Gating of the shaker potassium channel is modulated differentially by N-glycosylation and sialic acids

Abstract

N-linked glycans, including sialic acids, are integral components of ion channel complexes. To determine if N-linked sugars can modulate a rapidly inactivating K+ channel, the glycosylated Drosophila melanogaster Shaker K+ channel (ShB) and the N-glycosylation-deficient mutant (ShNQ), were studied under conditions of full and reduced sialylation. Through an apparent electrostatic mechanism, full sialylation induced uniform and significant hyperpolarizing shifts in all measured voltage-dependent ShB gating parameters compared to those measured under conditions of reduced sialylation. Steady-state gating of ShNQ was unaffected by changes in sialylation and was nearly identical to that observed for ShB under conditions of reduced sialylation, indicating that N-linked sialic acids were wholly responsible for the observed effects of sialic acid on ShB gating. Interestingly, the rates of transition among channel states and the voltage-independent rates of activation and inactivation were significantly slower for ShNQ compared to ShB. Both effects were independent of sialylation, indicating that N-linked sugars other than sialic acids alter ShB gating kinetics but have little to no effect on the steady-state distribution of channels among states. The effect of sialic acids on channel gating, particularly inactivation gating, and the impact of other N-linked sugars on channel gating kinetics are unique to the ShB isoform. Thus, ShB gating is modulated by two complementary but distinct sugar-dependent mechanisms, (1) an N-linked sialic acid-dependent surface charge effect and (2) a sialic acid-independent effect that is consistent with N-linked sugars affecting the stability of ShB among its functional states.

Regulated and aberrant glycosylation modulate cardiac electrical signaling

“…To determine whether the remodeled glycome modulates cardiac function, we observed the impact of the regulated and aberrant expression of a single glycogene on cardiomyocyte electrical activity. We studied the effect of the polysialyltransferase, ST8sia2 (responsible for addition of sialic acid polymers to N-and O-glycans), on cardiac function for several reasons that include: (i) Cardiac dysfunctions including arrhythmias and cardiomyopathy that are likely caused by changes in ion channel activity are prevalent in diseases of aberrant sialylation such as Chagas disease and some CDGs (16 -19); (ii) all our previous data indicated that changes in ion channel sialylation contribute significantly to the modulation of ion channel function (11)(12)(13)15); and (iii) cardiac ST8sia2 expression is regulated. ST8sia2 is expressed at much higher levels in the neonatal atrium than in the neonatal ventricle as determined through microarray ( Fig.…”

“…Previous reports indicated that the sugars attached to cardiac voltage-gated Na ϩ channels (Na v ) and K v may impact channel gating (9,10). Glycosylation of Na v and K v subunits were shown to alter channel gating in isoform-and subunit-dependent manners (11)(12)(13)(14). Most studies established that the sugar-dependent gating effects were imposed by the terminal residue attached to carbohydrate structures, sialic acid.…”

Sialic Acids Attached to O-Glycans Modulate Voltage-gated Potassium Channel Gating

“…Several studies detailed the isoform-specific effects of N-glycans on K v (13)(14)(15)(16)(17)20) and voltage-gated Na ϩ (Na v ) channel functions (10 -12). Negatively charged sialic acids (SA) are typically the terminal residues of glycoprotein glycan structures and were shown to impact gating of various voltage-gated ion channels differentially (10 -17, 20, 21).…”

Structure, Function, and Modification of the Voltage Sensor in Voltage-Gated Ion Channels

“…Incorrect glycosylation can lead to severe disorders like cardiac arrhythmias [236]. A clear relationship between the amount of sialic acid and voltage-dependence of channel activation is seen [237][238][239]. When increasing the glycosylation state, activation is shifted in hyperpolarizing direction.…”

“…When increasing the glycosylation state, activation is shifted in hyperpolarizing direction. Screening experiments with divalent cations suggests an electrostatic effect where the negatively charged sialic acid reduces the local surface potential experienced by the voltage sensor and thereby facilitates channel activation [237][238][239].…”