Current Alzheimer Research - Volume 13, Issue 4, 2016

From birth to death, neurons are dynamically accompanied by neuroglial cells in a very close morphological and functional relationship. Three families have been classically considered within the CNS: astroglia, oligodendroglia and microglia. Many types/subtypes (including NGR2+ cells), with a wide variety of physiological and pathological effects on neurons, have been described using morphological and immunocytochemical criteria. Glio-glial, glio-neuronal and neuro-glial cell signaling and gliotransmission are phenomena that are essential to support brain functions. Morphofunctional changes resulting from the plasticity of all the glial cell types parallel the plastic neuronal changes that optimize the functionality of neuronal circuits. Moreover, neuroglia possesses the ability to adopt a reactive status (gliosis) in which, generally, new functions arise to improve and restore if needed the neural functionality. All these features make neuroglial cells elements of paramount importance when attempting to explain any physiological or pathological processes in the CNS, because they are involved in both, neuroprotection/neurorepair and neurodegeneration. There exist diverse and profound, regional and local, neuroglial changes in all involutive processes (physiological and pathological aging; neurodegenerative disorders, including Alzheimer ´s disease –AD-), but today, the exact meaning of such modifications (the modifications of the different neuroglial types, in time and place), is not well understood. In this review we consider the different neuroglial cells and their responses in order to understand the possible role they fulfill in pathogenesis, diagnosis and treatment (preventive or palliative) of AD. The existence of differentiated and/or concurrent pathogenic and neuro-protective/neuro-restorative astroglial and microglial responses is highlighted.

Glial cells and their association with neurones are fundamental for brain function. The emergence of complex neurone-glial networks assures rapid information transfer, creating a sophisticated circuitry where both types of neural cells work in concert, serving different activities. All glial cells, represented by astrocytes, oligodendrocytes, microglia and NG2-glia, are essential for brain homeostasis and defence. Thus, glia are key not only for normal central nervous system (CNS) function, but also to its dysfunction, being directly associated with all forms of neuropathological processes. Therefore, the progression and outcome of neurological and neurodegenerative diseases depend on glial reactions. In this review, we provide a concise account of recent data obtained from both human material and animal models demonstrating the pathological involvement of glia in neurodegenerative processes, including Alzheimer's disease (AD), as well as physiological ageing.

Pathological remodelling of astroglia represents an important component of the pathogenesis of Alzheimer's disease (AD). In AD astrocytes undergo both atrophy and reactivity; which may be specific for different stages of the disease evolution. Astroglial reactivity represents the generic defensive mechanism, and inhibition of astrogliotic response exacerbates b-amyloid pathology associated with AD. In animal models of AD astroglial reactivity is different in different brain regions, and the deficits of reactive response observed in entorhinal and prefrontal cortices may be linked to their vulnerability to AD progression. Reactive astrogliosis is linked to astroglial Ca2+ signalling, this latter being widely regarded as a mechanism of astroglial excitability. The AD pathology evolving in animal models as well as acute or chronic exposure to β-amyloid induce pathological remodelling of Ca2+ signalling toolkit in astrocytes. This remodelling modifies astroglial Ca2+ signalling and may be linked to cellular mechanisms of AD pathogenesis.

Traditionally the brain has been viewed as being an immune-privileged organ. However, endogenous stimuli such as the presence of misfolded or aggregated proteins, as well as systemic inflammatory events may lead to the activation of microglial cells, the brain´s innate immune system, and, subsequently, to neuroinflammation. Alzheimer's disease, the leading cause of dementia, is characterized by amyloid beta deposition and tau hyperphosphorylation. Neuroinflammation in Alzheimer's disease has been identified as major contributor to disease pathogenesis. Once activated, microglia release several pro and anti-inflammatory mediators of which several affect the function and structure of the brain. Modulation of this microglial activation in Alzheimer's disease might open new therapeutic avenues.

Dysregulation of glutamate signalling is important in Alzheimer's disease and other pathologies. There has been a focus on changes in neuronal glutamate signalling, but microglia also express glutamate receptors (GluRs), which are known to modulate their responses to neuropathology. Microglia express both metabotropic and ionotropic GluRs. Among ionotropic GluRs, microglial AMPA (α-amino-hydroxy-5-methyl-isoxazole-4-propionate)-type of GluRs (AMPA-Rs) are Ca2+ impermeable due to the expression of subunit GluA2. Upon activation of microglia, expression level of surface GluA2 subunits significantly increase, while expression of GluA1, A3 and A4 subunits on membrane surface significantly decrease. Owing to the GluA2 subunits-dominant composition, AMPA-Rs in activated microglia show little response to Glu. On the other hand, microglia lacking GluA2 show higher Ca2+-permeability, consequently inducing a significant increase in the release of the pro-inflammatory cytokine, such as TNF-α. It is suggested that membrane translocation of GluA2-containing AMPA-Rs in activated microglia has functional importance. Thus, dysfunction or decreased expression of GluA2 reported in patients with neurodegenerative diseases such as Alzheimer’s and Creutzfeldt-Jakob disease may accelerate Glu neurotoxicity via excess release of proinflammatory cytokines from microglia, causing more neuronal death.

