General and specific interactions of the phospholipid bilayer with P-type ATPases

The bacterial magnesium transporter MgtA reveals highly selective interaction with specific cardiolipin species

The bacterial magnesium transporter A (MgtA) is a specialized P-type ATPase important for Mg2+ import into the cytoplasm; disrupted magnesium homeostasis is linked to intrinsic ribosome instability and antibacterial resistance in Salmonella strains. Here, we show that MgtA has functional specificity for cardiolipin 18:1. Still, it reaches maximum activity only in combination with cardiolipin 16:0, equivalent to the major components of native cardiolipin found in E. coli membranes. Native mass spectrometry indicates the presence of two binding sites for cardiolipin, agreeing with the kinetic studies revealing that a cooperative relationship likely exists between the two cardiolipin variants. This is the first experimental evidence of cooperative effects between lipids of the same class, with only minor variations in their acyl chain composition, acting on a membrane protein. In summary, our results reveal that MgtA exhibits a highly complex interaction with one cardiolipin 18:1 and one cardiolipin 16:0, affecting protein activity and stability, contributing to our understanding of the particular interactions between lipid environment and membrane proteins. Further, a better understanding of Mg2+ homeostasis in bacteria, due to its role as a virulence regulator, will provide further insights into the regulation and mechanism of bacterial infections.

Electrostatic switch mechanisms of membrane protein trafficking and regulation

Lipid-protein interactions are normally classified as either specific or general. Specific interactions refer to lipid binding to specific binding sites within a membrane protein, thereby modulating the protein's thermal stability or kinetics. General interactions refer to indirect effects whereby lipids affect membrane proteins by modulating the membrane's physical properties, e.g., its fluidity, thickness, or dipole potential. It is not widely recognized that there is a third distinct type of lipid-protein interaction. Intrinsically disordered N- or C-termini of membrane proteins can interact directly but nonspecifically with the surrounding membrane. Many peripheral membrane proteins are held to the cytoplasmic surface of the plasma membrane via a cooperative combination of two forces: hydrophobic anchoring and electrostatic attraction. An acyl chain, e.g., myristoyl, added post-translationally to one of the protein's termini inserts itself into the lipid matrix and helps hold peripheral membrane proteins onto the membrane. Electrostatic attraction occurs between positively charged basic amino acid residues (lysine and arginine) on one of the protein's terminal tails and negatively charged phospholipid head groups, such as phosphatidylserine. Phosphorylation of either serine or tyrosine residues on the terminal tails via regulatory protein kinases allows for an electrostatic switch mechanism to control trafficking of the protein. Kinase action reduces the positive charge on the protein's tail, weakening the electrostatic attraction and releasing the protein from the membrane. A similar mechanism regulates many integral membrane proteins, but here only electrostatic interactions are involved, and the electrostatic switch modulates protein activity by altering the stabilities of different protein conformational states.

Dynamic lipid interactions in the plasma membrane Na+,K+-ATPase

The function of ion-transporting Na+,K+-ATPases depends on the surrounding lipid environment in biological membranes. Two established lipid-interaction sites A and B within the transmembrane domain have been observed to induce protein activation and stabilization, respectively. In addition, lipid-mediated inhibition has been assigned to a site C, but with the exact location not experimentally confirmed. Also, possible effects on lipid interactions by disease mutants dwelling in the membrane-protein interface remain relatively uncharacterized. We simulated human Na+,K+-ATPase α1β1FXYD homology models in E1 and E2 states in an asymmetric, multicomponent plasma membrane to determine both wild-type and disease mutant lipid-protein interactions. The simulated wild-type lipid interactions at the established sites A and B were in agreement with experimental results thereby confirming the membrane-protein model system. The less well-characterized, proposed inhibitory site C was dominated by lipids lacking inhibitory properties. Instead, two sites hosting inhibitory lipids were identified at the extracellular side and also a cytoplasmic CHL-binding site that provide putative alternative locations of Na+,K+-ATPase inhibition. Three disease mutations, Leu302Arg, Glu840Arg and Met859Arg resided in the lipid-protein interface and caused drastic changes in the lipid interactions. The simulation results show that lipid interactions to the human Na+,K+-ATPase α1β1FXYD protein in the plasma membrane are highly state-dependent and can be disturbed by disease mutations located in the lipid interface, which can open up for new venues to understand genetic disorders.

