Background

Spinal cord injury (SCI) is a global disability, affecting several thousands of people [1,2,3,4]. The initial mechanical injury upon SCI can drive various pathogenic cascades, including inflammatory cell infiltration, microglia activation, and oxidative stress. In brief, the blood-brain barrier (BBB) leakage reflects an influx of inflammatory cells into the spinal cord. Lymphocytes release oxidative and proteolytic enzymes, reactive oxygen species (ROS), and pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) [5,6,7]. This acute pro-inflammatory response to damage can induce secondary damages in the neighboring tissues, resulting in neuronal dysfunction [5]. A reactive process of secondary damage was induced by the acute pro-inflammatory response to injury that surrounds the original injury site [5]. These secondary damage cascades may prolong for several weeks, reflecting an expanding matrix of proteins associated with neural cell apoptosis, such as soluble cluster of differentiation 95 (CD95) ligand (an initiator of the Fas apoptotic pathway) [8]. In addition, reactive astrocytes, responding to the damage at the traumatic region, secret several pro-inflammatory cytokines and neurotoxins that results in more neural cell death and tissue degeneration [9,10,11,12]. It mediates further glial scarring, mostly consist of reactive astrocytes and proteoglycans, resulting in a physical and chemical barrier against axonal regenerations [13,14,15].

The secondary responses are almost identical with traumatic brain injury (TBI) outcomes, such as cognitive decline and depression. However, the underlying mechanisms mediating SCI effects on the brain have remained elusive thus far. SCI can disorganize the cerebral cortex and thalamus circuits, likely through disrupted anterograde, and retrograde transports [16]. Previous research has reported that the thalamic nucleus sends some of its major cortical efferents to the hippocampus compartments [17]. Degenerated neurons upon SCI generate cysteine-cysteine chemokine ligand 21, released in distal spinal segments and the thalamus area, which trigger microglial activation [18, 19]. On the other hand, supratentorial alterations in particular brain areas are addressed to pain modulation upon SCI-related hyperesthesia [19,20,21,22,23]. It is clear that SCI-related hyperpathia results in sustained neuroinflammation in some brain areas regulating pain sensation, such as cortex, thalamus, and hippocampus [24]. Strikingly, there are reports demonstrating that there is no motor cortex dysfunction and neurodegeneration upon SCI [25,26,27]. However, there are remarkable neuropathological hallmarks triggered by SCI such as decreased number of cortical neurons [28,29,30,31,32,33].

Moreover, there is cognitive impairment in 60% of the SCI population. Also, individuals with SCI are 13 times more likely to have cognitive impairment than healthy subjects [34,35,36,37]. While this could be addressed to concomitant head injury [38], another possibility is encephalitis upon isolated SCI, observed in recent animal studies [39,40,41]. In addition, the BBB is disrupted for approximately 14 days upon either TBI or SCI [42,43,44], releasing the central nervous system (CNS) antigens into the systemic circulation.

Traumatic Brain Injury

Acute and chronic injuries are the two main categories of TBI. Acute brain injury contains mild TBI or concussion that may result in instant death. Chronic brain injury, named chronic traumatic encephalopathy (CTE), is considered a progressive degenerative disease results from repeated traumas on the brain and symptoms start years to decades after head trauma such as behavioral problems, and finally, it can lead to dementia [45]. TBI can be focal (a specific area of the brain) or diffuse (a more widespread area of the brain), in which focal injury is due to a severe direct impact on the brain upon severe TBI, and diffuse injury is due to tearing of the brain tissue upon mild TBI [45]. TBI drives a multifaceted cascade of neurochemical changes affecting brain function. The initiating event is stretching and disrupting of neuronal and axonal cell membranes [46], resulting in a deregulated flux of ions, including an efflux of potassium and influx of calcium, which accelerate glutamate release as an excitatory neurotransmitter. Glutamate binds to N-methyl-d-aspartate (NMDA) receptors and leads to calcium influx and neurons suppression with glucose hypometabolism [47, 48]. Increased glucose uptake followed by increased activity of membrane pumps causes calcium entry into mitochondria and oxidative metabolism impairment with lactate production, which leads to acidosis, and edema. Calcium influx triggers microtubule disassembly and some kinase activation [47, 48].