The central nervous system (CNS) shows dynamic immune and inflammatory responses to a variety of insults having crucial implications for reactive gliosis. Glial cells in the CNS serve not only as the source, but also as targets of proinflammatory mediators. Undoubtedly, these cells efficiently work towards the disposal of tissue debris and promotion of wound healing as well as tissue repair. However, these non-neuronal glial cells synthesize and release numerous inflammatory mediators, which can be detrimental to neurons, axons, myelin, and the glia themselves. While an acute insult is typically transient and unlikely to be detrimental to neuronal survival, chronic neuroinflammation is a long-standing and often self-perpetuating response, which persists even long after the initial injury or insult. It can serve as a point of origin for diverse neurological disorders including Alzheimer's disease. Accumulating evidence demonstrates the contribution of metabolic dysfunction and mitochondrial failure to the pathogenesis of neuroinflammatory and neurodegenerative diseases. Neurodegenerative conditions are also characterized by increased oxidative and endoplasmic reticulum stresses and autophagy defects. Furthermore, neuroinflammatory conditions are accompanied by an alteration in glial energy metabolism. Here, we comprehensively review the metabolic hallmarks of glia-mediated neuroinflammation and how the glial metabolic shift orchestrates the neuroinflammatory response and pathophysiology of diverse neurological disorders.

Microglia and astrocytes are the major source of cytokines in Alzheimer,s disease (AD). CX3CR1 is a delta chemokine receptor found in microglia and its neuronal ligand, Fractalkine, has two isoforms: an anchored-membrane isoform, and a soluble isoform. The reduced soluble fractalkine levels found in the brain (cortex/hippocampus) of aged rats, may be a consequence of neuronal loss. This soluble fractalkine maintains microglia in an appropiate state by interacting with CX3CR1. The ablation of the CX3CR1 gene in mice overexpressing human amyloid precursor protein (APP/PS-1) increased cytokine levels, enhanced Tau pathology and worsened behavioural performance in these mice. However, CX3CR1 deficiency resulted in a gene dose-dependent Aβ clearance in the brain, and induced microglial activation. In addition, CX3CR1 deficiency can have benefical effects by preventing neuronal loss in the 3xTg model. In fact, CX3CR1 deficiency increases microglial phagocytosome activity by inducing selective protofibrillar amyloid-beta phagocytosis in microglial cells in transgenic AD models. On the other hand, the fractalkine membrane isoform plays a differential role in amyloid beta clearance and Tau deposition. This anchored membrane FKN signalling might increase amyloid pathology while soluble fractalkine levels could prevent taupathies. However, in human AD, the only published study has reported higher systemic fractalkine levels in AD patients with cognitive impairment. In mouse models, inflammatory activation of microglia accelerates Tau pathology. Studies in transgenic mice with fractalkine null mice suggest that APP/PS-1 mice deficient for the anchored membrane-fractalkine isoform exhibited enhanced neuronal MAPT phosphorylation despite their reduced amyloid burden. The soluble fractalkine overexpression with adenoviral vectors reduced tau pathology and prevented neurodegeneration in a Tg4510 model of taupathy Finally, animals with Aβ (1-42) infused by lentivirus (cortex) or mice with the P301L mutation (frontotemporal dementia) had caspase-3 activation (8-fold) and higher proinflammatory TNF alpha levels and p-Tau deposits at 4 weeks postinfusion. Thus, the CX3CR1/Fractalkine axis regulates microglial activation, the clearance of amyloid plaque and plays a role in p-Tau intraneuronal accumulation in rodent models of AD.

Oligodendrocytes are specialised glial cells that myelinate CNS axons. Myelinated axons are bundled together into white matter tracts that interconnect grey matter areas of the brain and are essential for rapid, integrated neuronal communication and cognitive function. Life-long generation of oligodendrocytes is required for myelination of new neuronal connections and repair of myelin lost through natural ‘wear and tear’. This is the function of a substantial population of adult oligodendrocyte progenitors (OPs). Notably, there is white matter shrinkage and decreased myelination in the ageing brain, which is accelerated in dementia. The underlying causes of myelin loss in dementia are unresolved, but it implies a decline in the regenerative capacity of OPs. A feature of OPs is that they form neuron-glial synapses and respond to glutamate released by neurons via a range of glutamate receptors. Glutamate neurotransmission onto OPs is proposed to regulate their proliferation and differentiation into myelinating oligodendrocytes. Here, we discuss evidence that deregulation of glutamate neurotransmission in dementia and compromised generation of oligodendrocytes from OPs are key features of myelin loss and associated cognitive decline.

