Aberrant quality control in the endoplasmic reticulum impairs the biosynthesis of pulmonary surfactant in mice expressing mutant BiP

Abstract

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) induces the unfolded protein response (UPR), which alleviates protein overload in the secretory pathway. Although the UPR is activated under diverse pathological conditions, its physiological role during development and in adulthood has not been fully elucidated. Binding immunoglobulin protein (BiP) is an ER chaperone, which is central to ER function. We produced knock-in mice expressing a mutant BiP lacking the retrieval sequence to cause a defect in ER function without completely eliminating BiP. In embryonic fibroblasts, the UPR compensated for mutation of BiP. However, neonates expressing mutant BiP suffered respiratory failure due to impaired secretion of pulmonary surfactant by alveolar type II epithelial cells. Expression of surfactant protein (SP)-C was reduced and the lamellar body was malformed, indicating that BiP plays a critical role in the biosynthesis of pulmonary surfactant. Because pulmonary surfactant requires extensive post-translational processing in the secretory pathway, these findings suggest that in secretory cells, such as alveolar type II cells, the UPR is essential for managing the normal physiological ER protein overload that occurs during development. Moreover, failure of this adaptive mechanism may increase pulmonary susceptibility to environmental insults, such as hypoxia and ischemia, ultimately leading to neonatal respiratory failure.

Sublethal endoplasmic reticulum stress caused by the mutation of immunoglobulin heavy chain-binding protein induces the synthesis of a mitochondrial protein, pyrroline-5-carboxylate reductase 1

“…Detection of mRNA mRNAs were extracted from MEFs, and cDNAs were synthesized as previously described (Mimura et al 2007). RT-PCR analysis was performed with the following primers: 5′-AAGA…”

“…We used a homologous recombination to establish knock-in mice expressing BiP lacking the carboxyl-terminal KDEL sequence (Mimura et al 2007). The missing KDEL sequence was replaced by a hemagglutinin (HA) tag.…”

“…MEFs were prepared from 13.5-day-old embryos (Mimura et al 2007). MEFs were grown in a complete medium that consisted of Dulbecco's modified Eagle's medium (Sigma Chemical Co., Irvine, UK) with 10 % fetal bovine serum, 2 mM glutamine, 50 μg/ml streptomycin, and 50 U/ml penicillin G at 37°C in a 5 % CO 2 incubator.…”

“…Besides pathological states, the UPR also plays an essential role in the physiological development of multicellular organisms. The UPR accommodates the capacity of protein synthesis in secretory cells to cope with the expanding physiological demand, as reported in alveolar type II cells (Mimura et al 2007), Cajal-Retzius cells (Mimura et al 2008), pancreatic beta cells (Harding et al 2001), plasma cells (Reimold et al 2001), and hepatocytes (Wang et al 2012).…”

“…The homozygous mutant BiP mice died within several hours after birth because of respiratory failure with impaired biosynthesis of the pulmonary surfactant, especially surfactant protein C, by alveolar type II cells (Mimura et al 2007). In addition, we found that the mutant BiP mice displayed a disordered layer formation in the cerebral cortex and cerebellum; this is a neurological phenotype of the reeler mutant-like malformation (D'Arcangelo et al 1995).…”