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Serotonin: A real blast for T cells

April 2007
Blood 109(8):3130-3131

DOI:10.1182/blood-2007-01-069302

Authors:

University of Birmingham

[Comment on Leon-Ponte et al, page 3139][1]Mouse T lymphocytes unexpectedly produce the classic neurotransmitter serotonin, which—upon binding the constitutively expressed 5-HT7 Gs-coupled receptor-subtype—signals ERK1/2 phosphorylation and NFκB activation to boost their stimulation.

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doi:10.1182/blood-2007-01-069302

2007 109: 3130-3131

John Gordon and Nicholas M. Barnes

Serotonin: a real blast for T cells

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understood. Similarly, the application of

genomic technologies for identi�?cation of

novel, functionally-important platelet genes

and proteins remains a high priority.

Raslova and colleagues describe cellular

mRNA pro�?ling to speci�?cally dissect genetic

changes occurring during an in vitro model of

megakaryocyte differentiation and ploidiza-

tion. Cells were ﬂow-sorted by modal ploidy,

and gene changes were compared between 2

cellular subsets: aggregate 2N⫹4N MKs ver-

sus aggregate 8N⫹16N MKs. Of interest,

transcript changes were limited to approxi-

mately 350 genes across all the subsets, 106 of

which were consistently down-regulated and

248 of which were consistently up-regulated

between the 2 groups. Further analysis high-

lighted additional differences between the

up-regulated and down-regulated subsets;

speci�?cally, members of the latter subset not

infrequently (24/105) corresponded to genes

involved in DNA replication (arrest) and re-

combination repair, while a majority of the

former corresponded to genes important to

platelet biogenesis, viability, and function (ie,

actin and microtubule cytoskeleton, glycopro-

teins, and signaling/transport proteins). It is

important to note, however, that when the

gene subsets are carefully analyzed by gene

ontology functional classi�?cation, consider-

able overlap exists between these 2 groups,

confounding detailed interpretations. None-

theless, the data do support a role of ploidiza-

tion in modulating gene expression, although a

direct, regulatory role in platelet biogenesis

remains speculative.

The study by Macaulay and colleagues

adapts a nearly-identical in vitro strategy of

MK differentiation, coupled with a bioinfor-

matic strategy to speci�?cally identify novel

transmembrane domain– containing MK re-

ceptor proteins. An initial gene list of 151 tran-

scripts was assembled using paired, com-

parative expression pro�?ling with CD34

⫹

differentiated erythroblasts, and the list fur-

ther pared using strict criteria to identify puta-

tive, functionally-relevant platelet proteins.

Five of 8 highly-selected genes were charac-

terized by transcript and protein expression

studies, 3 of which were shown to be platelet

restricted (G6b, G6f, and LRRC32), and an-

other of which (SUCNR1) encoded the G

protein– coupled succinate receptor.

detailed functional studies established that

succinate (a key component of the citric acid

cycle) exhibited costimulatory effects on plate-

let aggregation induced by various platelet

agonists (adenosine diphosphate, thrombin

receptor activating peptide, and a glycoprotein

VI–speci�?c collagen peptide). The latter func-

tional data are especially insightful in that

they identify a novel, cocoupling signal trans-

duction pathway in platelets, while opening

new avenues of research linked to platelet

hyperreactivity.

While both study designs overlap in their

initial in vitro differentiation strategies using

megakaryocytes, the conclusions, future direc-

tions, and ability to compare data sets are dis-

tinct and limited. One restriction inherent in

cross-experimental microarray data-sharing is

the disparate platforms used among investiga-

tors, well exempli�?ed in these studies, that

used nonoverlapping oligonucleotide or

cDNA probe sets for their analyses.

5,6

This

limitation does not minimize results, although

it emphasizes the importance of validation

strategies of transcript differences initially

identi�?ed by microarray. The identi�?cation of

a costimulatory succinate receptor on platelets

represents a discrete end product of integrated

MK transcriptomic studies, coupled with a

concrete hypothesis and sophisticated experi-

mental design to characterize novel functional

receptors. Likewise, the application of mi-

croarray technology to dissect MK ploidiza-

tion is highly novel, and although the results

are less focused in scope, they are likely to

yield broader implications in the foreseeable

future. Finally, such unique data sets open up

exciting opportunities for sophisticated data

mining likely to provide unexpected insights

into molecular mechanisms of MK and plate-

let function.

The author declares no competing �?nancial

interests. ■

REFERENCES

1. Wright JH. The origin and nature of blood platelets.

Boston Med Surg J. 1906;154:643-645.

2. Gnatenko DV, Dunn JJ, McCorkle SR, Weissmann D,

Perrotta PL, Bahou WF. Transcript pro�?ling of human

platelets using microarray and serial analysis of gene expres-

sion. Blood. 2003;101:2285-2293.

3. He W, Miao FJ, Lin DC, et al. Citric acid cycle interme-

diates as ligands for orphan G-protein-coupled receptors.

Nature. 2004;429:188-193.

