The unfolded protein response (UPR), caused by stress, matches the folding capacity of endoplasmic reticulum (ER) to the load of client proteins in the organelle. In yeast, processing of HAC1 mRNA by activated Ire1 leads to synthesis of the transcription factor Hac1 and activation of the UPR. The responses to activated IRE1 in metazoans are less well understood. Here we demonstrate that mutations in either ire-1 or the transcription-factor-encoding xbp-1 gene abolished the UPR in Caenorhabditis elegans. Mammalian XBP-1 is essential for immunoglobulin secretion and development of plasma cells, and high levels of XBP-1 messenger RNA are found in specialized secretory cells. Activation of the UPR causes IRE1-dependent splicing of a small intron from the XBP-1 mRNA both in C. elegans and mice. The protein encoded by the processed murine XBP-1 mRNA accumulated during the UPR, whereas the protein encoded by unprocessed mRNA did not. Purified mouse IRE1 accurately cleaved XBP-1 mRNA in vitro, indicating that XBP-1 mRNA is a direct target of IRE1 endonucleolytic activity. Our findings suggest that physiological ER load regulates a developmental decision in higher eukaryotes.

Protein folding in the mitochondria is assisted by nuclear-encoded compartment-specific chaperones but regulation of the expression of their encoding genes is poorly understood. We found that the mitochondrial matrix HSP70 and HSP60 chaperones, encoded by the Caenorhabditis elegans hsp-6 and hsp-60 genes, were selectively activated by perturbations that impair assembly of multi-subunit mitochondrial complexes or by RNAi of genes encoding mitochondrial chaperones or proteases, which lead to defective protein folding and processing in the organelle. hsp-6 and hsp-60 induction was specific to perturbed mitochondrial protein handling, as neither heat-shock nor endoplasmic reticulum stress nor manipulations that impair mitochondrial steps in intermediary metabolism or ATP synthesis activated the mitochondrial chaperone genes. These observations support the existence of a mitochondrial unfolded protein response that couples mitochondrial chaperone gene expression to changes in the protein handling environment in the organelle.

The let-60 gene, an essential ras gene of the nematode Caenorhabditis elegans, acts as a switch in the inductive signalling pathway that initiates vulva formation. Recessive let-60 mutations that cause a vulvaless phenotype prevent let-60 function in response to the inductive signal. These mutations are clustered and define regions necessary either for the activation or for the action of the let-60 ras protein. Dominant let-60 mutations that cause a multivulva phenotype alter codon 13 and activate let-60 in vivo, rendering it independent of the inductive signal. The let-60 gene acts within an extensively defined genetic pathway, and other genes within this pathway seem likely to encode molecules that regulate let-60 function as well as molecules that are targets of let-60 action.

The induction of the hermaphrodite vulva and the migration of the sex myoblasts in the nematode Caenorhabditis elegans are both controlled by intercellular signalling. The gonadal anchor cell induces formation of the vulva from nearby hypodermal cells, and a set of somatic gonadal cells attract the migrating sex myoblasts to their final positions. Many genes required for vulval induction have been identified, including the let-23 receptor tyrosine kinase gene and the let-60 ras gene. We report here the identification and characterization of a new gene, sem-5 (sem, sex muscle abnormal), that acts both in vulval induction and in sex myoblast migration. On the basis of its DNA sequence, sem-5 encodes a novel 228-amino-acid protein which consists almost entirely of one SH2 (SH, src homology region) and two SH3 domains. SH2 and SH3 domains are present in many signalling proteins regulated by receptor and non-receptor tyrosine kinases. Mutations that impair sem-5 activity alter residues that are highly conserved among different SH2 and SH3 domains. Our results indicate that the sem-5 gene encodes a novel protein that functions in at least two distinct cell-signalling processes.

While endocytosis can regulate morphogen distribution, its precise role in shaping these gradients is unclear. Even more enigmatic is the role of retromer, a complex that shuttles proteins between endosomes and the Golgi apparatus, in Wnt gradient formation. Here we report that DPY-23, the C. elegans mu subunit of the clathrin adaptor AP-2 that mediates the endocytosis of membrane proteins, regulates Wnt function. dpy-23 mutants display Wnt phenotypes, including defects in neuronal migration, neuronal polarity, and asymmetric cell division. DPY-23 acts in Wnt-expressing cells to promote these processes. MIG-14, the C. elegans homolog of the Wnt-secretion factor Wntless, also acts in these cells to control Wnt function. In dpy-23 mutants, MIG-14 accumulates at or near the plasma membrane. By contrast, MIG-14 accumulates in intracellular compartments in retromer mutants. Based on our observations, we propose that intracellular trafficking of MIG-14 by AP-2 and retromer plays an important role in Wnt secretion.

Secreted Wnt proteins influence neural connectivity by regulating axon guidance, dendritic morphogenesis and synapse formation. We report a new role for Wnt and Frizzled proteins in establishing the anteroposterior polarity of the mechanosensory neurons ALM and PLM in C. elegans. Disruption of Wnt signaling leads to a complete inversion of ALM and PLM polarity: the anterior process adopts the length, branching pattern and synaptic properties of the wild-type posterior process, and vice versa. Different but overlapping sets of Wnt proteins regulate neuronal polarity in different body regions. Wnts act directly on PLM via the Frizzled LIN-17. In addition, we show that they are needed for axon branching and anteriorly directed axon growth. We also find that the retromer, a conserved protein complex that mediates transcytosis and endosome-to-Golgi protein trafficking, plays a key role in Wnt signaling. Deletion mutations of retromer subunits cause ALM and PLM polarity, and other Wnt-related defects. We show that retromer protein VPS-35 is required in Wnt-expressing cells and propose that retromer activity is needed to generate a fully active Wnt signal.

A set of conserved molecules guides axons along the metazoan dorsal-ventral axis. Recently, Wnt glycoproteins have been shown to guide axons along the anterior-posterior (A/P) axis of the mammalian spinal cord. Here, we show that, in the nematode Caenorhabditis elegans, multiple Wnts and Frizzled receptors regulate the anterior migrations of neurons and growth cones. Three Wnts are expressed in the tail, and at least one of these, EGL-20, functions as a repellent. We show that the MIG-1 Frizzled receptor acts in the neurons and growth cones to promote their migrations and provide genetic evidence that the Frizzleds MIG-1 and MOM-5 mediate the repulsive effects of EGL-20. While these receptors mediate the effects of EGL-20, we find that the Frizzled receptor LIN-17 can antagonize MIG-1 signaling. Our results indicate that Wnts play a key role in A/P guidance in C. elegans and employ distinct mechanisms to regulate different migrations.

The Philadelphia chromosome is present in more than 95% of chronic myeloid leukemia patients and 13% of acute lymphocytic leukemia patients. The Philadelphia translocation, t(9;22), fuses the BCR and ABL genes resulting in the expression of leukemia-specific, chimeric BCR-ABL messenger RNAs. To facilitate diagnosis of these leukemias, we have developed a method of amplifying and detecting only the unique mRNA sequences, using an extension of the polymerase chain reaction technique. Diagnosis of chronic myeloid and acute lymphocytic leukemias by this procedure is rapid, much more sensitive than existing protocols, and independent of the presence or absence of an identifiable Philadelphia chromosome.