DNA Damage as an Early Marker of Senescence in Tauopathies

Abstract

AbstractBackgroundHyperphosphorylated tau accumulation in neurons and astrocytes is a pathological hallmark of a class of neurodegenerative disorders known as “tauopathies”, such as progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Alzheimer’s disease (AD) and chronic traumatic encephalopathy (CTE). Previous research investigating the toxic effects of astrocytic tau pathology identified the DNA damage marker p16 in astrocytes from post‐mortem human cases of AD and FTD. DNA damage is one of many markers of cellular senescence, a state in which cells undergo irreversible cell cycle arrest. Senescence initiation can also be identified via the expression of phosphorylated histone H2AX (gamma‐H2AX) and cyclin‐dependent kinase inhibitor p21. Senescence in astrocytes leads to reduced functional support for neurons. While recognized, the role of senescent astrocytes in tauopathies is not well understood. Here, we sought to determine how expression of other DNA damage markers, such as gamma‐H2AX and p21, and the DAMP protein HMGB1 varies across different tauopathies.MethodImmunohistochemistry (IHC) was performed on postmortem human frontal cortex tissue of CTE (n = 3), AD (N = 3), PSP (n = 3), CBD (n = 3), Healthy (n = 1) and glioblastoma (GBM) (n = 1) cases with antibodies against markers of early senescence gamma H2A.x (yH2A.x) and p21, late senescence, p16, and a senescence‐associated secretory phenotype, HMGB1. Positively stained nuclei were quantified. Immunofluorescence (IF) was employed on postmortem cases using either glial fibrillary acidic protein (GFAP) or Neurofilament H (NF‐H) antibodies with yH2A.x, p16, p21 and HMGB1 markers in order to qualitatively and quantitatively measure the prevalence of senescent astrocytes and neurons in tauopathies.ResultImmunohistochemistry revealed yH2Ax expression was observed in AD, PSP, and CBD cases. Immunofluorescence experiments employing yH2AX co‐stained with astrocytic and neuronal markers revealed expression in AD, PSP and CBD cases. Minimal expression of yH2Ax was revealed in the CTE and healthy control cases. Results from p21 stains were inconclusive. Preliminary evidence reveals HMGB1 expression in CBD and PSP cases utilizing IF techniques, and p16 expression in cells employing IHC.ConclusionOverall, results indicate that DNA damage may be broadly associated with astrocytes in tauopathies. Utilizing alternative approaches to recognize and evaluate astrocyte senescence in post‐mortem human tissue will provide further insight into these findings.