Aging‐related tau astrogliopathy (ARTAG): not only tau phosphorylation in astrocytes

Abstract

Aging-related tau astrogliopathy (ARTAG) is defined by the presence of two types of tau-bearing astrocytes: thorn-shaped astrocytes (TSAs) and granular/fuzzy astrocytes in the brain of old-aged individuals. The present study is focused on TSAs in rare forms of ARTAG with no neuronal tau pathology or restricted to entorhinal and transentorhinal cortices, to avoid bias from associated tauopathies. TSAs show 4Rtau phosphorylation at several specific sites and abnormal tau conformation, but they lack ubiquitin and they are not immunostained with tau-C3 antibodies which recognize truncated tau at Asp421. Astrocytes in ARTAG have atrophic processes, reduced glial fibrillary acidic protein (GFAP) and increased superoxide dismutase 2 (SOD2) immunoreactivity. Gel electrophoresis and western blotting of sarkosyl-insoluble fractions reveal a pattern of phospho-tau in ARTAG characterized by two bands of 68 and 64 kDa, and several middle bands between 35 and 50 kDa which differ from what is seen in AD. Phosphoproteomics of dissected vulnerable regions identifies an increase of phosphorylation marks in a large number of proteins in ARTAG compared with controls. GFAP, aquaporin 4, several serine-threonine kinases, microtubule associated proteins and other neuronal proteins are among the differentially phosphorylated proteins in ARTAG thus suggesting a hyper-phosphorylation background that affects several molecules, including many kinases and proteins from several cell compartments and various cell types. Finally, present results show for the first time that tau seeding is produced in neurons of the hippocampal complex, astrocytes, oligodendroglia and along fibers of the corpus callosum, fimbria and fornix following inoculation into the hippocampus of wild type mice of sarkosyl-insoluble fractions enriched in hyper-phosphorylated tau from selected ARTAG cases. These findings show astrocytes as crucial players of tau seeding in tauopathies.

Astrogliopathy in Tauopathies

“…These observations indicate that astrocytes containing hyper-phosphorylated tau have the capability of seeding tau to neurons and glial cells, thus highlighting the putatively cardinal role of astrocytopathy in the pathogenesis of tauopathies [106]. Moreover, inoculation of ARTAG, containing 4Rtau astrocytes, produces 3Rtau seeding in neurons and glial cells in addition to 4Rtau deposition [106]. This also points to the likely involvement of astrocytes in the development of tau-containing neuronal processes in the aged brain [181].…”

“…More precise methods in several regions and different stages of disease have demonstrated the occurrence of very large numbers of phosphorylated proteins including kinases and synaptic proteins in areas with no relationship to β-amyloid deposits and NFTs [188]. Similar studies have recognized a number of phosphorylated proteins including phospho-kinases, neurofilaments, and synaptic and other neuronal proteins, in addition to phospho-GFAP and phosphorylated aquaporine-4 in the white matter in pure cases of ARTAG [106]. Although similar studies are not available in other tauopathies, these observations suggest that tau pathology in astrocytes should be interpreted not as an isolated process but in the context of a very particular environment which is hospitable to tau phosphorylation.…”

“…Immunohistochemistry utilizing tau antibodies directed against specific amino acids of tau (amino-terminal, carboxyl-terminal, middle segments) can be useful to uncover possible sites of tau truncation in astrocytes in tauopathies in addition to the characteristic truncation at aspartic acid 421 in astrocytes in minority tauopathies. Thorn-shaped astrocytes (TSAs) are stained with antibody 394 (amino acids 394-398, corresponding to the carboxyl-terminal); antibody 229 (against amino acids 229-233, middle region) and antibody 499 (directed against amino acids 14-26, amino-terminal) [106]. Tau-containing astrocytes in FTLD-tau 301T are stained with antibodies tau 7 (directed against amino acids 426-441), 394, 229 and 499 ( Figure 7A).…”

“…Tau-enriched fractions of brain homogenates from pure ARTAG (with no associated tauopathy) inoculated into the hippocampus (dentate gyrus and cornu ammonis (CA1) of wild-type mice generate intracytoplasmic hyper-phosphorylated tau inclusions in astrocytes, oligodendrocytes and neurons near the site of injection, and in nerve fiber tracts in the fimbria and corpus callosum [106] ( Figure 12). These observations indicate that astrocytes containing hyper-phosphorylated tau have the capability of seeding tau to neurons and glial cells, thus highlighting the putatively cardinal role of astrocytopathy in the pathogenesis of tauopathies [106]. Moreover, inoculation of ARTAG, containing 4Rtau astrocytes, produces 3Rtau seeding in neurons and glial cells in addition to 4Rtau deposition [106].…”

Astrocytic tau pathologies in aged human brain

“…ARTAG occurs mainly, but not exclusively, in individuals over 60 years of age . The two major cytomorphologies of ARTAG are thorn‐shaped astrocytes (TSA) and granular or fuzzy tau immunoreactivity in processes of astrocytes (GFA) . TSA are observed in subpial, subependymal, or perivascular areas, as well as white matter.…”