**9. The** *N***‐methyl‐d‐aspartate receptor hypofunction theory: anti‐NMDAr antibodies**

Encephalitis due to antibodies against the glutamatergic NMDA receptor (anti‐NMDAr antibodies) is an autoimmune disease caused when antibodies produced by the host immune system identify NMDA receptors as foreign antigens. This receptor forms a hetero‐ tetramer between two GluN1 and two GluN2 subunits and participates in essential func‐ tions for reality perception, memory, and the control of unconscious activities. The disease is characterized by the hypofunction of NMDA receptors, which would account for the psychotic symptoms, personality changes, memory impairment, and psychomotor agitation [141, 142]. It usually arises during the course of a paraneoplastic process and is frequently associated with the development of ovarian teratomas, explaining its higher incidence among females [143, 144]. Likewise, 14–75% of patients with systemic erythematous lupus, another autoimmune disease, have been reported to manifest psychiatric symptoms related to the presence of the same antibodies [145, 146]. This involvement of anti‐NMDAr antibod‐ ies (and other neurotransmission receptors) indicates an important link between immune abnormalities and altered neurotransmission in schizophrenia, major depression, or bipolar disorder [147, 148].

would give rise to structural alterations and neurochemical dysregulation that would have a global effect on all of these neurotransmitters (dopamine, glutamate, serotonin, GABA) and would become manifest from adolescence onward, inducing the appearance of the disease.

Outside the nervous system, GABAergic mechanisms have been observed in different tissues and peripheral organs, and GABA has also been found to exert a major role in the immune system, with important inter‐regulatory functions between this and the CNS [136]. It has been reported that *T. gondii* infection is followed by an increase in the motility and migratory capacity of infected dendritic cells, permitting propagation of the parasite to different tissues, including the brain [128]. Although dendritic cells are considered guardians of the immune system, they can also, paradoxically, mediate in the spread of the parasite. This mechanism is produced by the induction in these cells of the GAD enzyme and therefore of GABA pro‐ duction and secretion, which in turn activate GABA receptors expressed by these same cells, stimulating their motility [137]. In experimental mouse models, inhibition of the GABAergic pathway by blockade of GABAA receptors or inhibition of the GAD enzyme markedly reduced the hypermotility and spread of *T. gondii*‐infected dendritic cells and therefore of the parasite itself [137, 138]. Finally, it has also been reported that brain infection by *T. gondii* can interfere with the GABAergic system by inducing changes in the distribution of the GAD67 enzyme, although this event has been related more to possible neurological complications of toxoplasmic encephalitis, such as seizures [139], than to possible complications of latent

Accordingly, the inflammatory response of the host to parasitization, which aims to control parasite replication and alterations in differentiation and migration processes, can change levels of dopamine, tryptophan, kynurenic acid, serotonin, and GABA, leading to behavioral

In order to establish dopamine and other related neurotransmitters as a causal link between toxoplasmosis and schizophrenia development, it is necessary to confirm that this neurotrans‐ mitter is also involved in the disease genesis when there is infection by other pathogens [140], and this mechanism should also explain the possible contribution of *T. gondii* parasitization in

Encephalitis due to antibodies against the glutamatergic NMDA receptor (anti‐NMDAr antibodies) is an autoimmune disease caused when antibodies produced by the host immune system identify NMDA receptors as foreign antigens. This receptor forms a hetero‐ tetramer between two GluN1 and two GluN2 subunits and participates in essential func‐ tions for reality perception, memory, and the control of unconscious activities. The disease is characterized by the hypofunction of NMDA receptors, which would account for the psychotic symptoms, personality changes, memory impairment, and psychomotor agitation

Once more, infection by *T. gondii* may play an important role in this process.

toxoplasmosis, such as schizophrenia.

70 Schizophrenia Treatment - The New Facets

**anti‐NMDAr antibodies**

changes and giving rise to different psychotic symptoms.

other dopaminergic pathway diseases, e.g., Parkinson's disease [114].

