How to Translate Time: The Temporal Aspects of Rodent and Human Pathobiological Processes in Traumatic Brain Injury

Experimental studies

Experimental models of TBI have been used with ¹⁴ C- or ¹⁸ F-labeled 2-deoxyglucose autoradiography/microPET to determine the magnitude and time course of metabolic changes after injury. Mild fluid percussion injuries have shown immediate increase in glucose uptake, and by 6 h, glucose metabolic depression was detected, reaching their nadir at 1 day and recovery by 7–10 days post-injury in both the cortex and hippocampus. ²³ A similar pattern of CMRglc changes was also observed with ¹⁸ F-deoxyglucose PET (FDG-PET) in adult mild-fluid-percussion–injured rats with metabolic recovery by 16 days post-injury. ³⁰ Experimental data have shown that the magnitude and duration of CMRglc depression are dependent on injury severity. The temporal pattern appears to be similar in different models, varying from fluid percussion to controlled cortical impact (CCI). Whereas mild fluid percussion resulted in 20–30% decrease in CMRglc, more-severe CCI injury models produced 50–70% CMRglc depression, which continued to remain depressed at 10 days post-injury. ^16,31 Severe fluid percussion injuries have also been shown to induce chronic hypometabolism lasting 3–6 months after injury. ³² Experimental studies have also shown that the magnitude and duration of CMRglc depression is affected by age. Post-natal day 17 and adolescent rats showed faster metabolic recoveries than adult rats after fluid percussion injury and CCI injuries, especially in subcortical structures. ^16,33

Clinical observations

TBI-induced changes in CMRglc have also been analyzed in humans using ¹⁸ FDG-PET. Mild TBI has been examined with FDG-PET by several groups, but many of these were at chronic time points, the result of motor vehicle accidents, and did not report actual CMRglc, making it difficult to directly compare to experimental data. ^34,35 These studies do report CMRglc depression in regions associated with specific cognitive functional tests. A comprehensive review of these studies has been compiled by Byrnes and colleagues. ³⁰ Moderate-to-severe TBIs have also been studied with PET, but as with mild injury studies, there is often a variety of ages, time points, averaging of cases, as well as an absence of time-course profiles and reporting of metabolic rates. ³⁰ One study did show recovery of global cortical CMRglc depression after mild/moderate (Glasgow Coma Scale [GCS] 9–15) in adult human TBI patients by 3 months but 6–9 months after severe injury. ^24,36 Chronic CMRglc depression have also been reported after blast injuries, among comatose/vegetative severe TBI patients, and those with significant atrophy. ^{36 –38}

In animal studies, the nadir is reached within 24 h after impact, whereas the greatest CMRglc depression is attained within the first week in humans. CMRglc recovery after mild fluid percussion injuries is 10–14 days in adults and 21–30 days after CCI injury. These data suggest that TBI-induced CMRglc depression and recovery are ∼15 times faster in rodents than in humans. The differences between the rat and human timelines should be considered when trying to determine the estimated time for human CMRglc recovery as well as the therapeutic window for clinical trials.

Significance of identifying the temporal pattern of cerebral metabolic rates of glucose depression

From a metabolic point of view, the time of altered (depressed) CMRglc corresponds to the period of increased cerebral vulnerability. During this window of heightened cerebral vulnerability, repeated injury leads to substantially more-severe and longer-lasting symptoms in both experimental and clinical TBI. Accordingly, identifying the exact temporal relationship between rat and human CMRglc changes is important for civilian as well as military TBI, especially for mild TBI/concussion.

Experimental studies

How closely rodent models of inflammatory diseases mimic the human condition is the focus of intense and contentious debate. Comparing the genomic responses of human conditions with their relevant mouse models of trauma, burn, and endotoxemia has shown that at the whole genome level, there is very little similarity in human versus rodent transcriptional response and signaling pathways involved in the pathological response. ⁴¹ However, narrowing down the selection to genes involved in a subset of inflammatory responses showed more similarities in transcriptional response between rodent models and human cases. Importantly, the studies concurred that the temporal pattern of rodent versus human genomic response is very different. An important measure of the temporal pattern of the genomic response to inflammation is the time interval for the expression pattern to normalize. The time interval to return to normal (baseline) pattern of gene expression in the human burn cases averaged ∼5 months (1 month to 1 year). In the rodent model, average recovery time was 3 days (1 day up to 7 days), suggesting a ∼50-fold faster process in rats. Using a similar definition (return to normal pattern of gene expression), the recovery rate was found to be ∼30-fold faster in the rodent model of trauma than in humans. Interestingly, the recovery rate was similar for endotoxemia, around 1 day in both humans and rodents. However, the genomic responses themselves to trauma, burn, and endotoxemia were substantially different in the two species. ⁴¹

