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Low-voltage-activated Ca~v~3 channels that generate T-type currents display unique biophysical properties that allow them to operate near the resting membrane potential of nerve cells where they generate low-threshold calcium spikes leading to burst firing of action potentials and oscillatory discharges. They play an essential role in shaping the electrophysiological properties of thalamic, olivary, and cerebellar neurons, and alteration of Ca~v~3 channel activity is associated with a number of human neuronal disorders \[[@CR1], [@CR2]\]. However, the identification of specific physiological roles associated with each Ca~v~3 isoforms (Ca~v~3.1, Ca~v~3.2, and Ca~v~3.3) is often hampered for several reasons. First, Ca~v~3 channels are often coexpressed in nerve cells. Second, they present a similar electrophysiological signature which renders their molecular identification problematic in native neuronal systems. And third, selective pharmacological tools are not available. Therefore, there is a need for isoform-specific modulators of Ca~v~3 channels in order to better explore their respective physiological functions.

Structure-function studies have identified several channel molecular determinants that are responsible for shaping the unique gating properties of Ca~v~3 channels \[[@CR3]--[@CR8]\]. Recently, we reported the importance of the carboxy terminal domain and showed that the proximal region that is highly conserved across the three Ca~v~3 channel isoforms is essential for the functioning of Ca~v~3.3 channels \[[@CR9]\]. The question then arises as to whether an exogenous peptide corresponding to this proximal carboxy terminal region could potentially modulate the expression of Ca~v~3 channels.

To address this issue, we tested the effect of a TAT-based cell penetrating peptide containing the conserved carboxy proximal region of Ca~v~3.2 (TAT-C3P) on recombinant Ca~v~3 channels expressed in tsA-201 cells. Molecular modeling using Phyres^2^ \[[@CR10]\] predicted that this peptide may adopt a helical conformation (Fig. [1](#Fig1){ref-type="fig"}a). Cells were transfected with 5 μg of cDNA encoding for Ca~v~3.1, Ca~v~3.2, or Ca~v~3.3 channels. Twelve hours after transfection, cells were treated with 10 μg / mL of TAT-C3P peptide (GenScript), or with a control peptide containing a non-conserved distal region of the carboxy terminus of Ca~v~3.2 (TAT-C3D). The effect of the TAT peptide on T-type currents was assessed 48 h later in the whole cell configuration of the patch clamp technique. We observed that treatment of cell with the TAT-C3P produced a potent decrease of the T-type current in cells expressing Ca~v~3.3 channels (Fig. [1](#Fig1){ref-type="fig"}b). For instance, in response to a depolarizing pulse to − 30 mV, a 2.3-fold decrease (*p* \< 0.0001) in the mean peak T-type current density was observed in cells treated with TAT-C3P (− 12.1 ± 2.1 pA/pF, *n* = 31) compared to control (non-treated) cells (− 27.2 ± 2.9 pA/pF, *n* = 41) (Fig. [1](#Fig1){ref-type="fig"}c). The mean maximal slope conductance (*G*~max~) was decreased by 56% (*p* \< 0.0001) from 572 ± 53 pS/pF to 254 ± 39 pS/pF (Fig. [1](#Fig1){ref-type="fig"}d). This effect was not observed when cells were treated with the control TAT-C3D peptide indicating that TAT-C3P-induced inhibition of Ca~v~3.3 was specifically mediated by the carboxy proximal peptide and not from a non-specific effect that could have resulted from TAT itself (Fig. [1](#Fig1){ref-type="fig"}d). Inhibition of T-type currents by TAT-C3P was not associated with additional alteration of the voltage-dependence of activation and inactivation, nor of the recovery from inactivation, and only a slight acceleration of the inactivation kinetics of Ca~v~3.3 currents at hyperpolarized potentials was observed (supplemental Fig. S[1](#MOESM1){ref-type="media"}). Furthermore, this inhibition was not observed when TAT-C3P was acutely infused into the cells via the patch pipette (supplemental Fig. S[2](#MOESM1){ref-type="media"}) suggesting that TAT-C3P-induced inhibition of Ca~v~3.3 is likely to have occurred via a regulatory signaling pathway controlling the expression of the channel rather than via direct alteration of the channel activity itself. Finally, we did not observe any significant effect of TAT-C3P on cells expressing Ca~v~3.1 and Ca~v~3.2 channels indicating that this peptide is selective for Ca~v~3.3 channels (Fig. [1](#Fig1){ref-type="fig"}e and supplemental Fig. S[3](#MOESM1){ref-type="media"}). Considering that the proximal carboxy terminal region of Ca~v~3 channels is highly conserved across the three channel isoforms, the observation that TAT-C3P was effective only on Ca~v~3.3 channels suggests the existence of a distinct regulatory mechanism specific for Ca~v~3.3 that may be compromised by the peptide. Additional analysis will elucidate the detailed mechanisms underlying the effect of this peptide. Fig. 1Effect of TAT-C3P peptide on Ca~v~3 channels. **a** Schematic representation of the membrane topology of Ca~v~3 channels and protein sequence alignment showing the conserved proximal region of the carboxy terminus across all three Ca~v~3 channel isoforms (Ca~v~3.1, Ca~v~3.2, and Ca~v~3.3). Molecular modeling indicates that this region may adopt a helical conformation. **b** Representative T-type current traces recorded from cells expressing Ca~v~3.3 channels in response to 300 ms depolarizing steps to values ranging from − 80 mV to + 20 mV from a holding potential of − 100 mV for control (black traces) and TAT-C3P-treated cells (red traces). **c** Corresponding mean peak current density-voltage (*I*/*V*) relationship. **d** Corresponding mean maximal macroscopic conductance (*G*~max~) values obtained from the fit of the *I*/*V* curves with a modified Boltzmann equation for control (white dots), and cells treated with either TAT-C3P (red dots) or the control TAT-C3D peptide (grey dots). **e** Mean normalized *G*~max~ for cells expressing Ca~v~3.1, Ca~v~3.2, and Ca~v~3.3 channels in response to TAT-C3P treatment

While several pan Ca~v~3 channel blockers have been described, there is to date no molecule selective for one particular Ca~v~3 isoform \[[@CR11]\]. This lack of selective pharmacopeia not only hampered the identification of specific physiological roles for Ca~v~3 channels that in the absence of selective modulator requires the use of genetic or antisense nucleotide approaches, but also compromised the therapeutic development of Ca~v~3 channel modulators. Here, we reported the first non-genetic molecular tool to selectively inhibit Ca~v~3.3 channels in cells, and possibly in vivo. Although the molecular mechanism by which TAT-C3P inhibits Ca~v~3.3 channels remains to be explored in detail, the observation that discreet channel molecular determinants can be harnessed to selectively target a particular channel isoform represents an appealing strategy to study specific physiological functions, and to potentially enhance our understanding of Ca~v~3 channels in disease.

Supplementary information
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 {#Sec2}

**Additional file 1.** Extended methodology and supplemental data.

TAT-C3P

:   TAT-based peptide containing the carboxy proximal region of Ca~v~3 channels

TAT-C3D

:   TAT-based peptide containing the carboxy distal region of Ca~v~3 channels

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Supplementary information
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**Supplementary information** accompanies this paper at 10.1186/s13041-020-00636-y.

We thank Robin N. Stringer for copy-editing the manuscript and Charles University (Progres Q28).

LC performed electrophysiological recordings, analyzed and interpreted the results. NW designed and supervised the study, prepared figures and wrote the manuscript. All authors read and approved the final manuscript.

Not applicable.

All data generated or analyzed during this study are included in this published article and its supplementary information files.

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The authors declare that they have no competing interests.
