Spiral ganglion (SG) neurons of the cochlea convey all auditory inputs to the brain, yet the cellular and molecular complexity necessary to decode the various acoustic features in the SG has remained ...unresolved. Using single-cell RNA sequencing, we identify four types of SG neurons, including three novel subclasses of type I neurons and the type II neurons, and provide a comprehensive genetic framework that define their potential synaptic communication patterns. The connectivity patterns of the three subclasses of type I neurons with inner hair cells and their electrophysiological profiles suggest that they represent the intensity-coding properties of auditory afferents. Moreover, neuron type specification is already established at birth, indicating a neuronal diversification process independent of neuronal activity. Thus, this work provides a transcriptional catalog of neuron types in the cochlea, which serves as a valuable resource for dissecting cell-type-specific functions of dedicated afferents in auditory perception and in hearing disorders.
Type I spiral ganglion neurons (I-SGNs) of the mammalian cochlea convey all acoustic information from the sensory hair cells to second order neurons in the brainstem. Despite evidence supporting ...physiological diversity of I-SGNs and of its importance for encoding the various features of sounds, knowledge of their molecular diversity is only emerging. In this review, we outline the recent efforts in the identification of mammalian I-SGN types and summarize how genetic and anatomical features of each individual neuron type relate to functional aspects that characterize sound information processing in the primary auditory afferent system.
Abstract
Different types of spiral ganglion neurons (SGNs) are essential for auditory perception by transmitting complex auditory information from hair cells (HCs) to the brain. Here, we use deep, ...single cell transcriptomics to study the molecular mechanisms that govern their identity and organization in mice. We identify a core set of temporally patterned genes and gene regulatory networks that may contribute to the diversification of SGNs through sequential binary decisions and demonstrate a role for NEUROD1 in driving specification of a I
c
-SGN phenotype. We also find that each trajectory of the decision tree is defined by initial co-expression of alternative subtype molecular controls followed by gradual shifts toward cell fate resolution. Finally, analysis of both developing SGN and HC types reveals cell-cell signaling potentially playing a role in the differentiation of SGNs. Our results indicate that SGN identities are drafted prior to birth and reveal molecular principles that shape their differentiation and will facilitate studies of their development, physiology, and dysfunction.
Proprioceptive neurons (PNs) are essential for the proper execution of all our movements by providing muscle sensory feedback to the central motor network. Here, using deep single cell RNAseq of ...adult PNs coupled with virus and genetic tracings, we molecularly identify three main types of PNs (Ia, Ib and II) and find that they segregate into eight distinct subgroups. Our data unveil a highly sophisticated organization of PNs into discrete sensory input channels with distinct spatial distribution, innervation patterns and molecular profiles. Altogether, these features contribute to finely regulate proprioception during complex motor behavior. Moreover, while Ib- and II-PN subtypes are specified around birth, Ia-PN subtypes diversify later in life along with increased motor activity. We also show Ia-PNs plasticity following exercise training, suggesting Ia-PNs are important players in adaptive proprioceptive function in adult mice.
Somatic sensation is defined by the existence of a diversity of primary sensory neurons with unique biological features and response profiles to external and internal stimuli. However, there is no ...coherent picture about how this diversity of cell states is transcriptionally generated. Here, we use deep single cell analysis to resolve fate splits and molecular biasing processes during sensory neurogenesis in mice. Our results identify a complex series of successive and specific transcriptional changes in post-mitotic neurons that delineate hierarchical regulatory states leading to the generation of the main sensory neuron classes. In addition, our analysis identifies previously undetected early gene modules expressed long before fate determination although being clearly associated with defined sensory subtypes. Overall, the early diversity of sensory neurons is generated through successive bi-potential intermediates in which synchronization of relevant gene modules and concurrent repression of competing fate programs precede cell fate stabilization and final commitment.
The sensation of pain is essential for the preservation of the functional integrity of the body. However, the key molecular regulators necessary for the initiation of the development of pain-sensing ...neurons have remained largely unknown. Here, we report that, in mice, inactivation of the transcriptional regulator PRDM12, which is essential for pain perception in humans, results in a complete absence of the nociceptive lineage, while proprioceptive and touch-sensitive neurons remain. Mechanistically, our data reveal that PRDM12 is required for initiation of neurogenesis and activation of a cascade of downstream pro-neuronal transcription factors, including NEUROD1, BRN3A, and ISL1, in the nociceptive lineage while it represses alternative fates other than nociceptors in progenitor cells. Our results thus demonstrate that PRDM12 is necessary for the generation of the entire lineage of pain-initiating neurons.
