Tracking Endogenous Amelogenin and Ameloblastin In Vivo
Citation: Jacques J, Hotton D, De la Dure-Molla M, Petit S, Asselin A, et al. (
Tracking Endogenous Amelogenin and Ameloblastin In Vivo
Muriel De la Dure-Molla
Ashok B. Kulkarni
Carolyn Winters Gibson
Steven Joseph Brookes
Joseph Najbauer, University of Pecs Medical School, Hungary
Research on enamel matrix proteins (EMPs) is centered on understanding their role in enamel biomineralization and their bioactivity for tissue engineering. While therapeutic application of EMPs has been widely documented, their expression and biological function in non-enamel tissues is unclear. Our first aim was to screen for amelogenin (AMELX) and ameloblastin (AMBN) gene expression in mandibular bones and soft tissues isolated from adult mice (15 weeks old). Using RT-PCR, we showed mRNA expression of AMELX and AMBN in mandibular alveolar and basal bones and, at low levels, in several soft tissues; eyes and ovaries were RNA-positive for AMELX and eyes, tongues and testicles for AMBN. Moreover, in mandibular tissues AMELX and AMBN mRNA levels varied according to two parameters: 1) ontogenic stage (decreasing with age), and 2) tissue-type (e.g. higher level in dental epithelial cells and alveolar bone when compared to basal bone and dental mesenchymal cells in 1 week old mice). In situ hybridization and immunohistodetection were performed in mandibular tissues using AMELX KO mice as controls. We identified AMELX-producing (RNA-positive) cells lining the adjacent alveolar bone and AMBN and AMELX proteins in the microenvironment surrounding EMPs-producing cells. Western blotting of proteins extracted by non-dissociative means revealed that AMELX and AMBN are not exclusive to mineralized matrix; they are present to some degree in a solubilized state in mandibular bone and presumably have some capacity to diffuse. Our data support the notion that AMELX and AMBN may function as growth factor-like molecules solubilized in the aqueous microenvironment. In jaws, they might play some role in bone physiology through autocrine/paracrine pathways, particularly during development and stress-induced remodeling.
The specific properties of mineralized tissues result from their
unique extracellular matrix (ECM) composition. ECM has
multiple effects on the biological behavior of skeletal cells and
extracellular mineralization. As illustrated by the SIBLING family
of proteins , ECM proteins not only provide template for
ordered nucleation and crystal growth  but also control fate and
activity of cells responsible for odontogenesis and cells regulating
bone formation and turn-over. The organic matrix of bone, dentin
and cementum is based on type I collagen associated with number
of bone/tooth non-collagenous proteins . In contrast, enamel is
composed of specific enamel matrix proteins (EMPs) such as
amelogenin (AMELX) and ameloblastin (AMBN). Contrary to
bone, dentin or cementum ECM proteins, EMPs are ephemeral;
after their secretion in enamel ECM and their aggregation into
nanospheric structures, AMELX and AMBN are subject to
proteolytic processing [4,5].
In recent years, EMPs have been identified in root epithelial
cells  and non-enamel dental and bone cells . Presence of
EMPs RNA/proteins were also reported during early tooth
development at the pre-mineralization stage  and in organs
neither related to ectodermal appendages nor mineralized tissues,
such as brain . Based on these observations, AMELX 
and AMBN  might be functional in non-enamel tissues.
EMPs exhibit cell signaling properties that impact on a wide
range of cell activities. A commercially available enamel matrix
derivative (EMD) is used for periodontal regeneration as well as
epidermal wound healing (for review, ). More specifically,
using recombinant AMELX and AMBN and transgenic mice that
overexpressed EMPs and their splicing forms, previous studies
have demonstrated that EMPs control cell adhesion, proliferation,
polarity, commitment, differentiation and act on key-cellular
pathways . To date, nearly all the cells of
dentalperiodontal, epidermal and bone compartments have been found
to respond to EMPs (for review, ). Transgenic mouse studies
indicated that osteoblast and osteoclast cell activities are influenced
by AMELX and AMBN [7,24,25]. Thus, an extensive number of
investigations have documented in vitro and in vivo cell responses to
under- or over-expression of EMPs, knockdown of EMPs, ectopic
expression or addition of specific recombinants, synthetic peptides
or EMD fractions. Herein we describe the endogenous expression
of both AMELX and AMBN in mandibular bone and soft tissues.
We also report the potential mobility and diffusibility of AMELX
and AMBN in mandibular bone. This last point is an important
consideration when ascribing growth factor-like or cell signaling
attributes to AMELX and AMBN.
Materials and Methods
Animals and Tissue Sampling
The experimental animal protocol was reviewed and approved
by the French Ministry of Agriculture for care and use of
laboratory animals (B2 231010EA). All experiments were
performed in accordance with the French National Consultative
Bioethics Committee for Health and Life Science, following the
ethical guidelines for animal care. All procedures related to
AMELX KO and their Wild-Type (WT) littermates were
reviewed and approved by The Institutional Animal Care and
Use Committee (IACUC) of the University of Pennsylvania
(Protocol # 803067, Enamel Mineral Formation during Murine
WT Swiss male mice (Janvier, St Berthevin, France) at 1, 8 and
15 weeks of age and 1 and 8 week old AMELX KO mice 
As detailed in Fig. 1, alveolar and basal mandible bones and
dental epithelial and mesenchymal cells from 1 and 15 week old
WT mice were microdissected under a stereomicroscope (Leica
MZ FLIII, Leica Microscopy Systems, Ltd., Heerbrugg,
Switzerland). The molar alveolar bone (AB) was harvested after removal
of the mandibular soft tissues and molars. The exfoliation of
forming tooth germs and formed teeth was performed by careful
observation under stereomicroscope and using miniaturized
Gracey curette. Basal bone (BB) was collected from the mandible
angular process in order to exclude both dental and cartilage
tissues. The harvested bone tissues (AB and BB) were carefully
rinsed in 16 Dulbeccos phosphate-buffered saline (DPBS,
Invitrogen, Carlsbad, CA, USA) to avoid soft tissue
contamination. Dental epithelial cells (EP) and dental mesenchymal cells
(ME) were dissected as previously described . Briefly,
continuously erupting incisors were carefully extracted and the
dental epithelial cells (EP) were harvested by stripping the entire
epithelium tissue off the incisor buccal surface, excluding the
cervical loop to prevent contamination by dental stem cells (Fig. 1).
