Structural and Biophysical Analysis of Adeno-Associated Virus Serotype 2 Capsid Assembly Variants.

Adeno-associated virus (AAV) is a nonenveloped single-stranded DNA (ssDNA) icosahedral T=1 virus being developed as a vector for clinical gene delivery systems. Currently, there are approximately 160 AAV clinical trials, with AAV2 being the most widely studied serotype. To further understand the AAV gene delivery system, this study investigates the role of viral protein (VP) symmetry interactions on capsid assembly, genome packaging, stability, and infectivity. A total of 25 (seven 2-fold, nine 3-fold, and nine 5-fold symmetry interface) AAV2 VP variants were studied. Six 2-fold and two 5-fold variants did not assemble capsids based on native immunoblots and anti-AAV2 enzyme-linked immunosorbent assays (ELISAs). Seven of the 3-fold and seven of the 5-fold variants that assembled capsids were less stable, while the only 2-fold variant that assembled had ~2°C higher thermal stability (Tm) than recombinant wild-type AAV2 (wtAAV2). Three of the 3-fold variants (AAV2-R432A, AAV2-L510A, and N511R) had an approximately 3-log defect in genome packaging. Consistent with previous reports of the 5-fold axes, the region of the capsid is important for VP1u externalization and genome ejection, and one 5-fold variant (R404A) had a significant defect in viral infectivity. The structures of wtAAV2 packaged with a transgene (AAV2-full) and without a transgene (AAV2-empty) and one 5-fold variant (AAV2-R404A) were determined by cryo-electron microscopy and three dimensional (3D)-image reconstruction to 2.8, 2.9, and 3.6 Å resolution, respectively. These structures revealed the role of stabilizing interactions on the assembly, stability, packaging, and infectivity of the virus capsid. This study provides insight into the structural characterization and functional implications of the rational design of AAV vectors. IMPORTANCE Adeno-associated viruses (AAVs) have been shown to be useful vectors for gene therapy applications. Consequently, AAV has been approved as a biologic for the treatment of several monogenic disorders, and many additional clinical trials are ongoing. These successes have generated significant interest in all aspects of the basic biology of AAV. However, to date, there are limited data available on the importance of the capsid viral protein (VP) symmetry-related interactions required to assemble and maintain the stability of the AAV capsids and the infectivity of the AAV capsids. Characterizing the residue type and interactions at these symmetry-driven assembly interfaces of AAV2 has provided the foundation for understanding their role in AAV vectors (serotypes and engineered chimeras) and has determined the residues or regions of the capsid that can or cannot tolerate alterations.

Mutants at the 2-Fold Interface of Adeno-associated Virus Type 2 (AAV2) Structural Proteins Suggest a Role in Viral Transcription for AAV Capsids.

UNLABELLED: We previously reported that an amino acid substitution, Y704A, near the 2-fold interface of adeno-associated virus (AAV) was defective for transcription of the packaged genome (M. Salganik, F. Aydemir, H. J. Nam, R. McKenna, M. Agbandje-McKenna, and N. Muzyczka, J Virol 88:1071-1079, 2013, doi: http://dx.doi.org/10.1128/JVI.02093-13). In this report, we have characterized the defect in 6 additional capsid mutants located in a region ∼30 Šin diameter on the surface of the AAV type 2 (AAV2) capsid near the 2-fold interface. These mutants, which are highly conserved among primate serotypes, displayed a severe defect (3 to 6 logs) in infectivity. All of the mutants accumulated significant levels of uncoated DNA in the nucleus, but none of the mutants were able to accumulate significant amounts of genomic mRNA postinfection. In addition, wild-type (wt) capsids that were bound to the conformational antibody A20, which is known to bind the capsid surface in the region of the mutants, were also defective for transcription. In all cases, the mutant virus particles, as well as the antibody-bound wild-type capsids, were able to enter the cell, travel to the nucleus, uncoat, and synthesize a second strand but were unable to transcribe their genomes. Taken together, the phenotype of these mutants provides compelling evidence that the AAV capsid plays a role in the transcription of its genome, and the mutants map this functional region on the surface of the capsid near the 2-fold interface. This appears to be the first example of a viral structural protein that is also involved in the transcription of the viral genome that it delivers to the nucleus. IMPORTANCE: Many viruses package enzymes within their capsids that assist in expressing their genomes postinfection, e.g., retroviruses. A number of nonenveloped viruses, including AAV, carry proteases that are needed for capsid maturation or for capsid modification during infection. We describe here what appears to be the first example of a nonenveloped viral capsid that appears to have a role in promoting transcription. A total of six mutants at the AAV capsid 2-fold interface were shown to have a severe defect in expressing their genomes, and the defect was at the level of mRNA accumulation. This suggests that AAV capsids have a novel role in promoting the transcription of the genomes that they have packaged. Since wt virions could not complement the mutant viruses, and the mutant viruses did not effectively inhibit wt gene expression, our results suggest that the capsid exerts its effect on transcription in cis.

