Pharmacological inhibition of ENT1 enhances the impact of specific dietary fats on energy metabolism gene expression.

Medium chain fatty acids are commonly consumed as part of diets for endurance sports and as medical treatment in ketogenic diets where these diets regulate energy metabolism and increase adenosine levels. However, the role of the equilibrative nucleoside transporter 1 (ENT1), which is responsible for adenosine transport across membranes in this process, is not well understood. Here, we investigate ENT1 activity in controlling the effects of two dietary medium chain fatty acids (decanoic and octanoic acid), employing the tractable model system Dictyostelium. We show that genetic ablation of three ENT1 orthologues unexpectedly improves cell proliferation specifically following decanoic acid treatment. This effect is not caused by increased adenosine levels triggered by both fatty acids in the presence of ENT1 activity. Instead, we show that decanoic acid increases expression of energy-related genes relevant for fatty acid ß-oxidation, and that pharmacological inhibition of ENT1 activity leads to an enhanced effect of decanoic acid to increase expression of tricarboxylicacid cycle and oxidative phosphorylation components. Importantly, similar transcriptional changes have been shown in the rat hippocampus during ketogenic diet treatment. We validated these changes by showing enhanced mitochondria load and reduced lipid droplets. Thus, our data show that ENT1 regulates the medium chain fatty acid-induced increase in cellular adenosine levels and the decanoic acid-induced expression of important metabolic enzymes in energy provision, identifying a key role for ENT1 proteins in metabolic effects of medium chain fatty acids.

Developing a Tanshinone IIA Memetic by Targeting MIOS to Regulate mTORC1 and Autophagy in Glioblastoma.

Tanshinone IIA (T2A) is a bioactive compound that provides promise in the treatment of glioblastoma multiforme (GBM), with a range of molecular mechanisms including the inhibition of the mechanistic target of rapamycin complex 1 (mTORC1) and the induction of autophagy. Recently, T2A has been demonstrated to function through sestrin 2 (SESN) to inhibit mTORC1 activity, but its possible impact on autophagy through this pathway has not been investigated. Here, the model system Dictyostelium discoideum and GBM cell lines were employed to investigate the cellular role of T2A in regulating SESN to inhibit mTORC1 and activate autophagy through a GATOR2 component MIOS. In D. discoideum, T2A treatment induced autophagy and inhibited mTORC1 activity, with both effects lost upon the ablation of SESN (sesn-) or MIOS (mios-). We further investigated the targeting of MIOS to reproduce this effect of T2A, where computational analysis identified 25 novel compounds predicted to strongly bind the human MIOS protein, with one compound (MIOS inhibitor 3; Mi3) reducing cell proliferation in two GBM cells. Furthermore, Mi3 specificity was demonstrated through the loss of potency in the D. discoideum mios- cells regarding cell proliferation and the induction of autophagy. In GBM cells, Mi3 treatment also reduced mTORC1 activity and induced autophagy. Thus, a potential T2A mimetic showing the inhibition of mTORC1 and induction of autophagy in GBM cells was identified.

Decanoic acid inhibits mTORC1 activity independent of glucose and insulin signaling.

Low-glucose and -insulin conditions, associated with ketogenic diets, can reduce the activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway, potentially leading to a range of positive medical and health-related effects. Here, we determined whether mTORC1 signaling is also a target for decanoic acid, a key component of the medium-chain triglyceride (MCT) ketogenic diet. Using a tractable model system, Dictyostelium, we show that decanoic acid can decrease mTORC1 activity, under conditions of constant glucose and in the absence of insulin, measured by phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). We determine that this effect of decanoic acid is dependent on a ubiquitin regulatory X domain-containing protein, mediating inhibition of a conserved Dictyostelium AAA ATPase, p97, a homolog of the human transitional endoplasmic reticulum ATPase (VCP/p97) protein. We then demonstrate that decanoic acid decreases mTORC1 activity in the absence of insulin and under high-glucose conditions in ex vivo rat hippocampus and in tuberous sclerosis complex (TSC) patient-derived astrocytes. Our data therefore indicate that dietary decanoic acid may provide a new therapeutic approach to down-regulate mTORC1 signaling.

Perampanel and decanoic acid show synergistic action against AMPA receptors and seizures.

