Cellular and molecular mechanisms in vascular repair after traumatic brain injury: a narrative review

Abstract

Traumatic brain injury (TBI) disrupts normal brain function and is associated with high morbidity and fatality rates. TBI is characterized as mild, moderate or severe depending on its severity. The damage may be transient and limited to the dura matter, with only subtle changes in cerebral parenchyma, or life-threatening with obvious focal contusions, hematomas and edema. Blood vessels are often injured in TBI. Even in mild TBI, dysfunctional cerebral vascular repair may result in prolonged symptoms and poor outcomes. Various distinct types of cells participate in vascular repair after TBI. A better understanding of the cellular response and function in vascular repair can facilitate the development of new therapeutic strategies. In this review, we analyzed the mechanism of cerebrovascular impairment and the repercussions following various forms of TBI. We then discussed the role of distinct cell types in the repair of meningeal and parenchyma vasculature following TBI, including endothelial cells, endothelial progenitor cells, pericytes, glial cells (astrocytes and microglia), neurons, myeloid cells (macrophages and monocytes) and meningeal lymphatic endothelial cells. Finally, possible treatment techniques targeting these unique cell types for vascular repair after TBI are discussed.

Highlights.

• Cerebrovascular impairment is an important hallmark of traumatic brain injury.

• The consequences of cerebrovascular impairment after TBI are discussed.

• Various cell types contribute to vascular repair post-TBI.

Background

Traumatic brain injury (TBI) is caused by a bump, the brain’s movement in response to acceleration/deceleration accidents or a penetrating injury (such as a gunshot) to the head that disrupts brain function. People of all ages can be affected by TBI. The population most susceptible to TBI includes older adults, ethnic minorities, service members in the military and veterans, homeless people living in detention and correctional facilities or rural areas, and victims of domestic violence [1–4].

TBI causes primary vascular damage due to mechanical forces. Depending on the severity, location and injury frequency, blood vessel impairment may be limited to dura mater, with only transient symptoms, or may result in obvious tissue contusions, hematoma and subsequent brain damage involving the brain parenchyma, resulting in long-term repercussions. Vascular repair is important for the recovery of dural tissue, restoration of meningeal immune homeostasis [5] and the prevention of delayed hypoperfusion, edema, hemorrhage and other secondary injuries involving the parenchyma [6, 7]. TBI may induce chronic neurodegenerative changes due to the impairment of microvasculature [8]. Defective repair of cerebral vasculature may exacerbate brain injury symptom persistence and lead to poor outcomes.

Review

Classification of TBI

TBI is categorized based on the mechanism (closed vs open; impact vs blast), whereas clinical severity is based on the Glasgow coma scale, location (diffuse vs focal) and symptoms (post-concussive vs sub-concussive) [9, 10]. Traditionally, TBI has been classified as mild, moderate and severe based on severity. Different types of TBI share common pathophysiological changes induced by the primary injury. However, distinct cellular responses are observed due to varying degrees and mechanisms of injury.

The most common type of TBI is mild (mTBI). Most mTBIs occur in individuals >75 years old [11]. mTBI is typically induced by a knock or jolt to the head, with no discernible brain parenchymal abnormalities. The majority of head injury outcomes, such as skull fractures, structural abnormalities and hemorrhage, are undetected in routine magnetic resonance imaging (MRI) and computerized tomography (CT) scans [12]. Clinical and pre-clinical studies suggest that 7–33% of patients diagnosed with mTBI experience long-term and progressive cognitive impairment, despite the generally transient nature of the symptoms induced by mTBI [13].

A blow, bump or penetrating injury to the dura mater and cerebral parenchyma can induce moderate or severe TBI, which can have devastating long-term health effects. Macroscopic injuries to the cerebral parenchyma include shearing of white matter tracts, hematomas (intracerebral and extracerebral), focal contusions and diffuse swelling. Microscopic injuries to the cerebral parenchyma include microporation of membranes, leaky ion channels and abnormal stearic conformation of proteins inside the cells [14]. After the initial brain injury, secondary impairments such as calcium-mediated damage, free-radical production, inflammation, mitochondrial dysfunction and gene activation occur within hours or days. Direct structural damage of brain tissue and vasculature by primary injury is untreatable. Efforts should be made to limit the external impact by wearing a helmet and preventing head acceleration by immobilizing the head [9, 15]. In the case of moderate and severe TBIs, it is vital to reduce the deleterious effects of secondary injury, which can cause persistent symptoms. An impaired cerebral vasculature contributes significantly to both initial and subsequent damage following TBI.

