Astrocytic beta(2)-adrenergic receptors: from physiology to pathology

Importance of glucose and its metabolism in neurodegenerative disorder, as well as the combination of multiple therapeutic strategies targeting α-synuclein and neuroprotection in the treatment of Parkinson's disease

According to recent findings, Phosphoglycerate Kinase 1 (pgk-1) enzyme is linked to Parkinson's disease (PD). Mutations in the PGK-1 gene lead to decreases in the pgk-1 enzyme which causes an imbalance in the levels of energy demand and supply. An increase in glycolytic adenosine triphosphate (ATP) production would help alleviate energy deficiency and sustain the acute energetic need of neurons. Neurodegeneration is caused by an imbalance or reduction in ATP levels. Recent data suggest that medications that increase glycolysis and neuroprotection can be used to treat PD. The current study focuses on treatment options for disorders associated with the pgk-1 enzyme, GLP-1, and A2A receptor which can be utilized to treat PD. A combination of metformin and terazosin, exenatide and meclizine, istradefylline and salbutamol treatments may benefit parkinsonism. The review also looked at potential target-specific new techniques that might assist in satisfying unfulfilled requirements in the treatment of PD.

Vagus nerve stimulation enhances remyelination and decreases innate neuroinflammation in lysolecithin-induced demyelination

BACKGROUND

Current treatments for Multiple Sclerosis (MS) poorly address chronic innate neuroinflammation nor do they offer effective remyelination. The vagus nerve has a strong regulatory role in inflammation and Vagus Nerve Stimulation (VNS) has potential to affect both neuroinflammation and remyelination in MS.

OBJECTIVE

This study investigated the effects of VNS on demyelination and innate neuroinflammation in a validated MS rodent model.

METHODS

Lysolecithin (LPC) was injected in the corpus callosum (CC) of 46 Lewis rats, inducing a demyelinated lesion. 33/46 rats received continuously-cycled VNS (cVNS) or one-minute per day VNS (1minVNS) or sham VNS from 2 days before LPC-injection until perfusion at 3 days post-injection (dpi) (corresponding with a demyelinated lesion with peak inflammation). 13/46 rats received cVNS or sham from 2 days before LPC-injection until perfusion at 11 dpi (corresponding with a partial remyelinated lesion). Immunohistochemistry and proteomics analyses were performed to investigate the extend of demyelination and inflammation.

RESULTS

Immunohistochemistry showed that cVNS significantly reduced microglial and astrocytic activation in the lesion and lesion border, and significantly reduced the Olig2+ cell count at 3 dpi. Furthermore, cVNS significantly improved remyelination with 57.4 % versus sham at 11 dpi. Proteomic gene set enrichment analyses showed increased activation of (glutamatergic) synapse pathways in cVNS versus sham, most pronounced at 3 dpi.

CONCLUSION

cVNS improved remyelination of an LPC-induced lesion. Possible mechanisms might include modulation of microglia and astrocyte activity, increased (glutamatergic) synapses and enhanced oligodendrocyte clearance after initial injury.

Astrocyte β-Adrenergic Receptor Activity Regulates NMDA Receptor Signaling of Medial Prefrontal Cortex Pyramidal Neurons

Glutamate spillover from the synapse is tightly regulated by astrocytes, limiting the activation of extrasynaptically located NMDA receptors (NMDAR). The processes of astrocytes are dynamic and can modulate synaptic physiology. Though norepinephrine (NE) and β-adrenergic receptor (β-AR) activity can modify astrocyte volume, this has yet to be confirmed outside of sensory cortical areas, nor has the effect of noradrenergic signaling on glutamate spillover and neuronal NMDAR activity been explored. We monitored changes to astrocyte process volume in response to noradrenergic agonists in the medial prefrontal cortex of male and female mice. Both NE and the β-AR agonist isoproterenol (ISO) increased process volume by ∼20%, significantly higher than changes seen when astrocytes had G-protein signaling blocked by GDPβS. We measured the effect of β-AR signaling on evoked NMDAR currents. While ISO did not affect single stimulus excitatory currents of Layer 5 pyramidal neurons, ISO reduced NMDAR currents evoked by 10 stimuli at 50 Hz, which elicits glutamate spillover, by 18%. After isolating extrasynaptic NMDARs by blocking synaptic NMDARs with the activity-dependent NMDAR blocker MK-801, ISO similarly reduced extrasynaptic NMDAR currents in response to 10 stimuli by 18%. Finally, blocking β-AR signaling in the astrocyte network by loading them with GDPβS reversed the ISO effect on 10 stimuli-evoked NMDAR currents. These results demonstrate that astrocyte β-AR activity reduces extrasynaptic NMDAR recruitment, suggesting that glutamate spillover is reduced.

Promising biomarkers and therapeutic targets for the management of Parkinson's disease: recent advancements and contemporary research

Parkinson's disease (PD) is one of the progressive neurological diseases which affect around 10 million population worldwide. The clinical manifestation of motor symptoms in PD patients appears later when most dopaminergic neurons have degenerated. Thus, for better management of PD, the development of accurate biomarkers for the early prognosis of PD is imperative. The present work will discuss the potential biomarkers from various attributes covering biochemical, microRNA, and neuroimaging aspects (α-synuclein, DJ-1, UCH-L1, β-glucocerebrosidase, BDNF, etc.) for diagnosis, recent development in PD management, and major limitations with current and conventional anti-Parkinson therapy. This manuscript summarizes potential biomarkers and therapeutic targets, based on available preclinical and clinical evidence, for better management of PD.

Activation of β2-Adrenergic Receptors in Microglia Alleviates Neuropathic Hypersensitivity in Mice

Drugs enhancing the availability of noradrenaline are gaining prominence in the therapy of chronic neuropathic pain. However, underlying mechanisms are not well understood, and research has thus far focused on α2-adrenergic receptors and neuronal excitability. Adrenergic receptors are also expressed on glial cells, but their roles toward antinociception are not well deciphered. This study addresses the contribution of β2-adrenergic receptors (β2-ARs) to the therapeutic modulation of neuropathic pain in mice. We report that selective activation of β2-ARs with Formoterol inhibits pro-inflammatory signaling in microglia ex vivo and nerve injury-induced structural remodeling and functional activation of microglia in vivo. Systemic delivery of Formoterol inhibits behaviors related to neuropathic pain, such as mechanical hypersensitivity, cold allodynia as well as the aversive component of pain, and reverses chronically established neuropathic pain. Using conditional gene targeting for microglia-specific deletion of β2-ARs, we demonstrate that the anti-allodynic effects of Formoterol are primarily mediated by microglia. Although Formoterol also reduces astrogliosis at late stages of neuropathic pain, these functions are unrelated to β2-AR signaling in microglia. Our results underline the value of developing microglial β2-AR agonists for relief from neuropathic pain and clarify mechanistic underpinnings.

