Spooked by the pitfalls of γ-secretase drug development (see companion Sorrento story), BACE fans like to point out that their aspartyl protease may be a safer target. They take comfort in pointing out that compared to γ-secretase knockout mice, BACE knockout mice are doing just fine, thank you. (BACE cleaves APP before γ-secretase, and is therefore the first target in the pathogenic pathway of APP cleavage.) What's more, last week at the AD/PD 2005 conference in Sorrento, Sukanto Sinha of Elan suggested that if a safe BACE inhibitor could be found, restraining this protease slightly might be all that's needed for an AD treatment. The Elan team crossed PDAPP (i.e. APP-overexpressing) mice to heterozygous BACE knockout mice in order to reduce their BACE levels by half. These crosses have a mere 10 percent dent in their brain Aβ levels compared to PDAPP mice, yet show marked reductions in their soluble Aβ, amyloid deposition, and dystrophic neurites, Sinha reported.
What of it, then? Where are the inhibitors? Years of drug discovery effort have shown how difficult it is to try to fit small-molecule inhibitors into the inhospitable plane that is this enzyme's active site. Peptide inhibitors, such as those described in Sorrento by Jordan Tang, of the Oklahoma Medical Research Foundation in Oklahoma city, rarely become drugs, though they have been useful research tools in guiding structural studies and inhibitor design. Finally, small-molecule candidates are coming out and two are rumored to have entered phase 1 studies. Other, purely experimental, compounds have been reported in the literature (see e.g. Lefranc-Jullien et al., 2005, Hu et al. 2004, Hom et al., 2004.)
In Sorrento, Adam Simon of Merck Research Laboratories in West Point, Pennsylvania, reported on one promising molecule, dubbed Merck3. Simon noted that the present compound is selective for BACE1, inhibiting neither BACE 2 nor renin, which had been a problem with prior compounds (see also Stachel et al., 2004). The scientists tested this molecule in a mouse strain made by Bruce Lamb to express full-length human wild-type APP from a yeast artificial chromosome. They pumped the compound, which enters cells but appears not to cross the BBB effectively, into a brain ventricle and then measured Aβ40 and 42 levels. One hopes that Merck is developing more bioavailable compounds internally.
The Merck scientists also measured the APP fragment sAPPalpha, the product of the α-secretase, to test the underlying assumption that inhibiting BACE would shift APP processing toward the non-amyloidogenic α-secretase cleavage. At one, seven, and 14 days, the researchers saw a drop in Aβ42 and a concomitant increase in sAPPalpha in brain homogenates. This result suggests that α-secretase and BACE compete for APP in vivo, and it dovetails with a recent demonstration that turning up alpha-secretase even by a notch can reduce amyloid deposition and improve learning in APP-transgenic mice (Postina et al., 2004), Simon added.
Less Aα, More sAPPalpha: Buy One, Get One Free?
The goal of shifting APP cleavage away from β- and γ-secretase and toward α-secretase emerged as the field's hot new trail at the Sorrento meeting. Such a shift might yield the added bonus that the sAPPalpha fragment not only precludes the making of Aβ, but also does good in its own right. Various studies over the years have sung its praises for being neurotrophic, as well as for protecting against Aβ-induced LTP deficiency and against learning deficits induced by anti-muscarinic agents. In Sorrento, many informal conversations as well as formal presentations focused on it, most prominently Abraham Fisher's lecture reviewing the AF267B compound (previously covered in Alzforum story). Below is a summary of one other such approach, by Falk Fahrenholz of University of Mainz, Germany.
Fahrenholz believes ADAM-10 is one of several endogenous α-secretases, ADAM-17 being another. His group's paper last year (Postina et al., 2004) had raised the prospect of upregulating ADAM-10, and in Sorrento, he presented initial data on how one could go about it. Already, some γ-protein coupled receptors and their subsequent signaling pathways are known to control α-secretase expression. Certain non-tumorigenic protein kinase C activators, such as bryostatin, are reported to boost sAPPα levels (Etcheberrigaray et al., 2004). Fisher's M1-selective agonists do so, as well, by activating PKC molecules that are anchored to M1 acetylcholine receptors, Fisher said.
Another means of activating α-secretase could be found in endogenous hormones that modulate signal transduction pathways upstream of α secretase, Fahrenholz said. He is investigating the pituitary adenylate cyclase-activating polypeptide (PACAP) and its γ-protein coupled receptor PAC1. PACAP and its receptors occur in the human cerebral cortex and hippocampus and are implicated in neuroprotection, Fahrenholz noted. His most recent data show that in neuroblastoma cells expressing PACAP receptors, the hormone increases sAPPα levels at nanomolar doses, whereas an ADAM-10 inhibitor prevented the PACAP-induced α-secretase activity. Further dissection of the pathway confirmed the role of MAP kinase, protein kinase C, PiP3 and other players, Fahrenholz said, adding that he has begun testing PACAP in a mouse model.
Fahrenholz is hopeful this hormone could become a drug because it is known to reach the brain (for a review of PACAP transport, see Dogrukol-Ak et al. 2004.) Incidentally, PACAP is also thought to stimulate proliferation of neural stem cells in adult mouse brain (Mercer et al.,). Alternatively, other known PAC receptor agonists might work, or else, combing the ADAM-10 promoter for binding sites of other transcription factors that can be regulated by outside signals might prove fruitful. Finally, low cellular cholesterol appears to encourage α cleavage of APP, suggesting one mechanism by which statin treatment may help stave off AD, Fahrenholz concluded.—Gabrielle Strobel.