in enhancing the efficacy of different therapy approaches. In recent decades, mass spectrometry has made significant advancements in uncovering sphingolipid biomarkers and elucidating their impact on cancer development, progression, and resistance. Primary sphingolipids, such as ceramide and sphingosine-1-phosphate, exhibit contrasting roles in regulating cancer cell death and survival. The evasion of cell death is a characteristic hallmark of cancer cells, leading to treatment failure and a poor prognosis. The escape initiates with long-established apoptosis and extends to other programmed cell death (PCD) forms when patients experience chemotherapy, radiotherapy, and/or immunotherapy. Gradually, supportive evidence has uncovered the fundamental molecular mechanisms underlying various forms of PCD leading to the development of innovative molecular, genetic, and pharmacological tools that specifically target sphingolipid signaling nodes. In this study, we provide a comprehensive overview of the sphingolipid biomarkers revealed through mass spectrometry in recent decades, as well as an in-depth analysis of the six main forms of PCD (apoptosis, autophagy, pyroptosis, necroptosis, ferroptosis, and cuproptosis) in aspects of tumorigenesis, metastasis, and tumor response to treatments. We review the corresponding small-molecule compounds associated with these processes and their potential implications in cancer therapy.

Structure of the immunoregulatory sialidase NEU1

Sialic acids linked to glycoproteins and glycolipids are important mediators of cell and protein recognition events. These sugar residues are removed by neuraminidases (sialidases). Neuraminidase-1 (sialidase-1 or NEU1) is a ubiquitously expressed mammalian sialidase located in lysosomes and on the cell membrane. Because of its modulation of multiple signaling processes, it is a potential therapeutic target for cancers and immune disorders. Genetic defects in NEU1 or in its protective protein cathepsin A (PPCA, CTSA) cause the lysosomal storage diseases sialidosis and galactosialidosis. To further our understanding of this enzyme's function at the molecular level, we determined the three-dimensional structure of murine NEU1. The enzyme oligomerizes through two self-association interfaces and displays a wide substrate-binding cavity. A catalytic loop adopts an inactive conformation. We propose a mechanism of activation involving a conformational change in this loop upon binding to its protective protein. These findings may facilitate the development of selective inhibitor and agonist therapies.

Neuraminidase 4 (NEU4): new biological and physiological player

Sialidases are found in viruses, bacteria, fungi, avians, and mammals. Mammalian sialidases differ in their specificity, optimum pH, subcellular localization, and tissue expression. To date, four genes encoding mammalian sialidases (NEU1-4) have been cloned. This review examines the functional impact of NEU4 sialidase on complex physiological and cellular processes. The intracellular localization and trafficking of NEU4 and its potential target molecules are discussed along with its impact on cancer, lysosomal storage disease, and cellular differentiation. Modulation of NEU4 expression may be essential not only for the breakdown of sialylated glycoconjugates, but also in the activation or inactivation of functionally important cellular events.

3'-sulfated LewisA/C: An oncofetal epitope associated with metaplastic and oncogenic plasticity of the gastrointestinal foregut

Metaplasia, dysplasia, and cancer arise from normal epithelia via a plastic cellular transformation, typically in the setting of chronic inflammation. Such transformations are the focus of numerous studies that strive to identify the changes in RNA/Protein expression that drive such plasticity along with the contributions from the mesenchyme and immune cells. However, despite being widely utilized clinically as biomarkers for such transitions, the role of glycosylation epitopes is understudied in this context. Here, we explore 3'-Sulfo-Lewis A/C, a clinically validated biomarker for high-risk metaplasia and cancer throughout the gastrointestinal foregut: esophagus, stomach, and pancreas. We discuss the clinical correlation of sulfomucin expression with metaplastic and oncogenic transformation, as well as its synthesis, intracellular and extracellular receptors and suggest potential roles for 3'-Sulfo-Lewis A/C in contributing to and maintaining these malignant cellular transformations.

Neuraminidases-Key Players in the Inflammatory Response after Pathophysiological Cardiac Stress and Potential New Therapeutic Targets in Cardiac Disease

Glycoproteins and glycolipids on the cell surfaces of vertebrates and higher invertebrates contain α-keto acid sugars called sialic acids, terminally attached to their glycan structures. The actual level of sialylation, regulated through enzymatic removal of the latter ones by NEU enzymes, highly affects protein-protein, cell-matrix and cell-cell interactions. Thus, their regulatory features affect a large number of different cell types, including those of the immune system. Research regarding NEUs within heart and vessels provides new insights of their involvement in the development of cardiovascular pathologies and identifies mechanisms on how inhibiting NEU enzymes can have a beneficial effect on cardiac remodelling and on a number of different cardiac diseases including CMs and atherosclerosis. In this regard, a multitude of clinical studies demonstrated the potential of N-acetylneuraminic acid (Neu5Ac) to serve as a biomarker following cardiac diseases. Anti-influenza drugs i.e., zanamivir and oseltamivir are viral NEU inhibitors, thus, they block the enzymatic activity of NEUs. When considering the improvement in cardiac function in several different cardiac disease animal models, which results from NEU reduction, the inhibition of NEU enzymes provides a new potential therapeutic treatment strategy to treat cardiac inflammatory pathologies, and thus, administrate cardioprotection.

