Intrathecal long-term gene expression by self-complementary adeno-associated virus type 1 suitable for chronic pain studies in rats
© Storek et al; licensee BioMed Central Ltd. 2006
Received: 09 November 2005
Accepted: 30 January 2006
Published: 30 January 2006
Intrathecal (IT) gene transfer is an attractive approach for targeting spinal mechanisms of nociception but the duration of gene expression achieved by reported methods is short (up to two weeks) impairing their utility in the chronic pain setting. The overall goal of this study was to develop IT gene transfer yielding true long-term transgene expression defined as ≥ 3 mo following a single vector administration. We defined "IT" administration as atraumatic injection into the lumbar cerebrospinal fluid (CSF) modeling a lumbar puncture. Our studies focused on recombinant adeno-associated virus (rAAV), one of the most promising vector types for clinical use.
Conventional single stranded rAAV2 vectors performed poorly after IT delivery in rats. Pseudotyping of rAAV with capsids of serotypes 1, 3, and 5 was tested alone or in combination with a modification of the inverted terminal repeat. The former alters vector tropism and the latter allows packaging of self-complementary rAAV (sc-rAAV) vectors. Combining both types of modification led to the identification of sc-rAAV2/l as a vector that performed superiorly in the IT space. IT delivery of 3 × 10e9 sc-rAAV2/l particles per animal led to stable expression of enhanced green fluorescent protein (EGFP) for ≥ 3 mo detectable by Western blotting, quantitative PCR, and in a blinded study by confocal microscopy. Expression was strongest in the cauda equina and the lower sections of the spinal cord and only minimal in the forebrain. Microscopic examination of the SC fixed in situ with intact nerve roots and meninges revealed strong EGFP fluorescence in the nerve roots.
sc-rAAVl mediates stable IT transgene expression for ≥ 3 mo. Our findings support the underlying hypothesis that IT target cells for gene transfer lack the machinery for efficient conversion of the single-stranded rAAV genome into double-stranded DNA and favor uptake of serotype 1 vectors over 2. Experiments presented here will provide a rational basis for utilizing IT rAAV gene transfer in basic and translational studies on chronic pain.
The spinal cord (SC) is an important center of nociceptive signal processing and a notable anatomic target for analgesic therapy development. Delivery of drugs to the SC by common systemic routes, i.e. pills taken by mouth, patches applied to the skin, and injections given intravenously has two potential pitfalls: Firstly, the drug may not cross the blood brain barrier (BBB) if it is hydrophilic and/or has a high molecular weight. Secondly, once across the BBB, it may cause untoward effects by acting indiscriminately on supraspinal structures of the central nervous system (CNS). Intrathecal (IT) delivery addresses these issues by targeting drugs anatomically to the SC. While IT pharmacokinetics may vary depending on the drug's lipophilicity, its stability in the cerebrospinal fluid (CSF), the flow pattern of CSF in an individual animal or patient, and other modifying factors , this form of administration has in practice been found to allow specific spinal targeting of a variety of agents. It has been used in laboratory animals for decades [2, 3] and proven to be efficacious in patients . However, the need to implant and maintain an IT catheter and to infuse drugs through a pump system to achieve a long-term effect requires a specialized medical team  and can be complication-ridden [6–8]. This has effectively prevented the adoption of IT pain therapy in large groups of patients who could benefit, e.g. many with advanced cancer under the primary care of oncologists. In the investigational setting, targeting of a newly identified spinal pain mechanism may be curbed by the lack of an appropriate small molecule or recombinant protein. In many such instances the corresponding cDNAs are available and expressing them IT would suffice to proceed with a particular experiment or a translational clinical study.
