Schizophrenia is a psychiatric disorder that affects both cognition and behavior. The symptoms of schizophrenia are generally grouped into positive, negative, and cognitive symptoms including hallucinations, delusions, anhedonia, and reductions in attention and memory. The onset of schizophrenia typically occurs in late adolescence to early adulthood (Jablensky, 2000). However, schizophrenia symptoms can manifest differently for each individual and can vary in severity over that individual’s lifetime (Freedman, 2003). Certain genetic variations, stressful life circumstances, and altered brain structure or functions have been linked to a higher incidence of schizophrenia. Nevertheless, both the etiology and the pathology of schizophrenia remain elusive. No single gene is responsible for the development of the disorder and there are no biomarkers to aid in diagnosing patients. These limitations hinder our ability to both diagnose and properly treat schizophrenia. Antipsychotic drugs are typically prescribed to treat the symptoms of schizophrenia and have been a useful therapeutic. These drugs can help alleviate the positive symptoms, and to a lesser extent the negative symptoms of the disorder, but are ineffective in treating cognitive symptoms (Freedman, 2003). In addition, many of these drugs come with undesired side effects like movement disorders (Mentzel et al., 2017) and weight gain (Volavka et al., 2002); which can lead to inconsistent usage and reduced treatment effectiveness (Kane et al., 2013). Therefore, further investigation of the etiology and pathophysiology of schizophrenia can facilitate our understanding of this disorder and is critical for the development of more effective treatments. Several genes which show promise in helping us decipher the developmental risks and mechanisms behind schizophrenia belong to the 14-3-3 family.

The 14-3-3 family of proteins has been identified in all eukaryotic organisms (van Hemert et al., 2001). There are seven known mammalian 14-3-3 genes, each of which expresses a distinct protein isoform; beta (β), gamma (γ), epsilon (ε), zeta (ζ), eta (η), theta (θ), and sigma (σ) (Berg et al., 2003). The highest concentration of 14-3-3 proteins is found in the brain, where they encompass around 1% of the total soluble proteins (Boston et al., 1982). The crystalline structure of 14-3-3 shows that two L shaped monomers come together in a dimerized pair to form a cup-shaped structure (Liu et al., 1995). All 14-3-3 isoforms can form either hetero or homodimers (Takahashi, 2003). This allows for the binding of two regions of the same interacting protein (Berg et al., 2003) or the binding of two different ligands (Fu et al., 2000). Thus, 14-3-3 proteins have a diverse range of hundreds of binding partners. Inside the concave face of the 14-3-3 dimer, polar-charged and hydrophobic amino acids create an amphipathic groove that interacts with phosphoserine and phosphothreonine containing motifs located on its binding partners (Yaffe et al., 1997; Wang et al., 1998). Consequently, 14-3-3 can modulate the function or subcellular location of its binding partners through phosphorylation-dependent protein–protein interactions. Many cellular processes and pathways have been linked to 14-3-3 function and expression. In the nervous system, published studies have indicated the involvement of 14-3-3 proteins in intracellular signaling, cell division and differentiation, apoptosis, and ion channel function (Berg et al., 2003). In addition, both human and animal studies have implicated 14-3-3 in several neurodegenerative and psychiatric diseases, including schizophrenia (Foote and Zhou, 2012).

Several human genome-wide association studies (GWAS) have revealed a genetic link between the 14-3-3 family and schizophrenia. In addition, 14-3-3 knockout animal models have shown schizophrenia-like phenotypes, providing further evidence for this connection. This review will discuss the results of some of the genetic and animal model studies that provide evidence for the link between 14-3-3 and schizophrenia.

Genetic linkage and proteomic studies have sought to identify gene or protein expression that may be altered in individuals who are affected by schizophrenia. However, the genetics of the disorder can be heterogeneous; no single gene nor mutation has been identified as the sole cause of schizophrenia. Nevertheless, the 14-3-3 family has been implicated in several studies of schizophrenia patients (Table 1). These results suggest that 14-3-3 alterations may contribute to the development of the disorder; therefore, further investigation of this relationship is warranted.