Perampanel is an adjunctive treatment for epilepsy that works through the direct inhibition of AMPA receptors. The same molecular mechanism has recently been shown for a fatty acid, decanoic acid, prescribed in the medium chain triglyceride ketogenic diet for the treatment of patients with drug-resistant epilepsy. Because each compound has been proposed to act through a distinct AMPA receptor binding site, we predicted that perampanel and decanoic acid would act synergistically against AMPA receptors and, consequently, seizures. Here, we show a synergistic interaction between perampanel and decanoic acid in direct AMPA receptor inhibition, in an ex vivo model of seizure activity, and against seizure-induced activity in human brain slices. These data support a potential role for combination treatment using perampanel and dietary decanoic acid to provide enhanced seizure control.

Synapsin I phosphorylation is dysregulated by beta-amyloid oligomers and restored by valproic acid.

Alzheimer's disease is the most prevalent form of dementia in the elderly but the precise causal mechanisms are still not fully understood. Growing evidence supports a significant role for Aß42 oligomers in the development and progression of Alzheimer's. For example, intracellular soluble Aß oligomers are thought to contribute to the early synaptic dysfunction associated with Alzheimer's disease, but the molecular mechanisms underlying this effect are still unclear. Here, we identify a novel mechanism that contributes to our understanding of the reported synaptic dysfunction. Using primary rat hippocampal neurons exposed for a short period of time to Aß42 oligomers, we show a disruption in the activity-dependent phosphorylation cycle of SynapsinI at Ser9. SynapsinI is a pre-synaptic protein that responds to neuronal activity and regulates the availability of synaptic vesicles to participate in neurotransmitter release. Phosphorylation of SynapsinI at Ser9, modulates its distribution and interaction with synaptic vesicles. Our results show that in neurons exposed to Aß42 oligomers, the levels of phosphorylated Ser9 of SynapsinI remain elevated during the recovery period following neuronal activity. We then investigated if this effect could be targeted by a putative therapeutic regime using valproic acid (a short branch-chained fatty acid) that has been proposed as a treatment for Alzheimer's disease. Exposure of Aß42 treated neurons to valproic acid, showed that it restores the physiological regulation of SynapsinI after depolarisation. Our data provide a new insight on Aß42-mediated pathology in Alzheimer's disease and supports the use of Valproic acid as a possible pharmaceutical intervention for the treatment of Alzheimer's disease.

Seizure control by decanoic acid through direct AMPA receptor inhibition.

The medium chain triglyceride ketogenic diet is an established treatment for drug-resistant epilepsy that increases plasma levels of decanoic acid and ketones. Recently, decanoic acid has been shown to provide seizure control in vivo, yet its mechanism of action remains unclear. Here we show that decanoic acid, but not the ketones ß-hydroxybutryate or acetone, shows antiseizure activity in two acute ex vivo rat hippocampal slice models of epileptiform activity. To search for a mechanism of decanoic acid, we show it has a strong inhibitory effect on excitatory, but not inhibitory, neurotransmission in hippocampal slices. Using heterologous expression of excitatory ionotropic glutamate receptor AMPA subunits in Xenopus oocytes, we show that this effect is through direct AMPA receptor inhibition, a target shared by a recently introduced epilepsy treatment perampanel. Decanoic acid acts as a non-competitive antagonist at therapeutically relevant concentrations, in a voltage- and subunit-dependent manner, and this is sufficient to explain its antiseizure effects. This inhibitory effect is likely to be caused by binding to sites on the M3 helix of the AMPA-GluA2 transmembrane domain; independent from the binding site of perampanel. Together our results indicate that the direct inhibition of excitatory neurotransmission by decanoic acid in the brain contributes to the anti-convulsant effect of the medium chain triglyceride ketogenic diet.

An ancestral non-proteolytic role for presenilin proteins in multicellular development of the social amoeba Dictyostelium discoideum.

Mutations in either of two presenilin genes can cause familial Alzheimer's disease. Presenilins have both proteolysis-dependent functions, as components of the γ-secretase complex, and proteolysis-independent functions in signalling. In this study, we investigate a conserved function of human presenilins in the development of the simple model organism Dictyostelium discoideum. We show that the block in Dictyostelium development caused by the ablation of both Dictyostelium presenilins is rescued by the expression of human presenilin 1, restoring the terminal differentiation of multiple cell types. This developmental role is independent of proteolytic activity, because the mutation of both catalytic aspartates does not affect presenilin ability to rescue development, and the ablation of nicastrin, a γ-secretase component that is crucial for proteolytic activity, does not block development. The role of presenilins during Dictyostelium development is therefore independent of their proteolytic activity. However, presenilin loss in Dictyostelium results in elevated cyclic AMP (cAMP) levels and enhanced stimulation-induced calcium release, suggesting that presenilins regulate these intracellular signalling pathways. Our data suggest that presenilin proteins perform an ancient non-proteolytic role in regulating intracellular signalling and development, and that Dictyostelium is a useful model for analysing human presenilin function.