IT gene transfer has been investigated by several groups as an alternative to IT peptide or protein delivery [9–14]. The cells reached with current gene vectors in the IT space, e.g. meningeal fibroblasts, are not directly involved in nociceptive signal processing but can assume an active role by secreting a relevant peptide or protein (encoded by the gene vector) into the CSF. The peptide or protein can then act in a paracrine fashion on cells in the spinal cord as shown in IT gene transfer experiments for β-endorphin and interleukin (IL)-10 [10–12]. Different types of gene vectors have been used in the IT space: liposomes , plasmid DNA with electroporation [13, 14], plasmid DNA in hypertonic diluent , adenovirus [9–11], and adeno-associated virus (AAV) . While all studies reported some promising evidence of transgene activity, the duration of expression never exceeded two weeks (except for a protocol using repeat vector administration ). Thus, long-term gene expression has not been achieved with IT delivered vectors. For other body sites, the problem of limited duration of transgene expression has been a common finding and a favorite topic of gene therapy research. Few vector systems have emerged that can often overcome this problem, in particular vectors derived from AAV, lentiviruses, or herpes simplex viruses (HSV). Recombinant AAV (rAAV) is popular because it may be a particularly safe choice. It is derived from the adeno-associated virus that commonly infects humans without ever causing symptoms or a disease. rAAV vectors are engineered to be devoid of viral genes, integrate rarely into chromosomes, and appear to pose little risk to the genomic integrity and growth control mechanisms of primary cells. Clinical safety and efficacy data on rAAV is currently accumulating from several clinical trials for retinal degeneration, hemophilia, Parkinson's, and other diseases .
The overall goal of the present study was to develop IT gene transfer yielding true long-term expression. We defined this as stable expression for at least three months following a single vector administration. Many chronic pain models, e.g. the chronic constriction injury model of neuropathic pain induce alterations in nociceptive behaviour for approximately two months thereby exceeding the two-week limit of previous reports on IT gene transfer but staying within the time frame set here. We defined "IT" administration as atraumatic injection into the lumbar CSF, i.e. modelling a lumbar puncture (LP). A LP is a clinically safe procedure because it does not sever the SC. It thereby differs from reports "subarachnoid" vector delivery achieved by a laminectomy at a higher SC level followed by blind transdural vector injection with a sharp pipette , which leads to highly localized gene expression consistent an intraparenchymal injection into the SC . We focused on AAV based vectors even though a recent study in this journal had reported that IT rAAV2 expressed only for two weeks . We hypothesized that the IT performance of the conventional single-stranded rAAV serotype 2 used in the published studies was limited by a failure of the capsid protein to mediate uptake and intracellular virus particle processing and/or by a failure of the cellular machinery to mediate second-strand DNA synthesis, which is a requirement for gene expression from a single-stranded DNA parvovirus like AAV. We tested these possibilities using rAAV vectors pseudotyped with capsid proteins of the serotypes 1, 2, 3, and 5 and by employing a recently developed rAAV variant that is packaged as double-stranded DNA, a "self-complementary" rAAV (sc-rAAV). By combining both approaches, we found that sc-rAAV serotype 1 has superior IT gene transfer capability mediating reliable and stable IT gene expression for at least four months in rats.
Modification of rAAV capsid and ITR for improved IT gene transfer
rAAV2 is the best characterized type of rAAV vector and has been reported to mediate long-term gene expression in the brain, liver, muscle and other organs . We prepared rAAV2 expressing enhanced green fluorescent protein (EGFP) under the control of the CMV promoter and injected it IT into twenty rats at doses up to 1010 viral particles per animal. Ten control animals were injected with phosphate buffered saline (PBS). Animals were sacrificed after four weeks or four months, because rAAV2 has been reported to reach maximal expression levels in this time frame in other organs . SC tissue from half of the animals was prepared for Western blotting (fresh frozen at -80°C) and from the other half for microscopy (requiring formalin fixation). No EGFP could be detected by Western blotting of tissue pooled from the upper and lower half of the SC or by extensive review of sections from all SC segments including the cauda equina (CE) using laser scanning microscopy (data not shown).
