Adenosine and ADP prevent apoptosis in cultured rat cerebellar granule cells

Dysfunction of Mitochondria in Alzheimer’s Disease: ANT and VDAC Interact with Toxic Proteins and Aid to Determine the Fate of Brain Cells

Alzheimer’s disease (AD), certainly the most widespread proteinopathy, has as classical neuropathological hallmarks, two groups of protein aggregates: senile plaques and neurofibrillary tangles. However, the research interest is rapidly gaining ground in a better understanding of other pathological features, first, of all the mitochondrial dysfunctions. Several pieces of evidence support the hypothesis that abnormal mitochondrial function may trigger aberrant processing of amyloid progenitor protein or tau and thus neurodegeneration. Here, our aim is to emphasize the role played by two ‘bioenergetic’ proteins inserted in the mitochondrial membranes, inner and outer, respectively, that is, the adenine nucleotide translocator (ANT) and the voltage-dependent anion channel (VDAC), in the progression of AD. To perform this, we will magnify the ANT and VDAC defects, which are measurable hallmarks of mitochondrial dysfunction, and collect all the existing information on their interaction with toxic Alzheimer’s proteins. The pathological convergence of tau and amyloid β-peptide (Aβ) on mitochondria may finally explain why the therapeutic strategies used against the toxic forms of Aβ or tau have not given promising results separately. Furthermore, the crucial role of ANT-1 and VDAC impairment in the onset/progression of AD opens a window for new therapeutic strategies aimed at preserving/improving mitochondrial function, which is suspected to be the driving force leading to plaque and tangle deposition in AD.

Adenosine decreases oxidative stress and protects H2O2-treated neural stem cells against apoptosis through decreasing Mst1 expression

Overproduction of free radicals during oxidative stress induces damage to key biomolecules and activates programed cell death pathways. Neuronal cell death in the nervous system leads to a number of neurodegenerative diseases. The aim of the present study was to evaluate the neuroprotective effect of adenosine on inhibition of apoptosis induced by hydrogen peroxide (H₂O₂) in bone marrow-derived neural stem cells (B-dNSCs), with focus on its regulatory effect on the expression of mammalian sterile 20-like kinase 1 (Mst1), as a novel proapoptotic kinase. B-dNSCs were exposed to adenosine at different doses (2, 4, 6, 8 and 10 µM) for 48 h followed by 125 µM H₂O₂ for 30 min. Using MTT, terminal deoxynucleotidyl transferase dUTP nick-end labeling and real-time reverse transcription polymerase chain reaction assays, the effects of adenosine on cell survival, apoptosis and Mst1, nuclear factor (erythroid-derived 2)-like 2 and B-cell lymphoma 2 and adenosine A1 receptor expression were evaluated in pretreated B-dNSCs compared with controls (cells treated with H₂O₂ only). Firstly, results of the MTT assay indicated 6 µM adenosine to be the most protective dose in terms of promotion of cell viability. Subsequent assays using this dosage indicated that apoptosis rate and Mst1 expression in B-dNSCs pretreated with 6 µM adenosine were significantly decreased compared with the control group. These findings suggest that adenosine protects B-dNSCs against oxidative stress-induced cell death, and therefore, that it may be used to promote the survival rate of B-dNSCs and as a candidate for the treatment of oxidative stress-mediated neurological diseases.

Stimulation of the adenosine A3 receptor, not the A1 or A2 receptors, promote neurite outgrowth of retinal ganglion cells

Among candidate neuroprotective agents, adenosine is thought to be a possible treatment for central nervous system disorders. Adenosine elicits biological effects through four G protein-coupled receptors (A₁, A_2A, A_2B, and A₃). The A_2A and A_2B receptors stimulate adenylyl cyclase (AC) and increase cyclic adenosine monophosphate (cAMP) levels, whereas A₁ and A₃ receptors inhibit AC and decrease cAMP levels. Several studies have investigated the effects of adenosine receptors (AdoRs) in glaucoma, because modulation of A₁, A_2A, or A₃ receptor regulates intraocular pressure. In addition, AdoR-related phenomena may induce neuroprotective effects in retinal neurons. Notably, A₁, A_2A, and A₃ receptor agonists reportedly inhibit retinal ganglion cell (RGC) death in in vitro and in vivo glaucoma models. However, there is limited knowledge of the effects of AdoR activation on neurite outgrowth or the regeneration of RGCs. In this report, we described the role of an AdoR subtype in neurite outgrowth and RGC axonal regeneration. The distribution of AdoRs in the retina was evaluated by immunohistochemical analysis. Using primary cultured rat RGCs in vitro and an optic nerve crush model in vivo, neurite elongation was evaluated after stimulation by the following AdoR agonists: CHA, an A₁ receptor agonist; CGS21680, an A_2A receptor agonist; BAY60-6583, an A_2B receptor agonist; and 2-Cl-IB-MECA, an A₃ receptor agonist. To determine the mechanism of neurite promotion, the candidate molecules of signal transduction associated with the neurite elongation of AdoRs were evaluated by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis, respectively. All four AdoRs (A₁, A_2A, A_2B, and A₃) were present in the inner retinal layers. Among the agonists for AdoR, only 2-Cl-IB-MECA significantly promoted neurite outgrowth in primary cultured RGCs. Signaling pathway analyses showed that 2-Cl-IB-MECA caused upregulated phosphorylation of Akt in cultured RGCs. Additionally, LY294002, an inhibitor of Akt, suppressed the neurite-promoting effects of the A₃ receptor agonist in RGCs. Moreover, 2-Cl-IB-MECA increased the number of regenerating axons in the optic nerve crush model. Taken together, these data indicate that activation of the A₃ receptor, not the A₁ or A₂ receptors, promotes in vitro and in vivo neurite outgrowth during the regeneration of rat RGCs, which is caused by the activation of an Akt-dependent signaling pathway. Therefore, AdoR activation may be a promising candidate for the development of novel regenerative modalities for glaucoma and other optic neuropathies.

