Temporal and spatial distribution of murine placental and brain GLUT3-luciferase transgene as a readout of in vivo transcription

Neurodevelopment Is Dependent on Maternal Diet: Placenta and Brain Glucose Transporters GLUT1 and GLUT3

Glucose is the primary energy source for most mammalian cells and its transport is affected by a family of facilitative glucose transporters (GLUTs) encoded by the SLC2 gene. GLUT1 and GLUT3, highly expressed isoforms in the blood-brain barrier and neuronal membranes, respectively, are associated with multiple neurodevelopmental disorders including epilepsy, dyslexia, ADHD, and autism spectrum disorder (ASD). Dietary therapies, such as the ketogenic diet, are widely accepted treatments for patients with the GLUT1 deficiency syndrome, while ameliorating certain symptoms associated with GLUT3 deficiency in animal models. A ketogenic diet, high-fat diet, and calorie/energy restriction during prenatal and postnatal stages can also alter the placental and brain GLUTs expression with long-term consequences on neurobehavior. This review focuses primarily on the role of diet/energy perturbations upon GLUT isoform-mediated emergence of neurodevelopmental and neurodegenerative disorders.

Developing Brain Glucose Transporters, Serotonin, Serotonin Transporter, and Oxytocin Receptor Expression in Response to Early-Life Hypocaloric and Hypercaloric Dietary, and Air Pollutant Exposures

Perturbed maternal diet and prenatal exposure to air pollution (AP) affect the fetal brain, predisposing to postnatal neurobehavioral disorders. Glucose transporters (GLUTs) are key in fueling neurotransmission; deficiency of the neuronal isoform GLUT3 culminates in autism spectrum disorders. Along with the different neurotransmitters, serotonin (5-HT) and oxytocin (OXT) are critical for the development of neural connectivity. Serotonin transporter (SERT) modulates synaptic 5-HT levels, while the OXT receptor (OXTR) mediates OXT action. We hypothesized that perturbed brain GLUT1/GLUT3 regulated 5-HT-SERT imbalance, which serves as a contributing factor to postnatal neuropsychiatric phenotypes, with OXT/OXTR providing a counterbalance. Employing maternal diet restriction (intrauterine growth restriction [IUGR]), high-fat (HF) dietary modifications, and prenatal exposure to simulated AP, fetal (E19) murine brain 5-HT was assessed by ELISA with SERT and OXTR being localized by immunohistochemistry and measured by quantitative Western blot analysis. IUGR with lower head weights led to a 48% reduction in male and female fetal brain GLUT3 with no change in GLUT1, when compared to age- and sex-matched controls, with no significant change in OXTR. In addition, a ∼50% (p = 0.005) decrease in 5-HT and SERT concentrations was displayed in fetal IUGR brains. In contrast, despite emergence of microcephaly, exposure to a maternal HF diet or AP caused no significant changes. We conclude that in the IUGR during fetal brain development, reduced GLUT3 is associated with an imbalanced 5-HT-SERT axis. We speculate that these early changes may set the stage for altering the 5HT-SERT neural axis with postnatal emergence of associated neurodevelopmental disorders.

Dyslexia associated functional variants in Europeans are not associated with dyslexia in Chinese

Genome-wide association studies (GWAS) of developmental dyslexia (DD) often used European samples and identified only a handful associations with moderate or weak effects. This study aims to identify DD functional variants by integrating the GWAS associations with tissue-specific functional data and test the variants in a Chinese DD study cohort named READ. We colocalized associations from nine DD related GWAS with expression quantitative trait loci (eQTL) derived from brain tissues and identified two eSNPs rs349045 and rs201605. Both eSNPs had supportive evidence of chromatin interactions observed in human hippocampus tissues and their respective target genes ZNF45 and DNAH9 both had lower expression in brain tissues in schizophrenia patients than controls. In contrast, an eSNP rs4234898 previously identified based on eQTL from the lymphoblastic cell lines of dyslexic children had no chromatin interaction with its target gene SLC2A3 in hippocampus tissues and SLC2A3 expressed higher in the schizophrenia patients than controls. We genotyped the three eSNPs in the READ cohort of 372 cases and 354 controls and discovered only weak associations in rs201605 and rs4234898 with three DD symptoms (p < .05). The lack of associations could be due to low power in READ but could also implicate different etiology of DD in Chinese.

