Physiology of neuronal–glial networking

Astrocytes in the aging brain express characteristics of senescence-associated secretory phenotype

Cellular stress increases progressively with aging in mammalian tissues. Chronic stress triggers several signaling cascades that can induce a condition called cellular senescence. Recent studies have demonstrated that senescent cells express a senescence-associated secretory phenotype (SASP). Emerging evidence indicates that the number of cells expressing biomarkers of cellular senescence increases in tissues with aging, which implies that cellular senescence is an important player in organismal aging. In the brain, the aging process is associated with degenerative changes, e.g. synaptic loss and white matter atrophy, which lead to progressive cognitive impairment. There is substantial evidence for the presence of oxidative, proteotoxic and metabolic stresses in aging brain. A low-level, chronic inflammatory process is also present in brain during aging. Astrocytes demonstrate age-related changes that resemble those of the SASP: (i) increased level of intermediate glial fibrillary acidic protein and vimentin filaments, (ii) increased expression of several cytokines and (iii) increased accumulation of proteotoxic aggregates. In addition, in vitro stress evokes a typical senescent phenotype in cultured astrocytes and, moreover, isolated astrocytes from aged brain display the proinflammatory phenotype. All of these observations indicate that astrocytes are capable of triggering the SASP and the astrocytes in aging brain display typical characteristics of cellular senescence. Bearing in mind the many functions of astrocytes, it is evident that the age-related senescence of astrocytes enhances the decline in functional capacity of the brain. We will review the astroglial changes occurring during aging and emphasize that senescent astrocytes can have an important role in age-related neuroinflammation and neuronal degeneration.

Comparative study on the response of rat primary astrocytes and microglia to methylmercury toxicity

As the two major glial cell types in the brain, astrocytes and microglia play pivotal but different roles in maintaining optimal brain function. Although both cell types have been implicated as major targets of methylmercury (MeHg), their sensitivities and adaptive responses to this metal can vary given their distinctive properties and physiological functions. This study was carried out to compare the responses of astrocytes and microglia following MeHg treatment, specifically addressing the effects of MeHg on cell viability, reactive oxygen species (ROS) generation and glutathione (GSH) levels, as well as mercury (Hg) uptake and the expression of NF-E2-related factor 2 (Nrf2). Results showed that microglia are more sensitive to MeHg than astrocytes, a finding that is consistent with their higher Hg uptake and lower basal GSH levels. Microglia also demonstrated higher ROS generation compared with astrocytes. Nrf2 and its downstream genes were upregulated in both cell types, but with different kinetics (much faster in microglia). In summary, microglia and astrocytes each exhibit a distinct sensitivity to MeHg, resulting in their differential temporal adaptive responses. These unique sensitivities appear to be dependent on the cellular thiol status of the particular cell type.

Neuroglial roots of neurodegenerative diseases?

Neuroglia is critically important for controlling the brain homeostasis and for mounting the brain defence against pathological insults. Here, we overview recent data about the role of neuroglia in various types of neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, fronto-temporal dementia, Wernicke encephalopathy, amyotrophic lateral sclerosis and immunodeficiency virus-1-associated dementia). In all these forms of neurodegeneration, astroglia undergoes complex morphological and functional changes. The early and mid-term stages of neurodegenerative processes, and specifically of Alzheimer's disease, are associated with generalised atrophy of astroglia, whereas the later stages are characterised with an astrogliosis and microglial activation linked to neuropathological lesions such as senile plaques. Atrophic changes in astroglia may contribute to the initial cognitive deficits due to reduced glial synaptic coverage and decreased neuroprotection.

Is astrocyte calcium signaling relevant for synaptic plasticity?

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.