Regulation of Mct1 by cAMP-dependent internalization in rat brain endothelial cells

Enhanced oral and pulmonary delivery of biomacromolecules via amplified transporter targeting

Ligand-modified nanocarriers can promote oral or inhalative administration of macromolecular drugs across the intestinal or pulmonary mucosa. However, enhancing the unidirectional transport of the nanocarriers through "apical uptake→intracellular transport→basolateral exocytosis" route remains a hot topic and challenge in current research. Forskolin is a naturally occurring diterpenoid compound extracted from the roots of C. forskohlii. In our studies, we found that forskolin could increase the transcellular transport of butyrate-modified nanoparticles by 1.67-fold and 1.20-fold in Caco-2 intestinal epithelial cell models and Calu-3 lung epithelial cell models, respectively. Further mechanistic studies revealed that forskolin, on the one hand, promoted the cellular uptake of butyrate-modified nanoparticles by upregulating the expression of monocarboxylic acid transporter-1 (MCT-1) on the apical membrane. On the other hand, forskolin facilitated the binding of MCT-1 to caveolae, thereby mediating butyrate-modified nanoparticles hijacking caveolae to promote the basolateral exocytosis of butyrate-modified nanoparticles. Studies in normal mice model showed that forskolin could promote the transmucosal absorption of butyrate-modified nanoparticles by >2-fold, regardless of oral or inhalative administration. Using semaglutide as the model drug, both oral and inhalation delivery approaches demonstrated significant hypoglycemic effects in type 2 diabetes mice model, in which inhalative administration was more effective than oral administration. This study optimized the strategies aimed at enhancing the transmucosal absorption of ligand-modified nanocarriers in the intestinal or pulmonary mucosa.

Fasting upregulates the monocarboxylate transporter MCT1 at the rat blood-brain barrier through PPAR δ activation

BACKGROUND

The blood-brain barrier (BBB) is pivotal for the maintenance of brain homeostasis and it strictly regulates the cerebral transport of a wide range of endogenous compounds and drugs. While fasting is increasingly recognized as a potential therapeutic intervention in neurology and psychiatry, its impact upon the BBB has not been studied. This study was designed to assess the global impact of fasting upon the repertoire of BBB transporters.

METHODS

We used a combination of in vivo and in vitro experiments to assess the response of the brain endothelium in male rats that were fed ad libitum or fasted for one to three days. Brain endothelial cells were acutely purified and transcriptionaly profiled using RNA-Seq. Isolated brain microvessels were used to assess the protein expression of selected BBB transporters through western blot. The molecular mechanisms involved in the adaptation to fasting were investigated in primary cultured rat brain endothelial cells. MCT1 activity was probed by in situ brain perfusion.

RESULTS

Fasting did not change the expression of the main drug efflux ATP-binding cassette transporters or P-glycoprotein activity at the BBB but modulated a restrictive set of solute carrier transporters. These included the ketone bodies transporter MCT1, which is pivotal for the brain adaptation to fasting. Our findings in vivo suggested that PPAR δ, a major lipid sensor, was selectively activated in brain endothelial cells in response to fasting. This was confirmed in vitro where pharmacological agonists and free fatty acids selectively activated PPAR δ, resulting in the upregulation of MCT1 expression. Moreover, dosing rats with a specific PPAR δ antagonist blocked the upregulation of MCT1 expression and activity induced by fasting.

CONCLUSIONS

Altogether, our study shows that fasting affects a selected set of BBB transporters which does not include the main drug efflux transporters. Moreover, we describe a previously unknown selective adaptive response of the brain vasculature to fasting which involves PPAR δ and is responsible for the up-regulation of MCT1 expression and activity. Our study opens new perspectives for the metabolic manipulation of the BBB in the healthy or diseased brain.

Amyloid-Beta Peptides 40 and 42 Employ Distinct Molecular Pathways for Cell Entry and Intracellular Transit at the Blood-Brain Barrier Endothelium

