Apoptosis: its relevance to Parkinson's disease

Supramolecular organizing centers at the interface of inflammation and neurodegeneration

The pathogenesis of neurodegenerative diseases involves the accumulation of misfolded protein aggregates. These deposits are both directly toxic to neurons, invoking loss of cell connectivity and cell death, and recognized by innate sensors that upon activation release neurotoxic cytokines, chemokines, and various reactive species. This neuroinflammation is propagated through signaling cascades where activated sensors/receptors, adaptors, and effectors associate into multiprotein complexes known as supramolecular organizing centers (SMOCs). This review provides a comprehensive overview of the SMOCs, involved in neuroinflammation and neurotoxicity, such as myddosomes, inflammasomes, and necrosomes, their assembly, and evidence for their involvement in common neurodegenerative diseases. We discuss the multifaceted role of neuroinflammation in the progression of neurodegeneration. Recent progress in the understanding of particular SMOC participation in common neurodegenerative diseases such as Alzheimer's disease offers novel therapeutic strategies for currently absent disease-modifying treatments.

Apoptosis, Autophagy, Necrosis and Their Multi Galore Crosstalk in Neurodegeneration

The progression of neurodegenerative disorders is mainly characterized by immense neuron loss and death of glial cells. The mechanisms which are active and regulate neuronal cell death are namely necrosis, necroptosis, autophagy and apoptosis. These death paradigms are governed by a set of molecular determinants that are pivotal in their performance and also exhibit remarkable overlapping functional pathways. A large number of such molecules have been demonstrated to be involved in the switching of death paradigms in various neurodegenerative diseases. In this review, we discuss various molecules and the concurrent crosstalk mediated by them. According to our present knowledge and research in neurodegeneration, molecules like Atg1, Beclin1, LC3, p53, TRB3, RIPK1 play switching roles toggling from one death mechanism to another. In addition, the review also focuses on the exorbitant number of newer molecules with the potential to cross communicate between death pathways and create a complex cell death scenario. This review highlights recent studies on the inter-dependent regulation of cell death paradigms in neurodegeneration, mediated by cross-communication between pathways. This will help in identifying potential targets for therapeutic intervention in neurodegenerative diseases.

Preclinical Evidence and Possible Mechanisms of Baicalein for Rats and Mice With Parkinson's Disease: A Systematic Review and Meta-Analysis

Baicalein, a major bioactive flavone of Scutellaria baicalensis Georgi, has neuroprotective properties in several animal models of Parkinson's disease (PD). Here, we conducted a systematic review and meta-analysis to assess the available preclinical evidence and possible mechanisms of baicalein for animal models of PD. Ultimately, 20 studies were identified by searching 7 databases from inception to December 2019. Review Manager 5.3 was applied for data analysis. Meta-analyses showed baicalein can significantly improve neurobehavioral function in animal models with PD, including spontaneous motor activity test (n = 5), pole test (n = 2), rotarod test (n = 9), apomorphine-induced rotations test (n = 4), grid test (n = 2), and tremor test (n = 2). Compared with controls, the results of the meta-analysis showed baicalein exerted a significant effect in increasing the frequency of spontaneous activity, prolongating the total time for climbing down the pole, decreasing the number of rotations, prolongating the descent latency, reducing the amplitude, and the frequency in animal models with PD. The possible mechanisms of baicalein for PD are regulating neurotransmitters, adjusting enzyme activity, antioxidation, anti-inflammatory, inhibiting protein aggregation, restorating mitochondrial dysfunction, inhibiting apoptosis, and autophagy. In conclusion, these findings preliminarily demonstrated that baicalein exerts potential neuroprotective effects through multiple signaling pathways in animal models of PD.

