Intraspinal AAV Injections Immediately Rostral to a Thoracic Spinal Cord Injury Site Efficiently Transduces Neurons in Spinal Cord and Brain

PTEN knockout using retrogradely transported AAVs transiently restores locomotor abilities in both acute and chronic spinal cord injury

Restoring function in chronic stages of spinal cord injury (SCI) has often been met with failure or reduced efficacy when regenerative strategies are delayed past the acute or sub-acute stages of injury. Restoring function in the chronically injured spinal cord remains a critical challenge. We found that a single injection of retrogradely transported adeno-associated viruses (AAVrg) to knockout the phosphatase and tensin homolog protein (PTEN) in chronic SCI can effectively target both damaged and spared axons and transiently restore locomotor functions in near-complete injury models. AAVrg's were injected to deliver cre recombinase and/or a red fluorescent protein (RFP) under the human Synapsin 1 promoter (hSyn1) into the spinal cords of C57BL/6 PTENFloxΔ/Δ mice to knockout PTEN (PTEN-KO) in a severe thoracic SCI crush model at both acute and chronic time points. PTEN-KO improved locomotor abilities in both acute and chronic SCI conditions over a 9-week period. Regardless of whether treatment was initiated at the time of injury (acute), or three months after SCI (chronic), mice with limited hindlimb joint movement gained hindlimb weight support after treatment. Interestingly, functional improvements were not sustained beyond 9 weeks coincident with a loss of RFP reporter-gene expression and a near-complete loss of treatment-associated functional recovery by 6 months post-treatment. Treatment effects were also specific to severely injured mice; animals with weight support at the time of treatment lost function over a 6-month period. Retrograde tracing with Fluorogold revealed viable neurons throughout the motor cortex despite a loss of RFP expression at 9 weeks post-PTEN-KO. However, few Fluorogold labeled neurons were detected within the motor cortex at 6 months post-treatment. BDA labeling from the motor cortex revealed a dense corticospinal tract (CST) bundle in all groups except chronically treated PTEN-KO mice, indicating a potential long-term toxic effect of PTEN-KO to neurons in the motor cortex which was corroborated by a loss of β-tubulin III labeling above the lesion within spinal cords after PTEN-KO. PTEN-KO mice had significantly more β-tubulin III labeled axons within the lesion when treatment was delivered acutely, but not chronically post-SCI. In conclusion, we have found that using AAVrg's to knockout PTEN is an effective manipulation capable of restoring motor functions in chronic SCI and can enhance axon growth of currently unidentified axon populations when delivered acutely after injury. However, the long-term consequences of PTEN-KO on neuronal health and viability should be further explored.

The use of viral vectors to promote repair after spinal cord injury

Spinal cord injury (SCI) is a devastating event that can permanently disrupt multiple modalities. Unfortunately, the combination of the inhibitory environment at a central nervous system (CNS) injury site and the diminished intrinsic capacity of adult axons for growth results in the failure for robust axonal regeneration, limiting the ability for repair. Delivering genetic material that can either positively or negatively modulate gene expression has the potential to counter the obstacles that hinder axon growth within the spinal cord after injury. A popular gene therapy method is to deliver the genetic material using viral vectors. There are considerations when deciding on a viral vector approach for a particular application, including the type of vector, as well as serotypes, and promoters. In this review, we will discuss some of the aspects to consider when utilizing a viral vector approach to as a therapy for SCI. Additionally, we will discuss some recent applications of gene therapy to target extrinsic and/or intrinsic barriers to promote axon regeneration after SCI in preclinical models. While still in early stages, this approach has potential to treat those living with SCI.

Harnessing rAAV-retro for gene manipulations in multiple pathways that are interrupted after spinal cord injury

This paper explores the potential of rAAV2-retro to deliver gene modifying cargoes to the cells of origin of multiple pathways that are interrupted by spinal cord injury (SCI), summarizing data from previous studies and new data from additional experiments. rAAV-retro exhibits uniquely robust and reliable long-distance retrograde transport from pre-terminal axons and synapses back to neuronal bodies. Previous studies have documented that various AAV-based genetic modifications can enable axon regeneration after SCI, but these have targeted the cells of origin of one pathway at a time. In contrast, rAAV-retro can simultaneously transduce large numbers of neurons of origin of multiple spinal pathways with single injections into the spinal cord. Our initial studies use Rosa^tdTomato and double transgenic PTEN^f/f; Rosa^tdTomato mice in which transfection with rAAV-retro/Cre deletes PTEN and activates tdT expression in the same neurons. Injections of rAAV-retro/Cre into the cervical, thoracic and lumbar spinal cord led to topographically specific retrograde transduction in cortical motoneurons and neurons in subcortical regions that give rise to different spinal pathways. Our results confirm and extend previous studies indicating selective transduction of neurons that terminate at the level of the injection with minimal retrograde transduction of axons in transit to lower levels. We document feasibility of using rAAV-retro expressing shRNA against PTEN along with a GFP reporter (rAAV-retro-shPTEN/GFP) to effectively knock down PTEN in multiple populations of neurons, which can be used in any species. Some limitations and caveats of currently available rAAV-retros are discussed. Together, our results support the potential applications of rAAV-retro for AAV-based gene-modifications for SCI.

