Serotonin induces pulmonary artery smooth muscle cell migration

The Critical Role of the Shroom Family Proteins in Morphogenesis, Organogenesis and Disease

The Shroom (Shrm) family of actin-binding proteins has a unique and highly conserved Apx/Shrm Domain 2 (ASD2) motif. Shroom protein directs the subcellular localization of Rho-associated kinase (ROCK), which remodels the actomyosin cytoskeleton and changes cellular morphology via its ability to phosphorylate and activate non-muscle myosin II. Therefore, the Shrm-ROCK complex is critical for the cellular shape and the development of many tissues, including the neural tube, eye, intestines, heart, and vasculature system. Importantly, the structure and expression of Shrm proteins are also associated with neural tube defects, chronic kidney disease, metastasis of carcinoma, and X-link mental retardation. Therefore, a better understanding of Shrm-mediated signaling transduction pathways is essential for the development of new therapeutic strategies to minimize damage resulting in abnormal Shrm proteins. This paper provides a comprehensive overview of the various Shrm proteins and their roles in morphogenesis and disease.

The roles of serotonin in cell adhesion and migration, and cytoskeletal remodeling

Serotonin is well known as a neurotransmitter. Its roles in neuronal processes such as learning, memory or cognition are well established, and also in disorders such as depression, schizophrenia, bipolar disorder, and dementia. However, its effects on adhesion and cytoskeletal remodelling which are strongly affected by 5-HT receptors, are not as well studied with some exceptions for e.g. platelet aggregation. Neuronal function is strongly dependent on cell-cell contacts and adhesion-related processes. Therefore the role played by serotonin in psychiatric illness, as well as in the positive and negative effects of neuropsychiatric drugs through cell-related adhesion can be of great significance. In this review, we explore the role of serotonin in some of these aspects based on recent findings.

PINK1-induced phosphorylation of mitofusin 2 at serine 442 causes its proteasomal degradation and promotes cell proliferation in lung cancer and pulmonary arterial hypertension

Impaired mitochondrial fusion, due in part to decreased mitofusin 2 (Mfn2) expression, contributes to unrestricted cell proliferation and apoptosis-resistance in hyperproliferative diseases like pulmonary arterial hypertension (PAH) and non-small cell lung cancer (NSCLC). We hypothesized that Mfn2 levels are reduced due to increased proteasomal degradation of Mfn2 triggered by its phosphorylation at serine 442 (S442) and investigated the potential kinase mediators. Mfn2 expression was decreased and Mfn2 S442 phosphorylation was increased in pulmonary artery smooth muscle cells from PAH patients and in NSCLC cells. Mfn2 phosphorylation was mediated by PINK1 and protein kinase A (PKA), although only PINK1 expression was increased in these diseases. We designed a S442 phosphorylation deficient Mfn2 construct (PD-Mfn2) and a S442 constitutively phosphorylated Mfn2 construct (CP-Mfn2). The effects of these modified Mfn2 constructs on Mfn2 expression and biological function were compared with those of the wildtype Mfn2 construct (WT-Mfn2). WT-Mfn2 increased Mfn2 expression and mitochondrial fusion in both PAH and NSCLC cells resulting in increased apoptosis and decreased cell proliferation. Compared to WT-Mfn2, PD-Mfn2 caused greater Mfn2 expression, suppression of proliferation, apoptosis induction, and cell cycle arrest. Conversely, CP-Mfn2 caused only a small increase in Mfn2 expression and did not restore mitochondrial fusion, inhibit cell proliferation, or induce apoptosis. Silencing PINK1 or PKA, or proteasome blockade using MG132, increased Mfn2 expression, enhanced mitochondrial fusion and induced apoptosis. In a xenotransplantation NSCLC model, PD-Mfn2 gene therapy caused greater tumor regression than did therapy with WT-Mfn2. Mfn2 deficiency in PAH and NSCLC reflects proteasomal degradation triggered by Mfn2-S442 phosphorylation by PINK1 and/or PKA. Inhibiting Mfn2 phosphorylation has potential therapeutic benefit in PAH and lung cancer.

Monoamine oxidase A inhibition protects the myocardium after experimental acute volume overload

Objective:

The molecular pathway leading to myocardial cellular destruction after acute volume overload (AVO) may include monoamine oxidases. The aim of the present study was to investigate whether moclobemide (Mo), a monoamine oxidase inhibitor, protects the myocardium after AVO.

Methods:

Sixty syngeneic Fischer rats underwent surgical abdominal aortocaval fistula to induce AVO. Eighteen rats were treated with Mo 10 mg/kg/day and were compared with 42 untreated rats with AVO without treatment. Myocardial recovery was analyzed using quantitative reverse transcription polymerase chain reaction for hypoxia-inducible factor 1-alpha, inducible nitric oxide synthase, interleukin 6, E-selectin, atrial natriuretic peptide (ANP), brain natriuretic peptide, vascular endothelial growth factor-alpha, matrix metalloproteinase 9, chitinase 3-like protein (YKL-40), and transforming growth factor-beta.

Results:

After 3 days, the relative number of ischemic intramyocardial arteries in the left ventricle was lower in AVO treated with Mo than in without [0.04 (0.02–0.07) vs. 0.09 (0.07–0.14), point score unit]. After 1 day, ANP was lower in AVO treated with Mo than in without [0.95 (0.37–1.84) vs. 2.40 (1.33–3.09), fold changes from the baseline (FC), p=0.044], whereas after 1 and 3 days, YKL-40 was higher in AVO treated with Mo than in without [22.66 (14.05–28.83) vs. 10.06 (6.23–15.02), FC, p=0.006 and 6.03 (4.72–7.18) vs. 3.70 (2.62–5.35), FC, p=0.025].

