J. Neurochem. (2010) 112, 1099–1114. Abstract Among multiple structural and functional brain changes, aging is accompanied by an increase of inflammatory signaling in the nervous system as well as a dysfunction of the immune system elsewhere. Although the long‐held view that aging involves neurocognitive impairment is now dismissed, aging is a major risk factor for neurodegenerative diseases such as Alzheimer`s disease, Parkinson`s disease and Huntington’s disease, among others. There are many age‐related changes affecting the brain, contributing both to certain declining in function and increased frailty, which could singly and collectively affect neuronal viability and vulnerability. Among those changes, both inflammatory responses in aged brains and the altered regulation of toll like receptors, which appears to be relevant for understanding susceptibility to neurodegenerative processes, are linked to pathogenic mechanisms of several diseases. Here, we review how aging and pro‐inflammatory environment could modulate microglial phenotype and its reactivity and contribute to the genesis of neurodegenerative processes. Data support our idea that age‐related microglial cell changes, by inducing cytotoxicity in contrast to neuroprotection, could contribute to the onset of neurodegenerative changes. This view can have important implications for the development of new therapeutic approaches.

Summary Aging is the main risk factor for Alzheimer's disease. Among other characteristics, it shows changes in inflammatory signaling that could affect the regulation of glial cell activation. We have shown that astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by TGFβ1. However, whereas TGFβ1 is increased, glial cell activation persists in aging. To understand this apparent contradiction, we studied TGFβ1-Smad3 signaling during aging and their effect on microglial cell function. TGFβ1 induction and activation of Smad3 signaling in the hippocampus by inflammatory stimulation was greatly reduced in adult mice. We evaluated the effect of TGFβ1-Smad3 pathway on the regulation of nitric oxide (NO) and reactive oxygen species (ROS) secretion, and phagocytosis of microglia from mice at different ages with and without in vivo treatment with lipopolysaccharide (LPS) to induce an inflammatory status. NO secretion was only induced on microglia from young mice exposed to LPS, and was potentiated by inflammatory preconditioning, whereas in adult mice the induction of ROS was predominant. TGFβ1 modulated induction of NO and ROS production in young and adult microglia, respectively. Modulation was partially dependent on Smad3 pathway and was impaired by inflammatory preconditioning. Phagocytosis was induced by inflammation and TGFβ1 only in microglia cultures from young mice. Induction by TGFβ1 was also prevented by Smad3 inhibition. Our findings suggest that activation of the TGFβ1-Smad3 pathway is impaired in aging. Age-related impairment of TGFβ1-Smad3 can reduce protective activation while facilitating cytotoxic activation of microglia, potentiating microglia-mediated neurodegeneration.

Inflammatory cytokines and β amyloid (Aβ) induce activation of glial cells, leading to both protective and deleterious changes that are relevant for the pathogenesis of Alzheimer disease (AD). We have shown that astrocytes downregulate microglial cell cytotoxic activation through secretion of transforming growth factor-β (TGFβ1), and there is evidence that TGFβ1 modifies Aβ removal through the modulation of microglia. However, inflammatory activation of microglia is increased and Aβ clearance is reduced in AD patients, regardless of the fact that TGFβ1 is increased in their nervous system. We propose that changes in TGFβ Smad3 signal transduction could modify the regulation mediated by TGFβ1. Here we evaluated the participation of the TGFβ Smad3 pathway in regulation of the expression pattern of scavenger receptors (SR) and activation of microglia through nitric oxide (NO·) secretion and phagocytosis of Aβ. We found that TGFβ1 increased SR-A by 2.4-fold and decreased SR-BI expression by 79% at 48 hr, whereas it did not change SR-MARCO or CD36 expression. In addition, we observed a 51% increase of Aβ uptake and an 83% decrease of NO· production induced by lipopolysaccharide in microglial cell cultures. Increased expression of SR-A, phagocytosis, and downregulation of NO· by TGFβ1 were prevented by the inhibition of the TGFβ Smad3 pathway. Our results indicate that the modulation of microglial cell activation by TGFβ1, leading to increased clearance of Aβ and reduced cytotoxicity, is at least partially mediated by the Smad pathway.

The strong inflammatory response observed in neurodegenerative diseases can depend on the impairment of the endogenous control of microglial activation, triggering the release of potentially detrimental factors such as cytokines, nitric oxide (NO) and superoxide anion (O₂^–·). Our aim was to study the activation of microglial cells and the transduction pathways involved in their modulation by IL-1β and TNF-α. Microglial and mixed glial cell cultures from neonatal rats were exposed to IFN-γ and/or IL-1β and TNF-α. We analyzed NO secretion and the activation of ERK and STAT1. We found that astrocytes modulated microglial cell activation, decreasing production of NO. IFN-γ induced an 18- to 25-fold increase in NO, associated to a 3- to 5-fold increase in ERK phosphorylation in microglial cultures. IL-1β, but not TNF-α, inhibited IFN-γ-induced production of NO in microglia by 87%. It also reduced IFN-γ-induced phosphoERK (pERK) by 40%, without affecting phosphoSTAT1 (pSTAT1). In contrast, in microglial cultures exposed to media conditioned by astrocytes, IL-1β did not inhibit pERK, whereas it reduced activation of STAT1. Inducible NO synthase expression induced by IFN-γ in microglial cultures was reduced when the activation of ERK was prevented. We propose that IL-1β modulates IFN-γ-induced production of oxidative molecules through cross talk between STAT1 and MAPK pathways, regulating the amplitude and duration of microglial activation. Modulation of ERK was observed at 30 min, whereas inhibition of pSTAT was observed later (at 4 h), indicating that it was an early and transient phenomenon.

Galectin-8 (Gal-8) is a member of a glycan-binding protein family that regulates the immune system, among other functions, and is a target of antibodies in autoimmune disorders. However, its role in multiple sclerosis (MS), an autoimmune inflammatory disease of the central nervous system (CNS), remains unknown. We study the consequences of Gal-8 silencing on lymphocyte subpopulations and the development of experimental autoimmune encephalitis (EAE), to then assess the presence and clinical meaning of anti-Gal-8 antibodies in MS patients. Lgals8/Lac-Z knock-in mice lacking Gal-8 expression have higher polarization toward Th17 cells accompanied with decreased CCR6+ and higher CXCR3+ regulatory T cells (Tregs) frequency. These conditions result in exacerbated MOG35-55 peptide-induced EAE. Gal-8 eliminates activated Th17 but not Th1 cells by apoptosis and ameliorates EAE in C57BL/6 wild-type mice. β-gal histochemistry reflecting the activity of the Gal-8 promoter revealed Gal-8 expression in a wide range of CNS regions, including high expression in the choroid-plexus. Accordingly, we detected Gal-8 in human cerebrospinal fluid, suggesting a role in the CNS immune-surveillance circuit. In addition, we show that MS patients generate function-blocking anti-Gal-8 antibodies with pathogenic potential. Such antibodies block cell adhesion and Gal-8-induced Th17 apoptosis. Furthermore, circulating anti-Gal-8 antibodies associate with relapsing-remitting MS (RRMS), and not with progressive MS phenotypes, predicting clinical disability at diagnosis within the first year of follow-up. Our results reveal that Gal-8 has an immunosuppressive protective role against autoimmune CNS inflammation, modulating the balance of Th17 and Th1 polarization and their respective Tregs. Such a role can be counteracted during RRMS by anti-Gal-8 antibodies, worsening disease prognosis. Even though anti-Gal-8 antibodies are not specific for MS, our results suggest that they could be a potential early severity biomarker in RRMS.