A rapid and efficient immunoenzymatic assay to detect receptor protein interactions: G protein-coupled receptors

Functional Heterodimerization between the G Protein-Coupled Receptor GPR17 and the Chemokine Receptors 2 and 4: New Evidence

GPR17, a G protein-coupled receptor, is a pivotal regulator of myelination. Its endogenous ligands trigger receptor desensitization and downregulation allowing oligodendrocyte terminal maturation. In addition to its endogenous agonists, GPR17 could be promiscuously activated by pro-inflammatory oxysterols and chemokines released at demyelinating lesions. Herein, the chemokine receptors CXCR2 and CXCR4 were selected to perform both in silico modelling and in vitro experiments to establish their structural and functional interactions with GPR17. The relative propensity of GPR17 and CXCR2 or CXCR4 to form homo- and hetero-dimers was assessed by homology modelling and molecular dynamics (MD) simulations, and co-immunoprecipitation and immunoenzymatic assay. The interaction between chemokine receptors and GPR17 was investigated by determining receptor-mediated modulation of intracellular cyclic adenosine monophosphate (cAMP). Our data show the GPR17 association with CXCR2 or CXCR4 and the negative regulation of these interactions by CXCR agonists or antagonists. Moreover, GPR17 and CXCR2 heterodimers can functionally influence each other. In contrast, CXCR4 can influence GPR17 functionality, but not vice versa. According to MD simulations, all the dimers reached conformational stability and negative formation energy, confirming the experimental observations. The cross-talk between these receptors could play a role in the development of the neuroinflammatory milieu associated with demyelinating events.

Insulin-like Growth Factor 1 Promotes Cell Proliferation by Downregulation of G-Protein-Coupled Receptor 17 Expression via PI3K/Akt/FoxO1 Signaling in SK-N-SH Cells

Insulin-like growth factor 1 (IGF-1) not only regulates neuronal function and development but also is neuroprotective in the setting of acute ischemic stroke. G-protein-coupled receptor 17 (GPR17) expression in brain tissue serves as an indicator of brain damage. As whether IGF-1 regulates GPR17 expression remains unknown, the aim of this study is to investigate how IGF-1 regulates GPR17 expression in vitro. Human neuroblastoma SK-N-SH cells were used. Lentivirus-mediated short hairpin RNA (shRNA) was constructed to mediate the silencing of FoxO1, while adenoviral vectors were used for its overexpression. Verification of the relevant signaling cascade was performed using a FoxO1 inhibitor (AS1842856), a phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002), and a GPR17 antagonist (cangrelor). Cell proliferation was analyzed using EdU staining; immunofluorescence staining was used to detect the expression and subcellular localization of FoxO1. Chromatin immunoprecipitation was used to analyze the binding of FoxO1 to the GPR17 promoter in SK-N-SH cells. The expression of FoxO1, GPR17, and protein kinase B (also known as Akt) mRNA and protein as well as the levels of FoxO1 and Akt phosphorylation were investigated in this study. IGF-1 was found to downregulate FoxO1 and GPR17 expression in SK-N-SH cells while promoting cell viability and proliferation. Inhibition of FoxO1 and antagonism of GPR17 were found to play a role similar to that of IGF-1. Silencing of FoxO1 by lentivirus-mediated shRNA resulted in the downregulation of FoxO1 and GPR17 expression. The overexpression of FoxO1 via adenoviral vectors resulted in the upregulation of FoxO1 and GPR17 expression. Blocking of PI3K signaling by LY294002 inhibited the effect of IGF-1 on GPR17 suppression. Results from chromatin immunoprecipitation revealed that IGF-1 promotes FoxO1 nuclear export and reduces FoxO1 binding to the GPR17 promoter in SK-N-SH cells. Here, we conclude that IGF-1 enhances cell viability and proliferation in SK-N-SH cells via the promotion of FoxO1 nuclear export and reduction of FoxO1 binding to the GPR17 promoter via PI3K/Akt signaling. Our findings suggest that the enhancement of IGF-1 signaling to antagonize GPR17 serves as a potential therapeutic strategy in the management of acute ischemic stroke.

