Cell Cycle Progression and Cell Polarity Require Sphingolipid Biosynthesis in Aspergillus nidulans

Abstract

Sphingolipids are major components of the plasma membrane of eukaryotic cells and were once thought of merely as structural components of the membrane. We have investigated effects of inhibiting sphingolipid biosynthesis, both in germinating spores and growing hyphae of Aspergillus nidulans. In germinating spores, genetic or pharmacological inactivation of inositol phosphorylceramide (IPC) synthase arrests the cell cycle in G 1 and also prevents polarized growth during spore germination. However, inactivation of IPC synthase not only eliminates sphingolipid biosynthesis but also leads to a marked accumulation of ceramide, its upstream intermediate. We therefore inactivated serine palmitoyltransferase, the first enzyme in the sphingolipid biosynthesis pathway, to determine effects of inhibiting sphingolipid biosynthesis without an accumulation of ceramide. This inactivation also prevented polarized growth but did not affect nuclear division of germinating spores. To see if sphingolipid biosynthesis is required to maintain polarized growth, and not just to establish polarity, we inhibited sphingolipid biosynthesis in cells in which polarity was already established. This inhibition rapidly abolished normal cell polarity and promoted cell tip branching, which normally never occurs. Cell tip branching was closely associated with dramatic changes in the normally highly polarized actin cytoskeleton and found to be dependent on actin function. The results indicate that sphingolipids are essential for the establishment and maintenance of cell polarity via control of the actin cytoskeleton and that accumulation of ceramide is likely responsible for arresting the cell cycle in G 1 .

The Tip Growth Apparatus ofAspergillus nidulans

“…In previous studies, these have been difficult to image, but they have been visualized by immunofluorescence (Harris et al, 1994;Araujo-Bazán et al, 2008) and by using a GFP-tropomyosin fusion (Pearson et al, 2004). As reported previously (Harris et al, 1994;Torralba et al, 1998;Cheng et al, 2001), actin was also associated with forming septa (data not shown).…”

“…Actin has been visualized in fixed A. nidulans by immunofluorescence microscopy (Harris et al, 1994(Harris et al, , 1997McGoldrick et al, 1995;Torralba et al, 1998;Cheng et al, 2001), but it has not been imaged in living cells. We consequently constructed a strain expressing a fusion of GFP to the N terminus of actin (A. nidulans contains a single actin gene; Fidel et al, 1988).…”

Newly Discovered Neutral Glycosphingolipids in Aureobasidin A-resistant Zygomycetes

“…As shown in Fig. 1A, AbA strongly inhibits the growth of filamentous fungi such as A. oryzae, P. oxalicum, and Acremonium sp., previously reported for the yeast Saccharomyces cerevisiae (W303-1A) and other fungi (7)(8)(9)(10)13). However, growth inhibition by AbA was not observed for any Zygomycetes species such as M. hiemalis, R. microsporus, R. pusillus, and A. corymbifera (Fig.…”

“…These strains were cultivated in YPG medium (0.5% yeast extract, 0.5% peptone, 0.5% NaCl,1% glucose, pH 6.5) at 28°C for 48 h in 2-liter shaking flasks containing 700 ml of the medium on a reciprocal shaker at 120 rpm (12). For assaying of the growth inhibition by Aureobasidin A (AbA), fungal strains were grown on YPG plates containing 0.1-10 g/ml AbA for 48 h at 28°C (13).…”

Sphingolipids from the human fungal pathogen Aspergillus fumigatus

“…In S. cerevisiae, the deletion of AUR1 induced cell death. Similarly, AUR1 has been identified as an essential gene in A. fumigatus [44,45]. The inactivation of the IPC synthase by gene deletion or by aureobasidin A inhibition leads to ceramide accumulation and defect in cytokinesis in yeast [43,46].…”