Neonatal hypoxic–ischaemic encephalopathy (HIE) is a relatively common cause of death and disability in newborns. The incidence of neonatal HIE in full-term infants ranges from 1 to 2 per 1,000 infants (Wang et al., 2019). In countries with a relatively high prevalence of poverty, the incidence of neonatal HIE ranges from 10 to 20 per 1,000 live births (Greco et al., 2020); moreover, 15–20% of the afflicted neonates pass away during the perinatal stage. Furthermore, 25% of patients have neurological disabilities as children, which places a substantial burden on families and society (Filippi et al., 2018; Gunn and Thoresen, 2019; Novak et al., 2019).

In preclinical studies, neonatal HIE animal models have been established in postnatal day (P) 7–10 rodents via unilateral common carotid artery ligation followed by systemic hypoxia. This leads to injury in the cortex, hippocampus and striatum ipsilateral to the ligation site (Rice et al., 1981; Sheldon et al., 1998). Recent studies have shown that hypoxic-ischaemic (HI) injury causes a rapid decrease in N-methyl-D-aspartate receptor (NR) 2A levels, resulting in NR2B tyrosine phosphorylation in the rat brain at P7. Following HI, NR2B expression decreases at P21, but NR2A and NR2B tyrosine phosphorylation increases (Gurd et al., 2002). Moreover, Src family kinases (SFKs) are activated in response to HI injury in neonatal mice. SFK activity is correlated with increased NR2A and NR2B tyrosine phosphorylation. Additionally, Fyn is an SFK that is expressed in neurons and glia in the nervous system (Bare et al., 1993), and Fyn levels are strongly correlated with increased NR2A and NR2B phosphorylation in response to HI injury. Although it is uncertain whether SFKs are responsible for these phosphorylation events, previous studies have shown that, in mice, NR2B and Fyn are expressed at much higher levels in both synaptic and extrasynaptic membranes at P7 than in the adult brain, suggesting the importance of Fyn in regulating NR2B expression in the developing brain (Jiang et al., 2011). To address the specific contribution of Fyn, Fyn was overexpressed in the neurons of neonatal mice subjected to HI-induced injury. Compared with control mice, Fyn-overexpressing mice presented severe brain damage and increased mortality following HI-induced injury. This was associated with increased tyrosine phosphorylation of NR2A and NR2B as well as increased calpain activity. In addition, NR2B phosphorylation at Y1252 and Y1472 was greater in the synaptic fractions of Fyn-overexpressing mice than in those of control mice (Knox et al., 2013). These studies are consistent with the findings in the adult ischaemia literature, namely, that Fyn forms a complex with the NR during HI-induced injury.

In summary, future research on the functional effects of Fyn-mediated cell death on neonatal HI-induced injury and the related mechanisms will hasten the development of therapies that specifically target critical signaling nodes that mediate brain injury without impairing normal brain development, given the critical role of Fyn in neurodevelopment. Here, we review recent findings, highlight how Fyn might be involved in HIE, and provide further detail on how Fyn might be regulated under these circumstances. We draw attention to the necessity of investigating further Fyn-related molecular processes to deepen our understanding of HIE aetiology.

The primary mechanisms of HIE-induced injury to the neonatal brain include: (1) excitotoxicity, (2) oxidative stress, and (3) inflammation, which collectively contribute to neuronal cell death via either apoptosis or necrosis. The injury can be divided into four phases following a hypoxic-ischaemic (HI) insult just before, during, or immediately after labor (peripartum period): (1) the primary energy failure phase (0–6 h after), (2) the latent phase (6–12 h after the HI insult), (3) the secondary energy failure phase (12–72 h after the HI insult), if homeostasis is not effectively restored, and (4) the tertiary phase, where neuronal cell death may continue for days to months after the initial injury (Pedroza-García et al., 2022).

Specifically, the primary phase is characterized by primary energy failure during hypoxic-ischaemic events, leading to harmful reactions such as ATP-dependent pump blockage, lactic acidosis, intracellular accumulation of calcium ions, excitatory amino acid release, toxic oedema formation, and necrosis in the most sensitive areas of the brain (Nuñez et al., 2018). Prior to the secondary phase, there is an incubation period, the energy recovery phase of resuscitation (Yang et al., 2020). However, after 6–72 h, the brain’s energy depletion process reoccurs in areas with greater resistance, and maintenance of excitotoxicity, a large influx of ionic calcium, increased activation of neuronal NO synthase, oxidative stress and mitochondrial dysfunction lead to secondary energy failure and programmed neuronal death through activation of the caspase pathway. It is characterized by the deterioration of mitochondrial function and an acute inflammatory response, leading to apoptosis at this phase (Peeples and Genaro-Mattos, 2022). The tertiary phase involves a sustained inflammatory response and epigenetic changes. In this pathological process, oxidative stress can lead to direct central nervous system damage and activate a cascade of inflammatory responses that promote the development of the tertiary phase; persistent inflammation then exacerbates damage (Zhao et al., 2016; Nuñez et al., 2018).

