Irisin was first described by Böström et al. [1] in 2012 as a myokine that regulates the peripheral energy mechanism and is released from skeletal muscle with exercise. Irisin is a glycosylated polypeptide containing 112 amino acids, formed by proteolysis of the Fibronectin type III-containing protein (FNDC5) transmembrane protein. FNDC5 formation is stimulated by physical activity in skeletal muscle and converted to irisin by the action of peroxisome proliferator- activated receptor (PPAR)-gamma coactivator 1 alpha. Irisin formation is induced by increased expression of PPAR gamma-coactivator 1-alpha, a regulator of mitochondrial biogenesis [2].

Irisin takes part in the process of transforming white adipose tissue into brown adipose tissue. It regulates thermogenesis, energy metabolism, and glucose homeostasis. Animal studies have shown that increased plasma irisin levels increase energy expenditure [1].

Although the metabolic effects of irisin were demonstrated when it was first discovered, subsequent studies have shown that irisin is also expressed in the brain. The central nervous system (CNS) is an important extramuscular source of irisin. Expression of FNDC5, the precursor molecule of irisin, in various regions of the CNS, especially in astrocytes and microglia, the hippocampus, the olfactory bulb, the cerebellum, the pons, and the midbrain, has been shown [3,4].

There is evidence that irisin is involved in neurogenesis, neuronal proliferation, and neuronal differentiation in the CNS. It has been shown that irisin is required for neuronal differentiation in rat embryonic stem cells [5,6].

Current studies show that the central effects of irisin are mediated by multiple mechanisms. Irisin has been found to induce expression of brain-derived neurotrophic factor (BDNF) in the mouse hippocampus [7]. BDNF is one of the most found neurotrophins in the CNS. It is known to play a critical role in the regulation of synaptic functions, synaptic plasticity, and neuronal survival. BDNF plays an important role in hippocampal neurogenesis and long term potentialization formation. Recent studies have shown that the irisin-BDNF axis can enhance learning and memory functions [8,9]. Schizophrenia causes neurodevelop-mental changes, which alters the BDNF mediated hippocampal neuroplasticity, attributing to the cognitive deficits [10].

Irisin not only performs neuroprotective effects by inducing BDNF expression but also protects neurons against oxidative damage. It prevents neuronal damage caused by oxidative stress by activating the Akt/ERK1/2 pathway. It also protects neurons by reducing the secretion of proinflammatory cytokines such as tumor necrosis factor-alpha via the same pathway [11]. In addition, irisin has been shown to induce neuronal proliferation in the hippocampus through the STAT3 signaling pathway associated with hippocampal neurogenesis [12].

Moreover, irisin protects neurons in ischemia states by suppressing reactive oxygen species-NLRP3 inflammatory signaling [13]. Zhang et al. [14] showed that irisin contributes to neuronal differentiation by modulating metabolic responses in the CNS.

Cognitive dysfunctions are one of the main symptoms of schizophrenia and cognitive impairment is known to reduce treatment success. There are very few studies in the literature examining the relationship between irisin and schizophrenia. In a recent study examining irisin levels in schizophrenic patients and healthy controls, there was no difference between the groups in terms of age, gender, or comorbidity, but irisin levels were found to be lower in individuals with schizophrenia than in healthy controls [15]. However, there is no research examining the effect of irisin on cognitive functions in patients with schizophrenia.

Considering all these effects of irisin, it can be thought that it has a possible protective role against cognitive impairment. Previous studies exploring the effect of irisin on cognition were generally preclinical studies, and clinical studies were performed on healthy individuals, people with mild cognitive impairment, or patients diagnosed with dementia. The relationship between irisin levels and cognitive functions in patients with schizophrenia has not been examined. In this study, it was aimed to evaluate the relationship between irisin level and cognitive functions in patients with schizophrenia.