The blood-brain barrier (BBB) plays a critical role in maintaining the equilibrium between amyloid beta (Aβ) levels in blood and the brain by regulating Aβ transport. Our previous publications demonstrated that BBB trafficking of Aβ42 and Aβ40 is distinct and is disrupted under various pathophysiological conditions. However, the intracellular mechanisms that allow BBB endothelium to differentially handle Aβ40 and Aβ42 have not been clearly elucidated. In this study, we identified mechanisms of Aβ endocytosis in polarized human cerebral microvascular endothelial cell monolayers. Our studies demonstrated that Aβ peptides with fluorescent label (F-Aβ) were internalized by BBB endothelial cells via energy, dynamin, and actin-dependent endocytosis. Interestingly, endocytosis of F-Aβ40 but not F-Aβ42 was substantially reduced by clathrin inhibition, whereas F-Aβ42 but not F-Aβ40 endocytosis was reduced by half after inhibiting the caveolae-mediated pathway. Following endocytosis, both isoforms were sorted by the endo-lysosomal system. Although Aβ42 was shown to accumulate more in the lysosomes, which could lead to its higher degradation and/or aggregation at lower lysosomal pH, Aβ40 demonstrated robust accumulation in recycling endosomes, which may facilitate its exocytosis by the endothelial cells. These results provide a mechanistic insight into the selective ability of BBB endothelium to transport Aβ40 versus Aβ42. This knowledge contributes to the understanding of molecular pathways underlying Aβ accumulation in the BBB endothelium and associated BBB dysfunction. Moreover, it allows us to establish mechanistic rationale for altered Aβ40:Aβ42 ratios and anomalous amyloid deposition in the cerebral vasculature as well as brain parenchyma during Alzheimer's disease progression. SIGNIFICANCE STATEMENT: Differential interaction of Aβ40 and Aβ42 isoforms with the blood-brain barrier (BBB) endothelium may contribute to perturbation in Aβ42:Aβ40 ratio, which is associated with Alzheimer's disease (AD) progression and severity. The current study identified distinct molecular pathways by which Aβ40 and Aβ42 are trafficked at the BBB, which regulates equilibrium between blood and brain Aβ levels. These findings provide molecular insights into mechanisms that engender BBB dysfunction and promote Aβ accumulation in AD brain.

Distribution of Monocarboxylate Transporters in Brain and Choroid Plexus Epithelium

The choroid plexus (CP) plays central roles in regulating the microenvironment of the central nervous system by secreting the majority of cerebrospinal fluid (CSF) and controlling its composition. A monolayer of epithelial cells of CP plays a significant role in forming the blood-CSF barrier to restrict the movement of substances between the blood and ventricles. CP epithelial cells are equipped with transporters for glucose and lactate that are used as energy sources. There are many review papers on glucose transporters in CP epithelial cells. On the other hand, distribution of monocarboxylate transporters (MCTs) in CP epithelial cells has received less attention compared with glucose transporters. Some MCTs are known to transport lactate, pyruvate, and ketone bodies, whereas others transport thyroid hormones. Since CP epithelial cells have significant carrier functions as well as the barrier function, a decline in the expression and function of these transporters leads to a poor supply of thyroid hormones as well as lactate and can contribute to the process of age-associated brain impairment and pathophysiology of neurodegenerative diseases. In this review paper, recent findings regarding the distribution and significance of MCTs in the brain, especially in CP epithelial cells, are summarized.

The rete mirabile: a possible control site for swimbladder function

In a recent study, a large number of transport proteins was detected in the transcriptome and proteome of saline perfused rete mirabile tissue of the European eel. In this study, the data set was reanalyzed for the presence of receptor proteins and proteins involved in intracellular signaling pathways. A large number of expressed receptor proteins and proteins involved in intracellular signal transduction was detected. Several G-protein-coupled receptor signal pathways were significantly enriched in their expression level, in particular receptors and signaling pathways involved in the control of blood flow. The enriched signaling pathways also include pathways involved in trafficking of crucial transport proteins like, monocarboxylate transporters, V-ATPase, and aquaporin. The data, therefore, suggest that the rete mirabile has the capacity to control swimbladder function by regulating blood flow and by modifying countercurrent multiplication.

Spinal Astrocyte-Neuron Lactate Shuttle Contributes to the Pituitary Adenylate Cyclase-Activating Polypeptide/PAC1 Receptor-Induced Nociceptive Behaviors in Mice

We have previously shown that spinal pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP type 1 (PAC1) receptor signaling triggered long-lasting nociceptive behaviors through astroglial activation in mice. Since astrocyte-neuron lactate shuttle (ANLS) could be essential for long-term synaptic facilitation, we aimed to elucidate a possible involvement of spinal ANLS in the development of the PACAP/PAC1 receptor-induced nociceptive behaviors. A single intrathecal administration of PACAP induced short-term spontaneous aversive behaviors, followed by long-lasting mechanical allodynia in mice. These nociceptive behaviors were inhibited by 1,4-dideoxy-1,4-imino-d-arabinitol (DAB), an inhibitor of glycogenolysis, and this inhibition was reversed by simultaneous L-lactate application. In the cultured spinal astrocytes, the PACAP-evoked glycogenolysis and L-lactate secretion were inhibited by DAB. In addition, a protein kinase C (PKC) inhibitor attenuated the PACAP-induced nociceptive behaviors as well as the PACAP-evoked glycogenolysis and L-lactate secretion. Finally, an inhibitor for the monocarboxylate transporters blocked the L-lactate secretion from the spinal astrocytes and inhibited the PACAP- and spinal nerve ligation-induced nociceptive behaviors. These results suggested that spinal PAC1 receptor-PKC-ANLS signaling contributed to the PACAP-induced nociceptive behaviors. This signaling system could be involved in the peripheral nerve injury-induced pain-like behaviors.

