|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online November 26, 2007
doi: 10.1242/10.1242/dev.009118
1 Gonda Department of Cell and Molecular Biology, House Ear Institute, 2100 West
3rd Street, Los Angeles CA 90057, USA.
2 Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue,
Boston, MA 02115, USA.
3 Center for Basic Neuroscience, UT Southwestern Medical Center, Dallas, TX
75390, USA.
4 Department of Cell and Neurobiology, Keck School of Medicine, University of
Southern California, Los Angeles, CA 90033, USA.
* Authors for correspondence (e-mails: nsegil{at}hei.org; agroves{at}hei.org)
Accepted 19 September 2007
 |
SUMMARY |
|---|
 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Key words: Proneural gene, bHLH, Hair cells, Inner ear, Otocyst, Neurogenesis
|
INTRODUCTION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
;
Kiernan et al., 2002). Early
on, a subset of otic epithelial cells expresses neural fate markers
(Raft et al., 2004;
Cole et al., 2000;
Adam et al., 1998), delaminates
from the otocyst, and forms the VIIIth cranial ganglion rudiment with neural
crest-derived glial precursors
(D'Amico-Martel and Noden,
1983). In mice, hair cell production begins about 3.5 days after
the onset of neurogenic gene expression
(Shailam et al., 1999). Hair
cells and supporting cells remain epithelial and form six discrete sensory
patches along the inner ear labyrinth
(Sher, 1971). Five sensory
epithelia mediate the sense of balance (the utricular macula, saccular macula,
and three cristae) and one - the organ of Corti - detects sound.
Morphological and genetic homologies between the sensory organs of the
vertebrate ear and insects raise the question of whether neurons and hair
cells derive from a common progenitor cell type
(Fekete and Wu, 2002;
Adam et al., 1998). Direct
evidence for this is limited to one recent study in chicken that found clonal
relatives of VIIIth ganglion neurons within the utricular macula and adjacent
non-sensory epithelium (Satoh and Fekete,
2005). Existing evidence for such a relationship in mammals is
circumstantial (Matei et al.,
2005; Raft et al.,
2004).
Otic neurogenesis and hair cell generation are each dependent on an
Atonal-related bHLH transcription factor. neurogenin 1 (Ngn1; also
known as Neurog1 - Mouse Genome Informatics) is necessary for the
commitment of otocyst epithelial cells to a neural fate, as
Ngn1-/- embryos lack an VIIIth cranial ganglion and fail
to express neural fate markers in the otocyst
(Ma et al., 1998;
Ma et al., 2000). By contrast,
Math1 (also known as Atoh1 - Mouse Genome Informatics) is
necessary and sufficient for hair cell generation
(Bermingham et al., 1999;
Zheng and Gao, 2000;
Izumikawa et al., 2005). Thus,
during ear development, Ngn1 and Math1 function as
determination factors, but it is not known whether their expression is
regulated in a coordinated manner. In other neural systems, bHLH genes
cross-regulate to control the commitment of progenitor cells to alternative
fates (Bertrand et al., 2002).
Examples include the involvement of Mash/Math/Ngn1 genes in the
sequential production of retinal neurons and glia
(Inoue et al., 2002;
Akagi et al., 2004), and in the
simultaneous production of distinct neural subtypes in the forebrain and
spinal cord (Fode et al.,
2000; Gowan et al.,
2001).
Here, we identify a progenitor cell field that produces both neurons and hair cells and describe genetic interactions mediating a neural-hair cell fate decision. We show that neural precursor and hair cell production overlap in the otic epithelium for several days and demonstrate by fate mapping that the Ngn1-expressing neurogenic region is transformed into the sensory maculae of the utricle and saccule. We propose that this transformation is governed by a mutual antagonism between Ngn1 and Math1. We also show that Ngn1 negatively regulates its own expression through Notch-mediated lateral inhibition, whereas Math1 positively regulates its own expression. Differential autoregulation of Ngn1 and Math1 provides an explanation for the progression toward sensory epithelial development over time. Ngn1, as well as generating neural precursors, functions via lateral inhibition to maintain an uncommitted progenitor cell population for sensory epithelial development; Math1, in turn, irreversibly commits these progenitors to a hair cell fate.
|
MATERIALS AND METHODS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
),
Ngn1 (Ma et al.,
1998), or Pofut1 (Shi
and Stanley, 2003) loci were maintained on a CD1 background.
Ngn1-GFP and Ngn1-CreER BAC transgenic mice were constructed with
RPCI-23-457E22
(http://bacpac.CHORI.org/),
which contains a genomic insert of 184 kb with 113 kb 5' and 71 kb
3' of the Ngn1 coding sequence. The Math1-GFP BAC transgenic
was constructed with RPCI-23318G16, which contains a genomic insert of 181 kb
with 75 kb 5' and 104 kb 3' of the Math1 coding sequence.
Homologous recombination in bacteria (Yang
et al., 1997) was used to replace endogenous coding sequences with
coding sequence for either nuclear-localized EGFP (Clontech) or
CreERT2 (Feil et al.,
1997). Details of Ngn1CreER BAC transgene construction (E.J.K. and
L.V.G., unpublished) are available on request. Genotyping was accomplished by
PCR using the following oligos: Ngn1-GFP,
5'-CGAAGGCTACGTCCAGGAGCGCAC-3' and
5'-GCACGGGGCCGTCGCCGATGGGGGTGT-3'; Ngn1-CreER,
5'-AGCCCATTCACTCCCTGAG-3' and
5'-ATCAACGTTTTCTTTTCGGA-3'; Math1-GFP,
5'-CTGACCCTGAAGTTCATCTGCACC-3' and
5'-TGGCTGTTGTAGTTGTACTCCAGC-3'; Math1 mutants,
5'-GAACCCAAAGACCTTTTGCAC-3' and
5'-CACGAGACTAGTGAGACGTG-3'; Ngn1 mutants,
5'-AAGAGTGCCATGCCCGAAGG-3' and
5'-AAGGCCGACCTCCAAACCTC-3'; Pofut1 mutants,
5'-GGGTCACCTTCATGTACAAGTGAGTG-3' and
5'-ACCCACAGGCTGTGCAGTCTTTG-3'. The Math1-GFP transgenic line
carrying a 1.4 kb Math1 enhancer fragment was genotyped as previously
described (Chen et al., 2002;
Lumpkin et al., 2003). The
Ngn1-CreER transgene was maintained in males on a Z/EG reporter strain
(Novak et al., 2000);
double-transgenic embryos were obtained by breeding these to Z/EG reporter
females. Tamoxifen (Sigma T5648) in corn oil was administered by gavage twice
daily to alert, pregnant females (1.0 mg per 40 g of body weight per
administration).
