Effect of GM2 activator protein on the enzymatic hydrolysis of phospholipids and sphingomyelin

Expression of the GM2 activator protein in mouse testis

The GM2-activator protein (GM2-AP), revealed by Li et al. in 1973 in human liver, was initially identified as a protein cofactor that stimulated β-hexosaminidase A to hydrolyze N-acetylgalactosamine from GM2 ganglioside. This cofactor was found to be missing in human variant AB Tay-Sachs disease. Over the years, the GM2-AP has also been shown to be involved in kidney vesicular transport, lipid presentation by CD1 molecule to T-cells, and interaction of human sperm with zona pellucida. Since the expression of the GM2-AP via mRNA detection in mouse tissues was found to be the highest in testis, we became interested in the localization of the GM2-AP at cellular level in mouse testis during spermatogenesis. Using immunohistochemical analysis and electron microscopy, we found that the GM2-AP was predominantly localized in the basal cytoplasm and the attenuated processes of Sertoli cells. The stained structure appeared to be lysosomes. The most interesting finding was the association of the GM2-AP with the acrosomal apparatus in early spermatids. A modest to intense staining was observed in some acrosomal granules and acrosomal caps. The GM2-AP seemed to disappear from acrosomal caps in the later stage of spermatids, in which the nucleus became elongated and condensed. These results suggest that the GM2-AP may be involved in the normal functions of Sertoli cells and play important roles during the development of acrosomal caps in the early spermatids.

Association of neonatal necrotizing enterocolitis with myeloid differentiation-2 and GM2 activator protein genetic polymorphisms

The aim of the present study was to investigate the association of neonatal necrotizing enterocolitis (NEC) with myeloid differentiation-(MD-2) and GM2 activator protein (GM2A) genetic polymorphisms. Gene resequencing of the MD-2 and GM2A gene exons was performed on 42 neonates, diagnosed with NEC (NEC group), as well as in the rs11465996 locus, located in the MD-2 gene promoter region. The aim was to detect the genetic polymorphisms present in the neonates with NEC and compare the functional polymorphic loci with 83 neonates without NEC (control group), who had been born during the same period. A polymorphic locus with abnormal frequency was detected in the exon region of the MD-2 gene. In the NEC group, the frequency of genotypes carrying the low frequency allele (G) in the rs11465996 locus (MD-2 promoter region) was significantly higher compared with the control group (χ(2)=4.388, P=0.036). Furthermore, the frequencies of genotypes carrying the low frequency A and C alleles in the rs1048719 (GM2A gene exon 1) and rs2075783 loci (GM2A intron), respectively, were significantly higher in the NEC group compared with the control group (χ(2)=4.316, P=0.038; and χ(2)=13.717, P=0.000, respectively). In addition, the rs11465996 polymorphism in the MD-2 gene promoter region was found to be associated with the severity of NEC. Furthermore, the rs2075783 polymorphism in the GM2A gene exon 1 and the rs1048719 polymorphism in the intron region of this gene, were associated with the occurrence of NEC. The present study demonstrated that gene polymorphisms of MD-2 and GM2A were associated with the occurrence or severity of NEC; however, further in-depth exploration is required to clarify the associations between genetic predispositions to polymorphisms, and NEC.

Comparative gene expression profiling in human cumulus cells according to ovarian gonadotropin treatments

In in vitro fertilization cycles, both HP-hMG and rFSH gonadotropin treatments are widely used to control human follicle development. The objectives of this study are (i) to characterize and compare gene expression profiles in cumulus cells (CCs) of periovulatory follicles obtained from patients stimulated with HP-hMG or rFSH in a GnRH antagonist cycle and (ii) to examine their relationship with in vitro embryo development, using Human Genome U133 Plus 2.0 microarrays. Genes that were upregulated in HP-hMG-treated CCs are involved in lipid metabolism (GM2A) and cell-to-cell interactions (GJA5). Conversely, genes upregulated in rFSH-treated CCs are implicated in cell assembly and organization (COL1A1 and COL3A1). Interestingly, some genes specific to each gonadotropin treatment (NPY1R and GM2A for HP-hMG; GREM1 and OSBPL6 for rFSH) were associated with day 3 embryo quality and blastocyst grade at day 5, while others (STC2 and PTX3) were related to in vitro embryo quality in both gonadotropin treatments. These genes may prove valuable as biomarkers of in vitro embryo quality.

