Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder consisting 
of progressive loss of motor neurons. TDP-43 has been identified as a component 
of ubiquitin-immunoreactive inclusions of motor neurons in ALS. We focused on 
the diffuse cytoplasmic TDP-43 immunoreactivity in ALS neurons, and 
quantitatively assessed it in comparison with skein/round TDP-43 and ubiquitin 
immunostaining in motor neurons of 30 sporadic ALS cases. The percentage of 
spinal motor neurons with cytoplasmic TDP-43 immunoreactivity was higher than 
that of ubiquitin-immunoreactive ones. The percentage of TDP-43-positive motor 
neurons was independent of neuron counts in anterior horns, while the percentage 
of ubiquitinated neurons was inversely correlated. Aiming to define the 
cytosolic localization of TDP-43, the immunoblot analysis of spinal cord and 
frontal cortex showed that full-length TDP-43, the 45 kDa form and ubiquitinated 
TDP-43 are found in the soluble inclusion-free fraction. The present data 
suggest that delocalization, accumulation and ubiquitination of TDP-43 in the 
cytoplasm of motor neurons are early dysfunctions in the cascade of the events 
leading to motor neuron degeneration in ALS, preceding the formation of 
insoluble inclusion bodies. Being cytoplasmic accumulation an ongoing event 
during the course of the illness, a therapeutic approach to this incurable 
disease can be envisaged.