Introduction

The extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK-MAPK) pathway is a central regulator of tooth development. This cascade is typically initiated by the binding of a growth factor to a receptor tyrosine kinase (RTK), which triggers the phosphorylation of successive kinases and culminates in activation of effector kinases and the transcription of target genes (Fig. 1)1. The MAPK signalling pathway has been intensively studied by cancer biologists because of its effects on regulation of cell proliferation and survival1,2, but this pathway is also important throughout mouse embryogenesis3. The pathway has been investigated in numerous embryonic processes, including development of the central nervous system and mesodermal derivatives4,5, skeletal development6 and tooth development7,8,9,10,11.

Figure 1
figure 1

The FGF-activated ERK-MAPK cascade.

The phosphorylation cascade is activated by the fixation of a growth factor to its receptor tyrosine kinase and results in the activation of effector kinases and in the transcriptional activation of target genes. Molecular actors of this pathway that are focused on here are depicted in yellow (SPRY) and orange (RSK2).

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Tooth development is a well-documented example of ectodermal organ development. It is a tightly regulated process arising from the crosstalk between dental epithelium and its underlying mesenchyme12. The signalling networks responsible for properly building the dentition have been heavily investigated and numerous members of the ERK-MAPK signalling pathway are known to play a role in tooth development. Early studies examined the fibroblast growth factors (FGFs) that activate their receptors (FGFRs), thus triggering the ERK-MAPK phosphorylation cascade13,14. Subsequent investigations determined that additional components of the cascade were involved in tooth development8,15,16,17. An exciting current challenge is to understand the complexity of feedback regulation in this signalling pathway, which can be stage- and/or tissue-specific. In the present study, we compare the phenotype of molar teeth in mice carrying mutations in Sprouty1, Sprouty2, Sprouty4 and Rsk2 genes, which are involved at various levels in the MAPK cascade (Fig. 1).

The Sprouty (Spry) family of genes encodes general RTK inhibitors18,19. After stimulation by growth factors, the Sprouty proteins have been proposed to function by translocating to the plasma membrane, where their phosphorylation prevents the formation of an FGFR adaptor complex20; however, the biochemistry of the Sprouty proteins is still the subject of much debate. Spry1 is expressed in both the epithelium and the mesenchyme, with the exception of a cluster of non-proliferating epithelial cells that serve as a signalling centre called the enamel knot. Spry2 is expressed only in the epithelium adjacent to the dental mesenchyme, including the enamel knot and Spry4 is expressed in the dental mesenchyme7,21. Whereas the morphogenesis of molar teeth in Spry1−/− mice has not yet been examined, Spry2−/− and Spry4−/− mice are known to have abnormal dentition, which sometimes includes supernumerary teeth (ST) located immediately in front of the first lower molar7. These supernumerary teeth, which occur at differing frequencies depending on the genetic background7,22,23, are believed to derive from evolutionary vestigial tooth buds that normally undergo apoptosis in wild-type embryos24,25,26. Lagronova-Churava and colleagues (2013) showed that although all Spry2−/− and Spry4−/− embryos present a revitalisation of tooth rudiments at ED13.5, only 2% of Spry4−/− and 27% of Spry2−/− specimens had a lower ST. However, the role of Spry1, Spry2 and Spry4 in the development of upper molars is not known and the adult molar morphology has not been scrutinised in these mutants.

RSKs (90 kDa ribosomal S6 kinases) are effector kinases belonging to the eponymous family of highly conserved serine/threonine kinases27,28. Out of the four isoforms found in vertebrates, Rsk2 has been recently demonstrated to be involved in craniofacial development. Indeed Rsk2−/Y mice display a deformation of the nasal bone as well as diastemal ST which affect mesial parts of both upper and lower first molars8. Mutations in RSK2 have been associated with Coffin-Lowry syndrome (OMIM #303600), a condition characterized by mental and growth retardation along with craniofacial and other skeletal abnormalities29,30,31.

Phenotypic convergence across the ERK-MAPK signalling pathway remains poorly documented. By studying the dental phenotype resulting from mutations in genes located upstream (Sprouty) and downstream (Rsk2) of the ERK-MAPK cascade, we address the question of whether an ERK-MAPK signature phenotype exists. To answer this question, we have characterized the molar phenotype in several mutant populations in order to evaluate the distribution of the various changes affecting both upper and lower molar rows. We have also precisely quantified the occurrence of supernumerary teeth (ST) as well as their impact on the other teeth of the row. Finally, we have addressed the evolutionary role of the ERK-MAPK cascade by comparing specific dental traits of mutants with dental traits of other extant or extinct rodents.

