TRANSGENIC COMPLEMENTATION OF MARCKS DEFICIENCY WITH A NONMYRISTOYLATABLE, PSEUDO-PHOSPHORYLATED FORM OF MARCKS - EVIDENCE FOR SIMULTANEOUSPOSITIVE AND DOMINANT-NEGATIVE EFFECTS ON CENTRAL-NERVOUS-SYSTEM DEVELOPMENT

MARCKS is a widely expressed protein kinase C substrate that is essent ial for normal prenatal development of the central nervous system in m ice. MARCKS-deficient mice exhibit universal perinatal mortality and n umerous developmental abnormalities of the brain and retina. To determ ine which domains of the protein were important in complementing these neurodevelopmental anomalies, we have interbred MARCKS knockout mice with transgenic mice expressing an epitope-tagged human MARCKS transge ne that can completely correct the MARCKS-deficient phenotype. Previou s structure-function studies showed that a nonmyristoylatable form of MARCKS could correct all of the neuroanatomical abnormalities, and res ulted in approximately 25% viable pups that grew to adulthood and were fertile. The present experiment attempted a similar complementation s trategy in which a nonmyristoylatable, ''pseudo-phosphorylated'' form of the protein was used, which has been shown to be almost completely cytosolic in cell expression studies. Surprisingly, this transgene was able to complement almost all of the cerebral anatomical abnormalitie s characteristic of the knockout mice. However, these mice also exhibi ted a universal, novel phenotype: profound retinal ectopia, in which r etinal tissue was often found in the vitreous humor as well as extraoc ularly. Retrospective evaluation of the original MARCKS knockout pheno type revealed that this anomaly was present in about 43% of the knocko ut mice, and was clearly detectable as early as embryonic day 12.5, be fore retinal cell differentiation begins. These data suggest that a no nmyristoylatable, pseudo-phosphorylated form of MARCKS can complement most if not all cerebral aspects of the MARCKS-deficient phenotype, bu t that it appears to worsen a retinal phenotype, perhaps by exerting a dominant-negative effect on a coexpressed MARCKS homologue. (C) 1998 Academic Press.