Cerebral vascular impairment after TBI

The primary injury produced by the mechanical forces of TBI may directly affect meningeal and parenchymal blood vessels. Cerebrovascular dysfunction plays a role in the etiology of all types of TBI and is a significant feature of TBI. Structural abnormalities of cerebral vasculature after TBI include vacuolation, degeneration, discontinuity and necrosis of pericytes. Primary mechanical damage to the vessel walls accompanied by irreversible neuronal impairment can be observed early after the injury via ultrastructural analysis [16]. Changes in endothelial cells (ECs) can be seen in the acute phase of TBI by electron microscopy (EM) as early as 3 hours after TBI and up to at least a week [17]. EM images of the membrane reveal thickening, rarefaction, matrix disorder with reduplication and bifurcation, and basement membrane vacuolization. After a TBI, the cerebral vasculature undergoes ultrastructural alterations in the subacute stage (7–30 days later), including fibrous changes in the arterial wall, apoptotic ECs, internal elastic lamina corrugation, extracellular matrix degradation, condensed nuclei and heterophagy [18]. Technologies for the detection of cerebral vascular impairment after TBI in experimental models and clinical applications include functional MRI, laser Doppler flowmetry, in vivo microscopy, transcranial Doppler, CT and subtraction angiography, which are discussed thoroughly in other comprehensive reviews [18, 19].

Vascular impairment within the meninges

Meninges, including dura mater, arachnoid mater and pia mater, surround the brain. The tough leathery membrane called the dura mater lies on the outside and is vulnerable due to its perforated blood arteries and lack of tight junctions. Leptomeninges are composed of arachnoid and pia maters [20]. The meninges represent a highly vascularized physical barrier involved in the immune surveillance of the brain [21]. Meningeal injury can be observed in more than half of all patients with mTBI [22]. The TBI model also reveals a significant decrease in microvascular density within the area of brain contusion accompanied by an obvious inflammatory response [22, 23]. Persistent symptoms such as fatigue, headache, impaired concentration and memory, anxiety and irritability may result from inadequate meningeal vascular repair following mTBI [24].

Secondary injury following cerebral vascular impairment

Hemorrhage and coagulopathy

Nearly half of TBI patients experience hemorrhage, especially those with moderate and severe TBI [25, 26]. TBI can induce epidural, subdural, subarachnoid and parenchymal hemorrhages. Epidural hematomas develop between the dura mater and the inner lining of the skull. After a TBI, subdural hematomas form between the arachnoid and dura maters and are associated with poor prognosis [27]. Poor outcomes and higher mortality are linked to subarachnoid hemorrhage, which occurs between the pia and arachnoid mater [28]. Hemorrhage after TBI results in the accumulation of hemosiderin/ferritin, which is believed to be neurotoxic [29]. White matter impairment is reportedly associated with microbleeds following TBI [30]. Coagulopathy is associated with higher mortality and poor outcomes following TBI, with an incidence of 33% [31]. The mechanism of coagulopathy following TBI includes the release of tissue factors into the blood, coagulation factor depletion, platelet reduction, blood–brain barrier (BBB) disruption and an increase in circulating microparticles [32, 33]. A low platelet count increases the likelihood of hemorrhage [34].

B‌BB dysfunction and edema

The BBB, composed of ECs, astrocytes and pericytes, maintains the brain’s extremely limited environment and prohibits the free entry of circulating immune cells and blood-borne factors into the central nervous system (CNS) [35, 36]. BBB disruption occurs within hours following TBI [37, 38] and is associated with increased morbidity and mortality [39]. TBI disrupts the tight junction proteins connecting endothelial cells and sealing the BBB [40], followed by infiltration of immune cells and molecules such as neutrophils and albumin, aggravating the inflammatory response, edema and hypertension [41, 42]. In a study of individuals with severe non-penetrating TBI, a breach of the BBB was identified in 44% of patients and was associated with poor long-term consequences [43]. After a TBI, the altered BBB often recovers to normal within a few weeks. In some situations of mTBI, BBB dysfunction might linger for months or years. [44, 45].

Cerebral edema develops at the site of a TBI and eventually fuses with the surrounding tissue, resulting in death in 50% of all cases following severe TBI [46]. Cerebral edema is classified into vasogenic and cytotoxic types. After TBI, blood vessel components leak into the brain parenchyma, creating an osmotic gradient that induces water migration from the blood vessels into the extracellular space, resulting in vasogenic edema [47]. Vasogenic edema is characterized by tissue swelling, compression of brain structure, high intracranial pressure and altered blood flow [48]. Vasogenic edema occurs within the first few hours post-TBI. Cytotoxic edema occurs within a few days of the damage and can last up to 2 weeks [49, 50]. Cytotoxic edema results from the storage of excess water inside the cell due to defective sodium and potassium pumps in the cell membrane [47]. In the mouse model of ischemic stroke, the inflow of cerebral spinal fluid (CSF) into ischemic tissue triggered by spreading depolarization also contributes to brain swelling. The glymphatic system is also intimately involved in this process. Several hours after CSF influx, Na⁺ influx and edema in the endothelial cells of the brain are increased by the expression of sulfonylurea receptor-1-transient receptor potential cation channel subfamily M member 4 cation channels [51].

