Intracellular segment between transmembrane helices S0 and S1 of BK channel α subunit contains two amphipathic helices connected by a flexible loop

Identification and functional analysis of two new de novo KCNMA1 variants associated with Liang-Wang syndrome

AIM

Loss-of-function KCNMA1 variants cause Liang-Wang syndrome (MIM #618729), a newly identified multiple malformation syndrome with a broad spectrum of developmental and neurological phenotypes. However, the full spectrum of clinical features and underlying pathogenic mechanisms need full elucidation.

METHODS

Exome sequencing was used to identify pathogenic variants. Patch-clamp recordings were performed to access the effects of KCNMA1 variants on BK channels. Total and membrane protein expression levels of BK channels were characterized using Western blotting.

RESULTS

We report identification and functional characterization of two new de novo loss-of-function KCNMA1 variants p.(A172T) and p.(A314T) with characteristics of Liang-Wang syndrome. Variant p.(A172T) is associated with developmental delay, cognitive impairment and ataxia. Mechanistically, p.(A172T) abolishes BK potassium current, inhibits Mg²⁺ -dependent gating, but shifts conductance-voltage (G-V) curves to more positive potentials when complexed with WT channels. Variant p.(A314T) is associated with developmental delay, intellectual disability, cognitive impairment, mild ataxia and generalized epilepsy; suppresses BK current amplitude; and shifts G-V curves to more positive potentials when expressed with WT channels. In addition, two new patients with previously reported gain-of-function variants p.(N536H) and p.(N995S) are found to show epilepsy and paroxysmal dyskinesia as reported previously, but also exhibit additional symptoms of cognitive impairment and dysmorphic features. Furthermore, variants p.(A314T) and p.(N536H) reduced total and membrane levels of BK proteins.

CONCLUSION

Our findings identified two new loss-of-function mutations of KCNMA1 associated with Liang-Wang syndrome, expanded the spectrum of clinical features associated with gain-of-function KCNMA1 variants and emphasized the overlapping features shared by gain-of-function and loss-of-function mutations.

A New Splice Variant of Large Conductance Ca2+-activated K+ (BK) Channel α Subunit Alters Human Chondrocyte Function

Large conductance Ca²⁺-activated K⁺ (BK) channels play essential roles in both excitable and non-excitable cells. For example, in chondrocytes, agonist-induced Ca²⁺ release from intracellular store activates BK channels, and this hyperpolarizes these cells, augments Ca²⁺ entry, and forms a positive feed-back mechanism for Ca²⁺ signaling and stimulation-secretion coupling. In the present study, functional roles of a newly identified splice variant in the BK channel α subunit (BKαΔe2) were examined in a human chondrocyte cell line, OUMS-27, and in a HEK293 expression system. Although BKαΔe2 lacks exon2, which codes the intracellular S0-S1 linker (Glu-127-Leu-180), significant expression was detected in several tissues from humans and mice. Molecular image analyses revealed that BKαΔe2 channels are not expressed on plasma membrane but can traffic to the plasma membrane after forming hetero-tetramer units with wild-type BKα (BKαWT). Single-channel current analyses demonstrated that BKα hetero-tetramers containing one, two, or three BKαΔe2 subunits are functional. These hetero-tetramers have a smaller single channel conductance and exhibit lower trafficking efficiency than BKαWT homo-tetramers in a stoichiometry-dependent manner. Site-directed mutagenesis of residues in exon2 identified Helix2 and the linker to S1 (Trp-158-Leu-180, particularly Arg-178) as an essential segment for channel function including voltage dependence and trafficking. BKαΔe2 knockdown in OUMS-27 chondrocytes increased BK current density and augmented the responsiveness to histamine assayed as cyclooxygenase-2 gene expression. These findings provide significant new evidence that BKαΔe2 can modulate cellular responses to physiological stimuli in human chondrocyte and contribute under pathophysiological conditions, such as osteoarthritis.

