Microglia-derived extracellular vesicles in homeostasis and demyelination/remyelination processes

Microglia (MG) play a crucial role as the predominant myeloid cells in the central nervous system and are commonly activated in multiple sclerosis. They perform essential functions under normal conditions, such as actively surveying the surrounding parenchyma, facilitating synaptic remodeling, engulfing dead cells and debris, and protecting the brain against infectious pathogens and harmful self-proteins. Extracellular vesicles (EVs) are diverse structures enclosed by a lipid bilayer that originate from intracellular endocytic trafficking or the plasma membrane. They are released by cells into the extracellular space and can be found in various bodily fluids. EVs have recently emerged as a communication mechanism between cells, enabling the transfer of functional proteins, lipids, different RNA species, and even fragments of DNA from donor cells. MG act as both source and recipient of EVs. Consequently, MG-derived EVs are involved in regulating synapse development and maintaining homeostasis. These EVs also directly influence astrocytes, significantly increasing the release of inflammatory cytokines like IL-1β, IL-6, and TNF-α, resulting in a robust inflammatory response. Furthermore, EVs derived from inflammatory MG have been found to inhibit remyelination, whereas Evs produced by pro-regenerative MG effectively promote myelin repair. This review aims to provide an overview of the current understanding of MG-derived Evs, their impact on neighboring cells, and the cellular microenvironment in normal conditions and pathological states, specifically focusing on demyelination and remyelination processes.

Galectin-3 drives oligodendrocyte differentiation to control myelin integrity and function

Galectins control critical pathophysiological processes, including the progression and resolution of central nervous system (CNS) inflammation. In spite of considerable progress in dissecting their role within lymphoid organs, their functions within the inflamed CNS remain elusive. Here, we investigated the role of galectin-glycan interactions in the control of oligodendrocyte (OLG) differentiation, myelin integrity and function. Both galectin-1 and -3 were abundant in astrocytes and microglia. Although galectin-1 was abundant in immature but not in differentiated OLGs, galectin-3 was upregulated during OLG differentiation. Biochemical analysis revealed increased activity of metalloproteinases responsible for cleaving galectin-3 during OLG differentiation and modulating its biological activity. Exposure to galectin-3 promoted OLG differentiation in a dose- and carbohydrate-dependent fashion consistent with the 'glycosylation signature' of immature versus differentiated OLG. Accordingly, conditioned media from galectin-3-expressing, but not galectin-3-deficient (Lgals3(-/-)) microglia, successfully promoted OLG differentiation. Supporting these findings, morphometric analysis showed a significant decrease in the frequency of myelinated axons, myelin turns (lamellae) and g-ratio in the corpus callosum and striatum of Lgals3(-/-) compared with wild-type (WT) mice. Moreover, the myelin structure was loosely wrapped around the axons and less smooth in Lgals3(-/-) mice versus WT mice. Behavior analysis revealed decreased anxiety in Lgals3(-/-) mice similar to that observed during early demyelination induced by cuprizone intoxication. Finally, commitment toward the oligodendroglial fate was favored in neurospheres isolated from WT but not Lgals3(-/-) mice. Hence, glial-derived galectin-3, but not galectin-1, promotes OLG differentiation, thus contributing to myelin integrity and function with critical implications in the recovery of inflammatory demyelinating disorders.

Partial inhibition of proteasome activity enhances remyelination after cuprizone-induced demyelination

We have previously demonstrated that addition of low concentrations of lactacystin (a specific inhibitor of the proteasome) to oligodendroglial cell cultures containing a high percentage of precursor cells induces their exit from the cell cycle and their differentiation. On the other hand, we have recently shown that the mechanism of cuprizone toxicity on oligodendroglial cells involves the recruitment of microglia and their secretion of pro-inflammatory cytokines and in the increased production of oxidant species, which results in a decrease in the activities of the mitochondrial respiratory chain. In the present paper we investigated the effect of a decrease in proteasome activity induced by the injection of lactacystin in the corpus callosum in the remyelination process that normally occurs after cuprizone-induced demyelination. This treatment markedly improves the remyelination process that normally occurs in cuprizone-induced demyelination. It also attenuates the activation of NFkappaB and the recruitment of microglia and astrocytes, thus helping in the recovery of the mitochondrial respiratory chain activities that are affected by cuprizone treatment.

