Heteromultimers of DEG/ENaC subunits form H+-gated channels in mouse sensory neurons

Abstract

Acidic extracellular solution activates transient H(+)-gated currents in dorsal root ganglion (DRG) neurons. The biophysical properties of three degenerin/epithelial sodium (DEG/ENaC) channel subunits (BNC1, ASIC, and DRASIC), and their expression in DRG, suggest that they might underlie these H(+)-gated currents and function as sensory transducers. However, it is uncertain which of these DEG/ENaC subunits generate the currents, and whether they function as homomultimers or heteromultimers. We found that the biophysical properties of transient H(+)-gated currents from medium to large mouse DRG neurons differed from BNC1, ASIC, or DRASIC expressed individually, but were reproduced by coexpression of the subunits together. To test the contribution of each subunit, we studied DRG from three strains of mice, each bearing a targeted disruption of BNC1, ASIC, or DRASIC. Deletion of any one subunit did not abolish H(+)-gated currents, but altered currents in a manner consistent with heteromultimerization of the two remaining subunits. These data indicate that combinations of two or more DEG/ENaC subunits coassemble as heteromultimers to generate transient H(+)-gated currents in mouse DRG neurons.

Figure 1

Properties of acid-evoked currents in DRG neurons are not matched by any one DEG/ENaC subunit. (A) Representative acid-evoked currents in a wild-type DRG neuron and COS-7 cells expressing the indicated DEG/ENaC subunits. The bar above each current represents a fast solution change from pH 7.4 to pH 6, except for BNC1a, which is evoked by pH 5 (because of the relative pH insensitivity of BNC1a channels). Vertical scale bar: 2.71 nA DRG, 1.23 nA DRASIC, 1.78 nA ASICβ, 0.943 nA ASICα, 3 nA BNC1a. (B) Effect of pH on currents from DRG and COS cells expressing the indicated subunits from a holding pH of 8. Currents were normalized to those evoked by pH 5 (except BNC1a, which was normalized to pH 4 current). Lines are fit of Hill equation. n = at least four cells for all data points. (C) Time course of recovery from desensitization. Current was completely desensitized by a prolonged pulse to pH 6 (pH 4 for BNC1a). Cells then were bathed in pH 7.4 solution for the indicated times before they were stimulated again with a pH 6 (pH 4) solution. Recovery is percentage of current evoked by the second pulse compared with the first. Lines are fit of single exponentials. n = at least three cells for each data point. (D) Superimposed currents evoked by a solution change from pH 7.4 to pH 6 (pH 4 for BNC1a). Vertical scale bar: 1.27 nA DRG, 0.36 nA DRASIC, 1 nA ASICα, 4.95 nA BNC1a. (E) Mean time constants of desensitization as measured from single exponential fits to the falling phase of the currents evoked by pH 6 (pH 4 for BNC1a) application to wild-type DRG neurons (n = 29) and COS-7 cells expressing indicated subunits (n = at least five cells).

Figure 2

Coexpression of DRASIC, ASICα, and BNC1a mimic the properties of the pH-evoked current in wild-type DRG neurons. (A) Currents evoked by application of pH 6 from pH 7.4 solutions in a DRG neuron, and a COS-7 cell coexpressing DRASIC, ASICα, and BNC1a. Vertical scale bar: 1.32 nA DRG, 0.5 nA COS-7. (B) Mean time constant of desensitization of the current evoked by pH 6 application to COS-7 cells expressing all three subunits (n = 6; other bars represent data collected in Fig. 1 for comparison). (C) pH dose-response data for currents evoked from COS-7 cells expressing all three subunits (n = at least 4 for all data points) compared with wild-type DRG neurons. (D) Recovery from desensitization data from COS-7 cells expressing all three subunits (n = at least six cells for all data points) compared with wild-type DRG neurons. All data were collected and fit as per Fig. 1.

Figure 3

Targeted disruption of BNC1 gene alters pH-evoked current in DRG neurons in a manner consistent with expression of the remaining subunits. (A) Representative currents evoked by changing the solution from pH 7.4 to pH 6 in wild-type and BNC1-null DRG neurons demonstrating similar desensitization rates. Twenty-six percent of BNC1-null DRG (n = 82) responded to pH 5 with greater than 150 pA transient current, compared with 59% of wild type (n = 112). Vertical scale bar: 1 nA wild type, 0.38 nA BNC1 null. (B) pH dose-response and (C) recovery from desensitization data for BNC1-null DRG neurons (n = at least eight cells for all data points) compared with wild type. (D) Superimposed currents evoked by a solution change from pH 7.4 to pH 6 in a BNC1-null DRG neuron, and COS-7 cells expressing indicated subunits demonstrating that only coexpression of subunits reproduced the fast kinetics of the BNC1-null neuron. Vertical scale bar: 1.12 nA BNC1-null neuron, 0.283 nA DRASIC, 0.787 nA ASICα, 1 nA DRASIC + ASICα. (E) pH dose-response data and (F) recovery from desensitization data for DRASIC and ASICα coexpressed in COS-7 cells (n = at least six cells for all data points). Also shown are the BNC1-null DRG data from B and C and the fits of the data for subunits expressed individually for comparison. Data were collected and fit as per Fig. 1.

Figure 4

Targeted disruption of the ASIC gene alters pH-evoked current in DRG neurons in a manner predicted by expression of the remaining subunits. (A) Representative currents evoked by a solution change from pH 7.4 to pH 6 in wild-type and ASIC-null DRG neurons demonstrating slightly slower desensitization of the ASIC-null neurons. Forty-eight percent of ASIC-null DRG (n = 48) responded to pH 5 with greater than 150 pA transient current. Vertical scale bar: 1 nA wild type, 0.47 nA ASIC null. (B) pH dose-response and (C) recovery from desensitization data for ASIC-null DRG neurons (n = at least 12 cells for all data points) compared with wild type. (D) Currents evoked by application of pH 6 from pH 7.4 in an ASIC-null DRG neuron and COS-7 cells expressing indicated subunits demonstrates that only coexpression of subunits reproduced the fast kinetics of the ASIC-null DRG. Vertical scale bar: 0.33 nA ASIC-null neuron, 0.4 nA DRASIC, 5.5 nA BNC1a, 0.41 nA DRASIC + BNC1a. (E) pH dose-response data and (F) recovery from desensitization data for DRASIC and BNC1a coexpressed in COS-7 cells (n = at least seven cells for all data points). Also shown for comparison are the ASIC-null DRG data from B and C and the fits of the data for subunits expressed individually. Data were collected and fit as per Fig. 1.

Figure 5

Deletion of the DRASIC gene slows the desensitization kinetics of pH-evoked current in DRG neurons. (A) Superimposed currents evoked by a solution change from pH 7.4 to pH 5 in a wild-type and DRASIC-null DRG neurons, and a COS-7 cell coexpressing ASICα and BNC1a. Vertical scale bar: 4 nA wild-type DRG, 2.2 nA DRASIC-null DRG, 1.39 nA COS-7 cell. (B) Mean time constants of desensitization of the acid-evoked currents from wild-type and knockout DRG neurons (n = at least 13 cells), and DEG/ENaC subunits expressed individually (n = at least five cells), and coexpressed in COS-7 cells (n = at least six cells) demonstrates that only DRASIC coexpressed with other subunits reproduced the fast desensitization kinetics of the DRG neurons. Currents were evoked by pH 6, except BNC1a currents, which were evoked by pH 4.