GLUT1 Deficiency Links Nutrient Availability and Apoptosis during Embryonic Development

Abstract

GLUT1 is essential for human brain development and function, as evidenced by the severe epileptic encephalopathy observed in children with GLUT1 deficiency syndrome resulting from inherited loss-of-function mutations in the gene encoding this facilitative glucose transporter. To further elucidate the pathophysiology of this disorder, the zebrafish orthologue of human GLUT1 was identified, and expression of this gene was abrogated during early embryonic development, resulting in a phenotype of aberrant brain organogenesis consistent with the observed expression of Glut1 in the embryonic tectum and specifically rescued by human GLUT1 mRNA. Affected embryos displayed impaired glucose uptake concomitant with increased neural cell apoptosis and subsequent ventricle enlargement, trigeminal ganglion cell loss, and abnormal hindbrain architecture. Strikingly, inhibiting expression of the zebrafish orthologue of the proapoptotic protein Bad resulted in complete rescue of this phenotype, and this occurred even in the absence of restoration of apparent glucose uptake. Taken together, these studies describe a tractable system for elucidating the cellular and molecular mechanisms of Glut1 deficiency and provide compelling in vivo genetic evidence directly linking nutrient availability and activation of mitochondria-dependent apoptotic mechanisms during embryonic brain development.The cellular uptake of glucose is dependent in part upon the GLUT family of polytopic membrane transporters that facilitate the passive diffusion of this essential nutrient across membranes (1). GLUT1, the prototypic member of this protein family, is highly expressed in erythrocytes and the central nervous system and is believed to play an essential role in the homeostasis of brain glucose in the developing human infant (2). In support of this concept, infants with GLUT1 deficiency syndrome, a rare genetic disorder resulting from inherited heterozygous loss-of-function mutations in the gene encoding GLUT1, develop seizures, acquired microcephaly and profound developmental delay in association with profound hypoglycorrhachia (3). Despite considerable study of these patients, the neurochemical and neuropathologic consequences of GLUT1 deficiency during development are not well understood, and treatment of affected patients remains challenging.Although abundant in the extracellular milieu, the cellular uptake of glucose is precisely regulated by specific growth factors and signaling pathways (4). Cell culture studies reveal that inhibition of cellular glucose uptake dramatically increases apoptosis under conditions of growth factor restriction and that cell survival under such circumstances is dependent upon the regulation of glucose uptake and metabolism by the proto-oncogene Akt (5-7). A biochemical link between glucose homeostasis and apoptosis was further suggested by studies demonstrating an interaction between the proapoptotic protein, Bad, and the glycolytic enzyme, glucokinase (8). In support of these findings, a recent study now demonstrates a...

Akt2 Modulates Glucose Availability and Downstream Apoptotic Pathways during Development

“…Previous studies from our laboratory established zebrafish as a tractable system for defining the cellular and molecular mechanisms perturbed by impaired Glut1-dependent glucose transport (22). In this work, interference with Glut1 expression induced a striking neurodevelopmental syndrome that could be prevented by simultaneous inhibition of the proapoptotic protein bad.…”

“…Similarly, episodes of hypoglycemia during the neonatal period are known to cause permanent neurological impairment (3). Although these clinical scenarios suggest a causal relationship between glucose metabolism and neurodevelopment, the mechanisms integrating these processes remain poorly understood.Previous studies from our laboratory established zebrafish as a tractable system for defining the cellular and molecular mechanisms perturbed by impaired Glut1-dependent glucose transport (22). In this work, interference with Glut1 expression induced a striking neurodevelopmental syndrome that could be prevented by simultaneous inhibition of the proapoptotic protein bad.…”

“…In the present study, we extend our analysis of a previously described, Glut1-sensitive apoptotic pathway describing the molecular mechanism for this effect (22). Based on its role in the regulation of apoptosis and its dependence on glucose availability, we hypothesized that Akt2 is a critical component in this pathway.…”

“…With these differences in mind, we sought to define the role of Akt2 in embryonic development and test our hypothesis that this kinase is involved in activation of the previously described Glut1-sensitive apoptotic cascade (22).…”

“…This MO was identical to the Akt2 translational start site MO except for a 5-bp mismatch, indicated by lowercase letters. The inhibition of glut1 and bad was accomplished using MOs and conditions previously described (22). For knockdown experiments, 5 ng of each MO was microinjected into embryos at the 1-2-cell stage using pulled glass microcapillary pipettes attached to a micromanipulator (model P-97, Sutter Instrument Co.).…”