Islet ? cell mass in diabetes and how it relates to function, birth, and death.

0.75 MB·English

by  Weir, Gordon C

Ann.N.Y.Acad.Sci.ISSN0077-8923
ANNALS OF THE NEW YORK ACADEMY OF SCIENCES
Issue:TheYearinDiabetesandObesity
(cid:2)
Islet cell mass in diabetes and how it relates to function,
birth, and death
GordonC.WeirandSusanBonner-Weir
SectiononIsletCellBiologyandRegenerativeMedicine,ResearchDivision,JoslinDiabetesCenter,DepartmentofMedicine,
HarvardMedicalSchool,Boston,Massachusetts
Addressforcorrespondence:GordonC.Weir,M.D.,SectiononIsletCellBiologyandRegenerativeMedicine,JoslinDiabetes
Center,OneJoslinPlace,Boston,[email protected]
Intype1diabetes(T1D)(cid:2) cellmassismarkedlyreducedbyautoimmunity.Type2diabetes(T2D)resultsfrom
inadequate(cid:2)cellmassandfunctionthatcannolongercompensateforinsulinresistance.Thereductionof(cid:2)cell
mass in T2D may result from increased cell death and/or inadequate birth through replication and neogenesis.
Reductioninmassallowsglucoselevelstorise,whichplaces(cid:2)cellsinanunfamiliarhyperglycemicenvironment,
leadingtomarkedchangesintheirphenotypeandadramaticlossofglucose-stimulatedinsulinsecretion(GSIS),
whichworsensasglucoselevelsclimb.Toxiceffectsofglucoseon(cid:2)cells(glucotoxicity)appeartobetheculprit.
Thisdysfunctionalinsulinsecretioncanbereversedwhenglucoselevelsareloweredbytreatment,afindingwith
therapeuticsignificance.Restorationof(cid:2)cellmassinbothtypesofdiabetescouldbeaccomplishedbyeither(cid:2)cell
regenerationortransplantation.Learningmoreabouttherelationshipsbetween(cid:2)cellmass,turnover,andfunction
andfindingwaystorestore(cid:2)cellmassareamongthemosturgentprioritiesfordiabetesresearch.
Keywords: betacell;islets;diabetes;neogenesis;insulinsecretion
Introduction
tionindiabetes,andthecomplicatedissuesof(cid:2)cell
Whiletype1diabetes(T1D)clearlyresultsfroma birth,death,andreplenishment.
loss of (cid:2) cells, there were decades of uncertainty
(cid:2) cellmass
about the contribution of (cid:2) cell failure to type 2
diabetes(T2D).Inthefirsthalfofthe20thcentury In a normal adult human pancreas, (cid:2) cell mass is
itwasgenerallyassumedthat(cid:2)cellfailurewasim- approximately2%ofpancreaticweight.Aboutone
portantforalldiabetes.Inthe1960s,however,the millionisletsarescatteredthroughoutthepancreas,
development of the insulin radioimmunoassay led whichtranslatestoroughlyonebillion(cid:2)cells.Pan-
to the finding that plasma insulin levels were of- creasestypicallyweighbetween60gand100g,so(cid:2)
ten high in T2D, leading many to assume that the cellmassissomewherenear1–2g.
pathogenesis could be explained by insulin resis- In1955MacleanandOgilvie1reported,inacare-
tance alone. However, over the past 15 years there fulstudyonautopsypancreasesusinghistochemical
hasbeengeneralagreementthat(cid:2)cellinadequacyis staining, that (cid:2) cell mass in older people with di-
afundamentalpartofT2D.Thegrowingincidence abetes (presumably mostly T2D) was about 50%
ofT2Disdrivenbyinsulinresistance,whichislikely of controls, although this finding was largely ig-
theresultofatypicalWesternlifestylecharacterized nored. It was also found that (cid:3) cell mass was not
byobesityandminimalexercise.Akeypointisthat altered.Similarestimatesof(cid:2)cellmasswerefound
most people with insulin resistance will never de- in a few small studies, and then in 2003 a study
velopT2D:diabetesonlyresultsiftheir(cid:2) cellsfail led by Butler and Butler reported similar find-
toprovidesufficientinsulin.Thisreviewwillfocus ings using immunostaining in autopsy pancreases
upontherelationshipbetween(cid:2)cellmassandfunc- from better-characterized subjects.2 There was
doi:10.1111/nyas.12031
92 Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences.
Weir&Bonner-Weir Islet(cid:2)cellmassindiabetes
yearsanumberofthemhavediedanddonatedtheir
pancreases for study. At this writing, (cid:2) cells iden-
tifiedbyinsulinimmunostaininghavebeenfound
inall28subjects(S.Bonner-Weir,unpublished).It
also appears that those few with more residual C-
peptidehavemore(cid:2)cells.Thepresenceofthese(cid:2)
cellsraisesimportantquestionsastowhether(cid:2)cells
havespecialcharacteristicsthatresistdestructionor
whethernew(cid:2)cellsarecontinuallyformedbyrepli-
cationofexisting(cid:2)cellsand/orbyneogenesisand
thendestroyedbypersistingautoimmunity.