Calcium is uptake by the endoplasmic reticulum (ER) and mitochondria, which may result in increased ROS levels, significant structural damage, and dysfunction of organelles. Dysfunctional mitochondria release cytochrome C, which activates caspases, and throws out of calcium, which activates calpain [49,50,51]. Cytoskeletal proteins split by calpain and cause cytoskeletal degradation. Neuroinflammation, dysfunction of ER, and mitochondria, activation of ROS, caspase, and calpain, and cytoskeletal disruption result in tau pathology (Fig. 1). In return, pathologic tau contributes to neuroinflammation, ER/mitochondrial dysfunction, ROS generation, and cytoskeletal disruption [49,50,51,52].

Fig. 1
figure 1

TBI pathophysiology. TBI induces axonal injury, neuroinflammation, increases in intracellular Ca2+ and hyperphosphorylation of tau. Ca2+ uptake by the ER and mitochondria leads to organelle dysfunction and increases in ROS. Releasing of cytochrome c upon mitochondria dysfunction activates caspases and throws out of Ca2+ into the cytosol, which activates calpain. Cytoskeletal proteins (spectrin) splits by calpain. Neuroinflammation, ER/mitochondrial dysfunction, ROS, activation of caspase/calpain, and cytoskeletal disruption contribute to tau pathology. TBI: traumatic brain injury; ER: endoplasmic reticulum; ROS: reactive oxygen species

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Tau protein is a family of microtubule-associated proteins (MAPs) predominantly located in CNS axons, where it maintains the stability of microtubules and participates in anterograde transport within axons [53, 54]. Hyperphosphorylation of tau (P-tau) has been demonstrated in several neurodegenerative disorders, together known as tauopathies, such as Alzheimer’s disease (AD), CTE [53,54,55,56,57], TBI [58,59,60,61,62,63,64], and SCI [65,66,67] in animals and humans, and it is clear that neurodegeneration upon CNS injury spreads to other regions of the brain and/or spinal cord [68]. TBI and SCI studies showed an increase in cerebrospinal fluid (CSF) and serum P-tau as a biomarker [69,70,71,72,73], but there are a few studies on changes in P-tau levels following different severities of SCI.

SCI Pathological Hallmarks

SCI causes a breach in the BBB; resulting in various protein secretion, such as neurofilaments (NF) [71], S100 calcium-binding protein β (S100β) [74], neuron-specific enolase (NSE) [74], and glial fibrillary acidic protein (GFAP) [71] and tau [75]. Tau is moderately phosphorylated in physiological conditions but its abnormal hyperphosphorylation reflects pathogenicity. The human brain is enriched with lower molecular weight isoforms (48–68 kDa), while the peripheral nervous system (PNS) and spinal cord are full of high molecular weight species (110–120 kDa) [76]. Tau phosphorylated at pathogenic residues such as Tau-1, AT8 [Ser202/Thr205], AT180 [Thr231/Ser235], AT100 [Thr212/Ser214], 12E8, and paired helical filament 1 (PHF1) [77]. It has been suggested that neuronal cell death primes to tangle formation, meaning that some earlier intermediates are responsible for tau toxicity [78]. Soluble and oligomeric forms of hyperphosphorylated tau (P-tau) are the most pathologic tau species during neurodegenerative diseases [79]. Such oligomers are secreted and transmitted into the cells, reflecting tau pathology propagation [78, 80, 81]. In addition, they can lead to synaptic and mitochondrial impairment linked to memory deficits [82, 83].

Tau Hyperphosphorylation Cascades

Several protein kinases involved in hyperphosphorylation of tau that are divided into two groups of proline and non-proline-directed kinases [84,85,86]. Among the tau kinases, cyclin-dependent kinase 5 (CDK5), and glycogen synthase kinase 3β (GSK3β) are important. They phosphorylate tau at serine/threonine residues, so they cause tau pathology [87, 88]. Tau hyperphosphorylation at Thr231 and Ser262 is associated with its loss of functions [89], highly reduces tau microtubule-binding ability [90, 91]. Protein phosphatase 1 (PP1), PP2A, PP2B, and PP5 dephosphorylate tau protein [92]. PP2A has the most potent effect on tau dephosphorylating [93]. Tau phosphorylation can modulate with cellular stress such as thermal stress [94, 95] or oxidative stress [94, 96, 97]. Oxidative stress is earlier stages in tauopathies [98] that increases the activity of tau kinases and decreases the activity of tau phosphatase, which leads to aggregation of tau protein [94, 96, 97, 99].

