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

Abstract

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 (Ca(2+)-ATPase, V-H(+)-ATPase, H(+)-PPase), exchangers (Na(+)/H(+), Ca(2+)/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.

Figure 1.

Acidocalcisomes in trypanosomatids. (a) DAPI staining of epimastigotes of Trypanosoma cruzi. Overlay of the green (poly P) and blue (DNA) channels. Acidocalcisomes correspond to the punctate labelling (green; scale bar, 5 µm). (b) Differential interference contrast (DIC) image of procyclic forms of Trypanosoma brucei (scale bar, 10 µm). (c) Immunofluorescence analysis of the same cells in (b), after reaction with polyclonal antibodies against TbVP1 (V-H⁺-PPase, in red; scale bar, 10 µm). Figure 1a is adapted from Fang et al. (2007b).

Figure 2.

Thin section of procyclic forms of Trypanosoma brucei showing the morphology of acidocalcisomes. (a) acidocalcisomes appear as empty vacuoles or with an electron-dense inclusion (inclusion bodies, arrows). (b) A group of acidocalcisomes is observed at the anterior end of a cell with a thin layer of electron-dense material sticking to the inner phase of the membrane (arrows). Scale bar, 2 µm.

Figure 3.

Morphology of trypanosomatid acidocalcisomes. Electron spectroscopic imaging (contrast tuning) of whole cells (a–c) or fractions (d) adhered to formvar-coated grids showing the shape, size and distribution of acidocalcisomes (black spots) in different species. (a) Blastochritidia culicis; scale bar, 2 µm. (b) Herpetomonas anglusteri; scale bar, 3 µm. (c) Phytomonas serpens; scale bar, 0.5 µm. (d) Isolated acidocalcisomes from Trypanosoma cruzi; scale bar, 0.5 µm. Note the polymorphic nature of acidocalcisomes in (c). b,c are adapted from Miranda et al. (2004, Copyright Elsevier).

Figure 4.

Schematic of an early eukaryote acidocalcisome. Ca²⁺ uptake occurs in exchange for H⁺ by a reaction catalysed by a vacuolar Ca²⁺-ATPase. A H⁺ gradient is established by a vacuolar H⁺-ATPase and a vacuolar H⁺-pyrophosphatase (V-H⁺-PPase). An aquaporin allows water transport. Ca²⁺ release occurs in exchange of H⁺ and is favoured by sodium–proton exchange. Other transporters (for example, for Mg, Zn, inorganic phosphate (Pi), pyrophosphate (PPi), and basic amino acids) are probably present. The acidocalcisome is rich in pyrophosphate, short- and long-chain polyphosphate (poly P), magnesium, calcium, sodium and zinc. An exopolyphosphatase (PPX), a pyrophosphatase (PPase) and a polyphosphate kinase (PPK) may also be present. A question mark was added to indicate the lack of biochemical evidence for their presence.

Figure 5.

The contractile vacuole of Trypanosoma cruzi. Epimastigotes observed by transmission electron microscopy. Notations are flagellar pocket (FP), acidocalcisomes (Ac), kinetoplast (K), contractile vacuole (CV), tubules forming the spongiome (S). Note that similar electron-dense material (arrows) is observed in both the CV and the Ac. Scale bar, 0.25 µm. (b) is adapted from Montalvetti et al. (2004).