Emerging Therapeutic Strategies for Parkinson’s Disease and Future Prospects: A 2021 Update

Parkinson’s disease (PD) is a neurodegenerative disorder pathologically distinguished by degeneration of dopaminergic neurons in the substantia nigra pars compacta. Muscle rigidity, tremor, and bradykinesia are all clinical motor hallmarks of PD. Several pathways have been implicated in PD etiology, including mitochondrial dysfunction, impaired protein clearance, and neuroinflammation, but how these factors interact remains incompletely understood. Although many breakthroughs in PD therapy have been accomplished, there is currently no cure for PD, only trials to alleviate the related motor symptoms. To reduce or stop the clinical progression and mobility impairment, a disease-modifying approach that can directly target the etiology rather than offering symptomatic alleviation remains a major unmet clinical need in the management of PD. In this review, we briefly introduce current treatments and pathophysiology of PD. In addition, we address the novel innovative therapeutic targets for PD therapy, including α-synuclein, autophagy, neurodegeneration, neuroinflammation, and others. Several immunomodulatory approaches and stem cell research currently in clinical trials with PD patients are also discussed. Moreover, preclinical studies and clinical trials evaluating the efficacy of novel and repurposed therapeutic agents and their pragmatic applications with encouraging outcomes are summarized. Finally, molecular biomarkers under active investigation are presented as potentially valuable tools for early PD diagnosis.

Ca2+ as the prime trigger of aerobic glycolysis in astrocytes

Astroglial aerobic glycolysis, a process during which d-glucose is converted to l-lactate, a brain fuel and signal, is regulated by the plasmalemmal receptors, including adrenergic receptors (ARs) and purinergic receptors (PRs), modulating intracellular Ca²⁺ and cAMP signals. However, the extent to which the two signals regulate astroglial aerobic glycolysis is poorly understood. By using agonists to stimulate intracellular α₁-/β-AR-mediated Ca²⁺/cAMP signals, β-AR-mediated cAMP and P₂R-mediated Ca²⁺ signals and genetically encoded fluorescence resonance energy transfer-based glucose and lactate nanosensors in combination with real-time microscopy, we show that intracellular Ca²⁺, but not cAMP, initiates a robust increase in the concentration of intracellular free d-glucose ([glc]_i) and l-lactate ([lac]_i), both depending on extracellular d-glucose, suggesting Ca²⁺-triggered glucose uptake and aerobic glycolysis in astrocytes. When the glycogen shunt, a process of glycogen remodelling, was inhibited, the α₁-/β-AR-mediated increases in [glc]_i and [lac]_i were reduced by ∼65 % and ∼30 %, respectively, indicating that at least ∼30 % of the utilization of d-glucose is linked to glycogen remodelling and aerobic glycolysis. Additional activation of β-AR/cAMP signals aided to α₁-/β-AR-triggered [lac]_i increase, whereas the [glc]_i increase was unaltered. Taken together, an increase in intracellular Ca²⁺ is the prime mechanism of augmented aerobic glycolysis in astrocytes, while cAMP has only a moderate role. The results provide novel information on the signals regulating brain metabolism and open new avenues to explore whether astroglial Ca²⁺ signals are dysregulated and contribute to neuropathologies with impaired brain metabolism.

Astrocytes with TDP-43 inclusions exhibit reduced noradrenergic cAMP and Ca2+ signaling and dysregulated cell metabolism

Most cases of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have cytoplasmic inclusions of TAR DNA-binding protein 43 (TDP-43) in neurons and non-neuronal cells, including astrocytes, which metabolically support neurons with nutrients. Neuronal metabolism largely depends on the activation of the noradrenergic system releasing noradrenaline. Activation of astroglial adrenergic receptors with noradrenaline triggers cAMP and Ca²⁺ signaling and augments aerobic glycolysis with production of lactate, an important neuronal energy fuel. Astrocytes with cytoplasmic TDP-43 inclusions can cause motor neuron death, however, whether astroglial metabolism and metabolic support of neurons is altered in astrocytes with TDP-43 inclusions, is unclear. We measured lipid droplet and glucose metabolisms in astrocytes expressing the inclusion-forming C-terminal fragment of TDP-43 or the wild-type TDP-43 using fluorescent dyes or genetically encoded nanosensors. Astrocytes with TDP-43 inclusions exhibited a 3-fold increase in the accumulation of lipid droplets versus astrocytes expressing wild-type TDP-43, indicating altered lipid droplet metabolism. In these cells the noradrenaline-triggered increases in intracellular cAMP and Ca²⁺ levels were reduced by 35% and 31%, respectively, likely due to the downregulation of β₂-adrenergic receptors. Although noradrenaline triggered a similar increase in intracellular lactate levels in astrocytes with and without TDP-43 inclusions, the probability of activating aerobic glycolysis was facilitated by 1.6-fold in astrocytes with TDP-43 inclusions and lactate MCT1 transporters were downregulated. Thus, while in astrocytes with TDP-43 inclusions noradrenergic signaling is reduced, aerobic glycolysis and lipid droplet accumulation are facilitated, suggesting dysregulated astroglial metabolism and metabolic support of neurons in TDP-43-associated ALS and FTD.

Detecting neurodegenerative pathology in multiple sclerosis before irreversible brain tissue loss sets in

Background

Multiple sclerosis (MS) is a complex chronic inflammatory and degenerative disorder of the central nervous system. Accelerated brain volume loss, or also termed atrophy, is currently emerging as a popular imaging marker of neurodegeneration in affected patients, but, unfortunately, can only be reliably interpreted at the time when irreversible tissue damage likely has already occurred. Timing of treatment decisions based on brain atrophy may therefore be viewed as suboptimal.

Main body

This Narrative Review focuses on alternative techniques with the potential of detecting neurodegenerative events in the brain of subjects with MS prior to the atrophic stage. First, metabolic and molecular imaging provide the opportunity to identify early subcellular changes associated with energy dysfunction, which is an assumed core mechanism of axonal degeneration in MS. Second, cerebral hypoperfusion has been observed throughout the entire clinical spectrum of the disorder but it remains an open question whether this serves as an alternative marker of reduced metabolic activity, or exists as an independent contributing process, mediated by endothelin-1 hyperexpression. Third, both metabolic and perfusion alterations may lead to repercussions at the level of network performance and structural connectivity, respectively assessable by functional and diffusion tensor imaging. Fourth and finally, elevated body fluid levels of neurofilaments are gaining interest as a biochemical mirror of axonal damage in a wide range of neurological conditions, with early rises in patients with MS appearing to be predictive of future brain atrophy.

