Neuropathology of ALS: Spheroids

Spinal motor neuron protein supersaturation patterns are associated with inclusion body formation in ALS

Amyotrophic lateral sclerosis (ALS) is a heterogeneous degenerative motor neuron disease linked to numerous genetic mutations in apparently unrelated proteins. These proteins, including SOD1, TDP-43, and FUS, are highly aggregation-prone and form a variety of intracellular inclusion bodies that are characteristic of different neuropathological subtypes of the disease. Contained within these inclusions are a variety of proteins that do not share obvious characteristics other than coaggregation. However, recent evidence from other neurodegenerative disorders suggests that disease-affected biochemical pathways can be characterized by the presence of proteins that are supersaturated, with cellular concentrations significantly greater than their solubilities. Here, we show that the proteins that form inclusions of mutant SOD1, TDP-43, and FUS are not merely a subset of the native interaction partners of these three proteins, which are themselves supersaturated. To explain the presence of coaggregating proteins in inclusions in the brain and spinal cord, we observe that they have an average supersaturation even greater than the average supersaturation of the native interaction partners in motor neurons, but not when scores are generated from an average of other human tissues. These results suggest that inclusion bodies in various forms of ALS result from a set of proteins that are metastable in motor neurons, and thus prone to aggregation upon a disease-related progressive collapse of protein homeostasis in this specific setting.

Axonal swellings in cerebellar white matter of groggy mutant rat

In the groggy mutant rat, a number of axonal swellings appeared in the cerebellar white matter from 180 days of age onward. Since these axonal swellings were immunostained with an antibody against calbindin D28k, the axons forming these swellings were considered to belong to Purkinje cells. They were also immunostained with an anti-neurofilament antibody, and ultrastructurally characterized by the presence of myelin sheaths around them and abnormal accumulations of filamentous structures, mitochondria and smooth endoplasmic reticula (SER) in their axoplasm. The SER were considered to convey acid phosphatase (ACPase) to the swelling's axoplasm, where ACPase was set free from the SER throughout the axoplasm and engaged in the digestion of cytoplasmic organelles. The groggy rat is useful model model for the study of the mechanism of the age-related formation of axonal swellings.

Decreased synaptophysin immunoreactivity of the anterior horns in motor neuron disease

This study concerns the synaptophysin expression in anterior horn neurons of 15 patients with amyotrophic lateral sclerosis and of 4 patients with lower motor neuron disease, who had no upper motor neuron and corticospinal tract involvement. Immunohistochemical procedures were employed and specimens from 13 patients without neurological disease served as controls. A decrease in synaptophysin expression was observed in the anterior horn neuropil of all motor neuron disease patients and this reduction was correlated with the degree of degeneration or neuronal loss of anterior horn cells. Synaptophysin immunoreactivity was preserved in the proximal dendrites and around the somata of the remaining normal-appearing neurons, but it was reduced around the somata and dendrite, especially the distal portion of the proximal dendrites of small degenerated neurons with central chromatolysis, pigmentary atrophy, or simple neuronal atrophy. These observations suggest that presynaptic terminal loss is not secondary to motor cortical neuronal loss, but that the synaptic alterations in anterior horns occur in the wake of motor neuron degeneration.