Current Drug Targets - Volume 14, Issue 11, 2013

Astrocytes, classically considered as supportive cells for neurons without a direct role in brain information processing, are emerging as relevant elements in brain physiology through their ability to regulate neuronal activity and synaptic transmission and plasticity. In relation to the key role of astrocyte-neuron interactions in synaptic physiology, accumulating evidence suggests that dysfunctions of neuron-astrocyte signaling may be linked to the pathology of various neurological and neurodegenerative diseases. In this article, we summarize the evidence supporting the importance of astrocyte-neuron communication in synaptic physiology, which have led to reveal astrocytes as integral elements of synaptic function. We also discuss how this novel view of astrocytic functions on brain physiology is prompting us to reconsider the possible astrocytic roles in brain diseases, and specifically in depression.

The present paper reviews astrocyte pathology in major depressive disorder (MDD) and proposes that reductions in astrocytes and related markers are key features in the pathology of MDD. Astrocytes are the most numerous and versatile of all types of glial cells. They are crucial to the neuronal microenvironment by regulating glucose metabolism, neurotransmitter uptake (particularly for glutamate), synaptic development and maturation and the blood brain barrier. Pathology of astrocytes has been consistently noted in MDD as well as in rodent models of depressive-like behavior. This review summarizes evidence from human postmortem tissue showing alterations in the expression of protein and mRNA for astrocyte markers such as glial fibrillary acidic protein (GFAP), gap junction proteins (connexin 40 and 43), the water channel aquaporin-4 (AQP4), a calcium-binding protein S100B and glutamatergic markers including the excitatory amino acid transporters 1 and 2 (EAAT1, EAAT2) and glutamine synthetase. Moreover, preclinical studies are presented that demonstrate the involvement of GFAP and astrocytes in animal models of stress and depressive-like behavior and the influence of different classes of antidepressant medications on astrocytes. In light of the various astrocyte deficits noted in MDD, astrocytes may be novel targets for the action of antidepressant medications. Possible functional consequences of altered expression of astrocytic markers in MDD are also discussed. Finally, the unique pattern of cell pathology in MDD, characterized by prominent reductions in the density of astrocytes and in the expression of their markers without obvious neuronal loss, is contrasted with that found in other neuropsychiatric and neurodegenerative disorders.

Recently, mood disorders have been discussed to be characterized by glial pathology. The protein S100B, a growth and differentiation factor, is located in, and may actively be released by astro- and oligodendrocytes. This protein is easily assessed in human serum and provides a useful parameter for glial activation or injury. Here, we review studies investigating the glial marker S100B in serum of patients with mood disorders. Studies consistently show that S100B is elevated in mood disorders; more strongly in major depressive than bipolar disorder. Consistent with the glial hypothesis of mood disorders, serum S100B levels interact with age with higher levels in elderly depressed subjects. Successful antidepressive treatment has been associated with serum S100B reduction in major depression, whereas there is no evidence of treatment effects in mania. In contrast to the glial marker S100B, the neuronal marker protein neuron-specific enolase is unaltered in mood disorders. Recently, serum S100B has been linked to specific imaging parameters in the human white matter suggesting a role for S100B as an oligodendrocytic marker protein. In sum, serum S100B can be regarded as a promising in vivo biomarker for mood disorders deepening the understanding of the pathogenesis and plasticity-changes in these disorders. Future longitudinal studies combining serum S100B with other cell-specific serum parameters and multimodal imaging are warranted to further explore this serum protein in the development, monitoring and treatment of mood disorders.

Numerous clinical evidences support the notion that glial changes in fronto-limbic brain areas could contribute to the pathophysiology of mood disorders. Glial alterations have been reported not only in patients, but also in various kinds of animal models for depression. Molecular and cellular data suggest that all the major classes of glial cells are affected in these conditions, including astrocytes, oligodendrocytes, NG2-positive cells and microglia. The aim of this review was to summarize the currently available experimental results demonstrating alterations in glial morphology and functioning in animal models for mood disorders. Better understanding of these glial changes affecting neuronal activity could help us to identify novel targets for the development of antidepressant drugs.

Traditionally, microglia have been considered to act as macrophages of the central nervous system. While this concept still remains true it is also becoming increasingly apparent that microglia are involved in a host of nonimmunological activities, such as monitoring synaptic function and maintaining synaptic integrity. It has also become apparent that microglia are exquisitely sensitive to perturbation by environmental challenges. The aim of the current review is to critically examine the now substantial literature that has developed around the ability of acute, sub-chronic and chronic stressors to alter microglial structure and function. The vast majority of studies have demonstrated that stress promotes significant structural remodelling of microglia, and can enhance the release of pro-inflammatory cytokines from microglia. Mechanistically, many of these effects appear to be driven by traditional stress-linked signalling molecules, namely corticosterone and norepinephrine. The specific effects of these signalling molecules are, however, complex as they can exert both inhibitory and suppressive effects on microglia depending upon the duration and intensity of exposure. Importantly, research has now shown that these stress-induced microglial alterations, rather than being epiphenomena, have broader behavioural implications, with the available evidence implicating microglia in directly regulating certain aspects of cognitive function and emotional regulation.