Mice lacking Faim2 show increased cell death in the MPTP mouse model of Parkinson disease

The death receptor Fas/CD95 mediates apoptotic cell death in response to external stimuli. In neurons, Fas-induced apoptosis is prevented by Fas-apoptotic inhibitory molecule 2 (Faim2). Mice lacking Faim2 showed increased neurodegeneration in animal models of stroke and bacterial meningitis. We therefore tested the relevance of Faim2 in a classical animal model of Parkinson disease and determined the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in Faim2-deficient mice. Without MPTP treatment, there was no difference in the dopaminergic system between Faim2-deficient mice and control mice. MPTP was applied i.p. in doses of 30 mg per kg on five consecutive days. Fourteen days after the last MPTP injection, the number of dopaminergic neurons in the lateral substantia nigra, assayed by stereological counting, was reduced by 39% in control mice and 53% in Faim2-deficient mice. The density of dopaminergic fibers in the dorsal striatum was reduced by 36% in control mice and 69% in Faim2-deficient mice, in the ventral striatum 44% in control mice and 76% in Faim2-deficient mice. Fiber density recovered at 90 days after MPTP with similar density in both groups. Striatal catecholamine levels were reduced by 81-84% in both groups and recovered at 90 days. Faim2 expression was documented in mouse midbrain using quantitative reverse transcription-PCR (qRT-PCR) and found decreased after MPTP administration. Taken together, our findings demonstrate increased degeneration of dopaminergic neurons with Faim2 deficiency, indicating that Fas-induced apoptosis contributes to cell death in the MPTP mouse model. Along with the decreased expression of Faim2 after MPTP, this finding indicates that boosting Faim2 function might represent a therapeutic strategy for Parkinson disease.

Dopaminochrome induces caspase-independent apoptosis in the mesencephalic cell line, MN9D

Parkinson's disease is characterized by a deficiency in motor cortex modulation due to degeneration of pigmented dopaminergic neurons of the substantia nigra projecting to the striatum. These neurons are particularly susceptible to oxidative stress, perhaps because of their dopaminergic nature. Like all catecholamines, dopamine is easily oxidized, first to a quinone intermediate and then to dopaminochrome (DAC), a 5-dihydroxyindole tautomer, that is cytotoxic in an oxidative stress-dependent manner. Here we show, using the murine mesencephalic cell line MN9D, that DAC causes cell death by apoptosis, illustrated by membrane blebbing, Annexin V, and propidium iodide labeling within 3 h. In addition, DAC causes oxidative damage to DNA within 3 h, and positive terminal deoxynucleotidyl transferase dUTP nick end labeling fluorescence by 24 h. DAC, however, does not induce caspase 3 activation and its cytotoxic actions are not prevented by the pan-caspase inhibitor, Z-VAD-fmk. DAC-induced cytotoxicity is limited by the PARP1 inhibitor, 5-aminoisoquinolinone, supporting a role for apoptosis-inducing factor (AIF) in the apoptotic process. Indeed, AIF is detected in the nuclear fraction of MN9D cells 3 h after DAC exposure. Taken together these results demonstrate that DAC induces cytotoxicity in MN9D cells in a caspase-independent apoptotic manner, likely triggered by oxidative damage to DNA, and involving the translocation of AIF from the mitochondria to the nucleus.

Evidence of aberrant DNA damage response signalling but normal rates of DNA repair in dividing lymphoblasts from patients with schizophrenia

OBJECTIVES

Cancer incidence in schizophrenia is not increased commensurate with higher rates of risk exposures. Here we report an investigation of the DNA damage response, an anti-tumorigenic defence, in immortalised lymphoblasts from patients with schizophrenia.

METHODS

Unirradiated and irradiated (5Gy) lymphoblasts from schizophrenia patients (n = 28) and healthy controls (n = 28) were immunostained for the phosphorylated histone variant H2AX (γH2AX), an index of DNA double-strand breaks. Flow cytometry was used to assess cell cycle distribution and γH2AX immunofluorescence. Rate of DNA repair was quantified by determining the temporal change in γH2AX values following irradiation.

RESULTS

In unirradiated lymphoblasts, γH2AX levels were significantly increased in the schizophrenia group compared with controls (effect size = 0.86). This increase was most evident in patients with cognitive deficits. In irradiated lymphoblasts, peak radiation-induced γH2AX levels were significantly reduced in patients. No differences between patients and controls were found in the rate of DNA repair or in cell cycle distribution.

CONCLUSIONS

The significant differences in DNA damage response signalling observed involve modification of histone variant H2AX and thereby implicate regulatory processes determining chromatin structure in dividing lymphoblasts from patients with schizophrenia. The role that aberrant DNA damage response signalling plays in protecting patients from cancer is unclear.

