Increased cerebrospinal fluid ganglioside GM1 concentrations indicating neuronal involvement in all stages of HIV-1 infection

Are There Lipid Membrane-Domain Subtypes in Neurons with Different Roles in Calcium Signaling?

Lipid membrane nanodomains or lipid rafts are 10-200 nm diameter size cholesterol- and sphingolipid-enriched domains of the plasma membrane, gathering many proteins with different roles. Isolation and characterization of plasma membrane proteins by differential centrifugation and proteomic studies have revealed a remarkable diversity of proteins in these domains. The limited size of the lipid membrane nanodomain challenges the simple possibility that all of them can coexist within the same lipid membrane domain. As caveolin-1, flotillin isoforms and gangliosides are currently used as neuronal lipid membrane nanodomain markers, we first analyzed the structural features of these components forming nanodomains at the plasma membrane since they are relevant for building supramolecular complexes constituted by these molecular signatures. Among the proteins associated with neuronal lipid membrane nanodomains, there are a large number of proteins that play major roles in calcium signaling, such as ionotropic and metabotropic receptors for neurotransmitters, calcium channels, and calcium pumps. This review highlights a large variation between the calcium signaling proteins that have been reported to be associated with isolated caveolin-1 and flotillin-lipid membrane nanodomains. Since these calcium signaling proteins are scattered in different locations of the neuronal plasma membrane, i.e., in presynapses, postsynapses, axonal or dendritic trees, or in the neuronal soma, our analysis suggests that different lipid membrane-domain subtypes should exist in neurons. Furthermore, we conclude that classification of lipid membrane domains by their content in calcium signaling proteins sheds light on the roles of these domains for neuronal activities that are dependent upon the intracellular calcium concentration. Some examples described in this review include the synaptic and metabolic activity, secretion of neurotransmitters and neuromodulators, neuronal excitability (long-term potentiation and long-term depression), axonal and dendritic growth but also neuronal cell survival and death.

Dysfunctional stem and progenitor cells impair fracture healing with age

Successful fracture healing requires the simultaneous regeneration of both the bone and vasculature; mesenchymal stem cells (MSCs) are directed to replace the bone tissue, while endothelial progenitor cells (EPCs) form the new vasculature that supplies blood to the fracture site. In the elderly, the healing process is slowed, partly due to decreased regenerative function of these stem and progenitor cells. MSCs from older individuals are impaired with regard to cell number, proliferative capacity, ability to migrate, and osteochondrogenic differentiation potential. The proliferation, migration and function of EPCs are also compromised with advanced age. Although the reasons for cellular dysfunction with age are complex and multidimensional, reduced expression of growth factors, accumulation of oxidative damage from reactive oxygen species, and altered signaling of the Sirtuin-1 pathway are contributing factors to aging at the cellular level of both MSCs and EPCs. Because of these geriatric-specific issues, effective treatment for fracture repair may require new therapeutic techniques to restore cellular function. Some suggested directions for potential treatments include cellular therapies, pharmacological agents, treatments targeting age-related molecular mechanisms, and physical therapeutics. Advanced age is the primary risk factor for a fracture, due to the low bone mass and inferior bone quality associated with aging; a better understanding of the dysfunctional behavior of the aging cell will provide a foundation for new treatments to decrease healing time and reduce the development of complications during the extended recovery from fracture healing in the elderly.

NF-κB as a common signaling pathway in ganglioside-induced autophagic cell death and activation of astrocytes

We have previously shown that gangliosides induce autophagic cell death of brain astrocytes. As gangliosides are also known to induce inflammatory activation of astrocytes, we hypothesized that a canonical inflammatory signaling pathway NF-κB might be involved in the ganglioside-induced astrocyte cell death and activation. Using cultured mouse astrocytes and C6 rat glioma cell line, we determined the role of NF-κB in autophagic cell death and nitric oxide (NO) production in astrocytes. Gangliosides induced iNOS/GFAP expression and NF-κB activation. IKK inhibitor SC-514 and NF-κB inhibitor PDTC reduced ganglioside-induced astrocyte activation and cell death. Moreover, inhibition of NF-κB pathway also attenuated autophagy of astrocytes. Rho subfamily of small G proteins antagonized the ganglioside-induced astrocyte cell death as well as activation pathways. Taken together, IKK/NF-κB may constitute one of the common signaling pathways in ganglioside-induced astrocyte activation and autophagic cell death, and may play an important role in the ganglioside intracellular signaling that regulates astrocyte physiology and pathology.

Gangliosides induce autophagic cell death in astrocytes

BACKGROUND AND PURPOSE

Gangliosides, sialic acid-containing glycosphingolipids, abundant in brain, are involved in neuronal function and disease, but the precise molecular mechanisms underlying their physiological or pathological activities are poorly understood. In this study, the pathological role of gangliosides in the extracellular milieu with respect to glial cell death and lipid raft/membrane disruption was investigated.

