Table of Contents

Volume 6, Issue 2

  March/April 2012

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REVIEW

Ion channels in postnatal neurogenesis: Potential targets for brain repair

Leigh Anne Swayne and Leigh Wicki-Stordeur
Abstract | Full Text | PDF

Neural stem and progenitor cells (NSC/NPCs) are unspecialized cells found in the adult peri-ventricular and sub-granular zones that are capable of self-renewal, migration, and differentiation into new neurons through the remarkable process of postnatal neurogenesis. We are now beginning to understand that the concerted action of ion channels, multi-pass transmembrane proteins that allow passage of ions across otherwise impermeable cellular membranes tightly regulate this process. Specific ion channels control proliferation, differentiation and survival. Furthermore, they have the potential to be highly selective drug targets due to their complex structures. As such, these proteins represent intriguing prospects for control and optimization of postnatal neurogenesis for neural regeneration following brain injury or disease. Here, we concentrate on ion channels identified in adult ventricular zone NSC/NPCs that have been found to influence the stages of neurogenesis. Finally, we outline the potential of these channels to elicit repair, and highlight the outstanding challenges.




SHORT COMMUNICATION

Regulation of CREB signaling through L-Type Ca²⁺ channels by Nipsnap-2

Joel M. Brittain, Yuying Wang, Sarah Wilson and Rajesh Khanna
Abstract | Full Text

A recent study identified Nipsnap1 as an auxiliary protein inhibiting TRPV6 ion channel activity. Based upon this finding, we investigated the role of Nipsnap1, and the closely related Nipsnap2, in Ca²⁺ channel regulation. Here, we find that overexpression of Nipsnap2 caused a 45% increase in currents though L-type Ca²⁺ channels in a neuronal cell line, while siRNA knockdown of Nipsnap2 greatly reduced L-type currents. The increased influx through L-type Ca²⁺ channels due to Nipsnap2 overexpression led to increased phosphorylation of the transcription factor cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) along with enhanced expression of several transcription factors and CREB target genes. These experiments highlight a novel role of Nipsnap2 in transcriptional regulation via L-type Ca²⁺ channels.




RESEARCH PAPERS

Localization of Na_v1.7 in the normal and injured rodent olfactory system indicates a critical role in olfaction, pheromone sensing and immune function

Darshani B. Rupasinghe, Oliver Knapp, Linda V. Blomster, Annina B. Schmid, David J. Adams, Glenn F. King and Marc J. Ruitenberg
Abstract | Full Text | PDF

Loss-of-function mutations in the pore-forming α subunit of the voltage-gated sodium channel 1.7 (Na_v1.7) cause congenital indifference to pain and anosmia. We used immunohistochemical techniques to study Na_v1.7 localization in the rat olfactory system in order to better understand its role in olfaction. We confirm that Na_v1.7 is expressed on olfactory sensory axons and report its presence on vomeronasal axons, indicating an important role for Na_v1.7 in transmission of pheromonal cues. Following neuroepithelial injury, Na_v1.7 was transiently expressed by cells of monocytic lineage. These findings support an emerging role for Na_v1.7 in immune function. This sodium channel may provide an important pharmacological target for treatment of inflammatory injury and inflammatory pain syndromes.

Tryptophan scanning mutagenesis reveals distortions in the helical structure of the δM4 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor

Daniel Caballero-Rivera, Omar A. Cruz-Nieves, Jessica Oyola-Cintrón, David A. Torres-Nunez, Jose D. Otero-Cruz and José A. Lasalde-Dominicci
http://dx.doi.org/10.4161/chan.19540
Abstract | Full Text | PDF

The lipid-protein interface is an important domain of the nicotinic acetylcholine receptor (nAChR) that has recently garnered increased relevance. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, there is still a need to gain insight into the mechanism by which lipid-protein interactions regulate the function and conformational transitions of the nAChR. In this study, we extended the tryptophan scanning mutagenesis (TrpScanM) approach to dissect secondary structure and monitor the conformational changes experienced by the δM4 transmembrane domain (TMD) of the Torpedo californica nAChR, and to identify which positions on this domain are potentially linked to the regulation of ion channel kinetics. The difference in oscillation patterns between the closed- and open-channel states suggests a substantial conformational change along this domain as a consequence of channel activation. Furthermore, TrpScanM revealed distortions along the helical structure of this TMD that are not present on current models of the nAChR. Our results show that a Thr-Pro motif at positions 462–463 markedly bends the helical structure of the TMD, consistent with the recent crystallographic structure of the GluCl Cys-loop receptor which reveals a highly bent TMD4 in each subunit. This Thr-Pro motif acts as a molecular hinge that delineates two gating blocks in the δM4 TMD. These results suggest a model in which a hinge-bending motion that tilts the helical structure is combined with a spring-like motion during transition between the closed- and open-channel states of the δM4 TMD.

