Activation of the CRF(1) receptor causes ERK1/2 mediated increase in GRK3 expression in CATH.a cells

Early Chronic Stress Induced Changes within the Locus Coeruleus in Sporadic Alzheimer's Disease

Chronic exposure to stress throughout the lifespan has been the focus of many studies on Alzheimer's disease (AD) because of the similarities between the biological mechanisms involved in chronic stress and the pathophysiology of AD. In fact, the earliest abnormality associated with the disease is the presence of phosphorylated tau protein in locus coeruleus neurons, a brain structure highly responsive to stress and perceived threat. Here, we introduce allostatic load as a useful concept for understanding many of the complex, interacting neuropathological changes involved in the AD degenerative process. In response to chronic stress, aberrant tau proteins that begin to accumulate within the locus coeruleus decades prior to symptom onset appear to represent a primary pathological event in the AD cascade, triggering a wide range of interacting brain changes involving neuronal excitotoxicity, endocrine alterations, inflammation, oxidative stress, and amyloid plaque exacerbation. While it is acknowledged that stress will not necessarily be the major precipitating factor in all cases, early tau-induced changes within the locus coeruleus-norepinephrine pathway suggests that a therapeutic window might exist for preventative measures aimed at managing stress and restoring balance within the HPA axis.

Sex and the noradrenergic system

The central noradrenergic system comprises multiple brainstem nuclei whose cells synthesize and release the catecholamine transmitter norepinephrine (NE). The largest of these nuclei is the pontine locus coeruleus (LC), which innervates the vast majority of the forebrain. NE interacts with a number of pre- and postsynaptically expressed G protein-coupled receptors to affect a wide array of functions, including sensory signal processing, waking and arousal, stress responsiveness, mood, attention, and memory. Given the myriad functions ascribed to the locus coeruleus-noradrenergic (LC-NE) system, it is unsurprising that it is implicated in many disease states, including various mood, cognitive, neuropsychiatric, and neurodegenerative diseases. The LC-NE system is also notably sexually dimorphic with regard to its morphologic and anatomical features as well as how it responds to the peptide transmitter corticotropin releasing hormone (CRH), a major mediator of the central stress response. The sex-biased morphology and signaling that is observed in the LC could then be considered a potential contributor to the differential prevalence of various diseases between men and women. This chapter summarizes the primary differences between the male and female LC, based primarily on preclinical observations and how these disparities may relate to differential diagnoses of several diseases between men and women.

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Persistent Stress-Induced Neuroplastic Changes in the Locus Coeruleus/Norepinephrine System

Neural plasticity plays a critical role in mediating short- and long-term brain responses to environmental stimuli. A major effector of plasticity throughout many regions of the brain is stress. Activation of the locus coeruleus (LC) is a critical step in mediating the neuroendocrine and behavioral limbs of the stress response. During stressor exposure, activation of the hypothalamic-pituitary-adrenal axis promotes release of corticotropin-releasing factor in LC, where its signaling promotes a number of physiological and cellular changes. While the acute effects of stress on LC physiology have been described, its long-term effects are less clear. This review will describe how stress changes LC neuronal physiology, function, and morphology from a genetic, cellular, and neuronal circuitry/transmission perspective. Specifically, we describe morphological changes of LC neurons in response to stressful stimuli and signal transduction pathways underlying them. Also, we will review changes in excitatory glutamatergic synaptic transmission in LC neurons and possible stress-induced modifications of AMPA receptors. This review will also address stress-related behavioral adaptations and specific noradrenergic receptors responsible for them. Finally, we summarize the results of several human studies which suggest a link between stress, altered LC function, and pathogenesis of posttraumatic stress disorder.

Formin 2 links neuropsychiatric phenotypes at young age to an increased risk for dementia

Age-associated memory decline is due to variable combinations of genetic and environmental risk factors. How these risk factors interact to drive disease onset is currently unknown. Here we begin to elucidate the mechanisms by which post-traumatic stress disorder (PTSD) at a young age contributes to an increased risk to develop dementia at old age. We show that the actin nucleator Formin 2 (Fmn2) is deregulated in PTSD and in Alzheimer's disease (AD) patients. Young mice lacking the Fmn2 gene exhibit PTSD-like phenotypes and corresponding impairments of synaptic plasticity, while the consolidation of new memories is unaffected. However, Fmn2 mutant mice develop accelerated age-associated memory decline that is further increased in the presence of additional risk factors and is mechanistically linked to a loss of transcriptional homeostasis. In conclusion, our data present a new approach to explore the connection between AD risk factors across life span and provide mechanistic insight to the processes by which neuropsychiatric diseases at a young age affect the risk for developing dementia.

