Distribution of Notch1-expressing cells and proliferating cells in mouse vomeronasal organ

Notch signaling determines cell-fate specification of the two main types of vomeronasal neurons of rodents

The ability of terrestrial vertebrates to find food and mating partners, and to avoid predators, relies on the detection of chemosensory information. Semiochemicals responsible for social and sexual behaviors are detected by chemosensory neurons of the vomeronasal organ (VNO), which transmits information to the accessory olfactory bulb. The vomeronasal sensory epithelium of most mammalian species contains a uniform vomeronasal system; however, rodents and marsupials have developed a more complex binary vomeronasal system, containing vomeronasal sensory neurons (VSNs) expressing receptors of either the V1R or V2R family. In rodents, V1R/apical and V2R/basal VSNs originate from a common pool of progenitors. Using single cell RNA-sequencing, we identified differential expression of Notch1 receptor and Dll4 ligand between the neuronal precursors at the VSN differentiation dichotomy. Our experiments show that Notch signaling is required for effective differentiation of V2R/basal VSNs. In fact, Notch1 loss of function in neuronal progenitors diverts them to the V1R/apical fate, whereas Notch1 gain of function redirects precursors to V2R/basal. Our results indicate that Notch signaling plays a pivotal role in triggering the binary differentiation dichotomy in the VNO of rodents.

Analysis of the vomeronasal organ transcriptome reveals variable gene expression depending on age and function in rabbits

The vomeronasal organ (VNO) is a chemosensory organ specialized in pheromone detection that shows a broad morphofunctional and genomic diversity among mammals. However, its expression patterns have only been well-characterized in mice. Here, we provide the first comprehensive RNA sequencing study of the rabbit VNO across gender and sexual maturation stages. We characterized the VNO transcriptome, updating the number and expression of the two main vomeronasal receptor families, including 128 V1Rs and 67 V2Rs. Further, we defined the expression of formyl-peptide receptor and transient receptor potential channel families, both known to have specific roles in the VNO. Several sex hormone-related pathways were consistently enriched in the VNO, highlighting the relevance of this organ in reproduction. Moreover, whereas juvenile and adult VNOs showed significant transcriptome differences, male and female did not. Overall, these results contribute to understand the genomic basis of behavioural responses mediated by the VNO in a non-rodent model.

Smad4-dependent morphogenic signals control the maturation and axonal targeting of basal vomeronasal sensory neurons to the accessory olfactory bulb

The vomeronasal organ (VNO) contains two main types of vomeronasal sensory neurons (VSNs) that express distinct vomeronasal receptor (VR) genes and localize to specific regions of the neuroepithelium. Morphogenic signals are crucial in defining neuronal identity and network formation; however, if and what signals control maturation and homeostasis of VSNs is largely unexplored. Here, we found transforming growth factor β (TGFβ) and bone morphogenetic protein (BMP) signal transduction in postnatal mice, with BMP signaling being restricted to basal VSNs and at the marginal zones of the VNO: the site of neurogenesis. Using different Smad4 conditional knockout mouse models, we disrupted canonical TGFβ/BMP signaling in either maturing basal VSNs (bVSNs) or all mature VSNs. Smad4 loss of function in immature bVSNs compromises dendritic knob formation, pheromone induced activation, correct glomeruli formation in the accessory olfactory bulb (AOB) and survival. However, Smad4 loss of function in all mature VSNs only compromises correct glomeruli formation in the posterior AOB. Our results indicate that Smad4-mediated signaling drives the functional maturation and connectivity of basal VSNs.

Summary: Genetic disruption of TGFβ/BMP signaling in maturing basal vomeronasal sensory neurons (VSNs) or in all mature VSNs indicates that Smad4 signaling drives maturation and connectivity of basal VSNs.

The transcription factor Tfap2e/AP-2ε plays a pivotal role in maintaining the identity of basal vomeronasal sensory neurons

The identity of individual neuronal cell types is defined and maintained by the expression of specific combinations of transcriptional regulators that control cell type-specific genetic programs. The epithelium of the vomeronasal organ of mice contains two major types of vomeronasal sensory neurons (VSNs): 1) the apical VSNs which express vomeronasal 1 receptors (V1r) and the G-protein subunit Gαi2 and; 2) the basal VSNs which express vomeronasal 2 receptors (V2r) and the G-protein subunit Gαo. Both cell types originate from a common pool of progenitors and eventually acquire apical or basal identity through largely unknown mechanisms. The transcription factor AP-2ε, encoded by the Tfap2e gene, plays a role in controlling the development of GABAergic interneurons in the main and accessory olfactory bulb (AOB), moreover AP-2ε has been previously described to be expressed in the basal VSNs. Here we show that AP-2ε is expressed in post-mitotic VSNs after they commit to the basal differentiation program. Loss of AP-2ε function resulted in reduced number of basal VSNs and in an increased number of neurons expressing markers of the apical lineage. Our work suggests that AP-2ε, which is expressed in late phases of differentiation, is not needed to initiate the apical-basal differentiation dichotomy but for maintaining the basal VSNs' identity. In AP-2ε mutants we observed a large number of cells that entered the basal program can express apical genes, our data suggest that differentiated VSNs of mice retain a notable level of plasticity.

Pregnancy and estrogen enhance neural progenitor-cell proliferation in the vomeronasal sensory epithelium

Background

The hormonal state during the estrus cycle or pregnancy produces alterations on female olfactory perception that are accompanied by specific maternal behaviors, but it is unclear how sex hormones act on the olfactory system to enable these sensory changes.

