DTIC ADA561544: Cells, Biomarkers, and Posttraumatic Stress Disorder: Evidence for Peripheral Involvement in a Central Disease

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Cells, Biomarkers and Post-traumatic
Stress Disorder: Evidence for Peripheral
Involvement in A Central Disease

James A .Andrews
Kameran D. Neises

Naval Health Research Center

Report No. 10-27

The views expressed in this article are those of the authors and do not
necessarily reflect the official policy or position of the Department of the
Navy, Department of Defense, nor the U. S. Government..
Approved for public release, distribution unlimited..

Naval Health Research Center
140 Sylvester Road
San Diego, California 92106
JOURNAL OF NEUROCHEMISTRY | 2012 | 120 | 26–36 doi:10.1111/j.1471-4159.2011.07545.x
NavalHealthResearch Center, SanDiego, California, USA
Abstract traumatic events can trigger peripheral cells to migrate,
Post-traumatic stressdisorder(PTSD)isacomplicatedCNS mediateinflammation,anddecreaseneurogenesis,potentially
syndrome. Looking beyond the CNS, recent studies suggest leading to CNS volume loss. Biomarkers that associate with
that peripheral blood mononuclear cells could cause and/or PTSDsymptomshavethepotentialtodifferentiatePTSDfrom
exacerbate PTSD. This review summarizes the literature, traumatic brain injury, but more work needs to be done. Re-
describes associations between circulating peripheral blood searchexaminingthemechanismofhowtraumaticeventsare
cellsandPTSD,proposesanovelmechanism,andanalyzes linked to peripheral blood mononuclear cell functions and bi-
several biomarkers that appear to associate with PTSD omarkers may offer improved diagnoses and treatments for
symptoms.Several experimentalanimal modelshave shown PTSDpatients.
that peripheral blood mononuclear cell activity can cause Keywords:biomarkers,PBMC,post-traumaticstressdisorder,
hippocampal volume loss and PTSD-like symptoms. Data PTSD.
from these models suggest that a traumatic event and/or J.Neurochem.(2012)120,26–36.
Post-traumaticstressdisorder(PTSD)isasyndromecharac- vation,forthepurposeofthispaper,referstocellsthathave
terizedasaspectrumofsymptomsfollowingexposuretoan assumed a pro-inflammatory phenotype. Both microglial
extremely traumatic stressor. Based on the Diagnostic and activation and inflammation are associated with CNS cell
StatisticalManualofMentalDisorders,FourthEdition,Text death. This increased CNS cell death combined with
Revision, diagnostic criteria include re-experiencing the decreased neurogenesis lead to hippocampal volume loss.
traumatic event, avoidance of stimuli associated with the PBMCs, although very important for proper immune
event, and increased arousal that was not present before the function, can also injure healthy neurological tissue, if
trauma (American Psychology Association 2000). Despite inflammation progresses unchecked. PBMCs including
what is known about the symptomsof PTSD, little progress monocytes, macrophages, and lymphocytes, which are
has been made toward describing mechanisms or effective derived from the common hematopoietic stem cell lineage
diagnostic biomarkers. Through the study of the CNS, the in bone marrow and can be found within the circulatory
periphery and common disease mechanisms we can under- system. Some studies suggest that the number of circulating
stand the underlying etiology. This review centers on how
peripheral blood mononuclear cells (PBMCs) and biomar-
kers play a role in a putative mechanism and effective ReceivedMay25,2011,revisedmanuscriptreceivedOctober13,2011;
acceptedOctober16,2011.
diagnosis of PTSD.
Address correspondence and reprint requests to James A. Andrews,
Department166,NavalHealthResearchCenter140SylvesterRoad,San
Diego,CA92106-3521USA.E-mail:[email protected].
PBMCs affecting the CNS
Abbreviations used:BBB, blood–brain barrier; ICAM, intercellular
Theproposedmechanismisinitiated froma traumatic event adhesion molecule; IL, interleukin; iNOS, inducible nitric oxide syn-
thase; MRP8, myeloid-related protein-8; NGF, nerve growth factor;
and terminates with hippocampal volume loss. Figure 1
NPY, neuropeptide-Y; PBMC, peripheral blood mononuclear cells;
demonstrates the subsequent stages of microglial activation,
PTSD,post-traumaticstressdisorder;SDF-1,stromalcell-derivedfactor-
inflammation, and decreased neurogenesis. Microglial acti- 1;TBI,traumaticbraininjury;TNF,tumornecrosisfactor.
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26 J.Neurochem.(2012)120,26–36
Peripheralcells,biomarkers,andPTSD | 27
Traumatic event
PBMC activation
Migration
Inflammation
BBB
Accumulation
damage
Microglial Decreased
CNS cell death
activation neurogenesis
Hippocampal
volume loss
PTSD symptoms
Fig.1 TheproposedmechanismofPBMCsaffectonPTSD.PBMC,peripheralbloodmononuclearcells;CNS,centralnervoussyndrome;BBB,
blood–brainbarrier;PTSD,post-traumaticstressdisorder.