Conclusion:

Mo decreases intramyocardial arterial ischemia of the left ventricle after AVO while increases YKL-40, reflecting cellular protection during early cardiac remodeling. In the future, adding Mo may be a simple means for myocardial protection after AVO. (Anatol J Cardiol 2019; 21: 39-45)

Alpha-Catulin Co-Localizes With Vimentin Intermediate Filaments and Functions in Pulmonary Vascular Endothelial Cell Migration via ROCK

The ubiquitous α-catulin acts as a scaffold for distinct signalosomes including RhoA/ROCK; however, its function is not well understood. While α-catulin has homology to the cytoskeletal linkers α-catenin and vinculin, it appears to be functionally divergent. Here we further investigated α-catulin function in pulmonary vascular endothelial cells (VEC) on the premise that α-catulin has a unique cytoskeletal role. Examination of endogenous α-catulin intracellular localization by immunofluorescence revealed a highly organized cytosolic filamentous network suggestive of a cytoskeletal system in a variety of cultured VEC. Double-immunofluorescence analyses of VEC showed endogenous α-catulin co-localization with vimentin intermediate filaments. Similar to vimentin, α-catulin was found to distribute into detergent-soluble and -insoluble fractions. Treatment of VEC with withaferinA, an agent that targets vimentin filaments, disrupted the α-catulin network distribution and altered α-catulin solubility. Vimentin participates in cell migration, and withaferinA was found to inhibit VEC migration in vitro; similarly, α-catulin knock-down reduced VEC migration. Based on previous reports showing that ROCK modulates vimentin, we found that ROCK depletion attenuated VEC migration; furthermore, α-catulin depletion was shown to reduce ROCK-induced signaling. These findings indicate that α-catulin has a unique function in co-localization with vimentin filaments that contributes to VEC migration via a pathway that may involve ROCK signaling. J. Cell. Physiol. 231: 934-943, 2016. © 2015 Wiley Periodicals, Inc.

Serotonin enhances megakaryopoiesis and proplatelet formation via p-Erk1/2 and F-actin reorganization

Our previous studies have shown that serotonin (5-hydroxytryptamine; 5-HT) is a growth factor for hematopoietic stem/progenitor cells. In this study, we proposed a possible mechanism: 5-HT may enhance megakaryopoiesis and proplatelet formation via Erk1/2 pathway and cytoskeleton reorganization. Here, 5-HT(2B)R was first identified in megakaryocytic cells. 5-HT also promoted the megakaryocytes (MKs) proliferation and reduced the cell apoptosis via the activation of 5-HT(2B)R and Akt pathway. The effects were reduced by the 5-HT2B R inhibitor ketanserin. The effect of 5-HT on proplatelet formation in bone marrow MKs were further confirmed: the 5-HT treated group had more proplatelet bearing MKs compared with the control group. To determine whether 5-HT has effects on cytoskeleton reorganization of MKs, and whether these effects could be reduced by ketanserin or Erk1/2 inhibitor PD98059, MKs were stained with the F-actin specific binder rhodamine-phalloidin. The polymerized actin level was lower in the control group than the 5-HT group and was distributed throughout the cytoplasm with occasional aggregations. Our data demonstrated that Erk1/2 was activated in MKs treated with 5-HT. This study suggests that 5-HT has a potent effect on platelet formation and this effect is likely mediated via 5HT(2B)R with subsequent activation of p-Erk1/2 and consequent F-actin reorganization and proplatelet formation. We also demonstrated that melatonin, the metabolite of 5-HT, exerts a protective effect on MK and platelet recovery in the irradiated mouse model. This study suggested that 5-HT plays an important role in platelet formation via 5HT(2B)R, p-Erk1/2, and F-actin reorganization.

Inhibition of monoamine oxidase A increases recovery after experimental cardiac arrest

OBJECTIVES

Perioperative myocardial infarction (MI) with ischaemia-reperfusion injury (IRI) is a devastating entity occurring in 1-2% of patients after cardiac surgery. The molecular pathway leading to myocardial cellular destruction after MI may include monoamine oxidases. We experimentally investigated whether moclobemide, a monoamine oxidase inhibitor, enhances myocardial recovery after cardiac arrest and MI.

METHODS

Fifty-six syngeneic Fischer rats underwent heterotopic cardiac transplantation to induce reversible IRI after cardiac arrest. Twenty-eight rats also underwent permanent ligation of the left anterior descending coronary artery to induce MI after cardiac arrest. Twenty-eight rats with or without MI were treated with subcutaneous moclobemide 10 mg/kg/day. Methods used to study myocardial recovery were microdialysis for intramyocardial metabolism, histology and quantitative reverse-transcription polymerase chain reaction for high-mobility group box-1 (HMGB1), haeme oxygenase-1 (HO-1), interleukin-6, hypoxia-inducible factor 1α and macrophages (CD68).

RESULTS

Pyruvate increased in MI treated with moclobemide versus IRI with moclobemide (29.19 ± 7.64 vs 13.86 ± 8.49 µM, P = 0.028), reflecting metabolic activity after cardiac arrest and reperfusion. Myocardial inflammation increased in MI compared with IRI after 1 h (0.80 ± 0.56 vs 0, point score units [PSUs], P = 0.003), but decreased after 5 days in MI treated with moclobemide versus MI alone (0.80 ± 0.83 vs 2.00 ± 0.70, PSU, P = 0.033). Expressions of HMGB1, CD68 and HO-1 decreased in MI treated with moclobemide versus MI alone (1.33 ± 0.20 vs 1.75 ± 0.24, fold changes [FCs], P = 0.028; 5.15 ± 1.10 vs 9.59 ± 2.75, FC, P = 0.050; 10.41 ± 4.17 vs 21.28 ± 10.01, FC, P = 0.047), indicating myocardial recovery and increased cellularity of remote intramyocardial arteries.

