Mitochondrial function in neurodegeneration and ageing.

The mitochondrial respiratory chain and oxidative phosphorylation system are responsible for the production of ATP by aerobic metabolism. Defects of the respiratory chain are increasingly recognised as important causes of human disease, and neurodegenerative disorders in particular. This article will seek to review the clinical and biochemical effects of respiratory chain defects, and summarise what is known about the molecular mechanisms that underlie them. Increasing age is also associated with a decline in mitochondrial function. The biochemical correlates of this dysfunction and the possible molecular defects that may cause it will also be reviewed.     

Quality control of mitochondria: protection against neurodegeneration and ageing

Dysfunction of mitochondria has severe cellular consequences and is linked to ageing and neurodegeneration in human. Several surveillance strategies have evolved that limit mitochondrial damage and ensure cellular integrity. Intraorganellar proteases conduct protein quality control and exert regulatory functions, membrane fusion and fission allow mitochondrial content mixing within a cell, and the autophagic degradation of severely damaged mitochondria protects against apoptosis. Here, we will summarize the current knowledge on these surveillance strategies and their role in human disease.     

Early neurodegeneration progresses independently of microglial activation by heparan sulfate in the brain of mucopolysaccharidosis IIIB mice

Background

In mucopolysaccharidosis type IIIB, a lysosomal storage disease causing early onset mental retardation in children, the production of abnormal oligosaccharidic fragments of heparan sulfate is associated with severe neuropathology and chronic brain inflammation. We addressed causative links between the biochemical, pathological and inflammatory disorders in a mouse model of this disease.

Methodology/Principal Findings

In cell culture, heparan sulfate oligosaccharides activated microglial cells by signaling through the Toll-like receptor 4 and the adaptor protein MyD88. CD11b positive microglial cells and three-fold increased expression of mRNAs coding for the chemokine MIP1α were observed at 10 days in the brain cortex of MPSIIIB mice, but not in MPSIIIB mice deleted for the expression of Toll-like receptor 4 or the adaptor protein MyD88, indicating early priming of microglial cells by heparan sulfate oligosaccharides in the MPSIIIB mouse brain. Whereas the onset of brain inflammation was delayed for several months in doubly mutant versus MPSIIIB mice, the onset of disease markers expression was unchanged, indicating similar progression of the neurodegenerative process in the absence of microglial cell priming by heparan sulfate oligosaccharides. In contrast to younger mice, inflammation in aged MPSIIIB mice was not affected by TLR4/MyD88 deficiency.

Conclusions/Significance

These results indicate priming of microglia by HS oligosaccharides through the TLR4/MyD88 pathway. Although intrinsic to the disease, this phenomenon is not a major determinant of the neurodegenerative process. Inflammation may still contribute to neurodegeneration in late stages of the disease, albeit independent of TLR4/MyD88. The results support the view that neurodegeneration is primarily cell autonomous in this pediatric disease.     

LPS-induced protein oxidation and proteolysis in BV-2 microglial cells

Exposure of proteins to oxidants leads to increased oxidation followed by preferential degradation by the proteasomal system. The role of the biological oxidant production in microglial BV-2 cells in the oxidation and turnover of endogenous proteins was measured. It could be demonstrated, that BV-2 cells are relatively resistant to fluxes of oxidants, but nevertheless protein oxidation occurs due to activation by LPS. This protein oxidation is followed by an enhanced degradation of endogenous proteins. Using PBN, a free radical scavenger and antioxidant, we could demonstrate the involvement of free radicals in the increased proteolysis in BV-2 cells after LPS-treatment. A slight but significant up-regulation of the proteasomal system after LPS activation takes place, indicating the importance of his proteolytic system in the maintenance of the protein pool of microglial cells.     

