Alpha-Synuclein and GM1 Ganglioside Co-Localize in Neuronal Cytosol Leading to Inverse Interaction-Relevance to Parkinson's Disease.

Research on GM1 ganglioside and its neuroprotective role in Parkinson's disease (PD), particularly in mitigating the aggregation of α-Synuclein (aSyn), is well established across various model organisms. This essential molecule, GM1, is intimately linked to preventing aSyn aggregation, and its deficiency is believed to play a key role in the initiation of PD. In our current study, we attempted to shed light on the cytosolic interactions between GM1 and aSyn based on previous reports demonstrating gangliosides and monomeric aSyn to be present in neuronal cytosol. Native-PAGE and Western blot analysis of neuronal cytosol from mouse brains demonstrated the presence of both GM1 and monomeric aSyn in the neuronal cytosol of normal mouse brain. To demonstrate that an adequate level of GM1 prevents the aggregation of aSyn, we used NG108-15 and SH-SY5Y cells with and without treatment of 1-phenyl-2-palmitoyl-3-morpholino-1-propanol (PPMP), which inhibits the synthesis/expression of GM1. Cells treated with PPMP to reduce GM1 expression showed a significant increase in the formation of aggregated aSyn compared to untreated cells. We thus demonstrated that sufficient GM1 prevents the aggregation of aSyn. For this to occur, aSyn and GM1 must show proximity within the neuron. The present study provides evidence for such co-localization in neuronal cytosol, which also facilitates the inverse interaction revealed in studies with the two cell types above. This adds to the explanation of how GM1 prevents the aggregation of aSyn and onset of Parkinson's disease.

Synthetic GM1 improves motor and memory dysfunctions in mice with monoallelic or biallelic disruption of GM3 synthase.

This study attempts to answer the question of whether mice with biallelic and monoallelic disruption of the St3gal5 (GM3 synthase) gene might benefit from GM1 replacement therapy. The GM3 produced by this sialyltransferase gives rise to downstream GD3 and the ganglio-series of gangliosides. The latter includes the a-series (GM1 + GD1a), which has proved most essential for neuron survival and function (especially GM1, for which GD1a provides a reserve pool). These biallelic mice serve as a model for children with this relatively rare autosomal recessive condition (ST3GAL5-/-) who suffer rapid neurological decline including motor loss, intellectual disability, visual and hearing loss, failure to thrive, and other severe conditions leading to an early death by 2-5 years of age without supportive care. Here, we studied both these mice, which serve as a model for the parents and close relatives of these children who are likely to suffer long-term disabilities due to partial deficiency of GM1, including Parkinson's disease (PD). We find that the movement and memory disorders manifested by both types of mice can be resolved with GM1 application. This suggests the potential therapeutic value of GM1 for disorders stemming from GM1 deficiency, including GM3 synthase deficiency and PD. It was noteworthy that the GM1 employed in these studies was synthetic rather than animal brain-derived, reaffirming the therapeutic efficacy of the former.

Age-Related Decline in Gangliosides GM1 and GD1a in Non-CNS Tissues of Normal Mice: Implications for Peripheral Symptoms of Parkinson's Disease.

The purpose of this study was to determine whether the age-related decline in a-series gangliosides (especially GM1), shown to be a factor in the brain-related etiology of Parkinson's disease (PD), also pertains to the peripheral nervous system (PNS) and aspects of PD unrelated to the central nervous system (CNS). Following Svennerholm's demonstration of the age-dependent decline in a-series gangliosides (both GM1 and GD1a) in the human brain, we previously demonstrated a similar decline in the normal mouse brain. The present study seeks to determine whether a similar a-series decline occurs in the periphery of normal mice as a possible prelude to the non-CNS symptoms of PD. We used mice of increasing age to measure a-series gangliosides in three peripheral tissues closely associated with PD pathology. Employing high-performance thin-layer chromatography (HPTLC), we found a substantial decrease in both GM1 and GD1a in all three tissues from 191 days of age. Motor and cognitive dysfunction were also shown to worsen, as expected, in synchrony with the decrease in GM1. Based on the previously demonstrated parallel between mice and humans concerning age-related a-series ganglioside decline in the brain, we propose the present findings to suggest a similar a-series decline in human peripheral tissues as the primary contributor to non-CNS pathologies of PD. An onset of sporadic PD would thus be seen as occurring simultaneously throughout the brain and body, albeit at varying rates, in association with the decline in a-series gangliosides. This would obviate the need to postulate the transfer of aggregated α-synuclein between brain and body or to debate brain vs. body as the origin of PD.

