Brain function requires that neuronal structures established early in development persist throughout life. How is nervous system architecture maintained as an animal increases its body size, remodels parts of its anatomy and incorporates new neurons? Dedicated mechanisms maintain the precise position of axons in fascicles and of soma in ganglia. To date, four molecules have been implicated in neuronal maintenance: the two-Ig domain protein ZIG-4, the FGF receptor EGL-15(5A), the L1-like SAX-7 protein, and the giant protein DIG-1 that contains multiple cell adhesion and cell-cell interaction domains. Here we report the identification of two new mediators of neuronal maintenance,
zig-5 and
zig-8, which function by regulating the activity of
sax-7.
zig-5 and
zig-8 encode secreted two-Ig domain proteins.
sax-7 encodes an L1-like cell adhesion protein that functions within neurons to maintain neuronal position.
sax-7 is expressed as two isoforms that differ in the number of extracellular Ig domains and are differentially adhesive: a more adhesive short form (4 Ig domains) that is active in maintaining neurons, and a less adhesive long form (6 Ig domains). We find that
zig-5 zig-8 double mutants, but not single mutants, phenocopy the maintenance defects of
sax-7 mutants. In order to address whether
zig-5 and
zig-8 might be functionally related to
sax-7, we have analyzed their genetic interactions. In particular, a mutation of
sax-7, which depletes only the long isoform and displays no maintenance defects on its own, suppresses the defects of
zig-5 zig-8. In addition, overexpression of the short form of
sax-7 fully suppresses the defects of
zig-5 zig-8 double mutants. Previous work from the Mori and our own labs suggests that the long isoform of SAX-7 may adopt an autoinhibitory conformation, in which the activity of Ig domains #3 and #4 (crucial for adhesion) are inhibited by association with Ig domains #1 and #2, resulting in a reduction of adhesiveness. We speculate that ZIG-5 and ZIG-8 may help transform the autoinhibited form into an activated form, for example, by binding to Ig domain #1 or #2, thereby allowing Ig domains #3 and #4 to interact. Genetic removal of the long isoform would therefore abrogate the need of
zig-5 and
zig-8, thereby explaining our interaction data. We will present biochemical data that test this model. We have further identified that loss of
zig-1, another two-Ig domain protein, fully suppresses the defects of
zig-5 zig-8. Our current view is that
sax-7 and
zig-5/zig-8 promote cellular adhesion, which is opposed by
zig-1, creating a balanced adhesiveness of neurons to their environment to bring about robust maintenance.