Neuroprotection or sex bias; A protective response to traumatic brain injury in the females

Abstract

Traumatic brain injury (TBI) is a major healthcare problem and a common cause of mortality and morbidity. Clinical and preclinical research suggests sex-related differences in short- and long-term outcomes following TBI; however, males have been the main focus of TBI research. Females show a protective response against TBI. Female animals in preclinical studies and women in clinical trials have shown comparatively better outcomes against mild, moderate, or severe TBI. This reflects a favorable protective nature of the females compared to the males, primarily attributed to various protective mechanisms that provide better prognosis and recovery in the females after TBI. Understanding the sex difference in the TBI pathophysiology and the underlying mechanisms remains an elusive goal. In this review, we provide insights into various mechanisms related to the anatomical, physiological, hormonal, enzymatic, inflammatory, oxidative, genetic, or mitochondrial basis that support the protective nature of females compared to males. Furthermore, we sought to outline the evidence of multiple biomarkers that are highly potential in the investigation of TBI’s prognosis, pathophysiology, and treatment and which can serve as objective measures and novel targets for individualized therapeutic interventions in TBI treatment. Implementations from this review are important for the understanding of the effect of sex on TBI outcomes and possible mechanisms behind the favorable response in females. It also emphasizes the critical need to include female as a biological variable and in sufficient numbers in future TBI studies.

1. Introduction

Traumatic brain injury (TBI) is the leading cause of disability and death [1,2]. Globally 50 million people suffer TBI annually, and about 50% of the global population will suffer a TBI in their lifetime [3]. In the USA, 1.7 million people suffer TBI annually, and 5 million live with TBI-related complications, costing a healthcare cost of $60 to $221 billion annually [4,5]. Of the reported studies, around 80% of cases are mild TBI, with only 7% being based on sex differences as a variable, while 93% of those studies have only focused on males [6,7]. Male rodents have been predominantly used in preclinical studies [6]. Likewise, women are less likely to be included in clinical trials than men partly because TBI has a greater incidence in men than in women [5]. Thus, there is a sex-based bias denoted by the under-representation of females in these reports [8], which creates a critical need to include females as a biological variable in TBI-related studies [5].

The pathophysiology of TBI is complicated and is classified into primary and secondary brain injury [9]. The acute primary injury results from the initial mechanical insult and based on the severity, it can lead to focal or diffuse injury. Axonal and blood vasculature damage are primary brain insult hallmark features and can result in cognitive and behavioral deficits [10]. The pathophysiological cascades of the primary injury often progress into the delayed, degenerative, and prolonged secondary injury that can remain up to days, weeks, and even years after the injury [11]. Mechanistically, several biochemical, cellular, and molecular pathways mediate secondary brain injury. Multiple mechanisms are responsible for neuronal damage, including neuroinflammatory, oxidative stress, excitotoxicity, axonal degeneration, lipid peroxidation, mitochondrial dysfunction, and apoptotic cell death [12]. Manipulating the different factors involved in these mechanisms is considered a major therapeutic strategy to improve recovery and enhance functional outcomes following TBI [13].

Many studies predict that females perform better in prognosis and outcomes and have fewer complications following TBI than males [14]. However, there are quite a few sex-based variations in response and recovery to TBI in animals and humans [5,15]. Studies on moderate to severe TBI have revealed that females show comparatively better outcomes than males [5]. However, mild TBI studies based on subjective outcome measures have demonstrated better outcomes for females in preclinical TBI studies and worse outcomes for women in human clinical TBI studies [5,16]. As subjective measures are usually biased, including more objective measures may reveal more reliable outcomes [17,18]. A close analysis of clinical and preclinical studies shows similar trends, with females having better prospects. Concerning clinical studies, analysis of human studies showed that sample size distribution has a significant effect on sex-related outcomes [5]. In this context, 53% of studies with small sample sizes (less than 1000 patients) showed worse outcomes in women than men; however, that was reversed in studies with large sample sizes (more than 10,000 patients), where 67% of the studies described better outcomes in women than men [5]. In agreement with this evidence, sex-based differences have also been reported in other neurodegenerative diseases, such as multiple sclerosis [19]. Compared to women, multiple sclerosis in men have a worse prognosis, as demonstrated by poor relapse recovery, greater brain volume loss, and higher disability rates and cognitive dysfunctions [20,21].

