The Nuances of Hand Transplantation After Sepsis

Vascularized composite allotransplantation (VCA) of the upper extremity is an established restorative procedure for selected patients with acquired upper limb loss. The majority of upper limb VCAs performed worldwide have been for victims of various forms of trauma. However, in the developed world, amputation following severe sepsis seems to be an increasingly common indication for referral to hand transplant programs. Unlike trauma patients with isolated limb injuries, patients with amputations as a complication of sepsis have survived through a state of global tissue hypoperfusion and multisystem organ failure with severe, enduring effects on the entire body's physiology. This article reviews the unique considerations for VCA candidacy in postsepsis patients with upper limb amputation. These insights may also be relevant to postsepsis patients undergoing other forms of transplantation or to VCA patients requiring additional future solid organ transplants.

Sulfonamidoboronic Acids as "Cross-Class" Inhibitors of an Expanded-Spectrum Class C Cephalosporinase, ADC-33, and a Class D Carbapenemase, OXA-24/40: Strategic Compound Design to Combat Resistance in Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative organism listed as an urgent threat pathogen by the World Health Organization (WHO). Carbapenem-resistant A. baumannii (CRAB), especially, present therapeutic challenges due to complex mechanisms of resistance to β-lactams. One of the most important mechanisms is the production of β-lactamase enzymes capable of hydrolyzing β-lactam antibiotics. Co-expression of multiple classes of β-lactamases is present in CRAB; therefore, the design and synthesis of "cross-class" inhibitors is an important strategy to preserve the efficacy of currently available antibiotics. To identify new, nonclassical β-lactamase inhibitors, we previously identified a sulfonamidomethaneboronic acid CR167 active against Acinetobacter-derived class C β-lactamases (ADC-7). The compound demonstrated affinity for ADC-7 with a K_i = 160 nM and proved to be able to decrease MIC values of ceftazidime and cefotaxime in different bacterial strains. Herein, we describe the activity of CR167 against other β-lactamases in A. baumannii: the cefepime-hydrolysing class C extended-spectrum β-lactamase (ESAC) ADC-33 and the carbapenem-hydrolyzing OXA-24/40 (class D). These investigations demonstrate CR167 as a valuable cross-class (C and D) inhibitor, and the paper describes our attempts to further improve its activity. Five chiral analogues of CR167 were rationally designed and synthesized. The structures of OXA-24/40 and ADC-33 in complex with CR167 and select chiral analogues were obtained. The structure activity relationships (SARs) are highlighted, offering insights into the main determinants for cross-class C/D inhibitors and impetus for novel drug design.

Hand transplantation: can we balance the risks and benefits?

Asking 'can we balance the risks and benefits?' implies that a quantification of both risk and benefit in hand transplantation (here the terms hand transplant and hand transplantation refer to allotransplantation of the human hand or hand and part or all of the upper limb or limbs) is possible. Despite all we have learned in recent years about hand transplantation, much remains unknown. Even if reliable methods for quantification of risk and benefit were available, fundamental issues relating to effective communication across the gulf of lived experience between the (presumably) handed surgeon and the handless patient remain. Inherent complexities mean some consider hand transplantation an unsolved problem, but we believe the medical and technical considerations fall within the ambit of a competent multidisciplinary team, and that psychosocial and ethical challenges are open to management through robust frameworks for assessment and decision making, underpinned by an extended period of assessment and dialogue, with candid acknowledgement where uncertainty remains. This respects the patient's autonomy while addressing the need for a prolonged period of informing consent.Level of evidence: V.

Conformational flexibility in carbapenem hydrolysis drives substrate specificity of the class D carbapenemase OXA-24/40

The evolution of multidrug resistance in Acinetobacter spp. increases the risk of our best antibiotics losing their efficacy. From a clinical perspective, the carbapenem-hydrolyzing class D β-lactamase subfamily present in Acinetobacter spp. is particularly concerning because of its ability to confer resistance to carbapenems. The kinetic profiles of class D β-lactamases exhibit variability in carbapenem hydrolysis, suggesting functional differences. To better understand the structure–function relationship between the carbapenem-hydrolyzing class D β-lactamase OXA-24/40 found in Acinetobacter baumannii and carbapenem substrates, we analyzed steady-state kinetics with the carbapenem antibiotics meropenem and ertapenem and determined the structures of complexes of OXA-24/40 bound to imipenem, meropenem, doripenem, and ertapenem, as well as the expanded-spectrum cephalosporin cefotaxime, using X-ray crystallography. We show that OXA-24/40 exhibits a preference for ertapenem compared with meropenem, imipenem, and doripenem, with an increase in catalytic efficiency of up to fourfold. We suggest that superposition of the nine OXA-24/40 complexes will better inform future inhibitor design efforts by providing insight into the complicated and varying ways in which carbapenems are selected and bound by class D β-lactamases.

Arginine Modulates Carbapenem Deactivation by OXA-24/40 in Acinetobacter baumannii

The resistance of Gram-negative bacteria to β-lactam antibiotics stems mainly from β-lactamase proteins that hydrolytically deactivate the β-lactams. Of particular concern are the β-lactamases that can deactivate a class of β-lactams known as carbapenems. Carbapenems are among the few anti-infectives that can treat multi-drug resistant bacterial infections. Revealing the mechanisms of their deactivation by β-lactamases is a necessary step for preserving their therapeutic value. Here, we present NMR investigations of OXA-24/40, a carbapenem-hydrolyzing Class D β-lactamase (CHDL) expressed in the gram-negative pathogen, Acinetobacter baumannii. Using rapid data acquisition methods, we were able to study the "real-time" deactivation of the carbapenem known as doripenem by OXA-24/40. Our results indicate that OXA-24/40 has two deactivation mechanisms: canonical hydrolytic cleavage, and a distinct mechanism that produces a β-lactone product that has weak affinity for the OXA-24/40 active site. The mechanisms issue from distinct active site environments poised either for hydrolysis or β-lactone formation. Mutagenesis reveals that R261, a conserved active site arginine, stabilizes the active site environment enabling β-lactone formation. Our results have implications not only for OXA-24/40, but the larger family of CHDLs now challenging clinical settings on a global scale.

