Gangliosides in neurodegeneration*
KOUTSOURAKI EF., M.D.
1st Department of Neurology, Aristotelian University, Thessaloniki, Greece
Chairman: Professor S.J. Baloyannis
*Oral presentation in the 6th International Congress of the Improvement of the Quality of life on Dementia, Epilepsy, MS and Peripheral Neuropathies, Marseille, 2008.
Since their discovery in the late 1800s, gangliosides remain enigmatic, mostly because of their complexity, structural diversity, and cell specificity. They are composed of a common hydrophobic Cer moiety, which acts as a membrane anchor, and a hydrophilic oligosaccharide chain, which varies in length and composition and contains one or more sialic acid residues. Most gangliosides are amphipathic constituents of the outer leaflet of cell membranes, where they are vital for the maintenance of membrane structure and organization. A small proportion (10%) is localized in mitochondria and endoplasmic reticulum (ER). The amount and composition of gangliosides in a cell is species- and cell type-specific and may vary dramatically during development and changes in the metabolic state of the cell. Owing to their amphiphilic nature and the unique composition of their hydrophobic portion, gangliosides can either distribute asymmetrically or segregate with cholesterol and other membrane proteins in specialized microdomains or clusters (e.g., lipid rafts and caveolae) whose biological activities are greatly influenced by their lipid content. Encephalos 2009, 46(1):44-48.
Key words: Gangliosides, neurodegeneration.
The diversity and complexity of gangliosides suggest that they are not biologically redundant, but have unique functions as receptors or coreceptors for cytokines, toxins, viruses, and bacteria. Gangliosides are also key signaling molecules of pivotal biological processes, including cellular recognition and adhesion, receptor signal transduction, growth regulation, and differentiation. In addition, they are important messengers of the adaptive responses to stress such as apoptosis. Thus, a myriad of crucial cellular responses may be influenced or controlled by gangliosides and ultimately result in either cell growth and division, differentiation, or cell death. Nonetheless, the molecular mechanisms underlying many of these ganglioside-mediated responses remain largely unknown.
Many signal transduction events occur at the plasma membrane and are thought to proceed within caveolae or lipid rafts; these events are greatly influenced by the concentration and subtype of gangliosides. Evidence that gangliosides directly perturb membrane composition and permeability or affect the function of membrane components remains circumstantial. However, it is becoming increasingly clear that gangliosides also play crucial roles in subcellular compartments such as the ER and mitochondria. At these sites, gangliosides influence often opposite cell fate decisions (e.g., proliferation versus apoptosis), and this action appears to depend on their local concentration, structural characteristics, and sugar modifications.
Underlying many ganglioside-mediated effects is a change in intracellular calcium levels (Ca2+). Cytosolic Ca2+ concentration of resting cells is maintained at low levels by the concerted action of specialized channels, a Ca2+ pump, Ca2+-dependent enzymes, and Ca2+-binding proteins. These mechanisms are localized in the cytosol, ER, and mitochondria, the three compartments that control the traffic of Ca2+ across the plasma membrane or into intracellular stores. As Ca2+ regulates a plethora of physiological processes, it is not surprising that perturbation of Ca2+ homeostasis is a potent inducer of an ER stress response that, in turn, dictates the fate of the cells.
Recent studies on the metabolic turnover of gangliosides have demonstrated a variety of dynamic processes by which these molecules are modulated. Besides the typical biosynthetic and degradative pathways, specialized glycosidases can modify gangliosides at the plasma membrane; gangliosides can be directly recycled to the plasma membrane from early endosomes; they can be sorted to the Golgi apparatus from endosomes and subsequently reglycosylated; or they can be fully degraded in lysosomes and reused in the so-called salvage pathway. In the latter process, degradative products leave the lysosome and are subsequently modified and reused for the biosynthesis of new GSLs. In some cells, salvage pathways represent an important means of saving energy and can account for as much as 90% of overall ganglioside turnover. The existence of salvage pathways also potentially explains how certain cells cope with a fast turnover of complex gangliosides at the plasma membrane and intracellular membranes during cell division.
Under physiological conditions and at steady-state levels, pools of gangliosides (or ganglioside intermediates) are present in different subcellular compartments. The modulation of their concentration at those sites strictly depends on the coordinated regulation of the biosynthetic, degradative, and salvage/recycling pathways. However, the precise mechanisms by which the cell balances these pathways have not been fully elucidated.
The importance of gangliosides in cellular integrity and homeostasis is made apparent by the many catastrophic pathogenic conditions (e.g., neurodegenerative diseases and cancer) associated with the abnormal expression, degradation, or distribution of these molecules. Regulation of the metabolism of gangliosides in specific cell types is also imperative, as indicated by the numerous human genetic diseases known as GSL storage diseases or glycosphingolipidoses (Table 1). These monogenic disorders of metabolism that belong to the large group of the lysosomal storage diseases (LSDs) result from deficiency of any one of the lysosomal enzymes involved in GSL degradation and consequent accumulation of undigested GSLs or their intermediates in lysosomes. An account of the cellular consequences of ganglioside accumulation in lysosomes is given in Figure 1.