Consequently, dental epithelial cells isolated from incisors of 1
week old mice were mainly composed of enamel-forming cells
harvested from the secretion and maturation stages. Dental
epithelial cells were similarly harvested from 15 week old mice
but samples obtained from these older animals contained
proportionally less tissue from the secretory stage as the proportion
of epithelial cells present in the maturation stage and post
maturation stage (i.e. protective reduced enamel epithelium)
increases with age. Incisors were then cleaned and fractured to
collect dental mesenchymal cells (ME) from dental pulp. Finally, a
panel of non-mineralized tissues (eye, tongue, liver, heart, lung,
kidney, colon, ovary, testicle and striated muscle) were dissected
from 15 week old WT mice and washed in 16 DPBS. All
quantitative experiments were performed in triplicate with at least
three animals for each time point. After dissection, tissues were
immediately frozen and stored at 280uC.
RNA Extraction and RT-qPCR
Tissues were mechanically ground to a fine powder in liquid
nitrogen and RNA was extracted using the TriReagent kit
(Euromedex, Souffelweyersheim, France), following the
manufacturers instructions. In brief, total RNA was precipitated with
isopropanol and centrifuged at 12,000 g at 4uC. Then, the RNA
pellet was washed with 75% ethanol and resuspended in
RNasefree water. The concentration and purity of total RNA in each
sample were determined by the A260/A280 ratio. The integrity of
RNA was confirmed by electrophoresis on an agarose ethidium
bromide gel. One microgram of total RNA from each sample was
reverse transcribed into cDNA using 200 units of Superscript II
(Invitrogen) and 250 ng of random primers according the
manufacturers instructions. Real-time PCR reactions were
performed using a MiniOpticon Real-Time PCR Detection
System (Bio-Rad Laboratory, Hercules, CA, USA). According to
manufacturers recommendations, a 15 ml volume containing
7.5 ml of IQ SYBR Green Supermix (Bio-Rad Laboratory),
50 ng of cDNA as template and 0.3 mM of the appropriate
primer-pairs (Eurogentec, Lie`ge, Belgium) was used. The final
mixture was subjected to PCR under the following conditions:
denatured at 98uC for 10 s, followed by 40 amplification cycles
(98uC for 10 s, 60uC for 30 s, and 72uC for 20 s). AMBN and
AMELX PCR amplified products were resolved on a 2% agarose
gel and their specificity was confirmed by sequencing
(GATCBiotech, Mulhouse, France). mRNA levels of genes of interest were
normalized against gene expression of the housekeeping gene
GAPDH. The following specific primers sets were used: AMBN
In situ hybridization using Amelogenin oligonucleotidE
In situ hybridization using digoxygenin (DIG)-labeled
oligonucleotidic probes (sequence - gaggtggtaggggcatagcaaaa - Exiqon,
Vedbaek, Denmark) was performed as previously described .
Briefly, sections were deparaffinized using Clearene solvent (Leica
Microsystems, Nanterre, France) and rehydrated through ethanol
solutions diluted in in situ hybridization buffer (0.1% Diethyl
pyrocarbonate (DEPC, Sigma-Aldrich Co., St. Louis, MO, USA)
in DPBS). After three washes in in situ hybridization buffer,
sections were subjected to proteinase-K treatment in a humid
chamber. After washing with saline-sodium citrate (SSC) buffers,
sections were blocked using DIG blocking reagent in buffer
containing 10% heat-inactivated sheep serum, and hybridization
detected using alkaline phosphatase-conjugated anti-DIG, in
conjunction with 4-nitro-blue tetrazolium (NBT) and
5-bromo-4chloro-39-Indolylphosphate (BCIP) substrate (Roche, Mannheim,
Germany) together with 2 mM Levamisole (Dako, Glostrup,
Denmark). Sections were mounted with mounting resin (Eukit,
Agar Scientific, Freiburg, Germany) and observed using a Leica
DMRB microscope (Leica Microscopy Systems).
Tissue Preparation. After dissection, mandibles were fixed
for 24 h in 4% paraformaldehyde (PFA, Sigma-Aldrich Co.) in 16
DPBS (Invitrogen) and washed with 16 DPBS for 1 h. Samples
isolated from mice older than one week (i.e. 8 and 15 week old
mice) were then decalcified in buffered 4% ethylene
diaminetetraacetic acid (EDTA) solution at 4uC (under agitation from 1 up
to 15 weeks). Finally, the samples were dehydrated, cleared in
xylene and embedded in paraffin. Then, 810 mm sections were
Immunohistoperoxidase. After deparaffinization and
rehydration, endogenous peroxidase was inactivated by treatment with
3% H2O2 (Merck, Darmstadt, Germany) in 16 DPBS for 15 min.
Sections from 1 and 8 week old WT and AMELX KO mice were
then rinsed in 16 DPBS and blocked overnight at 4uC with
readyto-use (2.5%) normal horse blocking reagent (ImmPRESS reagent
kit, Vector Laboratories, Burlingame, CA, USA). Sections were
then probed with primary anti-AMBN antibody (1/100) (sc 50534
(M-300), Santa Cruz Biotechnology - rabbit polyclonal IgG to
AMBN - Immunogen: amino acids 108407 mapping at the
Cterminus of AMBN mouse origin - Reacts with mouse) and
antiAMELX antibody (1/100) (ab 59705, Abcam, Cambridge, MA,
USA - rabbit polyclonal IgG to AMELX - Immunogen: purified
full length native protein of AMELX cow origin Reacts with
mouse, rat and cow) for 1 h at room temperature. Tissue sections
were washed three times with 16 DPBS for 5 min and incubated
for 30 min with ImmPRESS reagent (ImmPRESS reagent
antirabbit Ig Kit, Vector Laboratories, Burlingame, CA, USA). After
three washes in DPBS 1X, immuno cross-reactivity was visualized
using diaminobenzidine peroxidase substrate (Novared, Vector
Laboratories). Sections were dehydrated and mounted in resin
(Eukit, Agar Scientific, Freiburg, Germany). Sections with no
primary antibodies were used as negative controls.
Tissues were dissected from 15 week old WT mice (n = 6 with n = 3 females and n = 3 males) and subjected to RT-PCR (see Materials and methods). Resulting products
were resolved on a 2% agarose gel. AMBN positive tissues show one amplicon band at 287 bp and AMELX positive tissues show at least one of the three amplicon
bands at 415 bp, 373 bp and 303 bp corresponding to transcript variants encoding different isoforms of AMELX described in the literature; at 415 bp (M217 M194
), 373 bp (M203 M180 ) and 303 bp (M179 M156 ). +, PCR products were repeatedly obtained; +/2, not all samples were positive; -, no PCR
products were visible. The overall high signal in mRNA levels (++) in mandibular mineralized tissues led to perform additional RT-qPCR analyses.
