Cryptdin-3 induces novel apical conductance(s) in Cl- secretory, including cystic fibrosis, epithelia

Opening ofanion-conductive pathways in apical membranes of secretory cells liningmucosal surfaces is a critical step in salt and water secretion and,thus, hydration of sites including airway and intestine. In intestine,Paneth cells are positioned at the base of the secretory gland (crypt)and release defensin peptide, in mice termed cryptdins, into the cryptlumen. Because at least some defensins have been shown to formanion-conductive channels in phospholipid bilayers, we tested whetherthese endogenous antimicrobial peptides could act as soluble inducersof channel-like activity when applied to apical membranes. To directlyevaluate the possibility of cryptdin-3-mediated apical anionconductance (G_ap), we have utilized amphotericinB to selectively permeabilize basolateral membranes of electricallytight monolayers of polarized human intestinal secretory epithelia (T84cells), thus isolating the apical membrane for study. Cryptdin-3induces G_ap that is voltage independent(G_ap = 1.90 ± 0.60 mS/cm²) and exhibits ion selectivity contrasting to thatelicited by forskolin or thapsigargin (for cryptdin-3,Cl = gluconate; for forskolin and thapsigargin,Cl gluconate). We cannot exclude the possibility thatthe macroscopic current induced by cryptdin could be the sum of cationand Cl currents. Cryptdin-3 induces a current inbasolaterally permeabilized epithelial monolayers derived from airwaycells harboring the F508 mutation of cystic fibrosis (CF;G_ap = 0.80 ± 0.06 mS/cm²), demonstrating that cryptdin-3 restores anionsecretion in CF cells; this occurs independently of the CFtransmembrane conductance regulator channel. These results support theidea that cryptdin-3 may associate with apical membranes ofCl-secreting epithelia and self-assemble into conductingchannels capable of mediating a physiological response.