Figure 1. (cid:2)cellrelativevolumeinautopsypancreasesfrom
Thequesttomeasure(cid:2) cellmasswith
individualswhowerenondiabetic(ND)hadimpairedfasting
glucose(IFTG)orhadtype2diabetesmellitus(TTDM).Figure imaging
takenfromRef.2,withpermissionfromtheAmericanDiabetes
Accuratemeasurementof(cid:2)cellmassinlivingsub-
Association.
jects would greatly enhance our understanding of
the pathogenesis of both T1D and T2D, as well as
considerable scatter in the data, but in both lean themonogenicformsofdiabetes,makingitama-
and obese individuals the relative (cid:2) cell volume, a jorresearchpriority.Forbothtypesitwouldbeof
surrogate for mass, was about 30–70% of nondi- greatvaluetofollowthedynamicsofthedeclinein
abetic controls. Importantly, in subjects with im- (cid:2)cellmassbecauseitmightbepossibletolinkthese
pairedfastingglucoselevels,the(cid:2)cellmasswasalso topathogeneticmechanisms.InT1Dthiscouldbe
reduced,butlessso,toabout60%ofnormal(Fig.1). enormouslyhelpfulinevaluatingtheefficacyofvar-
ThoroughstudiesfromKoreaandBelgiumreported iousimmuneinterventionsthatarebeingtestedin
similarreductionsin(cid:2)cellmassinT2D.3,4Another subjectswithprediabetesandnew-onsetdiabetes.In
finding important for understanding T2D is that bothtypesofdiabeteswewouldverymuchliketo
obesenondiabeticsubjectsinthreestudieshadonly understandtheeffectsofmedicationandglycemic
a20–40%increasein(cid:2)cellmasscomparedtolean controlon(cid:2)cellmass.
controls.2,4,5Thisonlymodestincreasewassurpris- Unfortunately,ithasprovedverydifficulttode-
ing,astheincreaseof(cid:2)cellmassininsulinresistant velop any methods that come close to being accu-
miceisproportionallymuchhigher.6 rate. There was great interest in the type 2 vesic-
InT1D,(cid:2) cellsaredecimatedbyautoimmunity ular monoamine transporter (VMAT2), which is
and the process usually occurs over several years. expressed by (cid:2) cells, can be labeled with [11C]-
Therearefewpathologicalstudiesand(cid:2)cellimag- dihydrotetrabenazine,andthenmeasuredwithPET
inghasnotadvancedenoughtobehelpful,soouras- scanning.Unfortunately,inhumansubjectsthisap-
sumptionsarelargelyextrapolatedfromC-peptide pearstobeinsufficientlyspecific.10NowtheGLP-1
secretion and insulin requirements. Because some receptor,whichmaybespecificenoughfor(cid:2)cells,
individuals with new-onset T1D have remissions isreceivingmuchattention.11 Anotherapproachis
that can last for several months, it is thought that totaginflammationinthepancreasofT1D,which
(cid:2) cell mass at diagnosis might be as high as 50% correlates with the activity of autoimmunity; the
ofnormalinsomeindividuals.7Weknowfromthe firstpublishedresultsinhumanslookpromising.12
DiabetesControlandComplicationsTrial(DCCT) Mostoftheapproachestoimaging(cid:2)cellsoverthe
thatC-peptidelevelscanbesubstantialinsomesub- pastseveralyearshavebeenrecentlyreviewed.13One
jectssoonafterdiagnosisbutthenfalltonegligible caveat is that to be useful a method will proba-
levels in just a few years.8 A new remarkable find- blyneedtoaccuratelymeasuredifferencesin(cid:2)cell
ing is that (cid:2) cells may never be completely wiped mass in the range of only 5%, because such small
outinT1D.TheJoslinMedalistStudyincludessub- changesarelikelytomakeadifferenceinglycemic
jectswhohavehadT1Dforover50years.9Inrecent control.
Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences. 93
Islet(cid:2)cellmassindiabetes Weir&Bonner-Weir
Normal(cid:2) cellfunctionandmaintenanceof itcouldhaveanegativeinfluenceonhepaticinsulin
mass action.
(cid:2)cellshavearemarkableabilitytomakeandstore βcellhypertrophyandatrophy
largequantitiesofinsulinandthensecretethisprod- Glucoseappearstobedominantamongthefactors
uctwithexquisitetimingandprecision.Glucoseis influencingthemaintenanceof(cid:2)cellmass.Changes
thedominantfactorcontrolling(cid:2)cellfunctionand in glucose levels seem to drive the major determi-
survival,andauniqueglucoserecognitionmecha- nantsof(cid:2)cellmass,hyperplasia,hypertrophy,and
nismallowscontrolbyextracellularconcentrations atrophy.Chronichypoglycemia,whichoccurswith
of glucose that usually range between 4 mM and insulin-secretingtumors,leadstomarked(cid:2)cellat-
8 mM (70–140 mg/dL). This is accomplished by rophy,27 whereas chronic hyperglycemia can lead
wide-openGLUT2glucosetransportersinrodents, to (cid:2) cell hyperplasia and hypertrophy.28,29 Using
althoughprobablymorebyGLUT1inhumans.14,15 partialpancreatectomytocreateamodelofhyper-
These transporters allow the extracellular and in- glycemiainrats,wefoundthatindividual(cid:2)cellsize
tracellular glucose concentrations to be essentially increasedby85%.29Increasesinglucoselevelsstim-
thesame.Thekeyglucosesignalingforinsulinse- ulatereplicationwithinaday,30 andovertimelead
cretion is done by glucose metabolism, the rate of tohyperplasia.Glucosestimulationof(cid:2) cellrepli-
whichiscontrolledbyglucosephosphorylationby cation has also been found in an in vivo model of
glucokinase,withaK ofabout7mM.16,17Glucose glucoseinfusioninmice31 andaninvivomodelof
m
metabolismatthemitochondriallevelturnsonse- humanislettransplantation.32
cretionviathereasonablywell-understoodK+-ATP-
Compensatoryβcellresponsetoinsulin
dependent pathway and the less well-understood
resistancewhenbloodglucoselevelsare
K+-ATPindependentpathway.18The(cid:2)cellalsore-
normal
spondstosignalsfromthebrainviabothbranches
There has been considerable debate about how (cid:2)
of the autonomic nervous system19 and the in-
cellsecretionandmasscanbeaugmentedininsulin
cretin hormones from the intestine, gastrointesti-
resistantstateswhenincreasesinglucoselevelscan-
nalinsulinotropicpeptide(GIP),andglucagon-like
notbedetermined.Wefavortheviewthatbecause
peptide1(GLP-1).20
glucose is such a dominant determinant of (cid:2) cell
The(cid:2)cellprovidesveryefficientcontrolofglu-
functionandgrowth,thesechangesaremainlycon-
coselevelswitheatingassecretedinsulinstartsac-
trolledbyextremelyefficientglucosefeedbackon(cid:2)
tivating the liver even before blood glucose levels
cells.6,33,34 Theremaybesubtlechangesinglucose
rise, due to vagal stimulation during the cephalic
levels that make a difference and there is evidence
phaseofinsulinsecretion17andsecretionofincretin
ofincreasedactivityofglucokinase,35whichmeans
hormones. As insulin secretion rises, glucagon se-
thata(cid:2)cellcanbemoreresponsiveatlowerglucose
cretion is suppressed, allowing this increased in-
concentrations. There is much interest in the pos-
sulin/glucagon ratio to enhance hepatic glucose
sibility that some important signals are produced
uptake at just the right time.21 Protection against
by the liver because of the impressive (cid:2) cell com-
hypoglycemiaisparticularlydependentuponrapid
pensation found with knockout of hepatic insulin
shutdownofinsulinsecretionbyfallingglucoselev-
receptorsinmice.34Thesearchcontinues.
