Tau Imaging in Head Injury.

proteins to MTs either acts to directly stabilize MTs or create a bridge to link MTs and other cyto-skeletal structures. 3 Tau proteins are required for the proper formation of postsynaptic structures, dendritic spines, and terminals where tau proteins act as neurotrophic factors in neurogenesis. 6 Certain domains of the tau protein have the capability of binding to the lipid bilayers in the cellular

membrane, and although the purpose of such membrane associations is speculated, the prevalence of tau-membrane binding has been correlated with tau aggregation.Tau proteins exhibit similar binding processes, although the most characterized binding interaction has been those involving MTs. 4 Tau proteins are natively unfolded, a paper-clip form of tau is present in the intracellular space as a result of intramolecular binding; otherwise, binding interactions with the MT structure alters the protein to expose the MT binding region of the tau proteins, which allows interactions between MT structures and the tau protein (Fig. 1). 3 Tau proteins exist in 8 different conformations and 6 particular isoforms derived from alternative splicing of the MT-associated protein tau (MAPT) gene. 3,7Of the 6 isoforms, each is differentiated by the number of N-terminal inserts (0N, 1N, 2N) and the number of MT binding repeats in the C-terminal (3R, 4R). 3 The ratio of isoforms varies over the developmental period and varies between regions of the human brain.For instance, the fetal brain expresses only 1 form of the tau isoform, whereas the fully developed brain confers all 6 isoforms.The 3R and 4R isoforms are equally expressed in the cerebral cortex of healthy adults 8 ; otherwise, there is a notable discrepancy in the prevalence of N-terminal variants; for example, 0N, 1N, and 2N tau represent 40%, 50%, and 10% of the total CNS tau, respectively. 9With respect to the proportions of tau protein isoform, there are significant differences between humans and other species. 3Once synthesized, tau proteins may undergo a variety of posttranslational modifications, including phosphorylation, glycation, acetylation, oxidation, polyamination, sumoylation, and ubiquitylation. 3au proteins are salient to the understanding of head injury.Head injuries, depending on the severity, can produce a series of complex and diverse neurophysiological consequences.The primary injury involves the immediate consequences of the physical insult, whereas secondary injury relates to a pathophysiological cascade.A common category of primary injuries is diffuse axonal injuries, which result from torsion and blunt forces.Specifically, external forces can harm the integrity of axonal structures such that axonal MT structures release previously bounded tau proteins into the parenchymal cerebrospinal fluid (CSF).Secondary injury has been associated with inflammatory pathways activation, neuronal metabolism and perfusion alteration, excitotoxicity, free radical generation, mitochondrial dysfunction, axonal degeneration, and neuronal dysfunction. 10Of note, secondary injury (eg, axonal degeneration) has been understood as the most contributory to the proliferation of tau proteins in the parenchymal space.
This relationship between free tau proteins as a result of head trauma has been examined by many studies in the context of fluid biomarkers.Following traumatic brain injuries (TBIs), biomarkers can be assayed primarily in CSF or peripheral blood, although CSF is often preferred. 11everal studies have thoroughly confirmed the association of several CSF biomarkers with axonal injury after mild (mTBI), moderate, and severe TBI (sTBI).More sensitive assays used by studies on sports-related mTBIs are associated with acute increases of tau in plasma where the concentration of tau correlated with the duration of postconcussive symptoms and the concentrations steadily declined during rehabilitation. 12sTBI events have correlated with greater concentrations of tau in CSF samples where tau protein levels in ventricular CSF have directly related to TBI severity, lesion size, hypoxia, and clinical outcomes. 13In those with repeated injury (eg, boxers, contact sport athletes), elevated levels of tau in CSF samples were observed more than a week after the sporting event, where normalization of tau levels occurred 2 to 3 months from incident. 13thin the context of head injury, the most serious consequences of the accumulation of tau proteins are seen in unique neurologic tauopathies, namely CTE, in which tau aggregation is linked to subsequent neurodegenerative processes. 14Epidemiologic studies have linked TBI, single event and repeat, to the development of tauopathies. 10Although the etiology of tauopathies are somewhat speculative, the pathologic characterization of neurologic tauopathies is generally agreed on. 15Tauopathies are defined by the intraneuronal presence of tau aggregates termed neurofibrillary tangles (NFT), which are composed of multiple units of hyperphosphorylated MT-associated tau isoforms.The particular form of neurodegeneration leads to a unique distribution and identity of prions 15 (Figs. 2 and 3, Table 1 16 ). 17n most tauopathy cases, tau proteins are hyperphosphorylated to become unbounded from MT Fig. 2. Stages of CTE.In stage I CTE, p-tau pathology is found in discrete foci in the cerebral cortex, most commonly in the superior or lateral frontal cortices, typically around small vessels at the depths of sulci.In stage II CTE, there are multiple foci of p-tau at the depths of the cerebral sulci and there is localized spread of neurofibrillary pathology from these epicenters to the superficial layers of adjacent cortex.The medial temporal lobe is spared neurofibrillary p-tau pathology.In stage III CTE, p-tau pathology is widespread; the frontal, insular, temporal and parietal cortices show widespread neurofibrillary degeneration with greatest severity in the frontal and temporal lobes, concentrated at the depths of the sulci.Also, in stage III CTE, the amygdala, hippocampus, and entorhinal cortex show substantial neurofibrillary pathology that is found in earlier CTE stages.In stage IV CTE, there is widespread severe p-tau pathology affecting most regions of the cerebral cortex and the medial temporal lobe, sparing calcarine cortex in all but the most severe cases.structures; these hyperphosphorylated proteins then accumulate within cells with MAPT mutations.However, changes in isoforms or phosphorylation patterns as a result of such mutations result in tau aggregation that is insoluble and harmful to neuronal function and axonal transport. 18Tau aggregates retain prion properties by way of seeding and spreading. 19Minimal exposure to tau seeds can further lead to misfolding and aggregation.This phenomenon is observed when tau proteins mislocalized into the soma and dendrites are transferred between the neurons. 10It has been observed that tau proteins can spread using the connectome network pattern and either spread preformed NFT or seed subsequent tau accumulation. 10Sparse tau aggregation generally develops naturally with age, however, and increased density and unique distributions of tau and other abnormal protein aggregates (eg, beta-amyloid plaques) in the context of clinical dementia become indicative of neurodegenerative disease. 15oncerning TBI, NFTs can be observed within 6 hours of the event, and postmortem studies of those with single-event moderate to sTBI have shown higher levels of NFTs than in controls. 10It is further understood that the risk of CTE is directly related to the number and severity of TBI events. 10nterestingly, although the distribution of tau aggregates and, to a lesser extent, beta-amyloid plaques, is unique in CTE, there is not unique phosphorylation or a specific isoform that differentiates CTE from other neurodegenerative conditions. 20Further, the ratio of tau proteins to beta-amyloid plaques is particularly elevated in CTE and is a unique characteristic of this neurodegenerative condition.