A proteomic pathway analysis revealed that changes in the hippocampus of schizophrenia patients prominently implicate 14-3-3 signaling (Schubert et al., 2015). Further, several studies have revealed that there are 14-3-3 isoform specific changes associated with schizophrenia. In an expression analysis of peripheral leukocytes of drug-naïve first-episode schizophrenia patients, there were four down and one upregulated 14-3-3 mRNA isoforms, and five downregulated protein isoforms (Qing et al., 2016). There was a positive correlation between these isoform specific expression changes and schizophrenia. In addition, there was a negative correlation between the expression of the ε, θ and ζ isoforms and the positive symptoms of schizophrenia. While in a more recent study of peripheral blood expression levels, five of the seven 14-3-3 family members showed significantly higher baseline expression and significant changes in expression in schizophrenia patients who converted to psychosis compared to those that did not convert (Demars et al., 2020). Interestingly, there also is evidence that the 14-3-3 isoform expression levels respond differentially to antipsychotic treatment (Middleton et al., 2005; Rivero et al., 2015). Together these findings suggest that each of the 14-3-3 isoforms may be involved in schizophrenia in different capacities and are worthy of individual investigation. In fact, the genetic link between schizophrenia and individual isoforms has been further studied, particularly the ε, η, and ζ isoforms.

The 14-3-3ε isoform is encoded by the YWHAE gene, which has been proposed to be a schizophrenia susceptibility gene. Gene-based analyses have shown that common variants in the YWHAE gene contribute to schizophrenia (Torrico et al., 2020). In the study of one Japanese population, the rs28365859 single nucleotide polymorphism (SNP) of the YWHAE gene showed a significant difference between schizophrenia patients and controls (Ikeda et al., 2008). The minor allele was more frequent in controls and corresponded to higher protein expression, indicating that a major allele may be a risk factor for schizophrenia. In subsequent MRI studies, the same SNP as well as several others were shown to be related to changes in the orbitofrontal sulcogyral pattern and changes in the volume of the insula, putamen, and hippocampus of schizophrenia patients, possible developmental abnormalities that could contribute to the disorder (Kido et al., 2014; Takahashi et al., 2014).

Among other chromosomal loci, susceptibility for schizophrenia has been identified at the 8p and 22q locations (Badner and Gershon, 2002). 14-3-3ζ is genetically encoded by the YWHAZ gene at the 8p23 location. In addition to being located on a susceptible locus, genetic studies have releveled associations between SNPs and ultra-rare variants of the YWHAZ gene and schizophrenia (Jia et al., 2004; Wong et al., 2005; Torrico et al., 2020). The YWHAH gene which encodes 14-3-3η is located at 22q12.3, another susceptibility locus. Interestingly, a deletion in 22q11.2 leads to 22q11.2 Deletion Syndrome and a phenotype that often includes schizophrenia (Bassett and Chow, 2008). An estimated 1% of schizophrenia patients have also been diagnosed with 22q11.2 Deletion Syndrome. The proximity of 22q11.2 to the YWHAH gene and the overlapping association with schizophrenia seems to indicate a strong genetic link between 14-3-3η and schizophrenia. In further support of this link, both SNPs and variable number tandem repeats (VNTRs) in YWHAH have been associated with schizophrenia in genetic microarray studies (Toyooka et al., 1999; Wong et al., 2003; Grover et al., 2009). In addition, several studies have shown that the 14-3-3η protein is differentially expressed in schizophrenia (Vawter et al., 2001; Altar et al., 2009; Wu et al., 2012). Thus, both YWHAZ and YWHAH genes have been considered as schizophrenia risk genes because they are located chromosomally close to loci that have been genetically associated with schizophrenia (English et al., 2011).

