**3.1. Total tau**

in biological fluids of neurodegenerative dementias and its relevance in the differential diagnostic context. Additionally, we explore how tau alterations in the brain tissue may

Tau is a microtubule-associated protein produced through alternative splicing of the MAPT (microtubule-associated protein tau) gene. Tau is highly abundant in the axons of nerve cells [1] where it plays a role in the stabilization and dynamics of the microtubules. To a lesser extent tau is also localized in the synaptic compartments [2] where it is suggested to modulate postsynaptic receptor activity by interaction with a broad range of synaptic proteins [3]. Besides its neuronal localization, tau is also expressed in oligodendrocytes, where it stabilizes microtubules during process outgrowth and myelination [4–6] and in astrocytes at trace levels [4], where it does not appear to be a major cytoskeletal protein. Although tau is mainly an intracellular protein, it can also be actively secreted by neurons to the brain interstitial fluid. The mechanisms of tau secretion under physiological conditions are not well understood, but it can be induced by neuronal hyperexcitability [7] and its release through ectosomal and exosomal vesicles [8, 9] or alternative secretory pathways [10] has been proposed. Additionally, under certain pathological conditions associated to neuronal degeneration tau can be released from the neurons to the

brain interstitial fluid, usually correlating with the degree of neuro-axonal damage.

Tau released into the interstitial fluid may drain into the cerebrospinal fluid (CSF) within the subarachnoid space as well as into blood. Therefore, alterations of tau levels in biological fluids may mirror the pathological state of the brain in those conditions associated to neuronal degeneration. Additionally, since tau is hyper-phosphorylated in patients suffering from primary tauopathies and Alzheimer's disease (AD), a tauopathy associated with beta-amyloid deposition, increased phospho-tau (p-tau) levels in biological fluids may reflect the undergo-

Consequently, the quantification of tau levels in biological fluids is extensively studied as a diagnostic and prognostic biomarker in a broad range of neurological conditions either associated or not to a concomitant tauopathy. Additionally, the analysis of different tau forms is also explored in the evaluation of the efficacy of disease-modifying therapies and to better understand the underlying molecular mechanisms associated to neuronal degeneration and

Tauopathies are a complex and heterogeneous group of neurodegenerative diseases characterized by the presence of hyper-phosphorylated and aggregated tau forms in neuronal and

**Keywords:** total tau, phospho-tau, neurodegenerative diseases, cerebrospinal fluid,

explain the etiology of its regulated levels in CSF and blood.

serum, plasma

ing tau pathology in the brain tissue.

**2. Tau pathology and neuro-axonal damage**

tau pathology.

**1. Introduction**

66 Cognitive Disorders

The presence of increased total-tau (t-tau) levels in the CSF is reported in several neurodegenerative diseases such as sporadic Creutzfeldt-Jakob disease (sCJD), AD and dementia with Lewy bodies (DLB) [16–18].

The highest t-tau levels are recurrently detected in sCJD, the most common form of human prion disease where deposition of prion protein, gliosis and massive neuronal damage accompanied with spongiform degeneration are common neuropathological hallmarks in the brain tissue [19]. The first report on the presence of high t-tau in the CSF of sCJD cases was in 1997 in a relatively small cohort of cases [20]. Further, several studies validated these observations in large populations [21–24]. A cut-off point of 1300 pg/mL t-tau was established for the discrimination of sCJD from non-CJD cases [21] and although t-tau is not included in the World Health Organization diagnostic criteria for sCJD, its quantification is used in several worldwide diagnostic centers as a supportive tool in the differential diagnosis of prion diseases.

In AD, CSF t-tau is considered as one of the three core biomarkers together with p-tau and amyloid beta-42. The first studies reporting elevated t-tau concentrations in AD patients compared to controls date from 1995 [37–40]. Since them these results have been replicated in

Tau Protein as a Biological Fluid Biomarker in Neurodegenerative Dementias

http://dx.doi.org/10.5772/intechopen.73528

69

Since t-tau is considered a marker of neurodegeneration, its levels are proposed to be changed latter during the progression of the disease and correlating with clinical symptom severity. This is in contrast to amyloid beta peptides which become abnormal before alterations in neurodegenerative biomarkers and cognitive symptoms are detected [43]. Importantly, as CSF t-tau reflects the intensity of acute neuronal damage and chronic neuronal degeneration elevated t-tau levels in MCI patients has been demonstrated to predict the progression to dementia [44], especially in short-term prognosis [45]. These observations are in agreement with a meta-analysis study that strongly supports the use of CSF t-tau in the identification of MCI-diagnosed subjects at higher risk of evolving to AD [46]. However, higher predictive values are reached when combined with p-tau, amyloid beta-42 and/or imaging biomarkers [45, 47]. Indeed data from longitudinal studies indicate that the combination of CSF t-tau, p-tau and amyloid beta-42 at baseline reach high prognostic values (95% sensitivity and 83%

