Rommy von Bernhardi

Microglial cell dysregulation in brain aging and neurodegeneration.

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergoes 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 (NO) 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 the reduction of protective activation and the facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegenerative diseases.

Aging-dependent changes of microglial cells and their relevance for neurodegenerative disorders.

J. Neurochem. (2010) 112, 1099–1114.

Expression of Scavenger Receptors in Glial Cells COMPARING THE ADHESION OF ASTROCYTES AND MICROGLIA FROM NEONATAL RATS TO SURFACE-BOUND β-AMYLOID

Astrocytes and microglia associate to amyloid plaques, a pathological hallmark of Alzheimer disease. Microglia are activated by and can phagocytose β-amyloid (Aβ). Scavenger receptors (SRs) are among the receptors mediating the uptake of fibrillar Aβ in vitro. However, little is known about the function of the astrocytes surrounding the plaques or the nature of their interaction with Aβ. It is unknown whether glial cells bind to nonfibrillar Aβ and if binding of astrocytes to Aβ depends on the same Scavenger receptors described for microglia. We determined the binding of glia to Aβ by an adhesion assay and evaluated the presence of scavenger receptors in glial cells by immunocytochemistry, immunohistochemistry of brain sections, and immunoblot. We found that astrocytes and microglia from neonatal rats adhered in a concentration-dependent manner to surfaces coated with fibrillar Aβ or nonfibrillar Aβ. Fucoidan and poly(I), known ligands for SR-type A, inhibited adhesion of microglia and astrocytes to Aβ and also inhibited Aβ phagocytosis. In contrast, a ligand for SR-type B like low density lipoprotein, did not compete glial adhesion to Aβ. Microglia presented immunodetectable SR-BI, SR-AI/AII, RAGE, and SR-MARCO (macrophage receptor with collagenous structure, a member of the SR-A family). Astrocytes presented SR-BI and SR-MARCO. To our knowledge, this is the first description of the presence of SR-MARCO in astrocytes. Our results indicate that both microglia and astrocytes adhere to fibrillar and nonfibrillar Aβ. Adhesion was mediated by a fucoidan-sensitive receptor. We propose that SR-MARCO could be the Scavenger receptor responsible for the adhesion of astrocytes and microglia to Aβ.

Alzheimer's Disease: Redox Dysregulation As a Common Denominator for Diverse Pathogenic Mechanisms

Alzheimers disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

Glial cell dysregulation: a new perspective on Alzheimer disease.

Alzheimer disease (AD) is a major cause of dementia. Several mechanisms have been postulated to explain its pathogenesis, beta-amyloid (Aß toxicity, cholinergic, dysfunction, Tau hyperphosphorylation, oxidative damage, synaptic dysfunction and inflammation secondary to senile plaques, among others. Glial cells are the major producers of inflammatory mediators, and cytotoxic activation of glial cells is linked to several neurodegenerative diseases; however, whether inflammation is a consequence or the cause of neurodegeneration is still unclear. I propose that inflammation and cellular stress associated with aging are key events in the development of AD through the induction of glial dysfunction. Dysregulated inflammatory response can elicit glial cell activation by compounds which are normally poorly reactive. Inflammation can also be the major cause of defective handling of Aß and the amyloid precursor protein (APP). Here I review evidence that support the proposal that dysfunctional glia and the resulting neuroinflammation can explain many features of AD. Evidence supports the notion that damage caused by inflammation is not only a primary cause of neurodegeneration but also an inducer for the accumulation of Aß in AD. Dysfunctional glia can result in im paired neuronal function in AD, as well as in many progressive neurodegenerative disorders. We show that microglial cell activation is enhanced under pro-inflammatory conditions, indicating that glial cell responses to Aß related proteins can be critically dependent on the priming of glial cells by pro-inflammatory factors.

Expression of α2‐macroglobulin receptor/low density lipoprotein receptor‐related protein (LRP) in rat microglial cells

Low density lipoprotein receptor‐related protein (LRP) participates in the uptake and degradation of several ligands implicated in neuronal pathophysiology including apolipoprotein E (apoE), activated α2 ‐macroglobulin (α2M*) and β‐amyloid precursor protein (APP). The receptor is expressed in a variety of tissues. In the brain LRP is present in pyramidal‐type neurons in cortical and hippocampal regions and in astrocytes that are activated as a result of injury or neoplasmic transformation. As LRP is expressed in the monocyte/macrophage cell system, we were interested in examining whether LRP is expressed in microglia. We isolated glial cells from the brain of neonatal rats and LRP was immunodetected both in microglial cells and in astrocytes expressing glial fibrillar acidic protein (GFAP). Microglial cells were able to bind and internalize LRP‐specific ligand, α2M*. The internalization was inhibitable by RAP, with a Kd of 1.7 nM. The expression of LRP was up‐regulated by dexamethasone, and down‐regulated by lipopolysaccharide (LPS), gamma interferon (IFN‐γ) or a combination of both. LRP was less sensitive to dexamethasone in activated astrocytes than in microglia. We provided the first analysis of LRP expression and regulation in microglia. Our results open the possibility that microglial cells could be related to the participation of LRP and its ligands in different pathophysiological states in brain. J. Neurosci. Res. 60:401–411, 2000

