APP Gene Family Alternative Splicing Generates Functionally Related Isoformsa

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Abstract
The Alzheimer's beta A4-amyloid protein precursor (APP) and the APP-like proteins (APLPs) are transmembrane glycoproteins with a similar modular domain structure. APP exists in 8 isoforms generated by alternative splicing of exons 7, 8, and 15, of which the L-APP mRNAs lacking exon 15 are ubiquitously expressed in rat tissues but not in neurons. Rat APLP2, the nearest relative of APP, is similarly expressed in 4 different isoforms due to alternative splicing of inserts encoding a Kunitz protease inhibitor domain (KPI, homologous to exon 7 of APP) and a divergent region of 12 amino acids on the NH2-terminal side of the transmembrane domain (12 aa exon). KPI-APLP2 transcripts are highly expressed in neurons, in contrast to KPI-APPs, while L-APLP2 mRNA isoforms lacking the 12 aa exon are predominantly expressed in non-neuronal rat tissues, similar to L-APPs. Further examination of the divergent domains in APP and APLP2 harboring the similarly alternatively spliced APP exon 15 and the 12 aa exon of APLP2 revealed some structural similarities of the amino acid sequences and the predicted secondary structures. In both L-APLP2 and L-APP, a putative xylosyl-transferase recognition site for chondroitin sulfate glycosaminoglycan attachment is present that is interrupted in APP and APLP2 isoforms expressing APP exon 15 or the 12 aa exon of APLP2. Thus, a related function of the divergent domains and the corresponding alternatively spliced APP and APLP2 isoforms in regulation of the binding properties of the ectodomain is suggested. Additionally, beta-secretase cleavage of APP might be sterically hindered selectively in proteoglycan L-APP but not in APP lacking the proteoglycan attachment site. Neurons which have a uniquely low portion of L-APP and high content of APP might therefore be especially susceptible to beta A4-protein liberation. This could explain the selective vulnerability of neurons that is observed in Alzheimer's disease.