|
). For combined in situ hybridization and GFP
immunohistochemistry (with anti-GFP rabbit polyclonal, Molecular Probes),
proteinase K digestion (5 µg/ml in PBS) was reduced to 3 minutes.
Digoxygenin-labeled RNA probes for NeuroD (Lee et al., 1994),
Math1 (Helms and Johnson,
1998), Ngn1 (Ma et
al., 1998) and Dll1
(Beckers et al., 1999) were
prepared by standard methods. TUNEL reactions were performed according to
manufacturer's instructions (Roche).
|
|
RESULTS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). NeuroD (also known as Neurod1 - Mouse
Genome Informatics), a neural differentiation gene
(Bertrand et al., 2002), is
expressed under the control of Ngn1 in the otic epithelium
(Ma et al., 1998) and marks
cells that are committed to the neural lineage. We also hybridized a
NeuroD RNA probe to tissue from a Math1-GFP transgenic reporter mouse
(Lumpkin et al., 2003).
Math1 is the earliest known specific marker of hair cells
(Bermingham et al., 1999).
The number of NeuroD+ cells within the epithelium
increases between E9 and E10.5 (Raft et
al., 2004) (Fig.
1A). Between E11.5 and E12.5, NeuroD expression split
into two distinct regions of neurogenesis that would ultimately lie within the
developing utricle and saccule (Fig.
1B,C, cyan). Neurogenesis declined from E11.5 onward
(Fig. 1D,E), but a few
delaminating NeuroD+ cells were still present in the
utricle as late as E17.5 (5±4 cells, n=3 ears; data not
shown). A comparison of NeuroD and Ngn1 expression in the
otic epithelium revealed no differences in their patterning (see Fig.
S1A,D,E,H in the supplementary material).
During the decline in neurogenesis, Math1 mRNA expression begins
and is maintained in all hair cells through to at least E17.5
(Shailam et al., 1999). At
E11.5, two Math1+ stripes appeared within the
NeuroD domain along its lateral and medial borders
(Fig. 1B,F,G). Between E11.5
and E12.5, Math1-GFP+ cells in these stripes increased in number by
8- to 10-fold and formed the nascent maculae of the utricle and saccule.
Initially, Math1-GFP+ and NeuroD+ cells
intermingled (Fig. 1H,I), but
later lay on either side of a border that delineates the macula and its
adjacent neurogenic domain (Fig.
1J). Math1 expression associated with the cristae first
appeared as separate foci outside the NeuroD+ domain at
around E12 (data not shown). We found no temporal overlap of Math1
and NeuroD expression in the cochlea (data not shown), suggesting
that neurogenesis and hair cell generation do not coincide in the auditory
end-organ. Taken together, our data reveal that neurogenesis is maintained
through stages of hair cell production in the utricle and saccule, but
declines sharply as Math1 expression and hair cell production
increase.
Maculae of the utricle and saccule derive from the Ngn1-expressing domain of the otocyst
Our expression analysis raised the possibility that sensory maculae of the
utricle and saccule, but not the cristae or organ of Corti, derive from the
neurogenic region of the otocyst. To test this, we permanently labeled cells
of the neurogenic region using a BAC transgenic mouse line (Ngn1-CreER)
(E.J.K. and L.V.G., unpublished) that expresses a tamoxifen-inducible form of
Cre recombinase (CreER) under the control of Ngn1 regulatory elements
(see Fig. S1B,F in the supplementary material). This allowed us to identify
cells transiently expressing Ngn1 and their progeny after sensory
epithelia have formed. As expected, when the Ngn-CreER mouse was crossed with
the Z/EG reporter line (Novak et al.,
2000) and tamoxifen administered, roughly 50% of VIIIth cranial
ganglion neurons were permanently labeled in double-transgenic embryos
(Fig. 2A; see Fig. S1I in the
supplementary material). Importantly, we also found Ngn1 derivatives to be
present in sensory and non-sensory inner ear epithelia of embryos that had
been sacrificed after neurogenesis was largely complete
(Fig. 2B-G).
To follow the fate of Ngn1-expressing cells in the ear, we administered tamoxifen twice daily from E8.5 until E13.5 to pregnant females of Ngn1-CreER x Z/EG matings. We analyzed 20 double-transgenic right ears from seven litters ranging in age from E13.5-16.5 and identified over 5000 labeled epithelial cells. Sensory epithelia were identified by the presence of myosin VIIa protein, and the resulting distribution pattern of epithelial Ngn1 derivatives is summarized in Fig. 2G. Supporting the hypothesis that maculae are the only sensory epithelia to derive from neurogenic epithelium, Ngn1 derivatives were present in the utricular and saccular maculae of all specimens analyzed (Fig. 2B,C). For embryos sacrificed at E14.5, tamoxifen administration from E8.5-13.5 yielded an average of 157±25 Ngn1 derivatives per utricular macula (n=6 ears from two litters). We found a lower occurrence of such cells in the saccular macula (82±24; n=6 ears from two litters). By contrast, only one ear out of the 20 analyzed had Ngn1 derivatives in the lateral crista (eight labeled supporting cells), and no such cells were detected in the other cristae in our cohort of specimens. No Ngn1 derivatives were detected in the organ of Corti. We were able to classify macular Ngn1 derivatives as differentiated myosin VIIa+ hair cells (Fig. 2H), undifferentiated myosin VIIa+ epithelial cells migrating within the apical-basal plane of the epithelium (Fig. 2I), or as myosin VIIa- pseudostratified epithelial cells (Fig. 2J). By E16.5, many of these myosin VIIa- Ngn1 derivatives exhibited morphological features of supporting cells (Fig. 2K, arrowheads).