Conclusions

Recent findings from the fields of advanced neuroradiology and neurochemistry provide the promising prospect of demonstrating degenerative brain pathology in patients with MS before atrophy has installed. Although the overall level of evidence on the presented topic is still preliminary, this Review may pave the way for further longitudinal and multimodal studies exploring the relationships between the abovementioned measures, possibly leading to novel insights in early disease mechanisms and therapeutic intervention strategies.

Salmeterol, a β2 Adrenergic Agonist, Promotes Adult Hippocampal Neurogenesis in a Region-Specific Manner

Neurogenesis persists in the subgranular zone of the hippocampal formation in the adult mammalian brain. In this area, neural progenitor cells (NPCs) receive both permissive and instructive signals, including neurotransmitters, that allow them to generate adult-born neurons which can be functionally integrated in the preexisting circuit. Deregulation of adult hippocampal neurogenesis (ahNG) occurs in several neuropsychiatric and neurodegenerative diseases, including major depression, and represents a potential therapeutic target. Of interest, several studies suggested that, both in rodents and in humans, ahNG is increased by chronic administration of classical monoaminergic antidepressant drugs, suggesting that modulation of this process may participate to their therapeutic effects. Since the established observation that noradrenergic innervations from locus coeruleus make contact with NPC in the dentate gyrus, we investigated the role of beta adrenergic receptor (β-AR) on ahNG both in vitro and in vivo. Here we report that, in vitro, activation of β₂-AR by norepinephrine and β₂-AR agonists promotes the formation of NPC-derived mature neurons, without affecting NPC survival or differentiation toward glial lineages. Additionally, we show that a selective β₂-AR agonist able to cross the blood-brain barrier, salmeterol, positively modulates hippocampal neuroplasticity when chronically administered in adult naïve mice. Indeed, salmeterol significantly increased number, maturation, and dendritic complexity of DCX⁺ neuroblasts. The increased number of DCX⁺ cells was not accompanied by a parallel increase in the percentage of BrdU⁺/DCX⁺ cells suggesting a potential prosurvival effect of the drug on neuroblasts. More importantly, compared to vehicle, salmeterol promoted ahNG, as demonstrated by an increase in the actual number of BrdU⁺/NeuN⁺ cells and in the percentage of BrdU⁺/NeuN⁺ cells over the total number of newly generated cells. Interestingly, salmeterol proneurogenic effects were restricted to the ventral hippocampus, an area related to emotional behavior and mood regulation. Since salmeterol is commonly used for asthma therapy in the clinical setting, its novel pharmacological property deserves to be further exploited with a particular focus on drug potential to counteract stress-induced deregulation of ahNG and depressive-like behavior.

Optogenetic manipulation of astrocytes from synapses to neuronal networks: A potential therapeutic strategy for neurodegenerative diseases

Astrocytes are the most widespread and heterogeneous glial cells in the central nervous system and key regulators for brain development. They are capable of receiving neurotransmitters produced by synaptic activities and regulating synaptic functions by releasing gliotransmitters as part of the tripartite synapse. In addition to communicating with neurons at synaptic levels, astrocytes can integrate into inhibitory neural networks to interact with neurons in neuronal circuits. Astrocytes are closely related to the pathogenesis and pathological processes of neurodegenerative diseases (NDs). Recently, optogenetics has now been applied to reveal the function of astrocytes in physiology and pathology. Herein, we discuss the possibility whether optogenetics could be used to control the release of gliotransmitters and regulate astrocytic membrane channels. Thus, the capability of modulating the bidirectional interactions between astrocytes and neurons in both synaptic and neuronal networks via optogenetics is evaluated. Furthermore, we discuss that manipulating astrocytes via optogenetics might be an effective way to investigate the potential therapeutic strategy for NDs.

Stress Pathway Modulation Is Detrimental or Ineffective for Functional Recovery after Spinal Cord Injury in Mice

A mounting body of evidence suggests that stress plays a major role in the injury progression after spinal cord injury (SCI). Injury activates the stress systems; this in turn may augment the generation of pro-inflammatory cytokines, stimulate pro-inflammatory immune cells, and alter the balance between the pro- and anti-inflammatory immune response. As a result, it is suggested that stress pathways may augment neuronal damage and loss after SCI. Considering these potential detrimental effects of stress after SCI, we hypothesized that inhibition of stress pathways immediately after SCI may offer protection from damage and improve recovery. To investigate the relevance of stress responses in SCI recovery, we investigated the effects of blocking three well-studied stress response axes in a mouse model of SCI. Propranolol, RU-486, and CP-99994 were administered to inhibit the sympathetic axis, the hypothalamus-pituitary-adrenal axis, and the neuropeptide axis, respectively. Surprisingly, assessing functional recovery by the Basso Mouse Scale revealed that RU-486 and CP-99994 did not affect functional outcome, indicating that these pathways are dispensable for neuroprotection or repair after SCI. Moreover, the beta-blocker propranolol worsened functional outcome in the mouse SCI model. In conclusion, immediate inhibition of three major stress axes has no beneficial effects on functional recovery after SCI in mice. These results suggest that injury-induced stress responses do not interfere with the healing process and hence, pharmacological targeting of stress responses is not a recommended treatment option for SCI. These findings are of great importance for other researchers to avoid unnecessary and potentially futile animal experiments.

Vascular aspects of multiple sclerosis: emphasis on perfusion and cardiovascular comorbidities

INTRODUCTION

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating, and neurodegenerative disease of the central nervous system. Over the last two decades, more favorable MS long-term outcomes have contributed toward increase in prevalence of the aged MS population. Emergence of age-associated pathology, such as cardiovascular diseases, may interact with the MS pathophysiology and further contribute to disease progression. Areas covered: This review summarizes the cardiovascular involvement in MS pathology, its disease activity, and progression. The cardiovascular health, the presence of various cardiovascular diseases, and their effect on MS cognitive performance are further explored. In similar fashion, the emerging evidence of a higher incidence of extracranial arterial pathology and its association with brain MS pathology are discussed. Finally, the authors outline the methodologies behind specific perfusion magnetic resonance imaging (MRI) and ultrasound Doppler techniques, which allow measurement of disease-specific and age-specific vascular changes in the aging population and MS patients. Expert opinion: Cardiovascular pathology significantly contributes to worse clinical and MRI-derived disease outcomes in MS. Global and regional cerebral hypoperfusion may be associated with poorer physical and cognitive performance. Prevention, improved detection, and treatment of the cardiovascular-based pathology may improve the overall long-term health of MS patients.

