DEVELOPMENTAL EXPRESSION OF MARCKS AND PROTEIN-KINASE-C IN MICE IN RELATION TO THE EXENCEPHALY RESULTING FROM MARCKS DEFICIENCY

The roles of protein kinase C and its substrates in development are po orly understood. Recently, we disrupted the mouse gene for a major cel lular substrate for protein kinase C, the MARCKS protein (Proc. Natl. Acad. Sci. USA, 92, 944-948, 1995). The resulting phenotype consisted of universal perinatal lethality, agenesis of the corpus callosum and other forebrain commissures, and neuronal ectopia and other cortical a nd retinal lamination disturbances. These mice also had high frequenci es of exencephaly (25% overall, 35% in females). In the present study, we have examined the normal expression of MARCKS and the various isoz ymes of protein kinase C at the time of cranial neural tube closure, i n an attempt to correlate MARCKS expression in time and anatomical loc ation with the exencephaly characteristic of MARCKS deficiency. Failur e of neural tube closure occurred at various sites in the cranial neur al tube, suggesting a cellular functional defect that was not limited Co a specific location. Non-exencephalic MARCKS-deficient embryos appe ared to be anatomically normal on embryonic day (E) 8.5-9.5. MARCKS an d PKC alpha were expressed at the plasma membrane of the neuroepitheli al cells comprising the future neural tube, as well as in the surface ectoderm and underlying mesenchyme. Endogenous protein kinase C specie s, comprising either or both alpha and delta, were capable of phosphor ylating MARCKS in intact E8.5 embryos. Thus, MARCKS is expressed at th e plasma membranes of the specific cell types involved In cranial neur ulation; its deficiency presumably results in a still-to-be-elucidated functional defect in these cells that leads to exencephaly in a high proportion of cases.