The multispecific thyroid hormone transporter OATP1C1 mediates cell-specific sulforhodamine 101-labeling of hippocampal astrocytes

Analysis of Functional NMDA Receptors in Astrocytes

Neuronal N-methyl-D-aspartate (NMDA) receptors are well known for their pivotal role in memory formation. Originally, they were thought to be exclusive to neurons. However, numerous studies revealed their functional expression also on various types of glial cells in the nervous system. Here, the methodology on how to study the physiology of NMDA receptors selectively on astrocytes will be described in detail. Astrocytes are the main class of neuroglia that control transmitter and ion homeostasis, which link cerebral blood flow and neuronal energy demands, but also affect synaptic transmission directly.

Dissecting the neurovascular unit in physiology and Alzheimer's disease: Functions, imaging tools and genetic mouse models

The neurovascular unit (NVU) plays an essential role in regulating neurovascular coupling, which refers to the communication between neurons, glia, and vascular cells to control the supply of oxygen and nutrients in response to neural activity. Cellular elements of the NVU coordinate to establish an anatomical barrier to separate the central nervous system from the milieu of the periphery system, restricting the free movement of substances from the blood to the brain parenchyma and maintaining central nervous system homeostasis. In Alzheimer's disease, amyloid-β deposition impairs the normal functions of NVU cellular elements, thus accelerating the disease progression. Here, we aim to describe the current knowledge of the NVU cellular elements, including endothelial cells, pericytes, astrocytes, and microglia, in regulating the blood-brain barrier integrity and functions in physiology as well as alterations encountered in Alzheimer's disease. Furthermore, the NVU functions as a whole, therefore specific labeling and targeting NVU components in vivo enable us to understand the mechanism mediating cellular communication. We review approaches including commonly used fluorescent dyes, genetic mouse models, and adeno-associated virus vectors for imaging and targeting NVU cellular elements in vivo.

Angiotensin II increases respiratory rhythmic activity in the preBötzinger complex without inducing astroglial calcium signaling

Angiotensin II (Ang II) is the primary modulator of the renin-angiotensin system and has been widely studied for its effect on the cardiovascular system. While a few studies have also indicated an involvement of Ang II in the regulation of breathing, very little is known in this regard and its effect on brainstem respiratory regions such as the preBötzinger complex (preBötC), the kernel for inspiratory rhythm generation, has not been investigated yet. This study reports that Ang II temporarily increases phrenic nerve activity in the working heart-brainstem preparation, indicating higher central respiratory drive. Previous studies have shown that the carotid body is involved in mediating this effect and we revealed that the preBötC also plays a part, using acute slices of the brainstem. It appears that Ang II is increasing the respiratory drive in an AT1R-dependent manner by optimizing the interaction of inhibitory and excitatory neurons of the preBötC. Thus, Ang II-mediated effects on the preBötC are potentially involved in dysregulating breathing in patients with acute lung injury.

Targeting gliovascular connexins prevents inflammatory blood-brain barrier leakage and astrogliosis

The blood-brain barrier is formed by capillary endothelial cells expressing Cx37, Cx40 and Cx43, and is joined by closely apposed astrocytes expressing Cx43 and Cx30. We investigated whether connexin-targeting peptides could limit barrier leakage triggered by LPS-induced systemic inflammation in mice. Intraperitoneal LPS increased endothelial and astrocytic Cx43 expression, elevated TNFα, IL1β, IFNγ and IL6 in plasma and IL6 in the brain, and induced barrier leakage recorded over 24h. Barrier leakage was largely prevented by global Cx43 knockdown and Cx43/Cx30 double-knockout in astrocytes, slightly diminished by endothelial Cx43 knockout and not protected by global Cx30 knockout. Intravenous administration of Gap27 or Tat-Gap19 just before LPS also prevented barrier leakage, and intravenous BAPTA-AM to chelate intracellular calcium was equally effective. Patch-clamp experiments demonstrated LPS-induced Cx43 hemichannel opening in endothelial cells, which was suppressed by Gap27, Gap19 and BAPTA. LPS additionally triggered astrogliosis that was prevented by intravenous Tat-Gap19 or BAPTA-AM. Cortically applied Tat-Gap19 or BAPTA-AM to primarily target astrocytes, also strongly diminished barrier leakage. In vivo dye uptake and in vitro patch-clamp showed Cx43 hemichannel opening in astrocytes that was induced by IL6 in a calcium-dependent manner. We conclude that targeting endothelial and astrocytic connexins is a powerful approach to limit barrier failure and astrogliosis.

Contributions of Astrocyte and Neuronal Volume to CA1 Neuron Excitability Changes in Elevated Extracellular Potassium

Rapid increases in cell volume reduce the size of the extracellular space (ECS) and are associated with elevated brain tissue excitability. We recently demonstrated that astrocytes, but not neurons, rapidly swell in elevated extracellular potassium (^∧[K⁺]_o) up to 26 mM. However, effects of acute astrocyte volume fluctuations on neuronal excitability in ^∧[K⁺]_o have been difficult to evaluate due to direct effects on neuronal membrane potential and generation of action potentials. Here we set out to isolate volume-specific effects occurring in ^∧[K⁺]_o on CA1 pyramidal neurons in acute hippocampal slices by manipulating cell volume while recording neuronal glutamate currents in 10.5 mM [K⁺]_o + tetrodotoxin (TTX) to prevent neuronal firing. Elevating [K⁺]_o to 10.5 mM induced astrocyte swelling and produced significant increases in neuronal excitability in the form of mixed α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-D-aspartate (NMDA) receptor mEPSCs and NMDA receptor-dependent slow inward currents (SICs). Application of hyperosmolar artificial cerebrospinal fluid (ACSF) by addition of mannitol in the continued presence of 10.5 mM K⁺ forced shrinking of astrocytes and to a lesser extent neurons, which resisted swelling in ^∧[K⁺]_o. Cell shrinking and dilation of the ECS significantly dampened neuronal excitability in 10.5 mM K⁺. Subsequent removal of mannitol amplified effects on neuronal excitability and nearly doubled the volume increase in astrocytes, presumably due to continued glial uptake of K⁺ while mannitol was present. Slower, larger amplitude events mainly driven by NMDA receptors were abolished by mannitol-induced expansion of the ECS. Collectively, our findings suggest that cell volume regulation of the ECS in elevated [K⁺]_o is driven predominantly by astrocytes, and that cell volume effects on neuronal excitability can be effectively isolated in elevated [K⁺]_o conditions.