Parkin regulates microglial NLRP3 and represses neurodegeneration in Parkinson's disease

Microglial hyperactivation of the NOD-, LRR-, and pyrin domain-containing 3 (NLRP3) inflammasome contributes to the pathogenesis of Parkinson's disease (PD). Recently, neuronally expressed NLRP3 was demonstrated to be a Parkin polyubiquitination substrate and a driver of neurodegeneration in PD. However, the role of Parkin in NLRP3 inflammasome activation in microglia remains unclear. Thus, we aimed to investigate whether Parkin regulates NLRP3 in microglia. We investigated the role of Parkin in NLRP3 inflammasome activation through the overexpression of Parkin in BV2 microglial cells and knockout of Parkin in primary microglia after lipopolysaccharide (LPS) treatment. Immunoprecipitation experiments were conducted to quantify the ubiquitination levels of NLRP3 under various conditions and to assess the interaction between Parkin and NLRP3. In vivo experiments were conducted by administering intraperitoneal injections of LPS in wild-type and Parkin knockout mice. The Rotarod test, pole test, and open field test were performed to evaluate motor functions. Immunofluorescence was performed for pathological detection of key proteins. Overexpression of Parkin mediated NLRP3 degradation via K48-linked polyubiquitination in microglia. The loss of Parkin activity in LPS-induced mice resulted in excessive microglial NLRP3 inflammasome assembly, facilitating motor impairment, and dopaminergic neuron loss in the substantia nigra. Accelerating Parkin-induced NLRP3 degradation by administration of a heat shock protein (HSP90) inhibitor reduced the inflammatory response. Parkin regulates microglial NLRP3 inflammasome activation through polyubiquitination and alleviates neurodegeneration in PD. These results suggest that targeting Parkin-mediated microglial NLRP3 inflammasome activity could be a potential therapeutic strategy for PD.     

Increased neurodegeneration during ageing in mice lacking high-affinity nicotine receptors

We have examined neuroanatomical, biochemical and endocrine parameters and spatial learning in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor (nAChR) during ageing. Aged beta2(-/-) mutant mice showed region-specific alterations in cortical regions, including neocortical hypotrophy, loss of hippocampal pyramidal neurons, astro- and microgliosis and elevation of serum corticosterone levels. Whereas adult mutant and control animals performed well in the Morris maze, 22- to 24-month-old beta2(-/-) mice were significantly impaired in spatial learning. These data show that beta2 subunit-containing nAChRs can contribute to both neuronal survival and maintenance of cognitive performance during ageing. beta2(-/-) mice may thus serve as one possible animal model for some of the cognitive deficits and degenerative processes which take place during physiological ageing and in Alzheimer's disease, particularly those associated with dysfunction of the cholinergic system.     

Connecting cell-cycle activation to neurodegeneration in Drosophila

Studies in cell-culture systems and in postmortem tissue from human disease have suggested a connection between cell-cycle activation and neurodegeneration. The fruit fly Drosophila melanogaster has recently emerged as a powerful model system in which to model neurodegenerative diseases. Here we review work in the fly that has begun to address some of the important questions regarding the relationship between cell-cycle activation and neurodegeneration in vivo, including recent data implicating cell-cycle activation as a downstream effector of tau-induced neurodegeneration. We suggest how powerful research tools in Drosophila might be utilized to approach fundamental questions that remain.     

Role of proteolysis in caspase-8 activation and stabilization

Caspase-8 is an apoptotic protease that is activated at the cytosolic face of the cell membrane. Activation relies on adaptor-induced dimerization of monomeric caspase-8 and is followed by specific limited autoproteolysis of the linker which separates the two subunits of the catalytic domain. However, the role of this autoproteolysis, which directly activates executioner caspases-3 and -7, is unknown for the apical caspase-8. We have generated linker mutants of caspase-8 that can be proteolyzed in a controlled manner by thrombin or tobacco etch mosaic virus protease, and we use these to define the role of proteolysis in the activation and stability of the enzyme. We show that proteolysis is insufficient for generating enzymatic activity in recombinant caspase-8. Kinetic activation studies using Hoffmeister salts demonstrate that activation is the result of caspase dimerization. However, linker proteolysis significantly enhances the equilibrium for caspase-8 dimerization, thereby increasing the stability of the dimer. Kinetic and fluorescence measurements demonstrate that caspase-8 activation by Hoffmeister salts is at least a two-step event, with the required step being dimerization, followed by an intramolecular event that further stabilizes the catalytic conformation. Autoproteolysis of caspase-8 may be a mechanism for increasing the lifetime of the dimeric enzyme following dissociation from its activating complex at the cell membrane.     

