Chapter Five - Localization and Targeting of GIRK Channels in Mammalian Central Neurons
Introduction
Ion channels are classified by their gating properties and ion selectivity for Na+, Ca2 +, Cl−, and K+ (Hille, 2001). Potassium (K+)-selective channels are key determinants of membrane excitability and regulate a variety of cellular processes including membrane potential, signal transduction, hormone release, vascular tone, cell volume, and immune responses (Curran, 1998). Four different subfamilies of K+ channels have been proposed based on their structural and phylogenetic relationship: voltage-gated K+ (Kv) channels, Ca2 +-activated K+ (KCa) channels, two-pore K+ (K2P) channels, and inwardly rectifying (Kir) channels (Gutman et al., 2005).
Kir channels are formed by 16 subunits falling into 7 subfamilies (Kir1–Kir7), including the Kir3 subfamily, which is directly coupled to heterotrimeric G proteins and mediates the inhibitory effect of activated G protein-coupled receptors (GPCRs) on neuronal excitability (Dascal, 1997, Kubo et al., 2005, Luján et al., 2014, Luján et al., 2009, Yamada et al., 1998). This subfamily of Kir channels, also known as G protein-gated inwardly rectifying K+ (GIRK) channels, is composed of four subunits, designated GIRK1–4 (Dascal, 1997, Luján et al., 2014, Lüscher and Slesinger, 2010).
The past decade has witnessed great progress in our understanding of the diverse functional roles that GIRK channels fulfill in different neuron populations and brain regions. Molecular cloning of GIRK channel genes has led to the design and synthesis of antibodies that, combined with several immunohistochemical techniques, facilitated studies on the distribution of GIRK channel subunits in the CNS. Indeed, immunohistochemical approaches at both the light and electron microscopic levels have yielded important insights into the distribution and precise subcellular localization of GIRK channels in neurons. In particular, the use of high-resolution immunohistochemical techniques at the electron microscopic level has allowed unparalleled precision, showing a highly regulated subcellular distribution patterns for GIRK channel subunits that are also region- and cell type-dependent (Luján et al., 2014, Luján et al., 2009). The demonstration of this precise subcellular compartmentalization provides a new understanding of the role of GIRK channels in information transfer and processing within neurons and neural networks under physiological and pathological conditions. In this chapter, we summarize current anatomical knowledge describing the regional and cellular distribution of the four GIRK channel subunits in the mammalian CNS. The precise subcellular distribution of GIRK channels at the surface of central neurons will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.
Section snippets
Molecular Organization and Heterogeneity of GIRK Channels
The structure and function of GIRK channels are reviewed in other chapters of this volume, and we refer the reader to those sections for further information. Here, we briefly describe basic molecular, biochemical, and physiological features of GIRK channels.
GIRK channels are composed of four different, but homologous, subunits (GIRK1–4) that are conserved in mouse, rat, and human (Hibino et al., 2010, Yamada et al., 1998). Each subunit contains hydrophilic amino- and carboxy-terminal domains
Neuroanatomical Approaches to the Study of GIRK Channel Distribution
The molecular definition of the mammalian GIRK family has led to the generation of subunit-specific molecular tools for investigating expression levels and cellular patterns of GIRK mRNAs in brain tissue with different techniques. Thus, northern blots, western blots, reverse transcription polymerase chain reaction (RT-PCR), in situ hybridization, histoblots, and immunohistochemical studies have been used to observe the regional, cellular, and subcellular distribution of GIRK channels in the CNS
Regional Distribution of GIRK Channel Subunits in the CNS
In situ hybridization, histoblot, and immunohistochemical studies have revealed that GIRK1, GIRK2, and GIRK3 channel subunits are widely expressed throughout the CNS of rodents, showing distinct although partially overlapping patterns of mRNA and protein distribution (Aguado et al., 2008, Chen et al., 1997, Fernández-Alacid et al., 2009, Fernández-Alacid et al., 2011, Inanobe et al., 1999, Karschin et al., 1996, Kobayashi et al., 1995, Koyrakh et al., 2005, Liao et al., 1996, Miyashita and
Cellular Distribution of GIRK Channel Subunits
Cell biological, biochemical, and electrophysiological approaches indicate that the dominant functional GIRK channel in the CNS contains GIRK1 and GIRK2 (Liao et al., 1996). This view is supported by data obtained using genetic studies, which show that ablation of GIRK2 results in loss of GIRK current in many brain regions, including the hippocampus (Koyrakh et al., 2005, Lüscher et al., 1997), cerebellum (Slesinger, Stoffel, Jan, & Jan, 1997), substantia nigra (Koyrakh et al., 2005), VTA (
Subcellular Distribution of GIRK Channel Subunits
The classical notion that the subcellular localization of ion channels in a particular cell type readily applies to all brain neurons is not well supported by available data. The emerging view is that each ion channel possesses its own unique subcellular distribution pattern in each cell type (Luján, 2010), suggesting that there are no simple rules concerning their subcellular organization. This is particularly true for GIRK channels. Indeed, available evidence suggests that GIRK channels can
Developmental Aspects of GIRK Channel Distribution
