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Translocation of enamel proteins from inner enamel epithelia to odontoblasts during mouse tooth development.

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THE ANATOMICAL RECORD 238:383-396 (1994)
Translocation of Enamel Proteins From Inner Enamel Epithelia to
Odontoblasts During Mouse Tooth Development
MASANORI NAKAMURA, PABLO BRINGAS, JR.,ANTONIO NANCI,
MARGARITA ZEICHNER-DAVID, BRIAN ASHDOWN, AND HAROLD C. SLAVKIN
Center For Craniofacial Molecular Biology, School of Dentistry, University of Southern
California, Los Angeles, California, USA (P.B., M.Z.-D.,B A . , H.C.S.), Department of
Anatomy, School of Dentistry, Tohoku University, Sendai, Japan (M.N.) and Department
of Stomatology, Faculty of Dentistry and Medicine, University of Montreal, Canada (A.N.)
ABSTRACT
The developmental problem of how dental epithelia and/or
dental papilla ectomesenchyme induce and/or up- or down-regulate tooth
formation are as yet unresolved issues. We have designed studies to map
the synthesis and fate pathways of secreted amelogenin proteins from Kallenbach differentiation zones 11-IV during in vivo and in vitro mouse mandibular first molar tooth development (MI). Tooth organs from cap, bell,
and crown stages were processed for reverse transcriptaselpolymerase
chain reaction (RT-PCR)and high resolution Protein A immunocytochemistry using anti-amelogenin and anti-peptide antibodies. Cap stage MI were
cultured for periods ranging from 10-21 days in vitro using either serumless, or 15% fetal calf sera-supplemented, chemically-defined medium.
Amelogenin transcripts are expressed in the mouse embryonic molar from
El5 through early postnatal development. Amelogenin antigens were first
detected in Kallenbach’s differentiation zone 11. Amelogenin proteins secreted from preameloblasts were identified along cell processes and cell
surfaces of odontoblasts adjacent to forming mantle dentine extracellular
matrix (ECM) prior to biomineralization. Amelogenin proteins were restricted to forming endocytotic vesicles, clathrin-coatedvesicles, and lysozomes within odontoblasts. At later stages (e.g. 2 days postnatal development), enamel proteins were not identified in odontoblasts or predentine
matrix following mineralization. Comparable observations for stages of development were noted for in vitro cultured tooth explants. Preameloblasts
synthesize and secrete amelogenin proteins which bind to odontoblast cell
surfaces possibly through the process of receptor-mediated endocytosis.
We conclude that amelogenin proteins secreted from preameloblasts, prior
to the initiation of biomineralization, were translocated to odontoblasts to
serve as yet unknown biological functions. o 1994 Wiey-Liss, Inc.
Key words: Tooth development, Mouse, Protein translocation, Amelogenin, Epithelial-mesenchymalinteractions, Intercellular communication, Immunocytochemistry, Differential gene expression,
In vitro organ culture
It has clearly been established that reciprocal epithelial-mesenchymal interactions mediate a number of
epidermal organ systems including tooth development
(Koch, 1967; Kollar, 1983; Lumsden, 1987; Lumsden
and Buchanan, 1986; Mina and Kollar, 1987; Ruch,
1985, 1988; Ruch et al., 1982; Saxen et al., 1978;
Slavkin, 1972, 1974, 1988; Thesleff, 1976, 1977;
Thesleff and Hurmerinta, 1981). Nucleopore transfilter
studies suggested that ectomesenchymal cell processes
mediate the transfer of putative instructive materials
(Saxen et al., 1978; Thesleff et al., 1978; Thesleff and
Hurmerinta, 1981). Direct mesenchymal-epithelial
cell-cell contacts have been described during Kallenbach differentiation zone IV and V in vivo (Kallenbach,
0
1994 WILEY-LISS, INC.
1971; Kallenbach and Piesco, 1979; Pannese, 1962;
Silva and Kailis, 1972; Slavkin and Bringas, 1976;
Slavkin et al., 1976), prior to the expression of amelogenin mRNA transcripts (Slavkin et al., 1984; Snead et
al., 1984, 1988). However, the physical-chemical characterization of these putative instructive signals are
not known. Therefore, when, where, and how hetero-
Received April 22, 1993; accepted September 20, 1993.
Address reprint requests to Professor Harold C. Slavkin, Center For
Craniofacial Molecular Biology, 2250 Alcazar Street, CSA 1st Floor,
University of Southern California, Los Angeles, California 90033.
384
M. NAKAMURA ET AL.
typic tissues communicate with one another remains
a n intriguing and significant problem area in developmental biology.
Most extracellular matrix (ECM) proteins are synthesized as larger precursors which are post-translationally processed (e.g. glycosylation, phosphorylation,
sulfation) and proteolytically cleaved on their way to
the plasma membrane where the mature product is
released from the cell by exocytosis (Farquhar, 1985).
Typical merocrine-like epithelial secretory cells synthesize approximately 103-104 different polypeptides,
the vast majority of which are directed for intracellular
functions whereas a few “luxury” proteins are secreted
for unique extracellular functions (Farquhar, 1985).
