Nature - USA (2020-01-16)

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fewer transcriptionally distinct neuronal populations in Ngf−/−Bax−/−
double mutants (Fig. 5a). Ntrk1-negative neurons (proprioceptors
and A-fibre mechanoreceptors) were not markedly transcriptionally
compromised in Ngf−/−Bax−/− mutants compared to Bax−/− controls
(Fig. 5b), as expected. However, the subtype-specific gene expression
patterns that are normally present in Ntrk1+ sensory neuron subtypes
were markedly altered in neurons from Ngf−/−Bax−/− mutants (Fig. 5c).
Notably, examination of the aforementioned subtype-restricted TFs
showed that the combinations of TFs expressed in the unidentified
neuronal clusters in Ngf−/−Bax−/− mutants bore no resemblance to the
TF combinations observed in neuronal subtypes in control mice, and
this was confirmed using smRNA-FISH (Extended Data Fig. 9a, b). Fur-
thermore, members of the NEUROD family of TFs, which are normally
extinguished during embryonic development, remained expressed at
P0 in Ngf−/−Bax−/− mutants (Extended Data Fig. 9b, c). These findings
indicate that the selection of somatosensory neuron subtype-restricted
TFs is controlled, at least in part, by extrinsic cues acting on nascent
sensory neurons.

Discussion

Our genome-wide transcriptomic analyses of cells traversing the devel-
opmental stages of somatosensory neurons support a model in which
newborn somatosensory neurons are unspecialized with respect to
expression of subtype-restricted TFs, and differential maintenance
of unique combinations of these subtype-restricted TFs enables nas-
cent sensory neurons to resolve into mature subtypes (Extended Data
Fig. 9d). Early co-expression, and subsequent resolution, of TFs has
been proposed to underlie the diversification of stem cells in the hae-
matopoietic lineage^36 –^38 , neural crest progenitors before lineage com-
mittment^39 , and developing spinal motor neurons^40 –^42 —although this

view has been challenged in the case of the haematopoietic system^43.

The diversification of somatosensory neurons shares commonalities

and differences with these systems. We propose that, unlike cells of the

early neural crest and the haematopoietic lineages, somatosensory

neuron subtypes emerge following cell-cycle exit and, unlike other

progenitor types, newborn, postmitotic Avil+ somatosensory neu-

rons are not migratory but rather are permanent residents of sensory

ganglia. Therefore, nascent sensory neurons cannot rely on cell divi-

sion or migration to encounter new environments. Rather, a feature

of nascent somatosensory neurons is that they immediately extend

axons along intermediate targets, such as large blood vessels, en route

to target organs, such as the skin, where they encounter extrinsic cues,

including NGF and other secreted factors. We propose a model in which

multiple distinct extrinsic cues act on axons of transcriptionally unspe-

cialized sensory neurons, depending on the timing and trajectories of

their projection patterns. These cues function, in part, to resolve TF

expression patterns from a co-expressed state to a subtype-restricted

state to promote the transcriptional specializations that underlie the

unique molecular, morphological, and physiological properties of

somatosensory neuron subtypes.

Online content

Any methods, additional references, Nature Research reporting sum-

maries, source data, extended data, supplementary information,

acknowledgements, peer review information; details of author con-

tributions and competing interests; and statements of data and code

availability are available at https://doi.org/10.1038/s41586-019-1900-1.

1. Abraira, V. E. & Ginty, D. D. The sensory neurons of touch. Neuron 79 , 618–639 (2013).


2. Julius, D. TRP channels and pain. Annu. Rev. Cell Dev. Biol. 29 , 355–384 (2013).

SST

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Fig. 5 | The extrinsic cue NGF is required for subtype-specif ic gene
expression patterns. a, t-SNE visualizations of scRNA-seq data for neurons
generated from P0 Bax−/− and Ng f−/−Bax−/− littermates. b, Cell-type-specific gene
expression in proprioceptor and A-fibre mechanoreceptor sensory neuron
subtypes in Bax−/− and Ng f−/−Bax−/− littermates. c, Cell-type-specific gene
expression in all other sensory neuron subtypes in Bax−/− and Ng f−/−Bax−/−
littermates. Boxes represent IQR, whiskers represent minimum and maximum
values, and notches represent the 95% confidence interval of the median.
b, c, All clusters are different from controls by two-sided Wilcoxon rank-sum
test with Bonferroni correction; P < 0.01. For n values, see Methods.