Offspring development is particularly vulnerable during the antenatal period and exposure of the fetus to adverse environments in utero during this critical period may impact on their health trajectories and disease susceptibility in the long term. This phenomenon, known as the “developmental origins of health and disease”, was described by David J Barker approximately 30 years ago. Further human and animal studies on fetal programming continued to support the pivotal role of the intrauterine environment in influencing the offspring phenotype and this transgenerational relationship may be mediated by modifications to the offspring epigenome.
Overnutrition and obesity during pregnancy has become a significant problem, mirroring the global obesity epidemic. Obesity during pregnancy is associated with an increased risk of maternal complications during the perinatal period, including gestational diabetes mellitus (GDM). Offspring born to obese and/or diabetic mothers are susceptible to short- and long-term morbidity and mortality, including an increased risk of obesity and cardiometabolic disorders in later life. Additionally, there is emerging evidence indicating an increased prevalence of offspring neurodevelopmental disorders and impaired cognitive function following exposure to maternal obesity and/or diabetes, although the mechanisms remain unclear.
I hypothesise that exposure of the developing brain to maternal obesity and diabetes during the antenatal and early postnatal period impacts on the epigenetic landscape, specifically DNA methylation, with downstream effects on genes associated with critical neurodevelopmental processes. I utilised a murine high fat diet-induced obesity in vivo model and demonstrated that exposure of dams to high fat high sugar diet (HFHS) recapitulated the phenotype seen in obese human pregnancies. Further, I showed that the HFHS offspring were hypoglycaemic and small-for-gestational age, which are known offspring complications associated with maternal obesity and hyperglycaemia during pregnancy. The expression of key enzymes involved in DNA methylation, specifically the Ten Eleven Translocase (TET) and DNA methyltransferase (DNMT) enzymes, were altered in male, but not female, HFHS offspring cortex and cerebellum, Additionally, I identified a sex-specific difference in cerebellar 5mC and 5hmC patterns, a novel finding that has not been described elsewhere. I have also demonstrated that exposure to maternal obesity and hyperglycaemia in the antenatal period associated with perturbations in metabolic and cell differentiation pathways.
Using an in vitro mixed-species cell culture system, I explored the impact of exposure of altered glucose availability on the transcriptomic and epigenetic landscape, as well as metabolic activity of 2 of the key cell types in the developing brain – astrocytes and neurons. This study confirmed that in silico analysis of mixed species astrocyte-neuron co-culture following MeDIP-sequencing was possible. I demonstrated that while exposure to high glucose concentrations had a modest impact on astrocyte and neuron transcriptome, there was evidence of perturbations in DNA methylation patterns, particularly in neurons. I identified that exposure of neurons to hyperglycaemic conditions impacted on differential methylation of promoters associated with cell signalling and synaptic function in neurons. I have also shown that changes in glucose availability impacts on mitochondrial respiration in both neurons and astrocytes.
Finally, using the in vitro cell culture model, I interrogated the non-cell autonomous regulation of the methylation and transcriptional landscape in astrocytes and neurons. I identified that astrocytes impact on neuronal DNA methylation with downstream effects on the transcription of key genes involved in neuronal synaptic activity and development, including immediate early response genes (Fos, Egr1 and Dusp5), as well as genes involved in key metabolic processes (Sucla1 and Pdk1). Further, the presence of neurons impacted on DNA methylation patterns and gene expression of a key astrocyte glutamate transporter, Slc1a2.
Collectively, the findings from this thesis highlights the role of epigenetic modification – specifically DNA methylation - in influencing the gene expression and pathways associated with key neurodevelopmental processes following the exposure to maternal obesity and diabetes. It has also demonstrated that altered glucose availability associates with changes in astrocyte and neuron transcriptome, methylome and energetics. Additionally, it has also contributed to our current knowledge of the non-cell autonomous relationship between astrocytes and neurons