Activity of Na+/K+- and Ca2+-ATPases in human erythrocyte membranes: Protocol improvement, relation to cholesterol content, and effects of polyphenols

It is known that the activities of Na+/K+- and Ca2+-ATPases in the plasma membrane with an excess of cholesterol are compromised. Our main goal was to find out whether quercetin, resveratrol, or caffeic acid, in the nano- and low micromolar concentration ranges, can improve the ATPase activity in human erythrocyte membranes with excess cholesterol. These molecules belong to different chemical classes of polyphenols and are widely present in plant foods. Also, due to some variations in the protocol for determining the ATPase activity, we first analyzed several key parameters of the protocol to improve the accuracy of the results. The activities of Na+/K+- and Ca2+-ATPases were reduced in membranes with moderate and high cholesterol levels compared to membranes from normocholesterolemic subjects (p < 0.01). All three polyphenols affected the ATPase activity in a similar biphasic manner. Namely, the ATPase activity gradually increased with increasing polyphenol concentration up to 80-200 nM, and then gradually decreased with further increase in polyphenol concentration. Moreover, the stimulating effect of the polyphenols was highest in membranes with high cholesterol content, making ATPase activity values close/equal to those in normal cholesterol membranes. In other words, quercetin, resveratrol, and caffeic acid at nanomolar concentrations were able to improve/restore the functioning of Na+/K+- and Ca2+-ATPases in erythrocyte membranes with high cholesterol levels. This suggests a common membrane-mediated mechanism of action for these polyphenols, related to the content of membrane cholesterol.

Anionic Phospholipids Stimulate the Proton Pumping Activity of the Plant Plasma Membrane P-Type H+-ATPase

The activity of membrane proteins depends strongly on the surrounding lipid environment. Here, we characterize the lipid stimulation of the plant plasma membrane H+-ATPase Arabidopsis thaliana H+-ATPase isoform 2 (AHA2) upon purification and reconstitution into liposomes of defined lipid compositions. We show that the proton pumping activity of AHA2 is stimulated by anionic phospholipids, especially by phosphatidylserine. This activation was independent of the cytoplasmic C-terminal regulatory domain of the pump. Molecular dynamics simulations revealed several preferential contact sites for anionic phospholipids in the transmembrane domain of AHA2. These contact sites are partially conserved in functionally different P-type ATPases from different organisms, suggesting a general regulation mechanism by the membrane lipid environment. Our findings highlight the fact that anionic lipids play an important role in the control of H+-ATPase activity.

Cooperativity in regulation of membrane protein function: phenomenological analysis of the effects of pH and phospholipids

Interaction between membrane proteins and ligands plays a key role in governing a wide spectrum of cellular processes. These interactions can provide a cooperative-type regulation of protein function. A wide variety of proteins, including enzymes, channels, transporters, and receptors, displays cooperative behavior in their interactions with ligands. Moreover, the ligands involved encompass a vast diversity and include specific molecules or ions that bind to specific binding sites. In this review, our particular focus is on the interaction between integral membrane proteins and ligands that can present multiple "binding sites", such as protons or membrane phospholipids. The study of the interaction that protons or lipids have with membrane proteins often presents challenges for classical mechanistic modeling approaches. In this regard, we show that, like Hill's pioneering work on hemoglobin regulation, phenomenological modeling constitutes a powerful tool for capturing essential features of these systems.

Na,K-ATPase: A murzyme facilitating thermodynamic equilibriums at the membrane-interface

The redox metabolic paradigm of murburn concept advocates that diffusible reactive species (DRS, particularly oxygen-centric radicals) are mainstays of physiology, and not mere pathological manifestations. The murburn purview of cellular function also integrates the essential principles of bioenergetics, thermogenesis, homeostasis, electrophysiology, and coherence. In this context, any enzyme that generates/modulates/utilizes/sustains DRS functionality is called a murzyme. We have demonstrated that several water-soluble (peroxidases, lactate dehydrogenase, hemogoblin, etc.) and membrane-embedded (Complexes I-V in mitochondria, Photosystems I/II in chloroplasts, rhodopsin/transducin in rod cells, etc.) proteins serve as murzymes. The membrane protein of Na,K-ATPase (NKA, also known as sodium-potassium pump) is the focus of this article, owing to its centrality in neuro-cardio-musculo electrophysiology. Herein, via a series of critical queries starting from the geometric/spatio-temporal considerations of diffusion/mass transfer of solutes in cells to an update on structural/distributional features of NKA in diverse cellular systems, and from various mechanistic aspects of ion-transport (thermodynamics, osmoregulation, evolutionary dictates, etc.) to assays/explanations of inhibitory principles like cardiotonic steroids (CTS), we first highlight some unresolved problems in the field. Thereafter, we propose and apply a minimalist murburn model of trans-membrane ion-differentiation by NKA to address the physiological inhibitory effects of trans-dermal peptide, lithium ion, volatile anesthetics, confirmed interfacial DRS + proton modulators like nitrophenolics and unsaturated fatty acid, and the diverse classes of molecules like CTS, arginine, oximes, etc. These explanations find a pan-systemic connectivity with the inhibitions/uncouplings of other membrane proteins in cells.