Insights into the regulation of podocyte and glomerular function by lactate and its metabolic sensor G-protein-coupled receptor 81

Podocytes and their foot processes are an important cellular layer of the renal filtration barrier that is involved in regulating glomerular permeability. Disturbances of podocyte function play a central role in the development of proteinuria in diabetic nephropathy. The retraction and effacement of podocyte foot processes that form slit diaphragms are a common feature of proteinuria. Correlations between the retraction of foot processes and the development of proteinuria are not well understood. Unraveling peculiarities of podocyte energy metabolism notably under diabetic conditions will provide insights into the pathogenesis of diabetic nephropathy. Intracellular metabolism in the cortical area of podocytes is regulated by glycolysis, whereas energy balance in the central area is controlled by oxidative phosphorylation and glycolysis. High glucose concentrations were recently reported to force podocytes to switch from mitochondrial oxidative phosphorylation to glycolysis, resulting in lactic acidosis. In this review, we hypothesize that the lactate receptor G-protein-coupled receptor 81 (also known as hydroxycarboxylic acid receptor 81) may contribute to the control of podocyte function in both health and disease.

Brain Endothelial Cells: Metabolic Flux and Energy Metabolism

The neurovascular unit (NVU) consists of multiple cell types including brain endothelial cells, pericytes, astrocytes, and neurons that function collectively to maintain homeostasis within the CNS microenvironment. As the principal barrier-forming component of the NVU, the endothelial cells perform an array of complex functions that require substantial energy resources. The principal metabolic pathways for producing ATP are glycolysis and mitochondrial oxidative phosphorylation. While previous studies have demonstrated that glycolysis is a primary pathway for most endothelial cells, details about the energy producing pathways of brain endothelial cells are not fully characterized. The contributions of glycolysis and mitochondrial respiration to energy metabolism are quantifiable using metabolic flux analysis that measures cellular oxygen consumption and acidification (proton production) in a closed microtiter plate format. ATP production rates are then calculated. The bioenergetics of the human brain microvascular endothelial cell line, hCMEC/D3, indicate that these cells exhibit relatively elevated rates of glycolytic flux and glycolytic ATP production, thus confirming their glycolytic nature even in the presence of abundant oxygen. Furthermore, energy producing pathways involving mitochondrial respiration are relatively low, although contributing significantly to total ATP production. Interestingly, the bioenergetics of the hCMEC/D3 cells are relatively similar to those of human primary brain microvascular endothelial cells (hBVECs). These findings allow a quantitative understanding of the bioenergetics of brain endothelial cells in a cultured and proliferative state and also provide a platform for comparative studies of disease states and conditions involving exposures to drugs or metabolic disruptors.

Immunoreactivity of receptor and transporters for lactate located in astrocytes and epithelial cells of choroid plexus of human brain

Glucose metabolism produces lactate and hydrogen ions in an anaerobic environment. Cerebral ischemia or hypoxia is believed to become progressively lactacidemic. Monocarboxylate transporters (MCTs) in endothelial cells are essential for the transport of lactate from the blood into the brain. In addition, it is considered that MCTs located in astrocytic and neuronal cells play a key role in the shuttling of energy metabolites between neurons and astrocytes. However, roles of lactate in the brain remain to be clarified. In this study, the localization of lactate transporters and a receptor for cellular uptake of lactate was immunohistochemically examined in autopsied human brains. Immunoreactivity for MCT1 was observed in the apical cytoplasmic membrane of some epithelial cells in the choroid plexus as well as astrocytes and the capillary wall, whereas that for MCT4 was found in the basolateral cytoplasmic membrane of small number of epithelial cells as well as astrocytes and the capillary wall. In addition, immunoreactivity for the hydroxy-carboxylic acid 1 receptor (HCA1 receptor), a receptor for cellular uptake of lactate, was also found on the basolateral cytoplasmic membrane of epithelial cells as well as astrocytic and neuronal cells. Immunoreactivity for lactate dehydrogenase (LDH)-B was observed in the cytoplasm of epithelial cells in the choroid plexus as well as astrocytes and the capillary wall. These immunohistochemical findings indicate the localization of MCT1, MCT4, the HCA1 receptor, and LDH-B in epithelial cells of the choroid plexus as well as astrocytes, and suggest the transport of intravascular lactate into the brain through epithelial cells of the choroid plexus as well as cerebral vessels and the possibility of lactate being utilized in epithelial cells.

