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Background and Objective Schizophrenia and Alzheimer’s disease are characterised by abnormalities in glutamatergic pathways related to N -methyl- d -aspartate receptor hypofunction. Glycine is an N -methyl- d -aspartate receptor co-agonist; inhibition of glycine transporter 1 may improve N -methyl- d -aspartate receptor function. This phase I, randomised, two-part study evaluated the safety, tolerability and pharmacokinetic profile of BI 425809, a novel glycine transporter 1 inhibitor, in healthy male and female volunteers. Methods Part 1 evaluated BI 425809 10, 25, 50 or 75 mg once daily or 75 mg twice daily in young subjects, and 25 mg or 50 mg once daily in elderly subjects. Each dose group comprised 12 subjects who received BI 425809 ( n  = 9) or placebo ( n  = 3) for 14 days (day 1: single dose; days 4–14: multiple dosing). Part 2 compared pharmacokinetic profiles in 12 subjects who received a single dose of BI 425809 25 mg in the morning and evening. Results Pharmacokinetic profiles were similarly shaped for all dose groups. Median time to maximum plasma concentration was 3.0–4.5 h with steady state being reached between days 6 and 10. Pharmacokinetic parameters demonstrated dose linearity at the predicted therapeutic exposure range of BI 425809 ≤ 25 mg once daily, but increased less than dose proportionally for ≥ 50 mg once daily. All reported adverse events were of mild-to-moderate intensity, 51/84 (61%; part 1) subjects had one or more treatment-related adverse event, no serious adverse events occurred and no dose dependency was observed. Conclusions Pharmacokinetic properties support both morning and evening dosing. BI 425809 was generally well tolerated at all tested doses. Clinicaltrials.gov identifier NCT02337283.

Glycine transporter 1 (GlyT1) is a key player in shaping extracellular glutamatergic signaling processes and holds promise for treating cognitive impairments associated with schizophrenia by inhibiting its activity and thus enhancing the function of NMDA receptors. Despite its significant role in physiological and pharmacology, its modulation mechanism by clinical drugs and internal lipids remains elusive. Here, we determine cryo-EM structures of GlyT1 in its apo state and in complex with clinical trial drugs iclepertin and sarcosine. The GlyT1 in its apo state is determined in three distinct conformations, exhibiting a conformational equilibrium of the transport cycle. The complex structures with inhibitor iclepertin and sarcosine elucidate their unique binding poses with GlyT1. Three binding sites of cholesterol are determined in GlyT1, two of which are conformation-dependent. Transport kinetics studies reveal that a delicate binding equilibrium for cholesterol is crucial for the conformational transition of GlyT1. This study significantly enhances our understanding of the physiological and pharmacological aspects of GlyT1. GlyT1 critically regulates excitatory neurotransmission and has thus emerged as a therapeutic target for schizophrenia. This study delineates the binding sites of the clinically trialed drugs iclepertin and sarcosine and elucidates how cholesterol modulates GlyT1 activity.

Neuronal human glycine transporter 2 (hGlyT2) plays a critical role in maintaining glycinergic neurotransmission via the reuptake of glycine into presynaptic neurons by using the driving force of sodium and chloride ion gradients. hGlyT2 represents an important drug target for analgesic purpose. However, its structure and the molecular mechanisms remain elusive. Here, we report structures of hGlyT2 in three functional states, including the apo state, the substrate glycine-bound state, and the inhibitor-bound states. The apo state of hGlyT2 adopts an inward conformation. The substrate glycine binds at the central pocket of hGlyT2 in its occluded conformation. Both inhibitors, ORG25543 and opiranserin, bind to an allosteric site, which is vertical to the extracellular tunnel, buried under the extracellular loop 4 (EL4) and near to the transmembrane helix 1b (TM1b). These inhibitors act as wedges to prevent the inward movement of TM1b and closure of the extracellular gate. Further structural analysis reveals both global and local conformational changes associated with the ions and glycine binding and release. These structures define the mechanisms governing transport and allosteric inhibition in hGlyT2, providing a blueprint for further development of non-opioid analgesics targeting hGlyT2. Neuronal human glycine transporter 2 (hGlyT2) is a target for non-opioid analgesics. Here, authors present four cryo-EM structures of hGlyT2 in three states, revealing transport and allosteric inhibition mechanisms for its safer pain drug design.