A comprehensive and contemporary "snapshot" of β-lactamases in carbapenem resistant Acinetobacter baumannii

Successful treatment of Acinetobacter baumannii infections require early and appropriate antimicrobial therapy. One of the first steps in this process is understanding which β-lactamase (bla) alleles are present and in what combinations. Thus, we performed WGS on 98 carbapenem-resistant A. baumannii (CR Ab). In most isolates, an acquired bla_OXA carbapenemase was found in addition to the intrinsic bla_OXA allele. The most commonly found allele was bla_OXA-23 (n = 78/98). In some isolates, bla_OXA-23 was found in addition to other carbapenemase alleles: bla_OXA-82 (n = 12/78), bla_OXA-72 (n = 2/78) and bla_OXA-24/40 (n = 1/78). Surprisingly, 20% of isolates carried carbapenemases not routinely assayed for by rapid molecular diagnostic platforms, i.e., bla_OXA-82 and bla_OXA-172; all had ISAba1 elements. In 8 CR Ab, bla_OXA-82 or bla_OXA-172 was the only carbapenemase. Both bla_OXA-24/40 and its variant bla_OXA-72 were each found in 6/98 isolates. The most prevalent ADC variants were bla_ADC-30 (21%), bla_ADC-162 (21%), and bla_ADC-212 (26%). Complete combinations are reported.

The Effect of MHC Antigen Matching Between Donors and Recipients on Skin Tolerance of Vascularized Composite Allografts

The emergence of skin-containing vascularized composite allografts (VCAs) has provided impetus to understand factors affecting rejection and tolerance of skin. VCA tolerance can be established in miniature swine across haploidentical MHC barriers using mixed chimerism. Because the deceased donor pool for VCAs does not permit MHC antigen matching, clinical VCAs are transplanted across varying MHC disparities. We investigated whether sharing of MHC class I or II antigens between donors and recipients influences VCA skin tolerance. Miniature swine were conditioned nonmyeloablatively and received hematopoietic stem cell transplants and VCAs across MHC class I (n = 3) or class II (n = 3) barriers. In vitro immune responsiveness was assessed, and VCA skin-resident leukocytes were characterized by flow cytometry. Stable mixed chimerism was established in all animals. MHC class II-mismatched chimeras were tolerant of VCAs. MHC class I-mismatched animals, however, rejected VCA skin, characterized by infiltration of recipient-type CD8⁺ lymphocytes. Systemic donor-specific nonresponsiveness was maintained, including after VCA rejection. This study shows that MHC antigen matching influences VCA skin rejection and suggests that local regulation of immune tolerance is critical in long-term acceptance of all VCA components. These results help elucidate novel mechanisms underlying skin tolerance and identify clinically relevant VCA tolerance strategies.

Creation of a Bioengineered Skin Flap Scaffold with a Perfusable Vascular Pedicle

Full-thickness skin loss is a challenging problem due to limited reconstructive options, demanding 75 million surgical procedures annually in the United States. Autologous skin grafting is the gold standard treatment, but results in donor-site morbidity and poor aesthetics. Numerous skin substitutes are available on the market to date, however, none truly functions as full-thickness skin due to lack of a vascular network. The creation of an autologous full-thickness skin analogue with a vascular pedicle would result in a paradigm shift in the management of wounds and in reconstruction of full-thickness skin defects. To create a clinically relevant foundation, we generated an acellular skin flap scaffold (SFS) with a perfusable vascular pedicle of clinically relevant size by perfusion decellularization of porcine fasciocutaneous flaps. We then analyzed the yielded SFS for mechanical properties, biocompatibility, and regenerative potential in vitro and in vivo. Furthermore, we assessed the immunological response using an in vivo model. Finally, we recellularized the vascular compartment of an SFS and reconnected it to a recipient's blood supply to test for perfusability. Perfusion decellularization removed all cellular components with preservation of native extracellular matrix composition and architecture. Biaxial testing revealed preserved mechanical properties. Immunologic response and biocompatibility assessed via implantation and compared with native xenogenic skin and commercially available dermal substitutes revealed rapid neovascularization and complete tissue integration. Composition of infiltrating immune cells showed no evidence of allorejection and resembled the inflammatory phase of wound healing. Implantation into full-thickness skin defects demonstrated good tissue integration and skin regeneration without cicatrization. We have developed a protocol for the generation of an SFS of clinically relevant size, containing a vascular pedicle, which can be utilized for perfusion decellularization and, ultimately, anastomosis to the recipient vascular system after precellularization. The observed favorable immunological response and good tissue integration indicate the substantial regenerative potential of this platform.

Skin grafts from genetically modified α-1,3-galactosyltransferase knockout miniature swine: A functional equivalent to allografts

Burn is associated with a considerable burden of morbidity worldwide. Early excision of burned tissue and skin grafting of the resultant wound has been established as a mainstay of modern burn therapy. However, in large burns, donor sites for autologous skin may be limited. Numerous alternatives, from cadaver skin to synthetic substitutes have been described, each with varying benefits and limitations. We previously proposed the use of genetically modified (alpha-1,3-galactosyl transferase knockout, GalT-KO) porcine skin as a viable skin alternative. In contrast to wild type porcine skin, which has been used as a biologic dressing following glutaraldehyde fixation, GalT-KO porcine skin is a viable graft, which is not susceptible to loss by hyperacute rejection, and undergoes graft take and healing, prior to eventual rejection, comparable to cadaver allogeneic skin. In the current study we aimed to perform a detailed functional analysis of GalT-KO skin grafts in comparison to allogeneic grafts for temporary closure of full thickness wounds using our baboon dorsum wound model. Grafts were assessed by measurement of fluid loss, wound infection rate, and take, and healed appearance, of secondary autologous grafts following xenograft rejection. Comparison was also made between fresh and cryopreserved grafts. No statistically significant difference was identified between GalT-KO and allogeneic skin grafts in any of the assessed parameters, and graft take and function was not adversely effected by the freeze-thaw process. These data demonstrate that GalT-KO porcine grafts are functionally comparable to allogeneic skin grafts for temporary closure of full thickness wounds, and support their consideration as an alternative to cadaver allogeneic skin in the emergency management of large burns.