Regions of non-sensory epithelium flanking the maculae of E13.5-16.5 ears also contain labeled cells (Fig. 2D,E), but we found no Ngn1 derivatives in the semicircular canals. In the auditory portion of the ear, Ngn1 derivatives were detected in 82% (14/17) of the cochleae analyzed (E13-16.5), but showed extreme variability in their numbers (93±146 cells, n=8 cochleae at E14.5). These cells commonly occupied the greater epithelial ridge (GER), a non-sensory region of the cochlea that is adjacent to the organ of Corti (Fig. 2F). However, as described above, no Ngn1 derivatives were detected in the organ of Corti itself.
Administration of tamoxifen only at placode/otocyst stages (E8.5 and E9.5) resulted in the same spatial distribution of Ngn1 derivatives as described above. Together, our results indicate that the utricular and saccular maculae, as well as some non-sensory epithelium flanking these structures, derive from the neurogenic region of the otocyst, and that Ngn1-expressing otocyst cells or their descendants can differentiate as hair cells, supporting cells, or as structural epithelial cells.
The Ngn1 domain contracts gradually and stereotypically in the primordia of the utricle and saccule
To understand how the sensory maculae and their surrounding tissue arise
from neurogenic tissue, we analyzed changes in Ngn1 expression over
time using two different transgenic lines. A series of Ngn1-CreER x Z/EG
litters received initial tamoxifen exposures at progressively later
developmental time points from E8.5 onwards; once begun, tamoxifen
administration was continued twice per day until E13.5 and all litters were
sacrificed on E14.5. By quantifying GFP+ cells in the utricle of
these embryos, we confirmed that starting tamoxifen administration at
progressively later times leads to diminishing numbers of labeled Ngn1
derivatives (Fig. 3C). From
these experiments, we mapped the distributions of Ngn1 derivatives in the
utricular macula and its flanking non-sensory tissue
(Fig. 3E-E'''). The
distribution of cells actively expressing Ngn1 at E14.5 was obtained
from a different BAC transgenic line (Ngn1-GFP) that reports directly on
Ngn1 promoter activity (see Fig. S1C,G,J in the supplementary
material).
Our results for the utricle are consistent with a stereotyped reduction in the area of Ngn1 expression over time. At E14.5, the active neurogenic domain of the utricle, defined by expression of the Ngn1-GFP reporter, lay medial to and centered along the anteroposterior axis of the macula (Fig. 3A,B). When tamoxifen was administered to Ngn1-CreER;Z/EG litters from E8.5-13.5 (Fig. 3E), Ngn1 derivatives were present throughout the neurogenic domain (white region), the macular sensory epithelium (gray region, defined by myosin VIIa expression), lateral non-sensory tissue (between the utriclar macula and lateral crista, yellow region), and posterior non-sensory tissue (between the utricular and saccular maculae, yellow region, bottom). When tamoxifen was administered from E10.5 or E11.5 to E13.5, Ngn1 derivatives were present at all these sites, except for posterior non-sensory tissue (between the maculae, Fig. 3E'). Finally, when tamoxifen was administered from E12.5-13.5, Ngn1 derivatives were present only within the neurogenic domain and a portion of the macular epithelium nearest its border with the neurogenic domain (Fig. 3E''). Contraction of the Ngn1 expression domain is therefore directional, occurring largely from lateral to medial in the utricle. We observed similar Ngn1 expression dynamics in the saccule (see Fig. S2 in the supplementary material), although contraction occurred largely along the anteroposterior axis of this structure, rather than along the medial-lateral axis as in the utricle. Importantly, all tamoxifen start times tested (ranging from E8.5-12.5) resulted in a cohort of macular Ngn1 derivatives comprising both hair cells and pseudostratified epithelial cells. These data, together with results described in the previous section, indicate that macular sensory cells can derive from cells that express Ngn1 at any time between E9 and E14.
Math1 suppresses neurogenesis in the developing utricle and saccule
Our results indicate that the domain of Ngn1+ precursor
cells is gradually transformed from a purely neurogenic region into sensory
epithelia of the utricular and saccular maculae. Since functional antagonism
between related bHLH transcription factors has been described in other systems
(Fode et al., 2000;
Gowan et al., 2001;
Akagi et al., 2004), we tested
whether Math1 (which is required for sensory epithelial differentiation)
(Bermingham et al., 1999)
suppresses neurogenesis by inhibiting Ngn1 transcription in otic
epithelial cells. We found that Math1 function is required for the
normal contraction of epithelial Ngn1 expression characterized in the
previous section (Fig. 4A,B;
data not shown). We quantified expression of the Ngn1-GFP reporter and
NeuroD in Math1-null homozygote embryos and observed large
(>6x) increases over wild type in the numbers of neural precursors
within the developing utricle and saccule; a less severe form of this
phenotype was found in Math1 heterozygotes
(Table 1). Excess neural
precursors were seen to delaminate and migrate away from the mutant epithelium
to form an VIIIth cranial ganglion that was larger than wild type (see Fig.
