Microglial cell dysregulation in brain aging and neurodegeneration

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergo phenotypic changes compatible with their activation. Glial activation can lead to neuroinflammation, which is increasingly accepted as part of the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). We hypothesize that in aging, aberrant microglia activation leads to a deleterious environment and neurodegeneration. In aged mice, microglia exhibit an increased expression of cytokines and an exacerbated inflammatory response to pathological changes. Whereas LPS increases nitric oxide secretion in microglia from young mice, induction of reactive oxygen species (ROS) predominates in older mice. Furthermore, there is accumulation of DNA oxidative damage in mitochondria of microglia during aging, and also an increased intracellular ROS production. Increased ROS activates the redox-sensitive nuclear factor kappa B, which promotes more neuroinflammation, and can be translated in functional deficits, such as cognitive impairment. Mitochondria-derived ROS and cathepsin B, are also necessary for the microglial cell production of interleukin-1β, a key inflammatory cytokine. Interestingly, whereas the regulatory cytokine TGFβ1 is also increased in the aged brain, neuroinflammation persists. Assessing this apparent contradiction, we have reported that TGFβ1 induction and activation of Smad3 signaling after inflammatory stimulation are reduced in adult mice. Other protective functions, such as phagocytosis, although observed in aged animals, become not inducible by inflammatory stimuli and TGFβ1. Here, we discuss data suggesting that mitochondrial and endolysosomal dysfunction could at least partially mediate age-associated microglial cell changes, and, together with the impairment of the TGFβ1-Smad3 pathway, could result in a reduction of protective activation and a facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegeneration

Age-dependent changes on fractalkine forms and their contribution to neurodegenerative diseases

The chemokine fractalkine (FKN, CX3CL1), a member of the CX3C subfamily, contributes to neuron–glia interaction and the regulation of microglial cell activation. Fractalkine is expressed by neurons as a membrane-bound protein (mCX3CL1) that can be cleaved by extracellular proteases generating several sCX3CL1 forms. sCX3CL1, containing the chemokine domain, and mCX3CL1 have high affinity by their unique receptor (CX3CR1) which, physiologically, is only found in microglia, a resident immune cell of the CNS. The activation of CX3CR1contributes to survival and maturation of the neural network during development, glutamatergic synaptic transmission, synaptic plasticity, cognition, neuropathic pain, and inflammatory regulation in the adult brain. Indeed, the various CX3CL1 forms appear in some cases to serve an anti-inflammatory role of microglia, whereas in others, they have a pro-inflammatory role, aggravating neurological disorders. In the last decade, evidence points to the fact that sCX3CL1 and mCX3CL1 exhibit selective and differential effects on their targets. Thus, the balance in their level and activity will impact on neuron–microglia interaction. This review is focused on the description of factors determining the emergence of distinct fractalkine forms, their age-dependent changes, and how they contribute to neuroinflammation and neurodegenerative diseases. Changes in the balance among various fractalkine forms may be one of the mechanisms on which converge aging, chronic CNS inflammation, and neurodegeneration

Unraveling the β-AMYLOID clearance by astrocytes: Involvement of metabotropic glutamate receptor 3, sAPPα, and class-A scavenger receptor

