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A gradient of Bmp7 specifies the tonotopic axis in the developing inner ear.

Mann ZF, Thiede BR, Chang W, Shin JB, May-Simera HL, Lovett M, Corwin JT, Kelley MW - Nat Commun (2014)

Bottom Line: During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics.Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7.Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Cochlear Development, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, 35A Convent Drive, Bethesda, Maryland 20892-3729, USA.

ABSTRACT
The auditory systems of animals that perceive sounds in air are organized to separate sound stimuli into their component frequencies. Individual tones then stimulate mechanosensory hair cells located at different positions on an elongated frequency (tonotopic) axis. During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics. Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7. Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics. Further, the effects of Bmp7 in determination of positional identity are shown to be mediated through activation of the Mapk, Tak1. These results indicate that graded, Bmp7-dependent, activation of Tak1 signalling controls the determination of frequency-specific hair cell characteristics along the tonotopic axis.

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Bmp7 is expressed in a gradient along the tonotopic axis of the BP(a)Differential expression of members of the Bmp signalling pathway in proximal and distal regions of the BP at E6.5. Data are presented as the average PMMR (sequences per million mapped reads) forBmp7, Follistatin-like 1& 4, chordin-like 1 and Twisted gastrulation (see text for further details). All experiments were run in triplicate (n=3). (b)Expression of Bmp7 in BP tissue from the indicated regions and time points based onquantitative real-timepolymerase chain reaction (qPCR) data. An increasing proximal-to-distal gradient is present at all four time points. (c)qPCR data for Chordin-like 1 (Chdl1) indicates the presence of a counter (decreasing proximal-to-distal) gradient to Bmp7 between E6.5 and E10.(d)Twisted gastrulation, which acts to enhance the effects of Chordin-like 1, is also present in a decreasing proximal-to-distal gradient between E6.5 and E10. For b,c and d, data are mean ± sem and n=4 for each developmental age and position along the BP. (e)In situ hybridization for Bmp7at E8 (top), E10 (middle) and Chdl1(bottom)in cross-sections through the BP at the indicated locations. Both Bmp7 and Chdl1are expressed in the developing sensory epithelium of the BP (brackets) and gradients of expression are evident. A sense control for Bmp7(bottom right) was negative.
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Figure 2: Bmp7 is expressed in a gradient along the tonotopic axis of the BP(a)Differential expression of members of the Bmp signalling pathway in proximal and distal regions of the BP at E6.5. Data are presented as the average PMMR (sequences per million mapped reads) forBmp7, Follistatin-like 1& 4, chordin-like 1 and Twisted gastrulation (see text for further details). All experiments were run in triplicate (n=3). (b)Expression of Bmp7 in BP tissue from the indicated regions and time points based onquantitative real-timepolymerase chain reaction (qPCR) data. An increasing proximal-to-distal gradient is present at all four time points. (c)qPCR data for Chordin-like 1 (Chdl1) indicates the presence of a counter (decreasing proximal-to-distal) gradient to Bmp7 between E6.5 and E10.(d)Twisted gastrulation, which acts to enhance the effects of Chordin-like 1, is also present in a decreasing proximal-to-distal gradient between E6.5 and E10. For b,c and d, data are mean ± sem and n=4 for each developmental age and position along the BP. (e)In situ hybridization for Bmp7at E8 (top), E10 (middle) and Chdl1(bottom)in cross-sections through the BP at the indicated locations. Both Bmp7 and Chdl1are expressed in the developing sensory epithelium of the BP (brackets) and gradients of expression are evident. A sense control for Bmp7(bottom right) was negative.

Mentions: To identify potential candidate molecules that could be instructive for tonotopic identity, differential expression of mRNAs along the tonotopic axis of the developing BP was analyzed using both Affymetrix microarrays and RNA-seq (Supplementary Data 1 and 2). Briefly, based on the results of the experiments described above, E6.5 BPs were dissected and separated into proximal and distal halves(microarray) or proximal, middle and distal thirds (RNA-seq) prior to isolation of mRNA. Microarray data were analyzed to identify transcripts that were expressed at levels at least two-fold higher in proximal versus distal halves and Gene Ontology (GO) analysis was used to identify relevant developmental pathways with differential activity in different halves of the BP. The pathways represented by the highest number of differentially expressed transcripts were those associated with cytoskeleton remodeling, in particular members of the transforming growth factor(TGF)-β/ bone morphogenetic protein (Bmp) signaling pathway. We were interested in the high number of differentially expressed transcripts related to TGF signaling (Fig. 2a), as this pathway has been shown to regulate developmental patterning in a number of tissues and several members have been suggested to act as morphogens21. Of the genes known to act within this pathway, transcripts for Bone morphogenetic protein 7 (Bmp7) showed the highest fold difference between proximal and distal regions (Fig. 2 and Supplementary Fig. 2) (2.9 fold difference in transcript expression measured by Affymetrix array and a 2-fold difference in transcript expression as measured by RNA-seq)(Fig. 2a, Supplementary Fig. 2). Consistent with this observation, two Bmp-immediate response genes, Id1 and Id3, were also enriched in the distal half of the BP (Supplementary Data 1,2). In addition, transcripts for several naturally-occurring antagonists of Bmp were found to be expressed at higher levels in the proximal region of the BP (Fig. 2a). Finally, the higher level of expression of transcripts forBmp7 in the distal region of the BP was also consistent with the location of a signaling center at the distal tip, as was suggested from the results of the separation experiments.