Gangliosides in Neurodegenerative Diseases.

The main purpose of this chapter is to summarize the chief findings on ganglioside changes/interactions with some of the neurodegenerative disorders. For the latter we have focused on three diseases that have seen especially intensive study in that regard: Parkinson's, Alzheimer's, and Huntington's diseases. Parkinson's disease (PD) has received the most intensive study with revelation of systemic deficiency of GM1 in brain and all peripheral tissues that have been analyzed to date; this pointed to GM1 replacement as a promising therapy which proved only partially successful when tried for reasons that are discussed. Huntington's disease resembles PD in also manifesting GM1 deficiency, which did, however, respond to GM1 replacement therapy - apparently due to GM1 being administered directly into the brain. Alzheimer's disease was more complex in relation to gangliosides, with b-series (GD1b, GT1b) apparently depressed along with a-series. GM1 administered in brain appeared to induce improvement, but in a limited number of patients. We summarize studies showing why GM1 is of critical importance in neuronal function, and we also briefly point to a few additional neurological disorders in which one or more ganglioside changes have been implicated.

The Key Role of GM1 Ganglioside in Parkinson's Disease.

We have endeavored in this review to summarize our findings, which point to a systemic deficiency of ganglioside GM1 in Parkinson's disease (PD) tissues. These include neuronal tissues well known to be involved in PD, such as substantia nigra of the brain and those of the peripheral nervous system, such as the colon and heart. Moreover, we included skin and fibroblasts in the study as well as peripheral blood mononuclear cells; these are tissues not directly involved in neuronal signaling. We show similar findings for ganglioside GD1a, which is the metabolic precursor to GM1. We discuss the likely causes of these GM1 deficiencies and the resultant biochemical mechanisms underlying loss of neuronal viability and normal functioning. Strong support for this hypothesis is provided by a mouse PD model involving partial GM1 deficiency based on mono-allelic disruption of the B4galnt1 gene. We point out that progressive loss of GM1/GD1a occurs in the periphery as well as the brain, thus obviating the need to speculate PD symptom transfer between these tissues. Finally, we discuss how these findings point to a potential disease-altering therapy for PD:GM1 replacement, as is strongly implicated in animal studies and clinical trials.

Systemic deficiency of GM1 ganglioside in Parkinson's disease tissues and its relation to the disease etiology.

Following our initial reports on subnormal levels of GM1 in the substantia nigra and occipital cortex of Parkinson's disease (PD) patients, we have examined additional tissues from such patients and found these are also deficient in the ganglioside. These include innervated tissues intimately involved in PD pathology such as colon, heart and others, somewhat less intimately involved, such as skin and fibroblasts. Finally, we have analyzed GM1 in peripheral blood mononuclear cells, a type of tissue apparently with no direct innervation, and found those too to be deficient in GM1. Those patients were all afflicted with the sporadic form of PD (sPD), and we therefore conclude that systemic deficiency of GM1 is a characteristic of this major type of PD. Age is one factor in GM1 decline but is not sufficient; additional GM1 suppressive factors are involved in producing sPD. We discuss these and why GM1 replacement offers promise as a disease-altering therapy.

Subnormal GM1 in PBMCs: Promise for Early Diagnosis of Parkinson's Disease?

The fact that Parkinson's disease (PD) pathologies are well advanced in most PD patients by the time of clinical elucidation attests to the importance of early diagnosis. Our attempt to achieve this has capitalized on our previous finding that GM1 ganglioside is expressed at subnormal levels in virtually all tissues of sporadic PD (sPD) patients including blood cells. GM1 is present in most vertebrate cells, is especially abundant in neurons where it was shown essential for their effective functioning and long term viability. We have utilized peripheral blood mononuclear cells (PBMCs) which, despite their low GM1, we found to be significantly lower in sPD patients compared to age-matched healthy controls. To quantify GM1 (and GD1a) we used high performance thin-layer chromatography combined with cholera toxin B linked to horseradish peroxidase, followed by densitometric quantification. GM1 was also deficient in PBMCs from PD patients with mutations in the glucocerebrosidase gene (PD-GBA), apparently even lower than in sPD. Reasons are given why we believe these results obtained with patients manifesting fully developed PD will apply as well to PD patients in preclinical stages-a topic for future study. We also suggest that these findings point to a potential disease altering therapy for PD once the early diagnosis is established.

