The term ‘glaucoma’ encompasses a group of ocular conditions characterized by progressive optic nerve damage associated with a gradual visual field loss due to a slowly progressive degeneration of retinal ganglion cells (RGCs) [1]. Although intraocular pressure (IOP) reduction represents the only proven treatable risk factor, in some glaucomatous patients, RGC loss may continue despite the reduction in IOP. Thus, independently or in addition to IOP, other factors can individually or collectively contribute to the death of retinal ganglion cells and optic nerve fibers in glaucoma [1].

In recent years, significant efforts have been made to fully understand the mechanisms underlying the pathophysiology of glaucoma. Even if it is unquestionable that much progress has been made, and that glaucoma is now recognized as a full-fledged neurodegenerative disease, several mechanisms underlying glaucomatous damage remain unknown.

Recently, the hypothesis of a correlation between glaucoma and protein tau (commonly associated with neurodegenerative disorders) has gained significant attention, suggesting that tau may also play a role in glaucomatous neurodegeneration ^[2][3][4][5][2,3,4,5].

Tau is a microtubule-stabilizing protein, and its function is intricately regulated through phosphorylation at numerous sites, thereby influencing its function and turnover based on its phosphorylation status. It is mainly expressed in neurons, playing a crucial role in the assembly of tubulin monomers into microtubules and constituting the neuronal microtubule network [6].

Hyperphosphorylation of tau depresses its biological activity, causing both microtubule instability and the formation of aberrant fibrillar polymers. Moreover, changes in microtubule organization can result in a disorganization of mitochondria or lysosomes ^{[7][8][9][10]}[7,8,9,10].

A dysregulation in the phosphorylation of tau protein is responsible for a group of neurodegenerative disorders referred to as tauopathies [11].

2. Ocular Structures and Glaucoma Pathophysiology

Although the pathophysiology of glaucoma is not entirely known, the loss of RGCs is known to be correlated with IOP. The IOP is balanced by the ciliary body’s production of aqueous humor (AH) and its drainage. Historically, aqueous humor outflow pathways have been classified into two main categories, the conventional (trabecular meshwork (TM)) and unconventional (uveoscleral) pathways [12]. In the conventional route, AH departs from the anterior chamber through the TM. Subsequently, it passes through the juxtacanalicular tissue (JCT), a loose connective tissue with an irregular network in which TM cells are surrounded by fibrillar elements of the extracellular matrix (ECM). Then, it is conveyed into the Schlemm canal, an endothelium-lined vessel that encircles the cornea, presenting features similar to both blood and lymphatic vasculature ^[13][14][13,14]. Then, through the network of collector channels continuous with the venous system, AH leaves the eye. To a lesser extent compared to this route discussed above, a portion of aqueous humor is drained through the unconventional pathway. In this pathway, AH is drained through the interstices of the ciliary muscle and ultimately through the choroid and sclera [14]. However, recent studies have shed light on the existence of previously unrecognized additional outflow routes. Notably, these newly identified pathways include the transcleral outflow pathway and uveolymphatic pathway, suggesting a more intricate and multifaceted network of aqueous humor drainage than was previously understood [15]. While in open-angle glaucoma there is an increased resistance to AH outflow, the drainage is typically obstructed in angle-closure glaucoma. In patients with elevated IOP, mechanical stress and tension on the posterior eye structures, particularly the lamina cribrosa, can lead to damage to retinal ganglion cells and axonal transport disruption ^[16][17][16,17]. Such changes can occur early in the development of glaucoma, leading to the accumulation of vesicles, and microtubule and neurofilament disorganization, in the prelaminar and postlaminar regions [12]. Glaucomatous optic neuropathy may also occur when IOP levels are within the normal range. This form is known as normal tension glaucoma, and the probable pathogenesis is vascular [18]. Other factors which may contribute to glaucoma pathogenesis include impaired microcirculation, altered immunity, and excitotoxicity [12].