PTMsnp: A Web Server for the Identification of Driver Mutations That Affect Protein Post-translational Modification

High-throughput sequencing technologies have identified millions of genetic mutations in multiple human diseases. However, the interpretation of the pathogenesis of these mutations and the discovery of driver genes that dominate disease progression is still a major challenge. Combining functional features such as protein post-translational modification (PTM) with genetic mutations is an effective way to predict such alterations. Here, we present PTMsnp, a web server that implements a Bayesian hierarchical model to identify driver genetic mutations targeting PTM sites. PTMsnp accepts genetic mutations in a standard variant call format or tabular format as input and outputs several interactive charts of PTM-related mutations that potentially affect PTMs. Additional functional annotations are performed to evaluate the impact of PTM-related mutations on protein structure and function, as well as to classify variants relevant to Mendelian disease. A total of 4,11,574 modification sites from 33 different types of PTMs and 1,776,848 somatic mutations from TCGA across 33 different cancer types are integrated into the web server, enabling identification of candidate cancer driver genes based on PTM. Applications of PTMsnp to the cancer cohorts and a GWAS dataset of type 2 diabetes identified a set of potential drivers together with several known disease-related genes, indicating its reliability in distinguishing disease-related mutations and providing potential molecular targets for new therapeutic strategies. PTMsnp is freely available at: http://ptmsnp.renlab.org.

Identification of osteoporosis markers through bioinformatic functional analysis of serum proteome

Osteoporosis is a severe chronic skeletal disorder that increases the risks of disability and mortality; however, the mechanism of this disease and the protein markers for prognosis of osteoporosis have not been well characterized. This study aims to characterize the imbalanced serum proteostasis, the disturbed pathways, and potential serum markers in osteoporosis by using a set of bioinformatic analyses. In the present study, the large-scale proteomics datasets (PXD006464) were adopted from the Proteome Xchange database and processed with MaxQuant. The differentially expressed serum proteins were identified. The biological process and molecular function were analyzed. The protein-protein interactions and subnetwork modules were constructed. The signaling pathways were enriched. We identified 209 upregulated and 230 downregulated serum proteins. The bioinformatic analyses revealed a highly overlapped functional protein classification and the gene ontology terms between the upregulated and downregulated protein groups. Protein-protein interactions and pathway analyses showed a high enrichment in protein synthesis, inflammation, and immune response in the upregulated proteins, and cell adhesion and cytoskeleton regulation in the downregulated proteins. Our findings greatly expand the current view of the roles of serum proteins in osteoporosis and shed light on the understanding of its underlying mechanisms and the discovery of serum proteins as potential markers for the prognosis of osteoporosis.

Cyclic adenosine monophosphate in acute ischemic stroke: some to update, more to explore

The development of effective treatment for ischemic stroke, which is a common cause of morbidity and mortality worldwide, remains an unmet goal because the current first-line treatment management interventional therapy has a strict time window and serious complications. In recent years, a growing body of evidence has shown that the elevation of intracellular and extracellular cyclic adenosine monophosphate (cAMP) alleviates brain damage after ischemic stroke by attenuating neuroinflammation in the central nervous system and peripheral immune system. In the central nervous system, upregulated intracellular cAMP signaling can alleviate immune-mediated damage by restoring neuronal morphology and function, inhibiting microglia migration and activation, stabilizing the membrane potential of astrocytes and improving the cellular functions of endothelial cells and oligodendrocytes. Enhancement of the extracellular cAMP signaling pathway can improve neurological function by activating the cAMP-adenosine pathway to reduce immune-mediated damage. In the peripheral immune system, cAMP can act on various immune cells to suppress peripheral immune function, which can alleviate the inflammatory response in the central nervous system and improve the prognosis of acute cerebral ischemic injury. Therefore, cAMP may play key roles in reducing post-stroke neuroinflammatory damage. The protective roles of the cAMP indicate that the cAMP enhancing drugs such as cAMP supplements, phosphodiesterase inhibitors, adenylate cyclase agonists, which are currently used in the treatment of heart and lung diseases. They are potentially able to be applied as a new therapeutic strategy in ischemic stroke. This review focuses on the immune-regulating roles and the clinical implication of cAMP in acute ischemic stroke.