S-acylation dependent post-translational cross-talk regulates large conductance calcium- and voltage- activated potassium (BK) channels

Mechanisms that control surface expression and/or activity of large conductance calcium-activated potassium (BK) channels are important determinants of their (patho)physiological function. Indeed, BK channel dysfunction is associated with major human disorders ranging from epilepsy to hypertension and obesity. S-acylation (S-palmitoylation) represents a major reversible, post-translational modification controlling the properties and function of many proteins including ion channels. Recent evidence reveals that both pore-forming and regulatory subunits of BK channels are S-acylated and control channel trafficking and regulation by AGC-family protein kinases. The pore-forming α-subunit is S-acylated at two distinct sites within the N- and C-terminus, each site being regulated by different palmitoyl acyl transferases (zDHHCs) and acyl thioesterases (APTs). S-acylation of the N-terminus controls channel trafficking and surface expression whereas S-acylation of the C-terminal domain determines regulation of channel activity by AGC-family protein kinases. S-acylation of the regulatory β4-subunit controls ER exit and surface expression of BK channels but does not affect ion channel kinetics at the plasma membrane. Furthermore, a significant number of previously identified BK-channel interacting proteins have been shown, or are predicted to be, S-acylated. Thus, the BK channel multi-molecular signaling complex may be dynamically regulated by this fundamental post-translational modification and thus S-acylation likely represents an important determinant of BK channel physiology in health and disease.

Size-to-charge dispersion of collision-induced dissociation product ions for enhancement of structural information and product ion identification

Ion mobility is used to disperse product ions formed by collision-induced dissociation (CID) on the basis of charge state and size-to-charge ratio. We previously described an approach for combining CID with ion mobility mass spectrometry (IM-MS) for dispersing fragment ions along charge state specific trend lines (Zinnel, N. F.; Pai, P. J.; Russell, D. H. Anal. Chem. 2012, 84, 3390; Sowell, R. A.; Koeniger, S. L.; Valentine, S. J.; Moon, M. H.; Clemmer, D. E. J. Am. Soc. Mass Spectrom. 2004, 15, 1341; McLean, J. A.; Ruotolo, B. T.; Gillig, K. J.; Russell, D. H. Int. J. Mass Spectrom. 2005, 240, 301), and this approach was used to assign metal ion binding sites for human metallothionein protein MT-2a (Chen, S. H.; Russell, W. K.; Russell, D. H. Anal. Chem. 2013, 85, 3229). Here, we use this approach to distinguish b-type N-terminal fragment ions from both internal fragment ions and y-type C-terminal fragment ions. We also show that in some cases specific secondary structural elements, viz., extended coils or helices, can be obtained for the y-type fragment ions series. The advantage of this approach is that product ion identity can be correlated to gas-phase ion structure, which provides rapid identification of the onset and termination of extended coil structure in peptides.

Intrinsic disorder in the BK channel and its interactome

The large-conductance Ca²⁺-activated K⁺ (BK) channel is broadly expressed in various mammalian cells and tissues such as neurons, skeletal and smooth muscles, exocrine cells, and sensory cells of the inner ear. Previous studies suggest that BK channels are promiscuous binders involved in a multitude of protein-protein interactions. To gain a better understanding of the potential mechanisms underlying BK interactions, we analyzed the abundance, distribution, and potential mechanisms of intrinsic disorder in 27 BK channel variants from mouse cochlea, 104 previously reported BK-associated proteins (BKAPS) from cytoplasmic and membrane/cytoskeletal regions, plus BK β- and γ-subunits. Disorder was evaluated using the MFDp algorithm, which is a consensus-based predictor that provides a strong and competitive predictive quality and PONDR, which can determine long intrinsically disordered regions (IDRs). Disorder-based binding sites or molecular recognition features (MoRFs) were found using MoRFpred and ANCHOR. BKAP functions were categorized based on Gene Ontology (GO) terms. The analyses revealed that the BK variants contain a number of IDRs. Intrinsic disorder is also common in BKAPs, of which ∼5% are completely disordered. However, intrinsic disorder is very differently distributed within BK and its partners. Approximately 65% of the disordered segments in BK channels are long (IDRs) (>50 residues), whereas >60% of the disordered segments in BKAPs are short IDRs that range in length from 4 to 30 residues. Both α and γ subunits showed various amounts of disorder as did hub proteins of the BK interactome. Our analyses suggest that intrinsic disorder is important for the function of BK and its BKAPs. Long IDRs in BK are engaged in protein-protein and protein-ligand interactions, contain multiple post-translational modification sites, and are subjected to alternative splicing. The disordered structure of BK and its BKAPs suggests one of the underlying mechanisms of their interaction.