Mice deficient in GM1 manifest both motor and non-motor symptoms of Parkinson's disease; successful treatment with synthetic GM1 ganglioside.

Parkinson's disease (PD) is a major neurodegenerative disorder characterized by a variety of non-motor symptoms in addition to the well-recognized motor dysfunctions that have commanded primary interest. We previously described a new PD mouse model based on heterozygous disruption of the B4galnt1 gene leading to partial deficiency of the GM1 family of gangliosides that manifested several nigrostriatal neuropathological features of PD as well as movement impairment. We now show this mouse also suffers three non-motor symptoms characteristic of PD involving the gastrointestinal, sympathetic cardiac, and cerebral cognitive systems. Treatment of these animals with a synthetic form of GM1 ganglioside, produced by transfected E. coli, proved ameliorative of these symptoms as well as the motor defect. These findings further suggest subnormal GM1 to be a systemic defect constituting a major risk factor in sporadic PD and indicate the B4galnt1(+/-) (HT) mouse to be a true neuropathological model that recapitulates both motor and non-motor lesions of this condition.

Parkinson's disease recovery by GM1 oligosaccharide treatment in the B4galnt1+/- mouse model.

Given the recent in vitro discovery that the free soluble oligosaccharide of GM1 is the bioactive portion of GM1 for neurotrophic functions, we investigated its therapeutic potential in the B4galnt1+/- mice, a model of sporadic Parkinson's disease. We found that the GM1 oligosaccharide, systemically administered, reaches the brain and completely rescues the physical symptoms, reduces the abnormal nigral α-synuclein content, restores nigral tyrosine hydroxylase expression and striatal neurotransmitter levels, overlapping the wild-type condition. Thus, this study supports the idea that the Parkinson's phenotype expressed by the B4galnt1+/- mice is due to a reduced level of neuronal ganglioside content and lack of interactions between the oligosaccharide portion of GM1 with specific membrane proteins. It also points to the therapeutic potential of the GM1 oligosaccharide for treatment of sporadic Parkinson's disease.

Gangliosides of the Nervous System.

This review begins by attempting to recount some of the pioneering discoveries that first identified the presence of gangliosides in the nervous system, their structures and topography. This is presented as prelude to the current emphasis on physiological function, about which much has been learned but still remains to be elucidated. These areas include ganglioside roles in nervous system development including stem cell biology, membranes and organelles within neurons and glia, ion transport mechanisms, receptor modulation including neurotrophic factor receptors, and importantly the pathophysiological role of ganglioside aberrations in neurodegenerative disorders. This relates to their potential as therapeutic agents, especially in those conditions characterized by deficiency of one or more specific gangliosides. Finally we attempt to speculate on future directions ganglioside research is likely to take so as to capitalize on the impressive progress to date.

Glycan Chains of Gangliosides: Functional Ligands for Tissue Lectins (Siglecs/Galectins).

Molecular signals on the cell surface are responsible for adhesion and communication. Of relevance in this respect, their chemical properties endow carbohydrates with the capacity to store a maximum of information in a minimum of space. One way to present glycans on the cell surface is their covalent conjugation to a ceramide anchor. Among the resulting glycosphingolipids, gangliosides are special due to the presence of at least one sialic acid in the glycan chains. Their spatial accessibility and the dynamic regulation of their profile are factors that argue in favor of a role of glycans of gangliosides as ligands (counterreceptors) for carbohydrate-binding proteins (lectins). Indeed, as discovered first for a bacterial toxin, tissue lectins bind gangliosides and mediate contact formation (trans) and signaling (cis). While siglecs have a preference for higher sialylated glycans, certain galectins also target the monosialylated pentasaccharide of ganglioside GM1. Enzymatic interconversion of ganglioside glycans by sialidase action, relevant for neuroblastoma cell differentiation and growth control in vitro, for axonogenesis and axon regeneration, as well as for proper communication between effector and regulatory T cells, changes lectin-binding affinity profoundly. The GD1a-to-GM1 "editing" is recognized by such lectins, for example, myelin-associated glycoprotein (siglec-4) losing affinity and galectin-1 gaining reactivity, and then translated into postbinding signaling. Orchestrations of loss/gain of affinity, of ganglioside/lectin expression, and of lectin presence in a network offer ample opportunities for fine-tuning. Thus glycans of gangliosides such as GD1a and GM1 are functional counterreceptors by a pairing with tissue lectins, an emerging aspect of ganglioside and lectin functionality.