CONCLUSIONS

Moclobemide enhances myocardial recovery after cardiac arrest and MI; inhibition of remote myocardial changes may be achieved by targeting treatment against monoamine oxidase.

Relationship of genetic variation in the serotonin transporter gene (SLC6A4) and congenital and acquired cardiovascular diseases

Recent reports have suggested an association between variation in the serotonin transporter and primary pulmonary hypertension and myocardial infarction. We set out to determine whether these associations were present in a population of patients who underwent SLC6A4 genotyping and to explore whether genetic variation in the serotonin transporter might be also associated with other cardiovascular functional and structural abnormalities. Included were 3473 patients who were genotyped for the SLC6A4 5HTTLPR polymorphism and a subset for rs25531 (n=816) and STin2 (n=819). An association was observed between 5HTTLPR and primary pulmonary hypertension (p=0.0130), anomalies of the cerebrovascular system (p<0.0001), and other anomalies of great veins (p=0.0359). The combined 5HTTLPR and rs25531 genotype was associated with tachycardia (p=0.0123). There was an association of the STin2 genotype with abnormal electrocardiogram (ECG) (p=0.0366) and abnormal cardiac study (0.0311). Overall, these results represent a step toward the understanding of the impact of SLC6A4 variation on cardiovascular pathology.

A role for actin polymerization in persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is defined as the failure of normal pulmonary vascular relaxation at birth. Hypoxia is known to impede postnatal disassembly of the actin cytoskeleton in pulmonary arterial myocytes, resulting in elevation of smooth muscle α-actin and γ-actin content in elastic and resistance pulmonary arteries in PPHN compared with age-matched controls. This review examines the original histological characterization of PPHN with attention to cytoskeletal structural remodeling and actin isoform abundance, reviews the existing evidence for understanding the biophysical and biochemical forces at play during neonatal circulatory transition, and specifically addresses the role of the cortical actin architecture, primarily identified as γ-actin, in the transduction of mechanical force in the hypoxic PPHN pulmonary circuit.

Cl⁻ channels in smooth muscle cells

In smooth muscle cells (SMCs), the intracellular chloride ion (Cl−) concentration is high due to accumulation by Cl−/HCO3− exchange and Na+–K+–Cl− cotransportation. The equilibrium potential for Cl− (ECl) is more positive than physiological membrane potentials (Em), with Cl− efflux inducing membrane depolarization. Early studies used electrophysiology and nonspecific antagonists to study the physiological relevance of Cl− channels in SMCs. More recent reports have incorporated molecular biological approaches to identify and determine the functional significance of several different Cl− channels. Both "classic" and cGMP-dependent calcium (Ca2+)-activated (ClCa) channels and volume-sensitive Cl− channels are present, with TMEM16A/ANO1, bestrophins, and ClC-3, respectively, proposed as molecular candidates for these channels. The cystic fibrosis transmembrane conductance regulator (CFTR) has also been described in SMCs. This review will focus on discussing recent progress made in identifying each of these Cl− channels in SMCs, their physiological functions, and contribution to diseases that modify contraction, apoptosis, and cell proliferation.

Serotonin receptor 3A controls interneuron migration into the neocortex

Neuronal excitability has been shown to control the migration and cortical integration of reelin-expressing cortical interneurons (INs) arising from the caudal ganglionic eminence (CGE), supporting the possibility that neurotransmitters could regulate this process. Here we show that the ionotropic serotonin receptor 3A (5-HT_3AR) is specifically expressed in CGE-derived migrating interneurons and upregulated while they invade the developing cortex. Functional investigations using calcium imaging, electrophysiological recordings and migration assays indicate that CGE-derived INs increase their response to 5-HT_3AR activation during the late phase of cortical plate invasion. Using genetic loss-of-function approaches and in vivo grafts, we further demonstrate that the 5-HT_3AR is cell autonomously required for the migration and proper positioning of reelin-expressing CGE-derived INs in the neocortex. Our findings reveal a requirement for a serotonin receptor in controlling the migration and laminar positioning of a specific subtype of cortical IN.

During brain development, neuronal excitability controls the laminar migration of cortical interneurons from the caudal ganglionic eminences (CGEs). Here the authors identify the 5-HT_3A receptor as a specific marker of CGE-derived cortical interneurons (cINs), and as a stimulator of cIN migration.

How the airway smooth muscle in cystic fibrosis reacts in proinflammatory conditions: implications for airway hyper-responsiveness and asthma in cystic fibrosis

Among patients with cystic fibrosis there is a high prevalence (40-70%) of asthma signs and symptoms such as cough and wheezing and airway hyper-responsiveness to inhaled histamine or methacholine. Whether these abnormal airway responses are due to a primary deficiency in the cystic fibrosis transmembrane conductance regulator (CFTR) or are secondary to the inflammatory environment in the cystic fibrosis lungs is not clear. A role for the CFTR in smooth muscle function is emerging, and alterations in contractile signalling have been reported in CFTR-deficient airway smooth muscle. Persistent bacterial infection, especially with Pseudomonas aeruginosa, stimulates interleukin-8 release from the airway epithelium, resulting in neutrophilic inflammation. Increased neutrophilia and skewing of CFTR-deficient T-helper cells to type 2 helper T cells creates an inflammatory environment characterised by high concentrations of tumour necrosis factor α, interleukin-8, and interleukin-13, which might all contribute to increased contractility of airway smooth muscle in cystic fibrosis. An emerging role of interleukin-17, which is raised in patients with cystic fibrosis, in airway smooth muscle proliferation and hyper-responsiveness is apparent. Increased understanding of the molecular mechanisms responsible for the altered smooth muscle physiology in patients with cystic fibrosis might provide insight into airway dysfunction in this disease.