Cryo-electron Microscopy Reconstruction and Stability Studies of the Wild Type and the R432A Variant of Adeno-associated Virus Type 2 Reveal that Capsid Structural Stability Is a Major Factor in Genome Packaging.

UNLABELLED: The adeno-associated viruses (AAV) are promising therapeutic gene delivery vectors and better understanding of their capsid assembly and genome packaging mechanism is needed for improved vector production. Empty AAV capsids assemble in the nucleus prior to genome packaging by virally encoded Rep proteins. To elucidate the capsid determinants of this process, structural differences between wild-type (wt) AAV2 and a packaging deficient variant, AAV2-R432A, were examined using cryo-electron microscopy and three-dimensional image reconstruction both at an ∼5.0-Å resolution (medium) and also at 3.8- and 3.7-Å resolutions (high), respectively. The high resolution structures showed that removal of the arginine side chain in AAV2-R432A eliminated hydrogen bonding interactions, resulting in altered intramolecular and intermolecular interactions propagated from under the 3-fold axis toward the 5-fold channel. Consistent with these observations, differential scanning calorimetry showed an ∼10°C decrease in thermal stability for AAV2-R432A compared to wt-AAV2. In addition, the medium resolution structures revealed differences in the juxtaposition of the less ordered, N-terminal region of their capsid proteins, VP1/2/3. A structural rearrangement in AAV2-R432A repositioned the ßA strand region under the icosahedral 2-fold axis rather than antiparallel to the ßB strand, eliminating many intramolecular interactions. Thus, a single amino acid substitution can significantly alter the AAV capsid integrity to the extent of reducing its stability and possibly rendering it unable to tolerate the stress of genome packaging. Furthermore, the data show that the 2-, 3-, and 5-fold regions of the capsid contributed to producing the packaging defect and highlight a tight connection between the entire capsid in maintaining packaging efficiency. IMPORTANCE: The mechanism of AAV genome packaging is still poorly understood, particularly with respect to the capsid determinants of the required capsid-Rep interaction. Understanding this mechanism may aid in the improvement of AAV packaging efficiency, which is currently ∼1:10 (10%) genome packaged to empty capsid in vector preparations. This report identifies regions of the AAV capsid that play roles in genome packaging and that may be important for Rep recognition. It also demonstrates the need to maintain capsid stability for the success of this process. This information is important for efforts to improve AAV genome packaging and will also inform the engineering of AAV capsid variants for improved tropism, specific tissue targeting, and host antibody escape by defining amino acids that cannot be altered without detriment to infectious vector production.

Adeno-associated virus serotype 1 (AAV1)- and AAV5-antibody complex structures reveal evolutionary commonalities in parvovirus antigenic reactivity.