α-Synuclein Heteromers in Red Blood Cells of Alzheimer's Disease and Lewy Body Dementia Patients

BACKGROUND

Red blood cells (RBCs) contain the majority of α-synuclein (α-syn) in blood, representing an interesting model for studying the peripheral pathological alterations proved in neurodegeneration.

OBJECTIVE

The current study aimed to investigate the diagnostic value of total α-syn, amyloid-β (Aβ1-42), tau, and their heteroaggregates in RBCs of Lewy body dementia (LBD) and Alzheimer's disease (AD) patients compared to healthy controls (HC).

METHODS

By the use of enzyme-linked immunosorbent assays, RBCs concentrations of total α-syn, Aβ1-42, tau, and their heteroaggregates (α-syn/Aβ1-42 and α-syn/tau) were measured in 27 individuals with LBD (Parkinson's disease dementia, n = 17; dementia with Lewy bodies, n = 10), 51 individuals with AD (AD dementia, n = 37; prodromal AD, n = 14), and HC (n = 60).

RESULTS

The total α-syn and tau concentrations as well as α-syn/tau heterodimers were significantly lower in the LBD group and the AD group compared with HC, whereas α-syn/Aβ1-42 concentrations were significantly lower in the AD dementia group only. RBC α-syn/tau heterodimers had a higher diagnostic accuracy for differentiating patients with LBD versus HC (AUROC = 0.80).

CONCLUSION

RBC α-syn heteromers may be useful for differentiating between neurodegenerative dementias (LBD and AD) and HC. In particular, RBC α-syn/tau heterodimers have demonstrated good diagnostic accuracy for differentiating LBD from HC. However, they are not consistently different between LBD and AD. Our findings also suggest that α-syn, Aβ1-42, and tau interact in vivo to promote the aggregation and accumulation of each other.

Prodromal Intestinal Events in Alzheimer's Disease (AD): Colonic Dysmotility and Inflammation Are Associated with Enteric AD-Related Protein Deposition

Increasing evidence suggests that intestinal dysfunctions may represent early events in Alzheimer’s disease and contribute to brain pathology. This study examined the relationship between onset of cognitive impairment and colonic dysfunctions in a spontaneous AD model before the full development of brain pathology. SAMP8 mice underwent Morris water maze and assessment of faecal output at four, six and eight months of age. In vitro colonic motility was examined. Faecal and colonic Aβ, tau proteins, α-synuclein and IL-1β were assessed by ELISA. Colonic citrate synthase activity was assessed by spectrophotometry. Colonic NLRP3, caspase-1 and ASC expression were evaluated by Western blotting. Colonic eosinophil density and claudin-1 expression were evaluated by immunohistochemistry. The effect of Aβ on NLRP3 signalling and mitochondrial function was tested in cultured cells. Cognitive impairment and decreased faecal output occurred in SAMP8 mice from six months. When compared with SAMR1, SAMP8 animals displayed: (1) impaired in vitro colonic contractions; (2) increased enteric AD-related proteins, IL-1β, active-caspase-1 expression and eosinophil density; and (3) decreased citrate synthase activity and claudin-1 expression. In THP-1 cells, Aβ promoted IL-1β release, which was abrogated upon incubation with caspase-1 inhibitor or in ASC^-/- cells. Aβ decreased mitochondrial function in THP-1 cells. In SAMP8, enteric AD-related proteins deposition, inflammation and impaired colonic excitatory neurotransmission, occurring before the full brain pathology development, could contribute to bowel dysmotility and represent prodromal events in AD.

High Levels of β-Amyloid, Tau, and Phospho-Tau in Red Blood Cells as Biomarkers of Neuropathology in Senescence-Accelerated Mouse