Neurogenesis is maintained in both neonatal and adult brain, although it is dramatically reduced in aged neurogenic brain region such as the subgranular layer and subventricular zone of the dentate gyrus (DG). Astrocytes play important roles for survival and maintenance of neurons as well as maintenance of neurogenic niche in quiescent state. Aβ can induce astrocyte activation which give rise to produce reactive oxygen species (ROS) and cytotoxic cytokines and chemokines, and subsequently induce neuronal death. Unfortunately, the current therapeutic medicines have been limited to reduce the symptoms and delay the pathogenesis of Alzheimer’s disease (AD), but not to cure it. Stem cells enhance neurogenesis and Aβ clearing as well as improved cognitive impairment. Neurotrophins and growth factors which are produced from both stem cells and astrocytes also have neuroprotective effects via neurogenesis. Secreted factors from both astrocytes and neural stem cells also are influenced in neurogenesis and neuron survival in neurodegenerative diseases. Transplanted stem cells overexpressing neurogenic factors may be an effective and therapeutic tool to enhance neurogenesis for AD.

Adrenomedullin (AM) is a potent vasodilator peptide highly expressed throughout the brain and originally isolated from pheochromocytoma cells. In addition to its vasoactive properties, AM is considered a neuromodulator that possesses antiapoptotic and antioxidant properties that suggest that this peptide can protect the brain from damage. In a previous study, we found that AM exerts a neuroprotective action in the brain and that this effect may be mediated by regulation of nitric oxide synthases, matrix metalloproteases, and inflammatory mediators. AM upregulation contributes to neuroprotection, but understanding the precise roles played by AM and its receptor (RAMP2) in neurodegenerative diseases including Alzheimer´s disease (AD), awaits further research. In search of Alzheimer´s biomarkers, the expression levels of peptides with endothelial vasodilatory action, including AM, were found to be significantly altered in mild AD or during pre-dementia stage of mild cognitive impairment. These studies concluded that ratio of AM or its precursor fragment mid-regional proAM in blood hold promise as diagnostic marker for AD. We are now presenting a study regarding the hypothesis that the AMRAMP2 system might be implicated in the pathophysiology of AD.

Alzheimer’s disease (AD) is a neurodegenerative disorder marked by progressive impairment of cognitive ability. Patients with AD display neuropathological lesions including plaques, neurofibrillary tangles, and neuronal loss in brain regions linked to cognitive functions. Despite progress in uncovering many of the factors that contribute to the etiology of this disease, the cause of neuronal death is largely unknown. Neuroinflammation seems to play a critical role in the pathogenesis of AD. Inflammatory processes in the brain are mainly mediated by the intrinsic innate immune system consisting of astrocytes and microglial cells, and cytokine, chemokine, and growth factor signaling molecules. However mounting evidence suggest that the Central Nervous System (CNS) is accessible to lymphocytes and monocytes from the blood stream, indicating that there is an intense crosstalk between the immune and the CN systems. On the other hand, some AD-specific brain-derived proteins or metabolites may enter the plasma through a deficient blood-brain barrier, and exert some measurable signaling properties in peripheral cells. The goals of this review are: 1) to explore the evidences of changes in signaling pathways that could mediate both central and peripheral manifestations of AD, and 2) to explore whether changes in immune cells, particularly lymphocytes, could contribute to AD pathogenesis.

New concepts about Alzheimer's disease (AD), considered as a clinical-biological entity, make essential the definition of biomarkers that could be used for the in vivo diagnosis of the disorder before dementia develops. Different types of genetic, biochemical and neuroimaging markers have been described, highlighting some of the changes that occur in the brain during the course of the disease, yet there is little proof of their pathognomonic and diagnostic value. Furthermore, many of the assays used are difficult to perform, the equipment/reagents are expensive or potentially hazardous (e.g.; use of radioactive compounds, CSF extraction). Thus, there is a need to define more suitable and convenient approaches, such as the determination of blood parameters that are easy to obtain and that can be repeated as necessary without contraindications. These data can be used by algorithms that combine specific and non-specific changes to classify patients at different stages of AD and/or distinguish AD from other related diseases with a greater specificity and reliability (> 80%). The blood parameters considered in this review are varied, including: β-amyloid, tau, apolipoproteins and proteins, as well as the metabolic behavior of blood cells, etc. Among the proteins, cytokines/chemokines and other cell factors related to both neuro-inflammatory and peripheral-inflammatory processes in AD are of prime importance. New technologies to detect and quantify these substances, reasonably priced such as the vibrational spectroscopy, panels of parameters and algorithms to assess the results, would be fundamental for the early AD diagnosis and to define new potential therapies.