Computational modeling and biomarker studies of pharmacological treatment of Alzheimer's disease (Review)

Alzheimer's disease (AD) is a complex and multifactorial disease. In order to understand the genetic influence in the progression of AD, and to identify novel pharmaceutical agents and their associated targets, the present study discusses computational modeling and biomarker evaluation approaches. Based on mechanistic signaling pathway approaches, various computational models, including biochemical and morphological models, are discussed to explore the strategies that may be used to target AD treatment. Different biomarkers are interpreted on the basis of morphological and functional features of amyloid β plaques and unstable microtubule‑associated tau protein, which is involved in neurodegeneration. Furthermore, imaging and cerebrospinal fluids are also considered to be key methods in the identification of novel markers for AD. In conclusion, the present study reviews various biochemical and morphological computational models and biomarkers to interpret novel targets and agonists for the treatment of AD. This review also highlights several therapeutic targets and their associated signaling pathways in AD, which may have potential to be used in the development of novel pharmacological agents for the treatment of patients with AD. Computational modeling approaches may aid the quest for the development of AD treatments with enhanced therapeutic efficacy and reduced toxicity.

Astroglial vesicular network: evolutionary trends, physiology and pathophysiology

Intracellular organelles, including secretory vesicles, emerged when eukaryotic cells evolved some 3 billion years ago. The primordial organelles that evolved in Archaea were similar to endolysosomes, which developed, arguably, for specific metabolic tasks, including uptake, metabolic processing, storage and disposal of molecules. In comparison with prokaryotes, cell volume of eukaryotes increased by several orders of magnitude and vesicle traffic emerged to allow for communication between distant intracellular locations. Lysosomes, first described in 1955, a prominent intermediate of endo- and exocytotic pathways, operate virtually in all eukaryotic cells including astroglia, the most heterogeneous type of homeostatic glia in the central nervous system. Astrocytes support neuronal network activity in particular through elaborated secretion, based on a complex intracellular vesicle network dynamics. Deranged homeostasis underlies disease and astroglial vesicle traffic contributes to the pathophysiology of neurodegenerative (Alzheimer's disease, Huntington's disease), neurodevelopmental diseases (intellectual deficiency, Rett's disease) and neuroinfectious (Zika virus) disorders. This review addresses astroglial cell-autonomous vesicular traffic network, as well as its into primary and secondary vesicular network defects in diseases, and considers this network as a target for developing new therapies for neurological conditions.

Co-Activation of Metabotropic Glutamate Receptor 3 and Beta-Adrenergic Receptors Modulates Cyclic-AMP and Long-Term Potentiation, and Disrupts Memory Reconsolidation

Activation of β-adrenergic receptors (βARs) enhances both the induction of long-term potentiation (LTP) in hippocampal CA1 pyramidal cells and hippocampal-dependent cognitive function. Interestingly, previous studies reveal that coincident activation of group II metabotropic glutamate (mGlu) receptors with βARs in the hippocampal astrocytes induces a large increase in cyclic-AMP (cAMP) accumulation and release of adenosine. Adenosine then acts on A₁ adenosine receptors at neighboring excitatory Schaffer collateral terminals, which could counteract effects of activation of neuronal βARs on excitatory transmission. On the basis of this, we postulated that activation of the specific mGlu receptor subtype that mediates this response could inhibit βAR-mediated effects on hippocampal synaptic plasticity and cognitive function. Using novel mGlu receptor subtype-selective allosteric modulators along with knockout mice we now report that the effects of mGlu_2/3 agonists on βAR-mediated increases in cAMP accumulation are exclusively mediated by mGlu₃. Furthermore, mGlu₃ activation inhibits the ability of the βAR agonist isoproterenol to enhance hippocampal LTP, and this effect is absent in slices treated with either a glial toxin or an adenosine A₁ receptor antagonist. Finally, systemic administration of the mGlu_2/3 agonist LY379268 disrupted contextual fear memory in a manner similar to the effect of the βAR antagonist propranolol, and this effect was reversed by the mGlu₃-negative allosteric modulator VU0650786. Taken together, these data suggest that mGlu₃ can influence astrocytic signaling and modulate βAR-mediated effects on hippocampal synaptic plasticity and cognitive function.

Astrocytic β2 Adrenergic Receptor Gene Deletion Affects Memory in Aged Mice

In vitro and in vivo studies suggest that the astrocytic adrenergic signalling enhances glycogenolysis which provides energy to be transported to nearby cells and in the form of lactate. This energy source is important for motor and cognitive functioning. While it is suspected that the β2-adrenergic receptor on astrocytes might contribute to this energy balance, it has not yet been shown conclusively in vivo. Inducible astrocyte specific β2-adrenergic receptor knock-out mice were generated by crossing homozygous β2-adrenergic receptor floxed mice (Adrb2^flox) and mice with heterozygous tamoxifen-inducible Cre recombinase-expression driven by the astrocyte specific L-glutamate/L-aspartate transporter promoter (GLAST-CreERT2). Assessments using the modified SHIRPA (SmithKline/Harwell/Imperial College/Royal Hospital/Phenotype Assessment) test battery, swimming ability test, and accelerating rotarod test, performed at 1, 2 and 4 weeks, 6 and 12 months after tamoxifen (or vehicle) administration did not reveal any differences in physical health or motor functions between the knock-out mice and controls. However deficits were found in the cognitive ability of aged, but not young adult mice, reflected in impaired learning in the Morris Water Maze. Similarly, long-term potentiation (LTP) was impaired in hippocampal brain slices of aged knock-out mice maintained in low glucose media. Using microdialysis in cerebellar white matter we found no significant differences in extracellular lactate or glucose between the young adult knock-out mice and controls, although trends were detected. Our results suggest that β2-adrenergic receptor expression on astrocytes in mice may be important for maintaining cognitive health at advanced age, but is dispensable for motor function.