TAU RADIOTRACERS IN HEAD INJURY
The strong link between the presence of NFT and neurocognitive decline has strongly motivated the development of tau radiotracers that can assess the magnitude and localization of abnormal protein aggregates (Table 2). 7In the context of most tauopathies, tau radiotracers are required to cross plasma cell membranes and the blood-brain barrier to reach intracellular tau proteins; tau NFT radiotracers must provide high selectivity given similar structures between NFT and b-amyloid aggregates; radiotracers must further account for the variation in NFT with respect to tertiary structures, posttranslational modifications, and isoforms. 7As such, there is a need for specificity and breadth when developing tau radiotracers.Interestingly, these challenges are less significant to the application of tau imaging in the context of head injury.Limitations in the concentration of tau proteins as compared with beta-amyloid plaques have posed a significant challenge to tau imaging in dementias, but this is not the case in CTE.In CTE, the prevalence of tau is significantly greater than that of beta- amyloid.As such, highly specific tau radiotracers can excel in binding to tau aggregates with less risk of off-site binding.In addition, there are higher concentrations of tau aggregates in perivascular space, which allows easier access of radiotracers to tau aggregates.As such, tau radiotracers in the context of CTE, as compared with other tauopathy dementias, are likely to perform well.Increased uptake in the cerebrum and white matter correlated to psychosis and other neuropsychiatric symptoms. 21Gorgraptis and colleagues 22 applied [18F]AV145-PET in 21 subjects with moderate to sTBI and 11 healthy control subjects; elevated whole brain and right occipital lobe uptake of [18F]AV145 were observed in the TBI group.Robinson and colleagues 23 observed increased white matter uptake of [18F]AV145 and notable uptake in the cerebellum, occipital lobe, inferior temporal lobe, and frontal lobe across 16 military veterans with a history of blast neurotrauma.However, no controls were used in this study as comparators.

