Abrogation of atypical neurogenesis and vascular-derived EphA4 prevents repeated mild TBI-induced learning and memory impairments

Abstract

Brain injury resulting from repeated mild traumatic insult is associated with cognitive dysfunction and other chronic co-morbidities. The current study tested the effects of aberrant neurogenesis in a mouse model of repeated mild traumatic brain injury (rmTBI). Using Barnes Maze analysis, we found a significant reduction in spatial learning and memory at 24 days post-rmTBI compared to repeated sham (rSham) injury. Cell fate analysis showed a greater number of BrdU-labeled cells which co-expressed Prox-1 in the DG of rmTBI-injured mice which coincided with enhanced cFos expression for neuronal activity. We then selectively ablated dividing neural progenitor cells using a 7-day continuous infusion of Ara-C prior to rSham or rmTBI. This resulted in attenuation of cFos and BrdU-labeled cell changes and prevented associated learning and memory deficits. We further showed this phenotype was ameliorated in EphA4f./f/Tie2-Cre knockout compared to EphA4f./f wild type mice, which coincided with altered mRNA transcript levels of MCP-1, Cx43 and TGFβ. These findings demonstrate that cognitive decline is associated with an increased presence of immature neurons and gene expression changes in the DG following rmTBI. Our data also suggests that vascular EphA4-mediated neurogenic remodeling adversely affects learning and memory behavior in response to repeated insult.

The absence of a neurogenic response to a repeated concussive-like injury in mice

“…Additionally, more recent studies using selective methods of ablating neurogenesis suggest post-TBI neurogenesis hinders memory outcome 42,43 ; Still, there are findings suggesting that post-TBI memory is impaired by the selective ablation of neurogenesis, indicating that post-TBI neurogenesis is benficial 39 . Some of these discrepancies may be model related, particularly if the “aberrant neurogenesis”, characterized by abnormal dendritic development, is influenced by the degree of injury 57 .…”

“…Furthermore, post-TBI neurons survive and functionally integrate into the hippocampal circuitry 38,44 . Nevertheless, there is compelling evidence from the epilepsy, stroke and TBI field that aberrant neurogenesis has a maladaptive role in cognitive recovery 30,42,43,56,57 . For example, abnormal synapse formation by immature neurons after seizures may promote recurrent excitation and subsequent seizures 58,59 .…”

Cerebrospinal Fluid MicroRNA Changes in Cognitively Normal Veterans With a History of Deployment-Associated Mild Traumatic Brain Injury

“…To increase rigor, we used two distinct mRNA target algorithms, TargetScan and miRDB, to identify significant canonical pathways common to both. For the miRNAs differentially expressed in DepTBI vs. ComC ( Table 2A ) there were 11 overlapping canonical pathways ( Figure 5A ), 4 of which are relevant to TBI: ephrin receptor, axonal guidance, BMP, and RhoA signaling ( Frugier et al, 2012 ; Patel et al, 2017 ; Bi et al, 2019 ; Divolis et al, 2019 ; Zhao et al, 2019 ; Greer et al, 2020 ; Mulherkar and Tolias, 2020 ; Duman et al, 2021 ). Likewise, for the miRNAs differentially expressed in DepTBI vs. DepC ( Table 2C ) there were 20 overlapping canonical pathways ( Figure 5C ), 8 of which are relevant to TBI: ERK/MAPK, TGF-β, senescence, reelin signaling in neurons, Wnt/β-catenin, eIF4 and p70S6K, PAK, and cyclins and cell cycle regulation ( Di Giovanni et al, 2005 ; Chen et al, 2007 ; Zhao et al, 2011 ; Salehi et al, 2018 ; Divolis et al, 2019 ; Sen, 2019 ; Dal Pozzo et al, 2020 ; Hascup and Hascup, 2020 ; Sharma et al, 2020 ; Tan et al, 2020 ; Schwab et al, 2021 ).…”

“…Decreases in peripheral levels of Eph4A reduces neuroinflammation and cortical damage of acute TBI ( Kowalski et al, 2019 ). In addition, in postmortem human brain from patients that died after acute closed head injury there is increased Eph4A expression in astrocytes ( Frugier et al, 2012 ), and in Eph4A knockout mice there is a decrease in TBI-induced neurogenesis and amelioration of cognitive impairments compared to wild-type mice ( Greer et al, 2020 ). Furthermore, neuroinflammation and decreases in neurogenesis are two hallmarks of cellular senescence, which was another pathway identified in DepTBI.…”

Atypical Neurogenesis, Astrogliosis, and Excessive Hilar Interneuron Loss Are Associated With the Development of Post-Traumatic Epilepsy

“…TBI produces direct and secondary damage to the brain that includes neuronal loss, cerebral blood flow disruption, neuroinflammation, reactive oxidative damage, and blood-brain barrier disruption [10][11][12][13]. These pathophysiological changes may contribute to circuit imbalances that favor excitatory over inhibitory synaptic function [14,15], formation of spontaneous epileptic foci, and PTE [15,16].…”