Cell-Type-Specific Mitochondrial Quality Control in the Brain: A Plausible Mechanism of Neurodegeneration.

Neurodegeneration is an age-dependent progressive phenomenon with no defined cause. Aging is the main risk factor for neurodegenerative diseases. During aging, activated microglia undergo phenotypic alterations that can lead to neuroinflammation, which is a well-accepted event in the pathogenesis of neurodegenerative diseases. Several common mechanisms are shared by genetically or pathologically distinct neurodegenerative diseases, such as excitotoxicity, mitochondrial deficits and oxidative stress, protein misfolding and translational dysfunction, autophagy and microglia activation. Progressive loss of the neuronal population due to increased oxidative stress leads to neurodegenerative diseases, mostly due to the accumulation of dysfunctional mitochondria. Mitochondrial dysfunction and excessive neuroinflammatory responses are both sufficient to induce pathology in age-dependent neurodegeneration. Therefore, mitochondrial quality control is a key determinant for the health and survival of neuronal cells in the brain. Research has been primarily focused to demonstrate the significance of neuronal mitochondrial health, despite the important contributions of non-neuronal cells that constitute a significant portion of the brain volume. Moreover, mitochondrial morphology and function are distinctly diverse in different tissues; however, little is known about their molecular diversity among cell types. Mitochondrial dynamics and quality in different cell types markedly decide the fate of overall brain health; therefore, it is not justifiable to overlook non-neuronal cells and their significant and active contribution in facilitating overall neuronal health. In this review article, we aim to discuss the mitochondrial quality control of different cell types in the brain and how important and remarkable the diversity and highly synchronized connecting property of non-neuronal cells are in keeping the neurons healthy to control neurodegeneration.

A2A R mediated modulation in IP3 levels altering the [Ca2+]i through cAMP-dependent PKA signalling pathway.

Stimulation of A2A receptors (A2A R) coupled to Gs/olf protein activates Adenylyl cyclase (AC) leading to the release of cAMP which activates the cAMP-dependent PKA phosphorylation. The possible role of A2A R in the modulation of free cytosolic Ca2+ concentration ([Ca2+]i) involving IP3, cAMP and PKA was investigated in HEK 293-A2A R. The levels of IP3 and cAMP were observed by enzyme immunoassay detection method and [Ca2+]i using Fluo-4 AM. Moreover, cAMP-dependent PKA was determined using the PKA Colorimetric Activity Kit. We observed that the cells pre-treated with A2A R agonist NECA showed increased levels of cAMP, PKA, IP3 and [Ca2+]i levels. However, the reverse effect was observed with A2A R antagonists (ZM241385 and caffeine). Blocking the Gαq/PLC/DAG/IP3 pathway with neomycin, a PLC inhibitor did not affect the modulation of IP3 and [Ca2+]i levels in HEK 293-A2A R cells. To investigate the Gαi/AC/cAMP/PKA, HEK 293-A2A R cells pre-treated with pertussis toxin followed by forskolin in the presence of A2A R agonist (NECA) showed no effect on cAMP levels. Further, Gαs/AC/cAMP/PKA pathway was investigated to elucidate the role of cAMP-dependent PKA in IP3 mediated [Ca2+]i modulation. In the HEK 293-A2A R cells pre-treated with PKA inhibitor KT5720 and treated with NECA led to inhibit the IP3 and [Ca2+]i levels. The study distinctly demonstrated that A2A R modulates IP3 levels to release the [Ca2+]i via cAMP-dependent PKA. The role of A2A R mediated Gαs pathway inducing IP3 mediated [Ca2+]i release may open new avenues in the therapy of neurodegenerative disorder.

BACE1 Inhibition Increases Susceptibility to Oxidative Stress by Promoting Mitochondrial Damage.