Changes in cerebral blood flow and ischemia

Depending on the location, magnitude and type of lesion, both global and regional alterations in cerebral blood flow (CBF) are seen in TBI. CBF often decreases within 12 h after a TBI [52, 53]. Humans typically have a CBF of 45–50 ml/100 ml/min, which can plummet to 18–20 ml/100 ml/min in the acute phase following severe TBI [52, 54]. The decline in CBF implies that ischemia plays a significant role in secondary damage following TBI and, in many cases, shares comparable pathophysiological pathways with stroke. In addition, an increase in CBF may occur in patients after the acute phase of TBI [55, 56]. Several studies have indicated that TBI may lead to subsequent stroke [57–59]. In a patient cohort of TBI, a decrease in CBF and cerebral oxygen metabolism was observed after TBI by oxygen positron emission tomography [60]. In addition, TBI leads to tissue hypoxia not only within the regions with structural abnormalities but also in the regions without apparent macrovascular ischemia [61]. Significant decrease in brain tissue oxygen in ischemia after TBI may lead to malignant cerebral edema and poor outcomes [62]. The alterations in CBF following TBI may be attributed to impaired autoregulation of CBF, resulting in adverse outcomes [63–66]. Vasospasm is defined as aberrant constriction of blood vessels, which occurs in ~19–68% of patients with TBI [67], reducing CBF in wounded tissue and possibly inducing ischemic injury. Vasospasms are normally temporary; however, they can linger for 14 to 30 days in some cases of closed head traumas and blast injuries [68, 69]. Potential vasospasm causes include hypertension, subarachnoid hemorrhage and the production of endothelin-1 by pericytes following TBI [70, 71].

Cellular response and function in vascular repair after TBI

Depending on the severity of the injury, the repair of cerebral vasculature following TBI can last several weeks [72, 73]. The main mechanisms of vascular repair after TBI are angiogenesis and vasculogenesis, which contribute to the development of a primitive vasculature during the embryonic phase and promote vascular repair in adulthood following TBI. These intricate yet distinct processes play a crucial role in repairing the damaged vasculature following TBI [74, 75]. Angiogenesis is characterized by EC proliferation, migration, tube formation, branching and anastomosis [76]. Vascular endothelial growth factor (VEGF), a soluble factor released by vascular cells and upregulated after TBI, mediates the proliferation of ECs from 12 h to 21 days after TBI [77, 78]. VEGF is a promising target for novel TBI therapeutic strategies due to its significant role in promoting angiogenesis [79, 80]. The proper organization of the vascular tree requires a balance between tubulogenesis, vascular branching, elongation and pruning. Imbalance in these processes leads to malformed and fragile vessels with dysregulated arteriovenous differentiation [81].

Rapid formation of new vasculature can be observed after TBI. Vascular repair is a complex process that involves many different types of cells. ECs, endothelial progenitor cells (EPCs), pericytes and macrophages participate in meningeal and parenchymal vascular repair after TBI. Meningeal lymphatic ECs and myeloid cells, including non-parenchymal macrophages and monocytes, play an important role in meningeal vascular repair. In moderate or severe TBI, glial cells (astrocytes and microglia) and neurons regulate parenchymal vascular repair. Schematic diagrams illustrating cerebral vascular impairment and cellular responses in vascular repair after TBI are presented in Figure 1 and 2.

Figure 1.

Schematic diagram showing an overview of cerebral vascular impairment and repair after traumatic brain injury (TBI). BBB blood–brain barrier, CBF cerebral blood flow

Figure 2.