CRHR1 promoter hypomethylation: An epigenetic readout of panic disorder?

The corticotropin releasing hormone receptor 1 (CRHR1) is crucially involved in the hypothalamic-pituitary-adrenal axis and thus a major regulator of the stress response. CRHR1 gene variation is associated with several mental disorders including anxiety disorders. Studies in rodents have demonstrated epigenetic regulation of CRHR1 gene expression to moderate response to stressful environment. In the present study, we investigated CRHR1 promoter methylation for the first time regarding its role in panic disorder applying a case-control approach (N=131 patients, N=131 controls). In an independent sample of healthy volunteers (N=255), CRHR1 methylation was additionally analyzed for association with the Beck Anxiety Inventory (BAI) score as a dimensional panic-related intermediate phenotype. The functional relevance of altered CRHR1 promoter methylation was investigated by means of luciferase-based reporter gene assays. In panic disorder patients, a significantly decreased CRHR1 methylation was discerned (p<0.001). Accordingly, healthy controls with high BAI scores showed significantly decreased CRHR1 methylation. Functional analyses revealed an increased gene expression in presence of unmethylated as compared to methylated pCpGl_CRHR1 reporter gene vectors. The present study identified a potential role of CRHR1 hypomethylation - conferring increased CRHR1 expression - in panic disorder and a related dimensional intermediate phenotype. This up-regulation of CRHR1 gene expression driven by de-methylation might constitute a link between the stress response and panic disorder risk.

Human kidney-2 cells harbor functional dopamine D1 receptors that require Giα for Gq/11α signaling

A recent study demonstrated that the dopamine D1 receptor (D1R) is nonfunctional in human kidney cells, HK2 cells, in terms of their inability to couple to Gs protein in response to the D1R agonist fenoldopam. Since D1R also couples to Gq protein, we tested whether D1R is functional in HK2 cells in terms of their ability to couple to Gq and produce downstream signaling. For comparison, we also studied another receptor, angiotensin II type 1 receptor (AT1R) known to couple to Gq. Protein kinase C (PKC) and (86)rubidium transport activities were determined as surrogate downstream signaling markers. Fenoldopam and angiotensin II increased PKC activity, which decreased in the presence of respective receptor antagonists (SCH23390 for D1R; candesartan for AT1R), PKC (chelerythrine chloride) and Gi protein (pertussis toxin) inhibitors and Gq/11α siRNA. Furthermore, fenoldopam and angiotensin II increased (35)S-GTPγS binding, an index of receptor-G protein coupling, which decreased with pertussis toxin and in Gq/11α-depleted cells. Also, fenoldopam-mediated inhibition of (86)rubidium transport (an index of Na-K-ATPase activity) was attenuated with SCH23390, chelerythrine chloride, pertussis toxin, and Gq/11α siRNA. Moreover, fenoldopam caused a decrease in cytosolic and increase in membranous abundance of Gq/11α. The immunoprecipitated levels of Gq/11α in the membranes were greater in fenoldopam-treated cells, and Giα coimmunoprecipitated with Gq/11α. Our results suggest that both D1R and AT1R are functional in HK2 cells, enabling Gq-mediated downstream signaling in a Gi dependent manner.

Novel role of RGS2 in regulation of antioxidant homeostasis in neuronal cells

Regulator of G-protein signaling protein (RGS)-2 is a modulator of anxiety and dysregulation of oxidative stress is implicated in anxiety. Also, RGS2 expression is reported to be induced by oxidative stress. Thus, if oxidative stress induces RGS2 expression and lack of RGS2 causes anxiety, then mechanisms that link RGS2 and oxidative stress potentially critical to anxiety must be revealed. Our study is the first to suggest role of RGS2 in regulation of enzymes involved in antioxidant defense namely glyoxalase-1 and glutathione reductase-1 via activation of p38 MAPK and PKC pathways in an Sp-1 dependent manner.