Previous studies reported that inner enamel epithelia
synthesize and secrete enamel proteins prior to initial
extracellular biomineralization (Inai et al., 1991;
Slavkin, 1988; Slavkin et al., 1988a, b, c; Snead et al.,
1984). The precise molecular characterization and fate
of these ectodermally-derived ECM molecules is not
known. We hypothesize that one fate could be to participate in the formation of the putative dentine/
enamel junction. A second fate might be translocation
from the epithelial cell surface to the adjacent ectomesenchyme-derived odontoblast cells. Such a n exocytosis
of newly synthesized secretory enamel protein and subsequent receptor-mediated endocytosis by adjacent ectomesenchyme-derived cells (Brown e t al., 1983; Goldstein et al., 1985; Helenius et al., 1983; Wileman et al.,
1985) may describe a route for the exchange of molecules which participate in regulatory processes required for developing tooth organogenesis.
This paper reports the detection of amelogenin transcripts from E l 5 (15 days gestation) through early
postnatal development and provides immunocytochemical evidence (based upon antigenic determinants
which are cross-reactive with anti-amelogenin and
anti-peptide antibodies) for the translocation of amelogenin proteins from preameloblasts to preodontoblasts during mouse molar tooth morphogenesis as visualized by immunohistology and high resolution
immunocytochemistry. The evidence supports the hypothesis that epithelial-derived proteins are translocated to adjacent ectomesenchymal cells during instructive stages of tooth development.
MATERIALS AND METHODS
Tooth Development In Vivo
Mandibular first molars (M,) and incisor tooth organs were dissected from timed-pregnant Swiss-Webster mice (a vaginal plug indicating day 0), staged according to Theiler (1972), and samples were taken each
day ( 2 12 hrs) from cap (E15) through late crown
stages of tooth formation (10 days postnatal development). The tooth organs were rinsed in cold PBS (phosphate-buffered saline, pH 7.4) and processed for subsequent light and transmission electron microscopy as
previously described (Evans et al., 1988; Nanci et al.,
1984, 1985, 1987; Slavkin et al., 1976, 1988a).
Amelogenin Transcript Detection
Tooth organs in vivo ( E l 4 through El91 and E l 5 explants cultured for periods up to 28 days were collected,
rinsed with phosphate buffered saline (PBS) and then
frozen with liquid nitrogen and stored a t -80°C. Total
RNA was extracted according to Evans and Kamdar
(1990). RT-PCR (reverse transcriptase/polymerase
chain reaction) was performed according to Rappolee et
al. (1989). Briefly, 3-7 tooth organs from each sample
group were homogenized with sterile pestle in buffer
containing 8 M guanidine hydrochloride, 3 M sodium
acetate at pH 5.2, and 10% sodium sarcosyl(8:l:l v/v/v)
on ice. The total RNA was extracted and precipitated
with absolute ethanol at -20°C to remove any contaminating DNA. The precipitated RNA pellet was then
dissolved in diethylpyrocarbonate (DEPC) water containing RNase inhibitor (Promega, Madison, WI).
Reverse-transcription was started by adding 0.1 M
methylmercury (11)hydroxide (Amresko, Solon, OH) to
the sample for 7 min. at room temperature, followed by
0.7 M 2-mercaptoethanol (Sigma, St. Louis, MO) for 5
min. A mixture of random primers, RNase inhibitor
(Pharmacia, Alameda, CA), M-MLV reverse transcriptase (GIBCO BRL, Gaithersburg, MD), dNTP
(Pharmacia, Alameda, CA), and RT buffer (GIBCO
BRL, Gaithersburg, MD) were added to the RNA extract. The samples were heated a t 42°C for one hour,
boiled at 95°C for 5 min. and immediately cooled on ice.
The reaction was repeated three times.
PCR reaction solution was prepared by adding PCR
buffer, 10 mM dNTP, and Thermus aquaticus (Taq)
polymerase (Perkin-Elmer Cetus, Norwalk, CT) in
DEPC water. Amelogenin (AMEL) primers were designed and synthesized based on the published sequence of the canonical mouse amelogenin (Snead et
al., 1985; Lau et al., 1992). The sequences of AMEL
primers used were as follows: 5‘ end GGT CTA GAA
TGC CGA AAT GGG GAC C’M! G, and 3’ G GTA CTG
AGG TI’G GGT TGT GGT. Beta-actin was used as a n
internal positive control with primers as follows: 5‘
CAT CGT GGG CCG CTC TAG GCA CCA, and 3’ G
GTA CTG AGG TTG GGT TGT GGT. A mixture containing 43 p1 PCR reaction solution, 2 pl of 3’ primer,
2 pl of 5’ primer, and 3 p1 RT reaction product was then
loaded in small Eppendorf tubes, sealed with mineral
oil and placed in a thermal cycler machine to be processed at 95°C for 5 min then 94°C for 1min, 55°C for
45 sec, and 72°C for 1 min in 60 cycles. Six microliters
of the 50 p1 PCR reaction product was fractionated by
electrophoresis in a 3% Nusieve (FMC, Rockland, ME)
and 1% agarose gel (GIBCO BRL, Gaithersburg, MD).