Transgene expression in the brain after IT vector delivery
Anatomical localization of transgene expression
We next localized EGFP expression in microscopic sections of the SC by laser-scanning microscopy. Using SC tissue from EGFP-transgenic and control mice as positive and negative controls, we established settings that suppressed background fluorescence and/or identified areas of unspecific fluorescence by their broader emission spectrum. Following an open label training period, the principal microscopist in the team (NMH) was given 15 slides in a blinded fashion that had been selected randomly from a collection of two hundred slides by a person not involved in the study and not knowledgeable of the animal's group assignment. Slides included sections from all parts of the SC and CE and were in roughly equal proportion from animals injected either with sc-rAAV2/l or with PBS. Examination of the slides under these conditions led to the correct identification of 7 of 7 tissue samples as positive, i.e. prepared from rats injected IT with sc-rAAV2/l and of identification of 8 of 8 samples as negative, i.e. prepared from rats injected with PBS. If the outcome of such an experiments is determined by chance, it follows a binomial distribution since the outcome is binary (correct/incorrect), the experiment consists of independent trials (each slide was evaluated independently of the outcome for the previous slide), the number of trials is fixed in advance, and an equal probability of success for each individual trial (each slide) exists, which is similar to the flip of a fair coin = 0.5. Thus, we calculated that the likelihood of 15 consecutive successes (= correct identification of slides) was (l/2)15 = 6.1 × 10-5, which means that the result is highly significant (p < 0.0001) and unlikely to consequence of chance. This argument provided us with confidence that the fluorescence observed under these conditions reflected a true gene transfer effect. The majority of specific fluorescence was observed in the nerve roots. A typical example from the lumbar SC is shown in figure 3B. This finding is consistent with the Western blot finding of figure 3A, namely that the portion of the SC with the greatest density of nerve roots, the CE, expresses EGFP at the highest level. We cannot exclude EGFP expression in other areas including possibly the DRG. In fact, on occasion we had the impression that EGFP could also be observed in meningeal fibroblasts and occasionally in cells intrinsic to the SC. These possibilities are currently under investigation with higher titer vectors preparations and improved detection techniques based on immunohistochemistry.
Long-term gene expression
The present study addressed the question if rAAV vectors constitute a notable improvement upon other gene vectors currently used for IT rat studies on chronic pain, in particular, if rAAV vectors can mediate efficient long-term gene expression in this paradigm. To overcome known inadequacies of conventional rAAV2 in the IT space, we tested alterations to its two principal components, the capsid and the genome. Specifically, we pseudotyped rAAV vectors with capsids from various serotypes and modified its genome to allow high-efficiency packaging of double-stranded (self-complementary, "sc-") instead of single-stranded recombinant genomes. While the parent vector, rAAV2 performed poorly yielding low expression levels that were detectable only by qPCR, a vector carrying both above modifications, sc-rAAV2/l performed favourably. EGFP marker gene expression was reproducibly detected in vivo by qPCR and Western blotting and under blinded conditions by confocal microscopy. Transgene levels were found by Western to be highest in the CE and the caudal SC, which was consistent with our microscopic observations that the nerve roots displayed strong specific EGFP fluorescence. Expression occurred as early as one week, increased subsequently, and remained stable between four and twelve weeks. These results supported the underlying hypothesis that the poor performance of rAAV2 vectors resulted from an inability of potential target cells in the rat IT space to mediate binding of serotype 2 capsids and/or to provide the machinery for efficient conversion of the single-stranded vector genome into a double-stranded DNA template ready to be transcribed. While further questions regarding the IT properties of these vectors could be addressed with additional marker gene studies as discussed below, the presented data should allow the efficient design of improved vectors for IT delivery in chronic pain models, e.g. since completion of the study we were able to construct an sc-rAAV2/l vector expressing a secreted analgesic gene product that we could detect in CSF from the lumbar IT space and the cisterna magna two months after IT vector application (data not shown).