Adipokine ganglioside GM2 activator protein stimulates insulin secretion

Recently, we identified ganglioside GM2 activator protein (GM2AP) as a novel adipokine, and revealed that treatment of cultured cells with GM2AP impairs insulin signal transduction. The aim of this study was to examine the impact of GM2AP on glucose metabolism in vivo. Injection of recombinant GM2AP in mice significantly lowered blood glucose levels in glucose tolerance tests. Administration of GM2AP to mice for 10 days increased serum insulin levels, whereas the contents of glucose, leptin and FFA were significantly decreased. Stimulation of calcium influx and insulin secretion by GM2AP was observed in hamster insulinoma HIT-T15 cells. Blockage of GM2AP function by specific antibodies inhibited GM2AP-induced insulin secretion. These results provide novel insights into the physiological functions of GM2AP in obesity.

A novel adipokine GM2AP impairs insulin signaling

In an attempt to discover novel adipokines, we performed proteomics analyses using culture medium from differentiated 3T3-L1 adipocytes, and first identified GM2AP. The levels of GM2AP mRNA and protein were augmented by adipogenesis in cultured adipocytes and expression in adipose tissue and serum of obese mice or human subjects was found to be significantly higher than in lean counterparts. Exposure of 3T3-L1 adipocytes to GM2AP protein accelerated dissociation of insulin receptor-beta (IRβ) from caveolin-1, and interrupted insulin signal transduction. Abrogation of GM2AP function by specific antibodies augmented glucose uptake. Furthermore, treatment of rat pheochromocytoma PC12 NS1 cells with GM2AP impaired NGF signal transduction. Taken together, these results provide novel insights into the physiological functions of GM2AP in obesity.

The Drosophila gene CheB42a is a novel modifier of Deg/ENaC channel function

Degenerin/epithelial Na⁺ channels (DEG/ENaC) represent a diverse family of voltage-insensitive cation channels whose functions include Na⁺ transport across epithelia, mechanosensation, nociception, salt sensing, modification of neurotransmission, and detecting the neurotransmitter FMRFamide. We previously showed that the Drosophila melanogaster Deg/ENaC gene lounge lizard (llz) is co-transcribed in an operon-like locus with another gene of unknown function, CheB42a. Because operons often encode proteins in the same biochemical or physiological pathway, we hypothesized that CHEB42A and LLZ might function together. Consistent with this hypothesis, we found both genes expressed in cells previously implicated in sensory functions during male courtship. Furthermore, when coexpressed, LLZ coprecipitated with CHEB42A, suggesting that the two proteins form a complex. Although LLZ expressed either alone or with CHEB42A did not generate ion channel currents, CHEB42A increased current amplitude of another DEG/ENaC protein whose ligand (protons) is known, acid-sensing ion channel 1a (ASIC1a). We also found that CHEB42A was cleaved to generate a secreted protein, suggesting that CHEB42A may play an important role in the extracellular space. These data suggest that CHEB42A is a modulatory subunit for sensory-related Deg/ENaC signaling. These results are consistent with operon-like transcription of CheB42a and llz and explain the similar contributions of these genes to courtship behavior.