Possible influence of free fatty acid receptors on pH regulation in the ruminal epithelium of sheep

High amounts of short-chain fatty acids (SCFAs) occur in the ovine rumen and constitute the animal's main energy source. However, they lead to an acidification of the ruminal epithelium. Therefore, effective intracellular pH (pH_i ) regulation by transport proteins like monocarboxylate transporter 1 (MCT1) and Na⁺ /H⁺ exchangers (NHEs) is pivotal to ruminants to avoid epithelial damage. SCFAs might function not only as nutrients but also as signalling molecules by activating free fatty acid receptors (FFARs) in the ruminal epithelium and thus influence pH_i regulation. FFARs work as nutrient sensors, transducing their information by modulating cyclic adenosine monophosphate (cAMP) levels. We hypothesized that (FFAR-modulated) decreases in cAMP levels stimulate the activity of MCT1 and NHEs in the ruminal epithelium of sheep. We detected two FFARs (GPR109A and FFAR2) immunohistochemically in the ovine ruminal epithelium. Administration of 10 mM butyrate to Ussing chamber-mounted epithelia provoked a significant reduction in intraepithelial cAMP levels. However, application of the GPR109A agonist niacin did not affect cAMP levels. MCT1 activity was analysed by measuring transepithelial ¹⁴ C-acetate fluxes, which were not inhibited by forskolin-induced increased cAMP levels. The recovery of pH_i after acidification was assessed as an indicator of NHE activity in primary cultured ruminal epithelial cells. Recovery was significantly reduced when cells with increased cAMP levels were subjected to the NHE inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (10 µM). Nonetheless, with augmented cAMP levels alone, NHE activity tended to decline. We hypothesize that modulation of cAMP levels by butyrate is accomplished by FFAR2 activation, regulating NHE activity for pH_i homoeostasis at least in part.

Dual Properties of Lactate in Müller Cells: The Effect of GPR81 Activation

Purpose

Besides being actively metabolized, lactate may also function as a signaling molecule by activation of the G-protein-coupled receptor 81 (GPR81). Thus, we aimed to characterize the metabolic effects of GPR81 activation in Müller cells.

Method

Primary Müller cells from mice were treated with and without 10 mM L-lactate in the presence or absence of 6 mM glucose. The effects of lactate receptor GPR81 activation were evaluated by the addition of 5 mM 3,5-DHBA (3,5-dihydroxybenzoic acid), a GPR81 agonist. Western blot analyses were used to determine protein expression of GPR81. Cell survival was assessed through 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) viability assays. Lactate release was quantified by commercially available lactate kits. 13C-labeling studies via mass spectroscopy and Seahorse analyses were performed to evaluate metabolism of lactate and glucose, and mitochondrial function. Finally, Müller cell function was evaluated by measuring glutamate uptake.

Results

The lactate receptor, GPR81, was upregulated during glucose deprivation. Treatment with a GPR81 agonist did not affect Müller cell survival. However, GPR81 activation diminished lactate release allowing lactate to be metabolized intracellularly. Furthermore, GPR81 activation increased metabolism of glucose and mitochondrial function. Finally, maximal glutamate uptake decreased in response to GPR81 activation during glucose deprivation.

Conclusions

The present study revealed dual properties of lactate via functioning as an active metabolic energy substrate and a regulatory molecule by activation of the GPR81 receptor in primary Müller cells. Thus, combinational therapy of lactate and GPR81 agonists may be of future interest in maintaining Müller cell survival, ultimately leading to increased resistance toward retinal neurodegeneration.

Loops and layers of post-translational modifications of drug transporters

Drug transporters encoded by solute carrier (SLC) family are distributed in multiple organs including kidney, liver, placenta, brain, and intestine, where they mediate the absorption, distribution, and excretion of a diverse array of environmental toxins and clinically important drugs. Alterations in the expression and function of these transporters play important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. While it is clear that the regulation of these transporters tightly depends on genetic mechanisms, many studies have demonstrated that these transporters are the target of various post-translational modifications. This review article summarizes the recent advances in identifying the posttranslational modifications underlying the regulation of the drug transporters of SLC family. Such mechanisms are pivotal not only in physiological conditions, but also in diseases.

The role of small GTPases and EPAC-Rap signaling in the regulation of the blood-brain and blood-retinal barriers

Maintenance and regulation of the vascular endothelial cell junctional complex is critical for proper barrier function of the blood-brain barrier (BBB) and the highly related blood-retinal barrier (BRB) that help maintain proper neuronal environment. Recent research has demonstrated that the junctional complex is actively maintained and can be dynamically regulated. Studies focusing on the mechanisms of barrier formation, maintenance, and barrier disruption have been of interest to understanding development of the BBB and BRB and identifying a means for therapeutic intervention for diseases ranging from brain tumors and dementia to blinding eye diseases. Research has increasingly revealed that small GTPases play a critical role in both barrier formation and disruption mechanisms. This review will summarize the current data on small GTPases in barrier regulation with an emphasis on the EPAC-Rap1 signaling pathway to Rho in endothelial barriers, as well as explore its potential involvement in paracellular flux and transcytosis regulation.