Longitudinal studies tracing microglial response to injury by PET imaging enables comparison of the neuroinflammatory timelines between human cases and rodent models of TBI. The most comprehensive experimental longitudinal study by Wang and colleagues used a combination of PET imaging with [ ¹⁸ F]-714, ⁴² a specific ligand to the 18-kD translocator protein (TSPO) for activated microglia, ex vivo autoradiography, and immunohistochemical verification of the PET and autoradiography data. An important caveat is that the various PET ligands also bind to TSPO in (activated) astrocytes, and thus TSPO should be considered a marker of activated astro- and microglia. ^43,44 The study showed that the neuroinflammatory response, as detected by all three methods, peaked at around day 6 after CCI injury. TSPO expression declined by day 10 and returned to the levels measured in the sham-treated group by day 28. This timeline was confirmed by in vitro autoradiography and also by immunohistochemical staining of activated microglia. These findings are in agreement with previous PET studies using different ligands ([ ¹¹ C]PK11195 and [ ¹⁸ F]FE-DAA1106) for activated microglia. ⁴⁵ These studies also showed maximum neuroinflammatory (microglia) response around day 6–10 post-injury that returned to normal levels by around 30 days post-impact. It should be noted that activated astroglia as well as peripheral macrophages also bind TSPO; thus, the protein is not entirely microglia specific. However, other cell types are also involved in mediating inflammatory response after TBI.

Clinical observations

There are significant challenges in comparing the temporal sequence of inflammatory events in rodent models versus human TBI. A key consideration in human neuroinflammation research is defining inflammation in the first instance. If inflammation is considered as the endogenous response to injury, it has to cover a wide spectrum of both humoral and cellular events that span both innate and adaptive immunity. ⁴⁶ Access to a number of biological compartments (plasma, cerebrospinal fluid [CSF], brain extracellular fluid) has allowed researchers to measure a range of inflammatory biomarkers after TBI, but each of these biocompartments has its limitations with respect to interpretation. For example, plasma sampling is confounded by extracranial trauma; CSF sampling is usually intermittent, and the CSF compartment may reflect a sump for clearance of inflammatory substances ^47,48 ; and microdialysate sampling from the brain extracellular space is limited by the proportion of a soluble mediator that crosses into the microdialysis catheter, which varies with molecular weight, charge, and methodological variations in microdialysis technique. ⁴⁹ The absolute concentrations of a given mediator are therefore difficult to relate directly to equivalent rodent studies when extrapolating data on the range of concentrations that may inflict neuronal injury. Human TBI research is necessarily opportunistic, and very early events that occur in the clinical resuscitative phase are beyond the reach of clinical studies that require sampling of biological fluids. However, an early post-mortem study carried out on patients succumbing to TBIs at various intervals has empirically demonstrated that at the messenger RNA and protein level, early proinflammatory drivers of innate inflammation, including interleukin (IL)1β and tumor necrosis factor (TNF), are significantly increased in the human brain within 24 h after TBI. ⁵⁰ Subsequent to this early period, once patients are admitted to specialist neurointensive care, invasive monitoring allows a more-detailed assessment of inflammatory biomarkers. Interestingly, serendipitous monitoring within the first 24 h picks up this early proinflammatory signal whereas subsequent monitoring time points do not. ⁴⁷ Beyond this time frame, a strong IL6 signal has been detected within the first 24–72 h in both microdialysate and CSF. ^{47,51 –53} IL6 is an interesting cytokine given that it may have both damaging and protective properties depending on the context in which it is produced. ⁵⁴ In absolute terms, IL6 is produced at concentrations several orders of magnitude higher than IL1β and TNF, ⁴⁷ which may introduce a selection bias into the literature given that it is easier to detect analytically.

