Influence of Folic Acid on Neural Connectivity during Dorsal Root Ganglion Neurogenesis

Abstract

The vitamin folic acid (FA) is essential for DNA synthesis, repair and methylation, and for methionine synthesis. Although it is necessary for neural development, recent studies suggest a possible link between excess maternal supplemental FA intake and adverse interferences with single-carbon metabolism and neural development. Insufficient FA early in brain development can lead to failure of the neural tube closure, but the consequences of too much intake have not been fully investigated. Plasma FA concentrations can increase greatly with dietary supplementation. To model the development of neural connectivity, we cultured dorsal root ganglia (DRGs) taken from 8-day-old chick embryos in a range of pteroylmonoglutamate (PteGlu, synthetic supplemental FA) concentrations. DRGs were cultured for 36 h, fixed and immunostained to reveal the locations of neural networks with synaptic vesicles. We found a concentration-dependent relationship with significant reduction in neurite length in PteGlu concentrations from 0.25 to 20 μ<smlcap>M</smlcap>. The average total of stained synaptic areas surrounding each cultured DRG was significantly reduced as well. To further characterize the effects, we carried out time-lapse imaging of growth cones at terminals of extending neurites. We found that PteGlu reduced the area-changing activity of the growth cone, hindering its exploratory capabilities, along with a tendency to inhibit overall advancement, thus altering the ability to extend and form synapses. Our results show that PteGlu at 250 n<smlcap>M</smlcap> and higher reduces neurite extension and synapse formation in a dose-dependent manner during neurogenesis, and that its effect is mediated through inhibition of growth cone motility.

Could folic acid influence growth cone motility during the development of neural connectivity?

“…Rowe and Ruddock 25 also showed that FA competes with glutamate to hyperpolarize retina horizontal cells of fish eyes. Our report that augmenting the glutamate concentration to 5 μM in our dorsal root ganglia cultures could overcome the inhibition of neurite extension caused by 5 μM FA 13 is consistent with this.…”

“…With regard to the inhibitory effects of FA on growth cones that we reported, 13 the role of neurotransmitters and their receptors is potentially relevant. The chemical structure of FA contains the exact structure of glutamate at one end.…”

“… 10-12 Yet the means by which excess FA could affect neural development has been lacking. Our recent study Influence of Folic Acid on Neural Connectivity During Dorsal Root Ganglia Neurogenesis published in Cells Tissues Organs 13 has provided some experimental evidence at the cellular behavior level for a mechanism by which excess FA may alter neurogenesis.…”

“…Our results showed that FA, at concentrations of 250 nM and higher reduces neurite extension and synapse formation in a dose-dependent manner during neurogenesis, and that its effect is likely mediated through inhibition of growth cone motility. 13 Although chick embryo DRG neurons are not human brain cortical neurons, they do form thin, straight neural processes (neurites) that elongate, led by exploratory growth cones. They do establish contact with other neurons to form neural networks.…”

“…FA added to the culture environment at 2.4 µM caused a 22% reduction of growth cone area changing activity (from 19 µm 2 per minute to 15 µm 2 per minute). 13 Clearly, the growth cone's complex infrastructure of actin with associated motility proteins was strongly affected by the presence of the added FA. But in what way was it affected?…”