Second-Messenger Gated Ion Channels

Membrane Biophysics, 2014

Ion Channel Presentation

Vehpi Yildirim and Joe McKenna

Overview

●Stimulus triggers iIntracelluar signal that modulates channel activity

●Examples

–G-protein coupled channels

–IP3-regulated channels

–Adenine nucleotide-sensitive channels

Examples

●G-protein coupled inward rectifying K+ channel

●IP3-regulated Ca2+ release from ER

Review Article

ATP-sensitive K+ Channels

●Link cellular energetics and excitability

●Gate efflux of K+

–Inward rectifier

–Shallow voltage-dependence

●Inhibited by ATP, activated by Mg2+

KATP Architecture

●Functional Octamer

●Kir 6.2: 4 sub-units

–Channel pore

–Site of ATP inhibition

●Sulphonylurea Receptor (SUR): 4 sub-units

–Site of Mg2+ activation

Kir 6.2/SUR Model

●Extrapolated from

–Bacteria K+ channel crystal

–Prokaryotic Kir

–Targeted mutation

●ATP binds at interface of SUR NBF1 & 2 (b. green)

Mechanism of Gating

●Fast ligand-independent gating by ion selectivity filter

●Ligand-dependent gating by hinged motion of M2

–Inhibited by ATP

–Activated by PIP2, MgADP

Gating Kinetic Model

●Fast ligand-independent gating and slow ligand-dependent gating

●One subunit in closed configuration → channel closed

●Two ways to achieve same half-maximal inhibition

KATP-related disease

●Pancreatic beta-cells

–Loss of function mutation →Hyperglycemia/diabetes

–Gain of function mutation → Hyperinsulinemia

●Coronoary cells

–Loss of function mutation → spontaneous contraction, early death

Identification and Properties of an ATP-Sensitive K+

Current in Rabbit Sino-Atrial Node Pacemaker Cells

X. Han, P. E. Light, W. R. Giles and R. J. French

Journal of Physiology (1996), 490.2, pp.337-350

INTRODUCTION

K(ATP) channles have been identified in many cell types.

Most studies use myocytes from atrium.

Here they use cells from sino-atrium node.

Questions to be Answered

Are K(ATP) channels present in SA node and, if so, what are their single channel properties?

Can physiological, pharmacological and pathological conditions which are known to activate K(ATP) channels alter SA-node activity?

METHODS

Isolated single cells from SA node of rabbit heart are studied by measuring spontaneous activity.

Both whole cell and single channel currents are measured.

Pharmacological blockers or openers are used.

Ventricular myocytes also isolated to compare results from different regions of heart.

Perforated patch technique for Whole cell.

Inside-out configuration for single channel.

RESULTS

Glibenclamide: K(ATP) channel blocker. Acts on SUR subunit.

Cromakalim and Pinacidil: K(ATP) channel openers. Act on SUR subunit.

Effects of glibenclamide on electrical activity and ion curents.

Effects of KATP channel openers on the Current

Effect of metabolic inhibition by NaCN

NaCN (Sodium Cyanide) : inhibits ATP production.

Properties of Single KATP Channels

Effects of drugs on single channel activity.

Effects with high ATP concentration.

Open and Close Times

Neonatal Diabetes (NDM) Overview

●Presents within first 3 months of life, requires insulin treatment

●Insulin response to sulphonylureas but not glucose or glucagon

●May result from Kir 6.2 gain of function mutations in pancreatic beta-cells

KATP Channels and NDM

●Glucose → ATP → channel closure → Ca2+ influx → Insulin secretion

NDM Patient Screening

●Patients with known diabetes-related mutations excluded

●Physical exam including insulin, sulphonylurea challenges

●Kir 6.2 gene sequenced

●Identified 6 novel mutations

●NDM seen only in patients with Kir6.2 mutations

Kir6.2 Affected Residues

●Highly conserved regions → functional role

●Near ATP-binding site or slide helix

Patient Response to Secretagogues

●3 patients with mutations in ATP binding site (ABS)

–No secretion from glucose

–Secretion from KATP channel opener

KATP Channels in Oocytes

●Channels with mutated ABS residues

–Larger current in steady [ATP]

–Current increased by sulphonylurea

–Weakly inhibition by ATP

KATP Channels in Oocytes

●NDM pathology more severe in homozygote mutants

–Significant difference in half-maximal activation by ATP

Conclusion

●Activating mutations in Kir6.2 causes NDM

●Found in 34% of patients with NDM

●Accompanying complications point to vital role of KATP channels in brain and muscle

●Potential therapy: channel blocker acting on SUR receptors

Defective Insulin Secretion and Enhanced Insulin Action in KATP Channel

Deficient Mice

Takashi Miki, Kazuaki Nagashima, Fumi Tashiro, Kazumi Kotake, Hideyuki Yoshitomi, Atsuko Tamamoto, Tohru Gonoi, Toshihiko Iwanaga, Jun-ichi Miyazaki, And Susumu Seino

PNAS Vol. 95, pp. 10402-10406, September 1998, Biochemistry

INTRODUCTION

KATP Channels in pancreatic Beta Cells comprise Kir6.2 and SUR1 subunits.

KATP Channels are ATP and ADP sensors and play a very important role in insulin secretion.

Mutations in regulatory genes cause hypoglycemia.

Here they use Kir6.2-/- mice to study the role of KATP

channels in insulin secretion.

Kir6.2+/+ and Kir6.2-/- cells are dialyzed with ATP-free pipette solution.

Glucose or Tolbutamide does not effect [Ca] in Kir6.2-\- cells.

AcetylCholine and High K+ does effect [Ca] like in wild type cells. Showing voltage gated Ca channels and IP3 sensitive Ca stores are functioning normally in Kir6.2-\- cells.

A rapid rise in Ca concentration is needed for glucose induced insulin secretion, rather than a continuous elevated [Ca].

In Kir6.2-/- mice, only a small first phase and no second phase secretion observed. (In Vitro)

Glucose induced insulin secretion is reduced in knock-out mice.

But surprisingly glucose lowering effect of insulin is significantly increased in knock-out mice.

Kir6.2+/+

Kir6.2-/-

Beta Alpha

CONCLUSION

KATP channels play a significant role in insulin secretion.

Glucose metabolism itself is insufficient for glucose-induced and sulfonylurea-induced insulin secretion, both of which require the rapid rise in [Ca2] caused by closure of the KATP channels.