[2] In situ hybridization techniques to map processing enzymes

Species-specific vesicular monoamine transporter 2 (VMAT2) expression in mammalian pancreatic beta cells: implications for optimising radioligand-based human beta cell mass (BCM) imaging in animal models

AIMS/HYPOTHESIS

Imaging of beta cell mass (BCM) is a major challenge in diabetes research. The vesicular monoamine transporter 2 (VMAT2) is abundantly expressed in human beta cells. Radiolabelled analogues of tetrabenazine (TBZ; a low-molecular-weight, cell-permeant VMAT2-selective ligand) have been employed for pancreatic islet imaging in humans. Since reports on TBZ-based VMAT2 imaging in rodent pancreas have been fraught with confusion, we compared VMAT2 gene expression patterns in the mouse, rat, pig and human pancreas, to identify appropriate animal models with which to further validate and optimise TBZ imaging in humans.

METHODS

We used a panel of highly sensitive VMAT2 antibodies developed against equivalently antigenic regions of the transporter from each species in combination with immunostaining for insulin and species-specific in situ hybridisation probes. Individual pancreatic islets were obtained by laser-capture microdissection and subjected to analysis of mRNA expression of VMAT2.

RESULTS

The VMAT2 protein was not expressed in beta cells in the adult pancreas of common mouse or rat laboratory strains, in contrast to its expression in beta cells (but not other pancreatic endocrine cell types) in the pancreas of pigs and humans. VMAT2- and tyrosine hydroxylase co-positive (catecholaminergic) innervation was less abundant in humans than in rodents. VMAT2-positive mast cells were identified in the pancreas of all species.

CONCLUSIONS/INTERPRETATION

Primates and pigs are suitable models for TBZ imaging of beta cells. Rodents, because of a complete lack of VMAT2 expression in the endocrine pancreas, are a 'null' model for assessing interference with BCM measurements by VMAT2-positive mast cells and sympathetic innervation in the pancreas.

TSH compensates thyroid-specific IGF-I receptor knockout and causes papillary thyroid hyperplasia

Although TSH stimulates all aspects of thyroid physiology IGF-I signaling through a tyrosine kinase-containing transmembrane receptor exhibits a permissive impact on TSH action. To better understand the importance of the IGF-I receptor in the thyroid in vivo, we inactivated the Igf1r with a Tg promoter-driven Cre-lox system in mice. We studied male and female mice with thyroidal wild-type, Igf1r(+/-), and Igf1r(-/-) genotypes. Targeted Igf1r inactivation did transiently reduce thyroid hormone levels and significantly increased TSH levels in both heterozygous and homozygous mice without affecting thyroid weight. Histological analysis of thyroid tissue with Igf1r inactivation revealed hyperplasia and heterogeneous follicle structure. From 4 months of age, we detected papillary thyroid architecture in heterozygous and homozygous mice. We also noted increased body weight of male mice with a homozygous thyroidal null mutation in the Igf1r locus, compared with wild-type mice, respectively. A decrease of mRNA and protein for thyroid peroxidase and increased mRNA and protein for IGF-II receptor but no significant mRNA changes for the insulin receptor, the TSH receptor, and the sodium-iodide-symporter in both Igf1r(+/-) and Igf1r(-/-) mice were detected. Our results suggest that the strong increase of TSH benefits papillary thyroid hyperplasia and completely compensates the loss of IGF-I receptor signaling at the level of thyroid hormones without significant increase in thyroid weight. This could indicate that the IGF-I receptor signaling is less essential for thyroid hormone synthesis but maintains homeostasis and normal thyroid morphogenesis.

Impact of monocarboxylate transporter-8 deficiency on the hypothalamus-pituitary-thyroid axis in mice

In patients, inactivating mutations in the gene encoding the thyroid hormone-transporting monocarboxylate transporter 8 (Mct8) are associated with severe mental and neurological deficits and disturbed thyroid hormone levels. The latter phenotype characterized by high T3 and low T4 serum concentrations is replicated in Mct8 knockout (ko) mice, indicating that MCT8 deficiency interferes with thyroid hormone production and/or metabolism. Our studies of Mct8 ko mice indeed revealed increased thyroidal T3 and T4 concentrations without overt signs of a hyperactive thyroid gland. However, upon TSH stimulation Mct8 ko mice showed decreased T4 and increased T3 secretion compared with wild-type littermates. Moreover, similar changes in the thyroid hormone secretion pattern were observed in Mct8/Trhr1 double-ko mice, which are characterized by normal serum T3 levels and normal hepatic and renal D1 expression in the presence of very low T4 serum concentrations. These data strongly indicate that absence of Mct8 in the thyroid gland affects thyroid hormone efflux by shifting the ratio of the secreted hormones toward T3. To test this hypothesis, we generated Mct8/Pax8 double-mutant mice, which in addition to Mct8 lack a functional thyroid gland and are therefore completely athyroid. Following the injection of these animals with either T4 or T3, serum analysis revealed T3 concentrations similar to those observed in Pax8 ko mice under thyroid hormone replacement, indicating that indeed increased thyroidal T3 secretion in Mct8 ko mice represents an important pathogenic mechanism leading to the high serum T3 levels.

PepT2 transporter protein expression in human neoplastic glial cells and mediation of fluorescently tagged dipeptide derivative β-Ala-Lys-Nε-7-amino-4-methyl-coumarin-3-acetic acid accumulation

Object

The present study was aimed at analyzing the accumulation of the fluorescently tagged dipeptide derivative, β-Ala-Lys-Nε-7-amino-4-methyl coumarin-3-acetic acid (AMCA), in primary cultures of human neoplastic glial cells. This molecule is a highly specific reporter used to investigate the dipeptide transport system hPepT2.

