Lipid droplets are central organelles for meiosis II progression during yeast sporulation.

Neutral lipids, predominantly triacylglycerol (TAG) and sterol ester, are stored within the cellular organelles termed lipid droplets (LDs). Although it is believed that the major function of LDs is to supply the cell with energy and membranes, little is known about the cellular events directly involving LDs and their contents. In this study, we provide cytological evidence that LDs form direct contacts with the prospore membrane (PSM) that is synthesized de novo during meiosis II to sequester the dividing nuclei in sporulating yeast. Lipidomic analyses indicate that TAG lipolysis releases free fatty acids at a time that correlates well with meiosis II progression, concomitant with phospholipid remodeling. Mutants lacking TAG or impaired of TAG hydrolysis show spore wall assembly defects, supporting a role for TAG and/or its metabolites in spore wall morphogenesis. Not only does LD integrity influence spore wall assembly, LDs are also essential for other aspects of spore development. Yeast cells lacking LDs are severely defective in PSM growth and organization and display disrupted spindles, producing dead spores or even failing to form spores. Together these results link LD physiology directly to a unique membrane morphogenesis process critical for development.

Lipid droplets maintain lipid homeostasis during anaphase for efficient cell separation in budding yeast.

The neutral lipids steryl ester and triacylglycerol (TAG) are stored in the membrane-bound organelle lipid droplet (LD) in essentially all eukaryotic cells. It is unclear what physiological conditions require the mobilization or storage of these lipids. Here, we study the budding yeast mutant are1Δ are2Δ dga1Δ lro1Δ, which cannot synthesize the neutral lipids and therefore lacks LDs. This quadruple mutant is delayed at cell separation upon release from mitotic arrest. The cells have abnormal septa, unstable septin assembly during cytokinesis, and prolonged exocytosis at the division site at the end of cytokinesis. Lipidomic analysis shows a marked increase of diacylglycerol (DAG) and phosphatidic acid, the precursors for TAG, in the mutant during mitotic exit. The cytokinesis and separation defects are rescued by adding phospholipid precursors or inhibiting fatty acid synthesis, which both reduce DAG levels. Our results suggest that converting excess lipids to neutral lipids for storage during mitotic exit is important for proper execution of cytokinesis and efficient cell separation.

Mouse sperm acquire a new structure on the apical hook during epididymal maturation.

Spermatozoa emerging from the testis undergo a maturation process in the epididymis during which they change morphologically, biochemically and physiologically to gain motility and the ability to fertilize ova. We examined mouse epididymal sperm with immunostaining and transmission electron microscopy (EM) and identified a previously unknown structure on the apical hook. The structure has a coiled configuration around 11 nm in thickness and is present at the tip of each corner of the triangular-rod shaped perforatorium. Surveying sperm isolated from various regions of the epididymis indicated that mouse sperm acquire the hook rim (HR) structure during its passage through the proximal two-thirds of the caput epididymidis. The structure withstands vigorous sonication and harsh chemical treatments and remains intact after the acrosome reaction. Its location and sturdiness suggest a function in protecting the apical hook from mechanical wear during fertilization. Our EM images of epididymal sperm also revealed additional novel structures as well as lateral asymmetry of the sperm head, indicating that mouse sperm head has a structure more complex than previously recognized.