A covalently linked probe to monitor local membrane properties surrounding plasma membrane proteins

Functional membrane proteins in the plasma membrane are suggested to have specific membrane environments that play important roles to maintain and regulate their function. However, the local membrane environments of membrane proteins remain largely unexplored due to the lack of available techniques. We have developed a method to probe the local membrane environment surrounding membrane proteins in the plasma membrane by covalently tethering a solvatochromic, environment-sensitive dye, Nile Red, to a GPI-anchored protein and the insulin receptor through a flexible linker. The fluidity of the membrane environment of the GPI-anchored protein depended upon the saturation of the acyl chains of the lipid anchor. The local environment of the insulin receptor was distinct from the average plasma membrane fluidity and was quite dynamic and heterogeneous. Upon addition of insulin, the local membrane environment surrounding the receptor specifically increased in fluidity in an insulin receptor-kinase dependent manner and on the distance between the dye and the receptor.

Conformational changes of a phosphatidylcholine flippase in lipid membranes

Type 4 P-type ATPases (P4-ATPases) actively and selectively translocate phospholipids across membrane bilayers. Driven by ATP hydrolysis, P4-ATPases undergo conformational changes during lipid flipping. It is unclear how the active flipping states of P4-ATPases are regulated in the lipid membranes, especially for phosphatidylcholine (PC)-flipping P4-ATPases whose substrate, PC, is a substantial component of membranes. Here, we report the cryoelectron microscopy structures of a yeast PC-flipping P4-ATPase, Dnf1, in lipid environments. In native yeast lipids, Dnf1 adopts a conformation in which the lipid flipping pathway is disrupted. Only when the lipid composition is changed can Dnf1 be captured in the active conformations that enable lipid flipping. These results suggest that, in the native membrane, Dnf1 may stay in an idle conformation that is unable to support the trans-membrane movement of lipids. Dnf1 may have altered conformational preferences in membranes with different lipid compositions.

Recent advances in research on biointerfaces: From cell surfaces to artificial interfaces

Biointerfaces are regions where biomolecules, cells, and organic materials are exposed to environmental media or come in contact with other biomaterials, cells, and inorganic/organic materials. In this review article, six research topics on biointerfaces are described to show examples of state-of-art research approaches. First, biointerface design of nanoparticles for molecular detection is described. Functionalized gold nanoparticles can be used for sensitive detection of various target molecules, including chemical compounds and biomolecules, such as DNA, proteins, cells, and viruses. Second, the interaction between bacterial cell surfaces and material surfaces, including the introduction of advances in analytical methods and theoretical calculations, are explained as well as their applications to bioprocesses. Third, bioconjugation technologies for localizing functional proteins at biointerfaces are introduced, in particular, by focusing the potential of enzymes as a catalytic tool for designing different types of bioconjugates that function at biointerfaces. Forth topics is focusing on lipid-protein interaction in cell membranes as natural biointerfaces. Examples of membrane lipid engineering are introduced, and it is mentioned how their compositional profiles affect membrane protein functions. Fifth topic is the physical method for molecular delivery across the biointerface being developed currently, such as highly efficient nanoinjection, electroporation, and nanoneedle devices, in which the key is how to perforate the cell membrane. Final topic is the chemical design of lipid- or polymer-based RNA delivery carriers and their behavior on the cell interface, which are currently attracting attention as RNA vaccine technologies targeting COVID-19. Finally, future directions of biointerface studies are presented.

