Activation of ganglioside GM3 biosynthesis in human monocyte/macrophages during culturing in vitro

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Homeostatic and pathogenic roles of GM3 ganglioside molecular species in TLR4 signaling in obesity

Innate immune signaling via TLR4 plays critical roles in pathogenesis of metabolic disorders, but the contribution of different lipid species to metabolic disorders and inflammatory diseases is less clear. GM3 ganglioside in human serum is composed of a variety of fatty acids, including long‐chain (LCFA) and very‐long‐chain (VLCFA). Analysis of circulating levels of human serum GM3 species from patients at different stages of insulin resistance and chronic inflammation reveals that levels of VLCFA‐GM3 increase significantly in metabolic disorders, while LCFA‐GM3 serum levels decrease. Specific GM3 species also correlates with disease symptoms. VLCFA‐GM3 levels increase in the adipose tissue of obese mice, and this is blocked in TLR4‐mutant mice. In cultured monocytes, GM3 by itself has no effect on TLR4 activation; however, VLCFA‐GM3 synergistically and selectively enhances TLR4 activation by LPS/HMGB1, while LCFA‐GM3 and unsaturated VLCFA‐GM3 suppresses TLR4 activation. GM3 interacts with the extracellular region of TLR4/MD2 complex to modulate dimerization/oligomerization. Ligand‐molecular docking analysis supports that VLCFA‐GM3 and LCFA‐GM3 act as agonist and antagonist of TLR4 activity, respectively, by differentially binding to the hydrophobic pocket of MD2. Our findings suggest that VLCFA‐GM3 is a risk factor for TLR4‐mediated disease progression.

Vascular Diseases and Gangliosides

Vascular diseases, such as myocardial infarction and cerebral infarction, are most commonly caused by atherosclerosis, one of the leading causes of death worldwide. Risk factors for atherosclerosis include lifestyle and aging. It has been reported that lifespan could be extended in mice by targeting senescent cells, which led to the suppression of aging-related diseases, such as vascular diseases. However, the molecular mechanisms underlying the contribution of aging to vascular diseases are still not well understood. Several types of cells, such as vascular (endothelial cell), vascular-associated (smooth muscle cell and fibroblast) and inflammatory cells, are involved in plaque formation, plaque rupture and thrombus formation, which result in atherosclerosis. Gangliosides, a group of glycosphingolipids, are expressed on the surface of vascular, vascular-associated and inflammatory cells, where they play functional roles. Clarifying the role of gangliosides in atherosclerosis and their relationship with aging is fundamental to develop novel prevention and treatment methods for vascular diseases based on targeting gangliosides. In this review, we highlight the involvement and possible contribution of gangliosides to vascular diseases and further discuss their relationship with aging.

The Role of Glycosphingolipids in Immune Cell Functions

Glycosphingolipids (GSLs) exhibit a variety of functions in cellular differentiation and interaction. Also, they are known to play a role as receptors in pathogen invasion. A less well-explored feature is the role of GSLs in immune cell function which is the subject of this review article. Here we summarize knowledge on GSL expression patterns in different immune cells. We review the changes in GSL expression during immune cell development and differentiation, maturation, and activation. Furthermore, we review how immune cell GSLs impact membrane organization, molecular signaling, and trans-interactions in cellular cross-talk. Another aspect covered is the role of GSLs as targets of antibody-based immunity in cancer. We expect that recent advances in analytical and genome editing technologies will help in the coming years to further our knowledge on the role of GSLs as modulators of immune cell function.

A mechanistic overview of dendritic cell-mediated HIV-1 trans infection: the story so far

Despite progress in antiretroviral therapy, HIV-1 rebound after cessation of antiretroviral therapy suggests that establishment of long-term cellular reservoirs of virus is a significant barrier to functional cure. There is considerable evidence that dendritic cells (DCs) play an important role in systemic virus dissemination. Although productive infection of DCs is inefficient, DCs capture HIV-1 and transfer-captured particles to CD4+ T cells, a mechanism of DC-mediated HIV-1 trans infection. Recent findings suggest that DC-mediated trans infection of HIV-1 is dependent on recognition of GM3, a virus-particle-associated host-derived ligand, by CD169 expressed on DCs. In this review, we describe mechanisms of DC-mediated HIV-1 trans infection and discuss specifically the role of CD169 in establishing infection in CD4+ T cells.