PBMCsincreasewithPTSDsymptoms.Inastudyperformed werealsoelevatedinthePBMCsofthesoldierswhojumped,
ontraumasurvivors,itwasshownthattherewasachangein indicatingthatPBMCswereactivated.Thereappearstobea
PBMC gene expression which correlated with the onset and temporal pattern of activation in this study where some
severity of PTSD symptoms (Segman et al. 2005). Blood markers were elevated the night prior to the jump and other
samplesweretakenuponarrivalatthehospitalaswellasone were elevated upon landing. These data suggest that NGF
and four months after the original traumatic event, and the maybepartofaPBMCmobilization/preparatoryresponseto
gene expression of the PBMCs in the blood samples was an approaching stressor. Activated PBMCs, possibly like
determined utilizing microarrays. PTSD symptoms were thosedescribedhere,mayaidinthedefenseagainstpossible
evaluatedusingDSMIVdiagnosticcriteriaatthesametime infectionencounteredduringfightorflight.Re-experiencing
asthebloodsamplesweredrawn.ThechangeinPBMCgene stressfuleventsisakeysymptomofPTSD,anditmaybean
expression correlated with the severity of PTSD symptoms. important source of frequent and chronic stress. Chronically
Moreover, genes associated with inflammation were of the elevatedserumlevelsofactivatedPBMCsmaybeakeylink
most markedly altered in patients with PTSD symptoms. in the mechanism of PTSD.
ThisstudyshowshowtheperipherycanbeinvolvedinCNS The impact of activated PBMCs on human CNS homeo-
disorders, and specifically demonstrates that PBMC activity stasis is not known, but most experiments performed in
correlates with PTSD. PTSD may involve a complex animalmodelssuggestthatPBMCsmayexacerbateexisting
interplay between the CNS and the activity of specific CNSinjuriesthroughinflammation.Thespleen,animportant
circulating PBMC populations, but more studies need to be source of PBMCs, appears to be involved in hyperinflam-
done to clarify this concept. matoryresponsesafterneurologicalinsults.Inarecentstudy,
A study performed with soldiers before a parachute jump itwasshownthatratswithoutspleenshadreducednumbers
providesanexampleofhowchronicstresscanaffectPBMC of activated macrophages, neutrophils and microglia in
functioninhumans.Inthisstudy,Aloeet al.(1994)analyzed comparison with those with spleens (Ajmo et al. 2008).
the blood of individuals the night before parachuting and After an ischemic stroke was induced, the rats with spleens
20 min after landing and compared the levels with those of showed more CNS tissue damage caused by inflammation
controls. Although the stress of jumping from an airplane compared with those without spleens. In other words, the
may appear acute, soldiers in paratrooper school are likely presence of more PBMCs may exacerbate CNS damage
undergoingchronicstressbecauseofthecontinuoustraining through a proinflammatory response. This study may also
prior to executing the actual parachute jump. In plasma, it have implications for other CNS disorders.
was found that nerve growth factor (NGF), cortisol, and Lymphocytes, a type of PBMC, are involved in many
adrenocorticotropichormonewereincreasedbeforeandafter different aspects of inflammation, infection, and disease.
landing. NGF receptor and tyrosine kinase receptor type 1 Similar to Ajmo et al. (2008), another study demonstrated
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28 | J.A.AndrewsandK.D.Neises
that the number of lymphocytes correlates with CNS extraction; cell counts affected by stressors may have
damage. After inducing ischemia in rats, it was found that temporal patterns, such as those seen during mechanical
after 24 h, severely immunodeficient mice, which lack T trauma or infections. For example, lymphocyte counts may
lymphocytes, had a smaller infarct region than animals with be higher immediately after traumatic incidents and may
intact immune systems (Hurn et al. 2007). This role of T decrease significantly, even below baseline, afterwards, but
lymphocytes may not be limited to infarction. studies to analyze these factors remain elusive.