TAU PET IMAGING STUDIES IN HEAD INJURY
Several studies have used National Football League (NFL) players as their study population, whereas most studies have aligned well with non-NFL populations with a few notable Tau Imaging in Head Injury exceptions.Dickstein and colleagues 24 examined one player with [18F]AV145-PET and found increased uptake in the gray matter-white junction along with the bilateral cingulate, occipital lobe, and orbitofrontal cortices, and temporal lobes.Mitsis and colleagues 25 observed increased [18F] AV1451 uptake in the 1 NFL player and 1 patient with sTBI in whom differential uptake patterns were observed.The NFL subject conferred higher uptake in the nigral and striatal regions, whereas the subcortical and hippocampal regions were more avid in the scans of the subject with sTBI. 25konkwo and colleagues 26 also observed elevated [18F]AV1451 uptake in 2 patients with TBI as compared with age-sex matched controls.Wooten and colleagues 27 assessed the [18F] AV1451-PET scans of 5 athletes, 2 veterans, and 1 vehicular accident patient as compared with 11 healthy subjects; regions with higher uptake in the TBI group were correlated with poor white matter function.
Larger studies with NFL players were performed by Barrio and colleagues 28 and Stern and colleagues 29 applied [18F] AV1451 to 16 NFL players and 31 healthy controls to find elevated uptake in the bilateral superior frontal, Fig. 5. Involvement of amygdala and midbrain areas in concussion-based mTBI is supported by both mechanistic concept of injury (I) and by the results of neuropathological examinations in deceased retired American football players with premortem complaints of functional impairments (II and III).(I) Rotation of the brain in the sagittal plane during a concussion, associated with significant accelerations and deceleration, will have significant negative effect on the brain tissue in the midbrain and thalamus (green shaded area) and on the affected cortical areas (red area).Stretching, compression, and shearing of axons during such sudden brain movements are hypothesized to be the cause of axonal injury.Similarly, rotation in the coronal plane has been shown to lead to consistent damage to midbrain region tracts (27).(II) (A-D) show results of tau immunohistochemistry (IHC) and demonstrate that in the mTBI group areas of increased [18F]FDDNP signal in amygdala and dorsal midbrain coincide with presence of dense tau deposits in periaqueductal gray in dorsal midbrain (A, B) and in amygdala (C, D). (III) Amygdala and medial temporal lobe (MTL) areas are affected in the brains of retired professional American football players who died due to suicide (left; 45-year-old retired player) or due to natural causes (right; 80year-old retired NFL player).Amygdala and MTL areas are the first areas with high density of tau deposits in the neocortex and remain one of the most affected cortical regions in most retired professional American football player cases.(From Barrio, J.R., et al., In vivo characterization of chronic traumatic encephalopathy using [F-18] FDDNP PET brain imaging.Proceedings of the National Academy of Sciences, 2015.112( 16): p. E2039-E2047; with permission.)bilateral medial temporal, and left parietal regions.Barrio and colleagues 28 used [18F] FDDNP-PET to study uptake patterns among 14 NFL players and 28 healthy controls.Although [18F]FDDNP is bound to beta-amyloid and tau aggregates, the limited prevalence of betaamyloid in CTE implies that much of the unique uptake in these populations as compared with the control are likely driven my tau aggregate accumulation and not beta-amyloid deposition. 28Nevertheless, Barrio and colleagues 28 noted increased uptake in the amygdala, anterior cingulate gyrus, and frontal cortex in the NFL players.Chen and colleagues 30 used [18F]FDDN-PET in a study population of 7 military veterans, 15 retired players with mTBI histories, and 28 healthy controls; findings were consistent with Barrio and colleagues, 28 but it was noted that military personnel had limited uptake in the amygdala and striatum relative to the player population.Vasilevskaya and colleagues 31 applied [18F] AV1451-PET to 38 former contact sport athletes.In this study, the presence of APOE4 alleles aligned with high cortical gray matter PET tau uptake such that the presence of APOE4 may incline individuals to accumulate tau aggregates more so than others 31 (Fig. 4, 32 Figs. 5 and 6, 28 Fig. 7 33 ).
Given that the present diagnosis of CTE is contingent on postmortem neuropathological examination, some of the most convincing tau-PET studies have attempted to confirm their imaging with postmortem analysis of the brain.Mantyh and colleagues 34 studied 1 former NFL player with [18F] AV1451-PET with subsequent postmortem analysis of the individual who was subsequently diagnosed with stage IV CTE, TDP 43 encephalopathy, and stage 3 Braak NFT.Uptake was most avid in degenerated and hypometabolic regions in the frontotemporal region; this overlapped postmortem tau aggregates in the left fusiform, inferior temporal gyri, and juxtacortical frontal white matter.High uptake with minimal tau deposition was noted in the basal ganglia, thalamus, motor cortex, and calcarine cortex. 34malu and colleagues 33 assessed the [18F] FDDNP-PET scan of 1 former NFL player and respective postmortem analysis to find that [18F] FDDNP-PET uptake correlated with tau deposition, most notably in the parasagittal and paraventricular regions of the brain and the brain stem.No correlation with amyloid or TDP-43 deposition was observed such that regions of the brain most involved in shearing and rotational forces were most linked to tau deposition; such deposition patterns would align with the unique patterns found in CTE. 33arquie ´and colleagues 35 did not perform any in vivo imaging, rather autoradiographic binding patterns of [18F]AV1451 were observed in 5 postmortem brains diagnosed with stage II through stage IV CTE.[18F]AV1451 binding observed in all NFT regions as confirmed by immunostaining and a limited signal was observed in white matter and other non-tangle-containing regions.Quantification of tau burden and tracer uptake was correlated. 35Previously mentioned in vivo studies have not found such consistent binding patterns and strong correlations, which may be indicative of a difference between the ex vivo and in vivo environments.