Methods

In this study the authors used immunocytochemical methods to determine the cell-specific accumulation of a small and fluorescently tagged reporter molecule named β-Ala-Lys-Nε-AMCA to detect dipeptide transport capacity of neoplastic glial cells. Furthermore, specific mRNA levels were quantified using Northern blot analysis and the tissue distribution of hPepT2 mRNA transcripts was demonstrated with in-situ hybridization histochemical analysis.

Results

Recent fluorescent immunocytochemical analyses have revealed that β-Ala-Lys-Nε-AMCA specifically accumulates within anaplastic cells of astrocytic lineage but not in anaplastic oligodendrocytes or neurons. Northern blot analysis demonstrated that human hPepT2 mRNA is specifically detected in primary cell cultures of human glioblastoma but not in oligodendroglioma. Moreover, in situ hybridization analyses revealed an astrocytic localization of hPepT2 transcripts in human glioblastoma and astrocytoma cells. The hPepT2 transcription levels were clearly dependent on the grade of glial cell differentiation: within low-grade gliomas (WHO Grade II), more hPepT2 mRNA was detected compared with tumors of a higher grade of dedifferentiation (WHO Grade IV). Analysis of expression levels of hPepT2 mRNA in human neoplastic glial cells xenografted into the brains of athymic rats (han rnu+/+) showed a markedly increased expression of hPepT2 after 2 weeks of growth in vivo compared with the primary counterparts grown in vitro.

Conclusions

The authors concluded that expression of the hPepT2 transporter protein is a characteristic of glial cells of astrocytic lineage, and is dependent on the grade of astroglial cell differentiation and the extracellular matrix (here brain neuropil). The authors found that β-Ala-Lys-Nε-AMCA is as an excellent reporter molecule for assessing neoplastic glial cell function and physiological characteristics.

Consequences of monocarboxylate transporter 8 deficiency for renal transport and metabolism of thyroid hormones in mice

Patients carrying inactivating mutations in the gene encoding the thyroid hormone transporting monocarboxylate transporter (MCT)-8 suffer from a severe form of psychomotor retardation and exhibit abnormal serum thyroid hormone levels. The thyroidal phenotype characterized by high-serum T(3) and low-serum T(4) levels is also found in mice mutants deficient in MCT8 although the cause of these abnormalities is still unknown. Here we describe the consequences of MCT8 deficiency for renal thyroid hormone transport, metabolism, and function by studying MCT8 null mice and wild-type littermates. Whereas serum and urinary parameters do not indicate a strongly altered renal function, a pronounced induction of iodothyronine deiodinase type 1 expression together with increased renal T(3) and T(4) content point to a general hyperthyroid state of the kidneys in the absence of MCT8. Surprisingly, accumulation of peripherally injected T(4) and T(3) into the kidneys was found to be enhanced in the absence of MCT8, indicating that MCT8 deficiency either directly interferes with the renal efflux of thyroid hormones or activates indirectly other renal thyroid hormone transporters that preferentially mediate the renal uptake of thyroid hormones. Our findings indicate that the enhanced uptake and accumulation of T(4) in the kidneys of MCT8 null mice together with the increased renal conversion of T(4) into T(3) by increased renal deiodinase type 1 activities contributes to the generation of the low-serum T(4) and the increase in circulating T(3) levels, a hallmark of MCT8 deficiency.

Interference of a mutant thyroid hormone receptor alpha1 with hepatic glucose metabolism

Mice expressing the mutant thyroid hormone receptor TRalpha1R384C, which has a 10-fold reduced affinity to the ligand T(3), exhibit hypermetabolism due to an overactivation of the sympathetic nervous system. To define the consequences in the liver, we analyzed hepatic metabolism and the regulation of liver genes in the mutant mice. Our results showed that hepatic phosphoenolpyruvate-carboxykinase was up-regulated and pyruvate kinase mRNA down-regulated, contrary to what observed after T(3) treatment. In contrast, mice expressing a mutant TRalpha1L400R specifically in the liver did not show a dysregulation of these genes; however, when the TRalpha1L400R was expressed ubiquitously, the hepatic phenotype differed from TRalpha1R384C animals, suggesting that the localization of the mutation plays an important role for its consequences on glucose metabolism. Furthermore, we observed that glycogen stores were completely depleted in TRalpha1R384C animals, despite increased gluconeogenesis and decreased glycolysis. Exposure of the mutant mice to high maternal levels of thyroid hormone during fetal development leads to a normal liver phenotype in the adult. Our results show how genetic and maternal factors interact to determine the metabolic setpoint of the offspring and indicate an important role for maternal thyroid hormone in the susceptibility to metabolic disorders in adulthood.

Neuronal nitric oxide synthase gene inactivation reduces the expression of vasopressin in the hypothalamic paraventricular nucleus and of catecholamine biosynthetic enzymes in the adrenal gland of the mouse

The impact of a lifelong absence of the neuronal nitric oxide synthase (nNOS) in the neuroendocrine stress response was investigated in nNOS knockout (KO) and wild type (WT) mice under basal conditions and in response to forced swimming. In the hypothalamic paraventricular nucleus oxytocin and corticotropin-releasing-hormone mRNA levels did not differ between these genotypes under resting conditions, whereas vasopressin mRNA levels were significantly lower in nNOS KO than in WT animals. Also, in the adrenal glands basal levels of tyrosine hydroxylase protein, the rate-limiting enzyme for catecholamine biosynthesis, and of phenylethanolamine N-methyltransferase, which converts norepinephrine to epinephrine, were significantly reduced in nNOS KO mice. Plasma adrenocorticotropin, corticosterone, norepinephrine and epinephrine levels were similar in the KO and WT genotypes under resting conditions. In response to forced swimming, a similar increase in plasma adrenocorticotropin and corticosterone was observed in KO and WT animals. Stressor exposure triggered also an increased epinephrine release in WT animals, but did not significantly alter plasma epinephrine levels in KO mice. These data suggest that the chronic absence of nNOS reduces the capacity of epinephrine synthesising enzymes in the adrenal gland to respond to acute stressor exposure with an adequate epinephrine release.

