Neurons in the Locus coeruleus (LC) play an important role in the central CO2 chemosensitivity. However, the molecular mechanisms for neuronal CO2 chemosensitivity remain unclear. To demonstrate the expression of pH/CO2 sensitive ion channels, we screened the inward rectifier K+ channels (Kir) and transient receptor protein (TRP) channels, as parallel studies in this lab suggested that certain Kir and TRP channels are involved in neuronal responses to high levels of CO2. Our results showed that several members of the Kir and TRP channel families were robustly expressed in the LC neurons at the mRNA level. Of particular interest are TRPC5, Kir4.1 and Kir5.1 channels that are all pH-sensitive. The rich expression of various pH-sensitive Kir and TRP channels suggests that these ion channels are likely to play a role in the chemosensitivity of LC neurons.

Degeneration of the noradrenergic locus coeruleus (LC) in aging and neurodegenerative diseases is well documented. Slowing or reversing this effect may have therapeutic implications. Phox2a and Phox2b are homeodomain transcriptional factors that function as determinants of the noradrenergic phenotype during embryogenesis. In the present study, recombinant lentiviral eGFP-Phox2a and -Phox2b (vPhox2a and vPhox2b) were constructed to study the effects of Phox2a/2b over-expression on dopamine β-hydroxylase (DBH) and norepinephrine transporter (NET) levels in central noradrenergic neurons. Microinjection of vPhox2 into the LC of adult rats significantly increased Phox2 mRNA levels in the LC region. Over-expression of either Phox2a or Phox2b in the LC was paralleled by significant increases in mRNA and protein levels of DBH and NET in the LC. Similar increases in DBH and NET protein levels were observed in the hippocampus following vPhox2 microinjection. In the frontal cortex, only NET protein levels were significantly increased by vPhox2 microinjection. Over-expression of Phox2 genes resulted in a significant increase in BrdU-positive cells in the hippocampal dentate gyrus. The present study demonstrates an upregulatory effect of Phox2a and Phox2b on the expression of DBH and NET in noradrenergic neurons of rat brains, an effect not previously shown in adult animals. Phox2 genes may play an important role in maintaining the function of the noradrenergic neurons after birth, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2- bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

The present study investigated the effects of forced overexpression of Phox2a/2b, two transcription factors, in the locus coeruleus (LC) of aged rats on noradrenergic and dopaminergic phenotypes in brains. Results showed that a significant increase in Phox2a/2b mRNA levels in the LC region was paralleled by marked enhancement in expression of DBH and TH per se. Furthermore, similar increases in TH protein levels were observed in the substantial nigra and striatum, as well as in the hippocampus and frontal cortex. Overexpression of Phox2 genes also significantly increased BrdU-positive cells in the hippocampal dentate gyrus and NE levels in the striatum. Moreover, this manipulation significantly improved the cognition behavior. The in vitro experiments revealed that norepinephrine treatments may increase the transcription of TH gene through the epigenetic action on the TH promoter. The results indicate that Phox2 genes may play an important role in improving the function of the noradrenergic and dopaminergic neurons in aged animals, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.

An important pathology in Parkinson's disease (PD) is the earlier and more severe degeneration of noradrenergic neurons in the locus coeruleus (LC) than dopaminergic neurons in the substantia nigra. However, the basis of such selective vulnerability to insults remains obscure. Using noradrenergic and dopaminergic cell lines, as well as primary neuronal cultures from rat LC and ventral mesencephalon (VM), the present study compared oxidative DNA damage response markers after exposure of these cells to hydrogen peroxide (H2O2). The results showed that H2O2 treatment resulted in more severe cell death in noradrenergic cell lines SK-N-BE(2)-M17 and PC12 than dopaminergic MN9D cells. Furthermore, there were higher levels of oxidative DNA damage response markers in noradrenergic cells and primary neuronal cultures from the LC than dopaminergic cells and primary cultures from the VM. It included increased tail moments and tail lengths in Comet assay, and increased protein levels of phosphor-p53 and γ-H2AX after treatments with H2O2. Consistent with these measurements, exposure of SK-N-BE(2)-M17 cells to H2O2 resulted in higher levels of reactive oxygen species (ROS). Further experiments showed that exposure of SK-N-BE(2)-M17 cells to H2O2 caused an increased level of noradrenergic transporter, reduced protein levels of copper transporter (Ctr1) and 8-oxoGua DNA glycosylase, as well as amplified levels of Cav1.2 and Cav1.3 expression. Taken together, these experiments indicated that noradrenergic neuronal cells seem to be more vulnerable to oxidative damage than dopaminergic neurons, which may be related to the intrinsic characteristics of noradrenergic neuronal cells.

