Jeworutzki E et al. (2014), GlialCAM, a CLC-2 Cl(-) channel subunit, activa...

XB-ART-51239

Biophys J 2014 Sep 02;1075:1105-1116. doi: 10.1016/j.bpj.2014.07.040.

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GlialCAM, a CLC-2 Cl(-) channel subunit, activates the slow gate of CLC chloride channels.

Jeworutzki E , Lagostena L , Elorza-Vidal X , López-Hernández T , Estévez R , Pusch M .

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GlialCAM, a glial cell adhesion molecule mutated in megalencephalic leukoencephalopathy with subcortical cysts, targets the CLC-2 Cl(-) channel to cell contacts in glia and activates CLC-2 currents in vitro and in vivo. We found that GlialCAM clusters all CLC channels at cell contacts in vitro and thus studied GlialCAM interaction with CLC channels to investigate the mechanism of functional activation. GlialCAM slowed deactivation kinetics of CLC-Ka/barttin channels and increased CLC-0 currents opening the common gate and slowing its deactivation. No functional effect was seen for common gate deficient CLC-0 mutants. Similarly, GlialCAM targets the common gate deficient CLC-2 mutant E211V/H816A to cell contacts, without altering its function. Thus, GlialCAM is able to interact with all CLC channels tested, targeting them to cell junctions and activating them by stabilizing the open configuration of the common gate. These results are important to better understand the physiological role of GlialCAM/CLC-2 interaction.

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Species referenced: Xenopus laevis
Genes referenced: bsnd clcn5