BACE1 is a key enzyme facilitating the generation of neurotoxic ß-amyloid (Aß) peptide. However, given that BACE1 has multiple substrates we explored the importance of BACE1 in the maintenance of retinal pigment epithelial (RPE) cell homeostasis under oxidative stress. Inhibition of BACE1 reduced mitochondrial membrane potential, increased mitochondrial fragmentation, and increased cleaved caspase-3 expression in cells under oxidative stress. BACE1 inhibition also resulted in significantly lower levels of mitochondrial fusion proteins OPA1 and MFN1 suggesting a higher rate of mitochondrial fission while increasing the levels of mitophagic proteins Parkin and PINK1 and autophagosome numbers. In contrast, BACE2 had minimal effect on cellular response to oxidative stress. In summary, our results emphasize the importance of BACE1 in augmenting cellular defense against oxidative stress by protecting mitochondrial dynamics.

Half-life of DISC1 protein and its pathological significance under hypoxia stress.

DISC1 (disrupted in schizophrenia 1) is an intracellular scaffolding molecule which regulates multiple signaling pathways for neural cell differentiation and function. Many biological studies utilizing animal models of DISC1 have indicated that loss of DISC1 functions are associated with pathological psychiatric conditions. Thus, DISC1 protein stability is a prerequisite to its goal in governing neural function, and modulating the protein stability of DISC1 may be a key target for understanding underlying pathology, as well promising drug discovery strategies. Nonetheless, a half-life of DISC1 protein has remained unexplored. Here, we determine for the first time the half-life of DISC1, which are regulated by ubiquitin-proteasome cascade. Overexpression of PDE4B2, a binding partner of DISC1, prolonged the half-life of DISC1, whereas NDEL1 does not alter DISC1 protein stability. Notably, the half-life of DISC1 is diminished under hypoxia stress by increasing protein degradation of DISC1, suggesting that alteration of DISC1 stability may be involved in hypoxia stress-mediated pathological conditions, such as ischemic stroke.

Psychostimulant pharmacological profile of paraxanthine, the main metabolite of caffeine in humans.

Caffeine induces locomotor activation by its ability to block adenosine receptors. Caffeine is metabolized to several methylxanthines, with paraxanthine being the main metabolite in humans. In this study we show that in rats paraxanthine has a stronger locomotor activating effect than caffeine or the two other main metabolites of caffeine, theophylline and theobromine. As previously described for caffeine, the locomotor activating doses of paraxanthine more efficiently counteract the locomotor depressant effects of an adenosine A(1) than an adenosine A(2A) receptor agonist. In drug discrimination experiments in rats trained to discriminate a maximal locomotor activating dose of caffeine, paraxanthine, unlike theophylline, generalized poorly to caffeine suggesting the existence of additional mechanisms other than adenosine antagonism in the behavioral effects of paraxanthine. Pretreatment with the nitric oxide inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) reduced the locomotor activating effects of paraxanthine, but not caffeine. On the other hand, pretreatment with the selective cGMP-preferring phosphodiesterase PDE9 inhibitor BAY 73-6691, increased locomotor activity induced by caffeine, but not paraxanthine. Ex vivo experiments demonstrated that paraxanthine, but not caffeine, can induce cGMP accumulation in the rat striatum. Finally, in vivo microdialysis experiments showed that paraxanthine, but not caffeine, significantly increases extracellular levels of dopamine in the dorsolateral striatum, which was blocked by l-NAME. These findings indicate that inhibition of cGMP-preferring PDE is involved in the locomotor activating effects of the acute administration of paraxanthine. The present results demonstrate a unique psychostimulant profile of paraxanthine, which might contribute to the reinforcing effects of caffeine in humans.

Dopamine-galanin receptor heteromers modulate cholinergic neurotransmission in the rat ventral hippocampus.