Circadian Volume Changes in Hippocampal Glia Studied by Label-Free Interferometric Imaging

Complex brain functions, including learning and memory, arise in part from the modulatory role of astrocytes on neuronal circuits. Functionally, the dentate gyrus (DG) exhibits differences in the acquisition of long-term potentiation (LTP) between day and night. We hypothesize that the dynamic nature of astrocyte morphology plays an important role in the functional circuitry of hippocampal learning and memory, specifically in the DG. Standard microscopy techniques, such as differential interference contrast (DIC), present insufficient contrast for detecting changes in astrocyte structure and function and are unable to inform on the intrinsic structure of the sample in a quantitative manner. Recently, gradient light interference microscopy (GLIM) has been developed to upgrade a DIC microscope with quantitative capabilities such as single-cell dry mass and volume characterization. Here, we present a methodology for combining GLIM and electrophysiology to quantify the astrocyte morphological behavior over the day-night cycle. Colocalized measurements of GLIM and fluorescence allowed us to quantify the dry masses and volumes of hundreds of astrocytes. Our results indicate that, on average, there is a 25% cell volume reduction during the nocturnal cycle. Remarkably, this cell volume change takes place at constant dry mass, which suggests that the volume regulation occurs primarily through aqueous medium exchange with the environment.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

Astrocyte reactivity in a mouse model of SCN8A epileptic encephalopathy

OBJECTIVE

SCN8A epileptic encephalopathy is caused predominantly by de novo gain-of-function mutations in the voltage-gated sodium channel Nav 1.6. The disorder is characterized by early onset of seizures and developmental delay. Most patients with SCN8A epileptic encephalopathy are refractory to current anti-seizure medications. Previous studies determining the mechanisms of this disease have focused on neuronal dysfunction as Nav 1.6 is expressed by neurons and plays a critical role in controlling neuronal excitability. However, glial dysfunction has been implicated in epilepsy and alterations in glial physiology could contribute to the pathology of SCN8A encephalopathy. In the current study, we examined alterations in astrocyte and microglia physiology in the development of seizures in a mouse model of SCN8A epileptic encephalopathy.

METHODS

Using immunohistochemistry, we assessed microglia and astrocyte reactivity before and after the onset of spontaneous seizures. Expression of glutamine synthetase and Nav 1.6, and Kir 4.1 channel currents were assessed in astrocytes in wild-type (WT) mice and mice carrying the N1768D SCN8A mutation (D/+).

RESULTS

Astrocytes in spontaneously seizing D/+ mice become reactive and increase expression of glial fibrillary acidic protein (GFAP), a marker of astrocyte reactivity. These same astrocytes exhibited reduced barium-sensitive Kir 4.1 currents compared to age-matched WT mice and decreased expression of glutamine synthetase. These alterations were only observed in spontaneously seizing mice and not before the onset of seizures. In contrast, microglial morphology remained unchanged before and after the onset of seizures.

SIGNIFICANCE

Astrocytes, but not microglia, become reactive only after the onset of spontaneous seizures in a mouse model of SCN8A encephalopathy. Reactive astrocytes have reduced Kir 4.1-mediated currents, which would impair their ability to buffer potassium. Reduced expression of glutamine synthetase would modulate the availability of neurotransmitters to excitatory and inhibitory neurons. These deficits in potassium and glutamate handling by astrocytes could exacerbate seizures in SCN8A epileptic encephalopathy. Targeting astrocytes may provide a new therapeutic approach to seizure suppression.

Calcium imaging and BAPTA loading of amygdala astrocytes in mouse brain slices

Astrocytes are glial cells that exhibit calcium signaling-mediated activity. Here, we present a protocol to monitor and manipulate astrocyte calcium activity from mouse amygdala slices. In the first part of this protocol, we describe the procedure of astrocyte calcium imaging. In the second part, we detail how to disrupt astrocyte calcium activity by patch-clamp-mediated loading of BAPTA. These two approaches are presented separately but they can also be used simultaneously to monitor the effects of disruption on an astrocyte network. For complete details on the use and execution of this protocol, please refer to Wahis et al. (2021).

Synergistic Control of Transmitter Turnover at Glycinergic Synapses by GlyT1, GlyT2, and ASC-1

In addition to being involved in protein biosynthesis and metabolism, the amino acid glycine is the most important inhibitory neurotransmitter in caudal regions of the brain. These functions require a tight regulation of glycine concentration not only in the synaptic cleft, but also in various intracellular and extracellular compartments. This is achieved not only by confining the synthesis and degradation of glycine predominantly to the mitochondria, but also by the action of high-affinity large-capacity glycine transporters that mediate the transport of glycine across the membranes of presynaptic terminals or glial cells surrounding the synapses. Although most cells at glycine-dependent synapses express more than one transporter with high affinity for glycine, their synergistic functional interaction is only poorly understood. In this review, we summarize our current knowledge of the two high-affinity transporters for glycine, the sodium-dependent glycine transporters 1 (GlyT1; SLC6A9) and 2 (GlyT2; SLC6A5) and the alanine–serine–cysteine-1 transporter (Asc-1; SLC7A10).

Amyloid-β plaques affect astrocyte Kir4.1 protein expression but not function in the dentate gyrus of APP/PS1 mice

Alzheimer pathology is accompanied by astrogliosis. Reactive astrocytes surrounding amyloid plaques may directly affect neuronal communication, and one of the mechanisms by which astrocytes impact neuronal function is by affecting K⁺ homeostasis. Here we studied, using hippocampal slices from 9‐month‐old Alzheimer mice (APP/PS1) and wild‐type littermates, whether astrocyte function is changed by analyzing Kir4.1 expression and function and astrocyte coupling in astrocytes surrounding amyloid‐β plaques. Immunohistochemical analysis of Kir4.1 protein in the dentate gyrus revealed localized increases in astrocytes surrounding amyloid‐β plaque deposits. We subsequently focused on changes in astrocyte function by using patch‐clamp slice electrophysiology on both plaque‐ and non‐plaque associated astrocytes to characterize general membrane properties. We found that Ba²⁺‐sensitive Kir4.1 conductance in astrocytes surrounding plaques was not affected by changes in Kir4.1 protein expression. Additional analysis of astrocyte gap junction coupling efficiency in the dentate gyrus revealed no apparent changes. Quantification of basic features of glutamatergic transmission to granule cells did not indicate disturbed neuronal communication in the dentate gyrus of APP/PS1 mice. Together, these results suggest that astrocytes in the dentate gyrus of APP/PS1 mice maintain their ability to buffer extracellular K⁺ and attempt to rectify imbalances in K⁺ concentration to maintain normal neuronal and synaptic function, possibly by localized increases in Kir4.1 protein expression. Our earlier transcriptomic data indicated that chronically activated astrocytes lose their neuronal support function. Here we show that, despite localized increased Kir4.1 protein expression, astrocyte Kir4.1 channel dysfunction is likely not involved in the pathogenesis of Alzheimer's disease.

The Role of Thyroid Hormone in the Regulation of Cerebellar Development

The proper organized expression of specific genes in time and space is responsible for the organogenesis of the central nervous system including the cerebellum. The epigenetic regulation of gene expression is tightly regulated by an intrinsic intracellular genetic program, local stimuli such as synaptic inputs and trophic factors, and peripheral stimuli from outside of the brain including hormones. Some hormone receptors are expressed in the cerebellum. Thyroid hormones (THs), among numerous circulating hormones, are well-known major regulators of cerebellar development. In both rodents and human, hypothyroidism during the postnatal developmental period results in abnormal morphogenesis or altered function. THs bind to the thyroid hormone receptors (TRs) in the nuclei and with the help of transcriptional cofactors regulate the transcription of target genes. Gene regulation by TR induces cell proliferation, migration, and differentiation, which are necessary for brain development and plasticity. Thus, the lack of TH action mediators may directly cause aberrant cerebellar development. Various kinds of animal models have been established in a bid to study the mechanism of TH action in the cerebellum. Interestingly, the phenotypes differ greatly depending on the models. Herein we summarize the actions of TH and TR particularly in the developing cerebellum.