els.22Thusglucoseandotherfactorscanturnonin-
sulinsecretionwithinminutesand,whenremoved, Dysfunctionalinsulinsecretionas
shutitoffasquickly. diabetesdevelops
Oscillationsofinsulinsecretion When glucose levels chronically rise to levels only
Insulin secretion exhibits oscillation with a peri- modestlyhigherthannormal,dramaticdysregula-
odicity of about five minutes,23,24 which appears tion of insulin secretion appears. This was shown
tobecoordinatedbyintrapanceaticneuralconnec- most impressively with a simple experiment pub-
tions.25Becausetheseoscillationsaresoprominent lished over 35 years ago (Fig. 2).36 Adult humans
intheportalvein,theylikelyhaveimportantinflu- withvariouslevelsoffastingglycemiareceivedrapid
enceoninsulin’seffectsupontheliver.Disruption infusions of glucose intravenously to elicit acute
oftheseoscillationshasbeenfoundinT2D,26 and glucose-simulated insulin secretion (GSIS). When
94 Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences.
Weir&Bonner-Weir Islet(cid:2)cellmassindiabetes
T2D,partialimprovementinGSISwasfoundafter
a 20-hour infusion of insulin40 and changes after
gastric surgery were found weeks to months later.
This is an important question because a thorough
understandingofthetimingandmagnitudeofthe
effectsofglucotoxitycouldhavetherapeuticvalue.
Thecasefortheimportanceof
glucotoxicityandlackofimportance
oflipotoxicityandglucolipotoxicity
Whileitisclearthatthediabeticmilieuisrespon-
sible for (cid:2) cell dysfunction, there has been much
discussionaboutthecontributionsofglucotoxicity,
Figure2. IncrementsofacuteGSISinsubjectswithincreasing lipotoxicity,andglucolipotoxicity.41–43Thecasefor
fastingplasmaglucoselevels.FiguretakenfromRef.36,with thedominanceofglucotoxicitycanbemadebecause
permissionfromtheEndocrineSociety.
oftheremarkablytightcorrelationbetweenglucose
levelsandlossofacuteGSIS.Moreover,changesin
the fasting glucose was normal at 4.5–5.6 mM insulinsecretionafterseveraldaysofglucoseinfu-
(80–100mg/dL)alargespikeofinsulinsecretionap- sionsinratsresemblethoseseenindiabetics,44 yet
pearedwithinjustafewminutes.However,themag- suchinfusionssuppressfreefattyacid(FFA)levels.
nitudeofGSISwasmuchlowerwhenglucoselevels Also,similarlossofGSIScanbefoundwhenisolated
rose above 5.6 mM and by the time they reached isletsareexposedtohigh-glucoseconcentrationsin
6.4 mM (115 mg/dL), a level in the range of im- culture.45,46
pairedfastingglucose(IFG),acuteGSIS,apredia- Thecaseforlipotoxicityisbasedalmostentirely
beticstateequatedwithfirst-phaseinsulinsecretion, uponinvitroexposureofprimary(cid:2)cellsand(cid:2)cell
wascompletelyobliterated.Nonetheless,the(cid:2)cells linestoFFAsinculture,especiallythesaturatedFFA
functionedwellenoughtomaintaintheprediabetic palmitate.NotonlydoespalmitatedisruptGSISand
statebecausetheycanrespondtomoreprolonged othertypesofinsulinsecretion,butalsoithasbe-
glucosestimulationwithsecondphaserelease37and comeafavoritetoolinthestudyof(cid:2) celldeath.It
to acute stimulation by incretin signals such as isnotatallclearthattheFFAconcentrationsused
GLP-1,aswellasaminoacids.Thesefindingshave fortheseinvitrostudiesaresimilartothoseseenby
nowbeenreproducedinmultiplehumanandani- (cid:2)cellsinvivo.Wehavelittleunderstandingofhow
malstudies. FFAs are handled in the interstitial space and the
Dysfunctionof (cid:2) cellsbecomesmoreseriousas environment of lipid membranes that surround (cid:2)
thediabeticstateworsensandfunctionalmassde- cells.Ithasalsobeenpostulatedthatpalmitategiven
teriorates.Agiven(cid:2)cellmassputsoutlessinsulin invitroto(cid:2)cellsmaybeconvertedtothetoxicprod-
in response to stimuli. In another old study, sub- ucttripalmitin,whichisnotexpectedtobeformed
jects with and without T2D received maximal (cid:2) in vivo.47 An apparent shortcoming in the use of
cell stimulation from prolonged infusions of glu- FFAs or high glucose concentrations48 to study (cid:2)
coseaugmentedwitharginine.38Itcanbeassumed celldeathisthatthefrequencyofdeathinsomany
thatthe(cid:2)cellmassoftheseT2Dsubjectswasinthe invitrostudiesisfarhigherthanthefrequencyseen
rangeof50%ofnormal,yettheirinsulinresponse invivo,whichraisesconcernsabouttherelevanceof
tothismaximalstimuluswasonly15%ofnormal someofthemechanismsthathavebeenidentified.
