Adeno-associated virus (AAV) cell entry: structural insights

Adeno-associated virus (AAV) is the leading vector in emerging treatments of inherited diseases. Higher transduction efficiencies and cellular specificity are required for broader clinical application, motivating investigations of virus-host molecular interactions during cell entry. High-throughput methods are identifying host proteins more comprehensively, with subsequent molecular studies revealing unanticipated complexity and serotype specificity. Cryogenic electron microscopy (cryo-EM) provides a path towards structural details of these sometimes heterogeneous virus-host complexes, and is poised to illuminate more fully the steps in entry. Here presented, is progress in understanding the distinct steps of glycan attachment, and receptor-mediated entry/trafficking. Comparison with structures of antibody complexes provides new insights on immune neutralization with implications for the design of improved gene therapy vectors.

Structure of the gene therapy vector, adeno-associated virus with its cell receptor, AAVR

Adeno-associated virus (AAV) vectors are preeminent in emerging clinical gene therapies. Generalizing beyond the most tractable genetic diseases will require modulation of cell specificity and immune neutralization. Interactions of AAV with its cellular receptor, AAVR, are key to understanding cell-entry and trafficking with the rigor needed to engineer tissue-specific vectors. Cryo-electron tomography shows ordered binding of part of the flexible receptor to the viral surface, with distal domains in multiple conformations. Regions of the virus and receptor in close physical proximity can be identified by cross-linking/mass spectrometry. Cryo-electron microscopy with a two-domain receptor fragment reveals the interactions at 2.4 Å resolution. AAVR binds between AAV's spikes on a plateau that is conserved, except in one clade whose structure is AAVR-incompatible. AAVR's footprint overlaps the epitopes of several neutralizing antibodies, prompting a re-evaluation of neutralization mechanisms. The structure provides a roadmap for experimental probing and manipulation of viral-receptor interactions.

The 2.8 Å Electron Microscopy Structure of Adeno-Associated Virus-DJ Bound by a Heparinoid Pentasaccharide

Atomic structures of adeno-associated virus (AAV)-DJ, alone and in complex with fondaparinux, have been determined by cryoelectron microscopy at 3 Å resolution. The gene therapy vector, AAV-DJ, is a hybrid of natural serotypes that was previously derived by directed evolution, selecting for hepatocyte entry and resistance to neutralization by human serum. The structure of AAV-DJ differs from that of parental serotypes in two regions where neutralizing antibodies bind, so immune escape appears to have been the primary driver of AAV-DJ's directed evolution. Fondaparinux is an analog of cell surface heparan sulfate to which several AAVs bind during entry. Fondaparinux interacts with viral arginines at a known heparin binding site, without the large conformational changes whose presence was controversial in low-resolution imaging of AAV2-heparin complexes. The glycan density suggests multi-modal binding that could accommodate sequence variation and multivalent binding along a glycan polymer, consistent with a role in attachment, prior to more specific interactions with a receptor protein mediating entry.

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

Electron microscopy analysis of a disaccharide analog complex reveals receptor interactions of adeno-associated virus

Mechanistic studies of macromolecular complexes often feature X-ray structures of complexes with bound ligands. The attachment of adeno-associated virus (AAV) to cell surface glycosaminoglycans (GAGs) is an example that has not proven amenable to crystallography, because the binding of GAG analogs disrupts lattice contacts. The interactions of AAV with GAGs are of interest in mediating the cell specificity of AAV-based gene therapy vectors. Previous electron microscopy led to differing conclusions on the exact binding site and the existence of large ligand-induced conformational changes in the virus. Conformational changes are expected during cell entry, but it has remained unclear whether the electron microscopy provided evidence of their induction by GAG-binding. Taking advantage of automated data collection, careful processing and new methods of structure refinement, the structure of AAV-DJ complexed with sucrose octasulfate is determined by electron microscopy difference map analysis to 4.8Å resolution. At this higher resolution, individual sulfate groups are discernible, providing a stereochemical validation of map interpretation, and highlighting interactions with two surface arginines that have been implicated in genetic studies. Conformational changes induced by the SOS are modest and limited to the loop most directly interacting with the ligand. While the resolution attainable will depend on sample order and other factors, there are an increasing number of macromolecular complexes that can be studied by cryo-electron microscopy at resolutions beyond 5Å, for which the approaches used here could be used to characterize the binding of inhibitors and other small molecule effectors when crystallography is not tractable.

