Neuronal intranuclear inclusions are ultrastructurally and immunologically distinct from cytoplasmic inclusions of neuronal intermediate filament inclusion disease

Abstract

Abnormal neuronal cytoplasmic inclusions (NCIs) containing aggregates of α-internexin and the neurofilament (NF) subunits, NF-H, NF-M, and NF-L, are the signature lesions of neuronal intermediate filament (IF) inclusion disease (NIFID). The disease has a clinically heterogeneous phenotype, including fronto-temporal dementia, pyramidal and extrapyramidal signs presenting at a young age. NCIs are variably ubiquitinated and about half of cases also have neuronal intranuclear inclusions (NIIs), which are also ubiquitinated. NIIs have been described in polyglutamine-repeat expansion diseases, where they are strongly ubiquitin immunoreactive. The fine structure of NIIs of NIFID has not previously been described. Therefore, to determine the ultrastructure of NIIs, immunoelectron microscopy was undertaken on NIFID cases and normal aged control brains. Our results indicate that the NIIs of NIFID are strongly ubiquitin immunoreactive. However, unlike NCIs which contain ubiquitin, α-internexin and NF epitopes, NIIs contain neither epitopes of α-internexin nor NF subunits. Neither NIIs nor NCIs were recognised by antibodies to expanded polyglutamine repeats. The NII of NIFID lacks a limiting membrane and contains straight filaments of 20 nm mean width (range 11-35 nm), while NCIs contain filaments with a mean width of 10 nm (range 5-18 nm; t-test, P<0.001). Biochemistry revealed no differences in neuronal IF protein mobilities between NIFID and normal brain tissue. Therefore, NIIs of NIFID contain filaments morphologically and immunologically distinct from those of NCIs, and both types of inclusion lack expanded polyglutamine tracts of the triplet-repeat expansion diseases. These observations indicate that abnormal protein aggregation follows separate pathways in different neuronal compartments of NIFID.

Fine structural analysis of the neuronal inclusions of frontotemporal lobar degeneration with TDP-43 proteinopathy

“…Some of these filaments were broader in the nuclear compartment than any TDP-43-positive filaments observed in the NCIs or DNs. Interestingly, we showed previously that the filaments of NIIs of neuronal intermediate filament inclusion disease (NIFID) were broader than those of the NCIs (20 nm compared with 10 nm; Mosaheb et al 2005). Whilst the granular component of the NII of FTLD with TDP-43 proteinopathy was relatively minor, this granular component predominated within most of the mature, swollen DNs and contained frequent TDP-43 epitopes.…”

Abundant FUS-immunoreactive pathology in neuronal intermediate filament inclusion disease

“…Chronic degenerative changes may affect a variety of cortical and subcortical regions, with the frontal and temporal lobes and caudate nucleus most consistently and severely involved. Several different types of neuronal cytoplasmic and intranuclear inclusions (NCI and NII, respectively) have been described, that vary in morphology, histochemical staining, immunoreactivity, ultrastructure and anatomical distribution [8, 29, 36]. By definition, these inclusions show no immunoreactivity for tau, α-synuclein or TDP-43 but at least some are immunoreactive for IFs.…”

“…Biochemical studies of post mortem brain tissue have failed to demonstrate any abnormal molecular modification of IFs in NIFID [11, 29] and no pathogenic variants of the corresponding genes have been identified [28]. More importantly, several published reports of NIFID have indicated that only a proportion of the inclusions are IF-ir [6, 18, 24, 36, 41].…”

“…More importantly, several published reports of NIFID have indicated that only a proportion of the inclusions are IF-ir [6, 18, 24, 36, 41]. The fact that IF IHC may also label the characteristic inclusion bodies of many other common neurodegenerative conditions, in which the primary molecular defect is known to be something other than IFs [1, 10, 23], leaves open the possibility that some other protein may play a more central role in the pathogenesis of NIFID [24, 29, 41]. …”

α-Internexin Is Structurally and Functionally Associated with the Neurofilament Triplet Proteins in the Mature CNS

“…Initially termed "neurofilament inclusion disease," this condition was renamed "neuronal IF inclusion disease" when ␣-internexin was discovered in the pathological inclusions, underscoring the prevailing view that ␣-internexin is not a component of the neurofilament (Cairns et al, 2004a,b,c;Momeni et al, 2005;Mosaheb et al, 2005). ␣-Internexin is also implicated in tropical spastic paraparesis/human T-cell leukemia virus type 1-associated myelopathy (Reddy et al, 1998), a degenerative CNS disorder in which neurofilaments accumulate in giant axonal spheroids (Wu et al, 1993;Liberski et al, 1994;Liberski et al, 1999).…”