The Role of Brain-derived Neurotrophic Factor (BDNF)-induced XBP1 Splicing during Brain Development

Abstract

Accumulation of unfolded proteins in the endoplasmic reticulum initiates intracellular signaling termed the unfolded protein response (UPR). Although Xbp1 serves as a pivotal transcription factor for the UPR, the physiological role of UPR signaling and Xbp1 in the central nervous system remains to be elucidated. Here, we show that Xbp1 mRNA was highly expressed during neurodevelopment and activated Xbp1 protein was distributed throughout developing neurons, including neurites. The isolated neurite culture system and time-lapse imaging demonstrated that Xbp1 was activated in neurites in response to brain-derived neurotrophic factor (BDNF), followed by subsequent translocation of the active Xbp1 into the nucleus. BDNF-dependent neurite outgrowth was significantly attenuated in Xbp1 ؊/؊ neurons. These findings suggest that BDNF initiates UPR signaling in neurites and that Xbp1, which is activated as part of the UPR, conveys the local information from neurites to the nucleus, contributing the neurite outgrowth. Endoplasmic reticulum (ER)2 is a site of synthesis, folding, and modification of secretory and cell surface proteins, and this organelle is widely distributed throughout neurons, including axons, dendrites, and growth cones (1). Various biological phenomena, such as increased protein synthesis, nutrient deprivation, and alteration in Ca 2ϩ homeostasis, hamper protein folding in the ER, causing unfolded proteins to accumulate in the ER lumen. This condition is designated as ER stress, which triggers an adaptive reaction known as the unfolded protein response (UPR) (2, 3). The protective signaling of the UPR acts transiently to maintain homeostasis within the ER, but sustained ER stress ultimately leads to apoptosis. Most of the previous studies on ER stress focused on its pathological aspect, as the pathogenesis of ischemic and neurodegenerative disorders is characterized by the accumulation of protein aggregates. Nevertheless, recent studies suggested that the UPR is required for normal development for certain cell lineages and that Xbp1, a pivotal transcription factor of the UPR, is essential for liver development (4) and plasma cell differentiation (5).Xbp1 is a basic leucine zipper type transcription factor; it is activated by spliceosome-independent mRNA splicing initiated by Ire1␣ on the cytosolic surface of the ER membrane (6). The endoribonuclease Ire1␣ cleaves a 26-nt fragment from an unspliced form of Xbp1 mRNA, inducing a frameshift of the open reading frame (ORF) of the message. Xbp1 protein translated from the unspliced mRNA (Xbp1u protein) has no transcriptional activity, whereas Xbp1 protein from the spliced mRNA (Xbp1s protein) is a potent transcription factor inducing expression of UPR-related genes. This type of transcription factor activation (the unconventional splicing of its mRNA in the cytoplasm not in the nucleus) is unique to Xbp1 in animals, and the mechanistic basis of Xbp1 splicing and its function has been studied intensively in non-neuronal cells (5-9). Although the mRNA of Xbp1 i...

Dual and opposing roles of the unfolded protein response regulated by IRE1α and XBP1 in proinsulin processing and insulin secretion

“…CPE and prohormone convertases also function in the biogenesis of secretory granules in neurons and the secretion of neuropeptides (45). XBP1 KO mice did not display any overt neurological defects (46), although it has been reported that axonal growth of developing neurons requires XBP1 (47). The possible role of XBP1 in the regulation of secretory pathways in other cell types, including neurons, awaits further investigation.…”

Induction of unfolded protein response during neuronal induction of rat bone marrow stromal cells and mouse embryonic stem cells

“…Roles of UPR during the neuronal development could be indirectly estimated from the studies using developing brain or knock-out mice. Recently, a few reports suggested the role of ER stress during brain development such as the role of XBP1 in neurite outgrowth (Hayashi et al, 2007), ER stress induction during embryonic development of the central nervous system (Zhang et al, 2007), and the critical role of BIP for the development of cerebral cortex (Mimura et al, 2008). Embryonic lethalities were also observed in IRE1α -/-, XBP1 -/-or eIF2 S51A knock-in mice, suggesting critical role of these genes in the development of mice (Reimold et al, 2000;Urano et al, 2000;Scheuner et al, 2001).…”

Consensus guidelines for the detection of immunogenic cell death

“…136,[254][255][256] For instance, the phosphorylation state of EIF2A and/or of the major EIF2A kinases, including EIF2A kinase 1 (EIF2AK1, best known as HRI), 257 EIF2AK2 (best known as PKR), 258 and EIF2AK3 (best known as PERK), [259][260][261] can be assessed by immunoblotting, flow cytometry or immunofluorescence microscopy with phosphoneoepitope-specific antibodies. 260 The splicing status of X-box binding protein 1 (XBP1) mRNA, reflecting the activation of the ER stress sensor endoplasmic reticulum to nucleus signaling 1 (ERN1, best known as IRE1a), can be monitored by quantitative real-time RT-PCR, 262 as well as by flow cytometry or fluorescence microscopy, either in cells that express a fluorescently-tagged version of XBP1 263 or upon the administration of a self-quenched RNA probe that can be cleaved by IRE1a. 264 Finally, the nuclear redistribution of activating transcription factor 6 (ATF6) can be easily evaluated by fluorescence microscopy in cells that constitutively express GFP-or RFP-tagged variants of ATF6.…”