A Distal Disulfide Bridge in OXA-1 β-Lactamase Stabilizes the Catalytic Center and Alters the Dynamics of the Specificity Determining Ω Loop

Widespread antibiotic resistance, particularly when mediated by broad-spectrum β-lactamases, has major implications for public health. Substitutions in the active site often allow broad-spectrum enzymes to accommodate diverse types of β-lactams. Substitutions observed outside the active site are thought to compensate for the loss of thermal stability. The OXA-1 clade of class D β-lactamases contains a pair of conserved cysteines located outside the active site that forms a disulfide bond in the periplasm. Here, the effect of the distal disulfide bond on the structure and dynamics of OXA-1 was investigated via 4 μs molecular dynamics simulations. The results reveal that the disulfide promotes the preorganized orientation of the catalytic residues and affects the conformation of the functionally important Ω loop. Furthermore, principal component analysis reveals differences in the global dynamics between the oxidized and reduced forms, especially in the motions involving the Ω loop. A dynamical network analysis indicates that, in the oxidized form, in addition to its role in ligand binding, the KTG family motif is a central hub of the global dynamics. As activity of OXA-1 has been measured only in the reduced form, we suggest that accurate assessment of its functional profile would require oxidative conditions mimicking periplasm.

The structure of a doripenem-bound OXA-51 class D β-lactamase variant with enhanced carbapenemase activity

OXA-51 is a class D β-lactamase that is thought to be the native carbapenemase of Acinetobacter baumannii. Many variants of OXA-51 containing active site substitutions have been identified from A. baumannii isolates, and some of these substitutions increase hydrolytic activity toward carbapenem antibiotics. We have determined the high-resolution structures of apo OXA-51 and OXA-51 with one such substitution (I129L) with the carbapenem doripenem trapped in the active site as an acyl-intermediate. The structure shows that acyl-doripenem adopts an orientation very similar to carbapenem ligands observed in the active site of OXA-24/40 (doripenem) and OXA-23 (meropenem). In the OXA-51 variant/doripenem complex, the indole ring of W222 is oriented away from the doripenem binding site, thereby eliminating a clash that is predicted to occur in wildtype OXA-51. Similarly, in the OXA-51 variant complex, L129 adopts a different rotamer compared to I129 in wildtype OXA-51. This alternative position moves its side chain away from the hydroxyethyl moiety of doripenem and relieves another potential clash between the enzyme and carbapenem substrates. Molecular dynamics simulations of OXA-51 and OXA-51 I129L demonstrate that compared to isoleucine, a leucine at this position greatly favors a rotamer that accommodates the ligand. These results provide a molecular justification for how this substitution generates enhanced binding affinity for carbapenems, and therefore helps explain the prevalence of this substitution in clinical OXA-51 variants.

Effects of Joint Exposures to Selected Peroxisome Proliferators on Hepatic Acyl-CoA Oxidase Activity in Male B6C3F1 Mice

The interaction potential of peroxisome proliferators of similar and dissimilar structure was examined in B6C3F1 mice. Mice were fed diets containing varying concentrations of ciprofibrate (Cipro), clofibrate (Clof) or di(2-ethylhexyl)phthalate (DEHP), or combinations of Cipro and Clof or Cipro and DEHP for 4 d. Induction of peroxisomal beta-oxidation, measured by increased acyl-CoA oxidase activity, was used as the endpoint for analysis. An additive response occurred following joint exposure to the structurally related compounds Cipro and Clof, whereas a possible synergistic response occurred at low dose combinations of the structurally dissimilar Cipro and DEHP. These findings represent the first report assessing the in-vivo interaction potential of structurally similar and dissimilar peroxisome proliferators and provides insight into the dose-response nature of joint exposures to certain non-genotoxic carcinogens.

Kinetic characterization of GES-22 β-lactamase harboring the M169L clinical mutation

The class A β-lactamase GES-22 has been identified in Acinetobacter baumannii isolates in Turkey, and subsequently shown to differ from GES-11 by a single substitution (M169L). Because M169 is part of the omega loop, a structure that is known to have major effects on substrate selectivity in class A β-lactamases, we expressed, purified and kinetically characterized this novel variant. Our results show that compared with GES-11^{6 × His}, GES-22^{6 × His} displays more efficient hydrolysis of penicillins, and aztreonam, but a loss of efficiency against ceftazidime. In addition, the M169L substitution confers on GES-22 more efficient hydrolysis of the mechanistic inhibitors clavulanic acid and sulbactam. These effects are highly similar to other mutations at the homologous position in other class A β-lactamases, suggesting that this methionine has a key structural role in aligning active site residues and in substrate selectivity across the class.

Engineered composite tissue as a bioartificial limb graft

The loss of an extremity is a disastrous injury with tremendous impact on a patient's life. Current mechanical prostheses are technically highly sophisticated, but only partially replace physiologic function and aesthetic appearance. As a biologic alternative, approximately 70 patients have undergone allogeneic hand transplantation to date worldwide. While outcomes are favorable, risks and side effects of transplantation and long-term immunosuppression pose a significant ethical dilemma. An autologous, bio-artificial graft based on native extracellular matrix and patient derived cells could be produced on demand and would not require immunosuppression after transplantation. To create such a graft, we decellularized rat and primate forearms by detergent perfusion and yielded acellular scaffolds with preserved composite architecture. We then repopulated muscle and vasculature with cells of appropriate phenotypes, and matured the composite tissue in a perfusion bioreactor under electrical stimulation in vitro. After confirmation of composite tissue formation, we transplanted the resulting bio-composite grafts to confirm perfusion in vivo.

Validation of multisociety combined task force definitions of abnormal disk morphology

BACKGROUND AND PURPOSE

The multisociety task force descriptively defined abnormal lumbar disk morphology. We aimed to use their definitions to provide a higher level of evidence for the validation of MR imaging in the evaluation of this pathology in patients who have undergone diskectomy by retrospectively classifying their preoperative MRI.

MATERIALS AND METHODS

This retrospective, institutional review board-approved study included 54 of 86 consecutive patients (47 men; average age, 44 years) enrolled in an ongoing prospective trial of surgically treated lumbar disk herniation who had preoperative MRI and documented intraoperative classification of the abnormal disk as protrusion, extrusion, or sequestration by the treating surgeon. Preoperative MRI was classified by 2 blinded radiologists; discrepancies were resolved by a third reader. Statistical analysis of interobserver agreement and imaging compared with surgical findings was performed.

