Signal transduction in red bloodcells of the lizards Ameiva ameiva and Tupinambis merianae (Squamata, Teiidae)

Revisiting the question of nucleated versus enucleated erythrocytes in birds and mammals

Erythrocyte enucleation is thought to have evolved in mammals to support their energetic cost of high metabolic activities. However, birds face similar selection pressure yet possess nucleated erythrocytes. Current hypotheses on the mammalian erythrocyte enucleation claim that the absence of cell organelles allows erythrocytes to 1) pack more hemoglobin into the cells to increase oxygen carrying capacity and 2) decrease erythrocyte size for increased surface area-to-volume ratio, and improved ability to traverse small capillaries. In this article, we first empirically tested current hypotheses using both conventional and phylogenetically informed analysis comparing literature values of mean cell hemoglobin concentration (MCHC) and mean cell volume (MCV) between 181 avian and 194 mammalian species. We found no difference in MCHC levels between birds and mammals using both conventional and phylogenetically corrected analysis. MCV was higher in birds than mammals according to conventional analysis, but the difference was lost when we controlled for phylogeny. These results suggested that avian and mammalian erythrocytes may employ different strategies to solve a common problem. To further investigate existing hypotheses or develop new hypothesis, we need to understand the functions of various organelles in avian erythrocytes. Consequently, we covered potential physiological functions of various cell organelles in avian erythrocytes based on current knowledge, while making explicit comparisons to their mammalian counterparts. Finally, we proposed by taking an integrative and comparative approach, using tools from molecular biology to evolutionary biology, would allow us to better understand the fundamental physiological functions of various components of avian and mammalian erythrocytes.

Might proton pump or sodium-hydrogen exchanger inhibitors be of value to ameliorate SARs-CoV-2 pathophysiology?

Discovering therapeutics for COVID-19 is a priority. Besides high-throughput screening of compounds, candidates might be identified based on their known mechanisms of action and current understanding of the SARs-CoV-2 life cycle. Using this approach, proton pump (PPIs) and sodium-hydrogen exchanger inhibitors (NHEIs) emerged, because of their potential to inhibit the release of extracellular vesicles (EVs; exosomes and/or microvesicles) that could promote disease progression, and to directly disrupt SARs-CoV-2 pathogenesis. If EVs exacerbate SARs-CoV-2 infection as suggested for other viruses, then inhibiting EV release by PPIs/NHEIs should be beneficial. Mechanisms underlying inhibition of EV release by these drugs remain uncertain, but may involve perturbing endosomal pH especially of multivesicular bodies where intraluminal vesicles (nascent exosomes) are formed. Additionally, PPIs might inhibit the endosomal sorting complex for transport machinery involved in EV biogenesis. Through perturbing endocytic vesicle pH, PPIs/NHEIs could also impede cleavage of SARs-CoV-2 spike protein by cathepsins necessary for viral fusion with the endosomal membrane. Although pulmonary epithelial cells may rely mainly on plasma membrane serine protease TMPRSS2 for cell entry, PPIs/NHEIs might be efficacious in ACE2-expressing cells where viral endocytosis is the major or a contributing entry pathway. These pharmaceutics might also perturb pH in the endoplasmic reticulum-Golgi intermediate and Golgi compartments, thereby potentially disrupting viral assembly and glycosylation of spike protein/ACE2, respectively. A caveat, however, is that facilitation not inhibition of avian infectious bronchitis CoV pathogenesis was reported in one study after increasing Golgi pH. Envelope protein-derived viroporins contributed to pulmonary edema formation in mice infected with SARs-CoV. If similar pathogenesis occurs with SARs-CoV-2, then blocking these channels with NHEIs could ameliorate disease pathogenesis. To ascertain their potential efficacy, PPIs/NHEIs need evaluation in cell and animal models at various phases of SARs-CoV-2 infection. If they prove to be therapeutic, the greatest benefit might be realized with the administration before the onset of severe cytokine release syndrome.

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.

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.

Melatonin triggers PKA activation in the rodent malaria parasite Plasmodium chabaudi

Calcium (Ca(2+) ) is a critical regulator of many aspects of the Plasmodium reproductive cycle. In particular, intra-erythrocyte Plasmodium parasites respond to circulating levels of the melatonin in a process mediated partly by intracellular Ca(2+) . Melatonin promotes the development and synchronicity of parasites, thereby enhancing their spread and worsening the clinical implications. The signalling mechanisms underlying the effects of melatonin are not fully established, although both Ca(2+) and cyclic AMP (cAMP) have been implicated. Furthermore, it is not clear whether different strains of Plasmodium use the same, or divergent, signals to control their development. The aim of this study was to explore the signalling mechanisms engaged by melatonin in P. chabaudi, a virulent rodent parasite. Using parasites at the throphozoite stage acutely isolated from mice erythrocytes, we demonstrate that melatonin triggers cAMP production and protein kinase A (PKA) activation. Interestingly, the stimulation of cAMP/PKA signalling by melatonin was dependent on elevation of Ca(2+) within the parasite, because buffering Ca(2+) changes using the chelator BAPTA prevented cAMP production in response to melatonin. Incubation with melatonin evoked robust Ca(2+) signals within the parasite, as did the application of a membrane-permeant analogue of cAMP. Our data suggest that P. chabaudi engages both Ca(2+) and cAMP signalling systems when stimulated by melatonin. Furthermore, there is positive feedback between these messengers, because Ca(2+) evokes cAMP elevation and vice versa. Melatonin more than doubled the observed extent of parasitemia, and the increase in cAMP concentration and PKA activation was essential for this effect. These data support the possibility to use melatonin antagonists or derivates in therapeutic approach.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

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.