A novel human receptor involved in bitter tastant detection identified using Dictyostelium discoideum.

Detection of substances tasting bitter to humans occurs in diverse organisms including the social amoeba Dictyostelium discoideum. To establish a molecular mechanism for bitter tastant detection in Dictyostelium, we screened a mutant library for resistance to a commonly used bitter standard, phenylthiourea. This approach identified a G-protein-coupled receptor mutant, grlJ(-), which showed a significantly increased tolerance to phenylthiourea in growth, survival and movement. This mutant was not resistant to a structurally dissimilar potent bitter tastant, denatonium benzoate, suggesting it is not a target for at least one other bitter tastant. Analysis of the cell-signalling pathway involved in the detection of phenylthiourea showed dependence upon heterotrimeric G protein and phosphatidylinositol 3-kinase activity, suggesting that this signalling pathway is responsible for the cellular effects of phenylthiourea. This is further supported by a phenylthiourea-dependent block in the transient cAMP-induced production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in wild-type but not grlJ(-) cells. Finally, we have identified an uncharacterized human protein γ-aminobutyric acid (GABA) type B receptor subunit 1 isoform with weak homology to GrlJ that restored grlJ(-) sensitivity to phenylthiourea in cell movement and PIP3 regulation. Our results thus identify a novel pathway for the detection of the standard bitter tastant phenylthiourea in Dictyostelium and implicate a poorly characterized human protein in phenylthiourea-dependent cell responses.

Seizure control by derivatives of medium chain fatty acids associated with the ketogenic diet show novel branching-point structure for enhanced potency.

The medium chain triglyceride (MCT) ketogenic diet is a major treatment of drug-resistant epilepsy but is problematic, particularly in adults, because of poor tolerability. Branched derivatives of octanoic acid (OA), a medium chain fat provided in the diet have been suggested as potential new treatments for drug-resistant epilepsy, but the structural basis of this functionality has not been determined. Here we investigate structural variants of branched medium chain fatty acids as new seizure-control treatments. We initially employ a series of methyl-branched OA derivatives, and using the GABAA receptor antagonist pentylenetetrazol to induce seizure-like activity in rat hippocampal slices, we show a strong, branch-point-specific activity that improves upon the related epilepsy treatment valproic acid. Using low magnesium conditions to induce glutamate excitotoxicity in rat primary hippocampal neuronal cultures for the assessment of neuroprotection, we also show a structural dependence identical to that for seizure control, suggesting a related mechanism of action for these compounds in both seizure control and neuroprotection. In contrast, the effect of these compounds on histone deacetylase (HDAC) inhibition, associated with teratogenicity, shows no correlation with therapeutic efficacy. Furthermore, small structural modifications of the starting compounds provide active compounds without HDAC inhibitory effects. Finally, using multiple in vivo seizure models, we identify potent lead candidates for the treatment of epilepsy. This study therefore identifies a novel family of fatty acids, related to the MCT ketogenic diet, that show promise as new treatments for epilepsy control and possibly other MCT ketogenic diet-responding conditions, such as Alzheimer disease.

New experimental therapies for status epilepticus in preclinical development.

Starting with the established antiepileptic drug, valproic acid, we have taken a novel approach to develop new antiseizure drugs that may be effective in status epilepticus. We first identified that valproic acid has a potent effect on a biochemical pathway, the phosphoinositide pathway, in Dictyostelium discoideum, and we demonstrated that this may relate to its mechanism of action against seizures in mammalian systems. Through screening in this pathway, we have identified a large array of fatty acids and fatty acid derivatives with antiseizure potential. These were then evaluated in an in vitro mammalian system. One compound that we identified through this process is a major constituent of the ketogenic diet, strongly arguing that it may be the fatty acids that are mediating the antiseizure effect of this diet. We further tested two of the more potent compounds in an in vivo model of status epilepticus and demonstrated that they were more effective than valproic acid in treating the status epilepticus. This article is part of a Special Issue entitled "Status Epilepticus".