Partial inhibition of the proteasome enhances the activity of the myelin basic protein promoter

We have previously shown that low concentrations of a specific proteasome inhibitor accelerate exit from the cell cycle and enhance oligodendroglial cell (OLGc) differentiation. To elucidate the mechanisms involved in this process, OLGcs of the N20.1 cell line, transfected with a reporter gene driven by the MBP promoter, were treated with proteasome inhibitors and/or inhibitors of different signaling pathways. Partial proteasome inhibition resulted in enhanced activation of the MBP promoter which involved the tyrosine kinase, PI3-Akt and PKC pathways, accompanied by an increase in the levels of p21(Cip1), p27(Kip1) and Sp1 and by a decrease in Nkx2.2. Binding of Sp1 to DNA was also increased. These results were not observed when the Sp1 binding site was mutated. We conclude that the enhanced activation of the MBP promoter induced by partial inhibition of the proteasome could be due, at least in part, to the stabilization of p27(Kip1) and Sp1.

The neurotoxic effect of cuprizone on oligodendrocytes depends on the presence of pro-inflammatory cytokines secreted by microglia

In order to further characterize the still unknown mechanism of cuprizone-induced demyelination, we investigated its effect on rat primary oligodendroglial cell cultures. Cell viability was not significantly affected by this treatment. However, when concentrations of IFNgamma and/or TNFalpha having no deleterious effects per se on cell viability were added together with cuprizone, cell viability decreased significantly. In mitochondria isolated from cuprizone-treated glial cells, we observed a marked decrease in the activities of the various complexes of the respiratory chain, indicating a disruption of mitochondrial function. An enhancement in oxidant production was also observed in cuprizone and/or TNFalpha-treated oligodendroglial cells. In in vivo experiments, inhibition of microglial activation with minocycline prevented cuprizone-induced demyelination. Based on the above-mentioned results we suggest that these microglial cells appear to have a very active role in cuprizone-induced oligodendroglial cell death and demyelination, through the production and secretion of pro-inflammatory cytokines.

Involvement of the ubiquitin-mediated proteolytic system in the signaling pathway induced by ceramide in primary astrocyte cultures

The selective degradation of abnormal or short half-life proteins in eukaryotic cells proceeds through the ubiquitin-mediated proteolytic system (UbPS). The signals that tag the proteins for their ubiquitination are well known. In the present study, our aim was to investigate the relationship between the action of ceramide and the changes in the expression of certain mRNAs of the Ub pathway and in the activation of the UbPS in cultured astrocytes (ASTs). Changes in the expression of components that are known to be substrates of the UbPS and that participate in the regulation of the cell death process were also studied. Addition of different concentrations of C2 ceramide to cultured ASTs produced an increase in the expression of the Ub gene and in the gene that encodes E1, one of the enzymes involved in the ubiquitination process, without any changes on cell viability. Immunocytochemical studies showed an increase in the expression of Bcl-2 with no changes in cytochrome c. Also, there was an increase in the nuclear reactivity of NFkappaB, suggesting a translocation of this factor towards the nucleus. Western blots showed a decrease in IkappaB and its phosphorylated form as well as an increase in Bcl-2 with no changes in cytochrome c. All of these compounds appear to be acting as possible modulators of AST responses to C2 ceramide. Our results suggest that in AST primary cultures, C2 ceramide, at the concentrations used in this study, does not produce apoptosis. However, it induces an activation of the UbPS, probably as a consequence of an activation of phosphatases and kinases, or through the generation of reactive oxygen species, which act as triggering signals of the UbPS. The fundamental role of NFkappaB and Bcl-2 as antiapoptotic factors is discussed.