A novel neurotransmitter-independent communication pathway between axons and glial cells

Recent studies have provided evidence that transmitters released by neurons can activate glial receptors and stimulate calcium signalling in glial cells. Glial calcium signalling, in turn, may affect neuronal performance such as long-term changes in synaptic efficacy. Olfactory ensheathing cells (OECs) are a special glial cell type in vertebrates and insects and promote axon growth in the developing and mature nervous system. Physiological properties of OECs, however, have not been studied so far in detail. We measured changes in the calcium concentration in OECs of the moth Manduca sexta, in situ and in vivo. Electrical stimulation of olfactory receptor neurons in pupae or odour stimulation of receptor neurons in adults resulted in calcium transients in OECs. Olfactory receptor axons release acetylcholine; however, application of acetylcholine or other transmitters such as glutamate, GABA or nitric oxide did not induce calcium transients in OECs. Upon nerve stimulation, extracellular potassium rose by several millimolar as measured with potassium-sensitive microelectrodes. When potassium in the perfusion saline was increased from 4 to 10 mM or higher, voltage-dependent calcium transients in OECs that resembled stimulation-induced calcium transients were evoked. Blocking neuronal potassium channels with TEA reduced both the stimulation-induced increases in extracellular potassium and the calcium transients in OECs, whereas calcium transients in receptor axons were augmented. Our results show for the first time that accumulation of potassium, released by electrically active axons, is sufficient to evoke voltage-dependent calcium influx into glial cells, whereas neurotransmitters appear not to be involved in this neuron-glia communication in Manduca.

In-depth analysis of the membrane and cytosolic proteome of red blood cells

In addition to transporting oxygen and carbon dioxide to and from the tissues, a range of other functions are attributed to red blood cells (RBCs) of vertebrates. Diseases compromising RBC performance in any of these functions warrant in-depth study. Furthermore, the human RBC is a vital host cell for the malaria parasite. Much has been learned from classical biochemical approaches about RBC composition and membrane organization. Here, we use mass spectrometry (MS)-based proteomics to characterize the normal RBC protein profile. The aim of this study was to obtain the most complete and informative human RBC proteome possible by combining high-accuracy, high-sensitivity protein identification technology (quadrupole time of flight and Fourier transform MS) with selected biochemical procedures for sample preparation. A total of 340 membrane proteins and 252 soluble proteins were identified, validated, and categorized in terms of subcellular localization, protein family, and function. Splice isoforms of proteins were identified, and polypeptides that migrated with anomalously high or low apparent molecular weights could be grouped into either ubiquitinylated, partially degraded, or ester-linked complexes. Our data reveal unexpected complexity of the RBC proteome, provide a wealth of data on its composition, shed light on several open issues in RBC biology, and form a departure point for comprehensive understanding of RBC functions.

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.

Exosome release is regulated by a calcium-dependent mechanism in K562 cells

Multivesicular bodies (MVBs) are endocytic structures that contain small vesicles formed by the budding of an endosomal membrane into the lumen of the compartment. Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles termed exosomes. K562 cells are a hematopoietic cell line that releases exosomes. The application of monensin (MON) generated large MVBs that were labeled with a fluorescent lipid. Exosome release was markedly enhanced by MON treatment, a Na+/H+ exchanger that induces changes in intracellular calcium (Ca2+). To explore the possibility that the effect of MON on exosome release was caused via an increase in Ca2+, we have used a calcium ionophore and a chelator of intracellular Ca2+. Our results indicate that increasing intracellular Ca2+ stimulates exosome secretion. Furthermore, MON-stimulated exosome release was completely eliminated by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), implying a requirement for Ca2+ in this process. We have observed that the large MVBs generated in the presence of MON accumulated Ca2+ as determined by labeling with Fluo3-AM, suggesting that intralumenal Ca2+ might play a critical role in the secretory process. Interestingly, our results indicate that transferrin (Tf) stimulated exosome release in a Ca2+-dependent manner, suggesting that Tf might be a physiological stimulus for exosome release in K562 cells.

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

We have previously reported that lizard red blood cells control their cytosolic calcium concentration by sequestering calcium ions in pools, which could be discharged by thapsigargin, by the Na+/H+ ionophore, monensin, by the K+/H+ ionophore, nigericin and by the proton pump inhibitor, bafilomycin A1 [1]. We have now demonstrated, with the aid of confocal microscopy, the presence in these cells of organelles, which accumulate the dye acridine orange and are thus by inference the sites of proton pools. We have found, moreover, that monensin, nigericin and bafilomycin all act to discharge these pools. We further show that calcium release ensues when the calcium ionophore, ionomycin, is added after thapsigargin and monensin; this implies the existence of a third pool, besides the acidic pool and the Endoplasmic Reticulum (ER), which participates in calcium homeostasis. The ER calcium pool can de discharged by the addition of the second messenger, IP3, and we present evidence, based on confocal microscopy, that the IP3 receptors are located in or close to the nucleus.