Three-Repeat Tau with Grain-Like Structures and Distribution in an 83-Year-Old Man

INTRODUCTION

Argyrophilic grain disease (AGD) is one of the diseases characterized by 4-repeat tau deposition [1, 2]. AGD involves characteristic structures of tau deposition called argyrophilic grains, which show characteristic distributions in brain structures including the ambient gyrus, amygdala, and dorsomedial temporal tip [3–7]. We encountered an 83-year-old man with 3-repeat dominant tau deposition with grain-like morphology and distribution in the ambient gyrus.

CASE REPORT

An 83-year-old man was admitted to the hospital because of pneumonia. He was in good health until approximately one year earlier, when he developed a gait disturbance, cough, and hemoptysis. Suspicion of lung carcinoma on chest computed tomography was confirmed by the presence of class V squamous cell carcinoma in the sputum cytology. He was readmitted because of ileus and volvulus of the sigmoid colon. He was able to understand the details of the operation but was unable to walk. Clinical details on cognitive and neurological statuses were not available. He presented with only disorientation. Clinical dementia rating was 0.5. Cognitive picture was consistent with mild cognitive impairment. Furthermore, he terminally showed dysphagia, falling tendency, dysuria (he also had prostatic hypertrophy), slow walking, lower extremity dominant weakness, thirst, anorexia, difficulty using right hand, slow movement of right upper and lower extremities, stiffness of whole body, and frequent complaint, sequentially. He had no family history of similar disease. After a temporary improvement with antibiotics, the patient developed cough and hemoptysis, followed by gradual exacerbation of his general condition and he died two months after admission.

Informed consent was obtained from the next of kin, and the protocol approval by the local Institutional Review Board was obtained.

The postmortem delay was 30 h 12 min. The brain was fixed in 10% formalin for 3 months.

A general pathological examination showed squamous cell carcinoma of the left lung, and bronchioloalveolar pneumonia. Neuropathological examination showed that brain weight was 1,470 g after fixation. The inferior horn of the left lateral ventricle at the anterior left temporal lobe was dilated and asymmetrical atrophy of the ambient gyrus was accentuated in the left side (Fig. 1A) [8]. Moderate depigmentation of the substantia nigra and the locus ceruleus were noted (Fig. 1B). Softening of the left putamen and arteriosclerosis of the basilar artery and the left internal carotid artery were observed.

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Fig.1

Asymmetrical atrophy of the ambient gyrus with abundant 3-repeat tau deposits resembling grains, tuft-shaped astrocytes and oligodendroglial coiled bodies. A) Inferior horn of the left lateral ventricle at the anterior left temporal lobe is dilated and asymmetrical atrophy of the ambient gyrus accentuated in the left side is observed: similar findings to the argyrophilic grain disease. B) Macroscopic findings of the brainstem. Moderate depigmentation of the substantia nigra (left) and the locus ceruleus (right). C) Abundant grain-like 3-repeat tau deposition in the ambient gyrus (RD3, bar = 20 μm). D) pTDP-43 positive grain-like deposition in the ambient gyrus (phosphorylated TDP-43, bar = 20 μm). E) No colocalization between 3-repeat tau (red) and pTDP43 (green) (bar = 20 μm). F) Ubiquitin positive grain-like deposition in the ambient gyrus (ubiquitin, bar = 20 μm). G) P62-positive grain-like deposition in the ambient gyrus (p62, bar = 20 μm). H) Partial colocalization between p62 and tau (red: p62, green: AT-8, bar = 20 μm). I) Some tau positive grain-like structures colocalizing with dendritic marker MAP2 (arrowhead, red: MAP2, green: tau, bar = 20 μm). Grain-like structures seems to be predominantly located at dendrites. J) Abundant macrophage infiltration and sponginess in the dorsomedial temporal tip (Haematoxylin-eosin, bar = 50 μm). K) 3-repeat tau-positive tuft-shaped astrocyte in the amygdala (RD3, bar = 50 μm). L) Abundant oligodendroglial 3-repeat tau immunoreactivities in the white matter of the temporal tip, often surrounding an oligodendroglial cell nucleus similar to coiled bodies in the AGD (RD3, bar = 50 μm).