Gels were run at 85 volt for 45-50 min, until the bromophenol blue dye front migrated 6 cm. Molecular
weight standard used was a mixture of ficoll, trypan
blue, and +xl74/HaeIII (GIBCO BRL, Gaithersburg,
MD). The gel was stained for 30 min in ethidium bromide, destained 10 min in water, viewed under UV
light box, and photographed using Polaroid film. The
positive control for AMEL production was 4 day postnatal mandibular molar tooth extracted mRNA from
Swiss Webster mice. The negative control used no tissue-derived RNA but all the reagents. Subsequent sequences for the amplified AMEL confirmed that these
PCR products were in fact amelogenin.
Tooth Development In Vitro
Cap stage molars were isolated and maintained in
organ culture for 28 days using either serum-supplemented or serumless, chemically-defined media as previously described (Bringas et al., 1987; Evans et al.,
385
TRANSLOCATION OF ENAMEL PROTEINS
1988; Yamada et al., 1980). The medium was FittonJackson’s modified BGJB (GIBCO, Staten Island, NY)
with or without 15% FCS (fetal calf sera), and supplemented with 50 mM ascorbic acid and 50 unitdm1
streptomycin and penicillin. Cultures were maintained
at 37°C in atmospheric conditions of 95% air and 5%
CO,. Medium was pH 7.4 at the start of each culture
and the medium was changed every other day.
Electroblotting and lmmunostaining
The electrophoretic transfer of proteins was based on
Burnette’s modifications (Burnette, 1981) of Towbin’s
method (Towbin et al., 1979). The transfers were conducted a t 4°C for 75 min at 0.6 A (60-90 V) after equilibration of the gels in the transfer buffer (0.192 M glycine; 0.025 M Tris, pH 8.3; 20% v/v methanol). It was
necessary to use at least 200 pg protein per sample for
Western blotting and immunodetection following proAntibodies Directed Against Amelogenin
tein separation of IEF-PAGE (Slavkin et al., 1988a).
Proteins of interest were determined to transfer only
The sensitivity and specificity for the rabbit antibody
toward
the anode. After transfer, the membranes were
to the major mouse amelogenin has been previously
removed and washed for 10 min in TBS (20 mM Tris,
reported (Slavkin et al, 1982; Slavkin et al., 1988a).
pH 7.5; 0.5 M NaC1) and then immersed in TBS containing 3% gelatin for 30 min. The membranes were
Synthetic Arnelogenin Anti-Peptide Antibodies
then incubated in a solution of TBS with 1% gelatin
Oligopeptides corresponding to the deduced amino (Hawkes et al., 1982) containing rabbit IgG antibodies
acid sequence for the major mouse amelogenin (Lau et to either mouse amelogenins or antipeptide antibodies,
al., 1992; Snead et al., 1985) were synthesized by Ap- in 1:2,000 dilution for 16 h at 30°C with gentle agitaplied Protein Technology (Cambridge, MA): (i) LP- tion. The incubated membranes were rinsed once with
PHPGHPGYIC (residues 3-13, designated peptide distilled water and twice for 10 min with TBS, and then
#l); (ii) IRQPYPSYGYERC (residues 30-40, desig- incubated with goat anti-rabbit horseradish-peroxinated peptide #2); (iii) SQQHPPSHTLQC (residues dase-conjugated (GAR/HRP) IgG at a 1:2,000 dilution
55-65, designated peptide #3); and (iv) HHQPNIPP- in 1% gelatidTBS for 1h. The filters were washed with
SAQQPFQC (residues 99-113, designated peptide #4). water and TBS as described above, and the peroxidase
The synthetic peptides were coupled to keyhole limpet color reaction was completed by placing membranes in
hemocyanin (KLH) (Kagan and Glick, 1979) and used freshly prepared TBS solution containing 16% methaas immunogens to immunize young female New nol, 0.05% 4-chloro-l-naphthol, and 0.05% hydrogen
Zealand White rabbits by multi-site subcutaneous in- peroxide for 15-30 min. The colorimetric reaction was
jections. Boosting was performed every 7 days, and im- terminated by washing the membranes twice for 10
mune sera collected after three boosts. The IgG fraction min in distilled water.
was prepared by DEAE-cellulose chromatography and
lmmunohistochemistry
used in enzyme-linked immunoabsorbent assays
Tooth
samples
from
E l 5 through two days postnatal
(ELISA) to determine the titer and specificity of the
antibody. A titer of 1:2,000 dilution was empirically as well a s from E l 5 molars cultured for periods up to 28
determined to be optimal for the immunocytochemistry days were fixed in Carnoy’s solution for 4 h and proand immunohistochemistry applications. The specific- cessed for paraffin embedding as recently described
ity and sensitivity of the anti-peptide amelogenin an- (Hu et al., 1992). Five micron serial sections were prepared from tissue blocks. Immunohistochemistry was
tibodies has been published (Slavkin et al., 1988a).
used to identify the cellular and extracellular distribution
of amelogenin protein antigens with the Zymed
High-Resolution Two-DimensionalGel Electrophoresis
streptavidin biotin system (Zymed, San Francisco, CA).
