hnRNPs Interacting with mRNA Localization Motifs Define Axonal RNA Regulons.

mRNA translation in axons enables neurons to introduce new proteins at sites distant from their cell body. mRNA-protein interactions drive this post-transcriptional regulation, yet knowledge of RNA binding proteins (RBP) in axons is limited. Here we used proteomics to identify RBPs interacting with the axonal localizing motifs of Nrn1, Hmgb1, Actb, and Gap43 mRNAs, revealing many novel RBPs in axons. Interestingly, no RBP is shared between all four RNA motifs, suggesting graded and overlapping specificities of RBP-mRNA pairings. A systematic assessment of axonal mRNAs interacting with hnRNP H1, hnRNP F, and hnRNP K, proteins that bound with high specificity to Nrn1 and Hmgb1, revealed that axonal mRNAs segregate into axon growth-associated RNA regulons based on hnRNP interactions. Axotomy increases axonal transport of hnRNPs H1, F, and K, depletion of these hnRNPs decreases axon growth and reduces axonal mRNA levels and axonal protein synthesis. Thus, subcellular hnRNP-interacting RNA regulons support neuronal growth and regeneration.

Anthrax Toxin Protective Antigen Variants That Selectively Utilize either the CMG2 or TEM8 Receptors for Cellular Uptake and Tumor Targeting.

The protective antigen (PA) moiety of anthrax toxin binds to cellular receptors and mediates the translocation of the two enzymatic moieties of the toxin to the cytosol. Two PA receptors are known, with capillary morphogenesis protein 2 (CMG2) being the more important for pathogenesis and tumor endothelial marker 8 (TEM8) playing a minor role. The C-terminal PA domain 4 (PAD4) has extensive interactions with the receptors and is required for binding. Our previous study identified PAD4 variants having enhanced TEM8 binding specificity. To obtain PA variants that selectively bind to CMG2, here we performed phage display selections using magnetic beads having bound CMG2. We found that PA residue isoleucine 656 plays a critical role in PA binding to TEM8 but has a much lesser effect on PA binding to CMG2. We further characterized the role of residue 656 in distinguishing PA binding to CMG2 versus TEM8 by substituting it with the other 19 amino acids. Of the resulting variants, PA I656Q and PA I656V had significantly reduced activity on TEM8-expressing CHO cells but maintained their activity on CMG2-expressing CHO cells. The preference of these PA mutants for CMG2 over TEM8 was further demonstrated using mouse embryonic fibroblast cells and mice deficient in the CMG2 and/or the TEM8 receptors. The structural basis of the alterations in the receptor binding activities of these mutants is also discussed.

Diphthamide modification on eukaryotic elongation factor 2 is needed to assure fidelity of mRNA translation and mouse development.

To study the role of the diphthamide modification on eukaryotic elongation factor 2 (eEF2), we generated an eEF2 Gly(717)Arg mutant mouse, in which the first step of diphthamide biosynthesis is prevented. Interestingly, the Gly(717)-to-Arg mutation partially compensates the eEF2 functional loss resulting from diphthamide deficiency, possibly because the added +1 charge compensates for the loss of the +1 charge on diphthamide. Therefore, in contrast to mouse embryonic fibroblasts (MEFs) from OVCA1(-/-) mice, eEF2(G717R/G717R) MEFs retain full activity in polypeptide elongation and have normal growth rates. Furthermore, eEF2(G717R/G717R) mice showed milder phenotypes than OVCA1(-/-) mice (which are 100% embryonic lethal) and a small fraction survived to adulthood without obvious abnormalities. Moreover, eEF2(G717R/G717R)/OVCA1(-/-) double mutant mice displayed the milder phenotypes of the eEF2(G717R/G717R) mice, suggesting that the embryonic lethality of OVCA1(-/-) mice is due to diphthamide deficiency. We confirmed that the diphthamide modification is essential for eEF2 to prevent -1 frameshifting during translation and show that the Gly(717)-to-Arg mutation cannot rescue this defect.

Capillary morphogenesis protein-2 is required for mouse parturition by maintaining uterine collagen homeostasis.

