Inorganic Polyphosphates in Eukaryotic Cells by Tatiana Kulakovskaya Evgeny Pavlov & Elena N. Dedkova

Author:Tatiana Kulakovskaya, Evgeny Pavlov & Elena N. Dedkova
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

9.2 PolyP Storage in Insect Oocytes and Eggs

Insects are oviparous animals, and their eggs are usually laid in a terrestrial environment protected by an impermeable sheath where the embryo development occurs isolated from the environment and maternal body. Thereby, all the nutrients and energy sources required for the growth, division, and differentiation of the embryo cells are previously stored inside the oocytes during oogenesis in the maternal ovary. It is believed that most of the synthesis of these storage components is performed by the fat body – a biosynthetic organ functionally similar to the human liver (Raikhel and Dhadialla 1992; Snigirevskaya et al. 1997). There, the proteins and other macromolecules are synthesized and secreted into the hemolymph (circulation fluid of the open circulatory system found in insects). Those macromolecules are incorporated into oocytes by receptor-mediated endocytosis and stored within the cytoplasm into different endocytic organelles collectively called yolk granules (YG). At the end of oogenesis, an insect oocyte can grow up to 4,000 times its original size, and most of the cytoplasm volume will be filled with YG.

In most insect species, vitellin (VT) is the main yolk protein synthesized by the fat body and accumulated into YG. It is believed that, as a phospholipidglycoprotein, VT provides the embryo cells with amino acids, phosphates, lipids, and sugars upon its mobilization. From a cell biology point of view, the YG have been compared to latent lysosomes. Hydrolytic activities are usually carried out by classic lysosomal enzymes regulated by proton-pump-dependent YG acidification, as usually observed during early embryogenesis of different species (Fagotto 1995). In this regard, modulation of the proton-pump activity allows the degradation of yolk macromolecules in a pace that meets the anabolic demand of the developing embryo and prevents early or excessive yolk mobilization. The presence of such regulatory mechanisms is reflected by the fact that VT hydrolysis and proteolytic activity are usually undetectable at the early steps of embryo development and are activated after the onset of gastrulation. Unfortunately, the molecular and cellular mechanisms underlying this regulated hydrolysis have remained elusive.

Traditionally, the literature refers to all of the oocyte/egg endocytic organelles as YG, but it has been shown that this comprises a heterogeneous population of organelles with distinct morphology and content, raising the question of whether phosphate and polyP-specialized stores could be identified among insect eggs. Accordingly, investigations on polyP storage using fluorescent probes led to the identification of a subset of distinguishable small vesicles containing high amounts of polyP in the eggs of the American cockroach Periplaneta americana and the kissing bug Rhodnius prolixus (Motta et al. 2009; Ramos et al. 2011). In addition to their small size (~500 nm), these polyP-rich granules also distinguished from the typical larger YG (1 to up to 100 μm) by their elemental content composed mainly by phosphorous and metallic atoms like calcium, iron, and magnesium (Fig. 9.1). This characteristic elemental profile is similar to a family of well-known polyP-rich granules previously named acidocalcisomes. Acidocalcisomes have been described in several models so far and have been proposed to be highly conserved during evolution (Docampo et al.

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