Phox2a and Phox2b are two homeodomain proteins that control the differentiation of noradrenergic neurons during embryogenesis. In the present study, we examined the possible effect of Phox2a/2b on the in vitro expression of the norepinephrine transporter (NET) and dopamine β-hydroxylase (DBH), two important markers of the noradrenergic system. SK-N-BE(2)C cells were transfected with cDNAs or short hairpin RNAs specific to the human Phox2a and Phox2b genes. Transfection of 0.1 to 5 μg of cDNAs of Phox2a or Phox2b significantly increased mRNA and protein levels of NET and DBH in a concentration-dependent manner. As a consequence of the enhanced expression of NET after transfection, there was a parallel increase in the uptake of [ 3H]norepinephrine. Co-transfection of Phox2a and Phox2b did not further increase the expression of noradrenergic markers when compared with transfection of either Phox2a or Phox2b alone. Transfection of shRNAs specific to Phox2a or Phox2b genes significantly reduced mRNA and protein levels of NET and DBH after shutdown of endogenous Phox2, which was accompanied by a decreased [3H]norepinephrine uptake. Furthermore, there was an additive effect after cotransfection with both shRNAs specific to Phox2a or Phox2b genes on NET mRNA levels. Finally, the reduced DBH expression caused by the shRNA specific to Phox2a could be reversed by transfection with Phox2b cDNA and vice versa. The present findings verify the determinant role of Phox2a and Phox2b on the expression and function of NET and DBH in vitro. Further clarifying the regulatory role of these two transcription factors on key proteins of the noradrenergic system may open a new avenue for therapeutics of aging-caused dysfunction of the noradrenergic system.

Dysfunction of the central noradrenergic and dopaminergic systems is the primary neurobiological characteristic of Parkinson’s disease (PD). Importantly, neuronal loss in the locus coeruleus (LC) that occurs in early stages of PD may accelerate progressive loss of dopaminergic neurons. Therefore, restoring the activity and function of the deficient noradrenergic system may be an important therapeutic strategy for early PD. In the present study, the lentiviral constructions of transcription factors Phox2a/2b, Hand2 and Gata3, either alone or in combination, were microinjected into the LC region of the PD model VMAT2 Lo mice at 12 and 18 month age. Biochemical analysis showed that microinjection of lentiviral expression cassettes into the LC significantly increased mRNA levels of Phox2a, and Phox2b, which were accompanied by parallel increases of mRNA and proteins of dopamine β-hydroxylase (DBH) and tyrosine hydroxylase (TH) in the LC. Furthermore, there was considerable enhancement of DBH protein levels in the frontal cortex and hippocampus, as well as enhanced TH protein levels in the striatum and substantia nigra. Moreover, these manipulations profoundly increased norepinephrine and dopamine concentrations in the striatum, which was followed by a remarkable improvement of the spatial memory and locomotor behavior. These results reveal that over-expression of these transcription factors in the LC improves noradrenergic and dopaminergic activities and functions in this rodent model of PD. It provides the necessary groundwork for the development of gene therapies of PD, and expands our understanding of the link between the LC-norepinephrine and dopamine systems during the progression of PD.

Stress has been reported to activate the locus coeruleus (LC)-noradrenergic system. In this study, corticosterone (CORT) was orally administrated to rats for 21 days to mimic stress status. In situ hybridization measurements showed that CORT ingestion significantly increased mRNA levels of norepinephrine transporter (NET) and dopamine β-hydroxylase (DBH) in the LC region. Immunofluorescence staining and western blotting revealed that CORT treatment also increased protein levels of NET and DBH in the LC, as well as NET protein levels in the hippocampus, the frontal cortex and the amygdala. However, CORT-induced increase in DBH protein levels only appeared in the hippocampus and the amygdala. Elevated NET and DBH expression in most of these areas (except for NET protein levels in the LC) was abolished by simultaneous treatment with combination of corticosteroid receptor antagonist mifepristone and spironolactone (s.c. for 21 days). Also, treatment with mifepristone alone prevented CORT-induced increases of NET expression and DBH protein levels in the LC. In addition, behavioral tasks showed that CORT ingestion facilitated escape in avoidance trials using an elevated T-maze, but interestingly, there was no significant effect on the escape trial. Corticosteroid receptor antagonists failed to counteract this response in CORT-treated rats. In the open-field task, CORT treatment resulted in less activity in a defined central zone compared to controls and corticosteroid receptor antagonist treatment alleviated this increase. In conclusion, this study demonstrates that chronic exposure to CORT results in a phenotype that mimics stress-induced alteration of noradrenergic phenotypes, but the effects on behavior are task dependent. As the sucrose consumption test strongly suggests CORT ingestion-induced depression-like behavior, further elucidation of underlying mechanisms may improve our understanding of the correlation between stress and the development of depression.