Isoelectric focusing-polyacrylamide gel electrophore- Sections were mounted on Histostik coated slides, desis (IEF-PAGE) was conducted as described (O’Farrell, paraffinized and rehydrated. Endogenous peroxidase
1975). The first-dimension gel used a mixture of am- activity was blocked by 30% hydrogen peroxide and
pholites (1.6% pH 5-8 and 0.4% pH 3-10) in a 10.5- absolute methanol (1:9, v/v), and slides were washed in
cm-long gube gel with a diameter of 2 mm. The sample phosphate-buffered saline (PBS). Slides were treated
buffer contained 9.5 M urea, 5% (v/v) 2-mercaptoetha- with a blocking solution, primary antibody, biotinynol, 2% (v/v) Nonidet P-40, and 2% (v/v) ampholites. lated secondary antibody, enzyme conjugate, substrate
Extracted proteins (100-200 pg) from mouse teeth chromogen mixture, and counter stained with diluted
were mixed with a n equal volume of lysis buffer after hematoxylin according to the specifications from the
the addition of solid urea. Isoelectric focusing was con- vendor.
ducted for 16 h at 400 V and 1 h at 800 V.
Primary antibody raised against mouse amelogenin
The second dimension was run according to the or synthetic amelogenin anti-peptide antibodies were
method of O’Farrell (1975)) employing the Laemmli used for indirect immunostaining. The optimal dilution
PAGE-system (Laemmli, 1970). Briefly, the IEF gels and incubation time for the amelogenin primary antiwere incubated in SDS equilibration buffer for 1-2 h body was determined to be 1:2,000 with 2 h or incubaand anchored into place with 1% agarose solution in tion at room temperature. Specificity controls included
the same buffer. After electrophoresis through the pre-immune sera instead of primary antibody, secondstacking gel (4% acrylamide) at 10 mA per gel, gels ary antibody only, and amelogenin or synthetic peptide
were run overnight through the resolving gel (10% antigen immunoadsorption of the primary antibody.
acrylamide) at 3 mA per gel. It was observed that slow
lmmunogold Cytochemistry
runs eliminated streaking and imDroved seDaration of
The protein A-gold technique was used to detect
proteins. Samples were silver-stained aftkr electroenamel Drotein antigens as Dreviouslv described
phoresis (Slavkin et al, 1988a).
u
v
386
M. NAKAMURA ET AL.
M I 2 3 4 M 5 6 7 8
bP
369246-
123the detection of two amelogenin transcripts at 364 and 256 base pairs.
The amelogenin transcript amplimer is barely detected a t E l 5 but is
readily apparent by E l 7 (in vivo data not shown) or E l 5 explants
cultured for 7 days in vitro (lane 6). Sequence data for RT-PCR amplified amelogenins confirmed that the detected amplimers were in
fact amelogenins and the pattern of transcription reflected the process of alternative splicing as suggested by Gibson et al. (1991),and
Lau et al. (1992).
Fig. 1. Amelogenin expression during E l 5 mouse molar tooth development in serumless, chemically-defined medium. Total RNA samples extracted from E l 5 molar explants cultured in serumless medium were amplified for amelogenin utilizing reverse transcriptase/
polymerase chain reactions (RT-PCR). Beta-actin mRNA was used as
an internal positive control. Lanes M designate molecular weight
markers 369,246, and 123 base pairs. Lanes 1 4 represent 0, 7, 12,
and 15 days in culture, respectively, and the presence of beta-actin.
Lanes 5-8 represent 0 , 7 , 1 2 , and 15 days in culture, respectively, and
7.5
I
kDa
6.5
I
5.5 7.5
1
6.5
I
IEF
PH
I
5.5 7.5
t
6.5
5.5
S
68-
D
S
43-
26-
Fig. 2. Comparison of anti-amelogenin and anti-peptide antibody
specificity to identify mouse enamel proteins separated and fractionated by two-dimensional gel electrophoresis, electroblotted, and immunostained. A Two days postnatal amelogenin proteins immunostained with anti-amelogenin antibody. B Two days postnatal
amelogenin proteins immunostained with anti-peptide #1 antibody.
C: Acetic acid-soluble proteins extracted from E l 5 mandibular first
molar explants cultured for 21 days in serumless, chemically-defined
medium and immunostained with anti-amelogenin antibody.
(Nanci et al., 1984, 1985, 1987, 1989). The antibody
used was polyclonal rabbit anti-mouse amelogenin IgG
antibody (Slavkin et al., 1982,1988a). As a n additional
control for tissue preservation and the localization of
enamel antigens, replicate samples were processed using anhydrous fixation (Diekwisch et al., 1993; Landis,
1983) and examined with transmission electron microscopy as previously described (Evans et al., 1988).