Alongside these key cytokines, several chemokines are detected and likely reflect recruitment or activation of inflammatory cells. Growth-related oncogene-alpha and IL8 are produced in the first 24–48 h and may reflect neutrophil recruitment. Beyond this, chemokines related to microglial activation, recruitment, activation, and expansion are found in an overlapping time frame, including monocyte chemoattractant protein 1/C-C motif chemokine ligand [CCL]2), regulated upon activation normal T-cell expressed and secreted (CCL5), and granulocyte colony-stimulating factor. When comparing studies, it is important to interpret the data in relation to time of injury rather than the time that monitoring or sampling commences. A late IL10 response at around days 4–7 may subsequently reflect a shift in microglial activation toward a reparatory or regulatory phenotype. ^47,55

Several researchers have considered the role of these inflammatory biomarkers in TBI pathophysiology and correlated concentrations with specific sequelae rather than considering the temporal sequence per se. ^56,57 Several studies relate this acute response to chronic sequelae ⁵⁸ and outcomes at 6 months after TBI, ^59,60 suggesting that this acute response plays a mechanistic role in inflicting injury.

In the chronic phase, there is evidence that the plasma cytokines (IL1β, IL6, IL8, and IL10) are raised for over 3 months after injury, and this may relate to clinical outcomes, albeit demonstrated in small groups of patients. ⁶⁰ Although no comprehensive study similar to Wang and colleagues is available for human TBI patients, ⁴² published works indicate that inflammatory response, as defined by the presence of activated microglia, persists for many years after the injury. Using the ligand, [ ¹¹ C]PK11195, Ramlackhansingh and colleagues have found activated microglia persisting in patients up to 17 years after TBI. ⁶¹ Interestingly, patients showed persistent activation of microglia distant from the lesion site, raising the possibility that this represents diaschisis or a long-lasting reorganization that underlies plasticity and recovery, distinct from the original injury. These findings are consistent with data obtained from post-mortem studies showing long-term presence of activated microglia ∼16 years after TBI as well as studies performed in non-human primates showing substantial neuroinflammation 1 year after injury (the latest time point studied), indicating that TBI-induced neuroinflammatory response can be 4 to 30 times faster in rats than in humans (summarized in Fig. 2). An additional, and often underappreciated, aspect of inflammation is the adaptive immune response. ⁶² There are a paucity of clinical studies in this context, but information about the additional phase of autoimmunization over a 10- to 14-day typical study period is missing.

Significance of identifying the temporal pattern of inflammation

The importance of defining and targeting an appropriate therapeutic window has been well described in the TBI literature. Inflammatory processes are a promising target, in that the time frames described above span several days, providing ample opportunity for clinical intervention. Nevertheless, there are several caveats that must be considered. A relatively small group of cytokines and chemokines has been measured in the clinical TBI literature, ⁴⁶ and because these mediators act in complex cascades and show a high degree of collinearity, it is impossible to draw conclusions about causality from observational correlative studies. When choosing putative therapeutic targets, an assessment of the biological consequences of administration should provide confirmation that the drug is exerting the desired effect. ⁶³ Further, with the heavy bias in the literature toward assessment of soluble mediators, there is ambiguity over the cellular origin of the measured cytokines and chemokines. Microglia, astrocytes, neutrophils, and neurons are all capable of producing these mediators. ⁴⁶ Considering the functional heterogeneity of these cells, immunomodulatory pharmacotherapies may require delivery at specific times to target distinct cellular subsets to have a beneficial effect.

From a therapeutic point of view, onset and extent of the inflammatory response to injury represent a window for anti-inflammatory therapies. Given the abundance and availability of anti-inflammatory therapies that target specific molecules of the inflammatory process, it is critical that the temporal differences between rodent and human inflammatory response are taken into account when trying to identify the therapeutic window for anti-inflammatory molecules in clinical trials.