An Approach to Maximize Retrograde Transport Based on the Spatial Distribution of Motor Endplates in Mouse Hindlimb Muscles

Knowledge regarding the relationship between muscles and the corresponding motor neurons would allow therapeutic genes to transport into specific spinal cord segments. Retrograde tracing technique by targeting the motor endplate (MEP), a highly specialized structure that offers direct access to the spinal motor neurons, has been used to elucidate the connectivity between skeletal muscles and the innervating motor neuron pools. However, current injection strategies mainly based on blind injection or the local MEP region might lead to an underestimation of the motor neuron number due to the uneven distribution of MEP in skeletal muscles. In this work, we proposed a novel intramuscular injection strategy based on the 3D distribution of the MEPs in skeletal muscles, applied the 3D intramuscular injection to the gastrocnemius and tibialis anterior for retrograde tracing of the corresponding motor neurons, and compared this with the existing injection strategy. The intramuscular diffusion of the tracer demonstrated that 3D injection could maximize the retrograde transport by ensuring a greater uptake of the tracer by the MEP region. In combination with optical clearing and imaging, we performed 3D mapping and quantification of the labeled motor neurons and confirmed that 3D injection could label more motor neurons than the current injection method. It is expected that 3D intramuscular injection strategy will help elucidate the connective relationship between muscles and motor neurons faithfully and becomes a promising tool in the development of gene therapy strategies for motor neuron diseases.

MicroRNA-7 promotes motor function recovery following spinal cord injury in mice

Spinal cord injury (SCI) is a devastating neurological condition for which there are no effective therapies. Following an initial injury, there is a cascade of multiple downstream events termed secondary injury. Thus, therapeutic approaches targeting a single pathway may not offer the best solution for treating SCI. One of the most attractive properties of microRNAs (miR) as potential therapeutics is that they are highly effective in regulating complex biological pathways by targeting multiple genes and pathways. The current study investigated the role of miR-7-5p (miR-7), which was previously shown to have neuroprotective functions, in promoting motor function recovery following SCI. We used an adeno-associated virus 1 (AAV1) vector to deliver the gene encoding miR-7 to the spinal cord of adult mice and found that this virus was mainly transduced into the neurons of the spinal cord. Transduction of AAV1-miR-7 improved hindlimb locomotor function following SCI over an 8-week observation period. This improvement was accompanied by reduced neuronal loss in the lesion. In addition, the beneficial effect of miR-7 was associated with enhanced levels of TH-positive axons in the lesion. Taken together, we suggest that miR-7 improves motor function recovery after SCI by protecting neuronal death and increasing axon levels. These findings suggest that miR-7 could be developed as a potential treatment for SCI in human.

Retrograde Axonal Transport Property of Adeno-Associated Virus and Its Possible Application in Future

Gene therapy has become a treatment method for many diseases. Adeno-associated virus (AAV) is one of the most common virus vectors, is also widely used in the gene therapy field. During the past 2 decades, the retrograde axonal transportability of AAV has been discovered and utilized. Many studies have worked on the retrograde axonal transportability of AAV, and more and more people are interested in this field. This review described the current application, influence factors, and mechanism of retrograde axonal transportability of AAV and predicted its potential use in disease treatment in near future.

