Clinical trials of all GSIs

Clinical trials of all GSIs have ended prematurely, raising the question on our understanding of their target, the γ-secretase/presenilin. Development of compounds to target γ-secretase and reduce Aβ production is complicated by the fact that γ-secretase/presenilin has critical biological function, and γ-secretase mediates the final proteolytic cleavage of Notch [60,61] and many substrates. There are over 100 γ-secretase substrates that are type I membrane proteins and have diverse functions. Notch is one of the most interesting and important substrates with diverse functions. Notch has a large extracellular domain, a single transmembrane (TM) domain and an intracellular domain. Notch is proteolyzed in the trans-Golgi as part of its maturation process into a heterodimeric cell surface receptor, then undergoes a second proteolysis upon ligand activation, leading to shedding of the extracellular domain of the receptor. The remaining membrane-bound C-terminal stub, like APP C-terminal fragment, is cleaved by γ-secretase at two sites (in the middle of its TM domain and at a residue close to the interface of the membrane and cytoplasm) to release the Notch-1-β peptide (Nβ, similar to Aβ of APP) and Notch intracellular domain (NICD), which translocates to the nucleus where it regulates gene Dimethyl Fumarate [[61], [62], [63]]. Notch ICD signaling is critical to a wide variety of cell fate determinations during embryonic development and adulthood. The cytosolic ICDs from known γ-secretase substrates represent a unique library of signaling molecules. Like Notch, they are generated by γ-secretase/PS1 cleavage of substrates [64,65]. These ICDs have different physiological functions linked to regulation of transcription of downstream genes, such as ICDs of alcadeins, CD44, DCC, Notch, Delta, Jagged, E- and N-cadherin, receptor-like protein tyrosine phosphatases, and leukocyte-common antigen related protein. They are involved in a variety of cellular pathways including regulation of cell fate and death, neurite outgrowth, transcriptional regulation, cell-cell adhesion, regulation of ion conductance, and neurotrophin signaling [64,65]. A key concern with GSI is their lack of selectivity among these γ-secretase substrates, e.g., GSIs have shown Notch-related toxicity in rats, including interference with maturation of B- and T-lymphocytes and gastrointestinal tract toxicity [66,67]. A similar requirement of γ-secretase for neuronal survival was found in zebrafish [68,69]. Treating zebrafish with a potent γ-secretase inhibitor, DAPT, causes Notch phenotypes with defects in somitogenesis and neurogenesis [[70], [71], [72], [73]]. The DAPT treated embryos exhibit suppression of Notch phenotypes after injection of Notch intracellular domains (NICD) mRNA [70]. In humans, a PS1 mutation that causes almost complete loss of γ-secretase activity was found in familial AD cases [74]. In mice, double conditional knockout (KO) of PS1 and its homolog PS2 showed progressive loss of synapses, dendrites and neurons, accompanying reduction of NMDA receptor mediated responses and synaptic levels of NMDA receptors [75,76]. Conditional KO of another γ-secretase component, nicastrin, in adult mice displays similar age-dependent cortical neuronal loss likely occurring through apoptosis [77]. With new knowledge of presenilin biology and γ-secretase substrates, it was apparent that development of GSIs for AD needed to be replaced with alternative approaches, such as γ-secretase modulators (GSM).
Modulating γ-secretase and neuroinflammation: one stone two birds? GSMs theoretically have a “regulated” inhibition of γ-secretase activity that could reduce Aβ42 production without obliterating Notch signaling [78], unlike non-selective inhibition of γ-secretase and unwanted side effects for AD therapy caused by GSIs. GSMs only modulate the γ-secretase cleavage site of APP instead of the downstream ε-cleavage site [79]. Competition studies indicate that GSMs have distinct binding sites [[80], [81], [82]]. Cross-linking probes have independently identified PS1-NTF as a specific target of some potent GSMs, while APP is targeted by R-flurbiprofen [83]. These studies have demonstrated that the enzyme γ-secretase itself can be modulated, rather than inhibited, and in such a way that can lower the ratio of Aβ42/40 [[80], [81], [82]]. Therefore, GSMs that have less of an effect on Notch or other substrates will cause fewer adverse side effects.