Abstract:
The reduced uptake of dietary micronutrients has been linked to both impaired cognitive function in the elderly and suboptimal neural development in infants, particularly for pre-term babies who miss the critical last weeks of gestation. The polyunsaturated fatty acid DHA and the carotenoid lutein are of particular interest in this regard as they are preferentially taken up by neural tissues. The current work explores the cellular mechanisms by which micronutrients promote the differentiation of neuronal progenitor cells. Specifically, we found that micronutrient supplementation encourages the growth and arborisation of neurites, the complexity of which is thought to determine the level of innervation and thus, the 'regional' interconnectedness and consequential functional capacity of the brain. Notably, we found that micronutrients promote neural differentiation by modulating underlying cellular metabolism. This supports an emerging paradigm which suggests that the developmental transition from a stem/precursor cell state is regulated by significant shifts in metabolic activity. A mechanistic prediction which arises from this is that mitochondrial activity and function will change, and subsequently influence patterns of nuclear gene expression through modulation of epigenetic processes. Using the established SY5Y cell line model of neuronal differentiation, the current work identifies the PI3K-Akt signalling pathway as the proximal intracellular mediator of micronutrient supplementation. Meanwhile, downstream epigenetic changes in cellular histone acetylation and the micronutrient-specific expression of microRNA species were found. Finally, transcriptomic analysis at a genome-wide level revealed micronutrient-specific changes in gene expression. Concomitant with these cellular and molecular manifestations, a micronutrient supplementation clearly changed the state of bioenergetics in the cells, increasing their glucose consumption, rates of glycolysis and enhancing mitochondrial respiration. Additional analysis suggests that the increased glycolytic function is due to modulation of enzyme activity, rather than altered gene expression. More intriguingly, the enhanced mitochondrial respiration is accompanied by elevated levels of reactive oxygen stress (ROS). As this was not found to stimulate an antioxidant response, it suggests that transient, localised production of ROS may be serving as a key signalling mediator of the mitochondrial response to micronutrient supplementation. In summary, the current work explores the molecular underpinnings by which micronutrients, known to enhance neuronal development, elicit their effects. As such, it attempts to detail the chain of causality through which dietary inputs promote differentiation by modulating intracellular signalling and consequential gene expression. Similarities in the phenomenological and molecular details, suggest that a substantial degree of mechanistic concordance may underpin exposures to micronutrients that are seemingly diverse (PUFAs vs carotenoids), with respect to both dietary bioavailability and biological functionality. Most significantly, evidence that changes in bioenergetics play a pivotal role to this causality reinforces a more general principle that cellular differentiation is regulated by significant shifts in metabolic state, but also highlights a potential signalling role for ROS in the context of neuronal development and function. With the gradual accumulation of evidence that neurogenesis may in fact continue throughout life, the potential benefits of key micronutrients to neural and cognitive well-being would extend well beyond the critical window of development and encompass the entire life history of an individual.