Alzheimer's Disease (AD) is the most common Neurodegenerative Disease (ND), primarily characterised by neuroinflammation, neuronal plaques of β-amyloid (Aβ), and neurofibrillary tangles of hyperphosphorylated tau. α-Synuclein (α-syn) and its heteroaggregates with Aβ and tau have been recently included among the neuropathological elements of NDs. These pathological traits are not restricted to the brain, but they reach peripheral fluids as well. In this sense, Red Blood Cells (RBCs) are emerging as a good model to investigate the biochemical alterations of aging and NDs. Herein, the levels of homo- and heteroaggregates of ND-related proteins were analysed at different stages of disease progression. In particular, a validated animal model of AD, the SAMP8 (Senescence-Accelerated Mouse-Prone) and its control strain SAMR1 (Senescence-Accelerated Mouse-Resistant) were used in parallel experiments. The levels of the aforementioned proteins and of the inflammatory marker interleukin-1β (IL-1β) were examined in both brain and RBCs of SAMP8 and SAMR1 at 6 and 8 months. Brain Aβ, tau, and phospho-tau (p-tau) were higher in SAMP8 mice than in control mice and increased with AD progression. Similar accumulation kinetics were found in RBCs, even if slower. By contrast, α-syn and its heterocomplexes (α-syn-Aβ and α-syn-tau) displayed different accumulation kinetics between brain tissue and RBCs. Both brain and peripheral IL-1β levels were higher in SAMP8 mice, but increased sooner in RBCs, suggesting that inflammation might initiate at a peripheral level before affecting the brain. In conclusion, these results confirm RBCs as a valuable model for monitoring neurodegeneration, suggesting peripheral Aβ, tau, and p-tau as potential early biomarkers of AD.

Potential Diagnostic Value of Red Blood Cells α-Synuclein Heteroaggregates in Alzheimer's Disease

A plethora of complex misfolded protein combinations have been found in Alzheimer disease (AD) brains besides the classical pathological hallmarks. Recently, α-synuclein (α-syn) and its heterocomplexes with amyloid-β (Aβ) and tau have been suggested to be involved in the pathophysiological processes of neurodegenerative diseases. These pathological features are not limited to the brain, but can be also found in peripheral fluids. In this respect, red blood cells (RBCs) have been suggested as a good model to investigate the biochemical alterations of neurodegeneration. Our aim is to find whether RBC concentrations of α-syn and its heterocomplexes (i.e., α-syn/Aβ and α-syn/tau) were different in AD patients compared with healthy controls (HC). The levels of homo- and heteroaggregates of α-syn, Aβ and tau, were analyzed in a cohort of AD patients at early stage either with dementia or prodromal symptoms (N = 39) and age-matched healthy controls (N = 39). All AD patients received a biomarker-based diagnosis (low cerebrospinal fluid levels of Aβ peptide combined with high cerebrospinal fluid concentrations of total tau and/or phospho-tau proteins; alternatively, a positivity to cerebral amyloid-PET scan). Our results showed lower concentrations of α-syn and its heterocomplexes (i.e., α-syn/Aβ and α-syn/tau) in RBCs of AD patients with respect to HC. RBC α-syn/Aβ as well as RBC α-syn/tau heterodimers discriminated AD participants from HC with fair accuracy, whereas RBC α-syn concentrations differentiated poorly the two groups. Although additional investigations are required, these data suggest α-syn heteroaggregates in RBCs as potential tool in the diagnostic work-up of early AD diagnosis.

α-Synuclein Heterocomplexes with β-Amyloid Are Increased in Red Blood Cells of Parkinson's Disease Patients and Correlate with Disease Severity

Neurodegenerative disorders (NDs) are characterized by abnormal accumulation/misfolding of specific proteins, primarily α-synuclein (α-syn), β-amyloid1-42 (Aβ1-42) and tau, in both brain and peripheral tissues. In addition to oligomers, the role of the interactions of α-syn with Aβ or tau has gradually emerged. Nevertheless, despite intensive research, NDs have no accepted peripheral markers for biochemical diagnosis. In this respect, Red Blood Cells (RBCs) are emerging as a valid peripheral model for the study of aging-related pathologies. Herein, a small cohort (N = 28) of patients affected by Parkinson's disease (PD) and age-matched controls were enrolled to detect the content of α-syn (total and oligomeric), Aβ1-42 and tau (total and phosphorylated) in RBCs. Moreover, the presence of α-syn association with tau and Aβ1-42 was explored by co-immunoprecipitation/western blotting in the same cells, and quantitatively confirmed by immunoenzymatic assays. For the first time, PD patients were demonstrated to exhibit α-syn heterocomplexes with Aβ1-42 and tau in peripheral tissues; interestingly, α-syn-Aβ1-42 concentrations were increased in PD subjects with respect to healthy controls (HC), and directly correlated with disease severity and motor deficits. Moreover, total-α-syn levels were decreased in PD subjects and inversely related to their motor deficits. Finally, an increase of oligomeric-α-syn and phosphorylated-tau was observed in RBCs of the enrolled patients. The combination of three parameters (total-α-syn, phosphorylated-tau and α-syn-Aβ1-42 concentrations) provided the best fitting predictive index for discriminating PD patients from controls. Nevertheless further investigations should be required, overall, these data suggest α-syn hetero-aggregates in RBCs as a putative tool for the diagnosis of PD.

