A Translational Regulatory Mechanism Mediated by Hypusinated Eukaryotic Initiation Factor 5A Facilitates β-Cell Identity and Function

As professional secretory cells, β-cells require adaptable mRNA translation to facilitate a rapid synthesis of proteins, including insulin, in response to changing metabolic cues. Specialized mRNA translation programs are essential drivers of cellular development and differentiation. However, in the pancreatic β-cell, the majority of factors identified to promote growth and development function primarily at the level of transcription. Therefore, despite its importance, the regulatory role of mRNA translation in the formation and maintenance of functional β-cells is not well defined. In this study, we have identified a translational regulatory mechanism mediated by the specialized mRNA translation factor eukaryotic initiation factor 5A (eIF5A), which facilitates the maintenance of β-cell identity and function. The mRNA translation function of eIF5A is only active when it is posttranslationally modified ("hypusinated") by the enzyme deoxyhypusine synthase (DHPS). We have discovered that the absence of β-cell DHPS in mice reduces the synthesis of proteins critical to β-cell identity and function at the stage of β-cell maturation, leading to a rapid and reproducible onset of diabetes. Therefore, our work has revealed a gatekeeper of specialized mRNA translation that permits the β-cell, a metabolically responsive secretory cell, to maintain the integrity of protein synthesis necessary during times of induced or increased demand.

ARTICLE HIGHLIGHTS

Deoxyhypusine synthase is required for the translational regulation of pancreatic beta cell maturation

As professional secretory cells, beta cells require adaptable mRNA translation to facilitate a rapid synthesis of proteins, including insulin, in response to changing metabolic cues. Specialized mRNA translation programs are essential drivers of cellular development and differentiation. However, in the pancreatic beta cell, the majority of factors identified to promote growth and development function primarily at the level of transcription. Therefore, despite its importance, the regulatory role of mRNA translation in the formation and maintenance of functional beta cells is not well defined. In this study, we have identified a translational regulatory mechanism in the beta cell driven by the specialized mRNA translation factor, eukaryotic initiation factor 5A (eIF5A), which facilitates beta cell maturation. The mRNA translation function of eIF5A is only active when it is post-translationally modified ("hypusinated") by the enzyme deoxyhypusine synthase (DHPS). We have discovered that the absence of beta cell DHPS in mice reduces the synthesis of proteins critical to beta cell identity and function at the stage of beta cell maturation, leading to a rapid and reproducible onset of diabetes. Therefore, our work has revealed a gatekeeper of specialized mRNA translation that permits the beta cell, a metabolically responsive secretory cell, to maintain the integrity of protein synthesis necessary during times of induced or increased demand.

ARTICLE HIGHLIGHTS

Pancreatic beta cells are professional secretory cells that require adaptable mRNA translation for the rapid, inducible synthesis of proteins, including insulin, in response to changing metabolic cues. Our previous work in the exocrine pancreas showed that development and function of the acinar cells, which are also professional secretory cells, is regulated at the level of mRNA translation by a specialized mRNA translation factor, eIF5A ^HYP . We hypothesized that this translational regulation, which can be a response to stress such as changes in growth or metabolism, may also occur in beta cells. Given that the mRNA translation function of eIF5A is only active when the factor is post-translationally modified ("hypusinated") by the enzyme deoxyhypusine synthase (DHPS), we asked the question: does DHPS/eIF5A ^HYP regulate the formation and maintenance of functional beta cells? We discovered that in the absence of beta cell DHPS in mice, eIF5A is not hypusinated (activated), which leads to a reduction in the synthesis of critical beta cell proteins that interrupts pathways critical for identity and function. This translational regulation occurs at weaning age, which is a stage of cellular stress and maturation for the beta cell. Therefore without DHPS/eIF5A ^HYP , beta cells do not mature and mice progress to hyperglycemia and diabetes. Our findings suggest that secretory cells have a mechanism to regulate mRNA translation during times of cellular stress. Our work also implies that driving an increase in mRNA translation in the beta cell might overcome or possibly reverse the beta cell defects that contribute to early dysfunction and the progression to diabetes.

Deoxyhypusine synthase, an essential enzyme for hypusine biosynthesis, is required for proper exocrine pancreas development

Pancreatic diseases including diabetes and exocrine insufficiency would benefit from therapies that reverse cellular loss and/or restore cellular mass. The identification of molecular pathways that influence cellular growth is therefore critical for future therapeutic generation. Deoxyhypusine synthase (DHPS) is an enzyme that post-translationally modifies and activates the mRNA translation factor eukaryotic initiation factor 5A (eIF5A). Previous work demonstrated that the inhibition of DHPS impairs zebrafish exocrine pancreas development; however, the link between DHPS, eIF5A, and regulation of pancreatic organogenesis remains unknown. Herein we identified that the conditional deletion of either Dhps or Eif5a in the murine pancreas results in the absence of acinar cells. Because DHPS catalyzes the activation of eIF5A, we evaluated and uncovered a defect in mRNA translation concomitant with defective production of proteins that influence cellular development. Our studies reveal a heretofore unappreciated role for DHPS and eIF5A in the synthesis of proteins required for cellular development and function.

Turmeric Extract Rescues Ethanol-Induced Developmental Defect in the Zebrafish Model for Fetal Alcohol Spectrum Disorder (FASD)

Prenatal ethanol exposure causes the most frequent preventable birth disorder, fetal alcohol spectrum disorder (FASD). The effect of turmeric extracts in rescuing an ethanol-induced developmental defect using zebrafish as a model was determined. Ethanol-induced oxidative stress is one of the major mechanisms underlying FASD. We hypothesize that antioxidant inducing properties of turmeric may alleviate ethanol-induced defects. Curcuminoid content of the turmeric powder extract (5 mg/mL turmeric in ethanol) was determined by UPLC and found to contain Curcumin (124.1 ± 0.2 μg/mL), Desmethoxycurcumin (43.4 ± 0.1 μg/mL), and Bisdemethoxycurcumin (36.6 ± 0.1 μg/mL). Zebrafish embryos were treated with 100 mM (0.6% v/v) ethanol during gastrulation through organogenesis (2 to 48 h postfertilization (hpf)) and supplemented with turmeric extract to obtain total curcuminoid concentrations of 0, 1.16, 1.72, or 2.32 μM. Turmeric supplementation showed significant rescue of the body length at 72 hpf compared to ethanol-treated embryos. The mechanism underlying the rescue remains to be determined.