The utility of gene therapy for a specific medical application depends critically on how well the vector properties match the requirements set forth by the molecular target. IT gene therapy for pain was proposed a decade ago in a report on an artificial opioid gene with paracrine spinal analgesic activity . When the gene was delivered IT by an adenovirus vector  or by grafting of retrovirally transduced fibroblasts  it suppressed nociceptive behaviour in rats. However, the approach had shortcomings that discouraged further exploration in pre-clinical studies. Firstly, it did not allow for regulation of the therapeutic gene following vector delivery, which may have been desirable in this case, because opioid medications typically require dose adjustments in the clinical setting. Secondly, gene expression and thus analgesic activity was not sustained beyond two weeks due to the gene transfer technology, namely an inflammatory response in the case of the adenoviral vectors and promoter shut-down in the case of the Moloney-type retroviral vectors. In the early era of gene therapy research cases like this – an appealing idea, support through a "proof of principle" study, then the realization that no available vector could do the job well enough – were common precipitating a call to pursue "The basic science of gene therapy" , i.e. a focus on studying the vectors in order to match their biological properties to a potential application or to improve their shortcomings. In the area of gene therapy for pain Drs. Glorioso, Fink, Yeomans et al. took the former approach by building their pre-pro-enkephalin gene transfer around the natural ability of HSV to infect peripheral sensory nerves and be retrogradely transported to the DRG [29–31]. Others including ourselves pursued the latter approach, i.e. searching for an improved vector system to achieve better gene delivery by the administration route that appears most appropriate for targeting spinal pain mechanisms, namely IT delivery. In a recent review article we provided an account of which vector types were tested by ourselves and others for IT use failing to provide sustained expression, why IT AAV appeared promising despite the limitations encountered in published studies, and which molecular targets characterized by others may be most suitable for IT therapy of chronic pain states without requiring regulation of the therapeutic gene or dose adjustments except through the delivered dose of vector . In the present article, we show the result of our search for a better vector providing the first report of long-term gene expression after a single IT administration of any vector type. The approach should be useful in its present form for studies in rats requiring chronic delivery of a secreted transgene product into the spinal CSF, e.g. IL-10 for the suppression of glial activation as proposed by Drs. Milligan, Watkins et al. [11, 12, 33, 34]. The AAV vector dose used in this study, 3 × l09 viral particles per animal could be titrated downwards if lower transgene levels are desired that are in line with levels obtained with naked DNA/plasmid gene transfer (unpublished). AAV vector doses can also be increased to ≥ 1011 viral particles per rat with well purified/concentrated AAV preparations while keeping the IT injection volume constant. This should provide flexibility for different therapeutic genes and experimental goals.
Extrapolating from the experience with rAAV delivery to other body sites (muscle, liver, brain), it can be expected that IT delivered rAAV does not induce cellular immunity to vector component in rodents and will have minimal or no effects ascribable to the gene transfer procedure thereby permitting a clean evaluation of the transgene of interest in mice or rats. rAAV appears to be among the safest gene transfer options for potential use in patients and is currently undergoing clinical evaluation for retinal degeneration, hemophilia, Parkinson's and other diseases . It may therefore be a candidate vector for future translational studies serving as a research tool to validate whether the manipulation of a specific spinal molecular mechanism that is operative in rodent models is similarly important in patients, e.g. glial activation in neuropathic pain. In order to evaluate rAAV for such use, a number of hypotheses need to be tested in large animals, which are related to the vector technology. Up-scaling experiments to large animals has been a critical benchmarks in other fields of gene therapy research, e.g. hemophilia, an area in which one of us (CEW) has been closely involved with this issue [35, 36]. Establishing in pigs, dogs, or non-human primates whether a serological and/or cellular immune response occurs to IT rAAV and, perhaps more importantly, to a specific transgene product of interest will be critical . In addition such experiments would allow to determine gene product levels, biodistribution, and target cells of IT rAAV in a model that appears more realistic for humans than rodents. Hopefully, experiments as presented and proposed here will provide a rational basis for utilizing gene transfer in basic and translational studies on chronic pain.
Animals and vector application through an IT catheter
All procedures involving animals were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) and followed national guidelines and requirements set forth by the National Institutes of Health (NIH), Bethesda, MD, USA. Adult male Sprague-Dawley rats of 300–350 g body weight were used.