RESULTS
AMEL Transcription During Cap Stage Molar
Tooth Development
Amelogenin transcripts were detected from E 15
through early postnatal stages of mouse mandibular
first molar tooth development in vivo and during tooth
culture in vitro using RT-PCR (Fig. 1).Whereas previ-
TRANSLOCATION O F ENAMEL PROTEINS
387
Fig. 3. Immunohistochemical localization of amelogenin proteins
using either anti-amelogenin or anti-peptide polyclonal antibodies
during translocation from inner enamel epithelium to dental papilla
ectomesenchyme. A At birth ameloblasts (Am) express and secrete
amelogenins (arrow) in juxtaposition to odontoblasts (Od) cell processes associated with forming dentin extracellular matrix. Bar line
= 50 pm in A, B & C. B: Comparable developmental stage of enamel
formation as shown in A but stained with anti-peptide #1 antibody
(arrows). C: A control using primary anti-peptide #I antibody immunoabsorbed with anti-peptide #1 immunogen. D Survey showing patterns of immunostaining with anti-peptide #I antibody in forming
cusps of E l 9 molar tooth organs. Positive immunolocalization is evident along the odontoblast cell bodies along the periphery of the dental papilla ectomesenchyme (Dpm). Bar line = 50 pm. E: Immuno-
localization of amelogenins within preameloblasts, ameloblasts and
adjacent odontoblast cell bodies (arrows) in E l 9 molars stained with
anti-peptide #1 antibody. Bar line = 50 pm in E-I. F: E l 5 molar
explants cultured for 12 days in vitro in serumless medium and
stained with anti-amelogenin antibody showing amelogenin along
odontoblast (Od) cell bodies (arrows). G: E l 5 molar explant cultured
for 12 days in vitro with medium supplemented with 15%fetal calf
serum and stained with anti-peptide #1 antibody. H Immunolocalization of amelogenins within preameloblasts, ameloblasts (Am), odontoblast cell processes (arrows), and adjacent odontoblast (Od) cell
bodies during E l 8 molar tooth development. I: A control using primary anti-amelogenin antibody immunoabsorbed with amelogenin
immunogen during E l 8 molar tooth development.
ous in situ hybridization studies demonstrated initial,
amelogenin transcription during newborn mouse molar development (Snead et al., 19881,the present studies report that amelogenin m
~ are detected
~
~ during
s
15-day gestation molar tooth development, E l 5 molar
explants cultured for periods up to 15 days (Fig. 1,
lanes 5-8), and throughout fetal and early postnatal
molar tooth formation (data not shown). Alternatively
spliced amelogenin products were detected a s 364 base
pairs and 256 base pairs, respectively (Fig. 1, lanes
7-8).
Two-Dimensional lrnrnunoblot Analysis of Enamel Protein
Expression Using Anti-Arnelogenin and
Anti-Peptide Antibodies
A comparision of acetic acid soluble enamel proteins
extracted from either in vivo or in vitro molar tooth
organs and separated by two-dimensional gel electro-
388
M. NAKAMURA ET AL.
Fig. 4. Odontoblast (Od) and ameloblast (Am) cytodifferentiation, and dentine (D) and enamel (En)
ECM formation in viva and in vitro. A: Secretory amelogenesis and enamel formation in vivo a t 2 days
postnatal development. B: Cap stage molar cultured for 12 days in BGJb medium supplemented with
15% FCS. Toluidine blue stained granules accumulate in predentine (pD) adjacent to odontoblast cell
surfaces (arrows). Bar line = 40 Fm.
FCS, cap stage tooth explants expressed crown morphogenesis and inner enamel epithelia differentiated into
ameloblasts (Fig. 5B).
Surveys of inner enamel epithelial differentiation in
vivo and in vitro (with or without serum supplementation), demonstrated the deposition of electron-dense
granular materials in differentiation zones 11-VI (Fig.
5A). During serumless cultures, both odontoblast and
preameloblasts extended cell processes (Fig. 5A). Electron-dense granular materials were deposited along epithelial cell surfaces, epithelial and odontoblast cell
processes, and also along the surfaces of odontoblasts
lmrnunohistochemical Localization of Enamel Proteins
(Fig. 5A,B, arrows). This electron-dense granular maDuring Epithelial Differentiation Into Ameloblasts
terial was also observed prior to and during initial
Mouse molar tooth development demonstrated ini- ECM biomineralization during in vivo development
tial amelogenin localization in preameloblasts and (Fig. 6). The material persisted during epithelial difameloblasts followed by localization with odontoblasts ferentiation zones 11-V, thereafter, the material was
(Fig. 3A-I). Immunohistochemistry was used to map not observed during in vivo or in vitro development.
Immunocytochemical localization of enamel proteins
the timing and positional patterns of amelogenin proteins. Both anti-amelogenin and anti-peptide antibod- using anti-mouse amelogenin antibodies identified the
ies demonstrated amelogenin immunostaining associ- electron-dense granular materials to contain enamel
ated with preameloblasts (Fig. 3A-B, arrows) in antigenic determinants (Fig. 6A-E). During tooth culdifferentiation zone I1 (Kallenbach, 19711, and associ- ture in serumless medium, enamel antigens were loated with odontoblasts (Fig. 3E-H, arrows). Ameloge- calized along both preameloblast and odontoblast cell
nin translation products were first localized within the surfaces (Fig. 6A, arrows). Enamel antigenic domains
inner enamel epithelium of E l 7 embryonic mouse were clustered along odontoblast cell surfaces and were
internalized by coated pits (Fig. 6B, arrows). During
mandibular first molars (data not shown).