Tau Abnormalities Triggered by SCI

Tau pathology triggered by traumatic SCI could be divided into two processes: the initial mechanical injury, which results in the axonal degenerations, and subsequent secondary events including ischemia, oxidative stress, edema, inflammation, and loss of ionic homeostasis, resulting in extensive neurodegeneration over the course of minutes to weeks [15, 100, 101]. The secondary events, particularly ischemia, result in excessive calcium entry into cells through NMDA receptors [102,103,104,105]. Activation of calcium-dependent proteases (calpains) and kinases is a critical step [106]. Activation of calpain results in loss of membrane potential, increased membrane permeability, and cell death [107]. Also, increased intracellular calcium levels activate various kinases, such as CDK5, Ca2+/calmodulin-dependent protein kinase (CaMK), microtubule affinity-regulating kinase (MARK), cyclic AMP-dependent protein kinase (PKA), GSK3, and casein kinase 1 [77, 108, 109]. CDK5 can phosphorylate the tau protein, and p35, and p25 as tau endogenous activators regulate its activity [110]. Conversion of p35 to p25 by calpain causes prolonged activation and mislocalization of CDK5. Improper calpain activation mediates neuronal death. Also, the CDK5-p25 complex causes tau hyperphosphorylation and tau pathology [111].

Tau in Animal Models of SCI

There is profound tau hyperphosphorylation upon SCI in a rat model of thoracic spinal cord hemisection, which phosphorylated at pathogenic residues, Tau-1 [112], and AT8 [Ser202/Thr205] [66, 67]. Based on the Hung et al. report, P-tau amounts in rats with traumatic spinal cord tissue showed a decrease following treatment with a calpain inhibitor; likely through attenuating p25 formation [66]. It has been reported that insulin-like growth factor I (IGF-I) (Box 1) reduces tau hyperphosphorylation, suppresses microglia and astrocyte activation, and prevents neurodegeneration upon SCI [67], so it can be addressed to p25 downregulation.

Box 1 IGF-I is a strong neurotrophic factor promoting projection neurons development, dendritic arborization, and synaptogenesis [113, 114]. IGF-I can act in the brain in an endocrine, paracrine, or autocrine manner for prooting glucose use, by phosphatidylinositol 3–kinase/Akt and the downstream GSK-3β routes. IGF-I has a neuroprotective effect that is coordinated through activating Akt, inhibiting GSK-3β, and subsequently inhibiting tau phosphorylation [115, 116].

Total-tau was assessed in a study of 51 dogs following intervertebral disc herniation (IVDH), which shows that CSF tau levels increased based on the trauma severity [72]. Therefore, CSF tau is possibly an appropriate biomarker for SCI. Rat models of spinal cord injuries were developed by Qi et al. [117]; they were injected with mouse hippocampal neural stem cells (NSC) via the tail vein. The results indicated that increased tau levels in the outer chamber did not affect NSC migration, but it showed a reduction after alterations in intracellular tau phosphorylation state. Also, migration at the trauma site was targeted by NSC. Tau is not a chemokine for targeted migration of NSC, but intracellular tau phosphorylation/dephosphorylation inhibits cell migration [117]. P-tau as a biomarker has been studied by Caprelli et al. [65]. They reported that severe impact compression SCI at T8 indicated P-tau existence in injured axons with the same time-course and distribution model to β–amyloid precursor protein (β-APP), a biomarker for axonal damage. P-tau and β-APP positive axons extended no more than 5000 μm rostral and caudal toward the injury epicenter and were at their maximum 1 day after SCI. CSF concentrations of P-tau and total tau showed a significant increase 1 day after the injury, but only serum levels of P-tau were significantly increased in SCI rats than naive rats. Accordingly, CSF and serum P-tau are probably appropriate biomarkers for severe traumatic SCI. Furthermore, at 1-day post-injury, CSF analysis of P-tau and total tau in SCI rats showed a 7.5-fold and 550-fold increase in P-tau and total tau levels, respectively, while the serum analysis showed a 7-fold and 1.2-fold increase in P-tau and total tau levels, respectively [65]. This indicates that total tau protein is a more sensitive biomarker for axonal injury within CSF; however, P-tau appeared to be a more sensitive biomarker within the serum.