**9. The** *N***‐methyl‐d‐aspartate receptor hypofunction theory:** 

The presence of anti‐NMDAr antibodies has been documented in schizophrenic patients in the absence of seizures, movement disorders, or other neurological signs or symptoms [149–151], although other researchers were unable to replicate these findings [152, 153]. For various reasons, the production of anti‐NMDAr antibodies is a plausible mechanism to explain at least a percentage of schizophrenic cases [149]: several studies reported that 5–10% of cases are associated with the presence of these antibodies in serum and cerebrospi‐ nal fluid [150, 151, 154]; kynurenic acid is an antagonist of glutamate *via* blockade of NMDA receptors, as commented in the previous section, suggesting that it contributes to the patho‐ genesis of schizophrenia [122]; persistent blockade of NMDA receptors in experimental ani‐ mals recreates clinical characteristics of schizophrenia [155]; selective elimination of subunit GluN1 of the NMDA receptor in neurons of the cortex and hippocampus in early postnatal development contributes to the pathophysiology of schizophrenia‐related disorders in mice [156]; some of the genes associated with schizophrenia are related to the NMDA receptor [157]; NMDA receptors are reduced in medication‐free schizophrenic patients [158]; block‐ ade of the receptor with ketamine or phencyclidine produces psychotic symptoms [159, 160]; and *de novo* mutations (large chromosomal copy number changes) affect genes that encode one or more nucleotides among the glutamatergic postsynaptic proteins that form part of the receptor, providing insight into possible etiological mechanisms underlying schizophrenia [161].

Maternal infection during brain development or infection during childhood may produce anti‐NMDAr antibodies, while other environmental or genetic factors may influence the age of disease onset [149]. Certain pathogens have been associated with elevated anti‐NMDAr antibodies [162, 163]. Thus, a *T. gondii*‐infected mouse model showed a significantly higher increase in serum GluN2A autoantibodies among juvenile‐ *versus* adult‐infected mice. Adolescence is a critical window in neurodevelopment, and the authors hypothesized that early infection would have greater effects on behavior and the brain in comparison with adult infection. It is possible that chronic infection with *T. gondii* affects pre‐ or postnatal brain development by altering synaptic maturation. An increase in NMDAr autoantibodies due to *T. gondii* exposure might underlie behavioral alterations in symptomatic individuals [164].

### **10. Studies on gene‐infection interaction**

Various studies have demonstrated the participation of numerous genes in schizophre‐ nia, providing firm evidence on the involvement of genetics in the etiology of the disease [165]. Some authors have described inheritability in >80% of cases, and schizophrenia has been associated with polymorphic variability in certain genes [21, 166–168]. However, the genetic hypothesis alone cannot explain the familial association of schizophrenia with other diseases, the seasonal peaks of schizophrenia births, the different prevalences among resi‐ dents of urban and rural areas, discordant results between monozygotic and dizygotic twins or between dizygotic twins and full siblings, or correlations in adopted children, which are, however, consistent with an infectious etiology [1]. Schizophrenia is likely a genetically complex disease that does not follow a Mendelian transmission pattern but rather involves multiple genes, each with a small effect, which act in combination with epigenetic and envi‐ ronmental factors [169]. Accordingly, epidemiological findings suggest that a combination of intrinsic (genetic) and extrinsic or environmental factors, including infections, may par‐ ticipate in the origin of this disease, operating during the development of the individual at some time between conception and adolescence [7]. Tomonaga [170] proposed that persistent chronic infections or the expression of microbial proteins may directly and/or indirectly affect CNS functions in infected individuals, changing the expressions of genes related to schizo‐ phrenia and increasing the risk of suffering this disease or at least some of its varied clinical phenotypes.