Use of cocultured cell systems to elucidate chemokine-dependent neuronal/microglial interactions: control of microglial activation

In order to understand processes involved in central nervous system inflammatory diseases, a critical appreciation of mechanisms involved in the control of immune function in the brain is needed. Microglial cells are watchful eyes for unusual events and detecting the presence of pathogens but are also alert to signals emanating from damaged neurons. Fractalkine (CX3CL1) is a chemokine which is expressed predominantly in the central nervous system, being localized on neurons, while its receptor, CX3CR1, is found on microglial cells. We have developed a strategy to investigate the role of this chemokine in neuronal-microglia interactions. Because fractalkine is expressed both as a soluble and as a membrane-attached protein, we have established various protocols involving different levels of cell-to-cell communication. Three experimental systems were instituted, including (1) a conditioned medium transfer system in which no cell-cell communication or contact is possible, (2) a transwell system that permits cell-contact-independent communication through diffusible soluble factors only, and (3) a coculture system allowing cell-to-cell communication via direct microglial-neuronal contacts. Using these in vitro cocultured systems, we have investigated the role of a soluble and/or cell-associated chemokine, such as fractalkine, in order to obtain insights into the role of glia-neuron interactions in cerebral inflammation.     

Microglial activation induced by neurodegeneration: a proteomic analysis

Neuroinflammation mediated by microglial activation appears to play an essential role in the pathogenesis of Parkinson disease; however, the mechanisms by which microglia are activated are not fully understood. Thus, we first evaluated the effects of two parkinsonian toxicants, manganese ethylene bisdithiocarbamate (Mn-EBDC) and 1-methyl-4-phenylpyridine (MPP+), on microglial activation as well as associated dopaminergic (DAergic) neurotoxicity in primary cell culture systems. The results demonstrated that, when rat primary mesencephalic neuron-enriched or neuron-microglia mixed cultures were treated with Mn-EBDC at 2-8 microm or MPP+ at 0.25-5 microm, respectively, for 7 days, both toxicants were capable of inducing DAergic neurodegeneration as well as activating microglia via a mechanism secondary to DAergic neurodegeneration. Furthermore activated microglia subsequently enhanced DAergic neurotoxicity induced by Mn-EBDC or MPP+. Detailed scrutiny of neuron-microglia interactions identified a fraction of the conditioned media derived from a DAergic cell line treated with Mn-EBDC or MPP+ that potently activated microglia. To further define potential mediators leading to microglial activation secondary to neurodegeneration, we utilized a quantitative proteomic technique termed SILAC (for stable isotope labeling by amino acids in cell culture) to compare the protein profiles of MPP+-treated cellular fraction that mediated microglial activation as compared with controls. The search revealed numerous novel proteins that are potentially important in neurodegeneration-mediated microglial activation, a process believed to be critical in Parkinson disease progression.     

Protein oxidation and proteolysis in RAW264.7 macrophages: effects of PMA activation

Macrophages are stimulable cells able to increase the production of reactive oxygen and nitrogen species dramatically for a short period of time. Free radicals and other oxidants are able to oxidize the intracellular protein pool. These oxidized proteins are selectively recognized and degraded by the intracellular proteasomal system. We used the mouse macrophage-like cell line RAW264.7 to test whether macrophagial cells are able to increase their protein turnover after oxidative stress and whether this is accompanied by an increased protein oxidation. Macrophagial cells are particularly susceptible to bolus additions of hydrogen peroxide and peroxynitrite. In further experiments we activated RAW264.7 cells with PMA to test whether the production of endogenous oxidants has analogous effects. A clear dependence of the protein turnover and protein oxidation on the oxidative burst could be measured. In further experiments the role of the proteasomal system in the selective removal of oxidized proteins could be revealed exploring the proteasome specific inhibitor lactacystin. Therefore, although oxidants are able to attack the intracellular protein pool in macrophages, these cells are able to remove oxidized proteins selectively and protect the intracellular protein pool from oxidation.     