Results

The mouse (Mus musculus) is a muroid rodent and like all the members of this superfamily, mice have a simplified dentition composed only of incisors and molars separated by a long toothless gap called a diastema. Three lower (M1 M2 M3) and three upper (M1 M2 M3) molar teeth are present in each mouth quadrant. The crowns of molar teeth bear a relatively stable number of cusps that are: 8 for M1, 6 for M2, 4 for M3, 7 for M1, 5 for M2 and 3 for M3 (Fig. 2, first column; Fig. 3a). Crowns of upper molars are made of rows of 3 cusps arranged in linguo-vestibular chevrons pointing mesially (except the third one, which is incomplete), whereas crowns of lower molars are made of rows of 2 cusps linked by linguo-vestibular and rather straight crests called lophs (Fig. 2, first column; Fig. 3a). The two mesial lophs of the M1 are also linked together by a mesio-distal connection.

Figure 2
figure 2

Dental character matrix.

(a) Upper dental row character matrix (c2–10); (b) Lower dental row character matrix (c11–17). On the first column are displayed WT upper (top) and lower (bottom) molar rows, with coloured arrowheads pointing to the localization of the most frequent defects seen in the 4 mutant backgrounds. Each character is listed with the occurrence frequency in all the mutant mice observed. (c1) is the occurrence of a ST; (c2–4) are M1 1st chevron defects, respectively lingual cusp disconnection, straight mesial cusp and absence of the vestibular cusp; (c5) is the presence of an extra lingual cusp between the 1st and 2nd chevrons; (c6) is the disconnection of the lingual cusp of the M1 2nd chevron; (c7) is the occurrence of a lingual crest; (c8) is the occurrence of an extra distal cusp; (c9) is the disconnection of the mesio-lingual cusp of the M2; (c10) is the connection of distal cusps. (c11–17) are defects of the lower mutant postcanine teeth. (c11) is the occurrence of a ST; c(12) is the absence of the mesio-vestibular cusp; (c13) is the display of very symmetric mesial-most cusps; (c14) is the split of the mesio-vestibular cusp; (c15) is the occurrence of an extra mesial cusp; (c16) is the strong disconnection of the mesial cusps of the M1; (c17) is the abnormal connection of the 2nd and 3rd lophs of the M2.

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Figure 3
figure 3

Abnormal phenotypes in the Spry1−/−, Spry2−/−, Spry4−/− and Rsk2−/Y mutant mice.

(a) WT dental rows; (b) Spry1−/− dental rows; (c) Spry2−/−dental rows; (d) Spry4−/− dental rows; (e) Rsk2−/Y dental rows. Coloured arrowheads correspond to the features displayed in the Fig. 2. Note that the dental rows are oriented similarly to what is displayed in Fig. 1. Scale bar: 0.75 mm.

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Diversity of the dental phenotypes in Sprouty mutants

We examined the arrangement and shape of the postcanine dentition in four populations comprising 25 Spry1−/−, 50 Spry2−/−, 50 Spry4−/− and 60 WT mice. Area measurements showed that the occlusal surface of molars in Spry1−/− and Spry4−/− mice is larger than in the WT mice, whereas the molar occlusal surface in Spry2−/− mice is smaller than in the WT mice (t test, p value < 0.05, Fig. 4a).

Figure 4
figure 4

Molar tooth proportions in the Spry1−/−, Spry2−/−, Spry4−/− and Rsk2−/Y mutant mice.

(a) Tooth surface (mm2); (b) tooth mesio-distal length (mm); (c) tooth vestibulo-lingual width (mm). First molars are coloured in green, second molars in pink and third molars in blue. Filled forms represent the WT for each background, squares represent the Spry1−/− mutants, Spry2−/− mutants without (blanked) or with (dot) ST, Spry4−/− mutants without (blanked) or with (dot) ST and Rsk2−/Y mutants without (blanked) or with (dot) ST. Significant differences in the tooth proportions are marked with an asterisk (t-test, p-value < 0.05).