G-protein coupled receptors as therapeutic targets for neurodegenerative and cerebrovascular diseases

Neurodegenerative and cerebrovascular diseases are frequent in elderly populations and comprise primarily of dementia (mainly Alzheimer's disease) Parkinson's disease and stroke. These neurological disorders (NDs) occur as a result of neurodegenerative processes and represent one of the most frequent causes of death and disability worldwide with a significant clinical and socio-economic impact. Although NDs have been characterized for many years, the exact molecular mechanisms that govern these pathologies or why they target specific individuals and specific neuronal populations remain unclear. As research progresses, many similarities appear which relate these diseases to one another on a subcellular level. Discovering these similarities offers hope for therapeutic advances that could ameliorate the conditions of many diseases simultaneously. G-protein coupled receptors (GPCRs) are the most abundant receptor type in the central nervous system and are linked to complex downstream pathways, manipulation of which may have therapeutic application in many NDs. This review will highlight the potential use of neurotransmitter GPCRs as emerging therapeutic targets for neurodegenerative and cerebrovascular diseases.

Adrenergic stimulation of single rat astrocytes results in distinct temporal changes in intracellular Ca(2+) and cAMP-dependent PKA responses

During the arousal and startle response, locus coeruleus neurons, innervating practically all brain regions, release catecholamine noradrenaline, which reaches neural brain cells, including astrocytes. These glial cells respond to noradrenergic stimulation by simultaneous activation of the α- and β-adrenergic receptors (ARs) in the plasma membrane with increasing cytosolic levels of Ca(2+) and cAMP, respectively. AR-activation controls a myriad of processes in astrocytes including glucose metabolism, gliosignal vesicle homeostasis, gene transcription, cell morphology and antigen-presenting functions, all of which have distinct temporal characteristics. It is known from biochemical studies that Ca(2+) and cAMP signals in astrocytes can interact, however it is presently unclear whether the temporal properties of the two second messengers are time associated upon AR-activation. We used confocal microscopy to study AR agonist-induced intracellular changes in Ca(2+) and cAMP in single cultured cortical rat astrocytes by real-time monitoring of the Ca(2+) indicator Fluo4-AM and the fluorescence resonance energy transfer-based nanosensor A-kinase activity reporter 2 (AKAR2), which reports the activity of cAMP via its downstream effector protein kinase A (PKA). The results revealed that the activation of α1-ARs by phenylephrine triggers periodic (phasic) Ca(2+) oscillations within 10s, while the activation of β-ARs by isoprenaline leads to a ∼10-fold slower tonic rise to a plateau in cAMP/PKA activity devoid of oscillations. Thus the concomitant activation of α- and β-ARs triggers the Ca(2+) and cAMP second messenger systems in astrocytes with distinct temporal properties, which appears to be tailored to regulate downstream effectors in different time domains.

ADRB2 polymorphisms predict the risk of myocardial infarction and coronary artery disease

Recently, the rs1042713 G > A and rs1042714 C > G polymorphisms in the beta-2 adrenergic receptor (ADRB2) gene were shown to be related to atherosclerosis diseases. Therefore, we performed a systemic meta-analysis to determine whether the two functional polymorphisms are related to the risk of myocardial infarction (MI) and coronary artery disease (CAD). We identified published studies that are relevant to our topic of interest. Seven case-control studies, with a total of 6,843 subjects, were incorporated into the current meta-analysis. Our analysis showed a higher frequency of rs1042713 G > A variant in patients with MI or CAD compared to healthy controls. A similar result was also obtained with the rs1042714 C > G variant under both the allele and dominant models. Ethnicity-stratified subgroup analysis suggested that the rs1042714 C > G variant correlated with an increased risk of the two diseases in both Asians and Caucasians, while rs1042713 G > A only contributes to the risk of two diseases in Asians. In the disease type-stratified subgroups, the frequencies of both the rs1042713 G > A and rs1042714 C > G variants were higher in the cases than in the controls in both the MI and CAD subgroups. Collectively, our data contribute towards understanding the correlation between the rs1042713 G > A and rs1042714 C > G polymorphisms in ADRB2 and the susceptibility to MI and CAD.

Adrenergic regulation of innate immunity: a review

The sympathetic nervous system has a major role in the brain-immune cross-talk, but few information exist on the sympathoadrenergic regulation of innate immune system. The aim of this review is to summarize available knowledge regarding the sympathetic modulation of the innate immune response, providing a rational background for the possible repurposing of adrenergic drugs as immunomodulating agents. The cells of immune system express adrenoceptors (AR), which represent the target for noradrenaline and adrenaline. In human neutrophils, adrenaline and noradrenaline inhibit migration, CD11b/CD18 expression, and oxidative metabolism, possibly through β-AR, although the role of α₁- and α₂-AR requires further investigation. Natural Killer express β-AR, which are usually inhibitory. Monocytes express β-AR and their activation is usually antiinflammatory. On murine Dentritic cells (DC), β-AR mediate sympathetic influence on DC-T cells interactions. In human DC β₂-AR may affect Th1/2 differentiation of CD4+ T cells. In microglia and in astrocytes, β₂-AR dysregulation may contribute to neuroinflammation in autoimmune and neurodegenerative disease. In conclusion, extensive evidence supports a critical role for adrenergic mechanisms in the regulation of innate immunity, in peripheral tissues as well as in the CNS. Sympathoadrenergic pathways in the innate immune system may represent novel antiinflammatory and immunomodulating targets with significant therapeutic potential.

Volume Transmission in Central Dopamine and Noradrenaline Neurons and Its Astroglial Targets

Already in the 1960s the architecture and pharmacology of the brainstem dopamine (DA) and noradrenaline (NA) neurons with formation of vast numbers of DA and NA terminal plexa of the central nervous system (CNS) indicated that they may not only communicate via synaptic transmission. In the 1980s the theory of volume transmission (VT) was introduced as a major communication together with synaptic transmission in the CNS. VT is an extracellular and cerebrospinal fluid transmission of chemical signals like transmitters, modulators etc. moving along energy gradients making diffusion and flow of VT signals possible. VT interacts with synaptic transmission mainly through direct receptor-receptor interactions in synaptic and extrasynaptic heteroreceptor complexes and their signaling cascades. The DA and NA neurons are specialized for extrasynaptic VT at the soma-dendrtitic and terminal level. The catecholamines released target multiple DA and adrenergic subtypes on nerve cells, astroglia and microglia which are the major cell components of the trophic units building up the neural-glial networks of the CNS. DA and NA VT can modulate not only the strength of synaptic transmission but also the VT signaling of the astroglia and microglia of high relevance for neuron-glia interactions. The catecholamine VT targeting astroglia can modulate the fundamental functions of astroglia observed in neuroenergetics, in the Glymphatic system, in the central renin-angiotensin system and in the production of long-distance calcium waves. Also the astrocytic and microglial DA and adrenergic receptor subtypes mediating DA and NA VT can be significant drug targets in neurological and psychiatric disease.

