Roles Of Glycogen Synthase Kinase3β In The Olfactory System

Glycogen synthase kinase3β (GSK3β) is widely expressed in almost all the mammalian tissues, with particularly abundant levels in the central nervous system. GSK3β is a multifaceted kinase, it is involved in many cellular processes, including glycometabolism, cell proliferation, differentiation and apoptosis, thus its activity must be precisely regulated. Its activity is regulated by tyrosine (stimulatory) and serine (inhibitory) phosphorylation, protein complex formation and intracellular localization. The activity of this kinase is primarily regulated by phosphorylation. Phosphorylation at Ser9(S9-GSK3β) and Tyr216(Y216-GSK3β) inhibits and activates the kinase activity, respectively. In resting cells, Y216-GSK3β is constitutively phosphorylated [3], and the activity of GSK3β is primarily regulated by inhibition. Recent studies suggested that GSK3β acts as an integrating protein in many signal pathways and dysregulation of GSK3β implicitly linked to diverse neurological diseases, i.e. Alzheimer’s disease and schizophrenia.The olfactory system is crucial for animals. Olfaction is involved in a wide range of behaviors, including emotional modulations, mate selection, sexual and parental behaviors, aggressive behavior, etc. It is initiated when odorants bind to their receptors in the olfactory sensory neurons (OSNs) to trigger a transduction cascade that results in the release of neurotransmitters, such as glutamate, in the glomeruli. Odorant information is further processed in the olfactory bulb (OB) and sent to the primary olfactory cortexes by mitral/tufted cells. Previous studies have reported that neurotransmitter synthesis and release in the OB are activity dependent and olfactory dysfunction is associated with many neurological diseases, such as Alzheimer’s disease, Parkinson’s disease and schizophrenia, although the roles of olfaction in these neurological diseases are not yet clear.In some neurological diseases, such as AD, olfactory dysfunction is known to be among the earliest symptoms. There are substantial studies strongly implicating that over-activation of GSK3β plays vital roles in AD, including the Aβ and tau pathogenesis. However, only limited studies have investigated the function of GSK3β in the olfactory system. Nevertheless, the functions of GSK3β in the olfactory system remain largely unknown. To gain insights into the potential functions of GSK3P in the olfactory system, it is necessary to explore the detailed dynamic expression patterns of the kinase in the system.Our results showed that:1. GSK3P was abundant in main and accessory olfactory systems, including the OE and VON, MOB and AOB at all different developmental stages. The expression level of GSK3β showed a decreasing tendency along with the developing olfactory system.2. Y216-GSK3β showed a dynamic expression pattern during the developing olfactory system, both in the expression level and cellular location. In embryonic stages, Y216-GSK3P was expressed in the whole olfactory bulb, as well as in the olfactory sensory neuron cell bodies and axons. In the adult mouse, Y216-GSK3β was not expressed in the olfactory epithelium; in the adult olfactory bulb, Y216-GSK3β immuno-signal could be detected in all layers except for the olfactory sensory neuron layer. Thus, GSK3β is constitutive active in the adult mouse olfactory bulb.3. S9-GSK3β also dynamically expressed in the olfactory system during development. In embryonic stages, S9-GSK3P was abundantly detected in the cell body of the olfactory sensory neuron; unlike Y216-GSK3β, S9-GSK3β was not detected in the axon bundle in the embryonic stages. In postnatal stages, S9-GSK3β was detected in the olfactory epithelium, including the cell bodies and axons. In the adult olfactory bulb, S9-GSK3β was detected in the olfactory sensory neuron layer and SEZ, the immature neurons cell pool.4. In the developing olfactory system, the activity of GSK3β may be regulated by the canonical Wnt/Dvl/β-catenin signaling pathway.Then we analyzed the possible functions of GSK3P in the adult olfactory bulb. The above results indicated that the expression and the activity (phosphorylation status) of GSK3β might be regulated by olfactory sensory inputs. We studied the function of GSK3β in the adult olfactory bulb by a combination of immunohistochemistry, Western blot, in vivo electrophysiological recording and animal behavior analysis. Our results showed that:1. Olfactory deficits (by CNGA2-/-knock out or zinc sulfate irrigation of the olfactory epithelium) increased the activity of GSK3β in the olfactory sensory neurons; however, the activity of GSK3β in the olfactory bulb was reduced. In contrast, odor exposure increased the activity of GSK3β in the olfactory bulb.2. When the activity of GSK3β was inhibited by a selective inhibitor, by electrophysiological recording we found that the spontaneous activity in the granule cell layer was decreased.3. We locally inhibited the activity of GSK3β by TDZD-8, by odor cross-habituation behavior analysis, we found that the mice could discriminate different odors; however, their ability to habituate and the short-term memory were significantly impaired.These results suggested that the activity of GSK3β was regulated by neuralactivity, on the other hand, the kinase activity of GSK3β could affect the spontaneous neural activity and the normal odor information processing procedures in the olfactory bulb. These results demonstrated that the kinase is involved in more general neural processes, deepening our understanding of the potential functions of the constitutively active GSK3β in the brain under physiological conditions.