Evolution of Synapses and Neurotransmitter Systems: The Divide-and-Conquer Model for Early Neural Cell-Type Evolution

Nervous systems evolved around 560 million years ago to coordinate and empower animal bodies. Ctenophores – one of the earliest-branching lineages – are thought to share few neuronal genes with bilaterians and may have evolved neurons convergently. Here we review our current understanding of the evolution of neuronal molecules in non-bilaterians. We also reanalyse single-cell sequencing data in light of new cell-cluster identities from a ctenophore and uncover evidence supporting the homology of one ctenophore neuron-type with neurons in Bilateria. The specific coexpression of the presynaptic proteins Unc13 and RIM with voltage-gated channels, neuropeptides and homeobox genes pinpoint a spiking sensory-peptidergic cell in the ctenophore mouth. Similar Unc13-RIM neurons may have been present in the first eumetazoans to rise to dominance only in stem Bilateria. We hypothesize that the Unc13-RIM lineage ancestrally innervated the mouth and conquered other parts of the body with the rise of macrophagy and predation during the Cambrian explosion.

Electric shock causes fear-like persistent behavioral response in the nematode Caenorhabditis elegans

Electricity is widely utilized as environmental stimulus to sense the world by many animal species. Despite its importance, however, molecular and physiological mechanisms for responding to electrical stimulus have been far less understood compared to other sensory stimuli. Here we report novel behavioral responses to electrical stimulus of the nematode C. elegans. When the animals on food are stimulated by alternating current, their movement speed suddenly increases more than 2-fold, which persists for a few minutes even after the electrical stimulation is terminated. Genetic analyses reveal that voltage-gated channels are required for the response, possibly as the sensors, and neuropeptide signaling suppresses the persistent response. Additional behavioral analysis reveals that, in addition to the persistence, the animal's response to electrical shock is scalable and has a negative valence, which are recently regarded as emotion primitives, suggesting that the response may reflect a primitive form of "fear" of animals.

Ion Channels as New Attractive Targets to Improve Re-Myelination Processes in the Brain

Multiple sclerosis (MS) is the most demyelinating disease of the central nervous system (CNS) characterized by neuroinflammation. Oligodendrocyte progenitor cells (OPCs) are cycling cells in the developing and adult CNS that, under demyelinating conditions, migrate to the site of lesions and differentiate into mature oligodendrocytes to remyelinate damaged axons. However, this process fails during disease chronicization due to impaired OPC differentiation. Moreover, OPCs are crucial players in neuro-glial communication as they receive synaptic inputs from neurons and express ion channels and neurotransmitter/neuromodulator receptors that control their maturation. Ion channels are recognized as attractive therapeutic targets, and indeed ligand-gated and voltage-gated channels can both be found among the top five pharmaceutical target groups of FDA-approved agents. Their modulation ameliorates some of the symptoms of MS and improves the outcome of related animal models. However, the exact mechanism of action of ion-channel targeting compounds is often still unclear due to the wide expression of these channels on neurons, glia, and infiltrating immune cells. The present review summarizes recent findings in the field to get further insights into physio-pathophysiological processes and possible therapeutic mechanisms of drug actions.

Mathematical Modeling of Ion Quantum Tunneling Reveals Novel Properties of Voltage-Gated Channels and Quantum Aspects of Their Pathophysiology in Excitability-Related Disorders

Voltage-gated channels are crucial in action potential initiation and propagation and there are many diseases and disorders related to them. Additionally, the classical mechanics are the main mechanics used to describe the function of the voltage-gated channels and their related abnormalities. However, the quantum mechanics should be considered to unravel new aspects in the voltage-gated channels and resolve the problems and challenges that classical mechanics cannot solve. In the present study, the aim is to mathematically show that quantum mechanics can exhibit a powerful tendency to unveil novel electrical features in voltage-gated channels and be used as a promising tool to solve the problems and challenges in the pathophysiology of excitability-related diseases. The model of quantum tunneling of ions through the intracellular hydrophobic gate is used to evaluate the influence of membrane potential and gating free energy on the tunneling probability, single channel conductance, and quantum membrane conductance. This evaluation is mainly based on graphing the mathematical relationships between these variables. The obtained mathematical graphs showed that ions can achieve significant quantum membrane conductance, which can affect the resting membrane potential and the excitability of cells. In the present work, quantum mechanics reveals original electrical properties associated with voltage-gated channels and introduces new insights and implications into the pathophysiology of excitability- related disorders. In addition, the present work sets a mathematical and theoretical framework that can be utilized to conduct experimental studies in order to explore the quantum aspects of voltage-gated channels and the quantum bioelectrical property of biological membranes.