Microscopic examination showed that, in addition to neuronal loss and gliosis, tau and phosphorylated TDP-43 (pTDP-43) deposition were observed in the ambient gyrus (Fig. 1C, D, Supplementary Figure 1C-E), amygdala, and dorsomedial temporal tip. Three-repeat dominant tau deposition in neurons in the CA2-3, entorhinal cortex, and nucleus accumbens was observed. Three-repeat dominant tau deposition in a few neurons and a few 3-repeat dominant grain-like structures were observed in dentate gyrus granular layer of hippocampus. Three-repeat tau dominant grain-like granular material deposition was noted in the neuropil of the ambient gyrus (Fig. 1C, Supplementary Figure 1C, D), amygdala, and dorsomedial temporal tip. Some of grain-like granular material deposition was Gallyas positive, but the numbers were less frequent than AT8 or RD3 (Supplementary Figure 1B). pTDP-43-positive grain-like structures were observed in the same regions (Fig. 1D, Supplementary Figure 1E), distributed in amygdala and medial temporal lobe. FTLD-TDP subtype was similar to type A. Double-labeling confocal microscope revealed, however, that most of the pTDP-43-positive structures did not colocalize with 3-repeat tau immunoreactivities (Fig. 1E). Ubiquitin/p62-positive grain-like structures were also observed in the same regions (Fig. 1F-H). The grain-like structures were not ubiquitinated (p62) or only small amount of them (Fig. 1H). Some of the grain-like structures colocalized with dendrite marker MAP 2 and the grain-like structures predominantly located at dendrites (Fig. 1I). Sponginess and abundant macrophage infiltrations were observed, and the macrophage infiltrations exhibited distribution similar to tau deposition (Fig. 1J, Supplementary Figure 2). A few tuft-shaped astrocytes were observed in the amygdala by AT-8. Tau in the astrocytes were partially RD3 positive (Fig. 1K, Supplementary Figure 1F). Diffuse granular astrocytic immunoreactivities were frequently observed in the amygdala by AT8 and RD3. Some astrocytes exhibited immunoreactivity for α-B crystallin. Abundant oligodendroglial 3-repeat tau immunoreactivities were noted in the white matter of the temporal tip (Fig. 1L). Only an achromatic neuron was observed in the precentral gyrus. Etat criblé or dilatations of Virchow-Robin space in the left putamen were observed. Acute ischemic focus in the right thalamus was also observed. Scattered neurofibrillary tangles were found in the subiculum and entorhinal cortex (Braak II). Several senile plaques were observed in the entorhinal cortex (Braak A, CERAD A). Amyloid-β deposition was at Thal phase Phase 1 (Thal phase). No Lewy bodies were observed on the α-synuclein stained sections. The distributions of sponginess and macrophage infiltrations were traced using a virtual slide (Supplementary Figure 2).

Neuronal loss and mild melanophagia were observed in the substantia nigra and the locus ceruleus. Substantia nigra showed Gallyas-positive materials in the neuropil and in the neurons. RD3-positive grain-like structures were scattered in the substantia nigra. Deposition of α-synuclein-positive materials in the dorsal vagal nucleus without typical Lewy body was observed. Epicardial sympathetic nerve fibers were normal.

DISCUSSION

Tau-positive lesions in the present case exhibited apparent similarity to argyrophilic grains in terms of their distribution in the ambient gyrus, amygdala, and dorsomedial temporal tip and the characteristic comma-like morphology (Fig. 1C, Supplementary Figure 1C, D) [3–7]. Surprisingly, these grain-like structures (Fig. 1C, Supplementary Figure 1C, D), as well as associated tuft-shaped astrocytes (Fig. 1K, Supplementary Figure 1F) and oligodendroglial tau immunoreactivities (Fig. 1L), exhibited predominant 3-repeat tau immunoreactivity, suggesting that grain-like structures and their characteristic distribution are mutually linked and not unique to 4-repeat tau deposition. Furthermore, pTDP immunoreactivity (Fig 1D, Supplementary Figure 1E), extensive macrophage infiltration (Fig. 1J), and spongiosis were associated with these 3-repeat tau deposits. Ambient gyrus of the present case macroscopically showed asymmetrical atrophy similar to the findings of argyrophilic grain disease (Fig. 1A) [8].

The deposited substances in the present case were predominantly 3-repeat tau (Fig. 1C, Supplementary Figure 1D) [9]. Moreover, they exhibited argyrophilic gain-like structures (Fig. 1C, Supplementary Figure 1C, D). In addition to grain-like structures, the abundant oligodendroglial tau immunoreactivities were also 3-repeat dominant (Fig. 1L) [3, 4]. Tuft-shaped astrocytes showed partial 3-repeat tau immunoreactivities (Fig. 1K, Supplementary Figure 1F). A few 3-repeat dominant neurofibrillary tangles and grains were observed in dentate gyrus granular layer of hippocampus. Both grain-like structures and neurofibrillary tangles in CA2/3 showed 3-repeat dominance. Only an achromatic neuron (ballooned neuron-like cell) was observed in the precentral gyrus.