The Donnan-dominated resting state of skeletal muscle fibers contributes to resilience and longevity in dystrophic fibers

Duchenne muscular dystrophy (DMD) is an X-linked dystrophin-minus muscle-wasting disease. Ion homeostasis in skeletal muscle fibers underperforms as DMD progresses. But though DMD renders these excitable cells intolerant of exertion, sodium overloaded, depolarized, and spontaneously contractile, they can survive for several decades. We show computationally that underpinning this longevity is a strikingly frugal, robust Pump-Leak/Donnan (P-L/D) ion homeostatic process. Unlike neurons, which operate with a costly “Pump-Leak–dominated” ion homeostatic steady state, skeletal muscle fibers operate with a low-cost “Donnan-dominated” ion homeostatic steady state that combines a large chloride permeability with an exceptionally small sodium permeability. Simultaneously, this combination keeps fiber excitability low and minimizes pump expenditures. As mechanically active, long-lived multinucleate cells, skeletal muscle fibers have evolved to handle overexertion, sarcolemmal tears, ischemic bouts, etc.; the frugality of their Donnan dominated steady state lets them maintain the outsized pump reserves that make them resilient during these inevitable transient emergencies. Here, P-L/D model variants challenged with DMD-type insult/injury (low pump-strength, overstimulation, leaky Nav and cation channels) show how chronic “nonosmotic” sodium overload (observed in DMD patients) develops. Profoundly severe DMD ion homeostatic insult/injury causes spontaneous firing (and, consequently, unwanted excitation–contraction coupling) that elicits cytotoxic swelling. Therefore, boosting operational pump-strength and/or diminishing sodium and cation channel leaks should help extend DMD fiber longevity.

Sterol Extraction from Isolated Plant Plasma Membrane Vesicles Affects H+-ATPase Activity and H+-Transport

Plasma membrane H+-ATPase is known to be detected in detergent-resistant sterol-enriched fractions, also called "raft" domains. Studies on H+-ATPase reconstituted in artificial or native membrane vesicles have shown both sterol-mediated stimulations and inhibitions of its activity. Here, using sealed isolated plasma membrane vesicles, we investigated the effects of sterol depletion in the presence of methyl-β-cyclodextrin (MβCD) on H+-ATPase activity. The rate of ATP-dependent ∆µH+ generation and the kinetic parameters of ATP hydrolysis were evaluated. We show that the relative sterols content in membrane vesicles decreased gradually after treatment with MβCD and reached approximately 40% of their initial level in 30 mM probe solution. However, changes in the hydrolytic and H+-transport activities of the enzyme were nonlinear. The extraction of up to 20% of the initial sterols was accompanied by strong stimulation of ATP-dependent H+-transport in comparison with the hydrolytic activity of enzymes. Further sterol depletion led to a significant inhibition of active proton transport with an increase in passive H+-leakage. The solubilization of control and sterol-depleted vesicles in the presence of dodecyl maltoside negated the differences in the kinetics parameters of ATP hydrolysis, and all samples demonstrated maximal hydrolytic activities. The mechanisms behind the sensitivity of ATP-dependent H+-transport to sterols in the lipid environment of plasma membrane H+-ATPase are discussed.

Structural basis for sarcolipin's regulation of muscle thermogenesis by the sarcoplasmic reticulum Ca2+-ATPase

The sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) plays a central role in muscle contractility and nonshivering thermogenesis. SERCA is regulated by sarcolipin (SLN), a single-pass membrane protein that uncouples Ca2+ transport from ATP hydrolysis, promoting futile enzymatic cycles and heat generation. The molecular determinants for regulating heat release by the SERCA/SLN complex are unclear. Using thermocalorimetry, chemical cross-linking, and solid-state NMR spectroscopy in oriented phospholipid bicelles, we show that SERCA’s functional uncoupling and heat release rate are dictated by specific SERCA/SLN intramembrane interactions, with the carboxyl-terminal residues anchoring SLN to the SR membrane in an inhibitory topology. Systematic deletion of the carboxyl terminus does not prevent the SERCA/SLN complex formation but reduces uncoupling in a graded manner. These studies emphasize the critical role of lipids in defining the active topology of SLN and modulating the heat release rate by the SERCA/SLN complex, with implications in fat metabolism and basal metabolic rate.