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The molar teeth of Spry1−/−and WT mice are globally similar in shape and 51% of the Spry1−/− dental rows display a WT-like phenotype. The changes are numbered as character# (c#) from the mesial to the distal part of the row and are mildly to moderately represented in the mutant cohorts. The main defects of the Spry1−/− postcanine dentition are: (c8) the occurrence of a supplementary distal cusp on the M1 (53%); (c9) the disconnection of the mesio-lingual cusp from the first chevron of the M2 (40%); and (c12) the absence of the mesio-vestibular cusp of the M1 (8%) (Figs 2 and 3b, Suppl. Fig. 1a). The postcanine dentition in Spry2−/− mice had stronger differences compared to WT samples and only 21% of Spry2−/− dental rows still display a WT-like phenotype. The main changes of the Spry2−/− postcanine dentition are: (c7) the connection between the two lingual cusps of the M1 (38%); (c8) the occurrence of a supplementary distal cusp on the M1 (46%); (c10) the connection between the two lingual cusps of the M2 (36%); (c11) the occurrence of a lower ST (27%); and (c13-14) an abnormal shape, number, and/or interconnection of the mesial cusps of the M1 (29%) (Figs 2 and 3c). Spry4−/− molar tooth phenotype is the most variable among the 3 Sprouty mutants. These teeth ranged from a WT-like phenotype (24%) to relatively severe anomalies, especially in the M1. The main defects of the Spry4−/− postcanine dentition are: (c1) the occurrence of an upper ST (17%); (c2–3–4) the presence of lingual cusp disconnection, straight mesial cusp and absence of the vestibular cusp affecting the first chevron of the M1 (66% combined); (c5) the occurrence of a supplementary lingual cusp between the first and the second chevron (14%); (c6) the disconnection of the lingual cusp from the second chevron of the M1 (20%); (c8) the occurrence of a supplementary distal cusp on the M1 (36%); (c9) the disconnection of the mesio-lingual cusp from the first chevron of the M2 (64%); (c11) the occurrence of a lower ST (3%); (c12–14) an abnormal number and/or interconnection of the mesial cusps of the M1 (16%) (Figs 2 and 3d).

Except for three characters, the dentition of Spry1−/− mice thus resembles that of WT mice, whereas Spry2−/− and Spry4−/− mice show extra cusps and crest disconnections, as well as severe reductions and defects in the mesial parts of the M1 (Spry4−/− only) and in the M1 (rarely in Spry4−/−, frequently in Spry2−/−). Spry4−/− mice develop ST in both upper and lower jaws, whereas Spry2−/− mice only display lower ST. Lower ST have been observed in Spry2−/− and Spry4−/− molar rows, whereas upper ST only occurred in Spry4−/− molar rows. Spry1−/− mice thus never develop any ST and only Spry4−/− mutants display ST in both upper and lower tooth rows. These findings suggest a potential relationship between the occurrence of ST and abnormal arrangement of the mesial parts of the first molars.

Similarities and differences of Rsk2 dental phenotype as compared to Sprouty dentitions

We next compared a cohort of 45 Rsk2−/Y mice with a cohort of 45 WT littermates. The postcanine occlusal surface area in Rsk2−/Y mice is smaller than in WT mice (M1 and M1,2 being significantly smaller, p-value < 0.05, Fig. 4a). Although many Rsk2−/Y specimens display relatively severe dental defects, 45% of the examined dental rows display a WT-like phenotype. The main defects of the Rsk2−/Y postcanine dentition are: (c1) the occurrence of a upper ST (14%); (c2–3–4) the presence of many defects on the first chevron of the M1 (71%); (c11) the occurrence of a lower ST (14%); (c12 + 14 + 16) an abnormal number and/or interconnection of the mesial cusps of the M1 (19%); (c17) abnormal mesio-distal connections between the second and the third lophs of the M2 (9%) (Figs 2 and 3e). The frequency of ST occurrence is lower than what has been previously reported8 and this may be explained by the examination of a larger sample or by shifts in the genetic background over time.

