Elusive roles for reactive astrocytes in neurodegenerative diseases

Abstract

Astrocytes play crucial roles in the brain and are involved in the neuroinflammatory response. They become reactive in response to virtually all pathological situations in the brain such as axotomy, ischemia, infection, and neurodegenerative diseases (ND). Astrocyte reactivity was originally characterized by morphological changes (hypertrophy, remodeling of processes) and the overexpression of the intermediate filament glial fibrillary acidic protein (GFAP). However, it is unclear how the normal supportive functions of astrocytes are altered by their reactive state. In ND, in which neuronal dysfunction and astrocyte reactivity take place over several years or decades, the issue is even more complex and highly debated, with several conflicting reports published recently. In this review, we discuss studies addressing the contribution of reactive astrocytes to ND. We describe the molecular triggers leading to astrocyte reactivity during ND, examine how some key astrocyte functions may be enhanced or altered during the disease process, and discuss how astrocyte reactivity may globally affect ND progression. Finally we will consider the anticipated developments in this important field. With this review, we aim to show that the detailed study of reactive astrocytes may open new perspectives for ND.

Keywords: Alzheimer's disease; Huntington's disease; Parkinson's disease; amyotrophic lateral sclerosis; astrocyte reactivity; neuroinflammation; neuron-astrocyte interactions.

Figure 1

Main extracellular stimuli and intracellular signaling pathways leading to astrocyte reactivity in ND. Dysfunctional neurons, activated microglia, and astrocytes themselves release a wide range of molecules, which bind specific receptors at the astrocyte plasma membrane. These signals activate intracellular pathways such as the JAK/STAT3 pathway (in red), the NF-κB pathway (in orange), the CN/NFAT pathway (in purple), or the MAPK pathway (in green). The JAK/STAT3 pathway is activated by interleukins such as IL-6 or CNTF. Upon cytokine binding, the kinase JAK is activated and STAT3 is recruited to the gp-130 receptor. JAK phosphorylates STAT3, which dimerizes and translocates to the nucleus, where it binds consensus sequences (STAT responsive element, SRE) in the promoter region of its target genes. In astrocytes, the JAK/STAT3 pathway regulates the transcription of gfap, vimentin, and stat3 itself. STAT3 also induces the expression of SOCS3, the endogenous inhibitor of the JAK/STAT3 pathway, which mediates an inhibitory feedback loop. The NF-κB pathway is activated by pro-inflammatory cytokines such as TNFα and IL-1β. The canonical NF-κB pathway involves the activation of the IKK complex by receptor-bound protein kinases, leading to the phosphorylation of IκBα, the master inhibitor of NF-κB. Upon phosphorylation, IκBα is polyubiquinated and targeted to the proteasome for degradation. The NF-κB subunits p50 and p65 then translocate to the nucleus, where they activate the transcription of various target genes such as inducible nitric oxide synthase and cox2. Like the JAK/STAT3 pathway, NF-κB induces the transcription of its own inhibitor, IκBα. The CN/NFAT pathway is activated by cytokines such as TNFα or by glutamate. CN is a Ca²⁺-dependent phosphatase with many regulatory effects on the NF-κB pathway depending on the initial trigger and cellular context. CN also activates NFAT by dephosphorylation. NFAT binds specific promoter sequences and activates the expression of target genes (cox2). The MAPK pathway is activated by growth factors and cytokines which initiate a phosphorylation cascade. Upon activation, ERK1/2, p38 and c-jun also regulate gene transcription through the activation of a specific set of transcription factors (TF). ND are characterized by intracellular and/or extracellular depositions of pathologic proteins (such as Aβ, Tau, and mHtt, which are shown in blue). In ND, pathological proteins can either be endogenously expressed or internalized by astrocytes. They represent “danger associated molecular patterns” that bind specific pattern recognition receptors (PRR) at the membrane or within astrocytes. These abnormal proteins can interfere with intracellular signaling pathways, activating or inhibiting various signaling proteins (represented as lightning, see Section Additional levels of Complexity and Figure 3). The precise molecular mechanisms involved in astrocytes are mostly unknown. These complex signaling cascades strongly affect the astrocyte transcriptome and lead to astrocyte reactivity. Abbreviations: NFAT RE, NFAT responsive element; GF-R, Growth factor receptor; GPCR, G-protein coupled receptor.

Figure 2

Activation of the JAK/STAT3 pathway is a common feature of astrocyte reactivity in various ND models. Images of brain sections from several models of AD and HD (APP/PS1dE9 mice, 3xTg-AD mice, Hdh140 mice, and the murine and primate lenti-Htt82Q-based models of HD). For all ND models, STAT3 (green) accumulates in the nucleus of reactive astrocytes labeled with GFAP (red) in specific vulnerable regions (as indicated at the bottom). The first line shows the merged STAT3 (green)/GFAP (red) staining in age-matched control animals for each model. Scale bars: 20 μm (mouse) and 40 μm (primate). Adapted from Ben Haim et al. (2015).

Figure 3

Complex interactions between intracellular cascades result in a unique pattern of astrocyte reactivity in ND. (1) Many signals can trigger astrocyte reactivity (see Section How do Astrocytes become Reactive? and Figure 1) such as cytokines, purines or abnormal epitopes like aggregated proteins. Astrocytes may also react to the absence of “resting signals” from neighboring cells, which occurs with neuronal death in ND. (2) These molecular signals are detected by specific receptor complexes at the astrocyte membrane, which activate several signaling cascades such as the JAK/STAT3 pathway, the NF-κB pathway, and the MAP kinase pathway (see also Figure 1). These pathways and their effectors interact either directly or indirectly, in the cytoplasm, in the nucleus, or on DNA promoter regions (see Section Additional Levels of Complexity). Recent studies support the idea that these cascades eventually converge on the JAK/STAT3 pathway, triggering a transcriptional program of reactivity in astrocytes. This program is modulated at several levels. (3) Disease-specific proteins (mHtt, mSOD1) endogenously expressed in astrocytes as well as internalized aggregated proteins (Aβ) can directly interfere with these signaling cascades or with transcriptional activity. (4) Several environmental factors may also affect the transcriptional program in reactive astrocytes, such as nuclear factors (other transcription factors, chromatin state) and environmental factors (age, sex). In the brain, dialog with other cell types or the specific brain regions involved is another potential level of modulation. These complex signaling cascades result in (5a) common features of astrocyte reactivity such as the upregulation of GFAP and cellular hypertrophy and (5b) disease-specific outcomes. This scheme illustrates how several signals can converge on a central signaling cascade that, in turn, is modulated in a disease- and environment-specific manner to produce a particular functional outcome.

Figure 4

The secretome of reactive astrocytes. Astrocytes secrete many active molecules that influence neuronal survival and synaptic activity. Reactivity affects the pattern of secreted molecules, and thus alters neuron-astrocyte communications. In ND, reactive astrocytes may secrete higher levels of antioxidants, such as glutathione and its precursors or metabolic substrates. These changes would promote neuron survival. However, reactive astrocytes may also release fewer trophic molecules such as cholesterol, growth factors or glutamine and produce more ROS than resting astrocytes. The regulation of glutamate and GABA homeostasis may also be altered by reactivity, due to a change in their release but also their uptake. Intracellular Ca²⁺ levels are deregulated in ND, which may stimulate the release of gliotransmitters such as glutamate and ATP. Reactive astrocytes also produce more cytokines, which activate microglial cells or act as paracrine factors, maintaining glial cells in a chronically reactive state.