Schematic diagram of cerebral vascular impairment and cellular responses in vascular repair after traumatic brain injury (TBI). Primary injuries due to mechanical forces directly damage blood vessels after TBI. Structural abnormalities of cerebral vasculature after TBI include vacuolation, degeneration, discontinuity and necrosis of pericytes. Thickening, rarefaction, matrix disorder with reduplication and bifurcation, and basement membrane vacuolization can be observed after TBI. Aside from the predominant injury within the meninges in mTBI, secondary injuries, including hemorrhage, coagulopathy, BBB dysfunction, edema, reduced cerebral blood flow (CBF) and ischemia adversely affect brain parenchyma in moderate-to-severe TBI. Hemorrhage can progress gradually and result in vasospasms, intracranial hypertension and compression of brain tissue. The release of tissue factors into the blood, coagulation-factor depletion, platelet reduction, BBBdisruption and increased circulating microparticles may lead to coagulopathy. BBB disruption occurs within hours following TBI, aggravating the inflammatory response and cerebral edema. Cerebral edema is classified into vasogenic and cytotoxic types. Vasogenic edema results from the leakage of blood-vessel components into the brain parenchyma, creating an osmotic gradient that induces water migration from the blood vessels into the extracellular space. Cytotoxic edema results from the storage of excess water inside the cell due to defective sodium and potassium pumps in the cell membrane. Both global and regional alterations in CBF are seen in TBI. The alterations in CBF following TBI may be attributed to impaired autoregulation of CBF and vasospasm. The decline in CBF may further lead to ischemic stroke and malignant cerebral edema. Distinct types of cells are involved in the process of vascular repair. Endothelial cells, endothelial progenitor cells, pericytes and macrophages participate in meningeal and parenchymal vascular repair after TBI. Meningeal lymphatic endothelial cells and non-parenchymal macrophages play an important role in meningeal vascular repair. Astrocytes, microglia and neurons mainly regulate parenchymal vascular repair in moderate or severe TBI. AQP4 aquaporin 4, VEGF vascular endothelial growth factor, HIF hypoxia-inducible factor, FGF fibroblast growth factor, CSF cerebral spinal fluid, BBB blood–brain barrier, BM basement membrane

ECs

ECs are essential components of blood vessels. The integrity and normal function of the BBB depend on tightly interconnected ECs. Injury to ECs may result in the production of inflammatory cytokines, oxidative stress, increased cellular adhesion molecules and vasogenic edema due to extravasation of plasma proteins [82–84]. The main mechanisms of vascular repair after TBI are angiogenesis and vasculogenesis. The first step of angiogenesis is vasodilation, which is regulated by nitric oxide. The permeability of vasculature increases in response to Src kinase and VEGF via the formation of vesiculo–vacuolar organelles, fenestrations, and redistribution of vascular endothelial cadherin and platelet endothelial cell adhesion molecule-1 [85]. Plasma proteins extravasate following the increased permeability of vasculature and form a transient migratory scaffold for ECs. Proteinases, including matrix metalloproteinase (MMP), heparinase or chymase, and plasminogen activator, mediate angiogenesis by releasing or activating growth factors, such as basic fibroblast growth factor (bFGF), VEGF and insulin-like growth factor-1 (IGF-1) secreted within the extracellular matrix, and by degrading the matrix itself [86]. VEGF drives the differentiation of ECs into distinct subtypes to restore nutrient supply and oxygenation by attracting new vessel sprouts [87]. In addition, ECs in dura mater exhibit intrinsic angiogenic potential via VEGF receptor 2 (VEGFR2) signaling [5].

After clearing the obstacles on the passage for ECs migration, proliferating ECs travel to distant locations. ECs frequently form solid cords before acquiring a lumen. Interactions between tip cells, stalk cells and phalangeal cells promote capillary sprouting. During capillary sprouting, the three subtypes of ECs play a highly specific role [88]. Tip cells are non-proliferative and highly migratory ECs that guide the new sprout in the right direction. Stalk cells exhibit a high proliferative capacity and elongate the new sprout. The highly mature area of the vessel is marked by quiescent phalangeal cells with their cobblestone shape [89]. Neighboring ECs are connected by tight junctions (TJs), adherens junctions and gap junctions. The proteins at endothelial junctions are remodeled dynamically under physiological and pathological conditions [90]. TJs are composed of claudins, occludin and junctional adhesion molecules [91]. Junctional proteins interact with the actin cytoskeleton dynamically and regulate the permeability of ECs [92]. Disruption of TJs is associated with leakage of BBB and edema in the brain after TBI [35, 90, 93].

Both remodeling and ‘pruning’ occur during the maturation of the endothelial network, forming a network of branched vessels [94]. The levels of MMP-9 are increased in the hypoxic angiogenic and post-hypoxic vascular pruning response during cerebrovascular remodeling under chronic mild hypoxia [95]. Pruning is designed to establish an efficient, mature vasculature [96–99]. Apoptosis of ECs was observed during vascular pruning in zebrafish internal carotid artery [100] and mouse retinal blood vessels [101]. Macrophages play a critical role in vascular pruning by regulating the apoptosis of ECs during the regression of hyaloid blood vessels in developing eyes [102].