RESULTS

The readers disagreed on only 1 of the 54 cases. The third reader resolved the disagreement. Eight protrusions and 46 extrusions were found on imaging, with no sequestrations. At surgery, there were 13 protrusions and 40 extrusions, with 2 of the extrusions also containing sequestrations; the remaining case had only sequestration. There were 16 discrepancies between imaging and surgery, resulting in 70% agreement.

CONCLUSIONS

This study, which was intended to validate the multisociety combined task force definitions of abnormal disk morphology by using MR imaging with a surgical criterion standard, found 70% agreement between imaging diagnosis and surgical findings. Although reasonable, this finding highlights differences that often exist between intraoperative and preoperative imaging findings of lumbar disk herniation.

Structural basis of activity against aztreonam and extended spectrum cephalosporins for two carbapenem-hydrolyzing class D β-lactamases from Acinetobacter baumannii

The carbapenem-hydrolyzing class D β-lactamases OXA-23 and OXA-24/40 have emerged worldwide as causative agents for β-lactam antibiotic resistance in Acinetobacter species. Many variants of these enzymes have appeared clinically, including OXA-160 and OXA-225, which both contain a P → S substitution at homologous positions in the OXA-24/40 and OXA-23 backgrounds, respectively. We purified OXA-160 and OXA-225 and used steady-state kinetic analysis to compare the substrate profiles of these variants to their parental enzymes, OXA-24/40 and OXA-23. OXA-160 and OXA-225 possess greatly enhanced hydrolytic activities against aztreonam, ceftazidime, cefotaxime, and ceftriaxone when compared to OXA-24/40 and OXA-23. These enhanced activities are the result of much lower Km values, suggesting that the P → S substitution enhances the binding affinity of these drugs. We have determined the structures of the acylated forms of OXA-160 (with ceftazidime and aztreonam) and OXA-225 (ceftazidime). These structures show that the R1 oxyimino side-chain of these drugs occupies a space near the β5-β6 loop and the omega loop of the enzymes. The P → S substitution found in OXA-160 and OXA-225 results in a deviation of the β5-β6 loop, relieving the steric clash with the R1 side-chain carboxypropyl group of aztreonam and ceftazidime. These results reveal worrying trends in the enhancement of substrate spectrum of class D β-lactamases but may also provide a map for β-lactam improvement.

Transgenic expression of human CD47 markedly increases engraftment in a murine model of pig-to-human hematopoietic cell transplantation

Mixed chimerism approaches for induction of tolerance of solid organ transplants have been applied successfully in animal models and in the clinic. However, in xenogeneic models (pig-to-primate), host macrophages participate in the rapid clearance of porcine hematopoietic progenitor cells, hindering the ability to achieve mixed chimerism. CD47 is a cell-surface molecule that interacts in a species-specific manner with SIRPα receptors on macrophages to inhibit phagocytosis and expression of human CD47 (hCD47) on porcine cells has been shown to inhibit phagocytosis by primate macrophages. We report here the generation of hCD47 transgenic GalT-KO miniature swine that express hCD47 in all blood cell lineages. The effect of hCD47 expression on xenogeneic hematopoietic engraftment was tested in an in vivo mouse model of human hematopoietic cell engraftment. High-level porcine chimerism was observed in the bone marrow of hCD47 progenitor cell recipients and smaller but readily measurable chimerism levels were observed in the peripheral blood of these recipients. In contrast, transplantation of WT progenitor cells resulted in little or no bone marrow engraftment and no detectable peripheral chimerism. These results demonstrate a substantial protective effect of hCD47 expression on engraftment and persistence of porcine cells in this model, presumably by modulation of macrophage phagocytosis.

Genetically modified porcine split-thickness skin grafts as an alternative to allograft for provision of temporary wound coverage: preliminary characterization

Temporary coverage of severely burned patients with cadaver allograft skin represents an important component of burn care, but is limited by availability and cost. Porcine skin shares many physical properties with human skin, but is susceptible to hyperacute rejection due to preformed antibodies to α-1,3-galactose (Gal), a carbohydrate on all porcine cells. Our preliminary studies have suggested that skin grafts from α-1,3-galactosyltransferase knock out (GalT-KO) miniature swine might provide temporary wound coverage comparable to allografts, since GalT-KO swine lack this carbohydrate. To further evaluate this possibility, eight non-human primates received primary autologous, allogeneic, GalT-KO, and GalT+xenogeneic skin grafts. Additionally, secondary grafts were placed to assess whether sensitization would affect the rejection time course of identical-type grafts. We demonstrate that both GalT-KO xenografts and allografts provide temporary coverage of partial- and full-thickness wounds for up to 11 days. In contrast, GalT+xenografts displayed hyperacute rejection, with no signs of vascularization and rapid avulsion from wounds. Furthermore, secondary GalT-KO transplants failed to vascularize, demonstrating that primary graft rejection sensitizes the recipient. We conclude that GalT-KO xenografts may provide temporary coverage of wounds for a duration equivalent to allografts, and thus, could serve as a readily available alternative treatment of severe burns.

A fluorescent carbapenem for structure function studies of penicillin-binding proteins, β-lactamases, and β-lactam sensors

By reacting fluorescein isothiocyanate with meropenem, we have prepared a carbapenem-based fluorescent β-lactam. Fluorescein-meropenem binds both penicillin-binding proteins and β-lactam sensors and undergoes a typical acylation reaction in the active site of these proteins. The probe binds the class D carbapenemase OXA-24/40 with close to the same affinity as meropenem and undergoes a complete catalytic hydrolysis reaction. The visible light excitation and strong emission of fluorescein render this molecule a useful structure-function probe through its application in sodium dodecyl sulfate-polyacrylamide gel electrophoresis assays as well as solution-based kinetic anisotropy assays. Its classification as a carbapenem β-lactam and the position of its fluorescent modification render it a useful complement to other fluorescent β-lactams, most notably Bocillin FL. In this study, we show the utility of fluorescein-meropenem by using it to detect mutants of OXA-24/40 that arrest at the acyl-intermediate state with carbapenem substrates but maintain catalytic competency with penicillin substrates.