Neural Deletion of Glucose Transporter Isoform 3 Creates Distinct Postnatal and Adult Neurobehavioral Phenotypes

We created a neural-specific conditional murine glut3 (Slc2A3) deletion (glut3 ^{flox/flox/nestin-Cre+}) to examine the effect of a lack of Glut3 on neurodevelopment. Compared with age-matched glut3 ^flox/flox = WT and heterozygotes (glut3 ^{flox/+/nestin-Cre+}), we found that a >90% reduction in male and female brain Glut3 occurred by postnatal day 15 (PN15) in glut3 ^{flox/flox/nestin-Cre+} This genetic manipulation caused a diminution in brain weight and cortical thickness at PN15, a reduced number of dendritic spines, and fewer ultrasonic vocalizations. Patch-clamp recordings of cortical pyramidal neurons revealed increased frequency of bicuculline-induced paroxysmal discharges as well as reduced latency, attesting to a functional synaptic and cortical hyperexcitability. Concomitant stunting with lower glucose concentrations despite increased milk intake shortened the lifespan, failing rescue by a ketogenic diet. This led to creating glut3 ^{flox/flox/CaMK2α-Cre+} mice lacking Glut3 in the adult male limbic system. These mice had normal lifespan, displayed reduced IPSCs in cortical pyramidal neurons, less anxiety/fear, and lowered spatial memory and motor abilities but heightened exploratory and social responses. These distinct postnatal and adult phenotypes, based upon whether glut3 gene is globally or restrictively absent, have implications for humans who carry copy number variations and present with neurodevelopmental disorders.SIGNIFICANCE STATEMENT Lack of the key brain-specific glucose transporter 3 gene found in neurons during early postnatal life results in significant stunting, a reduction in dendritic spines found on neuronal processes and brain size, heightened neuronal excitability, along with a shortened lifespan. When occurring in the adult and limited to the limbic system alone, lack of this gene in neurons reduces the fear of spatial exploration and socialization but does not affect the lifespan. These features are distinct heralding differences between postnatal and adult phenotypes based upon whether the same gene is globally or restrictively lacking. These findings have implications for humans who carry copy number variations pertinent to this gene and have been described to present with neurodevelopmental disorders.

Gestational food restriction decreases placental interleukin-10 expression and markers of autophagy and endoplasmic reticulum stress in murine intrauterine growth restriction

Intrauterine growth restriction (IUGR) affects up to 10% of pregnancies and often results in short- and long-term sequelae for offspring. The mechanisms underlying IUGR are poorly understood, but it is known that healthy placentation is essential for nutrient provision to fuel fetal growth, and is regulated by immunologic inputs. We hypothesized that in pregnancy, maternal food restriction (FR) resulting in IUGR would decrease the overall immunotolerant milieu in the placenta, leading to increased cellular stress and death. Our specific objectives were to evaluate (1) key cytokines (eg, IL-10) that regulate maternal-fetal tolerance, (2) cellular processes (autophagy and endoplasmic reticulum [ER] stress) that are immunologically mediated and important for cellular survival and functioning, and (3) the resulting IUGR phenotype and placental histopathology in this animal model. After subjecting pregnant mice to mild and moderate FR from gestational day 10 to 19, we collected placentas and embryos at gestational day 19. We examined RNA sequencing data to identify immunologic pathways affected in IUGR-associated placentas and validated messenger RNA expression changes of genes important in cellular integrity. We also evaluated histopathologic changes in vascular and trophoblastic structures as well as protein expression changes in autophagy, ER stress, and apoptosis in the mouse placentas. Several differentially expressed genes were identified in FR compared with control mice, including a considerable subset that regulates immune tolerance, inflammation, and cellular integrity. In summary, maternal FR decreases the anti-inflammatory effect of IL-10 and suppresses placental autophagic and ER stress responses, despite evidence of dysregulated vascular and trophoblast structures leading to IUGR.

Prenatal caloric restriction enhances DNA methylation and MeCP2 recruitment with reduced murine placental glucose transporter isoform 3 expression

Diminished transplacental glucose transport plays an important role in prenatal calorie restriction (CR) induced reduction in fetal growth. Fetal growth restriction (FGR) has an impact in shaping the adult phenotype with transgenerational implications. To understand the mechanisms underlying prenatal CR-induced transplacental glucose transport, we examined the epigenetic regulation of placental glucose transporter (Glut1 and Glut3) expression. We restricted calories by 50% in C57BL6 pregnant mice from gestational days 10 to 19 (CR; n=8) vs. controls (CON; n=8) and observed a 50% diminution in placental Glut3 expression (P<.05) with no effect on Glut1 expression by reverse transcription and quantitative real-time polymerase chain reaction (PCR). CR enhanced DNA methylation of a CpG island situated ~1000 bp upstream from the transcriptional start site of the glut3 gene, with no such effect on the glut1 gene as assessed by methylation-sensitive PCR and bisulfite sequencing. Chromatin immunoprecipitation (ChIP) assays demonstrated enhanced MeCP2 binding to the CpG island of the glut3 gene in response to CR vs. CON (P<.05). Sequential ChIP demonstrated that enhanced MeCP2 binding of the glut3-(m)CpG island enhanced histone deacetylase 2 recruitment (P<.05) but interfered with Sp1 binding (P<.001), although it did not affect Sp3 or Creb/pCreb interaction. We conclude that late-gestation CR enhanced DNA methylation of the placental glut3 gene. This epigenetic change augmented specific nuclear protein-DNA complex formation that was associated with prenatal CR-induced reduction of placental glut3 expression and thereby transplacental glucose transport. This molecular complex provides novel targets for developing therapeutic interventions aimed at reversing FGR.