4. Boos CJ, Lip GY. Platelet activation and cardiovascular

outcomes in acute coronary syndromes. J Thromb Hae-

most. 2006;4:2542-2543.

5. Brazma A, Hingamp P, Quackenbush J, et al. Minimum

information about a microarray experiment (MIAME)-

toward standards for microarray data. Nat Genet.

2001;29:365-371.

6. Gnatenko DV, Perrotta PL, Bahou WF. Proteomic ap-

proaches to dissect platelet function: half the story. Blood.

2006;108:3983-3991.

7. Denis MM, Tolley ND, Bunting M, et al. Escaping the

nuclear con�?nes: signal-dependent pre-mRNA splicing in

anucleate platelets. Cell. 2005;122:379-391.

�? �? �? IMMUNOBIOLOGY

Comment on Leo´n-Ponte et al, page 3139

Serotonin: a real blast for T cells

----------------------------------------------------------------------------------------------------------------

John Gordon and Nicholas M. Barnes UNIVERSITY OF BIRMINGHAM MEDICAL SCHOOL

Mouse T lymphocytes unexpectedly produce the classic neurotransmitter seroto-

nin, which— upon binding the constitutively expressed 5-HT

-coupled recep-

tor-subtype—signals ERK1/2 phosphorylation and NF�?�B activation to boost

their stimulation.

erotonin/5-hydroxytryptamine (5-HT),

often perceived as the brain’s “happy

chemical,�? is—increasingly—assuming a

prominent role in immune regulation. In fact,

this “neurotransmitter�? is primarily a product

of the periphery, with gut enterochromaf�?n

cells the principal factories. Intestinal lympho-

cytes could certainly be exposed directly to

their output, though platelets are typically

proffered as the source of the monoamine

within the immune system: these 5-HT–

loaded reservoirs delivering their potent cargo

at sights of inﬂammation and immunologic

reactivity.

1,2

In this issue of Blood, Leo´n-Ponte and col-

leagues not only consolidate serotonin’s im-

portance to T-cell activation but also tender a

paradigm whereby the monoamine is provided

by the immune cell itself. Critical to any out-

come from 5-HT exposure—whether the

source be autocrine or paracrine—is a capacity

for target cells to sense the monoamine. This

could be via the serotonin transporter (SERT)

or one or more of 14 receptor subtypes (13 in

3130 15 APRIL 2007 I VOLUME 109, NUMBER 8

blood

For personal use only. by guest on November 9, 2012. bloodjournal.hematologylibrary.orgFrom

rodents).

1,2

Using reverse transcription–poly-

merase chain reaction (RT-PCR), the authors

ruled out SERT and all but 3 receptor sub-

types: only 5-HT

, 5-HT

, and 5-HT

re-

ceptors remaining to deliver the 5-HT hit.

While not excluding contributions from

5-HT

and 5-HT

receptors, the authors

homed in on 5-HT

as the major player: this

7-transmembrane domain G protein– coupled

receptor (GPCR) assisted T-cell stimulation

by promoting ERK1/2 phosphorylation and

canonical NF�?�B activation. Not only were the

authors able to attenuate 5-HT– dependent

change with a selective 5-HT

receptor antag-

onist but, by using a 5-HT

receptor agonist,

were able to reverse otherwise defective T-cell

stimulation arising as a consequence of having

blocked tryptophan hydroxylase (TPH), the

rate-limiting enzyme in the conversion of

tryptophan to 5-HT. Tryptophan is also the

catabolic substrate for indolamine-2,3-dioxy-

genase (IDO), and the relative abundance/

activity of TPH, IDO, and, additionally, me-

tabolizing monoamine oxidases (MAOs) could

determine 5-HT output from immune cells.

While the present study marks a signi�?cant

advance, the authors themselves address

seeming inconsistencies between this and iso-

lated reports on the relative importance of

individual 5-HT receptor subtypes to T-cell

function, and rightly posit additional ques-

tions. Issues include strain and species differ-

ences, tissue and subset heterogeneity, and

methodological concerns.

Few studies have attempted such an exten-

sive survey of an immune cell’s 5-HT receptor

repertoire as the one here. The table summarizes

and compares the study’s salient features with 3

others of merit: a pioneering study on rat spleno-

cytes

and, from Idzko and colleagues, separate

analyses of receptor subtype distribution on hu-

man dendritic cells (Idzko et al

) and monocytes

(Durk et al

). Of the discordant and common

themes, the near-universal expression of 5-HT

receptors—irrespective of cell type or animal

species—is striking. A built-in knowledge of the

pharmacology and signaling properties of 5-HT

receptors (see table), driven by and gained

largely through their contribution to CNS path-

ways, holds promise for the ready translation of

5-HT receptor therapeutics to immunology and

hematology clinics once a more robust integra-

tion of their place in immune physiology—and

pathology— has been secured.