Abnormal thyroid hormone metabolism in mice lacking the monocarboxylate transporter 8

In humans, inactivating mutations in the gene of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8; SLC16A2) lead to severe forms of psychomotor retardation combined with imbalanced thyroid hormone serum levels. The MCT8-null mice described here, however, developed without overt deficits but also exhibited distorted 3,5,3'-triiodothyronine (T3) and thyroxine (T4) serum levels, resulting in increased hepatic activity of type 1 deiodinase (D1). In the mutants' brains, entry of T4 was not affected, but uptake of T3 was diminished. Moreover, the T4 and T3 content in the brain of MCT8-null mice was decreased, the activity of D2 was increased, and D3 activity was decreased, indicating the hypothyroid state of this tissue. In the CNS, analysis of T3 target genes revealed that in the mutants, the neuronal T3 uptake was impaired in an area-specific manner, with strongly elevated thyrotropin-releasing hormone transcript levels in the hypothalamic paraventricular nucleus and slightly decreased RC3 mRNA expression in striatal neurons; however, cerebellar Purkinje cells appeared unaffected, since they did not exhibit dendritic outgrowth defects and responded normally to T3 treatment in vitro. In conclusion, the circulating thyroid hormone levels of MCT8-null mice closely resemble those of humans with MCT8 mutations, yet in the mice, CNS development is only partially affected.

Fractalkine expression in the rhesus monkey brain during lentivirus infection and its control by 6-chloro-2',3'-dideoxyguanosine

Existing data concerning the role of the delta-chemokine fractalkine (CX3CL1) and its receptor (CX3CR1) in lentivirus-induced encephalitis are limited and controversial. We explored, by quantitative in situ hybridization and immunohistochemistry, the cell-specific changes of CX3CL1 and CX3CR1 in rhesus macaque brain during simian immunodeficiency virus (SIV) infection and antiretroviral treatment. Neuronal expression of CX3CL1 was significantly reduced in cortex and striatum of AIDS-diseased monkeys as compared with uninfected and asymptomatic SIV-infected monkeys. CX3CL1 mRNA was increased in some endothelial cells and newly induced in astrocytes and macrophages focally in areas of SIV burden and inflammatory infiltrates. In most CX3CL1-positive astrocytes and macrophages, the transcription factor NF-kappaB was translocated to the nucleus. CX3CR1 was upregulated in scattered, nodule, and giant cell-forming microglia/macrophages and mononuclear infiltrates close to CX3CL1-induced cells in the brain. Treatment of AIDS monkeys with the central nervous system-permeant 6-chloro-2',3'-dideoxyguanosine fully reversed SIV burden, productive inflammation, nuclear NF-kappaB translocation as well as focal induction of CX3CL1 in astrocytes and macrophages and downregulation in neurons. In contrast, diffuse CX3CR1-positive microgliosis and GFAP-positive astrogliosis were partially reversed by 6-chloro-2',3'-dideoxyguanosine. Thus, focally induced CX3CL1 may be a target for therapeutic intervention to limit ongoing inflammatory infiltration into brain in lentivirus infection.

Agouti-related protein is posttranslationally cleaved by proprotein convertase 1 to generate agouti-related protein (AGRP)83-132: interaction between AGRP83-132 and melanocortin receptors cannot be influenced by syndecan-3

Agouti-related protein (AGRP) plays a key role in energy homeostasis. The carboxyl-terminal domain of AGRP acts as an endogenous antagonist of the melanocortin-4 receptor (MC4-R). It has been suggested that the amino-terminal domain of AGRP binds to syndecan-3, thereby modulating the effects of carboxyl-terminal AGRP at the MC4-R. This model assumes that AGRP is secreted as a full-length peptide. In this study we found that AGRP is processed intracellularly after Arg(79)-Glu(80)-Pro(81)-Arg(82). The processing site suggests cleavage by proprotein convertases (PCs). RNA interference and overexpression experiments showed that PC1/3 is primarily responsible for cleavage in vitro, although both PC2 and PC5/6A can also process AGRP. Dual in situ hybridization demonstrated that PC1/3 is expressed in AGRP neurons in the rat hypothalamus. Moreover, hypothalamic extracts from PC1-null mice contained 3.3-fold more unprocessed full-length AGRP, compared with wild-type mice, based on combined HPLC and RIA analysis, demonstrating that PC1/3 plays a role in AGRP cleavage in vivo. We also found that AGRP(83-132) is more potent an antagonist than full-length AGRP, based on cAMP reporter assays, suggesting that posttranslational cleavage is required to potentiate the effect of AGRP at the MC4-R. Because AGRP is cleaved into distinct amino-terminal and carboxyl-terminal peptides, we tested whether amino-terminal peptides modulate food intake. However, intracerebroventricular injection of rat AGRP(25-47) and AGRP(50-80) had no effect on body weight, food intake, or core body temperature. Because AGRP is cleaved before secretion, syndecan-3 must influence food intake independently of the MC4-R.