Class D β-lactamases: a reappraisal after five decades

Despite 70 years of clinical use, β-lactam antibiotics still remain at the forefront of antimicrobial chemotherapy. The major challenge to these life-saving therapeutics is the presence of bacterial enzymes (i.e., β-lactamases) that can hydrolyze the β-lactam bond and inactivate the antibiotic. These enzymes can be grouped into four classes (A-D). Among the most genetically diverse are the class D β-lactamases. In this class are β-lactamases that can inactivate the entire spectrum of β-lactam antibiotics (penicillins, cephalosporins, and carbapenems). Class D β-lactamases are mostly found in Gram-negative bacteria such as Pseudomonas aeruginosa , Escherichia coli , Proteus mirabilis , and Acinetobacter baumannii . The active-sites of class D β-lactamases contain an unusual N-carboxylated lysine post-translational modification. A strongly hydrophobic active-site helps create the conditions that allow the lysine to combine with CO2, and the resulting carbamate is stabilized by a number of hydrogen bonds. The carboxy-lysine plays a symmetric role in the reaction, serving as a general base to activate the serine nucleophile in the acylation reaction, and the deacylating water in the second step. There are more than 250 class D β-lactamases described, and the full set of variants shows remarkable diversity with regard to substrate binding and turnover. Narrow-spectrum variants are most effective against the earliest generation penicillins and cephalosporins such as ampicillin and cephalothin. Extended-spectrum variants (also known as extended-spectrum β-lactamases, ESBLs) pose a more dangerous clinical threat as they possess a small number of substitutions that allow them to bind and hydrolyze later generation cephalosporins that contain bulkier side-chain constituents (e.g., cefotaxime, ceftazidime, and cefepime). Mutations that permit this versatility seem to cluster in the area surrounding an active-site tryptophan resulting in a widened active-site to accommodate the oxyimino side-chains of these cephalosporins. More concerning are the class D β-lactamases that hydrolyze clinically important carbapenem β-lactam drugs (e.g., imipenem). Whereas carbapenems irreversibly acylate and inhibit narrow-spectrum β-lactamases, class D carbapenemases are able to recruit and activate a deacylating water. The rotational orientation of the C6 hydroxyethyl group found on all carbapenem antibiotics likely plays a role in whether the deacylating water is effective or not. Inhibition of class D β-lactamases is a current challenge. Commercially available inhibitors that are active against other classes of β-lactamases are ineffective against class D enzymes. On the horizon are several compounds, consisting of both β-lactam derivatives and non-β-lactams, that have the potential of providing novel leads to design new mechanism-based inactivators that are effective against the class D enzymes. Several act synergistically when given in combination with a β-lactam antibiotic, and others show a unique mechanism of inhibition that is distinct from the traditional β-lactamase inhibitors. These studies will bolster structure-based inhibitor design efforts to facilitate the optimization and development of these compounds as class D inactivators.

Lower extremity soft tissue defect reconstruction with the serratus anterior flap

Reconstruction of limb-threatening lower extremity defects presents unique challenges. The selected method must provide adequate coverage of exposed bone, joints, and tendons while maximizing function of the limb. The traditional workhorse flaps, the free latissimus dorsi and rectus abdominis flaps, have been associated with donor site morbidity and bulkiness that can impair rehabilitation. We report a case series (n = 18) in which the free serratus anterior muscle flap and split thickness skin graft (STSG) was used for lower limb soft tissue coverage. Injuries were due to diabetes (9/18), trauma (7/18), and chronic venous stasis (2/18). A 94% flap survival rate was observed and all but one patient was ambulatory. No donor site morbidity was reported. Our series demonstrates that serratus anterior is an advantageous, reliable free flap with minimal donor site morbidity.

The gracilis myocutaneous free flap in swine: an advantageous preclinical model for vascularized composite allograft transplantation research

Vascularized composite allotransplantation (VCA) has become a clinical reality, prompting research aimed at improving the risk-benefit ratio of such transplants. Here, we report our experience with a gracilis myocutaneous free flap in Massachusetts General Hospital miniature swine as a preclinical VCA model. Fourteen animals underwent free transfer of a gracilis myocutaneous flap comprised of the gracilis muscle and overlying skin, each tissue supplied by independent branches of the femoral vessels. End-to-end anastomoses were performed to the common carotid artery and internal jugular vein, or to the femoral vessels of the recipients. Thirteen of fourteen flaps were successful. A single flap was lost due to compromise of venous outflow. This model allows transplantation of a substantial volume of skin, subcutaneous tissue, and muscle. The anatomy is reliable and easily identified and harvest incurs minimal donor morbidity. We find this gracilis myocutaneous flap an excellent pre-clinical model for the study of vascularized composite allotransplantation.

Site-saturation mutagenesis of position V117 in OXA-1 β-lactamase: effect of side chain polarity on enzyme carboxylation and substrate turnover

Class D β-lactamases pose an emerging threat to the efficacy of β-lactam therapy for bacterial infections. Class D enzymes differ mechanistically from other β-lactamases by the presence of an active-site N-carboxylated lysine that serves as a general base to activate the serine nucleophile for attack. We have used site-saturation mutagenesis at position V117 in the class D β-lactamase OXA-1 to investigate how alterations in the environment around N-carboxylated K70 affect the ability of that modified residue to carry out its normal function. Minimum inhibitory concentration analysis of the 20 position 117 variants demonstrates a clear pattern of charge and polarity effects on the level of ampicillin resistance imparted on Escherichia coli (E. coli). Substitutions that introduce a negative charge (D, E) at position 117 reduce resistance to near background levels, while the positively charged K and R residues maintain the highest resistance levels of all mutants. Treatment of the acidic variants with the fluorescent penicillin BOCILLIN FL followed by SDS-PAGE shows that they are active for acylation by substrate but deacylation-deficient. We used a novel fluorescence anisotropy assay to show that the specific charge and hydrogen-bonding potential of the residue at position 117 affect CO(2) binding to K70, which in turn correlates to deacylation activity. These conclusions are discussed in light of the mechanisms proposed for both class D β-lactamases and BlaR β-lactam sensor proteins and suggest a reason for the preponderance of asparagine at the V117-homologous position in the sensors.