Immunohistofluorescence. After deparaffinization and
rehydration, mandible tissue sections from 1 week old WT and
AMELX KO mice were permeabilized for 10 min in 1% Triton
X-100 (Thermo Fisher Scientific Inc, Waltham, MA, USA), then
rinsed with 16 DPBS 3 times for 5 min each under agitation.
Nonspecific binding sites were blocked by 30 min incubation in
1% bovine serum albumin (BSA, Euromedex, Mundolsheim,
France) diluted in DPBS. Sections were incubated overnight at
4uC with primary anti-AMBN antibody (1/200) (sc 50534
(M300), Santa Cruz Biotechnology) and anti-AMELX antibody (1/
200) (ab 59705, Abcam). After washes with 16 DPBS, Alexa Fluor
594 or 488 Goat Anti-Rabbit IgG antibodies (1/500) (Thermo
Fisher Scientific Inc) were applied for 1 h at room temperature
followed by DAPI nuclear staining (1/100,000) (Sigma-Aldrich
Co.). Sections were mounted using an aqueous mounting medium
(Fluoprep, Biomerieux, France).
Western Blot Analysis
To extract total proteins, mashed tissues from 1 and 15 week old
WT mice were homogenized in 300 mL of commercial protein
extraction reagent T-PER (Thermo Fisher Scientific Inc) which
contains the detergent bicine. Samples were sonicated and tissue
debris, removed by centrifugation for 5 min at 10,000 rpm.
Supernatants were recovered and protein concentration
determined using the BCA protein assay (Thermo Fisher Scientific Inc).
Samples were prepared for SDS PAGE by adding 100 mL of
Laemmli sample loading buffer containing 10% DTT (Bio-Rad
Laboratory). Samples were heated for 15 min at 70uC and stored
at 220uC. Samples were loaded at 10 mg protein per well on 12%
polyacrylamide gels (Prosieve 50, Lonza, Rockland, USA).
Extraction of proteins present in the fluid phase of the tissues
was carried out as previously described . Briefly, dissected
samples were immersed in 30 mL of TRIS solution (50 mM,
pH 7.4), crushed and centrifuged for 4 min to 20 000 g. Then, the
supernatant was recovered, diluted in 1:1 Laemmli buffer (v/v)
and stored at 280uC. Protein concentration in extracts was
determined using a ready-to-use assay compatible with samples
containing Laemmli SDS sample buffer, according to the
manufacturers instructions (Pierce 660 nm Assay, Thermo Fisher
Scientific Inc). Protein samples were thawed and heated 5 min at
100uC, before loading at 10 mg protein per wells on a 10%
Electrophoresis was carried out for 7590 min at 180200 V
prior to transfer onto PDVF membranes (Thermo Fisher Scientific
Inc) for 4560 min at 100 Volts. Membranes were stained to
check the efficiency of the electro-transfer (Novagen RedAlert,
EMD Millipore, Billerica, MA, USA). Membranes were blocked
overnight using 5% milk powder in 16 DPBS, then washed and
incubated 12 h with anti-AMBN antibody (1/400) (sc 50534
(M300), Santa Cruz Biotechnology) or anti-AMELX antibody (1/
400) (sc 32892 (FL-191), Santa Cruz Biotechnology - rabbit
polyclonal IgG to AMELX - Immunogen: amino acids 1191
representing full length AMELX isoform of human origin - Reacts
with mouse, rat and human). After rinsing, membranes were
incubated with a horseradish peroxidase (HRP)-conjugated goat
anti-rabbit IgG antibody (Sigma-Aldrich Co.) at 1/2,000 dilution.
Finally, immunocross-reactivity was visualized by
chemiluminescence using HRP substrate (Luminata Crescendo, Millipore Co.,
Billerica, USA) and a LAS 4000 Bioimager (ImageQuant,
Data are presented with values expressed as the mean 6
standard error of the mean (m 6 SEM). Statistical analyses of the
Q-PCR data were performed using Prism 5 statistical software
(GraphPad Software Inc., San Diego, CA). Non-parametric
twotailed tests were used to compare EMP expression; Mann-Whitney
(MW) test to investigate the EMP expression in each tissue
between different stages (1 week vs 15 week old mice) and
KruskalWallis (KW) test to compare the difference between tissues (EP,
ME, AB, BB) at one specific stage. The overall risk was fixed at p,
Figure 2. AMELX mRNA and protein distribution in mandible from 8 week old WT and AMELX KO mice. A-B. In situ hybridization was
performed using AMELX oligonucleotidic probes. B. WT mouse shows strong AMELX mRNA level in ameloblasts (Ambl) and odontoblasts (Odb).
AMELX mRNA is also detected in bone-lining cells (red arrows) and, with an apparent lower level, in dental follicle (DF) area. CD.
Immunohistodetection was performed using anti-AMELX antibody. D. AMELX protein expression in WT mouse was detected in ameloblasts,
odontoblasts and bone-lining cells (red arrows). Strong and diffuse protein signal was also observed in dental follicle. BD. No AMELX RNA and
protein signal is detected in striated muscle (Myo), a negative control tissue (see Table 1). AC. Neither AMELX mRNA nor protein expression is
detected in AMELX KO mice.
0.05 (*p,0.05, **p,0.01 ***p,0.001 (KW test), #p,0.05, #
#p,0.01 # # #p,0.001 (MW test), NS; not significant).
AMBN and AMELX RNA expression in mineralized and
Screening of AMBN and AMELX gene expression in murine
tissues revealed the expression of transcripts coding for matrix
enamel proteins in tooth, bone and soft tissues (Fig. 1 and Table 1).
Quantitative analysis of AMBN and AMELX mRNA expression
in the mandible tissues from 1 week old and 15 week old mice was
performed using RT-qPCR (Fig. 1). In mice of both age groups
AMBN and AMELX mRNA expression was not only detected in
dental epithelial cells (EP) and mesenchymal cells (ME) but also in
alveolar bone (AB) and basal bone (BB) (Fig. 1B-C). Overall,
AMBN and AMELX genes had similar expression patterns, with
RNA levels highly dependent on the investigated tissue and the
age of the donor mouse. In 1 week old WT mice, AMBN and
AMELX mRNA levels were significantly enhanced in EP when
compared to ME and BB (p,0.050.001), whereas mRNA levels
in EP and in AB remained comparable (NS). In 15 week old mice,
however, AMBN and AMELX mRNA levels were significantly
higher in EP when compared to the three other tissues
(p,0.050.001). Finally, when compared to 1 week old mice, AMBN and
AMELX mRNA expression was significantly decreased in 15 week
old mice in all investigated tissues (p,0.01), except in BB where
mRNA levels remained comparable (NS). A series of
nonmineralized extra-dental tissues from adult WT mice (15 week
old) were also screened for AMBN and AMELX mRNA
expression. Soft tissues were dissected and RNA was prepared
for RT-PCR. Dental epithelial cells (EP) harvested from incisors
were used as positive control tissue. RT-PCR results for these
tissues are listed in Table 1. It is important to note that, when
detected in soft tissues, AMBN and AMELX showed low mRNA
levels when compared to mandibular tissues. Among the 10 soft
tissues screened, only liver, lung and striated muscle showed no
AMBN and AMELX mRNA expression; identifying these tissues
as reliable negative controls for expression of these two EMP
genes. Based on these results, striated muscle tissue was
subsequently used as negative control tissue for both AMELX
and AMBN gene expression in in situ hybridization and
immunohistochemistry (IHC) performed on jaws.