Regulation of serotonin-induced trafficking and migration of eosinophils

Association of the neurotransmitter serotonin (5-HT) with the pathogenesis of allergic asthma is well recognized and its role as a chemoattractant for eosinophils (Eos) in vitro and in vivo has been previously demonstrated. Here we have examined the regulation of 5-HT-induced human and murine Eos trafficking and migration at a cellular and molecular level. Eos from allergic donors and bone marrow-derived murine Eos (BM-Eos) were found to predominantly express the 5-HT2A receptor. Exposure to 5-HT or 2,5-dimethoxy-4-iodoamphetamine (DOI), a 5-HT2A/C selective agonist, induced rolling of human Eos and AML14.3D10 human Eos-like cells on vascular cell adhesion molecule (VCAM)-1 under conditions of flow in vitro coupled with distinct cytoskeletal and cell shape changes as well as phosphorylation of MAPK. Blockade of 5-HT2A or of ROCK MAPK, PI3K, PKC and calmodulin, but not G_αi-proteins, with specific inhibitors inhibited DOI-induced rolling, actin polymerization and changes in morphology of VCAM-1-adherent AML14.3D10 cells. More extensive studies with murine BM-Eos demonstrated the role of 5-HT in promoting rolling in vivo within inflamed post-capillary venules of the mouse cremaster microcirculation and confirmed that down-stream signaling of 5-HT2A activation involves ROCK, MAPK, PI3K, PKC and calmodulin similar to AML14.3D10 cells. DOI-induced migration of BM-Eos is also dependent on these signaling molecules and requires Ca²⁺. Further, activation of 5-HT2A with DOI led to an increase in intracellular Ca²⁺ levels in murine BM-Eos. Overall, these data demonstrate that 5-HT (or DOI)/5-HT2A interaction regulates Eos trafficking and migration by promoting actin polymerization associated with changes in cell shape/morphology that favor cellular trafficking and recruitment via activation of specific intracellular signaling molecules (ROCK, MAPK, PI3K and the PKC-calmodulin pathway).

Involvement of TRPV1 and TRPV4 channels in migration of rat pulmonary arterial smooth muscle cells

Pulmonary hypertension, the main disease of the pulmonary circulation, is characterized by an increase in pulmonary vascular resistance, involving proliferation and migration of pulmonary arterial smooth muscle cells (PASMC). However, cellular and molecular mechanisms underlying these phenomena remain to be identified. In the present study, we thus investigated in rat intrapulmonary arteries (1) the expression and the functional activity of TRPV1 and TRPV4, (2) the PASMC migration triggered by these TRPV channels, and (3) the associated reorganization of the cytoskeleton. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis demonstrated expression of TRPV1 and TRPV4 mRNA in rat intrapulmonary arteries. These results were confirmed at the protein level by western blot. Using microspectrofluorimetry (indo-1), we show that capsaicin and 4α-phorbol-12,13-didecanoate (4α-PDD), selective agonists of TRPV1 and TRPV4, respectively, increased the intracellular calcium concentration of PASMC. Furthermore, stimulation of TRPV1 and TRPV4 induced PASMC migratory responses, as assessed by two different methods (a modified Boyden chamber assay and a wound-healing migration assay). This response cannot seem to be attributed to a proliferative effect as assessed by BrdU and Wst-1 colorimetric methods. Capsaicin- and 4α-PDD-induced calcium and migratory responses were inhibited by the selective TRPV1 and TRPV4 blockers, capsazepine and HC067047, respectively. Finally, as assessed by immunostaining, these TRPV-induced migratory responses were associated with reorganization of the F-actin cytoskeleton and the tubulin and intermediate filament networks. In conclusion, these data point out, for the first time, the implication of TRPV1 and TRPV4 in rat PASMC migration, suggesting the implication of these TRPV channels in the physiopathology of pulmonary hypertension.

The inhibitory effects of m-nisoldipine on the 5-hydroxytryptamine-induced proliferation of pulmonary artery smooth muscle cells via Ca2+ antagonism and antioxidant mechanisms

The excessive proliferation of pulmonary artery smooth muscle cells (PASMCs) plays a critical role in the development of pulmonary arterial hypertension. Recent studies indicate that Ca(2+) and reactive oxygen species are critically involved in the process of smooth muscle cell proliferation stimulated by mitogens, such as 5-hydroxytryptamine (5-HT). Because m-nisoldipine, a Ca(2+) channel blocker of the dihydropyridine class, possesses some calcium antagonistic and antioxidant properties, we investigated the effect of m-nisoldipine on PASMC proliferation. The results indicated that m-nisoldipine inhibited 5-HT-induced PASMC proliferation, evaluated by BrdU incorporation and the MTT assay, and this effect was associated with a decreased expression of proliferating cell nuclear antigen (PCNA). Flow cytometry analysis showed that m-nisoldipine blocked 5-HT-induced cell-cycle progression by arresting the cells in the G(0)/G(1) phase. Next, the production of reactive oxygen species and the levels of [Ca(2+)](i) in PASMCs were measured by laser scanning confocal microscopy; m-nisoldipine pretreatment attenuated the [Ca(2+)](i) elevation and the production of reactive oxygen species induced by 5-HT. In addition, m-nisoldipine significantly decreased the 5-HT-induced activation of extracellular signal-regulated kinase (ERK1/2) and c-Jun N-terminal kinase (JNK) and the subsequent c-fos and c-jun mRNA expression. Meanwhile, results also showed that N-acetylcysteine (a reactive oxygen species scavenger) suppressed the proliferation and the ERK1/2 and JNK activation induced by 5-HT. In summary, this study demonstrated that m-nisoldipine effectively suppressed the 5-HT-induced PASMC proliferation, ERK1/2 and JNK activation and subsequent c-fos and c-jun mRNA expression, all of which might be associated with the Ca(2+) antagonistic and antioxidant properties of m-nisoldipine.