The 1.4 A crystal structure of the class D beta-lactamase OXA-1 complexed with doripenem

The clinical efficacy of carbapenem antibiotics depends on their resistance to the hydrolytic action of beta-lactamase enzymes. The structure of the class D beta-lactamase OXA-1 as an acyl complex with the carbapenem doripenem was determined to 1.4 A resolution. Unlike most class A and class C carbapenem complexes, the acyl carbonyl oxygen in the OXA-1-doripenem complex is bound in the oxyanion hole. Interestingly, no water molecules were observed in the vicinity of the acyl linkage, providing an explanation for why carbapenems inhibit OXA-1. The side chain amine of K70 remains fully carboxylated in the acyl structure, and the resulting carbamate group forms a hydrogen bond to the alcohol of the 6alpha-hydroxyethyl moiety of doripenem. The carboxylate attached to the beta-lactam ring of doripenem is stabilized by a salt bridge to K212 and a hydrogen bond with T213, in lieu of the interaction with an arginine side chain found in most other beta-lactamase-beta-lactam complexes (e.g., R244 in the class A member TEM-1). This novel set of interactions with the carboxylate results in a major shift of the carbapenem's pyrroline ring compared to the structure of the same ring in meropenem bound to OXA-13. Additionally, bond angles of the pyrroline ring suggest that after acylation, doripenem adopts the Delta(1) tautomer. These findings provide important insights into the role that carbapenems may have in the inactivation process of class D beta-lactamases.

Mutation of the active site carboxy-lysine (K70) of OXA-1 beta-lactamase results in a deacylation-deficient enzyme

Class D beta-lactamases hydrolyze beta-lactam antibiotics by using an active site serine nucleophile to form a covalent acyl-enzyme intermediate and subsequently employ water to deacylate the beta-lactam and release product. Class D beta-lactamases are carboxylated on the epsilon-amino group of an active site lysine, with the resulting carbamate functional group serving as a general base. We discovered that substitutions of the active site serine and lysine in OXA-1 beta-lactamase, a monomeric class D enzyme, significantly disrupt catalytic turnover. Substitution of glycine for the nucleophilic serine (S67G) results in an enzyme that can still bind substrate but is unable to form a covalent acyl-enzyme intermediate. Substitution of the carboxylated lysine (K70), on the other hand, results in enzyme that can be acylated by substrate but is impaired with respect to deacylation. We employed the fluorescent penicillin BOCILLIN FL to show that three different substitutions for K70 (alanine, aspartate, and glutamate) lead to the accumulation of significant acyl-enzyme intermediate. Interestingly, BOCILLIN FL deacylation rates (t(1/2)) vary depending on the identity of the substituting residue, from approximately 60 min for K70A to undetectable deacylation for K70D. Tryptophan fluorescence spectroscopy was used to confirm that these results are applicable to natural (i.e., nonfluorescent) substrates. Deacylation by K70A, but not K70D or K70E, can be partially restored by the addition of short-chain carboxylic acid mimetics of the lysine carbamate. In conclusion, we establish the functional role of the carboxylated lysine in OXA-1 and highlight its specific role in acylation and deacylation.

DNA sequence-dependent folding determines the divergence in binding specificities between Maf and other bZIP proteins

Maf family transcription factors are atypical basic region-leucine zipper (bZIP) proteins that contain a variant basic region and an ancillary DNA-binding region. These proteins recognize extended DNA sequence elements flanking the core recognition element bound by canonical bZIP proteins. We have investigated the causes for the differences in DNA recognition between Maf and other bZIP family proteins through studies of Maf secondary structure, trypsin sensitivity, binding affinity, dissociation rate and DNA contacts. Our results show that specific DNA binding by Maf is coupled to a conformational change involving both the basic and ancillary DNA-binding regions that depends on the extended DNA sequence elements. Two basic region amino acid residues that differ between Maf and canonical bZIP proteins facilitate the conformational change required for Maf recognition of the extended elements. Nucleotide base contacts made by Maf differ from those made by canonical bZIP proteins. Taken together, our results suggest that the unusual DNA binding specificity of Maf family proteins is mediated by concerted folding of structurally unrelated DNA recognition motifs.

Lanosterol analogs: dual-action inhibitors of cholesterol biosynthesis

Drugs which suppress hepatic cholesterol biosynthesis are important therapeutic tools for lowering serum cholesterol, a major risk factor in coronary heart disease. With the goal of developing molecules that will effectively shut down cholesterol biosynthesis in hepatic tissue but allow for the buildup of the isoprenes needed for the biosynthesis of polyisoprenes other than sterols, we have designed and evaluated a series of lanosterol analogs to act as dual-action inhibitors of cholesterol biosynthesis. These sterols were predicted to act as competitive inhibitors of lanosterol 14alpha-methyl demethylase (P-450DM) and as partial suppressors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), the rate-limiting enzyme in the pathway. Compounds which have been identified as dual-action inhibitors of cholesterol biosynthesis include analogs of the intermediates generated during the removal of the 14alpha-methyl group of lanosterol by P-450DM, aminolanosterols with the amine nitrogen placed in the vicinity of C-32, and lanosterol analogs with a ketone or oxime functionality at C-15. While some dual-action inhibitors require an active P-450DM for suppression of HMGR activity, others do not. The inability of some compounds to suppress HMGR activity in cells which lack P-450DM activity suggests either that these compounds require P-450DM for conversion to an active metabolite which then suppresses HMGR activity, or that they cause the accumulation of the natural demethylation intermediates resulting in the suppression of HMGR activity. Lanosterol analogs, in contrast to 25-hydroxycholesterol, do not inhibit transcription of the HMGR gene. Rather, they inhibit translation of the HMGR mRNA, and in most cases also accelerate the degradation of enzyme protein. The potential pharmacological utility of cholesterol biosynthesis inhibitors may be determined at least in part by their effects on LDL receptor (LDLR) activity. The transcriptional regulator 25-hydroxycholesterol suppresses both HMGR and LDLR activities, while the post-transcriptional regulatory lanosterol analogs exhibit a more desirable profile, lowering HMGR levels without suppressing LDLR expression, and in some cases actually enhancing cellular LDL metabolism. Lanosterol analogs which function as dual-action inhibitors of cholesterol biosynthesis promise to be useful not only as tools for dissecting the cellular regulation of cholesterol metabolism, but also as models for the development of safe, effective hypocholesterolemic agents.