Besides the well documented elevated t-tau levels in sCJD, AD and DLB, several studies found slightly elevated t-tau levels in the spectrum of frontotemporal dementia (FTD)-related disorders [17] as well as in vascular dementia (VaD) [49, 50], although the reproducibility of these alterations are under discussion [17, 51]. Although meta-analysis studies support the presence of elevated t-tau levels in frontotemporal lobar-degeneration (FTLD) and VaD, these alterations are not as high as those detected in AD and CJD [52]. Importantly, alterations on t-tau levels in VaD and FTLD are dependent on the heterogenic presentation of these conditions. For instance, elevated t-tau is observed in VaD patients without progressive leukoaraiosis, while VaD patients with progressive leukoaraiosis have normal t-tau values [49]. Additionally, studies in VaD cases may have selection bias as co-occurrence of vascular and AD-related pathology is well-known [53]. In regard to FTLD cases, the two major subtypes, FTLD-TDP (FTLD with TAR DNA-binding protein 43 inclusions) and FTLD-tau (FTLD with tau inclusions) did not seem to differ on t-tau levels [54], although it has been recently shown that t-tau harbors a better discrimination rate than TDP-43 proteins in the differentiation of both FTLD subtypes [55]. Overall, in FTLD cases, despite the presence of slightly to moderate increased tau levels compared to controls in several studies, these differences, when observed, do not harbor clinical significance [56]. However, a potential prognostic role of t-tau levels in FTD has been suggested as non-inherited FTD patients with high t-tau levels (≥400 pg/mL), who had shorter survival than those with low levels. Instead, p-tau concentrations were not

In Parkinson's disease dementia (PDD) t-tau levels are also elevated compared controls or to Parkinson's disease patients with normal cognition, but lower than those detected in DLB [58, 59]. Whether elevated levels of t-tau in PDD are related to an underlying tau pathology participating in the development of dementia, similar to AD, or just reflecting increased neuro-

many studies [41, 42] with a predictive value around 90%.

specificity) in the detection of early AD cases with MCI [48].

associated with disease prognosis [57].

axonal damage compared to PD cases is still a matter of debate.

In sCJD, patient's molecular subtype is composed of two different kinds of information: (1) codon 129 genotype in the *PRNP* gene (MM, MV, or VV) and (2) prion protein (PrP) type (1 or 2) and it is a prognostic marker for disease progression and patient survival [19]. The different progression rates for each molecular subtype are associated to their differential neuropathological hallmarks, which in turn reflect the degree of neuronal degeneration. Accordingly, a significant association between CSF t-tau levels and sCJD molecular subtype was reported [25, 26]. Additionally, CSF t-tau levels are inversely associated with disease duration, suggesting a role for CSF t-tau as a prognostic marker for sCJD [27]. While t-tau protein tends to increase in sensitivity from onset to the advanced stage, these changes are only statistically significant in patients with methionine-valine (MV) heterozygosis at codon 129 in the *PRNP* gene [28].

Besides sporadic forms, prion disease can also present a genetic (or hereditary) etiology [29]. Hereditary prion diseases are a heterogeneous group of conditions classified as familial or genetic CJD prion diseases (gCJD), Gerstmann-Sträussler-Scheinker syndrome (GSS-S) and fatal familial insomnia (FFI), all of them associated to mutations in the prion protein gene and with an autosomal inheritance pattern. The most common gCJD types in the European population are those associated to mutations at codon 200 (E200K) and codon 210 (V210I). From a clinical and neuropathological point of view gCJD E200K and gCJD V210I resemble sCJD [30, 31]. Indeed, gCJD cases are often misclassified as sCJD in the absence of family history and genetic testing. In agreement with this, CSF t-tau levels in gCJD patients show levels comparable to those in sCJD [22, 32]. Instead, in GSS-S (associated to several mutations such as P102L, P105L, A117V, F198S) and FFI (associated to D178N mutation accompanied by a cis-129 M), CSF t-tau is usually below the sCJD cut-off point, [22, 32]. These differences are explained by the differential clinico-pathological hallmarks observed in these cases. First, GSS-S and FFI patients usually present a slower disease progression and prolonged disease duration compared to sCJD and gCJD cases [30]. Secondly, the brain regions affected are not overlapping those affected in sCJD. Indeed, FFI cases present a pathology which is usually restricted to the thalamic and olivary nucleus with spongiform changes in the cerebral cortex only in patients with long disease duration [30, 33, 34]. In contrast, neuropathological findings in GSS-S cases are heterogeneous depending on mutation type, but in general a widespread accumulation of PrP positive plaques in the cerebral and cerebellar cortices and the basal ganglia is observed [35, 36]. Although t-tau quantification lacks of clinical significance in the differential diagnosis of FFI and GSS-S, it is worth to mention that their mean CSF t-tau levels are increased compared to controls [22] indicating that, at some extent, t-tau may also reflect brain damage occurring in both conditions.

