Role of purinergic receptors in the Alzheimer's disease

P2X7 Receptor Antagonism as a Potential Therapy in Amyotrophic Lateral Sclerosis

This review focuses on the purinergic ionotropic receptor P2X7 (P2X7R) as a potential target for developing drugs that delay the onset and/or disease progression in patients with amyotrophic lateral sclerosis (ALS). Description of clinical and genetic ALS features is followed by an analysis of advantages and drawbacks of transgenic mouse models of disease based on mutations in a bunch of proteins, particularly Cu/Zn superoxide dismutase (SOD1), TAR-DNA binding protein-43 (TDP-43), Fused in Sarcoma/Translocated in Sarcoma (FUS), and Chromosome 9 open reading frame 72 (C9orf72). Though of limited value, these models are however critical to study the proof of concept of new compounds, before reaching clinical trials. The authors also provide a description of ALS pathogenesis including protein aggregation, calcium-dependent excitotoxicity, dysfunction of calcium-binding proteins, ultrastructural mitochondrial alterations, disruption of mitochondrial calcium handling, and overproduction of reactive oxygen species (ROS). Understanding disease pathogenic pathways may ease the identification of new drug targets. Subsequently, neuroinflammation linked with P2X7Rs in ALS pathogenesis is described in order to understand the rationale of placing the use of P2X7R antagonists as a new therapeutic pharmacological approach to ALS. This is the basis for the hypothesis that a P2X7R blocker could mitigate the neuroinflammatory state, indirectly leading to neuroprotection and higher motoneuron survival in ALS patients.

State-of-the-Art: Inflammatory and Metabolic Markers in Mood Disorders

Mounting evidence highlights the involvement of inflammatory/immune systems and their relationships with neurotransmitters and different metabolic processes in mood disorders. Nevertheless, there is a general agreement that available findings are still inconclusive. Therefore, further investigations are required, aimed at deepening the role of possible alterations of biomarkers in the pathophysiology of mood disorders that might lead to more focused and tailored treatments. The present study is a comprehensive review on these topics that seem to represent intriguing avenues for the development of real innovative therapeutic strategies of mood disorders.

A Role of Low-Density Lipoprotein Receptor-Related Protein 4 (LRP4) in Astrocytic Aβ Clearance

Amyloid-β (Aβ) deposition occurs years before cognitive symptoms appear and is considered a cause of Alzheimer's disease (AD). The imbalance of Aβ production and clearance leads to Aβ accumulation and Aβ deposition. Increasing evidence indicates an important role of astrocytes, the most abundant cell type among glial cells in the brain, in Aβ clearance. We explored the role of low-density lipoprotein receptor-related protein 4 (LRP4), a member of the LDLR family, in AD pathology. We show that Lrp4 is specifically expressed in astrocytes and its levels in astrocytes were higher than those of Ldlr and Lrp1, both of which have been implicated in Aβ uptake. LRP4 was reduced in postmortem brain tissues of AD patients. Genetic deletion of the Lrp4 gene augmented Aβ plaques in 5xFAD male mice, an AD mouse model, and exacerbated the deficits in neurotransmission, synchrony between the hippocampus and PFC, and cognition. Mechanistically, LRP4 promotes Aβ uptake by astrocytes likely by interacting with ApoE. Together, our study demonstrates that astrocytic LRP4 plays an important role in Aβ pathology and cognitive function.SIGNIFICANCE STATEMENT This study investigates how astrocytes, a type of non-nerve cells in the brain, may contribute to Alzheimer's disease (AD) development. We demonstrate that the low-density lipoprotein receptor-related protein 4 (LRP4) is reduced in the brain of AD patients. Mimicking the reduced levels in an AD mouse model exacerbates cognitive impairment and increases amyloid aggregates that are known to damage the brain. We show that LRP4 could promote the clearance of amyloid protein by astrocytes. Our results reveal a previously unappreciated role of LRP4 in AD development.

Zebrafish models: Gaining insight into purinergic signaling and neurological disorders

Zebrafish (Danio rerio) has been considered a complementary model for biomedical studies, especially due to advantages such as external and rapid development, and genetic manipulation. There is growing interest in this model in neuroscience research since the species has morphological and physiological similarities to mammals and a complex behavioral repertoire. The purinergic signaling has been described in zebrafish, and purinoceptors and nucleotide- and nucleoside-metabolizing enzymes have already been identified in the central nervous system (CNS) of this species. The involvement of the purinergic system in several models of neurological disorders, such as Alzheimers disease, Parkinson's disease, epilepsy, schizophrenia, and autism has been investigated in zebrafish. This mini review presents several studies describing purinergic signaling in the zebrafish CNS and the action of this neurotransmitter system in models of neurological disorders using this species as a biological model. The use of pharmacological approaches at different stages of development may be a useful tool for preclinical assays and the testing of purinergic compounds as new alternatives for the treatment of neurological disorders.