mouse mandibular first molar development differentiation zones 11-V, enamel protein antigens were locallmmunocyfochemical Localization of Extracellular Enamel
ized in electron-dense granular materials localized
Proteins on Odontoblast Cell Processes
along cell processes (Fig. 6C, arrows). One control for
Cap stage tooth explants produced enamel matrix antibody specificity used immunogen (amelogenin) adduring organ culture using medium either supple- sorbed primary antibody as shown in Figure 7.
mented with 15% FCS or in serumless, chemically-defined medium. By 12 days, initial biomineralization
Enamel Protein Antigen Localizations in
was detected in explants cultured in serum suppleOdontoblast Lysozornes
mented medium, and by 21 days enamel and dentine
In the later differentiation zones, enamel antigens
biomineralization was comparable to that observed
during postnatal development (Fig. 4A,B). Using 15% were localized within lysozomes in odontoblasts (Fig.
phoresis, electroblotted and immunostained with either anti-amelogenin or anti-synthetic peptide # 1 antibodies is shown in Figure 2. At 2 days postnatal
development, anti-amelogenin antibody identified a
larger number of epitopes (Fig. 2A) than the anti-peptide #1 antibody; both antibodies were used at 1:2,000
dilution (Fig. 2B). The sensitivity and specificity of the
anti-peptide antibody to identify specific amelogenin
epitopes during in vivo tooth development was also observed in E l 5 molar explants cultured for 21 days in
serumless medium (Fig. 2C).
Fig. 5.During epithelial-mesenchymal interactions, at Kallenbach
differentiation zone V, electron-dense materials were deposited along
the surfaces of extended odontohlast cell processes and adjacent to the
odontoblast cell body (Od). A Numerous odontoblast cell processes
approximate the undersurface of preameloblasts (pAm); electrondense granular materials were identified along epithelial and odon-
toblast cell surfaces (arrows). M, cultured in serumless medium after
12 days in uitro. Bar line = 2 pm. B: M, development a t 1day postnatal tooth formation. Basal lamina is discontinuous. Electron-dense
granular materials associated with cell processes were identified (arrows). Bar line = 1 pm.
Fig. 6
TRANSLOCATION OF ENAMEL PROTEINS
391
6D,E). No enamel antigen was detected in rER, Golgi
or mitochondria. Antigen was identified along odontoblast surfaces, aggregated in coated pits, and appeared
to be localized within intracellular vesicles and lysozomes (Fig. 6D,E). Analyses of serial sections (silver)
indicated that these antigens were localized within
lysozomal bodies. Odontoblasts showed intercellular
antigen labeling (Fig. 8A,B, arrows), continued accumulation along cell surfaces (Fig. 8B, arrows), and localization in lysozomes (Fig. 8B). No enamel antigen
was detected in odontoblasts after differentiation zone
VI .
DISCUSSION
We report the transcription of amelogenin during fetal tooth development. This finding provides additional
evidence that inner enamel epithelium expresses amelogenin well in advance of evidence for tissue-specific
enamel or dentine biomineralization. Further, we report a series of observations which suggest translocation of enamel polypeptides or peptides from
preameloblasts to odontoblasts prior to dentine biomineralization in uivo and in uitro. Analysis of the patterns of enamel antigenic determinant immunolocalization led us to suggest that ectodermally-derived
inner enamel epithelia secrete and transfer tissue-specific proteins to adjacent odontoblasts prior to the initiation of dentine biomineralization. Whereas, the epithelial-derived enamel protein(s) appeared to bind to
the cell surface suggestive of ligand binding to receptors, possibly endocytosed by odontoblasts and translocated to endosomal compartments (Goldstein et al.,
1985; Helenius et al., 1983; Lazarovits and Roth, 1988;
Verner and Schatz, 1988; Wileman e t al., 19851, extreme caution should be exercised since fluid phase
endocytosis would be undistinguished with the methods used in this study.
Previous studies with autoradiographic or immunocytochemical techniques have described the compartments within ameloblasts during enamel protein synthesis and secretion (Greulich and Slavkin, 1965;
Nanci et al., 1984, 1985, 1987; Slavkin et al., 1976;
Warshawsky, 1968; Weinstock, 1972; Weinstock and
Leblond, 1971). Our present studies confirm the immunohistochemical demonstration of amelogenin penetration from preameloblasts toward the dental pulp during early stages of rat molar tooth development (Inai et
al., 1991). However, whereas Inai and colleagues
Fig. 6. Immunocytochemical localization and suggested translocation of enamel protein antigenic determinants from inner enamel epithelia (preameloblasts, differentiation zone IV), along odontoblast
cell processes, and internalization by coated pits and localization in
lysozomes within odontoblasts. A: Differentiation zone IV during
tooth organ culture after 12 days. Odontoblast (Od) cell processes
extend towards preameloblasts (pAm). Arrows indicate enamel antigens along cell surface and associated with coated pits during internalization. Bar line = 1 pm. B Enamel antigens accumulate in regions (arrows) related to endocytosis. C: Suggested translocation of
enamel antigens (arrows) from preameloblasts to odontoblasts mediated by cell processes (cp) extended through the forming dentine matrix (D) at 1-day postnatal development. D Enamel protein antigens
were localized in odontoblast lysozomes. E: Odontoblast cell surface
with enamel antigens (arrow) and intracellular lysozome a t differentiation zone VI. Bar line for B-E = 200 pm.
