Immunolabel imaging of plasma membrane Ca++ pumps (PMCA-pan or PMCA2) implicates the apical cilium on receptor cells as the transduction site for electrosense in two Acipenseriformes fishes

Specialized receptor cells in ampulla of Lorenzini-like electroreceptors enable Acipenseriformes fishes to detect microvolt signals from aquatic prey. Candidate subcellular sites of electrosensory transduction include each receptor cell's apical face membrane, or its modified primary cilium, or sparse apical microvilli. We aimed to more precisely localize the apical transduction. Given its reported Ca⁺⁺ dependence, as proxy we immunolabeled electrosensory epithelia for Ca⁺⁺ ion pumps (PMCA: plasma membrane calcium ATPase), in frozen sections of ampullary organs from rostrum skin of freshwater paddlefish (Polyodon spathula) and shovelnose sturgeon (Scaphirhynchus platorynchus). We trialed two anti-PMCA antibodies: pan clone 5F10, or polyclonal anti-PMCA2. Most somatic plasma membrane of receptor cells was diffusely 5F10⁺, with dispersed bright punctate domains. Both anti-PMCA antibodies labeled the apical cilium on receptor cells, along most cilium length to a conical tip. Patterning of ciliary PMCA⁺ label varied: it was continuous on some shafts, or showed regional gradation with cilium length, or shafts could be banded or helically striped at 0.5-1 micron helix pitch. PMCA⁺ cilia had consistent cylindrical shape and dimensions in paddlefish: 5.6 micron total length, and 0.7-1 micron shaft OD (∼3x wider than other vertebrate primary cilia). By contrast, the apical face membrane of receptor cells expressed faint or nil 5F10⁺ label, imaged as a dark annulus encircled by zona occludens (anti-ZO1⁺). Microvilli were often absent apically. Marked expression of PMCAs on the apical cilium of receptor cells indicated Ca-dependent signaling there, consistent with ciliary transduction for electrosense in Acipenseriformes fishes.