In the present study, plasma levels of proteins in the tPA-BDNF pathway, cortisol and ACTH of drug-naïve FEP patients, UAS of these patients and HC were compared for possible endophenotypes.
The serum BDNF levels were found to be lower in the UAS than the FEP group, this level was lower in both FEP patients and their UAS compared to the HC. Since the discovery that BDNF can cross the blood brain barrier, several studies (some on SZ) have evaluated the blood levels of BDNF [20]. Although most of SZ studies showed a decrease in BDNF levels in chronic SZ and FEP patients, some studies reported an increase while others reported a lack of significant difference [21]. In the current study, no significant difference in serum pro-BDNF levels was found between the FEP group and HC while the levels were significantly lower in the UAS compared to FEP and HC. Similarly, some studies investigating the relationship between pro-BDNF and SZ reported a decrease in pro-BDNF values in SZ patients, while others reported no difference [22].
BDNF is synthesized in neurons via the formation of several precursor forms such as pre-pro-BDNF and pro-BDNF, the latter acts as a ligand for the TrkB receptor (Figure 1a) [23]. BDNF plays an important role in synaptic plasticity, neuronal differentiation, dendritic growth and branching by activating the PI3K/Akt/mTOR pathway [24]. BDNF can also contribute to brain development by mediating an anti-apoptotic effect via the activation of the MAPK/Ras signaling cascade [25]. It was also shown that pro-BDNF can trigger neuronal apoptosis by activating the c-Jun N terminal kinase (JNK) pathway via binding to the p75NTR receptor [26]. BDNF is thought to play a role in synaptic pruning and synaptic elimination, and therefore contributes towards cell elimination [27]. In addition, studies have shown that neurons with low m-BDNF and high pro-BDNF levels are eliminated, supporting a role of this protein in synaptic pruning [28].
A balance is maintained between the levels of m-BDNF and pro-BDNF, which can differ in different developmental stages of the brain [29]. An elevation of pro-BDNF was observed in the early postpartum period and adolescence, while m-BDNF was found to be high in adulthood [30]. Pro-BDNF, with high levels during the developmental period, contributes to brain development by eliminating excessively mature, damaged and malfunctioning neurons [29].
Previous studies on SZ have focused on the blood levels of these proteins with varying results reported. Pro-BDNF and m-BDNF are both active in the same pathway, are known to have opposite activities and can also affect each other when the pathway is activated [23]. Therefore, the balance between the blood levels of these proteins is thought to be more determinant for the pathogenesis of SZ compared to individual levels. Supporting this, we observed in a previous study that this balance was shifted towards pro-BDNF in SZ patients, activating apoptotis [18].
The literature provides strong evidence that excessive synaptic pruning, particularly occurring in early adulthood and adolescence, has an important role in the development of SZ [10]. Neuroimaging studies have shown that there is a much greater loss of gray matter volume in SZ patients compared to controls [11]. Excessive apoptosis may be implicated in the pathogenesis of SZ by causing more synaptic pruning than controls. In addition, neuroimaging studies performed on UAS suggest a decrease in volume in some parts of the brain compared to HC. However, this decrease was found to not be as high as in SZ patients, therfore, volume was considered to represent an endophenotype [11].
In the current study, although we did not find a significant difference in the ratio of pro-BDNF and m-BDNF between SZ patients and their UAS, the ratio was shifted towards pro-BDNF in both groups compared to HC. This supports the data obtained from neuroimaging studies. The apoptotic-neurotrophic balance may be impaired to an extent in UAS, albeit more modestly than SZ patients, which may cause volume loss in the brain. Considering that the balance was statistically similar in SZ patients and their UAS, the ratio of pro-BDNF/m-BDNF can be considered as an endophenotypic feature for SZ.
The BDNF levels were found to be lower in the UAS group compared to the FEP patients, the level was significantly lower in both groups compared to the HC group. Additionally, the pro-BDNF values were significantly lower in the UAS compared to HC and FEP patients, with no significant difference was found between FEP and HC groups. Activation of the tPA-BDNF pathway in a manner that favors apoptosis in SZ patients is likely to be reflected in a change in the balance compared to controls.