The comparison of the postcanine dental phenotypes between Rsk2−/Y and the three Sprouty mutant mice shows that Rsk2−/Y, Spry2−/− and Spry4−/− mice all develop ST, at varying frequencies. Spry2−/− mice never have an upper ST, but they frequently have a lower ST (27%). Spry4−/− mice frequently have an upper ST (17%), but they rarely have a lower ST (3%). Rsk2−/Y mice develop both upper and lower ST relatively frequently (14%). In addition to developing lower and upper ST, Rsk2−/Y and Spry4−/− postcanine dentitions share many similar defects affecting the mesial parts of both the upper and lower first molar. The occurrence of defects on mesial parts of the M1 is much more frequent than the occurrence of an upper ST, but the defects in the mesial parts of the M1 and M1 are also associated with a shortening in length of these teeth (Fig. 4). Finally, Rsk2−/Y mutants do not develop the supplementary distal cusp of the M1 common to all Sprouty mutants, nor do they share with Spry1−/− and Spry4−/− mutants the trend towards having larger teeth than the WT mice.

The presence of ST impacts both the shape and size of the other teeth in the row

The size and shape of ST range from small rounded monocuspid teeth to large complex multicuspid teeth that can comprise up to five cusps (Fig. 5). In complex upper ST, a large central cusp is always present surrounded by a variable number of cusps linked by an almost circular crest. The most complex upper ST tend to have a mesial chevron pointing mesially (especially in Spry4−/−) whereas the lower ST mainly have a bicuspid shape (especially in Spry2−/−). These features show that ST have a clear murine shape identity.

Figure 5
figure 5

ST phenotype and surface in Spry2−/−, Spry4−/− and Rsk2−/Y mutants.

ST range from small rounded monocuspid teeth to large multicuspid teeth. Right column indicates the mean surface in all the ST-displaying mutant mice, error bars represent the standard deviation. V and M respectively point towards the vestibular and mesial directions. Scale bar: 0.4 mm.

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The larger the ST is, the more the mesial parts of the neighbouring M1 and M1 are impacted. The occurrence of a ST leads to compression and flattening of the mesial part of the following teeth. This is particularly visible on M1 of Spry4−/− and on both M1 and M1 of Rsk2−/Y mutants. As a consequence, the overall occlusal area of the molar row is smaller in mutants displaying a ST, with the exception of the Spry4−/− upper row, in which the area of the M3 seems to increase in compensation for the decrease of M1 area (Fig. 3).

In the most severely impacted phenotypes, the mesial contour of the M1 becomes rounded, whereas it is triangular in WT mice (e.g. Fig. 4e without ST). In addition, the two most mesial cusps of the first chevron become extremely reduced into a simple crest (53% Spry4−/− and in Rsk2−/Y) and the mesio-vestibular cusp may completely disappear (20% Spry4−/− and 4% in Rsk2−/Y). Interestingly, the mesial shrinkage of the M1 is associated with a change in the tilt angle of both cusps and roots of the first chevron (Suppl Fig. 1b). In WT mice, the slope of the M1 mesial root is in continuity with the tilt of 50° that makes the central cusp of the first chevron with the dental neck. In mutants having a ST, as well as in some mutants that do not display any ST, the tilt between the root axis and the tooth neck tends to be more vertical (about 70°), as does the slope of the first chevron central cusp (about 60°). The same type of defects can be seen when M1 is preceded by a ST in the three mutants: the first loph of the M1 is shortened and cusps appear as crushed by the presence of the ST.

Discussion

Similar yet distinct dental phenotypes in Sprouty and Rsk2 mutants

Despite some variations in the penetrance of the dental defects in the mutants that we studied, which are likely to be accounted for by differences in the genetic background of our cohorts, some trends stand out. Morphological comparisons show that the occurrence of a supplementary cusp at the distal extremity of the M1 can be considered as a phenotypic signature of the Sprouty mutant dentitions and Rsk2 mutants never develop this supplementary cusp. These comparisons also highlight that Spry4−/− and Rsk2−/Ymutants share many phenotypic features, such as the occurrence of upper and lower ST as well as abnormal arrangements in the mesial parts of the first molars. Although mesial abnormalities in these teeth may result from the occurrence of ST, a question remains concerning the higher frequency of modifications of the M1 first chevron in Spry4−/− and Rsk2−/Ymutants (66–71%) when compared to the frequency of occurrence of ST (14–17%). It has already been suggested in EdaTa mutant mice that a supplementary dental germ can develop mesially to the M1 until advanced stages of odontogenesis without necessarily giving rise to a mineralized tooth32. In these cases, the development of a supplementary dental germ may be sufficient to cause disorders in the development of the mesial part of the following tooth. We can thus infer that about 70% of the specimens develop an upper ST germ that will impact on the phenotype of the M1.

