The putative cache (Ca channel and chemotaxis receptor) domain containing 1 (CACHD1) protein has predicted structural similarities to members of the α2δ voltage-gated Ca channel (VGCC) auxiliary subunit family. CACHD1 mRNA and protein were highly expressed in the male mammalian CNS, in particular in the thalamus, hippocampus and cerebellum, with a broadly similar tissue distribution to Ca3 subunits, in particular, Ca3.1. In expression studies, CACHD1 increased cell-surface localization of Ca3.1 and these proteins were in close proximity at the cell surface consistent with the formation of CACHD1-Ca3.1 complexes. In functional electrophysiological studies, co-expression of human CACHD1 with Ca3.1, Ca3.2 and Ca3.3 caused a significant increase in peak current density and corresponding increases in maximal conductance. By contrast, α2δ-1 had no effect on peak current density or maximal conductance in either Ca3.1, Ca3.2 or Ca3.3. Comparison of CACHD1-mediated increases in Ca3.1 current density and gating currents revealed an increase in channel open probability. In hippocampal neurons from male and female E19 rats, CACHD1 overexpression increased Ca3-mediated action potential (AP) firing frequency and neuronal excitability. These data suggest that CACHD1 is structurally an α2δ-like protein that functionally modulates Ca3 voltage-gated calcium channel activity.This is the first study to characterise the CACHD1 protein. CACHD1 is widely expressed in the CNS, in particular in the thalamus, hippocampus and cerebellum. CACHD1 distribution is similar to that of low-voltage-activated (Ca3, T-type) calcium channels, in particular to Ca3.1, a protein which regulates neuronal excitability and is a potential therapeutic target in conditions such as epilepsy and pain. CACHD1 is structurally a α2δ-like protein that functionally increases Ca3 calcium current. CACHD1 increases the presence of Ca3.1 at the cell surface, forms complexes with Ca3.1 at the cell-surface and causes an increase in channel open probability. In hippocampal neurons, CACHD1 causes increases in neuronal firing. Thus, CACHD1 represents a novel protein that modulates Ca3 activity.
In several types of neurons, firing is an intrinsic property produced by specific classes of ion channels. Low-voltage-activated T-type calcium channels (T-channels), which activate with small membrane depolarizations, can generate burst firing and pacemaker activity. Here we have investigated the specific contribution to neuronal excitability of cloned human T-channel subunits. Using HEK-293 cells transiently transfected with the human alpha(1G) (Ca(V)3.1), alpha(1H) (Ca(V)3.2) and alpha(1I) (Ca(V)3.3) subunits, we describe significant differences among these isotypes in their biophysical properties, which are highlighted in action potential clamp studies. Firing activities occurring in cerebellar Purkinje neurons and in thalamocortical relay neurons used as voltage clamp waveforms revealed that alpha(1G) channels and, to a lesser extent, alpha(1H) channels produced large and transient currents, while currents related to alpha(1I) channels exhibited facilitation and produced a sustained calcium entry associated with the depolarizing after-potential interval. Using simulations of reticular and relay thalamic neuron activities, we show that alpha(1I) currents contributed to sustained electrical activities, while alpha(1G) and alpha(1H) currents generated short burst firing. Modelling experiments with the NEURON model further revealed that the alpha(1G) channel and alpha(1I) channel parameters best accounted for T-channel activities described in thalamocortical relay neurons and in reticular neurons, respectively. Altogether, the data provide evidence for a role of alpha(1I) channel in pacemaker activity and further demonstrate that each T-channel pore-forming subunit displays specific gating properties that account for its unique contribution to neuronal firing.