3. The Role of Tau in Neurodegenerative Disorders

‘Tauopathies’ is an umbrella term referring to a group of neurodegenerative diseases characterized by the deposition of tau protein within the brain in the form of neurofibrillary tangles and paired helical filaments [19]. The deposition occurs primarily in neurons, but also in glial cells and in the extracellular space [19]. Under physiological conditions, the tau protein is primarily localized in axons [20] and plays a crucial role in microtubule assembly, stability, and dynamics ^[21][22][21,22]. However, several other functions of tau protein have been proposed ^[23][24][23,24]. It is encoded by the microtubule-associated protein tau (MAPT) gene [20]. Through alternative splicing of the exons 2, 3, and 10, the pre-mRNA can generate six distinct isoforms. Exon 10 encodes for the second microtubule-binding repeat domain (MTBD) in the C-terminal region [25]. The inclusion or exclusion of exon 10 determines the classification of these isoforms, based on the presence of three or four repeats of MTBDs [25]. Consequently, tau isoforms are categorized as either 3R or 4R isoforms. Both 3R and 4R isoforms are equally represented under physiological conditions in the adult human brain [26]. Tau binds to microtubules via repeat MTBDs in the C-terminus, and its phosphorylation plays a critical role in regulating its affinity to microtubules [27]. Hyperphosphorylation of tau, along with other tau alterations such as methylation, overexpression, and post-translational modifications other than phosphorylation, can lead to a decrease in binding affinity [28]. This reduction in binding affinity ultimately results in tau deposition and destabilization of microtubules [28]. Despite extensive research, the exact mechanism by which tau contributes to neurodegeneration remains to be fully elucidated. Various mechanisms have been proposed, including gain of function, loss of function, and mislocalization of tau [29]. These different hypotheses suggest that tau may exert its pathological effects through diverse mechanisms, further highlighting the complexity of its involvement in neurodegenerative processes. Based on the predominant tau isoform found in the aggregates, tauopathies are pathologically classified into three main categories: 3R tauopathies (frontotemporal dementia (FTD)), 4R tauopathies (progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), argyrophilic grain disease (AGD), and globular glial tauopathy (GGT)), and 3R/4R tauopathies (Alzheimer’s disease (AD), chronic traumatic encephalopathy (CTE)) and primary age-related tauopathy (PART) [19]. Clinical manifestations of tauopathies vary depending on the specific type and stage of the disease. Common symptoms observed across various tauopathies include memory impairment, behavioral changes, executive dysfunction, and motor impairment [19]. Motor symptoms, such as parkinsonism with rigidity and bradykinesia, are frequently seen in 4R tauopathies like PSP, corticobasal syndrome (CBS), AGD, and GGT ^[19][30][19,31]. The behavioral variant of frontotemporal dementia (bvFTD) is distinguished by isolated behavioral changes as its characteristic early manifestation, while the non-fuent variant primary progressive aphasia (nfvPPA) and semantic-variant primary progressive aphasia (svPPA) are typified by isolated speech disturbances ^[31][32][33][32,33,34]. Conversely, single or multidomain amnesia is identified as the predominant symptom marking the onset of Alzheimer’s disease (AD) ^[32][33]. However, as the disease progresses, these manifestations progressively overlap [19]. Concerning the clinicopathological correlation, a previous study ^[34][35] has delineated a stepwise progression of tau pathology through four defined phases in FTD. Beginning with Phase I, tau deposits initially emerge in the frontotemporal limbic/neocortex and angular gyrus. By Phase IV, even the primary visual cortex begins to exhibit a mild tau accumulation. Clinically, these tau depositions manifest in distinct ways. The majority of patients, right from the early tau accumulation stages, displayed hallmark features of bvFTD, especially social comportment disorders. This suggests that the initial tau accumulations in limbic and paralimbic regions, crucial for social behavior and emotion regulation, could be directly associated with these behavioral manifestations. Intriguingly, despite significant early tau presence in the hippocampus, a memory center, episodic memory difficulties often emerged as a later clinical symptom. This brings forth the possibility that other factors, possibly the involvement of prefrontal regions, might modulate the relationship between tau pathology and memory impairment. Adding another layer of complexity, one study’s neuroimaging findings ^[34][35] pinpointed early degeneration in the anterior insula and cingulate cortex, areas critical for emotion processing, further emphasizing the early behavioral changes seen in bvFTD. 