Athyroid Pax8-/- mice cannot be rescued by the inactivation of thyroid hormone receptor alpha1

The Pax8(-/-) mouse provides an ideal animal model to study the consequences of congenital hypothyroidism, because its only known defect is the absence of thyroid follicular cells. Pax8(-/-) mice are, therefore, completely athyroid in postnatal life and die around weaning unless they are substituted with thyroid hormones. As reported recently, Pax8(-/-) mice can also be rescued and survive to adulthood by the additional elimination of the entire thyroid hormone receptor alpha (TRalpha) gene, yielding Pax8(-/-)TRalpha(o/o) double-knockout animals. This observation has led to the hypothesis that unliganded TRalpha1 might be responsible for the lethal phenotype observed in Pax8(-/-) animals. In this study we report the generation of Pax8(-/-)TRalpha1(-/-) double-knockout mice that still express the non-T(3)-binding TR isoforms alpha2 and Deltaalpha2. These animals closely resemble the phenotype of Pax8(-/-) mice, including growth retardation and a completely distorted appearance of the pituitary with thyrotroph hyperplasia and hypertrophy, extremely high TSH mRNA levels, reduced GH mRNA expression, and the almost complete absence of lactotrophs. Like Pax8(-/-) mice, Pax8(-/-)TRalpha1(-/-) compound mutants die around weaning unless they are substituted with thyroid hormones. These findings do not support the previous interpretation that the short life span of Pax8(-/-) mice is due to the negative effects of the TRalpha1 aporeceptor, but, rather, suggest a more complex mechanism involving TRalpha2 and an unliganded TR isoform.

The monocarboxylate transporter 8 linked to human psychomotor retardation is highly expressed in thyroid hormone-sensitive neuron populations

Recent genetic analysis in several patients presenting a severe form of X-linked psychomotor retardation combined with abnormal thyroid hormone (TH) levels have revealed mutations or deletions in the gene of the monocarboxylate transporter 8 (MCT8). Because in vitro MCT8 functions as a TH transporter, the complex clinical picture of these patients indicated an important role for MCT8 in TH-dependent processes of brain development. To provide a clue to the cellular function of MCT8 in brain, we studied the expression of MCT8 mRNA in the murine central nervous system by in situ hybridization histochemistry. In addition to the choroid plexus structures, the highest transcript levels were found in neo- and allocortical regions (e.g. olfactory bulb, cerebral cortex, hippocampus, and amygdala), moderate signal intensities in striatum and cerebellum, and low levels in a few neuroendocrine nuclei. Colocalization studies revealed that MCT8 is predominantly expressed in neurons. Together with the spatiotemporal expression pattern of MCT8 during the perinatal period, these results strongly indicate that MCT8 plays an important role for proper central nervous system development by transporting TH into neurons as its main target cells.

Generation of Thyrotropin-Releasing Hormone Receptor 1-Deficient Mice as an Animal Model of Central Hypothyroidism

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1−/− mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1−/− mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T3, T4, and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1−/− animals TSHβ-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1−/− is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.

Colocalization of urocortin and neuronal nitric oxide synthase in the hypothalamus and Edinger-Westphal nucleus of the rat

Different lines of studies suggest that both the corticotropin-releasing hormone-related peptide Urocortin I (Ucn) and the neuromodulator nitric oxide (NO) are involved in the regulation of the complex mechanisms controlling feeding and anxiety-related behaviors. The aim of the present study was to investigate the possible interaction between Ucn and NO in the hypothalamic paraventricular nucleus (PVN), an area known to be involved in the modulation of these particular behaviors. Therefore, we mapped local mRNA and peptide/protein presence of both Ucn and the NO producing neuronal NO synthase (nNOS). This investigation was extended to include the hypothalamic supraoptic nucleus (SON) and the Edinger-Westphal nucleus area (EW), the latter being one of the major cellular Ucn-expressing sites. Furthermore, we compared the two predominantly used laboratory rat strains, Wistar and Sprague-Dawley. Ucn mRNA and immunoreactivity were detected in the SON and in the EW. A significant difference between Wistar and Sprague-Dawley rats was found in mRNA levels in the EW. nNOS was detected in all brain areas analyzed, showing a significantly lower immunoreactivity in the PVN and EW of Sprague-Dawley versus Wistar rats. Contrary to some previous reports, no Ucn mRNA and only a very low immunoreactivity were detectable in the PVN of either rat strain. Interestingly, double-labeling immunofluorescence revealed that in the SON approximately 75% of all cells immunoreactive for Ucn were colocalized with nNOS, whereas in the EW only approximately 2% of the Ucn neurons were found to contain nNOS. These findings suggest an interaction between Ucn and NO signaling within the SON, rather than the PVN, that may modulate the regulation of feeding, reproduction, and anxiety-related behaviors.

Relationship of pronociceptin/orphanin FQ and the nociceptin receptor ORL1 with substance P and calcitonin gene-related peptide expression in dorsal root ganglion of the rat

Recent evidence suggests a role of prepronociceptin/orphanin FQ (preproN/OFQ) derived neuropeptides in nociceptive signaling. Here, we examined the expression of preproN/OFQ and the nociceptin receptor ORL1 (opioid receptor like receptor 1) in the dorsal root ganglion (DRG) of the rat in relation to that of substance P (SP) and calcitonin gene-related peptide (CGRP). Double labeling in situ hybridization revealed a constitutive expression of preproN/OFQ in a distinct minor subpopulation of very small DRG neurons with no evidence for coexpression with either SP or CGRP. However, a major subpopulation of the preproN/OFQ-positive neurons showed direct juxtaposition to SP and CGRP containing neurons. ORL1 was abundantly expressed with a high degree of coexpression with SP (72%) and CGRP (82%) suggesting that N/OFQ may presynaptically modulate primary sensory nociceptive signaling. The DRG cell line F11 was found to express preproN/OFQ, but not ORL1, and, therefore, is well suited to study the mechanisms of N/OFQ gene regulation in vitro.