Suppressed NMDA receptor (NMDAR) function contributes to multiple brain disorders, including schizophrenia, autism spectrum disorder (ASD), and NMDAR encephalitis. Previous attempts to restore NMDAR activity by increasing ambient glycine, a critical co-agonist, through GlyT1 inhibition have yielded mixed outcomes, partly due to GlyT1’s extensive expression in essential brainstem regions. Slc6a20a, a glycine transporter widely expressed in cognition-relevant regions such as the cortex and hippocampus, offers a targeted alternative. Here we show that antisense oligonucleotide (ASO)-mediated Slc6a20a inhibition ( Slc6a20a -ASO) normalizes ASD-related phenotypes in male Shank2 - and Shank3 -mutant mice, with model-dependent rescue profiles. Slc6a20a -ASO rescues NMDAR hypofunction and synaptic phospho-proteomic profiles in the prefrontal cortex. Furthermore, ASO targeting human SLC6A20 rescues suppressed NMDAR function in cortical organoids harboring SHANK2 or SHANK3 mutations. These findings underscore the potential and limitations of Slc6a20a / SLC6A20 -ASO for treating disorders characterized by NMDAR hypofunction. Antisense oligonucleotides targeting the glycine transporter SLC6A20 restore NMDA -receptor function and rescue behavioral and synaptic phospho-proteomic deficits in Shank2- and Shank3-mutant mice and in human SHANK2/SHANK3 cortical organoids.

Chronic neuropathic pain, caused by nerve damage or disease, is increasing in prevalence, but current treatments are ineffective and over-reliant on opioids. The neuronal glycine transporter, GlyT2, regulates inhibitory glycinergic neurotransmission and represents a promising target for new analgesics. However, most GlyT2 inhibitors cause significant side effects, in part due to irreversible inhibition at analgesic doses. Here we develop a reversible inhibitor of GlyT2, RPI-GLYT2-82, and identify its binding site by determining cryo-EM structures of human GlyT2. We capture three fundamental conformational states of GlyT2 in the substrate-free state, and bound to either glycine, RPI-GLYT2-82 or the pseudo-irreversible inhibitor ORG25543. We demonstrate that RPI-GLYT2-82 dissociates from GlyT2 faster than ORG25543, providing analgesia in mouse neuropathic pain models without on-target side-effects or addiction liability. Our data provide a mechanistic understanding of allosteric inhibition of glycine transport, enabling structure-based design of non-opioid analgesics. Neuropathic pain is commonly treated with opioids due to limited alternatives. Here, authors determine cryo-EM structures of the neuronal glycine transporter GlyT2 and develop a reversible inhibitor that provides analgesia in vivo without side effects.

Accumulation of the light-reactive heme precursor protoporphyrin IX (PPIX) in blood causes protoporphyria, a disease characterized by severe pain resulting from sunlight exposure, as well as by the occurrence of liver failure in some patients. Thus, decreasing PPIX biosynthesis is a promising strategy to treat protoporphyria. In this issue of the JCI , Ducamp et al. report that inhibition of the glycine plasma membrane transporter GLYT1 using bitopertin decreased PPIX accumulation and ameliorated liver disease using human in vitro and mouse in vivo models. Their findings support the ongoing development of bitopertin to treat protoporphyria, while concurrently pointing to underexplored roles of glycine in erythroid cells.

Erythropoietic protoporphyria (EPP) is a genetic disorder typically resulting from decreased ferrochelatase (FECH) activity, the last enzyme in heme biosynthesis. Patients with X-linked protoporphyria (XLPP) have an overlapping phenotype caused by increased activity of 5-aminolevulinic acid synthase 2 (ALAS2), the first enzyme in erythroid heme synthesis. In both cases, protoporphyrin IX (PPIX) accumulates in erythrocytes and secondarily in plasma and tissues. Patients develop acute phototoxicity reactions upon brief exposure to sunlight. Some also experience chronic liver disease, and a small fraction develop acute cholestatic liver failure. Therapeutic options are limited, and none, save hematopoietic stem cell transplantation, directly targets erythroid PPIX accumulation. Bitopertin is an investigational orally available small-molecule inhibitor of the erythroid cell-surface glycine transporter GLYT1. We established the bitopertin PPIX inhibitory half-maximal effective concentration in a human erythroblast EPP model and confirmed a marked reduction of PPIX in erythroblasts derived from patients with EPP. We demonstrate that bitopertin also reduced erythrocyte and plasma PPIX accumulation in vivo in both EPP and XLPP mouse models. Finally, the reduction in erythroid PPIX ameliorated liver disease in the EPP mouse model. Altogether, these data support the development of bitopertin to treat patients with EPP or XLPP.

