Maternal imprinting of human SNRPN, a gene deleted in Prader-Willi syndrome.

Prader-Willi syndrome (PWS), a human neuroendocrine disorder, is associated with deficiencies of paternal chromosome 15q12. Small nuclear ribonucleoprotein polypeptide N (SNRPN) is the first expressed gene identified in the PWS critically deleted region. Following our demonstration that the murine homologue of SNRPN is imprinted, we have characterized a sequence polymorphism within expressed portions of human SNRPN and show that human SNRPN is monoallelically expressed in fetal brain and heart and in adult brain. Analysis of maternal DNA and SNRPN cDNA confirmed that the maternal allele of SNRPN is not expressed in fetal brain and heart. Maternal imprinting of SNRPN supports the hypothesis that paternal absence of SNRPN is responsible for the PWS phenotype.

Maternal imprinting of the mouse Snrpn gene and conserved linkage homology with the human Prader-Willi syndrome region.

Prader-Willi syndrome (PWS) is associated with paternal gene deficiencies in human chromosome 15q11-13, suggesting that PWS is caused by a deficiency in one or more maternally imprinted genes. We have now mapped a gene, Snrpn, encoding a brain-enriched small nuclear ribonucleoprotein (snRNP)-associated polypeptide SmN, to mouse chromosome 7 in a region of homology with human chromosome 15q11-13 and demonstrated that Snrpn is a maternally imprinted gene in mouse. These studies, in combination with the accompanying human mapping studies showing that SNRPN maps in the Prader-Willi critical region, identify SNRPN as a candidate gene involved in PWS and suggest that PWS may be caused, in part, by defects in mRNA processing.

A candidate mouse model for Prader-Willi syndrome which shows an absence of Snrpn expression.

The best examples of imprinting in humans are provided by the Angelman and Prader-Willi syndromes (AS and PWS) which are associated with maternal and paternal 15q11-13 deletions, respectively, and also with paternal and maternal disomy 15. The region of the deletions has homology with a central part of mouse chromosome 7, incompletely tested for imprinting effects. Here, we report that maternal duplication for this region causes a murine imprinting effect which may correspond to PWS. Paternal duplication was not associated with any detectable effect that might correspond with AS. Gene expression studies established that Snrpn is not expressed in mice with the maternal duplication and suggest that the closely-linked Gabrb-3 locus is not subject to imprinting. Finally, an additional new imprinting effect is described.

Expression in brain of a messenger RNA encoding a novel neuropeptide homologous to calcitonin gene-related peptide.

As a consequence of alternative RNA processing events, a single rat gene can generate messenger RNA's (mRNA's) encoding either calcitonin or a neuropeptide referred to as alpha-type calcitonin gene-related peptide (alpha-CGRP). An mRNA product of a related gene has been identified in rat brain and thyroid encoding the protein precursor of a peptide differing from alpha-CGRP by only a single amino acid. The RNA encoding this peptide, which is referred to as beta-CGRP, appears to be the only mature transcript of the beta-CGRP gene. Hybridization histochemistry reveals a similar distribution of alpha- and beta-CGRP mRNA's, but their relative levels of expression vary in different cranial nerve nuclei. Thus beta-CGRP is a new member of a family of related genes with potential functions in regulating the transduction of sensory and motor information.

Viral gene delivery selectively restores feeding and prevents lethality of dopamine-deficient mice.

Dopamine-deficient mice (DA-/- ), lacking tyrosine hydroxylase (TH) in dopaminergic neurons, become hypoactive and aphagic and die by 4 weeks of age. They are rescued by daily treatment with L-3,4-dihydroxyphenylalanine (L-DOPA); each dose restores dopamine (DA) and feeding for less than 24 hr. Recombinant adeno-associated viruses expressing human TH or GTP cyclohydrolase 1 (GTPCH1) were injected into the striatum of DA-/- mice. Bilateral coinjection of both viruses restored feeding behavior for several months. However, locomotor activity and coordination were partially improved. A virus expressing only TH was less effective, and one expressing GTPCH1 alone was ineffective. TH immunoreactivity and DA were detected in the ventral striatum and adjacent posterior regions of rescued mice, suggesting that these regions mediate a critical DA-dependent aspect of feeding behavior.

