A novel cell death pathway that is partially caspase dependent, but morphologically non‐apoptotic, elicited by proteasomal inhibition of rat sympathetic neurons

Abstract

Proteasomal dysfunction has been linked to neurodegeneration. Pharmacological proteasomal inhibitors may have pro‐survival or pro‐death effects in neuronal cells. We have previously found that application of such agents to mouse sympathetic neurons leads to activation of the intrinsic apoptotic pathway. We show here that in rat sympathetic neurons proteasomal inhibition leads to a form of death that is morphologically non‐apoptotic, with features of autophagy. The intrinsic apoptotic pathway is activated in a delayed fashion compared with mouse neurons, and is in part responsible for death, as evidenced by the partial protective effects of bcl‐xL and the general caspase inhibitor Boc‐aspartyl‐fluoromethylketone. Death is accompanied by induction of Bim and caspase activation, but caspase 3 activation is lacking; 3‐methyl‐adenine inhibits macroautophagy, but has a relatively small pro‐survival effect. We conclude that a complex array of pro‐ and anti‐apoptotic effects elicited by proteasomal inhibition in rat sympathetic neurons leads to partial engagement of the intrinsic apoptotic pathway and a morphologically non‐apoptotic, autophagic form of death. The species difference with mouse neurons is underscored by the fact that proteasomal inhibitors are protective against apoptosis elicited by nerve growth factor deprivation in rat, but not mouse, sympathetic neurons. The type of death described herein may be relevant to neurodegenerative diseases, where morphological evidence for apoptosis has been scant.

Inducible over‐expression of wild type α‐synuclein in human neuronal cells leads to caspase‐dependent non‐apoptotic death

“…We have recently shown (Lang‐Rollin et al. 2008) that in rat sympathetic neurons proteasomal inhibition leads to a form of death that is morphologically non‐apoptotic, but shows a number of the biochemical features of apoptosis, such as activation of the mitochondrial apoptotic pathway, resembling thus the form of death described here following WT ASYN over‐expression.…”

“…It is interesting to note that the death pathway described herein bears resemblance to a pathway we recently characterized in rat sympathetic neurons exposed to proteasomal inhibitors. In that setting as well, the intrinsic apoptotic pathway appeared to be activated, but then blocked at a point downstream of the mitochondria, leading to a morphologically non‐apoptotic form of death (Lang‐Rollin et al. 2008).…”

Cytoplasmic Inclusions of Htt Exon1 Containing an Expanded Polyglutamine Tract Suppress Execution of Apoptosis in Sympathetic Neurons

“…However, under similar conditions, rat SCG neurons may survive NGF deprivation during acute withdrawal (Sadoul et al, 1996) but ultimately die nonapoptotically (with some apoptotic features) (Lang-Rollin et al, 2008). In our hands, Q97 expression did not reproduce all the effects of proteasome inhibition because Lang- Rollin et al (2008) found that CsA had no protective effect on proteasome-inhibition-mediated death, whereas we found that Q97-mediated cyt c release was delayed by treatment with cyclosporine A. Hence, Q97 causes additional signals in SCG neurons.…”

Targeted disruption of neuronal 19S proteasome subunits induces the formation of ubiquitinated inclusions in the absence of cell death

“…4c). To evaluate the possibility that mutant 19S subunits may induce morphologically non‐apoptotic death, as can occur in certain settings following pharmacological proteasome inhibition (Lang‐Rollin et al. 2008), we also assessed cell death more globally, by incorporation of the dye ethidium homodimer.…”