Archives

  • 2018-07
  • 2018-10
  • 2018-11
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • Based on the collective experience to date

    2022-05-16

    Based on the collective experience to date, it is likely that a much more comprehensive assessment of GSI selectivity for inhibition of multiple substrates, as well as biomarkers to track inhibition of non-APP substrates in vivo will be needed to support future efforts to develop substrate selective GSIs. Similarly, development of GSIs designed to target a specific γ-secretase complex will be facilitated by development of such biomarkers that can facilely track inhibition of multiple substrates. The most likely biomarker candidates are the Aβ-like Z-DQMD-FMK australia produced by γ-secretase cleavage, as these are likely to be present in body fluids and detectable using sandwich ELISAs or mass-spectrometry. Alternatively one might collect peripheral blood and examine the accumulation of substrate derived carboxyl terminal fragments in peripheral leukocytes, but this will likely limit the number of substrates that could be assayed. Until such assays are available we will not know whether we can predict with any accuracy the differential in vivo activity of a given GSI using current preclinical models. With the repurposing of GSIs for cancer, one of the key findings from the early human trials is that subacute dosing with GSIs is reasonably well-tolerated especially when dosing regimens are altered so that dosing is not continuous but intermittent [123]. Alternatively, administration of glucocortocoids with GSI dramatically attenuated gastrointestinal toxicity [124]. In this regard, development of additional tools that better enable assessment of GSI activity on multiple substrates may help to optimize individual dosing so that maximal clinical benefit is achieved while minimizing side-effects. Of course in cancer as opposed to AD, there is generally a willingness to accept some level of toxicity if there are any signs of efficacy. One important question that remains is whether all current GSIs are biologically equivalent. Though many GSIs currently being used for cancer trials are considered “pan-GSI inhibitors” this labeling may be a misnomer. GSI inhibitory activity is often only established for Aβ and Notch 1 [16]. The net action of GSIs may be influenced by multiple factors within a target cell [9], [107], [109], [125], [126]. These factors not only include the variable subunit composition of the γ-secretase complexes but also a) the expression of the substrate in the target cell, b) the location of the substrate, c) sheddase expression, and d) activation of the sheddase. Thus, GSI action could be unexpectedly influenced by any of these factors. Clearly, given the investment in repurposing GSIs, additional studies directly comparing biological actions of various GSI used in clinical trials in various model systems are warranted. γ-Secretase cleavage is remarkably promiscuous, but somehow regulated [9]. To our knowledge there is no type 1 membrane protein which has been shown to be processed by a sheddase in which the membrane stub is not subsequently processed by γ-secretase. Furthermore, mutational studies and comparison of the TMD sequences cut by γ-secretase reveal that there is little sequence specificity to γ-cleavage [32], [127], [128], [129], [130]. Clearly, ligand binding induced ectodomain shedding is one regulatory step in γ-cleavage, but for many constitutively shed and processed proteins there must be other ways of regulating signaling [131]. Previously, we and others have shown that γ-secretase cleavage of APP was located in cholesterol rich buoyant membranes (lipid rafts) and that cholesterol depletion blocked cleavage [12], [132], [133], [134]. Subsequently, a number of labs have confirmed these findings for APP, and recently this was extended to show that γ-secretase interacts with tetraspanins and this interaction facilitates γ-secretase localization in raft domains [135], [136]. Another study suggested that Notch 1 γ-secretase cleavage occurs on the cell surface whereas APP occurs in intracellular compartments [131]. These data indicated that co-localization of ectodomain-shed substrate CTF and γ-secretase, helped to regulate cleavage. In contrast during development, γ-secretase appears to be active in both raft and non-raft membranes. How the potential altered localization of γ-secretase influences activity and response to a GSI is not known, but again an area worthy of further study.