The Relationship Between Development of Neuronal and Astrocytic Tau Pathologies in Subcortical Nuclei and Progression of Argyrophilic Grain Disease

Clinicopathological study of dementia with grains presenting with parkinsonism compared with a typical case

Argyrophilic grain disease (AGD) is one of the major pathological backgrounds of senile dementia. Dementia with grains refers to cases of dementia for which AGD is the sole background pathology responsible for dementia. Recent studies have suggested an association between dementia with grains and parkinsonism. In this study, we aimed to present two autopsy cases of dementia with grains. Case 1 was an 85-year-old man who exhibited amnestic dementia and parkinsonism, including postural instability, upward gaze palsy, and neck and trunk rigidity. The patient was clinically diagnosed with progressive supranuclear palsy and Alzheimer's disease. Case 2 was a 90-year-old man with pure amnestic dementia, clinically diagnosed as Alzheimer's disease. Recently, we used cryo-electron microscopy to confirm that the tau accumulated in both cases had the same three-dimensional structure. In this study, we compared the detailed clinical picture and neuropathological findings using classical staining and immunostaining methods. Both cases exhibited argyrophilic grains and tau-immunoreactive structures in the brainstem and basal ganglia, especially in the nigrostriatal and limbic systems. However, Case 1 had more tau immunoreactive structures. Considering the absence of other disease-specific structures such as tufted astrocytes, astrocytic plaques and globular glial inclusions, lack of conspicuous cerebrovascular disease, and no history of medications that could cause parkinsonism, our findings suggest an association between AGD in the nigrostriatal system and parkinsonism.

Pure argyrophilic grain disease revisited: independent effects on limbic, neocortical, and striato-pallido-nigral degeneration and the development of dementia in a series with a low to moderate Braak stage

Agyrophilic grains (AGs) are age-related limbic-predominant lesions in which four-repeat tau is selectively accumulated. Because previous methodologically heterogeneous studies have demonstrated inconsistent findings on the relationship between AGs and dementia, whether AGs affect cognitive function remains unclear. To address this question, we first comprehensively evaluated the distribution and quantity of Gallyas-positive AGs and the severity of neuronal loss in the limbic, neocortical, and subcortical regions in 30 cases of pure argyrophilic grain disease (pAGD) in Braak stages I-IV and without other degenerative diseases, and 34 control cases that had only neurofibrillary tangles with Braak stages I-IV and no or minimal Aβ deposits. Then, we examined whether AGs have independent effects on neuronal loss and dementia by employing multivariate ordered logistic regression and binomial logistic regression. Of 30 pAGD cases, three were classified in diffuse form pAGD, which had evident neuronal loss not only in the limbic region but also in the neocortex and subcortical nuclei. In all 30 pAGD cases, neuronal loss developed first in the amygdala, followed by temporo-frontal cortex, hippocampal CA1, substantia nigra, and finally, the striatum and globus pallidus with the progression of Saito AG stage. In multivariate analyses of 30 pAGD and 34 control cases, the Saito AG stage affected neuronal loss in the amygdala, hippocampal CA1, temporo-frontal cortex, striatum, globus pallidus, and substantia nigra independent of the age, Braak stage, and limbic-predominant age-related TDP-43 encephalopathy (LATE-NC) stage. In multivariate analyses of 23 pAGD and 28 control cases that lacked two or more lacunae and/or one or more large infarctions, 100 or more AGs per × 400 visual field in the amygdala (OR 10.02, 95% CI 1.12-89.43) and hippocampal CA1 (OR 12.22, 95% CI 1.70-87.81), and the presence of AGs in the inferior temporal cortex (OR 8.18, 95% CI 1.03-65.13) affected dementia independent of age, moderate Braak stages (III-IV), and LATE-NC. Given these findings, the high density of limbic AGs and the increase of AGs in the inferior temporal gyrus may contribute to the occurrence of dementia through neuronal loss, at least in cases in a low to moderate Braak stage.