Using Genetically Encoded Calcium Indicators to Study Astrocyte Physiology: A Field Guide

Ca²⁺ imaging is the most frequently used technique to study glial cell physiology. While chemical Ca²⁺ indicators served to visualize and measure changes in glial cell cytosolic Ca²⁺ concentration for several decades, genetically encoded Ca²⁺ indicators (GECIs) have become state of the art in recent years. Great improvements have been made since the development of the first GECI and a large number of GECIs with different physical properties exist, rendering it difficult to select the optimal Ca²⁺ indicator. This review discusses some of the most frequently used GECIs and their suitability for glial cell research.

Expression and Function of Organic Anion Transporting Polypeptides in the Human Brain: Physiological and Pharmacological Implications

The central nervous system (CNS) is an important pharmacological target, but it is very effectively protected by the blood–brain barrier (BBB), thereby impairing the efficacy of many potential active compounds as they are unable to cross this barrier. Among others, membranous efflux transporters like P-Glycoprotein are involved in the integrity of this barrier. In addition to these, however, uptake transporters have also been found to selectively uptake certain compounds into the CNS. These transporters are localized in the BBB as well as in neurons or in the choroid plexus. Among them, from a pharmacological point of view, representatives of the organic anion transporting polypeptides (OATPs) are of particular interest, as they mediate the cellular entry of a variety of different pharmaceutical compounds. Thus, OATPs in the BBB potentially offer the possibility of CNS targeting approaches. For these purposes, a profound understanding of the expression and localization of these transporters is crucial. This review therefore summarizes the current state of knowledge of the expression and localization of OATPs in the CNS, gives an overview of their possible physiological role, and outlines their possible pharmacological relevance using selected examples.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part II: how can key events of relevant adverse outcome pathways be addressed in toxicological assessments?

The current understanding of thyroid-related adverse outcome pathways (AOPs) with adverse neurodevelopmental outcomes in mammals has been reviewed. This served to establish if standard rodent toxicity test methods and in vitro assays allow identifying thyroid-related modes-of-action potentially leading to adverse neurodevelopmental outcomes, and the human relevance of effects - in line with the European Commission's Endocrine Disruptor Criteria. The underlying hypothesis is that an understanding of the key events of relevant AOPs provides insight into differences in incidence, magnitude, or species sensitivity of adverse outcomes. The rodent studies include measurements of serum thyroid hormones, thyroid gland pathology and neurodevelopmental assessments, but do not directly inform on specific modes-of-action. Opportunities to address additional non-routine parameters reflecting critical events of AOPs in toxicological assessments are presented. These parameters appear relevant to support the identification of specific thyroid-related modes-of-action, provided that prevailing technical limitations are overcome. Current understanding of quantitative key event relationships is often weak, but would be needed to determine if the triggering of a molecular initiating event will ultimately result in an adverse outcome. Also, significant species differences in all processes related to thyroid hormone signalling are evident, but the biological implications thereof (including human relevance) are often unknown. In conclusion, careful consideration of the measurement (e.g. timing, method) and interpretation of additional non-routine parameters is warranted. These findings will be used in a subsequent paper to propose a testing strategy to identify if a substance may elicit maternal thyroid hormone imbalance and potentially also neurodevelopmental effects in the progeny.

Brain Gene Expression in Systemic Hypothyroidism and Mouse Models of MCT8 Deficiency: The Mct8-Oatp1c1-Dio2 Triad

Background: The monocarboxylate transporter 8 (Mct8) protein is a primary thyroxine (T4) and triiodothyronine (T3) (thyroid hormone [TH]) transporter. Mutations of the MCT8-encoding, SLC16A2 gene alter thyroid function and TH metabolism and severely impair neurodevelopment (Allan-Herndon-Dudley syndrome [AHDS]). Mct8-deficient mice manifest thyroid alterations but lack neurological signs. It is believed that Mct8 deficiency in mice is compensated by T4 transport through the Slco1c1-encoded organic anion transporter polypeptide 1c1 (Oatp1c1). This allows local brain generation of sufficient T3 by the Dio2-encoded type 2 deiodinase, thus preventing brain hypothyroidism. The Slc16a2/Slco1c1 (MO) and Slc16a2/Dio2 (MD) double knockout (KO) mice lacking T4 and T3 transport, or T3 transport and T4 deiodination, respectively, should be appropriate models of AHDS. Our goal was to compare the cerebral hypothyroidism of systemic hypothyroidism (SH) caused by thyroid gland blockade with that present in the double KO mice. Methods: We performed RNA sequencing by using RNA from the cerebral cortex and striatum of SH mice and the double KO mice on postnatal days 21-23. Real-time polymerase chain reaction was used to confirm RNA-Seq results in replicate biological samples. Cell type involvement was assessed from cell type-enriched genes. Functional genomic differences were analyzed by functional node activity based on a probabilistic graphical model. Results: Each of the three conditions gave a different pattern of gene expression, with partial overlaps. SH gave a wider and highest variation of gene expression than MD or MO. This was partially due to secondary gene responses to hypothyroidism. The set of primary transcriptional T3 targets showed a tighter overlap, but quantitative gene responses indicated that the gene responses in SH were more severe than in MD or MO. Examination of cell type-enriched genes indicated cellular differences between the three conditions. Conclusions: The results indicate that the neurological impairment of AHDS is too severe to be fully explained by TH deprivation only.

O2-Dependent Protein Internalization Underlies Astrocytic Sensing of Acute Hypoxia by Restricting Multimodal TRPA1 Channel Responses

Hypoxia sensors are essential for regulating local oxygen (O₂) homeostasis within the body. This is especially pertinent within the CNS, which is particularly vulnerable to O₂ deprivation due to high energetic demand. Here, we reveal hypoxia-monitoring function exerted by astrocytes through an O₂-regulated protein trafficking mechanism within the CNS. Strikingly, cultured mouse astrocytes isolated from the parafacial respiratory group (pFRG) and retrotrapezoid nucleus (RTN) region are capable of rapidly responding to moderate hypoxia via the sensor cation channel transient receptor potential (TRP) A1 but, unlike multimodal sensory neurons, are inert to hyperoxia and other TRPA1 activators (carbon dioxide, electrophiles, and oxidants) in normoxia. Mechanistically, O₂ suppresses TRPA1 channel activity by protein internalization via O₂-dependent proline hydroxylation and subsequent ubiquitination by an E3 ubiquitin ligase, NEDD4-1 (neural precursor cell-expressed developmentally down-regulated protein 4). Hypoxia inhibits this process and instantly accumulates TRPA1 proteins at the plasma membrane, inducing TRPA1-mediated Ca²⁺ influx that triggers ATP release from pFRG/RTN astrocytes, potentiating respiratory center activity. Furthermore, astrocyte-specific Trpa1 disruption in a mouse brainstem-spinal cord preparation impedes the amplitude augmentation of the central autonomic respiratory output during hypoxia. Thus, reversible coupling of the TRPA1 channels with O₂-dependent protein translocation allows astrocytes to act as acute hypoxia sensors in the medullary respiratory center.