α-Synuclein Aggregates with β-Amyloid or Tau in Human Red Blood Cells: Correlation with Antioxidant Capability and Physical Exercise in Human Healthy Subjects

Neurodegenerative disorders (NDs) are characterized by abnormal accumulation/misfolding of specific proteins, primarily α-synuclein (α-syn), β-amyloid_1-42 (Aβ), and tau, in both brain and peripheral tissue. In addition to homo-oligomers, the role of α-syn interactions with Aβ or tau has gradually emerged. The altered protein accumulation has been related to both oxidative stress and physical activity; nevertheless, no correlation among the presence of peripheral α-syn hetero-aggregates, antioxidant capacity, and physical exercise has been discovered as of yet. Herein, the content of α-syn, Aβ, tau, and of their heterocomplexes was determined in red blood cells (RBCs) of healthy subjects (sedentary and athletes). Such parameters were related to the extent of the antioxidant capability (AOC), a key marker of oxidative stress in aging-related pathologies, and to physical exercise, which is known to play an important preventive role in NDs and to modulate oxidative stress. Tau content and plasma AOC toward hydroxyl radicals were both reduced in older or sedentary subjects; in contrast, α-syn and Aβ accumulated in elderly subjects and showed an inverse correlation with both hydroxyl AOC and the level of physical activity. For the first time, α-syn heterocomplexes with Aβ or tau were quantified and demonstrated to be inversely related to hydroxyl AOC. Furthermore, α-syn/Aβ aggregates were significantly reduced in athletes and inversely correlated with physical activity level, independent of age. The positive correlation between antioxidant capability/physical activity and reduced protein accumulation was confirmed by these data and suggested that peripheral α-syn heterocomplexes may represent new indicators of ND-related protein misfolding.

Osteogenesis Is Improved by Low Tumor Necrosis Factor Alpha Concentration through the Modulation of Gs-Coupled Receptor Signals

In the early phase of bone damage, low concentrations of the cytokine tumor necrosis factor alpha (TNF-α) favor osteoblast differentiation. In contrast, chronic high doses of the same cytokine contribute to bone loss, demonstrating opposite effects depending on its concentration and on the time of exposure. In the bone microenvironment, TNF-α modulates the expression/function of different G protein-coupled receptors (GPCRs) and of their regulatory proteins, GPCR-regulated kinases (GRKs), thus dictating their final biological outcome in controlling bone anabolic processes. Here, the effects of TNF-α were investigated on the expression/responsiveness of the A_2B adenosine receptor (A_2BAR), a Gs-coupled receptor that promotes mesenchymal stem cell (MSC) differentiation into osteoblasts. Low TNF-α concentrations exerted a prodifferentiating effect on MSCs, pushing them toward an osteoblast phenotype. By regulating GRK2 turnover and expression, the cytokine impaired A_2BAR desensitization, accelerating receptor-mediated osteoblast differentiation. These data supported the anabolic effect of TNF-α submaximal concentration and demonstrated that the cytokine regulates GPCR responses by interfering with the receptor desensitization machinery, thereby enhancing the anabolic responses evoked by A_2BAR ligands. Overall, these results indicated that GPCR desensitization plays a pivotal role in osteogenesis and that its manipulation is an effective strategy to favor bone remodeling.