Endothelial Progenitor Cells Physiology and Metabolic Plasticity in Brain Angiogenesis and Blood-Brain Barrier Modeling

Currently, there is a considerable interest to the assessment of blood-brain barrier (BBB) development as a part of cerebral angiogenesis developmental program. Embryonic and adult angiogenesis in the brain is governed by the coordinated activity of endothelial progenitor cells, brain microvascular endothelial cells, and non-endothelial cells contributing to the establishment of the BBB (pericytes, astrocytes, neurons). Metabolic and functional plasticity of endothelial progenitor cells controls their timely recruitment, precise homing to the brain microvessels, and efficient support of brain angiogenesis. Deciphering endothelial progenitor cells physiology would provide novel engineering approaches to establish adequate microfluidically-supported BBB models and brain microphysiological systems for translational studies.

Regulation of Monocarboxylic Acid Transporter 1 Trafficking by the Canonical Wnt/β-Catenin Pathway in Rat Brain Endothelial Cells Requires Cross-talk with Notch Signaling

The transport of monocarboxylate fuels such as lactate, pyruvate, and ketone bodies across brain endothelial cells is mediated by monocarboxylic acid transporter 1 (MCT1). Although the canonical Wnt/β-catenin pathway is required for rodent blood-brain barrier development and for the expression of associated nutrient transporters, the role of this pathway in the regulation of brain endothelial MCT1 is unknown. Here we report expression of nine members of the frizzled receptor family by the RBE4 rat brain endothelial cell line. Furthermore, activation of the canonical Wnt/β-catenin pathway in RBE4 cells via nuclear β-catenin signaling with LiCl does not alter brain endothelialMct1mRNA but increases the amount of MCT1 transporter protein. Plasma membrane biotinylation studies and confocal microscopic examination of mCherry-tagged MCT1 indicate that increased transporter results from reduced MCT1 trafficking from the plasma membrane via the endosomal/lysosomal pathway and is facilitated by decreased MCT1 ubiquitination following LiCl treatment. Inhibition of the Notch pathway by the γ-secretase inhibitorN-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycinet-butyl ester negated the up-regulation of MCT1 by LiCl, demonstrating a cross-talk between the canonical Wnt/β-catenin and Notch pathways. Our results are important because they show, for the first time, the regulation of MCT1 in cerebrovascular endothelial cells by the multifunctional canonical Wnt/β-catenin and Notch signaling pathways.

Glycolysis-mediated control of blood-brain barrier development and function

The blood-brain barrier (BBB) consists of differentiated cells integrating in one ensemble to control transport processes between the central nervous system (CNS) and peripheral blood. Molecular organization of BBB affects the extracellular content and cell metabolism in the CNS. Developmental aspects of BBB attract much attention in recent years, and barriergenesis is currently recognized as a very important and complex mechanism of CNS development and maturation. Metabolic control of angiogenesis/barriergenesis may be provided by glucose utilization within the neurovascular unit (NVU). The role of glycolysis in the brain has been reconsidered recently, and it is recognized now not only as a process active in hypoxic conditions, but also as a mechanism affecting signal transduction, synaptic activity, and brain development. There is growing evidence that glycolysis-derived metabolites, particularly, lactate, affect barriergenesis and functioning of BBB. In the brain, lactate produced in astrocytes or endothelial cells can be transported to the extracellular space via monocarboxylate transporters (MCTs), and may act on the adjoining cells via specific lactate receptors. Astrocytes are one of the major sources of lactate production in the brain and significantly contribute to the regulation of BBB development and functioning. Active glycolysis in astrocytes is required for effective support of neuronal activity and angiogenesis, while endothelial cells regulate bioavailability of lactate for brain cells adjusting its bidirectional transport through the BBB. In this article, we review the current knowledge with regard to energy production in endothelial and astroglial cells within the NVU. In addition, we describe lactate-driven mechanisms and action of alternative products of glucose metabolism affecting BBB structural and functional integrity in developing and mature brain.

Gene therapy for the nervous system: challenges and new strategies

Current clinical treatments for central nervous system (CNS) diseases, such as Parkinson's disease and glioblastoma do not halt disease progression and have significant treatment morbidities. Gene therapy has the potential to "permanently" correct disease by bringing in a normal gene to correct a mutant gene deficiency, knocking down mRNA of mutant alleles, and inducing cell-death in cancer cells using transgenes encoding apoptosis-inducing proteins. Promising results in clinical trials of eye disease (Leber's congenital aumorosis) and Parkinson's disease have shown that gene-based neurotherapeutics have great potential. The recent development of genome editing technology, such as zinc finger nucleases, TALENS, and CRISPR, has made the ultimate goal of gene correction a step closer. This review summarizes the challenges faced by gene-based neurotherapeutics and the current and recent strategies designed to overcome these barriers. We have chosen the following challenges to focus on in this review: (1) delivery vehicles (both virus and nonviral), (2) use of promoters for vector-mediated gene expression in CNS, and (3) delivery across the blood-brain barrier. The final section (4) focuses on promising pre-clinical/clinical studies of neurotherapeutics.

