Astrocytic dysfunction: Insights on the role in neurodegeneration

Inhibition of cytokine-induced connexin43 hemichannel activity in astrocytes is neuroprotective

Astrocytes express high levels of connexin43, a protein that forms two types of channels: gap junction channels for direct intercellular communication, and hemichannels for exchanges with the extracellular space. Inflammation induces connexin43 hemichannel activation, which has been proposed to be involved in neuroglial interactions. Here, we investigated the contribution of connexin43 to NMDA-induced excitotoxicity in neuron/astrocyte co-cultures, after treatment with a pro-inflammatory cytokine mixture, containing TNF-alpha and IL1-beta (Mix), that stimulated astroglial connexin43 hemichannel activity. Interestingly, NMDA treatment induced a higher amount of neurotoxicity in Mix-treated co-cultures than in untreated ones, whereas this extent of neurotoxicity was absent in enriched neuron cultures or in co-cultures with connexin43 knock-out astrocytes. Furthermore, application of connexin43 hemichannel blockers or a synthetic cannabinoid prevented the Mix-induced potentiated NMDA neurotoxicity. Altogether, these data demonstrate that inflammation-induced astroglial hemichannel activation plays a critical role in neuronal death and suggest a neuroprotective role of connexin43 hemichannel blockade.

NOV/CCN3 upregulates CCL2 and CXCL1 expression in astrocytes through beta1 and beta5 integrins

Increasing evidence suggests that CCN matricellular proteins play important roles in inflammation. One of the major cell types that handle inflammation in the brain is the astrocyte, which, upon activation, dramatically increases its production of cytokines and chemokines. Here, we report that NOV/CCN3, added to primary cultured rat brain astrocytes, markedly increased the expression of CCL2 and CXCL1 chemokines, as indicated by ELISA and RT-qPCR assays. This effect was selective, as the production of thirteen other cytokines and chemokines was not affected by NOV. NOV expression by astrocytes was demonstrated by immunocytochemistry and Western blot analysis, and astrocyte transfection with NOV small interfering RNA (siRNA) markedly decreased CXCL1 and CCL2 production, indicating that endogenous NOV played a major role in the control of astrocytic chemokine synthesis. NOV was shown to mediate several of its actions through integrins. Here, we observed that siRNAs against integrins beta1 and beta5 decreased basal and abrogated NOV-stimulated astrocyte expression of CCL2 and CXCL1, respectively. Using a panel of kinase inhibitors, we demonstrated that NOV action on CCL2 and CXCL1 production involved a Rho/ROCK/JNK/NF-kappaB and a Rho/qROCK/p38/NF-kappaB pathway, respectively. Thus, distinct integrins and signaling mechanisms are involved in NOV-induced production of CCL2 and CXCL1 in astrocytes. Finally, astrocytic expression of NOV was detected in rat brain tissue sections, and NOV intracerebral injection increased CCL2 and CXCL1 brain levels in vivo. Altogether, our data shed light on the signaling pathways operated by NOV and strongly suggest that NOV mediates astrocyte activation and, therefore, might play a role in neuroinflammation.

Astrocytes in Alzheimer's disease

The circuitry of the human brain is formed by neuronal networks embedded into astroglial syncytia. The astrocytes perform numerous functions, providing for the overall brain homeostasis, assisting in neurogenesis, determining the micro-architecture of the grey matter, and defending the brain through evolutionary conserved astrogliosis programs. Astroglial cells are engaged in neurological diseases by determining the progression and outcome of neuropathological process. Astrocytes are specifically involved in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and various forms of dementia. Recent evidence suggest that early stages of neurodegenerative processes are associated with atrophy of astroglia, which causes disruptions in synaptic connectivity, disbalance in neurotransmitter homeostasis, and neuronal death through increased excitotoxicity. At the later stages, astrocytes become activated and contribute to the neuroinflammatory component of neurodegeneration.

Recent advances in (patho)physiology of astroglia

Our view of astrocytes in the operation of the brain is changing dramatically over the last 3 decades. Astroglial calcium excitability controls the release of gliotransmitters, which can occur at the tripartite synapse. Astrocytes not only modulate synaptic transmission by releasing and taking up transmitters, but also receiving neuronal signals that act upon astrocytic plasma membrane receptors. This process represents the bidirectional neurone-glia communication. Additionally, astrocytes play role in the regulation of blood flow as well as ion and water homeostasis. Many of the brain dysfunctions are primary astropathies, including hepatic encephalopathy and Alexander disease, while other brain malfunctions, such as epilepsy and Alzheimer disease, may have substantial astrocytic contribution. Thus, these star-shaped cells by their roles in (patho)physiology of the brain seem to live up to the expectation one can have from their given name - astrocyte.

Alterations of glutamate release in the spinal cord of mice with experimental autoimmune encephalomyelitis

We have investigated the spontaneous and the depolarisation-induced release of [(3)H]D-aspartate ([(3)H]D-ASP), a non-metabolisable analogue of glutamate, in spinal cord slices, synaptosomes and gliosomes from mice with experimental autoimmune encephalomyelitis (EAE) at 13, 21 and 55 days post-immunisation (d.p.i.), representing onset, peak and chronic phases of the pathology. At 13 and 21 d.p.i., the KCl-evoked, calcium-dependent overflow of [(3)H]D-ASP in spinal cord slices was significantly lower (30-40%), whereas at 55 d.p.i. it was significantly higher (30%), than that elicited in matched controls. When the release was measured from spinal cord synaptosomes and gliosomes in superfusion, a different picture emerged. The spontaneous and the KCl(15 mM)-induced release of [(3)H]D-ASP were significantly increased both in synaptosomes (17% and 45%, respectively) and gliosomes (26% and 25%, respectively) at 21, but not at 13, d.p.i. At 55 d.p.i., the KCl-induced [(3)H]D-ASP release was significantly increased (40%) only in synaptosomes. Finally, uptake of [(3)H]D-ASP was markedly (50-60%) increased in spinal cord synaptosomes, but not in gliosomes, obtained from EAE mice at 21 d.p.i., whereas no differences could be detected at 13 d.p.i. Our data indicate that glutamatergic neurotransmission is altered in the spinal cord of EAE mice.