In another experimental study, rats were subcutaneously inoculated with saline or allogeneic spinal cord tissue on 1 or 2 occasions. Serum samples collected following the Morris water maze (MWM) test and animals were sacrificed after 8 weeks. Their findings indicate inflammation in the spinal cord and brain upon exposure to spinal cord antigens resulting in CNS cell degeneration, memory impairment, and tau production especially when this exposure is repeated [118]. There is one study of changes in P-tau levels upon different severities of SCI. Briefly, 160 female rats following a spinal cord trauma with dropping a 10-g rod upon the exposed spinal cord from different heights: 12.5 mm (mild SCI), 25 mm (moderate SCI), and 50 mm (severe SCI). Tau protein concentration was evaluated in serum and CSF specimens following surgery. There was a significant positive linear correlation between tau protein concentration and SCI severity in groups, as well as between the tau protein concentration and Basso, Beattie, and Bresnahan locomotor rating scale scores [119] (Table 1).

Table 1 Tau protein in animal models of SCI
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Table 2 American Spinal Injury Association Impairment Scale
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Tau in SCI Human

Pouw et al. measured tau concentrations in CSF obtained from 3 to 24 h after injury in motor-complete and motor-incomplete cases. Based on the results, tau levels were lower in the American Spinal Injury Association (ASIA) Impairment Scale (AIS) (Box 2) A patients that improved in their AIS scores than cases who remained AIS A. Interestingly, cases who remained AIS A during follow-up showed a higher tau level in AIS A than AIS A cases who neurologically converted to an AIS B [71]. Kwon et al. have reported that tau levels were significantly increased depending on the severity of injury in CSF sampled from complete or incomplete cases 24 h after injury [69, 70]. They plotted the tau level in the CSF between 8 and 120 h after SCI [69]. Surprisingly, the tau level remained higher in subjects from to 48 h following injury [69] (Table 3, Fig. 2).

Box 2 ASIA was established in 1973 for facilitating the exchange of investigations, information, and opinions among practitioners associated with the treatment of SCI cases. The Frankel scale had been introduced previously for categorizing spinal cord injuries, but it had several limitations [120]. In 1982, the ASIA published the International Standards for Neurological Classification of Spinal cord Injury (ISNCSCI), a scoring and categorizing model that would evolve into the present AIS (Table 2) [121]. The ISNCSCI classification is helpful in identifying crucial muscle groups and sensory points improving practitioners’ precision to identify the neurologic degree of injury. Moreover, it was reproducibly characterized by a comprehensive explanation about every sensory and motor grade, which leads to precise characterization of incomplete and complete spinal cord injuries [122]. Unfortunately, in the very acute stages after injury, it is often impossible to conduct the ISNCSCI examination due to associated comorbidities including TBI, pharmacologic sedation, intoxication, or multi-system trauma [123, 124]. In performing clinical research in acute SCI patients, because of this variability, large numbers of patients should be enrolled to obtain enough statistical power.

Table 3 Tau protein in human SCI
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Fig. 2
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Tau in healthy neurons and in tauopathies after SCI. Tau facilitates microtubule stabilization within cells and is particularly abundant in neurons. Tau function is compromised in SCI and other tauopathies. This probably results from both tau hyperphosphorylation, which reduces the binding of tau to microtubules, and the sequestration of hyperphosphorylated tau into NFTs, which reduces the amount of tau that is available to bind microtubules. SCI: spinal cord injury; CSF: cerebrospinal fluid; NFTs: neurofibrillary tangles

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Cistauosis: Novel Neurodegeneration Mechanism