Although the mechanisms behind TBI-related sex differences are not fully understood, sex-related physiological differences have confirmed that females have underlying protective mechanisms that might be responsible for providing them with better outcomes. The available literature hints at various morphological, cellular, and molecular basis of these mechanisms in females [14,15]. Studies have reported that females undergo reduced neuroinflammation compared to males after TBI, which suggests neuroprotection in females [22]. Hormonal differences play a major role in the neuroprotective effects observed in animal models of TBI [23]. Studies have shown that estrogen and progesterone, two essential hormones found primarily in women, have protective properties that help protect neurons from damage caused by TBI [24]. These hormones reduce inflammation and promote neuronal survival [24]. In addition, estrogen and progesterone can potentially improve cognitive function after TBI [25]. In this review, we have explored anatomical, physiological/biochemical [8,14], hormonal [26–28], enzymatic [28–31], anti-inflammatory [14,32,33], antioxidant [34–36], genetic [14,27,37] or mitochondrial [38,39] basis for protective mechanisms of females in comparison to males. However, considering the scarcity of literature on female sex-based studies, comparative evaluations, and contradictory revelations, there should be a future emphasis on including females as a biological variable in TBI studies to reveal the relevance of various findings. Further, this review details multiple markers/measures that can be objectively estimated to better interpret sex differences in TBI studies. It will also give insights on potential targets for developing therapeutic interventions for treating TBI.

2. Potential protective mechanisms

There is a morphological, cellular, and molecular basis of response to TBI in both males and females. However, due to the lack of research involving morphological, cellular, and subcellular criteria with sufficient sex-based numbers, sex differences in TBI studies are still not fully understood. To explore the differences in response or protective mechanisms, we summarized the available literature about sex-based responses to TBI and the factors that regulate these differences directly or indirectly. We came across various studies that provide anatomical, physiological/biochemical [5,8], pathobiological [15,40], hormonal [26,27], enzymatic [15,29,31], anti-inflammatory [14,32], antioxidant [34,35], genetic [37,41] and mitochondrial [36,38] basis of sex difference in protective response against TBI. There are interrelationships between these mechanisms and interdependence on one another. A structure (such as the blood-brain barrier (BBB) and mitochondria) may have anatomical, physio-biochemical, or pathobiological relevance to providing a protective response against TBI. Hence, it has been discussed under different headings with the relevant information. We also outlined all the possible limitations in these comparisons and the need for future explorations of sex-based TBI-related studies.

2.1. Anatomical/morphological response

There are subtle structural and functional differences in different parts of the brain of males and females that may have a bearing on disagreements in TBI outcomes [42]. Post-trauma variations in anatomy have been reported with larger volume and surface area of the insula’s anterior circular sulcus in males than in females with post-traumatic stress disorder (PTSD) [43]. Less damage in females than males on similar TBI has been reported [44]. Higher blood-brain barrier (BBB) disruption in males compared to females after TBI also reflects sex-based differences [44,45]. Differences in anatomy can cause discrepancies in circulating biomarkers after injury [8]. Smaller diameter axons in females may be associated with increased vulnerability to shear deformation [46]. Sex-based variations in cellular infiltration or response (macrophages and microglia), which have been reported, can also determine different post-TBI outcomes in males and females [47].

2.2. Physiological/Biochemical response

There are diverse post-trauma physiological/biochemical alterations that are sex-based. These alterations may provide a more favorable response to females. Post-traumatic brain edema and hyperthermia, the most critical prognosticator of impairment, affect males more than females [15]. Hypoperfusion is sex-specific and is another prominent indicator of TBI severity [48]. Better BBB integrity and delayed edema formation have been reported in females post-TBI [49]. Novel techniques, including “BBB on a chip”, neuroimaging or intravenous tracers, can elucidate BBB physiology and functions in normal and TBI-affected patients and can be explored to evaluate sex-based variations [50]. In females, dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis post-TBI occurs by improper regulation of pre-autonomic neurons.

