Reversible Tau Phosphorylation Induced by Synthetic Torpor in the Spinal Cord of the Rat

Abstract

Tau is a key protein in neurons, where it affects the dynamics of the microtubule system. The hyperphosphorylation of Tau (PP-Tau) commonly leads to the formation of neurofibrillary tangles, as it occurs in tauopathies, a group of neurodegenerative diseases, including Alzheimer's. Hypothermia-related accumulation of PP-Tau has been described in hibernators and during synthetic torpor (ST), a torpor-like condition that has been induced in rats, a non-hibernating species. Remarkably, in ST PP-Tau is reversible and Tau de-phosphorylates within a few hours following the torpor bout, apparently not evolving into pathology. These observations have been limited to the brain, but in animal models of tauopathies, PP-Tau accumulation also appears to occur in the spinal cord (SpCo). The aim of the present work was to assess whether ST leads to PP-Tau accumulation in the SpCo and whether this process is reversible. Immunofluorescence (IF) for AT8 (to assess PP-Tau) and Tau-1 (non-phosphorylated Tau) was carried out on SpCo coronal sections. AT8-IF was clearly expressed in the dorsal horns (DH) during ST, while in the ventral horns (VH) no staining was observed. The AT8-IF completely disappeared after 6 h from the return to euthermia. Tau-1-IF disappeared in both DH and VH during ST, returning to normal levels during recovery. To shed light on the cellular process underlying the PP-Tau pattern observed, the inhibited form of the glycogen-synthase kinase 3β (the main kinase acting on Tau) was assessed using IF: VH (i.e., in motor neurons) were highly stained mainly during ST, while in DH there was no staining. Since tauopathies are also related to neuroinflammation, microglia activation was also assessed through morphometric analyses, but no ST-induced microglia activation was found in the SpCo. Taken together, the present results show that, in the DH of SpCo, ST induces a reversible accumulation of PP-Tau. Since during ST there is no motor activity, the lack of AT8-IF in VH may result from an activity-related process at a cellular level. Thus, ST demonstrates a newly-described physiological mechanism that is able to resolve the accumulation of PP-Tau and apparently avoid the neurodegenerative outcome.

Synthetic torpor triggers a neuroprotective and regulated mechanism in the rat brain, favoring the reversibility of Tau protein hyperphosphorylation

“…Blood was collected transcardially from each animal after induction of deep anesthesia, and was centrifuged at 3000xg for 15 min at 4°C; plasma was then separated for subsequent ELISAs. Three animals per condition were transcardially perfused with paraformaldehyde 4% (w/v) as previously described (Luppi et al, 2019; Hitrec et al, 2021), and brain excised for immunofluorescence (IF); three animals per condition were sacrificed by decapitation, and the fresh brain was extracted for subsequent western-blot (WB) analysis. All samples were stored at −80 °C until assayed.…”

“…Thus, lowering of the temperature might allow GSK3β to act on Tau filaments without being counter-balanced by PP2A, at least for a while. However, this cannot explain the rather fast dephosphorylation of PPTau observed during recovery from ST (Luppi et al, 2019; Hitrec et al, 2021).…”

“…Notably, in all these cases, Tau hyperphosphorylation is reversible, not apparently evolving into an accumulation of neurofibrillary tangles that leads to neurodegeneration. The PPTau reversibility observed during ST in the non-hibernator (Luppi et al, 2019; Hitrec et al, 2021) appears of particular relevance from a translational point of view.…”

Association between Lower Body Temperature and Increased Tau Pathology in Cognitively Normal Older Adults

“…Temperature, an essential property of matter proportional to the average kinetic energy or speed at which molecules undergo Brownian motion, potently in uences tau phosphorylation. Planel et al, Arendt et al, and others showed in an elegant series of rodent experiments made over the last 20 years that a small (< 1°C) decrease in body temperature (Tb) within the human physiological range substantially increases neuronal tau phosphorylation-notably at phosphosites that are hyperphosphorylated in AD patients [36][37][38][39][40][41]. This effect was induced by experimental Tb decrease [36,37,42], seasonal torpor [43,44], as well as sleep-associated Tb decrease, which drove a robust increase in tau phosphorylation compared to waking in wild-type mouse [45].…”

“…To our knowledge, whether lower Tb is associated with tau pathology in human has not yet been reported, despite important clinical implications. While hypothermia-induced tau phosphorylation in rodent was reversible upon return to euthermia [40,41,43], it is plausible that persistent hyperphosphorylation in the setting of the persistent mild hypothermia, such as that known to be associated with aging [48][49][50][51][52][53], may contribute to NFT burden [39,44,54,55]. Further, age-related molecular pathway alterations may exacerbate hypothermia-induced tau phosphorylation or impair its reversibility [39,54,55].…”