Experimental studies

After experimental TBI, there is a rapid edema formation within the first hour, which diffusion-weighted magnetic resonance (MR) imaging studies have shown as vasogenic edema. ^{66 –68} Presence of a blood–brain barrier (BBB) that is more permeable to vascular proteins has confirmed the vasogenic nature of the edema. ⁶⁹ The initial increased permeability of the BBB to large vascular proteins was, however, limited to the first few hours after TBI, with the BBB becoming increasingly less permeable to vascular proteins and normalizing over the next week. ^69,70 As the BBB became less permeable, cytotoxic edema became more apparent, signifying a net movement of water from the extracellular to the intracellular compartment. Maximum cytotoxic edema was observed between days 7 and 14 after TBI, resolving thereafter. ^67,68

Some reports have suggested a second increase in BBB permeability to vascular proteins on day 3, ⁷¹ which may lead to a surge in intracranial pressure (ICP). Although not universally observed in all models of experimental trauma, this second opening of the BBB is consistent with the reported delayed tight junction breakdown, ⁷² most likely mediated by activation of matrix metalloproteinases. This is in contrast to the functional transcytotic upregulation of albumin transport, with tight junctions intact, observed in the early phase after TBI ^72,73 and indeed as early as 3 minutes after TBI. ⁷⁴ Such delayed opening of the BBB suggests the involvement of brain metabolic processes and inflammation that are initiated by the injury, in contrast to a largely neural/vascular interaction in the immediate post-injury period. ⁷⁵

Clinical observations

Studies of edema and BBB permeability in patients have been difficult given the inherent instability of TBI patients, particularly those that are severely injured, and when imaging studies are potentially complicated by mechanical ventilation. Nonetheless, there have been several studies, albeit most began many hours to days after the primary event. Both vasogenic and cytotoxic edema have been reported after TBI using MR imaging (MRI) techniques, ⁷⁶ although a predominance of cytotoxic edema is noted as time progresses with any subsequent tissue loss being replaced by extracellular water. ⁷⁷ MRI-detectable abnormalities in BBB permeability were more common in pericontusional areas after 2 days and frequently did not become apparent until 6 days after injury. ⁷⁸ This is consistent with the strong association between hemorrhage and activation of metalloproteinases, and the observation that persistent increases in metalloproteinases have been reported after 4 days in clinical TBI. ⁷⁹

An alternative to imaging studies is the use of CSF and serum inflammatory biomarkers or albumin distribution to indirectly determine increased BBB permeability. ^80,81 These studies have confirmed the MRI findings indicating BBB dysfunction in the days after TBI. Irrespective of the technique used, it has been noted that patients with increased BBB permeability have poorer outcomes, ⁸² with those with more-severe disruptions of BBB after 3 days being pre-disposed to developing substantially higher ICP values. ⁸⁰ Development of increased ICP in the presence of a permeable BBB implies a continuing role for vasogenic edema after clinical TBI. ⁸² BBB permeability is generally thought to return to normal within several weeks, although persistent BBB dysfunction has been reported months to years later in patients who develop post-traumatic epilepsy and even after mild TBI. ⁸³

Consistent with experimental studies, increased transcytosis of vascular protein while endothelial tight junctions are intact has been reported in the first hours after human TBI. ⁸⁴ However, this transcytosis persists for up to 2 years post-injury. ⁸⁵ More-severe edema at these later time points was also associated with opening of the tight junctions, perhaps reflecting the involvement of metabolic processes and inflammation initiated by the injury (once again) at these delayed time points.

Significance of identifying the temporal pattern of vascular permeability and edema

Although the immediate functional vascular response of rodents to TBI is very similar to that of humans, subsequent metabolic and inflammatory responses that underlie BBB permeability and edema formation at later time points are far more persistent in human TBI. Indeed, the majority of studies in experimental animals describe BBB restoration within a month after CNS lesions. Conversely, increased BBB permeability and edema formation has been noted years after the traumatic event in humans. ^{86 –88} Not only would this result in an extended period of injury development with a consequent lack of functional recovery in humans, but it also implies that the therapeutic opportunity for intervention at the metabolic and inflammatory level extends well beyond the weeks assumed from rodent studies. Important caveats include type and severity of the injury. Severe TBI is rarely modeled in rodents, and few studies focus on penetrating/ballistic injuries. Moreover, the methods used to monitor BBB function in rodents and humans are in most cases vastly dissimilar, contributing in part to the limited understanding of the temporal differences in vascular permeability that exist between the two. Additional difficulties include the unknown onset and extent of neovascularization after TBI observed in both rodent models and also in human TBI cases. ⁸⁹ Newly formed, immature vessels have increased permeability, and we currently do not know how long it takes until de novo vessels develop fully functional BBB in rodent models versus in human TBI cases. A recent study has shown that in a rodent model of mild TBI, the inflammatory process can trigger revascularization as part of the endogenous repair program repair, but only within a specific, delayed window period during which would-healing macrophages promote angiogenesis. ⁹⁰ This finding further underlines the critical importance of understanding the differences between rodent and human pathological processes.