The mechanics of β-amyloid (Aβ) clearance by astrocytes has not been univocally described, with different mediators appearing to contribute to this process under different conditions. Our laboratory has demonstrated neuroprotective effects of astroglial subtype 3 metabotropic glutamate receptor (mGlu3R), which are dependent on the secreted form of the amyloid precursor protein (sAPPα) as well as on Aβ clearance; however, the mechanism underlying mGlu3R-induced Aβ uptake by astrocytes remains unclear. The present study shows that conditioned medium from mGlu3R-stimulated astrocytes increased Aβ uptake by naïve astrocytes through a mechanism dependent on sAPPα, since sAPPα depletion from conditioned medium inhibited Aβ phagocytosis. Concordantly, recombinant sAPPα also increased Aβ uptake. Since we show that both sAPPα and the mGlu3R agonist LY379268 increased expression of class-A scavenger receptor (SR-A) in astrocytes, we next determined whether SR-A mediates mGlu3R- or sAPPα-induced Aβ uptake by using astrocyte cultures derived from SR-A knockout mice. We found that the effects of LY379268 as well as sAPPα on Aβ uptake were abolished in SR-A-deficient astrocytes, indicating a major role for this scavenger receptor in LY379268- and sAPPα-stimulated Aβ clearance by astrocytes. We also show results of coimmunoprecipitation and functional assays offering evidence of possible heterotrimerization of sAPPα with Aβ and SR-A which could allow Aβ to enter the astrocyte. In conclusion the present paper describes a novel pathway for Aβ clearance by astrocytes involving sAPPα as an enhancer of SR-A-dependent Aβ phagocytosis.Fil: Durand, Daniela Elizabeth. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Turati, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Rudi, María Julieta. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Ramírez, Delia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Saba, Julieta. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Caruso, Carla Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: Carniglia, Lila. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: von Bernhardi, Rommy. Pontificia Universidad Católica de Chile; ChileFil: Lasaga, Mercedes Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; Argentin

Commentaries on Viewpoint: The ongoing need for good physiological investigation: Obstructive sleep apnea in HIV patients as a paradigm

The final publication is available via http://dx.doi.org/10.1152/japplphysiol.00989.2014[Abstract] The intriguing paradigm put forth by Darquenne et al. (3) highlighted that improved therapy against human immunodeficiency virus (HIV) has come at the cost of elevated rates of chronic diseases, such as obstructive sleep apnea (OSA) and obesity, during the highly active antiretroviral therapy (HAART) era.Ministerio de Economía y Competitividad; TIN2013-40686-P

Role of Scavenger Receptors in Glia-Mediated Neuroinflammatory Response Associated with Alzheimer’s Disease

It is widely accepted that cells serving immune functions in the brain, namely, microglia and astrocytes, are important mediators of pathological phenomena observed in Alzheimer’s disease. However, it is unknown how these cells initiate the response that results in cognitive impairment and neuronal degeneration. Here, we review the participation of the immune response mediated by glial cells in Alzheimer’s disease and the role played by scavenger receptors in the development of this pathology, focusing on the relevance of class A scavenger receptor (SR-A) for Aβ clearance and inflammatory activation of glial cell, and as a potential target for Alzheimer’s disease therapy

Microglia ­ astrocyte interaction in Alzheimer's disease: friends or foes for the nervous system?

Brain glial cells secrete several molecules that can modulate the survival of neurons after various types of damage to the CNS. Activated microglia and astrocytes closely associate to amyloid plaques in Alzheimer Disease (AD). They could have a role in the neurotoxicity observed in AD because of the inflammatory reaction they generate. There is controversy regarding the individual part played by the different glial cells, and the interrelationships between them. Both astrocytes and microglia produce several cytokines involved in the inflammatory reaction. Moreover, the same cytokines may have different effects, depending on their concentration and the type of cells in the vicinity. In turn, the events occurring in response to injury may lead to changes in the nature and relative concentration of the various factors involved. To learn about these putative glial interrelationships, we examined some effects of astrocytes on microglial activatio

Repair of the central nervous system: Lessons from lesions in leeches

In contrast to the limited repair observed in the mammalian central nervous system (CNS), injured neurons in the leech reliably regenerate synapses and restore function with remarkable accuracy at the level of individual neurons. New and recent results reveal important roles for microglial cells and extracellular matrix components, including laminin, in repair. Tissue culture experiments have permitted isolation of neurons and manipulation of their environment, providing insights into the influence of substrate, electrical activity, and other cells, including microglia, on axon growth and synapse formation. The results account for distinctive features of successful repair in the adult leech, where axonal sprouting and target selection can be influenced by unequal competition between neurons. Differences between the formation of connections during embryonic development and repair in the adult include dissimilarities in the roles of glia and microglia in adults and embryos, suggesting that axon growth during regeneration in the CNS is not simply a recapitulation of processes observed during embryonic development. It may be possible in the future to improve mammalian CNS regeneration by recruiting cells whose counterparts in the leech have been identified as instrumental in repair. © 1995 John Wiley & Sons, Inc