Impairment of fast axonal transport in the proximal axons of anterior horn neurons in amyotrophic lateral sclerosis

Abstract

We studied the possible impairment of fast axonal transport in patients with amyotrophic lateral sclerosis (ALS) to gain some insight into the pathogenesis of the disease. We carried out an ultrastructural investigation of the proximal axons (axon hillock and initial segment) of the anterior horn neurons on samples from 11 ALS patients; specimens from 12 age-matched individuals who died of nonneurological diseases served as controls. Eighty-seven proximal axons that emanated directly from normal-appearing neurons were examined in each group of subjects. Increased smooth endoplasmic reticulum (SER) and the formation of bundles of fibrillary SER with a single unit membrane were not uncommonly observed in the initial segment of the patients with ALS. In some instances, there was loss of the parallel SER arrangement along the longitudinal axis. When viewed in transverse sections, the bundles had a tubular appearance. These morphologic changes of SER were exclusively demonstrated in patients with ALS. A marked increase or accumulation of mitochondria and lysosomes was more common in the proximal axons, particularly in the axon hillock, of ALS patients than of control subjects. The accumulation of these membrane-bounded cytoplasmic organelles suggests that fast axonal transport is impaired in the proximal axons of individuals with ALS. In addition, there were Lewy body-like hyaline inclusions, lipofuscin granules, and multiple membranous structures in the proximal axons. The presence of these unusual structures may also be a reflection of axonal transport dysfunction. By contrast, in the central chromatolytic neurons, there was not only a decrease in the number of neurofilaments in the axon hillock and initial segment, but also of mitochondria, lysosomes, and SER. In some instances, none of these cytoplasmic organelles was seen. These findings support the notion that the outflow of cytoplasmic constituents from the anterior horn cell body into the proximal axon may be impaired in central chromatolytic neurons.