The Fyn kinase is a 59-kDa protein that has 537 amino acids (Resh, 1998). The Fyn gene, which is located on chromosome 6q21, encodes the Fyn protein (Sherratt et al., 1997) and, as a member of the Src family, was initially identified in 1986 as Syn or Slk (Davidson et al., 1994). It is expressed mainly in the cytoplasmic leaflet of the plasma membrane, where it phosphorylates tyrosine residues on important targets involved in numerous signaling pathways (Saito et al., 2010). Three active isoforms of Fyn are known to exist: Fyn-B, Fyn-T, and Fyn-D7 (Goldsmith et al., 2002). Although Fyn-B is expressed throughout the body, the brain has a notably high concentration of Fyn-B (Goldsmith et al., 2002). Conversely, peripheral blood mononuclear cells and cells of hematopoietic origin are generally found to include Fyn-T and Fyn-D7, respectively (Meur and Karati, 2024). The catalytic domain (SHA1) of all Fyn isoforms is identical, but the linker sequences between the SH1 and SH2 domains are different in Fyn-B and Fyn-T, and unlike Fyn-B, Fyn-D7 does not have residues 233–287 (Meur and Karati, 2024). The primary cause of the structural organization differences between the canonical Fyn kinase and the Fyn-B, Fyn-T, and Fyn-D7 isoforms is alternative splicing. Exon 7A is added to Fyn-B, giving it a longer N-terminal unique domain; moreover, exon 7B is present in Fyn-T, whereas exon 7 is absent in Fyn-D7 (Goldsmith et al., 2002).

Fyn is composed of a unique domain that is distinct from Fyn and the four Src homology (SH) domains (SH1–SH4) that are highly conserved among SFK members (Resh, 1993). The capacity of Fyn to attach to the hydrophobic cellular membrane is made possible by the incredibly short SH4 domain at the N-terminus, which also controls the protein’s subcellular localization, stability, and activity through the process of palmitoylation (Resh, 1993; Zhang Y. et al., 2021). A unique domain consisting of the Tyr28 motif follows the SH4 domain. The Tyr28 motif is phosphorylated by the platelet-derived growth factor (PDGF) receptor, which activates Fyn (Yeo et al., 2011). The function of FYN is regulated by the dynamic phosphorylation of the Tyr531 motif at the C-terminus and the Tyr420 motif on the activation loop of the SH1 domain (Ponniah et al., 1999). When the Tyr531 motif is dephosphorylated, the intramolecular complex of Tyr531-SH2 disintegrates, leading to an open conformation that facilitates Fyn activation (Bhandari et al., 1998) (Figure 1). Because of its open conformation, Fyn can interact with a wide variety of substrates through the SH2 and SH3 domains, playing an important role as an upstream signaling mediator in numerous intracellular processes (Wu et al., 2007).

One of the most highly expressed SFKs in the brain, Fyn, is found in the striatum, cerebellum, and limbic regions of the brain (Umemori et al., 1992; Yagi et al., 1993b). Considerable amounts of Fyn have been found in the cerebral cortex, cerebellum, telencephalon, and brain stem throughout embryonic development (Yagi et al., 1994). This extensive Fyn distribution is evident during brain development and persists in the adult brain (Umemori et al., 1992). Fyn plays a crucial role in the brain, as evidenced by its widespread distribution. In fact, it modulates processes involved in learning and memory and interacts with excitatory and inhibitory synaptic transmission stimuli (Bixby and Jhabvala, 1993). Consistently, Fyn-deficient mice show an aberrant distribution of neocortical neurons at E16, specifically in layers II–III (Yuasa et al., 2004), with a diminished quantity of oligodendrocytes, axonal degeneration, and a thin cerebral cortex (Goto et al., 2008). Mice with a Fyn mutation in the regulatory residue Tyr531 (Y531F) exhibit a variety of abnormal locomotive behaviors, including hyperactivity leading to body weight loss, persistent tremors, lack of coordination, and early death (Yagi et al., 1993b). Furthermore, mice with the point mutation FynR176A in the SH2 domain exhibit poor neuronal migration in the cerebral cortex (Lu et al., 2015).

