Glycosylation of β1 subunit plays a pivotal role in the toxin sensitivity and activation of BK channels

Background:

The accessory β1 subunits, regulating the pharmacological and biophysical properties of BK channels, always undergo post-translational modifications, especially glycosylation. To date, it remains elusive whether the glycosylation contributes to the regulation of BK channels by β1 subunits.

Methods:

Herein, we combined the electrophysiological approach with molecular mutations and biochemical manipulation to investigate the function roles of N-glycosylation in β1 subunits.

Results:

The results show that deglycosylation of β1 subunits through double-site mutations (β1 N80A/N142A or β1 N80Q/N142Q) could significantly increase the inhibitory potency of iberiotoxin, a specific BK channel blocker. The deglycosylated channels also have a different sensitivity to martentoxin, another BK channel modulator with some remarkable effects as reported before. On the contrary to enhancing effects of martentoxin on glycosylated BK channels under the presence of cytoplasmic Ca²⁺, deglycosylated channels were not affected by the toxin. However, the deglycosylated channels were surprisingly inhibited by martentoxin under the absence of cytoplasmic Ca²⁺, while the glycosylated channels were not inhibited under this same condition. In addition, wild type BK (α+β1) channels treated with PNGase F also showed the same trend of pharmacological results to the mutants. Similar to this modulation of glycosylation on BK channel pharmacology, the deglycosylated forms of the channels were activated at a faster speed than the glycosylated ones. However, the V_1/2 and slope were not changed by the glycosylation.

Conclusion:

The present study reveals that glycosylation is an indispensable determinant of the modulation of β1-subunit on BK channel pharmacology and its activation. The loss of glycosylation of β1 subunits could lead to the dysfunction of BK channel, resulting in a pathological state.

Mutations in NaV1.5 Reveal Calcium-Calmodulin Regulation of Sodium Channel

Mutations in the SCN5A gene, encoding the cardiac voltage-gated sodium channel Na_V1.5, are associated with inherited cardiac arrhythmia and conduction disease. Ca²⁺-dependent mechanisms and the involvement of β-subunit (Na_Vβ) in Na_V1.5 regulation are not fully understood. A patient with severe sinus-bradycardia and cardiac conduction-disease was genetically evaluated and compound heterozygosity in the SCN5A gene was found. Mutations were identified in the cytoplasmic DIII-IV linker (K1493del) and the C-terminus (A1924T) of Na_V1.5, both are putative CaM-binding domains. These mutants were functionally studied in human embryonic kidney (HEK) cells and HL-1 cells using whole-cell patch clamp technique. Calmodulin (CaM) interaction and cell-surface expression of heterologously expressed Na_V1.5 mutants were studied by pull-down and biotinylation assays. The mutation K1493del rendered Na_V1.5 non-conductive. Na_V1.5_K1493del altered the gating properties of co-expressed functional Na_V1.5, in a Ca²⁺ and Na_Vβ1-dependent manner. Na_V1.5_A1924T impaired Na_Vβ1-dependent gating regulation. Ca²⁺-dependent CaM-interaction with Na_V1.5 was blunted in Na_V1.5_K1493del. Electrical charge substitution at position 1493 did not affect CaM-interaction and channel functionality. Arrhythmia and conduction-disease -associated mutations revealed Ca²⁺-dependent gating regulation of Na_V1.5 channels. Our results highlight the role of Na_V1.5 DIII-IV linker in the CaM-binding complex and channel function, and suggest that the Ca²⁺-sensing machinery of Na_V1.5 involves Na_Vβ1.

Chronic Prenatal Hypoxia Down-Regulated BK Channel Β1 Subunits in Mesenteric Artery Smooth Muscle Cells of the Offspring

BACKGROUND/AIMS

Chronic hypoxia in utero could impair vascular functions in the offspring, underlying mechanisms are unclear. This study investigated functional alteration in large-conductance Ca2+-activated K+ (BK) channels in offspring mesenteric arteries following prenatal hypoxia.

METHODS

Pregnant rats were exposed to normoxic control (21% O2, Con) or hypoxic (10.5% O2, Hy) conditions from gestational day 5 to 21, their 7-month-old adult male offspring were tested for blood pressure, vascular BK channel functions and expression using patch clamp and wire myograh technique, western blotting, and qRT-PCR.