Study Design and Participants

In this study, 96 individuals who were diagnosed with schizophrenia according to the Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5) criteria and which graduated from at least 8 years of primary education and applied our hospital outpatient unit were included. Pa-tients with conditions that could negatively affect neuropsychological test performance, such as vision and hearing loss, were excluded from the study. Individuals with mental retardation, a history of alcohol-substance abuse, electroconvulsive therapy in the last 6 months, and head trauma with amnesia were also excluded from the study, as neuropsychological test performance may be affected.

Ethics committee approval of the study was obtained from Dışkapı Yıldırım Beyazıt Training and Research Hospital Ethics Committee (2017-43/28).

Brief Psychiatric Rating Scale (BPRS) was used to assess disease severity. To evaluate the cognitive functions of the patients, the trail making test (TMT) was evaluated with the A and B forms, and the verbal memory processes scale.

Data Collection

Socio-demographic data form: The form containing age, gender, duration of illness, treatment and other infor-mation about the disease was prepared by the researchers.

Brief Psychiatric Rating Scale (BPRS): BPRS was used to evaluate the symptom severity of patients with schizo-phrenia. There are 18 questions in total and 0 to 6 scoring for each question. It is a semi- structured scale used to evaluate psychotic, depressive symptoms, mannerism and posture, motor slowdown in schizophrenia, and other psychotic disorders [16]. The validity and reliability study of the Turkish form of the scale has been demonstrated [17].

Trail making test: The test is believed to measure the cognitive domains of processing speed, sequencing, mental flexibility and visual–motor skills. The most widely used version of the TMT comprises parts A and B. Part A is generally presumed to be a test of visual search and motor speed skills, whereas part B is considered also to be a test of higher-level executive functions such as complex attention, planning and changing sets. Part B is more difficult because the distance between stimuli is greater and there are more visually interfering stimuli than part A. Before starting the test, a trial portion is given to the participant to make it easier for the participant to understand the test. Time is kept with the start of the test, and how long it takes the participant to complete the test is calculated. In addition, error and correction scores in both part A and part B are calculated and recorded separately [18]. The validity and reliability study in Turkish society was conducted by Türkeş et al. [19].

Verbal memory processes test (VMPT) is a test that evaluates many parameters related to memory. The first of these is the instant memory of the person, second is learning and the third process is remembering and recalling.

VMPT consists of fifteen unrelated words. Fifteen words are read to the participant at one-second intervals and then asked to say the words that remain in his/her mind. The number of correct answers is recorded the as the participant’s instant memory score. After the first attempt, the same list is read to the subject nine more times, and each time he/she is asked to say which is all rememdered.

This gives information about the learning function of the person. Approximately 30−40 minutes after, the participant is asked to remember the words read. Thus, the participant’s long-term memory and recall processes are evaluated. The recall function is also recorded in two ways as delayed recall and delayed recognition [20].

In our study, verbal instant memory score, learning score, total learning score, highest learning score, delayed recall and delayed recognition scores were calculated separately fort he VMPT. The Turkish validity and reliability study of the VMPT was performed by Öktem [21].

Blood Sampling and Irisin Analysis

After at least 12-hour night fast, 5-milliliter (ml) samples of fasting blood were obtained by the participants. Blood samples were isolated from sera within 30 to 60 minutes of collection and stored at 80°C until required for analysis. On the same day, BPRS was measured for each patient and the neuropsychological test battery was applied.

Levels of serum irisin were specified using enzyme- linked immunosorbent assay kits (2019; BioVendor) (Cat. No.: RAG018R, Lot No: X18-047) for quantitative determination in humans. The in-work CV% values were 4.863% and 6.748% for 0.678 mgr/ml and 1.539 mgr/ml concentrates, respectively, and the CV% values between the runs were 9.673% and 8.027% for 0.532 mgr/ml and 1.145 mgr/ml concentrates, respectively. The sensitivities were 1 ng/ml with a measurement range of 0.001−5 mgr/ml and reference range 0.2−2 mgr/ml.