Antidepressant drugs such as the serotonin (5-HT)/norepinephrine (NE) and dopamine (DA) reuptake inhibitors activate monoaminergic neurotransmission in various brain regions, such as the amygdala, the frontal cortex or the hippocampus. Although this property is well established, the post-synaptic mechanisms by which these pharmacological agents exert therapeutic activity in major depressive disorders (MDD) is not fully understood. Recent clinical and preclinical studies have indicated that the density and reactivity of glia and more particularly of astrocytes are reduced in MDD patients. These data along with the fact that astrocytes express monoaminergic transporters and receptors make these cells putative targets for antidepressant treatments. Accordingly, in vitro evidence has demonstrated that the application of various classes of antidepressant drugs on rodent primary astrocyte cultures elicits a wide spectrum of responses, from the rise in cytosolic calcium concentrations, as a marker of cellular activity, to the release of glucose metabolites, gliotransmitters and neurotrophic factors. Remarkably, antidepressant drugs also attenuate the release of inflammatory molecules from reactive astrocytes or microglia, suggesting that part of the beneficial effects in depressed patients or animal models of depression might result from the ability of antidepressants to regulate the synthesis and release of psychoactive substances acting on both pre- and post-synaptic neurons. Among the many long-term targets of antidepressant drugs, brainderived neurotrophic factor (BDNF) has been well studied because of the positive influence on adult hippocampal neurogenesis, synaptogenesis and the local serotonergic tone. This review will illustrate how the concept of the tripartite synapse, which is classically associated with different forms of plasticity involving glutamate, could be expanded to the monoaminergic systems to regulate antidepressant drug responses. The recent in vivo data supporting that hippocampal astrocytes act in concert with neurons to release BDNF under pharmacological conditions and thereby regulate different facets of anxiolytic-/antidepressant-like activities through neurogenesis-dependent and independent mechanisms will be emphasized.

With a lifetime prevalence of more than 16% worldwide, major depressive disorder is one of the most common psychiatric disorders. Only one third of patients experience a complete therapeutic improvement with the use of current antidepressant drugs, with a therapeutic effect appearing only after several weeks of treatment. Hence, a better understanding of the mechanisms of action of current antidepressant treatments is needed to ultimately identify new targets and enhance beneficial effects. Given the intimate relationships between astrocytes and neurons at synapses and the ability of astrocytes to “sense” neuronal communication and release gliotransmitters, an attractive hypothesis is emerging stating that the effects of antidepressants on brain function could be, at least in part, mediated by direct influences of astrocytes on neuronal networks. This review aims at highlighting the involvement of astrocytes and gliotransmission in the antidepressant effects of both non- and pharmacological therapies.

There is growing evidence that astrocytes are involved in the neuropathology of major depression. In particular, decreases in glial cell density observed in the cerebral cortex of individuals with major depressive disorder are accompanied by a reduction of several astrocytic markers suggesting that astrocyte dysfunction may contribute to the pathophysiology of major depression. In rodents, glial loss in the prefrontal cortex is sufficient to induce depressive-like behaviors and antidepressant treatment prevents the stress-induced reduction of astrocyte number in the hippocampus. Collectively, these data support the existence of a link between astrocyte loss or dysfunction, depressive-like behavior and antidepressant treatment. Astrocytes are increasingly recognized to play important roles in neuronal development, neurotransmission, synaptic plasticity and maintenance of brain homeostasis. It is also well established that astrocytes provide trophic, structural, and metabolic support to neurons. In this article, we review evidence that antidepressants regulate energy metabolism and neurotrophic factor expression with particular emphasis on studies in astrocytes. These observations support a role for astrocytes as new targets for antidepressants. The contribution of changes in astrocyte glucose metabolism and neurotrophic factor expression to the therapeutic effects of antidepressants remains to be established.

Activated glial cells are capable of generating various inflammatory mediators, including cytokines, nitric oxide and reactive oxygen species. These potentially neurotoxic molecules have been suggested to play a role in the etiology and development of depression. Accumulating evidence indicates that antidepressants have inhibitory effects on inflammatory activation of glial cells and confer neuroprotection under neuropathological conditions. Such efficacy of antidepressants appears to depend on suppressing microglial production of inflammatory substances and up-regulating both astrocytic secretion of neurotrophins and astrocytic glutamine synthase, which converts neurotoxic glutamate into non-toxic glutamine. Therefore, glial cells, both as source and target of inflammatory molecules, may represent a potential promising target involved in the pathophysiology of depression. Moreover, antidepressants have the possibility to be useful treatment, not only for depression, but for a broad spectrum of neuroinflammatory and neurodegenerative disorders where the pathogenesis is associated with glial activation.

