Deposition of mutant ubiquitin in parkinsonism–dementia complex of Guam

Guam parkinsonism–dementia complex (G-PDC) is an enigmatic neurodegenerative disease that affects the Chamorro residents of the Pacific island of Guam. G-PDC is clinically characterized by progressive cognitive impairment with extrapyramidal signs. Pronounced loss of neurons and abundant neurofibrillary tangles (NFTs) are observed throughout the brain of G-PDC patients [6, 7]. Although several hypotheses have been suggested for the cause of G-PDC, notably genetic predisposition and exposure to neurotoxins (e.g., β-N-methylamino-L-alanine (BMAA)), the etiology and pathogenesis remain elusive [10].

A frameshift mutant of ubiquitin, known as ubiquitin-B⁺¹ (UBB⁺¹), was previously found to accumulate in the neuropathological hallmarks of Alzheimer’s disease and several other disorders, including tauopathies and polyglutamine diseases [1, 3, 12] (Fig. 1a-b). UBB⁺¹ is a dose-dependent inhibitor of the ubiquitin-proteasome system (UPS) and its accumulation in cells an indicator of protein quality control failure. Impaired protein homeostasis is a frequent feature of neurodegenerative diseases and we hypothesized that accumulation of UBB⁺¹ might also be observed in G-PDC. To test whether UBB⁺¹ is detectable in G-PDC brains, immunohistochemical analyses were performed on G-PDC post-mortem brain tissue (Table 1). Immunohistochemistry confirmed the presence of numerous NFTs in G-PDC brains [5] (not shown), as well as other pathology that has been described to occur in G-PDC, i.e., TAR DNA-binding protein 43 (TDP-43)-positive inclusions [5] (Fig. 1f-h). Importantly, our results show that UBB⁺¹ is present in G-PDC brains. UBB⁺¹ deposits were found specifically in cytoplasm of pyramidal neurons and glia (astrocytes in the alveus and stratum oriens) in Ammon’s horn, showing a granular and tangle-like pattern of distribution (Fig. 1c-e). UBB⁺¹ was not detected in young control brains (n = 2, non-Guamanian cases, ages: 52 and 59 years old) [8]. Aggregate structures containing distinct components of the UPS, i.e., the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase L1 (UCH-L1) [9] (Fig. 1i-k) and the proteasomal ATPase subunit Rpt3/S6b [13] (Fig. 1l-n), were also present in these brains.

This demonstration of UBB⁺¹-immunoreactivity and accumulation of particular UPS components in G-PDC brains (n = 6) might have important implications for understanding of the pathological mechanisms underlying the disease. UBB⁺¹ has previously been shown to induce neuronal defects in in vitro and in vivo experimental models: long-term UPS inhibition due to UBB⁺¹ expression causes memory deficits and central breathing dysfunction in mice [4, 8, 11]. In addition, UBB⁺¹ might act as a modifier of other pathology in G-PDC. For example, UBB⁺¹ may enhance the aggregation and cellular toxicity of the RNA-binding protein TDP-43 through interfering with its degradation. It is striking that UBB⁺¹ accumulates in glial cells in G-PDC, because similar glial inclusions have been reported in progressive supranuclear palsy (PSP) [3], a disease that displays some similar topography of neurofibrillary degeneration [10]. Recognition of common mechanistic themes shared by neurodegenerative disorders, such as dysfunctional (ubiquitin-dependent) protein degradation and proteotoxic stress, may help in identifying therapeutic targets that prevent neurodegeneration. It will be interesting to investigate the potential contribution of disrupted proteostasis and UBB⁺¹ to G-PDC in more detail in future studies.