Spatiotemporal localization of AMBN and AMELX
Based on RT-PCR data (Fig. 1 and Table 1) and a range of
primary antibody dilutions tested against sections of mandible
containing EMPs-expressing ameloblasts (positive control tissue),
jaw bones and striated muscle (negative biological control, as
evidenced by RT-PCR described above - Table 1), we determined
two dilution thresholds for detection of EMPs; one for enamel and
one for bone (Fig. S1). Additionally, to avoid non-specific staining
we used a triple negative-control system including: 1- sections of
dental epithelium where primary antibody was omitted (data not
shown); 2- mandibular sections from WT mice including striated
muscle, and, for anti-AMELX IHC, 3- mandibular sections from
AMELX KO mice. AMELX labelling in AMELX KO tissues and
in striated muscle (Myo) from WT mice was negative (Fig. 2, Fig. 3,
Fig. S1 and Fig. S2).
In contrast to the age-dependent differential gene expression
revealed by RT-qPCR (Fig. 1), the distribution pattern of AMELX
and AMBN was identical at all studied ages in the mandibular
tissues surrounding the incisor. In situ hybridization in mandible
confirmed AMELX mRNA expression in ameloblasts (positive
control tissue) and, with lower intensity, in the alveolar bone area;
where bone-lining cells showed AMELX mRNA expression
(Fig. 2B). AMELX protein was visualized using IHC and showed
similar but more diffuse localization pattern when compared to
AMELX mRNA distribution (Fig. 2D). No RNA nor protein
signal was observed in mandible tissues from 8 week old AMELX
KO mice (Fig. 2AC).
AMBN and AMELX protein distribution in manbibular
sections from 1 week old WT and AMELX KO mice was further
investigated using immunohistofluorescence staining (Fig. 3-Fig.
S2). Besides their presence within enamel matrix, both AMBN and
AMELX proteins were detected in mandibular bone-lining cells
(Fig. 3.AB, white arrows, Fig. 3C, inset blue box) and in dental
follicle (DF) (Fig. 3A-B-C, inset yellow box) in WT mice.
Additionally, a slight diffuse expression of AMELX was detected
in the periosteum (Po) and bone (Bo) areas (Fig. 3B and Fig. 3C).
Western blot detection of AMELX and AMBN in
To analyze the protein expression profile of EMPs in alveolar
(AB) and basal bones (BB), we first studied total protein extracts
from 15 week old mice using a detergent-based dissociative
extraction procedure. Dental epithelium cells (EP) were used as
positive controls. Extracted proteins were separated by SDS
PAGE (10 mg total protein per well) and transferred to membranes
for western blotting. Anti-AMBN immunostaining of the
westernblot (Fig. 4A) revealed the presence of the nascent AMBN
molecule at 67 kDa and a range of lower molecular weight
processing products in dissociative extracts of dental epithelial
cells. A similar molecular weight profile was reported for
ameloblastin extracted from rat incisor enamel organ . The
nascent 67 kDa ameloblastin was also detected in dissociative
extracts of alveolar bone (AB) with lower molecular weight
processing products. AMBN immuno-cross reactivity in basal
bone (BB) was much weaker and was near the limit of detection.
Anti-AMBN immunostaining of blots of non-dissociative extracts
of EP, AB and BB (Fig. 4B) revealed a spectrum of AMBN staining
similar to the dissociative extracts. Anti-AMELX immune staining
of dissociative extracts showed the presence of multiple AMELX
proteins migrating below 26 kDa in the enamel epithelium
(Fig. 4C). The additional lower weight AMELX components
may be AMELX processing products from enamel matrix present
as a contaminant in dental epithelium cell extracts. The identity of
the stained bands migrating at 43 kDa and above is unclear
though amelogenin is known to form high molecular weight
complexes that are stable during SDS PAGE [13,31]. Dissociative
extracts of AB and BB exhibited a single band of amelogenin
crossreactivity at 26 kDa though at levels near the limits of detection.
Anti-AMELX immune staining of non-dissociative extracts
showed the presence of AMELX as a doublet at around 26 kDa
in enamel epithelium while alveolar bone exhibited a single band
of amelogenin cross-reactivity at 26 kDa. A similar band was
present in the basal bone extract but at such low levels that it was
only visible after digitally enhancing the image (not shown).
To the best of our knowledge, no previous studies have
quantified gene expression levels and examined the solubility states
of endogenously expressed EMPs in mandible bones using dental
epithelial cells as a reference in rodents. Using a similar strategy,
we previously demonstrated the regulation of EMPs gene
expression by nuclear effectors in vivo in dental [11,27,32] and
bone tissues [33,34]. Pioneer reports described AMELX [12,35]
and AMBN  in non-enamel mineralized tissues. Here we
confirm and extend these data; dental epithelial cells and alveolar
bone displayed higher levels of AMELX and AMBN mRNA
expression when compared to basal bone and dental mesenchymal
cells from post-natal mice (one week old).
By screening AMELX and AMBN gene expression in 10 soft
tissues, we were able to identify the lung, liver and striated muscle
as negative control tissues for in situ studies. In line with previous
studies [15,16], repeated or occasional mRNA expression of
AMBN and AMELX was detected in several soft tissues but at low
level when compared to mandibular tissues. This occasional
expression could result from endogenous oscillations of EMPs gene
expression orchestrated by clock genes; both AMELX and
enamelin being downstream targets of the clock genes family
of transcription factors [37,38]. Alternatively, these variations may
result from the existence of EMPs-positive circulating cells (e.g.
macrophages, megakaryocytes and some hematopoietic stem cells
Our RT-qPCR and western-blot results show that AMELX and
AMBN expression is higher in alveolar bone compared to basal
bone. This finding raises the possibility that AMELX and AMBN
are locally associated with high bone turn-over; this being a classic
characteristic of alveolar bone . Indeed, AMELX  and
AMBN  have been shown to impact on osteoblast and
osteoclast differentiation and activity. Consistently and in line with
previous studies , aging was associated with decreased gene
expression of EMPs in jaw bones. Recently, increased expression
of AMBN was also shown to be associated with mechanical bone
stimulation  and healing . Interestingly, MSX2, a
regulator of bone homeostasis  represses AMELX
transcription by targeting the AMELX promoter  and both AMELX
and AMBN inhibit Msx2 expression [44,45]. EMPs-Msx2
reciprocal cross-talk may thus play a part in localized jaw
modeling as supported by Msx2 and AMBN null-mutants
[34,44] and our preliminary biomechanical assay (Fig. S3).