Astroglial connexins and cognition: memory formation or deterioration?

Connexins are the membrane proteins that form high-conductance plasma membrane channels and are the important constituents of gap junctions and hemichannels. Among different types of connexins, connexin 43 is the most widely expressed and studied gap junction proteins in astrocytes. Due to the key involvement of astrocytes in memory impairment and abundant expression of connexins in astrocytes, astroglial connexins have been projected as key therapeutic targets for Alzheimer's disease. On the other hand, the role of connexin gap junctions and hemichannels in memory formation and consolidation has also been reported. Moreover, deletion of these proteins and loss of gap junction communication result in loss of short-term spatial memory. Accordingly, both memory formation and memory deteriorating functions of astrocytes-located connexins have been documented. Physiologically expressed connexins may be involved in the memory formation, while pathologically increased expression of connexins with consequent excessive activation of astrocytes may induce neuronal injury and cognitive decline. The present review describes the memory formation as well as memory deteriorating functions of astroglial connexins in memory disorders of different etiology with possible mechanisms.

The role of P2Y6R in cardiovascular diseases and recent development of P2Y6R antagonists

As a member of the P2Y receptor family with a typical 7-transmembrane structure, P2Y₆ purinergic receptor (P2Y₆R) belongs to the G-protein-coupled nucleotide receptor activating the phospholipase-C signaling pathway. P2Y₆R is widely involved in a range of human diseases, including atherosclerosis and other cardiovascular diseases, gradually attracting attention owing to its inappropriate or excessive activation. In addition, it was reported that P2Y₆R might regulate inflammatory responses by governing the maturation and secretion of proinflammatory cytokines. Hence, several P2Y₆R antagonists have been subjected to evaluation as new therapeutic strategies in recent years. This review was aimed at summarizing the role of P2Y₆R in the pathogenesis of cardiovascular diseases, with an insight into the recent progress on discovery of P2Y₆R antagonists.

Comprehensive quantification of purine and pyrimidine metabolism in Alzheimer's disease postmortem cerebrospinal fluid by LC-MS/MS with metal-free column

The metabolome presence of nucleobases, nucleosides, nucleotides and related phosphorylated metabolites has been examined for Alzheimer's disease (AD). Although reversed-phase liquid chromatography tandem mass spectrometry (LC-MS/MS) has been used for the determination of these analytes, they were limited in chromatographic signal intensity and reproducibility owing to significant peak tailing caused by complexing with metallic cations and phosphate groups. In this work, we applied LC-MS/MS analysis with a metal-free column for comprehensive quantification of 40 analytes regarding to purine and pyrimidine metabolism in postmortem cerebrospinal fluid (pCSF) from AD patients. For the analytical column, an InertSustain AQ-C₁₈ metal-free PEEK column was used. MS detection was by electrospray positive ionization. The metal-free column allowed for sharp peak detection of highly polar metabolites within a running time of 17 min. In validation, the limits of detection (LOD), the limit of quantitation (LOQ) and recovery value using a pooled pCSF sample are 1-500  nM, 0.5-250 nM and a range of 53.1-144.0% (RSD ranged from 0.4 to 19.6%). The developed LC-MS/MS method utilizing a metal-free column provides an accurate quantification of some metabolites regarding purine and pyrimidine metabolism in pCSF samples obtained from AD patients.