DNA bending determines Fos-Jun heterodimer orientation

Heterodimeric transcription factors can bind to palindromic recognition elements in two opposite orientations with potentially distinct effects on transcriptional activity. We have determined the orientation of Fos-Jun binding at different AP-1 sites using a novel gel-based fluorescence resonance energy transfer assay. The orientation preference of heterodimer binding varied over a greater than 10-fold range. Single base pair substitutions that alter bending of flanking sequences reversed the orientation of heterodimer binding. Single amino acid substitutions that reduce the difference in DNA bending between Fos and Jun also reduced the orientation preference. Consequently, indirect read-out mediated by differences in DNA structure can contribute to the structural organization of nucleoprotein complexes.

Molecular basis of cooperative DNA bending and oriented heterodimer binding in the NFAT1-Fos-Jun-ARRE2 complex

Cooperative DNA binding by transcription factors that bind to separate recognition sites is likely to require bending of intervening sequences and the appropriate orientation of transcription factor binding. We investigated DNA bending in complexes formed by the basic region-leucine zipper domains of Fos and Jun with the DNA binding region of nuclear factor of activated T cells 1 (NFAT1) at composite regulatory elements using gel electrophoretic phasing analysis. The NFAT1-Fos-Jun complex induced a bend at the ARRE2 site that was distinct from the sum of the bends induced by NFAT1 and Fos-Jun separately. We designate this difference DNA bending cooperativity. The bending cooperativity was directed toward the interaction interface between Fos-Jun and NFAT1. We also examined the influence of NFAT1 on the orientation of Fos-Jun heterodimer binding using a novel fluorescence resonance energy transfer assay. The interaction with NFAT1 could reverse the orientation of Fos-Jun heterodimer binding to the ARRE2 site. The principal determinants of both cooperative DNA bending and oriented heterodimer binding were localized to three amino acid residues at the amino-terminal ends of the leucine zippers of Fos and Jun. Consequently, interactions between transcription factors can remodel promoters by altering DNA bending and the orientation of heterodimer binding.

Biochemical studies of the mechanism of action of the Cdc42-GTPase-activating protein

The small GTP-binding proteins Rac, Rho, and Cdc42 were shown to mediate a variety of signaling pathways including cytoskeletal rearrangements, cell-cycle progression, and transformation. Key to the proper function of these GTP-binding proteins is an efficient shut-off mechanism that ensures the decay of the signal. Regulatory proteins termed GAPs (GTPase-activating proteins) enhance the intrinsic GTP hydrolysis of the GTP-binding proteins, thereby ensuring signal termination. We have used site-specific mutagenesis to elucidate the limit domain for GAP activity in Cdc42-GAP, and show that in addition to the known GAP-homology domain (three conserved boxes), a C-terminal region outside that domain is also essential for GAP activity. In addition, we have replaced the conserved arginine (Arg305), which was suggested by structural studies to be a key catalytic residue, with an alanine and found that the R305A Cdc42-GAP mutant has a greatly diminished catalytic capacity but is still able to bind Cdc42 with high affinity. Thus, a key catalytic role for this residue is confirmed. However, we also present evidence for the involvement of an additional residue(s), since the R305A Cdc42-GAP mutant still exhibits measurable activity. Some of this residual activity might result from a neighboring arginine, since a double mutant R305A/R306A shows a further decrease in catalytic activity.

The Drosophila gene asteroid encodes a novel protein and displays dosage-sensitive interactions with Star and Egfr

The asteroid gene of Drosophila was found to lie within 189 bp of Star. Asteroid cDNA clones were isolated and sequenced and a single putative open reading frame was identified that encodes a novel protein of 815 amino acids with a calculated molecular mass of 93 kilodaltons. Using cDNA probes, asteroid transcripts were localized to the proliferative tissues of embryos and to the mitotically active tissue anterior to the morphogenetic furrow in eye imaginal discs. Ribonuclease protection assays identified a mutation of asteroid that acts as a dominant enhancer of Star mutations and also enhances the Ellipse mutation, EgfrE1. Based on these data, a model for asteroid gene function in EGF receptor signaling is presented.

Interaction between Cdc42Hs and RhoGDI is mediated through the Rho insert region

Members of the Rho subfamily of GTP-binding proteins contain a region of amino acid sequence (residues 122-134) that is absent from other Ras-like proteins and is termed the Rho insert region. To address the functional role of this domain, we have constructed a Cdc42Hs/Ras chimera in which loop 8 from Ha-Ras was substituted for the region in Cdc42Hs that contains the 13-amino acid insert region. Our data indicate that the insert region of Cdc42Hs is not essential for its interactions with various target/effector molecules or for interactions with the guanine nucleotide exchange factor, Dbl, or the Cdc42 GTPase-activating protein (GAP). However, the regulation of GDP dissociation and GTP hydrolysis on Cdc42Hs by the Rho GDP-dissociation inhibitor (GDI) is extremely sensitive to changes in the insert region, such that a Cdc42Hs/Ha-Ras chimera that lacks this insert is no longer susceptible to a GDI-induced inhibition of GDP dissociation and GTP hydrolysis. The insensitivity to GDI activity is not due to the inability of the GDI molecule to bind to the Cdc42Hs/Ha-Ras chimera, and in fact, the GDI is fully capable of stimulating the release of this chimera from membranes.

Structural basis of DNA bending and oriented heterodimer binding by the basic leucine zipper domains of Fos and Jun

Interactions among transcription factors that bind to separate sequence elements require bending of the intervening DNA and juxtaposition of interacting molecular surfaces in an appropriate orientation. Here, we examine the effects of single amino acid substitutions adjacent to the basic regions of Fos and Jun as well as changes in sequences flanking the AP-1 site on DNA bending. Substitution of charged amino acid residues at positions adjacent to the basic DNA-binding domains of Fos and Jun altered DNA bending. The change in DNA bending was directly proportional to the change in net charge for all heterodimeric combinations between these proteins. Fos and Jun induced distinct DNA bends at different binding sites. Exchange of a single base pair outside of the region contacted in the x-ray crystal structure altered DNA bending. Substitution of base pairs flanking the AP-1 site had converse effects on the opposite directions of DNA bending induced by homodimers and heterodimers. These results suggest that Fos and Jun induce DNA bending in part through electrostatic interactions between amino acid residues adjacent to the basic region and base pairs flanking the AP-1 site. DNA bending by Fos and Jun at inverted binding sites indicated that heterodimers bind to the AP-1 site in a preferred orientation. Mutation of a conserved arginine within the basic regions of Fos and transversion of the central C:G base pair in the AP-1 site to G:C had complementary effects on the orientation of heterodimer binding and DNA bending. The conformational variability of the Fos-Jun-AP-1 complex may contribute to its functional versatility at different promoters.