Indeed, 72 h after altering occlusion, we observed a significant
increase in AMBN and AMELX mRNA expression associated
with decrease in Msx2 gene expression.
Protein occupies 2030% of the secretory stage enamel matrix
by volume and of this protein .90% is derived from the AMELX
gene whereas AMBN is present in the matrix at far lower
concentrations. AMELX is highly aggregative under physiological
conditions and acts as a structural scaffold during enamel
development. Nascent AMBN is solubilized in the developing
enamel matrix and is a mobile species. In contrast, AMBN
processing products are more aggregative . Identifying
solubilized and diffusible proteins in tissues using histology-based
techniques may be compromised by the fact that otherwise soluble
factors become immobilized by chemical fixation. Here AMELX
and AMBN proteins were detectable in the microenvironment
surrounding EMPs-producing (RNA-positive) cells. In addition,
high protein levels detected in the dental follicle suggest that dental
and bone cells may secrete these peptides toward this adjacent
tissue. If EMPs function in some capacity as signaling molecules
 then endogenously secreted EMPs would require a certain
degree of solubility in the extracellular environment in order for
them to diffuse and interact with nearby cells (paracrine signaling)
or receptors on the secreting cell itself (autocrine signaling) (Fig. 5).
To test this hypothesis, we extracted dental epithelial cells (EP),
alveolar bone and basal bone under dissociative and
nondissociative conditions and subjected equal amounts of total
protein extracted to western blot probing for AMBN and
AMELX. We provide evidence that AMBN, particularly the
nascent 67 kDa molecule, is readily detectable on blots of
dissociative extracts of alveolar bone; and it is present in these
extracts at a similar ratio relative to other components as found in
EP. The ratio of freely soluble 67 kDa AMBN in the
nondissociative extracts is also comparable to the ratio found in EP; i.e.
to some degree, AMBN is present in alveolar bone in a solubilized
state; or at least it is not tightly associated with the bone matrix.
AMELX was also detectable in blots of non-dissociative extracts of
alveolar bone but it comprised a far smaller ratio of the
dissociatively extracted proteins when compared to EP. Likewise,
AMELX was detectable in non-dissociative extracts of alveolar
bone but again at ratios far lower than the ratio of AMELX in EP.
We were unable to detect EMPs in serum (data not shown),
which suggests that, contrary to osteocalcin that behaves as
hormonal signal in general metabolism , EMPs might not play
a role in long-range endocrine signaling. Some matrix ligands,
omnipresent in mesenchymal extracellular compartments, could
perhaps entrap these secreted EMPs and limit their diffusion.
Indeed, fibronectin , heparan sulfate  and biglycan 
bind AMELX or AMBN in vitro. However, the inability to detect
EMPs in serum may simply be due to dilution of the bone-derived
EMP peptides once they enter the blood stream.
Autocrine and paracrine activities of growth factors control local
bone modeling and remodeling. Solubilized EMPs may behave in
the same way (Fig. 5). Indeed, distinct dysmorphologies
characterize bones in which AMELX [16,49] or AMBN [24,50]
expression are genetically modified, even though calcium and
phosphorus content and the degree of mineralization are normal
[51,52]. Disturbed EMP expression patterns are associated with
alveolar and jaw bone malformations [33,34]. These findings and
the present data suggest that solubilized EMPs may play a role in
controlling cell fate through short-distance paracrine and
autocrine functions and thus control local bone morphology via signals
previously evidenced for AMBN [19,53,54] and AMELX [55,56].
We thus propose a model in which EMPs play a role in
mandibular physiology (Fig. 5).
Based on our findings on the solubilized state and the
localization of AMELX and AMBN in mandibular bones, we
hypothesize that these EMPs play a dual role: one as components
of a structural extracellular matrix in developing enamel and
second as growth factor-like molecules in bones responsible for
supporting the teeth in the mandible.
Figure S1 Optimization of AMBN and AMELX
immunodetection in 1 week old WT and AMELX KO mice. A-B.
When using a classic dilution of antibody (1/1,000), cross
reactivity to AMELX and AMBN proteins is restricted to the
dental epithelial cells (EP) (positive control tissue) in WT mouse, a
pattern well documented in the literature. Dilutions of 1/500 show
the presence of AMBN and AMELX protein expression in EP, but
also in some bone stromal cells. Increased concentration of
antibody (1/100 for AMBN and 1/50 for AMELX) results in
stronger staining in osteoblasts, in osteocytes and in mandibular
bone matrix. At dilution 1/100 (AMBN) or 1/50 (AMELX),
striated muscle (negative control tissue) shows diffuse staining
indicating the limit of specificity for these antibodies. At dilution
1/10, the striated muscle was clearly stained for both AMBN and
AMELX, determining the non-specificity threshold. This
threshold was confirmed using AMELX KO mouse as a bona fide control,
non-specific cross reactivity signal being observed when using 1/
10 antibody dilution. For detailed immunohistoperoxidase
methods see File S1.
Figure S2 AMELX protein expression in 1 week old WT
and AMELX KO mice. A. When using 1/500 antibody
dilution, AMELX (green staining) is not detected in AMELX
KO mice. B. Using the same antibody dilution, WT section show
anti-AMELX cross reactivity in enamel (En) and ameloblasts
Figure S3 Impact of mechanical stimuli on AMBN and
AMELX mRNA expression in jaw. A. Right maxillary molar
cusps in 15 week old WT mice were ground flat to the level
indicated by the red dotted line. B. 72 h after grinding, significant
increase in mRNA levels of AMELX (x7) and AMBN (x3) is
observed in treated mandible alveolar bone. Concurrently, a
significant reduction in MSX2 mRNA level (x3) is observed;
MSX2 being a transcriptional repressor of AMELX gene. mRNA
expression of those genes are unaffected in basal bone (data not
shown). mRNA levels of genes of interest are normalized against
mRNA expression of the housekeeping gene RS15
probability was determined using KW test (**p,0.05). For
detailed molar grinding and flattening procedures see File S1.