In contrast, in males, it is due to the disruption of limbic pathways [51]. Sex variations in methionine metabolism, an essential proteinogenic amino acid that regulates many cellular functions, have been reported [52]. Estrogen and progesterone in females provide neuroprotection against cranial edema, pressure, and perfusion during trauma [53]. Similarly, modification of cellular pathophysiological responses, like microglia activation, can favor females, as a deteriorated response in males has been previously reported [22,54]. Alteration of mitochondrial physiology and the subsequent mitochondrial dysfunction post-TBI has a greater effect in males than females, further aiding poor outcomes following TBI in males [5,55]. So, there are physiological mechanisms underlying the different sex-based responses to TBI.

2.3. Pathobiological response

There are sex differences in pathobiological mechanisms initiated after TBI [15]. These are related to the injury-recovery processes. TBI-induced edema occurs more in males than in females [45,56]. This is attributed to the protective effect of hormones (estrogen, progesterone) in females or changes in BBB [15,57]. Sex differences in the BBB permeability in rodent models of TBI have been reported with more permeability in males than females [40,58]. Hyperthermia following TBI occurs more in males than females [59]. Another protective mechanism is less hypoperfusion in females than males [15,48]. This may be due to more arterial constriction in males following TBI than in females [60]. Vasoconstriction or dilation is found to be related to arterial K channel response [60,61] and expression of endothelin-1 (ET-1) [62] following TBI, with females showing better compliance [63,64]. Additionally, increased endothelial activation and expression of β-catenin in males post-injury is reported [65].

Females have mostly a protective advantage over males when considering numerous pathological features such as lesion size, cell death, and recovery/neurogenesis, with few aspects being related to the model used [15]. The smaller size of the brain lesion and lower mortality on similar TBI models in females compared to males is reported. In this context, females exhibited numerous protective mechanisms, including hormonal, anti-apoptotic, or regional regenerative capability [22,66,67]. Progesterone is reduced to its metabolites locally during brain injury, which plays a role in recovery [68]. Upregulation of the X-linked inhibitor of apoptosis (XIAP) [69] and reduced expression of caspase-3 [70] in females compared to males is considered a protective mechanism against cell death following TBI. Furthermore, cytoskeletal protein degradation and neurodegeneration occur rapidly in males compared to females after TBI [15].

2.4. Hormonal response

Hormones play an essential role in female response against TBI, with various hormones having been reported to provide a protective function through different mechanisms [14,27]. Steroid hormones like estrogen [71] and progesterone [72] are considered as main sex-specific mechanisms that protect females compared to males against TBI [73]. Other hormones having a notable role in TBI response include prolactin [74], cortisol [75], thyroid hormones, and thyroid stimulating hormone [76]. Dopamine [77,78] and growth hormone [79] have also shown alterations in post-traumatic brain injury; however, their role in TBI response is yet to be thoroughly investigated.

2.4.1. Estrogen

Estrogen provides neuroprotection to females through multiple mechanisms [71]. It promotes neurogenesis and neuronal recovery by attenuating neurite outgrowth and glial activity [80,81]. Following TBI, it prevents inflammatory cascade [82], apoptosis [83], and oxidative stress [84]. Estrogen decreases injury-induced neuroinflammation and increases cell survival and neuroprotection [81]. Moreover, estrogen helps reduce glutamate concentrations that are elevated due to trauma, thus preventing cellular excitotoxicity [24]. In this paradigm, it increases the expression of the astrocytic glutamate uptake transporters GLAST (glutamate and aspartate transporters) and GLT-1 (glutamate transporter) that mediate the reduction of glutamate concentrations in the brain [85,86]. Besides, estrogen enhances mitochondrial biogenesis, function, and oxidative protection through upregulation of peroxisome proliferator-activated receptor-γ coactivator-1 beta (PGC-1β and/or PGC-1 related coactivator (PRC) [87].