Despite the evidence discussed above for the genetic link between 14-3-3 and schizophrenia, there are some incongruencies in the literature. One study found no association between YWHAE SNPs and schizophrenia (Liu et al., 2011). While several studies have failed to find significant associations between YWHAH and schizophrenia (Hayakawa et al., 1998; Bell et al., 2000; Duan et al., 2005; Wang et al., 2005). In recognition of these discrepancies, Rivero et al. (2015) performed a western blot analysis of schizophrenia subjects and controls. This study indicated that the outcomes of genetic and proteomic studies in schizophrenia are likely influenced by gender, postmortem delay, age, and pharmacological differences in the subjects being tested. When all schizophrenia subjects were grouped together, no differences in 14-3-3 immunoreactivity were found in comparison to controls. However, when the subjects were more appropriately grouped, the results did show genetic linkages (Rivero et al., 2015). While the symptoms, demographics, and environmental factors of schizophrenia are widely varied, it is also important to consider the genetic heterogeneity of the disorder. Therefore, studying the genetic link between just one family of proteins with schizophrenia is further complicated by the fact that over 200 genetic loci have been identified in association with schizophrenia (Legge et al., 2021), and that interaction between two or more of these loci may contribute to the development of the disorder. These data suggest that the heterogeneity of schizophrenia may contribute to mixed results in the literature, especially when there is not proper grouping or when controls are not careful case matched. Thus, it is important to consider these limitations when interpreting genetic linkage studies and their discrepancies. Taking these considerations into account in future work will help further clarify the genetic link between the 14-3-3 family and schizophrenia.

Considering the evidence that 14-3-3 protein expressions are changed in the brain of schizophrenia patients, investigation of 14-3-3 levels in the cerebrospinal fluid (CSF) may serve as a promising new direction to take in the diagnosis of schizophrenia. Although the CSF is an indirect representation of neurochemistry, changes in mRNA and protein levels of various 14-3-3 isoforms could potentially be reflected in the CSF of schizophrenia patients. The 14-3-3 family has been implicated in several other neurodegenerative, neurodevelopmental, and neuropsychiatric disorders (Foote and Zhou, 2012). In fact, CSF levels of 14-3-3 have been used as a biomarker for several of these neurological diseases (Van Everbroeck et al., 2005; Antonell et al., 2020; Figgie and Appleby, 2021; Nilsson et al., 2021) as well as several other diseases (Neal and Yu, 2010; Zeng and Tan, 2018; Morales et al., 2012). Early diagnosis of schizophrenia is difficult, yet early treatment can often make a difference in the prognosis of disease progression and severity. The history of 14-3-3 being used as a biomarker suggests that it could potentially serve as an indicator of schizophrenia, thus further study into this diagnostic possibility is warranted.

Although human studies have provided valuable insight into the link between the 14-3-3 family and schizophrenia, they do not provide sufficient information about the role of 14-3-3 in the pathogenesis of schizophrenia. In order to better understand the pathology of schizophrenia and how 14-3-3 proteins may be involved; animal models are needed. Several animal models have been used to study the 14-3-3 family in the context of schizophrenia and have provided further support for the link suggested by human studies (Table 2).

Human genetic and proteomic linkage studies have provided several lines of evidence that the 14-3-3 protein family and their associated genes may be altered in schizophrenia. Because the 14-3-3 proteins mediate such a wide range of cellular and molecular processes, any mutation to or change in expression of these proteins may contribute to abnormalities in these processes and potentially to disease states such as schizophrenia.

Several neurotransmitter systems appear to be altered in schizophrenia, including the dopamine, glutamate, and GABA systems. For many years the dopamine hypothesis and the glutamate hypothesis were separate lenses in which researchers and physicians studied and tried to treat the neuropathology of schizophrenia. More recently, it has been proposed that dysfunction in the dopamine system may be a downstream consequence of dysregulated glutamate neurotransmission in the forebrain. As the major excitatory neurotransmitter, glutamate has influence all over the brain, including in the dopamine system. However, it is unknown how the two systems could be interacting in the disease state, making it difficult to merge the two hypotheses for schizophrenia. The 14-3-3 protein family may serve as a potential link between dopamine and glutamate dysfunction in schizophrenia, as there is evidence that changes in 14-3-3 have effects in both neurotransmitter systems. Alterations in NMDA receptor (NMDAR) activity is reported in several 14-3-3 knockout models, and decreased NMDAR activity can alter the excitation and inhibition balance in neural networks and circuits (Moghaddam and Javitt, 2012). Surface expression of NMDA receptors is regulated and promoted by 14-3-3 proteins, particularly the ζ and ε isoforms, through their interactions with particular NMDAR subunits (Chen and Roche, 2009; Lee et al., 2021). This relationship is further highlighted by the finding that knockdown of the NMDAR subunit NR1 leads to synaptic reduction of 14-3-3ε (Ramsey et al., 2011; Ferris et al., 2014). Further, a recent study from our lab has elucidated a polysynaptic pathway in which 14-3-3 dysfunction in the dorsal hippocampus underlies altered psychomotor behavior mediated by dopamine in the VTA (Zhang et al., 2022). These results offer evidence on how the glutamate and dopamine systems may be linked in schizophrenia, as well as a potential role for 14-3-3 dysfunction in the mechanism of the pathology.

