Enduring involvement of tau, beta-amyloid, alpha-synuclein, ubiquitin and TDP-43 pathology in the amyotrophic lateral sclerosis/parkinsonism-dementia complex of Guam (ALS/PDC)

Abnormal TDP-43 expression is identified in the neocortex in cases of dementia pugilistica, but is mainly confined to the limbic system when identified in high and moderate stages of Alzheimer's disease

The transactive response (TAR) DNA binding protein TDP-43 has been discovered to be a major ubiquitinated protein in frontotemporal lobar degeneration with ubiquitinated tau-negative inclusions (FTLD-U), which consequently has been renamed FTLD-TDP. However, TDP-43 has since been detected in conditions such as Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) but is often confined to the limbic region rather than the more widespread pattern seen in FTLD-TDP. Previous work has suggested some relationship between hippocampal sclerosis and TDP-43 expression. A number of AD cases of both moderate and high stage were examined to determine whether the pattern of TDP-43 immunohistochemical expression differed and whether any relationship to hippocampal sclerosis could be detected. Cases of hippocampal sclerosis from surgical epilepsy specimens were examined to determine whether hippocampal sclerosis alone could cause abnormal TDP-43 expression. To establish whether abnormal TDP-43 expression in other neurodegenerative diseases resembled the pattern and distribution in FTLD-TDP we examined multiple blocks from a variety of neurodegenerative conditions. In 75% of cases of high-stage AD there was abnormal TDP-43 positivity compared to 57% of moderate-stage AD. While the abnormal TDP-43 positivity was confined to the limbic regions in the moderate stages, occasional cases in the high stages showed neocortical positivity. Also amygdala and/or entorhinal positivity appeared to precede positivity in the dentate gyrus. No relationship could be established between abnormal TDP-43 expression and degree of hippocampal sclerosis either in the surgical or autopsy cases. The pattern of distribution of TDP-43 inclusions from cases of dementia pugilistica most closely resembled that in FTLD-TDP. This raises the question as to whether there may be some shared pathogenic mechanisms between the two conditions.

Selective occurrence of TDP-43-immunoreactive inclusions in the lower motor neurons in Machado-Joseph disease

Pathological transactivation-responsive DNA-binding protein 43 (TDP-43) has been identified as a component of ubiquitinated inclusions in frontotemporal lobar degeneration with motor neuron disease, as well as in sporadic and some forms of familial amyotrophic lateral sclerosis. To clarify whether pathological TDP-43 is present in other neurodegenerative diseases involving the motor neuron system, we immunohistochemically examined the brain and spinal cord affected by two CAG repeat (polyglutamine) diseases, Machado-Joseph disease (MJD) and spinal and bulbar muscular atrophy (SBMA), using polyclonal antibody against TDP-43. In all the MJD cases, TDP-43-immunoreactive (ir) neuronal cytoplasmic inclusions (NCIs), although few in number, were found only in the lower motor neurons in the brainstem and spinal cord. TDP-43-ir NCIs appeared as linear wisp-like, skein-like, or thick, somewhat rod-like bodies. These inclusions were also visualized with antibodies against phosphoserines 409 and 410 of TDP-43, and ubiquitin, but were not recognized by antibody against expanded polyglutamine stretches or ataxin-3. The ultrastructure of the TDP-43-ir NCIs was similar to that of the inclusions seen in sporadic ALS, consisting of bundles of parallel filaments. None of the SBMA cases showed abnormal TDP-43 immunoreactivity in any of the regions examined. Immunoblot analysis failed to recognize hyperphosphorylated TDP-43 at ~23 kDa in two MJD cases examined. However, the immunohistochemical findings strongly suggested that in MJD, in addition to the polyglutamine-dependent disease process, TDP-43-related pathogenesis is associated with degeneration and death of the lower motor neurons.

