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

Gliptins normalize posttraumatic hippocampal neurogenesis and restore cognitive function after controlled cortical impact on sensorimotor cortex

Traumatic brain injury (TBI) often leads to long-term neurocognitive dysfunctions. Adult neurogenesis in the hippocampal dentate gyrus (DG) serves critical functions in cognition but can be disrupted by brain injury and insult in serval forms. In the present study, we explore the cellular and molecular targets of DPP-4 inhibitors (or gliptins) as related to hippocampal function and TBI cognitive sequelae. Two structurally different gliptins, sitagliptin and vildagliptin, were examined using a controlled cortical impact (CCI) model of moderate TBI in mice. Sensorimotor CCI, although distal from the hippocampus, impaired hippocampal-dependent cognition without obvious hippocampal tissue destruction. Neurogenic cell proliferation in the DG was increased accompanied by large numbers of reactive astrocyte. Increased numbers of immature granule cells with abnormal dendritic outgrowth were ectopically localized in the outer granule cell layer (GCL) and hilus. Long-term potentiation of dentate immature granule cells was also impaired. Both sitagliptin and vildagliptin attenuated the CCI-induced ectopic migration of doublecortin-positive immature neurons into the outer GCL and hilus, restored the normal dendritic branching pattern of the immature neurons and prevented astrocyte reactivation. Both gliptins prevented loss of normal synaptic integration in the DG after sensorimotor CCI and improved cognitive behavior. Sensorimotor cortical injury thus results in an abnormal neurogenesis pattern and astrocyte reactivation in the distal hippocampus which appears to contribute to the development of cognitive dysfunction after TBI. DPP-4 inhibitors prevent astrocyte reactivation, normalize the posttraumatic hippocampal neurogenesis and help to maintain normal electrophysiology in the DG with positive behavioral effect in a mouse model.

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

BACKGROUND

Traumatic brain injury (TBI) remains a significant risk factor for post-traumatic epilepsy (PTE). The pathophysiological mechanisms underlying the injury-induced epileptogenesis are under investigation. The dentate gyrus-a structure that is highly susceptible to injury-has been implicated in the evolution of seizure development.

METHODS

Utilizing the murine unilateral focal control cortical impact (CCI) injury, we evaluated seizure onset using 24/7 EEG video analysis at 2-4 months post-injury. Cellular changes in the dentate gyrus and hilus of the hippocampus were quantified by unbiased stereology and Imaris image analysis to evaluate Prox1-positive cell migration, astrocyte branching, and morphology, as well as neuronal loss at four months post-injury. Isolation of region-specific astrocytes and RNA-Seq were performed to determine differential gene expression in animals that developed post-traumatic epilepsy (PTE+) vs. those animals that did not (PTE-), which may be associated with epileptogenesis.

RESULTS

CCI injury resulted in 37% PTE incidence, which increased with injury severity and hippocampal damage. Histological assessments uncovered a significant loss of hilar interneurons that coincided with aberrant migration of Prox1-positive granule cells and reduced astroglial branching in PTE+ compared to PTE- mice. We uniquely identified Cst3 as a PTE+-specific gene signature in astrocytes across all brain regions, which showed increased astroglial expression in the PTE+ hilus.

CONCLUSIONS

These findings suggest that epileptogenesis may emerge following TBI due to distinct aberrant cellular remodeling events and key molecular changes in the dentate gyrus of the hippocampus.

The endogenous progenitor response following traumatic brain injury: a target for cell therapy paradigms

Although there is ample evidence that central nervous system progenitor pools respond to traumatic brain injury, the reported effects are variable and likely contribute to both recovery as well as pathophysiology. Through a better understanding of the diverse progenitor populations in the adult brain and their niche-specific reactions to traumatic insult, treatments can be tailored to enhance the benefits and dampen the deleterious effects of this response. This review provides an overview of endogenous precursors, the associated effects on cognitive recovery, and the potential of exogenous cell therapeutics to modulate these endogenous repair mechanisms. Beyond the hippocampal dentate gyrus and subventricular zone of the lateral ventricles, more recently identified sites of adult neurogenesis, the meninges, as well as circumventricular organs, are also discussed as targets for endogenous repair. Importantly, this review highlights that progenitor proliferation alone is no longer a meaningful outcome and studies must strive to better characterize precursor spatial localization, transcriptional profile, morphology, and functional synaptic integration. With improved insight and a more targeted approach, the stimulation of endogenous neurogenesis remains a promising strategy for recovery following traumatic brain injury.