UNLABELLED: The clinical utility of the adeno-associated virus (AAV) gene delivery system has been validated by the regulatory approval of an AAV serotype 1 (AAV1) vector for the treatment of lipoprotein lipase deficiency. However, neutralization from preexisting antibodies is detrimental to AAV transduction efficiency. Hence, mapping of AAV antigenic sites and engineering of neutralization-escaping vectors are important for improving clinical efficacy. We report the structures of four AAV-monoclonal antibody fragment complexes, AAV1-ADK1a, AAV1-ADK1b, AAV5-ADK5a, and AAV5-ADK5b, determined by cryo-electron microscopy and image reconstruction to a resolution of ∼11 to 12 Å. Pseudoatomic modeling mapped the ADK1a epitope to the protrusions surrounding the icosahedral 3-fold axis and the ADK1b and ADK5a epitopes, which overlap, to the wall between depressions at the 2- and 5-fold axes (2/5-fold wall), and the ADK5b epitope spans both the 5-fold axis-facing wall of the 3-fold protrusion and portions of the 2/5-fold wall of the capsid. Combined with the six antigenic sites previously elucidated for different AAV serotypes through structural approaches, including AAV1 and AAV5, this study identified two common AAV epitopes: one on the 3-fold protrusions and one on the 2/5-fold wall. These epitopes coincide with regions with the highest sequence and structure diversity between AAV serotypes and correspond to regions determining receptor recognition and transduction phenotypes. Significantly, these locations overlap the two dominant epitopes reported for autonomous parvoviruses. Thus, rather than the amino acid sequence alone, the antigenic sites of parvoviruses appear to be dictated by structural features evolved to enable specific infectious functions. IMPORTANCE: The adeno-associated viruses (AAVs) are promising vectors for in vivo therapeutic gene delivery, with more than 20 years of intense research now realized in a number of successful human clinical trials that report therapeutic efficacy. However, a large percentage of the population has preexisting AAV capsid antibodies and therefore must be excluded from clinical trials or vector readministration. This report represents our continuing efforts to understand the antigenic structure of the AAVs, specifically, to obtain a picture of "polyclonal" reactivity as is the situation in humans. It describes the structures of four AAV-antibody complexes determined by cryo-electron microscopy and image reconstruction, increasing the number of mapped epitopes to four and three, respectively, for AAV1 and AAV5, two vectors currently in clinical trials. The results presented provide information essential for generating antigenic escape vectors to overcome a critical challenge remaining in the optimization of this highly promising vector delivery system.

Enhanced transgene expression from recombinant single-stranded D-sequence-substituted adeno-associated virus vectors in human cell lines in vitro and in murine hepatocytes in vivo.

UNLABELLED: We have previously reported that the removal of a 20-nucleotide sequence, termed the D sequence, from both ends of the inverted terminal repeats (ITRs) in the adeno-associated virus serotype 2 (AAV2) genome significantly impairs rescue, replication, and encapsidation of the viral genomes (X. S. Wang, S. Ponnazhagan, and A. Srivastava, J Mol Biol 250:573-580, 1995; X. S. Wang, S. Ponnazhagan, and A. Srivastava, J Virol 70:1668-1677, 1996). Here we describe that replacement of only one D sequence in either ITR restores each of these functions, but DNA strands of only single polarity are encapsidated in mature progeny virions. Since most commonly used recombinant AAV vectors contain a single-stranded DNA (ssDNA), which is transcriptionally inactive, efficient transgene expression from AAV vectors is dependent upon viral second-strand DNA synthesis. We have also identified a transcription suppressor sequence in one of the D sequences, which shares homology with the binding site for the cellular NF-κB-repressing factor (NRF). The removal of this D sequence from, and replacement with a sequence containing putative binding sites for transcription factors in, single-stranded AAV (ssAAV) vectors significantly augments transgene expression both in human cell lines in vitro and in murine hepatocytes in vivo. The development of these genome-modified ssAAV vectors has implications not only for the basic biology of AAV but also for the optimal use of these vectors in human gene therapy. IMPORTANCE: The results of the studies described here not only have provided novel insights into some of the critical steps in the life cycle of a human virus, the adeno-associated virus (AAV), that causes no known disease but have also led to the development of novel recombinant AAV vectors which are more efficient in allowing increased levels of gene expression. Thus, these studies have significant implications for the potential use of these novel AAV vectors in human gene therapy.

