Modificaciones relacionadas con la edad en el sistema ubiquitina-proteosoma (UPS) y en la respuesta a proteínas mal plegadas (UPR) en el hipocampo de rataRelación con procesos neurodegenerativos

Resumen

SUMMARY The present thesis is based on the fact that the accumulation of insoluble protein aggregates, as well as the development of neuroinflammatory processes, constitutes two cellular/molecular hallmarks involved in the aetiology and progression of neurodegenerative illness such as Alzheimer's disease (AD), for which aging represents the main risk factor. Here we have analysed the age-related alterations in the UPR (Unfolded protein response) and UPS (Ubiquitin-Protoeasome System), as mechanisms that could be directly implicated in neurodegeneration. This study aims to provide information that helps us in knowing how age-related changes may lead to protein accumulation and why under certain circumstances these processes can trigger pathological situations associated to ageing. Our findings demonstrated a different profile of protein accumulation, following proteasome inhibition, between the soluble and aggregate fractions. Ubiquitin-aggregates are preferentially located in an area surrounding the nucleus, probably in contact with the ER, mainly in older animals and young animals treated with lactacystin. These aggregates are much higher in older animals, especially after lactacystin injection. Among the possible causes that might explain the higher accumulation of proteins in the hippocampus of aged animals, we have focused on the study of protein degradation system associated with the ER (ERAD), which is part of the UPR, as well as in the biogenesis of proteasomes process, following lactacystin injection. Our results suggest that the ability to recognize and transport misfolded proteins from the ER to the cytosol for subsequent degradation by the proteasome, would be diminished in old animals, suggesting that the stressor would be more persistent in aged rats. This fact may explain why older animals are more vulnerable to a situation of ER stress occurring by proteasome inhibition. Also, proteasome inhibition induced the synthesis of constitutive proteasome in young rats, but immunoproteasome in aged animals. According to POMP expression, the rate of synthesis of proteasome was lower in aged with respect to young animals. Aged animals do not induce the expression of pro-survival factors (chaperones, Bcl-XL and Bcl-2), displaying a more sustained expression of proapoptotic markers (CHOP, Bax, Bak and JKN), and an increased caspase-3 processing. At the cellular level, proteasome inhibition induced neuronal damage in both, young and aged animals as assayed by Fluorojade-B staining. However, degenerating neurons were evident as soon as 24 h postinjection in aged rats, but it was delayed up to 3 days in young animals. Finally, data show that LPS injection also induced accumulation of ubiquitinated proteins, probably because of immunoproteasome induction, but induced the UPR in a non-canonical way, activating mostly the PERK pathway. Importantly, LPS injection increased the vulnerability of hippocampal neurons to protein accumulation. These data support that neuroinflammation could be acting synergistically with protein accumulation as detrimental factors for cell viability. In conclusion, our findings show evidence supporting age-related dysfunctions in the UPR activation as a potential mechanism linking protein accumulation to cell degeneration, which could be due, at least in part, to age-related neuroinflammatory processes. Present data add new molecular evidence supporting the aging process as the main risk factor to develop neurodegenerative diseases, such as sporadic Alzheimer.