Nucleolin down-regulation is involved in ADP-induced cell cycle arrest in S phase and cell apoptosis in vascular endothelial cells

High concentration of extracellular ADP has been reported to induce cell apoptosis, but the molecular mechanisms remain not fully elucidated. In this study, we found by serendipity that ADP treatment of human umbilical vein endothelial cells (HUVEC) and human aortic endothelial cells (HAEC) down-regulated the protein level of nucleolin in a dose- and time-dependent manner. ADP treatment did not decrease the transcript level of nucloelin, suggesting that ADP might induce nucleolin protein degradation. HUVEC and HAEC expressed ADP receptor P2Y13 receptor, but did not express P2Y1 or P2Y12 receptors. However, P2Y1, 12, 13 receptor antagonists MRS2179, PSB0739, MRS2211 did not inhibit ADP-induced down-regulation of nucleolin. Moreover, MRS2211 itself down-regulated nucleolin protein level. In addition, 2-MeSADP, an agonist for P2Y1, 12 and 13 receptors, did not down-regulate nucleolin protein. These results suggested that ADP-induced nucleolin down-regulation was not due to the activation of P2Y1, 12, or 13 receptors. We also found that ADP treatment induced cell cycle arrest in S phase, cell apoptosis and cell proliferation inhibition via nucleolin down-regulation. The over-expression of nucleolin by gene transfer partly reversed ADP-induced cell cycle arrest, cell apoptosis and cell proliferation inhibition. Furthermore, ADP sensitized HUVEC to cisplatin-induced cell death by the down-regulation of Bcl-2 expression. Taken together, we found, for the first time to our knowledge, a novel mechanism by which ADP regulates cell proliferation by induction of cell cycle arrest and cell apoptosis via targeting nucelolin.

Extracellular ADP prevents neuronal apoptosis via activation of cell antioxidant enzymes and protection of mitochondrial ANT-1

Apoptosis in neuronal tissue is an efficient mechanism which contributes to both normal cell development and pathological cell death. The present study explores the effects of extracellular ADP on low [K(+)]-induced apoptosis in rat cerebellar granule cells. ADP, released into the extracellular space in brain by multiple mechanisms, can interact with its receptor or be converted, through the actions of ectoenzymes, to adenosine. The findings reported in this paper demonstrate that ADP inhibits the proapoptotic stimulus supposedly via: i) inhibition of ROS production during early stages of apoptosis, an effect mediated by its interaction with cell receptor/s. This conclusion is validated by the increase in SOD and catalase activities as well as by the GSSG/GSH ratio value decrease, in conjunction with the drop of ROS level and the prevention of the ADP protective effect by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a novel functionally selective antagonist of purine receptor; ii) safeguard of the functionality of the mitochondrial adenine nucleotide-1 translocator (ANT-1), which is early impaired during apoptosis. This effect is mediated by its plausible internalization into cell occurring as such or after its hydrolysis, by means of plasma membrane nucleotide metabolizing enzymes, and resynthesis into the cell. Moreover, the findings that ADP also protects ANT-1 from the toxic action of the two Alzheimer's disease peptides, i.e. Aβ1-42 and NH2htau, which are known to be produced in apoptotic cerebellar neurons, further corroborate the molecular mechanism of neuroprotection by ADP, herein proposed.

Altered energy status of primary cerebellar granule neuronal cultures from rats exposed to lead in the pre- and neonatal period

This paper examines the effect of pre- and neonatal exposure of rats to lead (0.1% lead acetate in drinking water, resulting in rat offspring whole blood lead concentration (Pb-B) 4μg/dL) on the energy status of neuronal mitochondria by measuring changes in ATP, ADP, AMP, adenosine, TAN concentration, adenylate energy charge value (AEC) and mitochondrial membrane potential in primary cerebellar granule neurons (CGC) in dissociated cultures. Fluorescence studies were performed to imaging and evaluate mitochondria mass, mitochondrial membrane potential, intracellular and mitochondrial reactive oxygen species (ROS) production. The Na(+)/K(+) ATPase activity in intact CGC was measured spectrophotometrically. Our data shows that pre- and neonatal exposure of rats to Pb, even below the threshold of whole blood Pb value considered safe for people, affects the energy status of cultured primary cerebellar granule neurons through a decrease in ATP and TAN concentrations and AEC value, inhibition of Na(+)/K(+) ATPase, and increase in intracellular and mitochondrial ROS concentration. These observations suggest that even these low levels of Pb are likely to induce important alterations in neuronal function that could play a role in neurodegeneration.