Astrocytes of the early postnatal brain

In the rodent forebrain, the majority of astrocytes are generated during the early postnatal phase. Following differentiation, astrocytes undergo maturation which accompanies the development of the neuronal network. Neonate astrocytes exhibit a distinct morphology and domain size which differs to their mature counterparts. Moreover, many of the plasma membrane proteins prototypical for fully developed astrocytes are only expressed at low levels at neonatal stages. These include connexins and Kir4.1, which define the low membrane resistance and highly negative membrane potential of mature astrocytes. Newborn astrocytes moreover express only low amounts of GLT-1, a glutamate transporter critical later in development. Furthermore, they show specific differences in the properties and spatio-temporal pattern of intracellular calcium signals, resulting from differences in their repertoire of receptors and signalling pathways. Therefore, roles fulfilled by mature astrocytes, including ion and transmitter homeostasis, are underdeveloped in the young brain. Similarly, astrocytic ion signalling in response to neuronal activity, a process central to neuron-glia interaction, differs between the neonate and mature brain. This review describes the unique functional properties of astrocytes in the first weeks after birth and compares them to later stages of development. We conclude that with an immature neuronal network and wider extracellular space, astrocytic support might not be as demanding and critical compared to the mature brain. The delayed differentiation and maturation of astrocytes in the first postnatal weeks might thus reflect a reduced need for active, energy-consuming regulation of the extracellular space and a less tight control of glial feedback onto synaptic transmission.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Dopamine-induced calcium signaling in olfactory bulb astrocytes

It is well established that astrocytes respond to the major neurotransmitters glutamate and GABA with cytosolic calcium rises, whereas less is known about the effect of dopamine on astroglial cells. In the present study, we used confocal calcium imaging in mouse brain slices of the olfactory bulb, a brain region with a large population of dopaminergic neurons, to investigate calcium signaling evoked by dopamine in astrocytes. Our results show that application of dopamine leads to a dose-dependent cytosolic calcium rise in astrocytes (EC₅₀ = 76 µM) which is independent of neuronal activity and mainly mediated by PLC/IP₃-dependent internal calcium release. Antagonists of both D₁- and D₂-class dopamine receptors partly reduce the dopaminergic calcium response, indicating that both receptor classes contribute to dopamine-induced calcium transients in olfactory bulb astrocytes.

Astrocyte-Selective Volume Increase in Elevated Extracellular Potassium Conditions Is Mediated by the Na+/K+ ATPase and Occurs Independently of Aquaporin 4

Astrocytes and neurons have been shown to swell across a variety of different conditions, including increases in extracellular potassium concentration (^[K+]o). The mechanisms involved in the coupling of K+ influx to water movement into cells leading to cell swelling are not well understood and remain controversial. Here, we set out to determine the effects of ^[K+]o on rapid volume responses of hippocampal CA1 pyramidal neurons and stratum radiatum astrocytes using real-time confocal volume imaging. First, we found that elevating [K+]o within a physiological range (to 6.5 mM and 10.5 mM from a baseline of 2.5 mM), and even up to pathological levels (26 mM), produced dose-dependent increases in astrocyte volume, with absolutely no effect on neuronal volume. In the absence of compensating for addition of KCl by removal of an equal amount of NaCl, neurons actually shrank in ^[K+]o, while astrocytes continued to exhibit rapid volume increases. Astrocyte swelling in ^[K+]o was not dependent on neuronal firing, aquaporin 4, the inwardly rectifying potassium channel Kir 4.1, the sodium bicarbonate cotransporter NBCe1, , or the electroneutral cotransporter, sodium-potassium-chloride cotransporter type 1 (NKCC1), but was significantly attenuated in 1 mM barium chloride (BaCl2) and by the Na+/K+ ATPase inhibitor ouabain. Effects of 1 mM BaCl2 and ouabain applied together were not additive and, together with reports that BaCl2 can inhibit the NKA at high concentrations, suggests a prominent role for the astrocyte NKA in rapid astrocyte volume increases occurring in ^[K+]o. These findings carry important implications for understanding mechanisms of cellular edema, regulation of the brain extracellular space, and brain tissue excitability.

Tissue-Specific Function of Thyroid Hormone Transporters: New Insights from Mouse Models

Thyroid hormone (TH) transporters are required for cellular transmembrane passage of TH and are thus mandatory for proper TH metabolism and action. Consequently, inactivating mutations in TH transporters such as MCT8 or OATP1C1 can cause tissue- specific changes in TH homeostasis. As the most prominent example, patients with MCT8 mutations exhibit elevated serum T3 levels, whereas their CNS appear to be in a TH deficient state. Here, we will briefly summarize recent studies of mice lacking Mct8 alone or in combination with the TH transporters Mct10 or Oatp1c1 that shed light on many aspects and pathogenic events underlying global MCT8 deficiency and also underscore the contribution of Mct10 and Oatp1c1 in tissue-specific TH transport processes. Moreover, development of conditional knock-out mice that allow a cell-specific inactivation of TH transporters in distinct tissues, disclosed cell-specific changes in TH signaling, thereby highlighting the pathophysiological significance of local control of TH action.

Biological Assessment of a 18F-Labeled Sulforhodamine 101 in a Mouse Model of Alzheimer's Disease as a Potential Astrocytosis Marker

Neurodegenerative diseases have mainly been associated with neuronal death. Recent investigations have shown that astroglia may modulate neuroinflammation in the early and late stages of the disease. [¹¹C]Deuterodeprenyl ([¹¹C]DED) is a tracer that has been used for reactive astrocyte detection in Alzheimer’s disease, Creutzfeldt–Jakob disease and amyotrophic lateral sclerosis, among others, with some limitations. To develop a new radiotracer for detecting astrocytosis and overcoming associated difficulties, we recently reported the synthesis of a sulfonamide derivative of Sulforhodamine 101 (SR101), labeled with ¹⁸F, namely SR101 N-(3-[¹⁸F]Fluoropropyl) sulfonamide ([¹⁸F]2B-SRF101). The red fluorescent dye SR101 has been used as a specific marker of astroglia in the neocortex of rodents using in vivo models. In the present work we performed a biological characterisation of the new tracer including biodistribution and micro-PET/computed tomography (CT) images. PET/CT studies with [¹¹C]DED were also done to compare with [¹⁸F]2B-SRF101 in order to assess its potential as an astrocyte marker. Biodistribution studies with [¹⁸F]2B-SRF101 were carried out in C57BL6J black and transgenic (3xTg) mice. A hepatointestinal metabolization as well as the pharmacokinetic profile were determined, showing appropriate characteristics to become a PET diagnostic agent. Dynamic PET/CT studies were carried out with [¹⁸F]2B-SRF101 and [¹¹C]DED to evaluate the distribution of both tracers in the brain. A significant difference in [¹⁸F]2B-SRF101 uptake was especially observed in the cortex and hippocampus, and it was higher in 3xTg mice than it was in the control group. These results suggested that [¹⁸F]2B-SRF101 is a promising candidate for more extensive evaluation as an astrocyte tracer. The difference observed for [¹⁸F]2B-SRF101 was not found in the case of [¹¹C]DED. The comparative studies between [¹⁸F]2B-SRF101 and [¹¹C]DED suggest that both tracers have different roles as astrocytosis markers in this animal model, and could provide different and complementary information at the same time. In this way, by means of a multitracer approach, useful information could be obtained for the staging of the disease.

