Previous studies have shown that
rpm-1 regulates axon termination in the mechanosensory neurons, and synapse formation in the GABAergic motor neurons (1,2).
rpm-1 functions by positively regulating a Rab GTPase pathway that includes
glo-4 and
glo-1, and negatively regulating a MAP kinase cascade that includes
dlk-1,
mkk-4, and the
p38 MAP kinase
pmk-3 (3,4). RPM-1 acts as an ubiquitin ligase to target DLK-1 for degradation by the proteasome thereby inhibiting the MAP kinase cascade. While
rpm-1 is the only known functional regulator of the
dlk-1 MAP kinase cascade, in vitro biochemical studies have shown that the mammalian homologs of MKK-4 and PMK-3 can be inactivated by the Protein Phosphatase 2C (PP2C) alpha isoform. This observation suggests that PP2C phosphatases may function in neuronal development by negatively regulating the
dlk-1 MAP kinase cascade. This model is consistent with the observation that
dlk-1 overexpression leads to stronger phenotypes than
rpm-1 loss of function (lf) mutations (3) suggesting multiple negative regulatory mechanisms may keep this kinase cascade in check. We have obtained a loss of function allele,
ok578, for
tag-93 the C. elegans homolog of mammalian PP2Calpha and beta. The mec neurons of
tag-93 (lf) animals have similar defects in axon termination and synaptic branch formation as mutants for molecules that function downstream of
rpm-1, including
fsn-1,
glo-4, and
glo-1 (4). Importantly,
tag-93 (lf) phenotypes in mec neurons are significantly enhanced by loss of function in
fsn-1, a molecule that mediates RPM-1's ubiquitin ligase activity on DLK-1 (5). This observation is consistent with
tag-93 and
fsn-1 functioning in parallel to regulate the
dlk-1 MAP kinase cascade by acting at different points in the pathway. Synapse formation in the GABAergic motor neurons was analyzed using cell specific expression of SNB-1::GFP to mark presynaptic terminals. While
tag-93 (lf) mutants have no detectable defects in SNB-1::GFP patterns, loss of function in
tag-93 strongly enhances
fsn-1 (lf) leading to similar severity and penetrance of defects to those observed in
rpm-1 (lf) mutants. We are presently in the process of determining if
tag-93 functions cell autonomously in neurons, at what point
tag-93 may act on the
dlk-1 MAP kinase cascade, and if the phosphatase activity of
tag-93 is required for its function in neurodevelopment. 1. Zhen, et al. 2000 2. Schaefer et al. 2000 3. Nakata, et al. 2005 4. Grill, et al. 2007 5. Liao, et al. 2004