Molecular basis for functional connectivity between the voltage sensor and the selectivity filter gate in Shaker K+ channels

In Shaker K+ channels, the S4-S5 linker couples the voltage sensor (VSD) and pore domain (PD). Another coupling mechanism is revealed using two W434F-containing channels: L361R:W434F and L366H:W434F. In L361R:W434F, W434F affects the L361R VSD seen as a shallower charge-voltage (Q-V) curve that crosses the conductance-voltage (G-V) curve. In L366H:W434F, L366H relieves the W434F effect converting a non-conductive channel in a conductive one. We report a chain of residues connecting the VSD (S4) to the selectivity filter (SF) in the PD of an adjacent subunit as the molecular basis for voltage sensor selectivity filter gate (VS-SF) coupling. Single alanine substitutions in this region (L409A, S411A, S412A, or F433A) are enough to disrupt the VS-SF coupling, shown by the absence of Q-V and G-V crossing in L361R:W434F mutant and by the lack of ionic conduction in the L366H:W434F mutant. This residue chain defines a new coupling between the VSD and the PD in voltage-gated channels.

An advance about the genetic causes of epilepsy

Human hereditary epilepsy has been found related to ion channel mutations in voltage-gated channels (Na+, K+, Ca2+, Cl-), ligand gated channels (GABA receptors), and G-protein coupled receptors, such as Mass1. In addition, some transmembrane proteins or receptor genes, including PRRT2 and nAChR, and glucose transporter genes, such as GLUT1 and SLC2A1, are also about the onset of epilepsy. The discovery of these genetic defects has contributed greatly to our understanding of the pathology of epilepsy. This review focuses on introducing and summarizing epilepsy-associated genes and related findings in recent decades, pointing out related mutant genes that need to be further studied in the future.

N-Terminal Tagging with GFP Enhances Selectivity of Agitoxin 2 to Kv1.3-Channel Binding Site

Recently developed fluorescent protein-scorpion toxin chimeras (FP-Tx) show blocking activities for potassium voltage-gated channels of Kv1 family and retain almost fully pharmacological profiles of the parental peptide toxins (Kuzmenkov et al., Sci Rep. 2016, 6, 33314). Here we report on N-terminally green fluorescent protein (GFP)-tagged agitoxin 2 (GFP-L2-AgTx2) with high affinity and selectivity for the binding site of Kv1.3 channel involved in the pathogenesis of various (primarily of autoimmune origin) diseases. The basis for this selectivity relates to N-terminal location of GFP, since transposition of GFP to the C-terminus of AgTx2 recovered specific interactions with the Kv1.1 and Kv1.6 binding sites. Competitive binding experiments revealed that the binding site of GFP-L2-AgTx2 overlaps that of charybdotoxin, kaliotoxin 1, and agitoxin 2, the known Kv1.3-channel pore blockers. GFP-L2-AgTx2 was demonstrated to be applicable as a fluorescent probe to search for Kv1.3 pore blockers among individual compounds and in complex mixtures, to measure blocker affinities, and to visualize Kv1.3 distribution at the plasma membrane of Kv1.3-expressing HEK293 cells. Our studies show that definite combinations of fluorescent proteins and peptide blockers can result in considerable modulation of the natural blocker-channel binding profile yielding selective fluorescent ligands of certain channels.

An unconventional conductance is required for pacemaking of nigral dopamine neurons

ABSTRACTAlthough several ionic mechanisms are known to control rate and regularity of the pacemaker in dopamine (DA) neurons from the substantia nigra pars compacta (SNc), a conductance essential for pacing has yet to be defined. Here we provide pharmacological evidence that pacemaking of SNc DA neurons is enabled by an unconventional conductance. We found that 1-(2,4-xylyl)guanidine (XG), an established blocker of gating pore currents in mutant voltage gated sodium channels, selectively stops pacemaking of DA SNc neurons and is without effect on the main pore of their voltage-gated channels. We isolated a voltage-dependent, non-inactivating XG-sensitive current of 20-25 pA which operates in the relevant subthreshold range and is carried by both Na+ and Cl- ions. While the molecular identity of this conductance remains to be determined, we show that this XG-sensitive current is crucial to sustain pacemaking in these neurons.

Voltage-Gated Potassium Channels as Regulators of Cell Death

Ion channels allow the flux of specific ions across biological membranes, thereby determining ion homeostasis within the cells. Voltage-gated potassium-selective ion channels crucially contribute to the setting of the plasma membrane potential, to volume regulation and to the physiologically relevant modulation of intracellular potassium concentration. In turn, these factors affect cell cycle progression, proliferation and apoptosis. The present review summarizes our current knowledge about the involvement of various voltage-gated channels of the Kv family in the above processes and discusses the possibility of their pharmacological targeting in the context of cancer with special emphasis on Kv1.1, Kv1.3, Kv1.5, Kv2.1, Kv10.1, and Kv11.1.