Ligand extraction properties of the GM2 activator protein and its interactions with lipid vesicles

The GM2 activator protein (GM2AP) is an accessory protein required for the enzymatic conversion of GM2 to GM3 by hydrolases in the lysosomal compartments of cells. Here, GM2AP interactions with lipid vesicles are investigated by sucrose-loaded vesicle sedimentation and gel filtration assays, and the effects of pH and lipid composition on membrane binding and lipid extraction are characterized. The sedimentation experiments allow for facile quantification of the percentage of protein in solution and on the bilayer surface, with detailed analysis of the protein:lipid complex that remains in solution. Optimum binding and ligand extraction is found for pH 4.8 where <15% of the protein remains surface associated regardless of the lipid composition. In addition to extracting GM2, we find that GM2AP readily extracts dansyl-headgroup-labeled lipids as well as other phospholipids from vesicles. The ability of GM2AP to extract dansyl-DHPE from vesicles is altered by pH and the specific ligand GM2. Although the unique endosomal lipid, bis(monoacylglycero)phosphate, is not required for ligand extraction, it does enhance the extraction efficiency of GM2 when cholesterol is present in the vesicles.

Functional characterisation of a nematode secreted GM2-activator protein

We have identified a GM2-activator protein (GM2AP) with highly unusual properties secreted by the nematode parasite Trichinella spiralis. Expression in Pichia pastoris resulted in a hyperglycosylated protein of 28 kDa, but the 18 kDa native protein was not glycosylated. The parasite GM2AP does not facilitate degradation of GM2 ganglioside by N-acetyl-beta-hexosaminidase A, although it does inhibit phospholipase D activity. Lack of the former activity might be explained by the absence of a domain implicated in binding to hexosaminidase. In addition, and contrary to data on the human GM2AP, the nematode homologue does not inhibit platelet activating factor-induced calcium mobilisation in neutrophils, but actually enhances mediator-induced chemotaxis.

Identification and tissue-specific expression of amphioxus GM2 activator protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, AmphiGM2AP, encoding GM2 activator protein was isolated from the gut cDNA library of Branchiostoma belcheri. It is 907 bp long, and its longest open reading frame codes for a precursor protein consisting of 242 amino acid residues with a signal peptide of 14 amino acids. The deduced amino acid sequence includes a conserved domain typical of GM2APs between residues 53 and 224, a single N-linked glycosylation site at position 65 and 8 conserved cysteines. Phylogenetic analysis showed that amphiGM2AP forms a club together with invertebrate GM2APs, indicating that AmphiGM2AP is evolutionarily closely related to invertebrate GM2APs rather than vertebrate ones. Both Northern blotting and in situ hybridization histochemistry analyses revealed a tissue-specific expression pattern of AmphiGM2AP in adult amphioxus with the strongest expression in the digestive system, which is in contrast to the widespread expression pattern of human, mouse and sheep GM2AP genes. It is suggested that AmphiGM2AP is possibly involved in the take-in of digested food components like lipid molecules.

Saposin B binds and transfers phospholipids

Saposin B (Sap B) is a member of a family of four small glycoproteins, Sap A, B, C, and D. Like the other three saposins, Sap B plays a physiological role in the lysosomal degradation of sphingolipids (SLs). Although the interaction of Sap B with SLs has been investigated extensively, that with the main membrane lipid components, namely phospholipids and cholesterol (Chol), is scarcely known. Using large unilamellar vesicles (LUVs) as membrane models, we have now found that Sap B simultaneously extracts from the lipid surface neutral [phosphatidylcholine (PC)] and anionic [phosphatidylinositol (PI)] phospholipids, fewer SLs [ganglioside GM1 (GM1) or cerebroside sulfate (CS)], and no Chol. More PI than SL (GM1 or CS) was solubilized from LUVs containing equal amounts of PI and SLs. An increase in PI level had a poor effect on the Sap B-induced solubilization of GM1 or CS but strongly inhibited that of PC. Sap B was able not only to bind, but also to transfer phospholipids between lipid surfaces. Both the phospholipid binding and transfer activities were optimal at low pH values. These results represent the first biochemical analysis of the Sap B interaction with phospholipids. The capacity of Sap B to bind and transfer phospholipids occurs under conditions mimicking the interior of the late endosomal/lysosomal compartment and thus might have physiological relevance.