Sphingolipid-enriched domains in fungi.

Plasma membrane carries out multiple physiological functions that require its dynamic and tightly regulated organization into specialized domains of different size, stability, and lipid/protein composition. Sphingolipids are a group of lipids in which the plasma membrane is particularly enriched, thus being crucial for its structure and function. A specific type of sphingolipid-enriched plasma membrane domains, where ergosterol is depleted and lipids are tightly packed in a rigid gel phase, has recently been found in several fungal species, including yeasts and moulds. After presenting the main biophysical features of gel domains and the experimental method for their detection in the fungal plasma membrane, we review these sphingolipid-enriched gel domains and illustrate their importance to both unicellular and multicellular fungi. First, the biophysical properties of the fungal sphingolipid-enriched domains will be analysed taking into consideration the plasma membrane sphingolipidome. Next, their possible biological roles will be summarized, including their relations with plasma membrane compartments and involvement in stress responses. Moreover, since the plasma membrane is a target for several antifungal compounds, a biophysical connection between sphingolipid-enriched domains and antifungal action will be explored.

Sphingolipid hydroxylation in mammals, yeast and plants - An integrated view.

This review is focused on sphingolipid backbone hydroxylation, a small but widespread structural feature with profound impact on membrane biophysical properties. We start by summarizing sphingolipid metabolism in mammalian cells, yeast and plants, focusing on how distinct hydroxylation patterns emerge in different eukaryotic kingdoms. Then, a comparison of the biophysical properties in membrane model systems and cellular membranes from diverse organisms is made. From an integrative perspective, these results can be rationalized considering that superficial hydroxyl groups in the backbone of sphingolipids (by intervening in the H-bond network) alter the balance of favorable interactions between membrane lipids. They may strengthen the bonding or compete with other hydroxyl groups, in particular the one of membrane sterols. Different sphingolipid hydroxylation patterns can stabilize/disrupt specific membrane domains or change whole plasma membrane properties, and therefore be important in the control of protein distribution, function and lateral diffusion and in the formation and overtime stability of signaling platforms. The recent examples explored throughout this review unveil a potentially key role for sphingolipid backbone hydroxylation in both physiological and pathological situations, as it can be of extreme importance for the proper organization of cell membranes in mammalian cells, yeast and, most likely, also in plants.

Changes in the Biophysical Properties of the Cell Membrane Are Involved in the Response of Neurospora crassa to Staurosporine.

Neurospora crassa is a non-pathogenic filamentous fungus widely used as a multicellular eukaryotic model. Recently, the biophysical properties of the plasma membrane of N. crassa conidia were thoroughly characterized. They evolve during conidial germination at a speed that depends on culture conditions, suggesting an important association between membrane remodeling and the intense membrane biogenesis that takes place during the germinative process. Staurosporine (STS) is a drug used to induce programmed cell death in various organisms. In N. crassa, STS up-regulates the expression of the ABC transporter ABC-3, which localizes at the plasma membrane and pumps STS out. To understand the role of plasma membrane biophysical properties in the fungal drug response, N. crassa was subjected to STS treatment during early and late conidial development stages. Following 1 h treatment with STS, there is an increase in the abundance of the more ordered, sphingolipid-enriched, domains in the plasma membrane of conidia. This leads to higher fluidity in other membrane regions. The global order of the membrane remains thus practically unchanged. Significant changes in sphingolipid-enriched domains were also observed after 15 min challenge with STS, but they were essentially opposite to those verified for the 1 h treatment, suggesting different types of drug responses. STS effects on membrane properties that are more dependent on ergosterol levels also depend on the developmental stage. There were no alterations on 2 h-grown cells, clearly contrasting to what happens at longer growth times. In this case, the differences were more marked for longer STS treatment, and rationalized considering that the drug prevents the increase in the ergosterol/glycerophospholipid ratio that normally takes place at the late conidial stage/transition to the mycelial stage. This could be perceived as a drug-induced development arrest after 5 h growth, involving ergosterol, and pointing to a role of lipid rafts possibly related with an up-regulated expression of the ABC-3 transporter. Overall, our results suggest the involvement of membrane ordered domains in the response mechanisms to STS in N. crassa.