Overview
Zebrafish embryos offer an attractive model system for evaluating g-secretase inhibitors and modulators for unwanted interference with Notch pathway signaling. Loss of Notch activity in zebrafish embryos disrupts normal somite segmentation producing an easily observed early developmental phenotype.
Background
Notch signaling requires proteolytic processing of activated Notch receptors by the g-secretase complex. Because the g-secretase complex is also involved in conversion of Amyloid Precursor Protein (APP) to Amyloid beta (Ab), it has emerged as an important target for Alzheimer’s therapeutics. Although a large number of specific drug-like g-secretase inhibitors have been discovered, many produce unacceptable side effects in preclinical animal models due to inhibition of Notch signaling. Common side effects include abnormalities in the gastrointestinal tract (e.g. goblet cell metaplasia), thymus, and spleen. Consequently, recent discovery efforts have focused on g-secretase modulators, which inhibit processing of APP but do not block the cleavage of other g-secretase substrates such as Notch.
Zebrafish embryos offer an attractive model system for evaluating g-secretase inhibitors and modulators for unwanted interference with Notch signaling. Notch signaling has been extensively studied in zebrafish for over a decade. Zebrafish Notch homologs include Notch-1a, Notch-1b, Notch-2, Notch-3, Notch-5, and Notch-6; known zebrafish ligands include JAG-1a, JAG-1b, JAG-2, DeltaA, DeltaB, DeltaC, and DeltaD. Zebrafish mutants that are defective for Notch signaling display characteristic early developmental phenotypes1. The most obvious developmental phenotype arising from inhibition of Notch signaling is a disruption of normal somite segmentation. This defect is easily discerned by bright field microscopy within the first 24 hours of embryonic development.
Zebrafish also have homologs of all core components of the g-secretase complex, including PS12, PS23, Nicastrin, APH-1b4, and PEN-24. When transfected into cultured human cells, zebrafish PS1 is incorporated into the g-secretase complex and can functionally substitute for endogenous human PS1 and PS2 in APP and Notch processing. Treatment of zebrafish embryos with a known g-secretase inhibitor (DAPT) affects embryonic development in a manner indistinguishable from Notch signaling mutants (see below)5. g-secretase modulators that selectively inhibit APP processing without altering the Notch pathway (JLK inhibitors) do not disrupt somite formation in zebrafish6.
References
1 A. F. Schier, S. C. Neuhauss, M. Harvey et al., Development 123, 165 (1996); F. J. van Eeden, M. Granato, U. Schach et al., Development 123, 153 (1996).
2 U. Leimer, K. Lun, H. Romig et al., Biochemistry 38 (41), 13602 (1999).
3 C. Groth, S. Nornes, R. McCarty et al., Development genes and evolution 212 (10), 486 (2002).
4 R. Francis, G. McGrath, J. Zhang et al., Dev Cell 3 (1), 85 (2002).
5 A. Geling, H. Steiner, M. Willem et al., EMBO reports 3 (7), 688 (2002).
6 A. Petit, A. Pasini, C. Alves Da Costa et al., Journal of neuroscience research 74 (3), 370 (2003).