Regulation of gene expression in vivo following transduction by two separate rAAV vectors.

Control of gene expression is important to gene therapy for purposes of both dosing and safety. In vivo regulation of gene expression was demonstrated following co-injection of two separate recombinant adeno-associated virus vectors, one encoding an inducible murine erythropoietin transgene and the other a transcriptional activator, directly into the skeletal muscle of adult immunocompetent mice. Transcription was controlled by systemic administration or withdrawal of tetracycline over an 18 week period, demonstrating that the two vectors were capable of transducing the same cell. Cellular or humoral immune responses against the transactivator protein were not detected.

A tissue-specific enhancer in the rat-calcitonin/CGRP gene is active in both neural and endocrine cell types.

The calcitonin gene related peptide (CGRP) gene is a complex transcription unit that is expressed in a highly restricted pattern in both the nervous system, particularly in sensory ganglia and brainstem, and in the thyroid C cells of the endocrine system, with tissue-specific alternative RNA processing events generating transcripts encoding either the hormone, calcitonin, or the neuropeptide, CGRP. This pattern of expression in neural and endocrine tissues raises the question whether similar or distinct genomic elements are responsible for activation in both neural and endocrine cell types. We have identified a complex enhancer element, located more than 1 kilobase 5' of the transcription initiation site of the calcitonin/CGRP gene that functions in cells of neuronal or C cell origin, but not in any other cell type tested. At least two complementary regulatory sequences are required for the function of the cell-specific enhancer.

Calcium, dexamethasone, and the antiglucocorticoid RU-486 differentially regulate neuropeptide synthesis in a rat C cell line.

The differential regulation of neurotensin (NT), calcitonin (CT), and CT gene-related peptide (CGRP) production was studied in the clonal, rat C cell-derived, 44-2C cell line. Two experimental paradigms were used: cells were incubated with maximally effective concentrations of calcium (4.0 mM); alternatively, cells were treated with the synthetic glucocorticoid, dexamethasone (DEX). The specificity of the DEX-mediated response was assessed by using the synthetic antiglucocorticoid, RU-486. Calcium was not mitogenic in 44-2C cells and did not affect cell growth. Calcium increased the secretion and cellular accumulation of NT. In contrast, calcium treatment decreased CT content and release while it diminished the levels of CT- and CGRP-specific messenger RNA (mRNA) levels. DEX (10(-8) M) inhibited cell proliferation. NT content and secretion increased after DEX treatment, and this was potentiated by the addition of calcium. DEX-treated cells showed diminished CT content and secretion. The levels of CT- and CGRP-specific mRNA were significantly reduced in DEX-treated cultures. RU-486 antagonized the action of DEX and blocked DEX-inhibited cell proliferation. Inhibition of CT secretion by DEX was blocked by RU-486; CT- and CGRP-specific mRNA levels were increased in response to treatment with equimolar or 100-fold excess concentrations of RU-486. We conclude that NT secretion as well as CT/CGRP expression and release can be differentially regulated in the 44-2C cell line.

Induction of c-fos, calcitonin gene expression, and acidic fibroblast growth factor production in a multipeptide-secreting neuroendocrine cell line.

The multipeptide-secreting 44-2C cell line maintains differentiated function when grown in a serum-free, growth factor- and hormone-deprived milieu. The cells continue to synthesize and secrete calcitonin (CT), CT gene-related peptide, neurotensin, and somatostatin and respond to cellular secretagogues such as GRF and acidic and basic fibroblast growth factor. We designed experiments to ascertain the functional role(s) of cellular factors involved in the maintenance of the differentiated state in 44-2C cells. We report here the phenotypic transformation that occurs in these cells in the course of adjustment to the serum-free state. We also show the differential increase in CT-specific mRNA, the transient induction of c-fos, and the characterization of biologically active acidic fibroblast growth factor.

Age-related decreases in GTP-cyclohydrolase-I immunoreactive neurons in the monkey and human substantia nigra.

