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The extra cellular senile plaques and intra cellular neurofibrillary tangles associated with AD have been the major focus of research. The neurofibrillary tangles are mostly composed of the hyper phosphorylated tau protein(2) and the senile plaques are composed of deposited 42 amino acid long b-amyloid peptide(3). While the complete methods of synthesis of both structures are unknown, the production of the extra cellular amyloid plaques is one major defining point between familial and sporadic AD. Also mutations in the components that generate the b-amyloid peptide cause most cases of familial Alzheimer's disease.

The b-amyloid peptide exists in two predominant forms, one is the 40 amino acid long peptide and the other is the 42 amino acid long peptide. The differences in peptide length arise from differential cleavage of the amyloid precursor protein (APP) from which numerous forms of the b-amyloid peptide come(5). The 42 amino acid long b-amyloid peptide, which forms the senile plaques, comes from APP cleaved by both b- and g-secretases (Figure 1, modified figure from Sinha and Lieberburg 1999). The principal b-secretase in neurons is the aspartic protease BACE1 (b-site APP Cleavage Enzyme 1) which performs the first APP cleavage to release the NH2 terminus of the b-amyloid peptide from its precursor . Subsequent cleavage by the g-secretase releases the COOH terminus of the b-amyloid peptide(6). The g-secretase is a high molecular weight complex which is composed of Presenilin 1 (PS1), mature Nicastrin, APH-1, and Pen-2 (7). Elucidating the formation of this complex is key to finding pharmaceutical treatments for Alzheimer's disease because mutations in the gene that codes for presenilin 1 are the cause of half of all familial AD cases(8)(other causes are mutation in the APP substrate). The g-secretase is also thought to be involved in the cleavage of ErbB4 (9), intra cellular domains of Notch(10), and other similar types of proteins which show that this secretase is important in other pathways.

PS1 mutations have been shown to increase the amount of secreted 42 amino acid long b-amyloid peptide(11)(12). PS1 is an aspartyl protease (meaning that the active sites are two conserved aspartate residues, D257 and D385 that are located on the 6th and 8th hydrophobic region of PS1) and has between 6 to 8 transmembrane domains (most researchers believe there are eight transmembrane domains, Figure 2 from Kim and Schekman 2004) which are important to its function and interactions in the g-secretase complex (13). This protein is localized primarily in the ER (endoplasmic reticulum) and Golgi complexes. In the ER, PS1 exists as an uncleaved holoprotein( proteins that function in the presence of a non-protein cofactor) which is thought to be inactive, but in the Golgi region PS1 exists as a heterodimer with the NTF(N-terminal fragment) and CTF(C-terminal fragment) seperated, but closely associated in a 1:1 stoichiometry(14)(15). The mechanism by which PS1 is cleaved into its respective NTF and CTF is not known, but it is speculated that the other members of the g-secretase complex Nicastrin, APH-1, and Pen-2 are needed for formation of the stable g-secretase complex and for PS1 maturation(16). Nicastrin is a type 1 transmembrane protein that spans the membrane once and interacts in the g-secretase complex after it is N-glycosylated (this is the factor that is required to make mature Nicastrin) in the ER(17). In a low molecular weight sub complex, nicastrin interacts primarily with APH-1, which is predicted to transverse the membrane seven times(18). This nicastrin/APH-1 sub complex then is predicted to interact with PS1 CTF. Pen-2, which spans the membrane twice, is believed to interact with PS1 NTF and facilitates its maturation. In this model there are two sub complexes, one composed of nicastrin, APH-1, and PS1 CTF, and the other composed of Pen-2 and PS1 NTF (Figure 3 from Fraering et al. )(19). These sub complexes interact through the heterodimeric state of the PS1 NTF and CTF. In yeast, mammalian, and Drosophila cells, presence of PS1, nicastrin, APH-1, and Pen-2 were enough to reconstitute g-secretase activity (7)(20)(21). Once the stable g-secretase complex is formed it can cleave APP into the 42 amino acid long b-amyloid peptide. g-secretase activity is believe to happen in the ER, late golgi/TGN, endosomes, and plasma membrane. Depending on where APP is cleaved in the cell is thought to determine whether it is secreted or not. However it is debated what factors lead to the 42 amino acid long b-amyloid peptide plaques form. Also the role of non-secreted b-amyloid in AD is debated and some researchers think that intra cellular b-amyloid is generated by a distinct presenilin independent g-secretase (22).

One new avenue of research has opened up very recently, which is the role of a PS related protein called IMPAS 1 in presenilin 1 holoprotein cleavage. In cells transiently transfected with IMPAS 1 and PS1 holoprotein, however there was little to no indication of this possibly due to the disadvantages associated with Western Blot analysis (23), however it is highly possible that IMPAS 1 or one of the other proteins in its recently discovered family could possibly be responsible for PS1 holoprotein proteolysis. Further analysis must be performed in order to conclude any possible cleavage interaction between IMPAS 1 and PS1. Being that IMPAS 1 is thought to be able to cleave type 1 transmembrane domain proteins (23), it is possible that it may be part of other similar pathways. Other mechanisms have recently been proposed to be functional in AD such as Inositol triphosphate (IP3) ion-gated calcium ion channels because that PS1 is known to modulate IP3 mediated calcium ion liberation(24). It has been shown that in cells with familial AD linked mutations in the gene that codes for presenilin 1, there is an increase calcium ion transients which serve in many signaling functions. Recent studies have shown elevation in ER excitability due to calcium transient elevation caused by a specific PS1 mutation, but subsequent inhibition in the plasma membrane which will disrupt cell to cell signaling(24). This implies that PS1 not only affects AD through its role in the cleavage of the amyloid precursor protein, but also in elevating specific ion transients which disrupt responsiveness to certain synaptic signaling.

While many factors are thought to contribute to familial Alzheimer's Disease, the g-secretase complex is one of the most unknown and most researched components, due to its implications in other pathways and its novel interactions which have a substantial impact on the formation of the disease. Indeed further analysis of the interaction and stoichiometry of the components is needed in order to fully understand the complex and how it is functional in familial Alzheimer's Disease. By researching the mechanisms of the disease's formation, we can hope to apply this information one day to pharmaceutical treaments that can be used for familial Alzheimer's disease patients and to use this information to possibly elucidate the formation of similar neurodegenerative disorders.