Compound heterozygous mutations in the SUR1 (ABCC 8) subunit of pancreatic K_ATP channels cause neonatal diabetes by perturbing the coupling between Kir6.2 and SUR1 subunits

Yu-Wen Lin, Alejandro Akrouh, YeouChing Hsu, Nkecha Hughes, Diva De León and Colin G. Nichols
Abstract | Full Text | PDF

K_ATP channels regulate insulin secretion by coupling β-cell metabolism to membrane excitability. These channels are comprised of a pore-forming Kir6.2 tetramer which is enveloped by four regulatory SUR1 subunits. ATP acts on Kir6.2 to stabilize the channel closed state while ADP (coordinated with Mg²⁺) activates channels via the SUR1 domains. Aberrations in nucleotide-binding or in coupling binding to gating can lead to hyperinsulinism or diabetes. Here, we report a case of diabetes in a 7-mo old child with compound heterozygous mutations in ABCC8 (SUR1[A30V] and SUR1[G296R]). In unison, these mutations lead to a gain of K_ATP channel function, which will attenuate the β-cell response to increased metabolism and will thereby decrease insulin secretion. ⁸⁶Rb⁺ flux assays on COSm6 cells coexpressing the mutant subunits (to recapitulate the compound heterozygous state) show a 2-fold increase in basal rate of ⁸⁶Rb⁺ efflux relative to WT channels. Experiments on excised inside-out patches also reveal a slight increase in activity, manifested as an enhancement in stimulation by MgADP in channels expressing the compound heterozygous mutations or homozygous G296R mutation. In addition, the IC₅₀ for ATP inhibition of homomeric A30V channels was increased ~6-fold, and was increased ~3-fold for both heteromeric A30V+WT channels or compound heterozygous (A30V +G296R) channels. Thus, each mutation makes a mechanistically distinct contribution to the channel gain-of-function that results in neonatal diabetes, and which we predict may contribute to diabetes in related carrier individuals.





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REVIEW

A hot topic: Temperature sensitive sodium channelopathies

Csilla Egri and Peter C. Ruben
Abstract

Perturbations to body temperature affect almost all cellular processes and, within certain limits, results in minimal effects on overall physiology. Genetic mutations to ion channels or channelopathies can, however, shift the fine homeostatic balance resulting in a decreased threshold to temperature induced disturbances. This review summarizes the functional consequences of currently identified voltage-gated sodium (Na_V) channelopathies that lead to disorders with a temperature sensitive phenotype. A comprehensive knowledge of the relationships between genotype and environment is not only important for understanding the etiology of disease, but also for developing safe and effective treatment paradigms.




SHORT COMMUNICATION

The activity of the TRP-like channel depends on its expression system

Shaya Lev, Ben Katz and Baruch Minke
Abstract

The Drosophila light activated TRP and TRPL channels have been a model for TRPC channel gating. Several gating mechanisms have been proposed following experiments conducted on photoreceptor and tissue cultured cells. However, conclusive evidence for any mechanism is still lacking. Here, we show that the Drosophila TRPL channel expressed in tissue cultured cells is constitutively active in S2 cells but is silent in HEK cells. Modulations of TRPL channel activity in different expression system by pharmacology or specific enzymes, which change the lipid content of the plasma membrane, resulted in conflicting effects. These findings demonstrate the difficulty in elucidating TRPC gating, as channel behavior is expression system dependent. However, clues on the gating mechanism may arise from understanding how different expression systems affect TRPC channel activation.




RESEARCH PAPER

Chronic pregabalin inhibits synaptic transmission between rat dorsal root ganglion and dorsal horn neurons in culture

Janek Hendrich, Claudia S. Bauer and Annette C. Dolphin
Abstract

In this study, we have examined the properties of synaptic transmission between dorsal root ganglion (DRG) and dorsal horn (DH) neurons, placed in co-culture. We also examined the effect of the anti-hyperalgesic gabapentinoid drug pregabalin (PGB) at this pharmacologically relevant synapse. The main method used was electrophysiological recording of excitatory post synaptic currents (EPSCs) in DH neurons. Synaptic transmission between DRG and DH neurons was stimulated by capsaicin, which activates transient receptor potential vanilloid-1 (TRPV1) receptors on small diameter DRG neurons. Capsaicin (1 μM) application increased the frequency of EPSCs recorded in DH neurons in DRG-DH co-cultures, by about 3-fold, but had no effect on other measured properties of the EPSCs. There was also no effect of capsaicin in the absence of co-cultured DRGs. Application of PGB (100 μM) for 40–48 h caused a reduction in the capsaicin-induced increase in EPSC frequency by 57%. In contrast, brief preincubation of PGB had no significant effect on the capsaicin-induced increase in EPSC frequency. In conclusion, this study shows that PGB applied for 40–48 h, but not acute application inhibits excitatory synaptic transmission at DRG-DH synapses, in response to nociceptive stimulation, most likely by a presynaptic effect on neurotransmitter release from DRG presynaptic terminals.