Explore the vast range of titles available.

Lactate in the brain has long been associated with ischaemia; however, more recent evidence shows that it can be found there under physiological conditions. In the brain, lactate is formed predominantly in astrocytes from glucose or glycogen in response to neuronal activity signals. Thus, neurons and astrocytes show tight metabolic coupling. Lactate is transferred from astrocytes to neurons to match the neuronal energetic needs, and to provide signals that modulate neuronal functions, including excitability, plasticity and memory consolidation. In addition, lactate affects several homeostatic functions. Overall, lactate ensures adequate energy supply, modulates neuronal excitability levels and regulates adaptive functions in order to set the 'homeostatic tone' of the nervous system.

ATP13A2 (PARK9) is a late endolysosomal transporter that is genetically implicated in a spectrum of neurodegenerative disorders, including Kufor-Rakeb syndrome—a parkinsonism with dementia 1 —and early-onset Parkinson’s disease 2 . ATP13A2 offers protection against genetic and environmental risk factors of Parkinson’s disease, whereas loss of ATP13A2 compromises lysosomes 3 . However, the transport function of ATP13A2 in lysosomes remains unclear. Here we establish ATP13A2 as a lysosomal polyamine exporter that shows the highest affinity for spermine among the polyamines examined. Polyamines stimulate the activity of purified ATP13A2, whereas ATP13A2 mutants that are implicated in disease are functionally impaired to a degree that correlates with the disease phenotype. ATP13A2 promotes the cellular uptake of polyamines by endocytosis and transports them into the cytosol, highlighting a role for endolysosomes in the uptake of polyamines into cells. At high concentrations polyamines induce cell toxicity, which is exacerbated by ATP13A2 loss due to lysosomal dysfunction, lysosomal rupture and cathepsin B activation. This phenotype is recapitulated in neurons and nematodes with impaired expression of ATP13A2 or its orthologues. We present defective lysosomal polyamine export as a mechanism for lysosome-dependent cell death that may be implicated in neurodegeneration, and shed light on the molecular identity of the mammalian polyamine transport system. The lysosomal polyamine transporter ATP13A2 controls the cellular polyamine content, and impaired lysosomal polyamine export represents a lysosome-dependent cell death pathway that may be implicated in ATP13A2-associated neurodegeneration.

Key Points The regulation of intraneuronal Cl − by cation-chloride cotransporters (CCCs) has a key role in determining the reversal potential ( E GABA ) and driving force ( DF GABA ) of GABA A mediated currents and thereby controls the cellular and network-level actions of GABA. Changes in the spatiotemporal patterns of the expression of functional CCCs and consequent changes in Cl − regulation underlie major quantitative and qualitative changes in GABAergic signalling that are known to take place during neuronal development, plasticity and disease. GABAergic signalling exhibits ionic plasticity, which is based on short-term and long-term changes in the value of E GABA that are brought about by fast activity-dependent net ion fluxes as well as transcriptional and post-translational modifications of CCCs. Most of the basic effects of CCCs on GABA A R signalling also apply to the glycinergic system, and therefore CCCs are important in both the brain and the spinal cord. Recent evidence indicates that CCCs also have unexpected roles in neurons that go beyond their 'canonical' ion-transport functions. Notably, K + –Cl − cotransporter 2 (KCC2), the main neuronal Cl − -extruding CCC isoform, has an important morphogenic role in the formation of cortical dendritic spines. In addition to our increasing understanding of the roles of CCCs in the fundamental machinery underlying neuronal signalling and structure, their diverse roles in neurological diseases have attracted an increasing amount of attention in translational and clinical work. Emerging pharmacological strategies designed for combating various neurological disorders, including epilepsy and chronic pain, are based on targeting CCCs. Dynamic regulation of ion concentrations across the cellular membrane is vital for neuronal function. In this article, Kaila and colleagues review the contribution of members of the cation-chloride cotransporters to neuronal signalling, connectivity, plasticity and disease. Electrical activity in neurons requires a seamless functional coupling between plasmalemmal ion channels and ion transporters. Although ion channels have been studied intensively for several decades, research on ion transporters is in its infancy. In recent years, it has become evident that one family of ion transporters, cation-chloride cotransporters (CCCs), and in particular K + –Cl − cotransporter 2 (KCC2), have seminal roles in shaping GABAergic signalling and neuronal connectivity. Studying the functions of these transporters may lead to major paradigm shifts in our understanding of the mechanisms underlying brain development and plasticity in health and disease.