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	Figure 1. Interaction of GlialCAM with CLC-1. (A–D) Cellular distribution of CLC-1 in HeLa cells. CLC-1 alone is located uniformly in the plasma membrane and intracellularly (A, B) whereas GlialCAM changes CLC-1 localization when both proteins are co-transfected to regions of direct contact (C, arrow) or to cell-cell contact processes (D, arrow). (E–H) GlialCAM activates the slow gate of CLC-1 (inset: protocol to determine the open probability of the slow gate; see Methods). (E, F) typical currents from a patch from oocytes expressing CLC-1 alone (E) or CLC-1 with GlialCAM (F). (G) Initial (Io) tail currents from the patches shown were described with a Boltzmann function (plus offset) and the open probabilities of the example patches are plotted against voltage. The relative offset of the slow gate (H) is significantly larger with GlialCAM (∗∗p < 0.01, Student’s t-test, values are mean ±SE). Voltages of half-maximal activation were not significantly different (data not shown). To see this figure in color, go online.
	Figure 2. hCLC-Ka/Barttin mediated currents are modulated by GlialCAM. (A) Pulse protocol. (B–D) Typical currents from oocytes injected with CLC-Ka and GlialCAM (B), CLC-Ka and Barttin (C), or CLC-Ka, Barttin, and GlialCAM (D). Amount of cRNA for each construct was kept constant in the different co-injections. In (C) and (D) same scale bars as in (B). From the tail current after the –140 mV prepulse, the initial current (Imax), shown in (E) and the steady state current (Iss), shown in (F) were determined. A double exponential fit to this tail current yielded time constants and coefficients of the two exponential components (see Methods). Fast and slow time constants were for CLC-Ka/Barttin 43 ± 11 ms and 6.8 ± 1 ms, for CLC-Ka/Barttin/GlialCAM 36 ± 1 ms and 8.7 ± 0.3 ms, respectively (n = 10), i.e., not significantly different. (G) Ratio of slow (as) and fast coefficient (af) of the double exponential: the weight of the slow component is significantly increased by GlialCAM. (∗∗∗p < 0.001, Student’s t-test), values are mean ± SE. (H–I) Cellular distribution of CLC-K1 in HeLa cells. Barttin-CLC-K1 alone is located uniformly in the plasma membrane and intracellularly (H) whereas GlialCAM leads to CLC-K1/barttin localization in regions of cell-cell contacts (I, arrow). To see this figure in color, go online.
	Figure 3. Co-expression of GlialCAM with CLC-5 in HeLa cells. (A). Cellular distribution of CLC-5 alone. (B). GlialCAM does not modify CLC-5 localization when both proteins are co-transfected. To see this figure in color, go online.
	Figure 4. GlialCAM activates the slow gate of CLC-0. (A) Pulse protocol used to assay the slow gate. (B, C) Typical current traces obtained with this protocol for CLC-0 (B) and CLC-0/GlialCAM (C). (D) Example tail currents from the current traces shown in (B) and (C) plotted as a function of the prepulse voltage and fitted with a Boltzmann function with offset (lines). (E) Maximal current at 40 mV obtained from the Boltzmann analysis (F) Relative offset obtained from the Boltzmann analysis (∗∗∗p < 0.001, Student’s t-test). (G) GlialCAM-induced clustering of CLC-0-GFP in HEK293 cells. (H) Dose-response of increasing concentration of GlialCAM on relative offset of the slow gate. Amount of CLC-0 RNA was 0.25 ng/oocyte. Data were fitted by a “Boltzmann equation” resulting in a half maximal amount of 0.25 ng/oocyte GlialCAM RNA (qualitatively similar results were seen in a total of three batches of oocytes). To see this figure in color, go online.
	Figure 5. Temperature sensitivity of the deactivation kinetics of the slow gate. (A) Current decay at resting conditions, monitored by brief pulses every 2 s to 40 mV, after a long hyperpolarizing pulse (T = 33°C for this example). (B, C) Current deactivation was described by a single exponential function yielding the time constant (B) and the percentage of deactivation (C) as a function of temperature. Due to the very slow deactivation of CLC-0/GlialCAM at lower temperatures, it was not possible to obtain a reliable value for the time constant (n = 4 to 7 for each temperature from a total of 5 to 10 different oocyte batches; error bars represent SE).
	Figure 6. GlialCAM reduces Zn2+ sensitivity of CLC-0. (A) CLC-0 (closed circles) and CLC-0/GlialCAM (open circles) were activated by a long hyperpolarizing pulse and then deactivation at the resting voltage was monitored by brief pulses to 60 mV. 0.2 mM Zn2+ was applied at the indicated time point. At the end of the experiment, Cl- was replaced by iodide (gray arrow) to estimate leak currents. (B) Percentage of block by 0.2 mM Zn2+ of CLC-0 ± GlialCAM (n ≥ 4 different oocytes ± SE each; ∗∗∗∗p < 0.0001, Student’s t-test).
	Figure 7. Effect of GlialCAM on currents of CLC-0 gating mutants. (A) Voltage pulse that was repetitively applied to maximally activate the slow gate. (B) Currents from an uninjected control oocyte. (C) Example traces from a CLC-0 expressing oocyte before and after maximal activation of the slow gate. (D–H) Similar examples for CLC-0+GlialCAM (D), C212S (E), C212S+GlialCAM (F), E166C/C212S (G), and E166C/C212S+GlialCAM (H). (I) Maximal currents of the indicated constructs for one batch of oocytes, similar results were seen in four batches (∗p < 0.05, Bonferroni’s test). (J) Imin/Imax for the indicated constructs, representing the ratio of the currents at 60 mV before and after repetitive application of the pulse shown in (A) (n > 6 oocytes for each construct).
	Figure 8. CLC-2 double gating mutant E211V/H816A expressed in HEK cells and oocytes. (A, B) Typical current traces obtained by a voltage pulse to 60/–120/60 mV in oocytes (A) and HEK293 cells (B) for the double mutant without (top) or with (bottom) GlialCAM. (C) Fluorescence of HEK cells expressing the E211V/H816A mutant C-terminally fused to GFP without (top) or with (bottom) GlialCAM. (D) Imax at 60 mV was slightly increased for E211VH816A when co-expressing GlialCAM in oocytes (∗p < 0.05, Bonferroni’s test). Note, that in patch clamp experiments of HEK cells GlialCAM does not increase the currents of the double mutant. (E) In contrast to CLC-2 co-expressing the double gating mutant with GlialCAM does not alter its current characteristics as assayed by the ratio of steady state currents and Imax at 60 mV (n ≥ 6 cells from two different transfections/injections; error represents SEM). To see this figure in color, go online.

References [+] :

Accardi, Fast and slow gating relaxations in the muscle chloride channel CLC-1. 2000, Pubmed, Xenbase