P2Y2 nucleotide receptor-mediated responses in brain cells

Acute inflammation is important for tissue repair; however, chronic inflammation contributes to neurodegeneration in Alzheimer's disease (AD) and occurs when glial cells undergo prolonged activation. In the brain, stress or damage causes the release of nucleotides and activation of the G(q) protein-coupled P2Y(2) nucleotide receptor subtype (P2Y(2)R) leading to pro-inflammatory responses that can protect neurons from injury, including the stimulation and recruitment of glial cells. P2Y(2)R activation induces the phosphorylation of the epidermal growth factor receptor (EGFR), a response dependent upon the presence of a SH3 binding domain in the intracellular C terminus of the P2Y(2)R that promotes Src binding and transactivation of EGFR, a pathway that regulates the proliferation of cortical astrocytes. Other studies indicate that P2Y(2)R activation increases astrocyte migration. P2Y(2)R activation by UTP increases the expression in astrocytes of alpha(V)beta(3/5) integrins that bind directly to the P2Y(2)R via an Arg-Gly-Asp (RGD) motif in the first extracellular loop of the P2Y(2)R, an interaction required for G(o) and G(12) protein-dependent astrocyte migration. In rat primary cortical neurons (rPCNs) P2Y(2)R expression is increased by stimulation with interleukin-1beta (IL-1beta), a pro-inflammatory cytokine whose levels are elevated in AD, in part due to nucleotide-stimulated release from glial cells. Other results indicate that oligomeric beta-amyloid peptide (Abeta(1-42)), a contributor to AD, increases nucleotide release from astrocytes, which would serve to activate upregulated P2Y(2)Rs in neurons. Data with rPCNs suggest that P2Y(2)R upregulation by IL-1beta and subsequent activation by UTP are neuroprotective, since this increases the non-amyloidogenic cleavage of amyloid precursor protein. Furthermore, activation of IL-1beta-upregulated P2Y(2)Rs in rPCNs increases the phosphorylation of cofilin, a cytoskeletal protein that stabilizes neurite outgrowths. Thus, activation of pro-inflammatory P2Y(2)Rs in glial cells can promote neuroprotective responses, suggesting that P2Y(2)Rs represent a novel pharmacological target in neurodegenerative and other pro-inflammatory diseases.

Gi-coupled P2Y-ADP receptor mediates GSK-3 phosphorylation and beta-catenin nuclear translocation in granule neurons

Glycogen synthase kinase-3 (GSK-3) is a multifaceted enzyme involved in development, neurogenesis, and survival at the CNS. We investigated nucleotides signaling to GSK-3 in cerebellar granule neurons and found that the metabotropic agonist 2-methyl-thio-ADP (2MeSADP) was able to induce GSK-3 phosphorylation and inhibition of its catalytic activity. 2MeSADP could be acting through several P2Y-ADP receptors expressed in granule neurons, as RT-PCR expression was found for P2Y(1), P2Y(12), and P2Y(13) receptors, but the pharmacological data fitted well with a Gi-coupled P2Y(13) receptor: the effect was sensitive to pertussis toxin, was unaffected by specific antagonists of P2Y(1) and P2Y(12) receptors, such as 2'-deoxy-N(6)-methyl-adenosine 3',5'-diphosphate and 2-methyl-thio-AMP, respectively, and the EC(50) values for 2MeSADP and ADP were in the same low nanomolar range. 2MeSADP was able to phosphorylate and activate extracellular signal-regulated kinase (ERK)-1,2 and Akt proteins, but its effect on GSK-3 phosphorylation was primarily dependent on the phosphatidyl inositol-3 kinase (PI3-K)/Akt pathway, as it was abolished by the PI3-K inhibitor wortmannin. GSK-3 inactivation by 2MeSADP in granule neurons resulted in nuclear translocation of its substrate beta-catenin, which functions as a transcriptional regulator, this effect being lost with wortmaninn. The present study first describes the coupling of a Gi-coupled P2Y(13)-like receptor to GSK-3 and beta-catenin through PI3-K/Akt signaling.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Adenosine protects against suicidal erythrocyte death

Suicidal death of erythrocytes or eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. The cell membrane scrambling is triggered by an increase in cytosolic Ca(2+) activity and activation of protein kinase C (PKC). Phosphatidylserine exposure fosters adherence of affected erythrocytes to the vascular wall. Thus, microcirculation in ischemic tissues may be impaired by the appearance of eryptotic erythrocytes. Ischemia leads to release of adenosine, which in most tissues leads to vasodilation and protects against cell injury. The present experiments explored whether adenosine influences mechanisms underlying eryptosis. Erythrocyte phosphatidylserine exposure was estimated from annexin V binding, cell volume from forward scatter and cytosolic Ca(2+) activity from Fluo3 fluorescence. Glucose depletion (for 24 or 48 h) significantly increased annexin binding and decreased forward scatter, effects partially reversed by adenosine. The protective effect of adenosine reached statistical significance (s.d.) at > =30 microM. Low Cl(-) solution (Cl(-) exchanged by gluconate for 24 h) similarly increased annexin binding and decreased forward scatter, effects again reversed by adenosine (s.d. at > or =10 and 30 microM, respectively). Similarly, phosphatase inhibitor okadaic acid (OA, 1 microM) and PKC activator phorbol 12-myristate-13-acetate (PMA, 3 microM) significantly enhanced annexin binding and decreased forward scatter. Adenosine significantly blunted the effects of OA and PMA on annexin V binding (s.d. at > or =30 and 10 microM, respectively) and the effect of OA on forward scatter (s.d. at > or =10 microM). In conclusion, adenosine inhibits eryptosis by a mechanism presumably effective downstream of PKC. The effect may participate in the maintenance of microcirculation in ischemic tissue.

