Takeo Machida

Brain-specific Phgdh Deletion Reveals a Pivotal Role for l-Serine Biosynthesis in Controlling the Level of d-Serine, an N-methyl-d-aspartate Receptor Co-agonist, in Adult Brain

In mammalian brain, d-serine is synthesized from l-serine by serine racemase, and it functions as an obligatory co-agonist at the glycine modulatory site of N-methyl-d-aspartate (NMDA)-selective glutamate receptors. Although diminution in d-serine level has been implicated in NMDA receptor hypofunction, which is thought to occur in schizophrenia, the source of the precursor l-serine and its role in d-serine metabolism in adult brain have yet to be determined. We investigated whether l-serine synthesized in brain via the phosphorylated pathway is essential for d-serine synthesis by generating mice with a conditional deletion of d-3-phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95). This enzyme catalyzes the first step in l-serine synthesis via the phosphorylated pathway. HPLC analysis of serine enantiomers demonstrated that both l- and d-serine levels were markedly decreased in the cerebral cortex and hippocampus of conditional knock-out mice, whereas the serine deficiency did not alter protein expression levels of serine racemase and NMDA receptor subunits in these regions. The present study provides definitive proof that l-serine-synthesized endogenously via the phosphorylated pathway is a key rate-limiting factor for maintaining steady-state levels of d-serine in adult brain. Furthermore, NMDA-evoked transcription of Arc, an immediate early gene, was diminished in the hippocampus of conditional knock-out mice. Thus, this study demonstrates that in mature neuronal circuits l-serine availability determines the rate of d-serine synthesis in the forebrain and controls NMDA receptor function at least in the hippocampus.

Stimulatory effects of prostaglandin E2 on neurogenesis in the dentate gyrus of the adult rat.

Abstract Neurogenesis in the dentate gyrus of adult rodents is elicited by transient global ischemia. Cyclooxygenase (COX) -2, a rate-limiting enzyme for prostanoid synthesis, is also induced by ischemia. We recently found that the administration of a non-selective COX inhibitor to ischemic animals suppressed cell proliferation in the subgranular zone (SGZ) at the dentate gyrus of the hippocampus. To clarify whether prostaglandin E2 (PGE2) synthesis by COXs is involved in neurogenesis, sulprostone, an analogue of PGE2, was injected into the rat hippocampus. Sulprostone injection increased the number of 5-bromo-2′-deoxyuridine (BrdU)-positive cells in the SGZ. BrdU-positive cells also expressed polysialylated isoforms of neural cell adhesion molecule and neuronal nuclear antigen. These results suggest that PGE2 plays an important role in the proliferation of cells in the SGZ.

Neural recognition molecule NB-2 of the contactin/F3 subgroup in rat: Specificity in neurite outgrowth-promoting activity and restricted expression in the brain regions.

NB‐2, a neural cell recognition molecule of the contactin/F3 subgroup, promoted neurite outgrowth of the cerebral cortical neurons but not the hippocampal neurons. NB‐2 in rat became apparent after birth at protein level, reaching a maximum at postnatal day 14 in the cerebrum and postnatal day 3 in the cerebellum. NB‐2 in the cerebellum declined abruptly thereafter. In situ hybridization demonstrated that NB‐2 mRNA was highly expressed in regions implicated in the central auditory pathway, including the cochlear nuclei, superior olive, inferior colliculi, medial geniculate nuclei, and auditory cortex. In addition, a high level of NB‐2 expression was observed in the accessory olfactory bulb, thalamic nuclei, facial nucleus, and inferior olive. By immunohistochemistry, intense immunoreactivity against NB‐2 was also detected in the auditory pathway. Thus, NB‐2 is expressed in highly restricted brain regions, including the auditory system, suggesting that it plays specific roles in the development and/or maturation of the regions. J. Neurosci. Res. 65:100–110, 2001.

Acetylsalicylic acid reduces ischemia-induced proliferation of dentate cells in gerbils.

