GM1 ganglioside exerts protective effects against glutamate-excitotoxicity via its oligosaccharide in wild-type and amyotrophic lateral sclerosis motor neurons

Alterations in glycosphingolipid metabolism have been linked to the pathophysiological mechanisms of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease affecting motor neurons. Accordingly, administration of GM1, a sialic acid-containing glycosphingolipid, is protective against neuronal damage and supports neuronal homeostasis, with these effects mediated by its bioactive component, the oligosaccharide head (GM1-OS). Here, we add new evidence to the therapeutic efficacy of GM1 in ALS: Its administration to WT and SOD1G93A motor neurons affected by glutamate-induced excitotoxicity significantly increased neuronal survival and preserved neurite networks, counteracting intracellular protein accumulation and mitochondria impairment. Importantly, the GM1-OS faithfully replicates GM1 activity, emphasizing that even in ALS the protective function of GM1 strictly depends on its pentasaccharide.

GM1 structural requirements to mediate neuronal functions

Since the 1980s, it has been known that the administration of ganglioside GM1 to cultured cells induced or enhanced neuronal differentiation. GM1 mechanism of action relies on its direct interaction and subsequent activation of the membrane tyrosine kinase receptor, TrkA, which naturally serves as NGF receptor. This process is mediated by the sole oligosaccharide portion of GM1, the pentasaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal-(1-4)-β-Glc. Here we detailed the minimum structural requirements of the oligosaccharide portion of GM1 for mediating the TrkA dependent neuritogenic processing. By in vitro and in silico biochemical approaches, we demonstrated that the minimal portion of GM1 required for the TrkA activation is the inner core of the ganglioside's oligosaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal. The addition of a sialic acid residue at position 3 of the outer galactose of the GM1 oligosaccharide, which forms the oligosaccharide of GD1a, prevented the interaction with TrkA and the resulting neuritogenesis. On the contrary, the addition of a fucose residue at position 2 of the outer galactose, forming the Fucosyl-GM1 oligosaccharide, did not prevent the TrkA-mediated neuritogenesis.

SARS-CoV-2 hampers dopamine production in iPSC-derived dopaminergic neurons

An increasing number of patients experiences prolonged symptoms, whose profile and timeline remain uncertain, a condition that has been defined as post COVID. The majority of recovered hospitalized patients manifests at least one persistent symptom even sixty days after the first clinical manifestation's onset. Particularly, in light of the COVID-19-related symptomatology, it has been hypothesized that SARS-CoV-2 might affect the dopamine pathway. However, no scientific evidence has been produced so far. To this end, human iPSC-derived dopaminergic neurons were infected with EU, Delta and Omicron SARS-CoV-2 variants. The infection with EU and Delta variants, but not with Omicron, results in a reduced intracellular content and extracellular release of dopamine. Indeed, the tyrosine hydroxylase was found to be significantly upregulated at the mRNA level, while being greatly reduced at the protein level. The major downstream synthetic enzyme DOPA-decarboxylase and the dopamine transporter were significantly downregulated both at the mRNA and protein level. Notably, in vitro SARS-CoV-2 infection was also associated with an altered MAP2 and TAU expression and with an increased presence of neuronal stress markers. These preliminary observations suggest that the dopamine metabolism and production are affected by SARS-CoV-2, partially explaining some of the neurological symptoms manifested.

GM1 oligosaccharide efficacy against α-synuclein aggregation and toxicity in vitro

Fibrillary aggregated α-synuclein represents the neurologic hallmark of Parkinson's disease and is considered to play a causative role in the disease. Although the causes leading to α-synuclein aggregation are not clear, the GM1 ganglioside interaction is recognized to prevent this process. How GM1 exerts these functions is not completely clear, although a primary role of its soluble oligosaccharide (GM1-OS) is emerging. Indeed, we recently identified GM1-OS as the bioactive moiety responsible for GM1 neurotrophic and neuroprotective properties, specifically reverting the parkinsonian phenotype both in in vitro and in vivo models. Here, we report on GM1-OS efficacy against the α-synuclein aggregation and toxicity in vitro. By amyloid seeding aggregation assay and NMR spectroscopy, we demonstrated that GM1-OS was able to prevent both the spontaneous and the prion-like α-synuclein aggregation. Additionally, circular dichroism spectroscopy of recombinant monomeric α-synuclein showed that GM1-OS did not induce any change in α-synuclein secondary structure. Importantly, GM1-OS significantly increased neuronal survival and preserved neurite networks of dopaminergic neurons affected by α-synuclein oligomers, together with a reduction of microglia activation. These data further demonstrate that the ganglioside GM1 acts through its oligosaccharide also in preventing the α-synuclein pathogenic aggregation in Parkinson's disease, opening a perspective window for GM1-OS as drug candidate.