S3B asterisk, E-H, in the supplementary material). Ectopic neurogenesis in
Math1-/- epithelia localized specifically to parts of the
utricle and saccule that normally differentiate as sensory maculae, and was
not detected in the cristae, cochlea, or any non-sensory epithelia
(Fig. 4D,F; see Fig. S3A-D in
the supplementary material; data not shown); in Math1 heterozygotes,
it occurred only at the interface of neurogenic and sensory regions
(Fig. 4E, bracket). In
Math1-/- epithelia, marked excess neurogenesis at E14.5
and E15.5 followed a partial decline in neurogenic activity through E13.5
(Fig. 4N). This initial
declining trend in neurogenic activity in mutants, which is similar to that of
wild type, suggests that factors in addition to Math1 contribute to
early neurogenic suppression.
|
;
Ma et al., 2000). If
antagonism between Math1 and Ngn1 during ear development is
reciprocal, then Ngn1 loss-of-function should result in ectopic hair
cells. Our analysis of sensory marker expression in the
Ngn1-/- utricle confirmed this prediction. As indicated by
both Math1 and myosin VIIa expression, the
Ngn1-/- utricular macula at E13.5 and E14.5 was expanded
medially into the region that is normally neurogenic
(Fig. 4C,F,H,K,M). At E13.5,
the total area of Math1 expression in the Ngn1-/-
utricle exceeded that of wild type by 39% (P<0.002, n=3
epithelia per genotype), and there were more Math1+ cells
in the Ngn1-/- utricle than in wild type at E13.5 and
E14.5 (Table 1). Growth
abnormalities of the Ngn1-/- mutant ear
(Matei et al., 2005;
Ma et al., 2000) confounded
our analysis of other structures. The saccule was profoundly hypoplastic (see
Fig. S4A in the supplementary material), and we correlated this to a period of
intense, region-specific apoptosis in the otic epithelium between E11.5 and
E12.5 (see Fig. S4B,C in the supplementary material; data not shown).
Ngn1-/- cristae were also smaller than their wild-type
counterparts, which was evident from our quantification of
Math1+ cells in the Ngn1-null mutant lateral
crista (Table 1).
|
|
|
Ngn1 negatively regulates its own expression and is inhibited by Notch signaling
Ngn1 is thought to function analogously to the Drosophila
proneural genes (Ma et al.,
1996; Ma et al.,
1998; Cornell and Eisen,
2002). Consistent with a proneural function for Ngn1, we
found that Ngn1-GFP signal varies in intensity between neighboring cells, and
is strongest in delaminating cells (Fig.
5A). We therefore tested whether Ngn1 negatively
regulates its own transcription in the otic epithelium by comparing expression
patterns of the Ngn1-GFP BAC transgene in Ngn1-null homozygote and
wild-type littermates prior to the appearance of ectopic Math1
expression. At E12.5, Ngn1-GFP is normally expressed in a speckled pattern by
a subset of cells in the neurogenic region
(Fig. 5B). By contrast, in
Ngn1-/- embryos, the GFP signal was present in all cells
of the region, with little variability in signal strength between cells
(Fig. 5D). Loss of the speckled
expression pattern for the Ngn1-GFP transgene did not occur on a
Math1-/- background
(Fig. 5C), indicating that this
phenotype is specific to the loss of Ngn1. We also found excess
Ngn1-GFP+ and NeuroD+ epithelial cells in the
Ngn1+/- utricle and saccule compared with wild type
(Table 1). In
Ngn1-/- ears, expression of the Ngn1-GFP transgene was
completely abolished by E14.5, presumably owing to inhibition by ectopic
Math1 expression (Fig.
4H,M).
To investigate potential interactions of Ngn1 with Notch
signaling, we assayed expression of the Notch ligand Dll1 in the
Ngn1-null homozygote. We found reduced Dll1 expression in
the primitive utricle and saccule of mutants as compared with wild type
(Fig. 5E,F) (see also
Ma et al., 1998). To test
whether Notch activity suppresses Ngn1 transcription within the otic
epithelium, we analyzed Ngn1 expression in embryos lacking
Pofut1. This gene encodes the protein O-fucosyltransferase 1, which
glycosylates epidermal growth factor-like repeats within the extracellular
domain of the Notch receptor (Lei et al.,
2003; Okajima et al.,
2003; Okajima et al.,
2005). Pofut1 loss-of-function abolishes ligand-induced
Notch signaling and causes phenotypes similar to those of embryos lacking
downstream effectors of all Notch receptors, including mid-embryonic lethality
(Shi and Stanley, 2003). We
therefore assayed Ngn1 expression in early Pofut1 embryos
(E9-9.5), when the otic placode invaginates to first form an otocyst. At these
stages, Ngn1 mRNA signals were increased in
Pofut1-/- embryos as compared with wild-type littermates
in the otic epithelium, midbrain, trigeminal placode, epibranchial placodes
and spinal cord (Fig. 5G,H),
indicating that Ngn1 transcription in all these embryonic regions is
negatively regulated by canonical Notch signaling. Thus, the negative
autoregulation of Ngn1 in the otic epithelium might be controlled by
Notch-mediated lateral inhibition.
Positive autoregulation of Math1 in the inner ear epithelium
Math1 positively autoregulates its transcription at particular
sites in the embryo (Helms et al.,
2000). To determine whether Math1 is subject to positive
autoregulation during ear development, transgenic reporters of Math1
promoter activity were compared across wild-type and
Math1-/- backgrounds. A BAC that expresses GFP under the
control of Math1 regulatory elements mimics patterns of
Math1 mRNA expression in the developing ear
(Fig. 6A), hindbrain region
(Fig. 6A, inset), spinal cord,
and other sites in the embryo (J.E.J., unpublished). By contrast, we were
unable to detect GFP signal in the ears of
Math1-/-::Math1-GFP BAC embryos at stages E13.5 through
E15.5 (Fig. 6B; data not
shown), although GFP expression was clearly present at other sites of
expression, such as the hindbrain (Fig.
6B, inset). A second transgenic line, which carries a 1.4 kb
Math1 enhancer with an E-box site that is essential for Math1 binding
and autoregulation in other tissues (Helms
et al., 2000), also mimics Math1 expression in the
developing wild-type ear (Chen et al.,
2002; Lumpkin et al.,
2003). As with the Math1-GFP BAC, we found no GFP reporter signal
in the sensory epithelia of these Math1-/-::Math1-GFP
embryos at stages E13.5 through E15.5 (Fig.