Organic Anion-Transporting Polypeptide 1a4 (Oatp1a4/Slco1a4) at the Blood-Arachnoid Barrier is the Major Pathway of Sulforhodamine-101 Clearance from Cerebrospinal Fluid of Rats

The blood-arachnoid barrier (BAB), which is formed by arachnoid epithelial cells linked by tight junctions, has generally been considered impermeable to water-soluble substances. However, we recently demonstrated that organic anion transporters 1 and 3 (Oat1 and Oat3) play roles in drug clearance at the BAB. Here, we examined whether an organic anion-transporting polypeptide (Oatp) also plays a role, using the fluorescent organic anion sulforhodamine-101 (SR-101) as a model substrate. SR-101 was injected into the cisterna magna of rats in order to minimize the contribution of choroid plexus transport. The in vivo cerebrospinal fluid (CSF) elimination clearance of SR-101 after intracisternal administration was ninefold greater than that of fluorescein-labeled inulin, a bulk flow marker. In the case of pre-administration of taurocholate, a broad-spectrum inhibitor of Oatps, or digoxin, a strong substrate/inhibitor for Oatp1a4 but not for Oatp1a1, Oat1, and Oat3, the CSF elimination of SR-101 was significantly reduced, becoming similar to that of inulin, and thus indicating complete inhibition of SR-101 clearance from the CSF. The distribution of SR-101 fluorescence was restricted to the arachnoid mater in the absence of inhibitor, whereas the fluorescence was increased in the parenchyma of the spinal cord after co-injection of taurocholate or digoxin. Immunostaining confirmed the localization of Oatp1a4 in the arachnoid mater. These results indicate that Oatp1a4 at the BAB acts as an avid clearance pathway of SR-101 in the CSF to the blood. Thus, Oatp1a4 appears to play a major role in CSF detoxification by limiting the distribution of organic anions to the brain and spinal cord.

Visualizing the brain's astrocytes

Astrocytes are the most abundant cell type in the brain and are a crucial part of solving its mysteries. Originally assumed to be passive supporting cells, astrocyte's functions are now recognized to include active modulation and information processing at the neural synapse. The full extent of the astrocyte contribution to neural processing remains unknown. This is, in part, due to the lack of methods available for astrocyte identification and analysis. Existing strategies employ genetic tools like the astrocyte specific promoters glial fibrillary acidic protein (GFAP) or Aldh1L1 to create transgenic animals with fluorescently labeled astrocytes. Recently, small molecule targeting moieties have enabled the delivery of bright fluorescent dyes to astrocytes. Here, we review methods for targeting astrocytes, with a focus on a recently developed methylpyridinium targeting moiety's development, chemical synthesis, and elaboration to provide new features like light-based spatiotemporal control of cell labeling.

Gliotransmission: Beyond Black-and-White

Astrocytes are highly complex cells with many emerging putative roles in brain function. Of these, gliotransmission (active information transfer from glia to neurons) has probably the widest implications on our understanding of how the brain works: do astrocytes really contribute to information processing within the neural circuitry? "Positive evidence" for this stems from work of multiple laboratories reporting many examples of modulatory chemical signaling from astrocytes to neurons in the timeframe of hundreds of milliseconds to several minutes. This signaling involves, but is not limited to, Ca²⁺-dependent vesicular transmitter release, and results in a variety of regulatory effects at synapses in many circuits that are abolished by preventing Ca²⁺ elevations or blocking exocytosis selectively in astrocytes. In striking contradiction, methodologically advanced studies by a few laboratories produced "negative evidence," triggering a heated debate on the actual existence and properties of gliotransmission. In this context, a skeptics' camp arose, eager to dismiss the whole positive evidence based on a number of assumptions behind the negative data, such as the following: (1) deleting a single Ca²⁺ release pathway (IP3R2) removes all the sources for Ca²⁺-dependent gliotransmission; (2) stimulating a transgenically expressed Gq-GPCR (MrgA1) mimics the physiological Ca²⁺ signaling underlying gliotransmitter release; (3) age-dependent downregulation of an endogenous GPCR (mGluR5) questions gliotransmitter release in adulthood; and (4) failure by transcriptome analysis to detect vGluts or canonical synaptic SNAREs in astrocytes proves inexistence/functional irrelevance of vesicular gliotransmitter release. We here discuss how the above assumptions are likely wrong and oversimplistic. In light of the most recent literature, we argue that gliotransmission is a more complex phenomenon than originally thought, possibly consisting of multiple forms and signaling processes, whose correct study and understanding require more sophisticated tools and finer scientific experiments than done until today. Under this perspective, the opposing camps can be reconciled and the field moved forward. Along the path, a more cautious mindset and an attitude to open discussion and mutual respect between opponent laboratories will be good companions.Dual Perspectives Companion Paper: Multiple Lines of Evidence Indicate That Gliotransmission Does Not Occur under Physiological Conditions, by Todd A. Fiacco and Ken D. McCarthy.

Anisotropic Panglial Coupling Reflects Tonotopic Organization in the Inferior Colliculus

Astrocytes and oligodendrocytes in different brain regions form panglial networks and the topography of such networks can correlate with neuronal topography and function. Astrocyte-oligodendrocyte networks in the lateral superior olive (LSO)-an auditory brainstem nucleus-were found to be anisotropic with a preferred orientation orthogonally to the tonotopic axis. We hypothesized that such a specialization might be present in other tonotopically organized brainstem nuclei, too. Thus, we analyzed gap junctional coupling in the center of the inferior colliculus (IC)-another nucleus of the auditory brainstem that exhibits tonotopic organization. In acute brainstem slices obtained from mice, IC networks were traced employing whole-cell patch-clamp recordings of single sulforhodamine (SR) 101-identified astrocytes and concomitant intracellular loading of the gap junction-permeable tracer neurobiotin. The majority of dye-coupled networks exhibited an oval topography, which was preferentially oriented orthogonal to the tonotopic axis. Astrocyte processes showed preferentially the same orientation indicating a correlation between astrocyte and network topography. In addition to SR101-positive astrocytes, IC networks contained oligodendrocytes. Using Na⁺ imaging, we analyzed the capability of IC networks to redistribute small ions. Na⁺ bi-directionally diffused between SR101-positive astrocytes and SR101-negative cells-presumably oligodendrocytes-showing the functionality of IC networks. Taken together, our results demonstrate that IC astrocytes and IC oligodendrocytes form functional anisotropic panglial networks that are preferentially oriented orthogonal to the tonotopic axis. Thus, our data indicate that the topographic specialization of glial networks seen in IC and LSO might be a general feature of tonotopically organized auditory brainstem nuclei.