4. Tau Protein’s Role in Glaucoma: A Closer Look at the Evidence

Glaucoma is a neurodegenerative disease of the central nervous system characterized by the progressive degeneration of RGCs, damaging the optic nerve, and resulting in irreversible visual loss ^{[12][16][35][36][37]}[12,16,45,46,47]. Despite high IOP being amply demonstrated to be the most significant known risk factor for glaucoma development ^[38][39][48,49], the mechanism by which elevated pressure damages RGCs is still a matter under investigation [5]. At present, the hypothesis that retinal and brain degeneration share pathogenic mechanisms is gaining more traction, and the role of tau protein in the pathogenesis of glaucoma is a subject of ongoing debate ^{[40][41][42][43][44]}[50,51,52,53,54]. Tau intraretinal accumulation. Several studies have been conducted in animal models transgenic for P301S tau demonstrating the presence of phosphorylated tau protein in the retina. In 2011, Gasparini et al. ^[40][50] evaluated the retina of the mouse line transgenic for P301S tau, reporting a hyperphosphorylated transgenic tau accumulation in the nerve fiber layer (NFL). Symptoms and eye structural changes linking neurodegenerative diseases and glaucoma. Evidence from recent years shows how patients with AD often develop visual impairments, frequently associated with abnormalities in the eye ^[45][56]. Visual changes reported in the early stages of AD include difficulties in reading and finding objects, visual field loss, altered depth perception, abnormal color discrimination, and contrast sensitivity ^[46][47][48][57,58,59]. Although all of these issues were originally believed to originate exclusively from pathological cortex changes related to the underlying neurological disease, modern ocular imaging techniques have made it possible to prove a correlation between visual symptoms and structural ocular changes ^[49][60]. Ocular abnormalities found include a reduction in the number of optic nerve head axons, a reduced number of RGCs, and a decrease in the thickness of the retinal nerve fiber layer (RNFL) ^{[50][51][52][53][54][55]}[61,62,63,64,65,66] and thinning of the RGC layer ^[56][67]. These findings overlap with the actual tool for glaucoma diagnosis, as the same parameters are currently used for the early analysis of glaucomatous damage ^[57][68].  Glaucoma and cognitive impairment. Recent advancements in our understanding of cognitive impairment in individuals with glaucoma have shed new light on the intricate relationship between ocular health and cognitive function. Patients affected by glaucoma showed lower cognitive scores and were associated with an increased risk of dementia ^[58][59][60][76,77,78]. Maurano et al. reported that glaucoma patients had a reduction in cognition similar to the values reported in the literature for patients with AD ^[61][79]. These findings are highly intriguing, once again providing valuable avenues for framing the neurodegenerative aspect of glaucoma. However, it is important to note that these studies are all based on aggregate patient data, and there is a lack of data explaining the differences between patients with glaucoma and cognitive disorders and those with glaucoma who perform within normal ranges on cognitive tests. Tau accumulation in glaucoma and glaucoma models. These findings, together with the common features between neurodegenerative diseases and glaucoma, have led researchers to look for variations in the levels of certain specific characteristic proteins in the eyes of glaucoma patients. Among them, a few studies included the investigation of phosphorylated tau protein levels ^[41][42][62][51,52,84], suggesting that they may play a role in the degeneration of these cells. In 2005, Yoneda et al. measured tau protein concentrations in the vitreous fluid from patients with different ocular diseases, finding a significant increase in the tau level in patients with glaucoma compared with the control macular-hole patients ^[41][51].  Tau protein, axonal transport, and RGCs. Furthermore, evidence suggests that tau protein may have a role in the regulation of axonal transport, which is essential for the preservation of RGCs and their axons ^[63][64][86,87]. Reduced axonal transport results in a state of cellular strain, contributing to neurodegeneration and limiting neurons’ ability to resist damage ^[44][54]. RGCs have been proven to be particularly sensitive to the impairment of axonal transport, as the functioning of these neuronal cells considerably depends on retrograde trophic support ^[65][66][88,89]. Thus, dysregulation of tau protein may impair axonal transport and consequent RGC degeneration in glaucoma.  Tau protein, Aβ protein, and glaucoma. Furthermore, tau protein has been shown to interact with amyloid beta (Aβ) protein, which is also implicated in the pathogenesis of glaucoma ^[67][68][85,91]. Pathological deposits of Aβ have been shown to be a cause of RGC death and thinning of the RNFL associated with glaucomatous degeneration ^[68][91]. Deposits of Aβ have been proven to be present in all retinal layers, including the ganglion cell, nerve fiber, and photoreceptor layers ^[49][69][60,92].  Genetic factors linking glaucoma and tauopathies. In recent years, the potential for a genetic correlation between glaucoma and tauopathies has become a subject of investigation. Notably, in 2021, Gharahkhani et al. pinpointed three risk loci linked to AD and glaucoma (MAPT, CADM2, and APP) ^[70][97]. More recently, a systematic literature review uncovered 49 single nucleotide polymorphisms distributed across 11 risk loci associated with AD and glaucoma (AGBL2, CELF1, FAM180B, MTCH2, MYBPC3, NDUFS3, PSMC3, PTPMT1, RAPSN, SLC39A13, and SPI1) ^[71][98]. While this area of study remains ongoing, it provides further support for the notion of a connection between these pathologies.

