IER3IP1 is critical for maintaining glucose homeostasis through regulating the endoplasmic reticulum function and survival of β cells

Abstract

Recessive mutations in IER3IP1 (immediate early response 3 interacting protein 1) cause a syndrome of microcephaly, epilepsy, and permanent neonatal diabetes (MEDS). IER3IP1 encodes an endoplasmic reticulum (ER) membrane protein, which is crucial for brain development; however, the role of IER3IP1 in β cells remains unknown. We have generated two mouse models with either constitutive or inducible IER3IP1 deletion in β cells, named IER3IP1-βKO and IER3IP1-iβKO, respectively. We found that IER3IP1-βKO causes severe early-onset, insulin-deficient diabetes. Functional studies revealed a markedly dilated β-cell ER along with increased proinsulin misfolding and elevated expression of the ER chaperones, including PDI, ERO1, BiP, and P58IPK. Islet transcriptome analysis confirmed by qRT-PCR revealed decreased expression of genes associated with β-cell maturation, cell cycle, and antiapoptotic genes, accompanied by increased expression of antiproliferation genes. Indeed, multiple independent approaches further demonstrated that IER3IP1-βKO impaired β-cell maturation and proliferation, along with increased condensation of β-cell nuclear chromatin. Inducible β-cell IER3IP1 deletion in adult (8-wk-old) mice induced a similar diabetic phenotype, suggesting that IER3IP1 is also critical for function and survival even after β-cell early development. Importantly, IER3IP1 was decreased in β cells of patients with type 2 diabetes (T2D), suggesting an association of IER3IP1 deficiency with β-cell dysfunction in the more-common form of diabetes. These data not only uncover a critical role of IER3IP1 in β cells but also provide insight into molecular basis of diabetes caused by IER3IP1 mutations.

IER3IP1-mutations cause microcephaly by selective inhibition of ER-Golgi transport

“…ER stacks appear distended in cells lacking IER3IP1, suggesting a certain level of ER stress, but BiP protein levels are not increased. The ER stress sensors phospho-IRE1 and ATF6 are not activated, as previously observed in pancreatic b-cells 14 . IER3IP1 deficiency-related accumulation of enlarged ER cisternae is likely to activate cellular mechanisms that remove unnecessary membranes for maintaining cellular homeostasis 40 .…”

“…Among the IER3IP1 pathogenic variants, p.T79D is not expressed at the protein level, p.L78P negatively affects the organization of the secretory pathway, similarly to IER3IP1 depletion, whereas p.V21G causes no significant morphological changes, suggesting further investigations are necessary for its characterization. Although previous studies support the importance of IER3IP1 in ER function 10,14,15 , a direct role for IER3IP1 in ER to Golgi transport has not been demonstrated in mammalian cells, and its potential cargos have remained unknown.…”

“…To measure the number of objects per cell for the different markers analyzed, as well as the co-localization between GM130 and ST6GAL1 (object-based Pearson's correlation coefficient), images were processed with Fiji, and object thresholding, detection and measurement per image were performed with Cell Profiler (www.cellprofiler.org) 14 .…”

Proinsulin folding and trafficking defects trigger a common pathological disturbance of endoplasmic reticulum homeostasis

“…Among the many genetic variants linked to diabetes are genes expressed in β‐cells that impact on ER homeostasis and ER‐to‐Golgi trafficking (see Introduction). Some of these primary genetic deficiencies in β‐cells have already been reported to result in proinsulin misfolding, accompanied by ER stress (Zhu et al, 2019 ), including a recent report of IER3IP1 deficiency (Yang et al, 2022 ). To us, this suggests a ‘positive feedback loop’ in which primary proinsulin misfolding perturbs ER homeostasis and thus affects anterograde trafficking in the secretory pathway (Yuan et al, 2012 ), and primary impairment of anterograde trafficking in the secretory pathway perturbs ER homeostasis and thus affects the folding of proinsulin (Supplemental Figure S1 ).…”

Essential requirement for IER3IP1 in B cell development

“…Immediate early response 3 interacting protein 1 (IER3IP1) is a small ER membrane protein reported to function as an attenuator of the UPR ( 5 – 7 ). IER3IP1 is expressed in multiple tissues including brain, pancreas, heart, muscle, kidney, lung, and immune cells ( 8 ).…”