Traumatic brain injury (TBI) constitutes an acquired brain insult attributable to external mechanical forces, potentially resulting in acute or protracted impairment. As a leading contributor to global mortality and disability, TBI imposes a substantial strain on public health and economic systems worldwide (Maas et al., 2017; Stocchetti et al., 2017).The paucity of effective treatment modalities for TBI underscores the necessity of identifying additional therapeutic targets to safeguard or rehabilitate the injured brain.

The trajectory of TBI is heterogeneous and typically gives rise to intricate pathologies. Despite this, a sequential spatiotemporal pathology unfolds post-TBI, delineated into primary and secondary brain injuries (Bramlett and Dietrich, 2015; Adams et al., 1983; Kumar and Loane, 2012). Of these pathologic alterations following TBI, disruption of the blood-brain barrier (BBB) is deemed a critical contributor to elevated mortality and morbidity rates (Alluri et al., 2015). Disruption of the BBB transpires acutely post-TBI (Rinder and Olsson, 1968) but can endure for numerous years (Hay et al., 2015). Consequently, probing into the underlying mechanisms governing alterations in vascular endothelial function and BBB permeability subsequent to TBI is of paramount importance.

The integrity of the BBB is crucial for preserving cerebral homeostasis, achieved by limiting the entry of peripheral immune cells and enabling the delivery of nutrients and the removal of metabolic waste (Cash and Theus, 2020). BBB disruption encompasses two principal pathological processes. The first process involves the diminution of tight junction (TJ) proteins, resulting in augmented paracellular transport and permitting water-soluble molecules to infiltrate the brain parenchyma from the circulatory system. The second process is characterized by an escalation of transcytosis across endothelial cells (ECs), which typically exhibit minimal vesicle transport to constrain transcellular transport (Hawkins and Davis, 2005; Keaney and Campbell, 2015). Nevertheless, most of the prior research assessing BBB disruption post-TBI has concentrated on damage to the TJ complex through the paracellular pathway, rather than the transcytosis pathway (Li et al., 2021; Chen et al., 2020; Gao et al., 2017). Recent study investigation has revealed that reduced levels of caveolin-1 (CAV-1), a key component and indicator of caveolar vesicles, might be a ubiquitous phenomenon across central nervous system (CNS) barriers (Wang et al., 2020; Chow and Gu, 2017; Andreone et al., 2017; Nguyen et al., 2014; Knowland et al., 2014). Brian Wai Chow and colleagues discovered that in the immature vessels of the blood-retinal barrier (BRB), leakage depended entirely on transcytosis, despite functional tight junction (TJ) complexes from the onset of vessel incorporation into the CNS. Subsequently, transcytosis diminished, culminating in the establishment of a functional barrier (Chow and Gu, 2017). Likewise, CAV-1-mediated vesicle transcytosis emerges as an initial pathological event in the disruption of the BBB, with an observed increase following animal model of ischemic stroke and spreading depolarization (Knowland et al., 2014; Sadeghian et al., 2018). Moreover, our prior study also identified an upregulation of transcytosis in animal models post-TBI (Zhang et al., 2022). Major facilitator superfamily domain-containing 2a (Mfsd2a) is distinctively and persistently expressed in the ECs of the CNS (Betsholtz, 2014). Research indicates that the upregulation of Mfsd2a can alleviate BBB disruption by inhibiting CAV-1 in animal models of intracerebral hemorrhage (Yang et al., 2017), surgical brain injury (Eser Ocak et al., 2020), and subarachnoid hemorrhage (Zhao et al., 2020). Nonetheless, the mechanisms by which Mfsd2a impedes CAV-1 vesicular transport following TBI require further exploration.

The Wnt/β-catenin signaling pathway occupies a central role in the vascular invasion of the CNS and the development of the BBB, determined by either the elimination or stabilization of β-catenin in ECs (Wang et al., 2019; Logan and Nusse, 2004; Zhou et al., 2014). Under typical conditions, β-catenin is stabilized through its interaction with Norrin and Wnt receptors, thereby facilitating its biological functions. Conversely, in instances where the ligands or receptors are impaired, β-catenin may undergo phosphorylation and subsequent degradation (Moon et al., 2004). Historically, the Wnt signaling pathway has been associated with the disintegration of the BBB (Díaz-Coránguez et al., 2017). Prior research indicates that the Wnt signaling activator lithium chloride (LiCl) can preserve BBB integrity and mitigate neurodegeneration and cognitive decline (Li et al., 2020; Li et al., 2018). Recent findings indicate that the Wnt/β-catenin pathway can directly influence the expression of the CAV-1 inhibitory protein Mfsd2a, effectively curbing vesicular transcytosis and sustaining the blood-retinal barrier (BRB) (Wang et al., 2020). Furthermore, Wnt signals originating from astrocytes play a crucial role in sustaining Wnt/β-catenin activity within ECs, thus modulating CAV-1 expression and vesicular density. This regulation is essential for preserving both the structural and functional integrity of the neurovascular unit (NVU), and consequently, the physiological status of the BBB (Guerit et al., 2021). It can be deduced that WNT signaling not only directly governs the expression of CAV-1, consequently restraining the vesicle count in cerebral vascular ECs, but also directly modulates the expression of the CAV-1 inhibitory protein Mfsd2a. Nonetheless, the specific impact of Wnt signaling on CAV-1 expression and vesicular transcytosis following TBI remains ambiguous.

In summary, we hypothesize that the activation of Wnt signaling could play a protective role against BBB damage post-TBI by suppressing CAV-1 vesicular transcytosis through the upregulation of Mfsd2a expression.