Review sialidase NEU3 and its pathological significance

Sialidases (EC 3.2.1.18, also called neuraminidases) catalyze the removal of α-glycosidically linked sialic acid residues from glycoproteins and glycolipids; this is the initial step in the degradation of these glycoconjugates. Sialidases of mammalian origin have been implicated in not only lysosomal catabolism but also the modulation of functional molecules involved in many biological processes. To date, four types of mammalian sialidases have been cloned and designated as Neu1, Neu2, Neu3 and Neu4. These sialidases differ in their subcellular localization and enzymatic properties, as well as their chromosomal localization, and they are expressed in a tissue-specific manner. Among the sialidases, the plasma membrane-associated sialidase Neu3 appears to play particular roles in controlling transmembrane signaling through the modulation of gangliosides, and its aberrant expression is closely related to various pathogeneses, including that of cancer. Interestingly, the human orthologue NEU3 acts in two ways, catalytic hydrolysis of gangliosides and protein interactions with other signaling molecules. Aberrant NEU3 expression can induce various pathological conditions. This review briefly summarizes recent studies, focusing on the involvement of NEU3 in various pathological phenomena.

Human Neuraminidases: Structures and Stereoselective Inhibitors

This Perspective describes the classification, structures, substrates, mechanisms of action, and implications of human neuraminidases (hNEUs) in various pathologies. Some inhibitors have been developed for each isoform, leading to more precise interactions with hNEUs. Although crystal structure data are available for NEU2, most of the findings are based on NEU1 inhibition, and limited information is available for other hNEUs. Therefore, the synthesis of new compounds would facilitate the enrichment of the arsenal of inhibitors to better understand the roles of hNEUs and their mechanisms of action. Nevertheless, due to the already known inhibitors of human neuraminidase enzymes, a structure-activity relationship is presented along with different approaches to inhibit these enzymes for the development of potent and selective inhibitors. Among the different emerging strategies, one is the inhibition of the dimerization of NEU1 or NEU3, and the second is the inhibition of certain receptors located close to hNEU.

Sialidase neu4 deficiency is associated with neuroinflammation in mice

Sialidases catalyze the removal of sialic acid residues from glycoproteins, oligosaccharides, and sialylated glycolipids. Sialidase Neu4 is in the lysosome and has broad substrate specificity. Previously generated Neu4-/- mice were viable, fertile and lacked gross morphological abnormalities, but displayed a marked vacuolization and lysosomal storage in lung and spleen cells. In addition, we showed that there is an increased level of GD1a ganglioside and a markedly decreased level of GM1 ganglioside in the brain of Neu4-/- mice. In this study, we further explored whether sialidase Neu4 deficiency causes neuroinflammation. We demostrated that elevated level of GD1a and GT1b is associated with an increased level of LAMP1-positive lysosomal vesicles and Tunel-positive neurons correlated with alterations in the expression of cytokines and chemokines in adult Neu4-/- mice. Astrogliosis and microgliosis were also significantly enhanced in the hippocampus, and cerebellum. These changes in brain immunity were accompanied by motor impairment in these mice. Our results indicate that sialidase Neu4 is a novel mediator of an inflammatory response in the mouse brain due to the altered catabolism of gangliosides.

Inhibiting Sialidase-Induced TGF-β1 Activation Attenuates Pulmonary Fibrosis in Mice

The active form of transforming growth factor-β1 (TGF-β1) plays a key role in potentiating fibrosis. TGF-β1 is sequestered in an inactive state by a latency-associated glycopeptide (LAP). Sialidases (also called neuraminidases (NEU)) cleave terminal sialic acids from glycoconjugates. The sialidase NEU3 is upregulated in fibrosis, and mice lacking Neu3 show attenuated bleomycin-induced increases in active TGF-β1 in the lungs and attenuated pulmonary fibrosis. Here we observe that recombinant human NEU3 upregulates active human TGF-β1 by releasing active TGF-β1 from its latent inactive form by desialylating LAP. Based on the proposed mechanism of action of NEU3, we hypothesized that compounds with a ring structure resembling picolinic acid might be transition state analogs and thus possible NEU3 inhibitors. Some compounds in this class showed nanomolar IC₅₀ for recombinant human NEU3 releasing active human TGF-β1 from the latent inactive form. The compounds given as daily 0.1-1-mg/kg injections starting at day 10 strongly attenuated lung inflammation, lung TGF-β1 upregulation, and pulmonary fibrosis at day 21 in a mouse bleomycin model of pulmonary fibrosis. These results suggest that NEU3 participates in fibrosis by desialylating LAP and releasing TGF-β1 and that the new class of NEU3 inhibitors are potential therapeutics for fibrosis. SIGNIFICANCE STATEMENT: The extracellular sialidase NEU3 appears to be a key driver of pulmonary fibrosis. The significance of this report is that 1) we show the mechanism (NEU3 desialylates the latency-associated glycopeptide protein that keeps the profibrotic cytokine transforming growth factor-β1 (TGF-β1) in an inactive state, causing active TGF-β1 release), 2) we then use the predicted NEU3 mechanism to identify nM IC₅₀ NEU3 inhibitors, and 3) these new NEU3 inhibitors are potent therapeutics in a mouse model of pulmonary fibrosis.