Structure of AAV-DJ, a retargeted gene therapy vector: cryo-electron microscopy at 4.5 Å resolution

AAV-DJ, a leading candidate vector for liver gene therapy, was created through random homologous recombination followed by directed evolution, selecting for in vivo liver tropism and resistance to in vitro immune neutralization. Here, the 4.5 Å resolution cryo-EM structure is determined for the engineered AAV vector, revealing structural features that illuminate its phenotype. The heparan sulfate receptor-binding site is little changed from AAV-2, and heparin-binding affinity is similar. A loop that is antigenic in other serotypes has a unique conformation in AAV-DJ that would conflict with the binding of an AAV-2 neutralizing monoclonal antibody. This is consistent with increased resistance to neutralization by human polyclonal sera, raising the possibility that changed tropism may be a secondary effect of altered immune interactions. The reconstruction exemplifies analysis of fine structural changes and the potential of cryo-EM, in favorable cases, to characterize mutant or ligand-bound complexes.

Structure-function analysis of receptor-binding in adeno-associated virus serotype 6 (AAV-6)

Crystal structures of the AAV-6 capsid at 3Å reveal a subunit fold homologous to other parvoviruses with greatest differences in two external loops. The electrostatic potential suggests that receptor-attachment is mediated by four residues: Arg(576), Lys(493), Lys(459) and Lys(531), defining a positively charged region curving up from the valley between adjacent spikes. It overlaps only partially with the receptor-binding site of AAV-2, and the residues endowing the electrostatic character are not homologous. Mutational substitution of each residue decreases heparin affinity, particularly Lys(531) and Lys(459). Neither is conserved among heparin-binding serotypes, indicating that diverse modes of receptor attachment have been selected in different serotypes. Surface topology and charge are also distinct at the shoulder of the spike, where linear epitopes for AAV-2's neutralizing monoclonal antibody A20 come together. Evolutionarily, selection of changed side-chain charge may have offered a conservative means to evade immune neutralization while preserving other essential functionality.

Interaction between troponin and myosin enhances contractile activity of myosin in cardiac muscle

Ca(2+) signaling in striated muscle cells is critically dependent upon thin filament proteins tropomyosin (Tm) and troponin (Tn) to regulate mechanical output. Using in vitro measurements of contractility, we demonstrate that even in the absence of actin and Tm, human cardiac Tn (cTn) enhances heavy meromyosin MgATPase activity by up to 2.5-fold in solution. In addition, cTn without Tm significantly increases, or superactivates sliding speed of filamentous actin (F-actin) in skeletal motility assays by at least 12%, depending upon [cTn]. cTn alone enhances skeletal heavy meromyosin's MgATPase in a concentration-dependent manner and with sub-micromolar affinity. cTn-mediated increases in myosin ATPase may be the cause of superactivation of maximum Ca(2+)-activated regulated thin filament sliding speed in motility assays relative to unregulated skeletal F-actin. To specifically relate this classical superactivation to cardiac muscle, we demonstrate the same response using motility assays where only cardiac proteins were used, where regulated cardiac thin filament sliding speeds with cardiac myosin are >50% faster than unregulated cardiac F-actin. We additionally demonstrate that the COOH-terminal mobile domain of cTnI is not required for this interaction or functional enhancement of myosin activity. Our results provide strong evidence that the interaction between cTn and myosin is responsible for enhancement of cross-bridge kinetics when myosin binds in the vicinity of Tn on thin filaments. These data imply a novel and functionally significant molecular interaction that may provide new insights into Ca(2+) activation in cardiac muscle cells.

Gestational diabetes: postpartum glucose tolerance testing

OBJECTIVE

Our purpose was to determine the incidence of and evaluate risk factors for postpartum glucose intolerance among predominantly black gestational diabetic women.

STUDY DESIGN

One hundred forty-five gestational diabetics underwent a standard 2-hour glucose tolerance test in the early puerperium according to the criteria of the National Diabetes Data Group.

RESULTS

Fourteen patients (10%) were diabetic and eight (5%) showed impaired glucose tolerance. Maternal age, race, or obesity did not predict abnormal postpartum glucose tolerance testing. The requirement of insulin for glucose control during gestation and gestational age at diagnosis were significantly associated with abnormal postpartum glucose tolerance (p < 0.0001 and p = 0.012, respectively). Multivariate analysis showed that only a requirement for insulin for glucose control was significant (p < 0.001).

CONCLUSION

Pregnancies complicated by gestational diabetes are at increased risk of glucose intolerance during the early postpartum period. Abnormal glucose tolerance occurs predominantly among those patients requiring insulin therapy during gestation or those diagnosed before 24 weeks' gestation.

Ureteral injury during elective pregnancy termination. A case report

This case is the sixth reported one of ureteral injury following elective pregnancy termination. It reemphasizes that the anatomic relationship of the ureter to the lower genital tract can predispose to surgical injury.

Urinary dipstick protein: a poor predictor of absent or severe proteinuria

OBJECTIVE

Our purpose was to compare urinary protein dipstick values with standard 24-hour urinary protein excretion in women with hypertension in pregnancy.