EXPERIMENTAL APPROACH

We determined the effect of gangliosides on astrocyte death or survival using primary astrocyte cultures and astrocytoma/glioma cell lines as a model. Signalling pathways of ganglioside-induced autophagic cell death of astrocytes were examined using pharmacological inhibitors and biochemical and genetic assays.

KEY RESULTS

Gangliosides induced autophagic cell death in based on the following observations. Incubation of the cells with a mixture of gangliosides increased a punctate distribution of fluorescently labelled microtubule-associated protein 1 light chain 3 (GFP-LC3), the ratio of LC3-II/LC3-I and LC3 flux. Gangliosides also increased the formation of autophagic vacuoles as revealed by monodansylcadaverine staining. Ganglioside-induced cell death was inhibited by either a knockdown of beclin-1/Atg-6 or Atg-7 gene expression or by 3-methyladenine, an inhibitor of autophagy. Reactive oxygen species (ROS) were involved in ganglioside-induced autophagic cell death of astrocytes, because gangliosides induced ROS production and ROS scavengers decreased autophagic cell death. In addition, lipid rafts played an important role in ganglioside-induced astrocyte death.

CONCLUSIONS AND IMPLICATIONS

Gangliosides released under pathological conditions may induce autophagic cell death of astrocytes, identifying a neuropathological role for gangliosides.

Gangliosides trigger inflammatory responses via TLR4 in brain glia

Gangliosides participate in various cellular events of the central nervous system and have been closely implicated in many neuronal diseases. However, the precise molecular mechanisms underlying the pathological activity of gangliosides are poorly understood. Here we report that toll-like receptor 4 (TLR4) may mediate the ganglioside-triggered inflammation in glia, brain resident immune cells. Gangliosides rapidly altered the cell surface expression of TLR4 in microglia and astrocytes within 3 hours. Using TLR4-specific siRNA and a dominant-negative TLR4 gene, we clearly demonstrate the functional importance of TLR4 in ganglioside-triggered activation of glia. Inhibition of TLR4 expression by TLR4-siRNA suppressed nuclear factor (NF)-kappaB-binding activity, NF-kappaB-dependent luciferase activity, and transcription of inflammatory cytokines after exposure to gangliosides. Transient transfection of dominant-negative TLR4 also attenuated NF-kappaB-binding activity and interleukin-6 promoter activity. In contrast, these activities were slightly elevated in cells with wild-type TLR4. In addition, CD14 was required for ganglioside-triggered activation of glia, and lipid raft formation may be associated with ganglioside-stimulated signal propagation. Taken together, these results suggest that TLR4 may provide an explanation for the pathological ability of gangliosides to cause inflammatory conditions in the brain.

Gangliosides activate microglia via protein kinase C and NADPH oxidase

Microglia, the major immune effector cells in the central nervous system, are activated when the brain suffers injury. A number of studies indicate that gangliosides activate microglia. However, the signaling mechanisms involved in microglial activation are not yet to be elucidated. Our results show that gangliosides induce the expression of interleukin (IL)-1beta, tumor necrosis factor-alpha (TNF-alpha), and inducible nitric oxide synthase (iNOS) in rat brain microglia and BV2 murine microglia via protein kinase C (PKC) and NADPH oxidase. Expression of IL-1beta, TNF-alpha, and iNOS in ganglioside-treated cells was significantly reduced in the presence of inhibitors of PKC (GF109203X, Go6976, Ro31-8220, and rottlerin) and NADPH oxidase (diphenyleneiodonium chloride [DPI]). In response to gangliosides, PKC-alpha, betaII, and delta and NADPH oxidase p67(phox) translocated from the cytosol to the membrane. ROS generation was also activated within 5 min of ganglioside treatment. Ganglioside-induced ROS generation was blocked by PKC inhibitors. Furthermore, ganglioside-induced activation of NF-kappaB, an essential transcription factor that mediates the expression of IL-1beta, TNF-alpha, and iNOS, was reduced in the presence of GF109203X and DPI. Our results collectively suggest that gangliosides activate microglia via PKC and NADPH oxidase, which regulate activation of NF-kappaB.