RecentdatasuggestthatTlymphocytesplayanimportant TheabovementionedstudybyLewituset al.alsoprovides
roleintheCNSresponsetostressors,andthismaypresenta evidence of a possible T lymphocyte function for recovery
mechanism for CNS damage after experiencing stress that fromstress.Inthisstudy,micewereimmunizedagainsta20
leadstoPTSD.AstudybyLewituset al.(2008)foundthatT amino acid section of the myelin oligodendrocyte glycopro-
lymphocytes infiltrate the blood brain barrier (BBB) after tein (Lewitus et al. 2008). Myelin oligodendrocyte glyco-
bouts of stress. The infiltration was associated with an protein is located on the surface of some oligodendrocytes
increaseofintercellularadhesionmolecules(ICAM)-1onthe andtheoutersurfaceofmyelin.TheauthorsfoundthattheT
surfaceoftheBBB.ICAM-1isknowntoincreasemigration lymphocyte migration to the CNS after immunization was
ofcellsintotheCNSthroughtheBBB.Thisstudysuggested associated with an increase in brain-derived neurotropic
that choroid plexi adjacent to hippocampal regions demon- factor in the dentate gyrus of the hippocampus. The
stratethehighestincreaseinICAM-1surfaceexpressionand immunized animals exhibited reduced anxiety and were
adherence/infiltration of T lymphocytes. This may allow foundtobemoreresilienttostress.Cellularaccumulationin
accumulationofTlymphocyteswithintheCNSafterasingle the CNS may be beneficial for adult neurogenesis, memory
stress exposure, and more T lymphocytes can accumulate formation, and anxiolysis, but it is unknown which cell
after severe or longer duration stress exposures. It is not populations can deliver these favorable effects compared
knownwhetherornotthistypeofcellularaccumulationhas with the unfavorable effects. T lymphocyte activity is likely
an effect on PTSD symptoms. The ligand for ICAM-1, involved in the stress response, resiliency, and possibly
lymphocyte function-associated protein is displayed on the PTSD, but more research needs to be conducted to find out
surface of lymphocytes, macrophages, and neutrophils, howthesecellsandotherPBMCsaffectthecourseofPTSD.
where it binds and adheres to ICAM-1. It was shown that To our knowledge PTSD is solely a human disorder, but
blocking this interaction can reduce stress-induced inflam- animal studies, such as the abovementioned, provide impor-
mation (Joachim et al. 2008), so it is plausible that this tantevidencetosupportamechanisticlinkbetweenPBMCs
receptor/ligand combination can be augmented to inhibit and PTSD.
further CNS degradation. T lymphocytes may have multiple roles in the develop-
One study found that lymphocytes isolated from PTSD mentofPTSD,andsomespecificactionsmayrelyuponthe
patients exhibited decreased intracellular glucocorticoid particular T lymphocyte population that is activated. For
receptors levels compared with controls (Gotovac et al. example, T regulatory cells (CD4+ CD25+ cells) have been
2003), which indicates that lymphocytes may affect PTSD. shown to aid in the reduction of anxiety symptoms in an
Moreover,ithasbeenshownthatPTSDsymptomscorrelate animal model (Cohen et al. 2006). The T regulatory cells
withTlymphocytecountsandactivation.Inasimilarstudy, were collected from mice that were immunized against
war veterans diagnosed with PTSD were compared with myelin basic protein. The actions of these T lymphocytes
controls with no war experience, and the number of unlikeothersmaybenefitPTSD,butmoreresearchisneeded
circulating T lymphocytes were shown to be higher in to differentiate different T lymphocyte effects.
subjects with PTSD (Vidovic et al. 2007). InfurthersupportofthebeneficialactionsofPBMCs,the
In contrast to the evidence of negative effects of lympho- mononuclear fraction of human umbilical cord blood cells
cytes, a study showed that patients with a history of PTSD andthemononuclearfractionofratbonemarrowprogenitors
havedecreasednumbersofTlymphocytes(Kawamuraet al. appear to gain some of the characteristics of neurons when
2001). In this study, subjects diagnosed with PTSD accord- transplantedintoratbrains(Walczaket al.2004).Afterthese
ing to the DSM IV were compared with controls that had a cells were injected into brain parenchyma, a few cells were
similarhistoryofstressbutwithoutthesignsandsymptoms found to express nestin and endocortin, both markers of
of PTSD. The results showed that interferon-c and interleu- neural progenitor cells. Adult neurogenesis associated with
kin (IL)-4 are indicators of T helper cell type 1 and type 2 effective treatments and symptoms may help restore CNS
activities, respectively, were lower in patients with PTSD connectivity after a bout with PTSD, traumatic brain injury
when compared with controls. This contrasts with the (TBI), or other neurodegenerative disease. In adults, neuro-
Vidovic et al. (2007) study, which showed that patients blasts that arise from the subventricular zone of the
with PTSD had more T lymphocytes, demonstrating how hippocampus were shown to migrate along a specific
littleisknownaboutTlymphocyteactivityinPTSDpatients. pathway, via the rostral migratory stream adhering via b1-
Thesediscrepanciesmaybeassociatedwiththetimeofblood integrinconnectiontorostralmigratorystreamlamininalong
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Peripheralcells,biomarkers,andPTSD | 29
the surface of glia (Belvindrah et al. 2007). Walczak and CXCR4. These functions allow PBMCs to enter inflamed
colleaguesalsoshowedthatsomeinjectedmononuclearcells and injured regions, such as may occur in the CNS after
migrated along this tract toward the olfactory bulb, a injury from TBI, spinal cord injury, stroke, or infection.