Gangliosides and Neuroblastomas

The focus of this review is the ganglio-series of glycosphingolipids found in neuroblastoma (NB) and the myriad of unanswered questions associated with their possible role(s) in this cancer. NB is one of the more common solid malignancies of children. Five-year survival for those diagnosed with low risk NB is 90-95%, while that for children with high-risk NB is around 40-50%. Much of the survival rate reflects age of diagnosis with children under a year having a much better prognosis than those over two. Identification of expression of GD2 on the surface of most NB cells led to studies of the effectiveness and subsequent approval of anti-GD2 antibodies as a treatment modality. Despite much success, a subset of patients, possibly those whose tumors fail to express concentrations of gangliosides such as GD1b and GT1b found in tumors from patients with a good prognosis, have tumors refractory to treatment. These observations support discussion of what is known about control of ganglioside synthesis, and their actual functions in NB, as well as their possible relationship to treatment response.

Diversity of sialidases found in the human body - A review

Sialidases are enzymes essential for numerous organisms including humans. Hydrolytic sialidases (EC 3.2.1.18), trans-sialidases and anhydrosialidases (intramolecular trans-sialidases, EC 4.2.2.15) are glycoside hydrolase enzymes that cleave the glycosidic linkage and release sialic acid residues from sialyl substrates. The paper summarizes diverse sialidases present in the human body and their potential impact on development of antiviral compounds - inhibitors of viral neuraminidases. It includes a brief overview of catalytic mechanisms of action of sialidases and describes the origin of sialidases in the human body. This is followed by description of the structure and function of sialidase families with a special focus on the GH33 and GH34 families. Various effects of sialidases on human body are also briefly described. Modulation of sialidase activity may be considered a useful tool for effective treatment of various diseases. In some cases, it is desired to completely suppress the activity of sialidases by suitable inhibitors. Specific sialidase inhibitors are useful for the treatment of influenza, epilepsy, Alzheimer's disease, diabetes, different types of cancer, or heart defects. Challenges and future directions are shortly depicted in the final part of the paper.

Deregulation of sialidases in human normal and tumor tissues

BACKGROUND

Aberrant sialylation is a characteristic feature associated with cancer. The four types of mammalian sialidases identified to date have been shown to behave in different manners during carcinogenesis. While NEU1, NEU2 and NEU4 have been observed to oppose malignant phenotypes, the membrane-bound sialidase NEU3 was revealed to promote cancer progression.

OBJECTIVES

With the aim of improving the knowledge about sialidases deregulation in various cancer types, we investigated the amount of NEU1, NEU3 and NEU4 transcripts in paired normal and tumor tissues from 170 patients with 11 cancer types.

METHODS

mRNA was extracted from patients' tissue specimens and retrotranscribed into cDNA, which was quantified by Real-Time PCR.

RESULTS

We found NEU1 and NEU3 to be up regulated, while NEU4 was down regulated in most cancer types. In particular, colorectal cancer tissues showed the highest increase in NEU3 expression. Both NEU1 and NEU3 showed a strong up-regulation in ovarian cancer.

CONCLUSIONS

Our data show that human sialidases are expressed at different levels in healthy tissues and are strongly deregulated in tumors. Moreover, sialidases expression in our European cohort showed significant differences from Asian populations. Some of these peculiar features open potential applications of sialidases in cancer diagnosis and therapy.

Keeping it trim: roles of neuraminidases in CNS function

The sialylated glyconjugates (SGC) are found in abundance on the surface of brain cells, where they form a dense array of glycans mediating cell/cell and cell/protein recognition in numerous physiological and pathological processes. Metabolic genetic blocks in processing and catabolism of SGC result in development of severe storage disorders, dominated by CNS involvement including marked neuroinflammation and neurodegeneration, the pathophysiological mechanisms of which are still discussed. SGC patterns in the brain are cell and organelle-specific, dynamic and maintained by highly coordinated processes of their biosynthesis, trafficking, processing and catabolism. The changes in the composition of SGC during development and aging of the brain cannot be explained based solely on the regulation of the SGC-synthesizing enzymes, sialyltransferases, suggesting that neuraminidases (sialidases) hydrolysing the removal of terminal sialic acid residues also play an essential role. In the current review we summarize the roles of three mammalian neuraminidases: neuraminidase 1, neuraminidase 3 and neuraminidase 4 in processing brain SGC. Emerging data demonstrate that these enzymes with different, yet overlapping expression patterns, intracellular localization and substrate specificity play essential roles in the physiology of the CNS.