Experimental studies

Studies using rotational or acceleration/deceleration forces to cause axonal injury have shown that TAI occurs within minutes in the rat as shown by using dextrane as an indicator of increased axolemmal permeability. Impaired axonal transport can be detected as early as 15–30 min after TBI using APP immune-fluorescence. ^112,113 Intra-axonal changes begin immediately after the impact and axonal swelling and detachment develops within the first day (12–24 h) after injury. APP accumulation in the rodent TBI model can be detected up to 14 days after injury. ¹¹⁴ Hellewell and colleagues demonstrated a maximum number of retraction bulbs at day 1, with a gradual decrease at 14 days post injury. ¹¹¹ A key study by Mac Donald and colleagues established the temporal pattern of TAI in a mouse model of CCI using a combination of histology, electronmicroscopy, and DTI. ¹¹⁵ The study showed that the numbers of both APP and NF-L–positive axonal varicosities increased dramatically during the acute phase of injury defined as between 4–6 h and 4 days post-impact followed by a substantial decline during the subacute phase—1 week to 1 month. Importantly, quantitative analysis of diffusion tensor imaging (DTI) parameters—axial diffusivity (AD), radial diffusivity (RD), relative anisotropy (RA), and mean diffusivity—showed close correlation with histological findings. AD decreased during the acute phase and returned to control levels by 1 month whereas RA levels remained reduced during both the acute and subacute phase.

Li and colleagues observed an increase in APP-stained axonal varicosities up to 72 h. They also found that the temporal profile of immunohistochemical staining correlated very closely with that of changes detected by DTI. Both fractional anisotropy (FA) and AD were significantly reduced in the brain regions affected by the trauma between 3 and 72 h. ¹¹⁶ These findings are important because they suggest that changes detected by DTI, reduced FA and AD, can reflect the histological underpinning of TAI. ^115,117 There appears to be evidence of late demyelination in the rat at 4 weeks, ¹¹⁵ as compared to humans, where demyelination is still detectable at 1–4 years after the injury. As discussed above, activated glia cells and macrophages, indicating inflammatory processes, have been shown to peak around days 3–7 post-injury in the rodent model.

Clinical observations

DAI is a common pathological feature of closed head TBI, especially severe TBI. ^91,92,118 Clinically, DAI is characterized by early LOC and often exhibits disproportionately limited focal lesions in the brain on computed tomography (CT) scans. Nonetheless, DAI can be the sole pathology that causes coma in severe TBI patients. ^119,120 The brain regions that most commonly show DAI injury are the subcortical gray/white matter interface near the frontal and temporal lobes, corpus callosum (CC), and brainstem. ¹²¹ DAI results in impaired cognitive, autonomic, and sensory functions, depending on the extent of lesions and affected brain regions, with clinical severity generally increasing the deeper the structure(s) involved. Severity of DAI can be classified from: grade I (mild), characterized by subcortical and internal/external capsule lesions; grade II (moderate), in addition to the above regions the damage affects the CC; and grade III (severe), with additional lesions in the brainstem. ¹¹⁹ As with other pathobiologies triggered by TBI, the primary DAI is instantaneous whereas the secondary DAI can last for an extended period of time.