5. Oxidative Damage Is a Common Feature in Both Glaucoma and Tauopathies

The data discussed above, together with similar clinical history features and concurrent disease onset, show that glaucoma exhibits pathological traits typical of tauopathies, including tau accumulation, impaired phosphorylation, dysregulation of axonal transport, and the interaction of Aβ deposits. However, glaucoma and other tauopathies also exhibit persistent DNA damage ^[72][99]. DNA damage response (DDR) kinases, such as ataxia telangiectasia mutated (ATM), which may phosphorylate various substrates, are activated in conjunction with the beginning of the lesions. Reactive oxygen species (ROS) and tau oligomers are likely responsible for the lesions ^[73][100]. In glaucoma, oxidative DNA damage is a pathogenic factor ^[74][101]. In fact, high levels of 8-hydroxy-2′-deoxyguanosine (8-OHdG) have been found in glaucoma patients’ aqueous humor and serum ^[74][101]. The DNA base 8-OHdG, which has undergone oxidative modification, is a sign of oxidative DNA damage ^[75][102]. The amount of 8-OHdG is significantly more prevalent in the trabecular meshwork in patients with open-angle glaucoma and positively correlates with rising IOP and deteriorating visual field ^[76][103]. Additionally, POAG is linked to a rise in the amount of DNA breaks in the local trabecular meshwork and the regular circulating leukocyte ^[77][105]. Inflammation, apoptosis, senescence, and neural dysfunction can all result from the persistent activation of the DNA damage response, which can also lead to dysregulation of the cell cycle and re-entry into G1 ^[78][79][106,107]. In neurons of the lateral geniculate nucleus (LGN), primary visual cortex (V1), and secondary visual cortex (V2), laser-induced chronic glaucoma models in rhesus monkeys exhibited increased expressions of 8-hydroxyguanosine (8-OHG), indicating oxidative stress, and phosphorylated histone variant H2AX (γH2AX), indicating DNA double-strand breaks ^[80][108]. In the hippocampus and frontal cortex, people with mild cognitive impairment or Alzheimer’s disease have more histone H2AX-positive neurons than age-matched controls ^[81][109]; people with neurodegenerative diseases also have more oxidative DNA lesions in their nuclear and mitochondrial DNA ^[82][110]. This increase could be explained by the ability of amyloid oligomers to reduce DNA-PK activity and BRCA1 protein levels ^[83][111]. This vicious cycle between tau hyperphosphorylation and oxidative stress ^[84][113], as well as tau aggregate formation, can induce glial inflammation (astrogliosis) and neuropathology in Alzheimer’s disease-related mouse models via innate immune sensors (receptors) such as Tool-like receptors (TLRs) ^[85][119]. In fact, tau-containing astrocytes have been discovered in numerous tauopathies, including glaucoma ^[86][87][120,121].

6. Oxidative DNA Damage and Tau

Tau is primarily considered a cytosolic protein, but it has been found to localize to the nucleus of both neuronal and non-neuronal cells and interact with nucleic acids ^[88][126]. Tau is hypophosphorylated in the nucleus of neurons ^[89][127], and in vitro research has demonstrated that tau binds to the minor grooves of DNA to shield it from ROS ^[90][128]. When hyperphosphorylated, it becomes less able to bind DNA ^[91][129]. In physiological circumstances, tau binds chromatin in cultured murine primary neurons. Under stressful circumstances, this interaction can be altered due to its dynamic nature ^[92][131]. Tau most likely directly participates in DDR in cells in response to DNA lesions. In cells lacking tau, histone H2AX is more frequently phosphorylated on serine 139 (H2AX) ^[93][132], indicating either more DNA lesions or a slower rate of DNA repair. When expressed in the nucleus, human tau can reduce DNA breaks caused by cellular stress compared to wild-type neurons ^[89][127]. In the hippocampus of animals, tau deletion also results in a slower rate of DNA DSB repair ^[93][132], pointing to a dual role for tau in the brain regarding DNA protection and direct or indirect DNA repair. To reiterate, chromosomal aberrations are present in the peripheral cells of patients with mutated tau ^[94][133].