GM1 Oligosaccharide Efficacy in Parkinson's Disease: Protection against MPTP

Past evidence has shown that the exogenous administration of GM1 ganglioside slowed neuronal death in preclinical models of Parkinson's disease, a neurodegenerative disorder characterized by the progressive loss of dopamine-producing neurons: however, the physical and chemical properties of GM1 (i.e., amphiphilicity) limited its clinical application, as the crossing of the blood-brain barrier is denied. Recently, we demonstrated that the GM1 oligosaccharide head group (GM1-OS) is the GM1 bioactive portion that, interacting with the TrkA-NGF complex at the membrane surface, promotes the activation of a multivariate network of intracellular events regulating neuronal differentiation, protection, and reparation. Here, we evaluated the GM1-OS neuroprotective potential against the Parkinson's disease-linked neurotoxin MPTP, which destroys dopaminergic neurons by affecting mitochondrial bioenergetics and causing ROS overproduction. In dopaminergic and glutamatergic primary cultures, GM1-OS administration significantly increased neuronal survival, preserved neurite network, and reduced mitochondrial ROS production enhancing the mTOR/Akt/GSK3β pathway. These data highlight the neuroprotective efficacy of GM1-OS in parkinsonian models through the implementation of mitochondrial function and reduction in oxidative stress.

Metabolic Profile Variations along the Differentiation of Human-Induced Pluripotent Stem Cells to Dopaminergic Neurons

In recent years, the availability of induced pluripotent stem cell-based neuronal models has opened new perspectives on the study and therapy of neurological diseases such as Parkinson’s disease. In particular, P. Zhang set up a protocol to efficiently generate dopaminergic neurons from induced pluripotent stem cells. Although the differentiation process of these cells has been widely investigated, there is scant information related to the variation in metabolic features during the differentiation process of pluripotent stem cells to mature dopaminergic neurons. For this reason, we analysed the metabolic profile of induced pluripotent stem cells, neuronal precursors and mature neurons by liquid chromatography–tandem mass spectrometry. We found that induced pluripotent stem cells primarily rely on fatty acid beta-oxidation as a fuel source. Upon progression to neuronal progenitors, it was observed that cells began to shut down fatty acid β-oxidation and preferentially catabolised glucose, which is the principal source of energy in fully differentiated neurons. Interestingly, in neuronal precursors, we observed an increase in amino acids that are likely the result of increased uptake or synthesis, while in mature dopaminergic neurons, we also observed an augmented content of those amino acids needed for dopamine synthesis. In summary, our study highlights a metabolic rewiring occurring during the differentiation stages of dopaminergic neurons.

β-Glucocerebrosidase Deficiency Activates an Aberrant Lysosome-Plasma Membrane Axis Responsible for the Onset of Neurodegeneration

β-glucocerebrosidase is a lysosomal hydrolase involved in the catabolism of the sphingolipid glucosylceramide. Biallelic loss of function mutations in this enzyme are responsible for the onset of Gaucher disease, while monoallelic β-glucocerebrosidase mutations represent the first genetic risk factor for Parkinson’s disease. Despite this evidence, the molecular mechanism linking the impairment in β-glucocerebrosidase activity with the onset of neurodegeneration in still unknown. In this frame, we developed two in vitro neuronal models of β-glucocerebrosidase deficiency, represented by mouse cerebellar granule neurons and human-induced pluripotent stem cells-derived dopaminergic neurons treated with the specific β-glucocerebrosidase inhibitor conduritol B epoxide. Neurons deficient for β-glucocerebrosidase activity showed a lysosomal accumulation of glucosylceramide and the onset of neuronal damage. Moreover, we found that neurons react to the lysosomal impairment by the induction of their biogenesis and exocytosis. This latter event was responsible for glucosylceramide accumulation also at the plasma membrane level, with an alteration in lipid and protein composition of specific signaling microdomains. Collectively, our data suggest that β-glucocerebrosidase loss of function impairs the lysosomal compartment, establishing a lysosome–plasma membrane axis responsible for modifications in the plasma membrane architecture and possible alterations of intracellular signaling pathways, leading to neuronal damage.

Sphingolipids and plasma membrane hydrolases in human primary bronchial cells during differentiation and their altered patterns in cystic fibrosis

Human primary bronchial epithelial cells differentiated in vitro represent a valuable tool to study lung diseases such as cystic fibrosis (CF), an inherited disorder caused by mutations in the gene coding for the Cystic Fibrosis Transmembrane Conductance Regulator. In CF, sphingolipids, a ubiquitous class of bioactive lipids mainly associated with the outer layer of the plasma membrane, seem to play a crucial role in the establishment of the severe lung complications. Nevertheless, no information on the involvement of sphingolipids and their metabolism in the differentiation of primary bronchial epithelial cells are available so far. Here we show that ceramide and globotriaosylceramide increased during cell differentiation, whereas glucosylceramide and gangliosides content decreased. In addition, we found that apical plasma membrane of differentiated bronchial cells is characterized by a higher content of sphingolipids in comparison to the other cell membranes and that activity of sphingolipids catabolic enzymes associated with this membrane results altered with respect to the total cell activities. In particular, the apical membrane of CF cells was characterized by high levels of ceramide and glucosylceramide, known to have proinflammatory activity. On this basis, our data further support the role of sphingolipids in the onset of CF lung pathology.