KCNQ Channels

KCNQ channels, also known as Kv7 channels, are a diverse group of K+ channels that exhibit a wide variety of properties and perform various physiological functions in the body. This family comprises five members, Kv7.1-7.5. These channels exhibit unusual properties and serve unique physiological roles in the body, distinct from those of other voltage-gated K+ (Kv) channels. In addition, different KCNE beta subunits regulate some Kv7 members, drastically altering the functional properties of the channels. This chapter highlights the differences and uniqueness of KCNQ channels compared with other Kv channels. Mutations in KCNQ channels are associated with a range of diseases, such as cardiac arrhythmia, deafness, epilepsy and diabetes, emphasizing the important roles of this family of K+ channels in the body

Modeling of the axon plasma membrane structure and its effects on protein diffusion.

The axon plasma membrane consists of the membrane skeleton, which comprises ring-like actin filaments connected to each other by spectrin tetramers, and the lipid bilayer, which is tethered to the skeleton via, at least, ankyrin. Currently it is unknown whether this unique axon plasma membrane skeleton (APMS) sets the diffusion rules of lipids and proteins in the axon. To answer this question, we developed a coarse-grain molecular dynamics model for the axon that includes the APMS, the phospholipid bilayer, transmembrane proteins (TMPs), and integral monotopic proteins (IMPs) in both the inner and outer lipid layers. We first showed that actin rings limit the longitudinal diffusion of TMPs and the IMPs of the inner leaflet but not of the IMPs of the outer leaflet. To reconcile the experimental observations, which show restricted diffusion of IMPs of the outer leaflet, with our simulations, we conjectured the existence of actin-anchored proteins that form a fence which restricts the longitudinal diffusion of IMPs of the outer leaflet. We also showed that spectrin filaments could modify transverse diffusion of TMPs and IMPs of the inner leaflet, depending on the strength of the association between lipids and spectrin. For instance, in areas where spectrin binds to the lipid bilayer, spectrin filaments would restrict diffusion of proteins within the skeleton corrals. In contrast, in areas where spectrin and lipids are not associated, spectrin modifies the diffusion of TMPs and IMPs of the inner leaflet from normal to confined-hop diffusion. Overall, we showed that diffusion of axon plasma membrane proteins is deeply anisotropic, as longitudinal diffusion is of different type than transverse diffusion. Finally, we investigated how accumulation of TMPs affects diffusion of TMPs and IMPs of both the inner and outer leaflets by changing the density of TMPs. We showed that the APMS structure acts as a fence that restricts the diffusion of TMPs and IMPs of the inner leaflet within the membrane skeleton corrals. Our findings provide insight into how the axon skeleton acts as diffusion barrier and maintains neuronal polarity

The Calcium-Activated Slow AHP: Cutting Through the Gordian Knot

The phenomenon known as the slow afterhyperpolarization (sAHP) was originally described more than 30 years ago in pyramidal cells as a slow, Ca2+-dependent afterpotential controlling spike frequency adaptation. Subsequent work showed that similar sAHPs were widely expressed in the brain and were mediated by a Ca2+-activated potassium current that was voltage independent, insensitive to most potassium channel blockers, and strongly modulated by neurotransmitters. However the molecular basis for this current has remained poorly understood. The sAHP was initially imagined to reflect the activation of a potassium channel directly gated by Ca2+ but recent studies have begun to question this idea. The sAHP is distinct from the Ca2+-dependent fast and medium AHPs in that it appears to sense cytoplasmic [Ca2+]i and recent evidence implicates proteins of the neuronal calcium sensor family as diffusible cytoplasmic Ca2+ sensors for the sAHP. Translocation of Ca2+-bound sensor to the plasma membrane would then be an intermediate step between Ca2+ and the sAHP channels. Parallel studies strongly suggest that the sAHP current is carried by different potassium channel types depending on the cell type. Finally, the sAHP current is dependent on membrane PtdIns(4,5)P2 and Ca2+ appears to gate this current by increasing PtdIns(4,5)P2 levels. Because membrane PtdIns(4,5)P2 is essential for the activity of many potassium channels, these finding have led us to hypothesize that the sAHP reflects a transient Ca2+-induced increase in the local availability of PtdIns(4,5)P2 which then activates a variety of potassium channels. If this view is correct, the sAHP current would not represent a unitary ionic current but the embodiment of a generalized potassium channel gating mechanism. This model can potentially explain the cardinal features of the sAHP, including its cellular heterogeneity, slow kinetics, dependence on cytoplasmic [Ca2+], high temperature-dependence, and modulation