Guanosine triphosphate cyclohydrolase I (GTPCHI) is a critical enzyme in catecholamine function and is rate limiting for the synthesis of the catecholamine co-factor tetrahydrobiopterin. The present study assessed the distribution of GTPCHI immunoreactivity (-ir) within the monkey and human ventral midbrain and determined whether its expression is altered as a function of age. Light and confocal microscopic analyses revealed that young monkeys and humans displayed GTPCHI-ir within melanin-containing and tyrosine-hydroxylase-ir neurons in primate substantia nigra. Stereological counts revealed that there was a 67.4% reduction in GTPCHI-ir neuronal number, a 63.5% reduction in GTPCHI-ir neuronal density, and a 37.6% reduction in neuronal volume in aged monkeys relative to young cohorts. Similar age-related changes were seen in humans, in whom there were significant reductions in the number of GTPCHI-ir nigral neurons in middle age (58.4%) and aged (81.5%) cases relative to young cohorts. The density of GTPCHI-ir neurons within the nigra was similarly reduced in middle-aged (63.0%) and aged (81.8%) cases. In contrast to monkeys, aged humans did not display shrinkage in the volume of GTPCHI-ir nigral neurons. The presence of numerous melanin-positive, but GTPCHI-ir immunonegative, neurons in the aged monkey and human nigra indicates that these decreases represent an age-related phenotypic downregulation of this enzyme and not a loss of neurons per se. These data indicate that there is a dramatic decrease in GTPCHI-ir in nonhuman primates and humans as a function of age and that loss of this enzyme may be partly responsible for the age-related decrease in dopaminergic tone within nigrostriatal systems.

Sulpiride isomers exhibit reversed stereospecificity for D-1 and D-2 dopamine receptors in the CNS.

We have investigated the stereospecificity of the interaction of (-) and (+)sulpiride with [3H]cis-flupentixol and [3H]spiperone binding to D-1 and D-2 dopamine receptors respectively in rat striatum. Both isomers of sulpiride compete more potently at D-2 vs. D-1 dopamine receptors. (-)Sulpiride is 50-fold more potent than (+)sulpiride in blocking D-2 receptors, while (+)sulpiride is 3-fold more potent than (-)sulpiride at D-1 receptors. This reversed stereospecificity of sulpiride interactions with CNS D-1 and D-2 dopamine receptors is similar to the stereospecificity of sulpiride interactions at DA1 and DA2 dopamine receptors in peripheral vascular beds.

Long-term restoration of striatal L-aromatic amino acid decarboxylase activity using recombinant adeno-associated viral vector gene transfer in a rodent model of Parkinson's disease.

As a potential treatment for Parkinson's disease, viral vector-mediated over-expression of striatal L-aromatic amino acid decarboxylase was tested in an attempt to facilitate the production of therapeutic levels of dopamine after peripheral L-dihydroxyphenylalanine administration. The results of microdialysis and enzyme activity assays indicate that striatal decarboxylation of peripherally administered L-dihydroxyphenylalanine was enhanced by recombinant adeno-associated virus-mediated gene transfer of L-aromatic amino acid decarboxylase in unilateral 6-hydroxydopamine-lesioned rats. This gene transfer-induced increase in striatal decarboxylase activity was shown to remain undiminished over a six-month period and transgene expression was demonstrated to persist for at least one year. Unlike previous approaches involving delivery of either tyrosine hydroxylase, or tyrosine hydroxylase and L-aromatic amino acid decarboxylase transgenes together to accomplish unregulated dopamine delivery, the current study proposes a pro-drug strategy (peripheral L-dihydroxyphenylalanine administration after L-aromatic amino acid decarboxylase transduction). This strategy for dosage control could potentially allow lowered L-dihydroxyphenylalanine doses and potentially obviate complicated transcriptional regulation paradigms. These data suggest that the use of the non-pathogenic adeno-associated virus to transfer the L-aromatic amino acid decarboxylase gene into the striatum of Parkinson's disease patients may be an attractive gene therapy strategy.