In AD, CSF t-tau is considered as one of the three core biomarkers together with p-tau and amyloid beta-42. The first studies reporting elevated t-tau concentrations in AD patients compared to controls date from 1995 [37–40]. Since them these results have been replicated in many studies [41, 42] with a predictive value around 90%.

The highest t-tau levels are recurrently detected in sCJD, the most common form of human prion disease where deposition of prion protein, gliosis and massive neuronal damage accompanied with spongiform degeneration are common neuropathological hallmarks in the brain tissue [19]. The first report on the presence of high t-tau in the CSF of sCJD cases was in 1997 in a relatively small cohort of cases [20]. Further, several studies validated these observations in large populations [21–24]. A cut-off point of 1300 pg/mL t-tau was established for the discrimination of sCJD from non-CJD cases [21] and although t-tau is not included in the World Health Organization diagnostic criteria for sCJD, its quantification is used in several worldwide diagnostic centers as a supportive tool in the differential diagnosis of prion diseases.

68 Cognitive Disorders

In sCJD, patient's molecular subtype is composed of two different kinds of information: (1) codon 129 genotype in the *PRNP* gene (MM, MV, or VV) and (2) prion protein (PrP) type (1 or 2) and it is a prognostic marker for disease progression and patient survival [19]. The different progression rates for each molecular subtype are associated to their differential neuropathological hallmarks, which in turn reflect the degree of neuronal degeneration. Accordingly, a significant association between CSF t-tau levels and sCJD molecular subtype was reported [25, 26]. Additionally, CSF t-tau levels are inversely associated with disease duration, suggesting a role for CSF t-tau as a prognostic marker for sCJD [27]. While t-tau protein tends to increase in sensitivity from onset to the advanced stage, these changes are only statistically significant in patients with methionine-valine (MV) heterozygosis at codon 129 in the *PRNP* gene [28].

Besides sporadic forms, prion disease can also present a genetic (or hereditary) etiology [29]. Hereditary prion diseases are a heterogeneous group of conditions classified as familial or genetic CJD prion diseases (gCJD), Gerstmann-Sträussler-Scheinker syndrome (GSS-S) and fatal familial insomnia (FFI), all of them associated to mutations in the prion protein gene and with an autosomal inheritance pattern. The most common gCJD types in the European population are those associated to mutations at codon 200 (E200K) and codon 210 (V210I). From a clinical and neuropathological point of view gCJD E200K and gCJD V210I resemble sCJD [30, 31]. Indeed, gCJD cases are often misclassified as sCJD in the absence of family history and genetic testing. In agreement with this, CSF t-tau levels in gCJD patients show levels comparable to those in sCJD [22, 32]. Instead, in GSS-S (associated to several mutations such as P102L, P105L, A117V, F198S) and FFI (associated to D178N mutation accompanied by a cis-129 M), CSF t-tau is usually below the sCJD cut-off point, [22, 32]. These differences are explained by the differential clinico-pathological hallmarks observed in these cases. First, GSS-S and FFI patients usually present a slower disease progression and prolonged disease duration compared to sCJD and gCJD cases [30]. Secondly, the brain regions affected are not overlapping those affected in sCJD. Indeed, FFI cases present a pathology which is usually restricted to the thalamic and olivary nucleus with spongiform changes in the cerebral cortex only in patients with long disease duration [30, 33, 34]. In contrast, neuropathological findings in GSS-S cases are heterogeneous depending on mutation type, but in general a widespread accumulation of PrP positive plaques in the cerebral and cerebellar cortices and the basal ganglia is observed [35, 36]. Although t-tau quantification lacks of clinical significance in the differential diagnosis of FFI and GSS-S, it is worth to mention that their mean CSF t-tau levels are increased compared to controls [22] indicating that, at some extent, t-tau may also reflect

brain damage occurring in both conditions.