Functional alterations in synaptic contacts in specific brain areas are a hallmark of major depressive disorder (MDD). Antidepressant treatments not only readjust the aberrant concentrations of neurotransmitters in the synaptic clefts, but have the capacity to reshape neuronal circuits by affecting synaptogenesis and synaptic stabilization in specific regions of the brain. Nevertheless, the underlying molecular mechanisms are still unclear. Glial cells are active partners of neurons in orchestrating molecular signals that regulate the arrangement of neuronal circuits both in the developing and adult brain. Here, we present evidences indicating that glial cells might be substrates of antidepressant action for restructuring neuronal networks that has become miswired after the onset or progression of MDD. We aim to offer an alternative approach (a “gliocentric” view) to study this complex neuropsychiatric disorder and to identify alternative mechanisms of action for the currently available antidepressant therapies. Such knowledge may help to improve current treatment regimens or identify novel targets for the development of more efficacious antidepressant drugs.

The morphological and functional diversity of astrocytes, and their essential contribution in physiological and pathological conditions, are starting to emerge. However, experimental systems to investigate neuron-glia interactions and develop innovative approaches for the treatment of central nervous system (CNS) disorders are still very limited. Fluorescent reporter genes have been used to visualize populations of astrocytes and produce an atlas of gene expression in the brain. Knock-down or knock-out of astrocytic proteins using transgenesis have also been developed, but these techniques remain complex and time-consuming. Viral vectors have been developed to overexpress or silence genes of interest as they can be used for both in vitro and in vivo studies in adult mammalian species. In most cases, high transduction efficiency and long-term transgene expression are observed in neurons but there is limited expression in astrocytes. Several strategies have been developed to shift the tropism of lentiviral vectors (LV) and allow local and controlled gene expression in glial cells. In this review, we describe how modifications of the interaction between the LV envelope glycoprotein and the surface receptor molecules on target cells, or the integration of cell-specific promoters and miRNA post-transcriptional regulatory elements have been used to selectively express transgenes in astrocytes.

Radiation-induced lung fibrosis (RILF) is a severe side effect of radiotherapy in lung cancer patients that presents as a progressive pulmonary injury combined with chronic inflammation and exaggerated organ repair. RILF is a major barrier to improving the cure rate and well-being of lung cancer patients because it limits the radiation dose that is required to effectively kill tumor cells and diminishes normal lung function. Although the exact mechanism is unclear, accumulating evidence suggests that various cells, cytokines and regulatory molecules are involved in the tissue reorganization and immune response modulation that occur in RILF. In this review, we will summarize the general symptoms, diagnostics, and current understanding of the cells and molecular factors that are linked to the signaling networks implicated in RILF. Potential approaches for the treatment of RILF will also be discussed. Elucidating the key molecular mediators that initiate and control the extent of RILF in response to therapeutic radiation may reveal additional targets for RILF treatment to significantly improve the efficacy of radiotherapy for lung cancer patients.

The term non-alcoholic fatty liver disease (NAFLD) comprises at least four pathological entities (definite nonalcoholic steatohepatitis [NASH], borderline “zone 3” pattern, borderline “zone 1” pattern, not steatohepatitis with steatosis) with distinct patterns of lipid storage, fibrosis, and hepatocyte injury. Recent pathophysiological advances hold promise to provide much needed surrogate non-invasive biomarkers to detect steatohepatitis, fibrosis, and monitor NASH progression (or resolution, in the setting of clinical trials) without the cumbersome use of liver biopsy. Herein, we reviewed the current status of multimodal biomarker candidates derived from biochemical and genetic studies of NASH, as well as potential markers derived from imaging studies. A literature search was conducted in March 2013 on PubMed, Ovid Embase, Ovid Medline and Scopus using the following search terms: steatosis, non-alcoholic steatohepatitis, biomarker, genetics, imaging, clinical trials. Rather than to biopsy, the identification of steatohepatitis and fibrosis may originate primarily from prespecified multimodal biomarker data, including positive findings on serum or genetic biomarkers, and imaging tests like MR elastography or Fibroscan. In the setting of clinical trials, it seems recommendable to widen and expand the therapeutic vision beyond insulin resistance and focus on trials in very early NASH stages. The paradigm shift towards an earlier noninvasive characterization and diagnosis of NASH and fibrosis will be crucial to redefine and establish successful interventional trials.

Vitamin D (VitD) comes from sunlight exposure and food intake. Apart from regulating calcium homeostasis and bone function, its levels also associate with the presence of development of adenocarcinoma. VitD can interact with VitD receptor (VDR), which heterodimerizes with retinoic X receptor (RXR) and then induces transcription of proteins that function in cell proliferation, differentiation, apoptosis, and angiogenesis. We reviewed and discussed the genes and their associated polymorphisms involved in the correlation between development of adenocarcinoma and VitD deficiency to highlight how VitD may be instrumental in cancerization. Furthermore, pilot epidemiological data show that the detection of 25-hydroxy-Vitamin D3 ((36.5±10.7 nmol/L, n=129) vs (81.4±19.8 nmol/L, n=81)) can be a promising approach in cancer diagnosis. In this review, we suggest that 25-hydroxy-Vitamin D3 can act as an indicator and/or risk assessment factor in early diagnosis, prognosis and treatment of adenocarcinoma.