Key Points Aquaporins (AQPs) are water channel proteins that increase cell membrane water permeability and assemble in cell membranes as tetramers. AQP4, the main water channel in the CNS, is expressed in astrocytes and facilitates the formation and elimination of CNS oedema, modulates neuronal excitability and enhances astrocyte migration. AQP4 also has a role in sensory perception, including vision, hearing and olfaction. Autoantibodies against AQP4 cause neuromyelitis optica, an inflammatory demyelinating disease of the CNS. Aquaporins are also expressed in the peripheral and enteric nervous systems, although their functions at these sites are not known. The aquaporins (AQPs) are a family of integral membrane proteins that are involved in water movement across cell membranes. In this Review, Papadopoulos and Verkman examine the roles of AQPs in the functioning of the mammalian nervous system and in various neurological conditions. The aquaporins (AQPs) are plasma membrane water-transporting proteins. AQP4 is the principal member of this protein family in the CNS, where it is expressed in astrocytes and is involved in water movement, cell migration and neuroexcitation. AQP1 is expressed in the choroid plexus, where it facilitates cerebrospinal fluid secretion, and in dorsal root ganglion neurons, where it tunes pain perception. The AQPs are potential drug targets for several neurological conditions. Astrocytoma cells strongly express AQP4, which may facilitate their infiltration into the brain, and the neuroinflammatory disease neuromyelitis optica is caused by AQP4-specific autoantibodies that produce complement-mediated astrocytic damage.

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.The impairment of brain fluid homeostasis is a feature of various conditions, highlighting the need to better understand brain water transport for drug development. Here, Nanna MacAulay reviews the molecular mechanisms underlying transmembrane water movement in neurons and glia and across brain barriers, emphasizing the part played by water cotransporters in this process.

The serotonin transporter (SERT) terminates serotonergic signalling through the sodium- and chloride-dependent reuptake of neurotransmitter into presynaptic neurons. SERT is a target for antidepressant and psychostimulant drugs, which block reuptake and prolong neurotransmitter signalling. Here we report X-ray crystallographic structures of human SERT at 3.15 Å resolution bound to the antidepressants ( S )-citalopram or paroxetine. Antidepressants lock SERT in an outward-open conformation by lodging in the central binding site, located between transmembrane helices 1, 3, 6, 8 and 10, directly blocking serotonin binding. We further identify the location of an allosteric site in the complex as residing at the periphery of the extracellular vestibule, interposed between extracellular loops 4 and 6 and transmembrane helices 1, 6, 10 and 11. Occupancy of the allosteric site sterically hinders ligand unbinding from the central site, providing an explanation for the action of ( S )-citalopram as an allosteric ligand. These structures define the mechanism of antidepressant action in SERT, and provide blueprints for future drug design. X-ray crystal structures of the human serotonin transporter (SERT) bound to the antidepressants ( S )-citalopram or paroxetine show that the antidepressants lock the protein in an outward-open conformation, and directly block serotonin from entering its binding site; the structures define the mechanism of antidepressant action in SERT and pave the way for future drug design. Antidepressant structure/activity relationships Serotonin modulates the activity of the central nervous system, as well as many other processes throughout the body. These authors have solved X-ray structures of the human serotonin transporter (SERT) in complex with the selective serotonin reuptake inhibitors (SSRIs) ( S )-citalopram and paroxetine — two of the most widely prescribed antidepressants. The resulting structures reveal that the antidepressants lock the protein in an outward-open conformation, and directly block the entry of serotonin into its binding site. A previously unknown allosteric site is seen in the extracellular vestibule; binding of ligands to this site prevents dissociation from the central site, establishing a mechanism of antidepressant action in SERT and pointing the way for future drug design.

Organic cation transporters (OCTs) facilitate the translocation of catecholamines, drugs and xenobiotics across the plasma membrane in various tissues throughout the human body. OCT3 plays a key role in low-affinity, high-capacity uptake of monoamines in most tissues including heart, brain and liver. Its deregulation plays a role in diseases. Despite its importance, the structural basis of OCT3 function and its inhibition has remained enigmatic. Here we describe the cryo-EM structure of human OCT3 at 3.2 Å resolution. Structures of OCT3 bound to two inhibitors, corticosterone and decynium-22, define the ligand binding pocket and reveal common features of major facilitator transporter inhibitors. In addition, we relate the functional characteristics of an extensive collection of previously uncharacterized human genetic variants to structural features, thereby providing a basis for understanding the impact of OCT3 polymorphisms. The current work reports the structure of the human organic cation transporter 3 (OCT3 / SLC22A3) and provides the structural basis of its inhibition by two specific inhibitors, decynium-22 and corticosterone.