Morphometric analysis of astrocytes in brainstem respiratory regions

Astrocytes, the most abundant and structurally complex glial cells of the central nervous system, are proposed to play an important role in modulating the activities of neuronal networks, including respiratory rhythm‐generating circuits of the preBötzinger complex (preBötC) located in the ventrolateral medulla of the brainstem. However, structural properties of astrocytes residing within different brainstem regions are unknown. In this study astrocytes in the preBötC, an intermediate reticular formation (IRF) region with respiratory‐related function, and a region of the nucleus tractus solitarius (NTS) in adult rats were reconstructed and their morphological features were compared. Detailed morphological analysis revealed that preBötC astrocytes are structurally more complex than those residing within the functionally distinct neighboring IRF region, or the NTS, located at the dorsal aspect of the medulla oblongata. Structural analyses of the brainstem microvasculature indicated no significant regional differences in vascular properties. We hypothesize that high morphological complexity of preBötC astrocytes reflects their functional role in providing structural/metabolic support and modulation of the key neuronal circuits essential for breathing, as well as constraints imposed by arrangements of associated neurons and/or other local structural features of the brainstem parenchyma.

The First Postlesion Minutes: An In Vivo Study of Extravasation and Perivascular Astrocytes Following Cerebral Lesions in Various Experimental Mouse Models

The immediate alterations following lesions cannot be investigated by using fixed tissues. Here, we employed two-photon microscopy to study the alterations to the permeability of blood-brain barrier and to glio-vascular connections in vivo during the first minutes following cortical lesions in mice. Four models were used: (1) cryogenic lesion, (2) photodisruption using laser pulses, (3) photothrombosis, and (4) bilateral carotid ligation. Sulforhodamine101 was used for supravital labeling of astrocytes and dextran-bound fluorescein isothiocyanate for the assessment of extravasation. Transgenic mice, in which the endothelium and astrocytes expressed a yellow fluorescent protein, were also used. Astrocytic labeling in vivo was verified with postmortem immunostaining against glial fibrillary acidic protein (GFAP). Summary of results: (1) the glio-vascular connections were stable in the intact brain with no sign of spontaneous dynamic attachment/detachment of glial end-feet; (2) only direct vascular damage (photodisruption or cryogenic) resulted in prompt extravasation; (3) even direct damage failed to provoke a prompt astroglial response. In conclusion, the results indicate that a detachment of the astrocytic end-feet does not precede the breakdown of blood-brain barrier following lesions. Whereas vasogenic edema develops immediately after the lesions, this is not the case with cytotoxic edemas. Time-lapse recordings and three-dimensional reconstructions are presented as supplemental materials.

Visualizing the Brain's Astrocytes with Diverse Chemical Scaffolds

Astrocytes are the most abundant cells in the brain. They support neurons, adjust synaptic strength, and modulate neuronal signaling, yet the full extent of their functions is obscured by the dearth of methods for their visualization and analysis. Here, we report a chemical reporter that targets small molecules specifically to astrocytes both in vitro and in vivo. Fluorescent versions of this tag are imported through an organic cation transporter to label glia across species. The structural modularity of this approach will enable wide-ranging applications for understanding astrocyte biology.

A neuronal role of the Alanine-Serine-Cysteine-1 transporter (SLC7A10, Asc-1) for glycine inhibitory transmission and respiratory pattern

The Alanine-Serine-Cysteine-1 transporter (SLC7A10, Asc-1) has been shown to play a role in synaptic availability of glycine although the exact mechanism remains unclear. We used electrophysiological recordings and biochemical experiments to investigate the role of Asc-1 transporter in glycinergic transmission in the brainstem respiratory network. Using both the Asc-1 substrate and transportable inhibitor D-isoleucine (D-Ile), and the non-transportable Asc-1 blocker Lu AE00527 (Lu), we found that D-Ile reduces glycinergic transmission and increases glycine release via hetero-exchange, whereas Lu has no acute effect on glycinergic synaptic transmission. Furthermore, D-Ile increases the frequency and reduces amplitude of the phrenic nerve activity in the arterially-perfused working heart brainstem preparation. These results suggest a role of Asc-1 in modulating presynaptic glycine levels that can impact on the respiratory network.

Monitoring gap junctional communication in astrocytes from acute adult mouse brain slices using the gap-FRAP technique

Intercellular communication through gap junction channels plays a key role in cellular homeostasis and in synchronizing physiological functions, a feature that is modified in number of pathological situations. In the brain, astrocytes are the cell population that expresses the highest amount of gap junction proteins, named connexins. Several techniques have been used to assess the level of gap junctional communication in astrocytes, but so far they remain very difficult to apply in adult brain tissue. Here, using specific loading of astrocytes with sulforhodamine 101, we adapted the gap-FRAP (Fluorescence Recovery After Photobleaching) to acute hippocampal slices from 9 month-old adult mice. We show that gap junctional communication monitored in astrocytes with this technique was inhibited either by pharmacological treatment with a gap junctional blocker or in mice lacking the two main astroglial connexins, while a partial inhibition was measured when only one connexin was knocked-out. We validate this approach using a mathematical model of sulforhodamine 101 diffusion in an elementary astroglial network and a quantitative analysis of the exponential fits to the fluorescence recovery curves. Consequently, we consider that the adaptation of the gap-FRAP technique to acute brain slices from adult mice provides an easy going and valuable approach that allows overpassing this age-dependent obstacle and will facilitate the investigation of gap junctional communication in adult healthy or pathological brain.

Liver Zonation Index of Drug Transporter and Metabolizing Enzyme Protein Expressions in Mouse Liver Acinus

The purpose of the present study was to clarify the molecular basis of zonated drug distributions in mouse liver based on the protein expression levels of transporters and metabolizing enzymes in periportal (PP) and pericentral (PC) vein regions of mouse hepatic lobules. The distributions of sulforhodamine 101 (SR-101), a substrate of organic anion transporting polypeptides (Oatps), and ribavirin, a substrate of equilibrative nucleoside transporter 1 (Ent1), were elucidated in frozen liver sections of mice, to which each compound had been intravenously administered. Regions strongly positive for SR-101 (SR-101⁺) and regions weakly positive or negative for SR-101 (SR-101^-) were separated by laser microdissection. The zonated distribution of protein expression was quantified in terms of the liver zonation index. Quantitative targeted absolute proteomics revealed the selective expression of glutamine synthetase in the SR-101⁺ region, indicating predominant distribution of SR-101 in hepatocytes of the PC vein region. The protein levels of Oatp1a1, Oatp1b2, organic cation transporter 1 (Oct1), and cytochrome P450 (P450) 2e1 were greater in the PC vein regions, whereas the level of organic anion transporter 2 (Oat2) was greater in the PP vein regions. Mouse Oatp1a1 mediated SR-101 transport. On the other hand, there were no statistically significant differences in expression of Ent1, Na⁺-taurocholate cotransporting polypeptide, several canalicular transporters, P450 enzymes, and UDP-glucuronosyltransferases between the PP and PC vein regions. This is consistent with the almost uniform distribution of ribavirin in the liver. In conclusion, sinusoidal membrane transporters such as Oatp1a1, Oatp1b2, Oct1, and Oat2 appear to be determinants of the zonated distribution of drugs in the liver.