6C,D). The complete lack of reporter expression in the cristae of
both Math1-/-::Math1-GFP reporter lines indicates that the
phenotype is not due solely to inhibition by ectopic expression of
Ngn1.
|
|
DISCUSSION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
; Adam et al.,
1998; Cole et al.,
2000; Fekete and Wu,
2002), and a recent lineage study in the chicken shows that
vestibular and spiral (auditory) neurons of the VIIIth ganglion can be
clonally related to utricular epithelial cells (both macular and non-sensory)
(Satoh and Fekete, 2005). By
fate mapping in mouse, we show that Ngn1+ otic epithelial
cells can differentiate as vestibular or spiral ganglion neurons, hair or
supporting cells of the utricular and saccular maculae, or non-sensory
epithelial cells surrounding the maculae. Although the question remains open
as to whether, in mouse, these cell types descend clonally from a common
progenitor, our work unambiguously traces the origin of specific sensory
(macular) and non-sensory cells to the neurogenic domain of the otocyst.
Our fate mapping suggests that the other functional class of vestibular
sensory epithelia, the cristae, and their associated semicircular canals do
not derive from the Ngn1+ domain of the otocyst. This
confirms previous gene expression studies tracing the origin of cristae to a
Bmp4+ region outside of, and adjacent to, the neurogenic
domain (Morsli et al., 1998;
Raft et al., 2004)
(Fig. 7A'). The very rare
occurrence (a few cells in one of 20 ears) of Ngn1 derivatives in the lateral
crista suggests that mixing of cells between the neurogenic and Bmp4
domains occurs infrequently. Whether this lack of mixing is due to
differential affinity between the two regions, or whether neurogenesis is
actively suppressed within the Bmp4 domain, is not clear. In support
of the latter hypothesis, Tbx1, a T-box gene that inhibits
Ngn1 and maintains Bmp4 expression in the otocyst
epithelium, is expressed continuously and from very early stages in the
presumptive and definitive cristae (Arnold
et al., 2006; Raft et al.,
2004; Vitelli et al.,
2003).
Our mapping of Ngn1-GFP and NeuroD expression domains revealed no
evidence of active neurogenesis in the definitive cochlea. However, we did
find Ngn1 derivatives in a non-sensory region of the cochlea (the GER) in the
majority of ears analyzed. Based on its location in the ear, the GER might
derive from the most posteroventral-medial edge of the otocyst neurogenic
region. Interestingly, the GER lies immediately adjacent to the organ of
Corti, within which we found no Ngn1 derivatives. This result, the common
occurrence of Ngn1 derivatives in non-sensory tissue between the utricula
macula and the anterior/lateral cristae (but not in the cristae)
(Fig. 2E,G), and the initiation
of macular Math1 expression as stripes just within opposite borders
of the neurogenic domain, support the hypothesis that sensory epithelia are
induced at or near compartment boundaries in the otocyst
(Fekete, 1996;
Brigande et al., 2000).
Cross-inhibition between Math1 and Ngn1 segregates a progenitor field of dual competence into distinct neurogenic and sensory cell populations
We show that neurogenesis and hair cell production, long considered
strictly sequential, actually overlap in the developing utricle and saccule
for several days of gestation. During this period, neural precursors and
nascent hair cells initially intermingle and later sort out across
well-defined borders. Functionally, we show that Math1 and
Ngn1 mutants have complementary inner ear phenotypes, supporting the
hypothesis that mutual antagonism between these genes coordinates neurogenesis
and hair cell production (Matei et al.,
2005). Loss of Math1, which is normally expressed in all
sensory regions of the ear, leads to excess and ectopic neurogenesis only in
sensory regions with a history of Ngn1 expression (utricular and
saccular maculae). This effect is gene dose-sensitive, as Math1
heterozygotes exhibit a neurogenic phenotype intermediate to those of the
Math1-null homozygote and wild type. Conversely, Ngn1
hemizygosity causes excess and ectopic Math1 expression specifically
in the utricle and saccule, and although Ngn1-null homozygosity
causes growth abnormalities of the ear, the Ngn1-/-
utricle still shows a phenotype of excess and ectopic Math1
expression. These effects are seen only at sites in the developing ear where
Ngn1 and Math1 are co-expressed, and we propose that they
result from a disruption of close-range cross-inhibition. Cross-inhibition
might influence multiple steps in the process, whereby an
Ngn1+ progenitor field of dual competence (neural and
sensory epithelial) is gradually restricted to producing only sensory
epithelial cells. These include: (1) Math1 domain establishment
within opposite borders of the Ngn1+ region
(Fig. 7A'); (2)
Math1 domain expansion and decline in Ngn1 expression
(Fig. 7A',A''); and
(3) compartmentalization of the region into a pair of adjacent Math1
(sensory) and Ngn1 (neurogenic) domains
(Fig. 7A''). The potential
basis for the competitive advantage of Math1 over Ngn1 in
this system is discussed below.
Ngn1, but not Math1, functions as a proneural gene during mouse ear development
Criteria for proneural function include early, broad expression of
transcript in all cells of a germinal epithelium and subsequent refinement of
transcription to a subset of cells by lateral inhibition
(Jan and Jan, 1993;
Lewis, 1996). We find no
evidence of these features in our studies of Math1 expression in the
vestibular system of the mouse. Of the two genes relevant to this study, it is
Ngn1 and not Math1 that initially marks the prospective
maculae and exhibits the variegated expression among neighboring cells that is
characteristic of proneural genes (Fig.
