Keeping sphingolipid levels nORMal.

L ipids are essential for life as the principal components of biomembranes. In addition, many lipids act as signaling molecules both in and outside the cell. For these structural and regulatory functions it is crucial that the abundance of different lipids is controlled in a coordinated fashion to maintain the right balance between them. Aberrations in this equilibrium could result in altered membrane fluidity, increased membrane permeability, or depletion of signaling precursors. Particularly in the plasma membrane, a delicate balance between major lipid classes, namely glycerophospholipids, sterols, and sphingolipids, is maintained. Whereas important paradigms of sterol regulation have emerged principally from the work of the Brown and Goldstein laboratory, little is yet known about the regulation of sphingolipid levels. Two reports from the laboratories of Jonathan Weissman and Amy Chang published in Nature and PNAS, respectively, have started to uncover a fascinating regulatory mechanism for sphingolipid synthesis by the evolutionarily conserved family of endoplasmic reticulum (ER)-resident Orm proteins (1, 2). Starting from the observation a few years ago that deletion of ORM genes reduces fitness of yeast grown on media containing agents that induce ER stress (3), Han et al. found the unfolded protein response (UPR) that monitors the ER and homeostatically regulates its functions activated in ORM mutants. A functional link to sphingolipid metabolism is provided by a comprehensive genetic analysis generated in the Weissman laboratory, showing that overexpression of Orm proteins has a genetic interaction profile similar to the one of the hypomorphic alleles of LCB1 and LCB2, encoding subunits of serine– palmitoyl–transferase (SPT) that catalyze the committing step of sphingolipid synthesis in the ER. Conversely, deletion of the ORM genes resulted in a phenotypic signature opposite to that of lcb1 and lcb2. Interestingly, both groups found that this relationship is reflected in a physical complex of Orm proteins with SPT (1, 2). These results prompted the analysis of sphingolipid synthesis intermediates, which revealed highly elevated levels of long chain sphingoid bases such as phytosphingosine in the orm1Δorm2Δ mutants (1, 2). Breslow et al. (1) also found that Orm protein overexpression results in the opposite, namely reduction of long chain sphingoid base levels. Because long chain sphingoid bases are the product of SPT, the model emerging from these studies posits that Orm proteins negatively regulate sphingolipid synthesis at the committing step (Fig. 1). In this scenario, disruption of this negative regulation would relieve SPT from inhibition and lead to increased production of long chain sphingoid bases. This in turn would produce the pleiotropic phenotypes on ER stress, ER-to-Golgi trafficking, and inositol–phospholipid metabolism observed by Han et al. (2). In agreement with this model, most phenotypes of orm mutants are suppressed by nonlethal concentrations of SPT inhibitors that lower levels of long chain sphingoid bases (2). Interestingly, the regulation by Orm proteins might function as a homeostatic feedback loop. In an elegant experiment, Breslow et al. show that wild-type cells maintain normal long chain sphingoid base levels in the presence of increasing concentrations of SPT inhibitors up to a certain point where the system breaks. ORM mutants on the other hand respond by linearly decreasing long chain sphingoid base levels over the whole range of inhibitor concentrations (1). This is exactly the behavior expected from a homeostatic system, such as a climate control: In its presence, and as long as it is not overwhelmed by an external perturbation, it regulates a parameter, such as temperature or sphingolipid levels, in a narrow range. In its absence, the parameter that is controlled goes to an extreme, but responds directly to disturbances. How could such a feedback loop be achieved mechanistically? Initial insights from Breslow et al. (1) suggest that formation of SPT-containing complexes and a signal transduction cascade is involved. They found that Orm proteins associate together with SPT in higher-order assemblies and that the interactions involved are regulated by sphingolipids. Specifically, assembly is decreased after incubation with SPT inhibitors. In addition, Orm proteins are phosphorylated and the level of phosphorylation is altered in response to blocking sphingolipid synthesis. Importantly, mutation of phosphorylation sites in Orm proteins results in a reduction serine