In a recent article posted to the bioRxiv* preprint server, researchers investigated whether exposure to high fructose in early life directly affects essential microglia function to impair neonates' neurodevelopment, occasionally leading to anxiety-like behavior.
Study: Early life high fructose exposure disrupts microglia function and impedes neurodevelopment. Image Credit: fizkes/Shutterstock.com
*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Background
In mammals, microglia, the resident macrophages of the central nervous system (CNS), execute synaptic pruning and phagocytosis of dying neurons, a process well-recognized as efferocytosis.
Disruption to synaptic pruning, leading to non-removal of dying neurons, through any of the stages of efferocytosis likely adversely affects the neurodevelopment process.
Several studies have highlighted the health hazards of overconsumption of high fructose corn syrup (HFCS)-containing foods and beverages. Thus, in recent decades, it has emerged as one of the risk factors for obesity, diabetes, and cardiovascular disease (CVDs).
Experiments with laboratory animals have also shown that chronic high fructose consumption could specifically increase liver fructose and small intestine catabolism or fructolysis.
However, studies have not yet uncovered how high fructose affects CNS cellular function.
Nonetheless, as suggested by many epidemiological studies, chronic high fructose consumption raises the risk of neurodevelopmental deficits, particularly in pregnant females and adolescents, where it triggers anxiety and mood disorders.
About the study
In the present study, researchers fed wild-type C57BL/6J adult mice with control (CD) and high fructose (HF) diets at least one week before mating and maintained the same throughout gestation and lactation.
Likewise, they fed (intragastrically) 50 μL of sterile water with or without fructose at a prespecified dosage of 45 mg/mouse from the perinatal day (P)1 to P7 to dam neonates.
Notably, at P7, microglia-mediated synaptic pruning and dying neuron clearance peaks. This dosage was 45% of the daily dose for adult mice delivered by oral gavage and human intake of <12 oz of fructose-sweetened beverage.
Next, the researchers harvested neonatal mice brains for immunofluorescence and confocal microscopic examinations.
They obtained a minimum of 4 fields of view (FOV) from the prefrontal cortex and cortex, which they used for microglial morphological analysis.
Further, the researchers used a suite of approaches to identify microglia morphological changes indicative of cellular dysfunction.
For instance, they used the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay to identify uncleared dead neurons. Similarly, they analyzed microglia for postsynaptic density protein 95 (PSD95), a readout for synaptic pruning.
The team also investigated whether neonatal glucose transporter 5 (GLUT5), also known as SLC2A5, is necessary for the microglia phenotypes observed in HF-treated mice.
The researchers also assessed the effects of high fructose on human microglia using microglia-like cells derived from human pluripotent stem cells (hPSCs).
They used primary microglia isolated from GLUT5-deficient neonates to analyze phagocytosis capacity under HF conditions and whether this process was related to fructose transport. They assessed microglial phagocytosis of synaptic terminals and apoptotic neurons.
The team used [2- 13C]-fructose carbon nuclear magnetic resonance (NMR) to quantify fructose metabolism in vivo.
Finally, the team subjected wild-type or GLUT5-deficient mice raised by dams on CD or HF diet for three behavioral assessments: novel object recognition (NOR), modified Barnes maze, and fear extinction.
While the NOR broadly assessed animal cognition and recognition memory, the modified Barnes maze task assessed spatial and working memory, and fear extinction, an associative learning task, tested fear acquisition (through conditioning) and its reversal (extinction).
Studies have implicated fear extinction tasks in various anxiety disorders, including post-traumatic stress disorder (PTSD).
Results and conclusions
The study provided a mechanistic explanation for the observation that exposure to high fructose early in life could increase the likelihood of anxiety disorders in adolescence by directly impacting microglia phagocytosis and neonatal neurodevelopment.
How this factors into human adolescent development remains unknown, given the recent rise in mood and anxiety disorders globally, especially after the COVID-19 pandemic.
In this study, high fructose exposure drove microglia to catabolize fructose into fructose 6-phosphate using keto hexokinase in the liver and the small intestine, or hexokinase 2 (HK2).
Thus, HK2 deletion or pharmacological inhibition increased microglial phagocytic activity, and its supplementation reversed the process. Future studies should determine how HK2 enzymatic activity functions to suppress or enhance neonatal microglia phagocytosis.
With aging, microglial expression of GLUT5 increased in both mouse and human microglia. In vivo and in vitro NMR-based fructose tracing showed that neonatal microglia used GLUT5 to metabolize fructose, but it suppressed their phagocytic capacity.
HF-driven GLUT5-dependent fructose uptake and catabolism rewired microglia metabolism towards a hypo- or non-phagocytic state in adulthood.
Indeed, GLUT5 upregulation and increased fructose catabolism could be the key determinants of microglial longevity.
Thus, insights into how adult microglia use GLUT5 and fructose in diseased and healthy individuals are crucial, particularly given the wide use of HFCS as an additive in the food industry.
Furthermore, the behavioral test results suggested that early life exposure to high fructose affected the development of anxiety disorders (e.g., PTSD) but not memory.
Conclusion
To conclude, the study results have profound implications for pregnancy and adolescent development, particularly given that microglial synaptic pruning and phagocytosis continue throughout adolescence in humans.
It could be challenging as it broadens the window when HF consumption could be hazardous.
*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
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