2.4.2. Progesterone

Numerous mechanisms of progesterone protective response against TBI in females have been elucidated. Studies have shown that progesterone is involved in various pathways during TBI-induced primary and secondary brain damage [88]. In this context, progesterone prevents cerebral edema formation and BBB disruption[89], reduces neuroinflammation [90] and oxidative stress [88] and prevents demyelination [91]. It helps to regulate fluid and ion exchange through water channel proteins aquaporins and stabilize the BBB [92]. Furthermore, It regulates the synthesis of p-glycoproteins which are protein transporters associated with expulsing free radicals and inflammatory mediators [92]. Progesterone suppresses the synthesis of proinflammatory mediators like tumor necrosis factor alpha (TNF-α), interleukin (IL) 1 (IL-1), and IL-6 and influences the anti-inflammatory cytokines like transforming growth factor beta (TGF-β) [14], thus preventing the inflammatory cascade [93]. Moreover, progesterone suppresses the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2) and subsequently prevents oxidative disturbance [94]. The suppression of activated NF-κB through decreasing inhibitory κBα and NF-κB p65 phosphorylation and p65 nuclear translocation [94] reduces neuronal loss and glutamate levels [24,95]. Progesterone also reduces mitochondrial dysfunction and cell apoptosis [96] by inhibiting proapoptotic enzymes, cytochrome c, activated caspase-3, and the Bcl-2-associated death-promoter [92].

2.4.3. Prolactin

The protective role of prolactin in neuronal damage and neuroinflammation has been reviewed in detail [74]. Prolactin protects against brain damage, most likely through glial cells with some specific relevance to the female sex [97,98]. TBI affects the transport of prolactin inhibitory factors down the pituitary stalk into the gland resulting in disturbances of prolactin levels [99]. However, supplementation of prolactin in TBI subjects improves prolactin receptors, apolipoprotein E, and glial fibrillary acidic protein expression [98]. It decreases lesion-induced microgliosis, modifies the expression of microglial activation-related genes, and also protects from neuroinflammation by inhibiting NF-κB pathways via the extracellular regulated kinase 1/2 (ERK1/2) [97,100].

2.4.4. Cortisol

Glucocorticoids, including cortisol, are known for their life-saving roles in critical conditions like TBI. Their homeostatic, anti-inflammatory, and balancing hypothalamic-pituitary axis roles are well established; however, during TBI, their actions on metabolic, inflammatory, and neurochemical signaling are vital [75,101]. There is sex variation in cortisol-mediated response to TBI with some protective favor towards the female sex [3,51]. TBI disrupts limbic pathways involved in HPA axis coordination in males, and cortisol-induced immunosuppression is highly protective during the early stage of TBI as it decreases intracellular cAMP and decreases T cell infiltration and inflammatory responses in female mice [3].

2.4.5. Thyroid hormones and stimulating thyroid hormone

Thyroid hormones play a role in the protection and recovery after TBI [76]. They regulate neuronal plasticity processes, stimulate angiogenesis and neurogenesis, and modulate the dynamics of cytoskeletal elements and intracellular transport processes [102]. Nevertheless, few studies have demonstrated the sex-dependent and independent variation in thyroid hormones in TBI [103].

2.4.6. Dopamine

Similar to the mentioned hormones, neurotransmitters such as dopamine are affected during TBI [14]. A decrease in dopamine transporter (DAT) expression in males than in females following trauma has been reported [78]. Typically, females have a higher density of DAT binding sites than males [104], as estrogen regulates the dopamine system through genomic and nongenomic mechanisms, which are yet to be fully detailed [105].

Other neuroendocrine hormones like growth, adrenocorticotrophic (ACTH), TDH, and DHEA-S may have shown sex variation in TBI-affected individuals. Still, their role in neuroprotection needs more in-depth investigations [89,106].