Quantification of gene expression by Western blotting and reverse transcriptase PCR
Animals were sacrificed by carbon dioxide inhalation. The SC and CE were immediately removed with the meningeal linings intact and snap frozen on a metal block cooled I liquid nitrogen. The SC/CE tissue block was then subdivided into two or four portions along the rostral-caudal axis as indicated in the respective figure legend and pulverized in a ceramic grinder under liquid nitrogen.
For quantification of EGFP protein by Western blotting, an aliquot of the pulverized tissue was homogenized in RIPA buffer on ice with a high speed tissue homogenizer (Tissuemizer, Fisher Scientific) and centrifuged for 30 min at 13,000 rpm at 4°C allowing elimination of cell debris found in the pellet. The protein concentration of the supernatant was determined with the DC Protein Assay Kit II according to the manufacture's instruction (Bio-Rad). Spinal cord tissue of an EGFP transgenic mouse or recombinant EGFP protein (Clontech) was used as a positive control and quantitative standard. 20 ug total protein was electrophoresed in a 12% SDS-polyacrylamide gel and electroblotted onto a nitrocellulose membrane. The membranes were incubated in blocking solution containing 5% nonfat milk in 1× Tween Tris-buffered saline. Membranes were then incubated with the mouse monoclonal antibody against EGFP (JL-8, BD-Biosciences, dilution 1:1000) for 2 h at room temperature. Membranes were washed and incubated anti-mouse horse-reddish peroxidase-conjugated antibody (Amersham, dilution 1:2500) for 1 h at room temperature. Membranes were again washed and then covered with the Lumi-Light Western Blotting Substrate (Roche). The resulting chemiluminescence was then digitally recorded using the a cooled camera luminescent imaging system (LAS-3000, Fuji).
For quantification of EGFP RNA, an aliquot of the pulverized frozen tissue was dissolved and thereby thawed in RLT buffer followed by isolation of total RNA using a mini-column based kit (RNeasy Mini Kit, Qiagen). Single-stranded cDNA was synthesized in a 25 μl reaction from 1 μg total RNA using 2.5 ul 10× buffer, 10 ul MgC12, 2.5 ul dNTP, lul oligo dT, 0.5 ul Rnasin, 0.8 μl RT, and nuclease free water at 25°C for 10 min, and 45°C for 60 min followed by inactivation at 95°C for 5 min (Reserve Transcription Kit, Promega).
Quantitative PCR was performed in triplicates on an ABI PRISM 7900HT thermocycler (Applied Biosystems) using the fluorophore SYBR Green (Applied Biosystems). A cDNA amount corresponding to 15 ng of RNA was used per reaction in a total volume of 10 μl. The concentration of each primer was 0.4 μM. Primers for EGFP were: Forward 5'-AGC AAA GAC CCC AAC GAG AA-3', reverse 5'-GGC GGC GGT CAC GAA-3'. Primers for rat ubiquitin were: Forward 5'-GTG GCT ATT AAT TCT TCA GTC TGC AT-3', reverse 5'-GCA AAT GGC TAG AGT GCA GAG TAA-3'. Following incubation at 50°C for 2 min and denaturation at 95°C for 10 min 40 thermal cycles were executed as follows: 95°C × 15s, 60°C × 1 min.