Figures and data for paper titled "The periodic axon membrane skeleton leads to Na nanodomains but does not impact action potentials" 10.1016/j.bpj.2022.08.027

Data for the paper titled "The periodic axon membrane skeleton leads to Na nanodomains but does not impact action potentials" 10.1016/j.bpj.2022.08.027</p

EAAT2 (GLT-1; slc1a2) Glutamate Transporters Reconstituted in Liposomes Argues against Heteroexchange Being Substantially Faster than Net Uptake

The EAAT2 glutamate transporter, accounts for >90% of hippocampal glutamate uptake. Although EAAT2 is predominantly expressed in astrocytes, ∼10% of EAAT2 molecules are found in axon terminals. Despite the lower level of EAAT2 expression in glutamatergic terminals, when hippocampal slices are incubated with low concentration of d-aspartate (an EAAT2 substrate), axon terminals accumulate d-aspartate as quickly as astroglia. This implies an unexplained mismatch between the distribution of EAAT2 protein and of EAAT2-mediated transport activity. One hypothesis is that (1) heteroexchange of internal substrate with external substrate is considerably faster than net uptake and (2) terminals favor heteroexchange because of high levels of internal glutamate. However, it is currently unknown whether heteroexchange and uptake have similar or different rates. To address this issue, we used a reconstituted system to compare the relative rates of the two processes in rat and mice. Net uptake was sensitive to changes in the membrane potential and was stimulated by external permeable anions in agreement with the existence of an uncoupled anion conductance. By using the latter, we also demonstrate that the rate of heteroexchange also depends on the membrane potential. Additionally, our data further suggest the presence of a sodium leak in EAAT2. By incorporating the new findings in our previous model of glutamate uptake by EAAT2, we predict that the voltage sensitivity of exchange is caused by the voltage-dependent third Na(+) binding. Further, both our experiments and simulations suggest that the relative rates of net uptake and heteroexchange are comparable in EAAT2

Modeling of the axon membrane skeleton structure and implications for its mechanical properties

<div><p>Super-resolution microscopy recently revealed that, unlike the soma and dendrites, the axon membrane skeleton is structured as a series of actin rings connected by spectrin filaments that are held under tension. Currently, the structure-function relationship of the axonal structure is unclear. Here, we used atomic force microscopy (AFM) to show that the stiffness of the axon plasma membrane is significantly higher than the stiffnesses of dendrites and somata. To examine whether the structure of the axon plasma membrane determines its overall stiffness, we introduced a coarse-grain molecular dynamics model of the axon membrane skeleton that reproduces the structure identified by super-resolution microscopy. Our proposed computational model accurately simulates the median value of the Young’s modulus of the axon plasma membrane determined by atomic force microscopy. It also predicts that because the spectrin filaments are under entropic tension, the thermal random motion of the voltage-gated sodium channels (Na_v), which are bound to ankyrin particles, a critical axonal protein, is reduced compared to the thermal motion when spectrin filaments are held at equilibrium. Lastly, our model predicts that because spectrin filaments are under tension, any axonal injuries that lacerate spectrin filaments will likely lead to a permanent disruption of the membrane skeleton due to the inability of spectrin filaments to spontaneously form their initial under-tension configuration.</p></div

Young's moduli of rat hippocampal neuronal subcompartments determined by AFM.

<p>Histograms of Young’s moduli of rat hippocampal (A) soma, (B) dendrites, (C) axons, and (D) axons treated with 20μm Latrunculin B. The median Young's modulus of the soma is 0.7 ± 0.2 <i>kPa</i> (A), of dendrites is 2.5 ± 0.7 <i>kPa</i> (B). For the axon plasma membrane, the median Young’s modulus is 4.6 ± 1.5 <i>kPa</i> (C). When axons were treated with Latrunculin B (20μm, 1 hour) the median value of the axon plasma membrane Young’s modulus was reduced to 2.2 ± 0.6 <i>kPa</i>. Number of samples (N = 2), total number of tested neurons (n = 8). N = 1 and n = 6 for axon + Latrunculin B. (E) Box-whisker plots of mean Young's moduli of the soma, dendrites, axon, and axon treated with Latrunculin B. *** indicates statistical significance of <i>p</i> < 0.001 (Kruskal-Wallis test).</p