Age-Related Pathology in Corticobasal Degeneration

Elderly human brains are vulnerable to multiple proteinopathies, although each protein has a different transmission pathway. Tau-immunoreactive astrocytes are well-known in elderly brains. In contrast, astrocytic plaques, a hallmark in corticobasal degeneration (CBD), rarely occur in aging and neurodegenerative disease other than CBD. To elucidate the clinicopathological correlation of aging-related pathology in CBD, we examined 21 pathologically proven CBD cases in our institute (12 males and 9 females, with a mean age of death 70.6 years). All CBD cases showed grains and neurofibrillary tangles (NFTs). Fifteen cases (71.4%) showed beta-amyloid deposition such as senile plaques or cerebral amyloid angiopathy. Three cases (14.3%) had Lewy body pathology. One case was classified as amygdala-predominant Lewy body disease, although no cases met the pathological criteria for Alzheimer's disease. Five cases (23.8%) displayed Limbic-predominant and age-related TDP-43 encephalopathy (LATE). NFTs, grains, and TDP-43-positive neuronal inclusions were widely distributed throughout the limbic system of CBD patients, but their densities were low. CBD might a have similar cell vulnerability and transmission pathway to that of multiple proteinopathy in aging brains.

Neuropathological analysis of cognitive impairment in progressive supranuclear palsy

BACKGROUND

Cognitive impairment is an important symptom in progressive supranuclear palsy (PSP), but the pathological changes underlying the cognitive impairment are unclear. This study aimed to elucidate relationships between the severity of cognitive impairment and PSP-related pathology.

METHODS

We investigated the clinicopathological characteristics of 10 autopsy cases of PSP, including neuronal loss/gliosis and the burden of PSP-related tau pathology by using a semiquantitative score in 17 brain regions. Other concurrent pathologies such as Braak neurofibrillary tangle stage, Thal amyloid phase, Lewy-related pathology, argyrophilic grains, and TDP-43-related pathology were also assessed. We retrospectively divided the patients into a normal cognition group (PSP-NC) and cognitive impairment group (PSP-CI) based on antemortem clinical information about cognitive impairment and compared the pathological changes between these groups.

RESULTS

Seven patients were categorized into the PSP-CI group (men = 4) and three into the PSP-NC group (men = 3). The severity of neuronal loss/gliosis and concurrent pathologies were not different between the two groups. However, the total load of tau pretangles/neurofibrillary tangles was higher in the PSP-CI group than in the PSP-NC group. In addition, the burden of tufted astrocytes in the subthalamic nucleus and medial thalamus was higher in the PSP-CI group than in the PSP-NC group.

CONCLUSION

Cognitive impairment in PSP may be associated with the amount of tufted astrocyte pathology in the subthalamic nucleus and medial thalamus.

Four-repeat tauopathies and late-onset psychiatric disorders: Etiological relevance or incidental findings?

Argyrophilic grain disease (AGD), progressive supranuclear palsy (PSP) and corticobasal degeneration are four-repeat (4R) tauopathies that develop in the presenium or later. Whether these diseases are associated with the occurrence of late-onset psychiatric disorders remains unclear. To facilitate the accumulation of clinicopathological findings regarding this issue, we here present a selected series of 11 cases that clinically developed psychotic disorder (n = 7; age at onset: 41-75 years), depressive disorder (n = 1; 49 years), bipolar disorder (n = 2; 32 and 37 years) and somatoform disorder (n = 1; 88 years), and had at least one pathological hallmark of these tauopathies. The mean age at death was 74.3 years. No case showed dementia, at least in the early stage of the course. Nine cases had AGD. Granular fuzzy astrocytes in the amygdala were noted in all AGD cases and one non-AGD case. Two AGD cases had tufted astrocytes (TAs) in the amygdala but not in the frontal cortex and striatum. Three AGD and two non-AGD cases had TAs in the frontal cortex and/or striatum but not in the amygdala. One AGD case had a small number of astrocytic plaques in the frontal cortex, striatum and globus pallidus. Only one case was diagnosed as atypical PSP according to the NINDS-PSP neuropathological criteria. No case had high-level Alzheimer's disease pathology, Lewy body disease or limbic-predominant age-related TDP-43 encephalopathy. Two cases had mild neuronal loss in the hippocampus and substantia nigra, respectively. Clinicopathological studies focusing especially on early changes of 4R tauopathies, as well as the development of surrogate markers of these diseases, may be necessary for better understanding of the pathogenic backgrounds of late-onset psychiatric disorders.