Hyperekplexia is a neurologic disorder of marked perinatal significance. Affected neonates display generalized hypertonia and exaggerated startle reflex in response to innocuous stimuli, potentially leading to life-threatening apneic episodes. Although symptom severity typically diminishes during the first year of life, affected individuals often continue to exhibit disabling motor dysfunction and frequent unprotected falls throughout adulthood. Currently, no targeted therapeutic interventions are available. The pathophysiology involves partial or complete disruption of inhibitory glycinergic neurotransmission. Mutations in the gene encoding the neuronal glycine transporter GlyT2 (SLC6A5) represent the second-most frequent genetic etiology of human hyperekplexia. To investigate the mechanistic basis for the heightened severity of symptoms during the perinatal period, we examined the role of GlyT2 in neuronal differentiation using the PC12 cell model. Pharmacological induction of differentiation demonstrated that clones stably expressing GFP-GlyT2 exhibit increased expression of neuronal differentiation markers and enhanced neurite outgrowth—both in number and length—relative to parental PC12 cells. These clones also displayed elevated cytosolic calcium levels, which were attenuated by calmodulin overexpression, subsequently downregulating differentiation marker expression. We hereby proved that GlyT2 is clearly implicated in growth cone progression and differentiation of PC12 cells into neurons by increasing internal calcium and binding to growth cone proteins. Finally, our results were validated in primary neurons.

Hyperekplexia (OMIM 149400), a sensorimotor syndrome of perinatal clinical relevance, causes newborns to display an energic startle reflex in response to certain trivial stimuli. This condition can be lethal due to apnea episodes. The disease is caused by a blockade of glycinergic neurotransmission. Glycinergic interneurons preserve their identity by the activity of the surface glycine transporter GlyT2, which supplies glycine to presynaptic terminals to maintain glycine content in synaptic vesicles. Loss-of-function mutations in the GlyT2 gene (SLC6A5) cause a presynaptic form of human hyperekplexia. Here, we describe a new GlyT2 variant found in an infantile patient diagnosed with hyperekplexia. A missense mutation in the open reading frame of the GlyT2 gene inherited in homozygosity caused the substitution G449E in a residue highly conserved across the phylogenetic scale. The sequences of the glycine receptor genes GLRA1 and GLRB did not show abnormalities. We expressed the recombinant GlyT2 variant in heterologous cells and analyzed its pathogenic mechanism. The transporter was totally inactive, behaving as a bona fide loss-of-function mutant. Furthermore, the mutation promoted the abnormal insertion of the protein into the membrane, leading to its large incorporation into lipid rafts. However, there was no apparent alteration of wild-type trafficking upon mutant coexpression, as the mutant was prematurely degraded from the endoplasmic reticulum. Rescue with chemical chaperones was not possible for this mutant. Proteomics demonstrated that the expression of the mutant induced the unfolded protein response and interfered with raft-dependent processes. Therefore, the new variant causes a loss of function regarding GlyT2 activity but a gain of function as a cell proteostasis disturber.

Glycine transporters (GlyT1 and GlyT2) that regulate levels of brain glycine, an inhibitory neurotransmitter with co‐agonist activity for NMDA receptors (NMDARs), have been considered to be important targets for the treatment of brain disorders with suppressed NMDAR function such as schizophrenia. However, it remains unclear whether other amino acid transporters expressed in the brain can also regulate brain glycine levels and NMDAR function. Here, we report that SLC6A20A, an amino acid transporter known to transport proline based on in vitro data but is understudied in the brain, regulates proline and glycine levels and NMDAR function in the mouse brain. SLC6A20A transcript and protein levels were abnormally increased in mice carrying a mutant PTEN protein lacking the C terminus through enhanced β‐catenin binding to the Slc6a20a gene. These mice displayed reduced extracellular levels of brain proline and glycine and decreased NMDAR currents. Elevating glycine levels back to normal ranges by antisense oligonucleotide‐induced SLC6A20 knockdown, or the competitive GlyT1 antagonist sarcosine, normalized NMDAR currents and repetitive climbing behavior observed in these mice. Conversely, mice lacking SLC6A20A displayed increased extracellular glycine levels and NMDAR currents. Lastly, both mouse and human SLC6A20 proteins mediated proline and glycine transports, and SLC6A20 proteins could be detected in human neurons. These results suggest that SLC6A20 regulates proline and glycine homeostasis in the brain and that SLC6A20 inhibition has therapeutic potential for brain disorders involving NMDAR hypofunction. Synopsis This study reveals that SLC6A20A, an amino acid transporter previously known to transport proline, also transports glycine, a co‐agonist of NMDA receptors. SLC6A20A inhibition holds therapeutic potential for brain disorders with suppressed NMDAR function such as schizophrenia. SLC6A20A transports both glycine and proline in mammalian cells. PTEN C‐terminal deletion enhances SLC6A20A expression, brain glycine levels, and NMDA receptor function in mice. SLC6A20A haploinsufficient mice display increased brain glycine levels and enhanced NMDA receptor function. Graphical Abstract This study reveals that SLC6A20A, an amino acid transporter previously known to transport proline, also transports glycine, a co‐agonist of NMDA receptors. SLC6A20A inhibition holds therapeutic potential for brain disorders with suppressed NMDAR function such as schizophrenia.