(Fig.3). There are a few in vivo studies using infusions of
Importantly from a therapeutic perspective, the lipid,49buttheresultantFFAlevelsaresohighthat
severedysfunctioninducedbythediabeticstatecan theirclinicalrelevancemustbequestioned.
bereversedifglucoselevelsarebroughttonormal, Dataobtainedfrominvivostudiesonlipotoxic-
as best shown by the full restoration of secretion ity in humans and animal models are also uncon-
after bariatric surgery.39 It is surprising how little vincing. For example, FFA levels are high in obe-
we know about the timing of this restoration. In sity, which is associated with exuberant GSIS. The
Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences. 95
Islet(cid:2)cellmassindiabetes Weir&Bonner-Weir
humandiabetesoranimalmodelsispresentlyweak
tononexistent
Reductionofinsulincontentandabnormal
proinsulinsecretion
(cid:2) cells contain remarkable amounts of insulin,
about10%ofthetotalproteincontentwithanaver-
age(cid:2)cell,about20pgofinsulin.Insulinsynthesisis
balancedbyinsulinsecretionandbytheautophagic
destruction of secretory granules. Because insulin
mRNA has a long half-life and stable concentra-
tion,53thefine-tuningofreplenishmentaftermeals
seemsmostlydrivenbyglucose-stimulatedtransla-
tion.When(cid:2)cellsareexposedtothediabeticstate,
theybecomedegranulatedsuchthatinsomeexper-
imentalsystemsthecontentper(cid:2)cellmasscanbe
reduced to lower than 5% of normal.54 Data from
a variety of sources indicate that insulin depletion
ismuchmoreseverewithhighratherthanmodest
Figure 3. Subjects with noninsulin-dependent diabetes
(NIDDM,T2D)andcontrolsubjectswhoseglucoselevelswere elevationsinbloodglucose.Whilesecretedinsulin
increasedwithglucoseinfusionsfollowedbyacutestimulation accounts for some of this depletion, the reduction
ofinsulinsecretionwithintravenousarginine.Figuretakenfrom ofinsulinsynthesismustbeacriticalcomponent.55
Ref.38,withpermissionfromtheEndocrineSociety. Of secreted insulin immunoreactivity from (cid:2)
cells, about 2–4% is proinsulin, while circulating
increase of glucose levels in prediabetes correlates proinsulininbloodcanbe10–40%ofthetotaldue
verypreciselywithGSISdysfunction,36 butasimi- toitslongerhalf-life.Theproinsulintoinsulinratio
larcorrelationwithFFAhasyettobedemonstrated. inbloodisincreasedinT2Dandtoalesserextent
Ithasalsobeenpostulatedthatincreasedlipidstores IGT and has been used as a marker of (cid:2) cell dys-
intheformofdropletscausetoxicity,butacorrela- function. This proportional change is thought to
tionbetweenlipidstoresandsecretorydysfunction result from depletion of mature granules resulting
in(cid:2)cellshasyettobeshown.50Moreover,(cid:2)cellse- inincreasedsecretionoftheincompletelyprocessed
cretorydysfunctioncanbefoundindb/dbmicein- contentsofimmaturegranules.56
dependentofplasmalipidlevels.51TheobeseZucker
Fivestagesofdiabetes;clinical
diabeticfatty(ZDF)ratmodelofdiabeteshasbeen
consequencesof(cid:2) celldysfunction
proposedasanexampleoflipotoxicitybecausethe
inducedbyprediabeticanddiabeticstates
late stage islets contain large quantities of lipid as
diabetesworsens.52Theproblemisthatithasnever To understand both T1D and T2D, it is helpful
been shown that most of this lipid is located in (cid:2) to conceptualize five stages in their progression
cellsratherthanthefibroblastsandadipocytesthat (Table1).57 Stage 1 is successfulcompensation for
canbeseenintheseanatomicallydistortedislets. a modest fall in (cid:2) cell mass in T1D or for insulin
Proponentsoflipotoxicityfoundthatinvitroex- resistanceinT2D.The(cid:2) cellssecretemoreinsulin
posureof(cid:2)cellstoFFAsandhighglucosewaseven tomaintainanormoglycemicenvironment,which
moredamaging,whichledtothewidelyusedterm in turn allows acute GSIS because (cid:2) cells are not
glucolipotoxicity. The problem remains that FFAs exposedtoglucotoxicity.Withstage2,(cid:2) cellmass
may be toxic only in an artificial in vitro environ- drops a little more and/or insulin resistance wors-
ment. In conclusion, while lipotoxicity is a major ens,suchthatglucoselevelsrisetolevelsassociated
issue for the pathogenesis of insulin resistance in withamarkedlossofacuteGSIS,whichwepostu-
various insulin target tissues, evidence that lipo- lateiscausedbyglucotoxicity.Stage2isequivalent
toxicityandthelipidpartofglucolipotoxicitycon- toimpairedglucosetolerance(IGT)orIFG,which
tribute to (cid:2) cell secretory dysfunction or death in is stable because of maintenance of meaningful
96 Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences.