Non-small cell lung cancer (NSCLC), EGFR downstream pathway activation and TKI targeted therapies sensitivity: Effect of the plasma membrane-associated NEU3

Adenocarcinoma of Non-Small Cell Lung Cancer (NSCLC) is a severe disease. Patients carrying EGFR mutations may benefit from EGFR targeted therapies (e.g.: gefitinib). Recently, it has been shown that sialidase NEU3 directly interacts and regulates EGFR. In this work, we investigate the effect of sialidase NEU3 overexpression on EGFR pathways activation and EGFR targeted therapies sensitivity, in a series of lung cancer cell lines. NEU3 overexpression, forced after transfection, does not affect NSCLC cell viability. We demonstrate that NEU3 overexpression stimulates the ERK pathway but this activation is completely abolished by gefitinib treatment. The Akt pathway is also hyper-activated upon NEU3 overexpression, but gefitinib is able only to decrease, and not to abolish, such activation. These findings indicate that NEU3 can act directly on the ERK pathway through EGFR and both directly and indirectly with respect to EGFR on the Akt pathway. Furthermore, we provide evidence that a healthy mucosa cell line (with EGFR wild-type gene sequence) is slightly sensitive to gefitinib, especially in the presence of NEU3 overexpression, thus hypothesizing that NEU3 overexpressing patients may benefit from EGFR targeted therapies also in absence of EGFR point mutations. Overall, the expression of NEU3 may be a novel diagnostic marker in NSCLC because, by its ability to stimulate EGFR downstream pathways with direct and indirect mechanisms, it may help in the identification of patients who can profit from EGFR targeted therapies in absence of EGFR activating mutations or from new combinations of EGFR and Akt inhibitors.

Sphingolipids and mitochondrial apoptosis

The sphingolipid family of lipids modulate several cellular processes, including proliferation, cell cycle regulation, inflammatory signaling pathways, and cell death. Several members of the sphingolipid pathway have opposing functions and thus imbalances in sphingolipid metabolism result in deregulated cellular processes, which cause or contribute to diseases and disorders in humans. A key cellular process regulated by sphingolipids is apoptosis, or programmed cell death. Sphingolipids play an important role in both extrinsic and intrinsic apoptotic pathways depending on the stimuli, cell type and cellular response to the stress. During mitochondrial-mediated apoptosis, multiple pathways converge on mitochondria and induce mitochondrial outer membrane permeabilization (MOMP). MOMP results in the release of intermembrane space proteins such as cytochrome c and Apaf1 into the cytosol where they activate the caspases and DNases that execute cell death. The precise molecular components of the pore(s) responsible for MOMP are unknown, but sphingolipids are thought to play a role. Here, we review evidence for a role of sphingolipids in the induction of mitochondrial-mediated apoptosis with a focus on potential underlying molecular mechanisms by which altered sphingolipid metabolism indirectly or directly induce MOMP. Data available on these mechanisms is reviewed, and the focus and limitations of previous and current studies are discussed to present important unanswered questions and potential future directions.

Molecular cloning and biochemical characterization of medaka (Oryzias latipes) lysosomal neu4 sialidase

Glycoconjugates are known to be involved in many physiological events in vertebrates. Sialidase is one of the glycosidases, which removes sialic acid from glycoconjugates. In mammals, the properties and physiological functions of sialidases have been investigated, while there is little understanding of fish sialidase. Here, to investigate the significance of fish neu4 sialidase, neu4 gene was cloned from medaka brain mRNA and identified. Sialidase-specific motifs (GPG, YRVP and Asp-Box) were well conserved in the medaka neu4 polypeptide. Optimal pH of medaka neu4 sialidase was 4.6, but its activity was sustained even at neutral and weak alkaline pH. The neu4 considerably cleaved sialic acid from 4-methylumbelliferyl-N-acetyl-α-D-neuraminic acid and sialyllactose, but not from ganglioside and fetuin, which are good substrates for human NEU4. neu4 activity was mostly detected in mitochondria/lysosome fraction after biochemical fractionation, and indirect immunofluorescence assays revealed neu4 localization in lysosome in neu4 overexpressed cells. Next, developmental change in medaka neu4 and other sialidase mRNA levels were estimated by real-time PCR. Each sialidases showed different expression patterns in embryonic development: neu4 was up-regulated at late developmental stage in embryo, and neu3a mRNA level was quite high in 0.5 dpf. On the other hand, neu3b expression was drastically increased after hatching, suggesting that each sialidase may play a different role in embryonic development.

