3-n-butylphthalide (NBP), approved by the China Food and Drug Administration for the treatment of acute ischemic stroke, is a type of compound isolated from the seeds of Chinese celery (1). The molecular structure of NBP is presented in Fig. 1. Therapy using NBP has been recommended by Chinese guidelines for acute ischemic stroke (2). A randomized double-blind trial (clinical trial no. ChiCTR-TRC-09000483) reported that NBP significantly improves clinical outcomes, including the modified Rankin Scale (3) and National institute of Health Stroke Scale scores (4), of patients who experienced ischemic stroke (5). In addition, a study demonstrated that NBP therapy persistently increases the level of endothelial progenitor cells in peripheral blood, ameliorate cerebral blood flow and improve neuronal functions (6). Furthermore, NBP has been reported to be a safe treatment for cerebral ischemia stroke (5-7). A study has indicated that NBP exhibits protective effects in several neurodegenerative diseases (8). However, to the best of our knowledge, the neuroprotective mechanism of NBP remains unclear. Therefore, the present review discusses the potential mechanism of neuroprotective effects of NBP. The aim of the current review is to provide further understanding regarding the advances of NBP.

Inflammation, a complex biological response to injury, is associated with neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, traumatic brain injury (TBI) and more (9-11). NBP has exhibited anti-inflammatory effects in various models of these diseases and certain mechanisms have been identified. NBP has been reported to reduce the inflammatory reaction by inhibiting nucleotide binding oligomerization domain like receptor protein 3-inflammasome microglia activation and mitigating the Alzheimer's-like pathology via the nuclear factor erythroid-2-related factor 2-thioredoxin-interacting protein-TXNIP-thioredoxin axis in an APP/PS1 mouse model (12,13). Furthermore, NBP inhibited the inflammatory reaction in lipopolysaccharide (LPS)-induced rats via inhibition of c-Jun N-terminal kinase activation and the NF-κB pathway (14,15). NBP was reported to improve dyskinesia in a LPS-induced PD mouse model via a reduction in the loss of dopaminergic neurons, activation of mouse microglia, an increase in TNF-α levels and α-synuclein deposition in the black substantia of the mouse midbrain (16). Additionally, NBP-treatment reduces NF-κB activation following TBI (17), and NBP also inhibits the inflammatory reaction via the same pathway in spontaneously hypertensive rats (18). Notably, a number of studies have indicated that NBP inhibits the inflammatory reaction in other neuroassociated experimental models, such as an experimental model of autoimmune encephalomyelitis of microglia or autoimmune myositis in guinea pigs (19,20). In addition, NBP-treatment has been demonstrated to significantly ameliorate cerebral ischemia reperfusion-induced brain injury of Sprague-Dawley (SD) rats by inhibiting toll like receptor 4/NF-κB-associated inflammation (21). NBP attenuates advanced glycation end products-induced endothelial dysfunction by ameliorating inflammatory responses (22). In summary, there is some understanding regarding the mechanism of NBP in the inhibition of inflammation.

Mitochondria, the site of oxidative metabolism in eukaryotes, produce energy through the oxidation of carbohydrates, fats and amino acids (23). Therefore, mitochondrial dysfunction in the form of oxidative stress may contribute to the pathogenesis of various neurodegenerative diseases (24). Oxidative stress is considered a condition that is caused by an imbalance between pro- and antioxidant factors, which leads to molecular and cellular damage (25). Oxidative stress serves an essential role in the development of age-related diseases (26). NBP exhibits a cumulative beneficial effect on the process of mitochondrial damage (27). This section will discuss the mechanisms involved in mitochondrial oxidative stress.

Recently, NBP exhibited a powerful effect on antioxidant stress in some different models. NBP inhibited oxidative stress in K141N-induced SH-SY5Y cells and in LPS-induced rats through activation of the Kelch-like ECH-associating protein 1 Nrf2-related factor 2-antioxidant response element signaling pathway (15,28). Similarly, NBP reduced oxidative damage to provide neuroprotection in mice following TBI and in rats following carbon monoxide poisoning (29,30). In addition, NBP protects against cerebral ischemia-reperfusion injury by decreasing antioxidant stress via the ERK signaling pathway (31). NBP also protects against H₂O₂-induced injury in neural stem cells by activation of the PI3K/Akt and the Mash1 signaling pathways (32). Furthermore, NBP has been reported to increase superoxide dismutase and catalase activity, and reduce malondialdehyde activity in the experimental autoimmune myositis (EAM) model, NBP directly protects muscle mitochondria and muscle cells from oxidative damage (33). However, the protective effect of NBP on mitochondrial function is not only limited to neurodegeneration, but also appears in cardiovascular diseases. A study suggested that NBP exerts a cardioprotective effect on cardiac ischemic injury via the regulation of mitochondrial function both using in vivo and in vitro experiments (34). In summary, the antioxidant effect of NBP has been widely recognized.