A gradient of Bmp7 specifies the tonotopic axis in the developing inner ear.

Mann ZF, Thiede BR, Chang W, Shin JB, May-Simera HL, Lovett M, Corwin JT, Kelley MW - Nat Commun (2014)

Bmp7 is expressed in a gradient along the tonotopic axis of the BP(a)Differential expression of members of the Bmp signalling pathway in proximal and distal regions of the BP at E6.5. Data are presented as the average PMMR (sequences per million mapped reads) forBmp7, Follistatin-like 1& 4, chordin-like 1 and Twisted gastrulation (see text for further details). All experiments were run in triplicate (n=3). (b)Expression of Bmp7 in BP tissue from the indicated regions and time points based onquantitative real-timepolymerase chain reaction (qPCR) data. An increasing proximal-to-distal gradient is present at all four time points. (c)qPCR data for Chordin-like 1 (Chdl1) indicates the presence of a counter (decreasing proximal-to-distal) gradient to Bmp7 between E6.5 and E10.(d)Twisted gastrulation, which acts to enhance the effects of Chordin-like 1, is also present in a decreasing proximal-to-distal gradient between E6.5 and E10. For b,c and d, data are mean ± sem and n=4 for each developmental age and position along the BP. (e)In situ hybridization for Bmp7at E8 (top), E10 (middle) and Chdl1(bottom)in cross-sections through the BP at the indicated locations. Both Bmp7 and Chdl1are expressed in the developing sensory epithelium of the BP (brackets) and gradients of expression are evident. A sense control for Bmp7(bottom right) was negative.
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Figure 2: Bmp7 is expressed in a gradient along the tonotopic axis of the BP(a)Differential expression of members of the Bmp signalling pathway in proximal and distal regions of the BP at E6.5. Data are presented as the average PMMR (sequences per million mapped reads) forBmp7, Follistatin-like 1& 4, chordin-like 1 and Twisted gastrulation (see text for further details). All experiments were run in triplicate (n=3). (b)Expression of Bmp7 in BP tissue from the indicated regions and time points based onquantitative real-timepolymerase chain reaction (qPCR) data. An increasing proximal-to-distal gradient is present at all four time points. (c)qPCR data for Chordin-like 1 (Chdl1) indicates the presence of a counter (decreasing proximal-to-distal) gradient to Bmp7 between E6.5 and E10.(d)Twisted gastrulation, which acts to enhance the effects of Chordin-like 1, is also present in a decreasing proximal-to-distal gradient between E6.5 and E10. For b,c and d, data are mean ± sem and n=4 for each developmental age and position along the BP. (e)In situ hybridization for Bmp7at E8 (top), E10 (middle) and Chdl1(bottom)in cross-sections through the BP at the indicated locations. Both Bmp7 and Chdl1are expressed in the developing sensory epithelium of the BP (brackets) and gradients of expression are evident. A sense control for Bmp7(bottom right) was negative.
Mentions: To identify potential candidate molecules that could be instructive for tonotopic identity, differential expression of mRNAs along the tonotopic axis of the developing BP was analyzed using both Affymetrix microarrays and RNA-seq (Supplementary Data 1 and 2). Briefly, based on the results of the experiments described above, E6.5 BPs were dissected and separated into proximal and distal halves(microarray) or proximal, middle and distal thirds (RNA-seq) prior to isolation of mRNA. Microarray data were analyzed to identify transcripts that were expressed at levels at least two-fold higher in proximal versus distal halves and Gene Ontology (GO) analysis was used to identify relevant developmental pathways with differential activity in different halves of the BP. The pathways represented by the highest number of differentially expressed transcripts were those associated with cytoskeleton remodeling, in particular members of the transforming growth factor(TGF)-β/ bone morphogenetic protein (Bmp) signaling pathway. We were interested in the high number of differentially expressed transcripts related to TGF signaling (Fig. 2a), as this pathway has been shown to regulate developmental patterning in a number of tissues and several members have been suggested to act as morphogens21. Of the genes known to act within this pathway, transcripts for Bone morphogenetic protein 7 (Bmp7) showed the highest fold difference between proximal and distal regions (Fig. 2 and Supplementary Fig. 2) (2.9 fold difference in transcript expression measured by Affymetrix array and a 2-fold difference in transcript expression as measured by RNA-seq)(Fig. 2a, Supplementary Fig. 2). Consistent with this observation, two Bmp-immediate response genes, Id1 and Id3, were also enriched in the distal half of the BP (Supplementary Data 1,2). In addition, transcripts for several naturally-occurring antagonists of Bmp were found to be expressed at higher levels in the proximal region of the BP (Fig. 2a). Finally, the higher level of expression of transcripts forBmp7 in the distal region of the BP was also consistent with the location of a signaling center at the distal tip, as was suggested from the results of the separation experiments.

Bottom Line: During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics.Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7.Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Cochlear Development, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, 35A Convent Drive, Bethesda, Maryland 20892-3729, USA.

ABSTRACT
The auditory systems of animals that perceive sounds in air are organized to separate sound stimuli into their component frequencies. Individual tones then stimulate mechanosensory hair cells located at different positions on an elongated frequency (tonotopic) axis. During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics. Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7. Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics. Further, the effects of Bmp7 in determination of positional identity are shown to be mediated through activation of the Mapk, Tak1. These results indicate that graded, Bmp7-dependent, activation of Tak1 signalling controls the determination of frequency-specific hair cell characteristics along the tonotopic axis.

Show MeSH
Related in: MedlinePlus