Previous studies have shown that dopamine and galanin modulate cholinergic transmission in the hippocampus, but little is known about the mechanisms involved and their possible interactions. By using resonance energy transfer techniques in transfected mammalian cells, we demonstrated the existence of heteromers between the dopamine D(1)-like receptors (D(1) and D(5)) and galanin Gal(1), but not Gal(2) receptors. Within the D(1)-Gal(1) and D(5)-Gal(1) receptor heteromers, dopamine receptor activation potentiated and dopamine receptor blockade counteracted MAPK activation induced by stimulation of Gal(1) receptors, whereas Gal(1) receptor activation or blockade did not modify D(1)-like receptor-mediated MAPK activation. Ability of a D(1)-like receptor antagonist to block galanin-induced MAPK activation (cross-antagonism) was used as a "biochemical fingerprint" of D(1)-like-Gal(1) receptor heteromers, allowing their identification in the rat ventral hippocampus. The functional role of D(1)-like-Gal receptor heteromers was demonstrated in synaptosomes from rat ventral hippocampus, where galanin facilitated acetylcholine release, but only with costimulation of D(1)-like receptors. Electrophysiological experiments in rat ventral hippocampal slices showed that these receptor interactions modulate hippocampal synaptic transmission. Thus, a D(1)-like receptor agonist that was ineffective when administered alone turned an inhibitory effect of galanin into an excitatory effect, an interaction that required cholinergic neurotransmission. Altogether, our results strongly suggest that D(1)-like-Gal(1) receptor heteromers act as processors that integrate signals of two different neurotransmitters, dopamine and galanin, to modulate hippocampal cholinergic neurotransmission.

Novel 8-(furan-2-yl)-3-benzyl thiazolo [5,4-e][1,2,4] triazolo [1,5-c] pyrimidine-2(3H)-thione as selective adenosine A(2A) receptor antagonist.

Adenosine A(2A) receptor (A(2A)R) antagonists have emerged as potential drug candidates to alleviate progression and symptoms of Parkinson's disease (PD), and reduce the dopaminergic side effects. The synthesis of novel compound 8-(furan-2-yl)-3-benzyl thiazolo [5,4-e][1,2,4] triazolo [1,5-c] pyrimidine-2-(3H)-thione (BTTP) was carried out to evaluate the potential of BTTP as A(2A)R antagonist using SCH58261, a standard A(2A)R antagonist. The strong interaction of BTTP with A(2A)R (ΔG=-12.46kcal/mol and K(i)=0.6nM) in silico analysis was confirmed by radioligand receptor binding studies showing high affinity (K(i)=0.004nM) and selectivity with A(2A)R (A(2A)/A(1)=1155-fold). The effect of CGS21680 (selective A(2A)R agonist) induced cAMP concentration (0.1pmol/ml) in HEK293 cells was antagonized with BTTP (0.065pmol/ml) and SCH58261 (0.075pmol/ml). Furthermore, BTTP pre-treated (5, 10 and 20mg/kg) haloperidol-induced mice demonstrated significant attenuation in catalepsy and akinesia. BTTP induced elevation in the striatal dopamine concentration (2.90µM/mg of tissue) was comparable to SCH58261 (2.92µM/mg of tissue) at the dose of 10mg/kg. The results firmly articulate that BTTP possesses potential A(2A)R antagonist activity and can be further explored for the treatment of PD.

Novel 8-(furan-2-yl)-3-substituted thiazolo [5,4-e][1,2,4] triazolo[1,5-c] pyrimidine-2(3H)-thione derivatives as potential adenosine A(2A) receptor antagonists.

Novel thiazolotriazolopyrimidine derivatives (23-33) designed as potential adenosine A(2A) receptor (A(2A)R) antagonists were synthesized. Molecular docking studies revealed that all compounds (23-33) exhibited strong interaction with A(2A)R. The strong interaction of the compounds (23-33) with A(2A)R in silico was confirmed by their high binding affinity with human A(2A)R stably expressed in HEK293 cells using radioligand-binding assay. The compounds 24-26 demonstrated substantial binding affinity and selectivity for A(2A)R as compared to SCH58261, a standard A(2A)R antagonist. Decrease in A(2A)R-coupled release of endogenous cAMP in treated HEK293 cells demonstrated in vitro A(2A)R antagonist potential of the compounds 24-26. Attenuation in haloperidol-induced motor impairments (catalepsy and akinesia) in Swiss albino male mice pre-treated with compounds 24-26 further supports their role in the alleviation of PD symptoms.

Synthesis of novel 7-imino-2-thioxo-3,7-dihydro-2H-thiazolo [4,5-d] pyrimidine derivatives as adenosine A2A receptor antagonists.