2.5. Enzymatic response

Numerous enzymatic actions are identified as protective mechanisms in females against TBI [29,107]. Some of these enzymes are involved in hormone synthesis [108], antioxidant system [5], inflammatory mediator/protein synthesis, or apoptosis [38]. Enzymes involved in estrogen synthesis like estrogen synthase [81], estrogen-2-hydroxylase [109], phosphoinositide 3-kinase/Akt [108], p38α, Jun N-terminal kinase (JNK) and p38 [81,110], cycline-2 dependent kinases [111,112] and estrogen-metabolizing enzymes (EME) [81] including aromatase [81,113], steroid sulfatase [114], and estrogen steroid sulfotransferases [115] have a protective role in TBI. These estrogen metabolizing enzymes (EME) include aromatase, steroid sulfatase (STS), estrogen sulfotransferase (EST/SULT1E1), and some forms of 17β-hydroxysteroid dehydrogenase (HSD17B). These enzymes are involved in both the initial conversion and interconversion of estrogen from precursors [81] and thus are involved in the protective response mediated through estrogen in TBI. Similarly, progesterone-related enzymes, including 3β-hydroxysteroid dehydrogenase (3β-HSD) and 5α-reductase [116], play a role in mediating progesterone-linked neuroprotective actions, with many shown to be effective in females [90,116]. Several antioxidant/oxidant enzymes have been reported to have a role in aggravating or protecting against TBI. The enzymes that alleviate oxidative stress and thereby reduce the severity of TBI, including superoxide dismutase (SOD) [88], glutathione peroxidase, catalase, heme oxygenase, and thioredoxin [117], show sex variations. The enzymes that may aggravate TBI severity include nitric oxide (NO) synthase, xanthine dehydrogenase (XDH), xanthine oxidase (XO), phospholipases, cyclooxygenase, and 5-lipoxygenases which have also shown sex alterations [118]. Many studies have also reported higher oxidative stress in males compared to females post-TBI [5,14]. Males also have higher lipid peroxidation than females after severe TBI indicating sex-based aggravation in males [35]. Even brain cell mitochondria have shown less oxidative stress in females than males owing to more expression and enzymatic activity of the antioxidant system in females than males [36,39]. Mitochondrial-specific proapoptotic enzymes such as cytochrome c and activated caspase-3 [92] also showed sex variations, and their inhibition is beneficial in TBI [117]. Caspase mediates cell death in TBI is observed in females, whereas cell death is caspase-independent in males [38].

2.6. Neuroinflammatory response

The neuroinflammatory response is a major hallmark in TBI-induced injury, repair, and recovery pathophysiology. As a vital component of the secondary injury, glial cell activation and the infiltration of peripheral myeloid cells (macrophage, neutrophils, lymphocytes) play a major role in the acute neuroinflammatory cascade following TBI, which is directly associated with neuronal death [13]. The impact of sex differences on post-traumatic neuroinflammatory reactions has been disputed, and conflicting findings have been reported. While an earlier report showed no significant difference in microglial activation [119], recent studies have described a sex-based difference in preclinical models of TBI [120,121]. Villapol et al. have recently reported a completely divergent sex-based inflammatory response in the acute and subacute phases following TBI [22]. A rapid and significant increase and activation in microglial cells was found in male mice in different brain regions after TBI compared to female mice.

Moreover, rapid activation and infiltration of peripheral macrophage and other phagocytic cells were more pronounced in male mice but not in females. Acute astrogliosis measured by glial fibrillary acidic protein (GFAP) was also significantly higher in the ipsilateral cortex of male mice [22]. Glial and peripheral immune cells exert the neuroinflammatory response by producing inflammatory cytokines and chemokines [13]. Studying the sex difference in cytokine expression demonstrated that although females exhibit higher proinflammatory cytokines (IL-1β and TNFα) at an earlier time, a delayed anti-inflammatory phase was observed up to 30 days post-injury [22]. In agreement with this evidence, Doran et al. reported a sexually dimorphic acute neuroinflammatory response following TBI represented by rapid and exaggerated activation and infiltration of myeloid cells in males, but not in females [122]. These divergent inflammatory responses are directly translated into increased neuronal death, lesion volume, and behavioral impairments in males than females. Understanding the basis of how neuroinflammatory responses are affected by sex is a key step in the development of individualized and effective therapeutics for TBI.