Apoptosis in Schwann cell cultures is closely interrelated with the activity of the ubiquitin-proteasome proteolytic pathway

Although the participation of the ubiquitin-dependent pathway and of the proteasome in apoptosis has been proposed, its role in this process is not yet clearly defined. In previous studies, we have shown that in the central nervous system of the rat, programmed cell death and the ubiquitin-dependent proteolytic pathway are closely related to each other and that different types of neurons and of glial cells, shown different types of correlation between the two phenomena. In this work, we have used lactacystin, a highly specific inhibitor of the proteasome, to explore in Schwann cell cultures the relationship between the activity of the Ub-dependent pathway and apoptosis. Apoptosis was explored analyzing changes in nuclear morphology, using the Annexin V assay and by flow cytometry. Activity of caspase-3 was also measured. Changes in the levels of ubiquitin-protein conjugates and of the ubiquitin activating enzymes, E1, as well as expression of proteins that instruct the cells to apoptosis (p53, NFkappaB-IkappaB, Bcl2), or that participate in the control and regulation of the cell cycle, were also examined. Our results indicate that the decrease in the activity of the proteasome induced by lactacystin in Schwann cells, induces apoptotic cell death through changes in the concentration of certain key proteins that are involved in the apoptosis-signaling pathways.

Relationship between the ubiquitin-dependent pathway and apoptosis in different cells of the central nervous system: effect of thyroid hormones

We have recently shown that sustained neonatal hyperthyroidism in the rat activates apoptosis of oligodendroglial cells (OLGc) and that inhibition of the proteasome-ubiquitin (Ub) pathway by lactacystin produces increased apoptosis in cerebellar granule cells (CGC). In the present study we have analyzed the relationship between the activation of the Ub-dependent pathway, the expression of the Ub genes and programmed cell death in neurons of the rat cerebellum and cerebral cortex and in OLGc. This study was carried out in normal animals, in rats submitted to sustained neonatal hyperthyroidism and in cell cultures treated with an excess of thyroid hormones. In neurons of the cerebral cortex, thyroid hormone produces an increase of Ub-protein conjugates, an enhancement in the expression of the Ub genes and an increase in apoptosis, while the opposite results are obtained in CGC. These results indicate that in neurons, the changes in the cell death program produced by thyroid hormone run in parallel with those occurring in the Ub-dependent pathway. In OLGc, thyroid hormone increases apoptosis but does not produce changes in the Ub pathway. Preliminary studies indicate that in coincidence with what occurs in optic nerves, the sciatic nerves both in controls and in hyperthyroid animals are unable to form Ub-protein conjugates. These results indicate that in cells of the CNS such as neurons, in which the Ub-dependent pathway is actively expressed, it appears to be closely correlated with apoptosis.

Lactacystin, a specific inhibitor of the proteasome, induces apoptosis and activates caspase-3 in cultured cerebellar granule cells