Creb1-Mecp2-(m)CpG complex transactivates postnatal murine neuronal glucose transporter isoform 3 expression

The murine neuronal facilitative glucose transporter isoform 3 (Glut3) is developmentally regulated, peaking in expression at postnatal day (PN)14. In the present study, we characterized a canonical CpG island spanning the 5'-flanking region of the glut3 gene. Methylation-specific PCR and bisulfite sequencing identified methylation of this CpG ((m)CpG) island of the glut3 gene, frequency of methylation increasing 2.5-fold with a 1.6-fold increase in DNA methyl transferase 3a concentrations noted with advancing postnatal age (PN14 vs PN3). 5'-flanking region of glut3-luciferase reporter transient transfection in HT22 hippocampal neurons demonstrated that (m)CpGs inhibit glut3 transcription. Contrary to this biological function, glut3 expression rises synchronously with (m)CpGs in PN14 vs PN3 neurons. Chromatin immunoprecipitation (IP) revealed that methyl-CpG binding protein 2 (Mecp2) bound the glut3-(m)CpGs. Depending on association with specific coregulators, Mecp2, a dual regulator of gene transcription, may repress or activate a downstream gene. Sequential chromatin IP uncovered the glut3-(m)CpGs to bind Mecp2 exponentially upon recruitment of Creb1 rather than histone deacetylase 1. Co-IP and coimmunolocalization confirmed that Creb1 associated with Mecp2 and cotransfection with glut3-(m)CpG in HT22 cells enhanced glut3 transcription. Separate 5-aza-2'-deoxycytidine pretreatment or in combination with trichostatin A reduced (m)CpG and specific small interference RNAs targeting Mecp2 and Creb1 separately or together depleting Mecp2 and/or Creb1 binding of glut3-(m)CpGs reduced glut3 expression in HT22 cells. We conclude that Glut3 is a methylation-sensitive neuronal gene that recruits Mecp2. Recruitment of Creb1-Mecp2 by glut3-(m)CpG contributes towards transactivation, formulating an escape from (m)CpG-induced gene suppression, and thereby promoting developmental neuronal glut3 gene transcription and expression.

Hypoxic adaptation engages the CBP/CREST-induced coactivator complex of Creb-HIF-1α in transactivating murine neuroblastic glucose transporter

We have shown in vitro a hypoxia-induced time-dependent increase in facilitative glucose transporter isoform 3 (GLUT3) expression in N2A murine neuroblasts. This increase in GLUT3 expression is partially reliant on a transcriptional increase noted in actinomycin D and cycloheximide pretreatment experiments. Transient transfection assays in N2A neuroblasts using murine glut3-luciferase reporter constructs mapped the hypoxia-induced enhancer activities to -857- to -573-bp and -203- to -177-bp regions. Hypoxia-exposed N2A nuclear extracts demonstrated an increase in HIF-1α and p-Creb binding to HRE (-828 to -824 bp) and AP-1 (-187 to -180 bp) cis-elements, respectively, in electromobility shift and supershift assays, which was confirmed by chromatin immunoprecipitation assays. In addition, the interaction of CBP with Creb and HIF-1α and CREST with CBP in hypoxia was detected by coimmunoprecipitation. Furthermore, small interference (si)RNA targeting Creb in these cells decreased endogenous Creb concentrations that reduced by twofold hypoxia-induced glut3 gene transcription. Thus, in N2A neuroblasts, phosphorylated HIF-1α and Creb mediated the hypoxia-induced increase in glut3 transcription. Coactivation by the Ca⁺⁺-dependent CREST and CBP proteins may enhance cross-talk between p-Creb-AP-1 and HIF-1α/HRE of the glut3 gene. Collectively, these processes can facilitate an adaptive response to hypoxic energy depletion targeted at enhancing glucose transport and minimizing injury while fueling the proliferative potential of neuroblasts.