Weir&Bonner-Weir Islet(cid:2)cellmassindiabetes
Table1.Fivestagesofdiabetes57
Stage1:Normalbloodglucoselevels:compensationforinsulinresistancewithincreased(cid:2)cellinsulinsecretionand
mass.WithearlyautoimmunedestructionofT1D,increasedinsulinsecretioncompensatesfordecreased(cid:2)cell
mass.
Stage2:Stableadaptation:IGT,IFG,pre-T1D,islettransplantation.Inadequate(cid:2)cellmassallowsmodest
hyperglycemia,whichleadstodysfunctionalGSISfromglucotoxicity.
Stage3:Unstabledecompensation:T2DandearlyT1D;worseningglucotoxicityfor(cid:2)cellsandinsulintargettissues
causesarapidriseinbloodglucoselevels.
Stage4:Stabledecompensation:higherbloodglucoselevelsinT2DandearlyT1D,butenoughinsulinsecretionto
controlketosis.
Stage5:Decompensation:T1D;classicsymptomsofdiabetesandketosis;severereductionin(cid:2)cellmass.
insulinsecretion,suchthatonlyabout5%ofindi- (cid:2) cellbirthanddeath
vidualswithIGTprogresstoT2Dperyear.58 Stage To understand changes in (cid:2) cell mass one needs
3isanunstablephaseinwhichincreasingglucotox- to understand (cid:2) cell turnover, which is better un-
icityproducesmore(cid:2)celldysfunctionandinsulin
derstoodinrodentsthaninhumans.Birthcanre-
resistance, leading to a rapid rise in glucose levels sultfromreplicationofpreexisting(cid:2) cellsorfrom
tostage4,withmoreserioushyperglycemiainthe
neogenesis, the formation of new islets thought
rangeof9–17mM(160–300mg/dL).Thisrelatively
to originate from cells in the pancreatic duct ep-
rapid deterioration in control may not be caused
ithelium.Evidenceisaccumulatingthatsomecells
by a fall in (cid:2) cell mass but instead by increased
withductidentificationmarkerscanserveasmulti-
insulinresistancefromstressandovereating,forex- potent precursor cells.63,64 There has been debate
ample.Evidencesupportingtheexistenceofstages
and even controversy about the relative roles of
2–4inhumanswasrecentlyreportedbytheWhite-
replicationversusneogenesis.Replicationhasbeen
hall study group.59 Stage 5 has more severe (cid:2) cell shown to be particularly important in adult(cid:2) cell
depletionasfoundinT1D. expansion65 and for regeneration after (cid:2) cell de-
Theseconceptsaretherapeuticallysignificantfor pletion from diphtheria toxin in mice.66 In fetal
T2D because aggressive treatment of subjects in
life new islets are formed from cord- or duct-like
stage 4 may be able to reduce glucose levels to the
structures, and it is logical to think that postna-
more stable stage 2 of IGT, or to an even better
tal neogenesis is largely a recapitulation of this
stage1.Therearealreadysuggestionsthataggressive
process. In spite of discordant lineage tracing re-
early treatment of newly diagnosed patients with sults in mice,63 it is now better appreciated that
T2D with stage 4 glucose levels may lead to last-
substantial neogenesis occurs during the neona-
ingimprovementincontrol.60Thesestagesarealso tal period67,68 and in response to some forms of
important for T1D because the (cid:2) cell appears to injury.63,69,70
gothroughthesamedevelopmentofdysfunctional In humans the majority of (cid:2) cell expansion oc-
insulin secretion as autoimmunity reduces (cid:2) cell
curs in early childhood with probably a greater
mass.61 Forexample,subjectswithnew-onsetT1D
contribution from replication than from neogen-
maypresentwithhighglucoselevelsandthenhave esis.71 In adults, some studies find no evidence
aremissionthatiseitherspontaneousorinducedby of any (cid:2) cell replication;72,73 however these find-
immunosuppression.7DuringremissionC-peptide
ings are not straightforward. For example, when
levelsmightbeconsiderablyhigherthanwhenpa-
adulthumanisletsaretransplantedintoimmuno-
tientsinitiallypresentedwithhyperglycemia.62De- incompetent mice, they always have Ki67+ (cid:2) cells
spite hopes that this could be explained by (cid:2) cell intherangeof0.2–0.7%.74 Perhapsthereissome-
regeneration, relief from glucotoxicity leading to
thing about the mouse in vivo environment that
improved (cid:2) cell function seems like a better ex-
turnsonquiescentcells.Itmaybesignificantthatal-
planation.
mostallstudieshaveusedpancreasesobtainedfrom
Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences. 97
Islet(cid:2)cellmassindiabetes Weir&Bonner-Weir
Figure4. Sectionofpancreas(20×magnification)stainedforinsulin.Numerouspancreaticductsareshown,withinsulin-positive
cellspresentinandnexttoductwall,suggestingnewisletformationfromexocrineducts(neogenesis).FiguretakenfromRef.2,
withpermissionfromtheAmericanDiabetesAssociation.