Statistical Analysis

Kolmogorov–Smirnov test is used to determine if data has a normal distribution. Student’s ttest (for numeric variables with normal distribution) and Mann–Whitney Utest (for numeric variables without normal distribution) were used to identify the risk factors, which were differed in two groups, in independent samples.

The correlation analysis was made by Spearman correlation test. pvalues under 0.05 were accepted as statistically significant. Statistical Package for Social Sciences (SPSS) for Windows 20 (IBM SPSS Inc.) program was used for the statistical analysis.

Sociodemographic data of 96 patients included in the study are summarized in Table 1.

The schizophrenia group constituted 64 (66.7%) males and 32 (33.3%) females. The mean duration of the disease was found to be 14.7 years. The mean and standard deviations for age was 41.53 ± 10.85; 26% of the patients were working, 60.4% were not working, 2.1% were students, and 11.5% were retired. The mean and standard deviations of duration of illness was 14.76 ± 8.47 years and the mean number of hospitalization was been calculated as 2.11.

No significant relationship was found between irisin and sex in the sample group. There was no significant correlation between irisin, duration of disease and BPRS total score. The mean BPRS score of the individuals with schizophrenia was calculated as 21.6.

Table 2 shows the relationship between plasma irisin levels and cognitive test performances of the patients. The mean and standart deviations for blood irisin level was 3.8 ± 0.81 mgr/ml. Time, error score and correction scores were calculated separately for the A and B forms of the trailing test. Verbal instant memory score, learning score, total learning score, highest learning score, delayed recall and delayed recognition scores were calculated separately for the VMPT.

In the analyzes performed, a positive correlation was found only between the plasma irisin level and the error score of the TMT form B. Except that, no correlation was found between irisin level and cognitive performance in schizophrenia patients. Table 3 shows the correlation between irisin levels and neurocognitive test performances.

In addition, in subgroup analyzes between genders, it was found that the duration of the TMT B was longer in female schizophrenia patients (p = 0.021).

Cognitive dysfunctions are one of the main symptoms of schizophrenia [22]. Compared to other symptoms of the disease, deterioration in psychosocial functioning has a greater association with cognitive impairment and treatment success is lower [23]. In a previous study, irisin levels in individuals with schizophrenia were found to be lower than healthy individuals, regardless of activity [15]. When the current literature is examined, it is seen that there is no study examining whether there is a correlation between the irisin level and the severity of cognitive impairment in individuals with schizophrenia. In this study, we aimed to evaluate whether there is a relationship between plasma irisin levels and cognitive impairment in patients with schizophrenia.

As a result of many studies conducted in recent years, it has been shown that irisin both protects neurons against oxidative damage through different pathways and plays a role in neuronal survival by reducing proinflammatory cytokine release. In addition, it has been shown in different studies to induce neuronal proliferation and strengthen memory and learning functions by inducing BDNF expression [24-27]. With the demonstration that the irisin-BDNF axis may also play a role in the positive effects of exercise on cognition, studies investigating the effects of irisin on cognitive functions have begun. In a streptozocin-induced diabetic animal model, irisin administration has been shown to have a restorative effect on memory and cognitive functions in diabetic animals [28]. The indirect effect of irisin on BDNF-mediated hippocampal neurogenesis was replicated with the application of peripheral irisin [29]. In yet another preclinical study, it was shown that hippocampal neurogenesis in rodents is dose- dependently regulated by irisin [12].

A study by Belviranli et al. [30] compared the BDNF and irisin levels of sedentary people and athletes who regularly train; BDNF and irisin levels of athletes were found to be higher than sedentary people, and the result of this study showed that there was a positive correlation between irisin and BDNF levels. In the same study, mini- mental test (MMT) and verbal fluency tests were admini-stered to the participants, and a positive correlation was found between the MMT and verbal fluency tests and BDNF and irisin levels. In the study, it was concluded that higher BDNF and irisin levels were associated with higher verbal recognition, spatial, and episodic memory scores [30]. This research is important in that it shows the positive correlation between irisin and cognitive functions.

