The small heat shock protein HSP25 protects astrocytes against stress induced by proteasomal inhibition.

Proteasomal dysfunction has been implicated in neurodegenerative diseases, and molecular chaperones may provide a first line of defence against protein aggregate formation. We have shown before that oligodendrocytes respond to proteasomal inhibition by the onset of apoptotic cell death, whereas astrocytes have a higher capability to cope with stressful conditions that might be causally related to their high constitutive level of HSP25. This study was undertaken to investigate the effects of the proteasomal inhibitor MG-132 on aggregate formation in astrocytes, and to test if HSP25 exerts a protective means. Our data show that upon proteasomal inhibition aggresomes are formed in astrocytes that contain the small HSPs, HSP25 and alpha B-crystallin, and ubiquitinated proteins. HSP expression is induced and HSP25, alpha B-crystallin and ubiquitinated proteins are translocated from the soluble to the detergent-insoluble fraction. Simultaneously, the cytoskeletal organization is disturbed, microfilaments are fragmented, GFAP intermediate filaments and microtubules surround the aggresome, and mitochondria are assembled in these structures. Mitochondria membrane potential, however, stays intact. Aggresome formation and apoptotic cell death do not correlate. After the removal of MG-132, the observed effects are reversible. MG-132 promotes the formation of small oligomers of HSP25, which have been connected to the protection of the microfilament system. Downregulation of HSP25 by siRNA approach causes actin filament breakdown in control cells in the absence of stress stimuli, and sensitizes astrocytes against stress induced by proteasomal inhibition. Hence, HSP25 enables astrocytes to prevent irreversible damage and to recover after removal of the proteasomal inhibitor MG-132.