autopsiesorcadaverdonors,whichraisesquestions manscontainlipofuscin,whichtakesalongtimeto
astowhetherthewarmandcoldischemiaofthese accumulate.78
conditions dampens the Ki67 expression. This is Thepossiblesignificanceofalowrateofturnover
supportedbythefindingthatinpancreaticsamples shouldnotbeunderestimated.If0.5%of(cid:2)cellsare
obtainedatsurgeryKi67positivitywasfoundtobe Ki67+ andKi67positivitylastsfor12hours,(cid:2) cell
intherangeof0.5%.75 masscouldmorethandoubleinlessthanayear.The
Thereappearstobealowrateof(cid:2)celldeath,as changesinmasswillofcoursealsobeinfluencedby
judged by such techniques as TUNEL.2 Because (cid:2) thevariablesofdeath(fromapoptosisandnecrosis)
cellmassisreasonablywellmaintainedinnondia- andneogenesis,whichareevenhardertoquantify.
betics,itmakessensethattherewouldbesomelow
Whatcausesthereductionof(cid:2) cellmass
levelofturnover,withbirthbalancingdeath.Deter-
inT2D?
mining the contribution of neogenesis in humans
is very difficult, but the presence of small clusters Manyconcludethatthereduced(cid:2)cellmassinT2D
ofinsulin-containingcellswithin,protrudingfrom, reflectsanincreasedrateof(cid:2) celldeath,butwhile
and near ducts (Fig. 4), and the discovery of cells this must often be true, there may also be a (cid:2) cell
doublestainedforinsulinandtheductmarkercy- birthproblem.Forexample,T2Dmayresultfrom
tokeratin, are consistent with some level of neo- an intrauterine growth disturbance or genetic and
geneic activity.63 The severe hypoglycemia some- environmentalforcesthatslow(cid:2)cellreplicationor
timesseenafterbariatricsurgeryisassociatedwith neogenesis.
pancreaticpathologyconsistentwithneogenesis.76 Thereisalotofcircumstantialevidencethatthe
Thereisspeculationthatthisisdriven bythevery prediabetic/diabeticstate,ormorespecificallyglu-
highplasmalevelsofGLP-1seeninthesepatients.77 cotoxicity,leadstoanincreasedrateof(cid:2)celldeath
Regardless of how (cid:2) cells are born, they are cer- from either apoptosis or necrosis. However, over-
tainly long-lived; this is supported by the finding work in the absence of increases in glucose levels
that a very high percentage of (cid:2) cells in adult hu- seems to be associated with reduced (cid:2) cell mass.
98 Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences.
Weir&Bonner-Weir Islet(cid:2)cellmassindiabetes
TheobservationbytheButlergroupthatindividu- somehowcontributestocelldeathappearstohave
alswithIFG(stage2)have,onaverage,a40%reduc- been answered. Examination of pancreases from
tionin(cid:2)cellmass(Fig.1)2suggeststhatsomething subjects with T2D indicated that islets with large
deleteriousoccursduringthecompensatorystage1. amyloid deposits often have fewer (cid:2) cells.85 When
We know that glucose can stimulate replication,30 human IAPP, which can form amyloid unlike ro-
andwecanarguethatitstillhasaneffectininsulin dent IAPP, is overexpressed in (cid:2) cells of mice or
resistantstateswhenglucoselevelsappeartobenor- rats, the result is worsening of diabetes associated
mal,asinstage1.Wehypothesizethatduringthis withmoreamyloiddeposition.86–88 Itnowappears
stageofcompensationthereisincreasedreplication that the toxicity is exerted not by the extracellular
of(cid:2)cells,andsuspectthatthereissomelimitation depositsbutbysmallfibrilsortoxicoligomersthat
in the capacity for regeneration through glucose- candamagecellmembranes.89,90Thereisincreasing
drivenreplicationleadingtoslowingofbirth.Thus, evidencethatthesecancausedamagewhileinside
duringthistimeofoverworkalimitationinthebirth the(cid:2) cell;theymaywhenoutsideaswell.Itisstill
ofnewcellscouldleadtoanolderpopulationof(cid:2) puzzlingthatamyloiddepositsareseeninisletsof
cellsthathasahigherrateofcelldeath,whichcould T2Dandininsulinomasbutarefarlesscommonin
contributetotheimpressivereductionin(cid:2)cellmass obesity, in spite of the increased secretion of both
found in the earliest stages of diabetes of IGT and insulinandIAPP.