Connective tissue growth factor in tumor pathogenesis

Key roles for connective tissue growth factor (CTGF/CCN2) are demonstrated in the wound repair process where it promotes myofibroblast differentiation and angiogenesis. Similar mechanisms are active in tumor-reactive stroma where CTGF is expressed. Other potential roles include prevention of hypoxia-induced apoptosis and promoting epithelial-mesenchymal transistion (EMT). CTGF expression in tumors has been associated to both tumor suppression and progression. For example, CTGF expression in acute lymphoblastic leukemia, breast, pancreas and gastric cancer correlates to worse prognosis whereas the opposite is true for colorectal, lung and ovarian cancer. This discrepancy is not yet understood. High expression of CTGF is a hallmark of ileal carcinoids, which are well-differentiated endocrine carcinomas with serotonin production originating from the small intestine and proximal colon. These tumors maintain a high grade of differentiation and low proliferation. Despite this, they are malignant and most patients have metastatic disease at diagnosis. These tumors demonstrate several phenotypes potentially related to CTGF function namely: cell migration, absent tumor cell apoptosis, as well as, reactive and well vascularised myofibroblast rich stroma and fibrosis development locally and in distal organs. The presence of CTGF in other endocrine tumors indicates a role in the progression of well-differentiated tumors.

Excess of serotonin affects neocortical pyramidal neuron migration

The serotonin transporter (SERT) is a key molecule involved in the homeostasis of extracellular levels of serotonin and is regulated developmentally. Genetic deletion of SERT in rodents increases extracellular levels of serotonin and affects cellular processes involved in neocortical circuit assembly such as barrel cortex wiring and cortical interneuron migration. Importantly, pharmacological blockade of SERT during brain development leads to phenotypes relevant to psychiatry in rodents and to an increased risk for autism spectrum disorders in humans. Furthermore, developmental adversity interacts with genetically-driven variations of serotonin function in humans and nonhuman primates to increase the risk for a variety of stress-related phenotypes. In this study, we investigate whether an excess of serotonin affects the migration of neocortical pyramidal neurons during development. Using in utero electroporation combined with time-lapse imaging to specifically monitor pyramidal neurons during late mouse embryogenesis, we show that an excess of serotonin reversibly affects the radial migration of pyramidal neurons. We further identify that the serotonin receptor 5-HT(6) is expressed in pyramidal neuron progenitors and that 5-HT(6) receptor activation replicates the effects of serotonin stimulation. Finally, we show that the positioning of superficial layer pyramidal neurons is altered in vivo in SERT knockout mice. Taken together, these results indicate that a developmental excess of serotonin decreases the migration speed of cortical pyramidal neurons, affecting a fundamental step in the assembly of neural circuits. These findings support the hypothesis that developmental dysregulation of serotonin homeostasis has detrimental effects on neocortical circuit formation and contributes to increased vulnerability to psychiatric disorders.

Silencing of sodium-hydrogen exchanger 1 attenuates the proliferation, hypertrophy, and migration of pulmonary artery smooth muscle cells via E2F1

We previously found that deficiency of the sodium-hydrogen exchanger 1 (NHE1) gene prevented hypoxia-induced pulmonary hypertension and vascular remodeling in mice, which were accompanied by a significantly reduced proliferation of pulmonary artery smooth muscle cells (PASMCs), and which decreased the medial-wall thickness of pulmonary arteries. That finding indicated the involvement of NHE1 in the proliferation and hypertrophy of PASMCs, but the underlying mechanism was not fully understood. To define the mechanism by which the inhibition of NHE1 decreases hypoxic pulmonary hypertension and vascular remodeling, we investigated the role of E2F1, a nuclear transcription factor, in silencing the NHE1 gene-induced inhibition of the proliferation, hypertrophy, and migration of human PASMCs. We found that: (1) silencing of NHE1 by short, interfering RNA (siRNA) significantly inhibited PASMC proliferation and cell cycle progression, decreased hypoxia-induced hypertrophy (in terms of cell size and protein/DNA ratio) and migration (in terms of the wound-healing and migration chamber assays); (2) hypoxia induced the expression of E2F1, which was reversed by NHE1 siRNA; and (3) the overexpression of E2F1 blocked the inhibitory effect of NHE1 siRNA on the proliferation, hypertrophy, and migration of PASMCs. The present study determined that silencing the NHE1 gene significantly inhibited the hypoxia-induced proliferation, hypertrophy, and migration of human PASMCs via repression of the nuclear transcription factor E2F1. This study revealed a novel mechanism underlying the regulation of hypoxic pulmonary hypertension and vascular remodeling via NHE1.

