PgmNr W4092: A RhoGAP responds to axonal guidance signals to regulate actin nucleation during C. elegans morphogenesis.

Authors:
Andre Wallace; Martha Soto


Institutes
Rutgers - Robert Wood Johnson Medical School, Piscataway, NJ.


Keyword: Cell migration

Abstract:

The cell migrations that occur during C. elegans morphogenesis are meticulously regulated by rearrangement of the actin cytoskeleton.  To understand how this process is regulated requires identifying the molecules that function to maintain the balance of forces during F-actin nucleation. Our lab previously showed that three axonal guidance signals, UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph, function upstream of the CED-10/Rac-1 GTPase to modulate actin nucleation in a pathway centered on activation of the WAVE/SCAR complex. In an RNAi screen for regulators of the WAVE/SCAR pathway, we identified the C. elegans homolog of vertebrate Myosin IX, HUM-7. Loss of hum-7 alone results in a small percentage of dead embryos, most of which have morphogenesis defects similar to what is seen in WAVE mutants. Like its vertebrate homolog, Myo9, HUM-7 has a myosin domain in its head and a RhoGAP domain in its C-terminus. Loss of hum-7 suppressed embryonic defects in sax-3 mutants, enhanced defects in unc-40 and did not alter the embryonic defects observed in vab-1 mutants. The effects of hum-7 on these axonal guidance mutants is conserved post-embryonically since similar genetic interactions were observed in the guidance of AVM touch neuron. Loss of hum-7 enhanced unc-40 neuronal defects, suppressed sax-3 but did not alter vab-1 neuronal defects. To understand how an axonal signal acting on a RhoGAP results in morphogenesis defects, we tested which GTPase is being regulated by this candidate RhoGAP, and what loss of this protein does to F-actin levels and dynamics.  While Rac is essential for epidermal enclosure migrations, genetic and biochemical tests suggest that HUM-7 is a GAP for Rho-1.  Live imaging of F-actin in the migrating epidermis of the developing C. elegans embryo shows that loss of hum-7 results in elevated actin levels and increased F-actin protrusions at the leading edge of the ventral cells. Similar phenotypes were observed in other mutants known to increase Rho-1 activity suggesting that HUM-7 functions through Rho-1 to modulate actin events during embryonic morphogenesis. The increased F-actin levels observed in hum-7 mutants are similar to those observed in vab-1 mutants. This observation, coupled with the fact that loss of hum-7 causes no further impact on loss of vab-1 embryonically and post embryonically suggests that HUM-7 is functioning in a pathway with VAB-1 to control RHO-1 during the ventral enclosure step of C. elegans morphogenesis.



Wormbase Genetic Index
1. RHO-1
2. CED-10
3. HUM-7
4. SAX-3
5. VAB-1
6. UNC-40