5A,B). Furthermore, using two different transgenic reporter lines,
we show that Math1 is required for detectable levels of
Math1 reporter expression in the otic epithelium, suggesting that
Math1 promoter activity is amplified and maintained by positive
autoregulation. One possible consequence of this is a rapid and irreversible
commitment of progenitors to the hair-cell fate once Math1
transcription surpasses a threshold for positive autoregulation. Our results
thus support the view that Math1 functions as a hair-cell commitment
factor rather than a proneural (or `prosensory') gene
(Chen et al., 2002) (for a
review, see Kelley, 2006).
Interestingly, in zebrafish, which has two atoh1 genes, differences
in the timing and autoregulation of Math1/atoh1 genes from
that described here lead to the opposite conclusion
(Millimaki et al., 2007). For
example, zebrafish atoh1a and 1b are required for hair cell
generation, but their expression precedes that of ngn1
(Andermann et al., 2002) and
marks the prospective maculae from very early stages. Gene duplication and
evolutionary pressure on the regulatory genome might therefore dictate the
precise functions of Math1/atoh1 during ear development in
different species.
|
) and show a
pattern of increased Ngn1 expression in the early otic epithelium of
Pofut1-/- embryos, which are deficient in canonical Notch
signaling (Shi and Stanley,
2003). These results and the expression of Ngn1 in neural
and sensory progenitors from very early stages fulfill several criteria for
proneural function. Likely consequences of the proneural activity of
Ngn1 are control over the pace of neurogenesis during otocyst stages
and preservation of an uncommitted progenitor cell population for sensory
development. These functions are consistent with recently reported effects of
conditional Dll1 loss-of-function in the developing ear, which
include an enlarged ganglion rudiment and specific hypoplasia of the utricle
and saccule (Brooker et al.,
2006), and with blockade of Notch signaling in the chicken, which
causes excess neurogenesis at the expense of sensory epithelial precursors
(Daudet et al., 2007).
Differences in bHLH gene autoregulation and cell behavior may direct the transition towards sensory epithelial formation
Given our evidence for a mutual antagonism between Ngn1 and
Math1, how does Math1 exert the stronger inhibitory activity
so that sensory epithelia replace an active neurogenic region? Our model
states that Ngn1 promotes a neural fate cell-autonomously and keeps
its own expression low or off in neighboring cells through Notch-mediated
lateral inhibition (Fig. 7B,C).
Cells expressing high levels of Ngn1 delaminate from the epithelium
as neural precursors. Cells remaining within the neurogenic epithelium
constitute a dynamic mix of committed neural precursors and uncommitted
progenitors. The latter group may adopt a neural fate in subsequent rounds of
delamination or may remain uncommitted for several days, after which they
adopt hair or supporting cell fates in response to Math1 induction
within the region (Fig.
7B',B'',C'). This is supported by our fate
mapping results, as Ngn1 derivatives can have any of these identities. Once
Math1 transcription exceeds a particular threshold, positive
autoregulation irreversibly commits progenitors to a hair cell fate, and
committed hair cells may then induce the supporting cell phenotype through
intercellular signaling (Woods et al.,
2004). Since strongly Ngn1+ cells continuously
delaminate from the epithelium, Math1-expressing cells are left to
interact with epithelial progenitors expressing lower levels of Ngn1.
These features might bias the mutual antagonism between Ngn1 and
Math1, thereby promoting sensory epithelial differentiation at the
expense of continued neurogenesis.
Conclusion
We have implicated cross-regulation between bHLH genes and differential
autoregulation as mechanisms for converting a neurogenic epithelium into
specialized mechanosensory receptors. A novel aspect of this work - and one
that is potentially relevant to other systems - is the dynamic nature of the
patterning processes described. We show that progressive regionalization of
bHLH genes through cross-inhibition can result in a sequential and overlapping
production of distinct cell types, and that differential autoregulation might
provide the driving force for such a transition.
Many questions remain unanswered. For example, does cross-inhibition
between Ngn1 and Math1 occur within a single cell, through
intercellular signaling, or by a combination of these two mechanisms?
Cell-autonomous cross-inhibition might convert a weakly
Ngn1+ cell directly into a Math1+
nascent hair cell. Alternatively, if the antagonism occurs through
intercellular signaling, Ngn1+ cells might pass through a
`sensory-restricted progenitor' state before committing to the hair-cell fate
(Fig. 7C'). We find the
latter alternative attractive given that embryonic maculae contain many Ngn1
derivatives with a pseudostratified epithelial (non-hair-cell) phenotype.
Molecular and cellular mechanisms underlying the apparent compartmentalization
of sensory and neurogenic regions also warrant scrutiny, as there is abundant
evidence that Notch-mediated intercellular signaling occurs at nascent
boundaries during development (Irvine,
1999). In summary, our results form a basis for understanding how
progenitors are allocated to various cell fates during inner ear
development.
Supplementary material
Supplementary material for this article is available at
http://dev.biologists.org/cgi/content/full/134/24/4405/DC1
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Adam, J., Myat, A., Le Roux, I., Eddison, M., Henrique, D., Ish-Horowicz, D. and Lewis, J. (1998). Cell fate choices and the expression of Notch, Delta, and Serrate homologues in the chick inner ear: parallels with Drosophila sense-organ development. Development 125,4645 -4654.[Abstract]
Akagi, T., Inoue, T., Myoshi, G., Bessho, Y., Takahashi, M.,
Lee, J. E., Guillemot, F. and Kageyama, R. (2004).
Requirement of multiple basic Helix-loop-helix genes for retinal neuronal
subtype specification. J. Biol. Chem.
279,28492
-28498.
Andermann, P., Ungos, J. and Raible, D. W. (2002). Neurogenin1 defines zebrafish cranial sensory ganglia precursors. Dev. Biol. 251, 45-58.[CrossRef][Medline]
Arnold, J. S., Braunstein, E. M., Ohyama, T., Groves, A. K.,
Adams, J. C., Brown, M. C. and Morrow, B. E. (2006).
Tissue-specific roles of Tbx1 in the development of the outer, middle and
inner ear, defective in 22q11DS patients. Hum. Mol.