IFG,andcouldhelpexplaintheincreasedincidence
ofapoptosislaterinthedisease. Endoplasmicreticulumstress
While investigators may advocate for the domi- The potential contribution of endoplasmic retic-
nance of their favorite mechanism, it may be best ulum (ER) stress to (cid:2) cell apoptosis in T2D has
totakethepositionthatseveralmechanismsmight received much attention in the past few years.91–93
contribute. ER stress is part of the unfolded protein response
(UPR). This UPR is turned on when the cell has
Oxidativestress
problemswiththecomplexprocessoffoldingnewly
Therehasbeengreatinterestinthepossibilitythat
synthesizedproteins.Toprotectthecells,itrecruits
glucotoxicity leads to oxidative stress, which then
chaperoneproteinsthathelpwithfolding,acceler-
causessecretorydysfunctionandanincreasedrate
ates the degradation of unfolded proteins, and in-
of(cid:2)celldeath.79Certainlyglucosestimulationleads
hibitsthesynthesisofnewproteins.WhiletheUPR
to an increase in reactive oxygen species (ROS),
hasimportantbenefitsforfunctionandsurvival,a
but this is not necessarily bad. There is interest-
fully turned on stress response activates apoptotic
ingresearchunderwaytosortoutthecontributions
pathwaysthroughsuchmediatorsasC/EBPhomol-
of ROS to important (cid:2) cell signaling mechanisms
ogousprotein(CHOP)andc-JunN-terminalkinase
that are part of normal (cid:2) cell function and to (cid:2)
(JNK).Astudyofgeneexpressioninhuman(cid:2)cells
cell dysfunction and death.80 This has led to the
transplanted into mice showed that these cells, in
hopethatantioxidanttreatmentmightslowthead-
a transplant site exposed to mild hyperglycemia,
verseprocesses.81Indeed,theadministrationofN-
had an activated UPR but reduced expression of
acetyl-cysteinehasbeenfoundto slow theadverse
JNKandCHOP,94suggestingthatERstressismore
effectsofROSontheprogressionofdiabetesinZDF
helpfulthanharmfulinthatsituation.Inourstudy
rats.82
ofpancreasesfromcadaverdonorswithT2D,gene
Amyloid expressionwasperformedonlaser-captured(cid:2)cell-
Thefirstdescriptionofamyloiddeposits,calledhya- enrichedtissuewithlittlechangeinERstressgenes,
lineatthetime,intheisletsofpeoplewithdiabetes although there were a few changes suggesting that
was reported in 1900.83 It took until 1987 to de- partsoftheUPRwereactivated.95
termine that this amyloid was formed from the (cid:2) Butler’s group stained for CHOP in pancreases
cell secretory product islet associated polypeptide from subjects with T2D, and from lean and obese
(IAPP),84whichinhumansandafewotherspecies subjectswithoutdiabetes.96 The(cid:2)cellsofboththe
canformthe(cid:2)-pleatedsheetsthatmakeupamyloid. T2DandobeseindividualshadcytoplasmicCHOP
The question as to whether amyloid deposits are staininginabout15–20%oftheir(cid:2)cells.However,
just innocent bystanders or whether its formation nuclearstainingforCHOPwasseeninlessthan1%
Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences. 99
Islet(cid:2)cellmassindiabetes Weir&Bonner-Weir
ofthe(cid:2)cellfromT2D,andnuclearstaininginthe Needtobettercharacterize(cid:2) cellsto
obesesubjectswasevenlower.Thisisconsistentwith understandpathogenesis
theviewthatonlysmallnumbersof(cid:2)cellsinT2D
Most of our understanding comes from studying
are dying at any one time, but that a larger num-
largenumbersofisletcells,butmuchmorecouldbe
berareshowingsignsofstressorfragility.Thus,the
learnedaboutpathogenesisifwecouldlearnmore
fewcellswithnuclearCHOPcouldbeontheirway
abouttheheterogeneityof(cid:2) celldeath.Forexam-
todeathwhilethosewithcytoplasmicCHOPmay
ple,whyisitthatonlyoneinmanyhundredcellsis
bemorevulnerablebutstillfunctional.Likewise,it
chosentoenterthecellcycleandreplicate?Likewise,
maybethatinT2Dmany(cid:2)cellshavesomeUPRac-
whatisspecialabouttheonecellofmanythatdies?
tivation,whichprotectsthem,butinamorefragile,
We know from many studies there is considerable
perhaps older, population of (cid:2) cells,the proapop-
heterogeneity for the capacity to secrete insulin. A
toticERstresspathwaysaremoreactive.
human(cid:2)cellmayhavealifetimeofaseveralyears,
so there must be differences among cells that are
βcelldedifferentiation
new, in the early stages of maturation, in midlife,
Geneexpressionin(cid:2)cellsexposedtohyperglycemia
in the stage of postmitotic senescence, when they
canbestudiedaftersurgicalreductionof(cid:2)cellmass
are old and fragile, and finally when they are dy-
in rats with partial pancreatectomy.29,50,97–99 After
ing. Also, what are the differences between a new
variousperiodsoftime,the(cid:2)cellsinisolatedislets (cid:2) cell formed by neogenesis in a fetus versus one
andintissuesectionsfromtheremnantpancreascan
that formed during adulthood? And, what are the
be studied. Impressive changes in gene expression differencesbetween(cid:2)cellsformedfromneogenesis
werefoundin(cid:2)cellsexposedtoeitherverymodest
comparedwiththoseproducedfromthereplication
orseverehyperglycemia.Thesechangesincludedre-
ofexistingcells?Howimportantistopicallocation,
ducedexpressionofseveralkey(cid:2)celltranscription
suchastheisletcenterornexttothemantleofnon-
factors,changesrelatedtoglucosemetabolism,and (cid:2) cells? Could there be different fetal pathways of
upregulation of genes related to stress. It has been
developmenttheleadtothegenerationofdifferent
temptingtocallthischangein(cid:2)cellphenotypeded- kindsof(cid:2)cells?Whatmightthedifferencesbebe-
ifferentiation,butitisnotpossibletosaythatthese tween(cid:2)cellsinthedorsalversustheventrallobesof
cells reverted to a phenotype that occurred earlier thepancreas?Whathappensto(cid:2)cellsinisletsthat
indevelopment.Whileitmakessensethatsomeof
arelessactiveandhavelessbloodflow?102Onemight
thesechangescouldhaveadeleteriouseffectonse- guess that there is temporal cycling of (cid:2) cells that
cretion,littlecanbesaidabouthowtheymightbe
allowsthemtorestuntilneeded.Surelythediffer-
relatedtoafallin(cid:2)cellmass.
encesdescribedaboveareaccompaniedbychanges
A new look at phenotypic change was taken by
in gene and protein expression, so there must be
the Accili group, which obtained data from mice ways to find markers for many of the different (cid:2)
lacking FoxO1 and suggested the novel hypothesis
celltypes.Flowcytometryandhistochemicalstain-
that in T2D (cid:2) cells are dedifferentiated to a state
ingmaybethemostpromisingwaystoisolatethese
of immaturity whereby they express neurogenin3,
different cell types for study. Even if only some of
Oct4, Nanog, and L-Myc and no longer make in-
thesepossibilitiesarerealized,itisexcitingtothink
sulin.100 Someofthesecellsevenadoptedan(cid:3)cell of how much might be learned about how (cid:2) cells
fate. The argument was made that restoration of
function,areborn,anddie.
