Lipid microdomains or caveolae, small invaginations of plasma membrane, have emerged as important elements for lipid uptake and glucose homeostasis. Sphingomyelin (SM) is one of the major phospholipids of the lipid microdomains. In this study, we investigated the physiological function of sphingomyelin synthase 2 (SMS2) using SMS2 knock-out mice, and we found that SMS2 deficiency prevents high fat diet-induced obesity and insulin resistance. Interestingly, in the liver of SMS2 knock-out mice, large and mature lipid droplets were scarcely observed. Treatment with siRNA for SMS2 also decreased the large lipid droplets in HepG2 cells. Additionally, the siRNA of SMS2 decreased the accumulation of triglyceride in liver of leptin-deficient (ob/ob) mice, strongly suggesting that SMS2 is involved in lipid droplet formation. Furthermore, we found that SMS2 exists in lipid microdomains and partially associates with the fatty acid transporter CD36/FAT and with caveolin 1, a scaffolding protein of caveolae. Because CD36/FAT and caveolin 1 exist in lipid microdomains and are coordinately involved in lipid droplet formation, SMS2 is implicated in the modulation of the SM in lipid microdomains, resulting in the regulation of CD36/FAT and caveolae. Here, we established new cell lines, in which we can completely distinguish SMS2 activity from SMS1 activity, and we demonstrated that SMS2 could convert ceramide produced in the outer leaflet of the plasma membrane into SM. Our findings demonstrate the novel and dynamic regulation of lipid microdomains via conformational changes in lipids on the plasma membrane by SMS2, which is responsible for obesity and type 2 diabetes.

The Rho small G protein family members regulate various actin cytoskeleton-dependent cell functions. The Rho GDI (GDP dissociation inhibitor) family, consisting of Rho GDIa, -b, and -g, is a regulator that keeps the Rho family members in the cytosol as the GDP-bound inactive form and translocates the GDP-bound form from the membranes to the cytosol after the GTP-bound form accomplishes their functions. Rho GDIa is ubiquitously expressed in mouse tissues and shows GDI activity on all the Rho family members in vitro. We have generated mice lacking Rho GDIa by homologous recombination to clarify its in vivo function. Rho GDIa 7/7 mice showed several abnormal phenotypes. Firstly, Rho GDIa 7/7 mice were initially viable but developed massive proteinuria mimicking nephrotic syndrome, leading to death due to renal failure within a year. Histologically, degeneration of tubular epithelial cells and dilatation of distal and collecting tubules were readily detected in the kidneys. Secondly, Rho GDIa 7/7 male mice were infertile and showed impaired spermatogenesis with vacuolar degeneration of seminiferous tubules in their testes. Thirdly, Rho GDIa 7/7 embryos derived from Rho GDIa 7/7 female mice were defective in the postimplantation development. In addition, these morphological and functional abnormalities showed age-dependent progression. These results suggest that the signaling pathways of the Rho family members regulated by Rho GDIa play important roles in maintaining the structure and physiological function of at least kidneys and reproductive systems in adult mice.

Afadin is an actin filament–binding protein that binds to nectin, an immunoglobulin-like cell adhesion molecule, and is colocalized with nectin at cadherin-based cell–cell adherens junctions (AJs). To explore the function of afadin in cell–cell adhesion during embryogenesis, we generated afadin−/− mice and embryonic stem cells. In wild-type mice at embryonic days 6.5–8.5, afadin was highly expressed in the embryonic ectoderm and the mesoderm, but hardly detected in the extraembryonic regions such as the visceral endoderm. Afadin−/− mice showed developmental defects at stages during and after gastrulation, including disorganization of the ectoderm, impaired migration of the mesoderm, and loss of somites and other structures derived from both the ectoderm and the mesoderm. Cystic embryoid bodies derived from afadin−/− embryonic stem cells showed normal organization of the endoderm but disorganization of the ectoderm. Cell–cell AJs and tight junctions were improperly organized in the ectoderm of afadin−/− mice and embryoid bodies. These results indicate that afadin is highly expressed in the ectoderm- derived cells during embryogenesis and plays a key role in proper organization of AJs and tight junctions of the highly expressing cells, which is essential for proper tissue morphogenesis.

Rab GDP dissociation inhibitor ␣ (Rab GDI␣) is a regulator of the Rab small G proteins implicated in neurotransmission, and mutations of Rab GDI␣ cause human X-linked mental retardation associated with epileptic seizures. In Rab GDI␣-deficient mice, synaptic potentials in the CA1 region of the hippocampus displayed larger enhancement during repetitive stimulation, which was apparently opposite to the phenotype of Rab3A-deficient mice. Furthermore, the Rab GDI␣-deficient mice showed hypersensitivity to bicuculline, an inducer of epileptic seizures. These results suggest that Rab GDI␣ plays a specialized role in Rab3A recycling to suppress hyperexcitability via modulation of presynaptic forms of plasticity.