The Lbc Rho guanine nucleotide exchange factor α-catulin axis functions in serotonin-induced vascular smooth muscle cell mitogenesis and RhoA/ROCK activation

Serotonin (5-hydroxytryptamine, 5-HT) is mitogenic for several cell types including pulmonary arterial smooth muscle cells (PASMC), and is associated with the abnormal vascular smooth muscle remodeling that occurs in pulmonary arterial hypertension. RhoA/Rho kinase (ROCK) function is required for 5-HT-induced PASMC mitogenesis, and 5-HT activates RhoA; however, the signaling steps are poorly defined. Rho guanine nucleotide exchange factors (Rho GEFs) transduce extracellular signals to Rho, and we found that 5-HT treatment of PASMC led to increased membrane-associated Lbc Rho GEF, suggesting modulation by 5-HT. Lbc knockdown by siRNA attenuated 5-HT-induced thymidine uptake in PASMC, indicating a role in PASMC mitogenesis. 5-HT triggered Rho-dependent serum response factor-mediated reporter activation in PASMC, and this was reduced by Lbc depletion. Lbc knockdown reduced 5-HT-induced RhoA/ROCK activation, but not p42/44 ERK MAP kinase activation, suggesting that Lbc is an intermediary between 5-HT and RhoA/ROCK, but not ERK. 5-HT stimulation of PASMC led to increased association between Lbc, RhoA, and the α-catulin scaffold. Furthermore, α-catulin knockdown attenuated 5-HT-induced PASMC thymidine uptake. 5-HT-induced PASMC mitogenesis was reduced by dominant-negative G(q) protein, suggesting cooperation with Lbc/α-catulin. These results for the first time define a Rho GEF involved in vascular smooth muscle cell growth and serotonin signaling, and suggest that Lbc Rho GEF family members play distinct roles. Thus, the Lbc/α-catulin axis participates in 5-HT-induced PASMC mitogenesis and RhoA/ROCK signaling, and may be an interventional target in diseases involving vascular smooth muscle remodeling.

Role of protein transamidation in serotonin-induced proliferation and migration of pulmonary artery smooth muscle cells

Pulmonary hypertension is characterized by elevated pulmonary artery pressure and pulmonary artery smooth muscle cell (SMC) proliferation and migration. Clinical and experimental evidence suggests that serotonin (5-HT) is important in these responses. We previously demonstrated the participation of the 5-HT transporter and intracellular 5-HT (5-HTi) in the pulmonary vascular SMC-proliferative response to 5-HT. However, the mechanism underlying the intracellular actions of 5-HT is unknown. We speculated that 5-HTi activates SMC growth by post-translational transamidation of proteins via transglutaminase (TGase) activity, a process referred to as serotonylation. To test this hypothesis, serotonylation of pulmonary artery SMC proteins, and their role in 5-HT-induced proliferative and migratory responses, were assessed. 5-HT caused dose- and time-dependent increase in serotonylation of multiple proteins in both bovine and rat pulmonary artery SMCs. Inhibition of TGase with dansylcadaverin blocked this activity, as well as SMC-proliferative and migratory responses to 5-HT. Serotonylation of proteins also was blocked by 5-HT transporter inhibitors, and was enhanced by inhibition of monoamine oxidase, an enzyme known to degrade 5-HTi, indicating that 5-HTi levels regulate serotonylation. Immunoprecipitation assays and HPLC-mass spectral peptide sequencing revealed that a major protein serotonylated by TGase was fibronectin (FN). 5-HT-stimulated SMC serotonylation and proliferation were blocked by FN small interfering (si) RNA. These findings, together with previous observations that FN expression in the lung strongly correlates with the progression of pulmonary hypertension in both experimental animals and humans, suggest an important role of FN serotonylation in the pathogenesis of this disease.

Excess of serotonin affects embryonic interneuron migration through activation of the serotonin receptor 6

The discovery that a common polymorphism (5-HTTLPR, short variant) in the human serotonin transporter gene (SLC6A4) can influence personality traits and increase the risk for depression in adulthood has led to the hypothesis that a relative increase in the extracellular levels of serotonin (5-HT) during development could be critical for the establishment of brain circuits. Consistent with this idea, a large body of data demonstrate that 5-HT is a strong neurodevelopmental signal that can modulate a wide variety of cellular processes. In humans, serotonergic fibers appear in the developing cortex as early as the 10th gestational week, a period of intense neuronal migration. In this study we hypothesized that an excess of 5-HT could affect embryonic cortical interneuron migration. Using time-lapse videometry to monitor the migration of interneurons in embryonic mouse cortical slices, we discovered that the application of 5-HT decreased interneuron migration in a reversible and dose-dependent manner. We next found that 5-HT6 receptors were expressed in cortical interneurons and that 5-HT6 receptor activation decreased interneuron migration, whereas 5-HT6 receptor blockade prevented the migratory effects induced by 5-HT. Finally, we observed that interneurons were abnormally distributed in the cerebral cortex of serotonin transporter gene (Slc6a4) knockout mice that have high levels of extracellular 5-HT. These results shed new light on the neurodevelopmental alterations caused by an excess of 5-HT during the embryonic period and contribute to a better understanding of the cellular processes that could be modulated by genetically controlled differences in human 5-HT homeostasis.

