Red blood cells of the lizards Ameiva ameiva (Squamata, Teiidae) display multiple mechanisms to control cytosolic calcium

Extracellular ATP triggers proteolysis and cytosolic Ca²⁺ rise in Plasmodium berghei and Plasmodium yoelii malaria parasites

BACKGROUND

Plasmodium has a complex cell biology and it is essential to dissect the cell-signalling pathways underlying its survival within the host.

METHODS

Using the fluorescence resonance energy transfer (FRET) peptide substrate Abz-AIKFFARQ-EDDnp and Fluo4/AM, the effects of extracellular ATP on triggering proteolysis and Ca²⁺ signalling in Plasmodium berghei and Plasmodium yoelii malaria parasites were investigated.

RESULTS

The protease activity was blocked in the presence of the purinergic receptor blockers suramin (50 μM) and PPADS (50 μM) or the extracellular and intracellular calcium chelators EGTA (5 mM) and BAPTA/AM (25, 100, 200 and 500 μM), respectively for P. yoelii and P. berghei. Addition of ATP (50, 70, 200 and 250 μM) to isolated parasites previously loaded with Fluo4/AM in a Ca²⁺-containing medium led to an increase in cytosolic calcium. This rise was blocked by pre-incubating the parasites with either purinergic antagonists PPADS (50 μM), TNP-ATP (50 μM) or the purinergic blockers KN-62 (10 μM) and Ip5I (10 μM). Incubating P. berghei infected cells with KN-62 (200 μM) resulted in a changed profile of merozoite surface protein 1 (MSP1) processing as revealed by western blot assays. Moreover incubating P. berghei for 17 h with KN-62 (10 μM) led to an increase in rings forms (82% ± 4, n = 11) and a decrease in trophozoite forms (18% ± 4, n = 11).

CONCLUSIONS

The data clearly show that purinergic signalling modulates P. berghei protease(s) activity and that MSP1 is one target in this pathway.

Python erythrocytes are resistant to α-hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli lyses mammalian erythrocytes by creating nonselective cation pores in the membrane. Pore insertion triggers ATP release and subsequent P2X receptor and pannexin channel activation. Blockage of either P2X receptors or pannexin channels reduces HlyA-induced hemolysis. We found that erythrocytes from Python regius and Python molurus are remarkably resistant to HlyA-induced hemolysis compared to human and Trachemys scripta erythrocytes. HlyA concentrations that induced maximal hemolysis of human erythrocytes did not affect python erythrocytes, but increasing the HlyA concentration 40-fold did induce hemolysis. Python erythrocytes were more resistant to osmotic stress than human erythrocytes, but osmotic stress tolerance per se did not confer HlyA resistance. Erythrocytes from T. scripta, which showed higher osmotic resistance than python erythrocytes, were as susceptible to HlyA as human erythrocytes. Therefore, we tested whether python erythrocytes lack the purinergic signalling known to amplify HlyA-induced hemolysis in human erythrocytes. P. regius erythrocytes increased intracellular Ca²⁺ concentration and reduced cell volume when exposed to 3 mM ATP, indicating the presence of a P2X₇-like receptor. In addition, scavenging extracellular ATP or blocking P2 receptors or pannexin channels reduced the HlyA-induced hemolysis. We tested whether the low HlyA sensitivity resulted from low affinity of HlyA to the python erythrocyte membrane. We found comparable incorporation of HlyA into human and python erythrocyte membranes. Taken together, the remarkable HlyA resistance of python erythrocytes was not explained by increased osmotic resistance, lack of purinergic hemolysis amplification, or differences in HlyA affinity.

The role of melatonin in parasite biology

Regarded as the circadian hormone in mammals, melatonin is a highly conserved molecule, present in nearly all species. In this review, we discuss the role of this indolamine and its precursors in the cell biology of parasites and the role of the molecule in the physiology of the host. In Plasmodium, melatonin can modulate intracellular concentrations of calcium and cAMP, which in turn can regulate kinase activity and cell cycle. In Trypanosoma infections, modulation of the immune system by melatonin is extremely important in controlling the parasite population. Melatonin also contributes to the inflammatory response to Toxoplasma gondii infection. Thus, there are a number of unique adaptations involving intricate connections between melatonin and the biology of the parasite-host relationship.

Divergent calcium signaling in RBCs from Tropidurus torquatus (Squamata--Tropiduridae) strengthen classification in lizard evolution

Background

We have previously reported that a Teiid lizard red blood cells (RBCs) such as Ameiva ameiva and Tupinambis merianae controls intracellular calcium levels by displaying multiple mechanisms. In these cells, calcium stores could be discharged not only by: thapsigargin, but also by the Na⁺/H⁺ ionophore monensin, K⁺/H⁺ ionophore nigericin and the H⁺ pump inhibitor bafilomycin as well as ionomycin. Moreover, these lizards possess a P2Y-type purinoceptors that mobilize Ca²⁺ from intracellular stores upon ATP addition.