Synthesis of [18F]2B-SRF101: A Sulfonamide Derivative of the Fluorescent Dye Sulforhodamine 101

BACKGROUND

The red fluorescent dye Sulforhodamine 101 (SR101) has been used in neuroscience research as a useful tool for staining of astrocytes, since it has been reported as a marker of astroglia in the neocortex of rodents in vivo. The aim of this work is to label SR101 with positron emission radionuclides, in order to provide a radiotracer to study its biological behavior. This is the first attempt to label SR101 by [18F], using a chemical derivatization via a sulfonamidelinker and a commercially available platform.

METHODS

The synthesis of SR101 N-(3-Bromopropyl) sulfonamide and SR101 N-(3- Fluoropropyl) sulfonamide (2B-SRF101) was carried out. The radiosynthesis of SR101 N-(3- [18F]Fluoropropyl) sulfonamide ([18F]2B-SRF101) was performed in a TRACERlab® FX-FN. Different labeling conditions were tested. Three pilot batches were produced and quality control was performed. Lipophilicity, plasma protein binding and radiochemical stability of [18F]2BSRF101 in final formulation and in plasma were determined.

RESULTS

SR101 N-(3-Bromopropyl) sulfonamide was synthetized as a precursor for radiolabeling with [18F]. 2B-SRF101 was prepared for analytical purpose. [18F]2B-SRF101 was obtained with radiochemical purity of (97.0 ± 0.6%). The yield of the whole synthesis was (11.9 ± 1.7 %), nondecay corrected. [18F]2B-SRF101 was found to be stable in final formulation and in plasma. The octanol-water partition coefficient was (Log POCT = 1.88 ± 0.14). The product showed a high percentage of plasma protein binding.

CONCLUSIONS

The derivatization of SR101 via sulfonamide-linker and the first radiosynthesis of [18 F]2B-SRF101 were performed. It was obtained in accordance with quality control specifications. In vitro stability studies verified that [18F]2B-SRF101 was suitable for preclinical evaluations.

Hippocampal and Cortical Pyramidal Neurons Swell in Parallel with Astrocytes during Acute Hypoosmolar Stress

Normal nervous system function is critically dependent on the balance of water and ions in the extracellular space (ECS). Pathological reduction in brain interstitial osmolarity results in osmotically-driven flux of water into cells, causing cellular edema which reduces the ECS and increases neuronal excitability and risk of seizures. Astrocytes are widely considered to be particularly susceptible to cellular edema due to selective expression of the water channel aquaporin-4 (AQP4). The apparent resistance of pyramidal neurons to osmotic swelling has been attributed to lack of functional water channels. In this study we report rapid volume changes in CA1 pyramidal cells in hypoosmolar ACSF (hACSF) that are equivalent to volume changes in astrocytes across a variety of conditions. Astrocyte and neuronal swelling was significant within 1 min of exposure to 17 or 40% hACSF, was rapidly reversible upon return to normosmolar ACSF, and repeatable upon re-exposure to hACSF. Neuronal swelling was not an artifact of patch clamp, occurred deep in tissue, was similar at physiological vs. room temperature, and occurred in both juvenile and adult hippocampal slices. Neuronal swelling was neither inhibited by TTX, nor by antagonists of NMDA or AMPA receptors, suggesting that it was not occurring as a result of excitotoxicity. Surprisingly, genetic deletion of AQP4 did not inhibit, but rather augmented, astrocyte swelling in severe hypoosmolar conditions. Taken together, our results indicate that neurons are not osmoresistant as previously reported, and that osmotic swelling is driven by an AQP4-independent mechanism.

Astrocytic Vesicle-based Exocytosis in Cultures and Acutely Isolated Hippocampal Rodent Slices

Astrocytes are excitable neural cells that contribute to brain information processing via bidirectional communication with neurons. This involves the release of gliosignaling molecules that affect synapses patterning and activity. Mechanisms mediating the release of these molecules likely consist of non-vesicular and vesicular-based mechanisms. It is the vesicle-based regulated exocytosis that is an evolutionary more complex process. It is well established that the release of gliosignaling molecules has profound effects on information processing in different brain regions (e.g., hippocampal astrocytes contribute to long-term potentiation [LTP]), which has traditionally been considered as one of the cellular mechanisms underlying learning and memory. However, the paradigm of vesicle-based regulated release of gliosignaling molecules from astrocytes is still far from being unanimously accepted. One of the most important questions is to what extent can the conclusions obtained from cultured astrocytes be translated to in vivo conditions. Here, we overview the properties of vesicle mobility and their fusion with the plasma membrane in cultured astrocytes and compare these parameters to those recorded in astrocytes from acute brain hippocampal slices. The results from both experimental models are similar, which validates experiments on isolated astrocytes and further supports arguments in favor of in vivo vesicle-based exocytotic release of gliosignaling molecules. © 2017 Wiley Periodicals, Inc.

Disorder of thyroid hormone transport into the tissues

Transport of thyroid hormone (TH) across the plasma membrane is essential for intracellular TH metabolism and action, and this is mediated by specific transporter proteins. During the last two decades several transporters capable of transporting TH have been identified, including monocarboxylate transporter 8 (MCT8), MCT10 and organic anion transporting polypeptide 1C1 (OATP1C1). In particular MCT8 and OATP1C1 are important for the regulation of local TH activity in the brain and thus for brain development. MCT8 is a protein containing 12 transmembrane domains, and is encoded by the SLC16A2 gene located on the X chromosome. It facilitates both TH uptake and efflux across the cell membrane. Male subjects with hemizygous mutations in MCT8 are afflicted with severe intellectual and motor disability, also known as the Allan-Herndon-Dudley syndrome (AHDS), which goes together with low serum T4 and high T3 levels. This review concerns molecular and clinical aspects of MCT8 function.

Limitations of Sulforhodamine 101 for Brain Imaging

Since 2004, the red fluorescent dye Sulforhodamine 101 (SR101) has been boosting the functional analysis of astrocytes in a functional environment in an unprecedented way. However, two major limitations have been challenging the usefulness of this tool for cellular imaging: (i) SR101 is not as specific for astrocytes as previously reported; and (ii) discoveries of severe excitatory side effects of SR101 are bearing the risk of unwanted alteration of the system of interest. In this article, we summarize the current knowledge about SR101-labeling protocols and discuss the problems that arise from varying of the staining protocols. Furthermore, we provide a testable hypothesis for the observed hyper-excitability that can be observed when using SR101.