Since t-tau is considered a marker of neurodegeneration, its levels are proposed to be changed latter during the progression of the disease and correlating with clinical symptom severity. This is in contrast to amyloid beta peptides which become abnormal before alterations in neurodegenerative biomarkers and cognitive symptoms are detected [43]. Importantly, as CSF t-tau reflects the intensity of acute neuronal damage and chronic neuronal degeneration elevated t-tau levels in MCI patients has been demonstrated to predict the progression to dementia [44], especially in short-term prognosis [45]. These observations are in agreement with a meta-analysis study that strongly supports the use of CSF t-tau in the identification of MCI-diagnosed subjects at higher risk of evolving to AD [46]. However, higher predictive values are reached when combined with p-tau, amyloid beta-42 and/or imaging biomarkers [45, 47]. Indeed data from longitudinal studies indicate that the combination of CSF t-tau, p-tau and amyloid beta-42 at baseline reach high prognostic values (95% sensitivity and 83% specificity) in the detection of early AD cases with MCI [48].

Besides the well documented elevated t-tau levels in sCJD, AD and DLB, several studies found slightly elevated t-tau levels in the spectrum of frontotemporal dementia (FTD)-related disorders [17] as well as in vascular dementia (VaD) [49, 50], although the reproducibility of these alterations are under discussion [17, 51]. Although meta-analysis studies support the presence of elevated t-tau levels in frontotemporal lobar-degeneration (FTLD) and VaD, these alterations are not as high as those detected in AD and CJD [52]. Importantly, alterations on t-tau levels in VaD and FTLD are dependent on the heterogenic presentation of these conditions. For instance, elevated t-tau is observed in VaD patients without progressive leukoaraiosis, while VaD patients with progressive leukoaraiosis have normal t-tau values [49]. Additionally, studies in VaD cases may have selection bias as co-occurrence of vascular and AD-related pathology is well-known [53]. In regard to FTLD cases, the two major subtypes, FTLD-TDP (FTLD with TAR DNA-binding protein 43 inclusions) and FTLD-tau (FTLD with tau inclusions) did not seem to differ on t-tau levels [54], although it has been recently shown that t-tau harbors a better discrimination rate than TDP-43 proteins in the differentiation of both FTLD subtypes [55]. Overall, in FTLD cases, despite the presence of slightly to moderate increased tau levels compared to controls in several studies, these differences, when observed, do not harbor clinical significance [56]. However, a potential prognostic role of t-tau levels in FTD has been suggested as non-inherited FTD patients with high t-tau levels (≥400 pg/mL), who had shorter survival than those with low levels. Instead, p-tau concentrations were not associated with disease prognosis [57].

In Parkinson's disease dementia (PDD) t-tau levels are also elevated compared controls or to Parkinson's disease patients with normal cognition, but lower than those detected in DLB [58, 59]. Whether elevated levels of t-tau in PDD are related to an underlying tau pathology participating in the development of dementia, similar to AD, or just reflecting increased neuroaxonal damage compared to PD cases is still a matter of debate.

In corticobasal degeneration (CBD), initial reports suggested the presence of elevated t-tau levels [60, 61], but decreased or non-altered levels were later reported in other studies [62, 63]. Similarly, patients with progressive supranuclear palsy (PSP) showed total-tau levels comparable to those detected in controls [61, 63].

Contrary to t-tau, elevated CSF p-tau levels seem to be more restricted to AD cases [83]. This is an interesting recurrent observation as aberrant tau phosphorylation also occurs in the brain tissue of tauopathies such as FTD with Parkinsonism linked to chromosome 17 (FTDP-17), Pick disease (PiD), CBD and PSP leading to aggregation into neurofibrillary tangles (NFT)

Tau Protein as a Biological Fluid Biomarker in Neurodegenerative Dementias

http://dx.doi.org/10.5772/intechopen.73528

71

Neurofibrillary changes are composed of hyper-phosphorylated tau forms that correlate with disease duration and severity [87]. Indeed, the severity of cognitive impairment correlates better with the burden of neocortical neurofibrillary tangles than with amyloid pathology [87–89]. While meta-analysis studies indicate that CSF p-tau levels are a moderate prognostic marker in AD [90], longitudinal studies show that p-tau-181 levels correlated with the progression of cognitive decline [91]. Additionally, a low p-tau-181/tau ratio has also been reported as a strong predictor of cognitive decline [92]. In contrast, p-tau-231 levels correlate with the rate of hippocampal atrophy in AD cases, which are independent of disease duration and severity [93] as well as with a reduction hippocampal volume detected by magnetic resonance imaging (MRI) [94]. Since rates of hippocampal atrophy are suggested to reflect reduction of neuronal density, these observations suggest that p-tau231 is directly associated to extensive neuronal damage but not to disease stage. Additionally, p-tau-199 levels have been shown to be useful in the discrimination of AD patients from non-AD-related dementia