Transendothelial Transport and Its Role in Therapeutics

Present review paper highlights role of BBB in endothelial transport of various substances into the brain. More specifically, permeability functions of BBB in transendothelial transport of various substances such as metabolic fuels, ethanol, amino acids, proteins, peptides, lipids, vitamins, neurotransmitters, monocarbxylic acids, gases, water, and minerals in the peripheral circulation and into the brain have been widely explained. In addition, roles of various receptors, ATP powered pumps, channels, and transporters in transport of vital molecules in maintenance of homeostasis and normal body functions have been described in detail. Major role of integral membrane proteins, carriers, or transporters in drug transport is highlighted. Both diffusion and carrier mediated transport mechanisms which facilitate molecular trafficking through transcellular route to maintain influx and outflux of important nutrients and metabolic substances are elucidated. Present review paper aims to emphasize role of important transport systems with their recent advancements in CNS protection mainly for providing a rapid clinical aid to patients. This review also suggests requirement of new well-designed therapeutic strategies mainly potential techniques, appropriate drug formulations, and new transport systems for quick, easy, and safe delivery of drugs across blood brain barrier to save the life of tumor and virus infected patients.

Lactate transport and receptor actions in cerebral malaria

Cerebral malaria (CM), caused by Plasmodium falciparum infection, is a prevalent neurological disorder in the tropics. Most of the patients are children, typically with intractable seizures and high mortality. Current treatment is unsatisfactory. Understanding the pathogenesis of CM is required in order to identify therapeutic targets. Here, we argue that cerebral energy metabolic defects are probable etiological factors in CM pathogenesis, because malaria parasites consume large amounts of glucose metabolized mostly to lactate. Monocarboxylate transporters (MCTs) mediate facilitated transfer, which serves to equalize lactate concentrations across cell membranes in the direction of the concentration gradient. The equalizing action of MCTs is the basis for lactate’s role as a volume transmitter of metabolic signals in the brain. Lactate binds to the lactate receptor GPR81, recently discovered on brain cells and cerebral blood vessels, causing inhibition of adenylyl cyclase. High levels of lactate delivered by the parasite at the vascular endothelium may damage the blood–brain barrier, disrupt lactate homeostasis in the brain, and imply MCTs and the lactate receptor as novel therapeutic targets in CM.

Regulation of monocarboxylic acid transporter-1 by cAMP dependent vesicular trafficking in brain microvascular endothelial cells

In this study, a detailed characterization of Monocarboxylic Acid Transporter-1 (Mct1) in cytoplasmic vesicles of cultured rat brain microvascular endothelial cells shows them to be a diverse population of endosomes intrinsic to the regulation of the transporter by a brief 25 to 30 minute exposure to the membrane permeant cAMP analog, 8Br-cAMP. The vesicles are heterogeneous in size, mobility, internal pH, and co-localize with discreet markers of particular types of endosomes including early endosomes, clathrin coated vesicles, caveolar vesicles, trans-golgi, and lysosomes. The vesicular localization of Mct1 was not dependent on its N or C termini, however, the size and pH of Mct1 vesicles was increased by deletion of either terminus demonstrating a role for the termini in vesicular trafficking of Mct1. Using a novel BCECF-AM based assay developed in this study, 8Br-cAMP was shown to decrease the pH of Mct1 vesicles after 25 minutes. This result and method were confirmed in experiments with a ratiometric pH-sensitive EGFP-mCherry dual tagged Mct1 construct. Overall, the results indicate that cAMP signaling reduces the functionality of Mct1 in cerebrovascular endothelial cells by facilitating its entry into a highly dynamic vesicular trafficking pathway that appears to lead to the transporter's trafficking to autophagosomes and lysosomes.

