mTBI brains showed substantial morphological changes reflective of cellular senescence. Astrocytic cell bodies in the isocortex of mTBI brains were abnormally swollen and enlarged. Furthermore, sub-pial astrocytes in mTBI brains with DNA damage presented with beading of axonal processes (Figs.
10 and
11a-d). In contrast, brains without a history of trauma and with no DNA damage had normal astrocytic processes (Fig.
11d-e). This is a hallmark of senescence in astrocytes, suggesting that these cells have taken on a secretive function [
25]. Cellular senescence is detrimental to overall brain health, through its various effects on different cell types [
19]. For instance, when astrocytes become senescent they no longer provide trophic support to the neuron, disturbing normal neuronal function [
43]. Astrocytes comprise the majority of all brain cells, and are immediate responders to brain trauma, infections, and neurodegeneration [
142]. Indeed, in response to various stressors such as hydrogen peroxide [
9] and ionizing radiation [
173], astrocytes become senescent. Furthermore, astrocytic senescence has been reported in brains of aged animals [
118,
124], indicating that there is a correlation between brain ageing and astrocytic senescence. In the context of traumatic brain injury, post-mortem human brains after blast exposure show astrogliosis in sub-pial regions, specifically surrounding the grey-white matter junctions, in the subpial area and lining of ventricles [
139]. The morphology of astrocytes in these cases was similar to the cases illustrated here, in which astrocytic cell bodies are swollen and cellular processes are beaded (see figure 1 in reference [
139]). Furthermore, astrogliosis in these individuals was thought to be the basis of post-traumatic stress disorder (PTSD) in soldiers who had suffered blast exposure-related mTBI [
139]. Other important brain cells affected by senescence are oligodendrocytes, which may lose their ability to myelinate and therefore disrupt axonal health and signaling capabilities when they become senescent [
154]. Microglia also can become senescent, which affects their ability to mediate immune responses in the CNS in response to injury or infection [
23]. Indeed, microglia from aged human brains are generally swollen and show fragmentation of processes thought to contribute to neuronal death [
38]. Lastly, senescent endothelial cells may disrupt the integrity of the blood-brain barrier [
1], an effect which has been associated with age-related cognitive decline [
150]. Cellular senescence in glial cells therefore has vast repercussions on the integrity of neuronal function and global brain health, resulting in widespread tissue dysfunction and, inevitably, the emergence of neurological symptoms. The role of senescent cells in cognitive decline and p-tau pathology has been demonstrated in a transgenic mouse model of AD, in which eliminating senescent cells through senolytic intervention resulted in reduced tau phosphorylation, improved cognitive outcomes, and the prevention of the upregulation of senescence genes [
14]. In the context of TBI, markers of senescence have been shown to elevate in microglia and astrocytes following a controlled cortical impact protocol [
153]. Contrary to studies showing vascular dysfunction in mTBI [
2,
157], we did not see evidence of cellular senescence in endothelial cells. Furthermore, we did not specifically evaluate senescent microglia in this cohort. Future studies utilizing double-labeling techniques would help clarify the involvement of endothelial cells and microglia in mTBI-related senescence. Cellular senescence is therefore a powerful mechanism affecting mainly glial cells and capable of inducing overall brain dysfunction, in many instance without visible structural damage on histological examination, and leading to important neurocognitive deficits.
We suggest that senescent glial cells have detrimental effects on the function of neurons, and we have shown some preliminary markers of neuronal dysfunction. It is well known that glial cells are critical support cells for neurons [
159], and that their loss can induce neuronal dysfunction [
72]. In this study, we have revealed several changes in neurons suggesting changes in genome integrity, nuclear membrane structure, and axonal signalling. Indeed we found that cases with senescence presented with loss of nuclear proteins BRG1 and intranuclear tau. BRG1 is a transcription factor critical for healthy neuronal gene expression and functioning, including neuronal differentiation and the function of synapses [
97]. In studies on ALS for example, loss of crucial BRG1 subunits was found to cause dendritic attrition, which was delayed by overexpressing BRG1 [
152]. Furthermore, mutations in BRG1 led to reduced dendritic spine density, impaired synapse activity, and neurological deficits in a study on autism spectrum disorder [
171]. Together, the literature indicates that loss of BRG1 expression is an important marker of neuronal function, and may have implications for clinical manifestation. In addition to BRG1, we report translocation of neuronal intranuclear tau protein to the cytoplasm in cases with evidence of glial senenescence compared to controls. Tau protein, most well-known as a structural molecule of the cytoskeleton in the axons and for its involvement in AD pathogenesis in its hyperphosphorylated form, has been found to be normally expressed within the nucleus of neurons [
12]. Intranuclear tau has been shown to interact with nucleic acid proteins and other nuclear proteins, and various studies ranging from cell culture to human brain have suggested that it is essential for genome integrity [
12]. In particular, intranuclear tau is thought to bind to chromatin and stabilize it in response to cellular stressors such heat [
149]. A recent study on post-mortem AD brains revealed translocation of intranuclear tau to the soma in diseased brains compared to controls [
57], indicating that intranuclear tau may also play a role in its pathogenesis. In addition to changes in genome integrity, we found loss of the nuclear envelop protein emerin, an integral protein of the nuclear membrane which functions to tether chromatin and help stabilize the nuclear component, in cases with glial cell DNA damage. Loss of emerin can lead to disruption of signaling pathways critical for maintaining normal transcription [
78] and implies disruption of healthy structural integrity of the nuclear membrane. Lastly, we showed loss of MBP expression in neuronal axons and pallor of the white matter, yet intact neurofilament protein. These results suggest that neuronal axonal structure is maintained, but their myelination may be disrupted. We suggest that individuals with damage in oligodendrocytes may have a decreased ability to myelinate neuronal axons, potentially leading to disruption in neuronal communication. A significant decrease in myelination, even in small regions, may be sufficient to disrupt communications through large networks and therefore possibly lead to neurological dysfunction. Although these results are fairly preliminary, we believe that senescent glial cells may impact neuronal functioning such that their basic properties, namely genome integrity, nuclear membrane structure, and communication between cells and networks, may be disrupted. Further studies using experimental models will be critical in understanding the effects of glial cell senescence on neuronal function, and the present study should be considered an exploration into the neuronal involvement in glial cell senescence.