Seizure-induced reduction in PIP3 levels contributes to seizure-activity and is rescued by valproic acid.

Phosphatidylinositol (3-5) trisphosphate (PIP3) is a central regulator of diverse neuronal functions that are critical for seizure progression, however its role in seizures is unclear. We have recently hypothesised that valproic acid (VPA), one of the most commonly used drugs for the treatment of epilepsy, may target PIP3 signalling as a therapeutic mode of action. Here, we show that seizure induction using kainic acid in a rat in vivo epilepsy model resulted in a decrease in hippocampal PIP3 levels and reduced protein kinase B (PKB/AKT) phosphorylation, measured using ELISA mass assays and Western blot analysis, and both changes were restored following VPA treatment. These finding were reproduced in cultured rat hippocampal primary neurons and entorhinal cortex-hippocampal slices during exposure to the GABA(A) receptor antagonist pentylenetetrazol (PTZ), which is widely used to generate seizures and seizure-like (paroxysmal) activity. Moreover, VPA's effect on paroxysmal activity in the PTZ slice model is blocked by phosphatidylinositol 3-kinase (PI3K) inhibition or PIP2 sequestration by neomycin, indicating that VPA's efficacy is dependent upon PIP3 signalling. PIP3 depletion following PTZ treatment may also provide a positive feedback loop, since enhancing PIP3 depletion increases, and conversely, reducing PIP3 dephosphorylation reduces paroxysmal activity and this effect is dependent upon AMPA receptor activation. Our results therefore indicate that PIP3 depletion occurs with seizure activity, and that VPA functions to reverse these effects, providing a novel mechanism for VPA in epilepsy treatment.

The Rac GEF ZizB regulates development, cell motility and cytokinesis in Dictyostelium.

Dock (dedicator of cytokinesis) proteins represent a family of guanine nucleotide exchange factors (GEFs) that include the well-studied Dock180 family and the poorly characterised zizimin family. Our current understanding of Dock180 function is that it regulates Rho small GTPases and thus has a role in a number of cell processes, including cell migration, development and division. Here, we use a tractable model for cell motility research, Dictyostelium discoideum, to help elucidate the role of the related zizimin proteins. We show that gene ablation of zizA causes no change in development, whereas ablation of zizB gives rise to an aberrant developmental morphology and a reduction in cell directionality and velocity, and altered cell shape. Fluorescently labelled ZizA protein associates with the microtubule-organising centre (MTOC), whereas ZizB is enriched in the cortex. Overexpression of ZizB also causes an increase in the number of filopodia and a partial inhibition of cytokinesis. Analysis of ZizB protein binding partners shows that it interacts with Rac1a and a range of actin-associated proteins. In conclusion, our work provides insight into the molecular and cellular functions of zizimin GEF proteins, which are shown to have a role in cell movement, filopodia formation and cytokinesis.

Mutation Screening of Dictyostelium Restriction Enzyme-Mediated Integration (REMI) Libraries.

Identifying the mechanisms of action of existing and novel drugs is essential for the development of new compounds for therapeutic and commercial use. Here we provide a technique to identify these mechanisms through isolating mutant cell lines that show resistance to drug-induced phenotypes using Dictyostelium discoideum REMI libraries. This approach provides a robust and rapid chemical-genetic screening technique that enables an unbiased approach to identify proteins and molecular pathways that control drug sensitivity. Mutations that result in drug resistance often occur in target proteins thus identifying the specific protein targets for drugs and bioactive natural products. Following the identification of a list of putative molecular targets user selected compound targets can be analyzed to confirm and validate direct inhibitory effects.

Functional mechanism study of the allelochemical myrigalone A identifies a group of ultrapotent inhibitors of ethylene biosynthesis in plants.