Structural basis for substrate specificity of mammalian neuraminidases

The removal of sialic acid (Sia) residues from glycoconjugates in vertebrates is mediated by a family of neuraminidases (sialidases) consisting of Neu1, Neu2, Neu3 and Neu4 enzymes. The enzymes play distinct physiological roles, but their ability to discriminate between the types of linkages connecting Sia and adjacent residues and between the identity and arrangement of the underlying sugars has never been systematically studied. Here we analyzed the specificity of neuraminidases by studying the kinetics of hydrolysis of BODIPY-labeled substrates containing common mammalian sialylated oligosaccharides: 3′Sia-LacNAc, 3′SiaLac, SiaLe_x, SiaLe_a, SiaLe_c, 6′SiaLac, and 6′SiaLacNAc. We found significant differences in substrate specificity of the enzymes towards the substrates containing α2,6-linked Sia, which were readily cleaved by Neu3 and Neu1 but not by Neu4 and Neu2. The presence of a branching 2-Fuc inhibited Neu2 and Neu4, but had almost no effect on Neu1 or Neu3. The nature of the sugar residue at the reducing end, either glucose (Glc) or N-acetyl-D-glucosamine (GlcNAc) had only a minor effect on all neuraminidases, whereas core structure (1,3 or 1,4 bond between D-galactose (Gal) and GlcNAc) was found to be important for Neu4 strongly preferring β3 (core 1) to β4 (core 2) isomer. Neu3 and Neu4 were in general more active than Neu1 and Neu2, likely due to their preference for hydrophobic substrates. Neu2 and Neu3 were examined by molecular dynamics to identify favorable substrate orientations in the binding sites and interpret the differences in their specificities. Finally, using knockout mouse models, we confirmed that the substrate specificities observed in vitro were recapitulated in enzymes found in mouse brain tissues. Our data for the first time provide evidence for the characteristic substrate preferences of neuraminidases and their ability to discriminate between distinct sialoside targets.

Desialylation of surface receptors as a new dimension in cell signaling

Terminal sialic acid residues are found in abundance in glycan chains of glycoproteins and glycolipids on the surface of all live cells forming an outer layer of the cell originally known as glycocalyx. Their presence affects the molecular properties and structure of glycoconjugates, modifying their function and interactions with other molecules. Consequently, the sialylation state of glycoproteins and glycolipids has been recognized as a critical factor modulating molecular recognitions inside the cell, between the cells, between the cells and the extracellular matrix, and between the cells and certain exogenous pathogens. Until recently sialyltransferases that catalyze transfer of sialic acid residues to the glycan chains in the process of their biosynthesis were thought to be mainly responsible for the creation and maintenance of a temporal and spatial diversity of sialylated moieties. However, the growing evidence suggests that in mammalian cells, at least equally important roles belong to sialidases/neuraminidases, which are located on the cell surface and in intracellular compartments, and may either initiate the catabolism of sialoglycoconjugates or just cleave their sialic acid residues, and thereby contribute to temporal changes in their structure and functions. The current review summarizes emerging data demonstrating that mammalian neuraminidase 1, well known for its lysosomal catabolic function, is also targeted to the cell surface and assumes the previously unrecognized role as a structural and functional modulator of cellular receptors.

Inhibitors of the human neuraminidase enzymes

A review of known small molecule inhibitors and substrates of the human neuraminidase enzymes.

A proline-rich loop mediates specific functions of human sialidase NEU4 in SK-N-BE neuronal differentiation

Human sialidase NEU4 long (N4L) is a membrane-associated enzyme that has been shown to be localized in the outer mitochondrial membrane. A role in different cellular processes has been suggested for this enzyme, such as apoptosis, neuronal differentiation and tumorigenesis. However, the molecular bases for these roles, not found in any of the other highly similar human sialidases, are not understood. We have found that a proline-rich sequence of 81 amino acids, unique to NEU4 sequence, contains potential Akt and Erk1 kinase motifs. Molecular modeling, based on the experimentally determined three-dimensional structure of cytosolic human NEU2, showed that the proline-rich sequence is accommodated in a loop, thus preserving the typical beta-barrel structure of sialidases. In order to investigate the role of this loop in neuronal differentiation, we obtained SK-N-BE neuroblastoma cells stably overexpressing either human wild-type N4L or a deletion mutant lacking the proline-rich loop. Our results demonstrate that the proline-rich region can also enhance cell proliferation and retinoic acid (RA)-induced neuronal differentiation and it is also involved in NEU4 interaction with Akt, as well as in substrate recognition, modifying directly or through the interaction with other protein(s) the enzyme specificity toward sialylated glycoprotein(s). On the whole, our results suggest that N4L could be a downstream component of the PI3K/Akt signaling pathway required for RA-induced differentiation of neuroblastoma SK-N-BE cells.

Erythrophagocytosis of desialylated red blood cells is responsible for anaemia during Trypanosoma vivax infection

Trypanosomal infection-induced anaemia is a devastating scourge for cattle in widespread regions. Although Trypanosoma vivax is considered as one of the most important parasites regarding economic impact in Africa and South America, very few in-depth studies have been conducted due to the difficulty of manipulating this parasite. Several hypotheses were proposed to explain trypanosome induced-anaemia but mechanisms have not yet been elucidated. Here, we characterized a multigenic family of trans-sialidases in T. vivax, some of which are released into the host serum during infection. These enzymes are able to trigger erythrophagocytosis by desialylating the major surface erythrocytes sialoglycoproteins, the glycophorins. Using an ex vivo assay to quantify erythrophagocytosis throughout infection, we showed that erythrocyte desialylation alone results in significant levels of anaemia during the acute phase of the disease. Characterization of virulence factors such as the trans-sialidases is vital to develop a control strategy against the disease or parasite.

