The C. elegans protein HAM-1 is asymmetrically distributed in several cells in the dividing embryo and participates in the asymmetric divisions of at least five neuroblasts. In particular, we study the role of HAM-1 in the division of the HSN/PHB neuroblast. Antibody staining analyses show that HAM-1 is asymmetrically distributed in this neuroblast and inherited by its posterior daughter, the HSN/PHB precursor. Loss of
ham-1 function can cause the daughter cell that normally does not inherit HAM-1 to transform into an extra HSN/PHB precursor. This transformation results in duplications of the HSN and PHB neurons. Our current model is that HAM-1 acts as a tether for cell fate determinants and restricts the localization of these determinants to the HSN/PHB precursor. With the overall goal of identifying other players in this process, we conducted three genetic screens for mutants with Ham-1 phenotypes. Each screen was done in a gmIs12 background, a strain that expresses GFP in the PHB phasmid neuron, one of the cells duplicated in
ham-1 mutants. For each screen, we simply looked for mutants with abnormal numbers of GFP-expressing phasmid neurons. We isolated two main classes of mutants. The first class has extra phasmid neurons; we found 16 of these mutants, at least nine of which are in genes previously characterized:
ham-1 (
gm214 ,
gm267 , and
gm279 ),
ced-3 (
gm265 and
gm266 ),
egl-5 (
gm224 and
gm316 ),
lin-32 (
gm239 ), and
egl-27 (
gm314 ). Three more mutants in this class are allelic:
gm280 ,
gm300 , and
gm301 . The second class of mutants has fewer than normal phasmid neurons; we found 16 of these mutants. This class of mutants is interesting because this is the phenotype one would expect for the loss of a determinant that specifies HSN/PHB precursor fate. We were intrigued to find that
egl-27 mutants have extra phasmid neurons. EGL-27 is a homolog of MTA1, a member of the NURD complex (nucleosome remodeling and histone deacetylase).
egl-27 ; gmIs12 worms have extra phasmid neurons infrequently, but
egl-27 ; gmIs12 ;
ham-1 (
n1811 or
gm267 ) animals have a significantly enhanced PHB duplication defect.
egl-27 has previously been implicated as a possible transducer of Wnt signaling, a process known to affect cell polarity, and experiments are underway to explore what roles EGL-27 and other members of the NURD complex might be playing in the HSN/PHB lineage in conjunction with HAM-1. Another mutant we have been characterizing that affects this process is
gm34 .
gm34 mutants are frequently missing HSNs and PHBs, and this phenotype is epistatic to
ham-1 ,
ced-3 , and
lin-32 mutations. We cloned the defective gene by conventional mapping and injection rescue; its sequence (C18A3.8) predicts that it is a basic helix-loop-helix (bHLH) transcription factor. LIN-32 is another bHLH that plays a role in the HSN/PHB lineage, and since bHLH proteins are known to regulate transcription of target genes as dimers, we are currently exploring the possibility that this new bHLH and LIN-32 interact to determine the fates of cells in the HSN/PHB neuroblast lineage.