Roles of Sprouty and Rsk2 genes within the ERK-MAPK cascade

Sprouty genes are thought to encode proteins that inhibit signalling through the FGFR pathway18,20,33. Examination of the dental phenotypes in mutant mice led to the discovery of shared features. All three Sprouty mutants share the presence of a supernumerary distal cusp, arguing in favour of shared functions. But distinct characters were also observed in each mutant, pointing to some specific functions of each of these genes in tooth morphology. The phenotypic diversity in the Sprouty mutants may be ascribed to differences in spatial and temporal expression patterns of these genes. Our data complement previous findings concerning the generation of similar phenotypes with the loss of function of Spry2 and Spry4 genes. Spry4−/− phenotype penetrance was previously reported as lower than the one in Spry2 null mutants7. Our results show that the differences between the two proportions are even larger than previously reported. This trend is reversed for the upper rows, for which we did not observe ST in Spry2−/− mice, whereas these were found in 17% in Spry4−/− mice. The proportion of ST in mutant dental rows was comparable to what has been recently reported22, but lower than the first publication on the topic7. Such a change in penetrance could be explained by a shift in the genetic background over generations. Finally, neither of the previous reports7,22 studied the effect of loss of function of Spry1. Our work shows that Spry1−/− mice also display modifications in the shape of the M1 and M2, as well as in overall size of the dental row.

RSK2 is an effector kinase that acts downstream of the MAPK cascade, whereas Sprouty genes modulate the cascade through their inhibitory effect on the pathway. The possible negative feedback regulation from Rsk2 on the adaptor protein SOS, which recruits Ras27, may lead to similarities in the phenotypes of Sprouty and Rsk2 mutants. Both Spry4 and Rsk2 are expressed during odontogenesis in the dental papilla8,21,34,35, which may explain the high degree of similarity in the phenotypes associated with their loss of function. Our study thus provides an example of molecular and phenotypic convergence of various steps in the same signalling pathway.

Mutations in Sprouty genes and Rsk2 lead to more teeth and more cusps. This is the opposite trend of mutations in the Fgf genes (e.g. mutations in Fgf336,37 lead to fewer cusps). This contrast in phenotypic impacts of the mutations between Rsk2 and Fgf3 is concordant with the current knowledge of regulatory feedbacks acting on the whole pathway. Overall, in terms of phenotype penetration, one must note that the frequencies depend on the defect considered and that the penetrance is never complete. Variations in the initial genetic background of each line might be responsible for some of the differences observed. Structural conservation and functional redundancy of the actors of the pathway38 can however account for the shared dental defects.

Molecular basis of ST development

In addition to Sprouty and Rsk2 null mice, other mutants have been described with ST mesial to the first molars. Since some of these mutations are in genes encoding proteins that interact with the ERK-MAPK pathway, this pathway appears to be a central actor. The balance between SHH and Wnt signals has been demonstrated to be crucial for the development of the diastemal rudimentary tooth germ. The link between the two pathways involves regulation of expression of Fgf pathway members by the Wnt signalling pathway. Mice mutant for Gas1, Shh, Wnt123, Wise39, Lrp440, Apc, Ctnnb141, Eda or Edar42 genes are characterized by a disruption of this important molecular equilibrium and display ST. Apart from the Apc and Ctnnb1 null mutants, the ST occurs very similarly to what is observed in Sprouty and Rsk2 mutants. Thus, modulation of Wnt signalling upstream of the ERK-MAPK pathway could mimic the effect of loss of Sprouty gene or Rsk2 function, which might explain phenotypic similarities in the occurrence of ST in both pathways.

The Apc and Ctnnb1 null mutants display many more ST located in the vicinity of the incisor region41. These authors hypothesized that incisor stem cells might be migrating, thus generating odontogenesis-competent foci. The difference between one ST being displayed in a mesial position (compared to the molar row) and multiple ST being displayed in a disordered manner is likely influenced by the direct interaction with Ras and ERK. Indeed, such interactions have been demonstrated in the context of multiple cancer models in the mouse43,44.