Cell-permeable probe for identification and imaging of sialidases

Alkyne-hinged 3-fluorosialyl fluoride (DFSA) containing an alkyne group was shown to be a mechanism-based target-specific irreversible inhibitor of sialidases. The ester-protected analog DFSA (PDFSA) is a membrane-permeable precursor of DFSA designed to be used in living cells, and it was shown to form covalent adducts with virus, bacteria, and human sialidases. The fluorosialyl-enzyme adduct can be ligated with an azide-annexed biotin via click reaction and detected by the streptavidin-specific reporting signals. Liquid chromatography-mass spectrometry/mass spectrometry analysis on the tryptic peptide fragments indicates that the 3-fluorosialyl moiety modifies tyrosine residues of the sialidases. DFSA was used to demonstrate influenza infection and the diagnosis of the viral susceptibility to the anti-influenza drug oseltamivir acid, whereas PDFSA was used for in situ imaging of the changes of sialidase activity in live cells.

Unified theory of bacterial sialometabolism: how and why bacteria metabolize host sialic acids

Sialic acids are structurally diverse nine-carbon ketosugars found mostly in humans and other animals as the terminal units on carbohydrate chains linked to proteins or lipids. The sialic acids function in cell-cell and cell-molecule interactions necessary for organismic development and homeostasis. They not only pose a barrier to microorganisms inhabiting or invading an animal mucosal surface, but also present a source of potential carbon, nitrogen, and cell wall metabolites necessary for bacterial colonization, persistence, growth, and, occasionally, disease. The explosion of microbial genomic sequencing projects reveals remarkable diversity in bacterial sialic acid metabolic potential. How bacteria exploit host sialic acids includes a surprisingly complex array of metabolic and regulatory capabilities that is just now entering a mature research stage. This paper attempts to describe the variety of bacterial sialometabolic systems by focusing on recent advances at the molecular and host-microbe-interaction levels. The hope is that this focus will provide a framework for further research that holds promise for better understanding of the metabolic interplay between bacterial growth and the host environment. An ability to modify or block this interplay has already yielded important new insights into potentially new therapeutic approaches for modifying or blocking bacterial colonization or infection.

Sialidase significance for cancer progression

Aberrant glycosylation is a characteristic feature of cancer cells. In particular, altered sialylation is closely associated with malignant properties, including invasiveness and metastatic potential. To elucidate the molecular mechanisms underlying the aberrancy, our studies have focused on mammalian sialidase, which catalyzes the removal of sialic acid residues from glycoproteins and glycolipids. The four types of mammalian sialidase identified to date show altered expression and behave in different manners during carcinogenesis. The present review briefly summarizes results on altered expression of sialidases and their possible roles in cancer progression. These enzymes are indeed factors defining cancer malignancy and thus potential targets for cancer diagnosis and therapy.

NEU4L sialidase overexpression promotes β-catenin signaling in neuroblastoma cells, enhancing stem-like malignant cell growth

Neuroblastoma (NB) is a frequently lethal tumor that occurs in childhood and originates from embryonic neural crest cells. The malignant and aggressive phenotype of NB is strictly related to the deregulation of pivotal pathways governing the proliferation/differentiation status of neural crest precursor cells, such as MYCN, Delta/Notch and Wnt/β-catenin (CTNNB1) signaling. In this article, we demonstrate that sialidase NEU4 long (NEU4L) influences the differentiation/proliferation behavior of NB SK-N-BE cells by determining hyperactivation of the Wnt/β-catenin signaling pathway. NEU4L overexpression in SK-N-BE cells induced significant increases in active, nonphosphorylated β-catenin content, β-catenin/TCF transcriptional activity and β-catenin gene target expression including MYCN, MYC, CCND2 (cyclin D2) and CDC25A. In turn, these molecular features strongly modified the behavior of NEU4L SK-N-BE overexpressing cells, promoting the following: (1) an enhanced proliferation rate, mainly due to a faster transition from G1 to S phase in the cell cycle; (2) a more undifferentiated cell phenotype, which was similar to stem-like NB cells and possibly mediated by an increase of the expression of the pluripotency genes, MYC, NANOG, OCT-4, CD133 and NES (nestin); (3) the failure of NB cell differentiation after serum withdrawal. The molecular link between NEU4L and Wnt/β-catenin signaling appeared to rely most likely on the capability of the enzyme to modify the sialylation level of cell glycoproteins. These findings could provide a new candidate for therapeutic treatment.

Mammalian sialidases: physiological and pathological roles in cellular functions

Sialic acids are terminal acidic monosaccharides, which influence the chemical and biological features of glycoconjugates. Their removal catalyzed by a sialidase modulates various biological processes through change in conformation and creation or loss of binding sites of functional molecules. Sialidases exist widely in vertebrates and also in a variety of microorganisms. Recent research on mammalian sialidases has provided evidence for great importance of these enzymes in various cellular functions, including lysosomal catabolism, whereas microbial sialidases appear to play roles limited to nutrition and pathogenesis. Four types of mammalian sialidases have been identified and characterized to date, designated as NEU1, NEU2, NEU3 and NEU4. They are encoded by different genes and differ in major subcellular localization and enzymatic properties including substrate specificity, and each has been found to play a unique role depending on its particular properties. This review is an attempt to concisely summarize current knowledge concerning mammalian sialidases, with a special focus on their properties and physiological and pathological roles in cellular functions.