BKM120 alters the migration of doublecortin-positive cells in the dentate gyrus of mice

BKM120 is an inhibitor of class I phosphoinositide 3-kinases and its anti-cancer effects have been demonstrated in various solid cancer models. BKM120 is highly brain permeable and has been reported to induce mood disturbances in clinical trials. Therefore, we examined whether BKM120 produces anxiety- and depression-like behaviors in mice, as with patients receiving BKM120 in clinical trials. In this study, repeated BKM120 treatment (2.0 or 5.0mg/kg, i.p., five times at 12-h interval) significantly induced anxiety- and depression-like behaviors in mice. Although abnormal changes in hippocampal neurogenesis have been suggested to, at least in part, associated with the pathogenesis of depression and anxiety, BKM120 did not affect the incorporation of 5-bromo-2'-deoxyuridine or the expression of doublecortin (DCX); however, it significantly enhanced the radial migration of DCX-positive cells in the dentate gyrus. BKM120-induced changes in migration were not accompanied by obvious neuronal damage in the hippocampus. Importantly, BKM120-induced anxiety- and depression-like behaviors were positively correlated with the extent of DCX-positive cell migration. Concomitantly, p-Akt expression was significantly decreased in the dentate gyrus. Moreover, the expression of p-c-Jun N-terminal kinase (JNK), p-DCX, and Ras homolog family member A (RhoA)-GTP decreased significantly, particularly in aberrantly migrated DCX-positive cells. Together, the results suggest that repeated BKM120 treatment enhances the radial migration of DCX-positive cells and induces anxiety- and depression-like behaviors by regulating the activity of Akt, JNK, DCX, and RhoA in the dentate gyrus. It also suggests that the altered migration of adult-born neurons in the dentate gyrus plays a role in mood disturbances.

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

A history of traumatic brain injury (TBI) increases the odds of developing Alzheimer's disease (AD). The long latent period between injury and dementia makes it difficult to study molecular changes initiated by TBI that may increase the risk of developing AD. MicroRNA (miRNA) levels are altered in TBI at acute times post-injury (<4 weeks), and in AD. We hypothesized that miRNA levels in cerebrospinal fluid (CSF) following TBI in veterans may be indicative of increased risk for developing AD. Our population of interest is cognitively normal veterans with a history of one or more mild TBI (mTBI) at a chronic time following TBI. We measured miRNA levels in CSF from three groups of participants: (1) community controls with no lifetime history of TBI (ComC); (2) deployed Iraq/Afghanistan veterans with no lifetime history of TBI (DepC), and (3) deployed Iraq/Afghanistan veterans with a history of repetitive blast mTBI (DepTBI). CSF samples were collected at the baseline visit in a longitudinal, multimodal assessment of Gulf War veterans, and represent a heterogenous group of male veterans and community controls. The average time since the last blast mTBI experienced was 4.7 ± 2.2 years [1.5 - 11.5]. Statistical analysis of TaqMan^TM miRNA array data revealed 18 miRNAs with significant differential expression in the group comparisons: 10 between DepTBI and ComC, 7 between DepC and ComC, and 8 between DepTBI and DepC. We also identified 8 miRNAs with significant differential detection in the group comparisons: 5 in DepTBI vs. ComC, 3 in DepC vs. ComC, and 2 in DepTBI vs. DepC. When we applied our previously developed multivariable dependence analysis, we found 13 miRNAs (6 of which are altered in levels or detection) that show dependencies with participant phenotypes, e.g., ApoE. Target prediction and pathway analysis with miRNAs differentially expressed in DepTBI vs. either DepC or ComC identified canonical pathways highly relevant to TBI including senescence and ephrin receptor signaling, respectively. This study shows that both TBI and deployment result in persistent changes in CSF miRNA levels that are relevant to known miRNA-mediated AD pathology, and which may reflect early events in AD.