Low survival rate of young adult-born olfactory sensory neurons in the undamaged mouse olfactory epithelium

Abstract

Olfactory sensory neurons (OSNs) are generated throughout life from progenitor cells in the olfactory epithelium. OSN axons project in an odorant receptor-specific manner to the olfactory bulb (OB), forming an ordered array of glomeruli where they provide sensory input to OB neurons. The tetracycline transactivator (tTA) system permits developmental stage-specific expression of reporter genes in OSNs and has been widely used for structural and functional studies of the development and plasticity of the mouse olfactory system. However, the cellular ages at which OSNs stop expressing reporters driven by the immature OSN-specific Gγ8-tTA driver line and begin to express reporters driven by the mature OSN-specific OMP-tTA driver line have not been directly determined. We pulse-labeled terminally dividing cells in the olfactory epithelium of 28-day-old (P28) mice with EdU and analyzed EdU labeling in OSNs expressing fluorescent reporter proteins under control of either the Gγ8-tTA or OMP-tTA driver line 5-14 days later. Expression of OMP-tTA-driven reporters began in 6-day-old OSNs, while the vast majority of newborn OSNs did not express Gγ8-tTA-driven fluorescent proteins beyond 8 days of cellular age. Surprisingly, we also found a low survival rate for P28-born OSNs, very few of which survived for more than 14 days. We propose that OSN survival requires the formation of stable synaptic connections and hence may be dependent on organismal age.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

“…Our data are consistent with previous studies that used different approaches to label cells at the time of neurogenesis and analyze OMP expression in OSNs of different ages and, in some cases, following OE injury. In concordance with our data, these alternate approaches also found that OSNs do not express OMP before 6–7 d following basal cell division, regardless of the age of the animal or the OE condition (Miragall and Monti Graziadei, 1982; Schwob et al, 1992; Kondo et al, 2010; Kikuta et al, 2015; Rodriguez-Gil et al, 2015; Savya et al, 2019). Our findings show that the number of OMP + cells increases significantly after 8 d following basal cell division (Table 2, Fig.…”

“…2 E ) (cf. Suzuki and Takeda, 1993; Vedin et al, 2009; Kondo et al, 2010; Savya et al, 2019). Given that the total number of cells is determined by the balance between proliferation rate and cell death, we performed immunostaining for cleaved-caspase-3 (cCasp3), a marker for apoptotic cells, and for Ki67, a cellular marker for proliferation (Fig.…”

“…These precursor cells subsequently commit to a gene expression pattern that allows them to differentiate into the OSN (for review, see Suárez et al, 2012; Brann and Firestein, 2014). Previous studies reported that the presence of GAP 43 is transiently high in a temporal window between 3 and 9 d following injection of the thymidine analog EdU (Coleman et al, 2017), and between 5 and 10 d for Gγ8 following basal cell division (Savya et al, 2019), both of which are markers for iOSNs. Our results are overall in agreement with these previous findings; we show that GAP 43 is first expressed at DPI-Tx-3, peaks at DPI-Tx-5, maintains high expression through DPI-Tx-10, and decreases significantly afterward.…”

“…The OE is dynamic and develops throughout life, and may reach a point of equilibrium in which the OSN loss is counterbalanced by ongoing OSN neurogenesis. Our understanding of OSN neurogenesis has benefited from studies that have examined different aspects of the process, including OSN survival, regeneration, proliferation, and aging (Kondo et al, 2010; Holl, 2018; Ueha et al, 2018; Savya et al, 2019). Rodriguez-Gil et al (2015) previously characterized the maturation of perinatally generated OSNs, a period of exuberant neurogenesis that occurs before the maturation of the olfactory pathway.…”

Immature olfactory sensory neurons provide behaviourally relevant sensory input to the olfactory bulb

“…After terminal division of OE basal cells, Ascl1-expressing neuronally-committed intermediate cells briefly become nascent OSNs that express CXCR4 and DBN1 before transitioning to GAP43- and Gγ8-expressing immature OSNs (McIntyre et al, 2010; Ryba and Tirindelli, 1995; Schwob et al, 2017; Verhaagen et al, 1989). OSNs begin to express olfactory marker protein (OMP) and downregulate GAP43 expression 7-8 days after terminal cell division (Liberia et al, 2019; Miragall and Graziadei, 1982; Rodriguez-Gil et al, 2015; Savya et al, 2019). Mature OSNs express OMP but not GAP43 or Gγ8.…”