Regulation of death and survival in astrocytes by ADP activating P2Y1 and P2Y12 receptors

ADP is the endogenous agonist for both P2Y(1) and P2Y(12) receptors, which are important therapeutic targets. It was previously demonstrated that ADP and a synthetic agonist, 2-methylthioadenosine 5'-diphosphate (2MeSADP), can induce apoptosis by activating the human P2Y(1) receptor heterologously expressed in astrocytoma cells. However, it was not known whether the P2Y(12) receptor behaved similarly. We demonstrated here that, unlike with the G(q)-coupled P2Y(1) receptor, activation of the G(i)-coupled P2Y(12) receptor does not induce apoptosis. Furthermore, activation of the P2Y(12) receptor by either ADP or 2MeSADP significantly attenuates the tumor necrosis factor alpha (TNFalpha)-induced apoptosis in 1321N1 human astrocytoma cells. This protective effect was blocked by the P2Y(12) receptor antagonist 2-methylthioAMP and by inhibitors of phospholipase C (U73122) and protein kinase C (chelerythrin), but not by the P2Y(1) receptor antagonist MRS2179. Toward a greater mechanistic understanding, we showed that hP2Y(12) receptor activation by 10nM 2MeSADP, activates Erk1/2, Akt, and JNK by phosphorylation. However, at a lower protective concentration of 100pM 2MeSADP, activation of the hP2Y(12) receptor involves only phosphorylated Erk1/2, but not Akt or JNK. This activation is hypothesized as the major mechanism for the protective effect induced by P2Y(12) receptor activation. Apyrase did not affect the ability of TNFalpha to induce apoptosis in hP2Y(12)-1321N1 cells, suggesting that the endogenous nucleotides are not involved. These results may have important implications for understanding the signaling cascades that follow activation of P2Y(1) and P2Y(12) receptors and their opposing effects on cell death pathways.

The selective A2A receptor antagonist SCH 58261 protects from neurological deficit, brain damage and activation of p38 MAPK in rat focal cerebral ischemia

We investigated the protective effect of subchronic treatment of the A2A receptor antagonist, SCH 58261 (0.01 mg/kg, i.p.), administered 5 min, 6 h and 15 h after permanent right middle cerebral artery occlusion (MCAo). Twenty-four hours after ischemia, an extensive pallid area, evaluated by cresyl violet staining, is evident in the vascular territories supplied by the MCA, the striatum and the sensory motor cortex. The pallid area reflects the extent of necrotic neurons. Soon after waking, rats showed a definite contralateral turning behavior which was significantly reduced by SCH 58261 treatment. Twenty-four hours after MCAo, SCH 58261 significantly improved the neurological deficit and reduced ischemic damage in the striatum and cortex. Phospho-p38 mitogen-activated protein kinase (MAPK), evaluated by Western Blot, increased by 500% in the ischemic striatum 24 h after MCAo. SCH 58261 treatment significantly reduced phospho-p38 MAPK by 70%. Microglia was immunostained using the OX-42 antibody. Phospho-p38 MAPK and OX-42-immunoreactive cells are localized in the ventral striatum and frontoparietal cortex. Furthermore, both OX-42 and phospho-p38 MAPK-immunoreactive cells have overlapping morphological features, typical of reactive microglia. SCH 58261 reduced phospho-p38 MAPK immunoreactivity in the striatum and in the cortex without changing the microglial cell morphology. These results indicate that the protective effect of the adenosine antagonist SCH 58261 during ischemia is not due to reduced microglial activation but involves inhibition of phospho-p38 MAPK and suggest that treatment with the A2A antagonist from the first hour to several hours after ischemia may be a useful therapeutic approach in cerebral ischemia.

Molecular determinants of P2Y2 nucleotide receptor function: implications for proliferative and inflammatory pathways in astrocytes

In the mammalian nervous system, P2 nucleotide receptors mediate neurotransmission, release of proinflammatory cytokines, and reactive astrogliosis. Extracellular nucleotides activate multiple P2 receptors in neurons and glial cells, including G protein-coupled P2Y receptors and P2X receptors, which are ligand-gated ion channels. In glial cells, the P2Y2 receptor subtype, distinguished by its ability to be equipotently activated by ATP and UTP, is coupled to pro-inflammatory signaling pathways. In situ hybridization studies with rodent brain slices indicate that P2Y2 receptors are expressed primarily in the hippocampus and cerebellum. Astrocytes express several P2 receptor subtypes, including P2Y2 receptors whose activation stimulates cell proliferation and migration. P2Y2 receptors, via an RGD (Arg-Gly-Asp) motif in their first extracellular loop, bind to alphavbeta3/beta5 integrins, whereupon P2Y2 receptor activation stimulates integrin signaling pathways that regulate cytoskeletal reorganization and cell motility. The C-terminus of the P2Y2 receptor contains two Src-homology-3 (SH3)-binding domains that upon receptor activation, promote association with Src and transactivation of growth factor receptors. Together, our results indicate that P2Y2 receptors complex with both integrins and growth factor receptors to activate multiple signaling pathways. Thus, P2Y2 receptors present novel targets to control reactive astrogliosis in neurodegenerative diseases.