Gangliosides, α-Synuclein, and Parkinson's Disease.

This review addresses the role of α-synuclein (αSyn) in the etiopathology of Parkinson's disease (PD), with emphasis on its interaction with GM1 ganglioside. We begin with a brief review of some of the milestone discoveries that helped to elucidate PD neuropathology, including the fibrous inclusions of Lewy that characterize the degenerating dopaminergic neurons of the substantia nigra and the presence of αSyn as a major constituent of these Lewy bodies and neurites. This enabled Braak et al. to define the progressive nature of PD in developing their staging hypothesis which described the topographically predictable sequence of neuropathological changes giving rise to prodromal nonmotor symptoms that precede the classical motor dysfunctions. We recount recent studies demonstrating strong, specific binding of αSyn to GM1 that serves to inhibit fibril formation and the key role of N-acetylation of αSyn in enhancing GM1 binding and specificity. The consequences of insufficient GM1 are illustrated in a newly presented mouse model of PD based on partial deletion of this ganglioside due to heterologous disruption of B4galnt1 (GM2/GD2 synthase), such mice presenting accurate recapitulation of the PD phenotype. A key feature of these mice was marked elevation of αSyn aggregates which accompanied motor impairment, both aggregates and motor dysfunction being corrected by GM1 replacement therapy. Such therapy was achieved with high dosage of GM1 and more effectively with lower doses of LIGA20, a membrane permeable analog of GM1. The accuracy of this mouse model was emphasized by the finding that various central nervous system and noncentral nervous system tissues from PD patients manifested similar GM1 deficiency as the B4galnt1+/- mouse. A mechanism is proposed whereby the GM1 deficiency detected in PD patients gives rise to αSyn aggregation and facilitation by the latter in blocking glial cell-derived neurotrophic factor neuroprotection.

Galectins: their network and roles in immunity/tumor growth control.

One route of realizing the information of glycans involves endogenous receptors (lectins). Occurrence at branch ends renders galactosides particularly accessible. Thus, they are suited for such a recognition process. Fittingly, these epitopes serve as physiological ligands. The ga(lactoside-binding) lectins share the ß-sandwich fold with a sequence signature around a central tryptophan residue besides this specificity. Three modes of presentation of the carbohydrate recognition domain are known for galectins, and genome monitoring from fungi to mammals discloses that galectins form a network. The extent of its complexity varies considerably between organisms, for chicken reaching seven proteins, more for mammals. The current status of network analysis reveals overlapping and distinct expression profiles. Matching intra- and extracellular galectin presence, they have a broad range of functions at each site depending on their specific counterreceptor(s), with the possibility even for functional antagonism between family members. Orchestration of expression of galectin, the cognate glycan, its scaffold (protein or sphingolipid) and spatial aspects of glycoconjugate presentation has been detected to lead to growth regulation of immune and tumor cells. To delineate the factors that underlie the specificity of a galectin for its counterreceptor(s) in the cellular context and the details of structure-activity relationships by comparatively analyzing natural and rationally engineered proteins is the main challenge for ongoing research.

Functional interplay between ganglioside GM1 and cross-linking galectin-1 induces axon-like neuritogenesis via integrin-based signaling and TRPC5-dependent Ca²âº influx.