The authors thank Yong Li from the Department of Anatomy and Cell
Biology of the University of Pennsylvania, for assistance with the AMEL
KO mice and Dr. Didier Ferbus of the INSERM UMRS 1138 (Team
Berdal), for assistance with western blot analysis. This study represents part
of the dissertation of Jaime Jacquess PhD degree at Paris Diderot
University Paris 7, France.
Conceived and designed the experiments: JJ DH MDM SJB AB JI.
Performed the experiments: JJ DH MDM SP AA JI. Analyzed the data: JJ
DH SP ABK CWG SJB AB JI. Contributed reagents/materials/analysis
tools: ABK CWG. Wrote the paper: JJ SJB AB JI.
Figure 5. Proposed model for EMPs expression and signaling in extra-dental tissues. This model is based on the presently described RNA and protein patterns and published data. EMP-based autocrine and paracrine cell-cell communications would participate to distinct physiopathological events . They may play a role during tooth growth and alveolar bone modeling processes . In adults, EMPs would be involved in alveolar bone responses to mechanical stimuli . Supramolecular structures generated by self-assembly of EMPs might also intervene in these processes . Bibliographical references . 1. Haze , 2009 ; Tamburstuen, 2011 , 2. Zeichner-David , 2006 ; Fukae, 2006 ; Matsuzawa, 2009 ; Tambursten, 2010 ; Kakegawa, 2010 ; Zhang, 2011 ; Kitagawa, 2011 ; Kunimatsu, 2011 ; Izumikawa, 2012 ; Amin, 2013 , 3. Spahr , 2006 ; Haze, 2009 ; Iizuka, 2011 ; Tamburstuen, 2011 ; Atsawasuwan, 2013 , 4. Matsuzawa , 2009 ; Iizuka, 2011 ; Tamburstuen, 2010 ; Amin, 2013 ; Atsawasuwan, 2013 , 5. Haze , 2009 ; Tamburstuen, 2011 , 6. Tamburstuen , 2010 ; Lu, 2013 , 7. Haze , 2009 [8,20], 8. Saito , 2008 ; Wang, 2005 , 9. Wang , 2005 , 10. Beleyer, 2010 , 11. Fincham, 1994 , Wald, 2013 .
1. Staines KA , Macrae VE , Farquharson C ( 2012 ) The importance of the SIBLING family of proteins on skeletal mineralisation and bone remodelling . J Endocrinol 214 : 241 - 255 .
2. George A , Veis A ( 2008 ) Phosphorylated proteins and control over apatite nucleation, crystal growth, and inhibition . Chem Rev 108 : 4670 - 4693 .
3. Kawasaki K , Buchanan AV , Weiss KM ( 2009 ) Biomineralization in humans: making the hard choices in life . Annu Rev Genet 43 : 119 - 142 .
4. Iwata T , Yamakoshi Y , Hu JC , Ishikawa I , Bartlett JD , et al. ( 2007 ) Processing of ameloblastin by MMP-20 . J Dent Res 86 : 153 - 157 .
5. Bartlett JD ( 2013 ) Dental Enamel Development: Proteinases and Their Enamel Matrix Substrates . ISRN Dent 2013 : 684607 .
6. Bosshardt DD ( 2008 ) Biological mediators and periodontal regeneration: a review of enamel matrix proteins at the cellular and molecular levels . J Clin Periodontol 35 : 87 - 105 .
7. Hatakeyama J , Philp D , Hatakeyama Y , Haruyama N , Shum L , et al. ( 2006 ) Amelogenin-mediated regulation of osteoclastogenesis, and periodontal cell proliferation and migration . J Dent Res 85 : 144 - 149 .
8. Haze A , Taylor AL , Haegewald S , Leiser Y , Shay B , et al. ( 2009 ) Regeneration of bone and periodontal ligament induced by recombinant amelogenin after periodontitis . J Cell Mol Med 13 : 1110 - 1124 .
9. Nunez J , Sanz M , Hoz-Rodriguez L , Zeichner-David M , Arzate H ( 2010 ) Human cementoblasts express enamel-associated molecules in vitro and in vivo . J Periodontal Res 45 : 809 - 814 .
10. Tamburstuen MV , Reseland JE , Spahr A , Brookes SJ , Kvalheim G , et al. ( 2011 ) Ameloblastin expression and putative autoregulation in mesenchymal cells suggest a role in early bone formation and repair . Bone 48 : 406 - 413 .
11. Papagerakis P , Macdougall M , Hotton D , Bailleul-Forestier I , Oboeuf M , et al. ( 2003 ) Expression of amelogenin in odontoblasts . Bone 32 : 228 - 240 .
12. Veis A , Tompkins K , Alvares K , Wei K , Wang L , et al. ( 2000 ) Specific amelogenin gene splice products have signaling effects on cells in culture and in implants in vivo . J Biol Chem 275 : 41263 - 41272 .
13. Landin MA , Shabestari M , Babaie E , Reseland JE , Osmundsen H ( 2012 ) Gene Expression Profiling during Murine Tooth Development . Front Genet 3 : 139 .
14. Deutsch D , Haze-Filderman A , Blumenfeld A , Dafni L , Leiser Y , et al. ( 2006 ) Amelogenin, a major structural protein in mineralizing enamel, is also expressed in soft tissues: brain and cells of the hematopoietic system . Eur J Oral Sci 114 Suppl 1 : 183 - 189 ; discussion 201 - 182 , 381 .
15. Gruenbaum-Cohen Y , Tucker AS , Haze A , Shilo D , Taylor AL , et al. ( 2009 ) Amelogenin in cranio-facial development: the tooth as a model to study the role of amelogenin during embryogenesis . J Exp Zool B Mol Dev Evol 312B : 445 - 457 .
16. Li Y , Yuan ZA , Aragon MA , Kulkarni AB , Gibson CW ( 2006 ) Comparison of body weight and gene expression in amelogenin null and wild-type mice . Eur J Oral Sci 114 Suppl 1 : 190 - 193 ; discussion 201 - 192 , 381 .
17. Lyngstadaas SP , Wohlfahrt JC , Brookes SJ , Paine ML , Snead ML , et al. ( 2009 ) Enamel matrix proteins; old molecules for new applications . Orthod Craniofac Res 12 : 243 - 253 .
18. Iizuka S , Kudo Y , Yoshida M , Tsunematsu T , Yoshiko Y , et al. ( 2011 ) Ameloblastin regulates osteogenic differentiation by inhibiting Src kinase via cross talk between integrin beta1 and CD63 . Mol Cell Biol 31 : 783 - 792 .