Long lasting MDM2/Translocator protein modulator: a new strategy for irreversible apoptosis of human glioblastoma cells

The development of multi-target drugs and irreversible modulators of deregulated signalling proteins is the major challenge for improving glioblastoma multiforme (GBM) treatment. Reversible single-target drugs are not sufficient to sustain a therapeutic effect over time and may favour the activation of alternative signalling pathways and the onset of resistance phenomena. Thus, a multi-target compound that has a long-lasting mechanism of action might have a greater and longer life span of anti-proliferative activity. Recently, a dual-target indol-3ylglyoxyldipeptide derivative, designed to bind to the Translocator Protein (TSPO) and reactivate p53 function via dissociation from its physiological inhibitor, murine double minute 2 (MDM2), has been developed as a potent GBM pro-apoptotic agent. In this study, this derivative was chemically modified to irreversibly bind MDM2 and TSPO. The new compound elicited a TSPO-mediated mitochondrial membrane dissipation and restored p53 activity, triggering a long-lasting apoptosis of GBM cells. These effects were sustained over time, involved a stable activation of extracellular signal regulated kinases and were specifically observed in cancer cells, in which these protein kinases are deregulated. Dual-targeting and irreversible binding properties combined in the same molecule may represent a useful strategy to overcome the time-limited effects elicited by classical chemotherapies.

Cannabinoid Receptor Interacting Protein 1a Competition with β-Arrestin for CB1 Receptor Binding Sites

Cannabinoid receptor interacting protein 1a (CRIP1a) is a CB₁ receptor (CB₁R) distal C-terminal-associated protein that alters CB₁R interactions with G-proteins. We tested the hypothesis that CRIP1a is capable of also altering CB₁R interactions with β-arrestin proteins that interact with the CB₁R at the C-terminus. Coimmunoprecipitation studies indicated that CB₁R associates in complexes with either CRIP1a or β-arrestin, but CRIP1a and β-arrestin fail to coimmunoprecipitate with each other. This suggests a competition for CRIP1a and β-arrestin binding to the CB₁R, which we hypothesized could attenuate the action of β-arrestin to mediate CB₁R internalization. We determined that agonist-mediated reduction of the density of cell surface endogenously expressed CB₁Rs was clathrin and dynamin dependent and could be modeled as agonist-induced aggregation of transiently expressed GFP-CB₁R. CRIP1a overexpression attenuated CP55940-mediated GFP-CB₁R as well as endogenous β-arrestin redistribution to punctae, and conversely, CRIP1a knockdown augmented β-arrestin redistribution to punctae. Peptides mimicking the CB₁R C-terminus could bind to both CRIP1a in cell extracts as well as purified recombinant CRIP1a. Affinity pull-down studies revealed that phosphorylation at threonine-468 of a CB₁R distal C-terminus 14-mer peptide reduced CB₁R-CRIP1a association. Coimmunoprecipitation of CB₁R protein complexes demonstrated that central or distal C-terminal peptides competed for the CB₁R association with CRIP1a, but that a phosphorylated central C-terminal peptide competed for association with β-arrestin 1, and phosphorylated central or distal C-terminal peptides competed for association with β-arrestin 2. Thus, CRIP1a can compete with β-arrestins for interaction with C-terminal CB₁R domains that could affect agonist-driven, β-arrestin-mediated internalization of the CB₁R.

The ubiquitin ligase Mdm2 controls oligodendrocyte maturation by intertwining mTOR with G protein-coupled receptor kinase 2 in the regulation of GPR17 receptor desensitization

During oligodendrocyte precursor cell (OPC) differentiation, defective control of the membrane receptor GPR17 has been suggested to block cell maturation and impair remyelination under demyelinating conditions. After the immature oligodendrocyte stage, to enable cells to complete maturation, GPR17 is physiologically down-regulated via phosphorylation/desensitization by G protein-coupled receptor kinases (GRKs); conversely, GRKs are regulated by the "mammalian target of rapamycin" mTOR. However, how GRKs and mTOR are connected to each other in modulating GPR17 function and oligodendrogenesis has remained elusive. Here we show, for the first time, a role for Murine double minute 2 (Mdm2), a ligase previously involved in ubiquitination/degradation of the onco-suppressor p53 protein. In maturing OPCs, both rapamycin and Nutlin-3, a small molecule inhibitor of Mdm2-p53 interactions, increased GRK2 sequestration by Mdm2, leading to impaired GPR17 down-regulation and OPC maturation block. Thus, Mdm2 intertwines mTOR with GRK2 in regulating GPR17 and oligodendrogenesis and represents a novel actor in myelination.