Central administration of neuropeptide FF causes activation of oxytocin paraventricular hypothalamic neurones that project to the brainstem

Neuropeptide FF (NPFF), a morphine modulatory peptide, is emerging as an important neuromodulator in the context of central autonomic and neuroendocrine regulation. NPFF immunoreactivity and receptors have been identified in discrete autonomic regions within the brain and spinal cord, including the hypothalamic paraventricular nucleus (PVN). In this study, we examined the effects of intracerebroventricular (i.c.v.) administration of NPFF on activation of chemically identified PVN neurones that project to the brainstem nucleus of the solitary tract (NTS). In conscious rats, i.c.v. NPFF at a dose of 10 micro g, but not 8 micro g, caused an increase in arterial blood pressure. Immunohistochemical analysis revealed a dose-dependent increase in activated (Fos positive) PVN neurones following i.c.v. NPFF administration compared to controls receiving i.c.v. saline. Activated PVN neurones were located predominantly in the parvocellular compartment of the nucleus with relatively few Fos positive cells in the magnocellular subdivision. Chemical identification of activated neurones revealed significant number of activated cells to be oxytocin positive, whereas only few vasopressin, tyrosine hydroxylase (TH) or corticotrophin-releasing factor (CRF) neurones were double-labelled. Injection of the retrograde tracer fluorogold into the NTS resulted in labelling of significant numbers of parvocellular oxytocin, but not vasopressin, TH or CRF, PVN neurones. We conclude that centrally administered NPFF stimulates brainstem-projecting oxytocin PVN neurones. Oxytocin released from terminals within the NTS oxytocin thus modulate the activity of ascending visceral autonomic pathways that synapse initially within the NTS.

Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons

We have cloned and functionally characterized a third isoform of a vesicular glutamate transporter (VGLUT3) expressed on synaptic vesicles that identifies a distinct glutamatergic system in the brain that is partly and selectively promiscuous with cholinergic and serotoninergic transmission. Transport activity was specific for glutamate, was H(+)-dependent, was stimulated by Cl(-) ion, and was inhibited by Rose Bengal and trypan blue. Northern analysis revealed higher mRNA levels in early postnatal development than in adult brain. Restricted patterns of mRNA expression were observed in presumed interneurons in cortex and hippocampus, and projection systems were observed in the lateral and ventrolateral hypothalamic nuclei, limbic system, and brainstem. Double in situ hybridization histochemistry for vesicular acetylcholine transporter identified VGLUT3 neurons in the striatum as cholinergic interneurons, whereas VGLUT3 mRNA and protein were absent from all other cholinergic cell groups. In the brainstem VGLUT3 mRNA was concentrated in mesopontine raphé nuclei. VGLUT3 immunoreactivity was present throughout the brain in a diffuse system of thick and thin beaded varicose fibers much less abundant than, and strictly separated from, VGLUT1 or VGLUT2 synapses. Co-existence of VGLUT3 in VMAT2-positive and tyrosine hydroxylase -negative varicosities only in the cerebral cortex and hippocampus and in subsets of tryptophan hydroxylase-positive cell bodies and processes in differentiating primary raphé neurons in vitro indicates selective and target-specific expression of the glutamatergic/serotoninergic synaptic phenotype.

Gene expression of proprotein convertases in individual rat anterior pituitary cells and their regulation in corticotrophs mediated by glucocorticoids

The subtilisin-like proprotein convertases (SPCs) are a family of serine proteinases that process secreted proteins at single or paired basic residues. Each SPC has been localized in the rat anterior pituitary, implying their importance in precursor processing in this tissue. The cellular distribution of each SPC has not been established in each hormone-producing cell type. We used double labeling in situ hybridization histochemistry to examine the mRNA distribution of five SPCs in relation to corticotrophs, thyrotrophs, lactotrophs, gonadotrophs, and somatotrophs. Our data demonstrated that SPC expression patterns were distinct, with each SPC expressed in more than one cell type. We noted that overlapping SPC expressions were the rule rather than the exception, suggesting potential SPC redundant functions. We examined the effects of adrenalectomy on corticotroph SPC expression. Most corticotrophs expressed SPC1, SPC3, and SPC4, but few corticotrophs expressed SPC2 or SPC6. After adrenalectomy, we observed increased mRNA levels for SPC1, SPC3, SPC4, and SPC6, but not for SPC2, in POMC-positive anterior pituitary cells. These increased levels were reversed by dexamethasone treatment. These data demonstrate the plasticity of SPCs expression in corticotrophs. SPCs may be directly involved in the mammalian stress response and may be important in hypothalamic-pituitary-adrenal axis homeostasis.

Independent patterns of transcription for the products of the rat cholinergic gene locus

The cholinergic phenotype requires the expression of the vesicular acetylcholine transporter and choline acetyltransferase proteins. Both genes are encoded at one chromosomal location called the cholinergic gene locus. We have identified by in situ hybridization histochemistry distinct patterns of transcription from the cholinergic gene locus in the subdivisions of the rat cholinergic nervous system. The vesicular acetylcholine transporter and choline acetyltransferase are co-expressed in cholinergic neurons at all developmental stages in all major types of cholinergic neurons. The relative levels of vesicular acetylcholine transporter and choline acetyltransferase transcripts, however, change substantially during development in the CNS. They also differ dramatically in distinct subdivisions of the mature cholinergic nervous system, with vesicular acetylcholine transporter mRNA expressed at high levels relative to choline acetyltransferase mRNA in the peripheral nervous system, but at equivalent levels in the CNS. Expression of the R-exon, the presumptive first non-coding exon common to both the vesicular acetylcholine transporter and choline acetyltransferase, was not detectable at any developmental stage in any of the cholinergic neuronal subtypes in the rat nervous system. Thus, in contrast to less complex metazoan organisms, production of the vesicular acetylcholine transporter and choline acetyltransferase via a common differentially spliced transcript does not seem to occur to a significant extent in the rat. We suggest that separate transcriptional start sites within the cholinergic gene locus control vesicular acetylcholine transporter and choline acetyltransferase transcription, while additional elements are responsible for the specific transcriptional control of the entire locus in cholinergic versus non-cholinergic neurons. Independent transcription of the vesicular acetylcholine transporter and choline acetyltransferase genes provides a mechanism for regulating the relative expression of these two proteins to fine-tune acetylcholine quantal size in different types of cholinergic neurons, both centrally and peripherally.