In another study, the MMT was applied to people newly diagnosed with Alzheimer’s disease and healthy indivi-duals, and cerebrospinal fluid (CSF) irisin and BDNF levels were measured by taking CSF from the patient and control groups. As a result of the study, a significant positive correlation between CSF irisin levels and MMT scores was shown. A positive correlation was also found between CSF irisin and CSF BDNF levels [31]. This study is in line with previous studies showing that the level of CSF irisin correlates with the level of BDNF, indicating that irisin increases hippocampal BDNF expression in animals. The positive correlation between increased irisin and BDNF and cognitive functions suggests that irisin may be a suitable biomarker in the evaluation of pharmacological intervention responses to diseases such as Alzheimer’s disease, which degrade cognitive functions. It has been shown that increasing FNDC5 and irisin levels in experimentally created Alzheimer’s models reduces memory impairment [7]. Supporting brain FNDC5 and irisin levels pharmacologically or through exercise may be a new treatment strategy to protect or restore synapse function and prevent cognitive degradation in neurodegenerative diseases such as Alzheimer’s disease.

Previous studies examining the effects of irisin on cognitive functions have generally focused on memory functions, and it has been seen that more preclinical studies have been conducted. Clinical trials are generally quite limited in healthy individuals or individuals with mild cognitive impairment. The relationship between irisin levels and memory functions and other cognitive functions in patients with schizophrenia has not been examined. In this respect, we might point out that our study is the first to examine the relationship between irisin level and cognitive functions in schizophrenia.

There are various hypotheses as to why the irisin level is lower in patients with schizophrenia than in healthy individuals with no difference in age or gender. Decreased irisin levels in schizophrenia may be associated with oxidative damage. By reducing the irisin’s production of superoxide and peroxynitrite, and it has been found that it can protect neurons against oxidative damage by increasing the production of antioxidant enzymes such as glutathione peroxidase, catalase, and superoxide dismutase. The question of whether low irisin levels create a vulnerability to neurodegeneration and oxidative damage or are a result of neurochemical processes associated with the disease has not yet been given a clear answer. It can be thought that the more sedentary lifestyle of patients with schizophrenia also contributes to the low irisin levels.

In this study of plasma irisin level, disease severity, and cognitive functions, no correlation was found between plasma irisin level, duration of disorder, and severity of symptoms. We also determined that the gender difference did not cause a significant difference in the plasma irisin level. There is no evidence in the literature that the irisin level changes according to gender. In this respect, our study is compatible with the existing literature.

We have previously stated that the existing literature examining the relationship between irisin level and cognitive functions focuses specifically on memory functions, and a positive correlation was found between irisin and BDNF levels and memory functions [32]. In our study, a verbal memory processing test was applied to evaluate memory functions. The VMPT is a test that evaluates many memory functions, such as instant memory, learning, recall, and recognition. In this study, no significant correlation was found between serum irisin levels and memory functions in patients with schizophrenia. When the current literature data is examined, studies reporting a positive correlation between irisin levels and memory functions are in the majority.

There may be multiple reasons why the results of our study differ from the existing literature. First, the available data were mainly obtained from preclinical studies, and clinical studies were not conducted in the schizophrenia group. In studies conducted with healthy individuals and individuals with mild cognitive impairment, memory functions were examined with tests evaluating more general cognitive functions, such as mini-mental tests, and neuropsychological batteries that were more sensitive to memory functions were not used as in our study. Due to these differences, in our study, unlike the existing literature data, no correlation was found between irisin level and memory functions.

The TMT, another neuropsychological test applied in this study, evaluates attention, visuospatial processing, and processing speed. It is mainly a test to evaluate executive functions such as working memory, complex attention, planning, and set switching. In our study, both the A and B forms of the tracing test were applied to the patients, and the time, error, and correction scores were calculated separately for each form.