Mechanisms of adenosine-induced cytotoxicity and their clinical and physiological implications

Extracellular ATP (ATPo) and adenosine are cytotoxic to several cancer cell lines, suggesting their potential use for anticancer therapy. Adenosine causes cytotoxicity, either when added exogenously or when generated from ATPo hydrolysis, via mechanisms which are not mutually exclusive and which involve, adenosine receptor activation, pyrimidine starvation and/or increases in intracellular S-adenosylhomocysteine: S-adenosylmethionine ratio. Given that adenosine also appears to protect against cytotoxicity via mechanisms including immunity against damage by oxygen free radicals, an understanding of the contribution of adenosine to ATPo-induced cytotoxicity is thus crucial, when considering any potential therapeutic use for these compounds. However, such an understanding has been largely hindered by the fact that many studies have not focused enough on the possibility that both ATPo and adenosine may mediate cytotoxicity in the same system. Such studies can benefit from use a range of ATPo concentrations when assessing the contribution of adenosine to ATPo-induced cytotoxicity. Whilst future molecular and pharmacological studies are needed to establish the nature of the cytotoxic adenosine receptor, it is possible that more than just one adenosine receptor type is involved and that the cytotoxic receptor(s) type is more likely to have a low affinity for adenosine. Activation of the adenosine receptor(s) would thus lead to cytotoxicity only at relatively high adenosine concentrations, while lower adenosine concentrations mediate non-cytotoxic physiological effects.

The protective effect of adenosine A2A receptor antagonism in cerebral ischemia

OBJECTIVES

We reviewed our most recent work on the protective effect of adenosine A(2A)antagonism in cerebral ischemia.

METHODS

Focal ischemia was produced in rats by introducing a nylon monofilament pre-coated with silicone through the external carotid artery to occlude the right MCA at its origin.

RESULTS

A(2A) antagonism was found protective in the model of permanent focal ischemia induced by the monofilament technique. This methodology provides the possibility of evaluating the protection against the outflow of excitatory amino acids and against an acute motor disturbance, i.e.contralateral turning to the ischemic side in the first hours after ischemia in awake rats. Hours later, a definite neurological deficit and necrotic neuronal damage can be evaluated.

DISCUSSION

Our results suggest that A(2A) antagonism may be protective from the earliest up to several hours after the ischemic event.

Role of the autophagic-lysosomal system on low potassium-induced apoptosis in cultured cerebellar granule cells

Apoptotic and autophagic cell death have been implicated, on the basis of morphological and biochemical criteria, in neuronal loss occurring in neurodegenerative diseases and it has been shown that they may overlap. We have studied the relationship between apoptosis and autophagic cell death in cerebellar granule cells (CGCs) undergoing apoptosis following serum and potassium deprivation. We found that apoptosis is accompanied by an early and marked proliferation of autophagosomal-lysosomal compartments as detected by electron microscopy and immunofluorescence analysis. Autophagy is blocked by hrIGF-1 and forskolin, two well-known inhibitors of CGC apoptosis, as well as by adenovirus-mediated overexpression of Bcl-2. 3-Methyladenine (3-MA) an inhibitor of autophagy, not only arrests this event but it also blocks apoptosis. The neuroprotective effect of 3-MA is accompanied by block of cytochrome c (cyt c) release in the cytosol and by inhibition of caspase-3 activation which, in turn, appears to be mediated by cathepsin B, as CA074-Me, a selective inhibitor of this enzyme, fully blocks the processing of pro-caspase-3. Immunofluorescence analysis demonstrated that cathepsin B, normally confined inside the lysosomal-endosomal compartment, is released during apoptosis into the cytosol where this enzyme may act as an execution protease. Collectively, these observations indicate that autophagy precedes and is causally connected with the subsequent onset of programmed death.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Adenosine and glutamate extracellular concentrations and mitogen-activated protein kinases in the striatum of Huntington transgenic mice. Selective antagonism of adenosine A2A receptors reduces transmitter outflow

The basal ganglia and deep layers of cerebral cortex neurodegeneration typically characterize the postmortem brain of Huntington disease (HD) patients. In this study, we employed 10- to 11-week-old transgenic HD mice (R6/2 line), in which the striatal adenosine extracellular levels, measured using the microdialysis technique, are significantly increased in comparison to wild-type mice. An increase in striatal adenosine is probably a precocious index of mitochondrial dysfunction that is described in both the postmortem brain of HD patients and transgenic mice striatal cells. The adenosine increase is matched by activation of the p38 mitogen-activated protein kinase (MAPK) in the striatal neurons of R6/2 mouse but not in the cortex. This result indicates that p38 MAPK is a correlate of striatal damage and suggests a role for p38 in the striatal neuron suffering and apoptosis described in this disease. The selective adenosine A(2A) receptor antagonist SCH 58261, administered through microdialysis fiber into the striatum, significantly decreases the outflow of glutamate in R6/2 mice. Antagonism of adenosine A(2A) receptors might be regarded as potentially useful in the treatment of this disease to control striatal excitotoxicity.