IDA-1, a Caenorhabditis elegans homolog of the diabetic autoantigens IA-2 and phogrin, is expressed in peptidergic neurons in the worm

The closely related mammalian proteins IA-2 and phogrin are protein tyrosine phosphatase-like receptor proteins spanning the membrane of dense core vesicles of neuroendocrine tissues. They are of interest as molecular components of the secretory machinery and as major targets of autoimmunity in type I diabetes mellitus. The Caenorhabditis elegans genome has a single copy of an IA-2/phogrin homolog ida-1 III (islet cell diabetic autoantigen), which encodes the ida-1 (B0244.2) gene product as a series of 12 exons over a 10-kb region of chromosome III. The full-length sequence of the ida-1 cDNA encoded a 767-amino acid type 1 transmembrane protein of 87 kDa. The PTP catalytic site consensus sequence of IDA-1, like IA-2 and phogrin, diverged and would not be active. Expression of green fluorescent protein (GFP) under the ida-1 gene promoter showed activity in a subset of around 30 neurons with sensory functions and the uv1 cells of the vulva in hermaphrodites. Males showed additional expression in male-specific neurons. In situ experiments in rat brain showing the distribution of IA-2 and phogrin suggested a complimentary and overlapping pattern compared with the proprotein convertases PC1 and PC2. In C. elegans, IDA-1-expressing cells comprised a subset of those expressing the PC2 homolog KPC-2 (C51E3. 7), consistent with IDA-1 being a component of neuropeptide-containing dense core vesicles. The results support the hypothesis that C. elegans IDA-1 is the functional homolog of IA-2 and phogrin in mammals. Analysis of the function of IDA-1 should contribute to our understanding of the function of these proteins in signal transduction, vesicle locomotion, and exocytosis.

Neuroinvasion by human respiratory coronaviruses

Human coronaviruses (HCoV) cause common colds but can also infect neural cell cultures. To provide definitive experimental evidence for the neurotropism and neuroinvasion of HCoV and its possible association with multiple sclerosis (MS), we have performed an extensive search and characterization of HCoV RNA in a large panel of human brain autopsy samples. Very stringent reverse transcription-PCR with two primer pairs for both viral strains (229E and OC43), combined with Southern hybridization, was performed on samples from 90 coded donors with various neurological diseases (39 with MS and 26 with other neurological diseases) or normal controls (25 patients). We report that 44% (40 of 90) of donors were positive for 229E and that 23% (21 of 90) were positive for OC43. A statistically significant higher prevalence of OC43 in MS patients (35.9%; 14 of 39) than in controls (13.7%; 7 of 51) was observed. Sequencing of nucleocapsid protein (N) gene amplicons revealed point mutations in OC43, some consistently found in three MS patient brains and one normal control but never observed in laboratory viruses. In situ hybridization confirmed the presence of viral RNA in brain parenchyma, outside blood vessels. The presence of HCoV in human brains is consistent with neuroinvasion by these respiratory pathogens. Further studies are needed to distinguish between opportunistic and disease-associated viral presence in human brains.

Targeted infection of endothelial cells by avian influenza virus A/FPV/Rostock/34 (H7N1) in chicken embryos

The tissue tropism and spread of infection of the highly pathogenic avian influenza virus A/FPV/Rostock/34 (H7N1) (FPV) were analyzed in 11-day-old chicken embryos. As shown by in situ hybridization, the virus caused generalized infection that was strictly confined to endothelial cells in all organs. Studies with reassortants of FPV and the apathogenic avian strain A/chick/Germany/N/49 (H10N7) revealed that endotheliotropism was linked to FPV hemagglutinin (HA). To further analyze the factors determining endotheliotropism, the HA-activating protease furin was cloned from chicken tissue. Ubiquitous expression of furin and other proprotein convertases in the chick embryo indicated that proteolytic activation of HA was not responsible for restriction of infection to the endothelium. To determine the expression of virus receptors in embryonic tissues, histochemical analysis of alpha2,3- and alpha2,6-linked neuraminic acid was carried out by lectin-binding assays. These receptors were found on endothelial cells and on several epithelial cells, but not on tissues surrounding endothelia. Finally, we analyzed the polarity of virus maturation in endothelial cells. Studies on cultured human endothelial cells employing confocal laser scanning microscopy revealed that HA is specifically targeted to the apical surface of these cells, and electron microscopy of embryonic tissues showed that virus maturation occurs also at the luminar side. Taken together, these observations indicate that endotheliotropism of FPV in the chicken embryo is determined, on one hand, by the high cleavability of HA, which mediates virus entry into the vascular system, and, on the other hand, by restricted receptor expression and polar budding, which prevent spread of infection into tissues surrounding endothelia.

The Rat and Mouse Homologues of MASP-2 and MAp19, components of the Lectin Activation Pathway of Complement

Recently, we described two novel constituents of the multimolecular initiation complex of the mannan-binding lectin (MBL) pathway of complement activation, a serine protease of 76 kDa, termed MASP-2, and a MASP-2 related plasma protein of 19 kDa, termed MAp19. Upon activation of the MBL/MASPs/MAp19 complex, MASP-2 cleaves the fourth complement component C4, while the role of MAp19 within the MBL/MASP-1/MASP-2/MAp19 complex remains to be clarified. In humans, the mRNA species encoding MASP-2 (2.6 kb) and MAp19 (1.0 kb) arise by an alternative polyadenylation/splicing mechanism from a single structural MASP-2 gene. Here, we report the complete primary structures of the rat homologue of MASP-2 and of rat and mouse MAp19. We show that both MASP-2 and MAp19 are part of the rat MBL pathway activation complex and demonstrate their exclusively hepatic biosynthesis. Southern blot and PCR analyses of rat genomic DNA indicate that as in humans, rat MASP-2 and MAp19 are encoded by a single structural gene.

