A simple intrapipette salt bridge

The native TRPP2-dependent channel of murine renal primary cilia

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening monogenic renal disease. ADPKD results from mutations in either of two proteins: polycystin-1 (also known as PC1 or PKD1) or transient receptor potential cation channel, subfamily P, member 2 (TRPP2, also known as polycystin-2, PC2, or PKD2). Each of these proteins is expressed in the primary cilium that extends from many renal epithelial cells. Existing evidence suggests that the cilium can promote renal cystogenesis, while PC1 and TRPP2 counter this cystogenic effect. To better understand the function of TRPP2, we investigated its electrophysiological properties in the native ciliary membrane. We recorded directly from the cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin. In one-third of cilia examined, a large-conductance channel was observed. The channel was not permeable to Cl¯ but conducted cations with permeability ratios P_K:P_Ca:P_Na of 1:0.55:0.14. The single-channel conductance ranged from 97 pS in typical physiological solutions to 189 pS in symmetrical 145 mM KCl. Open probability of the channel was very sensitive to membrane depolarization or increasing cytoplasmic free Ca²⁺ in the low micromolar range, with the open probability increasing in either case. Knocking out TRPP2 by CRISPR/Cas9 genome editing eliminated the channel current, establishing it as TRPP2 dependent. Possible mechanisms for activating the TRPP2-dependent channel in the renal primary cilium are discussed.

A TRPM4-dependent current in murine renal primary cilia

Defects in primary cilia lead to a variety of human diseases. One of these, polycystic kidney disease, can be caused by defects in a Ca²⁺-gated ion channel (TRPP2) found on the cilium. Other ciliary functions also contribute to cystogenesis, and defects in apical Ca²⁺ homeostasis have been implicated. By recording directly from the native cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin, we have identified a second Ca²⁺-gated channel in the ciliary membrane: the transient receptor potential cation channel, subfamily M, member 4 (TRPM4). In excised primary cilia, TRPM4 was found to have a low sensitivity to Ca²⁺, with an EC₅₀ of 646 μM at +100 mV. It was inhibited by MgATP and by 9-phenanthrol. The channel was not permeable to Ca²⁺ or Cl⁻ and had a permeability ratio PK/PNa of 1.42. Reducing the expression of Trpm4 mRNA with short hairpin (sh) RNA reduced the TRPM4 current by 87% and shortened primary cilia by 43%. When phospholipase C was inhibited, the sensitivity to cytoplasmic Ca²⁺ greatly increased (EC₅₀ = 26 μM at +100 mV), which is consistent with previous reports that phosphatidylinositol 4,5-bisphosphate (PIP2) modulates the channel. MgATP did not restore the channel to a preinactivation state, suggesting that the enzyme or substrate necessary for making PIP2 is not abundant in primary cilia of mIMCD-3 cells. The function of TRPM4 in renal primary cilia is not yet known, but it is likely to influence the apical Ca²⁺ dynamics of the cell, perhaps in tandem with TRPP2.

Micro-agar salt bridge in patch-clamp electrode holder stabilizes electrode potentials

Maintaining a stable electrode potential is critical for patch-clamp measurements. The electrode potential of conventional patch electrode-holder assembly, where an Ag/AgCl wire is in direct contact with the patch pipette filling solution, is subject to drift if the pipette solution contains a low concentration of chloride ions (Cl-). We developed an agar bridge of 3 M KCl filled in a polyimide microtubing which forms an electrical connection between an Ag/AgCl wire and the pipette solution. We examined the offset potentials of the micro-agar salt bridge electrode assembly in parallel with a conventional electrode assembly in generic recording conditions (the pipette solution contained 5 mM NaCl). The junction potential between the Ag/AgCl wire and the pipette filling solution in the conventional electrode contributed to most of the offset potential drift observed during the course of 30 min recordings. The drift was up to 27.3 mV after several changes of the glass pipette. In contrast, the micro-agar salt bridge stabilized the electrode potential within typically 2 mV without affecting the patch electrode resistance, capacitance or noise level. Numerical simulations showed that Cl- diffusion from the agar bridge to the tip caused a negligible 0.4 microM Cl- concentration change at the pipette tip within 30 min. This method is easy to implement and provides long-term recording stability. The micro-agar salt bridge can fit in most commercial patch electrode holders and can be conveniently maintained.

Both external and internal calcium reduce the sensitivity of the olfactory cyclic-nucleotide-gated channel to CAMP

In vertebrate olfaction, odorous stimuli are first transduced into an electrical signal in the cilia of olfactory receptor neurons. Many odorants cause an increase in ciliary cAMP, which gates cationic channels in the ciliary membrane. The resulting influx of Ca2+ and Na+ produces a depolarizing receptor current. Modulation of the cyclic-nucleotide-gated (CNG) channels is one mechanism of adjusting olfactory sensitivity. Modulation of these channels by divalent cations was studied by patch-clamp recording from single cilia of frog olfactory receptor neurons. In accord with previous reports, it was found that cytoplasmic Ca2+ above 1 microM made the channels less sensitive to cAMP. The effect of cytoplasmic Ca2+ was eliminated by holding the cilium in a divalent-free cytoplasmic solution and was restored by adding calmodulin (CaM). An unexpected result was that external Ca2+ could also greatly reduce the sensitivity of the channels to cAMP. This reduction was seen when external Ca2+ exceeded 30 microM and was not affected by the divalent-free solution, by CaM, or by Ca2+ buffering. The effects of cytoplasmic and external Ca2+ were additive. Thus the effects of cytoplasmic and external Ca2+ are apparently mediated by different mechanisms. There was no effect of CaM on a Ca2+-activated Cl- current that also contributes to the receptor current. Increases in Ca2+ concentration on either side of the ciliary membrane may influence olfactory adaptation.

The cyclic nucleotide-activated conductance in olfactory cilia: effects of cytoplasmic Mg2+ and Ca2+

Olfactory receptor neurons depolarize in response to odorants. This depolarization is mediated by an increase in intracellular cyclic AMP, which directly gates channels in the membranes of the neuronal cilia. Previous evidence suggests that a Ca2+ influx during the odorant response may ultimately play a role in terminating the response. One way Ca2+ inside the cell could terminate the odorant response would be to directly inhibit the cAMP-gated channels. In this report the effects of cytoplasmic Ca2+ and Mg2+ on the cAMP-activated current were measured in single olfactory cilia. Near the neuronal resting potential, cytoplasmic Ca2+ and Mg2+ only slightly reduced the cAMP-activated current. Even at high levels (1.0 mM Ca2+ or 5.0 mM Mg2+), the average inhibition was only around 20%. It is therefore unlikely that an influx of divalent cations terminates the odorant response by a direct effect on the cAMP-gated channels.