Mild Traumatic Brain Injury Induces Transient, Sequential Increases in Proliferation, Neuroblasts/Immature Neurons, and Cell Survival: A Time Course Study in the Male Mouse Dentate Gyrus

Abstract

Mild traumatic brain injuries (mTBIs) are prevalent worldwide. mTBIs can impair hippocampal-based functions such as memory and cause network hyperexcitability of the dentate gyrus (DG), a key entry point to hippocampal circuitry. One candidate for mediating mTBI-induced hippocampal cognitive and physiological dysfunction is injury-induced changes in the process of DG neurogenesis. There are conflicting results on how TBI impacts the process of DG neurogenesis; this is not surprising given that both the neurogenesis process and the post-injury period are dynamic, and that the quantification of neurogenesis varies widely in the literature. Even within the minority of TBI studies focusing specifically on mild injuries, there is disagreement about if and how mTBI changes the process of DG neurogenesis. Here we utilized a clinically relevant rodent model of mTBI (lateral fluid percussion injury, LFPI), gold-standard markers and quantification of the neurogenesis process, and three time points post-injury to generate a comprehensive picture of how mTBI affects adult hippocampal DG neurogenesis. Male C57BL/6J mice (6-8 weeks old) received either sham surgery or mTBI via LFPI. Proliferating cells, neuroblasts/immature neurons, and surviving cells were quantified via stereology in DG subregions (subgranular zone [SGZ], outer granule cell layer [oGCL], molecular layer, and hilus) at short-term (3 days post-injury, dpi), intermediate (7 dpi), and long-term (31 dpi) time points. The data show this model of mTBI induces transient, sequential increases in ipsilateral SGZ/GCL proliferating cells, neuroblasts/immature neurons, and surviving cells which is suggestive of mTBI-induced neurogenesis. In contrast to these ipsilateral hemisphere findings, measures in the contralateral hemisphere were not increased in key neurogenic DG subregions after LFPI. Our work in this mTBI model is in line with most literature on other and more severe models of TBI in showing TBI stimulates the process of DG neurogenesis. However, as our DG data in mTBI provide temporal, subregional, and neurogenesis-stage resolution, these data are important to consider in regard to the functional importance of TBI-induction of the neurogenesis process and future work assessing the potential of replacing and/or repairing DG neurons in the brain after TBI.

Behavioral pattern separation and cognitive flexibility are enhanced in a mouse model of increased lateral entorhinal cortex-dentate gyrus circuit activity

“…Unbiased quantification of DCX+ cells (total as well as late progenitors and immature neurons in the superior [or suprapyramidal] and inferior [or infrapyramidal] blade of the DG) was performed via stereology (Latchney et al, 2014; Whoolery et al, 2017; Luna et al, 2019; Clark et al, 2021). The microscope used was a BX51 (Olympus America, Center Valley, PA, USA) with a 40X, 0.63 NA oil-immersion objective.…”

“…DCX+ cells in the DG subgranular zone (SGZ) were counted. The total DCX+ cell number was calculated by this formula (Yun et al, 2018b; Clark et al, 2021): where ssf is the section sampling fraction (DCX: 1/8), asf is the area sampling fraction (1 for these rare populations of cells; thus, all cells were counted in “ssf [e.g. 1/8]” sections), hsf is the height sampling fraction (1 given the minimal effect edge artifacts have in counting soma <10um with ssf 1/8) as described in prior work (Lagace et al, 2010).…”

“…DCX+ cells in the DG subgranular zone (SGZ) were counted. The total DCX+ cell number was calculated by this formula (Yun et al, 2018b;Clark et al, 2021):…”

“…Quantification of DCX+ Cells. Unbiased quantification of DCX+ cells (total as well as late progenitors and immature neurons in the superior [or suprapyramidal] and inferior [or infrapyramidal] blade of the DG) was performed via stereology (Latchney et al, 2014;Whoolery et al, 2017;Luna et al, 2019;Clark et al, 2021).…”

Identification of a distinct ductal subpopulation with self-renewal and differentiation potential from the adult murine pancreas

“…8b-c). One potential reason for the lack of proliferating cells in ductal rosettes could be that early injury conditions inhibit the activation of proliferation, which is known to occur in neurons after brain injury 44 . Thus, we analyzed injured pancreata 14-days post-injury (Fig.…”