Potential roles of Sprouty and Rsk2 genes in the evolution of rodent dentition

The most striking phenotypic trait of Spry2−/−, Spry4−/− and Rsk2−/Y mutants is the occurrence of ST located in front of the first molars at both the lower and the upper jaws. It has long been known that the rodent dentition has evolved towards a reduction in the number of teeth over the course of evolution in parallel with a specialization of the molar teeth45. Basal rodents had lower and upper premolars and it is only since the rise of muroid rodents that premolars have been completely lost in the lineage leading up to the murine rodents. It has however been demonstrated that rudimentary dental germs are maintained in both the upper and the lower diastema during mouse early dental development24,25. Because these rudimentary germs are located in the vicinity of the first molar germs, they have been considered to be rudiments of premolar germs lost over evolution26. These rudimentary germs rapidly abort by apoptosis and do not become autonomous mineralized teeth. It has been shown that they can contribute to the development of the M1 by merging with the M1 germ26. Although it has not yet been formally proved, it is likely that the same mechanism occurs with the upper rudimentary germ immediately adjacent to the M146 for participation in the development of the first chevron.

Here we deliver new insights into the role of the MAPK signalling pathway members beyond what has been previously reported7,8,22. Interesting, this signalling pathway is conserved in mammalian species and its signal transduction properties are even shared more broadly38,47, adding its plausible role in the evolution of mammalian dentition. Here, the loss of function of Spry2, Spry4 or Rsk2 allows the continuation of the development of some rudimentary premolar germs until the formation of autonomous mineralized teeth. The segmentation of the postcanine dental row is thus impacted, because it comprises four teeth instead of three and the mesial parts of the first molars are accordingly reduced (Fig. 4). We can thus assign the ST located immediately in front of the first molars to deciduous fourth premolar teeth (dP4 and dP4) that will not be replaced. Teeth located at the fourth premolar position are known in the earliest muroid rodents48 as well as in some extant dipodoids such as Sicista and Zapus49,50, the sister group of muroids. Fourth premolars are present in the squirrel- and in the guinea pig-related clades, which arose earlier in the history of rodents than the mouse-related clade51. The loss of function of Spry2, Spry4 or Rsk2 involves developmental mechanisms that revitalize autonomous premolar teeth and that simplify accordingly the mesial parts of the first molars. The dental phenotype in Spry2−/−, Spry4−/− and Rsk2−/Y mutants could thus be considered as a partial reversal in the evolutionary trends of the dental phenotype through the transition from pre-muroid to muroid rodents (Fig. 6a), which is documented within the fossil record to have probably occurred between 50 and 45 million years ago (Ma)48,52.

Figure 6
figure 6

Mutant dentition recapitulates the evolution of murine tooth characters.

(a) The dental phenotype arising from the loss of function of Spry2, Spry4 and Rsk2 genes mimics the reduction of tooth number in muroid rodent evolution. (b) Sprouty M1 mimic Progonomys debruijni M1, as depicted with the example of a Spry2−/− M1. M. auctor and P. debruijni molar pictures are courtesy of Y. Kimura.

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As discussed above, many other mouse mutants display ST mesially to first molars, but both the phenotypic and the developmental aspects of the postcanine row segmentation have solely been scrutinized in mutants of the EDA pathway32. From these studies, it is clear that dental phenotypes in EdaTa (Tabby) and Edardl-J (Downless) mice radically differ from those of Spry2−/−, Spry4−/− and Rsk2−/Y mice. EdaTa and Edardl-J mice have postcanine teeth with simplified occlusal patterns characterized by the absence of many cusps, as well as by abnormal arrangements of the lophs and the chevrons respectively in the lower and upper molars32. In contrast, Spry2, Spry4, or Rsk2 losses of function provoke rearrangements of the M1 and M1 mesial parts, but minimally impact the rest of the dental rows. From these arguments, we propose that Spry2, Spry4 or Rsk2 are candidate genes for a major role in the evolutionary trend towards reduction of the dental formula in muroid rodents 45 Ma.

Interestingly, a character that stands out from the mutant mice is the occurrence of a supplementary distal cusp, named the posterocone, on the M1 of the three Sprouty mutants (arrowhead in Fig. 6b). The posterocone is rarely present in extant species of murine rodents, whereas this cusp or the equivalent crest located at the same location, the posteroloph, were almost always present in the basal fossil murine genera such as Potwarmus, Antemus and Progonomys. However, neither the modern mouse (Mus musculus) nor its ancestor (Mus auctor) displays an individualized posterocone (Fig. 6b). In the lineage leading to the modern mouse, the posterocone has been lost during the transition from Progonomys debruijni to Mus auctor and this transition is documented to have occurred between 9.2 and 6.5 Ma by the fossil record in the Siwaliks of Pakistan53. Apart from the presence of a posterocone, Spry1−/−, Spry2−/− and Spry4−/− mice share other dental features with these basal murine genera such as the trend of the lingual cusps of the M1 to be less connected to the central cusps. All of these similarities indicate that Spry1, Spry2, or Spry4 could have played a major role during the transition from Progonomys to Mus, which constitutes a major transition in the history of murine rodents.