Fig. 7. Isolated mouse 26 kDa amelogenin (immunogen) adsorbed
primary antibody served a s one of the immunocytochemical controls
to illustrate antibody specificity. Odontoblast cell process (cp) with
electron-dense granular materials and forming dentine matrix. Bar
line = 200 pm.
(1991) described the localization of amelogenin protein
antigen in 0-5 day-old rat molar development, our
studies report for the first time, evidence for amelogenin expression and translocation during embryonic
mouse molar development in vivo and in vitro.
Epithelial-Ectomesenchyme Interactions During Tooth
Cytodifferentiationand Morphogenesis
The timing and position of instructive signal(s)
which regulate either epithelia or mesenchyme during
epidermal morphogenesis (e.g. thyroid gland, mammary gland, salivary gland, feather formation, hair for-
392
M. NAKAMURA ET AL.
Fig. 8. Enamel proteins translocated from epithelia to odontoblasts were localized either between
odontoblasts and/or within odontoblast lysozomes. A: Intercellular localization of enamel proteins (arrows) between odontoblasts. B: Differentiation zone VI showing surface antigens and odontoblast lysozomal localizations. (Arrows = enamel proteins) Bar line = 200 wm.
mation, tooth formation) are not known. What is evident is t h a t ectodermally-derived first branchial arch
epithelium appears to induce molar tooth formation
between E9-Ell (Lumsden, 1987; Lumsden and
Buchanan, 1986; Mina and Kollar, 1987). Thereafter,
ectomesenchyme-derived signals appear to mediate the
shapetform of tooth morphogenesis and perhaps serve
to induce enamel production (Koch, 1967; Kollar, 1983;
Lumsden, 1987; Lumsden and Buchanan, 1986; Ruch,
1985, 1988; Ruch et al., 1982; Saxen et al., 1976;
Slavkin, 1972, 1974, 1988; Slavkin et al., 1984, 1988a,
b, c; Snead et al., 1984,1988; Thesleff and Hurmerinta,
1981; Thesleff et al., 1978). Ultrastructural and transfilter studies have provided results which suggest that
preodontoblast cell processes and/or ECM molecules
“transfer” inductive signals from mesenchyme to epithelia (Kallenbach, 1971; Kallenbach and Piesco, 1979;
Koch, 1967; Pannese, 1962; Saxen e t al., 1976; Silva
and Kailis, 1972; Slavkin et al., 1976; Slavkin and
Bringas, 1976; Thesleff et al.; 1978). Evidence from
transfilter Millipore and Nucleopore filters (pore sizes
ranging from 0.1-0.8 pm and thicknesses of 25-100
pm) demonstrate that dental papilla ectomesenchyme
cell processes and/or epithelial microvilli protrude
through 0.2 pm pore size and 25 pm thick Nucleopore
filters to mediate instructive signals for epithelial cytodifferentiation (Koch, 1967; Thesleff and Hurmerinta, 1981; Thesleff et al., 1978); assayed according to
epithelial cell elongation and enamel ECM production.
Tritiated proline labeled enamel organ epithelia have
been demonstrated to synthesize and secrete 3H-proline-labeled proteins which diffuse from epithelia to
mesenchyme through Millipore transfilter tissue recombinations (Wolters, 1978). Despite the numerous
suggested molecular processes which might control
this heterotypic cell-cell communication (e.g. cell sur-
393
TRANSLOCATION OF ENAMEL PROTEINS
face proteoglycans, galactosyltransferases, differentiation cell surface antigens, short-range diffusible molecules, ion transport, transfer of nucleic acids, etc.)
(McMahon and West, 1976; Ruch, 1985; Saxen et al.,
1976; Slavkin, 1972; Slavkin et al., 1988a, b, c;
Thesleff, 1977; Thesleff and Hurmerinta, 1981;
Wolters, 19781, the molecular process for inductive epithelial-mesenchymal interactions during tooth formation is not known.
Translocation of Enamel Proteins: A Model for
Macromolecular Transfer From Epithelia to Odontoblasts
Am el o bl a st s
Preameloblast
Synthesis
Enamel
Proteins
,
Future DE+t
I
m,ii,
Translocation
l l
,cell
The scheme shown in Figure 9 illustrates the sugprocess
gested route of enamel protein translocation from
preameloblasts to odontoblasts during tooth development. Our studies confirm previous studies demonstrating the synthesis and secretion of enamel proteins
Elongation
by mouse preameloblasts (Nanci et al., 1984, 1985,
Coated
1987; Slavkin e t al., 1976, 1988a, b, c). The localization
Preodontoblast
of enamel antigenic determinants within the ECM is
clearly demonstrated by several types of evidence.