Tissue slide preparation and confocal microscopy
Animals were anesthetized with Chloral Hydrate 30% (Sigma, Lot no. 045K0664) dissolved in sterile water and injected intraperitoneally at a volume of 0.8–1.4 mL, which was repeated if needed to achieve deep anesthesia. Animals were then perfused transcardially for 1 min with freshly prepared ice-cold paraformaldehyde 1% (PFA, Electron Microscopy Sciences, catalogue number 19210) in PBS, followed by perfusion with PFA 4% and glutaraldehyde 0.125% (Electron Microscopy Sciences, 50% solution, cat. no. 16310) for 13 min. The spinal column was then removed by first exposing it in its entire length from the cisternal membrane to 14 cm caudally by anatomical dissection of the dorsal muscles and by then transecting of the ribs and other adjacent tissue. The spinal column with the SC, CE and meninges intact was then post-fixed in PFA 4% for 24 h. The spinal column was then subjected to a decalcification protocol consisting of slow-motion shaking of the tissue at 4°C in a buffer of 10% ethylene diamine tetra acetic acid (EDTA, Sigma, cat. no. E5134) prepared in distilled water and adjusted to a pH of approximately 7.4–7.6. The EDTA buffer was changed daily during the first two weeks followed by changes every other day until the bone of the spinal column was completely decalcified at 4 weeks. The tissue was then cryoprotected by immersion in increasing concentrations of sucrose (Sigma, Lot no. 62H0607) dissolved in PBS, namely at concentrations of 10% × 2 h, 20% × 2 h, and 30% overnight. The tissue was the dissected in smaller blocks, embedded in Optimal Cutting Temperature Compound (OCT, Tissue-Tek, 4583, Sukura) or Tissue Freezing Medium (TBS, Electron Microscopy Sciences, cat. no. 72592). Tissue sections of 50–60 microns were then cut on a Cryostat (2800 Frigocut E, Reichert-Jung), thaw-mounted on pre-treated glass slides (50 × 75 mm slides, Brain Research Laboratory), gently PBS-washed, and protected with a cover glass (48 × 60 mm no.l, cat. no. 4860-1, Brain Research Laboratory) using mounting medium (Vectashield hardset H-1400, Vector Laboratories).
Microscopic examination of tissue slides was performed with a confocal laser-scanning microscope (LSM 510 META, Zeiss Inc.). An excitation wavelength of 488 nm was used (Argon laser, intensity 5%). To detect EGFP, fluorescence images were recorded at an emission wavelength of 505–530 nm. Unspecific fluorescence was visualized at >560 nm. Settings for gain and offset were optimized using positive and negative control tissue and open label samples. Subsequently all settings were kept constant for the blinded examinations of slides.
All rAAV vectors were produced, purified, and titered as previously described . The sc-rAAV/EGFP DNA templates utilized for encapsidation in AAV 1, 2, and 5 capsids were derived from plasmid pHpa-trs-SK developed by one of the co-authors . The AAV1 and AAV5 packaging plasmids were generously supplied through the Gene Therapy Center, University of North Carolina, Chapel Hill (Dr. Jude Samulski).
The significance level for the scoring of slides by confocal microscopy under blinded conditions was calculated based on the binomial distribution, since the data type in this case was categorical (success/failure = correct/incorrect scoring of a slide), trials were independent (each slide was evaluated independently of the outcome for the previous slide), the number of trials was fixed in advance (15 coded slides handed to the microscopist), and the probability of success was uniformly 0.5 for each individual trial. If all trials n are successful as in our case, the significance level is calculated in a simplified way as (probability of success)n.
The data of all other experiments was continuous (i.e. non-categorical), unpaired, and of an unknown distribution type. Therefore a two-sided, non-paired Wilcoxon-Mann-Whitney test was used to assess differences between groups.
List of abbreviations
blood brain barrier
central nervous system
enhanced green fluorescent protein
herpes simplex viruses
Institutional Animal Care and Use Committee
inverted terminal repeats
phosphate buffered saline
quantitative polymerase chain reaction
recombinant adeno-associated virus
self-complementary recombinant adeno-associated virus
self-complementary recombinant adeno-associated virus type 2 pseudotyped with a capsid from serotype 1 (= packed in capsid protein from AAV1)
Beth Wittenberg coded microscopic slides for the blinded examination by confocal microscopy. We would like to acknowledge research support by the American Society for Clinical Oncology (ASCO), the T.J. Martell Foundation, the G&P Foundation for Cancer Research, and the National Institute for Neurological Disorders and Stroke (1K08NS046012) to A.S.B.
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