Serotonin induces Rho/ROCK-dependent activation of Smads 1/5/8 in pulmonary artery smooth muscle cells

Serotonin (5-HT) stimulates pulmonary artery smooth muscle cell proliferation and has been associated with pulmonary arterial hypertension (PAH). Bone morphogenetic protein receptor 2 (BMPR2) mutations similarly have been linked to PAH. However, possible crosstalk between 5-HT and BMPR signaling remains poorly characterized. We report here that 5-HT activates Smads 1/5/8 in bovine and human pulmonary artery smooth muscle cells (SMCs) and causes translocation of these Smads from cytoplasm to the nucleus. DN BMPR1A blocked 5-HT activation of Smads 1/5/8 by 5-HT and BMPR1A overexpression enhanced it. Activation of Smads by 5-HT occurred through the 5-HT 1B/1D receptor as it was blocked with the inhibitor GR 55562 but unaffected by inhibitors of the 5-HT transporter and a variety of 5-HT receptors. Activation of the Smads by 5-HT depended on Rho/Rho kinase signaling as it was blocked by Y27632, but unaffected by inhibitors of PI3K or MAPK. Transfection of cells with BMPR1A and ligation of the BMP receptor with BMP-2 also activated GTP-Rho A of these SMCs, while DN BMPR1A blocked the activation. 5-HT stimulated an increase in serine/threonine phosphorylation of BMPR1A, supporting the activation of BMPR1A by 5-HT in SMCs. Infusion of 5-HT into mice with miniosmotic infusion pumps caused activation of Smads 1/5/8 in lung tissue, demonstrating the effect in vivo. The studies support a unique concept that 5-HT transactivates the serine kinase receptor, BMPR 1A, to activate Smads 1/5/8 via Rho and Rho kinase in pulmonary artery SMCs. Rho and Rho kinase also participate in the activation of Smads by BMP.

Up-regulation of two actin-associated proteins prompts pulmonary artery smooth muscle cell migration under hypoxia

Hypoxia stimulates the migration of pulmonary artery smooth muscle cells (PASMCs), which contributes to the pathogenesis of pulmonary vessel structural remodeling in hypoxic pulmonary hypertension (HPH). In the present study, we found, using a proteomics-based method, that gelsolin-like actin-capping protein (CapG) and transgelin were preferentially expressed in human (h)PAMSCs under hypoxia compared with normoxia. These two actin-associated proteins, modulate a variety of physiologic processes, including motility of cells, by interacting differently with the actin cytoskeleton. Our study showed that these two genes were up-regulated at both mRNA and protein levels under hypoxia in hPASMCs. As a key transcriptional regulation factor under hypoxia, hypoxia-inducible factor 1alpha (HIF-1alpha) up-regulated CapG protein expression under normoxia, and knockdown of HIF-1alpha expression in hPASMCs also inhibited hypoxia induced CapG up-regulation. However, HIF-1alpha could not regulate transgelin expression. Reduction of CapG or transgelin expression in hPASMCs by RNA interference was accompanied by significantly impaired migration ability in vitro, especially under hypoxia. Our study demonstrates that CapG and transgelin were preferentially expressed in hPAMSCs under hypoxia compared with normoxia. Hypoxia stimulates expression of these two actin-associated proteins via HIF-1alpha-dependent and -independent pathways, respectively. The up-regulation of these two proteins may contribute to the increased motility of hPASMCs under hypoxia. These findings may contribute to the understanding of the pathogenesis of HPH.

Serotonin modifies cytoskeleton and brush-border membrane architecture in human intestinal epithelial cells

Serotonin or 5-hydroxytryptamine (5-HT) influences numerous functions in the gastrointestinal tract. We previously demonstrated that 5-HT treatment of Caco-2 cells inhibited Na(+)/H(+) exchangers (NHE) and Cl(-)/OH(-) exchange activities via distinct signaling mechanisms. Since regulation of several ion transporters such as NHE3 is influenced by intact cytoskeleton, we hypothesized that 5-HT modifies actin cytoskeleton and/or brush-border membrane architecture via involvement of signaling pathways. Ultrastructural analysis showed that 5-HT (0.1 muM, 1 h) treatment of Caco-2 cells caused the apical membrane to assume a convex dome shape that was associated with shortening of microvilli. To examine whether these cellular architecture changes are cytoskeleton driven, we analyzed actin cytoskeleton by fluorescence microscopy. 5-HT induced basal stress fibers with prominent cortical actin filaments via 5-HT3 and 5-HT4 receptor subtypes. This induction was partially attenuated by chelation of intracellular Ca(2+) and PKCalpha inhibition (Go6976). In vitro assays revealed that PKCalpha interacted with actin and this association was increased by 5-HT. Our data provide novel evidence that 5-HT-induced signaling via 5-HT3/4 receptor subtypes to cause Ca(2+) and PKCalpha-dependent regulation of actin cytoskeleton may play an important role in modulation of ion transporters that contribute to pathophysiology of diarrheal conditions associated with elevated levels of 5-HT.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Drosophila bestrophin-1 chloride current is dually regulated by calcium and cell volume

Mutations in the human bestrophin-1 (hBest1) gene are responsible for Best vitelliform macular dystrophy, however the mechanisms leading to retinal degeneration have not yet been determined because the function of the bestrophin protein is not fully understood. Bestrophins have been proposed to comprise a new family of Cl(-) channels that are activated by Ca(2+). While the regulation of bestrophin currents has focused on intracellular Ca(2+), little is known about other pathways/mechanisms that may also regulate bestrophin currents. Here we show that Cl(-) currents in Drosophila S2 cells, that we have previously shown are mediated by bestrophins, are dually regulated by Ca(2+) and cell volume. The bestrophin Cl(-) currents were activated in a dose-dependent manner by osmotic pressure differences between the internal and external solutions. The increase in the current was accompanied by cell swelling. The volume-regulated Cl(-) current was abolished by treating cells with each of four different RNAi constructs that reduced dBest1 expression. The volume-regulated current was rescued by transfecting with dBest1. Furthermore, cells not expressing dBest1 were severely depressed in their ability to regulate their cell volume. Volume regulation and Ca(2+) regulation can occur independently of one another: the volume-regulated current was activated in the complete absence of Ca(2+) and the Ca(2+)-activated current was activated independently of alterations in cell volume. These two pathways of bestrophin channel activation can interact; intracellular Ca(2+) potentiates the magnitude of the current activated by changes in cell volume. We conclude that in addition to being regulated by intracellular Ca(2+), Drosophila bestrophins are also novel members of the volume-regulated anion channel (VRAC) family that are necessary for cell volume homeostasis.

