Progranulin haploinsufficiency is a major cause in FTD pathogenesis (Baker et al., 2006
; Cruts et al., 2006
). The underlying mechanism remains poorly understood in part due to the lack of suitable model systems. Even in Grn
knockout mice, neuronal cell loss is limited and mechanistic studies are further complicated by the finding that progranulin levels may vary widely in patient brains in the later stages of disease (Chen-Plotkin et al., 2010
). The first iPSC-based human neuronal model of progranulin haploinsufficiency established in this report provides a platform for testing small molecules that can restore progranulin levels. It also serves as a valuable and more physiologically relevant model to better understand the disease mechanisms.
FTD is an age-dependent neurodegenerative disease, some intrinsic vulnerabilities of human neurons are more likely to manifest under stress conditions in culture. Indeed, this approach has been used recently to recapitulate some key features of major neurodegenerative diseases in human neurons derived from patient-specific iPSCs (e.g. Nguyen et al., 2011
). However, in contrast to well-studied Alzheimer’s disease (AD) and Parkinson’s disease (PD), little is known about neuronal defects in FTD patients that are caused by progranulin deficiency. Differential sensitivity of neurons to a particular stressor in culture within a short time window may reveal partially defective molecular pathways relevant to FTD pathogenesis.
Importantly, our data show that PGRN S116X neurons are more prone to reduced cell viability induced by specific protein kinase inhibitors, implicating the PI3K/Akt and MEK/MAPK signaling pathways in the molecular pathogenesis of FTD. This cellular defect is rescued by ectopic expression of progranulin in human PGRN S116X neurons, consistent with previous findings that progranulin promotes the survival of rodent primary neurons (Van Damme et al., 2008
; Ryan et al., 2009
; Xu et al., 2011
) and that it activates the PI3K/Akt/S6K pathway in cancer cells (Zanocco-Marani et al., 1999
). We also found that S6K2, a component in both PI3K/Akt and MEK/MAPK signaling pathways, is specifically downregulated in PGRN S116X neurons as part of a coordinated gene network, and its expression level can be restored to normal by ectopic progranulin expression. Interestingly, our re-analysis of the gene expression data published by Chen-Plotkin et al. (2008)
revealed that RPS6KB2
mRNA is dowregulated by about 40% in the frontal cortex, but not in the hippocampus or cerebellum, of FTD patients with progranulin mutations. Taken together, these findings reinforce the notion that the PI3K/Akt and MEK/MAPK signaling pathways are compromised in PGRN S116X neurons and highlight the primary role PGRN plays in promoting neuronal survival.
ER stress and mitochondrial impairment have both been closely linked to neurodegenerative diseases (Matus et al., 2011
; Schon and Przedborski, 2011
). Our finding that neither PGRN S116X nor sporadic FTD neurons show enhanced sensitivity to mitochondrial or oxidative stressors argues that these pathways are unlikely to be affected by reduced progranulin levels in cultured neurons. However, mitochondrial dysfunction and oxidative stress may develop at late stages of disease progression in FTD patients.
On the other hand, both PGRN S116X and sporadic FTD neurons are more susceptible to inducers of ER stress and inhibitors of proteasome function than control neurons. This cellular defect appears to be progranulin-independent since progranulin expression levels are normal in sporadic FTD neurons. In accordance with our findings, it was recently reported that ER stress and UPR activation contribute to both sporadic FTD and familial FTD caused by MAPT
mutations (Nijholt et al., 2012
). Moreover, both Aβ and increased levels of phosphorylated tau induce ER stress in AD (e.g. Hoozemans et al., 2009
), as does the accumulation of misfolded α-synuclein in PD (Colla et al., 2012
). Therefore, altered ER stress responses are likely to be a general feature in a variety of neurodegenerative diseases.
In summary, we establish new neuronal models of human PGRN deficiency and demonstrate specific and reversible defects affecting survival of these neurons. Our findings suggest that chronic weakening of pro-survival signaling pathways may render neurons more sensitive to environmental insults in FTD patients with progranulin deficiency. Thus, in addition to strategies to increase PGRN levels, therapeutic approaches that generally enhance neuronal survival through growth factor signaling may be beneficial in slowing disease progression in these patients.