The lipid phosphatase Sac1 was identified more than 25 years ago in a yeast genetic screen. Since then, investigators have learned much about its roles in the cell. In this review, we present our ...current understanding of Sac1 structure, regulation and function, and we describe how Sac1—as a phosphatidylinositol 4‐phosphatase—plays multifaceted roles in membrane homeostasis, vesicular and nonvesicular transport, cell signaling, development and disease.
The lipid phosphatase Sac1 dephosphorylates phosphatidylinositol 4‐phosphate (PI4P), thereby holding levels of this crucial membrane signaling molecule in check. Sac1 regulates multiple cellular processes, including cytoskeletal organization, membrane trafficking and cell signaling. Here, we review the structure and regulation of Sac1, its roles in cell signaling and development and its links to health and disease. Remarkably, many of the diverse roles attributed to Sac1 can be explained by the recent discovery of its requirement at membrane contact sites, where its consumption of PI4P is proposed to drive interorganelle transfer of other cellular lipids, thereby promoting normal lipid homeostasis within cells.
The complex functions of cellular membranes, and thus overall cell physiology, depend on the distribution of crucial lipid species. Sac1 is an essential, conserved, ER-localized phosphatase whose ...substrate, phosphatidylinositol 4-phosphate (PI4P), coordinates secretory trafficking and plasma membrane function. PI4P from multiple pools is delivered to Sac1 by oxysterol-binding protein and related proteins in exchange for other lipids and sterols, which places Sac1 at the intersection of multiple lipid distribution pathways. However, much remains unknown about the roles of Sac1 in subcellular homeostasis and organismal development. Using a temperature-sensitive allele (
), we show that Sac1 is required for structural integrity of the
retinal floor. The β
-integrin Myospheroid, which is necessary for basal cell adhesion, is mislocalized in
retinas. In addition, the adhesion proteins Roughest and Kirre, which coordinate apical retinal cell patterning at an earlier stage, accumulate within
retinal cells due to impaired endo-lysosomal degradation. Moreover, Sac1 is required for ER homeostasis in
retinal cells. Together, our data illustrate the importance of Sac1 in regulating multiple aspects of cellular homeostasis during tissue development.
Epithelial patterning in the developing
eye requires the Neph1 homolog Roughest (Rst), an immunoglobulin family cell surface adhesion molecule expressed in interommatidial cells (IOCs). Here, using a ...novel temperature-sensitive (ts) allele, we show that the phosphoinositide phosphatase Sac1 is also required for IOC patterning.
mutants have rough eyes and retinal patterning defects that resemble
mutants.
retinas exhibit elevated levels of phosphatidylinositol 4-phosphate (PI4P), consistent with the role of Sac1 as a PI4P phosphatase. Indeed, genetic rescue and interaction experiments reveal that restriction of PI4P levels by Sac1 is crucial for normal eye development. Rst is delivered to the cell surface in
mutants. However,
mutant IOCs exhibit severe defects in microtubule organization, associated with accumulation of Rst and the exocyst subunit Sec8 in enlarged intracellular vesicles upon cold fixation
Together, our data reveal a novel requirement for Sac1 in promoting microtubule stability and suggest that Rst trafficking occurs in a microtubule- and exocyst-dependent manner.
Type II phosphatidylinositol 4-kinase (PI4KII) produces the lipid phosphatidylinositol 4-phosphate (PI4P), a key regulator of membrane trafficking. Here, we generated genetic models of the sole ...Drosophila melanogaster PI4KII gene. A specific requirement for PI4KII emerged in larval salivary glands. In PI4KII mutants, mucin-containing glue granules failed to reach normal size, with glue protein aberrantly accumulating in enlarged Rab7-positive late endosomes. Presence of PI4KII at the Golgi and on dynamic tubular endosomes indicated two distinct foci for its function. First, consistent with the established role of PI4P in the Golgi, PI4KII is required for sorting of glue granule cargo and the granule-associated SNARE Snap24. Second, PI4KII also has an unforeseen function in late endosomes, where it is required for normal retromer dynamics and for formation of tubular endosomes that are likely to be involved in retrieving Snap24 and Lysosomal enzyme receptor protein (Lerp) from late endosomes to the trans-Golgi network. Our genetic analysis of PI4KII in flies thus reveals a novel role for PI4KII in regulating the fidelity of granule protein trafficking in secretory tissues.
Phosphatidylinositol (PI) phosphates (PIPs) are membrane lipids that regulate an array of cellular processes by recruiting effector proteins to specific membranes, as well as orchestrating membrane ...function and identity. Phosphatidylinositol 4-phosphase (PI4P) is one of the most abundant cellular PIPs. PI4P is produced by PI 4-kinases (PI4Ks) and kept in check by a sole, highly conserved phosphatase called Sac1. Studies in yeast and cultured mammalian cells have implicated Sac1 in a variety of cellular processes, including membrane trafficking, cytoskeletal dynamics, sphingolipid production and lipid homeostasis. However, because Sac1 is essential in multicellular organisms, it has been difficult to determine its contribution to development and homeostasis in vivo. To this end, I used a temperature-sensitive allele of sac1 (sac1ts) in Drosophila melanogaster to study the role of Sac1. Using this allele, I discovered that Sac1 is required for proper Drosophila eye formation and function, as well as for normal oogenesis. In the developing pupal retina, Sac1 is essential for patterning and specification of retinal cell precursors. Within these cells, Sac1 is important for microtubule (MT) stability and organization, transmembrane protein distribution and degradation, as well as endoplasmic reticulum (ER) homeostasis. Moreover, Sac1 loss leads to increased intracellular PI4P and decreased plasma membrane (PM) PI 4,5-bisphosphate PI(4,5)P2. The elevated pool of PI4P in sac1ts mutants is likely produced by the type II PI4K (PI4KII), since deletion of PI4KII can suppress Sac1 loss of function phenotypes, including defects in retinal cell patterning and eye formation. This indicates that Sac1 and PI4KII regulate a pool of PI4P that is required for Drosophila eye development. Proper regulation of PI4P by Sac1 and PI4KII is also required for eye pigmentation in adult flies. PI4KIIΔ or sac1ts mutants exhibit reduced pigment levels and fewer pigment granules (PGs) per ommatidium. Thus, PG biogenesis is highly sensitive to alterations in PI4P. Together, my results highlight the importance of Sac1 during development and provide a sensitized model for future studies.
STAT6 (signal transducer and activator of transcription 6) is a transcription factor that plays a central role in the pathophysiology of allergic inflammation. We have identified 16 patients from 10 ...families spanning three continents with a profound phenotype of early-life onset allergic immune dysregulation, widespread treatment-resistant atopic dermatitis, hypereosinophilia with esosinophilic gastrointestinal disease, asthma, elevated serum IgE, IgE-mediated food allergies, and anaphylaxis. The cases were either sporadic (seven kindreds) or followed an autosomal dominant inheritance pattern (three kindreds). All patients carried monoallelic rare variants in STAT6 and functional studies established their gain-of-function (GOF) phenotype with sustained STAT6 phosphorylation, increased STAT6 target gene expression, and TH2 skewing. Precision treatment with the anti-IL-4Rα antibody, dupilumab, was highly effective improving both clinical manifestations and immunological biomarkers. This study identifies heterozygous GOF variants in STAT6 as a novel autosomal dominant allergic disorder. We anticipate that our discovery of multiple kindreds with germline STAT6 GOF variants will facilitate the recognition of more affected individuals and the full definition of this new primary atopic disorder.
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