differentiation in these cells may be a therapeutic
pathtoincrease(cid:2)-cellmassinT2D.Thisworkwill
Restorationof(cid:2) cellmassindiabetes
no doubt stimulate new research, but at this early
stagequestionsmustberaisedastowhetheraFoxO1 We know that restoration of (cid:2) cell mass can
deficiencymodelisrelevanttoT2D.Certainlyinves- normalize blood glucose levels in both T1D and
tigatorswilltakeacloserlookathumanpancreatic T2D. The proof-of-principle has been established
T2Disletsforimmaturecells.Thepossibleconver- with islet transplantation in T1D103 and pancreas
sionof(cid:2)cellsto(cid:3)cellsisinterestingbut(cid:3)cellmass transplantation for T2D.104 Unfortunately current
inhumanT2Dhasnotbeenfoundtobeincreased approaches require cadaver pancreases and im-
intwothoroughstudies.1,101 munosuppression,whichgreatlylimitsthenumber
100 Ann.N.Y.Acad.Sci.1281(2013)92–105(cid:2)c 2013NewYorkAcademyofSciences.
Weir&Bonner-Weir Islet(cid:2)cellmassindiabetes
of diabetics that can be treated. To make (cid:2) cell conformalcoatingusingpolyethyleneglycol(PEG)
replacement therapy more accessible, we need an orsimilarmaterials.121
abundant supply of well-characterized (cid:2) cells and Finally,thereisthedreamofstimulatingregener-
must find safe ways to prevent autoimmune and ationof(cid:2) cellsinthepancreas.122 Drugsmightbe
allogeneic killing of the newly provided (cid:2) cells. developedthatcouldstimulate(cid:2)cellreplicationor
Possibilitiesforreplacementincludecellulartrans- neogenesis.Anotherpossibilityisthattheexocrine
plantationandregenerationofpancreaticendocrine pancreas, or possibly even cells in the liver, could
cells. bereprogrammedtobecome(cid:2) cells.123 Inastrik-
Forasourceoftransplantablecells,isletcellsde- ing example of reprogramming, a single injection
rivedfromhumanembryonicstemcellsorinduced ofadenovirusescarryinggenesforthreetranscrip-
pluripotentstem(IPS)cellsareparticularlyattrac- tionfactorsimportantfor(cid:2)celldevelopment(Pdx1,
tive because there have been impressive scientific neurogenin3,andMafA)intoamousepancreascan
advances in recent years.105–107 Another hope is to producelargenumbersofcellswith(cid:2) cellcharac-
findwaystoexpandhumanpancreaticcellsinvitro teristicsthatcanstoreandreleaseenoughinsulinto
sotheycanbetransplanted.Thereareafewencour- reversethediabeticstate.124
aging studies suggesting that (cid:2) cells or pancreatic
Summary
ductcellscanbedirectedtoamesenchymalpheno-
type,allowingexpansionandthenredifferentiation Reduction of (cid:2) cell mass is fundamental to the
toisletcells.108,109 Thepossibility ofstimulating (cid:2) pathogenesis of both T1D and T2D and leads to
cellreplicationhasalsobeenreceivingconsiderable severe dysfunction of insulin secretion. These two
attention,withtheidentificationofnewcompounds abnormalitiesleadtotheconceptofthereduced(cid:2)
that can stimulate division110,111 and the elucida- cellfunctionalmass.Weneedtoknowmoreabout
tionofmechanismsthatmightbeexploitedinthe how reduced mass results from unbalanced (cid:2) cell
future.112–116 Anotherapproachistocreatea(cid:2)cell turnover,andunderstandhowthisleadstosecretory
linewithgeneticengineeringthatcanberedifferen- dysfunction.Diabetescanbepreventedorreversed
tiatedbeforetransplantation,butsafetyconcernsare by inhibiting the fall of (cid:2) cell mass or by restor-
likelytoremain.117 Finally,thepossibilityofxeno- ing mass through regeneration or transplantation.
transplantation with porcine islets from adult or Theseissuesareamongthehighestprioritiesforthe
perinatalpigscontinuestobeexplored.118,119 field.
Findingwaystolimitautoimmunityandallore-
Acknowledgments
jection has been a daunting endeavor. With stem
cells,itshouldbepossibletouselinesthatwillallow Our work has been supported by the National In-
favorable human leukocyte antigen (HLA) match- stitutes of Health [R01 DK 66056 and DK 93909
ing. Transplantation of islet cells derived from iPS (S.B.-W.),P30DK36836 (JoslinDiabetes Research
cells generated from people with T2D should face Center)], the Juvenile Diabetes Research Founda-
noimmuneattack,butthosederivedfromsubjects tion, the American Diabetes Association, and the
with T1D should face a strong autoimmune reac- Diabetes Research and Wellness Foundation. We
tion. There are many approaches to control allo- thank the many collaborators, trainees, and lab-
andautoreactivitythatarebeyondthescopeofthis oratory workers who made our work in this area
review, but the possibility of protecting cells with possible.
immunobarriermembranesisreceivingrenewedat-
Conflictsofinterest
tention.120 It seems likely that the first transplants
ofisletcellsderivedfromstemcellswillemploythis Theauthorsdeclarenoconflictsofinterest.
protectionmethod.Thegeneralapproachistouse
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