Title

Electric field-guided neurite growth: A proposed mechanism involving localized membrane depolarizations, localized calcium increases and biased filopodia

Date of Completion

January 1993

Keywords

Biology, Neuroscience|Biology, Cell|Health Sciences, Medicine and Surgery

Degree

Ph.D.

Abstract

Proper function of the nervous system depends upon the patterns of neural connections established through directed neural process (neurite) growth. Many factors may take part in directing neurite growth, including mechanical forces, soluble and substrate-bound chemical gradients and electric fields. The present study addressed electric field-guided neurite growth (neurite galvanotropism); specifically, it tried to elucidate the mechanism underlying this phenomenon. The hypothesis was that the galvanotropic mechanism involved changes in filopodia, intraneuronal calcium concentration ((Ca$\sp{2+}\rbrack \sb{\rm i}$) and cell membrane potential.^ Direct current electric fields were applied to N1E-115 mouse neuroblastoma cells. Phase contrast microscopy was used to demonstrate that the fields directed N1E-115 neurite initiation, elongation, sparing from retraction and turning toward the negative pole (cathode). Differential interference contrast and fluorescence microscopy helped confirm that field-guided N1E-115 growth was accompanied by cathode-biased filopodial protrusions, transient cathode-localized ($\rm Ca\sp{2+}\rbrack \sb{i}$ elevations and persistent cathode-facing plasma membrane depolarizations. Further, temporal and spatial correlations, experiments preventing field-evoked calcium influx and experiments mimicking field-induced depolarizations or field-evoked calcium influx suggested that these newly identified events comprised a connected series which explained N1E-115 galvanotropism. However, transient exposures to fields (for time periods equal to the duration of the observed ($\rm Ca\sp{2+}\rbrack \sb{i}$ elevations) did not result in directed neurite growth.^ A galvanotropic mechanism consistent with the results of this and other studies is proposed. In it, the observed electrostatically-induced cathode-facing membrane depolarizations evoke persistent cathode-localized L-channel activation and persistent calcium influx, thus explaining the observed ($\rm Ca\sp{2+}\rbrack \sb{i}$ elevations. ($\rm Ca\sp{2+}\rbrack \sb{i}$ gradually normalizes in the central region of the cell, but not in the periphery (because of its high surface-to-volume ratio and lack of calcium-buffering organelles). Persistent, cathode-localized ($\rm Ca\sp{2+}\rbrack \sb{i}$ elevations elicit actin polymerization, leading to the observed cathode-biased filopodial activity; this then directs neurite initiation, elongation, sparing from retraction and turning toward the cathode. ^