FBXL5 Inactivation in Mouse Brain Induces Aberrant Proliferation of Neural Stem Progenitor Cells

Abstract

FBXL5 is the substrate recognition subunit of an SCF-type ubiquitin ligase that serves as a master regulator of iron metabolism in mammalian cells. We previously showed that mice with systemic deficiency of FBXL5 fail to sense intracellular iron levels and die in utero at embryonic day 8.5 (E8.5) as a result of iron overload and subsequent oxidative stress. This early embryonic mortality has thus impeded study of the role of FBXL5 in brain development. We have now generated mice lacking FBXL5 specifically in nestin-expressing neural stem progenitor cells (NSPCs) in the brain. Unexpectedly, the mutant embryos manifested a progressive increase in the number of NSPCs and astroglia in the cerebral cortex. Stabilization of iron regulatory protein 2 (IRP2) as a result of FBXL5 deficiency led to accumulation of ferrous and ferric iron as well as to generation of reactive oxygen species. Pharmacological manipulation suggested that the phenotypes of FBXL5 deficiency are attributable to aberrant activation of mammalian target of rapamycin (mTOR) signaling. Our results thus show that FBXL5 contributes to regulation of NSPC proliferation during mammalian brain development.

mTOR signaling in stem and progenitor cells

“…The oxidation of PTEN and activation of PI3K-AKT signaling in Fbxl5-KO mice indicates that ROS-mediated inhibition of PTEN causes mTORC1 activation (Yamauchi et al, 2017). Moreover, this study also reported that the chemical inhibition of AKT and mTORC1 is sufficient to promote NSC differentiation in vitro and in vivo (Yamauchi et al, 2017). The functions of mTORC1 activity in aging NSC populations have also been investigated.…”

“…A number of other factors also appear to regulate mTOR activity in NSCs and during neural development and ageing. The NSCspecific deletion of F box and leucine-rich repeat protein 5 (Fbxl5), for example, increases mTORC1 activation and impairs NSC differentiation and brain development (Yamauchi et al, 2017) (Fig. 4A).…”

“…It is known that FBXL5 regulates iron homeostasis by promoting iron uptake in low iron conditions and the degradation of iron response proteins, and that excess cellular iron produces reactive oxygen species (ROS) that inhibit PTEN (Leslie et al, 2003). The oxidation of PTEN and activation of PI3K-AKT signaling in Fbxl5-KO mice indicates that ROS-mediated inhibition of PTEN causes mTORC1 activation (Yamauchi et al, 2017). Moreover, this study also reported that the chemical inhibition of AKT and mTORC1 is sufficient to promote NSC differentiation in vitro and in vivo (Yamauchi et al, 2017).…”

SCFFbxw7 ubiquitylates KLF7 for degradation in a manner dependent on GSK‐3‐mediated phosphorylation

“…F‐box proteins constitute a large family of >70 eukaryotic proteins (Jin et al, ), but the substrates of each F‐box protein have remained largely unknown. To identify substrates for a given F‐box protein, we previously developed a method termed differential proteomics‐based identification of ubiquitylation substrates (DiPIUS; Yumimoto, Matsumoto, Oyamada, Moroishi, & Nakayama, ), and we have applied this method to several systems (Nakajima et al, ; Nakayama, Yumimoto, Kawamura, & Nakayama, ; Nishiyama, Nita, Yumimoto, & Nakayama, ; Watanabe, Yumimoto, & Nakayama, ; Yamauchi, Nishiyama, Moroishi, Kawamura, & Nakayama, ; Yumimoto, Matsumoto, Onoyama, Imaizumi, & Nakayama, ).…”

“…Genes to Cells SUGIYAMA et Al. (Nakajima et al, 2015;Nakayama, Yumimoto, Kawamura, & Nakayama, 2018;Nishiyama, Nita, Yumimoto, & Nakayama, 2015;Watanabe, Yumimoto, & Nakayama, 2015;Yamauchi, Nishiyama, Moroishi, Kawamura, & Nakayama, 2017;Yumimoto, Matsumoto, Onoyama, Imaizumi, & Nakayama, 2013).…”