Basophilic inclusions and neuronal intermediate filament inclusions in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Basophilic inclusions (BIs) and neuronal intermediate filament inclusions (NIFIs) are key structures of basophilic inclusion body disease and neuronal intermediate filament inclusion disease (NIFID), respectively. BIs are sharply‐defined, oval or crescent neuronal intracytoplasmic inclusions that appear pale blue‐gray in color with HE staining and purple in color with Nissl but are stained poorly with silver impregnation techniques. Immunohistochemically BIs are negative for tau, trans‐activation response DNA 43 (TDP‐43), α‐synuclein, neurofilament (NF) and α‐internexin, positive for p62, and variably ubiquitinated. Noticeably, BIs are consistently fused in sarcoma (FUS) positive. NIFIs are by definition immuno‐positive for class IV IFs including three NF triplet subunit proteins and α‐internexin but negative for tau, TDP‐43, and α‐synuclein. In NIFID cases several types of inclusions have been identified. Among them, hyaline conglomerate‐like inclusions are the only type that meets the above immunohistochemical features of NIFIs. This type of inclusion appears upon HE staining as multilobulated, faintly eosinophilic or pale amphophilic spherical masses with a glassy appearance. These hyaline conglomerates appear strongly argyrophilic, and robustly and consistently immuno‐positive for IFs. In contrast, this type of inclusion shows no or only occasional dot‐like FUS immunoreactivity. Therefore, BIs and NIFIs are distinct from each other in terms of morphological, tinctorial and immunohistochemical features. However, basophilic inclusion body disease (BIBD) and NIFID are difficult to differentiate clinically. Moreover, Pick body‐like inclusions, the predominant type of inclusions seen in NIFID, are considerably similar to the BIs of BIBD in that this type of inclusion is basophilic, poorly argyrophilic, negative for IFs and intensely immuno‐positive for FUS. As BIBD and NIFID share FUS accumulation as the most prominent molecular pathology, whether these two diseases are discrete entities or represent a pathological continuum remains a question to be answered.