Risk of neurodegenerative disease due to tau phosphorylation changes and arsenic exposure via drinking water

It is estimated that around 140 million people are drinking highly contaminated water with arsenic (As) as a natural earth's crust component. On the other hand, the prevalence of neurodegenerative disorders, especially Alzheimer's disease, is constantly increasing. The aim of the present study was to investigate the correlation between oral arsenic trioxide exposure and its impact on tau protein phosphorylation at Ser262. Fifty-four male mice were randomly divided into three groups and were freely accessed to food and contaminated water of 1 and 10 ppm arsenic trioxide for 3 months, except for control subjects. At the end of each month, As concentration and tau phosphorylation were checked with graphite furnace atomic absorption spectrometer and western blot analysis, respectively. Surprisingly, it was observed that the amount of measured brain arsenic in 10 ppm-exposed subjects was significantly increased after 3 months (P-value ˂ 0.0001). The significant changes in tau phosphorylation were not seen in the 1 ppm-exposed subjects, and it was observed that Ser262 phosphorylation significantly increased after 2 and 3 months in the 10 ppm group (P-value < 0.05). Our results demonstrated that arsenic accumulated in the brain time-dependently and increased Ser262 tau phosphorylation, which is very important in several tauopathies. In conclusion, it could be inferred that environmental arsenic exposure even at very low concentrations could be considered as a reason for increasing the risk of developing neurodegenerative disease.

Factors associated with development and distribution of granular/fuzzy astrocytes in neurodegenerative diseases

Granular/fuzzy astrocytes (GFAs), a subtype of “aging‐related tau astrogliopathy,” are noted in cases bearing various neurodegenerative diseases. However, the pathogenic significance of GFAs remains unclear. We immunohistochemically examined the frontal cortex, caudate nucleus, putamen and amygdala in 105 cases composed of argyrophilic grain disease cases (AGD, N = 26), and progressive supranuclear palsy (PSP, N = 10), Alzheimer’s disease (AD, N = 20) and primary age‐related tauopathy cases (PART, N = 18) lacking AGD, as well as 31 cases bearing other various neurodegenerative diseases to clarify (i) the distribution patterns of GFAs in AGD, and PSP, AD and PART lacking AGD, (ii) the impacts of major pathological factors and age on GFA formation and (iii) immunohistochemical features useful to understand the formation process of GFAs. In AGD cases, GFAs consistently occurred in the amygdala (100%), followed by the putamen (69.2%) and caudate nucleus and frontal cortex (57.7%, respectively). In PSP cases without AGD, GFAs were almost consistently noted in all regions examined (90–100%). In AD cases without AGD, GFAs were less frequent, developing preferably in the putamen (35.0%) and caudate nucleus (30.0%). PART cases without AGD had GFAs most frequently in the amygdala (35.3%), being more similar to AGD than to AD cases. Ordered logistic regression analyses using all cases demonstrated that the strongest independent factor of GFA formation in the frontal cortex and striatum was the diagnosis of PSP, while that in the amygdala was AGD. The age was not significantly associated with GFA formation in any region. In GFAs in AGD cases, phosphorylation and conformational change of tau, Gallyas‐positive glial threads indistinguishable from those in tufted astrocytes, and the activation of autophagy occurred sequentially. Given these findings, AGD, PSP, AD and PART cases may show distinct distributions of GFAs, which may provide clues to predict the underlying processes of primary tauopathies.