Concluding Remarks

Spry1, Spry2, Spry4, and Rsk2, which encode components of the ERK-MAPK signalling pathway are identified here as potentially major actors in the evolution of the dentition in rodents. These genes may have played a pivotal role in the reduction of the dental formula and in the rearrangement of the mesial parts of the first molars during the transition from pre-muroid to muroid rodents prior to 45 Ma. They also may have acted in the disappearance of the posterocone and in the connections of the lingual cusps to the chevrons during the transition from basal murines to Mus auctor between 9.2 and 6.5 Ma. Both morphological transformations happened independently, separated by about 35 Ma, suggesting that the premolar loss might have been triggered by Rsk2 gene dosage augmentation. The posterocone would then have been lost because of an increase in Sprouty gene expression. An interesting issue remains open. Indeed the potential role of Sprouty or Fgf genes in the morphogenesis of the connections between lingual cusps and central cusps of the M1 chevrons needs further investigations. Here we report that Sprouty mice, especially Spry2−/− and Spry4−/− mice, often have M1 with disconnected lingual cusps. As Sprouty and Fgf genes are known to play antagonist roles during tooth morphogenesis, it is interesting to note that the loss of function of the two antagonist results in an equivalent impact on the lingual cusps of the M1. The role of this central pathway could be further studied using other genetically engineered mice, notably over-expressing the same molecules to understand the plausible evolutionary implications of dosage modulations. Focusing on the molecular redundancy allowing vestigial tooth buds to develop into fully erupted teeth is likely to bring new insights into the molecular networks involved in both tooth development and evolution.

Material And Methods

Sprouty mutant mice

All the Sprouty mutant mice were generated in backgrounds resulting from crossing between several lineages. The three mutants and the wild type mice were generated by breeding at UCSF. The sample set was composed of homozygous mice as indicated: 25 Spry1−/−, 50 Spry2−/−, 50 Spry4−/− and 60 WT individuals (littermates of the various mutants). For each specimen, left and right, upper and lower tooth rows were studied independently. The age of the specimens ranged from 1 month to 2.5 months. Animal experimentation was carried out in compliance with the policies and procedures established by the UCSF Institutional Animal Care and Use Committee.

Rsk2−/Y mice

The Rsk2 mutant mouse line was generated as previously described54. Since Rsk2 is located on the X chromosome, analyses were performed on Rsk2−/Y males, on a C57BL/6J background. The sample set was composed of 45 Rsk2−/Y and 45 WT littermates (Rsk2+/Y). Mouse protocols were complied with the 2010/63/UE directive and the 2013/02/01 French decree and were thus approved by the CERBM-GIE: ICS/IGBMC Ethical Research Board.

Observation and imaging of dental rows

All the heads were prepared in order to remove all non-mineralized tissues to allow good observation and measurement of the dental rows. They were examined and photographed using a Leica stereomicroscope. The measurements were obtained by following the outline of each tooth from the photos of occlusal view of the row. Thus, the length, width and area of the tooth were produced by Leica software. Some Sprouty mutant dental rows were imaged using X-ray-synchrotron Radiation Facility (ESRF, Grenoble, France), beamline ID 19 and BM5, with a monochromatical beam at energy of 25 keV. X-ray synchrotron microtomography has been demonstrated to bring high-quality results for accurate imaging of small teeth55. A cubic voxel of 7.46 μm was used. Some Rsk2−/Y mutant dental rows were imaged using the X-ray cone-beam computed microtomography with a Nanotom machine (GE) with a source tension of 100 kV with a cubic voxel of 3 μm. All 3D renderings were performed using VGStudiomax software.

Statistics

Student’s t-tests were used to verify the significance of differences in tooth size between the mutant mice but also between the mutant and the WT mice. A threshold value of 0.05 (p-value) was used to assess the significance of the observed differences.

Additional Information

How to cite this article: Marangoni, P. et al. Phenotypic and evolutionary implications of modulating the ERK-MAPK cascade using the dentition as a model. Sci. Rep. 5, 11658; doi: 10.1038/srep11658 (2015).