Recovery from penetrating brain injury in adult spiny mice and C57BL/6 mice: a quantitative comparison

Traumatic brain injury (TBI) can be diffuse or localised, penetrating or non-penetrating and differ in the severity of structural damage. Penetrating brain injury has the highest rate of mortality and survivors suffer from significant life-long morbidity (Vakil and Singh, 2017, Heslot et al., 2021). Immediate treatment includes resuscitation, triage and imaging to determine the severity of injury, post-traumatic complications and overall prognosis (Algattas and Huang, 2013, Puig et al., 2020). Current treatments aim to limit further secondary insults arising from the initial penetrating brain injury, but these are ineffective at reducing mortality and morbidity following injury, highlighting the need for novel therapeutic developments. Furthermore, despite many preclinical studies in rodent TBI models, no interventions have been effectively translated to the clinic (Margulies et al., 2009, Chang et al., 2016). Since accidents involving penetrating brain injury are unpredictable in their location and physical extent, laboratory models of penetrating brain injury seek to make lesions of reproducible size and location, so that quantitative assessments can be made of the cellular responses to injury over time. One widely-used traumatic injury models involves the penetration of the cortex and hippocampus by a hypodermic needle under stereotaxic control (Persson et al., 1976, Baumgart et al., 2012, Acaz-Fonseca et al., 2015, Vakil and Singh, 2017, Meng et al., 2021). Penetrating brain injury has commonly served as a primary experimental model to study CNS response mechanisms to injury and repair in lower and higher order vertebrates (Heinicke, 1982, Hampton et al., 2004, Acaz-Fonseca et al., 2015). The underlying wound repair mechanisms following penetrating brain injury include necrosis, apoptosis, inflammation and astroglial scarring, which are fundamental cellular responses conserved across many forms of brain injury and amongst higher order mammalian species, including humans (Ekdahl et al., 2003, Rolls et al., 2009, Sofroniew and Vinters, 2010, Burda et al., 2016). Recent studies in the spiny mouse (Acomys genus) have shown scar-free healing in the spinal cord, skeletal muscle and skin (Seifert et al., 2012, Brant et al., 2016, Maden, 2018, Maden et al., 2018, Streeter et al., 2020). This ability to almost completely heal a wound is not observed in conventional laboratory rodents or adult mammals, including humans. However, the fundamental reparative and neurogenic response to brain injury in the adult spiny mouse has not yet been characterised. The studies in this thesis investigated the reparative and neurogenic responses following a penetrating brain injury in the male and female adult spiny mouse, and compared this to the adult male and female C57BL/6J mouse. We present evidence of a different reparative and neurogenic response in the wild-type adult spiny mouse brain following injury in comparison to the conventional laboratory mouse. The experiments enabled characterisation of the cellular and structural consequences of penetrating injury in the spiny mouse brain and found that significant differences exist from 1 to 10 days post-injury in comparison to the C57BL/6J mouse. Chapter 3 investigated the fundamental neuro-immune response of microglia and astrocytes adult spiny mouse and C57BL/6J mouse brain. Microglia are the resident immune-responsive cells of the CNS and are implicated in the brain's response to injury (Lannes et al., 2017, Alam et al., 2020). The inflammatory signals initiated by the microglial response trigger astrocytes to transform into a `reactive' phenotype (Hanisch, 2002, Burda et al., 2016). Reactive astrocytes form an astroglial scar, a physical barrier that prevents local neuronal repair and results in the redirection of axons and dendrites around the injury site. Astroglial scarring is one of the main barriers preventing the functional restoration of lost neural tissue in the human brain (Sofroniew and Vinters, 2010). The evidence presented in Chapter 3 suggests that the spiny mouse has a different neuro-immune response in comparison to the C57BL/6J mouse, with reduced microglial reactivity and astroglial scar development following a penetrating brain injury. Chapter 4 examined neuronal apoptosis and the response of axons and myelin to a penetrating brain injury in the adult spiny mouse and C57BL/6J mouse brain. Penetrating brain injury causes damage to neural tissue, including neurons, axons and myelin (McIntosh et al., 1994, Akamatsu and Hanafy, 2020). Primary neural tissue damage is caused by mechanical insult at impact and is considered irreversible, while secondary damage occurs as a result of delayed neurochemical processes and signalling pathways, and is considered reversible and a potential target for clinical treatments and therapies (Akamatsu and Hanafy, 2020). Secondary neural tissue damage is associated with the disturbance of cellular metabolism and loss of neurons via apoptosis (Colicos and Dash, 1996, Bramlett et al., 1997). The evidence presented in this chapter suggests that the spiny mouse cortex had reduced neuronal apoptosis and greater infiltration of axons into the injury site over the 10 days following injury. Chapter 5 explored the response of neural progenitor cells in the dorsal subventricular zone (dSVZ) and at the cortical injury site in the adult spiny mouse and C57BL/6J mouse. Neural progenitor cells are self-renewing multipotent cells that generate new neuronal and glial cells in the brain (Taupin, 2006, Navarro Negredo et al., 2020). Adult stem cells in the mammalian brain are found in the SVZ (Ihrie et al., 2011). In response to injury, these endogenous progenitors may proliferate, migrate to the sites of injury, and differentiate to support the repair of lost neural tissue (Yu et al., 2013). Herein, it was found that spiny mice had an increased density and proliferation of neural progenitor cells and neuroblasts/immature neurons in the dSVZ and an increased density of neural progenitor cells in the cortical injury site over the 10 days following injury. Overall, the studies outlined in this thesis present the first investigation of the reparative and neurogenic response following penetrating brain injury in the male and female adult spiny mouse over 10 days following injury. The spiny mouse has a different reparative response that leads to accelerated wound closure, reduced microglial and astroglial accumulation, and reduced neuronal apoptosis. Furthermore, the spiny mouse displays reduced astroglial scarring and a greater recovery of axons within and adjacent to the injury site over the 10 days following injury. In parallel with this reparative response, the spiny mouse display an increase in the number of proliferating neural progenitor cells in both the dSVZ and cortical injury site over the 10 days following injury. These distinct differences between the spiny mouse and C57BL/6J mouse, a conventional laboratory rodent, suggests a new approach to discovering how brain injury can be repaired, and further studies with the spiny mouse have the potential to unveil new targets for the clinical treatment of various forms of brain injury.