Lipid body biogenesis and the role of microtubules in lipid synthesis in Ornithogalum umbellatum lipotubuloids

Abstract

Lipid bodies present in lipotubuloids of Ornithogalum umbellatum ovary epidermis take the form of a lens between leaflets of ER (endoplasmic reticulum) membrane filled with a highly osmiophilic substance. The two enzymes, DGAT1 [DAG (diacylglycerol) acyltransferase 1] and DGAT2 (DAG acyltransferase 2), involved in this process are synthesized on rough ER and localized in the ER near a monolayer surrounding entities like lipid bodies. After reaching the appropriate size, newly formed lipid bodies transform into mature spherical lipid bodies filled with less osmiophilic content. They appear to be surrounded by a half-unit membrane, with numerous microtubules running adjacently in different directions. The ER, no longer continuous with lipid bodies, makes contact with them through microtubules. At this stage, lipid synthesis takes place at the periphery of lipid bodies. This presumption, and a hypothesis that microtubules are involved in lipid synthesis delivering necessary components to lipid bodies, is based on strong arguments: (i) silver grains first appear over microtubules after a short [3H]palmitic acid incubation and before they are observed over lipid bodies; (ii) blockade of [3H]palmitic acid incorporation into lipotubuloids by propyzamide, an inhibitor of microtubule function; and (iii) the presence of gold grains above the microtubules after DGAT1 and DGAT2 reactions, as also near microtubules after an immunogold method that identifies phospholipase D1.

Immunogold method evidences that kinesin and myosin bind to and couple microtubules and actin filaments in lipotubuloids of Ornithogalum umbellatum ovary epidermis

“…Gold grains decorating microtubules which indicate the presence of PLD were observed with the immunogold technique in O. umbellatum lipotubuloids (Kwiatkowska et al 2012).…”

“…The way a lipotubuloid moves in a cell (see Fig. 2) suggests that this movement is not accidental but is controlled by signals from a cell directing this lipotubuloid to the site, where nutrients and cuticle building blocks are most necessary (Kwiatkowska et al 2012). It seems possible since microtubules and actin filaments play a receptor-like role for Rho proteins which are involved in many important biological processes, e.g.…”

“…On the surface of lipotubuloid lipid bodies, similarly as of lipid droplets in oleate-loaded COS-7-cells (Kuerschner et al 2008;Stone et al 2009) in macrophages (Buers et al 2009) and murine cells (McFie et al 2011) there is DGAT2. This enzyme is involved in triacylglyceride synthesis which was evidenced by autoradiographic studies with 3 H-palmitic acid which is incorporated into the surface layer of lipotubuloid lipid bodies (Kwiatkowska et al 2012). Lipids forming in them became lipolyzed by lipase and the resulting products were transported out of a lipotubuloid into cytoplasm and following appropriate biochemical reactions were deposited into cuticle (Kwiatkowska et al 2011b).…”

“…The latter idea is in agreement with our finding showing gold grains, thus indicating myosin presence in Golgi cisternae. A hypothesis can be put forward that dictyosomes move within a lipotubuloid, being a set of cooperating organelles, which plays a key role in O. umbellatum ovary epidermis development (Kwiatkowska et al 2012).…”

“…These actin filaments, lying parallel to the surface of microtubules, are sensitive to cytochalasin B and are connected with rotary motion of lipotubuloids, i.e. a cytoplasmic domain containing aggregates of lipid bodies surrounded by microtubules which also include abundance of ribosome and endoplasmic reticulum, as well as single dictyosomes, mitochondria, microbodies and autolytic vacuoles (Kwiatkowska 1972;Kwiatkowska et al 2009Kwiatkowska et al , 2012. Lipotubuloids do not have their own membrane but on the large area they are surrounded by a tonoplast as they invaginate into vacuoles.…”