TABLE 1-FIGURE 1
Glycosphingolipidoses represent one of the most frequent causes of neurodegeneration and mental retardation in children. These diseases are complex and, in most cases, present with a progressive and severe neurodegenerative course and a broad spectrum of systemic abnormalities. The neurologic symptoms include mental retardation or dementia, motor dysfunction, sensory deficits, increased startle response, and seizures. In some disorders, cerebellar signs predominate, whereas in others, the cerebral cortex, basal ganglia, or spinal cord neurons are the most affected. The variations in symptoms associated with different glycosphingolipidoses may reflect differences in the metabolic needs of individual cell types that depend on the selective nature of the primary defect. Pathophysiologic studies of patients and animal models have identified changes in neuronal connectivity in the cerebral cortex, including degeneration of axons and synapses of inhibitory neurons (axon swelling or "spheroids"), regrowth of dendrites (ectopic dendrites or meganeurites), and formation of new synapses of pyramidal neurons. Many of these phenomena have been attributed to ganglioside storage, albeit the underlying molecular effectors are still unknown. Neuronal cell death and demyelination occur in some of these LSDs and are often accompanied by astrogliosis and microgliosis that appear mostly in areas of severe neuronal vacuolation. The presence of reactive astrocytes is indicative of an elicited neuroinflammatory response, and the biochemical heterogeneity and adaptive plasticity may reflect differences in microenvironmental cues, such as the combination of cytokines, growth factors, adhesion molecules, and other signals emanating from injured neurons, activated microglia, endothelial cells, and vascular components.
In addition to the altered intracellular concentration or distribution of gangliosides that occur as a consequence of lysosomal storage, the presence of free pools of gangliosides in the blood plasma, cerebrospinal fluid (CSF), and other body fluids could be part of the molecular mechanisms of disease. Under physiologic conditions, a constant exchange appears to occur between cell-associated and non-cell-associated gangliosides. However, increased turnover of cell membranes resulting from cell degeneration or cell growth leads to an increased release of gangliosides into the extracellular milieu by a process called "shedding".
Several studies have suggested that gangliosides in the CSF are shed from plasma membranes of neural cells. Degenerative processes in the CNS have been linked to increased shedding of membrane fragments into the intercellular space. For example, ganglioside GD3, which is present only in trace amounts in the normal adult brain, is expressed at high levels in activated microglia and in reactive astrocytes. GD3 is released by primary murine microglial cells under neuroinflammatory conditions and is directly responsible for the induction of apoptosis in oligodendrocytes (GD3-mediated apoptotic response). Increased GD3 expression has also been detected in brain tissue from patients with various neurodegenerative disorders such as Creutzfeld-Jacob disease and multiple sclerosis. Moreover, elevated CSF levels of GM3 and to a lesser extent GD3 have been associated with pronounced dysfunction of the blood-brain barrier. On the basis of these observations, we predict that altered intracellular expression of GM3 and GD3 and their release into the extracellular milieu occur in other neurodegenerative and neuroinflammatory conditions including some LSDs. We also speculate that these gangliosides actively participate in cell degeneration.
Increased GM1 concentration at the ER may trigger the ER stress response either by directly affecting Ca2+ transport across the membrane or by indirectly influencing the activity of other membrane components, including the Ca2+ pump and channels. The latter possibility would be consistent with the observed effect of GM2 accumulation in microsomal membranes from Sandhoff disease mice and the cholesterol-induced ER stress response in macrophages from Niemann-Pick A/B mice. Ultimately, the disruption of intracellular Ca2+ homeostasis in ganglioside-accumulating cells could affect protein folding in the ER and induce the ER stress response through the conventional route, thereby suggesting a novel mechanism of neuronal apoptosis that could also occur in other neurodegenerative diseases.
The finding that gangliosides are active mediators of apoptotic programs has opened a remarkable and interesting field of research that has improved our knowledge of the function of these molecules in physiologic and pathologic conditions. Understanding the hierarchy in interorganelle crosstalk and the way that ER and mitochondria interact in response to stress will help to unravel the multifaceted roles of gangliosides in the apoptosis and signal transduction pathways. So far, Ca2+ signaling appears to be the key player in the apoptotic arena, but the molecular mechanism(s) by which gangliosides perturb intracellular Ca2+ flux that ultimately result in cell death need to be further clarified. The Bcl-2 family of proteins could serve as intermediate components of the ganglioside-mediated apoptotic program induced by disturbance of the ER Ca2+ homeostasis. This hypothesis does not necessarily exclude the possibility that gangliosides may also directly affect the MMP and cause mitochondrial damage. In either of the scenarios, gangliosides integrate a plethora of cellular responses by impinging on the functional integrity of intracellular membranes. A full understanding of the involvement of gangliosides in the cell death program will allow us to target molecular effectors that would potentially prevent apoptosis-modulating signals in numerous conditions where gangliosides appear to be harmful.