Interactions of agonists with D-2 dopamine receptors: evidence for a single receptor population existing in multiple agonist affinity-states in rat striatal membranes.

Several lines of evidence previously indicated that [3H]spiroperidol (SPIRO) or [3H]domperidone (DOMP) might label heterogeneous populations of striatal dopamine receptors in radioligand binding studies. We have examined this possibility in rat striatum using computerized non-linear curve fitting and a method to block the unwanted [3H]SPIRO binding to S-2 serotonergic receptors. In the absence of a serotonergic antagonist, [3H]SPIRO saturation data produce curved Scatchard plots which are best computer fit by assuming the presence of two sites of different [3H]SPIRO affinities. In the presence of appropriate concentrations of ketanserin to block S-2 serotonergic binding, Scatchard plots are linear, with data modeling best to a single population of homogeneous binding sites. The D-2 dopamine receptor Bmax and [3H]SPIRO KD determined in this fashion are indistinguishable from that obtained for the higher affinity binding site by computer analysis of data obtained in the absence of ketanserin. [3H]DOMP produced indistinguishable values for D-2 receptor Bmax as well. Competitions by (-)sulpiride, metoclopramide, and DOMP for [3H]SPIRO binding sites in the presence of ketanserin are steep (Hill slope approximately 1), demonstrating that the previously observed heterogeneity of these sites is due entirely to serotonergic [3H]SPIRO binding. In contrast, agonist/3H-antagonist competition curves in the presence of ketanserin are best computer fit by assuming two independent receptor sites of high (RH) and low (RL) agonist affinity. With the addition of 5'-guanylylimidodiphosphate [Gpp(NH)p] computer analyses of agonist/3H-antagonist competition curves show an increased ratio RL/RH concomitant with apparent decreases in agonist affinities for both sites. Under some conditions, some agonist/3H-antagonist competition curves are best fit by a single site model in which agonist affinity is indistinguishable from the agonist's affinity at RL determined in the absence of Gpp(NH)p. These data are consistent with the presence of a single dopaminergic 3H-butyrophenone binding site, representing the D-2 receptor, which exists in two interconverting states differing in agonist affinity.

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors.

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.

Interactions of dopaminergic agonists and antagonists with dopaminergic D3 binding sites in rat striatum. Evidence that [3H]dopamine can label a high affinity agonist-binding state of the D1 dopamine receptor.

The interactions of dopaminergic agonists and antagonists with 3H-agonist labeled D3 dopaminergic binding sites of rat striatum have been characterized by radioligand-binding techniques. When the binding of [3H]dopamine and [3H]apomorphine to D2 dopamine receptors is blocked by the inclusion of D2 selective concentrations of unlabeled spiroperidol or domperidone, these ligands appear to label selectively the previously termed "D3" binding site. Antagonist/[3H]dopamine competition curves are of uniformly steep slope (nH = 1.0), suggesting the presence of a single D3 binding site. The relative potencies of antagonists to inhibit D3 specific [3H]dopamine binding are significantly correlated with their potencies to block D1 dopamine receptors as measured by the inhibition of both dopamine-stimulated adenylate cyclase and [3H]flupentixol-binding activities. The affinities of agonists to inhibit D3 specific [3H]dopamine binding are also correlated with estimates of these agonists' affinities for the high affinity binding component of agonist/[3H]flupentixol competition curves. Both D3 specific [3H] dopamine binding and the high affinity agonist-binding component of dopamine/[3H]flupentixol competition curves show a similar sensitivity to guanine nucleotides. Taken together, these data strongly suggest that the D3 binding site is related to a high affinity agonist-binding state of the D1 dopamine receptor.

Dopaminergic D-3 binding sites are not presynaptic autoreceptors.

