Mutant huntingtin expression in clonal striatal cells: Dissociation of inclusion formation and neuronal survival by caspase inhibition

Neuronal intranuclear inclusions are found in the brains of patients with H untington's disease and form from the polyglutamine-expanded N-terminal reg ion of mutant huntingtin. To explore the properties of inclusions and their involvement in cell death, mouse clonal striatal cells were transiently tr ansfected with truncated and full-length human wild-type and mutant hunting tin cDNAs. Both normal and mutant proteins localized in the cytoplasm, and infrequently, in dispersed and perinuclear vacuoles. Only mutant huntingtin formed nuclear and cytoplasmic inclusions, which increased with polyglutam ine expansion and with time after transfection. Nuclear inclusions containe d primarily cleaved N-terminaI products, whereas cytoplasmic inclusions con tained cleaved and larger intact proteins. Cells with wild-type or mutant p rotein had distinct apoptotic features (membrane blebbing, shrinkage, cellu lar fragmentation), but those with mutant huntingtin generated the most cel l fragments (apoptotic bodies). The caspase inhibitor Z-VAD-FMK significant ly increased cell survival but did not diminish nuclear and cytoplasmic inc lusions. In contrast, Z-DEVD-FMK significantly reduced nuclear and cytoplas mic inclusions but did not increase survival. A series of N-terminal produc ts was formed from truncated normal and mutant proteins and from full-lengt h mutant huntingtin but not from full-length wild-type huntingtin. One prom inent N-terminal product was blocked by Z-VAD-FMK. In summary, the formatio n of inclusions in clonal striatal cells corresponds to that seen in the HD brain and is separable from events that regulate cell death. N-terminal cl eavage may be linked to mutant huntingtin's role in cell death.