Novel TREM2 splicing isoform that lacks the V-set immunoglobulin domain is abundant in the human brain
TREM2 is an immunoglobulin-like receptor expressed by certain myeloid cells, such as macrophages, dendritic cells, osteoclasts and microglia. In the brain, TREM2 plays an important role in the immune function of microglia, and its dysfunction is linked to various neurodegenerative conditions in humans. Ablation of TREM2 or its adaptor protein TYROBP causes Polycystic Lipo-Membranous Osteodysplasia with Sclerosing Leukoencephalopathy (also known as Nasu-Hakola disorder) with early onset of dementia, while some missense variants in TREM2 are associated with an increased risk of late-onset Alzheimers disease. Human TREM2 gene is a subject to alternative splicing, and its major, full-length canonical transcript encompasses 5 exons. Herein, we report a novel alternatively spliced TREM2 isoform without exon 2 ({Delta}e2), which constitutes a sizable fraction of TREM2 transcripts and has highly variable inter-individual expression in the human brain (average frequency 10%; range 3.7-35%). The protein encoded by {Delta}e2 lacks a V-set immunoglobulin domain from its extracellular part but retains its transmembrane and cytoplasmic domains. We demonstrated {Delta}e2 protein expression in TREM2-positive THP-1 cells, in which the expression of full-length transcript was precluded by CRISPR/Cas9 disruption of the exon 2 coding frame. In "add-back" experiments, overexpression of full-length, but not {Delta}e2 TREM2, restored phagocytic capacity and promoted interferon type I response in the knockout cells. Our findings suggest that expression of a {Delta}e2 splice isoform may modify the dosage of full-length transcript potentially weakening some TREM2 receptor functions in the human brain.