Serum levels of the m-BDNF receptor TrkB was found to be low in SZ patients, serum levels of the pro-BDNF receptor p75NTR was found to be statistically similar between patients and controls in some studies or lower in SZ compared to controls in others. We have previously reported lower TrkB levels in FEP patients with no significant difference in the serum levels of p75NTR, supporting the findings from the current study [18]. The p75NTR level in the current study was found to be lower in the UAS compared to both HC and FEP patients while the TrkB values in the FEP group was lower than only the HC group. These receptors mediate the functions of pro-BDNF and m-BDNF, therefore, the balance between receptors may be similarly related to the balance between their ligands. Moreover, the activation of these receptors may vary depending on the ratio of pro-BDNF to m-BDNF. Our data indicate a higher p75NTR/TrkB ratio in FEP patients and their UAS compared to the HC group. A shift of this balance towards an activation of the apoptotic receptor is highly consistent with a shift in the ratio towards pro-BDNF and the pathogenesis of synaptic pruning (Figure 1b). However, although the pro-BDNF/mature BDNF ratio was higher in UAS compared to HC, the absence of a significant difference between the FEP group and the HC may suggest the presence of apoptosis and thus synaptic pruning in the UAS, albeit not severe enough to cause disease (Figure 1b). These data support the use of imaging as an endophenotype for SZ, along with evaluation of the balance between neurotrophic-apoptotic proteins.
One of the leading hypotheses in the etiology of SZ is the stress diathesis hypothesis [31]. The relationship between FEP and stress hormones has been extensively studied and has held [32]. Previous studies have reported high cortisol levels in drug-naive FEP patients [33]. Walsh et al. showed that both ACTH and cortisol levels were high in patients with FEP [34]. In the current study, we found that stress hormones were higher in the FEP group compared to the HC, consistent with previous findings. Additionally, there was no significant difference in cortisol levels in the UAS compared to the HC group, ACTH levels were significantly higher in SZ. Additionally, the ACTH levels of the UAS group was significantly lower than the FEP group. Stress hormones are directly involved in the synthesis of BDNF by inhibiting the conversion of pro-BDNF to m-BDNF [22]. In the stress diathesis hypothesis, it has been suggested that exposure to a major stressful event can trigger SZ [32]. Thus, exposure of SZ patients to greater stress compared to the UAS may trigger the disease. The lack of an significant increase in pro-BDNF and p75NTR levels of the UAS group in the current study may have resulted from the low cortisol levels identified. This may prevent skweing of the pro-BDNF/m-BDNF balance in the direction of apoptotsis.
In addition to its role in the coagulation pathway, tPA also has important functions in the brain. tPA synthesized in the oligodendrocytes and microglia specifically contributes to synaptic plasticity and neuronal regeneration [35]. Moreover, by inducing BDNF synthesis, tPA also plays a role in the formation of reward and fear- related memory reconsolidation in the amygdala, hypothalamus, prefrontal cortex (Figure 1a) [36]. Previous studies have shown a decrease in tPA levels in SZ patients, which may contribute towards pathologies related to memory in SZ [35]. In the current study, no significant difference in tPA levels was identified between FEP patients and UAS or HC, the level was significantly lower in the UAS compared to the HC group. PAI-1 is secreted from endothelial cells and reduces the activity of tPA, playing a negative role in the formation of BDNF [22]. Although some studies found a decrease of PAI-1 in SZ patients, other studies did not find a significant difference compared to HC [37]. In the current study, tPA levels were found to be significantly lower in the UAS compared to the other two groups. Contrary to previous studies, tPA levels were not significantly different between FEP patients and the HC. However, the PAI-1 levels were significantly lower in both FEP and UAS groups than HC, moreover, it was lower in UAS compared to FEP patients. Considering the tPA-PAI levels together, the low PAI-1 levels in UAS may inhibit the formation of m-BDNF.
The current study has some limitations that need to be considered while evaluating the results. We did not investigate any other apoptotic or anti-apoptotic proteins, and did not evaluate polymorphisms in the BDNF-related genes. Moreover, the lack of confirmation of the data in postmortem tissues and the small sample size can be counted as other limitations.