Adeno-associated virus capsid proteins may play a role in transcription and second-strand synthesis of recombinant genomes.

A group of four interacting amino acids in adeno-associated virus type 8 (AAV8) called the pH quartet has been shown to undergo a structural change when subjected to acidic pH comparable to that seen in endosomal compartments. We examined the phenotypes of mutants with mutations in these amino acids as well as several nearby residues in the background of AAV2. We found that three of the mutations in this region (Y704A, E562A, and E564A) produce normal titers of mature capsids but are extremely defective for transduction (>10(7)-fold). The remaining mutants were also defective for transduction, but the defect in these mutants (E563A, E561A, H526A, and R389A) is not as severe (3- to 22-fold). Two other mutants (Y700A and Y730A) were found to be defective for virus assembly. One of the extremely defective mutants (Y704A) was found to enter the cell, traffic to the nucleus, and uncoat its DNA nearly as efficiently as the wild type. This suggested that some step after nuclear entry and uncoating was defective. To see if the extremely defective mutants were impaired in second-strand synthesis, the Y704A, E562A, and E564A mutants containing self-complementary DNA were compared with virus containing single-stranded genomes. Two of the mutants (Y704A and E564A) showed 1-log and 3-log improvements in infectivity, respectively, while the third mutant (E562A) showed no change. This suggested that inhibition of second-strand synthesis was responsible for some but not most of the defect in these mutants. Comparison of Y704A mRNA synthesis with that of the wild-type capsid showed that accumulation of steady-state mRNA in the Y704A mutant was reduced 450-fold, even though equal genome numbers were uncoated. Our experiments have identified a novel capsid function. They suggest that AAV capsids may play a role in the initiation of both second-strand synthesis and transcription of the input genome.

Overcoming the cystic fibrosis sputum barrier to leading adeno-associated virus gene therapy vectors.

Gene therapy has not yet improved cystic fibrosis (CF) patient lung function in human trials, despite promising preclinical studies. In the human CF lung, inhaled gene vectors must penetrate the viscoelastic secretions coating the airways to reach target cells in the underlying epithelium. We investigated whether CF sputum acts as a barrier to leading adeno-associated virus (AAV) gene vectors, including AAV2, the only serotype tested in CF clinical trials, and AAV1, a leading candidate for future trials. Using multiple particle tracking, we found that sputum strongly impeded diffusion of AAV, regardless of serotype, by adhesive interactions and steric obstruction. Approximately 50% of AAV vectors diffused >1,000-fold more slowly in sputum than in water, with large patient-to-patient variation. We thus tested two strategies to improve AAV diffusion in sputum. We showed that an AAV2 mutant engineered to have reduced heparin binding diffused twice as fast as AAV2 on average, presumably because of reduced adhesion to sputum. We also discovered that the mucolytic N-acetylcysteine could markedly enhance AAV diffusion by altering the sputum microstructure. These studies underscore that sputum is a major barrier to CF gene delivery, and offer strategies for increasing AAV penetration through sputum to improve clinical outcomes.

Structure and dynamics of adeno-associated virus serotype 1 VP1-unique N-terminal domain and its role in capsid trafficking.

The importance of the phospholipase A2 domain located within the unique N terminus of the capsid viral protein VP1 (VP1u) in parvovirus infection has been reported. This study used computational methods to characterize the VP1 sequence for adeno-associated virus (AAV) serotypes 1 to 12 and circular dichroism and electron microscopy to monitor conformational changes in the AAV1 capsid induced by temperature and the pHs encountered during trafficking through the endocytic pathway. Circular dichroism was also used to monitor conformational changes in AAV6 capsids assembled from VP2 and VP3 or VP1, VP2, and VP3 at pH 7.5. VP1u was predicted (computationally) and confirmed (in solution) to be structurally ordered. This VP domain was observed to undergo a reversible pH-induced unfolding/refolding process, a loss/gain of α-helical structure, which did not disrupt the capsid integrity and is likely facilitated by its difference in isoelectric point compared to the other VP sequences assembling the capsid. This study is the first to physically document conformational changes in the VP1u region that likely facilitate its externalization from the capsid interior during infection and establishes the order of events in the escape of the AAV capsid from the endosome en route to the nucleus.