behavior similar to newly arisen neuroblasts (Walczak et al. Althoughchemoattractivesubstancesincreasethenumber
2004). This suggests that PBMCs may play a direct role in ofPBMCsthatmigratethroughtheBBB,morePBMCscan
adult neurogenesis, a role that is beneficial to CNS homeo- transverse a BBB with issues. A malfunctioning BBB can
stasis. Therefore, it is important to sustain or increase allow for the accumulation of PBMCs in the brain
neurogenic functions when attempting to inhibit the PBMC parenchyma under normal conditions, which damage CNS
functionsthatappeartobedamagingandneurodegenerative. tissueduringtheirmigration(Wunderet al.2009).Ithasalso
Inflammationis definedasacollection ofsymptomssuch beensuggestedthatplasmaproteinscaninduceBBBdamage
as edema, loss of function, pain, elevated temperature, and (StolpandDziegielewska2009),andadditionaldamagefrom
redness. Inflammation is highly complex, so the aspects of migrating cells and their secreted products can lead to a
inflammation most specific to neurodegeneration will be the further decline in CNS tissue health. An example of
focus of this review. Inflammation contributes to CNS cell substances that can induce such damage are the matrix
death and is linked to the development of many CNS metalloproteinases, a class of enzymes (Rosenberg 2009).
diseasesanddisordersincluding;Alzheimer’s disease(Mac- Theseproteins degrade intercellular linkages that are impor-
cioni et al. 2009), Parkinson’s disease (Hirsch and Hunot tant for BBB integrity, and it is this an action by which the
2009), spinal cord injury (Chan 2008), multiple sclerosis BBB and surrounding CNS tissue can be damaged.
(Frohman et al. 2008), and it may also contribute to PTSD. Another mechanism which CNS cell death occurs is by
Importantly,thisisamechanismbywhichtheperipherycan microgliaactivity,whichaffectsthecourseoftissuelossand
affect CNS function. patient function in several neurodegenerative diseases (Liu
Inflammatory processes have been correlated with emo- and Hong 2003). Microglia can damage surrounding tissue
tion,andsomedatashowthattherelationshipsexistbetween via several mechanisms which include release of cytokines,
personality types and the cytokine levels which control the chemokines, other toxic substances, and migration (Raivich
levelofinflammation.Forexample,inastudybyMommer- et al. 1999; Stoll et al. 2000); this role is shared with
steeget al.(2008)individualswhoarehostileappeartohave monocytes and macrophages in the progression of other
more inducible inflammatory activity. The levels of 11 inflammatory diseases in the periphery.
cytokines and chemokines were determined and compared The CNS is populated with microglia during embryogen-
with the hostility score of each subject. The levels of esis, and relatively little repopulation of CNS microglia
inflammatory markers correlated with the hostility score. occurs during adulthood (Chan et al. 2007). Some recent
This subsequent increase in inflammation may translate into findingsdemonstratethatadultCNSmicroglialrepopulation
diseases such as cardiovascular disease and a similar may have a significant effect on CNS function (Bechmann
mechanism may contribute to CNS damage reported in et al. 2005), suggesting that these few cells have large
PTSD. Work performed in rats has shown that stress may consequences for PTSD. In this study, the authors showed
causeincreasedCNSdamageafteraninflammatorystimulus that PBMCs can infiltrate inflamed CNS tissue and acquire
(de Pablos et al. 2006). In this study, inflammation was the microglial phenotype. The entorhinal cortex projects
induced in stressed and non-stressed rats and inflammatory axonsintothe hippocampus; whensevered in rodents,these
markers were subsequently measured. It was shown that axons degrade. Using this animal model, Bechmann and
stress strengthens the changes by the induced inflammation colleaguesfoundthatPBMCsmigratedthroughtheBBBand
including an increase in proinflammatory cytokines and a into the area of axonal degradation and injury. The authors
decrease in the number of neurons. This study and others concluded that the damage and axonal degradation were
show that a growing body of descriptive data suggests that sufficient to induce release of pro-inflammatory substances.
stress, anxiety, and inflammation are correlated, but empir- Unfortunately, no mechanistic investigation of PBMC
ical studies demonstrating cause-and-effect relationships are recruitment or function followed the proposal of this
lacking. hypothesis. However, with the growing body of literature,
PBMCs escape the circulating blood by way of attaching wecandeducethatPBMCsmaybeinvolvedintheneuronal
toadhesionmolecules,rolling,andmigration(vanBuuland damage that occurs during PTSD and other neurodegener-
Hordijk2004).ThemigrationthroughtheBBBendothelium ative diseases.
and subsequent tissue layers toward chemoattractive sub- CD14 is a marker of microglia and when present in the
stancessuchasmonocytechemotacticprotein-1andstromal CNS, is associated with decreased CNS function after TBI
cell-derived factor-1 (SDF-1) may cause additional injury. and ischemic damage (Beschorner et al. 2002a; b). This is
SDF-1 (or CXCL12), is highly expressed in damaged or evidence ofmicroglial involvement in recovery from neuro-
hypoxic tissues (Ceradini et al. 2004). SDF-1 is a powerful logical insults. Although microglia can damage CNS tissue,
chemoattractive substance for PBMCs via its receptor theyalsoprotectagainstencephalitisandinvasionofforeign
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substancespasttheBBB.Modulationoftheiractivitymaybe hippocampi in PTSD patients are nearly 7% smaller than
the key to reducing CNS inflammatory activity and altering controls (Smith 2005), while some individual studies have
the course of neuronal cell death. More research should be reported over twice that level of reduction (Gurvits et al.