Analysis of Functional NMDA Receptors in Astrocytes

Neuronal N-methyl-D-aspartate (NMDA) receptors are well known for their pivotal role in memory formation. Originally, they were thought to be exclusive to neurons. However, numerous studies revealed their functional expression also on various types of glial cells in the nervous system. Here, the methodology how to study the physiology of NMDA receptors selectively on astrocytes will be described in detail. Astrocytes are the main class of neuroglia that control transmitter and ion homeostasis, which link cerebral blood flow and neuronal energy demands, but also affect synaptic transmission directly.

Inducible targeting of CNS astrocytes in Aldh1l1-CreERT2 BAC transgenic mice

Background: Studying astrocytes in higher brain functions has been hampered by the lack of genetic tools for the efficient expression of inducible Cre recombinase throughout the CNS, including the neocortex. Methods: Therefore, we generated BAC transgenic mice, in which CreERT2 is expressed under control of the Aldh1l1 regulatory region. Results: When crossbred to Cre reporter mice, adult Aldh1l1-CreERT2 mice show efficient gene targeting in astrocytes. No such Cre-mediated recombination was detectable in CNS neurons, oligodendrocytes, and microglia. As expected, Aldh1l1-CreERT2 expression was evident in several peripheral organs, including liver and kidney. Conclusions: Taken together, Aldh1l1-CreERT2 mice are a useful tool for studying astrocytes in neurovascular coupling, brain metabolism, synaptic plasticity and other aspects of neuron-glia interactions.

The role of organic anion transporting polypeptides in drug absorption, distribution, excretion and drug-drug interactions

INTRODUCTION

The in vivo fate and effectiveness of a drug depends highly on its absorption, distribution, metabolism, excretion and toxicity (ADME-Tox). Organic anion transporting polypeptides (OATPs) are membrane proteins involved in the cellular uptake of various organic compounds, including clinically used drugs. Since OATPs are significant players in drug absorption and distribution, modulation of OATP function via pharmacotherapy with OATP substrates/inhibitors, or modulation of their expression, affects drug pharmacokinetics. Given their cancer-specific expression, OATPs may also be considered anticancer drug targets. Areas covered: We describe the human OATP family, discussing clinically relevant consequences of altered OATP function. We offer a critical analysis of published data on the role of OATPs in ADME and in drug-drug interactions, especially focusing on OATP1A2, 1B1, 1B3 and 2B1. Expert opinion: Four members of the OATP family, 1A2, 1B1, 1B3 and 2B1, have been characterized in detail. As biochemical and pharmacological knowledge on the other OATPs is lacking, it seems timely to direct research efforts towards developing the experimental framework needed to investigate the transport mechanism and substrate specificity of the poorly described OATPs. In addition, elucidating the role of OATPs in tumor development and therapy response are critical avenues for further research.

Unspecific labelling of oligodendrocytes by sulforhodamine 101 depends on astrocytic uptake via the thyroid hormone transporter OATP1C1 (SLCO1C1)

The red fluorescent dye Sulforhodamine 101 (SR101) is often used as a marker for astrocytes, although variations of the staining protocol have been shown to influence the preferentially labeled cell type. Here we analyzed SR101-labeling of oligodendrocytes in the hippocampal slices preparation of PLP-EGFP mice. Using different staining protocols, we found robust SR101-labeled oligodendrocytes in the CA1 stratum radiatum of the hippocampus. Application of L-thyroxin, which is known to block SR101 transport into astrocytes via competitive inhibition of the multi-specific OATP1C1 (SLCO1C1) transporter, significantly reduced oligodendrocyte labeling. Since OATP1C1 is not expressed in oligodendrocytes, we conclude that oligodendrocyte labeling with SR101 requires SR101-uptake by astrocytes, which then diffuses to oligodendrocytes via heterotypic gap junctions of the pan-glial network. In summary, unequivocal identification of a particular cell type is not possible by SR101 only, hence caution is recommended when using SR101 in future studies.

Sulforhodamine 101, a widely used astrocyte marker, can induce cortical seizure-like activity at concentrations commonly used

Sulforhodamine 101 (SR101) is a preferential astrocyte marker widely used in 2-photon microscopy experiments. Here we show, that topical loading of two commonly used SR101 concentrations, 100 μM and 250 μM when incubated for 10 min, can induce seizure-like local field potential (LFP) activity in both anaesthetized and awake mouse sensori-motor cortex. This cortical seizure-like activity develops in less than ten minutes following topical loading, and when applied longer, these neuronal discharges reliably evoke contra-lateral hindlimb muscle contractions. Short duration (<1 min) incubation of 100 μM and 250 μM SR101 or application of lower concentrations 25 μM and 50 μM of SR101, incubated for 30 and 20 min, respectively, did not induce abnormal LFP activity in sensori-motor cortex, but did label astrocytes, and may thus be considered more appropriate concentrations for in vivo astrocyte labeling. In addition to label astrocytes SR101 may, at 100 μM and 250 μM, induce abnormal neuronal activity and interfere with cortical circuit activity. SR101 concentration of 50 μM or lower did not induce abnormal neuronal activity. We advocate that, to label astrocytes with SR101, concentrations no higher than 50 μM should be used for in vivo experiments.

Presynaptic facilitation of glycinergic mIPSC is reduced in mice lacking α3 glycine receptor subunits

Glycinergic neurons provide an important mechanism to control excitation of motoneurons in the brainstem and a reduction or loss of glycinergic inhibition can be deleterious by leading to hyperexcitation such as in hyperekplexia or neurodegeneration and neuronal death as in amyotrophic lateral sclerosis (ALS). Second messenger systems that change cyclic AMP and lead to phosphorylation of the α3 subunit of the glycine receptor (GlyR α3) have been shown to be potent modulators of synaptic inhibition in the spinal cord and brain stem. In this study we analyzed the role of GlyR α3 in synaptic inhibition to the hypoglossal nucleus using Glra3 (the gene encoding the glycine receptor α3 subunit) knockout mice. We observed that baseline glycinergic synaptic transmission to nucleus of hypoglossal motoneurons is rather normal in Glra3 knockout mice. Interestingly, we found that the modulation of synaptic transmission by cAMP-mediated pathways appeared to be reduced in Glra3 knockout mice. In the second postnatal week the forskolin-induced increase of miniature inhibitory postsynaptic potential (mIPSC) frequency was significantly larger in control as compared to Glra3 knockout mice suggesting that presynaptic glycine release in the hypoglossal nucleus is partially depending on GlyR α3.

Synthetic Fluorophores for Visualizing Biomolecules in Living Systems

The last decade has witnessed significant advance in the imaging of living systems using fluorescent markers. This progress has been primarily associated with the discovery of different spectral variants of fluorescent proteins. However, the fluorescent protein technology has its own limitations and, in some cases, the use of low-molecular-weight fluorophores is preferable. In this review, we describe the arsenal of synthetic fluorescent tools that are currently in researchers' hands and span virtually the entire spectrum, from the UV to visible and, further, to the near-infrared region. An overview of recent advances in site-directed introduction of synthetic fluorophores into target cellular objects is provided. Application of these fluorescent probes to the solution of a wide range of biological problems, in particular, to the determination of local ion concentrations and pH in living systems, is discussed.