Deciphering the Astrocyte Reaction in Alzheimer's Disease

Abstract

Reactive astrocytes were identified as a component of senile amyloid plaques in the cortex of Alzheimer's disease (AD) patients several decades ago. However, their role in AD pathophysiology has remained elusive ever since, in part owing to the extrapolation of the literature from primary astrocyte cultures and acute brain injury models to a chronic neurodegenerative scenario. Recent accumulating evidence supports the idea that reactive astrocytes in AD acquire neurotoxic properties, likely due to both a gain of toxic function and a loss of their neurotrophic effects. However, the diversity and complexity of this glial cell is only beginning to be unveiled, anticipating that astrocyte reaction might be heterogeneous as well. Herein we review the evidence from mouse models of AD and human neuropathological studies and attempt to decipher the main conundrums that astrocytes pose to our understanding of AD development and progression. We discuss the morphological features that characterize astrocyte reaction in the AD brain, the consequences of astrocyte reaction for both astrocyte biology and AD pathological hallmarks, and the molecular pathways that have been implicated in this reaction.

Keywords: Alzheimer’s disease; amyloid plaques; astrocytes; glia; microglia; neurofibrillary tangles.

Figure 1

Astrocyte morphological features in the normal brain. (A) Protoplasmic astrocyte in layer II of the occipital neocortex with its typical bushy appearance depicted with immunohistochemistry for the glutamate transporter GLT-1/EAAT2. (B) Fibrous astrocytes in the white matter of the temporal lobe are rich in glial fibrillary acidic protein (GFAP). (C) Subpial interlaminar astrocytes in the frontal association neocortex of a healthy control individual form a palisade of processes which extend towards deep layers perpendicularly to the cortical surface and are GFAP-immunoreactive. (D) Perivascular astrocytes with their vascular endfeet wrapping a capillary vessel, here shown with immunohistochemistry for aldehyde dehydrogenase 1 L1 (constitutively present in all astrocytes). Scale bars: 10 μm in (A,C,D); 20 μm in (B).

Figure 2

Astrocyte-neuron glutamate-glutamine cycle and lactate shuttle. (A) Cartoon illustrating a tripartite excitatory synapse with the glutamate-glutamine cycle. (B) Cartoon illustrating a tripartite excitatory synapse and a glioneurovascular unit with the lactate shuttle. Abbreviations: AMPAR, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; ATP, adenosine triphosphate; EAAT, excitatory amino acid transporter; Gln-NH3, glutamine; Glu, glutamate; GLUT, glucose transporter; LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; mGluR, metabotropic glutamate receptor; NMDAR, N-methyl-D-aspartate receptor.

Figure 3

Astrocyte reaction to amyloid plaques in the Alzheimer’s brain. (A–D) Double fluorescent immunohistochemistry for GFAP (red) and the enzyme glutamine synthetase (green) showing the progression of astrocyte reaction (defined by GFAP immunoreactivity) in the temporal association neocortex (Brodmann’s area 38) of a healthy control subject without Alzheimer’s disease (AD) changes (A), another healthy control individual with sparse neuritic plaques (B), and two patients with an AD diagnosis (C,D). Arrowheads point to increasingly numerous clusters of GFAP-immunoreactive astrocytes, which become confluent in advanced stages of the disease. (E) Clusters of GFAP-immunoreactive astrocytes (red) surrounding dense-core amyloid plaques (Thioflavine-S positive, green) in the temporal association neocortex (Brodmann’s area 38) of an AD patient. (F) GFAP-immunoreactive astrocytes penetrate and surround dense-core amyloid plaques with their processes. Scale bars: 50 μm in (A–D), 100 μm in (E), and 10 μm in (F).

Figure 4

Astrocyte reaction to neurofibrillary tangles (NFTs) in the Alzheimer’s brain. (A–D) Fluorescent immunohistochemical staining for GFAP (A, red) with Thioflavine-S (B, green) and DAPI (C, blue) staining in the temporal association neocortex (Brodmann’s Area 38) of an individual with a diagnosis of AD. Reactive (GFAP+) astrocytes surround and penetrate extracellular “ghost” NFTs, so that these late-stage extracellular NFTs become GFAP-immunoreactive (D, merge). (E) Peroxidase-DAB immunohistochemical staining for total tau showing thorn-shaped tau-immunoreactive astrocytes in the subpial (inset 2) and mid layers (insets 1 and 3) of the frontal association cortex (Brodmann’s areas 8/9) of a subject with a diagnosis with AD. Scale bars: 10 μm in (A–D), 200 μm in (E), and 50 μm in insets 1–3.

Figure 5

Molecular signaling pathways involved in astrocyte reaction in AD. Cartoon illustrating the JAK/STAT3, MAPK, CN/NFAT and NFκB signaling cascades. JAK/STAT3 pathway: the JAKs (JAK 1, 2, 3 and Tyk2) are tyrosine-kinases associated with the intracellular domain of cytokine receptors. The binding of a cytokine ligand to its receptor triggers a dimerization or oligomerization of the associated JAK and this leads to its transphosphorylation and activation. The activated JAK then phosphorylates key tyrosine residues in the cytokine receptor, which leads to the recruitment of STAT (STAT1, 2, 3, 4, 5A, 5B or 6) and a single tyrosine phosphorylation at the Y of its C-terminus. Once phosphorylated, STAT forms homo or heterodimers and is imported to the nucleus, where it complexes with other transcription factors, notably NFkB, to bind its target genes. MAPK pathway: the p38 mitogen-activating protein kinase (p38 MAPK) is a family of serine-threonine kinases of which p38α is the most studied due to its ubiquitous expression at high levels. p38α phosphorylates multiple substrates including transcription factors, DNA and RNA binding proteins, other serine-threonine kinases (including GSK3β), and cell cycle and pro-apoptotic proteins. Calcineurin/NFAT pathway: the phosphatase function of CN is activated upon the binding of calcium and calmodulin to its regulatory subunit. Once activated, CN removes several phosphate residues from the N-terminus of the transcription factors NFATc (nuclear factor of activated T cells) proteins, named NFATc1 to 4, and exposes their nuclear localization motif, leading to their rapid entry into the nucleus. NFATc targets are largely genes encoding cytokines, growth factors and their receptors, and cell adhesion proteins, as well as many microRNAs. NFκB pathway: NFκB is a heterodimeric transcription factor composed of the subunits p65/RelA and p50, which belong to the Rel family of proteins. It is normally located in the cytoplasm in an inactive form due to its binding to the inhibitor IκB, which masks its nuclear localization signal located in the p65 subunit. Activation of IKK complex by certain extracellular signals can dissociate NFκB and IκB by promoting the phosphorylation of the IκB-α, which is a necessary step for IκB-α ubiquitination and subsequent degradation by the proteasome. Once free from IκB, NFκB can translocate to the nucleus and bind to the 10 base pair consensus-sequence GGGACTTTCC GGGRNNYYCC in the promoter of its target genes (NFκB response elements). Abbreviations: CN, calcineurin; COX, cyclo-oxygenase; ERK, extracellular signal-regulated kinases; IL, interleukin; IL1-R, interleukin 1 receptor; iNOS, inducible nitric oxide synthetase; IP3, inositol-3-phosphate; JAK, Janus kinase; MAPK, mitogen-activated protein kinase; MMPs, matrix metalloproteases; NFAT, nuclear factor of activated T cells; PL3, phospholipase 3; STAT, signal transducer and activator of transcription; TLR, toll-like receptor; TNF-α, tumor necrosis factor α; TNF-R, tumor necrosis factor receptor.