Use of a fluorescence spectroscopic readout to characterize the interactions of Cdc42Hs with its target/effector, mPAK-3

The family of p21-activated kinases (PAKs) has been shown to contain a domain that can independently bind to the Ras-like proteins Cdc42Hs and Rac. We have expressed a 72 amino acid recombinant form of this p21-binding domain (PBD) from mPAK-3 in Escherichia Coli for use in structure-function studies. The protein can be purified on a nickel affinity resin due to a hexa-His tag that is incorporated onto the amino terminus of the domain. PBD binds to Cdc42Hs in a guanine nucleotide-dependent manner as demonstrated by a novel fluorescence assay that takes advantage of the spectroscopic properties of N-methylanthraniloyl (Mant)-guanine nucleotides. Ionic strength has little effect on the affinity of PBD for Cdc42Hs, but alkaline pH values tend to weaken the interaction. We have shown that the inhibition of the GTPase activity of Cdc42Hs, as well as a previously undescribed inhibition of guanine nucleotide dissociation, is mediated by the PBD portion of the mPAK-3 molecule. These findings suggest that PBD binding alters the geometry of the guanine nucleotide binding site on Cdc42Hs, perhaps as an outcome of the target/effector molecule binding in close proximity to the nucleotide domain. We therefore tested if mutations in the effector region of Cdc42Hs (32-40), which in Ras are very close to the guanine nucleotide binding site, had any effect on PBD binding. Changing tyrosine 32 to lysine (Y32K) resulted in a small (5-fold) inhibition of PBD binding, but the very conservative mutation D38E yielded at least a 50-fold decrease in affinity. Finally, the catalytic domain of the GTPase activating protein, Cdc42-GAP, was shown to inhibit PBD binding in a competitive manner, indicating that this target molecule and the negative regulator (GAP) bind to overlapping sites on the Cdc42Hs molecule.

Investigation of the GTP-binding/GTPase cycle of Cdc42Hs using extrinsic reporter group fluorescence

The overall goal of these studies was to examine the applicability of extrinsic reporter group fluorescence in monitoring the GTP-binding/GTPase cycle of a Ras-like GTP-binding protein. Toward this end, we have labeled the GTP-binding protein Cdc42Hs with the environmentally sensitive fluorophore succinimidyl 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate (sNBD) at a single reactive lysine residue. We find that the sNBD-labeled Cdc42Hs undergoes a fluorescence enhancement at 545 nm when Cdc42Hs exchanges bound GDP for GTP. This enhancement is then fully reversed upon GTP hydrolysis. The specific GTPase-activating protein for Cdc42Hs, the Cdc42Hs-GAP, strongly stimulates the rate of reversal of the fluorescence enhancement at 545 nm, consistent with its ability to fully catalyze the GTPase reaction of Cdc42Hs. Conversely, the specific guanine nucleotide exchange factor (GEF), Cdc24, strongly stimulates the fluorescence enhancement that accompanies GTP binding, consistent with its ability to stimulate the GDP-GTP exchange reaction on Cdc42Hs. Resonance energy transfer measurements yielded a distance of approximately 32 A for the sNBD moiety and the guanine nucleotide binding site occupied with either N-methylanthraniloyl- (Mant) dGDP or MantdGTP. Taken together, these results identify a conformationally sensitive reporter site on the Cdc42Hs molecule that is located some distance away from the guanine nucleotide binding site but nonetheless provides a highly sensitive monitor for GTP-binding, GTPase activity, and the interactions of key regulatory proteins.

The influence model of sponsorship. A Catholic hospital is now part of the "Mayo Clinic"

Saint Marys Hospital was founded in Rochester, MN, in 1889. Constructed by the Sisters of St. Francis, it was staffed by physician members of the local Mayo family. The Mayo practice grew into an association of many physicians and medical residents who later began to staff Rochester Methodist Hospital also; the three healthcare institutions became collectively known as the "Mayo Clinic." By the mid-1980s, billing was so complex for the three still-independent facilities that their leaders decided to integrate more formally. This was done in three phases and resulted in the creation of a single institution known as the Mayo Medical Center. From Saint Marys' standpoint, the facilitating document in this process was a "Sponsorship Agreement" whose purpose was to maintain the sponsor's interests and obligations in the integrated structure. A Sponsorship Board was created to continue the hospital's Catholic tradition, including maintaining its chaplaincy, chapels, religious symbols, and special funds. The Sponsorship Board views the new environment as a special challenge. Its members know that Catholic sponsorship: Comforts patients, who realize they are in the hands of people motivated by the Christian ethic Creates an atmosphere in which patients and their families can seek the spiritual support that often aids healing Strengthens a sense of community among physicians, hospital staff, and administrators The Sponsorship Board hopes the sponsor's influence may come to affect the whole Mayo Medical Center, bringing patients, family members, and staff an "added dimension" of care.

Potency ranking of methemoglobin-forming agents

This study represents the first systematic attempt to rank methemoglobin-forming agents. It is a quantitative potency ranking study utilizing linear regression analysis of dose-response data for comparative purposes. Six agents that are direct-acting and eight that require bioactivation were tested for their ability to induce methemoglobin formation in Dorset sheep erythrocytes under defined in vitro conditions. The agents were then ranked according to three complementary methods based on the slope of the linear regression, the calculated dose expected to induce a given amount of methemoglobin formation and the calculated percentage methemoglobin response induced by 1 mmol l-1 of the agent. The direct-acting agents, ranked from most to least potent inducers of methemoglobin formation, are: p-dinitrobenzene > o-dinitrobenzene > copper = nitrite > chlorite > chlorate. The ranking from most to least potent inducers of the bioactivated agents are: alpha-naphthol > p-nitroaniline > m-nitroaniline, o-nitroaniline > p-nitrotoluene = aniline > m-nitrotoluene = o-nitrotoluene. The ranking procedures are discussed and issues of interindividual variation and agent-specific sensitivities are addressed.