Adult neurogenic process in the subventricular zone-olfactory bulb system is regulated by Tau protein under prolonged stress

Abstract

Objectives: The area of the subventricular zone (SVZ) in the adult brain exhibits the highest number of proliferative cells, which, together with the olfactory bulb (OB), maintains constant brain plasticity through the generation, migration and integration of newly born neurons. Despite Tau and its malfunction is increasingly related to deficits of adult hippocampal neurogenesis and brain plasticity under pathological conditions [e.g. in Alzheimer's disease (AD)], it remains unknown whether Tau plays a role in the neurogenic process of the SVZ and OB system under conditions of chronic stress, a well-known sculptor of brain and risk factor for AD.

Materials and methods: Different types of newly born cells in SVZ and OB were analysed in animals that lack Tau gene (Tau-KO) and their wild-type littermates (WT) under control or chronic stress conditions.

Results: We demonstrate that chronic stress reduced the number of proliferating cells and neuroblasts in the SVZ leading to decreased number of newborn neurons in the OB of adult WT, but not Tau-KO, mice. Interestingly, while stress-evoked changes were not detected in OB granular cell layer, Tau-KO exhibited increased number of mature neurons in this layer indicating altered neuronal migration due to Tau loss.

Conclusions: Our findings suggest the critical involvement of Tau in the neurogenesis suppression of SVZ and OB neurogenic niche under stressful conditions highlighting the role of Tau protein as an essential regulator of stress-driven plasticity deficits.

Keywords: Tau protein; chronic stress; neurogenesis; olfactory bulb; oligodendrogenesis; subventricular zone.

FIGURE 1

Chronic stress suppresses the number of proliferating cells and neuroblasts in the adult subventricular zone of WT, but not Tau‐KO, animals. A, Schematic representation of the experimental design where wild‐type (WT) and Tau‐knockout (Tau‐KO) mice were divided into control (CON) and chronic stress (STR) groups. Animals of all groups were randomly divided into two groups receiving 5‐bromo‐2′‐deoxyuridine (BrdU) injections before sacrifice (left panel) and 4 weeks before sacrifice (right panel). B,C, Schematic illustration of the mouse brain (B) highlighting the neurogenic areas of the subventricular zone (SVZ) and olfactory bulb (OB) as well as different types of newly born SVZ cells analysed and the markers used for their monitoring. D‐F, Representative microphotograph of BrdU/DCX double‐labelled cells (arrow head) in the SVZ (D). Chronic stress evoked a decrease in BrdU‐positive cell density (reflecting proliferating cells) in WT, but not Tau‐KO, animals. Note that stressed Tau‐KO animals present higher number of proliferating cells when compared to stressed WT animals (E). Similarly, stress reduced the percentage of DCX/BrdU double‐labelled cells (reflecting neuroblasts) only in WT animals (F). All numerical data are shown as mean ± s.e.m (*P < .05). CON, control‐non‐stressed; STR, stressed; BrdU, 5‐bromo‐2′‐deoxyuridine; DCX, doublecortin; WT, wild type; Tau‐KO, Tau‐knockout

FIGURE 2

Levels of neural stem cells and oligodendrocytes in the subventricular zone are not altered by chronic stress. A‐E, GFAP/BrdU and Olig2/BrdU immunofluorescent staining of SVZ in animals injected with BrdU before sacrifice. B, Representative microphotograph of BrdU/GFAP double‐labelled cells (arrow head) in the SVZ. C, Chronic stress does not affect the percentage of GFAP/BrdU double‐labelled cells (reflecting neural stem cells) in WT and Tau‐KO animals. D, Olig2/BrdU microphotograph of double‐labelled cells (arrow head) in the SVZ. E, Exposure to chronic stress does not affect the percentage of Olig2/BrdU double‐labelled cells (reflecting oligodendrocytes progenitor cells) in WT and Tau‐KO animals. All numerical data are shown as mean ± SEM; GFAP, glial fibrillary acidic protein; Olig2, oligodendrocyte transcription factor 2; WT, wild type; Tau‐KO, Tau‐knockout

