Mild traumatic brain injury (mTBI) is the most common form of central nervous system (CNS) injury and is often characterized by persistent neuroinflammation, metabolic dysregulation, and oxidative stress. Repetitive injuries compound these pathologies and lead to multifocal axonal injuries and long-term functional deficits. Despite the prevalence of mTBIs, the cellular mechanisms that facilitate or prevent recovery following injury remain poorly defined. Here, we extend our previous work on the role of the protein transglutaminase 2 (TG2) in CNS injury and we hypothesize that transcriptional regulation by TG2 restricts metabolic versatility in astrocytes following TBI, thereby impairing neuronal energetic support and worsening pathological outcomes. We utilized an established weight-drop model of repetitive mTBI followed by multi-parametric analysis of TBI pathology in complete TG2 knockout (TG2-/-) and wild type mice. At 28 days post-injury, TG2-/- mice showed marked attenuation of TBI pathology, compared to wild type mice, in vulnerable white matter and default mode network (DMN) regions, as assessed by diffusion magnetic resonance imaging (MRI), resting-state functional MRI, and immunohistochemistry. Integrated epigenomic, proteomic, and metabolomic profiling of cortical astrocytes isolated 28 days after injury revealed a pronounced metabolic restriction in wild type astrocytes which was remarkably attenuated in the TG2-/- mice. This rescue was associated with a de-repression of gene networks involved in glutamate recycling, lipid metabolism, and metabolic homeostasis. Together, these studies provide novel mechanistic insights into the metabolic dysregulation that characterizes persistent TBI pathology, and establish a foundation for evaluating TG2 as a therapeutic target for TBI.

Astrocytes are the primary support cells of the central nervous system (CNS) that help maintain the energetic requirements and homeostatic environment of neurons. CNS injury causes astrocytes to take on reactive phenotypes with an altered overall function that can range from supportive to harmful for recovering neurons. The characterization of reactive astrocyte populations is a rapidly developing field, and the underlying factors and signaling pathways governing which type of reactive phenotype that astrocytes take on are poorly understood. Our previous studies suggest that transglutaminase 2 (TG2) has an important role in determining the astrocytic response to injury. Selectively deleting TG2 from astrocytes improves functional outcomes after CNS injury and causes widespread changes in gene regulation, which is associated with its nuclear localization. To begin to understand how TG2 impacts astrocytic function, we used a neuron-astrocyte co-culture paradigm to compare the effects of TG2−/− and wild-type (WT) mouse astrocytes on neurite outgrowth and synapse formation. Neurons were grown on a control substrate or an injury-simulating matrix comprised of inhibitory chondroitin sulfate proteoglycans (CSPGs). Compared to WT astrocytes, TG2−/− astrocytes supported neurite outgrowth to a significantly greater extent only on the CSPG matrix, while synapse formation assays showed mixed results depending on the pre- and post-synaptic markers analyzed. We hypothesize that TG2 regulates the supportive functions of astrocytes in injury conditions by modulating gene expression through interactions with transcription factors and transcription complexes. Based on the results of a previous yeast two-hybrid screen for TG2 interactors, we further investigated the interaction of TG2 with Zbtb7a, a ubiquitously expressed transcription factor. Co-immunoprecipitation and colocalization analyses confirmed the interaction of TG2 and Zbtb7a in the nucleus of astrocytes. Overexpression or knockdown of Zbtb7a levels in WT and TG2−/− astrocytes revealed that Zbtb7a robustly influenced astrocytic morphology and the ability of astrocytes to support neuronal outgrowth, which was significantly modulated by the presence of TG2. These findings support our hypothesis that astrocytic TG2 acts as a transcriptional regulator to influence astrocytic function, with greater influence under injury conditions that increase its expression, and Zbtb7a likely contributes to the overall effects observed with astrocytic TG2 deletion.

Transglutaminase 2 (TG2) is widely expressed across mammalian tissues, including the central nervous system (CNS). TG2 has long been associated with neurodegenerative diseases and CNS injury; however, its primary mechanistic contributions to these pathologies have been difficult to distill due to its complex regulation and multifunctional nature. While it is best known as a cytosolic protein with transamidating activity which catalyzes the formation of isopeptide bonds, TG2 is found in multiple cellular compartments and displays localization-and-conformation-specific effects that can influence cell function and survival in response to stress. Further still, these effects are dependent on the type of injury stimuli and CNS cell type. Recently, multiple studies have highlighted TG2's role in transcription regulation as a potentially important component of transcript changes associated with pathology, opening new avenues of investigation of TG2's role in neurological disease.

