Characterization and isolation of immature neurons of the adult mouse piriform cortex

Abstract

Physiological studies indicate that the piriform or primary olfactory cortex of adult mammals exhibits a high degree of synaptic plasticity. Interestingly, a subpopulation of cells in the layer II of the adult piriform cortex expresses neurodevelopmental markers, such as the polysialylated form of neural cell adhesion molecule (PSA-NCAM) or doublecortin (DCX). This study analyzes the nature, origin, and potential function of these poorly understood cells in mice. As previously described in rats, most of the PSA-NCAM expressing cells in layer II could be morphologically classified as tangled cells and only a small proportion of larger cells could be considered semilunar-pyramidal transitional neurons. Most were also immunoreactive for DCX, confirming their immature nature. In agreement with this, detection of PSA-NCAM combined with that of different cell lineage-specific antigens revealed that most PSA-NCAM positive cells did not co-express markers of glial cells or mature neurons. Their time of origin was evaluated by birthdating experiments with halogenated nucleosides performed at different developmental stages and in adulthood. We found that virtually all cells in this paleocortical region, including PSA-NCAM-positive cells, are born during fetal development. In addition, proliferation analyses in adult mice revealed that very few cells were cycling in layer II of the piriform cortex and that none of them was PSA-NCAM-positive. Moreover, we have established conditions to isolate and culture these immature neurons in the adult piriform cortex layer II. We find that although they can survive under certain conditions, they do not proliferate in vitro either. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 748-763, 2016.

Distribution and fate of DCX/PSA-NCAM expressing cells in the adult mammalian cortex: A local reservoir for adult cortical neuroplasticity?

“…In addition, electron microscopic analysis by Shapiro and colleagueś (2007) revealed that DCX labeled cells had ultrastructural features of immature migrating cells. Third, DCX/PSA-NCAM-expressing cells are negative for mature oligodendroglial, astroglial and microglial markers (Gómez-Climent et al, 2008;Luzzati et al, 2009;Rubio et al, 2015), and very few DCX/PSA-NCAM-expressing cells express NG2 within the piriform cortex (Gómez-Climent et al, 2008;Luzzati et al, 2009;Rubio et al, 2015). Fourth, the expression of DCX/ PSA-NCAM in cortical cells overlaps with other early developmental markers typically found in immature neurons during development, such as TUC-4, TUJ-1, and CNGA3 (Gómez-Climent et al, 2008;Xiong et al, 2008;Luzzati et al, 2009).…”

“…Whether or not some pyramidal neurons in the adult murine piriform cortex are derived from nonproliferating NG2-expressing cells (Guo et al, 2010;Richardson et al, 2011), does not exclude the contribution of subventricular zone-derived neuroblasts to the generation of neuron-specific nuclear protein (NeuN)-expressing cells in very young rodents (Shapiro et al, 2009;Shapiro et al, 2007a), nor does it exclude that prenatally produced DCX/ PSA-NCAM-expressing cells may also be present in cortical layer II of adult mammals Luzzati et al, 2009;Zhang et al, 2009;Varea et al, 2011;Rossi et al, 2014;Rubio et al, 2015). Given previous disagreements, it is possible that these three cell populations, with an assumed neuronal fate, are present in the healthy adult piriform cortex in different quantities according to age and species (e.g.…”

“…The first evidence is that most of these cells are negative for markers of mature principal neurons and interneurons (Gómez-Climent et al, 2008;Cai et al, 2009;Luzzati et al, 2009;Varea et al, 2011;Rubio et al, 2015). Second, they are devoid of ultrastructural features that indicate synaptic inputs, as shown by transmission electron microscopy (Gómez-Climent et al, 2008).…”

“…Notably, it has been recently suggested that post-mitotic cells mature into newly formed neurons in some noncanonical neurogenic regions of the adult mammalian brain (Gómez-Climent et al, 2008;Luzzati et al, 2009;GomezCliment et al, 2010;Bonfanti and Peretto, 2011;Bonfanti and Nacher, 2012;Clarke et al, 2012;Rubio et al, 2015), which raises questions regarding the origin, fate, and functional relevance of these post-mitotic cells in the adult cortex.…”

“…Cortical cells which co-express both DCX and PSA-NCAM (DCX/PSA-NCAM cells), are likely to constitute a population of immature and post-mitotic neurons in the adult brain, which are not tagged by short-term BrdU pulse-labeling experiments (Gómez-Climent et al, 2008;Luzzati et al, 2009;Varea et al, 2011;Bonfanti and Nacher, 2012;Rubio et al, 2015;Yang et al, 2015). Nevertheless, these cells maybe a source of new inhibitory (Xiong et al, 2008;Cai et al, 2009;Zhang et al, 2009) or excitatory neurons (Luzzati et al, 2009;Varea et al, 2011;Rubio et al, 2015). Until now, the origin, fate, and function of cortical DCX/PSA-NCAM-expressing cells in non-canonical neurogenic niches remain controversial.…”