Human neurons derived from induced pluripotent stem cells as a new platform for preclinical drug screening and development.

Alzheimerdisease (AD) is themostcommondementia,withan estimated prevalence of 30 million people worldwide. Currently, there is no effective treatment that delays the onset or slows the progression of AD. β-Amyloid (Aβ), the key component of plaques, is thought to play a pivotal role in the initiation of AD pathogenesis.1 Previously,all clinical trialsofAβ-targetedtherapeuticshavefailed. There are several possible reasons for this, oneofwhich is that the preclinical drug discovery and development have relied mostly on cell lines or transgenic animals that overexpressAβ. Testingdrugs inhumanneuronswasdifficultbecausebrain tissue is inaccessible through biopsies. After the finding that somaticcells canbereprogrammedinto inducedpluripotentstem cells (iPSCs),which canbe furthermodified into specific types ofcells,2,3neurons inducedfrompatientswith familial andsporadic AD have been regenerated, characterized,4 and used to study aspects relevant to AD biology such as toxicity of Aβ oligomers.5 In this issue of JAMA Neurology, Liu et al6 present their work applying neurons derived fromhuman iPSCs as amodel system to test a new generation of highly potent γ-secretase– targetingdrugs, semagacestatandγ-secretasemodulator (GSM) 4. γ-Secretase, a multisubunit protease that cleaves amyloid precursor protein (APP) at the last step ofAPPproteolysis and generates Aβ, has been considered as a direct target to decrease Aβ levels in the brain. Over the last 20 years, effort has been made to seek efficient γ-secretase–targeting drugs. γ-Secretase inhibitors (GSIs) and GSMs are 2 major groups of compounds that affect γ-secretase. γ-Secretase inhibitors inhibit γ-secretase activity and lower overall Aβ levels. Besides APP, γ-secretase cleaves many other type 1 transmembrane proteins including Notch-1, which plays an important role in embryonicdevelopment.Therefore, a significant challenge regarding the use of a GSI as a therapeutic strategy is targetmediated toxicity. Compounds known as GSMs modify the cleavage sites of γ-secretase within APP and change the profileofdifferentAβspecieswithminimalknowneffectsonother γ-secretase substrates. Because Aβ42 is more prone to aggregation than shorter Aβ peptides, GSMs are considered promising AD therapeutics to selectively lower Aβ42 or the Aβ42: Aβ40 ratio without the likely adverse effects of a GSI. TheGSI testedbyLiuet al,6 semagacestat,was the firstGSI tobetaken intophase3clinical trialsbasedonitsability to lower Aβ during preclinical tests. However, the phase 3 clinical trial carried out by Eli Lillywas terminated before completion owing to anoverallworsening of cognition and an increase in adverse effects including skin cancers and infections in those treatedwith theGSI.7 The adverse effectswere thought to occur becauseNotch signaling or other non-APPγ-secretase targets was altered by semagacestat. However, although the plasma Aβ was reduced by semagacestat, the cerebrospinal fluid Aβ at the time selected for assessment was unaltered. In the article by Liu et al,6 neurons derived from human iPSCs fromnondemented control individuals and presenilin 1 mutationcarriers responded to semagacestatwith lowerAβ42, Aβ40, Aβ38, and total Aβ levels in a dose-dependentmanner, which is consistentwithprevious studies.However, the semagacestat 50% inhibitory concentration values using human iPSC-derived neurons was about 5-fold higher than previous values obtained using neurons from APPswe-overexpressing transgenic mice.6 This discrepancy between human iPSCderivedneuronsandAPP-overexpressingmouseneuronsmay help to explain the lack of significant reduction in cerebrospinal fluidAβ in the clinical studyat the timeassessed. Thehalflife of semagacestat is short and there were small but significant effects of it on reducing Aβ synthesis when its effects in cerebrospinal fluid were assessed bymetabolic labeling studiesatearlierpoints.8γ-Secretasemodulator4 isapotentsecondgenerationnon–nonsteroidal anti-inflammatorydruglikeGSM that has been tested and characterized inAPP-overexpressing cellsandtransgenicmice.Accordingtopreviousstudies,GSM-4 lowers Aβ40 and Aβ42, while increasing Aβ37 and Aβ38without altering total Aβ inTg2576micemixedbrain cultures.9 Liu et al6 found that Aβ40 and Aβ42 were decreased, consistent with previous studies. However, Aβ38 was decreased as opposedtothe increaseseen inthepreviousreport.Giventhehigh degreeofheterogeneityofγ-secretasecomplexsubunitsacross different animal species, neurons derived from human iPSCs may serve better to predict the effects in patients than primary neurons obtained from rodents that overexpress APP. Besidesprimaryneurons cultured fromAPP-overexpressing transgenic mice, nonneuronal tumor-derived human cell lines, such as human embryonic kidney and Helen Lake cells overexpressing APP, were used as drug screening platforms. Data collected from these cells have provided useful information. Liu et al6 found that the 50% inhibitory concentration of GSM-4 from human iPSC-derived neurons is within the same rangeas thatobtainedfromHelenLakecells.9Apotential shortcomingwith using nonneuronal cells to test neuron-targeting Related article page 1481 Opinion