ALFY-Controlled DVL3 Autophagy Regulates Wnt Signaling,Determining Human Brain Size

Primary microcephaly is a congenital neurodevelopmental disorder of reduced head circumference and brain volume, with fewer neurons in the cortex of the developing brain due to premature transition between symmetrical and asymmetrical cellular division of the neuronal stem cell layer during neurogenesis. We now show through linkage analysis and whole exome sequencing, that a dominant mutation in ALFY, encoding an autophagy scaffold protein, causes human primary microcephaly. We demonstrate the dominant effect of the mutation in drosophila: transgenic flies harboring the human mutant allele display small brain volume, recapitulating the disease phenotype. Moreover, eye-specific expression of human mutant ALFY causes rough eye phenotype. In molecular terms, we demonstrate that normally ALFY attenuates the canonical Wnt signaling pathway via autophagy-dependent removal specifically of aggregates of DVL3 and not of Dvl1 or Dvl2. Thus, autophagic attenuation of Wnt signaling through removal of Dvl3 aggregates by ALFY acts in determining human brain size.     

Intercellular transfer of oncogenic H-Ras at the immunological synapse

Immune cells establish dynamic adhesive cell-cell interactions at a specific contact region, termed the immunological synapse (IS). Intriguing features of the IS are the formation of regions of plasma membrane fusion and the intercellular exchange of membrane fragments between the conjugated cells. It is not known whether upon IS formation, intact intracellular proteins can transfer from target cells to lymphocytes to allow the transmission of signals across cell boundaries. Here we show by both FACS and confocal microscopy that human lymphocytes acquire from the cells they scan the inner-membrane protein H-Ras, a G-protein vital for common lymphocyte functions and a prominent participant in human cancer. The transfer was cell contact-dependent and occurred in the context of cell-conjugate formation. Moreover, the acquisition of oncogenic H-RasG12V by natural killer (NK) and T lymphocytes had important biological functions in the adopting lymphocytes: the transferred H-RasG12V induced ERK phosphorylation, increased interferon-gamma and tumor necrosis factor-alpha secretion, enhanced lymphocyte proliferation, and augmented NK-mediated target cell killing. Our findings reveal a novel mode of cell-to-cell communication-allowing lymphocytes to extend the confines of their own proteome-which may moreover play an important role in natural tumor immunity.     

Agonist-specific regulation of delta-opioid receptor trafficking by G protein-coupled receptor kinase and beta-arrestin

Opioid receptors mediate multiple biological functions through their interaction with endogenous opioid peptides as well as opioid alkaloids including morphine and etorphine. Previously we have reported that the ability of distinct opioid agonists to differentially regulate mu-opioid receptor (mu OR) responsiveness is related to their ability to promote G protein-coupled receptor kinase (GRK)-dependent phosphorylation of the receptor (1). In the present study, we further examined the role of GRK and beta-arrestin in agonist-specific regulation of the delta-opioid receptor (delta OR). While both etorphine and morphine effectively activate the delta OR, only etorphine triggers robust delta OR phosphorylation followed by plasma membrane translocation of beta-arrestin and receptor internalization. In contrast, morphine is unable to either elicit delta OR phosphorylation or stimulate beta-arrestin translocation, correlating with its inability to cause delta OR internalization. Unlike for the mu OR, overexpression of GRK2 results in neither the enhancement of delta OR sequestration nor the rescue of delta OR-mediated beta-arrestin translocation. Therefore, our findings not only point to the existence of marked differences in the ability of different opioid agonists to promote delta OR phosphorylation by GRK and binding to beta-arrestin, but also demonstrate differences in the regulation of two opioid receptor subtypes. These observations may have important implications for our understanding of the distinct ability of various opioids in inducing opioid tolerance and addiction.     

