ADP-ribosylation of arginine.

Arginine adenosine-5'-diphosphoribosylation (ADP-ribosylation) is an enzyme-catalyzed, potentially reversible posttranslational modification, in which the ADP-ribose moiety is transferred from NAD(+) to the guanidino moiety of arginine. At 540 Da, ADP-ribose has the size of approximately five amino acid residues. In contrast to arginine, which, at neutral pH, is positively charged, ADP-ribose carries two negatively charged phosphate moieties. Arginine ADP-ribosylation, thus, causes a notable change in size and chemical property at the ADP-ribosylation site of the target protein. Often, this causes steric interference of the interaction of the target protein with binding partners, e.g. toxin-catalyzed ADP-ribosylation of actin at R177 sterically blocks actin polymerization. In case of the nucleotide-gated P2X7 ion channel, ADP-ribosylation at R125 in the vicinity of the ligand-binding site causes channel gating. Arginine-specific ADP-ribosyltransferases (ARTs) carry a characteristic R-S-EXE motif that distinguishes these enzymes from structurally related enzymes which catalyze ADP-ribosylation of other amino acid side chains, DNA, or small molecules. Arginine-specific ADP-ribosylation can be inhibited by small molecule arginine analogues such as agmatine or meta-iodobenzylguanidine (MIBG), which themselves can serve as targets for arginine-specific ARTs. ADP-ribosylarginine specific hydrolases (ARHs) can restore target protein function by hydrolytic removal of the entire ADP-ribose moiety. In some cases, ADP-ribosylarginine is processed into secondary posttranslational modifications, e.g. phosphoribosylarginine or ornithine. This review summarizes current knowledge on arginine-specific ADP-ribosylation, focussing on the methods available for its detection, its biological consequences, and the enzymes responsible for this modification and its reversal, and discusses future perspectives for research in this field.

Single domain antibodies: promising experimental and therapeutic tools in infection and immunity.

Antibodies are important tools for experimental research and medical applications. Most antibodies are composed of two heavy and two light chains. Both chains contribute to the antigen-binding site which is usually flat or concave. In addition to these conventional antibodies, llamas, other camelids, and sharks also produce antibodies composed only of heavy chains. The antigen-binding site of these unusual heavy chain antibodies (hcAbs) is formed only by a single domain, designated VHH in camelid hcAbs and VNAR in shark hcAbs. VHH and VNAR are easily produced as recombinant proteins, designated single domain antibodies (sdAbs) or nanobodies. The CDR3 region of these sdAbs possesses the extraordinary capacity to form long fingerlike extensions that can extend into cavities on antigens, e.g., the active site crevice of enzymes. Other advantageous features of nanobodies include their small size, high solubility, thermal stability, refolding capacity, and good tissue penetration in vivo. Here we review the results of several recent proof-of-principle studies that open the exciting perspective of using sdAbs for modulating immune functions and for targeting toxins and microbes.

Limited impact of homeostatic plasticity on motor learning in humans.

Neuroplasticity is the adaptive modification of network connectivity in response to environmental demands and has been identified as a major physiological correlate of learning. Since unrestricted neuroplastic modifications of network connectivity will result in a de-stabilization of the system, metaplastic modification rules have been proposed for keeping plastic connectivity changes within a useful dynamic range. In this connection, the modification threshold to achieve synaptic strengthening is thought to correlate negatively with the history of activity of the respective neurons, i.e. high previous activity enhances the threshold for synaptic strengthening and vice versa. However, the relevance of metaplasticity for actual learning processes has not been tested so far. We reduced or enhanced motor cortex excitability before performance of the serial reaction time task (SRTT), a sequential motor learning paradigm, and a reaction time task (RTT) by transcranial direct current stimulation (tDCS). If homeostatic rules apply, excitability-diminishing cathodal tDCS should improve subsequent motor learning, especially if combined with the partial NMDA receptor-agonist d-cycloserine, which selectively enhances efficacy of active receptors, while excitability-enhancing anodal tDCS should reduce it. Only the results for anodal tDCS, when combined with d-cycloserine, were in accordance with the rules of homeostatic plasticity. We conclude that homeostatic plasticity, as tested here, has a limited influence on implicit sequential motor learning.

Nanobodies effectively modulate the enzymatic activity of CD38 and allow specific imaging of CD38+ tumors in mouse models in vivo.