JAK-STAT signaling mediates gangliosides-induced inflammatory responses in brain microglial cells

Neuronal cell membranes are particularly rich in gangliosides, which play important roles in brain physiology and pathology. Previously, we reported that gangliosides could act as microglial activators and are thus likely to participate in many neuronal diseases. In the present study we provide evidence that JAK-STAT inflammatory signaling mediates gangliosides-stimulated microglial activation. Both in rat primary microglia and murine BV2 microglial cells, gangliosides stimulated nuclear factor binding to GAS/ISRE elements, which are known to be STAT-binding sites. Consistent with this, gangliosides rapidly activated JAK1 and JAK2 and induced phosphorylation of STAT1 and STAT3. In addition, gangliosides increased transcription of the inflammation-associated genes inducible nitric-oxide synthase, ICAM-1, and MCP-1, which are reported to contain STAT-binding elements in their promoter regions. AG490, a JAK inhibitor, reduced induction of these genes, nuclear factor binding activity, and activation of STAT1 and -3 in gangliosides-treated microglia. AG490 also inhibited gangliosides-induced release of nitric oxide, an inflammation hallmark. Furthermore, AG490 markedly reduced activation of ERK1/2 MAPK, indicating that ERKs act downstream of JAK-STAT signaling during microglial activation. However, AG490 did not affect activation of p38 MAPK. We also report that the sialic acid residues present on gangliosides may be one of the essential components in activation of JAK-STAT signaling. The present study indicates that JAK-STAT signaling is an early event in gangliosides-induced brain inflammatory responses.

The reverse transcriptase sequence of human immunodeficiency virus type 1 is under positive evolutionary selection within the central nervous system

The human immunodeficiency virus type 1 (HIV-1) enters the central nervous system (CNS) during the acute phase of infection and causes AIDS-related encephalitis and dementia in 30% of individuals. Previous studies show that HIV-1 sequences derived from the CNS of infected patients, including the sequence encoding reverse transcriptase (RT), are genetically distinct from sequences in other tissues. The hypothesis of the current study is that the RT sequence of HIV-1 is under positive selection within the CNS. Multiple alignments of non-CNS-derived and CNS-derived HIV-1 RT sequences were constructed using the ClustalW 1.8 program. The multiple alignments were analyzed with the Synonymous/Nonsynonymous Analysis Program. Codon positions 122-125, 135-149, and 166-212 of the CNS-derived RT sequences underwent a greater accumulation of nonsynonymous than synonymous substitutions, which was markedly different from the analysis results of the non-CNS-derived RT sequences. These residues are located in the finger and palm subdomains of the RT protein structure, which encodes the polymerase active site. The analysis of CNS-derived partial-length RT sequences that encompass these regions yielded similar results. A comparison of CNS-derived RT sequences to a non-CNS-derived RT consensus sequence revealed that a majority of the nonsynonymous substitutions resulted in a specific amino acid replacement. These results indicate that reverse transcriptase is under positive selection within the CNS. The amino acid replacements were visualized on a three-dimensional structure of HIV-1 RT using the Sybyl software suite. The protein structure analysis revealed that the amino acid replacements observed among the CNS-derived sequences occurred in areas of known structural and functional significance.

Heightened intrathecal release of axonal cytoskeletal proteins in multiple sclerosis is associated with progressive disease and clinical disability

The pathologic basis of disease progression in multiple sclerosis (MS) is thought to involve axonal degeneration, which contributes to the accumulation of neurological disability. Recent reports suggest that intrathecal concentrations of the neurofilament protein in relapsing remitting MS correlate with disease activity and the degree of disability. We sought to investigate the intrathecal levels of other cytoskeletal components of axons, primarily actin, tubulin and the light subunit of neurofilament (NFL) in patients with progressive MS and relevant controls and correlate results with clinical parameters of disease severity. Cerebrospinal fluid (CSF) concentrations of actin, tubulin and NFL were significantly increased in MS patients when compared to corresponding levels in patients with other inflammatory or non-inflammatory neurological diseases. Moreover, the intrathecal release of actin and tubulin, and to a lesser extent NFL, was significantly more marked in patients with primary and secondary progressive MS when compared to patients with relapsing remitting disease and was correlated with clinical disability. Our findings suggest that progressive MS is associated with the heightened intrathecal release of axonal cytoskeletal proteins, and that CSF actin, tubulin and NFL are reliable markers of axonal damage.

Intrathecal immune activation is associated with cerebrospinal fluid markers of neuronal destruction in AIDS patients

We analysed the relationship between cerebrospinal fluid (CSF) concentrations of the light subunit of the neurofilament protein (NFL, a marker of neurons, mainly axons), neopterin (a marker of immune activation), and quantitative HIV RNA levels in 47 patients with HIV-1 infection, 25 of whom had AIDS. In the AIDS patients, the mean levels of CSF NFL were high indicating neuronal destruction. The CSF NFL and the CSF neopterin concentrations were correlated in the subgroup of patients without CNS opportunistic infection (p < 0.05). There was no significant correlation between NFL and HIV RNA levels in CSF. In HIV seropositive patients without AIDS, only 3/22 had CSF NFL concentrations above the upper normal reference value. The results suggest that CNS neuronal destruction occurs frequently in patients with AIDS but rarely in those without AIDS, and that immune activation rather than the HIV viral load is associated with neurochemical signs of axonal destruction.