Differential effects of inescapable stress on locus coeruleus GRK3, alpha2-adrenoceptor and CRF1 receptor levels in learned helpless and non-helpless rats: a potential link to stress resilience

Exposure of rats to unpredictable, inescapable stress results in two distinct behaviors during subsequent escape testing. One behavior, suggestive of lack of stress resilience, is prolonged escape latency compared to non-stressed rats and is labeled learned helplessness (LH). The other behavior suggestive of stress resilience is normal escape latency and is labeled non-helpless (NH). This study examines the effects of unpredictable, inescapable tail-shock stress (TSS) on alpha(2)-adrenoceptor (α(2A)-AR) and corticotropin-releasing factor 1 receptor (CRF(1)-R) regulation as well as protein levels of G protein-coupled receptor kinase 3 (GRK3), GRK2, tyrosine hydroxylase (TH) plus carbonylated protein levels in locus coeruleus (LC), amygdala (AMG), cortex (COR) and striatum (STR). In NH rats, α(2A)-AR and CRF(1)-R were significantly down-regulated in LC after TSS. No changes in these receptor levels were observed in the LC of LH rats. GRK3, which phosphorylates receptors and thereby contributes to α(2A)-AR and CRF(1)-R down-regulation, was reduced in the LC of LH but not NH rats. GRK2 levels were unchanged. In AMG, GRK3 but not GRK2 levels were reduced in LH but not NH rats, and receptor regulation was impaired in LH rats. In STR, no changes in GRK3 or GRK2 levels were observed. Finally, protein carbonylation, an index of oxidative stress, was increased in the LC and AMG of LH but not NH rats. We suggest that reduced stress resilience after TSS may be related to oxidative stress, depletion of GRK3 and impaired regulation of α(2A)-AR and CRF(1)-R in LC.

Role of CRF receptor signaling in stress vulnerability, anxiety, and depression

Markers of hyperactive central corticotropin releasing factor (CRF) systems and CRF-related single nucleotide polymorphisms (SNPs) have been identified in patients with anxiety and depressive disorders. Designing more effective antagonists may now be guided by data showing that small molecules bind to transmembrane domains. Specifically, CRF(1) receptor antagonists have been developed as novel anxiolytic and antidepressant treatments. Because CRF(1) receptors become rapidly desensitized by G protein-coupled receptor kinase (GRK) and beta-arrestin mechanisms in the presence of high agonist concentrations, neuronal hypersecretion of synaptic CRF alone may be insufficient to account for excessive central CRF neurotransmission in stress-induced affective pathophysiology. In addition to desensitizing receptor function, GRK phosphorylation and beta-arrestin binding can shift a G protein-coupled receptor (GPCR) to signal selectively via the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK-MAPK) or Akt pathways independent of G proteins. Also, Epac-dependent CRF(1) receptor signaling via the ERK-MAPK pathway has been found to potentiate brain-derived neurotrophic factor (BDNF)-stimulated TrkB signaling. Thus, genetic or acquired abnormalities in GRK and beta-arrestin function may be involved in the pathophysiology of stress-induced anxiety and depression.

Role and action in the pituitary corticotroph of corticotropin-releasing factor (CRF) in the hypothalamus

Corticotropin-releasing factor (CRF), produced in the hypothalamic paraventricular nucleus (PVN) in response to stress, stimulates the synthesis and secretion of adrenocorticotropin (ACTH) via CRF receptor type 1 (CRF(1) receptor) in the anterior pituitary (AP) of mammals. CRF is critical for the circadian rhythmicity of the hypothalamic-pituitary-adrenal axis and the augmented release of ACTH from the pituitary in response to the stress. A higher molecular weight form of immunoreactive beta-endorphin, putative proopiomelanocortin (POMC), is increased in CRF-knockout mice (CRF KO), suggesting the important role of CRF in the processing of POMC. In fact, CRF is able to modulate the processing of POMC through changes in prohormone convertase (PC)-1 expression levels. Multiple forms of ACTH-related peptides containing unprocessed ones are present in some cases of ACTH-producing tumors, presumably without action of PC-1 under the control of CRF. Following CRF-activated stimulation of the receptor signaling, CRF(1) receptor is down-regulated and desensitized. In fact, CRF facilitates the degradation of CRF(1) receptor mRNA via the protein kinase A pathway. Prolonged agonist activation of CRF(1) receptor leads to a loss of responsiveness, or desensitization of the receptor. G protein-coupled receptor kinase 2 is involved in desensitization of CRF(1) receptor by CRF in the corticotroph.