Glucose deprivation increases monocarboxylate transporter 1 (MCT1) expression and MCT1-dependent tumor cell migration

The glycolytic end-product lactate is a pleiotropic tumor growth-promoting factor. Its activities primarily depend on its uptake, a process facilitated by the lactate-proton symporter monocarboxylate transporter 1 (MCT1). Therefore, targeting the transporter or its chaperon protein CD147/basigin, itself involved in the aggressive malignant phenotype, is an attractive therapeutic option for cancer, but basic information is still lacking regarding the regulation of the expression, interaction and activities of both proteins. In this study, we found that glucose deprivation dose-dependently upregulates MCT1 and CD147 protein expression and their interaction in oxidative tumor cells. While this posttranslational induction could be recapitulated using glycolysis inhibition, hypoxia, oxidative phosphorylation (OXPHOS) inhibitor rotenone or hydrogen peroxide, it was blocked with alternative oxidative substrates and specific antioxidants, pointing out at a mitochondrial control. Indeed, we found that the stabilization of MCT1 and CD147 proteins upon glucose removal depends on mitochondrial impairment and the associated generation of reactive oxygen species. When glucose was a limited resource (a situation occurring naturally or during the treatment of many tumors), MCT1-CD147 heterocomplexes accumulated, including in cell protrusions of the plasma membrane. It endowed oxidative tumor cells with increased migratory capacities towards glucose. Migration increased in cells overexpressing MCT1 and CD147, but it was inhibited in glucose-starved cells provided with an alternative oxidative fuel, treated with an antioxidant, lacking MCT1 expression, or submitted to pharmacological MCT1 inhibition. While our study identifies the mitochondrion as a glucose sensor promoting tumor cell migration, MCT1 is also revealed as a transducer of this response, providing a new rationale for the use of MCT1 inhibitors in cancer.

Increased brain transport and metabolism of acetate in hypoglycemia unawareness

CONTEXT

Intensive insulin therapy reduces the risk for long-term complications in patients with type 1 diabetes mellitus (T1DM) but increases the risk for hypoglycemia-associated autonomic failure (HAAF), a syndrome that includes hypoglycemia unawareness and defective glucose counterregulation (reduced epinephrine and glucagon responses to hypoglycemia).

OBJECTIVE

The objective of the study was to address mechanisms underlying HAAF, we investigated whether nonglucose fuels such as acetate, a monocarboxylic acid (MCA), can support cerebral energetics during hypoglycemia in T1DM individuals with hypoglycemia unawareness.

DESIGN

Magnetic resonance spectroscopy was used to measure brain transport and metabolism of [2-(13)C]acetate under hypoglycemic conditions.

SETTING

The study was conducted at the Yale Center for Clinical Investigation Hospital Research Unit, Yale Magnetic Resonance Research Center.

PATIENTS AND OTHER PARTICIPANTS

T1DM participants with moderate to severe hypoglycemia unawareness (n = 7), T1DM controls without hypoglycemia unawareness (n = 5), and healthy nondiabetic controls (n = 10) participated in the study.

MAIN OUTCOME MEASURE(S)

Brain acetate concentrations, (13)C percent enrichment of glutamine and glutamate, and absolute rates of acetate metabolism were measured.

RESULTS

Absolute rates of acetate metabolism in the cerebral cortex were 1.5-fold higher among T1DM/unaware participants compared with both control groups during hypoglycemia (P = .001). Epinephrine levels of T1DM/unaware subjects were significantly lower than both control groups (P < .05). Epinephrine levels were inversely correlated with levels of cerebral acetate use across the entire study population (P < .01), suggesting a relationship between up-regulated brain MCA use and HAAF.

CONCLUSION

Increased MCA transport and metabolism among T1DM individuals with hypoglycemia unawareness may be a mechanism to supply the brain with nonglucose fuels during episodes of acute hypoglycemia and may contribute to the syndrome of hypoglycemia unawareness, independent of diabetes.

Lactate receptor sites link neurotransmission, neurovascular coupling, and brain energy metabolism

The G-protein-coupled lactate receptor, GPR81 (HCA1), is known to promote lipid storage in adipocytes by downregulating cAMP levels. Here, we show that GPR81 is also present in the mammalian brain, including regions of the cerebral neocortex and hippocampus, where it can be activated by physiological concentrations of lactate and by the specific GPR81 agonist 3,5-dihydroxybenzoate to reduce cAMP. Cerebral GPR81 is concentrated on the synaptic membranes of excitatory synapses, with a postsynaptic predominance. GPR81 is also enriched at the blood-brain-barrier: the GPR81 densities at endothelial cell membranes are about twice the GPR81 density at membranes of perivascular astrocytic processes, but about one-seventh of that on synaptic membranes. There is only a slight signal in perisynaptic processes of astrocytes. In synaptic spines, as well as in adipocytes, GPR81 immunoreactivity is located on subplasmalemmal vesicular organelles, suggesting trafficking of the protein to and from the plasma membrane. The results indicate roles of lactate in brain signaling, including a neuronal glucose and glycogen saving response to the supply of lactate. We propose that lactate, through activation of GPR81 receptors, can act as a volume transmitter that links neuronal activity, cerebral energy metabolism and energy substrate availability.