2.7. Oxidative response

There is a differential oxidative/antioxidant stress response in males and females against TBI [14,115]. In addition to the oxidative-related enzymatic response mentioned earlier, males have more oxidative stress owing to more lipid peroxidation [35] and less expression and enzymatic activity of the antioxidant system in mitochondria [36,39]. An earlier clinical study showed that women had remarkably lower F₂-isoprostane levels, a marker of lipid peroxidation, than men in cerebrospinal fluid samples following severe TBI [35]. Likewise, a recent preclinical study reported higher post-TBI peroxide production in the pericontusional cortex in male rats, while no significant change was noticed in females [123]. Moreover, female and male rats have a distinct increase in protein carbonylation after TBI in the injured area [14]. Progesterone and estrogen also provide antioxidant effects by regulating mitochondrial oxidative metabolism in the brain and reducing protein carbonylation. Interaction of sex hormones like estrogen [87] or progesterone [94] with the antioxidant system reduces oxidative stress in females as compared to males [87,94].

2.8. Genetic response

Differential expression of genes and pathways associated with brain injury and recovery in males and females highlights sex-specific response towards TBI [15,124]. Genetics and sex hormones influence inflammation, edema, oxidative stress, excitotoxicity, and mitochondrial function [29,38]. Neuronal trophic factors, such as brain-derived neurotrophic factor (BDNF) is essential for neuronal growth and function but show differential expression in males and females following TBI [125]. The expression of dopamine transporter (DAT), important in the brain’s dopaminergic system and involved in cellular function and CNS inflammation, is reduced more in males than females [78,126]. Females carrying the APOE4 allele have shown worse Glasgow Outcome Scale-Extended (GOS-E) scores than males [127]. Race and sex affect race-specific single nucleotide polymorphisms in genes affecting inflammation and predisposition to genetic diseases [128].

2.9. Mitochondrial response

There are sex differences in mitochondrial function or dysfunction that influence the neuroprotective response in males and females [38] and might contribute to differential sex-dependent outcomes following TBI [5]. Mitochondria play a sex-specific role in metabolism, inflammation, oxidative stress, and apoptosis and are sex-specific [55]. Reduced expression of mitochondrial-specific proapoptotic caspase-3 [70] in females compared to males is considered a protective mechanism against cell death following TBI, similar to cytochrome C [92]. Mitochondria and sex hormone interaction and the defensive response against TBI in females have also revealed favorable outcomes by supporting mitochondria biogenesis, reducing oxidative stress and apoptosis [87].

3. Discrepancies in sex-based response to TBI; reasons and solutions

As reported by many studies, females are suggested to have a greater degree of protection against TBI. We provided the possible detailed mechanisms with scientific evidence. However, we came across numerous studies wherein either nonsignificant sex differences to TBI or more vulnerability of females compared to males or more protective response by males were found. Limited studies have shown unfavorable results or poor outcomes for females or no significant difference compared to males like lesion volume [129], edema [45], neurodegeneration [129], increased neuroinflammation, astrocytosis [130], microglia activation [22,129], BBB permeability [58,129], and inflammatory cytokines (TNF-α, IL-1β, and IL-17) [40]. After analyzing the literature, we came across various possible reasons for the sex differences in TBI. Less or no inclusion of females in TBI studies is one of the main reasons for getting inconclusive sex-based differences. Many studies have used male subjects, but studies on female subjects are limited. Hence, including the female sex as a biological variable in TBI studies can help remove sex-based bias [7].