Genes whose variants or polymorphisms have been associated with the risk of schizophre‐ nia include some that encode proteins with important functions in neurodevelopment or neurodegeneration and in neuronal neurotransmission circuits. This is the case of the gene that encodes neuregulin 1 (NRG1), a key molecule in maintaining brain synaptic plasticity in adults, which has been related to schizophrenia etiology [171, 172], and the genes that encode catechol‐O‐methyltransferase (COMT) [173], proline dehydrogenase (PRODH) [174], dysbin‐ din protein (DTNBP1) [175], a regulator of G4 protein (RGS4) [176], a regulator of potassium calcium channels (KCNN3) [177], and d‐amino‐oxidase complex (G72, DAAO) [178], among others [179]. The genes that encode these proteins are located in chromosomal regions that have been described as relevant for the study of schizophrenia, and many of these proteins participate in glutamatergic, dopaminergic, or serotonergic neurotransmission circuits.

Genetic polymorphisms that increase susceptibility to schizophrenia, including some of the above, have also been related to resistance or susceptibility to certain infections through their important role in the life cycle of some pathogens, including *T. gondii* [169, 179, 180]. Schizophrenia may possibly correspond to a model in which various genes may interact with microbial agents in a process that is probably mediated by the inflammatory and immune response of the individual, increasing the risk of developing psychiatric disease [169, 179– 182]. It appears reasonable to assume that infections may interact, thereby changing the expressions of schizophrenia‐related genes and increasing the risk of suffering this condition.

Various rodent [79, 183, 184] and human [185, 186] studies have supported the existence of genetic susceptibility to *T. gondii* parasitization, suggesting that if the parasite were one of the possible causes underlying schizophrenia development, this genetic susceptibility might also explain familial cases of schizophrenia [1]. As commented above, some *T. gondii* genes encode proteins with a similar activity to that of enzymes (e.g., tyrosine hydroxylase) in the cells of their intermediate hosts. Therefore, this parasite has genes that allow it to "manipulate" the behavior of the host and facilitate its capture by the cat, its definitive host, thereby favoring parasite survival. The presence of these genes is consequently an evolutionary advantage of *T. gondii* [19].

[165]. Some authors have described inheritability in >80% of cases, and schizophrenia has been associated with polymorphic variability in certain genes [21, 166–168]. However, the genetic hypothesis alone cannot explain the familial association of schizophrenia with other diseases, the seasonal peaks of schizophrenia births, the different prevalences among resi‐ dents of urban and rural areas, discordant results between monozygotic and dizygotic twins or between dizygotic twins and full siblings, or correlations in adopted children, which are, however, consistent with an infectious etiology [1]. Schizophrenia is likely a genetically complex disease that does not follow a Mendelian transmission pattern but rather involves multiple genes, each with a small effect, which act in combination with epigenetic and envi‐ ronmental factors [169]. Accordingly, epidemiological findings suggest that a combination of intrinsic (genetic) and extrinsic or environmental factors, including infections, may par‐ ticipate in the origin of this disease, operating during the development of the individual at some time between conception and adolescence [7]. Tomonaga [170] proposed that persistent chronic infections or the expression of microbial proteins may directly and/or indirectly affect CNS functions in infected individuals, changing the expressions of genes related to schizo‐ phrenia and increasing the risk of suffering this disease or at least some of its varied clinical

Genes whose variants or polymorphisms have been associated with the risk of schizophre‐ nia include some that encode proteins with important functions in neurodevelopment or neurodegeneration and in neuronal neurotransmission circuits. This is the case of the gene that encodes neuregulin 1 (NRG1), a key molecule in maintaining brain synaptic plasticity in adults, which has been related to schizophrenia etiology [171, 172], and the genes that encode catechol‐O‐methyltransferase (COMT) [173], proline dehydrogenase (PRODH) [174], dysbin‐ din protein (DTNBP1) [175], a regulator of G4 protein (RGS4) [176], a regulator of potassium calcium channels (KCNN3) [177], and d‐amino‐oxidase complex (G72, DAAO) [178], among others [179]. The genes that encode these proteins are located in chromosomal regions that have been described as relevant for the study of schizophrenia, and many of these proteins participate in glutamatergic, dopaminergic, or serotonergic neurotransmission circuits.

