Here we present new findings that suggest a novel pro-apoptotic role for Pin1 during programmed cell death in developing neurons. First, ectopic expression of catalytically active Pin1 is sufficient to override NGF-derived survival signals and promote caspase-dependent cell death. Second, expression of Pin1 in the presence of NGF results in elevated levels of Ser63-phosphorylated c-Jun. Third, down regulating Pin1 expression prior to NGF withdrawal inhibits the accumulation of phosphorylated c-Jun and the subsequent release of cytochrome c from mitochondria that occurs during trophic factor deprivation. Fourth, knockdown of Pin1 expression inhibits NGF deprivation-induced death to the same extent as a dominant negative form of c-Jun. Finally, Pin1-induced death is blocked by expression of dominant negative c-JunbZIP, indicating that c-Jun functions downstream of Pin1 in these cells. Together, these observations suggest that Pin1 functions during trophic factor deprivation in a pathway that leads to activation of c-Jun.
Precedence for Pin1 as a regulator of c-Jun comes from studies in tumor cells, where the ability of Pin1 to enhance cyclin D1 transcription was shown to be partially dependent on c-Jun (Wulf et al. 2001
). In these cells, JNK phosphorylation of c-Jun promotes its binding to Pin1, resulting in greater c-Jun transcription factor activity. Whether Pin1 binds to phosphorylated c-Jun in NGF-deprived neurons remains to be determined. Pin1 could also enhance levels of phosphorylated c-Jun by stimulating the activity of JNK or JNK-activating kinases. Recently, Pin1 was shown to interact with the JNK scaffolding protein JIP3 in neurons (Becker and Bonni 2006
) suggesting that a subset of the Pin1 in cells could be in close proximity to JNK. A third possibility is that Pin1 could act on proteins other than c-Jun to enhance AP-1 transcription factor activity, resulting in increased c-Jun transcription and thus c-Jun protein. Along these lines, a role for Pin1 as a transcriptional co-activator has been described in breast cancer cells (Yi et al. 2005
Becker and Bonni (2006)
recently provided the first evidence that Pin1 functions as a positive regulator of apoptosis in neurons. Using a model involving postnatal cerebellar granule neurons deprived of survival factors, they showed that Pin1 can act by binding and stabilizing JNK-phosphorylated forms of BimEL
. As outlined in the Introduction
is also important for cell death in NGF-dependent sympathetic neurons. Furthermore, transcription of the Bim
gene after NGF withdrawal is partially regulated by AP-1 (Biswas et al. 2007
). However, our finding that inhibiting Pin1 expression results in greater protection from cell death than disruption of Bim
indicates the existence of Pin1 targets other than BimEL
that contribute to apoptosis caused by NGF deprivation. This is further supported by the observation that Pin1 expression increases c-Jun phosphorylation even in Bim
(−/−) neurons. By affecting c-Jun activity, Pin1 would be expected to influence the expression of other pro-apoptotic AP-1 targets in addition to BimEL
. These might include cyclin D1, the BH3-only protein Dp5, and the prolyl hydroxylase EGLN3 (Freeman et al. 1994
; Park et al. 1997
; Imaizumi et al. 1997
; Lee et al. 2005
; Ma et al. 2007
While Pin1 has a pro-apoptotic function in developing neurons, it may serve a predominantly protective role in the adult nervous system. A variety of studies including post-mortem analyses in humans suggest that decreased Pin1 function may contribute to Alzheimer’s disease and certain frontotemporal dementias. Strong support for this hypothesis comes from the observation that mice lacking Pin1
develop Tau and Aβ-related neurodegeneration (reviewed by Balastik et al. 2007
). A protective role for Pin1 in adult oligodendrocytes was also recently described. In a mouse model of spinal cord injury, the ability of Pin1 to bind and stabilize the anti-apoptotic Bcl-2 family protein Mcl-1 helps serve to protect oligodendrocytes from apoptosis (Li et al. 2007
). Ultimately, the importance of Pin1 for cell death and cell survival will likely depend on the developmental stage, cellular context, and type of insult that is under investigation.
In conclusion, our study provides new evidence supporting a role for Pin1 in cell death induced by trophic factor deprivation. Based on these results, we propose a model in which Pin1 stimulates pro-apoptotic signaling in developing neurons at the level of JNK/c-Jun activation.