Allelochemicals represent a class of natural products released by plants as root, leaf, and fruit exudates that interfere with the growth and survival of neighboring plants. Understanding how allelochemicals function to regulate plant responses may provide valuable new approaches to better control plant function. One such allelochemical, Myrigalone A (MyA) produced by Myrica gale, inhibits seed germination and seedling growth through an unknown mechanism. Here, we investigate MyA using the tractable model Dictyostelium discoideum and reveal that its activity depends on the conserved homolog of the plant ethylene synthesis protein 1-aminocyclopropane-1-carboxylic acid oxidase (ACO). Furthermore, in silico modeling predicts the direct binding of MyA to ACO within the catalytic pocket. In D. discoideum, ablation of ACO mimics the MyA-dependent developmental delay, which is partially restored by exogenous ethylene, and MyA reduces ethylene production. In Arabidopsis thaliana, MyA treatment delays seed germination, and this effect is rescued by exogenous ethylene. It also mimics the effect of established ACO inhibitors on root and hypocotyl extension, blocks ethylene-dependent root hair production, and reduces ethylene production. Finally, in silico binding analyses identify a range of highly potent ethylene inhibitors that block ethylene-dependent response and reduce ethylene production in Arabidopsis. Thus, we demonstrate a molecular mechanism by which the allelochemical MyA reduces ethylene biosynthesis and identify a range of ultrapotent inhibitors of ethylene-regulated responses.

Molecular pharmacology in a simple model system: implicating MAP kinase and phosphoinositide signalling in bipolar disorder.

Understanding the mechanisms of drug action has been the primary focus for pharmacological researchers, traditionally using rodent models. However, non-sentient model systems are now increasingly being used as an alternative approach to better understand drug action or targets. One of these model systems, the social amoeba Dictyostelium, enables the rapid ablation or over-expression of genes, and the subsequent use of isogenic cell culture for the analysis of cell signalling pathways in pharmacological research. The model also supports an increasingly important ethical view of research, involving the reduction, replacement and refinement of animals in biomedical research. This review outlines the use of Dictyostelium in understanding the pharmacological action of two commonly used bipolar disorder treatments (valproic acid and lithium). Both of these compounds regulate mitogen activated protein (MAP) kinase and inositol phospholipid-based signalling by unknown means. Analysis of the molecular pathways targeted by these drugs in Dictyostelium and translation of discoveries to animal systems has helped to further understand the molecular mechanisms of these bipolar disorder treatments.

Identifying an uptake mechanism for the antiepileptic and bipolar disorder treatment valproic acid using the simple biomedical model Dictyostelium.

Valproic acid (VPA) is the most highly prescribed epilepsy treatment worldwide and is also used to prevent bipolar disorder and migraine. Surprisingly, very little is known about its mechanisms of cellular uptake. Here, we employ a range of cellular, molecular and genetic approaches to characterize VPA uptake using a simple biomedical model, Dictyostelium discoideum. We show that VPA is taken up against an electrochemical gradient in a dose-dependent manner. Transport is protein-mediated, dependent on pH and the proton gradient and shows strong substrate structure specificity. Using a genetic screen, we identified a protein homologous to a mammalian solute carrier family 4 (SLC4) bicarbonate transporter that we show is involved in VPA uptake. Pharmacological and genetic ablation of this protein reduces the uptake of VPA and partially protects against VPA-dependent developmental effects, and extracellular bicarbonate competes for VPA uptake in Dictyostelium. We further show that this uptake mechanism is likely to be conserved in both zebrafish (Danio rerio) and Xenopus laevis model systems. These results implicate, for the first time, an uptake mechanism for VPA through SLC4-catalysed activity.

The search for better epilepsy treatments: from slime mould to coconuts.

Drug-resistant epilepsy has remained a problem since the inception of antiepileptic drug development, despite the large variety of antiepileptic drugs available today. Moreover, the mechanism-of-action of these drugs is often unknown. This is due to the widespread screening of compounds through animal models. We have taken a different approach to antiepileptic drug discovery and have identified a biochemical pathway in Dictyostelium discoideum (a 'slime mould') that may relate to the mechanism-of-action of valproate, one of the most commonly used and effective antiepileptic drugs. Through screening in this pathway, we have been able to identify a whole host of fatty acids and fatty acid derivatives with potential antiepileptic activity; this was then confirmed in in vitro and in vivo mammalian seizure models. Some of these compounds are more potent than valproate and potentially lack many of the major side effects of valproate (including birth defects and liver toxicity). In addition, one of the compounds that we have identified is a major constituent of the ketogenic diet, strongly arguing that it may be the fatty acids and not the ketogenesis that are mediating the effect of this diet.