SUMMARY
In reviewing this literature, there are several apparent takeaways.There are variations in the binding patterns between different tau radiotracers.Nevertheless, there is consistently uptake in certain regions across studies; aberrant binding is expected given the variation in small studies and potential off-site radiotracer binding.However, the literature suggests that larger studies may be more consistent in finding uptake in regions where tau aggregates are normally observed in CTE populations.Overall, the evidence suggests that tau-PET imaging will continue to play a significant role in TBI and CTE.These studies have shown considerable promise in the imaging of tau and prospective larger studies may substantiate the use of a particular radiotracer in the assessment of long-term TBI ramifications and the diagnosis of CTE.In particular, there is a notable need for future studies that incorporate in vivo imaging and postmortem pathologic study.

CLINICS CARE POINTS
Patients with a history of head trauma should be assessed for long-term sequalae associated with head injury.
Tau deposition is associated with chronic ramifications of head trauma.
Tau-PET has shown promise in assessing the progression of chronic symptoms and degenerative conditions associated with head trauma.

Fig. 1 .
Fig.1.Binding of tau to MTs.Tau associates with MTs primarily through the MT binding domain, comprising either 3 or 4 repeats.The N and C termini of tau are closely associated when tau is free in the cytoplasm, giving rise to the proposed "paper-clip" model of tau conformation.On binding to MTs, the terminal regions of tau become separated and the N terminus of tau projects away from the MT surface.(From Guo, T., W. Noble, and D.P. Hanger, Roles of tau protein in health and disease.Acta neuropathologica, 2017.133(5): p. 665-704.; with permission.)