Dual-Viral Transduction Utilizing Highly Efficient Retrograde Lentivirus Improves Labeling of Long Propriospinal Neurons

The nervous system coordinates pathways and circuits to process sensory information and govern motor behaviors. Mapping these pathways is important to further understand the connectivity throughout the nervous system and is vital for developing treatments for neuronal diseases and disorders. We targeted long ascending propriospinal neurons (LAPNs) in the rat spinal cord utilizing Fluoro-Ruby (FR) [10kD rhodamine dextran amine (RDA)], and two dual-viral systems. Dual-viral tracing utilizing a retrograde adeno-associated virus (retroAAV), which confers robust labeling in the brain, resulted in a small number of LAPNs being labeled, but dual-viral tracing using a highly efficient retrograde (HiRet) lentivirus provided robust labeling similar to FR. Additionally, dual-viral tracing with HiRet lentivirus and tracing with FR may preferentially label different subpopulations of LAPNs. These data demonstrate that dual-viral tracing in the spinal cord employing a HiRet lentivirus provides robust and specific labeling of LAPNs and emphasizes the need to empirically optimize viral systems to target specific neuronal population(s).

AAV2-mediated and hypoxia response element-directed expression of bFGF in neural stem cells showed therapeutic effects on spinal cord injury in rats

Neural stem cell (NSCs) transplantation has been one of the hot topics in the repair of spinal cord injury (SCI). Fibroblast growth factor (FGF) is considered a promising nerve injury therapy after SCI. However, owing to a hostile hypoxia condition in SCI, there remains a challenging issue in implementing these tactics to repair SCI. In this report, we used adeno-associated virus 2 (AAV2), a prototype AAV used in clinical trials for human neuron disorders, basic FGF (bFGF) gene under the regulation of hypoxia response element (HRE) was constructed and transduced into NSCs to yield AAV2-5HRE-bFGF-NSCs. Our results showed that its treatment yielded temporally increased expression of bFGF in SCI, and improved scores of functional recovery after SCI compared to vehicle control (AAV2-5HRE-NSCs) based on the analyses of the inclined plane test, Basso-Beattie-Bresnahan (BBB) scale and footprint analysis. Mechanistic studies showed that AAV2-5HRE-bFGF-NSCs treatment increased the expression of neuron-specific neuronal nuclei protein (NeuN), neuromodulin GAP43, and neurofilament protein NF200 while decreased the expression of glial fibrillary acidic protein (GFAP) as compared to the control group. Further, the expressions of autophagy-associated proteins LC3-II and Beclin 1 were decreased, whereas the expression of P62 protein was increased in AAV2-5HRE-bFGF-NSCs treatment group. Taken together, our data indicate that AAV2-5HRE-bFGF-NSCs treatment improved the recovery of SCI rats, which is accompanied by evidence of nerve regeneration, and inhibition of SCI-induced glial scar formation and cell autophagy. Thus, this study represents a step forward towards the potential use of AAV2-5HRE-bFGF-NSCs for future clinical trials of SCI repair.

Deubiquitinase USP18 regulates reactive astrogliosis by stabilizing SOX9

Reactive astrogliosis is a pathological feature of spinal cord injury (SCI). The ubiquitin-proteasome system plays a crucial role in maintaining protein homeostasis and has been widely studied in neuroscience. Little, however, is known about the underlying function of deubiquitinating enzymes in reactive astrogliosis following SCI. Here, we found that ubiquitin-specific protease 18 (USP18) was significantly upregulated in astrocytes following scratch injury, and in the injured spinal cord in mice. Knockdown of USP18 in vitro and conditional knockout of USP18 in astrocytes (USP18 CKO) in vivo significantly attenuated reactive astrogliosis. In mice, this led to widespread inflammation and poor functional recovery following SCI. In contrast, overexpression of USP18 in mice injected with adeno-associated virus (AAV)-USP18 had beneficial effects following SCI. We showed that USP18 binds, deubiquitinates, and thus, stabilizes SRY-box transcription factor 9 (SOX9), thereby regulating reactive astrogliosis. We also showed that the Hedgehog (Hh) signaling pathway induces expression of USP18 through Gli2-mediated transcriptional activation after SCI. Administration of the Hh pathway activator SAG significantly increased reactive astrogliosis, reduced lesion area and promoted functional recovery in mice following SCI. Our results demonstrate that USP18 positively regulates reactive astrogliosis by stabilizing SOX9 and identify USP18 as a promising target for the treatment of SCI.