Medium-Chain Fatty Acids Rescue Motor Function and Neuromuscular Junction Degeneration in a Drosophila Model of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterised by progressive degeneration of the motor neurones. An expanded GGGGCC (G4C2) hexanucleotide repeat in C9orf72 is the most common genetic cause of ALS and frontotemporal dementia (FTD); therefore, the resulting disease is known as C9ALS/FTD. Here, we employ a Drosophila melanogaster model of C9ALS/FTD (C9 model) to investigate a role for specific medium-chain fatty acids (MCFAs) in reversing pathogenic outcomes. Drosophila larvae overexpressing the ALS-associated dipeptide repeats (DPRs) in the nervous system exhibit reduced motor function and neuromuscular junction (NMJ) defects. We show that two MCFAs, nonanoic acid (NA) and 4-methyloctanoic acid (4-MOA), can ameliorate impaired motor function in C9 larvae and improve NMJ degeneration, although their mechanisms of action are not identical. NA modified postsynaptic glutamate receptor density, whereas 4-MOA restored defects in the presynaptic vesicular release. We also demonstrate the effects of NA and 4-MOA on metabolism in C9 larvae and implicate various metabolic pathways as dysregulated in our ALS model. Our findings pave the way to identifying novel therapeutic targets and potential treatments for ALS.

Enhanced Sestrin expression through Tanshinone 2A treatment improves PI3K-dependent inhibition of glioma growth.

Glioblastomas are a highly aggressive cancer type which respond poorly to current pharmaceutical treatments, thus novel therapeutic approaches need to be investigated. One such approach involves the use of the bioactive natural product Tanshinone IIA (T2A) derived from the Chinese herb Danshen, where mechanistic insight for this anti-cancer agent is needed to validate its use. Here, we employ a tractable model system, Dictyostelium discoideum, to provide this insight. T2A potently inhibits cellular proliferation of Dictyostelium, suggesting molecular targets in this model. We show that T2A rapidly reduces phosphoinositide 3 kinase (PI3K) and protein kinase B (PKB) activity, but surprisingly, the downstream complex mechanistic target of rapamycin complex 1 (mTORC1) is only inhibited following chronic treatment. Investigating regulators of mTORC1, including PKB, tuberous sclerosis complex (TSC), and AMP-activated protein kinase (AMPK), suggests these enzymes were not responsible for this effect, implicating an additional molecular mechanism of T2A. We identify this mechanism as the increased expression of sestrin, a negative regulator of mTORC1. We further show that combinatory treatment using a PI3K inhibitor and T2A gives rise to a synergistic inhibition of cell proliferation. We then translate our findings to human and mouse-derived glioblastoma cell lines, where both a PI3K inhibitor (Paxalisib) and T2A reduces glioblastoma proliferation in monolayer cultures and in spheroid expansion, with combinatory treatment significantly enhancing this effect. Thus, we propose a new approach for cancer treatment, including glioblastomas, through combinatory treatment with PI3K inhibitors and T2A.

Quantitative determination of SLC2A1 variant functional effects in GLUT1 deficiency syndrome.

OBJECTIVE: The goal of this study is to demonstrate the utility of a growth assay to quantify the functional impact of single nucleotide variants (SNVs) in SLC2A1, the gene responsible for Glut1DS. METHODS: The functional impact of 40 SNVs in SLC2A1 was quantitatively determined in HAP1 cells in which SLC2A1 is required for growth. Donor libraries were introduced into the endogenous SLC2A1 gene in HAP1-Lig4KO cells using CRISPR/Cas9. Cell populations were harvested and sequenced to quantify the effect of variants on growth and generate a functional score. Quantitative functional scores were compared to 3-OMG uptake, SLC2A1 cell surface expression, CADD score, and clinical data, including CSF/blood glucose ratio. RESULTS: Nonsense variants (N = 3) were reduced in cell culture over time resulting in negative scores (mean score: -1.15 ± 0.17), whereas synonymous variants (N = 10) were not depleted (mean score: 0.25 ± 0.12) (P < 2e-16). Missense variants (N = 27) yielded a range of functional scores including slightly negative scores, supporting a partial function and intermediate phenotype. Several variants with normal results on either cell surface expression (p.N34S and p.W65R) or 3-OMG uptake (p.W65R) had negative functional scores. There is a moderate but significant correlation between our functional scores and CADD scores. INTERPRETATION: Cell growth is useful to quantitatively determine the functional effects of SLC2A1 variants. Nonsense variants were reliably distinguished from benign variants in this in vitro functional assay. For facilitating early diagnosis and therapeutic intervention, future work is needed to determine the functional effect of every possible variant in SLC2A1.