Inhibition of serotonin-induced mitogenesis, migration, and ERK MAPK nuclear translocation in vascular smooth muscle cells by atorvastatin

The HMG-CoA reductase inhibitors, statins, have pleiotropic effects which may include interference with the isoprenylation of Ras and Rho small GTPases. Statins have beneficial effects in animal models of pulmonary hypertension, although their mechanisms of action remain to be determined. Serotonin [5-hydroxytryptamine (5-HT)] is implicated in the process of pulmonary artery smooth muscle (PASM) remodeling as part of the pathophysiology of pulmonary hypertension. We examined the effect of atorvastatin on 5-HT-induced PASM cell responses. Atorvastatin dose dependently inhibits 5-HT-induced mitogenesis and migration of cultured bovine PASM cells. Inhibition by atorvastatin was reversed by mevalonate and geranylgeranylpyrophosphate (GGPP) supplement, suggesting that the statin targets a geranylgeranylated protein such as Rho. Concordantly, atorvastatin inhibits 5-HT-induced cellular RhoA activation, membrane localization, and Rho kinase-mediated phosphorylation of myosin phosphatase-1 subunit. Atorvastatin reduced activated RhoA-induced serum response factor-mediated reporter activity in HEK293 cells, indicating that atorvastatin inhibits Rho signaling, and this was reversed by GGPP. While 5-HT-induced ERK MAP and Akt kinase activation were unaffected by atorvastatin, 5-HT-induced ERK nuclear translocation was attenuated in a GGPP-dependent fashion. These studies suggest that atorvastatin inhibits 5-HT-induced PASM cell mitogenesis and migration through targeting isoprenylation which may, in part, attenuate the Rho pathway, a mechanism that may apply to statin effects on in vivo models of pulmonary hypertension.

The 5-HT transporter transactivates the PDGFbeta receptor in pulmonary artery smooth muscle cells

Serotonin (5-HT) stimulates smooth muscle cell growth through 5-HT receptors and the 5-HT transporter (5-HTT), and has been associated with pulmonary hypertension (PH). Platelet-derived growth factor receptors (PDGFR) have also been associated with PH. We present evidence for the first time that 5-HT transactivates PDGFRbeta through the 5-HTT in pulmonary artery (PA) SMCs. Inhibition of PDGFR kinase with imatinib or AG1296 blocks 5-HT-stimulated PDGFRbeta phosphorylation. 5-HTT inhibitors and the Na+/K+-ATPase inhibitor ouabain, but not 5-HT2 and 5-HT1B/1D receptor inhibitors, block PDGFRbeta activation by 5-HT. Notably, 5-HTT binds the PDGFRbeta upon 5-HT stimulation and the 5-HTT inhibitor fluoxetine blocks both the binding and PDGDRbeta activation. Activation of PDGFRbeta may occur through oxidation of a catalytic cysteine of tyrosine phosphatase. 5-HT-activated PDGFRbeta phosphorylation is blocked by the antioxidant N-acetyl-L-cysteine and the NADPH oxidase inhibitor, DPI. Inhibition of PDGFR kinase with imatinib or AG1296 significantly inhibits SMC proliferation and migration induced by 5-HT in vitro. Infusion of 5-HT by miniosmotic pumps enhances PDGFRbeta activation in mouse lung in vivo. In summary, these results demonstrate that 5-HT transactivates PDGFRbeta in PASMCs leading to SMC proliferation and migration, and may be an important signaling pathway in the production of PH in vivo.

Pulmonary arterial hypertension: evaluation and management

Pulmonary arterial hypertension (PAH), a rare disease involving the pulmonary vascular circuit, is defined as an elevation in pulmonary arterial pressures and is characterized by symptoms of dyspnea, chest pain, and syncope. If left untreated, the disease carries a high mortality rate, with the most common cause of death being decompensated right heart failure. Over the past 5 years, there have been significant advances in this field in regards to understanding the pathogenesis, diagnosis, and classification of PAH. The availability of newer drugs has resulted in a radical change in the management of this disease with significant improvement in both quality of life and mortality. Ongoing research promises to lead to a more comprehensive understanding of the genetics, etiology, and pathogenesis of pulmonary arterial hypertension, which may ultimately translate into more effective therapeutic options.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.

Molecular Biology of Chronic Thromboembolic Pulmonary Hypertension

Recent efforts have seen major advances in elucidating the mechanisms underlying pulmonary arterial hypertension. However, chronic thromboembolic pulmonary hypertension (CTEPH) often has been excluded from these studies. Consequently, whereas the clinical, radiographic, and hemodynamic characteristics of CTEPH have been well described, there remains a deficit in our understanding of the cellular, molecular, and genetic mechanisms underlying CTEPH. Furthermore, although prior venous thromboembolism may act as the inciting event, it is still unclear what predisposes some patients to develop CTEPH. CTEPH has two major pathogenic components. The first is the primary obstruction of central pulmonary arteries by accumulation of thrombotic material. The second is characterized by severe pulmonary vascular remodeling, similar to that seen in idiopathic pulmonary arterial hypertension. Other articles in this series describe the pathological, surgical, and therapeutic aspects of CTEPH. Here, we review the potential molecular and cellular mechanisms that may contribute to the pathogenesis of CTEPH.