Transient global ischemia causes neurogenesis in the dentate gyrus of adult rodents. Ischemic insults to rodents also induce cyclooxygenase-2 (COX-2), an isoform of cyclooxygenases (COXs) and a rate-limiting enzyme for prostanoid synthesis. In the present experiments, adult Mongolian gerbils were chronically treated with acetylsalicylic acid (ASA), a non-selective COX inhibitor, and the proliferation of cells in the dentate gyrus was examined under ischemia. It was proved that BrdU-labeled cells in the dentate gyrus were significantly reduced in number following ASA treatment after 10 min global ischemia. The result strongly suggests that COX, probably COX-2, and prostanoids play an important role in the proliferation of neural cells after ischemia in gerbils.

Lipid rafts enriched in phosphatidylglucoside direct astroglial differentiation by regulating tyrosine kinase activity of epidermal growth factor receptors

Membrane lipid rafts provide a specialized microenvironment enriched with sphingolipids and phospholipids containing saturated fatty acids and serve as a platform for various intracellular signalling pathways. PtdGlc (phosphatidylglucoside) is a type of glycophospholipid localized in the outer leaflet of the plasma membrane. Owing to PtdGlcs unique fatty acid composition, exclusively composed of C(18:0) at sn-1 and C(20:0) at sn-2 of the glycerol backbone, it tends to form PGLRs (PtdGlc-enriched lipid rafts). Previously, we demonstrated that PGLRs reside on the cell surface of astroglial cells from fetal rat brain [Nagatsuka, Horibata, Yamazaki, Kinoshita, Shinoda, Hashikawa, Koshino, Nakamura and Hirabayashi (2006) Biochemistry 45, 8742-8750]. In the present study, we observed PGLRs in astroglial lineage cells at mid-embryonic to early-postnatal stages of developing mouse cortex. This suggests that PGLRs are developmentally correlated with astroglial differentiation during fetal cortical development. Our cell culture studies with multipotent neural progenitor cells prepared from fetal mouse telencephalon demonstrated that treatment with EGF (epidermal growth factor) or anti-PtdGlc antibody caused recruitment of EGFRs (EGF receptors) into lipid raft compartments, leading to activation of EGFRs. Moreover, the activation of EGFRs by antibody triggered downstream tyrosine kinase signalling and induced marked GFAP (glial fibrillary acidic protein) expression via the JAK (Janus kinase)/STAT (signal transducer and activator of transcription) signalling pathway. These findings strongly suggest that PGLRs are physiologically coupled to activated EGFRs on neural progenitor cells during fetal cortical development, and thereby play a distinct role in mediating astrogliogenesis.

Impaired neurogenesis in the growth-retarded mouse is reversed by T3 treatment.

Triiodothyronine (T3) regulates neuronal development, and T3 deficiency impairs the formation of the central nervous system during childhood. Here, we used the hypothyroidal growth-retarded (grt) mouse to investigate whether the generation of new neurons is affected by T3 deficiency. Cell proliferation in the hippocampus, as measured by 5-bromo-2′-deoxyuridine (BrdU) incorporation, was markedly decreased in the grt mouse while the number of BrdU-positive cells in T3-treated grt mice was equivalent to that of wild type mice. The cellular differentiation rate did not differ among the experimental groups. These results suggest that T3 plays an important role during neurogenesis.

Impaired insulin secretion from the pancreatic islets of hypothyroidal growth-retarded mice

The growth-retarded (grt) mouse shows thyroid dysfunction-related hyporesponsiveness to TSH. Thyroid hormone is a critical regulator of metabolism in many cells; thus, derangement of thyroid function affects many organs and systems. Experiments were conducted focusing on the function of the pancreatic islets in grt mice. We showed occurrence of a fasting hyperglycemia and a decreased plasma insulin level response to a glucose load in grt mice, despite normal insulin molecules being stored in secretory granules of pancreatic islets. We also demonstrated a reduction of insulin secretion in response to glucose administration from islets of grt mice in vitro, while the insulin release in response to KCl stimulation was comparable to that in normal mice, indicating that the isolated islets from grt mice have normal ATP-sensitive K(+) channels and postchannel activity. The mRNA expression levels of glucose transporter 2 and glucokinase in the islets of grt mice were similar to those in normal mice. Triiodothyronine administration to grt mice improved insulin secretion very slightly. On the other hand, mRNA for tyrosylprotein sulfotransferase 2 (Tpst2) was found to be expressed in the pancreatic islets of grt mice. Considering that Tpst2 is the responsible gene of grt mice, mutation of which is associated with a poor function of TSH receptor, the findings raise a possibility of involvement of factors including Tpst2 in the insulin hyposecretion in grt mice.