FIGURE 3

Impact of chronic stress and Tau on newborn neurons in differential sublayers of the olfactory bulb (OB). A, For olfactory bulb (OB) analysis, mice were injected with BrdU 4 weeks before sacrifice. B‐C, Schematic illustration of the mouse brain highlighting the olfactory bulb (OB) level of analysis followed by an olfactory bulb coronal section (C) and the different sublayers analysed; granular cell layer (GCL), mitral cell layer (MCL), glomerular cell layer (GL). D, Representative microphotograph of BrdU/NeuN double‐labelled cells (arrow head) in the OB. E, Tau‐KO animals exhibited increased number of BrdU‐positive cells and NeuN/BrdU double‐labelled cells in the GCL; no differences were found in the percentage of BrdU‐positive cells that are NeuN/BrdU double‐labelled. F, In MCL of OB, Tau‐KO animals exhibited reduced levels of NeuN/BrdU double‐labelled cells compared to WTs and chronic stress reduced NeuN/BrdU cells in WT, but not Tau‐KO, animals. G, In GL, exposure to chronic stress reduced the density of BrdU‐positive cells and NeuN/BrdU double‐labelled cells, as well as, the percentage of NeuN/BrdU cells in GL of WT animals; this stress effect was not found in Tau‐KO animals. All numerical data are shown as mean ± SEM (*P < .05). OB, olfactory bulb; GCL, granular cell layer; MCL, mitral cell layer; GL, glomerular cell layer; BrdU, 5‐bromo‐2′‐deoxyuridine; WT, wild type; Tau‐KO, Tau‐knockout

FIGURE 4

Tau deletion does not interfere with endocrine response to stress but attenuates stress‐induced behavioural impairment. A‐B, Stressed animals of both WT and Tau‐KO genotype exhibited reduced body weight (A) and increased levels of corticosterone, the main stress hormones (B), when compared with their corresponding control animals. C, Total distance travelled in the open‐field arena was not different among groups. D, Chronic stress decreased the number of ultrasonic vocalizations (USVs) in WT animals indicating deficits of emotional status; no effect of stress was found in Tau‐KO animals. E, Preference index in the Novel Object Recognition test was reduced in stressed WTs when compared to WT controls; this stress effect was not found in Tau‐KOs suggesting a Tau‐dependent cognitive impairment caused by chronic stress. All numerical data are shown as mean ± SEM (*P <.05). USVs, ultrasonic vocalizations; WT, wild type; Tau‐KO, Tau‐knockout

FIGURE 5

Summary of the impact of chronic stress on different cell populations in the subventricular zone ‐ olfactory bulb neurogenic niche. Chronic stress suppresses proliferation, neuronal, but not oligodendrocytic, differentiation and maturation of newly born cells in the subventricular zone (SVZ) and olfactory bulb (OB) of the adult brain as assessed by decreased levels of proliferating cells and neuroblasts in SVZ and reduced newborn neurons in the mitral cell and glomerular cell lB neurogenic niche of the adult brainayers (MCL and GL, respectively) of OB. However, the stress impact on the above cell populations was blocked in animals lacking Tau (Tau‐KO). These findings suggest that Tau protein is essentially involved in the neurogenesis‐suppressing role of chronic stress on the SVZ‐OB neurogenic niche of the adult brain in line with previously reported reduction of the hippocampal neurogenesis by stress. ²² , ³³ Interestingly, compared to wild type (WT), newborn neurons of Tau‐KO animals seem to be accumulated in the first layer of the OB, the granular cell layer (GCL), indicating delayed migration of newborn neurons in the other OB cell layers and subsequently, decreased number of these cells in the mitral cell layer (MCL). GCL, granular cell layer; MCL, mitral cell layer; GL, glomerular cell layer; NSCs, neural stem cells; OPCs, oligodendrocytes progenitor cells; WT; wild‐type, Tau‐KO, Tau‐knockout