6-Hydroxydopamine increases the level of TNFα and bax mRNA in the striatum and induces apoptosis of dopaminergic neurons in hemiparkinsonian rats

This study was focused on the apoptosis (programmed cell death) induction involved in the loss of dopaminergic (DA-ergic) neurons in 6-hydroxydopamine (6-OHDA) hemiparkinsonian rats. The apoptosis in the striatum and substantia nigra pars compacta (SNpc), was examined 6, 24 h and 7 days after the 6-OHDA lesions employing the TUNEL method. The changes in mRNA levels of pro-apoptotic protein tumor necrosis factor alpha (TNFalpha) and its "death receptor" TNFalphaRI and then bax mRNA, as an important regulator of apoptotic neurodegeneration were followed by RT-PCR procedure. In situ analysis revealed an increased number of TUNEL-positive neurons in 6-OHDA-treated animals in all examined time points. The highest number of apoptotic neurons was detected 24 h after the lesion, both in the ipsilateral striatum (3.41+/-0.18) and SNpc (5.8+/-0.79). A significant increase in the level of TNFalpha mRNA was observed in 6-OHDA-lesioned striatum, with maximal value after 24 h (46%) comparing to the control. In contrast, 6-OHDA did not significantly change the level of TNFalphaRI mRNA in any time point. Six and 24 h post-operatively, we observed a significant increase of bax mRNA expression (40% and 45%, respectively) in the ipsilateral striatum of treated animals in comparison with the right striatum of the controls. However, the highest level of the bax mRNA expression was reached 7 days after the surgery (94%) in the ipsilateral striatum of 6-OHDA-treated animals. These results suggest that striatal injection of 6-OHDA can induce early changes that would be an important regulator of apoptotic neurodegeneration of dopamine-producing neurons, during the first post-operative week.

Autosomal recessive juvenile parkinsonism Cys212Tyr mutation in parkin renders lymphocytes susceptible to dopamine- and iron-mediated apoptosis

Mutations in parkin are implicated in the pathogenesis of autosomal recessive juvenile parkinsonism (AR-JP) disease. We show that homozygote Cys212Tyr parkin mutation in AR-JP patients renders lymphocytes sensitive to dopamine, iron and hydrogen peroxide stimuli. Indeed, dopamine-induced apoptosis by four alternative mechanisms converging on caspase-3 activation and apoptotic morphology: (1) NF-kappaB-dependent pathway; mitochondrial dysfunction either by (2) H(2)O(2) or (3) hydroxyl exposure and (4) increase of unfolded-protein stress. We also demonstrate that 17beta-estradiol and testosterone prevent homozygote lymphocytes from oxidative stressors-evoked apoptosis. These results may contribute to understanding the relationship between genetic and environmental factors and iron in AR-JP.

Neuroprotective action of flavopiridol, a cyclin-dependent kinase inhibitor, in colchicine-induced apoptosis

Flavopiridol was developed as a drug for cancer therapy due to its ability to inhibit cell cycle progression by targeting cyclin-dependent kinases (CDKs). In this study, we show that flavopiridol may also have a neuroprotective action. We show that at therapeutic dosage (or at micromolar range), flavopiridol almost completely prevents colchicine-induced apoptosis in cerebellar granule neurones. In agreement with this, flavopiridol inhibits both the release of cyt c and the activation of caspase-3 induced in response to colchicine treatment. We demonstrate that in this cellular model for neurotoxicity, neither re-entry in the cell cycle nor activation of stress-activated protein kinases, such as c-Jun N-terminal kinase (JNK) or p38 MAP kinase, is involved. In contrast, we show that colchicine-induced apoptosis correlates with a substantial increase in the expression of cdk5 and Par-4, which is efficiently prevented by flavopiridol. Accordingly, a cdk5 inhibitor such as roscovitine, but not a cdk4 inhibitor such as 3-ATA, was also able to protect neurons from apoptosis as well as prevent accumulation of cdk5 and Par-4 in response to colchicine. Our data suggest a potential therapeutic use of flavopiridol in disorders of the central nervous system in which cytoskeleton alteration mediated by cdk5 activation and Par-4 expression has been demonstrated, such as Alzheimer's disease.