The cell surface ecto-enzyme CD38 is a promising target antigen for the treatment of hematological malignancies, as illustrated by the recent approval of daratumumab for the treatment of multiple myeloma. Our aim was to evaluate the potential of CD38-specific nanobodies as novel diagnostics for hematological malignancies. We successfully identified 22 CD38-specific nanobody families using phage display technology from immunized llamas. Crossblockade analyses and in-tandem epitope binning revealed that the nanobodies recognize three different non-overlapping epitopes, with four nanobody families binding complementary to daratumumab. Three nanobody families inhibit the enzymatic activity of CD38 in vitro, while two others were found to act as enhancers. In vivo, fluorochrome-conjugated CD38 nanobodies efficiently reach CD38 expressing tumors in a rodent model within 2 hours after intravenous injection, thereby allowing for convenient same day in vivo tumor imaging. These nanobodies represent highly specific tools for modulating the enzymatic activity of CD38 and for diagnostic monitoring CD38-expressing tumors.

Immuno-targeting the multifunctional CD38 using nanobody.

CD38, as a cell surface antigen is highly expressed in several hematologic malignancies including multiple myeloma (MM) and has been proven to be a good target for immunotherapy of the disease. CD38 is also a signaling enzyme responsible for the metabolism of two novel calcium messenger molecules. To be able to target this multifunctional protein, we generated a series of nanobodies against CD38 with high affinities. Crystal structures of the complexes of CD38 with the nanobodies were solved, identifying three separate epitopes on the carboxyl domain. Chromobodies, engineered by tagging the nanobody with fluorescence proteins, provide fast, simple and versatile tools for quantifying CD38 expression. Results confirmed that CD38 was highly expressed in malignant MM cells compared with normal white blood cells. The immunotoxin constructed by splicing the nanobody with a bacterial toxin, PE38 shows highly selective cytotoxicity against patient-derived MM cells as well as the cell lines, with half maximal effective concentration reaching as low as 10(-11) molar. The effectiveness of the immunotoxin can be further increased by stimulating CD38 expression using retinoid acid. These results set the stage for the development of clinical therapeutics as well as diagnostic screening for myeloma.

Neuronal Na+/K+ ATPase is an autoantibody target in paraneoplastic neurologic syndrome.

OBJECTIVES: To identify an autoreactivity in a 66-year-old woman who presented with combined brainstem and cerebellar syndrome including vertical gaze palsy, severe progressive ataxia, and spastic tetraparesis, an acute deterioration of vision, dysarthria, and dysphagia with concurrent diagnosis of a colon adenocarcinoma. METHODS: Patient's serum and CSF underwent comprehensive autoantibody screening by indirect immunofluorescence assay and immunoblot. For autoantigen purification, a histo-immunoprecipitation technique was developed followed by mass spectrometrical analysis. Recombinant candidate antigens were expressed in HEK293 and used to verify the identification. RESULTS: Indirect immunofluorescence assay screening revealed strong immunoglobulin G reactivity with neural tissues in serum and CSF, but not with a panel of 28 recombinantly expressed established neural autoantigens. The hitherto unknown target antigen was identified as the neuronal Na(+)/K(+) ATPase. Epitope mapping and competitive inhibition experiments showed that the autoantibodies were directed against the membrane-spanning alpha 3 subunit (ATP1A3) of the enzyme but did not bind to extracellular epitopes. Immunohistochemical analysis revealed overexpression of this subunit in the patient's tumor. CONCLUSIONS: We describe a case of an anti-ATP1A3-associated neurologic disorder. Mutations in the gene encoding this neuronal surface protein have already been recognized as the cause of infantile alternating hemiplegia, rapid-onset dystonia parkinsonism, and CAPOS syndrome. Although the autoantibodies are unlikely to be pathogenic, they are likely to be rare biomarkers for the apparently paraneoplastic neurologic syndrome or for the tumor itself.

Selection of nanobodies that block the enzymatic and cytotoxic activities of the binary Clostridium difficile toxin CDT.

The spore-forming gut bacterium Clostridium difficile is the leading cause of antibiotic-associated diarrhea in hospitalized patients. The major virulence factors are two large glucosylating cytotoxins. Hypervirulent strains (e.g. ribotype 027) with higher morbidity and mortality additionally produce the binary CDT toxin (Clostridium difficile transferase) that ADP-ribosylates actin and induces microtubule-based cell protrusions. Nanobodies are robust single domain antibodies derived from camelid heavy chain antibodies. Here we report the generation of functional nanobodies against the enzymatic CDTa and the heptameric receptor binding subunit CDTb. The nanobodies were obtained from a variable-domain repertoire library isolated from llamas immunized with recombinant CDTa or CDTb. Five CDTa-specific nanobodies blocked CDTa-mediated ADP-ribosylation of actin. Three CDTa-specific and two CDTb-specific nanobodies neutralized the cytotoxicity of CDTa+b. These nanobodies hold promise as new tools for research, diagnosis and therapy of C. difficile associated disease.