This article summarizes a Symposium on 'Radiation risks of the central nervous system' held virtually at the 67th Annual Meeting of the Radiation Research Society, 3-6 October 2021. Repeated low-dose radiation exposure over a certain period could lead to reduced neuronal proliferation, altered neurogenesis, neuroinflammation and various neurological complications, including psychological consequences, necessitating further research in these areas. Four speakers from radiation biology, genetics and epidemiology presented the latest data from their studies seeking insights into this important topic. This symposium highlighted new and important directions for further research on mental health disorders, neurodegenerative conditions and cognitive impairment. Future studies will examine risks of mental and behavioral disorders and neurodegenerative diseases following protracted radiation exposures to better understand risks of occupational exposures as well as provide insights into risks from exposures to galactic cosmic rays. The Million Person Study of Low-Dose Health Effects (MPS) is evaluating the risk of cognitive dysfunction and dementia following intakes of radionuclides and after low-LET external radiation in the workplace. A recent study of Mayak workers in Russia suggested a link with Parkinson's disease following low-LET radiation. High-LET radiation from galactic cosmic ray simulations have reported the potential to accelerate the development of Alzheimer's disease, dementia and anxiety disorders in experimental studies. To date, seven MPS cohorts have evaluated Parkinson's disease at the level of mortality among 515,857 workers and veterans. The excess relative risks (ERR) per 100 mGy dose to brain was estimated for nuclear power plant workers (NPP), industrial radiographers (IR), medical radiation workers (MRW), nuclear weapons test participants and former DOE workers at Mallinckrodt Chemical Works, Mound facility in Ohio, and Los Alamos National Laboratory. There was a general tendency for the risk of Parkinson's disease to increase with increasing estimates of radiation dose to brain. The pooled ERR per 100 mGy for the combined NPP + IR + MRW cohorts was 0.30 (95% CI 0.08, 0.56). Confirmation of these associations are being sought from ongoing studies of an additional 300,000 workers and from linkages within the Centers for Medicare & Medicaid Services (CMS) databases for incidence data on Parkinson's disease for 600,000 workers. Further, a wide range of nonfatal conditions related to dementia and depression are being identified, as are cognitive impairment scores among 600,000 cohort workers alive in 1999 when the CMS data systems became available. Neuroinflammation and synaptic loss have been implicated in cognitive dysfunction seen after brain radiation exposure. Based on evidence that elements of the complement cascade are critical for synaptic pruning by microglia during development and are upregulated in neurodegenerative diseases and aging where synapse density is reduced, we previously demonstrated that deletion of complement receptor 3 (CR3), which is expressed on microglia, protected male mice from dendritic spine loss in the molecular layer of the hippocampus after radiation exposure. Using Thy1-eYFP mice with or without the CR3 receptor, we also found a clear correlation between microglial activation, spine loss and cognitive dysfunction 30 d after 10 Gy cranial irradiation, only in male mice, that is dependent on CR3 expression. Moreover, treatment with leukadherin-1 (LA1), which engages the CR3 receptor, prevents spine loss and cognitive dysfunction due to radiation exposure. To guide application of potential therapeutic interventions, we carried out time course analyses of synaptic density, microglial activation and complement deposition and found that these processes are initiated in the first few days following radiation exposure, suggesting a relatively acute response with long-lasting consequences.

Serial-section electron microscopy such as FIB-SEM (focused ion beam scanning electron microscopy) has become an important tool for neuroscientists to trace the trajectories and global architecture of neural circuits in the brain, as well as to visualize the 3D ultrastructure of cellular organelles in neurons. In this study, we examined 3D features of mitochondria in electron microscope images generated from serial sections of four regions of mouse brains: nucleus accumbens (NA), hippocampal CA1, somatosensory cortex and dorsal cochlear nucleus (DCN). We compared mitochondria in the presynaptic terminals to those in the postsynaptic/dendritic compartments, and we focused on the shape and size of mitochondria. A common feature of mitochondria among the four brain regions is that presynaptic mitochondria generally are small and short, and most of them do not extend beyond presynaptic terminals. In contrast, the majority of postsynaptic/dendritic mitochondria are large and many of them spread through significant portions of the dendrites. Comparing among the brain areas, the cerebral cortex and DCN have even larger postsynaptic/dendritic mitochondria than the NA and CA1. Our analysis reveals that mitochondria in neurons are differentially sized and arranged according to their subcellular locations, suggesting a spatial organizing principle of mitochondria at the synapse.

Traumatic brain injury (TBI) is becoming an increasing public health issue. With an annually estimated 1.7 million TBIs in the United States (U.S) and nearly 70 million worldwide, the injury, isolated or compounded with others, is a major cause of short- and long-term disability and mortality. This, along with no specific treatment, has made exploration of TBI therapies a priority of the health system. Age and sex differences create a spectrum of vulnerability to TBI, with highest prevalence among younger and older populations. Increased public interest in the long-term effects and prevention of TBI have recently reached peaks, with media attention bringing heightened awareness to sport and war related head injuries. Along with short-term issues, TBI can increase the likelihood for development of long-term neurodegenerative disorders. A growing body of literature supports the use of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and glucagon (Gcg) receptor (R) agonists, along with unimolecular combinations of these therapies, for their potent neurotrophic/neuroprotective activities across a variety of cellular and animal models of chronic neurodegenerative diseases (Alzheimer's and Parkinson's diseases) and acute cerebrovascular disorders (stroke). Mild or moderate TBI shares many of the hallmarks of these conditions; recent work provides evidence that use of these compounds is an effective strategy for its treatment. Safety and efficacy of many incretin-based therapies (GLP-1 and GIP) have been demonstrated in humans for the treatment of type 2 diabetes mellitus (T2DM), making these compounds ideal for rapid evaluation in clinical trials of mild and moderate TBI.