Role of cortical filamentous actin in the melanotrope cell of Xenopus laevis.
In secretory cells filamentous actin (f-actin) is mostly present subjacent to the plasma membrane, referred to as cortical actin. While the function of cortical actin in the secretory processes has been extensively studied, little attention has been given to the role of actin in signal transduction and intracellular second messenger dynamics. Analysis with the fluorescent f-actin probe Alexa-phalloidin shows that Xenopus laevis pituitary melanotrope cells possess a thick cortical actin ring. This cell is a good model to study the possible function(s) of f-actin in signal transduction processes. Regulation of the release of alpha-MSH from this cell involves a convergence of various receptor mechanisms to regulate the activity of voltage-operated Ca2+ channels. We have considered three potential functions for the cortical actin ring in the signaling process of the melanotrope: (1) it functions as a barrier for access of secretory granules to the membrane for exocytosis, (2) it is involved in anchoring components of the Ca2+ signalling machinery of the cell, and/or (3) it helps to form a scaffold for components of the signal transduction machinery used by the various neurotransmitters and neuropeptides that regulate the activity of the cell. To test these possibilities we have examined the effect of the f-actin depolymerising toxin latrunculin B on Ca2+ signaling, signal transduction and alpha-MSH secretion in the melanotrope. We show that while the toxin is effective in disrupting the cortical actin ring, this treatment has no effect on either Ca2+ signaling or the signal transduction processes studied. The toxin does induce an increase in alpha-MSH release, indicating that the cortical actin ring acts as a barrier for secretory granule access to the membrane.
D000199:Actins / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D003599:Cytoskeleton / Q000502:physiology; D009490:Neurosecretory Systems / Q000502:physiology; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D014982:Xenopus laevis / Q000502:physiology*; D000521:alpha-MSH / Q000378:metabolism*
D000199:Actins; D000521:alpha-MSH; D002118:Calcium