EPCs

The level of circulating EPCs was found to be associated with clinical severity and outcome in patients after TBI [103]. In patients with mTBI, the percentage of EPCs expressing C-X-C chemokine receptor type 4 at 7 days after TBI was found to be related to clinical outcome at 3 months. Changes in circulating EPCs were also found to be related to platelets and blood glucose levels [104, 105]. In a rat TBI model, CD34⁺ cells increased significantly in both peripheral circulation and brain tissue, with a close relation to angiogenesis after TBI [106]. EPCs in circulation are associated with hemangioblast stem cells and contribute to the development of postnatal vasculature via angiogenesis and vasculogenesis [107]. EPCs facilitate neovascularization in vivo via proliferation and differentiation into ECs. It has been shown that EPCs derived from peripheral blood mononuclear cells integrate into sites of myocardial neovascularization [108]. In patients with acute myocardial infarction, intracoronary infusion of bone marrow or peripheral blood-derived progenitors resulted in significant benefits in post-infarction remodeling [109, 110]. Statin treatment in a model of hindlimb ischemia led to an increase in circulating EPCs, angiogenic cytokines and angiogenesis [111]. An increase in new blood vessels and blood perfusion were observed after the mobilization of EPCs to promote the recovery of ischemic tissue [112, 113]. Increased numbers of circulating EPCs and angiogenesis were observed around the injured area after TBI. EPC transplantation or mobilization of circulating EPCs improves neurological outcomes after TBI by promoting angiogenesis [114–116]. Activation of Notch signaling was found to enhance the angiogenic ability of EPCs and promote vascular and tissue repair after mTBI [117]. In addition to neovascularization, EPCs promote the survival and proliferation of ECs by secreting various cytokines. EPCs are regulated by cytokines and growth factors released by injured tissues [118]. During angiogenesis in tumor or ischemic injury, the circulating level of VEGF increases [119–121], mobilizing EPCs from bone marrow. Conversely, Ang-1 inhibits the release of EPCs from bone marrow [122, 123].

Pericytes

Pericytes play an important role in CBF regulation [124, 125], maintenance of BBB integrity [126, 127], formation of new blood vessels [128], scar formation [112] and transendothelial vesicle trafficking [126, 127]. Disruption of platelet-derived growth factor-B/PDGF receptor signaling after TBI results in the loss of pericyte–EC interaction and neurovascular dysfunction [35]. After TBI, pericytes are crucial for restoring the vasculature [129]. Pericytes line the exterior of cerebral blood vessels and are in direct contact with ECs [130]. Within hours of TBI in a rat model of controlled cortical impact, nearly 40% of pericytes relocated to the injured basal lamina area [71]. The recruitment of pericytes is important for angiogenesis and stabilization of new vessels [131]. Reduced pericyte numbers were found in the early stages post-injury in a controlled cortical impact model [132].

To allow ECs to form vessel sprouts, pericytes must first detach from the vessel wall [71]. To separate pericytes from the vessels, several angiogenic factors are necessary, including the VEGF-A/VEGFR2 signal and the VEGF120 isoform [133, 134]. In the later stages of TBI, a process known as pericytosis—a dramatic increase in reactive pericytes racked up around the lesion—occurs. Angiogenic growth factors secreted by activated pericytes mediate EC activity and maintain blood-vessel integrity [130].

Macrophages

Bone marrow-derived monocytes detect blood vessel damage during vascular injury and repair. They accumulate at the injury site due to the chemokine gradient. Vascular repair by macrophages occurs via the following mechanisms: (1) guiding the sprouting and branching of blood vessels for neovascularization by bridging tip cells directly; (2) enhancing smooth muscle cells and EC proliferation by angiogenic factors (FGF, VEGF and hypoxia-inducible factor); (3) necrotic cell debris and pathogen elimination by phagocytosis; (4) the release of pro-inflammatory chemokines and cytokines during initial leukocyte infiltration; (5) release of MMPs to mediate extracellular matrix remodeling; and (6) regulation of vascular pruning by pro-apoptotic factors [77, 102,135–137].