Structural insights into adeno-associated virus serotype 5.

The adeno-associated viruses (AAVs) display differential cell binding, transduction, and antigenic characteristics specified by their capsid viral protein (VP) composition. Toward structure-function annotation, the crystal structure of AAV5, one of the most sequence diverse AAV serotypes, was determined to 3.45-Å resolution. The AAV5 VP and capsid conserve topological features previously described for other AAVs but uniquely differ in the surface-exposed HI loop between ßH and ßI of the core ß-barrel motif and have pronounced conformational differences in two of the AAV surface variable regions (VRs), VR-IV and VR-VII. The HI loop is structurally conserved in other AAVs despite amino acid differences but is smaller in AAV5 due to an amino acid deletion. This HI loop is adjacent to VR-VII, which is largest in AAV5. The VR-IV, which forms the larger outermost finger-like loop contributing to the protrusions surrounding the icosahedral 3-fold axes of the AAVs, is shorter in AAV5, creating a smoother capsid surface topology. The HI loop plays a role in AAV capsid assembly and genome packaging, and VR-IV and VR-VII are associated with transduction and antigenic differences, respectively, between the AAVs. A comparison of interior capsid surface charge and volume of AAV5 to AAV2 and AAV4 showed a higher propensity of acidic residues but similar volumes, consistent with comparable DNA packaging capacities. This structure provided a three-dimensional (3D) template for functional annotation of the AAV5 capsid with respect to regions that confer assembly efficiency, dictate cellular transduction phenotypes, and control antigenicity.

Capsid antibodies to different adeno-associated virus serotypes bind common regions.

Interactions between viruses and the host antibody immune response are critical in the development and control of disease, and antibodies are also known to interfere with the efficacy of viral vector-based gene delivery. The adeno-associated viruses (AAVs) being developed as vectors for corrective human gene delivery have shown promise in clinical trials, but preexisting antibodies are detrimental to successful outcomes. However, the antigenic epitopes on AAV capsids remain poorly characterized. Cryo-electron microscopy and three-dimensional image reconstruction were used to define the locations of epitopes to which monoclonal fragment antibodies (Fabs) against AAV1, AAV2, AAV5, and AAV6 bind. Pseudoatomic modeling showed that, in each serotype, Fabs bound to a limited number of sites near the protrusions surrounding the 3-fold axes of the T=1 icosahedral capsids. For the closely related AAV1 and AAV6, a common Fab exhibited substoichiometric binding, with one Fab bound, on average, between two of the three protrusions as a consequence of steric crowding. The other AAV Fabs saturated the capsid and bound to the walls of all 60 protrusions, with the footprint for the AAV5 antibody extending toward the 5-fold axis. The angle of incidence for each bound Fab on the AAVs varied and resulted in significant differences in how much of each viral capsid surface was occluded beyond the Fab footprints. The AAV-antibody interactions showed a common set of footprints that overlapped some known receptor-binding sites and transduction determinants, thus suggesting potential mechanisms for virus neutralization by the antibodies.

Evidence for pH-dependent protease activity in the adeno-associated virus capsid.