Genet. 15,1629
-1639.
Barald, K. F. and Kelley, M. W. (2004). From
placode to polarization: new tunes in inner ear development.
Development 131,4119
-4130.
Beckers, J., Clark, A., Wunsch, K., Hrabe De Angelis, M. and Gossler, A. (1999). Expression of the mouse Delta1 gene during organogenesis and fetal development. Mech. Dev. 84,165 -168.[CrossRef][Medline]
Ben-Arie, N., Bellen, H. J., Armstrong, D. L., McCall, A. E., Gordadze, P. R., Guo, Q., Matzuk, M. M. and Zoghbi, H. Y. (1997). Math1 is essential for genesis of cerebellar granule neurons. Nature 390,167 -172.
Bermingham, N. A., Hassan, B. A., Price, S. D., Vollrath, M. A.,
Nissim, B.-A., Eatock, R. A., Bellen, H. J., Lysakowski, A. and Zoghbi, H.
Y. (1999). Math1: An essential gene for the generation of
inner ear hair cells. Science
284,1837
-1841.
Bertrand, N., Castro, D. S. and Guillemot, F. (2002). Proneural genes and the specification of neural cell types. Nat. Rev. Neurosci. 3, 517-530.[CrossRef][Medline]
Brigande, J. V., Kiernan, A. E., Gao, X., Iten, L. E. and
Fekete, D. M. (2000). Molecular genetics of pattern formation
in the inner ear: Do compartment boundaries play a role? Proc.
Natl. Acad. Sci. USA 97,11700
-11706.
Brooker, R., Hozumi, K. and Lewis, J. (2006).
Notch ligands with contrasting functions: Jagged1 and Delta1 in the mouse
inner ear. Development
133,1277
-1286.
Chen, P., Johnson, J. E., Zoghbi, H. Y. and Segil, N. (2002). The role of Math1 in inner ear development: uncoupling the establishment of the sensory primordium from hair cell fate determination. Development 129,2495 -2505.[Medline]
Cole, L. K., Le Roux, I., Nunes, F., Laufer, E., Lewis, J. and Wu, D. K. (2000). Sensory organ generation in the chicken inner ear: contributions of Bone Morphogenetic Protein 4, Serrate1, and Lunatic Fringe. J. Comp. Neurol. 424,509 -520.[CrossRef][Medline]
Cornell, R. A. and Eisen, J. S. (2002).
Delta/Notch signaling promotes formation of zebrafish neural crest by
repressing Neurogenin 1 function. Development
129,2639
-2648.
D'Amico-Martel, A. and Noden, D. M. (1983). Contributions of placodal and neural crest cells to avian cranial peripheral ganglia. Am. J. Anat. 166,445 -468.[CrossRef][Medline]
Daudet, N., Ariza-McNaughton, L. and Lewis, J.
(2007). Notch signaling is needed to maintain, but not to
initiate, the formation of prosensory patches in the chick inner ear.
Development 134,2369
-2378.
Feil, R., Wagner, J., Metzger, D. and Chambon, P. (1997). Regulation of Cre recombinase activity by mutated estrogen receptor ligand-binding domains. Biochem. Biophys. Res. Commun. 28,752 -757.[CrossRef]
Fekete, D. M. (1996). Cell fate specification in the inner ear. Curr. Opin. Neurobiol. 6, 533-541.[CrossRef][Medline]
Fekete, D. M. and Wu, D. K. (2002). Revisiting cell fate specification in the inner ear. Curr. Opin. Neurobiol. 12,35 -42.[CrossRef][Medline]
Fode, C., Ma, Z., Casarosa, S., Ang, S.-L., Anderson, D. J. and
Guillemot, F. (2000). A role for neural determination genes
in specifying the dorsoventral identity of telencephalic neurons.
Genes Dev. 14,67
-80.
Fritzsch, B., Beisel, K. W., Jones, K., Farinas, I., Maklad, A., Lee, J. and Reichardt, L. F. (2002). Development and evolution of inner ear sensory epithelia and their innervation. J. Neurobiol. 53,143 -156.[CrossRef][Medline]
Gowan, K., Helms, A. W., Hunsaker, T. L., Collisson, T., Ebert, P. J., Odom, R. and Johnson, J. E. (2001). Crossinhibitory activities of Ngn1 and Math1 allow specification of distinct dorsal interneurons. Neuron 31,219 -232.[CrossRef][Medline]
Helms, A. W. and Johnson, J. E. (1998). Progenitors of dorsal commissural interneurons are defined by MATH1 expression. Development 125,919 -925.[Abstract]
Helms, A. W., Abney, A. L., Ben-Arie, N., Zoghbi, H. Y. and Johnson, J. E. (2000). Autoregulation and multiple enhancers control Math1 expression in the developing nervous system. Development 127,1185 -1196.[Abstract]
Inoue, T., Hojo, M., Sessho, Y., Tano, Y., Lee, J. E. and Kageyama, R. (2002). Math3 and NeuroD regulate amacrine cell fate specification in the retina. Development 129,831 -842.[Medline]
Irvine, K. D. (1999). Fringe, Notch, and making developmental boundaries. Curr. Opin. Genet. Dev. 9, 434-441.[CrossRef][Medline]
Izumikawa, M., Minoda, R., Kawamoto, K., Abrashkin, K. A., Swederski, D. L., Dolan, D. F., Brough, D. E. and Raphael. Y. (2005). Auditory hair cell replacement and hearing improvement by Atoh1 gene therapy in deaf mammals. Nat. Med. 11,271 -276.[CrossRef][Medline]
Jan, Y. N. and Jan, L. Y. (1993). HLH proteins, fly neurogenesis, and vertebrate myogenesis. Cell 75,827 -830.[CrossRef][Medline]
Kelley, M. W. (2006). Regulation of cell fate in the sensory epithelia of the inner ear. Nat. Rev. Neurosci. 7,837 -849.[CrossRef][Medline]
Kiernan, A. E., Steel, K. P. and Fekete, D. M. (2002). Development of the mouse inner ear. In Mouse Development: Patterning, morphogenesis, and Organogenesis (ed. J. Rossant and P. L. Tam), pp. 539-566. New York: Academic Press.