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•PRDM12 is expressed in neural crest cells (NCCs) and all nociceptive lineage neurons•Inactivation of PRDM12 results in the absence of the entire nociceptive lineage•Forced expression of PRDM12 in NCCs represses non-nociceptor fates•PRDM12 regulates progenitor proliferation and the sensory neurogenesis program
The sensation of pain, temperature, and itch by neurons of the nociceptive lineage is essential for animal survival. Bartesaghi et al. report that the transcriptional regulator PRDM12 is indispensable in neural crest cells (NCCs) for the initiation of the sensory neuronal differentiation program that generates the entire nociceptive lineage.
Developmental cell death plays an important role in the construction of functional neural circuits. In vertebrates, the canonical view proposes a selection of the surviving neurons through stochastic ...competition for target-derived neurotrophic signals, implying an equal potential for neurons to compete. Here we show an alternative cell fitness selection of neurons that is defined by a specific neuronal heterogeneity code. Proprioceptive sensory neurons that will undergo cell death and those that will survive exhibit different molecular signatures that are regulated by retinoic acid and transcription factors, and are independent of the target and neurotrophins. These molecular features are genetically encoded, representing two distinct subgroups of neurons with contrasted functional maturation states and survival outcome. Thus, in this model, a heterogeneous code of intrinsic cell fitness in neighboring neurons provides differential competitive advantage resulting in the selection of cells with higher capacity to survive and functionally integrate into neural networks.
Hearing, one of our main senses, allows us to socialize, listen and enjoy soundsaround us. The critical transmitters of the sound information are the spiralganglion neurons (SGNs); located in the ...cochlea, they transmit the auditory signalsfrom the hair cells to the brain. This thesis aims to extend our presentunderstanding of the diversity (study I and II) and the development (study III) ofthe SGNs. To contextualize, this thesis first reviews the relevant literature in theIntroduction chapter, followed by the presentation of the significant findings.In studies I and II, we use single-cell RNA sequencing to analyze the molecularprofiles of adult SGNs in mice. We identify three new subtypes of type I SGNs (Ia,Ib, and Ic) and new markers for type II neurons that we confirm withimmunological and in situ hybridization labeling. We also correlate those newsubtypes to previously known physiologically different subtypes of SGNs. Finally,we observe that those neuronal subtypes can already be identified soon after birthin mice. Results of study I and of previous research in the field are summarized instudy II.In study III, we use single-cell RNA sequencing to analyze the molecular profilesof mouse embryonic SGNs during their early development. We observe themolecular diversification of the different SGN lineages, starting with anunspecialized population at E14.5, giving rise, through successive bifurcations, todifferent developmental trajectories leading to Ic neurons, then the type IIneurons (E15.5-16.5), followed by Ib and Ia (E16.5-E17.5). The sequencinganalysis also revealed the dynamic change of genes and gene regulatory networksof potential importance for the SGNs diversification, among which Neurod1 wasidentified as essential for the Ic pathway differentiation program.Altogether, the data included in this thesis add new insights into the crucialmolecular aspects regulating the development and maturation of the SGNs andevidence regarding the existence of molecular types of SGNs.
The collection of human biological samples is of major importance for future research in France and Europe. In recent years, new regulatory procedures have been designed to monitor these activities; ...but they are somewhat complex and some clarifications are needed. The law needs also to be amended. The definition of biobanking activities should be clarified, and regulatory procedures, including consultation of the Ethics Committee, declarations to the Ministry of Research and the protection of personal data, should be simplified. It is also of great importance to correctly define the modalities in which Biobanks are granted their authorisations. The role of Ethics Committees regarding the evaluation of information and the consent procedures should also be clarified, particularly when samples from children are used, or when the samples are used for genetic analyses. As well as scientific and public health aspects, the storage of human biological samples may also have important economic consequences. It is hence crucial to adapt the procedure for submitting patents, particularly when several public or private partners are working together. The possible changes to both French and European laws planned in the next months would be an ideal time to introduce these changes.
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