Role of N-terminal tau domain integrity on the survival of cerebellar granule neurons

Although the role of the microtubule-binding domain of the tau protein in the modulation of microtubule assembly is widely established, other possible functions of this protein have been poorly investigated. We have analyzed the effect of adenovirally mediated expression of two fragments of the N-terminal portion - free of microtubule-binding domain - of the tau protein in cerebellar granule neurons (CGNs). We found that while the expression of the tau (1-230) fragment, as well as of full-length tau, inhibits the onset of apoptosis, the tau (1-44) fragment exerts a powerful toxic action on the same neurons. The antiapoptotic action of tau (1-230) is exerted at the level of Akt-mediated activation of the caspase cascade. On the other hand, the toxic action of the (1-44) fragment is not prevented by inhibitors of CGN apoptosis, but is fully inhibited by NMDA receptor antagonists. These findings point to a novel, physiological role of the N-terminal domain of tau, but also underlay that its possible proteolytic truncation mediated by apoptotic proteases may generate a highly toxic fragment that could contribute to neuronal death.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Chemical anoxia delays germ cell apoptosis in the human testis

An understanding of testicular physiology and pathology requires knowledge of the regulation of cell death. Previous observation of suppression of apoptosis by hypoxia suggested a role for ATP in germ cell death. However, the exact effects of ATP production on germ cell death and of apoptosis on the levels of ATP and other adenine nucleotides (ANs) have remained unclear. We investigated the levels of ANs during human testicular apoptosis (analyzed by HPLC) and the role of chemical anoxia in germ cell death (detected by Southern blot analysis of DNA fragmentation, in situ end labeling of DNA, and electron microscopy). Incubation of seminiferous tubule segments under serum-free conditions induced apoptosis and concomitantly decreased the levels of ANs. Chemical anoxia, induced with potassium cyanide (KCN), an inhibitor of mitochondrial respiration, dropped ATP levels further and suppressed apoptosis at 4 h. After 24 h, many of the testicular cells underwent delayed apoptosis despite ATP depletion. Some cells showed signs of necrosis or toxicity. The addition of 2-deoxyglucose, an antimetabolite of glycolysis, did not alter the results obtained with KCN alone, whereas a toxic concentration of hydrogen peroxide switched apoptosis to necrosis. In most of the testicular cells, mitochondrial respiration appears to play a crucial role in controlling primary cell death cascades. In the human testis, there seem to be secondary apoptotic pathways that do not require functional respiration (or ATP).

The apoptosis/necrosis transition in cerebellar granule cells depends on the mutual relationship of the antioxidant and the proteolytic systems which regulate ROS production and cytochrome c release en route to death

We investigate the death route induced by potassium depletion in cerebellar granule cells in 0-15 h time range and study whether and how mutual relationship occurs between the cell antioxidant and proteolytic system. To achieve this, we incubated cells in the absence or presence of inhibitors of the antioxidant system, including superoxide dismutase and catalase, and of the proteolytic system, consisting of proteasomes and caspases, and investigated whether and how (i) cell survival, (ii) reactive oxygen species (ROS) production and (iii) antioxidant enzyme and caspase-3 activity change as a function of time after the apoptotic stimulus. The involvement of both antioxidant and proteolytic system on cytochrome c release was also investigated. Cell survival was found to increase in the presence of either proteasome or caspase inhibitors. On the contrary, as a result of the antioxidant system impairment, shift from apoptosis to necrosis occurs. We show that the antioxidant system, which exhibits a huge activity increase up to 3 h after apoptosis induction, is subjected to the proteasome-dependent proteolysis and that the increase in the antioxidant system found in the absence of proteasome activity is accompanied by ROS production decrease. Consistently, the early ROS-dependent release of cytochrome c was found to be prevented when the activity of the antioxidant system increased. Finally, caspase-3 activation was prevented by the inhibitors of both antioxidant system and proteasome.

Development of novel EDG3 antagonists using a 3D database search and their structure-activity relationships

Sphingosine-1-phosphate (S1P) is an intracellular second messenger and an extracellular mediator through endothelial differentiation gene (EDG) receptors, which are a novel class of G-protein-coupled receptors. Although EDG has attracted much attention because of its various roles, no selective agonists or antagonists have yet been developed. This could account for the delay in clarifying the physiological roles of members of the EDG family. Because precise structural information on EDG receptors is not yet available, pharmacophore models were generated based on structural information for S1P using the rational drug design software Catalyst. Novel antagonists, 2-alkylthiazolidine-4-carboxylic acids, were retrieved from a three-dimensional database search using the pharmacophore models, and these showed activity for EDG3. On the basis of their nonphosphoric acid structure, more potent antagonists, 2-(m- or p-heptylphenyl)thiazolidine-4-carboxylic acid, were developed.