Order-disorder transitions of cytoplasmic N-termini in the mechanisms of P-type ATPases

Membrane protein structure and function are modulated via interactions with their lipid environment. This is particularly true for integral membrane pumps, the P-type ATPases. These ATPases play vital roles in cell physiology, where they are associated with the transport of cations and lipids, thereby generating and maintaining crucial (electro-)chemical potential gradients across the membrane. Several pumps (Na⁺, K⁺-ATPase, H⁺, K⁺-ATPase and the plasma membrane Ca²⁺-ATPase) which are located in the asymmetric animal plasma membrane have been found to possess polybasic (lysine-rich) domains on their cytoplasmic surfaces, which are thought to act as phosphatidylserine (PS) binding domains. In contrast, the sarcoplasmic reticulum Ca²⁺-ATPase, located within an intracellular organelle membrane, does not possess such a domain. Here we focus on the lysine-rich N-termini of the plasma-membrane-bound Na⁺, K⁺- and H⁺, K⁺-ATPases. Synthetic peptides corresponding to the N-termini of these proteins were found, via quartz crystal microbalance and circular dichroism measurements, to interact via an electrostatic interaction with PS-containing membranes, thereby undergoing an increase in helical or other secondary structure content. As well as influencing ion pumping activity, it is proposed that this interaction could provide a mechanism for sensing the lipid asymmetry of the plasma membrane, which changes drastically when a cell undergoes apoptosis, i.e. programmed cell death. Thus, polybasic regions of plasma membrane-bound ion pumps could potentially perform the function of a "death sensor", signalling to a cell to reduce pumping activity and save energy.

Deciphering ion transport and ATPase coupling in the intersubunit tunnel of KdpFABC

KdpFABC, a high-affinity K⁺ pump, combines the ion channel KdpA and the P-type ATPase KdpB to secure survival at K⁺ limitation. Here, we apply a combination of cryo-EM, biochemical assays, and MD simulations to illuminate the mechanisms underlying transport and the coupling to ATP hydrolysis. We show that ions are transported via an intersubunit tunnel through KdpA and KdpB. At the subunit interface, the tunnel is constricted by a phenylalanine, which, by polarized cation-π stacking, controls K⁺ entry into the canonical substrate binding site (CBS) of KdpB. Within the CBS, ATPase coupling is mediated by the charge distribution between an aspartate and a lysine. Interestingly, individual elements of the ion translocation mechanism of KdpFABC identified here are conserved among a wide variety of P-type ATPases from different families. This leads us to the hypothesis that KdpB might represent an early descendant of a common ancestor of cation pumps.

Long chain sphingomyelin depletes cholesterol from the cytoplasmic leaflet in asymmetric lipid membranes

The transbilayer distribution of cholesterol (CHL) in complex asymmetric lipid membranes remains controversial, with contrasting investigations suggesting that there is more CHL either in the exoplasmic, outer leaflet (OL) or the cytoplasmic, inner leaflet (IL) depending on cell type or model, membrane composition, and method of investigation. Here, we launch systematic coarse-grained molecular dynamics simulations to investigate the impact of the sphingomyelin (SM) acyl chain length upon CHL distribution in asymmetric lipid membrane mixtures which account for the variation of the most abundant headgroups and acyl chain unsaturation in the two membrane leaflets. We find that there is always more CHL in the OL, but longer chain SM depletes more CHL from the IL than short chain SM in simple membrane mixtures containing SM and 16 : 0, 18 : 1 phospholipids. The difference between longer and shorter chain SM is neutralised in a more complex asymmetric membrane, where there are more saturated tails in the outer leaflet. We propose that interdigitation of long-chain SM into the opposing IL pushes cytoplasmic CHL towards the OL, but higher chain saturation of the outer leaflet compensates for the effect of SM chain length.

Long acyl chain sphingomyelin and saturated phospholipid tails in the outer membrane leaflet deplete cholesterol from the inner leaflet in mammalian membranes.

Protein Adsorption on Solid Supported Membranes: Monitoring the Transport Activity of P-Type ATPases

P-type ATPases are a large family of membrane transporters that are found in all forms of life. These enzymes couple ATP hydrolysis to the transport of various ions or phospholipids across cellular membranes, thereby generating and maintaining crucial electrochemical potential gradients. P-type ATPases have been studied by a variety of methods that have provided a wealth of information about the structure, function, and regulation of this class of enzymes. Among the many techniques used to investigate P-type ATPases, the electrical method based on solid supported membranes (SSM) was employed to investigate the transport mechanism of various ion pumps. In particular, the SSM method allows the direct measurement of charge movements generated by the ATPase following adsorption of the membrane-bound enzyme on the SSM surface and chemical activation by a substrate concentration jump. This kind of measurement was useful to identify electrogenic partial reactions and localize ion translocation in the reaction cycle of the membrane transporter. In the present review, we discuss how the SSM method has contributed to investigate some key features of the transport mechanism of P-type ATPases, with a special focus on sarcoplasmic reticulum Ca²⁺-ATPase, mammalian Cu⁺-ATPases (ATP7A and ATP7B), and phospholipid flippase ATP8A2.

Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities

Sarcoendoplasmic reticulum calcium ATPase (SERCA), a member of the P-type ATPase family of ion and lipid pumps, is responsible for the active transport of Ca²⁺ from the cytoplasm into the sarcoplasmic reticulum lumen of muscle cells, into the endoplasmic reticulum (ER) of non-muscle cells. X-ray crystallography has proven to be an invaluable tool in understanding the structural changes of SERCA, and more than 70 SERCA crystal structures representing major biochemical states (defined by bound ligand) have been deposited in the Protein Data Bank. Consequently, SERCA is one of the best characterized components of the calcium transport machinery in the cell. Emerging approaches in the field, including spectroscopy and molecular simulation, now help integrate and interpret this rich structural information to understand the conformational transitions of SERCA that occur during activation, inhibition, and regulation. In this review, we provide an overview of the crystal structures of SERCA, focusing on identifying metrics that facilitate structure-based categorization of major steps along the catalytic cycle. We examine the integration of crystallographic data with different biophysical approaches and computational methods to link biochemical and structural states of SERCA that are populated in the cell. Finally, we discuss the challenges and new opportunities in the field, including structural elucidation of functionally important and novel regulatory complexes of SERCA, understanding the structural basis of functional divergence among homologous SERCA regulators, and bridging the gap between basic and translational research directed toward therapeutic modulation of SERCA.

Mechanical stress triggers nuclear remodeling and the formation of transmembrane actin nuclear lines with associated nuclear pore complexes

Mechanical stimulation of fibroblasts induces changes in the actin cytoskeleton including stress fiber (SF) reinforcement and realignment. Here we characterize the nuclear response to mechanical stimulation (uniaxial cyclic stretch). Using fluorescence microscopy and quantitative image analysis we find that stretch-induced nuclear elongation and alignment perpendicular to the stretch vector are dependent on formin-regulated actin polymerization. The mechanosensitive transcription factors Yes-associated protein/Transcriptional coactivator with PDZ domain (YAP/TAZ) and myocardin-related transcription factor (MRTF-A, also known as MKL1 and MAL1) accumulate in the nucleus and activate their target genes in response to uniaxial cyclic stretch. We show that transmembrane actin nuclear (TAN) lines are induced by stretch stimulation and nuclear envelope (NE) proteins including nesprins, SUN2, and lamins form Linkers of the Nucleoskeleton and Cytoskeleton (LINC) complexes aligned with actin SFs. These NE structures are altered by pharmacological treatments (Cytochalasin D and Jasplakinolide) or genetic disruption (zyxin gene deletion) that alter actin, and their persistence requires maintenance of stretch stimulation. Nuclear pore complexes (NPCs) accumulate at TAN lines providing a potential mechanism for linking mechanical cues to NPC function.

The solute carrier transporters and the brain: Physiological and pharmacological implications

Solute carriers (SLCs) are the largest family of transmembrane transporters that determine the exchange of various substances, including nutrients, ions, metabolites, and drugs across biological membranes. To date, the presence of about 287 SLC genes have been identified in the brain, among which mutations or the resultant dysfunctions of 71 SLC genes have been reported to be correlated with human brain disorders. Although increasing interest in SLCs have focused on drug development, SLCs are currently still under-explored as drug targets, especially in the brain. We summarize the main substrates and functions of SLCs that are expressed in the brain, with an emphasis on selected SLCs that are important physiologically, pathologically, and pharmacologically in the blood-brain barrier, astrocytes, and neurons. Evidence suggests that a fraction of SLCs are regulated along with the occurrences of brain disorders, among which epilepsy, neurodegenerative diseases, and autism are representative. Given the review of SLCs involved in the onset and procession of brain disorders, we hope these SLCs will be screened as promising drug targets to improve drug delivery to the brain.