Incubation of highly purified adeno-associated virus (AAV) capsids in vitro at pH 5.5 induced significant autocleavage of capsid proteins at several amino acid positions. No autocleavage was seen at pH 7.5. Examination of other AAV serotypes showed at least two different pH-induced cleavage patterns, suggesting that different serotypes have evolved alternative protease cleavage sites. In contrast, incubation of AAV serotypes with an external protease substrate showed that purified AAV capsid preparations have robust protease activity at neutral pH but not at pH 5.5, opposite to what is seen with capsid protein autocleavage. Several lines of evidence suggested that protease activity is inherent in AAV capsids and is not due to contaminating proteins. Control virus preparations showed no protease activity on external substrates, and filtrates of AAV virus preparations also showed no protease activity contaminating the capsids. Further, N-terminal Edman sequencing identified unique autocleavage sites in AAV1 and AAV9, and mutagenesis of amino acids adjacent to these sites eliminated cleavage. Finally, mutation of an amino acid in AAV2 (E563A) that is in a conserved pH-sensitive structural region eliminated protease activity on an external substrate but did not seem to affect autocleavage. Taken together, our data suggested that AAV capsids have one or more protease active sites that are sensitive to pH induction. Further, it appears that acidic pHs comparable to those seen in late endosomes induce a structural change in the capsid that induces autolytic protease activity. The pH-dependent protease activity may have a role in viral infection.

Structural insight into the unique properties of adeno-associated virus serotype 9.

Adeno-associated virus serotype 9 (AAV9) has enhanced capsid-associated tropism for cardiac muscle and the ability to cross the blood-brain barrier compared to other AAV serotypes. To help identify the structural features facilitating these properties, we have used cryo-electron microscopy (cryo-EM) and three-dimensional image reconstruction (cryo-reconstruction) and X-ray crystallography to determine the structure of the AAV9 capsid at 9.7- and 2.8-Å resolutions, respectively. The AAV9 capsid exhibits the surface topology conserved in all AAVs: depressions at each icosahedral two-fold symmetry axis and surrounding each five-fold axis, three separate protrusions surrounding each three-fold axis, and a channel at each five-fold axis. The AAV9 viral protein (VP) has a conserved core structure, consisting of an eight-stranded, ß-barrel motif and the αA helix, which are present in all parvovirus structures. The AAV9 VP differs in nine variable surface regions (VR-I to -IX) compared to AAV4, but at only three (VR-I, VR-II, and VR-IV) compared to AAV2 and AAV8. VR-I differences modify the raised region of the capsid surface between the two-fold and five-fold depressions. The VR-IV difference produces smaller three-fold protrusions in AAV9 that are less "pointed" than AAV2 and AAV8. Significantly, residues in the AAV9 VRs have been identified as important determinants of cellular tropism and transduction and dictate its antigenic diversity from AAV2. Hence, the AAV9 VRs likely confer the unique infection phenotypes of this serotype.

Construction, expression, and purification of recombinant αVß5 integrin.

A recombinant integrin expression system has been created for the large-scale production of αVß5 integrin extracellular domains that take advantage of Fos and Jun dimerization for expression in bacterial, insect, and mammalian cells. This utilizes an all-in-one vector, pQE-TriSystem, with molecular machinery for parallel expression without the need of additional subcloning. Optimal expression in HEK293 cells was determined by a time course analysis. The heterodimer was purified in a one-step nickel column purification scheme, and the sequence and functional state were confirmed by mass spectrometry and inhibition assays, respectively. The yields of αVß5 integrin obtained are in quantities suitable for multiple applications including structural biology and functional assays.

Molecular cloning, overexpression, and an efficient one-step purification of α5ß1 integrin.

The α5ß1 integrin heterodimer is involved in many cellular processes and is an anti-cancer therapeutic target. Therefore, access to quantities of protein suitable for studies aimed at understanding its biological functions is important. To this end, a large-scale protein expression system, utilizing the recombinant baculovirus/SF9 insect cell expression system, was created to produce the extracellular domain of the α5ß1 integrin. An incorporated 8X-histidine tag enabled one-step nickel-column purification. Following sequence confirmation by LC-MS/MS, the conformation of the heterodimer was characterized by native dot blot and negative stain electron microscopy. Cellular transduction inhibition studies confirmed biological activity. The system allows expression and purification of α5ß1 integrin in quantities suitable for an array of different experiments including structural biology.

