Evolution of acidocalcisomes and their role in polyphosphate storage and osmoregulation in eukaryotic microbes

Acidocalcisomes are acidic electron-dense organelles, rich in polyphosphate (poly P) complexed with calcium and other cations. While its matrix contains enzymes related to poly P metabolism, the membrane of the acidocalcisomes has a number of pumps (Ca²⁺-ATPase, V-H⁺-ATPase, H⁺-PPase), exchangers (Na⁺/H⁺, Ca²⁺/H⁺), and at least one channel (aquaporin). Acidocalcisomes are present in both prokaryotes and eukaryotes and are an important storage of cations and phosphorus. They also play an important role in osmoregulation and interact with the contractile vacuole complex in a number of eukaryotic microbes. Acidocalcisomes resemble lysosome-related organelles (LRO) from mammalian cells in many of their properties. They share similar morphological characteristics, acidic properties, phosphorus contents and a system for targeting of their membrane proteins through adaptor complex-3 (AP-3). Storage of phosphate and cations may represent the ancestral physiological function of acidocalcisomes, with cation and pH homeostasis and osmoregulatory functions derived following the divergence of prokaryotes and eukaryotes.     

生物细胞中颗粒状多聚磷酸盐细胞器的结构与功能

多聚磷酸盐(polyphosphat,poly P)是一种由数十个或上百个磷酸根聚合而成的生物大分子,以颗粒状、胶体状和溶解状等多种状态存在于各类生物细胞中。生物体中的poly P能够通过分解提供能量;鳌合金属离子来调节细胞内渗透压,维持质膜稳定;与蛋白质或DNA结合稳定其结构,减轻细胞应激损伤。颗粒状多聚磷酸盐细胞器主要指细胞中用于贮存颗粒状poly P、金属阳离子以及蛋白质、氨基酸和少量水等物质的细胞器。在寄生虫细胞中颗粒状聚磷细胞器常称为酸性钙体,而细菌或者其他微生物细胞中则称为异染颗粒,但是随着研究的不断深入,发现酸性钙体和异染颗粒都具有相似的结构特征,遂将其统一定义为颗粒状多聚磷酸盐细胞器。颗粒状聚磷细胞器的发现拓展了生物共同祖先(last universal common ancestor,LUCA)的学说,丰富了原核生物细胞器认知,我们相信该细胞器在生命起源、抗环境胁迫、生物互作和代谢调控等方面具有重要功能,在疾病治疗以及磷生物地球化学循环过程中发挥重要作用。     

Acidocalcisome is required for autophagy in Trypanosoma brucei

Lysosomes play important roles in autophagy, not only in autophagosome degradation, but also in autophagy initiation. In Trypanosoma brucei, an early divergent protozoan parasite, we discovered a previously unappreciated function of the acidocalcisome, a lysosome-related organelle characterized by acidic pH and large content of Ca²⁺ and polyphosphates, in autophagy regulation. Starvation- and chemical-induced autophagy is accompanied with acidocalcisome acidification, and blocking the acidification completely inhibits autophagosome formation. Blocking acidocalcisome biogenesis by depleting the adaptor protein-3 complex, which does not affect lysosome biogenesis or function, also inhibits autophagy. Overall, our results support the role of the acidocalcisome, a conserved organelle from bacteria to human, as a relevant regulator in autophagy.     

钙酸体的结构特征和功能

钙酸体是一类存在于原生寄生虫中的特殊的细胞器。它的结构及元素成分特殊,在寄生虫的生长、生存及毒性方面具有重要作用。本文主要介绍了钙酸体的发现和命名、结构特征及其在寄生虫中的功能。     

Acidocalcisome is required for autophagy in Trypanosoma brucei

Lysosomes play important roles in autophagy, not only in autophagosome degradation, but also in autophagy initiation. In Trypanosoma brucei, an early divergent protozoan parasite, we discovered a previously unappreciated function of the acidocalcisome, a lysosome-related organelle characterized by acidic pH and large content of Ca²⁺ and polyphosphates, in autophagy regulation. Starvation- and chemical-induced autophagy is accompanied with acidocalcisome acidification, and blocking the acidification completely inhibits autophagosome formation. Blocking acidocalcisome biogenesis by depleting the adaptor protein-3 complex, which does not affect lysosome biogenesis or function, also inhibits autophagy. Overall, our results support the role of the acidocalcisome, a conserved organelle from bacteria to human, as a relevant regulator in autophagy.     

Calcium‐ and polyphosphate‐containing acidocalcisomes in chicken egg yolk

Background information. Poly P (inorganic polyphosphate) is a polymer formed by P_i residues linked by high‐energy phosphoanhydride bonds. The presence of poly P in bacteria, fungi, algae and protists has been widely recognized, but the distribution of poly P in more complex eukaryotes has been poorly studied. Poly P accumulates, together with calcium, in acidic vesicles or acidocalcisomes in a number of organisms and possesses a diverse array of functions, including roles in stress response, blood clotting, inflammation, calcification, cell proliferation and apoptosis. Results. We report here that a considerable amount of phosphorus in the yolk of chicken eggs is in the form of poly P. DAPI (4′,6‐diamidino‐2‐phenylindole) staining showed that poly P is localized mainly in electron‐dense vesicles located inside larger vacuoles (compound organelles) that are randomly distributed in the yolk. These internal vesicles were shown to contain calcium, potassium, sodium, magnesium, phosphorus, chlorine, iron and zinc, as detected by X‐ray microanalysis and elemental mapping. These vesicles stain with the acidophilic dye Acridine Orange. The presence of poly P in organellar fractions of the egg yolk was evident in agarose gels stained with Toluidine Blue and DAPI. Of the total phosphate (P_i) of yolk organelles, 16% is present in the form of poly P. Total poly P content was not altered during the first 4 days of embryogenesis, but poly P chain length decreased after 1 day of development. Conclusions. The results of the present study identify a novel organelle in chicken egg yolk comprising acidic vesicles with a morphology, physiology and composition similar to those of acidocalcisomes, within larger acidic vacuoles. The elemental composition of these acidocalcisomes is proportionally similar to the elemental composition of the yolk, suggesting that most of these elements are located in these organelles, which might be an important storage compartment in eggs.