Genetic polymorphisms that increase susceptibility to schizophrenia, including some of the above, have also been related to resistance or susceptibility to certain infections through their important role in the life cycle of some pathogens, including *T. gondii* [169, 179, 180]. Schizophrenia may possibly correspond to a model in which various genes may interact with microbial agents in a process that is probably mediated by the inflammatory and immune response of the individual, increasing the risk of developing psychiatric disease [169, 179– 182]. It appears reasonable to assume that infections may interact, thereby changing the expressions of schizophrenia‐related genes and increasing the risk of suffering this condition. Various rodent [79, 183, 184] and human [185, 186] studies have supported the existence of genetic susceptibility to *T. gondii* parasitization, suggesting that if the parasite were one of the possible causes underlying schizophrenia development, this genetic susceptibility might also explain familial cases of schizophrenia [1]. As commented above, some *T. gondii* genes encode proteins with a similar activity to that of enzymes (e.g., tyrosine hydroxylase) in the cells of their intermediate hosts. Therefore, this parasite has genes that allow it to "manipulate" the

phenotypes.

72 Schizophrenia Treatment - The New Facets

Genetic studies (in animals and humans) currently center on the possible presence of genes or specific allelic variants that interact with the genes of microorganisms that can infect the patient (gene‐infection interaction hypothesis), increasing the risk of schizophrenia [187–189]. Thus, it has been demonstrated that a critical role in human congenital *T. gondii* infection is played by the *ALOX12* gene, which encodes arachidonate 5‐lipoxygenase enzyme, which is involved in fatty acid metabolism and has been related to schizophrenia, at least in a Korean population [190, 191]. HLA‐related genes such as *SGK1* on chromosome 6, which plays a role in regulating different brain functions [192] and mediates the effects of cortisol on hippocam‐ pal neurogenesis [193], have a modulating effect on some infectious agents, including *T. gondii*, consistent with the proposition that parasitization may modify the risk of schizophrenia [187]. In a study of mice parasitized with *T. gondii*, heterozygous deletion of the *Nurr1* gene (Nurr1 ± genotype), an orphan nuclear receptor essential for the development of mesence‐ phalic dopamine neurons [194], predisposed the animals to behavioral disorders that involve dopamine neurotransmission associated with schizophrenia symptoms [195].

A further example in support of this hypothesis is the Akt cell signaling system. The *Akt* gene encodes a serine‐threonine kinase with three isoforms (Akt1, 2, and 3), whose activa‐ tion mediates cell survival processes and whose inhibition favors apoptosis. As commented above, the innate immune system induces a range of processes after infection of brain cells by *T. gondii*, including antimicrobial activity and the generation of ROS to assist in the destruc‐ tion of foreign pathogens. However, increases in ROS concentrations activate the Akt system, which guarantees cell survival and allows the pathogen to persist and replicate within the infected cell. Akt is above all activated in pathophysiological situations in which ROS increase as the result of ischemia‐reperfusion, playing an important role in the protection of the differ‐ ent cells and tissues involved, including nerve tissue [196]. On the other hand, Akt is known to affect dopaminergic signaling, and polymorphisms of the *Akt1* gene have been found to increase the risk of developing schizophrenia through its relationship with dopaminergic pathways of the prefrontal cortex [197].

Other researchers reported similar associations between schizophrenia risk and other human pathogens, supporting the gene‐infection interaction hypothesis [198–201]. This research line on the effects of interaction between genes or genetic variants on the risk of schizophrenia related to *T. gondii* parasitization is highly likely to establish the true causes of the disease, at least in some types of patient.