RESULTS

Prenatal hypoxia increased pressor responses and vasoconstrictions to phenylephrine in the offspring. Whole-cell currents density of BK channels and amplitude of spontaneous transient outward currents (STOCs), not the frequency, were significantly reduced in Hy vascular myocytes. The sensitivity of BK channels to voltage, Ca2+, and tamoxifen were reduced in Hy myocytes, whereas the number of channels per patch and the single-channel conductance were unchanged. Prenatal hypoxia impaired NS1102- and tamoxifen-mediated relaxation in mesenteric arteries precontracted with phenylephrine in the presence of Nω-nitro-L-arginine methyl ester. The mRNA and protein expression of BK channel β1, not the α-subunit, was decreased in Hy mesenteric arteries.

CONCLUSIONS

Impaired BK channel β1-subunits in vascular smooth muscle cells contributed to vascular dysfunction in the offspring exposed to prenatal hypoxia.

Effects of 4,9-anhydrotetrodotoxin on voltage-gated Na+ channels of mouse vas deferens myocytes and recombinant NaV1.6 channels

Molecular investigations were performed in order to determine the major characteristics of voltage-gated Na⁺ channel β-subunits in mouse vas deferens. The use of real-time quantitative PCR showed that the expression of Scn1b was significantly higher than that of other β-subunit genes (Scn2b - Scn4b). Immunoreactivity of Scn1b proteins was also detected in the inner circular and outer longitudinal smooth muscle of mouse vas deferens. In whole-cell recordings, the actions of 4,9-anhydroTTX on voltage-gated Na⁺ current peak amplitude in myocytes (i.e., native I_Na) were compared with its inhibitory potency on recombinant Na_V1.6 channels (expressed in HEK293 cells). A depolarizing rectangular voltage-pulse elicited a fast and transient inward native I_Na and recombinant Na_V1.6 expressed in HEK293 cells (i.e., recombinant I_Na). The current decay of native I_Na was similar to the recombinant Na_V1.6 current co-expressed with β₁-subunits. The current-voltage (I-V) relationships of native I_Na were similar to those of recombinant Na_V1.6 currents co-expressed with β₁-subunits. Application of 4,9-anhydroTTX inhibited the peak amplitude of native I_Na (K _i  = 510 nM), recombinant I_Na (K _i  = 112 nM), and recombinant I_Na co-expressed with β₁-subunits (K _i  = 92 nM). The half-maximal (V_half) activation and inactivation of native I_Na values were similar to those observed in recombinant I_Na co-expressed with β₁-subunits. These results suggest that β₁-subunit proteins are likely to be expressed mainly in the smooth muscle layers of murine vas deferens and that 4,9-anhydroTTX inhibited not only native I_Na but also recombinant I_Na and recombinant I_Na co-expressed with β₁-subunits in a concentration-dependent manner.

β1-subunit-induced structural rearrangements of the Ca2+- and voltage-activated K+ (BK) channel

Large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels are involved in a large variety of physiological processes. Regulatory β-subunits are one of the mechanisms responsible for creating BK channel diversity fundamental to the adequate function of many tissues. However, little is known about the structure of its voltage sensor domain. Here, we present the external architectural details of BK channels using lanthanide-based resonance energy transfer (LRET). We used a genetically encoded lanthanide-binding tag (LBT) to bind terbium as a LRET donor and a fluorophore-labeled iberiotoxin as the LRET acceptor for measurements of distances within the BK channel structure in a living cell. By introducing LBTs in the extracellular region of the α- or β1-subunit, we determined (i) a basic extracellular map of the BK channel, (ii) β1-subunit-induced rearrangements of the voltage sensor in α-subunits, and (iii) the relative position of the β1-subunit within the α/β1-subunit complex.

On the multiple roles of the voltage gated sodium channel β1 subunit in genetic diseases

Voltage-gated sodium channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are composed of a pore-forming α-subunit and associated β-subunits. The β1-subunit was the first accessory subunit to be cloned. It can be important for controlling cell excitability and modulating multiple aspects of sodium channel physiology. Mutations of β1 are implicated in a wide variety of inherited pathologies, including epilepsy and cardiac conduction diseases. This review summarizes β1-subunit related channelopathies pointing out the current knowledge concerning their genetic background and their underlying molecular mechanisms.