done to investigate microglial activity after stress and/or 1996;Hedgeset al.2003).Afewreportsmaysuggestthata
PTSD. Evidence of successful pharmaceutical inhibition of smallhippocampuspre-disposespeopletoPTSD,butatthis
PBMC/microglia function was reported in an animal model time, the majority of the literature favors hippocampal
of blast-induced TBI (Moochhala et al. 2004). In this atrophy as a consequence of PTSD or chronic stress
investigation, the activity of inducible nitric oxide synthase (Sapolsky 2000; Bremner et al. 2008). The hippocampal
(iNOS) was inhibited by administration of aminoguanidine. tissue loss associated with PTSD may be due to several
The rats with peripherally administered aminoguanidine did physiological mechanisms, microglial activation, inflamma-
not have as many blast-related injuries or functional impair- tion and decreased neurogenesis. Therefore, a balance of
ments. Interestingly, it appeared to not matter if the tissuegrowthandatrophymaybepartofthehomeostasisof
aminoguanidine was administered before or after the injury. the hippocampus. In PTSD and under chronic stress
As iNOS is only known to be expressed in immune cells conditions, this homeostatic ratio may become unbalanced,
such as PBMCs (Chabrier et al. 1999), these findings resulting in loss of tissue volume.
reinforce the role of PBMCs in neurodegeneration. PBMC
involvementinCNStissuelossmaybeactivatedafterbouts
Potential biomarkers for PTSD
ofstress,andevidencesuggeststhatwemaybeabletoalter
PBMC function to reduce neurodegeneration. Biomarkers for PTSD (Table 1) are an area of continuous
Although PTSD may be caused by an increased level of interest because they may provide some insight into the
neuronaldeath,itmayalsobecausedbyadecreasedlevelof mechanism and an objective diagnosis. Objective diagnosis
neuronalcellgrowth.Selectiveserotoninreuptakeinhibitors, of PTSD is important because it may provide improved
which are used as a treatment for PTSD symptoms, have detection for a disease that is highly stigmatized and
beenshowntoincreaseneurogenesis(Santarelliet al.2003). misdiagnosed among service members (Gaylord 2006). The
Therefore, neurogenesis may counteract the neuronal cell markers associated with other CNSdisorders can give some
deathinPTSDpatients.Studiesperformedinanimalmodels insightintowhichwillbemosteffectiveasdiagnostictools.
suggest that PTSD/traumatic events decrease neurogenesis. Eight potential PTSD biomarkers are reviewed below and
Recently, itwasshownthat rats exhibitPTSD-likebehavior many more will follow in the future. There may not be a
after treatment with inescapable tail shock, which was singlebiomarker,butratheracombinationthatwillprovidea
dispensed as a chronic stressor (Kikuchi et al. 2008). In conclusive diagnosis of PTSD.
addition to PTSD-like behaviors, stressed rats had less
bromodeoxyuridine (a marker for replicating DNA) incor- Neuropeptide-Y
porated into their hippocampal subgranular zone, further Neuropeptide-Y (NPY), a sympathetic co-transmitter and
indicating that chronic stress correlates with decreased hormone, is present in the circulation, and it is present at
neurogenesis. Neurogenesis within the hippocampus occurs higher concentrations after sympathetic activation, support-
from the subgranular zone of the dentate gyrus. Currently, ing the connection between the sympatho-adrenomedullary
there is a lack of postmortem neuropathological studies on system, bone marrow cells, and circulation (Zukowska-
human PTSD populations to show more specific morpho- Grojec 1995). Moreover, NPY is important for the proper
logical and histological details of hippocampal atrophy or functioningofPBMCs.NPYisdetectableinPBMCs,andit
amygdaloidhypertrophy;thus,thisisanareaofgreatinterest isalteredduringinflammation(Holleret al.2008).NPYhas
and demand. a bimodal effect on T lymphocyte function. It mitigates the
The loss of CNS tissue during the development of PTSD activityofmatureTcellsand,conversely,NPYisintegralto
symptoms may be directed by inflammation, microglial mature T lymphocyte development (Wheway et al. 2005).