Increased cerebrospinal fluid protein tau concentration in neuro-AIDS

OBJECTIVES

Assessment of cerebrospinal fluid (CSF) levels of protein tau in human immunodeficiency virus type 1 (HIV-1) infection.

MATERIAL AND METHODS

CSF tau levels were analyzed in 52 HIV-1-infected patients, 37 of whom had no neurological symptoms, eight had aquired immunodeficiency syndrome (AIDS) dementia complex (ADC), and seven had AIDS with other neurological complications.

RESULTS

A significantly higher mean CSF tau concentration was found in patients with ADC (380 pg/ml) compared with patients with neuroasymptomatic HIV-1 infection (120 pg/ml, P<0.01) and HIV-negative controls (150 pg/ml, P<0.05). No difference in CSF tau levels was found between patients with ADC and patients with AIDS with other neurological complications.

CONCLUSION

CSF tau might be used as a biochemical marker for axonal degeneration and might be of use to identify HIV-1-infected patients with ADC and other neurological complications, but it cannot discriminate between ADC and other neurological complications in HIV-1-infection.

Gangliosides activate cultured rat brain microglia

Microglia, brain resident macrophages, are activated in brain injuries and several neurodegenerative diseases. However, microglial activators that are produced in the brain are not yet defined. In this study, we showed that gangliosides, sialic acid-containing glycosphingolipids, could be a microglial activator. Gangliosides induced production of nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) and expression of cyclooxygenase-2 (COX-2). The effect of gangliosides on NO release increased dose-dependently in the range of 10-100 microgram/ml; however, the effect decreased at concentrations higher than 200 microgram/ml. Specific types of gangliosides showed differential effects on microglial activation. Similar to gangliosides, GT1b induced production of NO and TNF-alpha and expression of COX-2. However, GM1 and GD1a induced expression of COX-2 but had little effect on NO and TNF-alpha release. The effect of gangliosides and GT1b on NO release was reduced in the presence of neuraminidase, which removes sialic acid residues from gangliosides and GT1b. Gangliosides activated extracellular signal-regulated kinase significantly but activated c-jun N-terminal kinase/stress-activated protein kinase and p38 relatively weakly. The inhibition of extracellular signal-regulated kinase by PD98059 reduced NO release from both gangliosides- and GT1b-treated microglia whereas inhibition of p38 by SB203580 increased it rather slightly. Gangliosides activated NF-kappaB, and N-acetyl cystein, an inhibitor of NF-kappaB, reduced NO release. These results suggest that gangliosides could be a microglial activator that functions via activation of mitogen-activated protein kinase and NF-kappaB.

Gangliosides in cerebrospinal fluid in children with autism spectrum disorders

Gangliosides are sialic acid-containing glycolipids found in all cells, especially abundant in nerve cells and mainly situated on outer-membrane surfaces. The aim of this study was to provide data on the concentration of gangliosides in the CSF of children and adolescents with autism spectrum disorders (ASD) - 66 with autistic disorder, and 19 with other autism spectrum disorders. The comparison group consisted of 29 children and adolescents, whose CSF had been sampled to exclude acute infectious CNS disorder. The concentrations of the gangliosides GM1, GD1a, GD1b, and GT1b were determined using a microimmunoaffinity technique. The ASD group had a significantly higher concentration of ganglioside GM1 compared with the comparison group. The GM1 increase could not be explained as secondary to other clinical factors. Mean ganglioside levels did not differentiate subgroups with autistic disorder and those with a more atypical clinical picture, nor subgroups with known medical disorders and those with idiopathic autism. Altered patterns of gangliosides in the CNS might reflect important correlates of pathogenesis in autism.

Glycosphingolipids as potential diagnostic markers and/or antigens in neurological disorders

Glycosphingolipids are most abundant in the nervous system within which are developmental, regional, structural and cellular differences regarding their composition. The are shedded to the cerebrospinal fluid and thus potential markers for pathogenic alterations in the brain, such as developmental abnormalities, demyelination, gliosis, neuronal cell destruction. The glycosphingolipids have also been found to be antigens in autoimmune processes involving the nervous system, in particular in peripheral neuropathies like Guillain Barré syndrome, multifocal motor neuropathy etc. The immune response might have been triggered by infectious agents with an antigen epitope which mimic the glycosphingolipid or by a primary nerve tissue damage leading to release of glycosphingolipids. There is a series of support for a clinical significance of cerebrospinal fluid glycosphingolipid determinations and the presence of anti-glycosphingolipid antibodies but this has to be further explored. This paper is a mini review of the state of the art and discuss methodological aspects and improvements that might help to explore the relevance of glycosphingolipids in neurological disorders.