Disruption of Intracellular Calcium Homeostasis as a Therapeutic Target Against Trypanosoma cruzi

Abstract

There is no effective cure for Chagas disease, which is caused by infection with the arthropod-borne parasite, Trypanosoma cruzi. In the search for new drugs to treat Chagas disease, potential therapeutic targets have been identified by exploiting the differences between the mechanisms involved in intracellular Ca²⁺ homeostasis, both in humans and in trypanosomatids. In the trypanosomatid, intracellular Ca²⁺ regulation requires the concerted action of three intracellular organelles, the endoplasmic reticulum, the single unique mitochondrion, and the acidocalcisomes. The single unique mitochondrion and the acidocalcisomes also play central roles in parasite bioenergetics. At the parasite plasma membrane, a Ca²⁺-^-ATPase (PMCA) with significant differences from its human counterpart is responsible for Ca²⁺ extrusion; a distinctive sphingosine-activated Ca²⁺ channel controls Ca²⁺ entrance to the parasite interior. Several potential anti-trypansosomatid drugs have been demonstrated to modulate one or more of these mechanisms for Ca²⁺ regulation. The antiarrhythmic agent amiodarone and its derivatives have been shown to exert trypanocidal effects through the disruption of parasite Ca²⁺ homeostasis. Similarly, the amiodarone-derivative dronedarone disrupts Ca²⁺ homeostasis in T. cruzi epimastigotes, collapsing the mitochondrial membrane potential (ΔΨ_m), and inducing a large increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) from this organelle and from the acidocalcisomes in the parasite cytoplasm. The same general mechanism has been demonstrated for SQ109, a new anti-tuberculosis drug with potent trypanocidal effect. Miltefosine similarly induces a large increase in the [Ca²⁺]_i acting on the sphingosine-activated Ca²⁺ channel, the mitochondrion and acidocalcisomes. These examples, in conjunction with other evidence we review herein, strongly support targeting Ca²⁺ homeostasis as a strategy against Chagas disease.

Keywords: calcium; new drugs candidates; signaling; therapeutic target; trypanosomatids.

Figure 1

Schematic representation of the mechanisms involved in the intracellular Ca²⁺ regulation in Trypanosoma cruzi. (1) Sphingosine-activated Ca²⁺ channel, responsible for Ca²⁺ entry. (2) Calmodulin-regulated plasma membrane Ca²⁺ Pump, responsible for Ca²⁺ extrusion. (3) Mitochondrial Ca²⁺ Uniporter (MCU) and a Na⁺/ Ca²⁺ exchanger at the unique parasit mitochondrion. (4) SERCA type Ca²⁺ Pump at the endoplasmic reticulum and (5) a Ca²⁺ channel for Ca²⁺ release. (6) A PMCA type Ca²⁺-ATPase, responsible for Ca²⁺ accumulation in acidocalcisomes and (7) an IP₃ Receptor for Ca²⁺ release from the acidocalcisomes to the cytoplasm (See text for detailed explanations).

Figure 2

Expanded model of Ca²⁺ regulating mechanisms within the parasites plasma membrane. Left: The sphingosine-stimulated Ca²⁺ channel, where the activation by Bay K 8466 and miltefosine and the inhibition by nifedipine and verapamil are depicted. Right: The CaM-stimulated PMCA, where the inhibition by pentamidine and crystal violet are shown (See text for explanations).

Figure 3

Structural depiction of main compounds known to exert trypanocidal effect through intracellular Ca²⁺ homeostasis disruption.

Figure 4

Schematic representation of the ergosterol synthesis pathway, showing target points for the main inhibitors.