High resolution autoradiographic studies demonstrate
isotopically-labeled tryptophan-containing enamel
proteins secreted by preameloblasts and associated
?
with extended odontoblast cell processes (Slavkin et
al., 1976). Nanci and colleagues have shown the exOdontoblasts
pression of enamel proteins in vivo (Nanci et al., 1985,
1987) and during in vitro mouse molar tooth developFig. 9. Scheme illustrating suggested translocation of enamel proment in serumless as well a s serum-supplemented me- teins from preameloblasts to odontoblasts. The suggested pathway is
dium (Nanci et al., 1984). More recently, Inai and col- as follows: (i) synthesis and secretion of enamel proteins from inner
epithelia in differentiation zones 11-IV; (ii) enamel protein(s)/
leagues (1991) demonstrated that the presecretory enamel
type I collagen interactions associated with the formation of the puameloblasts in their early stages of differentiation both tative dentinoenamel junction (DEJ); (iii) translocation of enamel prosynthesized amelogenin and secreted this protein tein along odontoblast cell processes; (iv) suggested binding of enamel
through a classical merocrine secretory pathway; im- protein(s1 to odontoblast cell surfaces and subsequent internalization
coated vesicles; (v) coated vesicles form endosomes and lysozomes
munostaining was also localized in the predentin a s by
within odontoblasts; and (vi) enamel protein antigen is also localized
well a s the intercellular spaces of odontoblasts. Fi- between odontoblasts and between odontoblasts and adjacent pulpal
nally, a number of ultrastructural studies have re- fibroblast cells. Possible biological responses to the translocation inported that the enamel precursor-like material re- clude odontoblast up- or down-regulation of differential gene expression (e.g. dentine phosphoprotein, dentin sialoglycoprotein, growth
ferred to as stippled material was observed in factors,
growth factor receptors, fibronectin, etc.).
predentin a s well as in dentin and along odontoblast
cell surfaces (Reith, 1967; Suga, 1960; Watson, 1960;
Yamamoto et al., 1980). Curiously, Yamamoto et al.
(1980) were the first to suggest that enamel-like stip- translocated to adjacent odontoblasts during tooth depled material found in predentin originated from velopment.
ameloblasts.
Epithelial-derived enamel proteins participate in the
Isolated enamel organ epithelia labeled with 3H- determination of the dentine-enamel junction. It is evproline for one hour an> recombined in transfilter stud- ident that odontoblast cell processes extend to the denies with isolated dental papilla ectomesenchyme, dem- tine-enamel interface (Kallenbach, 1971; Kallenbach
onstrated the synthesis, secretion, and diffusion of and Piesco, 1979; Kelley et al., 1981; Ruch, 1988; Sigal
isotopically labeled molecules from epithelia to mesen- et al., 1984; Thesleff and Hurmerinta, 1981; Thesleff et
chyme (Wolters, 1978); proline is a major amino acid al., 1978). Both inner enamel epithelia and dental paconstituent of amelogenins and enamelins (Deutsch, pilla mesenchyme synthesize and secrete collagens
1989; Deutsch et al., 1991; Ogata et al., 1988; Termine (Trelstad and Slavkin, 1974).The dentine-enameljuncet al., 1980). One possibility is that the enamel organ tion may represent the coprecipitation of the collageepithelia synthesized and secreted types I and IV col- nous and non-collagenous extracellular matrix prolagens as previously demonstrated by Trelstad and teins (Arsenault and Robinson, 1989). The timing and
Slavkin (Trelstad and Slavkin, 1974). However, in position for this supramolecular assembly may be
Wolter’s study (Wolters, 1978), approximately 40% of controlled by the initially secreted enamel proteins
the proline-labeled material which accumulated be- (Deutsch, 1989; Diekwisch et al., 1993; Silva and Kaltween previously labeled epithelia and non-labeled lis, 1972; Slavkin, 1988), and/or molecular interactions
mesenchyme was digested by type I collagenase; the between anionic enamel proteins and extended odontoremaining material being epithelial-derived non-col- blast cell processes.
lagenous extracellular matrix molecules. We suggest
The present studies have defined the time- and posithat the material described in the present study repre- tion-restricted expression of enamel antigenic determisents epithelial-derived enamel proteins which were nants according to the convention of Kallenbach and
I 441 I
I
394
M. NAKAMURA ET AL.
recently applied to mouse tooth development (Slavkin Research, National Institutes of Health, United States
et al., 1988a, b, c). From Kallenbach’s differentiation Public Health Service, DE-02848 and DE-06425.
zones 11-V we observed the fate and spatial pathway of
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(Figs. 6, 8). We also observed material with enamel
Slavkin 1987 Ultrastructural analysis of enamel formation during in vitro development using chemically-defined medium. Scanantigenic determinants between odontoblast cells.
ning Microscopy Intl., lt1103-1108.
These results support recent observations from Inai Brown,
M.S., R.G.W. Anderson, and J.L. Goldstein 1983 Recycling
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close proximity to immunolabeled amelogenin along Burnette, W.N. 1981 “Western blotting”: Electrophoretic transfer of
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cise molecular characteristics of the transferred Evans,
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ACKNOWLEDGMENTS
We thank Mrs. Helen Fludd for preparing this manuscript. These studies were supported in part by research grants from the National Institute for Dental
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