Acute synaptic modulation by nicotinic agonists in developing cerebellar Purkinje cells of the rat

The synaptic properties of the immature mammalian cerebellum were studied with a focus on the nicotinic modulation of synaptic transmission. Synaptic currents in Purkinje neurones were recorded using whole-cell patch electrodes applied to cerebellar slices (200 microm thick) obtained from newborn rats at postnatal days 5-10 (P5-P10). When the membrane potential of a Purkinje cell was held at -40 mV, spontaneous synaptic currents occurring in the cell comprised both inward and outward components. The former was glutamatergic and the latter was GABAergic, as confirmed by measuring reversal potentials and by using the specific glutamate and GABA blockers, 6-cyano-7-nitroquinoziline-2,3-dione and bicuculline, respectively. Application of ACh (0.1-1000 microM) from a 'Y tube' enhanced the occurrence of both glutamatergic and GABAergic synaptic currents in Purkinje cells. These responses appeared within 1 s after the application of ACh, and they were mimicked by nicotinic agonists (10 microM nicotine, 10 microM cytisine, 10 microM 1,1-dimethyl-4-phenyl-piperazinium iodide, or 10 nM epibatidine), but were sensitive to a specific nicotinic antagonist (1 microM dihydro-beta-erythroidine). When the generation of action potentials by cerebellar neurones in the slice preparation was blocked by the addition of TTX (1 microM) to the external saline, these ACh-induced responses almost disappeared. This indicates that the enhanced synaptic activities in Purkinje cells are induced via presynaptic nicotinic receptors on the excitatory and inhibitory interneurones, presumably on the proximal axons or somatodendritic domains of granule cells and basket cells in the cerebellar cortex. Interestingly, these nicotinic effects were remarkable in immature rats (P5-P10), but were barely detectable in older rats (more than 10 days of age), indicating that nicotinic ACh receptors are regulated developmentally and may play a novel role in the maturing cerebellum.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

Intracellular, nonreceptor-mediated signaling by adenosine: induction and prevention of neuronal apoptosis

Inhibitory effect of adenosine on the isolated heart muscle and vascular system were first described in 1929. Since then, numerous reviews have been published on the diverse actions of this nucleoside on a wide variety of cell types. Essentially all effects of adenosine in neurons and non-neuronal cells are mediated by activation of nucleoside membrane receptors coupled to specific intracellular second messenger pathways. This brief review describes two novel actions of adenosine in peripheral sympathetic neurons, which are not mediated by adenosine receptors. First is described how adenosine and related nucleosides are able to induce apoptosis during the initial stages of neuronal growth and development in vitro and in vivo. Second is discussed how adenosine is able to prevent or delay apoptosis in more mature sympathetic neurons subjected to nerve growth factor deprivation in culture. Both the induction and prevention of apoptosis are independent of receptor activation, and totally dependent on the intracellular accumulation and subsequent phosphorylation of adenosine. The physiological significance and mechanisms by which adenosine can induce apoptosis in one situation, and rescue from apoptosis in another, are described in this article.

Adenosine attenuates reperfusion-induced apoptotic cell death by modulating expression of Bcl-2 and Bax proteins

This study tests the hypothesis that infarct reduction with adenosine (Ado) is associated with inhibition of apoptotic cell death by modulating expression of anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins and reducing neutrophil accumulation. In three groups of dogs, the left anterior descending coronary artery was occluded for 60 min and reperfused for 6 h. Either saline (Control, n=8), Ado (140 microg/kg/min, n=8) or CGS21680, an adenosine A2A receptor analogue, (0.2 microg/kg/min, n=7) were infused during the first 2 h of reperfusion. Myocardial apoptosis was detected by histological TUNEL staining and DNA laddering. Expression of Bcl-2 and Bax proteins was analyzed using Western blot assay. Neutrophil localization was detected by immunohistochemistry with monoclonal anti-neutrophil CD18 antibody. There was no group difference in collateral blood flow (colored microspheres) during ischemia. Intra-left atrial administration of Ado and CGS21680 significantly decreased infarct size from 26+/-2% in Control to 13+/-1%* and 16+/-3%*, respectively. TUNEL positive cells in the peri-necrotic zone of the ischemic myocardium were also significantly reduced from 16+/-2% in Control group to 9+/-1%* and 10+/-2%*, respectively, consistent with the absence of DNA laddering in these two groups. Densitometrically, Ado and CGS21680 at reperfusion significantly increased the expression (% of normal myocardium) of downregulated Bcl-2 from 45+/-6% in Control group to 78+/-12%* and 69+/-10%*, respectively, and attenuated expression of upregulated Bax from 198+/-16% in Control group to 148+/-10%* and 158+/-12%*, respectively. Furthermore, the number of positive CD18 cells (mm(2) myocardium), which was significantly correlated with TUNEL positive cells in peri-necrotic zone, was significantly reduced from 403+/-42 in Control group to 142+/-18* in Ado group and 153+/-20%* in CGS21680 group, respectively. In conclusion, the present study suggests that inhibition of apoptosis by Ado at reperfusion involves alterations in anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins and neutrophil accumulation, primarily mediated by an adenosine A2A receptor. * P<0.05 v Control group.