Activation of ganglioside GM3 biosynthesis in human blood mononuclear cells in atherosclerosis

Using blood monocytes and lymphocytes from atherosclerotic patients and healthy subjects we have investigated activity of GM3 synthase, cellular levels of ganglioside GM3 and its role in monocyte adhesion to cultured human umbilical vein endothelial cells (HUVEC). The results showed that activity of GM3 synthase and cellular levels of ganglioside GM3 in blood mononuclear cells from atherosclerotic patients were several-fold higher than those from healthy subjects. In monocytes the activity of GM3 synthase was one an order of magnitude higher than in lymphocytes from both groups studied; this suggests the major contribution of monocytes to enhanced biosynthesis and levels of GM3 in mononuclear cells in atherosclerosis. Enrichment of monocytes from healthy subjects with ganglioside GM3 by incubation in medium containing this ganglioside increased adherence of these monocytes to HUVEC up to the values typical for monocytes from atherosclerotic patients. In addition, an increase in CD11b integrin expression was observed that was comparable to that seen in lipopolysaccharide-activated monocytes. It is suggested that in atherosclerosis the enhanced cellular levels of GM3 in monocytes and lymphocytes may be an important element of cell activation that facilitates their adhesion to endothelial cells and penetration into intima.

Role of glycosphingolipids in dendritic cell-mediated HIV-1 trans-infection

Glycosphingolipids (GSLs) are components of the cell membrane that comprise a membrane bound lipid, ceramide, coupled to an extracellular carbohydrate. GSLs impact numerous aspects of membrane biology, including membrane fluidity, curvature, and organization. The role of these molecules in both chronic inflammation and infectious disease and underlying pathogenic mechanisms are just starting to be recognized. As a component of the cell membrane, GSLs are also incorporated into lipid bilayers of diverse enveloped viruses as they bud out from the host cell and can go on to have a significant influence on viral pathogenesis. Dendritic cell (DC) subsets located in the peripheral mucosal tissues are proposed to be one of the earliest cell types that encounter transmitted viruses and help initiate adaptive immune responses against the invading pathogen by interacting with T cells. In turn, viruses, as obligatory intracellular parasites, rely on host cells for completing their replication cycle, and not surprisingly, HIV has evolved to exploit DC biology for the initial transmission event as well as for its dissemination and propagation within the infected host. In this review, we describe the mechanisms by which GSLs impact DC-mediated HIV trans-infection by either modulating virus infectivity, serving as a direct virus particle-associated host-derived ligand for specific interactions with DCs, or modulating the T cell membrane in such a way as to impact viral entry and thereby productive infection of CD4(+) T cells.

HIV-1 incorporation of host-cell-derived glycosphingolipid GM3 allows for capture by mature dendritic cells

The interaction between HIV and dendritic cells (DCs) is an important early event in HIV-1 pathogenesis that leads to efficient viral dissemination. Here we demonstrate a HIV gp120-independent DC capture mechanism that uses virion-incorporated host-derived gangliosides with terminal α2-3-linked sialic acid linkages. Using exogenously enriched virus and artificial liposome particles, we demonstrate that both α2-3 gangliosides GM1 and GM3 are capable of mediating this interaction when present in the particle at high levels. In the absence of overexpression, GM3 is the primary ligand responsible for this capture mechanism, because siRNA depletion of GM3 but not GM1 from the producer cell and hence virions, resulted in a dramatic decrease in DC capture. Furthermore, HIV-1 capture by DCs was competitively inhibited by targeting virion-associated GM3, but was unchanged by targeting GM1. Finally, virions were derived from monocytoid THP-1 cells that constitutively display low levels of GM1 and GM3, or from THP-1 cells induced to express high surface levels of GM1 and GM3 upon stimulation with the TLR2/1 ligand Pam3CSK4. Compared with untreated THP-1 cells, virus produced from Pam3CSK4-stimulated THP-1 cells incorporated higher levels of GM3, but not GM1, and showed enhanced DC capture and trans-infection. Our results identify a unique HIV-1 DC attachment mechanism that is dependent on a host-cell-derived ligand, GM3, and is a unique example of pathogen mimicry of host-cell recognition pathways that drive virus capture and dissemination in vivo.

Ganglioside GM3 and its biological functions

Metabolism, topology, and possible mechanisms for regulation of the ganglioside GM3 content in the cell are reviewed. Under consideration are biological functions of GM3, such as involvement in cell differentiation, proliferation, oncogenesis, and apoptosis.