Astrovirus encephalitis in boy with X-linked agammaglobulinemia

Encephalitis is a major cause of death worldwide. Although >100 pathogens have been identified as causative agents, the pathogen is not determined for up to 75% of cases. This diagnostic failure impedes effective treatment and underscores the need for better tools and new approaches for detecting novel pathogens or determining new manifestations of known pathogens. Although astroviruses are commonly associated with gastroenteritis, they have not been associated with central nervous system disease. Using unbiased pyrosequencing, we detected an astrovirus as the causative agent for encephalitis in a 15-year-old boy with agammaglobulinemia; several laboratories had failed to identify the agent. Our findings expand the spectrum of causative agents associated with encephalitis and highlight unbiased molecular technology as a valuable tool for differential diagnosis of unexplained disease.

Concomitant astroglial atrophy and astrogliosis in a triple transgenic animal model of Alzheimer's disease

Astrocytes are fundamental for brain homeostasis and are at the fulcrum of neurological diseases including Alzheimer's disease (AD). Here, we monitored changes in astroglia morphology throughout the age-dependent progression of AD. We used an immunohistochemical approach that allows us to determine the domain of glial cytoskeleton, by measuring the surface, volume, and the relationship between astrocytes and neuritic plaques. We investigated astroglia in the hippocampus of a triple transgenic mouse model of AD (3xTg-AD) that mimics the progression of the human disease. The numerical density of astrocytes is affected neither by AD nor by age. We found reduction of surface and volume of GFAP profiles from early ages (6 months; 43.84 and 52.76%, respectively), persisting at 12 (40.73 and 45.39%) and 18 months (64.80 and 71.95%) in the dentate gyrus (DG) of 3xTg-AD, whereas in CA1 it appears at 18 months (29.42 and 32.74%). This cytoskeleton atrophy is accompanied by a significant reduction of glial somata volume in DG at 12 and 18 months (40.46 and 75.55%, respectively), whereas in CA1 it is significant at 18 months (42.81%). However, while astroglial atrophy appears as a generalized process, astrocytes surrounding plaques are clearly hypertrophic as revealed by increased surface (48.06%; 66.66%), and volume (57.10%; 71.06%) of GFAP profiles in DG and CA1, respectively, at 18 months. We suggest differential effects of AD on astroglial populations depending on their association with plaques accounting for the progressive disruption of neural networks connectivity and neurotransmitters imbalance which underlie mnesic and cognitive impairments observed in AD.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

Long-term pain, neuroinflammation and glial activation

Nociceptive and neuropathic pain signals are known to result from noxious stimuli, which are converted into electrical impulses within tissue nociceptors. There is a complex equilibrium of pain-signalling and pain-relieving pathways connecting PNS and CNS. Drugs against long-term pain are today directed against increased neuronal excitability, mostly with less success. An injury often starts with acute physiological pain, which becomes inflammatory, nociceptive, or neuropathic, and may be transferred into long-term pain. Recently a low-grade inflammation was identified in the spinal cord and along the pain pathways to thalamus and the parietal cortex. This neuroinflammation is due to activation of glial cells, especially microglia, with production of cytokines and other inflammatory mediators within the CNS. Additionally, substances released to the blood from the injured region influence the blood-brain barrier, and give rise to an increased permeability of the tight junctions of the capillary endothelial cells, leading to passage of blood cells into the CNS. These cells are transformed into reactive microglia. If the inflammation turns into a pathological state the astrocytes will be activated. They are coupled into networks and respond to substances released by the capillary endothelial cells, to cytokines released from microglia, and to neurotransmitters and peptides released from neurons. As the astrocytes occupy a strategic position between the vasculature and synapses, they monitor the neuronal activity and transmitter release. Increased release of glutamate and ATP leads to disturbances in Ca2+ signalling, increased production of cytokines and free radicals, attenuation of the astrocyte glutamate transport capacity, and conformational changes in the astrocytic cytoskeleton, the actin filaments, which can lead to formation and rebuilding of new synapses. New neuronal contacts are established for maintaining and spreading pain sensation with the astrocytic networks as bridges. Thereby the glial cells can maintain the pain sensation even after the original injury has healed, and convert the pain into long-term by altering neuronal excitability. It can even be experienced from other parts of the body. As astrocytes are intimate co-players with neurons in the CNS, more knowledge on astrocyte responses to inflammatory activators may give new insight in our understanding of mechanisms of low-grade inflammation underlying long-term pain states and pain spreading. Novel treatment strategies would be to restore glial cell function and thereby attenuate the neuroinflammation.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

Neuroglia in neurodegeneration

Neuroglial cells are fundamental for control of brain homeostasis and they represent the intrinsic brain defence system. All forms in neuropathology therefore inevitably involve glia. The neurodegenerative diseases disrupt connectivity within brain circuits affecting neuronal-neuronal, neuronal-glial and glial-glial contacts. In addition neurodegenerative processes trigger universal and conserved glial reactions represented by astrogliosis and microglial activation. The complex of recently acquired knowledge allows us to regard the neurodegenerative diseases as primarily gliodegenerative processes, in which glial cells determine the progression and outcome of neuropathological process.