Microglial reactivity to β-amyloid is modulated by astrocytes and proinflammatory factors

The brains of Alzheimers disease (AD) patients present activated glial cells, amyloid plaques and dystrophic neurites. The core of amyloid plaques is composed of aggregated amyloid peptide (Abeta), a peptide known to activate glial cells and to have neurotoxic effects. We evaluated the capability of glial cells to mediate Abeta(1-42) cytotoxicity in hippocampal cultures. Conditioned media obtained from microglial cultures exposed to Abeta induced apoptosis of hippocampal cells. This pro-apoptotic effect was not observed in hippocampal cultures exposed to conditioned media obtained from mixed glial (astrocytes and microglia) cultures that had been exposed to Abeta. Microglia exposed to Abeta responded with reactive morphological changes, induction of iNOS, elevated nitric oxide production and decreased reductive metabolism. All these responses were attenuated by the presence of astrocytes. This astrocyte modulation was however, not observed when glial cells were exposed to proinflammatory factors (LPS+Interferon-gamma) alone or in combination with Abeta. Our results suggest that astrocytes and proinflammatory molecules are determining factors in the response of microglia to Abeta.

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 activation.

Prenatal to Early Postnatal Nicotine Exposure Impairs Central Chemoreception and Modifies Breathing Pattern in Mouse Neonates: A Probable Link to Sudden Infant Death Syndrome

Nicotine is a neuroteratogen and is the likely link between maternal cigarette smoking during pregnancy and sudden infant death syndrome (SIDS). Osmotic minipumps were implanted in 5–7 d CF1 pregnant mice to deliver nicotine bitartrate (60 mg Kg−1 day−1) or saline (control) solutions for up to 28 d. Prenatal to early postnatal nicotine exposure did not modify the number of newborns per litter or their postnatal growth; however, nicotine-exposed neonates hypoventilated and had reduced responses to hypercarbia (inhalation of air enriched with 10% CO2 for 20 min) and hypoxia (inhalation of 100% N2 for 20 s) at postnatal days 0–3 (P0–P3). In contrast, at postnatal day 8, nicotine-exposed neonates were indistinguishable from controls. Isolated brainstem–spinal cord preparations obtained from P0 to P3 nicotine-exposed neonates showed fictive respiration with respiratory cycles longer and more irregular than those of controls, as indicated by high short- and long-term variability in Poincaré plots. In addition, their responses to acidification were reduced, indicating compromise of central chemoreception. Furthermore, the cholinergic contribution to central chemosensory responses switched from muscarinic receptor to nicotinic receptor-based mechanisms. No significant astrogliosis was detectable in the ventral respiratory group of neurons with glial fibrillary acidic protein immunohistochemistry. These results indicate that nicotine exposure affects the respiratory rhythm pattern generator and causes a decline in central chemoreception during early postnatal life. Consequently, breathing would become highly vulnerable, failing to respond to chemosensory demands. Such impairment could be related to the ventilatory abnormalities observed in SIDS.

Mannose receptor is present in a functional state in rat microglial cells.

We studied the expression of the mannose receptor (ManR) in rat microglial cells. Microglial cells are the central nervous system resident macrophages, key participants of the innate immune response. ManR is a differentiation marker and a relevant glycoprotein for the phagocytic and endocytic function of macrophages. Because there is evidence suggesting that ManR could mediate some of the nonenzymatic effects of acetilcholinesterase (AchE) and the enzyme seems to be involved in Alzheimers disease (AD), we looked for ManR in microglia, evaluating the functionality of the receptor. We isolated microglial cells from the brain of 2‐day‐old neonatal rats. Microglial cells, identified by their specific staining with the lectin Griffonia simplicifolia, expressed ManR, being detected by immunocytochemistry, Western blot, and immunoprecipitation. Microglial ManR was downregulated by lipopolysaccharide (LPS) and upregulated by dexamethasone, as described for peripheral macrophages. Microglial ManR was functional and able to internalize horseradish peroxidase (HRP), a known ManR ligand, in a mannan‐inhibitable manner. The presence of a functional ManR in microglia opens the possibility that ManR could participate in multiple physiologic and pathologic conditions in the central nervous system (CNS), including inflammation, ischaemia, and neurodegerative diseases such as AD. J. Neurosci. Res. 58:387–395, 1999.