Capillary morphogenesis protein-2 (CMG2) functions as an anthrax toxin receptor that plays an essential role in anthrax pathogenesis. Although mutations in CMG2 have been identified to cause two human autosomal recessive disorders, Juvenile Hyaline Fibromatosis and Infantile Systemic Hyalinosis, both characterized by excess hyaline material deposition in connective tissues, the physiologic function of CMG2 remains elusive. To study the roles of CMG2 in normal physiology, here we performed detailed histological analyses of the CMG2-null mice we generated previously. While no morphological or histological defects were observed in CMG2(-/-) male mice, CMG2(-/-) female mice were unable to produce any offspring due to a defect in parturition. We found that deletion of CMG2 resulted in a diffuse deposition of collagen within the myometrium of CMG2(-/-) females, causing remarkable morphological changes to their uteri. This collagen accumulation also led to loss of smooth muscle cells in the myometrium of CMG2(-/-) mice, apparently disabling uterine contractile function during parturition. As a consequence, even though pregnant CMG2(-/-) mice were able to carry the gestation to full term, they were unable to deliver pups. However, the fully-developed fetuses could be successfully delivered by Cesarean section and survived to adulthood when fostered. Our results demonstrate that CMG2 is not required for normal mouse embryonic development but is indispensable for murine parturition. In parallel to its role in anthrax toxin binding and internalization, herein we provide evidence that CMG2 may function as a collagen receptor which is essential for maintaining collagen homeostasis in the uterus.

Susceptibility to anthrax lethal toxin-induced rat death is controlled by a single chromosome 10 locus that includes rNlrp1.

Anthrax lethal toxin (LT) is a bipartite protease-containing toxin and a key virulence determinant of Bacillus anthracis. In mice, LT causes the rapid lysis of macrophages isolated from certain inbred strains, but the correlation between murine macrophage sensitivity and mouse strain susceptibility to toxin challenge is poor. In rats, LT induces a rapid death in as little as 37 minutes through unknown mechanisms. We used a recombinant inbred (RI) rat panel of 19 strains generated from LT-sensitive and LT-resistant progenitors to map LT sensitivity in rats to a locus on chromosome 10 that includes the inflammasome NOD-like receptor (NLR) sensor, Nlrp1. This gene is the closest rat homolog of mouse Nlrp1b, which was previously shown to control murine macrophage sensitivity to LT. An absolute correlation between in vitro macrophage sensitivity to LT-induced lysis and animal susceptibility to the toxin was found for the 19 RI strains and 12 additional rat strains. Sequencing Nlrp1 from these strains identified five polymorphic alleles. Polymorphisms within the N-terminal 100 amino acids of the Nlrp1 protein were perfectly correlated with LT sensitivity. These data suggest that toxin-mediated lethality in rats as well as macrophage sensitivity in this animal model are controlled by a single locus on chromosome 10 that is likely to be the inflammasome NLR sensor, Nlrp1.

Capillary morphogenesis protein-2 is the major receptor mediating lethality of anthrax toxin in vivo.

Anthrax toxin, a major virulence factor of Bacillus anthracis, gains entry into target cells by binding to either of 2 von Willebrand factor A domain-containing proteins, tumor endothelium marker-8 (TEM8) and capillary morphogenesis protein-2 (CMG2). The wide tissue expression of TEM8 and CMG2 suggest that both receptors could play a role in anthrax pathogenesis. To explore the roles of TEM8 and CMG2 in normal physiology, as well as in anthrax pathogenesis, we generated TEM8- and CMG2-null mice and TEM8/CMG2 double-null mice by deleting TEM8 and CMG2 transmembrane domains. TEM8 and CMG2 were found to be dispensable for mouse development and life, but both are essential in female reproduction in mice. We found that the lethality of anthrax toxin for mice is mostly mediated by CMG2 and that TEM8 plays only a minor role. This is likely because anthrax toxin has approximately 11-fold higher affinity for CMG2 than for TEM8. Finally, the CMG2-null mice are also shown to be highly resistant to B. anthracis spore infection, attesting to the importance of both anthrax toxin and CMG2 in anthrax infections.

MyD88-dependent signaling protects against anthrax lethal toxin-induced impairment of intestinal barrier function.