Senescence Marker Protein‐30 as a Novel Antiaging Molecule

Abstract: Senescence marker protein‐30 (SMP30), composed of 299 amino acids, has an approximate molecular mass of 32‐34 kDa and has a pI 4.9 in charge. The amino acid alignment from various animal species revealed a highly conserved structure. SMP30 has an enzyme activity hydrolyzing sarin, soman, and tabun, known as lethal toxic nerve chemicals. We analyzed the organophosphatase activity of SMP30 using DFP as a substrate. This DFPase activity is revealed in a dose‐dependent manner in the presence of magnesium ions. We investigated the intracellular localization of SMP30. It is localized in both the cytoplasm and nucleus. To confirm the presence of SMP30 in the nucleus, we prepared nuclear and cytoplasmic extracts from isolated cultured hepatocytes. Western blotting showed that SMP30 was detected in both extracts. Because the expression is reduced by carbon tetrachloride, one can speculate that the expression is modulated by oxidative stress increased with aging.

Association between bone loss and promoter polymorphism in the IL-6 gene in elderly Japanese women with hip fracture

Abstract Recently, a G/C polymorphism was found at position −573 of the interleukin-6 (IL-6) gene promoter. We investigated how this genetic polymorphism relates to IL-6 production and osteoporosis in elderly Japanese women. Genomic DNA was extracted from an aliquot of monocytes in the bone marrow; the monocytes were simultaneously used to form osteoclast-like multinucleated cells (MNCs) and to produce IL-6. Of the 47 subjects with fractures, 96% had a C allele at position −573 of the IL-6 gene. Only 2 subjects possessed homozygotes of G at that position. We investigated IL-6 levels, MNC formation in bone marrow culture, and femoral neck bone mineral density (BMD) in the subjects with the GC and CC genotypes. There were no significant differences between these genotypes as regards IL-6 levels, MNC formation, and femoral neck BMD. However, in the CC genotype, there was a negative relationship between femoral neck BMD and IL-6 levels, and between femoral neck BMD and MNC formation, whereas in the GC genotype and combined (GC + CC) genotypes, femoral neck BMD tended to be related to IL-6 levels and MNC formation. Moreover, the stromal cells in the CC genotype showed higher IL-1α-stimulated IL-6 production than did the stromal cells in the GC genotype. Our findings suggest that important information might be obtained not only by continued comparison of different genotypes but also by comparative study within each particular genotype.

D2 Dopamine receptor subtype mediates the inhibitory effect of dopamine on TRH-induced prolactin release from the bullfrog pituitary.

Dopamine receptors in mammals are known to consist of two D1-like receptors (D1 and D5) and three D2-like receptors (D2, D3 and D4). The aim of this study was to determine the dopamine receptor subtype that mediates the inhibitory action of dopamine on the release of prolactin (PRL) from the amphibian pituitary. Distal lobes of the bullfrog (Rana catesbeiana) were perifused and the amount of PRL released in the effluent medium was measured by means of a homologous enzyme-immunoassay. TRH stimulated the release of PRL from perifused pituitaries. Dopamine suppressed TRH-induced elevation of PRL release. Quinpirole (a D2 receptor agonist) also suppressed the stimulatory effect of TRH on the release of PRL, whereas SKF-38393 (a D1 receptor agonist) exhibited no such an effect. The inhibitory action of dopamine on TRH-induced PRL release from the pituitary was nullified by the addition of L-741,626 (a selective D2 receptor antagonist) to the medium, but not by the addition of SCH-23390 (a selective D1 receptor antagonist). These data indicate that the inhibitory effect of dopamine on TRH-evoked PRL release from the bullfrog pituitary gland is mediated through D2 dopamine receptors.