Cell-type-resolved NRXN1 isoforms across human brain tissues and hiPSC organoids

ABSTRACT NRXN1 undergoes extensive alternative splicing that generates a highly diverse repertoire of isoforms, diversifying protein–protein interactions, shaping synaptic specialization, and contributing to neuropsychiatric disease when disrupted. However, the splicing landscape of NRXN1 across distinct human brain cell types remains poorly defined. Because NRXN1 expression level is relatively low in adult brains and human induced pluripotent stem cell (hiPSC) derived neurons, single-cell Iso-seq RNA sequencing often provides insufficient coverage to capture its full isoform diversity, particularly across heterogeneous cell populations. To address this gap, we developed an integrative sequencing strategy that combines single-cell transcriptomics, targeted enrichment of NRXN1 transcripts, and long-read sequencing. This approach reveals a comprehensive catalog of cell-type resolved NRXN1 isoforms across adult and fetal human postmortem brains as well as hiPSC-derived cortical organoids. From the adult prefrontal cortex (PFC) region, our analyses reveal distinct splicing programs across interneuron subtypes, pyramidal neurons, and glial lineages. Comparisons between prenatal and adult brains indicate that NRXN1 isoform profiles are established during early development and remain stable throughout neuronal maturation. In hiPSC organoid models, splicing profiles partially mirror both developmental and mature brain patterns, with a subset of splice sites exhibiting cell type - and stage-specific divergence. In the cerebellum of an autism case and in organoids derived from schizophrenia patients, carrying non-recurrent heterozygous NRXN1 deletions, we identify disrupted isoform expression and mutant NRXN1α isoforms and characterize their distribution across diverse cell types. These findings help understand the complex alternative splicing of NRXN1 in adult human brain and hiPSC-derived models and establish a framework for decoding cell type-specific isoform diversity for important genes with relatively low gene expression levels.