Astroglia and Tau: New Perspectives

Astrocytes contribute to the pathogenesis of neurodegenerative proteinopathies as influencing neuronal degeneration or neuroprotection, and also act as potential mediators of the propagation or elimination of disease-associated proteins. Protein astrogliopathies can be observed in different forms of neurodegenerative conditions. Morphological characterization of astrogliopathy is used only for the classification of tauopathies. Currently, at least six types of astrocytic tau pathologies are distinguished. Astrocytic plaques (AP), tufted astrocytes (TAs), ramified astrocytes (RA), and globular astroglial inclusions are seen predominantly in primary tauopathies, while thorn-shaped astrocytes (TSA) and granular/fuzzy astrocytes (GFA) are evaluated in aging-related tau astrogliopathy (ARTAG). ARTAG can be seen in the white and gray matter and subpial, subependymal, and perivascular locations. Some of these overlap with the features of tau pathology seen in Chronic traumatic encephalopathy (CTE). Furthermore, gray matter ARTAG shares features with primary tauopathy-related astrocytic tau pathology. Sequential distribution patterns have been described for tau astrogliopathies. Importantly, astrocytic tau pathology in primary tauopathies can be observed in brain areas without neuronal tau deposition. The various morphologies of tau astrogliopathy might reflect a role in the propagation of pathological tau protein, an early response to a yet unidentified neurodegeneration-inducing event, or, particularly for ARTAG, a response to a repeated or prolonged pathogenic process such as blood-brain barrier dysfunction or local mechanical impact. The concept of tau astrogliopathies and ARTAG facilitated communication among research disciplines and triggered the investigation of the significance of astrocytic lesions in neurodegenerative conditions.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.

Astrocytic tau pathologies in aged human brain

Argyrophilic and tau-positive abnormal structures in astrocytes are frequent in aged brains, with a new nomenclature of aging-related tau astrogliopathy (ARTAG) proposed. The two major cytomorphologies of ARTAG are thorn-shaped astrocytes (TSA) and granular or fuzzy tau immunoreactivity in processes of astrocytes (GFA). We selected 28 cases in which many AT8-identified astrocytic tauopathies were observed in the central nervous system from 330 routine aged autopsied cases, including Alzheimer's disease. AT8-identified and Gallyas silver staining-positive TSA were observed in subpial, subependymal, perivascular areas as well as white matter. These TSA were 4-repeat (4R) tau-positive. In contrast, 3-repeat (3R)-tau was negative in TSA, but positive in short thick cell processes, likely neuropil threads, in subpial and subependymal areas. The frequency of 3R-tau-positive processes was variable. Small dot-like AT8-identified astrocytic processes surrounding vessels in the neuropil were also positive for 4R-tau, but negative for 3R-tau. GFA in cerebral gray matter were AT8-identified and Gallyas-positive, and positive for 4R-tau but negative for 3R-tau. In this study, we did not identify 3R-tau+/4R-tau+ or 3R-tau+/4R-tau- astrocytes. Further studies are needed to clarify the nature and progression of glial tau-positive structures in ARTAG.

Protein astrogliopathies in human neurodegenerative diseases and aging

Neurodegenerative diseases are characterized by progressive dysfunction and loss of neurons associated with depositions of pathologically altered proteins showing hierarchical involvement of brain regions. The role of astrocytes in the pathogenesis of neurodegenerative diseases is explored as contributors to neuronal degeneration or neuroprotection pathways, and also as potential mediators of the transcellular spreading of disease-associated proteins. Protein astrogliopathy (PAG), including deposition of amyloid-β, prion protein, tau, α-synuclein, and very rarely transactive response DNA-binding protein 43 (TDP-43) is not unprecedented or unusual in neurodegenerative diseases. Morphological characterization of PAG is considered, however, only for the neuropathological diagnosis and classification of tauopathies. Astrocytic tau pathology is seen in primary frontotemporal lobar degeneration (FTLD) associated with tau pathologies (FTLD-Tau), and also in the form of aging-related tau astrogliopathy (ARTAG). Importantly, ARTAG shares common features with primary FTLD-Tau as well as with the astroglial tau pathologies that are thought to be hallmarks of a brain injury-related tauopathy known as chronic traumatic encephalopathy (CTE). Supported by experimental observations, the morphological variability of PAG might reflect distinct pathogenic involvement of different astrocytic populations. PAG might indicate astrocytic contribution to spreading or clearance of disease-associated proteins, however, this might lead to astrocytic dysfunction and eventually contribute to the degeneration of neurons. Here, we review recent advances in understanding ARTAG and other related forms of PAG.