activation, and decreased neurogenesis. Another possible NPYactivitiesarecarriedoutthroughmultiplereceptors,and
reason for CNS volume loss in PTSD patients is apoptosis. NPY receptors modulate release of catecholamines via Y1
Cytokines including tumor necrosis factor-alpha (TNF-a), receptors (Cavadas et al. 2006) and pre-synaptic Y2 autore-
IL-1, and interferon-c can act on their receptors to initiate ceptors(PotterandTripovic2006).TheactionsofNPYand
apoptosis and subsequent neurodegeneration (Hitomi et al. its multiple receptors are gaining importance in the field of
2008).Thisisonemorepossiblelinkbetweentheperiphery PTSD, where NPY might act as an anxiolytic. Circulating
and PTSD. NPY levels have been associated with more positive
PTSD patients have reduced hippocampal volume when outcomestostressinsomestudies,furthersupportingNPY’s
compared with healthy patients. This has also been docu- function as an anxiolytic (Morgan et al. 2002). Circulating
mentedinindividualswithatraumaticchildhoodexperience levels of NPY may be indicative of CNS levels and
(Stein et al. 1997). A meta-analysis reported that the sympatho-adrenomedullary system tone. NPY has multiple
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Peripheralcells,biomarkers,andPTSD | 31
Table1 BiomarkersthatmayindicatedevelopmentofPTSDsymptoms
Biomarker Humangene Findings Knownfunctions References
NPY NPY Releasedfromsympathetic Ismitogenicandhaspleiotropic Zukowska-Grojec(1995),
neuronsduringchronicand effectsonperipheralcell Whewayetal.(2005)
sustainedstress function
CD14 CD14 IntheCNS,associateswithTBI, Anextracellularsurfacereceptor Beschorneretal.(2002a,b)
stroke,andmicroglialactivity thatinitiatesaninnateimmune
response
MRP8;S100A8 S100A8 TBIpatientshaveelevated Markerofmicrogliaand Beschorneretal.(2000)
numberinCNStissue macrophages.Asanumber
subunitofcalprotectin,it
functionstostembacterial
growthbybindingdivalent
cations
pll;S100A10 CLP11 Lowerlevelswerefoundin Functionsasanadapterprotein, Svenningssonetal.(2006),
PBMCsfromPTSDpatients andittrafficstheserotonin Suetal.(2009)
comparedwithcontrolsand receptor5HTIBtotheplasma
othermentaldisorders membrane
TNF-a TNF AssociatedwithPTSD Widerangeofeffects,including LorzandMehmet(2009)
apoptosis.Averypotent
initiatoroftheinflammatory
response
IL-1 ILIAandIL1B ElevatedinPTSDpatients Releasedfrominjuredor Spivaketal.(1997),Tucker
inflamedtissue,andcausesa etal.(2004)
widerangeofeffects,including
localandsystemic
inflammationandapoptosis
IL-6 IL-6 ElevatedinPTSDpatients Pleiotropic;initiatesacutephase Maesetal.(1999),von
responseanddecreasessome Kaneletal.(2010)
aspectsofinflammation
Gsa GNAS1 Higherlevelsinlipidrafts Thisproteinfunctionsinsignal Donatietal.(2008)
associatewithdepressionand transduction,communicating
suicide activityfromreceptorsto
intracellularmolecules
ThistablesummarizesthebiomarkersthatmayindicatedevelopmentofPTSD.Correspondingreferencesalsoindicated.PTSD,post-traumatic
stressdisorder;TBI,traumaticbraininjury;IL,interleukin;MRP,myeloid-relatedprotein;NPY,neuropeptide-Y;PBMC,peripheralbloodmono-
nuclearcells;TNF;tumornecrosisfactor.
functions, and it associates with many diseases. Moreover, ActivationofCD14stimulatesaninnateimmuneresponse.It
NPY appears to contribute to resiliency, and its roles in isactivatedintheCNSduringbacterialmeningitis,butitalso
peripheral cell activity, inflammation, and chronic stress functionsasagoodmarkerofmicroglialactivityintheCNS.
suggest that we further examine the role of NPY in PTSD. Inasimilarinvestigation,TBIpatientsshowedthatCD14+
cell counts within the CNS increased over the week
CD14 following the TBI-inducing incident and remained elevated
Extensive work by Beschorner and colleagues showed that for weeks afterward. Neuropathological evaluation of 25
biomarkers may associate with PBMC/microglia-induced cases and five controls showed that CD14+ cell counts are
inflammationanddamageafterCNSinsults.Inastudyof18 elevatedintheregionofthelesionandthesurroundingarea.
strokevictims,follow-upneuropathologicalanalysisshowed CD14+ cells appeared to accumulate within 1–2 days of the
that the number of CD14+ (read as ‘CD14-positive’) cells injury (Beschorner et al. 2002a). These findings were
had increased in local tissue (Beschorner et al. 2002b). Co- replicated in another study that focused on ischemic stroke
staining experimentation showed that microglia (CD68+) and showed again that CD14+ cells were elevated in and
displayed surface CD14, but no CD14 was found on T aroundtheinfarctlesionarea(Beschorneret al.2002b).The
lymphocytes (CD3+), astrocytes (GFAP+), or endothelial authors recommended that anti-CD14 therapies be consid-
cells (CD34+). In conjunction with Toll-like receptor 4, eredforCNSinflammatorydiseases.Studiesbyothergroups
CD14 is a co-receptor for bacterial membrane components. have supported this hypothesis, because it was shown that
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soluble CD14 in cerebral spinal fluid is elevated after CNS should have the potential to provide diagnostic information
inflammatory states such as multiple sclerosis (Kraus et al. and possible mechanistic insight, but more work is needed.