Noradrenergic regulation of glial activation: molecular mechanisms and therapeutic implications

It has been known for many years that the endogenous neurotransmitter noradrenaline (NA) exerts anti-inflammatory and neuroprotective effects both in vitro and in vivo. In many cases the site of action of NA are beta-adrenergic receptors (βARs), causing an increase in intracellular levels of cAMP which initiates a broad cascade of events including suppression of inflammatory transcription factor activities, alterations in nuclear localization of proteins, and induction of patterns of gene expression mediated through activity of the CREB transcription factor. These changes lead not only to reduced inflammatory events, but also contribute to neuroprotective actions of NA by increasing expression of neurotrophic substances including BDNF, GDNF, and NGF. These properties have prompted studies to determine if treatments with drugs to raise CNS NA levels could provide benefit in various neurological conditions and diseases having an inflammatory component. Moreover, increasing evidence shows that disruptions in endogenous NA levels occurs in several diseases and conditions including Alzheimer's disease (AD), Parkinson's disease (PD), Down's syndrome, posttraumatic stress disorder (PTSD), and multiple sclerosis (MS), suggesting that damage to NA producing neurons is a common factor that contributes to the initiation or progression of neuropathology. Methods to increase NA levels, or to reduce damage to noradrenergic neurons, therefore represent potential preventative as well as therapeutic approaches to disease.

Molecular basis of impaired glycogen metabolism during ischemic stroke and hypoxia

Background

Ischemic stroke is the combinatorial effect of many pathological processes including the loss of energy supplies, excessive intracellular calcium accumulation, oxidative stress, and inflammatory responses. The brain's ability to maintain energy demand through this process involves metabolism of glycogen, which is critical for release of stored glucose. However, regulation of glycogen metabolism in ischemic stroke remains unknown. In the present study, we investigate the role and regulation of glycogen metabolizing enzymes and their effects on the fate of glycogen during ischemic stroke.

Results

Ischemic stroke was induced in rats by peri-vascular application of the vasoconstrictor endothelin-1 and forebrains were collected at 1, 3, 6 and 24 hours post-stroke. Glycogen levels and the expression and activity of enzymes involved in glycogen metabolism were analyzed. We found elevated glycogen levels in the ipsilateral hemispheres compared with contralateral hemispheres at 6 and 24 hours (25% and 39% increase respectively; P<0.05). Glycogen synthase activity and glycogen branching enzyme expression were found to be similar between the ipsilateral, contralateral, and sham control hemispheres. In contrast, the rate-limiting enzyme for glycogen breakdown, glycogen phosphorylase, had 58% lower activity (P<0.01) in the ipsilateral hemisphere (24 hours post-stroke), which corresponded with a 48% reduction in cAMP-dependent protein kinase A (PKA) activity (P<0.01). In addition, glycogen debranching enzyme expression 24 hours post-stroke was 77% (P<0.01) and 72% lower (P<0.01) at the protein and mRNA level, respectively. In cultured rat primary cerebellar astrocytes, hypoxia and inhibition of PKA activity significantly reduced glycogen phosphorylase activity and increased glycogen accumulation but did not alter glycogen synthase activity. Furthermore, elevated glycogen levels provided metabolic support to astrocytes during hypoxia.

Conclusion

Our study has identified that glycogen breakdown is impaired during ischemic stroke, the molecular basis of which includes reduced glycogen debranching enzyme expression level together with reduced glycogen phosphorylase and PKA activity.

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta-protein (Ab) levels. However, there are problems with the cholesterol-AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD.Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well-established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Ab. The purpose of this review, therefore, was to examine the above-mentioned issues, discuss the pros and cons of the cholesterol-AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

β₂-adrenergic agonists modulate TNF-α induced astrocytic inflammatory gene expression and brain inflammatory cell populations

BACKGROUND

The NF-κB signaling pathway orchestrates many of the intricate aspects of neuroinflammation. Astrocytic β₂-adrenergic receptors have emerged as potential regulators in central nervous system inflammation and are potential targets for pharmacological modulation. The aim of this study was to elucidate the crosstalk between astrocytic β₂-adrenergic receptors and the TNF-α induced inflammatory gene program.

METHODS

Proinflammatory conditions were generated by the administration of TNF-α. Genes that are susceptible to astrocytic crosstalk between β₂-adrenergic receptors (stimulated by clenbuterol) and TNF-α were identified by qPCR-macroarray-based gene expression analysis in a human 1321 N1 astrocytoma cell line. Transcriptional patterns of the identified genes in vitro were validated by RT-PCR on the 1321 N1 cell line as well as on primary rat astrocytes. In vivo expression patterns were examined by intracerebroventricular administration of clenbuterol and/or TNF-α in rats. To examine the impact on the inflammatory cell content of the brain we performed extensive FACS analysis of rat brain immune cells after intracerebroventricular clenbuterol and/or TNF-α administration.

RESULTS

Parallel transcriptional patterns in vivo and in vitro confirmed the relevance of astrocytic β₂-adrenergic receptors as modulators of brain inflammatory responses. Importantly, we observed pronounced effects of β2-adrenergic receptor agonists and TNF-α on IL-6, CXCL2, CXCL3, VCAM1, and ICAM1 expression, suggesting a role in inflammatory brain cell homeostasis. Extensive FACS-analysis of inflammatory cell content in the brain demonstrated that clenbuterol/TNF-α co-administration skewed the T cell population towards a double negative phenotype and induced a shift in the myeloid brain cell population towards a neutrophilic predominance.

CONCLUSIONS

Our results show that astrocytic β₂-adrenergic receptors are potent regulators of astrocytic TNF-α-activated genes in vitro and in vivo, and ultimately modulate the molecular network involved in the homeostasis of inflammatory cells in the central nervous system. Astrocytic β₂-adrenergic receptors and their downstream signaling pathway may serve as potential targets to modulate neuroinflammatory responses.