Glucose regulated protein 78 diminishes α-synuclein neurotoxicity in a rat model of Parkinson disease.

Accumulation of human wild-type (wt) α-synuclein (α-syn) induces neurodegeneration in humans and in experimental rodent models of Parkinson disease (PD). It also leads to endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). We overexpressed glucose regulated protein 78, also known as BiP (GRP78/BiP), to test the hypothesis that this ER chaperone modulates the UPR, blocks apoptosis, and promotes the survival of nigral dopamine (DA) neurons in a rat model of PD induced by elevated level of human α-syn. We determined that α-syn activates ER stress mediators associated with pancreatic ER kinase-like ER kinase (PERK) and activating transcription factor-6 (ATF6) signaling pathways as well as proaoptotic CCAAT/-enhancer-binding protein homologous protein (CHOP) in nigral DA neurons. At the same time, overexpression of GRP78/BiP diminished α-syn neurotoxicity by down regulating ER stress mediators and the level of apoptosis, promoted survival of nigral tyrosine hydroxylase (TH) positive cells and resulted in higher levels of striatal DA, while eliminating amphetamine induced behavioral asymmetry. We also detected a complex between GRP78/BiP and α-syn that may contribute to prevention of the neurotoxicity caused by α-syn. Our data suggest that the molecular chaperone GRP78/BiP plays a neuroprotective role in α-syn-induced Parkinson-like neurodegeneration.

Restoration of visual function in P23H rhodopsin transgenic rats by gene delivery of BiP/Grp78.

The P23H mutation within the rhodopsin gene (RHO) causes rhodopsin misfolding, endoplasmic reticulum (ER) stress, and activates the unfolded protein response (UPR), leading to rod photoreceptor degeneration and autosomal dominant retinitis pigmentosa (ADRP). Grp78/BiP is an ER-localized chaperone that is induced by UPR signaling in response to ER stress. We have previously demonstrated that BiP mRNA levels are selectively reduced in animal models of ADRP arising from P23H rhodopsin expression at ages that precede photoreceptor degeneration. We have now overexpressed BiP to test the hypothesis that this chaperone promotes the trafficking of P23H rhodopsin to the cell membrane, reprograms the UPR favoring the survival of photoreceptors, blocks apoptosis, and, ultimately, preserves vision in ADRP rats. In cell culture, increasing levels of BiP had no impact on the localization of P23H rhodopsin. However, BiP overexpression alleviated ER stress by reducing levels of cleaved pATF6 protein, phosphorylated eIF2alpha and the proapoptotic protein CHOP. In P23H rats, photoreceptor levels of cleaved ATF6, pEIF2alpha, CHOP, and caspase-7 were much higher than those of wild-type rats. Subretinal delivery of AAV5 expressing BiP to transgenic rats led to reduction in CHOP and photoreceptor apoptosis and to a sustained increase in electroretinogram amplitudes. We detected complexes between BiP, caspase-12, and the BH3-only protein BiK that may contribute to the antiapoptotic activity of BiP. Thus, the preservation of photoreceptor function resulting from elevated levels of BiP is due to suppression of apoptosis rather than to a promotion of rhodopsin folding.

Structural studies of adeno-associated virus serotype 8 capsid transitions associated with endosomal trafficking.

The single-stranded DNA (ssDNA) parvoviruses enter host cells through receptor-mediated endocytosis, and infection depends on processing in the early to late endosome as well as in the lysosome prior to nuclear entry for replication. However, the mechanisms of capsid endosomal processing, including the effects of low pH, are poorly understood. To gain insight into the structural transitions required for this essential step in infection, the crystal structures of empty and green fluorescent protein (GFP) gene-packaged adeno-associated virus serotype 8 (AAV8) have been determined at pH values of 6.0, 5.5, and 4.0 and then at pH 7.5 after incubation at pH 4.0, mimicking the conditions encountered during endocytic trafficking. While the capsid viral protein (VP) topologies of all the structures were similar, significant amino acid side chain conformational rearrangements were observed on (i) the interior surface of the capsid under the icosahedral 3-fold axis near ordered nucleic acid density that was lost concomitant with the conformational change as pH was reduced and (ii) the exterior capsid surface close to the icosahedral 2-fold depression. The 3-fold change is consistent with DNA release from an ordering interaction on the inside surface of the capsid at low pH values and suggests transitions that likely trigger the capsid for genome uncoating. The surface change results in disruption of VP-VP interface interactions and a decrease in buried surface area between VP monomers. This disruption points to capsid destabilization which may (i) release VP1 amino acids for its phospholipase A2 function for endosomal escape and nuclear localization signals for nuclear targeting and (ii) trigger genome uncoating.