STUDY DESIGN

Urinary protein dipstick determinations and concurrent 24-hour urinary protein excretion measurements were compared by review of 300 urine samples obtained from women with hypertension in pregnancy.

RESULTS

One hundred twenty-three samples had negative to trace protein on dipstick on two occasions at least 6 hours apart. Eight-one (66%) of these patients had significant proteinuria (> or = 300 mg per 24 hours). Seventy-six samples revealed 3+ to 4+ protein on dipstick in at least two samples. Of these, 27 (36%) had heavy proteinuria (> or = 5 gm per 24 hours), and 42 (55%) had nephrotic range proteinuria of > or = 3.5 gm per 24 hours. One hundred one patients had urine dipstick values of 1+ to 2+, of whom 89 (88%) had significant proteinuria.

CONCLUSION

Urinary protein dipstick values > or = 1+ have a positive predictive value of 92% (162/177) for predicting > or = 300 mg per 24 hours. In contrast, a dipstick of negative to trace should not be used to rule out significant proteinuria because its negative predictive value is only 34% (42/123) in hypertensive patients. Moreover, urine dipstick values of 3+ to 4+ should not be used to diagnose severe preeclampsia because their positive predictive value is only 36% (27/76).

Positive serum screening for fetal Down syndrome does not predict adverse pregnancy outcome in absence of fetal aneuploidy

OBJECTIVE

The purpose of this study was to determine whether false-positive maternal serum screening for fetal Down syndrome is predictive of poor pregnancy outcome.

METHODS

The pregnancy outcomes of 99 women having positive serum screening for fetal Down syndrome (study group)--based upon maternal serum alpha-fetoprotein (MSAFP), unconjugated estriol (uE3), hCG, and maternal age--were compared to the outcomes of matched control patients having negative serum screening results (control group). The outcome indices analyzed were fetal death, intrauterine growth retardation (IUGR), preeclampsia, and fetal anomalies.

RESULTS

Between the study group and the control group, there were no statistically significant differences in pregnancy outcome with respect to fetal death, IUGR, preeclampsia, or fetal anomalies.

CONCLUSIONS

Our findings demonstrate no apparent increase in the adverse perinatal outcomes analyzed in women having unexplained positive serum screening for fetal Down syndrome. Although further investigation is needed, these results provide no evidence to support increased antepartum surveillance in such patients.

Adhesion formation associated with the use of absorbable staples in comparison to other types of peritoneal injury

Adhesion formation was assessed after inducing peritoneal injury with absorbable staples, absorbable suture, surgical excision or thermal injury. In addition, adhesion formation was assessed in the presence of Hyskon. Absorbable staples were associated with increased adhesion formation when compared to the other methods of injury. Hyskon appears to significantly reduce the formation of peritoneal adhesions at sites of peritoneal suturing, excision, and thermal injury, but did not reduce adhesion formation in the area of stapling.

A histologic evaluation of peritoneal injury and repair: implications for adhesion formation

A study of peritoneal healing after specific types of peritoneal injury was conducted in rabbits. Histologic specimens were examined at four, eight, 12, 24, and 48 hours, and at five, seven, and 14 days postperitoneal injury. Comparison of types of injury and attempts at peritoneal repair revealed differences in healing mechanisms. Histologic evidence of rapid reperitonealization was associated with those types of repair that involved minimal tissue necrosis and absence of foreign body reaction.

Cytological events associated with in vitro aged and fertilized rabbit eggs

The morphogenetic events associated with rabbit eggs aged in vitro for 12 to 50 hours prior to mixing with sperm have been examined by light and electron microscopy. After 12 hours in culture, morphological alterations of the meiotic spindle and the cortex of unfertilized eggs were evident. By 24 to 50 hours in culture, unfertilized eggs contained subnuclei, structures which formed when individual and/or groups of meiotic chromosomes dispersed and becmae invested by a double-laminated structure reminiscent of a nuclear envelope. Although most eggs obtained 11.5 to 12 hours after induced ovulation and in vitro fertilized displayed morphogenetic patterns similar to those described for in vivo fertilized ova, some (10%) contained three pronuclei. Many eggs obtained 13 to 15 hours after induced ovulation and subsequently mixed with sperm in vitro appeared to undergo processes of fertilization typical of in vivo fertilized eggs, however, approximately 30% contained subnuclei in association with the male pronucleus. Few eggs (15%) aged 12 hours prior to in vitro fertilization displayed patterns of pronuclear development and association typical of fertilized unaged ova. Subnuclei developed in many of the fertilized ova. Supernumerary sperm nuclei, which did not develop into male pronuclei, were observed in some zygotes. Cleavage of eggs aged 12 hours prior to fertilization was abnormal or retarded. After 24 hours in culture approximately 16% of the eggs fertilized. Seventy percent of the fertilized eggs failed to support the development of a male or female pronucleus.