Dynamics of β-adrenergic/cAMP signaling and morphological changes in cultured astrocytes

The morphology of astrocytes, likely regulated by cAMP, determines the structural association between astrocytes and the synapse, consequently modulating synaptic function. β-Adrenergic receptors (β-AR), which increase cytosolic cAMP concentration ([cAMP]i ), may affect cell morphology. However, the real-time dynamics of β-AR-mediated cAMP signaling in single live astrocytes and its effect on cell morphology have not been studied. We used the fluorescence resonance energy transfer (FRET)-based cAMP biosensor Epac1-camps to study time-dependent changes in [cAMP]i ; morphological changes in primary rat astrocytes were monitored by real-time confocal microscopy. Stimulation of β-AR by adrenaline, noradrenaline, and isoprenaline, a specific agonist of β-AR, rapidly increased [cAMP]i (∼15 s). The FRET signal response, mediated via β-AR, was faster than in the presence of forskolin (twofold) and dibutyryl-cAMP (>35-fold), which directly activate adenylyl cyclase and Epac1-camps, respectively, likely due to slow entry of these agents into the cytosol. Oscillations in [cAMP]i have not been recorded, indicating that cAMP-dependent processes operate in a slow time domain. Most Epac1-camps expressing astrocytes revealed a morphological change upon β-AR activation and attained a stellate morphology within 1 h. The morphological changes exhibited a bell-shaped dependency on [cAMP]i . The 5-10% decrease in cell cross-sectional area and the 30-50% increase in cell perimeter are likely due to withdrawal of the cytoplasm to the perinuclear region and the appearance of protrusions on the surface of astrocytes. Because astrocyte processes ensheath neurons, β-AR/cAMP-mediated morphological changes can modify the geometry of the extracellular space, affecting synaptic, neuronal, and astrocyte functions in health and disease.

Xylazine induced central antinociception mediated by endogenous opioids and μ-opioid receptor, but not δ-or κ-opioid receptors

Endogenous opioids have been implicated in compound-induced antinociception, and our group previously suggested that xylazine induces peripheral antinociception by releasing endogenous opioids that act on their respective receptors. In this study, we investigated the involvement of endogenous opioids in α2-adrenoceptor agonist xylazine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Probabilities less than 5% (p<0.05) were considered to be statistically significant (ANOVA/Bonferroni's test). Our results demonstrated that opioid receptor antagonist naloxone and μ-opioid receptor antagonist clocinnamox, but not δ-opioid receptor antagonist naltrindole and κ-opioid receptor antagonist nor-binaltorphimine, antagonized xylazine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ-opioid receptors in xylazine-induced central antinociception. In contrast, δ- and κ-opioid receptors do not appear to be involved in this effect. The results contribute to a greater understanding of the central antinociceptive mechanisms of a drug widely used in veterinary therapy.

β-adrenergic receptor inhibition affects cerebral glucose metabolism, motor performance, and inflammatory response after traumatic brain injury

BACKGROUND

The purpose of this study was to evaluate how β-adrenergic receptor inhibition after traumatic brain injury (TBI) alters changes in early cerebral glucose metabolism and motor performance, as well as cerebral cytokine and heat shock protein (HSP) expression.

METHODS

Mouse cerebral glucose metabolism was measured by microPET fluorodeoxyglucose uptake and converted into standardized uptake values (SUV). Four groups of C57/Bl6 mice (wild type [WT]) were initially evaluated: sham or TBI, followed by tail vein injection of either saline or a nonselective β-adrenergic receptor inhibitor (propranolol, 4 mg/kg). Then motor performance, cerebral cytokine, and HSP70 expression were studied at 12 hours and 24 hours after sham injury or TBI in WT mice treated with saline or propranolol and in β1-adrenergic/β2-adrenergic receptor knockout (BARKO) mice treated with saline.

RESULTS

Cerebral glucose metabolism was significantly reduced after TBI (mean SUV TBI, 1.63 vs. sham 1.97, p < 0.01) and propranolol attenuated this reduction (mean SUV propranolol, 1.89 vs. saline 1.63, p < 0.01). Both propranolol and BARKO reduced motor deficits at 24 hours after injury, but only BARKO had an effect at 12 hours after injury. TBI WT mice treated with saline performed worse than propranolol mice at 24 hours after injury on rotarod (23 vs. 44 seconds, p < 0.01) and rearing (130 vs. 338 events, p = 0.01) results. At 24 hours after injury, sham BARKO and TBI BARKO mice were similar on rotarod (21 vs. 19 seconds, p = 0.53), ambulatory testing (2,891 vs. 2,274 events, p = 0.14), and rearing (129 vs. 64 events, p = 0.09) results. Interleukin 1β expression was affected by BARKO and propranolol after TBI; attenuation of interleukin 6 and increased HSP70 expression were noted only with BARKO.

CONCLUSION

β-adrenergic receptor inhibition affects cerebral glucose metabolism, motor performance, as well as cerebral cytokine and HSP expression after TBI.

β2-adrenergic receptor and astrocyte glucose metabolism

Astrocyte glucose metabolism functions to maintain brain activity in both normal and stress conditions. Dysregulation of astrocyte glucose metabolism relates to development of neuronal disease, such as multiple sclerosis and Alzheimer's disease. In response to acute stress, beta2-adrenergic receptor is activated and initiates multiple signaling events mediated by Gs, Gi, arrestin, or other effectors depending on specific cellular contexts. In astrocytes, beta2-adrenergic receptor promotes glucose uptake through GLUT1 and accelerates glycogen degradation via coupling to Gs and second messenger cAMP-dependent pathway. Beta2-adrenergic receptor may regulate other steps in astrocyte glucose metabolism, such as lactate production or transduction. Inappropriate regulation of beta2-adrenergic receptor activity can disrupt normal glucose metabolism, and leads to accelerate neuronal disease development. It was demonstrated that the absence of beta2-adrenergic receptor in astrocytes occurred in multiple sclerosis patients, and the increased beta2-adrenergic receptor activity relates to Alzheimer's disease. A clear view of beta2-adrenergic receptor-mediated signaling pathways in regulating astrocyte glucose metabolism could help us to develop neuronal diseases treatment by targeting to the beta2-adrenergic receptor.

Neurotransmitter receptors and cognitive dysfunction in Alzheimer's disease and Parkinson's disease

Cognitive dysfunction is one of the most typical characteristics in various neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease (advanced stage). Although several mechanisms like neuronal apoptosis and inflammatory responses have been recognized to be involved in the pathogenesis of cognitive dysfunction in these diseases, recent studies on neurodegeneration and cognitive dysfunction have demonstrated a significant impact of receptor modulation on cognitive changes. The pathological alterations in various receptors appear to contribute to cognitive impairment and/or deterioration with correlation to diversified mechanisms. This article recapitulates the present understandings and concepts underlying the modulation of different receptors in human beings and various experimental models of Alzheimer's disease and Parkinson's disease as well as a conceptual update on the underlying mechanisms. Specific roles of serotonin, adrenaline, acetylcholine, dopamine receptors, and N-methyl-D-aspartate receptors in Alzheimer's disease and Parkinson's disease will be interactively discussed. Complex mechanisms involved in their signaling pathways in the cognitive dysfunction associated with the neurodegenerative diseases will also be addressed. Substantial evidence has suggested that those receptors are crucial neuroregulators contributing to cognitive pathology and complicated correlations exist between those receptors and the expression of cognitive capacities. The pathological alterations in the receptors would, therefore, contribute to cognitive impairments and/or deterioration in Alzheimer's disease and Parkinson's disease. Future research may shed light on new clues for the treatment of cognitive dysfunction in neurodegenerative diseases by targeting specific alterations in these receptors and their signal transduction pathways in the frontal-striatal, fronto-striato-thalamic, and mesolimbic circuitries.