Vascular repair is controlled by two types of activated macrophages: anti-angiogenic M1 macrophages during the inflammatory phase induced by damage-associated molecular patterns or pathogen-associated molecular patterns; and pro-angiogenic M2 macrophages (including the M2a and M2c subsets) at later stages [138]. M2 macrophages play a role in vascular repair after neutrophils and macrophages remove harmful stimuli [139]. Interleukin (IL)-4 induces M2a macrophages, which increase the expression of FGF2, IGF-1 and C-C motif chemokine ligand to promote angiogenesis. M2c is induced by IL-10 and mainly expresses the placental growth factor [140]. Meanwhile, M1 macrophages also produce pro-angiogenic factors, including VEGF and FGF2 [141]. The perivascular space contains tissue-resident macrophages that are in close proximity to the vessel walls [142]. Following weight drop injury, one study reported a substantial increase in CD163⁺ perivascular macrophages [143]. The antimicrobial reaction of myelomonocytic cells has been reported to impede vascular repair via interferon signaling [144].

Meningeal lymphatics and macrophages in meningeal vascular repair after mTBI

Meningeal lymphatic ECs (mLECs) on the brain surface participate in CSF drainage and neuroimmune function [145–149]. Meningeal lymphatic vessels in mice regulate the amount of CSF and their loss is linked to cognitive impairments [150]. Edema was decreased by improving the function of meningeal lymphatics after TBI [151]. Recent evidence suggests that meningeal lymphatics are modified after brain injury and may actively engage in brain tissue and vascular repair [152–154]. In a mouse model of TBI, dysfunctional meningeal lymphatic drainage was detected within hours and persisted for at least 1 month [153]. In a zebrafish model of cerebrovascular injury, the drastic ingrowth of meningeal lymphatics into the injured brain parenchyma was regulated by VEGF-C. Meningeal lymphatic repair of cerebrovascular damage appears to be mediated via drainage of interstitial fluid from the injured tissue via the lumenized ingrowth meningeal lymphatics and acting as “growing tracks” for nascent blood vessels. However, they promote the development of blood vasculature [155, 156].

A heterogeneous population of myeloid cells, including parenchymal microglia and non-parenchymal perivascular macrophages (PVMs), meningeal macrophages and choroid plexus macrophages populate the meninges in addition to mLECs [157–159]. Meningeal macrophages exhibit amoeboid morphology and movement. PVMs are long, highly oriented cells that extend and retract their protrusions along the blood vessel wall [160]. Non-parenchymal PVMs are important effectors and regulators at the CNS borders in many neurological diseases. The LEC marker hyaluronan receptor (LYVE)-1 is also expressed by non-parenchymal macrophages [161]. More than half of angiogenic and wound-localizing CD206⁺ macrophages express LYVE1. Meningeal angiogenesis is preceded by localization and proliferation of CD206⁺LYVE1⁺ wound-healing macrophages [162, 163]. Unique myeloid cell subsets were demonstrated to support meningeal vascular network repair in a mouse model of mTBI. Increased CD206⁺ macrophages around the lesion site stimulate angiogenesis by removing fibrin through MMP-2 production [161]. However, another study involving mTBI indicated that macrophages recruited to the injured dura mater have a minor impact on vascular regeneration. Compared with highly upregulated VEGF-A from macrophages in the pia mater, the expression of VEGF-A by macrophages in the dura mater did not significantly increase. These macrophages express low levels of pro-angiogenic molecules and high levels of pro-inflammatory molecules [5]. Further studies investigating the mechanism of meningeal vascular repair regulated by mLECs and other cell subtypes are warranted.

Glial cells

Astrocytes are crucial for maintaining the integrity of the BBB. Mechanical tissue damage caused by TBI initiates a rapid increase in ATP released from astrocytes, followed by an increase in cytoplasmic calcium in reactive astrocytes surrounding the injured tissue. The calcium gradient recruits microglia to the lesion area, resulting in astrocyte and microglial reactivity [164–166]. After TBI, reactive astrocytes regulate the BBB [167] by producing and releasing VEGF, which increases the permeability of the BBB and leukocyte extravasation [168, 169]. By secreting apolipoprotein E and ramping up BBB permeability, astrocytes inhibit the cyclophilin A–nuclear factor kappa-light-chain-enhancer of activated B cells–MMP-9 pathway in pericytes [170]. After CNS injury, Sonic hedgehog released by astrocytes exerts neuroprotective effects on multiple cell types [171, 172], and retinoic acid can also decrease BBB permeability [173]. In addition to impairing perivascular aquaporin (AQP)-4 polarization along endothelial surfaces after TBI, reactive astrocytes also alter the homeostasis of tissue fluid, resulting in edema [174, 175].