2000), human immunodeficiency virus dementia (Fischer-
Smith et al. 2001), and meningitis (Cauwels et al. 1999). Tumor necrosis factor-alpha
TNF-a has an important role in immunity and it can
Myeloid-related protein-8 eliminatetumorswhenpresentathighenoughconcentrations
Another protein, myeloid-related protein-8 (MRP8) (Pennicaet al.1984;Shiraiet al.1985).Inadditiontothese
(S100A8), previously has been implicated as a potential beneficial effects, there are many known pathophysiological
biomarkerforTBI(Beschorneret al.2000).Beschornerand effects of TNF-a in the CNS. The release of TNF-a from
colleagues’ work showed that MRP8 microglia accumulate activated macrophages/microglia and subsequent cell death
afterTBIininjuredCNStissue.MRP8ispartofanimmune have been suggested to contribute to the mechanisms of
response; it binds free divalent cations and inhibits the Alzheimer’s Disease, Parkinson’s Disease, Multiple Sclero-
growthofbacteria(Corbinet al.2008).ThestudiesonCD14 sis,andischemicstrokelesionformation(LorzandMehmet
and MRP8 suggest that microglia, macrophages, and mono- 2009).TNF-aactivatescellsurfacereceptorsonneuronsand
cytesplayanimportantroleinTBIandstroke,andthatsome glia of the CNS, which can initiate the apoptotic process
biomarkers are specific to CNS inflammation. Although (Idriss and Naismith 2000). TNF-a also causes injury
these markers correlate well with stroke and TBI, it is through increased activation of iNOS synthase and sub-
unknown if they correlate with PTSD symptoms. Inflamma- sequent release of damaging amounts of nitric oxide
tion may exacerbate PTSD-related neurodegeneration, and (Gonzalez-Gay et al. 2009).
more studies should be conducted on these biomarkers to
investigate mechanistic links between microglial activity, Interleukins
inflammation, and PTSD. IL-1 may also be an important link between cellular action
andthehippocampalvolumelossassociatedwithPTSD.Ina
p11 study, serum IL-1b was elevated in (n = 19) PTSD patients
p11(S100A10)messengerRNAisapotentialbiomarkerfor and (n = 19) controls, and IL-1b levels also correlated with
PTSD. In a recent investigation by Su et al. (2009), PBMC length of PTSD symptoms (Spivak et al. 1997). Another
p11 levels were examined in PTSD, major depressive study showed similar findings. Patients (n = 58) had higher
disorder, bipolar disorder, and schizophrenia patients and IL-1b levels than controls (n = 21) and were treated with
compared with controls (2009a). Real-time PCR, cortisol SSRIs. After treatment, levels were lower than initial serum
levels in blood and saliva, using radioimmunoassay were concentrations(Tuckeret al.2004).Thus,itappearsthatIL-
compared.PTSDpatientshadlowerp11messengerRNAin 1 may be an important marker for PTSD, but more studies
PBMCs than controls. The other disorders that were should focus on possible mechanistic links between hippo-
examined had significantly higher p11 levels than controls, campal cell death and PTSD.
suggesting that PTSD may be particularly associated with IL-1 and TNF-a are released from PBMCs and other
specific alterations in p11 expression levels. Interestingly, immune cells. Peripheral cytokine actions during inflamma-
basallevelsofcortisolofPTSDpatientswerenotstatistically tionareverycomplex.Theactionsofothercytokinessuchas
different from controls, thus suggesting that p11 gene IL-10 and IL-4 appear to be anti-inflammatory and thus
expression in PBMCs may function as a specific marker possibly anti-neurodegenerative. In contrast, the actions of
for PTSD. IL-6 are pleiotropic, presenting both anti- and pro-inflam-
p11hasbeenshowntoaffectdepressivestates.Inarodent matory properties (Tilg et al. 1997). In one study, PTSD
model, it increased after antidepression treatments and was patients with depression had higher levels of IL-6 than
found to cause increased numbers of the serotonin receptor control patients, and patients with PTSD and no depression
5-hydroxytryptamine receptor 1B (Svenningsson et al. (Maes et al. 1999). A more recent study showed that PTSD
2006). In this study, p11 functions as the adaptor protein for patientshavehigherlevelsofIL-6thancontrolpatients(von
5-hydroxytryptamine receptor 1B. Behaviorally, it was shown Kanel et al. 2010). These studies support the association
that p11-null mice exhibit the symptoms of depression. To between cytokines and PTSD.