AGD has been reported as the only tauopathy lacking tau acetylation at lysine residue 274, although tau inclusions in AD and the majority of 3-repeat and 4-repeat tauopathies have been acetylated at lysine residue 274 [10]. On the other hand, AGD had significant AC-K280 (a rabbit-generated polyclonal antibody specific for tau modified with an acetyl group at the 280 lysine (K) residue) reactivity [11, 12]. We tried a commercially available anti-tau antibody (Acetyl K280), rabbit, polyclonal (AnaSpec), which demonstrated ambiguous immunoreactivity to our 3-repeat positive grain-like structures. Furthermore, the acetylated K280 epitope is located in the second microtubule-binding repeat, and is specific for 4-repeat tau isoforms. It was a study limitation that testing for acetylated tau was not possible. Different acetylation epitopes may behave differently in neurodegenerative diseases. We think that tau inclusions in AGD may be acetylated at lysine residue 280 but may not be acetylated at lysine residue 274.

Some grain-like structures colocalized with dendritic marker MAP2 (Fig. 1I). Therefore, 3-repeat dominant tau and the argyrophilic grains seemed to be similarly deposited in the dendrites. Argyrophilic grains are preferentially localized in the dendrites and dendritic branches [7, 13]. In contrast with the common 4-repeat tau AGD, there were few ballooned neurons. Oligodendroglial inclusions (Fig. 1L), neuronal tau (Fig. 1C, Supplementary Figure 1C, D), and localization of the grain-like structure in dendrite (Fig. 1I) of the present case resembled a typical 4-repeat tau AGD. Despite the difference in tau isoform, this 3-repeat tau deposition is similar to that of AGD in terms of its cytopathology and distribution, suggesting that the underlying mechanism for deposition is shared between the 4-repeat tau (AGD) and its 3-repeat counterpart (this case).

In addition, pTDP-43-positive grain-like structures were observed (Fig. 1D, Supplementary Figure 1E). Most of these pTDP-43-positive structures, however, were not colocalized with 3-repeat tau (Fig. 1E), similar to previous findings describing AGD [14]. pTDP-43-positive small structures corresponding to the argyrophilic grains were observed in only one of 37 ALS cases [15]. However, 9 of 15 cases of AGD showed TDP-43 immunoreactivities mainly in the limbic regions and the lateral occipitotemporal cortex [14]. Furthermore, TDP-43-positive structures included neuronal cytoplasmic inclusions, dystrophic neurites, glial cytoplasmic inclusions, grain-like dot-shaped structures, and neurofibrillary tangle (NFT)-like structures [14]. However, pTDP-43-positive structures were not colocalized with AT8 immunoreactivity [14]. Colocalization of pTDP-43 and AT-8 immunoreactivity was observed only in parts of neuronal cytoplasmic inclusions, grain-like structures, and NFT-like structures [14]. Findings of this study are similar to the findings on AGD.

Macrophage infiltration in the present case is also unusual (Fig. 1J). Vascular lesions such as cerebral infarctions also cause macrophage infiltration a few days to a month after ischemic insult; however, softening, ischemic foci, ischemic neurons, and proliferation of gemistocytic astrocytes were not observed in the present case. Furthermore, abundant macrophage infiltrations exhibited a similar distribution to tau deposition (Supplementary Figure 2). Therefore, vascular lesion was ruled out as the cause of macrophage infiltration in the present case.

Such a predominance of 3-repeat tau in grain-like structures has not been reported previously. For example, Case 4 reported by Kovacs et al. was characterized by the deposition of 3-repeat tau and pTDP-43, frontotemporal superficial layer spongiosis, and diffuse granular astrocytic tau immunoreactivity. Some tufted astrocytes and depigmentation of the substantia nigra and locus ceruleus were also noted; however, grain-like structures and massive infiltration of macrophages were not observed [16]. Although clinical details and frozen samples for biochemical analysis are unfortunately unavailable, this deposition of 3-repeat tau in grain-like structures and its distribution with the involvement of pTDP provides a novel combination of atypical tau deposition.