Participation of protease-activated receptor-1 in thrombin-induced microglial activation

Activation of microglia, the resident macrophages in the CNS, plays a significant role in neuronal death or degeneration in a broad spectrum of CNS disorders. Recent studies indicate that nanomolar concentrations of the serine protease, thrombin, can activate microglia in culture. However, in contrast to other neural cells responsive to thrombin, the participation of novel protease-activated receptors (PARs), such as the prototypic thrombin receptor PAR1, in thrombin-induced microglial activation was cast in doubt. In this report, by utilizing primary microglial cultures from PAR1 knockout (PAR1-/-) mice, application of the PAR1 active peptide TRAP-6 (SFLLRN) in comparison to a scrambled peptide (LFLNR), we have unambiguously demonstrated that murine microglia constitutively express PAR1 mRNA that is translated into fully functional protein. Activation of the microglial PAR1 induces a rapid cytosolic free [Ca2+]i increase and transient activation of both p38 and p44/42 mitogen-activated protein kinases. Moreover, although in part, this PAR1 activation directly contributes to thrombin-induced microglial proliferation. Furthermore, although not directly inducing tumor necrosis factor-alpha (TNF-alpha) release, PAR1 activation up-regulates microglial CD40 expression and potentiates CD40 ligand-induced TNF-alpha production, thus indirectly contributing to microglial activation. Taken together, these results demonstrate an essential role of PAR1 in thrombin-induced microglial activation. In addition, strategies aimed at blocking thrombin signaling through PAR1 may be therapeutically valuable for diseases associated with cerebral vascular damage and significant inflammation with microglial activation.     

Fractalkine-induced activation of the phosphatidylinositol-3 kinase pathway attentuates microglial activation in vivo and in vitro

Several neurodegenerative disorders are associated with evidence of inflammation, one feature of which is increased activation of microglia, the most likely cellular source of inflammatory cytokines like interleukin-1β. It is now recognized that interaction of microglia with other cells contributes to maintenance of microglia in a quiescent state and the complementary distribution of the chemokine, fractalkine (CX₃CL1) on neurons and its receptor (CX₃CR1) on microglia, suggests that this interaction may play a role in modulating microglial activation. Here we demonstrate that both soluble and membrane-bound fractalkine attenuate lipopolysaccharide-induced microglial activation in vitro. We also show that fractalkine expression is reduced in the brain of aged rats and this is accompanied by an age-related increase in microglial activation. Treatment of aged rats with fractalkine attenuates the age-related increase in microglial activation and the evidence indicates that fractalkine-induced activation of the phosphatidylinositol-3 kinase pathway is required to maintain microglia in a quiescent state both in vivo and in vitro .     

Tobacco carcinogen induces microglial activation and subsequent neuronal damage

4-Methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) is a tobacco-specific procarcinogen. We have investigated whether NNK causes inflammatory upheaval in the brain by activation of resident microglia and astrocyte and result in bystander neuronal damage. We have carried out the work in both in vitro and in vivo models. We have found that treatment with NNK causes significant activation of mouse microglial (BV2) cell line as evident by increase in reactive oxygen species and nitric oxide level. Western blot analysis has showed increase in proinflammatory signaling proteins, proinflammatory effector proteins, and other stress-related proteins. Interestingly, increased levels of proinflammatory cytokines like interleukin (IL)-6, tumor necrosis factor-α, monocyte chemoattractant protein 1 (MCP1), and IL-12p70 are also detected. Work from our in vivo studies has demonstrated similar increase in proinflammatory signaling and effector molecules along with the proinflammatory cytokine levels, following NNK treatment. Immunohistochemical staining of the brain sections of NNK-treated mice reveals massive microglial and astrocyte activation along with distinct foci of neuronal damage. Both in vitro and in vivo results provide strong indication that NNK causes significant upheaval of the inflammatory condition of brain and inflicts subsequent neuronal damage.