The adrenergic system plays an important role in the regulation of blood pressure. In the spontaneously hypertensive rat, the most studied model of essential hypertension, many components of the adrenergic system are altered. Changes in expression level of any catecholamine biosynthetic enzymes or any adrenergic receptor subtypes could be one of the causes of hypertension development. In this work, the expression of adrenergic system genes was measured in adrenal gland, renal cortex and renal medulla of the spontaneously hypertensive (SHR), Wistar-Kyoto and Brown Norway rats at the age of thirteen weeks. In adrenal gland of SHR, all four catecholamine biosynthetic enzymes (tyrosine hydroxylase, DOPA decarboxylase, dopamine β-hydroxylase and phenylethanolamine-N- methyltransferase) and almost all subtypes of adrenergic receptors (with the exception of Adra1a and Adra1d) were underexpressed. This generally decreased expression in adrenal gland of SHR suggests that at least a part of regulation of adrenergic system gene expression is common. The mechanism of this downregulation in SHR could be a negative feedback through adrenergic receptors stimulated by high plasma noradrenaline concentration. In the kidney of SHR, there were no differences in the expression of most of adrenergic receptor subtypes with the...

<p> Exposure to chronic low oxygen (hypoxia) leads to a series of adaptive responses involving changes in gene expression that are critical for cell, tissue, and organismal survival. These changes are mediated by an important set of regulators belonging to the hypoxia inducible factor (HIF) family of transcription factors (e.g. HIF-lα, HIF-2α, HIF3α) which undergo rapid degradation during normal oxygen (normoxia) but are rapidly stabilized during hypoxia. While the role of HIF-1α has been extensively studied in many cell types, there have been relatively few studies on the role of HIF-2α, though recent evidence suggests its function maybe tissue specific. This thesis examined the hypothesis that HIF-2α plays a central role in the development and function of catecholaminergic cells of the sympathoadrenal (SA) lineage. The study was aided by use of an immortalized line of rat adrenomedullary chromaffin cells (i.e. MAH cells), derived from fetal SA progenitors, which express several hypoxia-sensitive properties characteristic of native cells in the adrenal gland. In Chapter 2, I investigated the potential contributions of mitochondrial reactive oxygen species (ROS) and 0 2 consumption to HIF-2α induction in MAH cells exposed to chronic hypoxia (2% O(2); 24 hr). In MAH cells, chronic hypoxia caused an increase in HIF-2α induction which was blocked by inhibition of any of the mitochondrial complexes using pharmacological agents, or by specific inhibition of complexes III and IV using RNAi techniques. It was found that in this 0 2-sensitive chromaffin cell line mitochondrial O(2) consumption, rather than changes in ROS, regulated HIF-2α induction during hypoxia. In Chapter 3, I investigated the hypothesized role of HIF-2α in the development of the catecholaminergic phenotype in cells of the SA lineage using the MAH cell line as a model. Mutant MAH cells, with depleted HIF-2α due to siRNA knock-down, showed dramatically lower levels of dopamine and noradrenaline compared to untransfected and scrambled control cells, regardless of whether the cells were cultured under normoxia or chronic hypoxia. This was correlated with a marked reduction in the expression of DOPA decarboxylase (DDC) and dopamine B hydroxylase (DBH), though the expression of tyrosine hydroxylase (TH) was unaffected. Moreover, HIF-2α was able to bind to a region of the DDC gene promoter which contains two putative hypoxia response elements (HREs). These data suggest that a basal level of HIF-2α function is required for the normal developmental expression of DDC and DBH in SA progenitor cells, and that loss of this function leads to impaired catecholamine (CA) biosynthesis. In Chapter 4, I investigated genes regulated by chronic hypoxia in MAH cells, with a focus on those involved in CA metabolism, storage, and secretion. Using microarray analysis combined with QPCR and RNAi knock-down methodology I uncovered several genes, involved in amine vesicular packaging, trafficking and secretion, which were upregulated during chronic hypoxia. One gene specifically, the adenosine A(2A) receptor (A(2A)R) gene, which appears to modulate CA secretion via autocrine or paracrine actions of extracellular adenosine, was dramatically upregulated in chronic hypoxia. Interestingly, this effect was completely abolished in HIF-2α knockdown MAH cells, suggesting a critical involvement of HIF-2α. Chromatin immunoprecipitation (ChIP) assays revealed that HIF-2α bound to the promoter region of the A(2A)R gene which contains a putative hypoxia response element (HRE) immediately upstream of exon 1. Ratiometric fluorescence measurements of intracellular Ca(2+) revealed that adenosine (50 μM) potentiated the high K(+)-evoked rise in [Ca(2+)]i in MAH cells. This effect of adenosine was further enhanced after chronic hypoxia, but was abolished in HIF-2α knock-down cells. In conclusion, these data suggest that HIF-2α is a key regulator of several genes involved in CA biosynthesis, and of others that mediate the facilitatory effects of chronic hypoxia on CA secretion in sympathoadrenal derivatives. / Thesis / Doctor of Philosophy (PhD)