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Although the participation of the ubiquitin-dependent pathway and of the proteasome in apoptosis has been proposed, its role in this process is not yet clearly defined. In previous studies, we have shown that in the central nervous system of the rat, programmed cell death and the ubiquitin-dependent proteolytic pathway are closely related to each other and that different types of neurons and of glial cells, shown different types of correlation between the two phenomena. In this work, we have used lactacystin, a highly specific inhibitor of the proteasome, to explore in Schwann cell cultures the relationship between the activity of the Ub-dependent pathway and apoptosis. Apoptosis was explored analyzing changes in nuclear morphology, using the Annexin V assay and by flow cytometry. Activity of caspase-3 was also measured. Changes in the levels of ubiquitin-protein conjugates and of the ubiquitin activating enzymes, E1, as well as expression of proteins that instruct the cells to apoptosis (p53, NFkappaB-IkappaB, Bcl2), or that participate in the control and regulation of the cell cycle, were also examined. Our results indicate that the decrease in the activity of the proteasome induced by lactacystin in Schwann cells, induces apoptotic cell death through changes in the concentration of certain key proteins that are involved in the apoptosis-signaling pathways.

Recent findings have led to a better understanding of the roles of the ubiquitin (Ub) proteasome system in the progression of different human degenerative diseases. The neuropathological hallmarks of Alzheimer's disease (AD) are the neurofibrillary tangles (NFT) and the senile plaques (SP). NFT accumulate within neurons and are composed of filamentous aggregates (PHF) of the microtubule‐associated protein tau (Layfield 2001). The intraneuronal inclusions contain deposits of ubiquitylated proteins, indicating that perturbations of Ub‐dependent proteolysis may be occurring. We have demonstrated the existence of a defective ubiquitination in the cytosol of AD brains, due to a decrease in normal Ub activating enzyme (E1) activity in the cytosol as a consequence of its de‐localization to the particulate fraction (Lopez Salon 2000). Immunohistochemical experiments using an antibody against PHF, showed a great number of NFT in AD. Using an anti E1 antibody we demonstrated the presence of this enzyme in the NFT. In order to corroborate the above mentioned results, we have carried out biochemical studies to investigate the pattern of solubilization of the proteins present in NFT in different fractions isolated by differential centrifugation. In AD almost 60% of E1 is obtained in a particulate fraction, while in control samples E1 is mainly located at the supernatant. The de‐localization of E1 could be a late event occurring during the progression of AD. A secondary impairment of Ub‐dependent degradation due to a decrease in normal E1 activity in the brain cytosol may contribute to the abnormal accumulation of proteins in AD and deserves future research as a possible disease mechanism.