alpha-Internexin aggregates are abundant in neuronal intermediate filament inclusion disease (NIFID) but rare in other neurodegenerative diseases

Abstract

Abnormal neuronal aggregates of alpha-internexin and the three neurofilament (NF) subunits, NF-L, NF-M, and NF-H have recently been identified as the pathological hallmarks of neuronal intermediate filament (IF) inclusion disease (NIFID), a novel neurological disease of early onset with a variable clinical phenotype including frontotemporal dementia, pyramidal and extrapyramidal signs. alpha-Internexin, a class IV IF protein, a major component of inclusions in NIFID, has not previously been identified as a component of the pathological protein aggregates of any other neurodegenerative disease. Therefore, to determine the specificity of this protein, alpha-internexin immunohistochemistry was undertaken on cases of NIFID, non-tau frontotemporal dementias, motor neuron disease, alpha-synucleinopathies, tauopathies, and normal aged control brains. Our results indicate that class IV IF proteins are present within the pleomorphic inclusions of all cases of NIFID. Small subsets of abnormal neuronal inclusions in Alzheimer's disease, Lewy body diseases, and motor neuron disease also contain epitopes of alpha-internexin. Thus, alpha-internexin is a major component of the neuronal inclusions in NIFID and a relatively minor component of inclusions in other neurodegenerative diseases. The discovery of alpha-internexin in neuronal cytoplasmic inclusions implicates novel mechanisms of pathogenesis in NIFID and other neurological diseases with pathological filamentous neuronal inclusions.

Fig. 1

Low-power photomicrograph showing neuronal intracytoplasmic inclusions in the subiculum of the temporal lobe of a case of neuronal intermediate filament inclusion disease (NIFID). The inclusions are compact, pleomorphic, and intensely stained. α-Internexin immunohistochemistry; bar 100 μm

Fig. 2

Neuronal cytoplasmic inclusions in NIFID contain epitopes of class IV IF proteins. Intracytoplasmic inclusions are labeled by: (a), anti-pNF-H (RMO 24) and anti-ubiquitin (UBQ) antibodies (b) in the subiculum of case NIFID9. Intracytoplasmic inclusions are variably ubiquitinated (b; arrows Pick body-like inclusions, arrowheads pale or unstained inclusions) Ubiquitin immunohistochemistry. c–e Consecutive sections of a compact neuronal inclusion labeled by ubiquitin (c), pNF-H (RMO 24) (d), and α-internexin (e, INA) antibodies (IF intermediate filament, pNF phosphorylated neurofilament). Bars a 20 μm; b–e 10 μm

Fig. 3

a Many neurons in the dentate gyrus contain cytoplasmic ubiquitinated inclusions (arrows) in a case of FTLD-MND. b In an adjacent section, only a small subset of neuronal inclusions in a contains aggregates of α-internexin (INA; arrows). The α-internexin aggregate (arrow) occupies a small fraction of the total volume of the inclusion. Immunohistochemistry for a ubiquitin, b α-Internexin. Bars 10 μm

Fig. 4

Epitopes of α-internexin are present in subsets of neuronal inclusions of neurodegenerative diseases and normal aging. Dystrophic neurites of a neuritic plaque (a), a neurofibrillary tangle in the hippocampus (b), and a swollen achromatic neuron in the frontal lobe of a case of AD (c) contain epitopes of α-internexin. The core regions of a subset of Lewy bodies in DLB (d) and PD (e) contain intense α-internexin immunoreactivity. A neuromelanin-containing neuron contains a classical Lewy body with an intensely stained core and lightly stained ring (arrowhead), and a pale body is unstained (arrow; e) within the same cell. A swollen achromatic neuron or “Pick cell” in the frontal lobe of a case of Pick's disease (f) is immunostained. A swollen axon is present in the granule cell layer of the cerebellum of a normal (NL) aged subject (g), while in the same field a Purkinje cell is unstained. A swollen axon and axonal spheroid in the medulla of a case of MND contain α-internexin epitopes (h). A swollen achromatic neuron in the medulla of a case of BIBD contains diffuse α-internexin staining (i) (AD Alzheimer's disease, DLB dementia with Lewy bodies, PD Parkinson's disease, MND motor neuron disease, BIBD basophilic inclusion body disease). α-Internexin immunohistochemistry; bars 10 μm