It has been demonstrated some post phosphorylation modifications may lead to tau pathogenicity. In particular, phosphorylated tau at Thr-Pro motifs may exist in two distinct cis and trans conformations in which that cis P-tau is almost neurotoxic and prone to aggregation [125, 126]. Cis to trans isomerization is being conducted by peptidyl-prolyl cis/trans isomerase (Pin1). It is clear that Pin1 suppression reflects cis P-tau accumulation and tau pathology [127,128,129]. Moreover, it has been recently demonstrated that phosphorylated tau at Th231 in the cis conformation is extremely neurotoxic and early driver of neurodegeneration. Cis pT231-tau has a prion nature and spreads into brain areas and CSF. Cis P-tau levels increase as Alzheimer’s disease develops. Also, cis pT231-tau is being increased and spread upon TBI in a timely manner. Moreover, the more severe trauma, the more cis pT231-tau. Cis P-tau accumulates and makes aggregates inside the neurons and consequently, induces cell death upon either TBI, oxidative stress, or nutrition depletion; a process termed cistauosis [68]. In TBI, cis P-tau appears earlier than other P-tau epitopes such as PHF [68, 130, 131]. Furthermore, cis P-tau can cause and spread axonal degeneration in the acute and chronic stages following TBI. TBI induces cis P-tau in a dose-dependent fashion [68]. Although mild TBI is able to cause transient and modest cis P-tau induction, repetitive mild (rmTBI) or single severe TBI (ssTBI) causes ongoing and potent cis P-tau induction prior to tau oligomerization and tangle formation [68]. Cis P-tau is diffusely localized to axons, which leads to microtubule impairment and mitochondrial transport deficiency, apoptosis, and it expands within the brain that all subsequently cause and spread cistauosis phenotypes in the brain [68]. In addition, like in vitro studies, cistauosis induction is blocked after treatment with cis P-tau monoclonal antibody (mAb) following ssTBI in murine brains that completely suppresses the growth and expansion of TBI-oriented tau pathologic events, deposition, impaired long-term potentiation, and anxiety or risk-taking behaviors, and brain atrophy, whereas using cis mAb can improve cognitive impairment [68, 125, 126].

Albayram et al. studied cis P-tau in autopsy samples and indicated cis P-tau importance in developing and treatment of short-term and long-term outcomes of TBI, such as several CTE-like neurodegenerative features, like axonal, tau, and cerebellar pathology, neuroinflammation, neuronal loss, white matter degeneration, and also common clinical functional impairments, such as imbalanced sensorimotor coordination, urinary incontinence, and cognitive deficit. However, humans exposed to repetitive head trauma detected by CTE at autopsy showed robust cis P-tau in the cerebral cortex and deeper brain areas, such as the thalamus. Six months following injury, cis mAb has found to eliminate and block the spread of cis P-tau, pathology in axons, and astrogliosis into the hippocampus, as well as prevent other mechanisms of neuropathology, including tau oligomerization, tangle generation, gliosis, and APP assembly, and preventing sensorimotor coordination impairments and urinary incontinence [130] (Fig. 3). Therefore, cis P-tau is probably a precursor of tau pathology and a major driver of neurodegeneration.

Fig. 3
figure 3

A model for the roles of cis P-tau and its mAb in the development and treatment of TBI. Upon phosphorylation of tau, Pin1 converts cis to trans of P-tau. Cis P-tau mainly localizes to axons and causes and spreads axonal pathology, contributing to the development and progression of a range of neuropathological and functional outcomes. Treatment of TBI mice with cis mAb not only eliminates cis P-tau and blocks its spreading, but also prevents the development and progression of neuropathological and functional outcomes, but cistauosis after SCI is unknown. TBI: traumatic brain injury; Pin1: peptidyl-prolyl cis/trans isomerase; SCI: spinal cord injury

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Conclusion

In recent years, aggregation intermediates, like soluble cis pT231-tau has been considered a possible option for the toxic species in frequent tauopathies [68, 125, 126, 130, 132]. Several studies have been conducted on tau aggregation intermediates making its prion-like nature apparent. Cases with TBI have high levels of CSF cis pT231-tau and toxic tau can seed and spread within connected areas of the brain and displays systematic and predictable models of diffusion [68, 130]. Using mouse monoclonal antibodies, which distinguishes trans from cis P-tau, can be an appropriate strategy to characterize and treat tau pathology. Cis pT231-tau removal suppressed neurodegeneration in TBI models [68].

On the other hand, clinical and animal research has found that serum and CSF P-tau is a feasible candidate biomarker for axonal injury upon SCI. These data indicate that serum and CSF levels of tau have a positive correlation with the severity of the injury and negative correlation with the locomotor function [65,66,67, 69,70,71,72]. Therefore, tau protein levels have been suggested as a novel biomarker for assessing SCI severity.

This review has shown that the identification of the prion-like behaviors of the tau protein is a critical step to discover novel and beneficial therapeutic methods for a large number of prevalent neurodegenerative disorders. Secondary injury cascades, as active pathological processes, seem to be a promising option for treating SCI, and future studies on cistauosis upon SCI are therefore recommended.