Developing in vivo assays for investigation of p75NTR and NRH1 transmembrane domain cleavage events using zebrafish embryos

{gamma}-secretase is an important protease complex responsible for the cleavage of over 100 substrates within their transmembrane domains. {gamma}-secretase acts in Alzheimers disease by cleavage of AMYLOID BETA (A4) PRECURSOR PROTEIN to produce aggregation-prone Amyloid beta peptide. Other {gamma}-secretase substrates such as p75NTR are also relevant to Alzheimers disease. How {gamma}-secretase cleavage site specificity is determined is still unclear. A previous study using Xenopus laevis to investigate the proteolytic processing of p75NTR and its homolog NRH1 found that transmembrane cleavage of NRH1 was not sensitive to the {gamma}-secretase inhibitor DAPT, suggesting that it is not processed by {gamma}-secretase. To investigate this further, we identified zebrafish orthologues of the genes p75NTR and NRH1 and developed in vivo assays to assess cleavage of the resultant p75NTR and Nrh1 proteins. Our observations from these assays in zebrafish are consistent with the Xenopus laevis study. Inhibition of {gamma}-secretase by DAPT treatment results in accumulation of uncleaved p75NTR substrate, while cleavage of Nrh1 is not affected. This supports that p75NTR is cleaved by {gamma}-secretase while Nrh1 is cleaved by a separate {gamma}-secretase-like activity. We extended our approach by generating a chimeric Nrh1 protein in which the Nrh1 transmembrane domain was replaced by that of p75NTR, in an attempt to determine whether it is the p75NTR TMD that confers susceptibility for {gamma}-secretase cleavage. Our results from analysis of this chimeric protein revealed that the p75NTR transmembrane domain alone is insufficient to confer {gamma}-secretase cleavage susceptibility. This is not completely unexpected, as there is evidence to suggest that other factors are crucial for selection/cleavage by the {gamma}-secretase complex. We have established a system in which we can now attempt to dissect the structural basis for {gamma}-secretase cleavage specificity and evolution.