Alternatively Spliced Isoforms of TRIP8b Differentially Control h Channel Trafficking and Function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) are the molecular basis for the current, I-h, which contributes crucially to intrinsic neuronal excitability. The subcellular localization and biophysical properties of h channels govern their function, but the mechanisms controlling these characteristics, and especially the potential role of auxiliary subunits or other binding proteins, remain unclear. We focused on TRIP8b, an h channel-interacting protein that colocalizes with HCN1 in cortical and hippocampal pyramidal neuron dendrites, and found that it exists in multiple alternative splice variants with distinct effects on h channel trafficking and function. The developmentally regulated splice variants of TRIP8b all shared dual, C terminus-located interaction sites with HCN1. When coexpressed with HCN1 in heterologous cells individual TRIP8b isoforms similarly modulated gating of I-h, causing a hyperpolarizing shift in voltage dependence of channel activation, but differentially upregulated or downregulated I-h current density and HCN1 surface expression. In hippocampal neurons, coexpression of TRIP8b isoforms with HCN1 produced isoform-specific changes of HCN1 localization. Interestingly, the TRIP8b isoforms most abundant in the brain are those predicted to enhance h channel surface expression. Indeed, shRNA knockdown of TRIP8b in hippocampal neurons significantly reduced native I-h. Thus, although TRIP8b exists in multiple splice isoforms, our data suggest that the predominant role of this protein in brain is to promote h channel surface expression and enhance I-h. Because I-h expression is altered in models of several diseases, including temporal lobe epilepsy, TRIP8b may play a role in both normal neuronal function and in aberrant neuronal excitability associated with neurological disease.