Fig. 3 .
Fig. 3. Microscopic changes in stage IV CTE. Whole-mount coronal sections in stage IV CTE show widespread p-tau pathology affecting most regions of the cerebral cortex and medial temporal lobe.Astrocytic tangles are prominent and there is marked neuronal loss in the cortex, amygdala, and hippocampus.There are also widespread pTDP-43 abnormalities.All images: 50 m tissue sections, CP-13, or p-TDP-43 immunostain.(From McKee, A.C., et al., The neuropathology of chronic traumatic encephalopathy.Brain pathology, 2015.25(3): p. 350-364; with permission.) Using many of the noted radiotracers, many studies have applied tau PET to TBI populations.Most tau-PET imaging of individuals with singleevent TBIs takes place long after the original insult, whereas most studies use patients with years of mTBI experiences through sports or military combat.Takahata and colleagues 21 used [11C]PBB3-PET to assess tau patterns in 27 individuals with either repeat mTBI or sTBI as compared with 15 healthy control subjects.

Fig. 4 .
Fig. 4. [18F]FDDNP-PET results for NFL players and a control.Coronal and transaxial [18F]FDDNP-PET scans of the retired NFL players include the following: NFL1: 59-year-old linebacker with mild cognitive impairment (MCI), who experienced momentary loss of consciousness after each of 2 concussions; NFL2: 64-year-old quarterback with ageconsistent memory impairment, who experienced momentary loss of consciousness and 24-hour amnesia following 1 concussion; NFL3: 73year-old guard with dementia and depression, who suffered brief loss of consciousness after 20 concussions, and a 12-hour coma following 1 concussion; NFL4: 50-year-old defensive lineman with MCI and depression, who suffered 2 concussions and loss consciousness for 10 minutes following one of them; NFL5: 45year-old center with MCI, who suffered 10 concussions and complained of light sensitivity, irritability, and decreased concentration after the last two.The players' scans show consistently high signals in the amygdala and subcortical regions and a range of cortical binding from extensive to limited, whereas the control subject shows limited binding in these regions.Red and yellow areas indicate high [18F]FDDNP binding signals.(From Small, G.W., et al., PET scanning of brain tau in retired national football league players: preliminary findings.The American Journal of Geriatric Psychiatry, 2013.21(2): p. 138-144; with permission.)

Fig. 6 .
Fig. 6. [18F]FDDNP distribution volume ratios (DVR) parametric images showing patterns T1 to T4 of increased [18F]FDDNP signal observed in the mTBI group compared with cognitive control subjects (left).The T1 pattern shows involvement of 2 core areas that have consistently increased [18F]FDDNP signal in all 4 patterns: amygdala (limbic) and dorsal midbrain (subcortical).Patterns T2 to T4 are marked by increase of [18F]FDDNP signal in these 2 core regions and progressively larger number of subcortical, limbic, and cortical areas.Although more complex patterns (eg, T4) overlap with AD in the cortex, midbrain and amygdala signals are elevated above the levels in AD.An AD case is shown in the right column for comparison.(Lower) (A) is a 2-dimensional scatter plot showing [18F]FDDNP DVR values in 2 core areas consistently involved in CTE (subcortical structures [dorsal midbrain] and limbic structures [amygdala]), clearly demonstrating separation of mTBI and control (CTRL) groups.(B, C) demonstrate similar separation effect when dorsal midbrain is compared with cortical areas typically associated with CTE and its mood disorders, namely anterior cingulate gyrus (ACG) (B) and frontal lobe (C).Subjects with mTBI are represented by green circles, and CTRL subjects are represented by blue circles.(From Barrio, J.R., et al., In vivo characterization of chronic traumatic encephalopathy using [F-18] FDDNP PET brain imaging.Proceedings of the National Academy of Sciences, 2015.112(16): p. E2039-E2047; with permission.)

Table 1
Pathologic classification of chronic traumatic encephalopathyFrom Ayubcha, C., et al., A critical review of radiotracers in the positron emission tomography imaging of traumatic brain injury: FDG, tau, and amyloid imaging in mild traumatic brain injury and chronic traumatic encephalopathy.European Journal of Nuclear Medicine and Molecular Imaging, 2020; with permission.