Ion pumps establish homeostasis across the membranes of living cells. Hearing is a mechanotransduction event that takes place in a closed compartment containing a fluid high in K+ concentrations. In Drosophila melanogaster, this closed compartment is formed by a scolopale cell that wraps around the dendrite of sensory neurons. The receptor lymph is maintained by the scolopale cell. The lumenal membrane of the scolopale cell is the wall of the compartment containing the receptor lymph, the scolopale space. The ablumenal membrane of the scolopale cell creates the border of the scolopidium.
The Na/K pump is located on the ablumenal membrane of the scolopale cell, bringing K+ into the scolopale cell cytoplasm and extruding K electrogenically (Roy et al, 2013). We explored other primary and secondary ion pumps that are involved in creating a K+-rich lumen in the Malpighian tubule (Day et al, 2008; Rodan et al, 2012). We used RNAi technology to knockdown one gene at a time and electrophysiology to measure a sound evoked potential (SEP) that reflects the fly’s ability to hear.
We found that knocking down V-ATPase, a proton pump, subunits involved in proton extrusion significantly reduces the SEP of knockdown flies. The involvement of cation chloride cotransporters (CCCs) and cation proton antiporter (CPAs), both secondary ion pumps that use the gradients created by the Na/K pump and V-ATPase respectively to pump other ions up their gradient, is less clear. We found that knocking down Nhe3, a CPA, significantly reduced the SEP when knocked down in the scolopale cell, suggesting it as a partner to the V-ATPase. Knocking down CG31547, a CCC, statistically increased the SEP, possibly a type1 statistical error.