MyD88-deficient mice were previously shown to have increased susceptibility to Bacillus anthracis infection relative to wild-type animals. To determine the mechanism by which MyD88 protects against B. anthracis infection, knockout mice were challenged with nonencapsulated, toxigenic B. anthracis or with anthrax toxins. MyD88-deficient mice had increased susceptibility to B. anthracis and anthrax lethal toxin but not to edema toxin. Lethal toxin alone induced marked multifocal intestinal ulcers in the knockout animals, compromising the intestinal epithelial barrier. The resulting enteric bacterial leakage in the knockout animals led to peritonitis and septicemia. Focal ulcers and erosion were also found in MyD88-heterozygous control mice but with far lower incidence and severity. B. anthracis infection also induced a similar enteric bacterial septicemia in MyD88-deficient mice but not in heterozygous controls. We show that lethal toxin and B. anthracis challenge induce bacteremia as a result of intestinal damage in MyD88-deficient mice. These results suggest that loss of the intestinal epithelial barrier and enteric bacterial septicemia may contribute to sensitizing MyD88-deficient mice to B. anthracis and that MyD88 plays a protective role against lethal toxin-induced impairment of intestinal barrier.

Deacetylation of Miro1 by HDAC6 blocks mitochondrial transport and mediates axon growth inhibition.

Inhibition of histone deacetylase 6 (HDAC6) was shown to support axon growth on the nonpermissive substrates myelin-associated glycoprotein (MAG) and chondroitin sulfate proteoglycans (CSPGs). Though HDAC6 deacetylates α-tubulin, we find that another HDAC6 substrate contributes to this axon growth failure. HDAC6 is known to impact transport of mitochondria, and we show that mitochondria accumulate in distal axons after HDAC6 inhibition. Miro and Milton proteins link mitochondria to motor proteins for axon transport. Exposing neurons to MAG and CSPGs decreases acetylation of Miro1 on Lysine 105 (K105) and decreases axonal mitochondrial transport. HDAC6 inhibition increases acetylated Miro1 in axons, and acetyl-mimetic Miro1 K105Q prevents CSPG-dependent decreases in mitochondrial transport and axon growth. MAG- and CSPG-dependent deacetylation of Miro1 requires RhoA/ROCK activation and downstream intracellular Ca2+ increase, and Miro1 K105Q prevents the decrease in axonal mitochondria seen with activated RhoA and elevated Ca2+ These data point to HDAC6-dependent deacetylation of Miro1 as a mediator of axon growth inhibition through decreased mitochondrial transport.

Axonal G3BP1 stress granule protein limits axonal mRNA translation and nerve regeneration.

Critical functions of intra-axonally synthesized proteins are thought to depend on regulated recruitment of mRNA from storage depots in axons. Here we show that axotomy of mammalian neurons induces translation of stored axonal mRNAs via regulation of the stress granule protein G3BP1, to support regeneration of peripheral nerves. G3BP1 aggregates within peripheral nerve axons in stress granule-like structures that decrease during regeneration, with a commensurate increase in phosphorylated G3BP1. Colocalization of G3BP1 with axonal mRNAs is also correlated with the growth state of the neuron. Disrupting G3BP functions by overexpressing a dominant-negative protein activates intra-axonal mRNA translation, increases axon growth in cultured neurons, disassembles axonal stress granule-like structures, and accelerates rat nerve regeneration in vivo.

Anthrax toxin targeting of myeloid cells through the CMG2 receptor is essential for establishment of Bacillus anthracis infections in mice.

Bacillus anthracis kills through a combination of bacterial infection and toxemia. Anthrax toxin working via the CMG2 receptor mediates lethality late in infection, but its roles early in infection remain unclear. We generated myeloid-lineage specific CMG2-deficient mice to examine the roles of macrophages, neutrophils, and other myeloid cells in anthrax pathogenesis. Macrophages and neutrophils isolated from these mice were resistant to anthrax toxin. However, the myeloid-specific CMG2-deficient mice remained fully sensitive to both anthrax lethal and edema toxins, demonstrating that targeting of myeloid cells is not responsible for anthrax toxin-induced lethality. Surprisingly, the myeloid-specific CMG2-deficient mice were completely resistant to B. anthracis infection. Neutrophil depletion experiments suggest that B. anthracis relies on anthrax toxin secretion to evade the scavenging functions of neutrophils to successfully establish infection. This work demonstrates that anthrax toxin uptake through CMG2 and the resulting impairment of myeloid cells are essential to anthrax infection.