The authors are directors and cofounders of

Celentyx Ltd, a spinout company whose activities

include the pro�?ling of serotonergic compounds

against immune cells. ■

REFERENCES

1. Meredith EJ, Chamba A, Holder MJ, et al. Close en-

counters of the monoamine kind: immune cells betray their

nervous disposition. Immunology. 2005;115:289-295.

2. Gordon J, Barnes NM. Lymphocytes transport seroto-

nin and dopamine: agony or ecstasy? Trends Immunol.

2003;24:438-443.

3. Stefulj J, Jernej B, Cicin-Sain L, et al. mRNA expression

of serotonin receptors in cells of the immune tissues of the

rat. Brain Behav Immun. 2000;14:219-224.

4. Idzko M, Panther E, Stratz C, et al. The serotoninergic

receptors of human dendritic cells: identi�?cation and cou-

pling to cytokine release. J Immunol. 2004;172:6011-6019.

5. Durk T, Panther E, Muller T, et al. 5-Hydroxytrypta-

mine modulates cytokine and chemokine production in

LPS-primed human monocytes via stimulation of different

5-HTR subtypes. Int Immunol. 2005;17:599-606.

�? �? �? NEOPLASIA

Comment on Zhao et al, page 3432

The CBFB-MYH11 butterﬂy effect

in hematopoiesis

----------------------------------------------------------------------------------------------------------------

Andre J. van Wijnen and Gary S. Stein UNIVERSITY OF MASSACHUSETTS MEDICAL SCHOOL

As the butterﬂy ﬂaps its wings, so can a small latent change within hematopoietic

cells have major effects on a pathological outcome such as leukemia.

he paper by Zhao and colleagues in this

issue of Blood indicts a leukemia-related

chimeric protein for selectively killing one cell

type while sparing another to cause a cell type–

speci�?c disease.

The runt-related transcrip-

tion factor 1 (RUNX1/AML1) and its cofac-

tor core-binding factor subunit �?� (CBF�?�) are

essential regulators of hematopoiesis. Genetic

Reported expression of 5-HT receptor transcripts in immune cells. Synthesis of �?ndings from studies by

*Leo´n-Ponte et al in this issue of Blood and from Stefulj et al,

Idzko et al,

and Durk et al

highlighting the

repertoire of 5-HT receptor subtype of mRNA expressed in immune cells. R indicates resting; A, activated; I,

immature; M, mature; Ion chan., ligand-gated ion channel; ⴚ, negative; ⴙ, strong signal; and ⴞ, weak sig-

nal. §5-HT1B protein increased on activation. ¶Absent from mouse and rat. #Truncated transcript in human.

Illustration by Frank Forney.

blood

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The origin and nature of the blood plates

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The neurotransmitter 5-hydroxytryptamine (5-HT), commonly known as serotonin, is released at peripheral sites from activated enterochromaffin cells, mast cells and platelets. In this study we analyzed the biological activity and intracellular signaling of 5-HT in human monocytes. By reverse transcription (RT) and PCR, messenger RNA (mRNA) expression of 5-HT receptor 1E (5-HTR(1E)), 5-HTR(2A), 5-HTR(3), 5-HTR(4) and 5-HTR(7) could be revealed. Functional studies showed that 5-HT modulates the release of IL-1beta, IL-6, IL-8/CXCL8, IL-12p40 and tumor necrosis factor-alpha (TNF-alpha), while it has no effect on the production of IL-18 and IFN-gamma in LPS-stimulated human blood monocytes. Moreover, RT and PCR revealed that 5-HT modulated mRNA levels of IL-6 and IL-8/CXCL8, but did not influence mRNA levels of IL-1beta and TNF-alpha. Pharmacological studies with isotype-selective receptor agonists allowed us to show that 5-HTR(3) subtype up-regulates the LPS-induced production of IL-1beta, IL-6 and IL-8/CXCL8, while it was not involved in TNF-alpha and IL-12p40 secretion. Furthermore, activation of the G(s)-coupled 5-HTR(4) and 5-HTR(7) subtypes increased intracellular cyclic AMP (cAMP) and secretion of IL-1beta, IL-6, IL-12p40 and IL-8/CXCL8, while, on the contrary, it inhibited LPS-induced TNF-alpha release. Interestingly, 5-HTR(1) and 5-HTR(2) agonists did not modulate the LPS-induced cytokine production in human monocytes. Our results point to a new role for 5-HT in inflammation by modulating cytokine production in monocytes via activation of 5-HTR(3), 5-HTR(4) and 5-HTR(7) subtypes.

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Aug 2005
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Here we review the evidence for immune cells expressing multiple components of the serotonergic and dopaminergic systems that are more commonly associated with the central nervous system (CNS). We discuss where and how peripheral encounters with these biogenic monoamines occur and posit reasons as to why the immune system would wish to deploy these pathways. A full taxonomy of serotonergic and dopaminergic constituents and their workings in component cells of the immune system should facilitate the formulation of novel therapeutic approaches in diseases characterized by immune dysfunction and potentially provide a range of surrogate peripheral markers for registering and monitoring disturbances within the CNS.