Microglia are parenchymal macrophages. Several studies have found that reactive microglia are crucial in boosting the angiogenic response to TBI. Angiogenic remodeling occurs within the penumbra in the first few days following ischemic insult [176, 177], and macrophages and microglia frequently surround angiogenic vasculature [178, 179]. M2 macrophages contribute to wound healing and vascular repair [140, 180, 181]. The mechanisms of M2 macrophage-mediated angiogenesis include the production of growth factors and pro-angiogenic cytokines (such as VEGF and FGF2) [140], releasing MMP-9 propeptide [181] and MMP-2 [161]. Reactive microglia enhance EC proliferation by changing the balance between tumor necrosis factor alpha (TNF-α) and transforming growth factor beta (TGF-β) [182]. A conditioned medium from M2 microglia treated with metformin stimulates angiogenesis in vitro by acting on ECs [183]. These findings suggest that microglia control angiogenesis and vascular repair following TBI [184].

Neurons

Under both physiological [185] and pathological [186] settings, the neurological and circulatory systems are highly interdependent and share several regulatory molecules [187, 188]. In addition to the central role of neurons in functional recovery after TBI, they regulate vascular repair. Increased neurogenesis can be observed in the sub-ventricular zone around perilesional blood vessels after TBI [189, 190]. In a rat model of TBI, the administration of neural progenitor cells resulted in a transitory surge in angiogenesis adjacent to the lesion margin [191]. Trophic factors such as FGF2 and VEGF secreted by neural progenitors and stem cells promote angiogenesis [192].

Cytokines and chemokines regulate cellular responses in vascular repair after TBI

Distinct cell types regulate differential events of cerebral vascular repair after TBI, including angiogenesis and vasculogenesis, the proliferation of ECs and smooth muscle cells, degradation of extracellular matrix and debris, stabilization of blood vessels, maintenance of BBB integrity, pruning of vasculature and neovascularization. These events are under the control of various cytokines and chemokines [9, 193, 194]. VEGFs, including at least five members (VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E), are key regulators of angiogenesis and vasculogenesis in vascular repair after TBI through interacting with VEGF receptors (VEGFR1, VEGFR2, and VEGFR3) [76, 195, 196]. VEGFs regulate angiogenesis by promoting the proliferation, migration and survival of ECs [197, 198] and incorporating EPCs into neovascularization foci [199]. However, VEGF-A, originally known as the vascular permeability factor, may be detrimental after TBI due to its role in facilitating edema and BBB permeability [200, 201]. FGF2 binds to FGF receptor-1 (FGFR-1) and FGFR-2 and regulates the processes of proliferation, migration survival of ECs and maintenance of the BBB [202–204]. MMPs are necessary for both pro- and anti-angiogenic processes through promoting ECs migration and tube formation, enhancing ECs growth, breaking intercellular adhesions, and cleavage of extracellular matrix [161, 205, 206]. IGF-1 promotes angiogenesis and neovascularization through stimulating migration and tube formation of ECs, regulating the inflammatory and vasodilatory response of ECs, and acting as a mitogen and antiapoptotic factor for vascular smooth muscle cells [207]. TGF-β regulates vascular repair through its effects on ECs and mural cells. TGF-β secreted by ECs can recruit mesenchymal cells to differentiate into smooth muscle cells and pericytes in perivascular space [208]. TGF-β exerts an angiogenic effect and promotes angiogenesis via binding to the complex of two cell-surface receptors, type II (TβRII) with activin receptor-like kinase-1 and endoglin [209]. TGF-β also exerts an anti-angiogenic effect via TβRII and activin receptor-like kinase-5 signaling to keep a quiescent state of ECs, which is required for the maturation of the vascular network [210]. TGF-β also influences vascular repair by regulating the proliferation and differentiation of smooth muscle cells and pericytes [211]. TNF-α is an inflammatory factor and interacts with TNF receptor 1/2. TNF-α regulates angiogenesis mediated by endothelial cells [212], macrophages [213] and endothelial progenitor cells [214]. Nitric oxide (NO) can exert both pro-angiogenic and anti-angiogenic functions by regulating the expression of FGF2, VEGF, endostatin and MMPs [215–218]. In CNS, NO is mainly synthesized by nitric oxide synthase (NOS), such as neuronal NOS (nNOS) [219], endothelial NOS (eNOS) [220] and inducible NOS (iNOS) [221]. Increased eNOS and eNOS phosphorylation have been reported to mediate EC migration and reendothelialization induced by high-density lipoprotein cholesterol [222]. In the brain stimulated by VEGF, iNOS has been reported to mediate an increase in MMP activity and angiogenesis [223]. Vascular cell adhesion molecule-1 (VCAM-1) is expressed by ECs on the luminal and lateral sides and mediates adhesion and rolling of leukocytes under inflammatory conditions [224, 225]. An increase in VCAM-1 was observed after stimulating ECs with VEGF [226, 227]. Soluble forms of VCAM-1 and E-selectin can exert an angiogenic effect by acting on ECs through a sialyl Lewis-X-dependent mechanism [228].