Results

Herein, we show that the production of neuronal progenitors is stimulated in the vomeronasal organ (VNO) epithelium of female mice during a late phase of pregnancy. Using a wide range of molecular markers that cover the whole VNO cell maturation process in combination with Ca²⁺ imaging in early postmitotic neurons, we show that newly generated VNO cells adopt morphological and functional properties of mature sensory neurons. A fraction of these newly generated cells project their axons to the olfactory forebrain, extend dendrites that contact the VNO lumen, and can detect peptides and urinary proteins shown to contain pheromone activity. High-throughput RNA-sequencing reveals concomitant differences in gene expression in the VNO transcriptomes of pregnant females. These include relative increases in expression of 20 vomeronasal receptors, of which 17 belong to the V1R subfamily, and may therefore be considered as candidate receptors for mediating maternal behaviors. We identify the expression of several hormone receptors in the VNO of which estrogen receptor α (Esr1) is directly localized to neural progenitors. Administration of sustained high levels of estrogen, but not progesterone, is sufficient to stimulate vomeronasal progenitor cell proliferation in the VNO epithelium.

Conclusions

Peripheral olfactory neurogenesis driven by estrogen may contribute to modulate sensory perception and adaptive VNO-dependent behaviors during pregnancy and early motherhood.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0211-8) contains supplementary material, which is available to authorized users.

A lifetime of neurogenesis in the olfactory system

Neurogenesis continues well beyond embryonic and early postnatal ages in three areas of the nervous system. The subgranular zone supplies new neurons to the dentate gyrus of the hippocampus. The subventricular zone supplies new interneurons to the olfactory bulb, and the olfactory neuroepithelia generate new excitatory sensory neurons that send their axons to the olfactory bulb. The latter two areas are of particular interest as they contribute new neurons to both ends of a first-level circuit governing olfactory perception. The vomeronasal organ and the main olfactory epithelium comprise the primary peripheral olfactory epithelia. These anatomically distinct areas share common features, as each exhibits extensive neurogenesis well beyond the juvenile phase of development. Here we will discuss the effect of age on the structural and functional significance of neurogenesis in the vomeronasal and olfactory epithelia, from juvenile to advanced adult ages, in several common model systems. We will next discuss how age affects the regenerative capacity of these neural stem cells in response to injury. Finally, we will consider the integration of newborn neurons into an existing circuit as it is modified by the age of the animal.

Regeneration of new neurons is preserved in aged vomeronasal epithelia

During normal and diseased aging, it is thought the capacity for tissue regeneration and repair in neuronal tissues diminishes. In the peripheral olfactory system, stem cell reservoirs permit regeneration of olfactory and vomeronasal sensory neurons, a unique capacity among neurons. Following injury, a large number of new neurons can be regenerated in a young animal. However, it is unknown whether this capacity for renewal exists in aged proliferative populations. Here, we report that neuronal replacement-associated proliferation continues in the vomeronasal organ of aged (18-24 months) mice. In addition, the potential for the aged stem cell to yield a mature neuron persisted at the same rate as that observed in young animals. Furthermore, the robust regenerative capacity to respond to both acute and sustained injury following olfactory bulbectomy remains intact even in very old animals. Hence, the neuronal epithelium lining the vomeronasal organ is unique in that it contains stem cells capable of generating functional neurons throughout life and in the aged animal in particular. This persistent regenerative capacity provides hope for neuronal replacement therapies in the aged nervous system.

Embryonic development of Python sebae – II: Craniofacial microscopic anatomy, cell proliferation and apoptosis

This study explores the microscopic craniofacial morphogenesis of the oviparous African rock python (Python sebae) spanning the first two-thirds of the post-oviposition period. At the time of laying, the python embryo consists of largely undifferentiated mesenchyme and epithelium with the exception of the cranial base and trabeculae cranii, which are undergoing chondrogenesis. The facial prominences are well defined and are at a late stage, close to the time when lip fusion begins. Later (11-12d), specializations in the epithelia begin to differentiate (vomeronasal and olfactory epithelia, teeth). Dental development in snakes is different from that of mammals in several aspects including an extended dental lamina with the capacity to form 4 sets of generational teeth. In addition, the ophidian olfactory system is very different from the mammalian. There is a large vomeronasal organ, a nasal cavity proper and an extraconchal space. All of these areas are lined with a greatly expanded olfactory epithelium. Intramembranous bone differentiation is taking place at stage 3 with some bones already ossifying whereas most are only represented as mesenchymal condensations. In addition to routine histological staining, PCNA immunohistochemistry reveals relatively higher levels of proliferation in the extending dental laminae, in osseous mesenchymal condensations and in the olfactory epithelia. Areas undergoing apoptosis were noted in the enamel organs of the teeth and osseous mesenchymal condensations. We propose that localized apoptosis helps to divide a single condensation into multiple ossification centres and this is a mechanism whereby novel morphology can be selected in response to evolutionary pressures. Several additional differences in head morphology between snakes and other amniotes were noted including a palatal groove separating the inner and outer row of teeth in the upper jaw, a tracheal opening within the tongue and a pharyngeal adhesion that closes off the pharynx from the oral cavity between stages 1 and 4. Our studies on these and other differences in the python will provide valuable insights into in developmental, molecular and evolutionary mechanisms of patterning.