Mechanisms of action of Phoneutria nigriventer toxins on ion channels

BEIRÃO, P.S.L.(1)

(1)Department of Biochemistry and Immunology, Instituto de Ciências Biológicas da UFMG. Caixa Postal 486 - 30161.970 Belo Horizonte, MG, Brazil

The Phoneutria nigriventer (family Ctenidae, suborder Labdognatha) venom is rich in polypeptide toxins, with molecular weights that range from 2 to 9 kDa. It can be factionated into three major fractions that exert neurotoxic effects when injected into mouse central nervous system. In our laboratory we have investigated the effects on ion channels of toxins purified from the venom of P. nigriventer, using the patch clamp technique, in the modalities of whole cell, perforated patch and loose patch clamp. The toxins were purified by the group of Professor Carlos Diniz. Fraction PhTx2 has a marked effect on Na⁺ channels, decreasing the rate of inactivation and shifting to hyperpolarizing potentials the voltage-dependences of the conductance and of the steady-state inactivation. Fraction PhTx2 contains 9 toxins, and two of them (toxins Tx2-5 and Tx2-6) reproduce the major effects of the fraction. These toxins are expressed as preprotoxins, and their amino acid sequences (and of their precursors) have no significant identity with any known peptide, except among themselves. P. nigriventer Tx2-6, whose precursor has 82 amino acids, exerts complex effects. Its predominant effect is similar to -scorpion toxins, but also cause a negative shift of the conductance voltage-dependence and a decrease of Na⁺ current similar to -scorpion toxins. P. nigriventer Tx1 and fraction PhTx3 have no significant effect on Na⁺ currents.

PhTx3 exerts marked inhibitory effect on Ca²⁺ currents. By using cells that express the different types of Ca²⁺ channels, and using appropriate pharmacological tools to selectively inhibit unwanted currents, it was possible to isolate T-, L-, N-, P/Q- and R-type Ca²⁺ channels. We have observed that toxin Tx3-3 inhibited P/Q- and R-type currents, with high affinity. It also inhibits, albeit less effectively, L- and N-type channels, and has no significant effect on T-type currents. Thus, this toxin can be classified as an -type toxin. We also observed that toxin Tx3-2 inhibits L-type Ca²⁺ currents. On the other hand, toxin Tx3-1 has no direct effect on Ca²⁺ currents, but inhibits A-type K⁺ currents, thus affecting Ca²⁺ oscillation frequency in GH3 cells.

Our results show that Phoneutria nigriventer venom is a rich source of neuroactive peptides that act on different types of ion channels.

Supported by FAPEMIG and CNPq. 

CORRESPONDENCE TO:

BEIRÃO, P.S.L., Department of Biochemistry and Immunology, Instituto de CiênciasBiológicasda UFMG. CaixaPostal 486, CEP 30161.970 Belo Horizonte, MG, Brazil, Email: pslb@mono.icb.ufmg.br