A small sample size of the population or group can be another reason. In most of the TBI studies, females are under-represented [17]; as a result, there are inappropriate results, and data interpretation becomes difficult. Increasing the sample size or the number of participants in TBI studies can produce reliable results without bias. The types of TBI, viz. mild, moderate, or severe to be studied, also reflect variations. Studying similar TBI types simultaneously in different sexes can reveal differences in response. TBI of a moderate to severe nature generates an equivocal response in females compared to males in both animals and humans. Sex differences in animals and humans result in variable responses and recovery against TBI of mild nature. On subjective measures, mild injuries tend to have a better outcome in female animals and worst in women reflecting variation between responses; however, on closer observation, similar trends of preclinical and clinical studies are noted concerning the female sex [14,15]. In animal models, a difference in sex-based outcomes has been revealed depending on the model used and the parameter assessed. In animal studies, contrary to human studies, most females have better outcomes than males after TBI. Of the studies conducted by the controlled cortical impact (CCI) model of TBI, 55% have shown that females have better outcomes in post-injury evaluation, including behavioral and motor functions [5,40]. In the closed head injury (CHI) model of TBI, 31% reported better outcomes in females based on the evaluation of cytoskeletal degradation, reactive astrogliosis, and cortical blood flow. Of the TBI fluid percussion injury (FPI) model, 60% noticed that females have worse survival and regulation of cell apoptosis [5,40]. This shows parameters to be evaluated pre-, or post-TBI also determine the nature of sex variations, while females have favor in some parameters and males in others. For example, when choosing hormones as parameters, females have specific or more relevant hormones, and males have their own set of hormones. So, evaluating only one kind of hormone may provide a biased result. Just like estrogen and progesterone, the male hormone testosterone has shown neuroprotective actions, such as increased levels of antioxidants, BNDF [131], decreased reactive astroglia and microglia [132], favoring neurogenesis [131], protecting mitochondrial function [133], and regulating the expression of neuroglobin in astrocytes [134].

Similarly, subjective outcomes like cognitive and psychosocial outcomes may raise bias concerns. The selection of parameters based on objective measures would be helpful. Age, puberty, duration, differential patterning, social involvement, or environmental interaction also affects the sex-specific evaluation of TBI [5,8,15]. In addition, understanding sex-based differences in neurogenesis post-TBI may reveal some hidden facets, as there are pieces of evidence for such differences in normal genders. In this context, there are sex differences in neurogenesis, with females having more favor than males in most of the studies [135]. Neurogenesis is reflected by cell proliferation, migration, differentiation, and survival. Some of these events can be independent or interdependent [136]. Variation in neurogenesis steps post-TBI is rarely explored but may have sex-based influence at different levels, including angiogenesis or vasculature [102,129], neurogenesis [80], mitochondrial biogenesis [87], or antioxidant defense [131]. Lastly, the use of experimental TBI models, advanced techniques, preferably non-invasive ones, and specific biomarkers can help remove sex-related biases.

4. Conclusion

Although considered to have protective mechanisms against TBI, the female sex needs to be studied in detail concerning various TBI-related models and mechanisms. Multiple factors affect the outcome of TBI studies, and normalizing these factors against different variables can help to uniform the investigation. In addition to subjective outcomes against TBI, the female sex needs to be evaluated on objective measures for various protective mechanisms discussed here. This review outlines these mechanisms from morphological, cellular, and molecular aspects to provide an understanding of the demonstrated protective nature of females against TBI. It also reveals an interrelation among these mechanisms wherein they supplement each other.

Furthermore, it summarises various biomarkers based on physiological, pathobiological, biochemical, hormonal, hematological/serological, oxidative, inflammatory, genetic, or cellular backgrounds that can be explored for diagnostic and prognostic purposes of TBI (Figure 1). These biomarkers provide a set of objective measures that can be monitored over time to assess discrete sex variations in the TBI response being investigated. These can also serve as potential molecular targets for designing future individualized therapeutic interventions against TBI. Ultimately, the foremost requirement in future research is to include the female sex as a biological variable in sufficient numbers to overcome the under-representation in TBI research.

Figure 1.

The potential mechanisms involved in the sex-related difference in response to TBI. Various basis of responsive mechanisms is represented in the pathophysiology of TBI and some of them are found to be sexually distinct. Several components are included under each mechanical response which can be investigated as essential biomarkers for the development of individualized therapeutic strategies for the treatment of TBI (created with BioRender.com).

Data availability

No data was used for the research described in the article.