Impact of intracellular hemin on N-type inactivation of voltage-gated K+ channels

Structural insights into the function, dysfunction and modulation of Kv3 channels

The third subfamily of voltage-gated K+ (Kv) channels includes four members, Kv3.1, Kv3.2, Kv3.3 and Kv3.4. Fast gating and activation at relatively depolarized membrane potentials allows Kv3 channels to be major drivers of fast action potential repolarization in the nervous system. Consequently, they help determine the fast-spiking phenotype of inhibitory interneurons and regulate fast synaptic transmission at glutamatergic synapses and the neuromuscular junction. Recent studies from our group and a team of collaborators have used cryo-EM to demonstrate the surprising gating role of the Kv3.1 cytoplasmic T1 domain, the structural basis of a developmental epileptic encephalopathy caused by the Kv3.2-C125Y variant and the mechanism of action of positive allosteric modulators involving unexpected interactions and conformational changes in Kv3.1 and Kv3.2. Furthermore, our recent work has shown that Kv3.4 regulates use-dependent spike broadening in a manner that depends on gating modulation by phosphorylation of the channel's N-terminal inactivation domain, which can impact activity-dependent synaptic facilitation. Here, we review and integrate these studies to provide a perspective on our current understanding of Kv3 channel function, dysfunction and pain modulation in the nervous system.

Signal transduction mechanisms in heme-based globin-coupled oxygen sensors with a focus on a histidine kinase (AfGcHK) and a diguanylate cyclase (YddV or EcDosC)

Heme is a vital cofactor of proteins with roles in oxygen transport (e.g. hemoglobin), storage (e.g. myoglobin), and activation (e.g. P450) as well as electron transfer (e.g. cytochromes) and many other functions. However, its structural and functional role in oxygen sensing proteins differs markedly from that in most other enzymes, where it serves as a catalytic or functional center. This minireview discusses the mechanism of signal transduction in two heme-based oxygen sensors: the histidine kinase AfGcHK and the diguanylate cyclase YddV (EcDosC), both of which feature a heme-binding domain containing a globin fold resembling that of hemoglobin and myoglobin.

Extracellular hemin is a reverse use-dependent gating modifier of cardiac voltage-gated Na+ channels

Heme (Fe²⁺-protoporphyrin IX) is a well-known protein prosthetic group; however, heme and hemin (Fe³⁺-protoporphyrin IX) are also increasingly viewed as signaling molecules. Among the signaling targets are numerous ion channels, with intracellular-facing heme-binding sites modulated by heme and hemin in the sub-µM range. Much less is known about extracellular hemin, which is expected to be more abundant, in particular after hemolytic insults. Here we show that the human cardiac voltage-gated sodium channel hNa_V1.5 is potently inhibited by extracellular hemin (IC ₅₀ ≈ 80 nM), while heme, dimethylhemin, and protoporphyrin IX are ineffective. Hemin is selective for hNa_V1.5 channels: hNa_V1.2, hNa_V1.4, hNa_V1.7, and hNa_V1.8 are insensitive to 1 µM hemin. Using domain chimeras of hNa_V1.5 and rat rNa_V1.2, domain II was identified as the critical determinant. Mutation N803G in the domain II S3/S4 linker largely diminished the impact of hemin on the cardiac channel. This profile is reminiscent of the interaction of some peptide voltage-sensor toxins with Na_V channels. In line with a mechanism of select gating modifiers, the impact of hemin on Na_V1.5 channels is reversely use dependent, compatible with an interaction of hemin and the voltage sensor of domain II. Extracellular hemin thus has potential to modulate the cardiac function.

Intracellular hemin is a potent inhibitor of the voltage-gated potassium channel Kv10.1

Heme, an iron-protoporphyrin IX complex, is a cofactor bound to various hemoproteins and supports a broad range of functions, such as electron transfer, oxygen transport, signal transduction, and drug metabolism. In recent years, there has been a growing recognition of heme as a non-genomic modulator of ion channel functions. Here, we show that intracellular free heme and hemin modulate human ether à go-go (hEAG1, Kv10.1) voltage-gated potassium channels. Application of hemin to the intracellular side potently inhibits Kv10.1 channels with an IC₅₀ of about 4 nM under ambient and 63 nM under reducing conditions in a weakly voltage-dependent manner, favoring inhibition at resting potential. Functional studies on channel mutants and biochemical analysis of synthetic and recombinant channel fragments identified a heme-binding motif CxHx₈H in the C-linker region of the Kv10.1 C terminus, with cysteine 541 and histidines 543 and 552 being important for hemin binding. Binding of hemin to the C linker may induce a conformational constraint that interferes with channel gating. Our results demonstrate that heme and hemin are endogenous modulators of Kv10.1 channels and could be exploited to modulate Kv10.1-mediated cellular functions.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

Distinct Mechanisms Account for In Vitro Activation and Sensitization of TRPV1 by the Porphyrin Hemin

TRPV1 mediates pain occurring during sickling episodes in sickle cell disease (SCD). We examined if hemin, a porphyrin released during intravascular hemolysis modulates TRPV1. Calcium imaging and patch clamp were employed to examine effects of hemin on mouse dorsal root ganglion (DRG) neurons and HEK293t cells expressing TRPV1 and TRPA1. Hemin induced a concentration-dependent calcium influx in DRG neurons which was abolished by the unspecific TRP-channel inhibitor ruthenium red. The selective TRPV1-inhibitor BCTC or genetic deletion of TRPV1 only marginally impaired hemin-induced calcium influx in DRG neurons. While hTRPV1 expressed in HEK293 cells mediated a hemin-induced calcium influx which was blocked by BCTC, patch clamp recordings only showed potentiated proton- and heat-evoked currents. This effect was abolished by the PKC-inhibitor chelerythrine chloride and in protein kinase C (PKC)-insensitive TRPV1-mutants. Hemin-induced calcium influx through TRPV1 was only partly PKC-sensitive, but it was abolished by the reducing agent dithiothreitol (DTT). In contrast, hemin-induced potentiation of inward currents was not reduced by DTT. Hemin also induced a redox-dependent calcium influx, but not inward currents on hTRPA1. Our data suggest that hemin induces a PKC-mediated sensitization of TRPV1. However, it also acts as a photosensitizer when exposed to UVA-light used for calcium imaging. The resulting activation of redox-sensitive ion channels such as TRPV1 and TRPA1 may be an in vitro artifact with limited physiological relevance.