The ubiquitin proteasome system in neuropathology

The ubiquitin proteasome system (UPS) orchestrates the turnover of innumerable cellular proteins. In the process of ubiquitination the small protein ubiquitin is attached to a target protein by a peptide bond. The ubiquitinated target protein is subsequently shuttled to a protease complex known as the 26S proteasome and subjected to degradative proteolysis. The UPS facilitates the turnover of proteins in several settings. It targets oxidized, mutant or misfolded proteins for general proteolytic destruction, and allows for the tightly controlled and specific destruction of proteins involved in development and differentiation, cell cycle progression, circadian rhythms, apoptosis, and other biological processes. In neuropathology, alteration of the UPS, or mutations in UPS target proteins may result in signaling abnormalities leading to the initiation or progression of tumors such as astrocytomas, hemangioblastomas, craniopharyngiomas, pituitary adenomas, and medulloblastomas. Dysregulation of the UPS may also contribute to tumor progression by perturbation of DNA replication and mitotic control mechanisms, leading to genomic instability. In neurodegenerative diseases caused by the expression of mutant proteins, the cellular accumulation of these proteins may overload the UPS, indirectly contributing to the disease process, e.g., sporadic Parkinsonism and prion diseases. In other cases, mutation of UPS components may directly cause pathological accumulation of proteins, e.g., autosomal recessive Parkinsonism and spinocerebellar ataxias. Defects or dysfunction of the UPS may also underlie cognitive disorders such as Angelman syndrome, Rett syndrome and autism, and muscle and nerve diseases, e.g., inclusion body myopathy and giant axon neuropathy. This paper describes the basic biochemical mechanisms comprising the UPS and reviews both its theoretical and proven involvement in neuropathological diseases. The potential for the UPS as a target of pharmacological therapy is also discussed.

Absence of alpha-synuclein pathology in postencephalitic parkinsonism

Postencephalitic parkinsonism (PEP), a chronic complication of encephalitis lethargica, is a tauopathy characterized by multisystem neuronal loss and gliosis with widespread neurofibrillary lesions composed of both 3- and 4-repeat (3R and 4R) tau isoforms. Previous immunohistochemical studies in a small number of PEP cases demonstrated absence of Lewy bodies as well as the lack of other alpha-synuclein pathology, classifying PEP as a "pure" tauopathy. Neuropathologic examination of 10 brains with clinico-pathologically verified PEP confirmed widespread neurodegeneration in subcortical and brainstem areas associated with multifocal neurofibrillary pathology comprising both 3R and 4R tau. Very rare beta-amyloid deposits were observed in two elderly patients, while Lewy bodies and neurites or any other alpha-synuclein deposits were completely absent. The causes and molecular background of total absence of alpha-synuclein pathology in PEP, in contrast to most other tauopathies, remain as unknown as the pathogenesis of PEP.

TDP-43 pathology in familial British dementia

Trans-activation-responsive DNA-binding protein 43 (TDP-43) is a component of pathological inclusions in amyotrophic lateral sclerosis and several forms of sporadic and familial frontotemporal lobar degeneration. This has suggested defining a new class of diseases known as TDP-43 proteinopathies. However, it has been reported more recently that TDP-43 positive inclusions occur in other neurodegenerative disorders such as Alzheimer's disease, Dementia with Lewy Bodies and Parkinsonism dementia complex of Guam. Here we report the occurrence of TDP-43 inclusions in one other neurodegenerative disorder: familial British dementia. Using a variety of antibodies against phosphorylated and non-phosphorylated TDP-43 epitopes, we found intense accumulation occurred in the form of dystrophic neurites, neuronal cytoplasmic inclusions and was also occasionally associated with neurofibrillary tangles. Double immunostaining revealed that TDP-43 and tau aggregates were rarely directly colocalized, but co-existed in the same neurons as separate inclusions. Double staining with ubiquitin showed a direct colocalization with TDP-43. The phosphorylation-dependent TDP-43 antibodies proved superior to phosphorylation-independent antibodies in revealing pathological inclusions since the former did not stain non-phosphorylated TDP-43 in normal nuclei. Our results support the concept that TDP-43 pathology is not narrowly restricted, but is involved in the etiology of many neurodegenerative disorders.