Shinichi Aizawa

PPARγ Mediates High-Fat Diet–Induced Adipocyte Hypertrophy and Insulin Resistance

Abstract Agonist-induced activation of peroxisome proliferator-activated receptor γ (PPARγ) is known to cause adipocyte differentiation and insulin sensitivity. The biological role of PPARγ was investigated by gene targeting. Homozygous PPARγ -deficient embryos died at 10.5–11.5 dpc due to placental dysfunction. Quite unexpectedly, heterozygous PPARγ -deficient mice were protected from the development of insulin resistance due to adipocyte hypertrophy under a high-fat diet. These phenotypes were abrogated by PPARγ agonist treatment. Heterozygous PPARγ -deficient mice showed overexpression and hypersecretion of leptin despite the smaller size of adipocytes and decreased fat mass, which may explain these phenotypes at least in part. This study reveals a hitherto unpredicted role for PPARγ in high-fat diet–induced obesity due to adipocyte hypertrophy and insulin resistance, which requires both alleles of PPARγ .

Impairment of motor coordination, Purkinje cell synapse formation, and cerebellar long-term depression in GluRδ2 mutant mice

Of the six glutamate receptor (GluR) channel subunit families identified by molecular cloning, five have been shown to constitute either the AMPA, kainate, or NMDA receptor channel, whereas the function of the delta subunit family remains unknown. The selective localization of the delta 2 subunit of the GluR delta subfamily in cerebellar Purkinje cells prompted us to examine its possible physiological roles by the gene targeting technique. Analyses of the GluR delta 2 mutant mice reveal that the delta 2 subunit plays important roles in motor coordination, formation of parallel fiber-Purkinje cell synapses and climbing fiber-Purkinje cell synapses, and long-term depression of parallel fiber-Purkinje cell synaptic transmission. These results suggest a close relationship between synaptic plasticity and synapse formation in the cerebellum.

Cellular commitment to oncogene-induced transformation or apoptosis is dependent on the transcription factor IRF-1

The transcriptional activator interferon regulatory factor 1 (IRF-1) and its antagonistic repressor IRF-2 are regulators of the interferon (IFN) system and of cell growth. Here we report that embryonic fibroblasts (EFs) from mice with a null mutation in the IRF-1 gene (IRF-1-/- mice) can be transformed by expression of an activated c-Ha-ras oncogene. This property is not observed in EFs from wild-type or IRF-2-/- mice but is still observed in EFs from mice deficient in both genes. The transformed phenotype of ras-expressing IRF-1-/- EFs could be suppressed by the expression of the IRF-1 cDNA. Thus, IRF-1 functions as a tumor suppressor. Furthermore, expression of the c-Ha-ras oncogene causes wild-type but not IRF-1-/- EFs to undergo apoptosis when combined with a block to cell proliferation or treated by anticancer drugs or ionizing radiation. Hence, IRF-1 may be a critical determinant of oncogene-induced cell transformation or apoptosis.

Notch2 Is Preferentially Expressed in Mature B Cells and Indispensable for Marginal Zone B Lineage Development

The Notch genes play a key role in cellular differentiation. The significance of Notch1 during thymocyte development is well characterized, but the function of Notch2 is poorly understood. Here we demonstrate that Notch2 but no other Notch family member is preferentially expressed in mature B cells and that conditionally targeted deletion of Notch2 results in the defect of marginal zone B (MZB) cells and their presumed precursors, CD1d(hi) fraction of type 2 transitional B cells. Among Notch target genes, the expression level of Deltex1 is prominent in MZB cells and strictly dependent on that of Notch2, suggesting that Deltex1 may play a role in MZB cell differentiation.

Impairment of suckling response, trigeminal neuronal pattern formation, and hippocampal LTD in NMDA receptor ε2 subunit mutant mice

Multiple epsilon subunits are major determinants of the NMDA receptor channel diversity. Based on their functional properties in vitro and distributions, we have proposed that the epsilon 1 and epsilon 2 subunits play a role in synaptic plasticity. To investigate the physiological significance of the NMDA receptor channel diversity, we generated mutant mice defective in the epsilon 2 subunit. These mice showed no suckling response and died shortly after birth but could survive by hand feeding. The mutation hindered the formation of the whisker-related neuronal barrelette structure and the clustering of primary sensory afferent terminals in the brainstem trigeminal nucleus. In the hippocampus of the mutant mice, synaptic NMDA responses and longterm depression were abolished. These results suggest that the epsilon 2 subunit plays an essential role in both neuronal pattern formation and synaptic plasticity.