The multicatalytic protease complex or proteasome is a fundamental nonlysosomal tool that the cell uses to process or degrade proteins at a fast rate through the ubiquitin and ATP-dependent proteolytic pathway. Examples of these important proteins include the tumor suppressor protein p53, various cyclins, the cyclin-dependent kinase inhibitor p27, NFkappaB, IkappaB, c-fos, and c-jun. The activation of proteolytic enzymes, including certain cystein-proteases of the ced-3/ICE (interleukin-1beta-converting enzyme) family, is a characteristic feature of the apoptotic program. However, the role of the multicatalytic protease complex in apoptosis is not well known. In order to obtain further information regarding the participation of the ubiquitin-mediated pathway in the decision of the cell to execute the cell death program, we have used a specific inhibitor of the multicatalytic protease complex, lactacystin, in cultured cerebellar granule cells. Cells were obtained from the cerebellum of 6- to 8-day-old Wistar rats and cultured in Neurobasal medium supplemented with B-27. Addition of lactacystin to the cultures induced apoptosis of the granule cells in a time-dependent fashion. The morphological changes produced by the proteasome inhibitor included nuclear condensation and DNA fragmentation measured by the diphenylamine test, as well as a positive labeling by the TUNEL (terminal deoxynucleotidyltransferase mediated-dUTP nick end labeling) assay, all of them typical features of apoptosis. Concomitant with apoptosis, there were changes in the expression of the ubiquitin mRNA, a progressive depletion in the free ubiquitin pool, and an increase in the high molecular weight ubiquitin-protein conjugates. Caspase-3, a member of the ced-3/ICE family of cystein-proteases, showed a marked increase in activity in the lactacystin-treated cells. In flow cytometry studies, the amount of cells in the S phase of the cell cycle was smaller in the lactacystin-treated cells than in controls, suggesting that apoptosis could be due, in part, to an alteration of the cell cycle.

Effect of oxidant systems on the ubiquitylation of proteins in the central nervous system

Ubiquitin (Ub) modification of different proteins plays an important role in many cellular processes. However, the best studied function of Ub is the labeling of proteins committed to rapid degradation, by an ATP-dependent pathway. We previously found that this pathway is operative in the central nervous system (CNS) of adult rats (Adamo et al. [1994] J. Neurosci. Res. 38:358-364). In the present study, we examined the changes in the capacity to form high-molecular-weight Ub protein conjugates (UbPC) and the changes in the production of 2-thiobarbituric acid-reactive substances (TBARS), in the content of protein-associated carbonyl groups (PAC), and in the activity of glutamine synthetase produced by in vitro peroxidation of the cell cytosolic proteins and of the mitochondrial fraction isolated from rat brain. Under these experimental conditions, there was an increase in PAC and TBARS in the cytosol, indicating that damage to certain cellular components had occurred. Simultaneously there was a marked increase in UbPC in comparison with the nonoxidized controls. These conjugates showed an active turnover and accumulated when Ub-mediated proteolysis was inhibited. In vitro peroxidation of the mitochondrial fractions resulted in an increase in the production of PAC and in an enhancement in the formation of UbPC. These results demonstrate that the oxidized proteins can be recognized by the ubiquitylating system and that in the CNS the Ub-dependent proteolytic pathway is one of the possible mechanisms involved in the removal of cytosolic and mitochondrial fraction damaged proteins.

Effect of oxidant systems on the ubiquitylation of proteins in the central nervous system

Ubiquitin (Ub) modification of different proteins plays an important role in many cellular processes. However, the best studied function of Ub is the labeling of proteins committed to rapid degradation, by an ATP-dependent pathway. We previously found that this pathway is operative in the central nervous system (CNS) of adult rats (Adamo et al. [1994] J. Neurosci. Res. 38:358-364). In the present study, we examined the changes in the capacity to form high-molecular-weight Ub protein conjugates (UbPC) and the changes in the production of 2-thiobarbituric acid-reactive substances (TBARS), in the content of protein-associated carbonyl groups (PAC), and in the activity of glutamine synthetase produced by in vitro peroxidation of the cell cytosolic proteins and of the mitochondrial fraction isolated from rat brain. Under these experimental conditions, there was an increase in PAC and TBARS in the cytosol, indicating that damage to certain cellular components had occurred. Simultaneously there was a marked increase in UbPC in comparison with the nonoxidized controls. These conjugates showed an active turnover and accumulated when Ub-mediated proteolysis was inhibited. In vitro peroxidation of the mitochondrial fractions resulted in an increase in the production of PAC and in an enhancement in the formation of UbPC. These results demonstrate that the oxidized proteins can be recognized by the ubiquitylating system and that in the CNS the Ub-dependent proteolytic pathway is one of the possible mechanisms involved in the removal of cytosolic and mitochondrial fraction damaged proteins.