Carboxypeptidase D is a potential candidate to carry out redundant processing functions of carboxypeptidase E based on comparative distribution studies in the rat central nervous system

Post-translational processing is essential for the biological activation of many proteins and peptides. After precursor cleavage at specific single residues or pairs of basic residues by the proprotein convertases, the C-terminal basic residues are removed. Carboxypeptidase E was thought to be the only enzyme responsible. Recent studies with carboxypeptidase E-deficient mice, Cpe(fat)/Cpe(fat), indicated the existence of carboxypeptidase E-like carboxypeptidases, such as carboxypeptidase D. In order to define potential redundant functions in vivo, we compared the distributions of both carboxypeptidases in the rat central nervous system and selected endocrine tissues. Carboxypeptidase D messenger RNA was abundantly expressed in glial cells in the gray and white matter, while neurons in several brain regions, such as the piriform cortex, basolateral amygdala and hippocampus, also expressed high levels of carboxypeptidase D messenger RNA. Co-localization of carboxypeptidases E and D messenger RNAs was observed in many brain regions, the spinal cord and endocrine tissues. Immunohistochemistry showed the intracellular distribution of carboxypeptidase D with a perinuclear pattern. The extensive distribution of carboxypeptidase D in both glial and neuronal cells indicates the important role of carboxypeptidase D in peptide processing, possibly working together with furin, a ubiquitously expressed proprotein convertase. The co-localization of carboxypeptidases D and E suggests that carboxypeptidase D may, at least partially, compensate for carboxypeptidase E processing functions in Cpe(fat)/Cpe(fat) mice.

The peptide transporter PepT2 is expressed in rat brain and mediates the accumulation of the fluorescent dipeptide derivative beta-Ala-Lys-Nepsilon-AMCA in astrocytes

We describe the synthesis of a fluorescent dipeptide derivative, beta-Ala-Lys-Nepsilon-AMCA, which could be used as an excellent reporter molecule for studying the oligopeptide transport system in brain cell cultures. Fluorescence microscopic and immunocytochemical studies revealed that the reporter peptide specifically accumulated in astrocytes (type I and II) and O-2A progenitor cells but not in neurons or differentiated oligodendrocytes. In astroglia-rich cell culture the dipeptide derivative is taken up in unmetabolized form by an energy dependent, saturable process with apparent kinetic constants of KM = 28 microM and Vmax = 6 nmol x h(-1) x mg protein(-1) at pH 7.2. Competition studies revealed that the accumulation of beta-Ala-Lys-Nepsilon-AMCA is strongly inhibited by dipeptides and pseudopeptides such as bestatin, arphamenine A and B. The biochemical data indicated that the properties of this high-affinity oligopeptide carrier closely resemble those of the renal peptide transport system PepT2 and Northern blot analysis demonstrated that PepT2 mRNAis expressed in glial but not in neuronal cell cultures. In situ hybridization histochemistry also revealed a non-neuronal localization of PepT2 transcripts and a diffuse, widespread distribution of PepT2 signals throughout the entire rat brain. The selective accumulation of the fluorescent reporter molecule by brain cells under viable conditions may provide a useful tool for studying peptide uptake systems and other aspects of astroglial physiology.

Neuroendocrine protein 7B2 is essential for proteolytic conversion and activation of proprotein convertase 2 in vivo

The 7B2 protein is widely distributed in neural and endocrine tissues. Its biological function was found to be related to the processing enzyme proprotein convertase 2 (PC2), a mammalian subtilisin/kexin-like endoproteinase that cleaves at specific single or multiple basic amino-acid residues. In order to examine the proposed function of 7B2 on PC2 in in vivo models, we first compared the distribution of 7B2 and PC2 mRNAs in the rat brain. Expression of 7B2 mRNA was found to be pan-neuronal, but additionally, we observed 7B2 mRNA in ependymal cells and in the subcommissural organ. Although the expression of PC2 mRNA was exclusively neuronal, it was more restricted, sparing some regions expressing high levels of 7B2. This finding suggests that 7B2 has an additional function in non-PC2-expressing cells. No evidence of PC2-positive/7B2-negative cells could be obtained in the adult rat brain. However, in the developing rat brain (E17), such regions were easily observed, showing higher levels of pro-PC2 (75 kD). Similarly, in the animal model of insulin-induced hypoglycemic shock, where adrenomedullary 7B2 expression is decreased, the ratio of pro-PC2 to mature PC2 (75 kD:68 kD) was observed to be increased. Finally, the human neuroepithelioma SK-N-MCIXC expresses PC2 but not 7B2. Accordingly, only inactive pro-PC2 forms were observed: 75-kD intracellular and 71-kD extracellular. After stable transfection of SK-N-MCIXC cells with 27-kD pro-7B2, mature and active (68-kD) PC2 was secreted into the medium. Our data demonstrate a critical role of 7B2 in the proteolytic conversion and activation of PC2 in vivo.

Neuronal expression of fractalkine in the presence and absence of inflammation

Fractalkine is the only as yet known member of a novel class of chemokines. Besides its novel Cys-X-X-X-Cys motif, fractalkine exhibits features which have not been described for any other member of the chemokine family, including its unusual size (397 amino acids human, 395 mouse) and the possession of a transmembrane anchor, from which a soluble form may be released by extracellular cleavage. This report demonstrates the abundant mRNA and fractalkine protein expression in neuronal cells. The neuronal expression of fractalkine mRNA is unaffected by experimentally induced inflammation of central nervous tissue.