Lee, J. E., Hollenberg, S. M., Snider, L., Turner, D. L.,
Lipnick, N. and Wientraub, J. (1995). Conversion of Xenopus
ectoderm into neurons by NeuroD, a basic helix-loop-helix protein.
Science 268,836
-844.
Lei, L., Xu, A., Panin, V. M. and Irvine, K. D.
(2003). An O-fucose site in the ligand binding domain inhibits
Notch activation. Development
130,6411
-6421.
Lewis, J. (1996). Neurogenic genes and vertebrate neurogenesis. Curr. Opin. Neurobiol. 6, 3-10.[CrossRef][Medline]
Lumpkin, E. A., Collisson, T., Parab, P., Omer-Abdalla, A., Haeberle, H., Chen, P., Doetzlhofer, A., White, P., Groves, A. K., Segil, N. et al. (2003). Math1-driven GFP expression in the developing nervous system of transgenic mice. Gene Expr. Patterns 3, 389-395.[CrossRef][Medline]
Ma, Q., Kintner, C. and Anderson, D. J. (1996). Identification of neurogenin, a vertebrate neuronal determination gene. Cell 87,43 -52.[CrossRef][Medline]
Ma, Q., Chen, Z., del Barco Barantes, I., de la Ponpa, J. L. and Anderson, D. J. (1998). neurogenin1 is essential for the determination of neuronal precursors for proximal cranial sensory ganglia. Neuron 20,469 -482.[CrossRef][Medline]
Ma, Q., Anderson, D. J. and Fritzsch, B. (2000). Neurogenin1 null mutant ears develop fewer, morphologically normal hair cells in smaller sensory epithelia devoid of innervation. J. Assoc. Res. Otolaryngol. 1, 129-143.[Medline]
Matei, V., Pauley, S., Kaing, S., Rowitch, D., Beisel, K. W., Morris, K., Feng, F., Jones, K., Lee, J. and Fritzsch, B. (2005). Smaller inner ear epithelia in Neurog 1 null mice are related to earlier hair cell cycle exit. Dev. Dyn. 234,633 -650.[CrossRef][Medline]
Millimaki, B. B., Sweet, E. M., Dhason, M. S. and Riley, B.
B. (2007). Zebrafish atoh1 genes: classic proneural activity
in the inner ear and regulation by Fgf and Notch.
Development 134,295
-305.
Morsli, H., Choo, D., Ryan, A., Johnson, R. and Wu, D. K.
(1998). Development of the mouse inner ear and origin of its
sensory organs. J. Neurosci.
18,3327
-3335.
Novak, A., Guo, C., Yang, W., Nagy, A. and Lobe, C. G. (2000). Z/EG, a double reporter mouse line that expresses enhanced green fluorescent protein upon Cre-mediated excision. Genesis 28,147 -155.[CrossRef][Medline]
Okajima, T. A., Xu, A. and Irvine, K. D.
(2003). Modulation of notch-ligand binding by protein
O-fucosyltransferase. J. Biol. Chem.
278,42340
-42345.
Okajima, T. A., Xu, A., Lei, L. and Irvine, K. D.
(2005). Chaperone activity of protein O-fucosyltransferase 1
promotes Notch receptor folding. Science
307,1599
-1603.
Raft, S., Nowotschin, S., Liao, J. and Morrow, B.
(2004). Suppression of neural fate and control of inner ear
morphogenesis by Tbx1. Development
131,1801
-1812.
Satoh, T. and Fekete, D. M. (2005). Clonal
analysis of the relationships between mechanosensory cells and the neurons
that innervate them in the chicken ear. Development
132,1687
-1697.
Shailam, R., Lanford, P. J., Dolinsky, C. M., Norton, C. R., Gridley, T. and Kelley, M. W. (1999). Expression of proneural and neurogenic genes in the embryonic mammalian vestibular system. J. Neurocytol. 28,809 -819.[CrossRef][Medline]
Sher, A. E. (1971). The embryonic and postnatal development of the inner ear of the mouse. Acta Otolaryngol. Suppl. 285,5 -77.
Shi, S. and Stanley, P. (2003). Protein
O-fucosyltransferase 1 is an essential component of Notch signaling pathways.
Proc. Natl. Acad. Sci. USA
100,5234
-5239.
Stern, C. D. (1998). Detection of multiple gene products simultaneously by in situ hybridization and immunohistochemistry in whole mounts of avian embryos. Curr. Top. Dev. Biol. 36,223 -243.[Medline]
Vitelli, F., Viola, A., Morishima, M., Pramparo, T., Baldini, A.
and Linsay, E. A. (2003). TBX1 is required for inner ear
morphogenesis. Hum. Mol. Genet.
12,2041
-2048.
Woods, C., Montcouquiol, M. and Kelley, M. W. (2004). Math1 regulates development of the sensory epithelium in the mammalian cochlea. Nat. Neurosci. 7,1310 -1318.[CrossRef][Medline]
Yang, X. W., Model, P. and Heintz, N. (1997). Homologous recombination based modification in Escherichia coli and germline transmission in transgenic mice of a bacterial artificial chromosome. Nat. Biotechnol. 15,859 -865.[CrossRef][Medline]
Zheng, J. L. and Gao, W. Q. (2000). Overexpression of Math1 induces robust production of extra hair cells in postnatal rat inner ears. Nat. Neurosci. 3, 580-586.[CrossRef][Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  E. J. Koundakjian, J. L. Appler, and L. V. Goodrich Auditory Neurons Make Stereotyped Wiring Decisions before Maturation of Their Targets J. Neurosci., December 19, 2007; 27(51): 14078 - 14088. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