Brain development results from a temporospatial pattern of events, beginning with neuronal proliferation, followed by migration and differentiation and ending with synapse formation and circuit refinements. A growing body of evidence suggests that each step in the developmental sequence of the CNS involves the appropriate expression and function of neurotransmitters, their receptors, and ion channels (Luján, 2010, Luján et al., 2005). For example, Kv channels are involved in the control of
Conclusions and Future Perspectives
The electrical signaling of neurons depends largely on the abundance and function of a large diversity of neurotransmitter receptors and ion channels located at specific sites in neuronal somata, dendrites, and axons. The selective placement of GIRK channel subunits at precise locations in mammalian neurons and their dynamic regulation through specific signaling pathways allows for a wide variety of neuronal function in the brain. However, the cellular diversity of the brain and the
Acknowledgments
The authors would like to thank Alexandra Salewski, M.Sc., for the English revision of the manuscript and Mercedes Gil for her excellent technical assistance. We also thank the Spanish Ministry of Education and Science (BFU-2012-38348; Consolider-Ingenio CSD2008-00005), the European Union (HBP—Project Ref. 604102), and the Junta de Comunidades de Castilla-La Mancha (PPII-2014-005-P) for their generous support of our research. Additionally, our gratitude extends to all members of our laboratory
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Cited by (46)
Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system
2021, Pharmacology and TherapeuticsCitation Excerpt :Additionally, GIRK1 and GIRK2 subunits are found in suprachiasmatic nucleus (SCN) with circadian rhythm of GIRK2 expression, suggesting the possible role in the circadian rhythm control, since GirK channels exert the activity of melatonin and neuropeptide Y (Hablitz, Molzof, Paul, Johnson, & Gamble, 2014; Hall, Earle-Cruikshanks, & Harrington, 1999; Sosulina, Schwesig, Seifert, & Pape, 2008). Finally, with the help of immunoelectron microscopy, in situ hybridization and immunofluorescence techniques, we know that GirK channels are predominately expressed in neurons (Aguado et al., 2008; Booker et al., 2013; del Burgo et al., 2008; Labouebe et al., 2007; Lujan & Aguado, 2015; Slesinger & Wickman, 2015). There is little evidence confirming the expression of GirK channels in glial cells, mainly in Bergman glia in the cerebellum (GIRK3 expressed) (Fernandez-Alacid et al., 2009), in Müller retina cells (Kofuji et al., 2002; Newman, 2015) as well as in cultured astrocytes (Perillán et al., 2000) and oligodendrocytes (Karschin & Wischmeyer, 1995; Karschin, Wischmeyer, Davidson, & Lester, 1994).
Advances in Targeting GIRK Channels in Disease
2021, Trends in Pharmacological SciencesCitation Excerpt :Under physiological conditions, the small outward K+ current through GIRK channels decreases the excitability of the cell [1]. Three types of GIRK subunits (GIRK1, GIRK2, and GIRK3) are expressed in the brain and form either homotetramers of GIRK2 or heterotetramers (e.g., GIRK1, GIRK2) in various brain regions [2–4]. Although the GIRK1/GIRK2 combination is abundant in the majority of brain regions, dopaminergic neurons in the ventral tegmental area (VTA) express only GIRK2 and GIRK3 subunits and substantia nigra pars compacta (SNc) dopaminergic neurons express only GIRK2 [4–6].
I<inf>h</inf>, GIRK, and KCNQ/Kv7 channels differently modulate sharp wave - ripples in the dorsal and ventral hippocampus
2020, Molecular and Cellular NeuroscienceCitation Excerpt :These observations suggest that KCNQ channels-dependent memory modulation may involve regulation of synaptic plasticity and SPW-Rs clustering specifically in the dorsal hippocampus. Finally, Ih, GIRK, and KCNQ/Kv7 channels are targets of several neuromodulators in the brain; see reviews by (He et al., 2014; Lujan and Aguado, 2015; Vogalis et al., 2003) whose actions can vary along the hippocampal long axis (Strange et al., 2014). For instance, Ih and KCNQ2/3 channels are modulated by muscarinic cholinergic transmission (Fisahn et al., 2002; Sparks and Chapman, 2014), and GIRK channels are targets of adenosine A1 receptors (Kim and Johnston, 2015).
Identification of a G-Protein-Independent Activator of GIRK Channels
2020, Cell ReportsCitation Excerpt :Conversely, augmentation of GIRK channel activity in an epilepsy model may exert a protective effect, especially if focused on GIRK1/GIRK2 channels, the predominant form in the brain (Liao et al., 1996). In the current study, we show that GiGA1 preferentially activates GIRK1/GIRK2 channels, as well as those channels natively expressed in hippocampal pyramidal neurons (Koyrakh et al., 2005; Luján and Aguado, 2015). Studies of temporal lobe epilepsy have reported the involvement of the hippocampus and suggest that the hippocampus plays an essential role in epilepsy occurring and spreading (McIntyre and Racine, 1986; Diehl et al., 2004; Cascino, 2005).
The small molecule GAT1508 activates brain-specific GIRK1/2 channel heteromers and facilitates conditioned fear extinction in rodents
2020, Journal of Biological ChemistryCitation Excerpt :Because GIRK2 expression is restricted to the nervous system and GAT1508 fails to strengthen channel–PIP2 interactions and activate peripheral GIRK1/4 channels, its effects would be expected to also be restricted to neuronal tissues. Detailed studies in adult rodent brains have suggested co-expression of the GIRK1/2 subunits in multiple brain regions, including the olfactory bulb, neocortex, hippocampus, cerebellum, thalamus, hypothalamus, and amygdala (34). Given this widespread pattern of expression, we decided to explore the effects of GAT1508 in the amygdala and in fear-conditioning paradigms, where the role of the physiological relevance and therapeutic potential of GIRK1/2 have not been studied in depth.