Electrophysiology whole cell recordings

Human embryonic kidney 293 (HEK-293) cells permanently transfected with full length P2X2 (kindly provided by Drs. Annmarie Surprenant and Dr. Alan North) (P2X2 cells) were transiently transfected with full length Fe65 (P2X2-Fe65 cells) and the properties of the currents under patch clamp configuration were measured.

Current amplitude, desensitization and current recovery

Transfected cells were measured under the patch clamp configuration at a constant membrane potential of -70 mV. Under these conditions, the fast application of 100 µM ATP elicited currents through P2X2 receptors that varied in amplitude from -2 to -15 nA (n=25) for P2X2 cells and from -1.5 to -12 nA (n=20) for P2X2-Fe65 cells. Application of higher concentrations of ATP (250 and 500 μM) elicited currents of the same amplitude, indicating that 100 µM is the supramaximal agonist concentration both for P2X2 and P2X2-Fe65 receptors. Currents obtained did not desensitize during the application period of 1-3 seconds, but the channel did close when the application of ATP was discontinued, both for P2X2 and P2X2-Fe65 receptors. After a washing time of 2 minutes in the absence of the agonist, the current was recovered to 70-100 % of the initial current. Elicited currents from P2X2 and P2X2-Fe65 receptors obtained under three consecutives applications of 100 μM ATP followed by washing steps of 2 min are shown in figure 4.19.

Figure 4.19. Representative currents of P2X2 and P2X2-Fe65 receptors. Whole cell recordings of P2X₂ and P2X2-Fe65 cells. (A) Currents evoked by P2X₂ cells upon application of 100 µM ATP during 1s with 2 min wash steps between each application (B) Currents evoked by P2X₂-Fe65 cells subjected to application of 100 µM ATP during 1s with 2 min wash steps between applications.

Once the initial conditions for recording of the currents were obtained, concentration-response and current-voltage relationships were measured.

Sensitivity to ATP, concentration-response curves.

Response to ATP for P2X2 subunits in the absence and presence of Fe65 (P2X2

receptors and P2X2-Fe65 receptors, respectively) was first studied by applying different concentration of ATP (5, 10, 24, 50, 75 and 100 μM ATP) in order to determine the amount of ATP that elicited 50% of the maximal current obtained at 100 µM ATP, the so-called EC50

value (Figure 4.20).

Figure 4.20. Interaction with Fe65 does not change the current kinetics or the ATP sensitivity of P2X2 receptors. Whole cell recordings of P2X₂ and P2X₂-Fe65 cells. (A) currents evoked by 100 µM ATP on P2X₂ (black) or P2X₂-Fe65 cells (red). (B) concentration-response curves for ATP in P2X₂ (black) or P2X2-Fe65 cells (red). A concentration-response curve was fitted to the data using IgorPro software. The EC50s obtained are 23.7 ± 1.6 and 24.4 ± 0.9 µM, for P2X2 and P2X2-Fe65 respectively.

Hill slopes for P2X₂ is 2.1 ± 0.1 and 1.8 ± 0.2 for P2X₂-Fe65. Data are mean ± S.E.M.

The results revealed equal pharmacological properties between P2X2 receptors alone or in the presence of Fe65. Thus, no differences in ATP EC50 values were found: 23.7 ± 1.6 and 24.4 ± 0.9 µM with Hill slopes of 2.1 ± 0.1 and 1.8 ± 0.2, for P2X2 receptors and P2X2 -Fe65 receptors respectively. In addition, similar current amplitude and kinetics were observed (Figure 4.20). Thus, the formation of the complex between Fe65 and P2X2 does not change the response of the receptor to ATP, its natural ligand.

Current-voltage relationship.

Additionally, measurements of currents-voltage (I-V) relationships of ATP-gated currents were carried out. To determine the I-V relations curves, P2X2 cells or P2X2-Fe65 cells were maintained at a initial holding potential of -70mV and during the 100 µM ATP application, the cells were clamped at various holding potentials from -100 to 80 mV, in 20 mV steps (Figure 4.21B) . This protocol allows the evaluation of the potential where the inward and outward currents through the channel are identical so there is net zero current flowing to the channel. This is the so called reversal potential (Erev) of the current, and depends on the permeability of the channel to the different ions present in the intra- and extracellular solution. The shape of the I-V curve gives an idea on the gating of the channel at the different voltages.

As observed in figure 4.21, both in the presence and absence of Fe65, the P2X2

receptor displays a similar behavior concerning its current-voltage dependence. Only at very negative voltages (-70 to -100 mV), the binding of Fe65 seems to exert a slight decrease (15 to 30 % reduction) in the amount of current that it is being carried through the channel. As expected the obtained reversal potentials in both cases were close to zero, -2.7 ± 0.2 mV (n=12) for P2X2 receptors, and -2.4 ± 2.0 mV (n=10) for the complex P2X2-Fe65, respectively. Notably, both exhibited a similar degree of inward rectification as previously

reported (Evans et al., 1996), supporting the notion that the response to ATP a different holding voltages essentially not affected (Figure 4.21).

Figure 4.21. P2X2 present similar current-voltage relationships in the presence and the absence of Fe65. (A) I-V relation curves from voltage steps (-100 to 80 mV, in 20 mV steps) of P2X₂ receptors (black, top panel) and P2X₂-Fe65 receptors (red, lower panel). On the left, representative currents evoked by one cell when the I-V protocol sowed in (B) was applied. (B) Voltage steps protocol, the cells were voltage-clamped at the indicated holding potential during ATP application.

(C) I-V relation curves in (A) but displayed together for comparison.

The absence of changes in the functional properties so far investigated upon formation of the complex between Fe65 and P2X2, pointed our attention towards a recent discovery on the mechanism by which P2X2 channels undergopermeability changes. It was found that P2X channels open to a small pore in a second or less, and during maintained or repetitive ATP applications, they open a larger pore (Khakh et al., 1999; Virginio et al., 1999). Results suggest that the selectivity filter in these channels is dynamic, and that the channel has to necessary undergo conformational rearrangements to switch its preference between ions.

What was recently observed is that motions on the cytosolic domain of P2X2 may controlion channel permeability, which was indeed prevented by immobilization of the C-terminal domain (Fisher et al., 2004). Consequently it is possible that Fe65 binding restrains the mobility of the P2X2 C-terminus and alters the changes in ionic selectivity of the channel.

In order to measure permeability changes that might indicate a pore dilation, extracellular Na⁺ was substituted by N-methyl-D-glucamine (NMDG⁺), a bulky organic cation which permeates poorly through P2X2 because it is larger than the narrowest part of the channel pore (Evans et al., 1996). It was shown that NMDG⁺ was only uptaked by P2X2

receptors upon prolonged exposure to ATP (Virginio et al., 1999), so we attempted to measure permeability changes on HEK293 cells transfected with P2X2 receptors in the presence or absence of Fe65.

When we attempted to measure NMDG⁺ uptake in HEK293 cells, a significant leak, higher than 25% of the total current, was detected, which made measurements not reliable.

This feature could be attributed to the effect that the absence of Ca⁺² provokes on the homeostasis of the membrane from cultured cells. Unfortunately, this was an important limitation that prompts us to change the method of choice for recording the electrophysiological properties of the channel. Instead of HEK293 cells we had to employ

two electrode voltage-clamp recordings in Xenopus laevis oocytes, which are easier to measure in the absence of extracellular Ca²⁺.