The N-terminal domain of the regulatory subunit is sufficient for complete activation of acetohydroxyacid synthase III from Escherichia coli

We have previously proposed a model for the fold of the N-terminal domain of the small, regulatory subunit (SSU) of acetohydroxyacid synthase isozyme III. The fold is an alpha-beta sandwich with betaalphabetabetaalphabeta topology, structurally homologous to the C-terminal regulatory domain of 3-phosphoglycerate dehydrogenase. We suggested that the N-terminal domains of a pair of SSUs interact in the holoenzyme to form two binding sites for the feedback inhibitor valine in the interface between them. The model was supported by mutational analysis and other evidence. We have now examined the role of the C-terminal portion of the SSU by construction of truncated polypeptides (lacking 35, 48, 80, 95, or 112 amino acid residues from the C terminus) and examining the properties of holoenzymes reconstituted using these constructs. The Delta35, Delta48, and Delta80 constructs all lead to essentially complete activation of the catalytic subunits. The Delta80 construct, corresponding to the putative N-terminal domain, has the highest level of affinity for the catalytic subunits and leads to a reconstituted enzyme with k(cat)/K(M) about twice that of the wild-type enzyme. On the other hand, none of these constructs binds valine or leads to a valine-sensitive enzyme on reconstitution. The enzyme reconstituted with the Delta80 construct does not bind valine, either. The N-terminal portion (about 80 amino acid residues) of the SSU is thus necessary and sufficient for recognition and activation of the catalytic subunits, but the C-terminal half of the SSU is required for valine binding and response. We suggest that the C-terminal region of the SSU contributes to monomer-monomer interactions, and provide additional experimental evidence for this suggestion.     

Role of the cytoplasmic tails of CXCR1 and CXCR2 in mediating leukocyte migration,activation, and regulation

IL-8 (or CXCL8) activates the receptors CXCR1 (IL-8RA) and CXCR2 (IL-8RB) to induce chemotaxis in leukocytes, but only CXCR1 mediates cytotoxic and cross-regulatory signals. This may be due to the rapid internalization of CXCR2. To investigate the roles of the intracellular domains in receptor regulation, wild-type, chimeric, phosphorylation-deficient, and cytoplasmic tail (C-tail) deletion mutants of both receptors were expressed in RBL-2H3 cells and studied for cellular activation, receptor phosphorylation, desensitization, and internalization. All but one chimeric receptor bound IL-8 and mediated signal transduction, chemotaxis, and exocytosis. Upon IL-8 activation, the chimeric receptors underwent receptor phosphorylation and desensitization. One was resistant to internalization, yet it mediated normal levels of beta-arrestin 2 (beta arr-2) translocation. The lack of internalization by this receptor may be due to its reduced association with beta arr-2 and the adaptor protein-2 beta. The C-tail-deleted and phosphorylation-deficient receptors were resistant to receptor phosphorylation, desensitization, arrestin translocation, and internalization. They also mediated greater phosphoinositide hydrolysis and exocytosis and sustained Ca(2+) mobilization, but diminished chemotaxis. These data indicate that phosphorylation of the C-tails of CXCR1 and CXCR2 are required for arrestin translocation and internalization, but are not sufficient to explain the rapid internalization of CXCR2 relative to CXCR1. The data also show that receptor internalization is not required for chemotaxis. The lack of receptor phosphorylation was correlated with greater signal transduction but diminished chemotaxis, indicating that second messenger production, not receptor internalization, negatively regulates chemotaxis.     

Acetohydroxyacid synthase from Mycobacterium avium and its inhibition by sulfonylureas and imidazolinones

Tuberculosis (TB) remains one of the world's leading causes of death from infectious disease. It is caused by infection with Mycobacterium tuberculosis or sometimes, particularly in immune-compromised patients, Mycobacterium avium. The aim of this study was to create a tool that could be used in the search for new anti-TB drugs that inhibit branched-chain amino acid (BCAA) biosynthesis, as these are essential amino acids that are not available to a mycobacterium during growth in an infected organism. To this end, we cloned, overexpressed, purified and characterised for the first time an acetohydroxyacid synthase (AHAS), a key enzyme in the pathway to the biosynthesis of the BCAAs, from the genus Mycobacterium. Nine commercial herbicides of the sulfonylurea and imidazolinone classes were tested for their influence on this enzyme. Four of the sulfonylureas were potent inhibitors of the enzyme. The relative potency of the different inhibitors towards the M. avium enzyme was unlike their potency towards other AHASs whose inhibitor profile has been reported, emphasising the advantage of using a mycobacterial enzyme as a tool in the search for new anti-TB drugs.