The β₁ subunit of the Ca²⁺-sensitive K⁺ channel protects against hypertension

Abstract

Previous animal studies have demonstrated that the loss of the β₁ subunit of the large-conductance Ca²⁺-activated K⁺ (BK) channel leads to hypertension. A new study demonstrates that a gain in β₁ subunit function is associated with protection against diastolic hypertension in humans, underscoring the importance of the β₁ subunit and the BK channel in the regulation of vascular resistance.

Elevated blood pressure — hypertension — is a major risk factor for brain, heart, and kidney diseases and affects at least 50 million people in the USA, and about 1 billion people worldwide. Despite the devastating consequences of this disease, the underlying cause is not known in 90 to 95% of all cases. The known genetic causes of hypertension involve mutations that disrupt salt regulation (1).

Blood pressure is determined by the amount of blood ejected by the heart (cardiac output) and by the resistance to blood flow, which is regulated by the vasculature. Blood pressure exhibits characteristic fluctuations that correspond to the initial ejection (systolic) and filling (diastolic) phases of cardiac contractions. Blood pressures below 120 mmHg systolic and 80 mmHg diastolic are considered desirable.

Mice have been engineered in which genes key to blood vessel development and the regulation of vascular resistance have been deleted. For example, ablation or suppression of the genes for eNOS (2, 3), cyclic GMP–dependent kinase I (4), an isoform — SK3 — of the small-conductance Ca²⁺-activated potassium (SK) channel (5), or the β₁ subunit of the large-conductance Ca²⁺-activated K⁺ (BK) channel (6–8) leads to a chronic elevation of blood pressure. The β₁ subunit of the BK channel is particularly interesting, since it appears to be exclusively expressed in smooth muscle (6, 9, 10), where it acts to increase the apparent Ca²⁺- and voltage-sensitivity of the BK channel (11).

β₁ subunit of the BK channel regulates vascular tone

BK channels, which are activated by both intracellular Ca²⁺ ions ([Ca²⁺]_i) and membrane potential depolarization, regulate the membrane potential of arterial smooth muscle cells. BK channels are activated in arterial smooth muscle by local Ca²⁺ release events (“Ca²⁺ sparks”) caused by the opening of a cluster of ryanodine receptors in the sarcoplasmic reticulum membrane adjacent to the cell membrane (12). In pressurized arteries under normal physiological membrane potentials (∼–40 mV) and [Ca²⁺]_i (∼200 nM) these channels have an exceedingly low activity. A Ca²⁺ spark increases the activity of nearby BK channels 10⁴- to 10⁶-fold, resulting in an efflux of K⁺ that is sufficient to hyperpolarize the membrane potential by 10 to 20 mV (13–15). Blocking BK channels or ryanodine receptors in arterial smooth muscle can cause membrane potential depolarization, an elevation of arterial wall Ca²⁺, and vasoconstriction (12, 16). The Ca²⁺ spark–BK channel pathway thus functions as a negative feedback loop to limit membrane depolarization and contraction (12, 14) (Figure 1A).

Figure 1.

Proposed role for the β_1E65K subunit, resulting from a single point mutation in the β₁ subunit and leading to a gain-of-function of the BK channel and vasodilation. (A) The BK channels in smooth muscle are composed of α pore-forming subunits and β₁ subunits. Local Ca²⁺ release (Ca²⁺ sparks) through a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) membrane activates nearby BK channels leading to membrane potential hyperpolarization, decreased influx of Ca²⁺ through voltage-dependent Ca²⁺ channels (VDCCs), and less contraction. β₁ subunits play a crucial role in the Ca²⁺ spark–BK channel negative feedback loop, since they increase the Ca²⁺-sensitivity of the pore-forming α subunit of the BK channel. (B) The mutant form of the β₁ subunit, the β_1E65K subunit, which reflects a single amino acid substitution, has an even higher efficacy in enhancing the Ca²⁺-sensitivity of BK channels resulting in their gain-of-function. Hence, the mutant β_1E65K subunit enhances the role of the Ca²⁺ spark–BK channel negative feedback mechanism in limiting vasoconstriction and effectively provides protection against diastolic hypertension.

A wide variety of vasodilators exert their actions through activation of BK channels (17–19). BK channels in smooth muscle are regulated by multiple second messenger signaling pathways, including cAMP- and cGMP-dependent protein kinases (PKA and PKG, respectively). PKA and PKG activate BK channels directly through channel phosphorylation, but also activate BK channels indirectly through an elevation of Ca²⁺ spark frequency and amplitude (14).