19. Lu X , Ito Y , Atsawasuwan P , Dangaria S , Yan X , et al. ( 2013 ) Ameloblastin modulates osteoclastogenesis through the integrin /ERK pathway. Bone 54 : 157 - 168 .
20. Saito K , Konishi I , Nishiguchi M , Hoshino T , Fujiwara T ( 2008 ) Amelogenin binds to both heparan sulfate and bone morphogenetic protein 2 and pharmacologically suppresses the effect of noggin . Bone 43 : 371 - 376 .
21. Warotayanont R , Zhu D , Snead ML , Zhou Y ( 2008 ) Leucine-rich amelogenin peptide induces osteogenesis in mouse embryonic stem cells . Biochem Biophys Res Commun 367 : 1 - 6 .
22. Stahl J , Nakano Y , Kim SO , Gibson CW , Le T , et al. ( 2013 ) Leucine rich amelogenin peptide alters ameloblast differentiation in vivo . Matrix Biol 32 : 432 - 442 .
23. Grandin HM , Gemperli AC , Dard M ( 2012 ) Enamel matrix derivative: a review of cellular effects in vitro and a model of molecular arrangement and functioning . Tissue Eng Part B Rev 18 : 181 - 202 .
24. Atsawasuwan P , Lu X , Ito Y , Zhang Y , Evans CA , et al. ( 2013 ) Ameloblastin inhibits cranial suture closure by modulating MSX2 expression and proliferation . PLoS One 8 : e52800 .
25. Hatakeyama J , Sreenath T , Hatakeyama Y , Thyagarajan T , Shum L , et al. ( 2003 ) The receptor activator of nuclear factor-kappa B ligand-mediated osteoclastogenic pathway is elevated in amelogenin-null mice . J Biol Chem 278 : 35743 - 35748 .
26. Gibson CW , Yuan ZA , Hall B , Longenecker G , Chen E , et al. ( 2001 ) Amelogenin-deficient mice display an amelogenesis imperfecta phenotype . J Biol Chem 276 : 31871 - 31875 .
27. Berdal A , Hotton D , Pike JW , Mathieu H , Dupret JM ( 1993 ) Cell- and stagespecific expression of vitamin D receptor and calbindin genes in rat incisor: regulation by 1,25-dihydroxyvitamin D3 . Dev Biol 155 : 172 - 179 .
28. Nielsen Bs , Jorgensen S , Fog JU , Sokilde R , Christensen IJ , et al. ( 2011 ) High levels of microRNA-21 in the stroma of colorectal cancers predict short diseasefree survival in stage II colon cancer patients . Clin Exp Metastasis 28 : 27 - 38 .
29. Brookes SJ , Kirkham J , Shore RC , Wood SR , Slaby I , et al. ( 2001 ) Amelin extracellular processing and aggregation during rat incisor amelogenesis . Arch Oral Biol 46 : 201 - 208 .
30. Uchida T , Murakami C , Dohi N , Wakida K , Satoda T , et al. ( 1997 ) Synthesis, secretion, degradation, and fate of ameloblastin during the matrix formation stage of the rat incisor as shown by immunocytochemistry and immunochemistry using region-specific antibodies . J Histochem Cytochem 45 : 1329 - 1340 .
31. Limeback H , Simic A ( 1989 ) Porcine high molecular weight enamel proteins are primarily stable amelogenin aggregates and serum albumin-derived proteins . In: ed. FR, editor. Tooth Enamel V. Yokohama , Japan: Florence Publishers. pp. 269 - 273 .
32. Jedeon K , De La Dure-Molla M , Brookes SJ , Loiodice S , Marciano C , et al. ( 2013 ) Enamel defects reflect perinatal exposure to bisphenol A . Am J Pathol 183 : 108 - 118 .
33. Aioub M , Lezot F , Molla M , Castaneda B , Robert B , et al. ( 2007 ) Msx2 -/- transgenic mice develop compound amelogenesis imperfecta, dentinogenesis imperfecta and periodental osteopetrosis . Bone 41 : 851 - 859 .
34. Molla M , Descroix V , Aioub M , Simon S , Castaneda B , et al. ( 2010 ) Enamel protein regulation and dental and periodontal physiopathology in MSX2 mutant mice . Am J Pathol 177 : 2516 - 2526 .
35. Haze A , Taylor AL , Blumenfeld A , Rosenfeld E , Leiser Y , et al. ( 2007 ) Amelogenin expression in long bone and cartilage cells and in bone marrow progenitor cells . Anat Rec (Hoboken) 290 : 455 - 460 .
36. Spahr A , Lyngstadaas SP , Slaby I , Pezeshki G ( 2006 ) Ameloblastin expression during craniofacial bone formation in rats . Eur J Oral Sci 114 : 504 - 511 .
37. Athanassiou-Papaefthymiou M , Kim D , Harbron L , Papagerakis S , Schnell S , et al. Molecular and circadian controls of ameloblasts . Eur J Oral Sci 119 Suppl 1 : 35 - 40 .
38. Zheng L , Seon YJ , Mourao MA , Schnell S , Kim D , et al. Circadian rhythms regulate amelogenesis . Bone 55 : 158 - 165 .
39. Sodek J , McKee MD ( 2000 ) Molecular and cellular biology of alveolar bone . Periodontol 2000 24: 99 - 126 .
40. Atsawasuwan P , Lu X , Ito Y , Chen Y , Gopinathan G , et al. ( 2013 ) Expression and function of enamel-related gene products in calvarial development . J Dent Res 92 : 622 - 628 .
41. Alikhani M , Khoo E , Alyami B , Raptis M , Salgueiro JM , et al. ( 2012 ) Osteogenic effect of high-frequency acceleration on alveolar bone . J Dent Res 91 : 413 - 419 .
42. Tamburstuen MV , Reppe S , Spahr A , Sabetrasekh R , Kvalheim G , et al. ( 2010 ) Ameloblastin promotes bone growth by enhancing proliferation of progenitor cells and by stimulating immunoregulators . Eur J Oral Sci 118 : 451 - 459 .
43. Zhou YL , Snead ML ( 2000 ) Identification of CCAAT/enhancer-binding protein alpha as a transactivator of the mouse amelogenin gene . J Biol Chem 275 : 12273 - 12280 .
44. Fukumoto S , Kiba T , Hall B , Iehara N , Nakamura T , et al. ( 2004 ) Ameloblastin is a cell adhesion molecule required for maintaining the differentiation state of ameloblasts . J Cell Biol 167 : 973 - 983 .
45. Sonoda A , Iwamoto T , Nakamura T , Fukumoto E , Yoshizaki K , et al. ( 2009 ) Critical role of heparin binding domains of ameloblastin for dental epithelium cell adhesion and ameloblastoma proliferation . J Biol Chem 284 : 27176 - 27184 .
