TRIP8b Splice Variants Form a Family of Auxiliary Subunits that Regulate Gating and Trafficking of HCN Channels in the Brain

Abstract

Summary Hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels, which generate the Ih current, mediate a number of important brain functions. The HCN1 isoform regulates dendritic integration in cortical pyramidal neurons, and provides an inhibitory constraint on both working memory in prefrontal cortex and spatial learning and memory in the hippocampus. Altered expression of HCN1 following seizures may contribute to the development of temporal lobe epilepsy. Yet the regulatory networks and pathways governing HCN channel expression and function in the brain are largely unknown. Here we report the presence of nine alternative N-terminal splice forms of the brain-specific cytoplasmic protein TRIP8b and demonstrate the differential effects of six isoforms to downregulate or upregulate HCN1 surface expression. Furthermore, we find that all TRIP8b isoforms inhibit channel opening by shifting activation to more negative potentials. TRIP8b thus functions as an auxiliary subunit that provides a mechanism for the dynamic regulation of HCN1 channel expression and function.

HCN Channels: From the Role in Chronic Cerebral Hypoperfusion-Induced Cognitive Impairments to a New Therapeutic Target for Vascular Dementia

“…A quantitative realtime PCR analysis demonstrated that TRIP8b (1a-4) and TRIP8b (1a) are the most abundant TRIP8b mRNA species in the brain, representing 30%-40% and 25%-30% of total TRIP8b mRNA, respectively. While TRIP8b (1b-2) expressed at somewhat lower levels, accounts for 10%-15% of total brain TRIP8b mRNA [51]. TRIP8b (1a-4) strongly increases HCN1 surface expression [51,52] and decreases the surface expression of HCN2 [53].…”

“…While TRIP8b (1b-2) expressed at somewhat lower levels, accounts for 10%-15% of total brain TRIP8b mRNA [51]. TRIP8b (1a-4) strongly increases HCN1 surface expression [51,52] and decreases the surface expression of HCN2 [53]. While TRIP8b (1b-2) has been reported to produce a potent downregulation in the surface expression of HCN1 and HCN2 [43].…”

Degeneracy in hippocampal physiology and plasticity

“…For instance, A-type K + channels in the hippocampus could be assembled by the main subunits from the Kv1 or Kv4 families and auxiliary subunits from the KChIP and DPP families (Amarillo et al, 2008;Birnbaum et al, 2004;Jerng et al, 2004;Kim et al, 2007;Kim et al, 2005;Vacher and Trimmer, 2011), whereas auxiliary subunits MiRP1, KCR1 and TRIP8b have been implicated in regulating trafficking and properties of h channels assembled with main subunits from the HCN family of proteins. Additionally, the properties of h channels, in terms of their voltage-dependence, their kinetics and modulation by cyclic nucleotides, are critically regulated by the specific isoforms that are expressed in conjunction with the specific stoichiometry of such expression (Biel et al, 2009;He et al, 2014;Lewis et al, 2011;Much et al, 2003;Robinson and Siegelbaum, 2003;Santoro et al, 2000;Santoro et al, 2009;Santoro et al, 2004;Ulens and Siegelbaum, 2003;Ulens and Tytgat, 2001;Zolles et al, 2009). …”

“…http://dx.doi.org/10.1101/203943 doi: bioRxiv preprint first posted online Oct. 16, 2017; Zukin, 2007; Lewis et al, 2011;Lujan et al, 2009;Nusser, 2012;Santoro et al, 2009;Shah et al, 2010;Shepherd and Huganir, 2007;Steward and Schuman, 2001;Turrigiano, 2008;Vacher et al, 2008;Wenthold et al, 2003), and it is now clear that all protein molecules undergo modulation in response to activity patterns or behavioral experience or pathology (Kim and Linden, 2007). In addition to these changes in cytosolic and membrane proteins, it has been shown that hippocampal spines undergo continuous structural changes, apart from demonstrations of distinct forms of structural plasticity in spines, dendrites and axons (Attardo et al, 2015;Chen et al, 2014;Emoto, 2011;Engert and Bonhoeffer, 1999;Ghiretti and Paradis, 2014;Grubb and Burrone, 2010a, b;Grubb et al, 2011;Ikegaya et al, 2001;Johnston et al, 2016;Luo and O'Leary, 2005;Matsuzaki et al, 2004;Nagerl et al, 2004;Tonnesen et al, 2014;Yuste and Bonhoeffer, 2001).…”

Dynamic measurements for funny channels

“…1A) HCN in a 1:1 stoichiometry and has profound effects on channel trafficking and gating (8)(9)(10). The C-terminal tetratricopeptide repeat (TPR) domain of TRIP8b interacts with the final three amino acids of HCN (SNL in HCN1, -2, and -4; ANM in HCN3) and this interaction affects channel trafficking (9,11,12).…”