Chronic Traumatic Encephalopathy: Role of Astrocytes

Miami Va Health Care System, Miami FL

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Abstract

? DESCRIPTION (provided by applicant): Chronic traumatic encephalopathy (CTE) is a devastating neurodegenerative disorder that develops over months-years, due to multiple episodes of head trauma. CTE is frequently diagnosed in boxers, and in athletes who have experienced multiple brain concussions. CTE is also an important neurological complication in our military veterans who have been engaged in the Iraq and Afghanistan wars. It is estimated that more than 500,000 military personnel have experienced one or more concussive episodes. CTE is associated with a progressive dementia, neurobehavioral and cognitive deficits, confusion, alterations in sleep patterns, depression and suicidal tendencies. However, little is known about the molecular and cellular events contributing to CTE pathogenesis, and there are no effective therapies currently available. A few studies have identified the hyperphosphorylation, ubiquitination and aggregation of the Transactivating DNA binding Protein, molecular weight 43 kDa (TDP-43) in neurons from postmortem human tissue. Precisely how TDP-43 is regulated and contributes to the pathogenesis of CTE is not known. In preliminary studies, we found that cultured neurons subjected to single or multiple trauma, showed hyperphosphorylation of TDP-43 (p-TDP-43), along with its ubiquitination (the TDP-43 proteinopathy). We also observed a loss of synaptic proteins following traumatic injury. While astrocytes are well known to be critically involved in the maintenance of many CNS activities, including the synthesis and release of various growth factors, their involvement in the mechanism of CTE has thus far not been investigated. In preliminary studies, we found that trauma to cultured astrocytes caused time-dependent alterations in the synthesis and release of the astrocytic protein thrombospondin-1 (TSP-1). In the early stages of trauma (1-3d post-trauma), astrocytes exhibited an enhanced synthesis and release of this protein, whereas at later stages (10-15d), a marked reduction in the synthesis and release of TSP-1 was observed. Noteworthy, conditioned media (CM) from astrocytes in the early stages following trauma, when added to traumatized cultured neurons, mitigated the enhanced levels of p-TDP-43, as well as prevented the loss of various synaptic proteins. However, at 10-15 d following trauma, astrocytes failed to exert a protective effect on neurons. We also found increased p-TDP-43 in cortical neurons following traumatic brain injury (TBI) in rats, while TSP-1 knock-out (KO) mice displayed a worsening in neurobehavioral outcomes 1-2 weeks post-trauma, as compared to WT mice post-TBI. Moreover, a several fold-increase in p-TDP-43 levels was observed in TSP-1 KO mice as compared to WT mice. These data strongly suggest that astrocytes exert neuroprotective effects in the early stages of trauma, whereas at later stages, such protection is absent. We postulate that such loss of neuroprotection by astrocytes in the later phases of trauma contributes to the pathogenesis of CTE. Our proposal will: 1) investigate alterations in TDP-43 in cultured neurons following trauma, and the associated loss of key synaptic proteins; 2) examine the role of astrocytes and microglia in the early and late stages of trauma on neuronal/synaptic integrity; 3) identify mechanisms (e.g., oxidative/nitrative stress; activation of NF-?B) which may result in a failure of astrocytes to synthesize and secrete sufficient amounts of TSP-1, and investigate the potential therapeutic benefits of agents known to enhance astrocytic TSP-1 synthesis and release, e.g., metformin, N-acetylcysteine (NAC); 4) investigate in a rat model of repetitive traumatic injury, whether various events (TDP-43 phosphorylation, loss of synaptic proteins, and a reduction in astrocytic TSP-1), all of which were identified in vitro; also occur in vivo, and 5) examine whether agents (e.g., metformin, NAC) capable of enhancing astrocytic TSP-1 synthesis and release, will also diminish the cellular and neurobehavioral abnormalities observed in vivo following TBI. We believe that our studies aimed at a better understanding the molecular, cellular and neurobehavioral events associated with CTE will greatly facilitate the identification of agents capable of ameliorating this debilitating neurological condition.