our knowledge, p11 does not appear to have a role in
inflammation.ItappearstoformcomplexeswithannexinA2, Gsa
and they function together to traffic proteins to membranes. This protein subunit is vital for G-coupled protein receptor
More relevant, its presence in PBMCs presents a potential function.ItwashypothesizedthatGsaplaysaroleinPTSD
pathwayfortreatmentofCNSdisorderswithperipheralcells. symptomology (Gurguis et al. 1999), but only recently has
Asdiscussedearlier,PBMCscandiffusetoregionsofinjury, itsrolebecomemoreunderstood.Recentworkhasshownthe
and increased concentration of p11 may, in turn, increase close linkage between Gsa and suicide and depression
seratonergic signaling. Therefore, p11 and other biomarkers (Donatiet al.2008).Gsamigratesfromthelipidraftstothe
Published2011.ThisarticleisaUSGovernmentworkandisinthepublicdomainintheUSA
J.Neurochem.(2012)120,26–36
Peripheralcells,biomarkers,andPTSD | 33
membrane regions with more access to adenylate cyclase, occursinthedaysfollowingaxonsevering,necrosis,orother
and depressed individuals appear to have more Gsa in their insult.SomeofthecurrentlyutilizedassessmentsofTBImay
lipid rafts than non-depressed. This molecule’s presence in alsobeindicatorsofPTSD.Therefore,biomarkersmaybea
lipid rafts can be used as an indicator for G-coupled protein clearandlessexpensivemethodtodiagnoseanddifferentiate
receptor functions, which appear to play important roles in these disorders.
PTSD symptoms. Therefore, studies need to be performed In conclusion, stressful events may lead to PTSD, and
assess the effectiveness of this potential diagnostic tool for diagnostic biomarkers are being investigated. Current find-
PTSD. ings suggest that PBMCs are involved in PTSD because
concentrations of them and their secreted substances are
Glucocorticoids associated with PTSD symptoms. Many PBMC-derived
A large body of evidence suggests that elevated glucocor- substances affect the function of CNS tissue and may
ticoids contribute to hippocampal volume reduction (Mc contribute to hippocampal degradation during PTSD. The
Ewen 1997, 2001). In particular, chronically elevated literaturesupportsthepotentialtouseexternallyderivedcells
glucocorticoids have been shown to increase susceptibility for therapy; they can be modified, injected, home to the
toexcitatorydeath(RoyandSapolsky2003).Muchworkhas injured CNS tissue, and deliver payloads of neurotrophic
been performed from this perspective, and most of this has factors or inhibitors of apoptosis. The overlap of PTSD and
been effectively reviewed elsewhere (Sapolsky et al. 2000). TBI symptoms and their comorbidity suggests that common
The use of glucocorticoids as a PTSD biomarker should be mechanisticpathwaysmayexist,andthismakesitdifficultto
explored further. differentiate between them. At this time, more work should
focusonPBMCsandtheirroleinneurodegenerativediseases
such as PTSD. Potential outcomes of this research may
Conclusion
include better mitigation of neurodegeneration, PTSD, and
As long as a diagnosis for PTSD is elusive, treatment sequelae of traumatic brain injury.
remains a serious problem (McAllister 2009). An important
issue will be to define which populations of PBMCs are
Acknowledgements
associated with beneficial and negative effects in PTSD
patients. Dichotomous actions of PBMCs may reveal Report No.10-27 supported bytheNavy Bureau of Medicine and
potential pathways of PTSD treatment with specific cell SurgeryPsychologicalHealthandTraumaticBrainInjuryProgram,
populations. underWorkUnitNo.60818.Theviewsexpressedinthisarticleare
TBI is categorized as either severe or mild based on the thoseoftheauthoranddonotnecessarilyreflecttheofficialpolicy
orpositionoftheDepartmentoftheNavy,DepartmentofDefense,
initial symptoms following the trauma (Park et al. 2008).
ortheU.S.Government.Approvedforpublicrelease;distributionis
Mild TBIs are difficult to diagnose because they can be
unlimited.Thisresearchhasbeenconductedincompliancewithall
causedbyminoracceleration-decelerationforces.Symptoms
applicable federal regulations governing the protection of human
ofTBIcanvary,butsomecommonsymptomsareheadache,
subjects.Nohumansubjectsweredirectlyinvolvedinthisresearch.
tinnitus, dizziness, lack of concentration, and memory
Special thanks to David Metzgar, PhD, for his assistance in
problems. preparationofthismanuscript.
Diagnosing PTSD is a difficult task, but it is more
daunting to effectively diagnose TBI versus PTSD (Hoge
Conflict of interest
et al. 2008; Elder and Cristian 2009). Therefore, many
individuals can be misdiagnosed, indicating that there is a Nonedeclared.
currentneedforspecificbiomarkerstodifferentiatethesetwo
syndromes.Inmilitarypopulations,thereisarecentincrease
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