A comparative study on p-tau231, p-tau181 and p-tau199 performance showed that the three p-tau forms were significantly elevated in patients with AD compared with FTD, DLB, VaD and control cases [96]. This study also indicated that p-tau231 and p-tau181 assays performed similarly in the discrimination of AD from non-demented controls, whereas the p-tau199 assay showed a weaker discriminatory value. However, the combinations of the three measurements did not add discriminative power compared to single measurements. Although alternative p-tau epitopes have been studied in the context of AD pathogenesis, it is generally considered that p-tau181, 199 and 231 are those more characteristic to AD. Indeed, most studies have focused on p-tau231 and p-tau181 [14], which in turn, are the most standardized

For p-tau-181, a cut-off of >60 pg/mL is generally used to define pathological levels due to AD

While other p-tau species have also been measured such as p-tau199, p-tau199 + 202, as well as p-tau396 + 404, importantly, p-tau levels in AD are increased not only compared to controls, but also compared to other tauopathies and neurodegenerative diseases [98]. Measurement of them becomes a useful marker in the differentiation of AD from its most relevant differential diagnoses. Phosphorylation at Thr-231 is helpful in the discrimination of AD from FTD, and its levels are correlated with disease progression, whereas p-tau-181 (the most established p-tau assay [99]) improves the differentiation between AD and DLB [96, 100, 101]. Additionally, p-tau, when used in combination with amyloid beta-42 (p-tau/amyloid beta-42 ratio), shows the best diagnostic performance in the discrimination of AD from FTLD [102]. This meta-analysis study also reports that p-tau alone would be more useful for high Mini-Mental-State-Examination (MMSE scores), while p-tau/amyloid beta-42 would be preferable

and axonal transport dysfunction [84–86].

assays for p-tau quantification.

for low MMSE scores and younger patients.

pathogenesis [97].

and non-demented patients in a large-scale multicenter study [95].

Increased CSF t-tau levels in normal pressure hydrocephalus (NPH) patients compared to controls have also been reported [64]. Interestingly, t-tau levels correlated with the severity of dementia, urinary incontinence, and gait disturbance. However, further studies have not been able to replicate the presence of elevated t-tau in NPH patients [65, 66].

In Huntington's disease (HD), a recent study reported elevated levels of CSF t-tau in gene expansion carriers compared with their control subjects. Additionally, t-tau concentrations were associated with phenotypic variability, thus a role for t-tau as a biomarker of disease progression was proposed [67]. However, a diagnostic role for t-tau in HD is excluded as no differences between HD patients compared with pre-manifest gene expansion carriers have been reported [68].

As a marker of neuronal damage, elevated t-tau levels have been reported in acute stroke [69] and head trauma [70]. CSF t-tau levels in acute brain injury present a transitory peak. For instance in traumatic brain injury (TBI), t-tau levels increased shortly after contusion, peaking in the second week post-trauma, slowly decreasing after this period and reaching basal values at 43 days [70]. Instead, in Olympic boxers subjected to repetitive trauma t-tau levels increased after boxing and remained elevated after a rest period of at least 14 days [71]. In acute stroke, t-tau levels are associated to disease severity and long-term outcome [72]. The sizes and localization of the lesion generally affects the profile of CSF t-tau levels after stroke which vary to a great extent, generally reaching maximum levels after 3 weeks to 1 month and returning to normal levels after 3–5 months [69, 73]. Other neurological conditions with transiently elevated t-tau levels include acute Wernicke's disease [74, 75], after chemotherapy treatment for hematologic malignancies [76], and patients with temporary neurologic dysfunction after aortic surgery [77]. Altogether, these studies support the idea of CSF t-tau reflects the degree of brain injury and harbors a prognostic value in transient acute neuronal damage syndromes.

An important confounding factor in the biomarker field is the age of subjects in the study. In this regard, the discriminative power of CSF t-tau is higher in young old (<70 years) than in old (≥70 years) control and AD cases, indicating that CSF t-tau loses its discriminative power along the aging process [78]. Therefore, the age effect should be considered when establishing the diagnostic parameters of t-tau quantification as CSF biomarker. Increased t-tau levels during aging in healthy individuals have been validated in several independent cohorts [79], which in turn were associated to ApoE genotype [80, 81]. Consequently, age-dependent t-tau cut-off values in neurologically and psychiatrically healthy individuals have been established [82].