It was long believed that DAI was part of the acute to subacute pathological response to TBI; however, there is strong indication that DAI can persist years after the insult. The original focus on immediate shearing forces and retraction bulbs of severed axons followed by Wallerian degeneration has been expanded by a growing appreciation of the extended metabolic disturbance initiated. ^122,123 This is thought to involve calcium dysregulation, disruption of ionic homeostasis, energetics and cytoskeleton breakdown leading to axonal swelling and glial activation, followed by an extended period of inflammation, possibly lasting for years. ¹²³ Histological evidence of DAI has been observed in human forensic material as early as 35 min after injury, ¹¹⁸ and axonal swelling and beading size have been noted to progress up to 24–48 h, ¹²⁴ where milder trauma exhibits a slower swelling. Additionally, this has been shown to be a non-homogeneous process with much local variation. Temporal progression of DAI on MRI has been investigated, but the great variability in timing within and between MRI studies makes a clear sequential picture hard to delineate. TBI is, however, associated with general white matter loss over time, in excess of the natural loss observed in age-matched controls. The time course of human DAI as observed on MRI has been suggested to be divided into four stages: acute (<24 h), early subacute (1–13 days), late subacute (14–21 days), and chronic (>21 days), where early lesions are generally superseded by local edema in the subacute phases and by general white matter loss in the late chronic stage. ^{125 –128} DTI detection of DAI focuses on non-uniform or directional flow (anisotropy) along tracts and is quantified as FA or mean diffusivity. Early changes over the first week may thus be confounded by edema, exhibiting either restricted (intracellular, days) or increased (extracellular, 1–2 weeks) water movement. Late chronic stages may show a continuation of white matter loss for years. Gray matter loss may be a faster process and has been shown to diminish at 3 months, ¹²⁹ whereas late volume loss appears to be white-matter related. ¹³⁰ Bendlin and colleagues identified continued white matter loss at 1 year post-trauma. ¹³¹ In a follow-up study of this cohort of mild-severe TBI patients, white matter changes in the CC using DTI were shown to progress most the first year after trauma, but continue up to 4 years. ¹³⁰ Importantly, reactive microglia were also detected in white matter tracts that showed axonal pathology, suggesting that persistent axonal/white matter pathology may be the result of a chronic neuroinflammatory process. ¹³² Interestingly, in this late follow-up, RD (water movement perpendicular to axonal tracts) was more affected than axial diffusivity (along tracts), leading the authors to conclude that late pathology was driven more by myelin changes than axonal changes. However, a recent study that followed moderate to severe TBI patients longitudinally for at least at 3 time points, ¹³³ from the acute through chronic phase (medians: 1 week, 7 and 21 months), found late alterations to be driven by changes in both radial and axonal diffusivity. Additionally, the inflammation process may persist for years. ^61,134 Although candidate protein biomarkers exist, none have yet been established to follow the evolution of DAI. NF-L is shown to increase for days to weeks after injury, and levels have been related to outcome. However, NF-L levels have not been clearly related to MRI findings. In aggregate, the time course of DAI in humans is a process with early and late pathophysiological components where changes can be ongoing for years.

Significance of identifying the temporal pattern of axonal injury

The temporal pattern of axonal injury appears to be comparable between rodents and humans during the acute phase of the injury. It is the subchronic and chronic phase of TBI where the temporal patterns differ. In the absence of a new physical impact, the late phase of axonal injury is likely caused and maintained by other TBI-induced metabolic and inflammatory abnormalities. ^92,96,132 Mitochondrial abnormalities, oxidative stress, and lipid peroxidation can render structurally intact axons functionally impaired along with elevated intra-axonal Ca++ level. Neuroinflammation that can persist for years in humans after TBI can cause similarly lasting, chronic axonal degeneration. ¹³⁵

Given the limited ability of the mammalian CNS to regrow lost or dysfunctional axons, the most beneficial therapeutic interventions are likely aimed at restoring/normalizing cerebral metabolism, buffering neurotoxicity, and modifying the neuroinflammatory process. ⁹² There are important caveats that include the relative low-fidelity modeling of axonal injury in rodents attributed to anatomical differences discussed above and the unknown contributions of the distinct secondary injury processes to the overall response to injury. These, in combination with the type of injury (i.e., closed head or penetrating, focal, or diffuse) can majorly determine the relative contribution of axonal injury to the overall secondary injury process. DAI in clinical practice is a complex injury that is frequently associated with bleeding or microbleeding and often accompanied by other lesions. Bleeding, an indicator of vascular injury detectable by imaging, is an important (if not critical) comorbidity that further complicates the secondary injury process. Additionally, aging can affect the outcome of axonal injury. There are indications that the vulnerability of axons to physical impact is affected by age, and age can affect recovery. ^{136 –138}

The duration of ongoing axonal pathology reflects the period of increased cerebral vulnerability, that is, when reinjury can trigger disproportionally severe consequences by rendering changes to axons irreversible and resulting in axonal losses. Rehabilitation therapies, when optimally timed, can result in substantial functional recovery. Better knowledge of the duration and trajectory of axonal pathologies in rodents versus humans will help to identify the window for such interventions. In summary, the experimental relationship between human and rodent DAI/TAI may not be immediately comparable, but evidence suggests that the temporal changes in the acute phase may be similar between rodents and human cases. However, the late and chronic phases of DAI suggest that the process may be 10–12 times faster in rodents than in humans.