It was determined that there is a positive correlation between the TMT B-form error score and the insulin level. It can be said that as the plasma irisin level increases, the number of errors in the TMT B form increases. An increase in the number of errors in the TMT B form is associated with impaired executive functions [33]. It has been shown in repeated studies that executive functions are impaired in schizophrenia patients compared to healthy indivi-duals [34-37]. This relationship between impaired executive functions and irisin levels may suggest that the irisin level is increased as compensation for the impairment in executive functions. In the literature, findings similar to our study were obtained in a clinical study evaluating the effect of irisin on executive functions. In this study conducted on individuals with obesity, a negative correlation was found between irisin level and executive functions, and plasma irisin levels were higher in participants with impaired executive functions [38]. It seems that more research is needed in this area to examine whether there is an increase in compensatory iris due to the existing impairment in executive functions.

Another significant result in our study is that the completion time of the B form of the TMT is longer in female individuals with schizophrenia. While the TMT evaluates executive functions, it also evaluates the processing speed, and the prolongation of the test time suggests that the processing speed is prolonged. Although there are studies in the literature stating that the duration scores of the TMT is not affected by gender [39,40], some researchers have reported that there is a gender difference in the completion time of the A and B forms of the TMT [41]. When the results of the study were examined, it was seen that in general, men completed the TMT form B in a shorter time compared to women [42]. In our study, it was observed that female patients completed the TMT form B in a longer time, in line with the current data.

There are some methodological difficulties and limitations in this study, which examines the relationship between cognitive functions and irisin levels in patients with schizophrenia. First, there are some difficulties in the evaluation of cognitive functions arising from the nature of those functions. Although cognitive functions are examined under separate headings, they are all intertwined processes. It is not possible to evaluate a single cognitive function in isolation. For example, it is possible that an impairment in verbal memory may also lead to impairment in executive functions, and attention-related problems may lead to memory problems.

One of the limitations of our study is that none of the neuropsychological tests used were developed for indivi-duals with schizophrenia. The neuropsychological tests we used in this study can be used to identify many neuropsychiatric syndromes, and almost all of them have been developed to distinguish patients from healthy individuals. The lack of a control group can also be considered a limitation of the study. When we compared irisin levels in individuals with schizophrenia and healthy individuals in a study we conducted in our clinic recently, we found that irisin levels were lower in schizophrenic patients [15]. Therefore, the control group was not included in this study, as we mainly aimed to examine the relationship of the irisin with cognitive functions in patients with schizophrenia.

Although there are some methodological limitations, our study evaluating the effect of irisin, a molecule that has been widely researched in recent years due to its critical role in neurogenesis and synaptogenesis, on cognitive functions in patients with schizophrenia is valuable as it is the first study in the literature on this subject as far as we know. In this study, only a positive correlation was found between the plasma irisin level and the error score of the TMT form B. Examining the relationship of the irisin with other cognitive functions in individuals with schizophrenia may also be a valuable research topic. A study evaluating irisin levels between unipolar and bipolar depression patients and a control group has been done before [43], but it has not been investigated whether there is a relationship between depressive symptoms and irisin levels in individuals with schizophrenia. We believe that examining the relationship between depressive symp-toms in individuals with schizophrenia and irisin may be an important research topic in future studies.

More research is needed to understand the role of irisin in cognitive impairment and schizophrenia.

None.

No potential conflict of interest relevant to this article was reported.

Conceptualization: Gamze Erzin, Sibel Örsel. Data acquisition: Hatice Ayça Kaloğlu. Formal analysis: Sibel Örsel. Funding: Sibel Örsel, Gamze Erzin. Supervision: Gamze Erzin. Writing—original draft: Hatice Ayça Kaloğlu, Sibel Örsel, Gamze Erzin. Writing—review & editing: Gamze Erzin, Sibel Örsel, Hatice Ayça Kaloğlu.