Results

Here we report, that RBCs from the tropidurid lizard Tropidurus torquatus store Ca²⁺ in endoplasmic reticulum (ER) pool but unlike in the referred Teiidae, these cells do not store calcium in monensin-nigericin sensitive pools. Moreover, mitochondria from T. torquatus RBCs accumulate Ca²⁺. Addition of ATP to a calcium-free medium does not increase the [Ca²⁺]_c levels, however in a calcium medium we observe an increase in cytosolic calcium. This is an indication that purinergic receptors in these cells are P2X-like.

Conclusion

T. torquatus RBCs present different mechanisms from Teiid lizard red blood cells (RBCs), for controlling its intracellular calcium levels. At T. torquatus the ion is only stored at endoplasmic reticulum and mitochondria. Moreover activation of purinergic receptor, P2X type, was able to induce an influx of calcium from extracelullar medium. These studies contribute to the understanding of the evolution of calcium homeostasis and signaling in nucleated RBCs.

Calcium signaling in lizard red blood cells

The ion calcium is a ubiquitous second messenger, present in all eukaryotic cells. It modulates a vast number of cellular events, such as cell division and differentiation, fertilization, cell volume, decodification of external stimuli. To process this variety of information, the cells display a number of calcium pools, which are capable of mobilization for signaling purposes. Here we review the calcium signaling on lizards red blood cells, an interesting model that has been receiving an increasing notice recently. These cells possess a complex machinery to regulate calcium, and display calcium responses to extracellular agonists. Interestingly, the pattern of calcium handling and response are divergent in different lizard families, which enforces the morphological data to their phylogenetic classification, and suggest the radiation of different calcium signaling models in lizards evolution.

Involvement of Na+/H+exchanger in the calcium signaling in epimastigotes ofTrypanosoma cruzi

We studied the effect of Na(+) extracellular on Ca(2+) mobilization from intracellular store evoked by carbachol in Trypanosoma cruzi. We report that slow component of Ca(2+) signaling evoked by agonist is dependent on extracellular Na(+) but not on InsP(3) increase. Moreover, this Ca(2+) signaling progressively increased when pH of the medium changed from 7.0 to 7.8. In addition, we found that it was regulated by PKC. The agonist was also able to induce the alkalinization of the acidic compartment, and both Ca(2+) signaling and alkalinization were inhibited by the EIPA-inhibitor of the Na(+)/H(+) exchanger. These results demonstrated the alkalinization of acidic vacuoles and PKC are involved in the triggering of the epimastigote Ca(2+) signaling.

Products of tryptophan catabolism induce Ca2+ release and modulate the cell cycle of Plasmodium falciparum malaria parasites

Intraerythrocytic malaria parasites develop in a highly synchronous manner. We have previously shown that the host hormone melatonin regulates the circadian rhythm of the rodent malaria parasite, Plasmodium chabaudi, through a Ca2+-based mechanism. Here we show that melatonin and other molecules derived from tryptophan, i.e. N-acetylserotonin, serotonin and tryptamine, also modulate the cell cycle of human malaria parasite P. falciparum by inducing an increase in cytosolic free Ca2+. This occurs independently of the extracellular Ca2+ concentration, indicating that these molecules induce Ca2+ mobilization from intracellular stores in the trophozoite. This in turn leads to an increase in the proportion of schizonts. The effects of the indolamines in increasing cytosolic free Ca2+ and modulating the parasite cell cycle are both abrogated by an antagonist of the melatonin receptor, luzindole, and by the phospholipase inhibitor, U73122.

Activation of a P2Y4-like purinoceptor triggers an increase in cytosolic [Ca2+] in the red blood cells of the lizard Ameiva ameiva (Squamata, Teiidae)

An increasing number of pathophysiological roles for purinoceptors are emerging, some of which have therapeutic potential. Erythrocytes are an important source of purines, which can be released under physiological and physiopathological conditions, acting on purinergic receptors associated with the same cell or with neighboring cells. Few studies have been conducted on lizards, and have been limited to ATP agonist itself. We have previously shown that the red blood cells (RBCs) of the lizard Ameiva ameiva store Ca2+ in the endoplasmic reticulum (ER) and that the purinergic agonist ATP triggers a rapid and transient increase of [Ca2+]c by mobilization of the cation from internal stores. We also reported the ability of the second messenger IP3 to discharge the ER calcium pool of the ER. Here we characterize the purinoceptor present in the cytoplasmic membrane of the RBCs of the lizard Ameiva ameiva by the selective use of ATP analogues and pyrimidine nucleotides. The nucleotides UTP, UDP, GTP, and ATPgammaS triggered a dose-dependent response, while interestingly 2MeSATP, 2ClATP, alpha, ss-ATP, and ADP failed to do so in a 1- to 200-microm con- centration. The EC50 obtained for the compounds tested was 41.77 microM for UTP, 48.11 microM for GTP, 53.11 microM for UDP, and 30.78 microM for ATPgammaS. The present data indicate that the receptor within the RBCs of Ameiva ameiva is a P2Y4-like receptor due to its pharmacological similarity to the mammalian P2Y4 receptor.