Role of GPCR signaling and calcium dysregulation in Alzheimer's disease

Alzheimer's disease (AD), a late onset neurodegenerative disorder is characterized by the loss of memory, disordered cognitive function, caused by accumulation of amyloid-β (Aβ) peptide and neurofibrillary tangles (NFTs) in the neocortex and hippocampal brain area. Extensive research has been done on the findings of the disease etiology or pathological causes of aggregation of Aβ and hyperphosphorylation of tau protein without much promising results. Recently, calcium dysregulation has been reported to play an important role in the pathophysiology of AD. Calcium ion acts as one of the major secondary messengers, regulates many signaling pathways involved in cell survival, proliferation, differentiation, transcription and apoptosis. Calcium signaling is one of the major signaling pathways involved in the formation of memory, generation of energy and other physiological functions. It also can modulate function of many proteins upon binding. Dysregulation in calcium homeostasis leads to many physiological changes leading to neurodegenerative diseases including AD. In AD, GPCRs generate secondary messengers which regulate calcium homeostasis inside the cell and is reported to be disturbed in the pathological condition. Calcium channels and receptors present on the plasma membrane and intracellular organelle maintain calcium homeostasis through different signaling mechanisms. In this review, we have summarized the different calcium channels and receptors involved in calcium dysregulation which in turn play a critical role in the pathogenesis of AD. Understanding the role of calcium channels and GPCRs to maintain calcium homeostasis is an attempt to develop effective AD treatments.

Differential role of adenosine signaling cascade in acute and chronic pain

Adenosine is a signaling molecule induced under stress such as energy insufficiency and ischemic/hypoxic conditions. Adenosine controls multiple physiological and pathological cellular and tissue function by activation of four G protein-coupled receptors (GPCR). Functional role of adenosine signaling in acute pain has been widely studied. However, the role of adenosine signaling in chronic pain is poorly understood. At acute levels, adenosine can be beneficial to anti-pain whereas a sustained elevation of adenosine can be detrimental to promote chronic pain. In recent years, extensive progress has been made to define the role of adenosine signaling in chronic pain and to dissect molecular new insight underlying the development of chronic pain. In this review, we summarize the differential role of adenosine signaling cascade in acute and chronic pain with a major focus on recent studies revealing adenosine ADORA2B receptor activation in the pathology of chronic pain. We further provide a therapeutic outlook of how multiple adenosine signaling components can be useful to treat chronic pain.

Mapping the Site of Action of Human P2X7 Receptor Antagonists AZ11645373, Brilliant Blue G, KN-62, Calmidazolium, and ZINC58368839 to the Intersubunit Allosteric Pocket

The P2X7 receptor is a trimeric ligand-gated ion channel activated by ATP. It is implicated in the cellular response to trauma/disease and considered to have significant therapeutic potential. Using chimeras and point mutants we have mapped the binding site of the P2X7R-selective antagonist AZ11645373 to the known allosteric binding pocket at the interface between two subunits, in proximity to, but separated from the ATP binding site. Our structural model of AZ11645373 binding is consistent with effects of mutations on antagonist sensitivity, and the proposed binding mode explains variation in antagonist sensitivity between the human and rat P2X7 receptors. We have also determined the site of action for the P2X7R-selective antagonists ZINC58368839, brilliant blue G, KN-62, and calmidazolium. The effect of intersubunit allosteric pocket “signature mutants” F88A, T90V, D92A, F103A, and V312A on antagonist sensitivity suggests that ZINC58368839 comprises a binding mode similar to AZ11645373 and other previously characterized antagonists. For the larger antagonists, brilliant blue G, KN-62, and calmidazolium, our data imply an overlapping but distinct binding mode involving the central upper vestibule of the receptor in addition to the intersubunit allosteric pocket. Our work explains the site of action for a series of P2X7R antagonists and establishes “signature mutants” for P2X7R binding-mode characterization.

Protein misfolding, aggregation and mechanism of amyloid cytotoxicity: An overview and therapeutic strategies to inhibit aggregation

Protein and peptides are converted from their soluble forms into highly ordered fibrillar aggregates under various conditions inside the cell. Such transitions confer diverse neurodegenerative diseases including Alzheimer's disease, Huntington's disease Prion's disease, Parkinson's disease, polyQ and share abnormal folding of potentially cytotoxic protein species linked with degeneration and death of precise neuronal populations. Presently, major advances are made to understand and get detailed insight into the structural basis and mechanism of amyloid formation, cytotoxicity and therapeutic approaches to combat them. Here we highlight classifies and summarizes the detailed overview of protein misfolding and aggregation at their molecular level including the factors that promote protein aggregation under in vivo and in vitro conditions. In addition, we describe the recent technologies that aid the characterization of amyloid aggregates along with several models that might be responsible for amyloid induced cytotoxicity to cells. Overview on the inhibition of amyloidosis by targeting different small molecules (both natural and synthetic origin) have been also discussed, that provides important approaches to identify novel targets and develop specific therapeutic strategies to combat protein aggregation related neurodegenerative diseases.