Novel bicyclic thiazolopyrimidine compounds (15-26) were synthesized to develop adenosine A(2A) receptor (A(2A)R) antagonist for the treatment of Parkinson's disease (PD). The binding affinity of the compounds (15-26) with A(2A)R was evaluated using radioligand binding assay on isolated membranes from stably transfected HEK293 cells. Selectivity of the compounds towards A(2A)R was assessed by comparing their binding affinities with A(1) receptors (A(1)R). cAMP concentrations were measured from HEK293 cells treated with compounds (15-26) as compared to NECA (A(2A)R agonist). The compound (16) possessed strongest A(2A)R binding affinity (K(i) value=0.0038 nM) and selectivity (737-fold) versus A(1)R. Decrease in A(2A)R-coupled release of endogenous cAMP from HEK293 cells treated with compounds (15-26) is evocative of their potential as A(2A)R antagonist.

In silico study of naphtha [1, 2-d] thiazol-2-amine with adenosine A 2A receptor and its role in antagonism of haloperidol-induced motor impairments in mice.

Loss of dopaminergic nigrostriatal neurons in the substantia nigra leads to Parkinson's disease (PD). Adenosine A(2A) receptors (A(2A)Rs) have been anticipated as novel therapeutic target for PD. A(2A)Rs potentiate locomotor behavior and are predominantly expressed in striatum. Naphtha [1, 2-d] thiazol-2-amine (NATA), a tricyclic thiazole have been studied as new anti-Parkinsonian compound. AutoDock analysis and pharmacophore study of NATA with known A(2A)R antagonists explicit its efficacy as a possible adenosine receptor antagonist. In vivo pharmacology of NATA showed reduction of haloperidol (HAL)-induced motor impairments in Swiss albino male mice. Relatively elevated levels of dopamine in NATA pre-treated mice are suggestive of its possible role as neuromodulator in PD.

Antagonism of haloperidol-induced swim impairment in L-dopa and caffeine treated mice: a pre-clinical model to study Parkinson's disease.

Parkinson's disease (PD) exhibits symptoms of motor dysfunction such as tremor, akinesia and rigidity. Agents that selectively disrupt or destroy catecholaminergic systems, such as reserpine, methamphetamine, 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine, have been used to develop PD models and to study the animal behavior like catalepsy, akinesia, swim-test, etc. The major apprehension while working with these chemicals is their irreversible neuro-toxic effect. Haloperidol is a classical antipsychotic drug, which produces extra-pyrimidal Parkinson's symptoms (EPS). Measuring catalepsy and akinesia in the treated mice monitored the haloperidol-induced EPS. Alternatively, swimming disability was tested as a new parameter to monitor haloperidol-induced EPS. The results showed that the restoration of swimming disability in haloperidol-induced L-dopa and caffeine pre-treated mice could be used as pre-clinical model to study PD.

Neuroprotective effect of naphtha[1,2-d]thiazol-2-amine in an animal model of Parkinson's disease.

Increased oxidative stress has been implicated in the pathogenesis of dopaminergic neurodegeneration leading to the development of Parkinson's disease. In this study, we investigated whether naphtha[1,2-d]thiazol-2-amine (NTA) may ameliorate haloperidol-induced catalepsy and oxidative damage in mice brain. Haloperidol-induced catalepsy was measured with the standard bar test. The extent of oxidative stress has been evaluated by measuring levels of MDA, GSH and activities of antioxidant enzymes (SOD and GSH-Px) from brain homogenate. Haloperidol treatment significantly induced the catalepsy as observed from increased descent time measured in the bar test. Pretreatment with NTA significantly reduced the catalepsy induced by haloperidol in a dose-dependent manner. The elevated level of MDA in haloperidol-treated mice was significantly decreased by NTA pretreatment. The decreased level of GSH as well as SOD and GSH-Px activities in haloperidol-treated mice were significantly increased by NTA pretreatment. NTA reduces the oxidative stress allowing recovery of detoxifying enzyme activities and controlling free radical production, suggesting a potential role of the drug as an alternative/adjuvant drug in preventing and treating the neurodegenerative diseases, such as Parkinson's disease.