Neurogliogenesis in the mature olfactory system: A possible protective role against infection and toxic dust

Abstract

The outpost position of the olfactory bulb (OB) between the direct inputs from sensory neurons of the nasal epithelium and other parts of the brain suggests its highest vulnerability among all brain structures to penetration of exogenous agents. A number of neurotropic viruses have been found to invade the brain through the OB. There is growing evidence that microscopic particles of toxic dusts can propagate from the nasal epithelium to the OB and further into the brain. These harmful agents impair cellular elements of the brain. Apparently, cells in the OB are the most affected, as they are the first to encounter viral infections and toxic particles. It is well known that neuronal and glial progenitors are continuously generated from neuronal stem cells in the subventricular zone of the adult brain and then migrate predominantly into the OB. Therefore, it is feasible to suggest that substitution of injured or dead cells in the OB by new-born neurons, differentiating from progenitors, plays a role in protecting the OB neuronal microcircuits from destruction. Furthermore, some cytokines and chemokines released in response to infection and/or intoxication can modulate different stages of neurogenesis (proliferation, migration, and differentiation). We hypothesize that continuous neurogenesis in the olfactory system throughout adulthood evolved as a protective mechanism to prevent impairment of the most ancient but vitally important sensory system. In addition, differentiation of a substantial portion of progenitors to glial cells, including macrophages and microglia, may create an additional barrier to exogenous agents on their way deep to the brain.

Canonical Wnt signaling promotes the proliferation and neurogenesis of peripheral olfactory stem cells during postnatal development and adult regeneration

“…The olfactory epithelium (OE) offers a unique opportunity to study the postnatal and adult stem cell biology and neural regeneration as a consequence of the robustness of stem cell activation and neuronal production throughout the lifespan of the animal (Calof et al, 2002;Loseva et al, 2009;Murdoch and Roskams, 2007;Schwob, 2002). Two groups of cells within the basal layer of the OE function as neuronal precursors, multipotent progenitors and/or stem cells: the globose basal cells (GBCs) and the horizontal basal cells (HBCs) (Iwai et al, 2008;Jang et al, 2003;Leung et al, 2007).…”

Transolfactory neuroinvasion by viruses threatens the human brain

“…Finally, similar to other epithelial cells, but unlike other neurons, ORNs undergo apoptosis and regeneration as a part of their normal turnover process that continues throughout adult life and they have an average life span ranging from 30 to 120 days (Oboti et al, 2011). Furthermore, neuronal stem cells in the subventricular zone of the adult brain predominantly migrate into the olfactory bulb, likely protecting the olfactory bulb neuronal circuits from damage by infectious and toxic agents (Loseva et al, 2009). Th e physiological processes of uptake and transsynaptic transport of molecules from the nasal cavity to the olfactory bulb potentially pose great risks for the CNS, namely neurological damage induced by environmental factors including viruses.…”

Smell with new neurons

“…These new neurons might also participate in odor memory formation and fine odor discrimination processes, because of their increased plasticity (Ortega-Perez et al 2007). Moreover, the consistent birth and death of new cells in the bulb might protect against dust and viruses that would otherwise invade the brain through the olfactory nerve pathway; the apoptosis of infected neurons have been demonstrated to stop the further transmission of pathogens in the brain (Loseva et al 2009). …”