Fig 2: Anti-NCAM1 autoantibodies disrupt NCAM1-NCAM1 and NCAM1-GDNF interactions(A) Pull-down assay confirming that IgG purified from a patient with schizophrenia who was positive for anti-NCAM1 autoantibodies disrupts NCAM1-NCAM1 interactions. His-tagged proteins were pulled down by Ni-NTA-agarose, and GST-tagged proteins were pulled down by Glutathione Sepharose.(B) Pull-down assay showing that IgG purified from a patient with schizophrenia who was anti-NCAM1 autoantibody-positive disrupts the NCAM1-GDNF interaction. His-tagged proteins were pulled down by Ni-NTA-agarose, and GST-tagged proteins were pulled down by Glutathione Sepharose.

Fig 3: Injection of anti-NCAM1 autoantibodies from a patient with schizophrenia into mice(A) Experimental protocol for IgG injection. AAV1-SYN1-EGFP and AAV2-VAMP2-mCherry were injected into the frontal cortex of mice aged 6 weeks, and purified IgG was injected into the CSF of mice aged 8 weeks. Molecular, histological, two-photon microscopy, and behavioral analyses were performed in 9-week-old mice. IHC, immunohistochemistry; IP, immunoprecipitation; WB, western blot.(B) Immunoprecipitation analysis of tissue from the frontal cortex of mice revealed that the NCAM1-Fyn interaction was inhibited by anti-NCAM1 autoantibodies acquired from patients with schizophrenia. CT, computed tomography.(C) Effect of IgG purified from patient 1 with schizophrenia on FAK, MEK1, and ERK1 phosphorylation in the frontal cortex. Removal of anti-NCAM1 antibodies from purified IgG reversed the decrease in pFAK, pMEK, and pERK1.(D) Quantitative analyses of western blots with five mice per group. **p < 0.01 (n = 5, Tukey’s honest significant difference [HSD] test). Data are expressed as the mean ± SEM.(E) Two-photon microscopic images of dendritic spines in the first layer of the frontal cortex of mice injected with AAV1-SYN1-EGFP and IgG purified from patient 1 with schizophrenia or IgG purified from a healthy control. Removal of anti-NCAM1 antibodies from the purified IgG reversed the decrease in the number of spines. The graph on the right shows quantitative analysis of spine number. **p < 0.01 (n = 5 mice per group; 50 dendrites/mouse, 500 spines/mouse; Tukey’s HSD test). Data are expressed as the mean ± SEM. Scale bar: 5 µm.(F) Two-photon microscopic images showing contact between axon terminals and dendritic spines in the first layer of the frontal cortex of mice injected with AAV2-VAMP2-mCherry, AAV1-SYN1-EGFP, IgG purified from patient 1 with schizophrenia, or IgG from a healthy control. The graph on the right shows quantitative analysis of axon terminals merged with spines. **p < 0.01 (n = 5 mice per group; 50 dendrites/mouse, 500 spines/mouse; Tukey’s HSD test). Data are expressed as the mean ± SEM. Scale bar: 5 µm.(G) Alteration ratios in the Y maze test after injection of purified IgG from patient 1 with schizophrenia or from a healthy control. Removal of anti-NCAM1 antibodies from purified IgG reversed the decrease in the alteration ratios. **p < 0.01 (n = 9 mice per group; Tukey’s HSD test). Data are expressed as the mean ± SEM.(H) Pre-pulse inhibition rates of mice injected with IgG purified from patient 1 with schizophrenia or a healthy control. Removal of anti-NCAM1 antibodies from purified IgG reversed the deficiency in pre-pulse inhibition. *p < 0.05 and **p < 0.01 (n = 9 mice per group; Tukey’s HSD test). Data are expressed as the mean ± SEM.

Fig 4: Identification of anti-NCAM1 autoantibodies(A) Titers of anti-NCAM1 autoantibodies in serum by ELISA. **p < 0.01 (n = 201, healthy controls; n = 223, patients with schizophrenia; Mann-Whitney U test).(B) Immunocytochemistry using a commercial anti-NCAM1 antibody, serum and CSF from schizophrenia patient 1, and serum from healthy controls. NCAM1 and EGFP were expressed from a plasmid. Confocal images show antibodies bound to the membrane of EGFP-positive HeLa cells. Similar results were obtained for all anti-NCAM1 antibody-positive patients with schizophrenia (Figure S1B). Antibodies in serum did not react with EGFP because (1) they did not react with EGFP in the nucleus and (2) they did not react with cells transfected with an empty plasmid expressing only EGFP (data not shown). Scale bar: 10 µm. Ab, antibody; Sz, schizophrenia.(C) Titers of anti-NCAM1 autoantibodies in serum by cell-based assay. **p < 0.01 (n = 201, healthy controls; n = 223, patients with schizophrenia; Mann-Whitney U test).(D) ELISA analysis of serum-soluble NCAM in healthy controls (n = 201) and patients with schizophrenia (n = 223). **p < 0.01 (Mann-Whitney U test).(E) ELISA analysis of serum-soluble NCAM in schizophrenia patients with (n = 211) or without (n = 12) anti-NCAM1 autoantibodies defined by cell-based assay. *p < 0.05 (Mann-Whitney U test).(F) ELISA analysis of serum-soluble NCAM in healthy controls (n = 201) and schizophrenia patients without anti-NCAM1 autoantibodies (n = 211) defined by cell-based assay. **p < 0.01 (Mann-Whitney U test).

Fig 5: Anti-NCAM1 autoantibodies from patients with schizophrenia cause schizophrenia-related behavior and changes in synapse numbers in mice(A) Experimental protocol for IgG injection. AAV1-SYN1-EGFP and AAV2-VAMP2-mCherry were injected into the frontal cortex of mice aged 6 weeks, and purified IgG was injected into the CSF of mice aged 8 weeks. Two-photon microscopy and behavioral analyses were performed in 9-week-old mice.(B) Two-photon microscopic analysis of dendritic spines in the first layer of the frontal cortex of mice injected with AAV1-SYN1-EGFP and IgG purified from a healthy control and patient 2 and patient 3 with schizophrenia. **p < 0.01 (n = 5 mice per group; 50 dendrites/mouse, 500 spines/mouse; Tukey’s HSD test). Data are expressed as the mean ± SEM.(C) Two-photon microscopic analysis of axon terminals merged with spines in the first layer of the frontal cortex of mice injected with AAV2-VAMP2-mCherry, AAV1-SYN1-EGFP, and IgG purified from a healthy control and patient 2 and patient 3 with schizophrenia. **p < 0.01 (n = 5 mice per group; 50 dendrites/mouse, 500 spines/mouse; Tukey’s HSD test). Data are expressed as the mean ± SEM. Scale bar: 5 µm.(D) Alteration ratios in the Y maze test after injection of IgG purified from a healthy control and patient 2 and patient 3 with schizophrenia. **p < 0.01 (n = 9 mice per group; Tukey’s HSD test). Data are expressed as the mean ± SEM.(E) Pre-pulse inhibition rates of mice injected with IgG purified from a healthy control and patient 2 and patient 3 with schizophrenia. *p < 0.05 and **p < 0.01 (n = 9 mice per group; Tukey’s HSD test). Data are expressed as the mean ± SEM.