Postsynaptic dopamine (DA) receptors have been classified biochemically and pharmacologically into two types: D-1 receptors mediate adenylate cyclase stimulation, demonstrating micromolar affinity for DA and butyrophenone antagonists; D-2 receptors mediate adenylate cyclase inhibition, demonstrating nanomolar affinity for DA and butyrophenone antagonists. D-1 receptors are labelled by 3H-thioxanthene antagonists, while D-2 receptors are labelled by both 3H-agonists and all 3H-antagonists. A third class of dopaminergic binding site, termed D-3, represents high-affinity 3H-agonist binding sites demonstrating low, micromolar, affinity for butyrophenones. In the rat striatum, D-3 sites were decreased 50% by 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal DA pathway, suggesting that such D-3 binding labels presynaptic DA autoreceptors on nigrostriatal terminals. However, nigrostriatal denervation produces a concomitant depletion of striatal DA. Here we demonstrate that a reserpine-induced depletion of DA produces a decrease in D-3 binding comparable to that seen with nigrostriatal denervation, independent of presynaptic terminal degeneration. This loss in binding, or that caused by 6-OHDA lesions, is recovered by preincubating the striatal membranes with DA or with the supernatant from control striatal membrane preparations. We therefore suggest that the loss of D-3 binding following 6-OHDA lesions results from the depletion of endogenous DA rather than the degeneration of terminals and their putatively associated autoreceptors.

Agonist interactions with dopamine receptors: focus on radioligand-binding studies.

Radioligand-binding experiments have demonstrated that both D-1 (adenylate cyclase stimulatory) and D-2 (adenylate cyclase inhibitory) dopamine receptors demonstrate high- and low-affinity agonist-binding states. Under appropriate conditions, both D-1 and D-2 high-affinity binding states can be labeled directly with 3H-labeled agonists. Previous studies that identified a portion of high-affinity 3H-labeled agonist binding as a distinct dopamine receptor subclass (D-3 or autoreceptor) were, in fact, labeling the high-affinity agonist-binding state of postsynaptic D-1 receptors.

Dopamine receptor turnover rates in rat striatum are age-dependent.

The time course of recovery of [3H]spiperone binding in the rat striatum after a single injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) shows that a slower rate of regeneration/turnover of D-2 dopamine receptors occurs in mid-life-mature versus young male rats. This slower receptor recovery reflects relatively slower rates of both receptor synthesis and degradation. Studies using cycloheximide indicate that protein synthesis plays a significant role in the reappearance of [3H]spiperone-binding sites. Other experiments indicate that chronic reserpine treatment, which produces dopamine receptor up regulation, also produces accelerated receptor recovery after EEDQ blockade. An age-related decline in dopamine receptor turnover, if present in humans and progressive into senescence, could be responsible for the increased risk of developing Parkinson disease and drug-induced parkinsonian-like extrapyramidal side effects with age. On the other hand, the more rapid receptor turnover rates seen in young rats may be a biochemical feature related to plasticity in the striatum during development.

Interactions of dopamine agonists with brain D1 receptors labeled by 3H-antagonists. Evidence for the presence of high and low affinity agonist-binding states.

The interactions of dopaminergic agonists and antagonists with 3H-antagonist labeled D1 dopamine receptors of rat striatum have been characterized. [3H]Flupentixol has been found to selectively label D1 dopamine receptors when its binding to D2 dopamine receptors is blocked by the inclusion of D2 selective concentrations of unlabeled spiroperidol or domperidone. Antagonist/3H-antagonist competition curves are of uniformly steep slope (nH = 1.0) suggesting the presence of a single D1 dopamine receptor. Agonist/3H-antagonist competition curves are extremely shallow (nH less than or equal to 0.5) for agonists of high relative efficacy, suggesting the presence of heterogeneous populations of agonist-binding states of the D1 dopamine receptor. Computer-modeling techniques were used to estimate affinities and relative site densities for these heterogeneous binding states. This analysis indicates that the ratio of agonist affinities for low and high affinity agonist-binding states is correlated with agonist relative efficacies in activating adenylate cyclase in membrane homogenates. Under the assay conditions employed, the addition of saturating concentrations of guanine nucleotides reduced, but did not abolish, the relative density of high affinity agonist-binding sites. These binding data can, at least in part, be explained by postulating two states of the D1 dopamine receptor, inducible by agonists but not by antagonists and modulated by guanine nucleotides.