Sequential stages and distribution patterns of aging-related tau astrogliopathy (ARTAG) in the human brain

Aging-related tau astrogliopathy (ARTAG) describes tau pathology in astrocytes in different locations and anatomical regions. In the present study we addressed the question of whether sequential distribution patterns can be recognized for ARTAG or astroglial tau pathologies in both primary FTLD-tauopathies and non-FTLD-tauopathy cases. By evaluating 687 postmortem brains with diverse disorders we identified ARTAG in 455. We evaluated frequencies and hierarchical clustering of anatomical involvement and used conditional probability and logistic regression to model the sequential distribution of ARTAG and astroglial tau pathologies across different brain regions. For subpial and white matter ARTAG we recognize three and two patterns, respectively, each with three stages initiated or ending in the amygdala. Subependymal ARTAG does not show a clear sequential pattern. For grey matter (GM) ARTAG we recognize four stages including a striatal pathway of spreading towards the cortex and/or amygdala, and the brainstem, and an amygdala pathway, which precedes the involvement of the striatum and/or cortex and proceeds towards the brainstem. GM ARTAG and astrocytic plaque pathology in corticobasal degeneration follows a predominantly frontal-parietal cortical to temporal-occipital cortical, to subcortical, to brainstem pathway (four stages). GM ARTAG and tufted astrocyte pathology in progressive supranuclear palsy shows a striatum to frontal-parietal cortical to temporal to occipital, to amygdala, and to brainstem sequence (four stages). In Pick’s disease cases with astroglial tau pathology an overlapping pattern with PSP can be appreciated. We conclude that tau-astrogliopathy type-specific sequential patterns cannot be simplified as neuron-based staging systems. The proposed cytopathological and hierarchical stages provide a conceptual approach to identify the initial steps of the pathogenesis of tau pathologies in ARTAG and primary FTLD-tauopathies.

Tau Proteolysis in the Pathogenesis of Tauopathies: Neurotoxic Fragments and Novel Biomarkers

With predictions showing that 131.5 million people worldwide will be living with dementia by 2050, an understanding of the molecular mechanisms underpinning disease is crucial in the hunt for novel therapeutics and for biomarkers to detect disease early and/or monitor disease progression. The metabolism of the microtubule-associated protein tau is altered in different dementias, the so-called tauopathies. Tau detaches from microtubules, aggregates into oligomers and neurofibrillary tangles, which can be secreted from neurons, and spreads through the brain during disease progression. Post-translational modifications exacerbate the production of both oligomeric and soluble forms of tau, with proteolysis by a range of different proteases being a crucial driver. However, the impact of tau proteolysis on disease progression has been overlooked until recently. Studies have highlighted that proteolytic fragments of tau can drive neurodegeneration in a fragment-dependent manner as a result of aggregation and/or transcellular propagation. Proteolytic fragments of tau have been found in the cerebrospinal fluid and plasma of patients with different tauopathies, providing an opportunity to develop these fragments as novel disease progression biomarkers. A range of therapeutic strategies have been proposed to halt the toxicity associated with proteolysis, including reducing protease expression and/or activity, selectively inhibiting protease-substrate interactions, and blocking the action of the resulting fragments. This review highlights the importance of tau proteolysis in the pathogenesis of tauopathies, identifies putative sites during tau fragment-mediated neurodegeneration that could be targeted therapeutically, and discusses the potential use of proteolytic fragments of tau as biomarkers for different tauopathies.

Neuropathological comorbidity associated with argyrophilic grain disease

Argyrophilic grain disease (AGD) is a common four-repeat tauopathy in elderly people. While dementia is a major clinical picture of AGD, recent studies support the possibility that AGD may be a pathological base in some patients with mild cognitive impairment, late-onset psychosis, bipolar disorder and depression. AGD often coexists with various other degenerative changes. The frequency of AGD in progressive supranuclear palsy (PSP) cases was reported to range from 18.8% to 80%. The frequency of AGD in corticobasal degeneration (CBD) cases tends to be higher than that in PSP cases, ranging from 41.2% to 100%. Conversely, in our previous study of the frequencies of mild PSP and CBD pathologies in AGD cases, five of 20 AGD cases (25%) had a few Gallyas-positive tufted astrocytes, six cases (30%) had a few granular/fuzzy astrocytes, and one case (5.0%) had a few Gallyas-positive astrocytic plaques in the putamen, caudate nucleus and/or superior frontal gyrus. Both Gallyas-positive tufted astrocytes and Gallyas-negative tau-positive granular/fuzzy astrocytes preferentially developed in the putamen, caudate nucleus and superior frontal cortex in AGD cases, being consistent with the predilection sites of Gallyas-positive tufted astrocytes in PSP cases. Further, in AGD cases, the quantities of Gallyas-positive tufted astrocytes, overall tau-positive astrocytes, and tau-positive neurons in the subcortical nuclei and superior frontal cortex were significantly correlated with Saito AGD stage, respectively. The frequency of AGD in AD cases was reported to reach up to 25% when using four-repeat tau immunohistochemistry. Pretangles are essential pathologies in AGD; however, the Braak stage of three-repeat tau-positive NFTs, which may indicate mild AD pathology or primary age-related tauopathy, was not correlated with Saito AGD stage. Clinicians should be aware of the possibility that coexisting AGD may impact clinical and radiological features in cases of other degenerative diseases.

