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Studies on the mechanism of embryonic frog nerve orientation in a small applied electric field
1Department of Physiology, Marischal College, University of Aberdeen, Scotland.
Abstract:
The mechanism of nerve orientation in an applied electric field has been investigated using a number of pharmacological agents. Galvanotropism may depend on redistribution within the plasma membrane of integral membrane proteins (IMP); blocking this with concanavalin A inhibited orientation. Orientation may depend also on an influx of Ca2+; Co2+ and La3+ blockade of calcium channels inhibited turning in an electric field. Organic blockers of calcium channels did not influence orientation, suggesting that L-type Ca2+ channels may not be present at the growth cone. Procedures that may induce asymmetric entry of Ca2+ on the anodal side of cells caused a reversal of normal galvanotropism, with growth directed towards the anode. This may implicate local levels of cytoplasmic Ca2+ within the growth cone in controlling turning behaviour. An asymmetric distribution of filopodia precedes and may predict the direction of nerve growth in an electric field. Various pharmacological agents perturbed the distribution of filopodia in such a way that this did not reflect subsequent orientation. It is suggested that, normally, local Ca2+ increases and an asymmetry of filopodia operate together in determining orientation, but that filopodial activity is subordinate to and can be overriden by local Ca2+ levels in the growth cone. In addition, two of the drug treatments markedly increased rates of nerve growth, which may be of importance in nerve regeneration.
Insights
Nerve cells orient towards an electric field through calcium ion influx and integral membrane protein redistribution. Local calcium levels and filopodia dynamics control nerve growth direction, impacting nerve regeneration.
Area of Science:
- Neuroscience
- Cell Biology
- Biophysics
Background:
- Nerve cells exhibit galvanotropism, orienting in response to electric fields.
- The precise molecular mechanisms underlying this directional growth are not fully understood.
Purpose of the Study:
- To investigate the roles of integral membrane proteins (IMPs) and calcium ions (Ca2+) in nerve cell galvanotropism.
- To elucidate the signaling pathways controlling growth cone turning behavior in electric fields.
Main Methods:
- Utilized pharmacological agents to block IMP redistribution and calcium channels.
- Applied electric fields to cultured nerve cells and observed orientation responses.
- Manipulated calcium entry to induce asymmetric signaling and reversed galvanotropism.
Main Results:
- Blocking IMPs with concanavalin A inhibited orientation.
- Ca2+ channel blockers (Co2+, La3+) inhibited turning, while organic blockers did not, suggesting non-L-type channels are involved.
- Asymmetric Ca2+ entry reversed normal galvanotropism, indicating local cytoplasmic Ca2+ levels control turning.
- Filopodial asymmetry, while often predictive, was overridden by Ca2+ signaling.
- Certain drug treatments enhanced nerve growth rates.
Conclusions:
- Nerve orientation in electric fields depends on both IMP redistribution and Ca2+ influx.
- Local Ca2+ levels within the growth cone are critical for controlling turning direction.
- Filopodial dynamics are subordinate to Ca2+ signaling in galvanotropism.
- Findings suggest potential therapeutic strategies for enhancing nerve regeneration.