46. Rached MT , Kode A , Silva BC , Jung DY , Gray S , et al. ( 2010 ) FoxO1 expression in osteoblasts regulates glucose homeostasis through regulation of osteocalcin in mice . J Clin Invest 120 : 357 - 368 .
47. Beyeler M , Schild C , Lutz R , Chiquet M , Trueb B ( 2010 ) Identification of a fibronectin interaction site in the extracellular matrix protein ameloblastin . Exp Cell Res 316 : 1202 - 1212 .
48. Wang H , Tannukit S , Zhu D , Snead ML , Paine ML ( 2005 ) Enamel matrix protein interactions . J Bone Miner Res 20 : 1032 - 1040 .
49. Coxon TL , Brook AH , Barron MJ , Smith RN ( 2012 ) Phenotype-genotype correlations in mouse models of amelogenesis imperfecta caused by Amelx and Enam mutations . Cells Tissues Organs 196 : 420 - 430 .
50. Wazen RM , Moffatt P , Zalzal SF , Yamada Y , Nanci A ( 2009 ) A mouse model expressing a truncated form of ameloblastin exhibits dental and junctional epithelium defects . Matrix Biol 28 : 292 - 303 .
51. Kuroda S , Wazen R , Sellin K , Tanaka E , Moffatt P , et al. ( 2011 ) Ameloblastin is not implicated in bone remodelling and repair . Eur Cell Mater 22 : 56 - 66 ; discussion 66 - 57 .
52. Smith CE , Wazen R , Hu Y , Zalzal SF , Nanci A , et al. ( 2009 ) Consequences for enamel development and mineralization resulting from loss of function of ameloblastin or enamelin . Eur J Oral Sci 117 : 485 - 497 .
53. Kitagawa M , Kitagawa S , Nagasaki A , Miyauchi M , Uchida T , et al. ( 2011 ) Synthetic ameloblastin peptide stimulates differentiation of human periodontal ligament cells . Arch Oral Biol 56 : 374 - 379 .
54. Zhang Y , Zhang X , Lu X , Atsawasuwan P , Luan X ( 2011 ) Ameloblastin regulates cell attachment and proliferation through RhoA and p27 . Eur J Oral Sci 119 Suppl 1 : 280 - 285 .
55. Huang YC , Tanimoto K , Tanne Y , Kamiya T , Kunimatsu R , et al. ( 2010 ) Effects of human full-length amelogenin on the proliferation of human mesenchymal stem cells derived from bone marrow . Cell Tissue Res 342 : 205 - 212 .
56. Wen X , Cawthorn WP , MacDougald OA , Stupp SI , Snead ML , et al. ( 2011 ) The influence of Leucine-rich amelogenin peptide on MSC fate by inducing Wnt10b expression . Biomaterials 32 : 6478 - 6486 .
57. Zeichner-David M , Chen LS , Hsu Z , Reyna J , Caton J , et al. ( 2006 ) Amelogenin and ameloblastin show growth-factor like activity in periodontal ligament cells . Eur J Oral Sci 114 Suppl 1 : 244 - 253 ; discussion 254 - 246 , 381 - 242 .
58. Fukae M , Kanazashi M , Nagano T , Tanabe T , Oida S , et al. ( 2006 ) Porcine sheath proteins show periodontal ligament regeneration activity . Eur J Oral Sci 114 Suppl 1 : 212 - 218 ; discussion 254 - 216 , 381 - 212 .
59. Matsuzawa M , Sheu TJ , Lee YJ , Chen M , Li TF , et al. ( 2009 ) Putative signaling action of amelogenin utilizes the Wnt/beta-catenin pathway . J Periodontal Res 44 : 289 - 296 .
60. Kakegawa A , Oida S , Gomi K , Nagano T , Yamakoshi Y , et al. ( 2010 ) Cytodifferentiation activity of synthetic human enamel sheath protein peptides . J Periodontal Res 45 : 643 - 649 .
61. Kunimatsu R , Tanimoto K , Tanne Y , Kamiya T , Ohkuma S , et al. ( 2011 ) Amelogenin enhances the proliferation of cementoblast lineage cells . J Periodontol 82 : 1632 - 1638 .
62. Izumikawa M , Hayashi K , Polan MA , Tang J , Saito T ( 2012 ) Effects of amelogenin on proliferation, differentiation, and mineralization of rat bone marrow mesenchymal stem cells in vitro . ScientificWorldJournal 2012 : 879731 .
63. Amin HD , Olsen I , Knowles JC , Dard M , Donos N ( 2013 ) Effects of enamel matrix proteins on multi-lineage differentiation of periodontal ligament cells in vitro . Acta Biomater 9 : 4796 - 4805 .
64. Amin HD , Olsen I , Knowles JC , Donos N ( 2012 ) Differential effect of amelogenin peptides on osteogenic differentiation in vitro: identification of possible new drugs for bone repair and regeneration . Tissue Eng Part A 18 : 1193 - 1202 .
65. Fincham AG , Moradian-Oldak J , Simmer JP , Sarte P , Lau EC , et al. ( 1994 ) Selfassembly of a recombinant amelogenin protein generates supramolecular structures . J Struct Biol 112 : 103 - 109 .
66. Wald T , Osickova A , Sulc M , Benada O , Semeradtova A , et al. ( 2013 ) Intrinsically disordered enamel matrix protein ameloblastin forms ribbon-like supramolecular structures via an N-terminal segment encoded by exon 5 . J Biol Chem 288 : 22333 - 22345 .
67. Bartlett Jd , Skobe Z , Lee Dh , Wright Jt , Li Y , et al. ( 2006 ) A developmental comparison of matrix metalloproteinase-20 and amelogenin null mouse enamel . Eur J Oral Sci 114 Suppl 1 : 18 - 23 ; discussion 39 - 41 , 379 .
68. Simmer JP , Hu CC , Lau EC , Sarte P , Slavkin HC , et al. ( 1994 ) Alternative splicing of the mouse amelogenin primary RNA transcript . Calcif Tissue Int 55 : 302 - 310 .
69. Li W , Mathews C , Gao C , DenBesten PK ( 1998 ) Identification of two additional exons at the 39 end of the amelogenin gene . Arch Oral Biol 43 : 497 - 504 .
70. Lau EC , Simmer JP , Bringas P , Jr., Hsu DD , Hu CC , et al. ( 1992 ) Alternative splicing of the mouse amelogenin primary RNA transcript contributes to amelogenin heterogeneity . Biochem Biophys Res Commun 188 : 1253 - 1260 .