SRSF1 and PTBP1 Are -Acting Factors That Suppress the Formation of a CD33 Splicing Isoform Linked to Alzheimer's Disease Risk.

A single nucleotide polymorphism (SNP) in exon 2 of the CD33 gene is associated with reduced susceptibility to late-onset Alzheimer's disease (AD) and causal for elevated mRNA lacking exon 2. In contrast to full-length CD33， transcripts lacking exon 2 result in CD33 protein unable to suppress activation responses in myeloid cells， including microglia. Currently， little is known about the regulation of CD33 exon 2 splicing. Using functional genomics and proteomic approaches， we found that SRSF1 and PTBP1 act as splicing enhancers to increase CD33 exon 2 inclusion in mRNA. Binding of PTBP1 to RNA sequences proximal to the intron 1-exon 2 splice junction is altered by the SNP and represents a potential mechanism behind the SNP-genotype dependent alternative splicing. Our studies also reveal that binding of SRSF1 to the CD33 RNA is not altered by the SNP genotype. Instead， a putative SRSF1 binding sequence at the 3' end of exon 2 directs CD33 exon 2 inclusion into the mRNA， indicating that PTBP1 and SRSF1 promote full-length isoform expression through different mechanisms. Our findings shed light on molecular interactions that regulate CD33 exon 2 splicing， ultimately impacting receptor expression on the cell surface. These data aid in the understanding of CD33's regulation of microglial signaling underpinning the AD genetic associations.