Structure and Function of Mammalian Sialidases

The removal of sialic acids, catalyzed by sialidase, is the initial step in degradation of oligosaccharides, glycoproteins, and glycolipids. The catalytic reaction may greatly influence biological processes through changing the conformation of glycoproteins and create or mask binding sites of functional molecules. Recent progress in sialidase research has clarified that mammalian sialidases indeed contribute to the regulation of various cellular functions as well as lysosomal catabolism, unlike the sialidases of microbial origin that probably play roles limited to nutrition and pathogenesis. However, the mammalian enzymes contain consensus sequences in the six-blade β-propeller structural organization typical of microbial sialidases, despite the low degree of similarity to the amino acid sequences of the microbial enzymes. The present review briefly summarizes structural and functional features of mammalian sialidases.

Where catabolism meets signalling: neuraminidase 1 as a modulator of cell receptors

Terminal sialic acid residues are found in abundance in glycan chains of glycoproteins and glycolipids on the surface of all live cells forming an outer layer of the cell originally known as glycocalyx. Their presence affects the molecular properties and structure of glycoconjugates, modifying their function and interactions with other molecules. Consequently, the sialylation state of glycoproteins and glycolipids has been recognized as a critical factor modulating molecular recognitions inside the cell, between the cells, between the cells and the extracellular matrix, and between the cells and certain exogenous pathogens. Sialyltransferases that attach sialic acid residues to the glycan chains in the process of their initial synthesis were thought to be mainly responsible for the creation and maintenance of a temporal and spatial diversity of sialylated moieties. However, the growing evidence also suggests that in mammalian cells, at least equally important roles belong to sialidases/neuraminidases, which are located on the cell surface and in intracellular compartments, and may either initiate the catabolism of sialoglycoconjugates or just cleave their sialic acid residues, and thereby contribute to temporal changes in their structure and functions. The current review summarizes emerging data demonstrating that neuraminidase 1 (NEU1), well known for its lysosomal catabolic function, can be also targeted to the cell surface and assume the previously unrecognized role as a structural and functional modulator of cellular receptors.

Primer on genes encoding enzymes in sialic acid metabolism in mammals

Sialic acid, a nine-carbon sugar acid usually is present in the non-reducing terminal position of free oligosaccharides and glycoconjugates. Sialylated conjugates in mammals perform important roles in cellular recognition, signaling, host-pathogen interaction and neuronal development. Metabolism of sialylated conjugates involves a complex pathway consisting of enzymes distributed among the different compartments in the cell. These enzymes are encoded by 32 genes diversely distributed throughout the mammalian genome. Genetic variants in some of these genes are associated with embryonic lethality and abnormal phenotypes in mice and neuromuscular diseases, carcinomas and immune-mediated diseases in humans. In humans, the CMP-NeuAc-hydroxylase (CMAH) enzyme is inactivated due to a deletion mutation in the encoded enzyme. This lack of Neu5Gc phenotype makes humans unique among mammals. This review focuses on genes encoding enzymes in sialic acid metabolism pathways in mammalian cells with special emphasis on the human, mouse and cow.

Regulation of sialyl Lewis antigen expression in colon cancer cells by sialidase NEU4

Sialyl Lewis antigens, sialyl Lewis a and sialyl Lewis x, are utilized as tumor markers, and their increase in cancer is associated with tumor progression by enhancement of cancer cell adhesion to endothelial E-selectin. However, regulation mechanisms are not fully understood. We previously demonstrated that NEU4 is the only sialidase efficiently acting on mucins and it is down-regulated in colon cancer. To elucidate the significance of NEU4 down-regulation, we investigated sialyl Lewis antigens as endogenous substrates for the sialidase. NEU4 was found to hydrolyze the antigens in vitro and decrease cell surface levels much more effectively than other sialidases. Western blot, thin layer chromatography, and metabolic inhibition studies of desialylation products revealed NEU4 to preferentially catalyze sialyl Lewis antigens expressed on O-glycans. Cell adhesion to and motility and growth on E-selectin were significantly reduced by NEU4. E-selectin stimulation of colon cancer cells enhanced cell motility through activation of the p38/Hsp27/actin reorganization pathway, whereas NEU4 attenuated the signaling. On immunocytochemical analysis, some NEU4 molecules were localized at cell surfaces. Under hypoxia conditions whereby the antigens were increased concomitantly with several sialyl- and fucosyltransferases, NEU4 expression was markedly decreased. These results suggest that NEU4 plays an important role in control of sialyl Lewis antigen expression and its impairment in colon cancer.