Apoptosis and autophagy are basic biological phenomena of cells, which serve essential roles in removing abnormal cells in multicellular organisms. Disorders in the apoptosis and autophagy processes may cause the occurrence of neuropathy (35). The neuroprotective effect of NBP via the regulation of apoptosis and autophagy has been demonstrated. Treatment with NBP has been reported to reduce apoptotic cell death by increasing the levels of cleaved caspase-3 and caspase-9 following TBI (17). Furthermore, NBP blocks neural apoptosis in areas surrounding cortical contusions on the brain that are induced by TBI (29). The neuroprotective mechanism of NBP involves the mitochondrial apoptotic pathway. NBP inhibits HSPB8 K141N mutation-induced neurotoxicity, attenuates β-amyloid-induced toxicity in SH-SY5Y cells, and protects rat cardiomyocytes from ischemia or reperfusion through regulating mitochondrion-mediated apoptosis (28,36,37). Furthermore, certain studies have demonstrated the inhibition of apoptosis by NBP via the Akt pathway. One study reported that NBP activates Akt/mTOR signaling to inhibit neuronal apoptosis and autophagy in mice with repeated cerebral ischemia reperfusion injury (38). Another study demonstrated that NBP improves cognitive impairment of APP/PS1 mice by inhibiting apoptosis via the PI3K/AKT pathway (39). Additionally, NBP reduces the number of apoptotic cells by regulating Bcl-2 in HUVECs and an EAM model (22,33).

ERS is characterized by incorrect folding and aggregation of unfolded proteins in the endoplasmic reticulum lumen and a disturbance of the calcium balance, which can activate the unfolded protein response and lead to disturbance of the cell function and cell death (40). In recent years, certain studies have reported an anti-ERS effect of NBP. One study demonstrated that NBP inhibits doxorubicin-induced ERS in SD rats (41). In addition, NBP alleviates vascular cognitive impairment by regulating ERS and the Sonic hedgehog/Patched homolog 1 signaling pathway in SD rats (42). Both of these studies agreed that NBP attenuates ERS through regulating the expression of 78-kDa glucose-regulated protein (GRP78), CCAAT-enhancer binding protein homologous protein (CHOP) and caspase-12. Furthermore, NBP also inhibits ERS by attenuating activating transcription factory (ATF)-4, ATF-6, X-box binding protein 1, protein disulfide isomerase, GRP78, CHOP and cleaved-caspase-12 in a spinal cord injury (SCI) model, which may improve functional recovery and prevent disruption of the blood-spinal cord barrier (43,44). However, this mechanism has only recently been identified; therefore, there is limited literature about it. Further research on this mechanism may lead to new findings.

Abnormal protein deposition is closely associated with numerous neurodegenerative diseases (45), such as Alzheimer's disease, which is associated with amyloid-β (Aβ) and tau proteins; and PD, which is associated with α-synuclein (46). A study has demonstrated that NBP significantly reduces total cerebral Aβ plaque deposition and lowers Aβ levels in brain homogenates in a triple-transgenic mouse model of Alzheimer's disease via directing amyloid precursor protein processing toward a non-amyloidogenic pathway (47). Furthermore, NBP treatment inhibited tau hyperphosphorylation in AβPP/PS1 mice, which may improve cognitive impairment (48). NBP enhances a 1-methyl-4-phenylpyridiniumion-induced cellular model and a LPS-induced mice model of PD via reducing the accumulation of α-synuclein (16,49). However, the molecular mechanisms of how NBP reduces the accumulation of α-synuclein and inhibits tau hyperphosphorylation remain unclear. Furthermore, to the best of our knowledge, there is no associated study that provides the clinical evidence that NBP is effective in multiple sclerosis or Lewy body dementia via attenuating abnormal protein deposition. Potentially, new findings can be revealed in additional neurodegenerative diseases.

In summary, current studies suggest that NBP serves a neuroprotective role through inhibiting inflammation, protecting mitochondrial function, alleviating oxidative stress, regulating apoptosis, resisting ERS and decreasing the abnormal protein deposition (Fig. 2). Details on specific molecular mechanisms are presented in Table I. Taken together, it is suggested that NBP provides a promising therapeutic strategy for neurodegenerative diseases. In further studies, the mechanism of action of NBP may be further clarified, and the understanding regarding its potential uses may be expanded.