AP-2β regulates amyloid beta-protein stimulation of apolipoprotein E transcription in astrocytes

Two key players involved in Alzheimer's disease (AD) are amyloid beta protein (Aβ) and apolipoprotein E (apoE). Aβ increases apoE protein levels in astrocytes which is associated with cholesterol trafficking, neuroinflammatory responses and Aβ clearance. The mechanism for the increase in apoE protein abundance is not understood. Based on different lines of evidence, we propose that the beta-adrenergic receptor (βAR), cAMP and the transcription factor activator protein-2 (AP-2) are contributors to the Aβ-induced increase in apoE abundance. This hypothesis was tested in mouse primary astrocytes and in cells transfected with an apoE promoter fragment with binding sites for AP-2. Aβ(42) induced a time-dependent increase in apoE mRNA and protein levels which were significantly inhibited by βAR antagonists. A novel finding was that Aβ incubation significantly reduced AP-2α levels and significantly increased AP-2β levels in the nuclear fraction. The impact of Aβ-induced translocation of AP-2 into the nucleus was demonstrated in cells expressing AP-2 and incubated with Aβ(42). AP-2 expressing cells had enhanced activation of the apoE promoter region containing AP-2 binding sites in contrast to AP-2 deficient cells. The transcriptional upregulation of apoE expression by Aβ(42) may be a neuroprotective response to Aβ-induced cytotoxicity, consistent with apoE's role in cytoprotection.

Lactate produced by glycogenolysis in astrocytes regulates memory processing

When administered either systemically or centrally, glucose is a potent enhancer of memory processes. Measures of glucose levels in extracellular fluid in the rat hippocampus during memory tests reveal that these levels are dynamic, decreasing in response to memory tasks and loads; exogenous glucose blocks these decreases and enhances memory. The present experiments test the hypothesis that glucose enhancement of memory is mediated by glycogen storage and then metabolism to lactate in astrocytes, which provide lactate to neurons as an energy substrate. Sensitive bioprobes were used to measure brain glucose and lactate levels in 1-sec samples. Extracellular glucose decreased and lactate increased while rats performed a spatial working memory task. Intrahippocampal infusions of lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis impaired memory and this impairment was reversed by administration of lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. Pharmacological block of the monocarboxylate transporter responsible for lactate uptake into neurons also impaired memory and this impairment was not reversed by either glucose or lactate. These findings support the view that astrocytes regulate memory formation by controlling the provision of lactate to support neuronal functions.

Adrenergic modulation of immune cells: an update

Sympathoadrenergic pathways are crucial to the communication between the nervous system and the immune system. The present review addresses emerging issues in the adrenergic modulation of immune cells, including: the specific pattern of adrenoceptor expression on immune cells and their role and changes upon cell differentiation and activation; the production and utilization of noradrenaline and adrenaline by immune cells themselves; the dysregulation of adrenergic immune mechanisms in disease and their potential as novel therapeutic targets. A wide array of sympathoadrenergic therapeutics is currently used for non-immune indications, and could represent an attractive source of non-conventional immunomodulating agents.

Studying G protein-coupled receptor activation using split-tobacco etch virus assays

G protein-coupled receptors (GPCRs) constitute the largest receptor family in mammals and represent important drug targets. Signaling through GPCRs mediates physiological effects that are strongly dependent on the cellular context. Therefore, the availability of assays monitoring GPCR activation applicable in different cell types could help to better understand GPCR functions and to realize the potential of known substances as well as novel ones. Here we introduce a split-TEV (tobacco etch virus) assay to monitor GPCR activation through the stimulation-dependent recruitment of β-arrestin 2. Inactive N- and C-terminal fragments of the TEV protease are coupled to a GPCR and β-arrestin 2, respectively. Ligand-dependent interaction of the two fusion proteins leads to functional complementation of the TEV protease, followed by the cleavage of an artificial transcription factor and successive reporter gene activation. The presented split-TEV assay system is highly sensitive and was successfully applied in heterologous cell lines as well as in primary cultured neuronal and glial cells. We show that assay performance strongly depends on the endogenous properties of different cell types. The sensitivity and flexibility make split-TEV assays a valuable tool to analyze GPCR activation in different cell types in a rapid and cost-effective way.

The dopamine β-hydroxylase -1021C/T polymorphism is associated with the risk of Alzheimer's disease in the Epistasis Project

Background

The loss of noradrenergic neurones of the locus coeruleus is a major feature of Alzheimer's disease (AD). Dopamine β-hydroxylase (DBH) catalyses the conversion of dopamine to noradrenaline. Interactions have been reported between the low-activity -1021T allele (rs1611115) of DBH and polymorphisms of the pro-inflammatory cytokine genes, IL1A and IL6, contributing to the risk of AD. We therefore examined the associations with AD of the DBH -1021T allele and of the above interactions in the Epistasis Project, with 1757 cases of AD and 6294 elderly controls.

Methods

We genotyped eight single nucleotide polymorphisms (SNPs) in the three genes, DBH, IL1A and IL6. We used logistic regression models and synergy factor analysis to examine potential interactions and associations with AD.

Results

We found that the presence of the -1021T allele was associated with AD: odds ratio = 1.2 (95% confidence interval: 1.06-1.4, p = 0.005). This association was nearly restricted to men < 75 years old: odds ratio = 2.2 (1.4-3.3, 0.0004). We also found an interaction between the presence of DBH -1021T and the -889TT genotype (rs1800587) of IL1A: synergy factor = 1.9 (1.2-3.1, 0.005). All these results were consistent between North Europe and North Spain.

Conclusions

Extensive, previous evidence (reviewed here) indicates an important role for noradrenaline in the control of inflammation in the brain. Thus, the -1021T allele with presumed low activity may be associated with misregulation of inflammation, which could contribute to the onset of AD. We suggest that such misregulation is the predominant mechanism of the association we report here.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.