Recommendation 1

We recommend conducting more longitudinal studies, collecting and reporting data at multiple post-injury time points. This will increase the data density necessary to characterize the profile and evolution of injury-induced changes. Specifically, data should be collected at 2–3 times the rate of the expected information content of the temporal signal studied, as expected by the experimental hypotheses (Nyquist frequency). ¹⁴² In the absence of such a hypothesis, and without the possibility of high-frequency sampling, an initial sample frequency along a geometric time series multiple of 2 (0.5, 1, 2, 4, 8, and 16 time-units) can help to identify the required sampling frequencies/ time points for a specific early biomarker.

Developing an algorithm for “translating time” between rodent and human TBI pathobiologies is further hindered by two inter-related issues: different classifications of injury severity and different outcome measures in rodent TBI studies versus clinical cases. Classification has played an essential role in our understanding and treatment of diseases. Subdividing complex diseases into smaller, more homogeneous sets of pathologies has increasingly enabled accurate diagnosis, prognosis, and the development of specific treatments. However, TBI greatly lags behind other much better characterized diseases, such as cancer, when it comes to classification. Cancer is now subdivided into smaller subclasses, each with more homogenous molecular pathologies, increasingly enabling personalized, evidence-based treatments. TBI is an acquired, sudden-onset disease with dynamically changing pathobiological processes. Therefore, knowing the identity, onset, and extent of the various pathobiological processes is fundamental for developing a molecular-level classification system and evidence-based treatments for TBI. Current classification is still based on rather subjective clinical observations, and neuroimaging findings to a lesser extent, but not on molecular criteria. Experimental TBI studies generally lack even this subjective classification system. In clinical TBI, the GCS, length of LOC, alteration in mental/conscious sate (AOC), and post-traumatic amnesia (PTA) are used for classification. Because of their ease of use, practically every clinical TBI report contains GCS (and some LOC, AOC, and PTA) scores that are used for triaging, assessing disease progression, and selecting patients for clinical trials and epidemiology studies. ^143,144 Similarly, simple and subjective tests, such as the Glasgow Outcome Scale, are used to assess outcome and efficacy of the pharmacological or other intervention in most clinical trials. In contrast, most experimental TBI studies do not report injury severity; available neurobehavioral classifications, for example the Neurology Severity Scale (NSS), ¹⁴⁵ or the duration of post-injury apnea, regaining of the self-righting reflex, and so forth are rarely used and/or reported. Although the use of anesthesia is a major confounding factor in experimental TBI, classifying injury severity using standardized neurobehavioral assessments could provide critical information to better compare between experimental and clinical TBI cases.

Recommendation 2

We recommend that relevant organizations, such as funding agencies and scientific journals, encourage or even require the reporting of NSS or similar acute assessment of neurobehavioral deficits after experimental TBI. This simple step, combined with increased identification and utilization of clinically relevant, molecular-level proxy outcome measures (e.g., biochemical or imaging markers (see Recommendation #3 below), will increase our ability to directly compare injury severity between clinical and experimental cases and will increase our ability to more directly compare the temporal patterns of various molecular pathologies as a function of injury severity.

TBI triggers an array of pathobiological responses, depending on the type and severity of impact. To better understand the temporal differences between rodent and human TBI-induced pathological processes, the identity, onset, intensity, and extent of the various injury-induced pathological changes should be determined using objective, quantifiable outcome measures that are identical (or at least comparable) to clinical TBI studies. In clinical TBI research, the severe form of TBI is the most studied, and the disease process is best documented. Severely injured patients are admitted to neurointensive care and are evaluated and constantly monitored by neurocritical care specialists. ^{146 –148} Neurocritical care monitoring provides real-time data on physiological parameters, such as blood pressure, breath, heart and pulse rates, blood oxygen saturation, and ICP, in addition to local and global methods of assessing cerebral blood flow, oxygenation, and metabolism. No equivalent pre-clinical data exist, although there are simple tools available for the physiological monitoring of rodents. ^149,150 For ethical reasons, modeling severe TBI in animal models is virtually non-existent; therefore, we cannot really compare the most data-rich clinical studies (severe TBI), where the main outcome measure is survival, to experimental TBI data. Conversely, experimental studies mostly model mild-to-moderate forms of TBI, making these types of pre-clinical studies the most data rich. Clinical studies are conducted on heterogeneous populations where confounders, such as age, sex, comorbidities, medications, and pre-hospital/hospital care, are not replicated in the animal-based experimental setting. This heterogeneity is hard to overcome and resolve in an experimental setting, but at least the effects of age and sex may be investigated. Moreover, the most clinically studied, albeit least frequent, forms of TBI (i.e., moderate to severe) are typically part of polytraumas that cause other organ injuries, bone fractures, and so forth. In experimental TBI, however, practically every model only targets the head or brain.

