Rhomboids, evolutionarily conserved integral membrane proteases, participate in crucial signaling pathways. Presenilin-associated rhomboid-like (PARL) is an inner mitochondrial membrane rhomboid of unknown function, whose yeast ortholog is involved in mitochondrial fusion. Parl-/- mice display normal intrauterine development but from the fourth postnatal week undergo progressive multisystemic atrophy leading to cachectic death. Atrophy is sustained by increased apoptosis, both in and ex vivo. Parl-/- cells display normal mitochondrial morphology and function but are no longer protected against intrinsic apoptotic death stimuli by the dynamin-related mitochondrial protein OPA1. Parl-/- mitochondria display reduced levels of a soluble, intermembrane space (IMS) form of OPA1, and OPA1 specifically targeted to IMS complements Parl-/- cells, substantiating the importance of PARL in OPA1 processing. Parl-/- mitochondria undergo faster apoptotic cristae remodeling and cytochrome c release. These findings implicate regulated intramembrane proteolysis in controlling apoptosis.

The γ-secretase complex plays a role in Alzheimer’s disease (AD) and cancer progression. The development of clinical useful inhibitors, however, is complicated by the role of the γ-secretase complex in regulated intramembrane proteolysis of Notch and other essential proteins. Different γ-secretase complexes containing different Presenilin or Aph1 protein subunits are present in various tissues. Here we show that these complexes have heterogeneous biochemical and physiological properties. Specific inactivation of the Aph1B γ-secretase in a murine Alzheimer’s disease model led to improvements of Alzheimer’s disease-relevant phenotypic features without any Notch-related side effects. The Aph1B complex contributes to total γ-secretase activity in the human brain, thus specific targeting of Aph1B-containing γ-secretase complexes may be helpful in generating less toxic therapies for Alzheimer’s disease.

␥-Secretase is the protease responsible for amyloid ␤ peptide release and is needed for Notch, N-Cadherin, and possibly other signaling pathways. The protease complex consists of at least four subunits, i.e., Presenilin, Aph1, Pen2, and Nicastrin. Two different genes encode Aph1A and Aph1B in man. A duplication of Aph1B in rodents has given rise to a third gene, Aph1C. Different mixes of ␥-secretase subunits assemble in at least four human and six rodent complexes but it is not known whether they have different activities in vivo. We report here the inactivation of the three Aph1 genes in mice. Aph1A ؊/؊ embryos show a lethal phenotype characterized by angiogenesis defects in the yolk sac, neuronal tube malformations, and mild somitogenesis defects. Aph1B ؊/؊ or C ؊/؊ or the combined Aph1BC ؊/؊ mice (which can be considered as a model for total Aph1B loss in human) survive into adulthood. However, Aph1BC ؊/؊ deficiency causes a mild but significant reduction in amyloid ␤ percursor protein processing in selective regions of the adult brain. We conclude that the biochemical and physiological repercussions of genetically reducing ␥-secretase activity via the different Aph1 components are quite divergent and tissue specific. Our work provides in vivo evidence for the concept that different ␥-secretase complexes may exert different biological functions. In the context of Alzheimer's disease therapy, this implies the theoretical possibility that targeting specific ␥-secretase subunit combinations could yield less toxic drugs than the currently available general inhibitors of ␥-secretase activity.Alzheimer ͉ intramembrane cleavage ͉ Presenilin ͉ knockout T he multimolecular complex ␥-secretase cleaves proteins in their transmembrane domain. The complex consists of at least four subunits called Presenilin (Psen), Nicastrin (Nct), Pen2, and Aph1 (1-3). The Psens provide the catalytic subunits of the complex (4), although the precise functional contribution of the other subunits remains to be clarified. Mutations in the genes encoding presenilin 1 (PSEN1) or its homologue presenilin 2 (PSEN2) cause familial Alzheimer's disease (5, 6). Besides amyloid ␤ (A␤) precursor protein (APP), ␥-secretase cleaves an increasing list of type I transmembrane proteins including Notch (7) and N-Cadherin (8) (for a full review, see ref. 9).Until now, ␥-secretase has largely been considered as a homogenous activity, but especially in mammals the situation is probably more complicated (10). Two different Psen genes and two (human) or three (rodent) Aph1 genes that can be alternatively spliced have been identified. Aph1A or Aph1B and Psen1 or Psen2 are incorporated in a mutually exclusive way into different complexes as demonstrated recently, providing formal proof that at least four different complexes in man (and six in mouse) can be generated (11,12). The question remains however whether those different complexes have also different physiological functions. Because ␥-secretase is considered a potential drug target in Alzheimer's disease, a...