Axon-like neuritogenesis in neuroblastoma (NG108-15) cells and primary cerebellar granular neurons is furthered by the presence of ganglioside GM1. We describe here that galectin-1 (Gal-1), a homobivalent endogenous lectin, is an effector by cross-linking the ganglioside and its associated glycoprotein α5 ß1 -integrin. The thereby triggered signaling cascade involves autophosphorylation of focal adhesion kinase and activation of phospholipase Cγ and phosphoinositide-3 kinase. This leads to a transient increase in the intracellular Ca(2+) concentration by opening of TRPC5 channels, which belong to the signal transduction-gated cation channels. Controls with GM1-defective cells (NG-CR72 and neurons from ganglio-series KO mice) were retarded in axonal growth, underscoring the relevance of GM1 as functional counterreceptor for Gal-1. The lectin's presence was detected in the NG108-15 cells, suggesting an autocrine mechanism of action, and in astrocytes in situ. Gal-1, as cross-linking lectin, can thus translate metabolic conversion of ganglioside GD1a to GM1 by neuraminidase action into axon growth. Galectin-1 (Gal-1) was shown an effector of axonogenesis in cerebellar granule neurons (CGNs) and NG108-15 cells by cross-linking GM1 ganglioside and its associated glycoprotein α5 ß1 -integrin. The resulting signaling led to a transient increase in intracellular Ca(2+) by opening TRPC5 channels. CGNs deficient in GM1 showed retarded axonogenesis, underscoring the relevance of GM1 as functional counterreceptor for Gal-1 in this process. This Gal-1/GM1-induced signaling was manifest only at the earliest, initiating stage of axon development.

Ganglioside accumulation in activated glia in the developing brain: comparison between WT and GalNAcT KO mice.

Our previous studies have shown accumulation of GM2 ganglioside during ethanol-induced neurodegeneration in the developing brain, and GM2 elevation has also been reported in other brain injuries and neurodegenerative diseases. Using GM2/GD2 synthase KO mice lacking GM2/GD2 and downstream gangliosides, the current study explored the significance of GM2 elevation in WT mice. Immunohistochemical studies indicated that ethanol-induced acute neurodegeneration in postnatal day 7 (P7) WT mice was associated with GM2 accumulation in the late endosomes/lysosomes of both phagocytic microglia and increased glial fibrillary acidic protein (GFAP)-positive astrocytes. However, in KO mice, although ethanol induced robust neurodegeneration and accumulation of GD3 and GM3 in the late endosomes/lysosomes of phagocytic microglia, it did not increase the number of GFAP-positive astrocytes, and the accumulation of GD3/GM3 in astrocytes was minimal. Not only ethanol, but also DMSO, induced GM2 elevation in activated microglia and astrocytes along with neurodegeneration in P7 WT mice, while lipopolysaccharide, which did not induce significant neurodegeneration, caused GM2 accumulation mainly in lysosomes of activated astrocytes. Thus, GM2 elevation is associated with activation of microglia and astrocytes in the injured developing brain, and GM2, GD2, or other downstream gangliosides may regulate astroglial responses in ethanol-induced neurodegeneration.

The multi-tasked life of GM1 ganglioside, a true factotum of nature.

GM1 ganglioside occurs widely in vertebrate tissues, where it exhibits many essential functions, both in the plasma membrane and intracellular loci. Its essentiality is revealed in the dire consequences resulting from genetic deletion. This derives from its key roles in several signalosome systems, characteristically located in membrane rafts, where it associates with specific proteins that have glycolipid-binding domains. Thus, GM1 interacts with proteins that modulate mechanisms such as ion transport, neuronal differentiation, G protein-coupled receptors (GPCRs), immune system reactivities, and neuroprotective signaling. The latter occurs through intimate association with neurotrophin receptors, which has relevance to the etiopathogenesis of neurodegenerative diseases and potential therapies. Here, we review the current state of knowledge of these GM1-associated mechanisms.

GDNF signaling implemented by GM1 ganglioside; failure in Parkinson's disease and GM1-deficient murine model.

GDNF is indispensible for adult catecholaminergic neuron survival, and failure of GDNF signaling has been linked to loss of dopaminergic neurons in Parkinson's disease (PD). This study demonstrates attenuated GDNF signaling in neurons deficient in ganglio-series gangliosides, and restoration of such signaling with LIGA20, a membrane permeable analog of GM1. GM1 is shown to associate in situ with GFRα1 and RET, the protein components of the GDNF receptor, this being necessary for assembly of the tripartite receptor complex. Mice wholly or partially deficient in GM1 due to disruption of the B4galnt1 gene developed PD symptoms based on behavioral and neuropathological criteria which were largely ameliorated by gene therapy with AAV2-GDNF and also with LIGA20 treatment. The nigral neurons of PD subjects that were severely deficient in GM1 showed subnormal levels of tyrosine phosphorylated RET. Also in PD brain, GM1 levels in the occipital cortex, a region of limited PD pathology, were significantly below age-matched controls, suggesting the possibility of systemic GM1 deficiency as a risk factor in PD. This would accord with our finding that mice with partial GM1 deficiency represent a faithful recapitulation of the human disease. Together with the previously demonstrated age-related decline of GM1 in human brain, this points to gradual development of subthreshold levels of GM1 in the brain of PD subjects below that required for effective GDNF signaling. This hypothesis offers a dramatically different explanation for the etiology of sporadic PD as a manifestation of acquired resistance to GDNF.