Recommendation 3

Funding agencies, scientific journals, and other regulatory bodies should encourage or even require the use of clinically relevant outcome measures in experimental TBI, such as neurobehavioral monitoring, structural imaging, and biochemical markers, in the peripheral blood and/or CSF at multiple post-injury time points. In combination with outcome measures that are unique to experimental TBI research (e.g., histopathology), these clinically relevant data will help to better identify the pathological processes and their temporal patterns.

Experimental and clinical TBI studies generally use different tools and methodologies to identify underlying pathologies. In the absence of identical or at least comparable outcome measures, the temporal patterns of pathobiological processes in rodents versus humans cannot be precisely determined. In vivo imaging—mostly CT, but also MRI/DTI—are key determinants of structural damage in the clinical setting. Serial neuroimaging is often used to assess disease progression. However, imaging is currently infrequently used in pre-clinical TBI studies to generate enough data for comparison, and serial imaging studies are extremely rare in experimental TBI research. Structural information in experimental TBI has been overwhelmingly obtained by terminal histopathology, which is not very useful for determining the evolution of pathological changes in clinical TBI. The proxy outcome measures that are most commonly used in both experimental and clinical TBI are blood-based protein biomarkers. However, serial sampling in experimental TBI is rarely performed, and in the absence of comparable injury severity classifications in experimental and clinical TBI, and lack of established relations to functional outcome, comparison of available protein biomarker data is challenging.

Translating time?

Based on the available data, some of it discussed in this review, it appears that there is no universal “X factor” that can be used to “translate time” between rodent and human pathobiological changes. Normal biological and pathobiological processes seem to run faster in the rodent than in the human. ¹⁴ Normal metabolic, physiological, and developmental processes, such as RNA turnover, respiratory rate, gestation, and senescence, are approximately 2.5–84 times faster in rodents than in humans. Thus, 1 human day can roughly equal 2 rat h during gestation or 0.3 rat h during the period of sexual maturity. How can we then take this “time factor” into consideration when attempting to model neurodegenerative diseases like Alzheimer's or CTE in rodents when the life expectancies of the two species are so vastly different? As illustrated above, the rate for rodents can be 15 or 100 times higher, respectively, in TBI-induced changes in CMRglc and neuroinflammation. In other words, there is no universal conversion rate.

The most probable way to successfully “translate time” is to use a big data (BD) approach. ^151,152 However, successful BD approaches are critically dependent on the volume of data. In theory, BD could generate new knowledge by mining the available ∼40,000 legacy TBI publications. Neurolinguistic programming is rapidly improving and can be used to identify repeatable patterns such as relationships and correlations between the temporal patterns of rodent and human pathobiological processes. However, it appears that published TBI data are not (yet) sufficiently big for BD. ¹⁵³ It has been calculated that only a fraction of the data generated during experimental or clinical TBI studies are ever published, deposited, or made available in databases or other archives. ¹⁵⁴ Conservative estimates put the amount of unpublished, so-called dark data to more than 50% of all data generated. Accordingly, publications only represent a fraction of all data and, importantly, the published data are typically “curated” (i.e. “selected” from a much larger pool of data), making the BD approach prone to subjectivity and bias. Efforts by the National Institutes of Health and other agencies, requiring the use of common data elements and the deposition of all experimental data, will increase the availability of experimental data for mining and the accuracy of potential algorithms that could be used to “translate time.” Such a BD approach could and will go beyond translating time, given that it will enable more-precise predictions of relationships between various pathological processes, species, and diseases, thus guiding hypothesis generation and clinical trials based on compounded and weighted evidence.