Akt2 Modulates Glucose Availability and Downstream Apoptotic Pathways during Development

Abstract

Glucose is the primary energy substrate for eukaryotic cells and the predominant substrate for the brain. Studies suggest that glucose serves an additional role in the regulation of cellular functions, including viability. Zebrafish is a tractable system for defining the cellular and molecular mechanisms perturbed by impaired glucose transport and metabolism. Previously, we demonstrated a critical role for the facilitative glucose transporter, Glut1, in the regulation of embryonic brain development. In this study, we aim to identify mediators in this Glut1-sensitive process by investigating the role of the antiapoptotic kinase, Akt2. Results show that abrogating expression of akt2 causes a phenotype strikingly similar to that observed when glut1 expression is inhibited. akt2-deficient embryos exhibit increased neuronal apoptosis, impaired glucose uptake, and death by 72 h postfertilization. Similar to what was observed in the glut1 morphants, inhibiting the expression of the proapoptotic protein, bad, in the context of impaired akt2 expression results in the inhibition of apoptosis and rescue of the morphant embryos. Intriguingly, overexpression of glut1 in the akt2 morphants was also able to rescue these embryos. Quantitative reverse transcription-PCR analysis revealed decreased glut1 transcript expression in akt2 morphant embryos. Taken together, these data suggest that Akt2 modulates glucose availability by regulating Glut1 expression at the transcript level. These data support a role for akt2 in an integrative pathway directly linking glucose, Glut1 expression, and activation of apoptosis and demonstrate the dependence of akt2 on glucose availability for the maintenance of cellular viability, particularly in the central nervous system.The acute metabolic transition that a neonate experiences at birth predisposes it to the development of hypoglycemia. The sudden switch from total reliance on maternal nutrients to dependence on endogenous fuel stores requires a rapid, coordinated, and integrated hormonal and enzymatic response by the neonate (1, 2). Proper homeostatic control is critical during this period because the consequences of repeated and/or prolonged episodes of hypoglycemia can result in neurodevelopmental impairment (3). Although it is clear that dysregulation of mechanisms controlling glucose homeostasis, including deficiencies in growth hormone or cortisol and hyperinsulinism, can cause hypoglycemia during this volatile period, there is virtually nothing known about the mechanisms underlying the irreversible neurologic sequelae. Although there are substantial data defining possible mechanisms underlying injury associated with hypoxia-ischemia, most of these studies have been performed in adult animals, making conclusions regarding neonates with isolated hypoglycemia indirect at best (4).Investigators studying a variety of cell systems have established glucose uptake and metabolism as pivotal mediators of cell survival. Cancer cells are dependent on glycolysis for the generation of ATP even in the ...

GLUT3 gene expression is critical for embryonic growth, brain development and survival

“…Additionally, a MO targeting exon 4 and 5 splice sites within the GLUT3 transcript ( 5’ –TTGCGTCTGTGGAAAGAAAAACAGA- 3’ ) was generated. The inhibition of glut1 and glut3 was accomplished using MOs and conditions previously described (9). For knockdown experiments each MO was microinjected into embryos at the 1–2 cell stage in the amounts indicated using pulled glass micro-capillary pipettes attached to a micromanipulator, Model P-97 (Sutter Instrument Co., Novato, CA).…”

Protective Role of Akt2 in Salmonella enterica Serovar Typhimurium-Induced Gastroenterocolitis

“…In the present study, we have demonstrated that Salmonella-induced apoptosis at the sites of infection in mice lacking Akt2 and Akt2 deficiency renders these mice more susceptible to infection, suggesting that apoptosis may play an important role in the pathogenesis of Salmonella-induced colitis. Although Akt2 has been shown to play an antiapoptotic role (17), the enhanced level of apoptosis observed in Salmonella-infected Akt2 KO mice may also be due to the decrease in glucose availability, as Akt2 modulates glucose homeostasis and downstream apoptotic pathways during development (28) and Akt2 knockout mice have been shown to develop a type 2 diabetic phenotype (8).…”

Signal Transduction in Cancer

“…The Akt substrate AS160 plays an undefined role in insulin-stimulated GLUT4 translocation and glucose transport through its Rab-GTPase-activating domain (Miinea et al 2005), and phosphorylation of the protein synip by Akt2 triggers its dissociation from the trafficking regulator syntaxin 4 and assembly of a protein complex that mediates translocation of GLUT4 vesicles to the plasma membrane (Yamada et al 2005). Akt2 also regulates transcription, accumulation (Barthel et al 1999;Jensen et al 2010), and trafficking of GLUT1, which is the principle glucose transporter expressed in most cell types (Wieman et al 2007). Phosphorylation of TSC2 by Akt affects metabolism through mTORC1-mediated regulation of glycolysis; however, the mechanism of regulation is not known.…”