Elsevier

Life Sciences

Volume 91, Issues 7–8, 17 September 2012, Pages 258-263
Life Sciences

Distinct cellular distributions of Kv4 pore-forming and auxiliary subunits in rat dorsal root ganglion neurons

https://doi.org/10.1016/j.lfs.2012.07.007Get rights and content

Abstract

Aims

Dorsal root ganglia contain heterogeneous populations of primary afferent neurons that transmit various sensory stimuli. This functional diversity may be correlated with differential expression of voltage-gated K+ (Kv) channels. Here, we examine cellular distributions of Kv4 pore-forming and ancillary subunits that are responsible for fast-inactivating A-type K+ current.

Main methods

Expression pattern of Kv α-subunit, β-subunit and auxiliary subunit was investigated using immunohistochemistry, in situ hybridization and RT-PCR technique.

Key findings

The two pore-forming subunits Kv4.1 and Kv4.3 show distinct cellular distributions: Kv4.3 is predominantly in small-sized C-fiber neurons, whereas Kv4.1 is seen in DRG neurons in various sizes. Furthermore, the two classes of Kv4 channel auxiliary subunits are also distributed in different-sized cells. KChIP3 is the only significantly expressed Ca2 +-binding cytosolic ancillary subunit in DRGs and present in medium to large-sized neurons. The membrane-spanning auxiliary subunit DPP6 is seen in a large number of DRG neurons in various sizes, whereas DPP10 is restricted in small-sized neurons.

Significance

Distinct combinations of Kv4 pore-forming and auxiliary subunits may constitute A-type channels in DRG neurons with different physiological roles. Kv4.1 subunit, in combination with KChIP3 and/or DPP6, form A-type K+ channels in medium to large-sized A-fiber DRG neurons. In contrast, Kv4.3 and DPP10 may contribute to A-type K+ current in non-peptidergic, C-fiber somatic afferent neurons.

Introduction

Voltage-gated K+ (Kv) currents in sensory neurons are divided into two major categories; sustained delayed rectifier (KDR) and transient A-type K+ (KA) currents (Kostyuk et al., 1981, Hall et al., 1994, Gold et al., 1996, Yoshimura et al., 1996). KA current is activated at subthreshold of action potential and rapidly inactivates. Thus, this current is important to determine the initiation and interval of action potentials. KA current in sensory neurons may be carried by a number of Kv pore-forming subunits including Kv1.4 and any of Kv4 subunits (Kv4.1, Kv4.2, and Kv4.3). It has been shown that Kv1.4 is localized in small-sized C-fiber DRG neurons (Rasband et al., 2001). Furthermore, KA current in small-sized C-fiber neurons exhibits slower inactivation and sensitivity to α-dendrotoxin, a blocker of Kv1-family channels. In addition, reduced KA current and Kv1.4 proteins are associated with hyperexcitability of DRG neurons in animal models of bladder pain (Hayashi et al., 2009). Therefore, Kv1.4 significantly contributes to the formation of A-type channels in a subset of C-fiber neurons. In contrast to Kv1.4 subunits, relatively less is known about cellular distributions of Kv4 channel subunits in DRGs. Previous studies showed that Kv4.3 protein is predominantly expressed in non-peptidergic, small-sized DRG neurons (Chien et al., 2007). PCR analysis also detected Kv4.1 mRNA in DRG tissue and a large number of isolated, small to medium-sized DRG neurons (Phuket and Covarrubias, 2009). These findings support differential expression of the two Kv4 pore-forming subunits in distinct DRG neurons. Yet, the cell-size distribution of Kv4.1 in the entire DRG neuronal population remains unclear.

Kv4 pore-forming proteins are known to form complexes with two distinct types of auxiliary subunits that markedly alter channel expression and gating. The first type of Kv4 auxiliary subunits are small cytosolic Ca2 +-binding proteins, namely Kv channel interacting proteins (KChIPs) (An et al., 2000), whereas the other type contains one transmembrane domain with a large extracellular portion similar to dipeptidyl peptidase (DPP6/10) (Jerng et al., 2004, Nadal et al., 2003, Ren et al., 2005). Diverse KChIPs are generated by the presence of four genes (An et al., 2000, Morohashi et al., 2002) and alternative splicing of transcripts (Rosati et al., 2001, Takimoto et al., 2002, Holmqvist et al., 2002, Patel et al., 2002, Boland et al., 2003). However, less is known about the distribution of KChIPs and DPPs in DRG neurons.

We wished to determine cellular distributions and subunit compositions of Kv4 channel complexes in distinct DRG neurons. We utilized PCR analysis, in-situ hybridization and immunohistochemistry to examine the expression and cellular distributions of Kv4 pore-forming and auxiliary subunits in rat DRG neurons.

Section snippets

Materials and methods

Experiments were performed using female Sprangue–Dawley rats (220–250 g). Care and handling of animals were in accordance with institutional guidelines and were approved by the Animal Care and Use Committees of the Nara Medical University and University of Pittsburgh Institutional Animal Care and Use Committees.

Cellular distributions of Kv4 pore-forming subunits in DRGs

Previous PCR analysis suggested significant expression of Kv4.1 and Kv4.3 mRNAs in DRG neurons (Phuket and Covarrubias, 2009). We also observed abundant Kv4.1 and long-isoform of Kv4.3 transcripts, but not Kv4.2 mRNA, in L6-S1 DRGs (unpublished observation). Thus, we first examined the cellular distributions of the two pore-forming subunits, Kv4.1 and Kv4.3. Since commercial anti-Kv4.1 antibodies appeared less suitable for immunohistochemistry, we used in situ hybridization to test for

Discussion

DRGs contain cell bodies for heterogeneous populations of primary afferent neurons. These neurons may be categorized by cell body sizes and innervating tissues. Aα/β-fiber neurons with large-sized cell bodies generally carry mechanical information, whereas small-sized cell bodies correspond to C and Aδ-fiber neurons that are responsible for pain sensation. The latter small-sized neurons are also implicated in the development of chronic pain. In this study, we determined the cellular

Conclusion

Kv4 channel complexes in small-sized, somatic DRG neurons consist of Kv4.1/Kv4.3 and DPP6/10, but not any KChIPs, whereas Kv4 channel complexes in medium to large-sized DRG neurons consist of.Kv4.1, DPP6 and KChIP3.

Conflict of interest statement

None.

Acknowledgements

This work was supported by grants from the National Institute of Health (DK057267 and DK088836), the Department of Defense (SC100134 and PR110326) and the Ministry of Education, Science, Sports and Culture of Japan (22591798).

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