Extracellular tau induces microglial phagocytosis of living neurons in cell cultures

Abstract

Tau is a microtubule-associated protein, found at high levels in neurons, and the aggregation of which is associated with neurodegeneration. Recently it was found that tau can be actively secreted from neurons, but the effects of extracellular tau on neuronal viability are not clear. In this study, we investigated whether extracellular tau 2N4R can cause neurotoxicity in primary cultures of rat brain neurons and glial cells. Cell cultures were treated with recombinant fulllength tau 2N4R isoform, and neuronal numbers, death and phosphatidylserine exposure, as well as microglial phagocytosis were analysed by fluorescent microscopy. Aggregation of tau 2N4R was assests by atomic force microscopy. We found that extracellular addition of tau induced a gradual loss of neurons over 1-2 days, without neuronal necrosis or apoptosis, but accompanied by proliferation of microglia in the neuronal-glial co-cultures. Tau addition caused exposure of the "eat-me" signal phosphatidylserine on the neuronal cell surface (without apparent apoptosis or necrosis), and this was prevented by elimination of the microglia or by inhibition of neutral sphingomyelinase. Tau also increased the phagocytic activity of pure microglia, and this was blocked by inhibitors of neutral sphingomyelinase or protein kinase C. The neuronal loss induced by tau was prevented by inhibitors of neutral sphingomyelinase, protein kinase C or the phagocytic receptor MerTK, or by eliminating microglia from the cultures. The data suggest that extracellular tau induces primary phagocytosis of stressed neurons by activated microglia, and identifies multiple ways in which the neuronal loss induced by tau can be prevented.

Tau Pathology Drives Dementia Risk-Associated Gene Networks toward Chronic Inflammatory States and Immunosuppression

“…This is particularly salient based upon the recent observation that pathological tau drives chromatin destabilization (Guo et al, 2018;Sun et al, 2018), a known source of endogenous dsRNA that can activate Ifih1 (MDA5) and trigger an IFN response outside of the CNS (Ahmad et al, 2018;Cuellar et al, 2017). These intracellular events contrast with extracellular tau seeds and fibrils, which experimental evidence suggests activate microglia NAct pathways (Pampuscenko et al, 2020;Stancu et al, 2019).…”

Microglia become hypofunctional and release metalloproteases and tau seeds after phagocytosing live neurons with P301S tau aggregates

“…The presence of intraneuronal aggregates of tau best correlate with the neuronal cell death that is associated with the clinical signs and symptoms of disease (2). The mechanism of cell death in tauopathy remains unclear, however several studies implicate microglia in non-cell-autonomous routes (3)(4)(5)(6)(7)(8).…”

“…This process is accompanied by secretion of the opsonin milk fat globule EGF 8 (MFGE8) and nitric oxide production, and leads to the transfer of tau aggregates inside the phagocytosed neurons to the microglia (17). Microglia have also been shown to phagocytose extracellular tau and become activated to phagocytose neurons in a PKC dependant manner (4). These data implicate microglia in the pathological loss of neurons and synapses in tauopathy by intensifying phagocytic activity to damaging levels.…”

Pattern Recognition to Objectively Differentiate the Etiology of Cognitive Decline: Analysis of the Impact of Stroke and Alzheimer’s Disease

“…An increasing body of research has highlighted the importance of mixed AD and stroke as a crucial area of interest as many individuals lacking cerebrovascular disease do not progress in AD despite evidence of amyloidosis [38]. One explanation for this effect that has been offered is that microglia, which are activated in cerebrovascular disease [39], are also central to more rapid proliferation of protein-tau [40]. Additionally, researchers have posited that the amyloid cascade may require both the deposition of β-amyloid 1–42 throughout the cerebrum and the inclusion of β-amyloid 1–40 in the vasculature in order to cause neurodegeneration [41].…”