The BK channel in smooth muscle is composed of α pore-forming subunits and β₁ subunits (Figure 1A). The β₁ subunit is highly expressed in smooth muscle, but not in other tissues (6, 10). The β₁ subunit has been shown to increase the apparent voltage- and Ca²⁺-sensitivity of the pore-forming α subunit in heterologous expression systems (20–22). The importance of the β₁ subunit in arterial smooth muscle physiology is just emerging (6). Disruption of the gene encoding the β₁ subunit (Kcnmb1) in mice functionally uncouples Ca²⁺ sparks from activation of BK channels, leading to membrane potential depolarization, vasoconstriction, an elevation of blood pressure, and left ventricular hypertrophy (6–8). In commonly used rat models of hypertension, including spontaneously hypertensive rats and rats made hypertensive by chronic angiotensin II infusion, elevated blood pressure is associated with a downregulation of the β₁ subunit, but not the α subunit, of the BK channel (23, 24). Significantly, blocking the BK channel has a diminished effect on vasoconstriction in these models, suggesting that the observed decrease in expression is functionally relevant. Estradiol has also been shown to activate BK channels through binding to the β₁ subunit (25), an observation that may provide a mechanistic basis for well-characterized gender differences in resting vascular tone and myogenic responses. Collectively, these studies support the concept that the β₁ subunit of BK channels plays an important role in regulating vascular resistance.

Association between genetic variants of the β₁ subunit (KCNMB1) gene and human hypertension

The human gene that encodes the β₁ subunit of the BK channel, KCNMB1, maps to chromosome 5q34 and had not been linked to human hypertension (26). In this issue of the JCI, a report based on the results of a large genetic epidemiological study (3,876 participants) by Fernández-Fernández et al. describes a new single nucleotide substitution (G352A) in the third exon of the KCNMB1 gene (27). The resulting mutant protein (β_1E65K) contains a glutamic acid to lysine substitution at position 65 in the β₁ subunit. Interestingly, the investigators found that the frequency of the β_1E65K polymorphism is dramatically lower (3.2%) among individuals in the population with severe elevations in diastolic blood pressure (>110 mmHg) compared to that in the normotensive population (21.6%). There was no relationship between β_1E65K allele frequency and systolic blood pressure. These results suggest that the β_1E65K variant provides a protective effect against diastolic hypertension, which would be consistent with a gain-of-function of the BK channel that increases opposition to constriction of resistance arteries.

To test this hypothesis, the investigators examined the effects of β_1E65K on the Ca²⁺- and voltage-activation of human BK channel α subunit (hSlo1), expressed in HEK-293 cells (27). As has been previously shown, expression of the wild-type β₁ subunit increased the apparent Ca²⁺- and voltage-sensitivity of the pore-forming α subunit. Remarkably, β_1E65K alone or in combination with wild-type β₁ subunits further increased the apparent α- and voltage-sensitivity (Figure 1B). This effect was equivalent to a negative 30 mV shift in the activation curve of the BK channel in the presence of 10 μM Ca²⁺, a concentration that the BK channel would experience during a Ca²⁺ spark. Furthermore, the E65K mutation had a dominant-positive effect in combination with the wild-type β₁ subunit. Finally, the authors used the allosteric model of Horrigan and Aldrich (28) to explain the increase in apparent Ca²⁺- and voltage-sensitivity, without a change in channel kinetics.

Future directions

In mouse studies, the loss of the BK channel β₁ subunit leads to an increase in vascular tone and hypertension. The study by Fernández-Fernández et al. (27) provides an exciting complementary view, demonstrating that a gain-of-function BK channel β₁ subunit variant exerts a protective effect against diastolic hypertension in humans. The combined use of human genetic epidemiological studies and functional studies on exogenously expressed channels in vitro has established the basic mechanism that connects BK channel function with a form of human hypertension. An examination of vascular tone control and BK channel function in resistance arteries from people exhibiting the β_1E65K variant is the next logical step. Additional insights might also be gained by exploring the effects of the E65K mutation on β₁ subunit modulation of BK channel function, vascular tone, and blood pressure control in a mouse model. Nonetheless, this study elevates the potential importance of the β₁ subunit and the BK channel in the regulation of blood pressure, and provides significant motivation for the development of BK channel–regulating therapeutic agents that specifically target the β₁ subunit.