Glycobiology of ion transport in the nervous system.

The nervous system is richly endowed with large transmembrane proteins that mediate ion transport, including gated ion channels as well as energy-consuming pumps and transporters. Transport proteins undergo N-linked glycosylation which can affect expression, location, stability, and function. The N-linked glycans of ion channels are large, contributing between 5 and 50 % of their molecular weight. Many contain a high density of negatively charged sialic acid residues which modulate voltage-dependent gating of ion channels. Changes in the size and chemical composition of glycans are responsible for developmental and cell-specific variability in the biophysical and functional properties of many ion channels. Glycolipids, principally gangliosides, exert considerable influence on some forms of ion transport, either through direct association with ion transport proteins or indirectly through association with proteins that activate transport through appropriate signaling. Examples of both pumps and ion channels have been revealed which depend on ganglioside regulation. While some of these processes are localized in the plasma membrane, ganglioside-regulated ion transport can also occur at various loci within the cell including the nucleus. This chapter will describe ion channel and ion pump structures with a focus on the functional effects of glycosylation on ion channel availability and function, and effects of alterations in glycosylation on nervous system function. It will also summarize highlights of the research on glycolipid/ganglioside-mediated regulation of ion transport.

Deficiency of ganglioside GM1 correlates with Parkinson's disease in mice and humans.

Several studies have successfully employed GM1 ganglioside to treat animal models of Parkinson's disease (PD), suggesting involvement of this ganglioside in PD etiology. We recently demonstrated that genetically engineered mice (B4galnt1(-/-) ) devoid of GM1 acquire characteristic symptoms of this disorder, including motor impairment, depletion of striatal dopamine, selective loss of tyrosine hydroxylase-expressing neurons, and aggregation of α-synuclein. The present study demonstrates similar symptoms in heterozygous mice (HTs) that express only partial GM1 deficiency. Symptoms were alleviated by administration of L-dopa or LIGA-20, a membrane-permeable analog of GM1 that penetrates the blood-brain barrier and accesses intracellular compartments. Immunohistochemical analysis of paraffin sections from PD patients revealed significant GM1 deficiency in nigral dopaminergic neurons compared with age-matched controls. This was comparable to the GM1 deficiency of HT mice and suggests that GM1 deficiency may be a contributing factor to idiopathic PD. We propose that HT mice with partial GM1 deficiency constitute an especially useful model for PD, reflecting the actual pathophysiology of this disorder. The results point to membrane-permeable analogs of GM1 as holding promise as a form of GM1 replacement therapy.

Beyond glycoproteins as galectin counterreceptors: tumor-effector T cell growth control via ganglioside GM1 [corrected].

Glycoprotein glycan chains, by virtue of structure, topology of presentation and connection to signal-inducing units, are functional galectin counterreceptors. As example, cross-linking of the α(5)ß(1) integrin by galectin-1 on carcinoma cells leads to G(1) arrest or anoikis. Contact-dependent switching from proliferation to differentiation in cultured neuroblastoma cells (SK-N-MC) also utilizes galectin-1. Activity enhancement of a cell surface sialidase underlies the shift in glycan display to ganglioside GM1. Its pentasaccharide within microdomains becomes the target. Similarly, this recognition pair is upregulated upon T cell activation. Cross-linking of GM1 along with associated α(4)/α(5)ß(1) integrins elicits Ca(2+)-influx via TRPC5 channels as the relevant response for T effector cell (T(eff)) suppression. Unlike T(eff) cells from wild-type mice, those from genetically altered mice lacking GM1 are not suppressed by galectin-1 or regulatory T cells. Similarly, in the context of GM1 deficiency in NOD mice, T(eff) cells are associated with resistance to regulatory T cell suppression, which is reversed by applied GM1. The broad array of glycosphingolipid structures suggests the possible existence of several novel counterreceptors targeted to endogenous lectins, with sulfatide-galectin-4 interplay within apical delivery serving as recent example.