Otx2 homeobox gene controls retinal photoreceptor cell fate and pineal gland development

Understanding the molecular mechanisms by which distinct cell fate is determined during organogenesis is a central issue in development and disease. Here, using conditional gene ablation in mice, we show that the transcription factor Otx2 is essential for retinal photoreceptor cell fate determination and development of the pineal gland. Otx2-deficiency converted differentiating photoreceptor cells to amacrine-like neurons and led to a total lack of pinealocytes in the pineal gland. We also found that Otx2 transactivates the cone-rod homeobox gene Crx, which is required for terminal differentiation and maintenance of photoreceptor cells. Furthermore, retroviral gene transfer of Otx2 steers retinal progenitor cells toward becoming photoreceptors. Thus, Otx2 is a key regulatory gene for the cell fate determination of retinal photoreceptor cells. Our results reveal the key molecular steps required for photoreceptor cell-fate determination and pinealocyte development.

Increased insulin sensitivity and hypoglycaemia in mice lacking the p85α subunit of phosphoinositide 3-kinase

The hallmark of type 2 diabetes, the most common metabolic disorder, is a defect in insulin–stimulated glucose transport in peripheral tissues. Although a role for phosphoinositide–3–kinase (PI3K) activity in insulin–stimulated glucose transport and glucose transporter isoform 4 (Glut4) translocation has been suggested in vitro, its role in vivo and the molecular link between activation of PI3K and translocation has not yet been elucidated. To determine the role of PI3K in glucose homeostasis, we generated mice with a targeted disruption of the gene encoding the p85α regulatory subunit of PI3K (Pik3r1; refs 3, 4, 5). Pik3r1−/− mice showed increased insulin sensitivity and hypoglycaemia due to increased glucose transport in skeletal muscle and adipocytes. Insulin–stimulated PI3K activity associated with insulin receptor substrates (IRSs) was mediated via full–length p85α in wild–type mice, but via the p50α alternative splicing isoform of the same gene in Pik3r1−/− mice. This isoform switch was associated with an increase in insulin–induced generation of phosphatidylinositol(3,4,5)triphosphate (PtdIns(3,4,5)P 3) in Pik3r1−/− adipocytes and facilitation of Glut4 translocation from the low–density microsome (LDM) fraction to the plasma membrane (PM). This mechanism seems to be responsible for the phenotype of Pik3r1−/− mice, namely increased glucose transport and hypoglycaemia. Our work provides the first direct evidence that PI3K and its regulatory subunit have a role in glucose homeostasis in vivo.

Constitutive activation of Src family kinases in mouse embryos that lack Csk

Csk is a novel cytoplasmic protein-tyrosine kinase that has been shown to inactivate members of the Src family of protein-tyrosine kinases in vitro. To examine the function of Csk in vivo, Csk-deficient mouse embryos were generated by gene targeting in embryonic stem cells. These embryos were developmentally arrested at the 10 to 12 somite stage and exhibited growth retardation and necrosis in the neural tissues. The kinase activity of p60c-src, p59fyn, and p53/56lyn in these embryos was greatly enhanced as an apparent consequence of enhanced specific activity. The increase in kinase activity was associated with an increase in tyrosine phosphorylation of several proteins, especially those around 85 and 120 kd. Thus, these results suggest that Csk indeed acts as an indispensable negative regulator of Src family kinases in vivo.

Glucokinase and IRS-2 are required for compensatory β cell hyperplasia in response to high-fat diet–induced insulin resistance

Glucokinase (Gck) functions as a glucose sensor for insulin secretion, and in mice fed standard chow, haploinsufficiency of beta cell-specific Gck (Gck(+/-)) causes impaired insulin secretion to glucose, although the animals have a normal beta cell mass. When fed a high-fat (HF) diet, wild-type mice showed marked beta cell hyperplasia, whereas Gck(+/-) mice demonstrated decreased beta cell replication and insufficient beta cell hyperplasia despite showing a similar degree of insulin resistance. DNA chip analysis revealed decreased insulin receptor substrate 2 (Irs2) expression in HF diet-fed Gck(+/-) mouse islets compared with wild-type islets. Western blot analyses confirmed upregulated Irs2 expression in the islets of HF diet-fed wild-type mice compared with those fed standard chow and reduced expression in HF diet-fed Gck(+/-) mice compared with those of HF diet-fed wild-type mice. HF diet-fed Irs2(+/-) mice failed to show a sufficient increase in beta cell mass, and overexpression of Irs2 in beta cells of HF diet-fed Gck(+/-) mice partially prevented diabetes by increasing beta cell mass. These results suggest that Gck and Irs2 are critical requirements for beta cell hyperplasia to occur in response to HF diet-induced insulin resistance.

Type III secretion systems and bacterial flagella: Insights into their function from structural similarities

Type III secretion systems and bacterial flagella are broadly compared at the level of their genetic structure, morphology, regulation, and function, integrating structural information, to provide an overview of how they might function at a molecular level.