Therapeutic perspective

Numerous drugs have exhibited protective benefits in pre-clinical studies by reducing vascular damage and encouraging angiogenesis. Glibenclamide may reduce edema and subsequent hemorrhage in the CNS by acting on cerebral microvessels and stimulating neurogenesis by inhibiting sulfonylurea receptor-1 [229]. Atorvastatin may preserve vascular repair after TBI by reducing secondary injury via anti-inflammatory and anti-oxidative mechanisms and increasing circulating EPCs levels [115]. Further, earlier studies showed that atorvastatin improved vascular remodeling via EPC mobilization in models of coronary heart disease and hindlimb ischemia [111, 230]. In models of ischemic stroke, sildenafil boosted CBF and promoted angiogenesis, neurogenesis and axonal remodeling surrounding the infarct site [231–233]. In a transgenic Alzheimer’s disease mouse model and in in vitro studies, sunitinib was found to blunt the expression of thrombin, TNF-α, IL-1β, MMP-9 and IL-6 by cerebrovasculature and improve cognitive behaviors in Alzheimer’s disease mice by targeting VEGF and PDGF receptors [234]. Increased microvascular density, and decreased BBB leakage and brain water content can be observed after exogenous administration of mesenchymal stem cells and EPCs after TBI [235, 236]. Exogenous administration of angiogenic cytokines or chemokines was found to preserve BBB integrity [204] and promote the generation of tissue repair macrophages [237] and angiogenesis [238, 239]. These findings suggest that drugs that modulate key cells and molecules in vascular repair have the potential to improve clinical outcomes following TBI. A major focus of TBI studies is biomarkers that may help to define TBI severity (such as detecting concussion, detecting intracranial injury, which needs a head CT scan, and predicting delayed recovery and adverse outcome after TBI [240]) and guide a more precise strategy for the treatment of the heterogeneous condition of TBI. Fluid biomarkers of TBI are derived from cerebrospinal fluid, blood, saliva, urine and tears. Neuron-specific enolase, ubiquitin C-terminal hydrolase-L1, brain-derived neurotrophic factor, tau, amyloid–β, αII-spectrin breakdown products, myelin basic protein and S100-B can be used to reflect neuronal, axonal and astroglial injuries [241]. The CSF serum albumin ratio [242], protein components of tight junctions [243], VEGF and circulating extracellular vesicles are reported to be related to cerebral vascular injury and BBB leakage [241, 244]. Besides fluid biomarkers, vascular injury, microhemorrhages, CBF changes and BBB impairment can be indicated by neuroimaging biomarkers, such as functional MRI, arterial spin labeling, susceptibility weighted imaging, quantitative susceptibility mapping and contrast-enhanced imaging of MRI techniques [244]. For more information about biomarkers of TBI and vascular-directed therapeutic strategies, readers are encouraged to refer to recent reviews [8, 244, 245].

Conclusions

TBI is a life-threatening condition, and even mTBI may induce long-lasting symptoms related to defective vascular repair. Various cells contribute to vascular repair under the control of cytokines and chemokines released following TBI. Therapeutic approaches that target vascular repair by controlling key cells are promising in TBI treatment.

Abbreviations

B‌BB: Blood–brain barrier; CBF: Cerebral blood flow; CNS: Central nervous system; CSF: Cerebral spinal fluid; CT: Computerized tomography; EC: Endothelial cell; EM: Electron microscopy; FGF: fibroblast growth factor; IL-4: Interleukin 4; LYVE1: Lymphatic endothelial cell marker hyaluronan receptor 1; mLEC: Meningeal lymphatic endothelial cells; MMP: Matrix metalloproteinase; MRI: Magnetic resonance imaging; mTBI: Mild traumatic brain injury; PVM: Perivascular macrophage; TBI: Traumatic brain injury; TGF-β; Transforming growth factor beta; TJ: Tight junction; TNF-α: Tumor necrosis factor alpha; VCAM-1: Vascular cell adhesion molecule-1; VEGF: Vascular endothelial growth factor.

Funding

The work was supported by Macao Young Scholars Program (AM2020032), Macau Science and Technology Development Fund (0061/2021/A2) and (EF026/ICMS-SHX/2022/SZSTIC).

Authors’ contributions

ZAZ, LLY, JW and FY drafted and revised the manuscript. ZAZ, YL and LLY carried out the literature search and categorization of references. ZAZ contributed to the Figure drawing. HXS contributed to the design and critically edited the manuscript. All authors read and approved the final manuscript.

Conflict of interest

The authors declare that they have no conflict of interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.