Multisite Assessment of Aging-Related Tau Astrogliopathy (ARTAG)

Aging-related tau astrogliopathy (ARTAG) is a recently introduced terminology. To facilitate the consistent identification of ARTAG and to distinguish it from astroglial tau pathologies observed in the primary frontotemporal lobar degeneration tauopathies we evaluated how consistently neuropathologists recognize (1) different astroglial tau immunoreactivities, including those of ARTAG and those associated with primary tauopathies (Study 1); (2) ARTAG types (Study 2A); and (3) ARTAG severity (Study 2B). Microphotographs and scanned sections immunostained for phosphorylated tau (AT8) were made available for download and preview. Percentage of agreement and kappa values with 95% confidence interval (CI) were calculated for each evaluation. The overall agreement for Study 1 was >60% with a kappa value of 0.55 (95% CI 0.433-0.645). Moderate agreement (>90%, kappa 0.48, 95% CI 0.457-0.900) was reached in Study 2A for the identification of ARTAG pathology for each ARTAG subtype (kappa 0.37-0.72), whereas fair agreement (kappa 0.40, 95% CI 0.341-0.445) was reached for the evaluation of ARTAG severity. The overall assessment of ARTAG showed moderate agreement (kappa 0.60, 95% CI 0.534-0.653) among raters. Our study supports the application of the current harmonized evaluation strategy for ARTAG with a slight modification of the evaluation of its severity.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Behavioral variant of frontotemporal dementia: Fundamental clinical issues associated with prediction of pathological bases

Behavioral variant of frontotemporal dementia (bvFTD) is a clinical syndrome characterized mainly by behavioral symptoms due to frontal dysfunction. Major neurodegenerative bases of bvFTD include Pick's disease, frontotemporal lobar degeneration with trans-activation response DNA protein 43-positive inclusions, corticobasal degeneration, and progressive supranuclear palsy. Early disinhibition characterized by socially inappropriate behaviors, loss of manners, and impulsive, rash and careless actions is the most important clinical feature of bvFTD. On the other hand, it was reported that clinical presentations of some Alzheimer's disease cases and patients with psychiatric disorders (e.g., addictive disorders, gambling disorder and kleptomania) often resemble that of bvFTD. Although clinical differentiation of 'true' bvFTD cases with frontotemporal lobar degeneration (FTLD) pathology from mimicking cases without it is not always easy, evaluation of the following features, which were noted in autopsy-confirmed FTLD cases and/or clinical bvFTD cases with circumscribed lobar atrophy, may often provide clues for the diagnosis. (i) The initial symptoms frequently develop at 65 years or younger, and (ii) 'socially inappropriate behaviors' can be frequently interpreted as contextually inappropriate behaviors prompted by environmental visual and auditory stimuli. Taking a detailed history usually reveals various kinds of such behaviors in various situations in everyday life rather than the repetition of a single kind of behavior (e.g., repeated shoplifting). (iii) A correlation between the distribution of cerebral atrophy and neurological and behavioral symptoms is usually observed, and the proportion of FTLD cases with right side-predominant cerebral atrophy may be higher in a psychiatric setting than a neurological setting. Finally, (iv) whether the previous course and the combination of symptoms observed at the first medical visit can be explained by major evolution patterns of clinical syndromes in pathologically confirmed FTLD cases should be considered. These views may provide clues to differentiate FTLD from Alzheimer's disease and to predict a subsequent clinical course and therapeutic interventions needed in the future.