APH-1 (anterior pharynx defective) along with nicastrin and PEN-2 (presenilin enhancer) are essential components of the presenilin (PS)-dependent gamma-secretase complex. There exist three murine Aph-1 alleles termed Aph-1a, Aph-1b, and Aph-1c that encode four distinct APH-1 isoforms: APH-1aL and APH-1aS derived from differential splicing of Aph-1a, APH-1b, and APH-1c. To determine the contributions of mammalian APH-1 homologs in formation of functional gamma-secretase complexes, we generated Aph-1a-/- mice and derived immortalized fibroblasts from these embryos. Compared with littermate controls, the development of Aph-1a-/- embryos was dramatically retarded by embryonic day 9.5 and exhibited patterning defects that resemble, but are not identical to, those of Notch1, nicastrin, or PS null embryos. Moreover, in immortalized Aph-1a-/- fibroblasts, the levels of nicastrin, PS fragments, and PEN-2 were dramatically decreased. Consequently, deletion of Aph-1a resulted in significant reduction in levels of high-molecular-weight gamma-secretase complex and secretion of beta-amyloid (Abeta). Importantly, complementation analysis revealed that all mammalian APH-1 isoforms were capable of restoring the levels of nicastrin, PS, and PEN-2, as well as Abeta secretion in Aph-1a-/- cells. Together, our findings establish that APH-1a is the major mammalian APH-1 homolog present in PS-dependent gamma-secretase complexes during embryogenesis and support the view that mammalian APH-1 isoforms define a set of distinct functional gamma-secretase complexes.
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