Antibody-mediated sialidase activity in blood serum of patients with multiple myeloma

Cell surface sialylation is known to be tightly connected with tumorigenicity, invasiveness, metastatic potential, clearance of aged cells, while the sialylation of IgG molecules determines their anti-inflammatory properties. Four sialidases - hydrolytic enzymes responsible for cleavage of sialic residues - were described in different cellular compartments. However, sialidases activity in body fluids, and specifically in blood serum, remains poorly studied. Here, we characterize first known IgG antibodies possessing sialidase-like activity in blood serum of multiple myeloma (MM) patients. Ig fractions were precipitated with ammonium sulfate (50% of saturation) from blood serum of 12 healthy donors and 14 MM patients, and screened for the presence of sialidase activity by using 4-MUNA (2'-(4-methylumbelliferyl)-α-D-N-acetylneuraminic acid) as substrate. High level of sialidase activity was detected in the MM patients, but not in healthy donors. Subsequent antibody purification by protein-G affinity chromatography and HPLC size exclusion chromatography at acidic conditions demonstrated that sialidase activity was attributable to IgG molecules. Sialidase activity was also specific for (Fab)(2) fragment of IgG and blocked by sialidase inhibitor DANA. Sialidase activity of IgG molecule was also confirmed by in gel assay for cleavage of sialidase substrate. Kinetic parameters of the catalysis reaction were described by Michaelis-Menten equation with K(m)  = 44.4-108 µM and k(cat) = 2.7-23.1 min(-1). The action of IgG possessing sialidase-like activity towards human red blood cells resulted in a subsequent increase in their agglutination by the peanut agglutinin, that confirms their desialylation by the studied IgG. This is the first demonstration of the intrinsic sialidase activity of IgG isolated from blood serum of MM patients.

Thymoquinone from nutraceutical black cumin oil activates Neu4 sialidase in live macrophage, dendritic, and normal and type I sialidosis human fibroblast cells via GPCR Galphai proteins and matrix metalloproteinase-9

Anti-inflammatory activities of thymoquinone (TQ) have been demonstrated in in vitro and in vivo studies. However, the precise mechanism(s) of TQ in these anti-inflammatory activities is not well understood. Using a newly developed assay to detect sialidase activity in live macrophage cells (Glycoconj J doi: 10.1007/s10719-009-9239-8 ), here we show that TQ has no inhibitory effect on endotoxin lipopolysaccharide (LPS) induced sialidase activity in live BMC-2 macrophage cells. In contrast, the parent black seed oil (BSO) and another constituent of BSO para-cymene (p-CY) completely block LPS induced sialidase activity. All of these compounds had no effect on cell viability. On the other hand, TQ induces a vigorous sialidase activity in live BMC-2 macrophage cells in a dose dependent manner as well in live DC-2.4 dendritic cells, HEK-TLR4/MD2, HEK293, SP1 mammary adenocarcinoma cells, human WT and 1140F01 and WG0544 type I sialidosis fibroblast cells. Tamiflu (oseltamivir phosphate) inhibits TQ-induced sialidase activity in live BMC-2 cells with an IC(50) of 0.0194 microM compared to an IC(50) of 19.1 microM for neuraminidase inhibitor DANA (2-deoxy-2,3-dehydro-N-acetylneuraminic acid). Anti-Neu1, -2 and -3 antibodies have no inhibition of TQ-induced sialidase activity in live BMC-2 and human THP-1 macrophage cells but anti-Neu4 antibodies completely block this activity. There is a vigorous sialidase activity associated with TQ treated live primary bone marrow (BM) macrophage cells derived from WT and hypomorphic cathepsin A mice with a secondary Neu1 deficiency (NeuI KD), but not from Neu4 knockout (Neu4 KO) mice. Pertussis toxin (PTX), a specific inhibitor of Galphai proteins of G-protein coupled receptor (GPCR) and the broad range inhibitors of matrix metalloproteinase (MMP) galardin and piperazine applied to live BMC-2, THP-1 and primary BM macrophage cells completely block TQ-induced sialidase activity. These same inhibitory effects are not observed with the GM1 ganglioside specific cholera toxin subunit B (CTXB) as well as with CTX, tyrosine kinase inhibitor K252a, and the broad range GPCR inhibitor suramin. The specific inhibitor of MMP-9, anti-MMP-9 antibody and anti-Neu4 antibody, but not the specific inhibitor of MMP-3 completely block TQ-induced sialidase activity in live THP-1 cells, which express Neu4 and MMP-9 on the cell surface. Neu4 sialidase activity in cell lysates from TQ-treated live THP-1 cells desialylates natural gangliosides and mucin substrates. RT-PCR and western blot analyses reveal no correlation between mRNA and protein values for Neu3 and Neu4 in human monocytic THP-1 cells, suggesting for the first time a varied post-transcriptional mechanism for these two mammalian sialidases independent of TQ activation. Our findings establish an unprecedented activation of Neu4 sialidase on the cell surface by thymoquinone, which is derived from the nutraceutical black cumin oil. The potentiation of GPCR-signaling by TQ via membrane targeting of Galphai subunit proteins and matrix metalloproteinase-9 activation may be involved in the activation process of Neu4 sialidase on the cell surface.