The 14-3-3 proteins have many binding partners and are involved in several critical molecular and cellular pathways; thus, the disruption of these proteins can have many potential impacts. One pathway affected by 14-3-3 disruption is the catenin/Rho GTPase/Limk1/cofilin signaling pathway, which plays a significant role in regulating actin cytoskeleton dynamics during brain development. Moreover, the Ndel1/Lis1/14-3-3ε complex has been proposed to be critical for neuronal migration (Foote and Zhou, 2012). Interestingly, the localization of the Ndel1/Lis1/14-3-3ε complex to axons is regulated by the schizophrenia related protein DISC1 (Taya et al., 2007). 14-3-3ε binds to the DISC1 binding region of Ndel1, maintaining its phosphorylation (Toyo-oka et al., 2003; Johnson et al., 2010), and deficiency of 14-3-3ε leads to mis-localization of Ndel1 and Lis1 (Toyo-oka et al., 2003). Additionally, 14-3-3 proteins have been shown to interact with many other cytoskeleton and dendritic spine related proteins. For example, 14-3-3ζ interacts with microtubule-associated protein/microtubule affinity-regulating kinase 3 (MARK3) (Angrand et al., 2006). The proper regulation of microtubules is necessary for neuronal migration and spine formation. In fact, knockout of microtubule associated protein 6 (MAP6) in mice results in many similar schizophrenia-like phenotypes as 14-3-3 knockout models. These include deficits in synaptic plasticity, abnormal glutamatergic signaling, and locomotor hyperactivity (Cuveillier et al., 2021). The similarities between MAP6 and 14-3-3 knockout models along with the physical interaction between 14-3-3 proteins and microtubule related proteins suggest that 14-3-3 are key players in appropriate cytoskeletal regulation. The association between 14-3-3 proteins and these specific binding partners and pathways may underlie the neuronal migration and synaptic defects observed in the 14-3-3 knockout animal models discussed above, as well as provide potential mechanistic insight into the pathogenesis of schizophrenia.

Several of these processes are critical to neurodevelopment, suggesting that any changes in 14-3-3 proteins during critical periods could potentially lead to abnormal structural and functional connections in the brain. In fact, the viability of knockout animals and the severity of their abnormalities in the brain are influenced by the timing of the 14-3-3 knockout. Similarly, the timing of the progression of schizophrenia in humans also points to the importance of these critical developmental periods. The molecular changes that underlie schizophrenia require further study to create a more holistic understanding of the disease and how 14-3-3 dependent regulatory pathways may be involved. The diverse and important roles for 14-3-3 proteins serve as promising points from which to study the mechanisms underlying the disorder.

Schizophrenia is a complicated mental disorder that greatly affects those who are diagnosed with it. The heterogeneity of the disorder makes it difficult to decipher the neurobiological basis of schizophrenia. One interesting and promising route of study in schizophrenia is the role of 14-3-3 proteins. The genetic link between 14-3-3 and schizophrenia suggests that studying schizophrenia through the 14-3-3 family can offer valuable insight to the disease. The results from 14-3-3 knockout models validate this idea and have allowed for a better understanding of the pathology of schizophrenia and how the 14-3-3 family may contribute to its progression. The 14-3-3 protein family has a wide variety of binding partners and functions, the exact mechanisms behind how alterations in 14-3-3 proteins can lead to schizophrenia-like phenotypes is not fully understood. However, some potential mechanisms include abnormal neural development and neuronal signaling following altered neurotransmitter receptor levels, mis-localized protein complexes, and interrupted cellular pathways. Future study will be needed to fully elucidate the causes of these observed phenotypes but will undoubtedly provide further understanding of the complex pathology of schizophrenia.

MN was responsible for drafting, writing, reviewing, and editing this manuscript. YZ was responsible for writing, reviewing, and editing the manuscript with suggestive ideas on formatting and outlining this manuscript. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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