P2-type ATPase sodium-potassium pumps (Na + /K + -ATPases) are ion-transporting enzymes that use ATP to transport Na + and K + on opposite sides of the lipid bilayer against their electrochemical gradients to maintain ion concentration gradients across the membranes in all animal cells. Despite the available molecular architecture of the Na + /K + -ATPases, a complete molecular mechanism by which the Na + and K + ions access into and are released from the pump remains unknown. Here we report five cryo-electron microscopy (cryo-EM) structures of the human alpha3 Na + /K + -ATPase in its cytoplasmic side-open (E1), ATP-bound cytoplasmic side-open (E1•ATP), ADP-AlF 4 − trapped Na + -occluded (E1•P-ADP), BeF 3 − trapped exoplasmic side-open (E2P) and MgF 4 2− trapped K + -occluded (E2•P i ) states. Our work reveals the atomically resolved structural detail of the cytoplasmic gating mechanism of the Na + /K + -ATPase. Through cryo-EM analysis, here authors reveal conformational rearrangements that are critical for the gating mechanism of the human alpha3 Na+/K+−ATPase

The serotonin transporter (SERT) regulates neurotransmitter homeostasis through the sodium- and chloride-dependent recycling of serotonin into presynaptic neurons 1 – 3 . Major depression and anxiety disorders are treated using selective serotonin reuptake inhibitors—small molecules that competitively block substrate binding and thereby prolong neurotransmitter action 2 , 4 . The dopamine and noradrenaline transporters, together with SERT, are members of the neurotransmitter sodium symporter (NSS) family. The transport activities of NSSs can be inhibited or modulated by cocaine and amphetamines 2 , 3 , and genetic variants of NSSs are associated with several neuropsychiatric disorders including attention deficit hyperactivity disorder, autism and bipolar disorder 2 , 5 . Studies of bacterial NSS homologues—including LeuT—have shown how their transmembrane helices (TMs) undergo conformational changes during the transport cycle, exposing a central binding site to either side of the membrane 1 , 6 – 12 . However, the conformational changes associated with transport in NSSs remain unknown. To elucidate structure-based mechanisms for transport in SERT we investigated its complexes with ibogaine, a hallucinogenic natural product with psychoactive and anti-addictive properties 13 , 14 . Notably, ibogaine is a non-competitive inhibitor of transport but displays competitive binding towards selective serotonin reuptake inhibitors 15 , 16 . Here we report cryo-electron microscopy structures of SERT–ibogaine complexes captured in outward-open, occluded and inward-open conformations. Ibogaine binds to the central binding site, and closure of the extracellular gate largely involves movements of TMs 1b and 6a. Opening of the intracellular gate involves a hinge-like movement of TM1a and the partial unwinding of TM5, which together create a permeation pathway that enables substrate and ion diffusion to the cytoplasm. These structures define the structural rearrangements that occur from the outward-open to inward-open conformations, and provide insight into the mechanism of neurotransmitter transport and ibogaine inhibition. Cryo-electron microscopy reveals three conformations of the serotonin transporter in complex with ibogaine, detailing the structural rearrangements that occur between the different stages of its transport cycle.

The authors find that glutamate release increases the diffusion of the astrocytic glutamate transporter GLT-1 in the plasma membrane. This activity-dependent increase in mobility facilitates glutamate clearance from the synaptic cleft, which influences the kinetics of excitatory post-synaptic events in rat hippocampal neurons. Control of the glutamate time course in the synapse is crucial for excitatory transmission. This process is mainly ensured by astrocytic transporters, high expression of which is essential to compensate for their slow transport cycle. Although molecular mechanisms regulating transporter intracellular trafficking have been identified, the relationship between surface transporter dynamics and synaptic function remains unexplored. We found that GLT-1 transporters were highly mobile on rat astrocytes. Surface diffusion of GLT-1 was sensitive to neuronal and glial activities and was strongly reduced in the vicinity of glutamatergic synapses, favoring transporter retention. Notably, glutamate uncaging at synaptic sites increased GLT-1 diffusion, displacing transporters away from this compartment. Functionally, impairing GLT-1 membrane diffusion through cross-linking in vitro and in vivo slowed the kinetics of excitatory postsynaptic currents, indicative of a prolonged time course of synaptic glutamate. These data provide, to the best of our knowledge, the first evidence for a physiological role of GLT-1 surface diffusion in shaping synaptic transmission.