Region-specific expression of thyrotrophin-releasing hormone-degrading ectoenzyme in the rat central nervous system and pituitary gland

Thyrotrophin-releasing hormone (TRH), a hypothalamic neuropeptide hormone and a putative neuromodulator/ neurotransmitter in the central nervous system is inactivated by the TRH-degrading ectoenzyme (TRH-DE), a TRH-specific metallopeptidase localized on the surface of neuronal brain cells in culture and on lactotrophic cells of the pituitary. After succeeding in cloning the cDNA of TRH-DE we now report on the cellular distribution pattern of this enzyme in rat brain, spinal cord and pituitary gland using in situ hybridization histochemistry. In the pituitary, TRH-DE mRNA was found both in the anterior and the neural lobe but not in the intermediate lobe. After treatment with triiodothyronine (T3) a dramatic increase in the mRNA levels of the TRH-DE and a decrease in the intensity of the TRH receptor could be observed in the anterior lobe of the pituitary. In brain, TRH-DE transcripts were predominantly found in neo- and allocortical regions with strongest signals in the olfactory bulb, the piriform cortex, the cerebral cortex, the granular layer of the cerebellar cortex and the pyramidal cells of the Ammon's horn. In the diencephalon, the highest TRH-DE mRNA levels were observed in the medial habenulae followed by several hypothalamic subregions. In the mesencephalon and brainstem, moderate signals were present in the superior colliculi, substantia nigra, dorsal raphe and in the periolivar region. In the spinal cord, TRH-DE mRNA positive neurons were present in all layers. The very distinct distribution of TRH-DE in the brain and the hormonal regulation of the adenohypophyseal enzyme support the concept that this peptidase serves very specialized functions.

Axonal transports of Boc-Gly-Arg-Arg-MCA hydrolysing enzyme in rat sciatic nerves

Study on neural axon transport is a very useful method to find a neuron-specific protease. In the present study, the enzyme activity (release of 7-amino-4-methyl-coumarin from t-butyloxycarbonyl-glycyl-L-arginyl-L-arginine-4-methylcoumaryl-7-amide) was measured in the proximal, middle, and distal segments between 12 and 120 h after double ligations of rat sciatic nerves to find precursor processing enzyme specific for pair of basic amino acid residue. The enzyme activity was significantly increased not only in the proximal but also in the distal segments 12-120 h after the ligation, and the maximal enzyme activity was found in both segments at 72 h. The enzyme activity eluted by anion exchange chromatography of the proximal segment showed at least three peaks, and was slightly higher than the activity of the distal one. The activity in the middle segment was very low in comparison with the activity in the proximal and distal segments. These data indicate that some of the enzymes specific for pair of basic amino acid residue are transported by both anterograde and retrograde axonal flow, and may undergo a neuron-specific processing.

Prodynorphin processing by proprotein convertase 2. Cleavage at single basic residues and enhanced processing in the presence of carboxypeptidase activity

Endoproteolytic processing of the 26-kDa protein precursor prodynorphin (proDyn) at paired and single basic residues is most likely carried out by the proprotein convertases (PCs); however, the role of PCs at single basic residues is unclear. In previous studies we showed that limited proDyn processing by PC1/PC3 at both paired and single basic residues resulted in the formation of 8- and 10-kDa intermediates. Because PC2 is colocalized with proDyn, we examined the potential role of this convertase in cleaving proDyn. PC2 cleaved proDyn to produce dynorphin (Dyn) A 1-17, Dyn B 1-13, and alpha-neo-endorphin, without a previous requirement for PC1/PC3. PC2 also cleaved at single basic residues, resulting in the formation of the C-peptide and Dyn A 1-8. Only PC2, but not furin or PC1/PC3, could cleave the Arg-Pro bond to yield Dyn 1-8. Structure-activity studies with Dyn A 1-17 showed that a P4 Arg residue is important for single basic cleavage by PC2 and that the P1' Pro residue impedes processing. Conversion of Dyn A 1-17 or Dyn B 1-13 into leucine-enkephalin (Leu-Enk) by PC2 was never observed; however, Dyn AB 1-32 cleavage yielded small amounts of Leu-Enk, suggesting that Leu-Enk can be generated from the proDyn precursor only through a specific pathway. Finally, PC2 cleavages at single and paired basic residues were enhanced when carried out in the presence of carboxypeptidase (CP) E. Enhancement was blocked by GEMSA, a specific inhibitor of CPE activity, and could be duplicated by other carboxypeptidases, including CPD, CPB, or CPM. Our data suggest that carboxypeptidase activity enhances PC2 processing by the elimination of product inhibition caused by basic residue-extended peptides.

Prohormone convertases in mouse submandibular gland: co-localization of furin and nerve growth factor

Nerve growth factor (NGF) in mouse submandibular glands (SGs) is generated from a 35-kD precursor by proteolytic enzymes that have yet to be identified. Prohormone convertases (PCs) cleave the NGF precursor in vitro, and in this study we questioned whether PCs could process salivary NGF in vivo. mRNA coding for PC2 (but not PC1) was detected on Northern blots of SG mRNA and also by in situ hybridization within parasympathetic neurons of intralobular ganglia. Northern blot and in situ hybridization analyses also detect mRNA coding for furin. In SGs of male mice, furin mRNA levels are high at birth and remain high throughout development. In glands from female mice, levels decline during postnatal development and are lower in adults than in newborns. Immunocytochemistry detects furin immunoreactivity in pro-acinar and ductal cells of glands from newborn and pubescent mice. In glands of adults, furin immunoreactivity is detectable in acinar cells but highest levels are present in NGF-containing granular convoluted tubule cells. These data, taken together with those from previous studies, suggest that furin is a candidate processing enzyme for NGF in mouse submandibular glands.