Enhanced expression of Pctk1, Tcf12 and Ccnd1 in hippocampus of rats: Impact on cognitive function, synaptic plasticity and pathology.

We previously identified a set of 50 genes that were differentially transcribed in the hippocampal CA1 region of aged, learning-impaired rats compared to aged, superior learning animals during a Morris water maze paradigm. In the current study, we expressed three of these genes (Pctk1, Tcf12 and Ccnd1), which had shown increased transcription in aged, learning impaired rats, in the hippocampus of young rats using viral gene transfer and tested for learning and memory deficits at age 7-14months. Pctk1 injected animals displayed a modest deficit in acquiring latency in both the Morris water maze and the reverse Morris maze. In the radial arm water maze paradigm, Pctk1, Tcf12 and Ccnd1 expressing animals all showed significant deficits in spatial working memory compared to controls. Rats injected with Ccnd1 and Tcf12, but not Pctk1, also showed a significant deficit in spatial reference memory in the radial arm water maze. Electrophysiological experiments revealed no difference in LTP in Ccnd1 and Pctk1 animals. However, LTD induced by low frequency stimulation was observed in control and Ccnd1 animals, but not in Pctk1 treated animals. In addition, neither Ccnd1 nor Pctk1 expression produced any detectable neuropathology. In contrast Tcf12 expressing animals displayed significant neurodegeneration in both CA1 and dentate gyrus. Several Tcf12 animals also developed tumors that appeared to be glioblastomas, suggesting that aberrant Tcf12 expression in the hippocampus is tumorigenic. Thus, behavioral experiments suggested that overexpression of Pctk1 and Ccnd1 produce a deficit in learning and memory, but electrophysiological experiments do not point to a simple mechanism. In contrast, the learning and memory deficits in Tcf12 animals are likely due to neuropathology associated with Tcf12 gene expression.

α-Synuclein expression in rat substantia nigra suppresses phospholipase D2 toxicity and nigral neurodegeneration.

We present genetic evidence that an in vivo role of α-synuclein (α-syn) is to inhibit phospholipase D2 (PLD2), an enzyme that is believed to participate in vesicle trafficking, membrane signaling, and both endo- and exocytosis. Overexpression of PLD2 in rat substantia nigra pars compacta (SNc) caused severe neurodegeneration of dopamine (DA) neurons, loss of striatal DA, and an associated ipsilateral amphetamine-induced rotational asymmetry. Coexpression of human wild type α-syn suppressed PLD2 neurodegeneration, DA loss, and amphetamine-induced rotational asymmetry. However, an α-syn mutant defective for inhibition of PLD2 in vitro also failed to inhibit PLD toxicity in vivo. Further, reduction of PLD2 activity in SNc, either by siRNA knockdown of PLD2 or overexpression of α-syn, both produced an unusual contralateral amphetamine-induced rotational asymmetry, opposite to that seen with overexpression of PLD2, suggesting that PLD2 and α-syn were both involved in DA release or reuptake. Finally, α-syn coimmunoprecipitated with PLD2 from extracts prepared from striatal tissues. Taken together, our data demonstrate that α-syn is an inhibitor of PLD2 in vivo, and confirm earlier reports that α-syn inhibits PLD2 in vitro. Our data also demonstrate that it is possible to use viral-mediated gene transfer to study gene interactions in vivo.