Granule neuron DNA damage following deafferentation in adult rats cerebellar cortex: a lesion model

Neuronal programmed cell death is regulated by a neurotrophic supply from targets and afferent inputs. The relative contribution of each component varies according to neuronal type and age. We have previously reported that primary cultures of cerebellar granule cells undergo apoptosis when deprived of depolarising KCl concentrations, suggesting a significant role of afferent inputs in the control of cerebellar granule cells survival. This issue was investigated by setting up various in vivo lesional paradigms in order to obtain partial or total deafferentation of the cerebellar granule layer in adult rats. At different times after surgery, cerebellar sections were subjected to TUNEL staining in order to detect possible DNA damage. One week after unilateral pedunculotomy, few scattered groups of apoptotic granule neurons were observed in the homolateral hemisphere. On the contrary, total deafferentation obtained by a new experimental paradigm based on an "L-cut" lesion induced massive and widespread apoptotic death in the granule layer of the deafferentated area. The time window of DNA fragmentation in granule layer was one to seven days after the "L-cut". Selective Purkinje cell deafferentation obtained by 3-acetylpyridine injection did not result in TUNEL staining in the cerebellar cortex. The current finding that mossy fiber axotomy induces granule cell apoptotic death points out for the first time the crucial role of afferent inputs in mature granule cell survival. Moreover, the in vivo lesional model described here may prove to be an useful tool for investigating cellular and molecular mechanisms of neuronal death triggered by deafferentation.

Cyclopentyladenosine improves cell proliferation, wound healing, and hair growth

BACKGROUND

N(6)-Cyclopentyladenosine (CPA), a structural analog of adenosine, is a vasodilator with extensive pharmacological effects. However, little is known about the effect of CPA on wound healing and hair growth.

METHODS

Cellular responses to CPA were measured in vitro by tetrazolium dye reduction and in vivo by bromodeoxyuridine (BrdU) uptake. The effect of CPA on healing of incisional and excisional wounds on the dorsum of diabetic (db/db, n = 94) and nondiabetic (db/+, n = 20) mice and hair growth along the wound margin was evaluated with wound breaking strength, wound closure rate, and quantitative histology.

RESULTS

CPA stimulated proliferation of BALB/3T3 fibroblasts and human dermal microvascular endothelial cells in both quiescent and nonquiescent phases. Wounds treated with CPA at 10 microM showed a significant increase in the number of BrdU-labeled cells, including keratinocytes, fibroblasts, endothelial cells, and cells in sebaceous glands and the outer root sheath of hair follicles, compared with controls (P < 0.05). CPA application (5.1 microg/daily for 12 days) significantly increased the breaking strength of incisional wounds at day 24 postwound (P < 0.05). Excisional wound closure rate in the CPA-treated group (3.4 microg/daily for 15 days) was accelerated starting at day 10 postwound compared with controls (P < 0.01). Tissue sections from CPA-treated wounds showed a sevenfold increase in hair follicle number, compared with controls (P < 0.01). Enhanced hair growth along the wound margin was revealed in CPA-treated groups.

CONCLUSION

CPA stimulated proliferation of many cell types in vivo and in vitro and enhanced wound healing and hair growth. Therefore, CPA could be an interesting candidate for clinical application.

Early release and subsequent caspase-mediated degradation of cytochrome c in apoptotic cerebellar granule cells

Cytochrome c (cyt c) release was investigated in cerebellar granule cells used as an in vitro neuronal model of apoptosis. We have found that cyt c is released into the cytoplasm as an intact, functionally active protein, that this event occurs early, in the commitment phase of the apoptotic process, and that after accumulation, this protein is progressively degraded. Degradation, but not release, is fully blocked by benzyloxycarbonyl-Val-Ala-Asp-fluoromethylchetone (z-VAD-fmk). On the basis of previous findings obtained in the same neuronal population undergoing excitotoxic death, it is hypothesized that release of cyt c may be part of a cellular attempt to maintain production of ATP via cytochrome oxidase, which is reduced by cytosolic NADH in a cytochrome b5-soluble cyt c-mediated fashion.

Tau cleavage and dephosphorylation in cerebellar granule neurons undergoing apoptosis

Cerebellar granule cells undergo apoptosis in culture after deprivation of potassium and serum. During this process we found that tau, a neuronal microtubule-associated protein that plays a key role in the maintenance of neuronal architecture, and the pathology of which correlates with intellectual decline in Alzheimer's disease, is cleaved. The final product of this cleavage is a soluble dephosphorylated tau fragment of 17 kDa that is unable to associate with microtubules and accumulates in the perikarya of dying cells. The appearance of this 17 kDa fragment is inhibited by both caspase and calpain inhibitors, suggesting that tau is an in vivo substrate for both of these proteases during apoptosis. Tau cleavage is correlated with disruption of the microtubule network, and experiments with colchicine and taxol show that this is likely to be a cause and not a consequence of tau cleavage. These data indicate that tau cleavage and change in phosphorylation are important early factors in the failure of the microtubule network that occurs during neuronal apoptosis. Furthermore, this study introduces new insights into the mechanism(s) that generate the truncated forms of tau present in Alzheimer's disease.