Fyn is implicated in cell growth, motility and adhesion; ion channel function; growth factor receptor signaling; platelet activation; and immune system regulation by modulating the proliferation and function of T cells, natural killer cells, bone marrow–derived macrophages (BMDMs), and mast cells (Schenone et al., 2011; Poli et al., 2018). For example, Fyn is also involved in T-cell differentiation, specifically proinflammatory cytokine release via Th17 cells (Ueda et al., 2012). In the central nervous system (CNS), Fyn participates in several processes related to brain development, including the differentiation of oligodendrocytes, myelination, neurite outgrowth, axon–glial signal transduction and synaptic plasticity (Nakamura et al., 2001; Schenone et al., 2011; Poli et al., 2018). For example, pharmaceutical upregulation of Fyn activity may neutralize the inhibitory effects of myelin debris on remyelination processes. Furthermore, this upregulation may be beneficial in several crucial aspects of oligodendrogenesis promotion, with increased Fyn activity required for the migration of OPCs to damaged sites (Wolf et al., 2001; Rajasekharan et al., 2010), the facilitation of actin dynamics permitting oligodendrocyte maturation and contact with axons (Liang et al., 2004), and the subsequent production of myelin proteins (Peckham et al., 2016). In addition, Fyn knockout mice display impaired long-term potentiation (LTP) and memory formation as well as ethanol sensitivity (Schenone et al., 2011). Fyn overexpression and dysregulation have been associated with various tumors, including melanomas, breast cancer, and gliomas (Tintori et al., 2015).

Currently, emerging evidence points toward its crucial role in neurodegenerative disorders, including Alzheimer’s disease (AD) and Parkinson’s disease (PD) (Guglietti et al., 2021). In both AD and PD, the current body of evidence promotes the inhibition of Fyn as a therapeutic target for pathological protein-induced toxicity and neuroinflammation. Specifically, inhibition of Fyn may interrupt Fyn-PrPc-ɑ-syn/Aβ signaling, perturbing protein aggregation and oligomeric binding, which would otherwise result in impaired neuronal communication and cell death (Kudo et al., 2012; You et al., 2012). Furthermore, given the key role that Fyn may play in the upregulation of microglial and astrocytic activation, Fyn kinase may represent a universal target, not only in AD and PD but also across multiple conditions known to be associated with a chronic neuroinflammatory response, including amyotrophic lateral sclerosis (ALS) (Liu and Wang, 2017), stroke (Zhang W. et al., 2021), traumatic brain injury (Schimmel et al., 2017) and status epilepticus (Sharma et al., 2021).

Fyn has been extensively linked to neurodevelopment, and changes in the gene expression of Fyn have been connected to the emergence of deficiencies in the behavioral and emotional domains. Indeed, aberrant hippocampal development has been observed in mice lacking Fyn (Grant et al., 1992). Mice in which the Fyn gene is replaced also present increased sensitivity to stress and emotional abnormalities, which can be explained by the presence of lesions in the limbic system (Yagi et al., 1993a). Moreover, increased Fyn expression in the prefrontal cortex appears to be a consistent factor in the pathophysiology of schizophrenia (Ohnuma et al., 2003), and the Fyn gene sequence has been linked to numerous diseases that affect development (Schafer et al., 2019). In this respect, the Fyn polymorphisms rs6916861, rs3730353, and rs706895 are risk factors for schizophrenia in the Chinese–Han community (Wu et al., 2013), and rs6916861 and rs3730353 are risk factors for bipolar disorder (Szczepankiewicz et al., 2009). Additionally, Fyn regulates the NGF/TrkA and BDNF/TrkB pathways, both of which are implicated in neurodegenerative and neurodevelopmental processes (Peckham et al., 2016; Houlton et al., 2019). Notably, BDNF gene polymorphisms may contribute to the pathophysiology of autism spectrum disorder (ASD) via Fyn activity dysregulation (Iwasaki et al., 1998; Yoo et al., 2014). Fascinatingly, Fyn increases the phosphorylation of the semaphorin receptor (plexin) and facilitates the interaction between semaphorin and plexin (Renaud et al., 2008; Suda et al., 2011), which is crucial for mediating events related to neuronal polarization and migration (Renaud et al., 2008), synapse formation (Low et al., 2008; Orr et al., 2017), axonal pruning (Low et al., 2008) and dendritic arborization (Makihara et al., 2016; Ziak et al., 2020) during brain development.

In conclusion, a growing body of convincing evidence indicates that Fyn plays a critical role in regulating the physiologic pathways and growth of neurons. These findings highlight the issues of whether and which changes to these molecular pathways cause neuronal deficits and dysfunctions following HIE, as well as whether and to what extent Fyn targeting can ameliorate these disorders. Research from numerous groups has provided answers to these questions, indicating that pharmacologically targeting Fyn may be therapeutically effective in treating brain damage where Fyn activity is compromised. Moreover, targeting established and new components of Fyn signaling cascades appears to be a promising therapeutic approach for preventing neuronal dysfunction following HIE. Given its widespread expression throughout the body and strong homology with other Src family kinases, which could result in unintentional off-target effects, Fyn is still a challenging factor to target.