Cerebral Organoids: A Human Model for AAV Capsid Selection and Therapeutic Transgene Efficacy in the Brain

The development of gene therapies for central nervous system disorders is challenging because it is difficult to translate preclinical data from current in vitro and in vivo models to the clinic. Therefore, we developed induced pluripotent stem cell (iPSC)-derived cerebral organoids as a model for recombinant adeno-associated virus (rAAV) capsid selection and for testing efficacy of AAV-based gene therapy in a human context. Cerebral organoids are physiological 3D structures that better recapitulate the human brain compared with 2D cell lines. To validate the model, we compared the transduction efficiency and distribution of two commonly used AAV serotypes (rAAV5 and rAAV9). In cerebral organoids, transduction with rAAV5 led to higher levels of vector DNA, transgenic mRNA, and protein expression as compared with rAAV9. The superior transduction of rAAV5 was replicated in iPSC-derived neuronal cells. Furthermore, rAAV5-mediated delivery of a human sequence-specific engineered microRNA to cerebral organoids led to a lower expression of its target ataxin-3. Our studies provide a new tool for selecting and deselecting AAV serotypes, and for demonstrating therapeutic efficacy of transgenes in a human context. Implementing cerebral organoids during gene therapy development could reduce the usage of animal models and improve translation to the clinic.

The adeno-associated virus rh10 vector is an effective gene transfer system for chronic spinal cord injury

Treatment options for chronic spinal cord injury (SCI) remain limited due to unfavourable changes in the microenvironment. Gene therapy can overcome these barriers through continuous delivery of therapeutic gene products to the target tissue. In particular, adeno-associated virus (AAV) vectors are potential candidates for use in chronic SCI, considering their safety and stable gene expression in vivo. Given that different AAV serotypes display different cellular tropisms, it is extremely important to select an optimal serotype for establishing a gene transfer system during the chronic phase of SCI. Therefore, we generated multiple AAV serotypes expressing ffLuc-cp156, a fusion protein of firefly luciferase and Venus, a variant of yellow fluorescent protein with fast and efficient maturation, as a reporter, and we performed intraparenchymal injection in a chronic SCI mouse model. Among the various serotypes tested, AAVrh10 displayed the highest photon count on bioluminescence imaging. Immunohistological analysis revealed that AAVrh10 showed favourable tropism for neurons, astrocytes, and oligodendrocytes. Additionally, with AAVrh10, the area expressing Venus was larger in the injury epicentre and extended to the surrounding tissue. Furthermore, the fluorescence intensity was significantly higher with AAVrh10 than with the other vectors. These results indicate that AAVrh10 may be an appropriate serotype for gene delivery to the chronically injured spinal cord. This promising tool may be applied for research and development related to the treatment of chronic SCI.

miRNA-26a-5p Accelerates Healing via Downregulation of PTEN in Fracture Patients with Traumatic Brain Injury

Patients who sustain a traumatic brain injury (TBI) are known to have a significantly quicker fracture healing time than patients with isolated fractures, but the underlying mechanism has yet to be identified. In this study, we found that the upregulation of miRNA-26a-5p induced by TBI correlated with a decrease in phosphatase and tensin homolog (PTEN) in callus formation. In vitro, overexpressing miRNA-26a-5p inhibited PTEN expression and accelerated osteoblast differentiation, whereas silencing of miRNA-26a-5p inhibited osteoblast activity. Reduction of PTEN facilitated osteoblast differentiation via the PI3K/AKT signaling pathway. Through luciferase assays, we found evidence that PTEN is a miRNA-26a-5p target gene that negatively regulates the differentiation of osteoblasts. Moreover, the present study confirmed that preinjection of agomiR-26a-5p leads to increased bone formation. Collectively, these results indicate that miRNA-26a-5p overexpression may be a key factor governing the improved fracture healing observed in TBI patients after the downregulation of PTEN and PI3K/AKT signaling. Upregulation of miRNA-26a-5p may therefore be a promising therapeutic strategy for promoting fracture healing.

Astrocyte-selective AAV gene therapy through the endogenous GFAP promoter results in robust transduction in the rat spinal cord following injury

Adeno-associated viral (AAV) vectors are a promising system for transgene delivery into the central nervous system (CNS) based on their safety profile and long-term gene expression. Gene delivery to the CNS has largely been neuron centric but advances in AAV technology are facilitating the development of approaches to enable transduction of glial cells. Considering the role of astrocytes in the on-going secondary damage in spinal cord injury (SCI), an AAV vector that targets astrocytes could show benefit as a potential treatment. Transduction efficiency, transgene expression and cellular tropism were compared for the AAV serotypes AAV5, AAV9 and AAVRec2 whereby destabilised yellow fluorescent protein (dYFP) was controlled by the GFAP or the truncated GfaABC₁D promoter. The vectors were tested in primary spinal cord astrocyte cell culture, spinal cord slice culture and an in vivo model of SCI contusion. AAV5 resulted in greater transduction efficiency, transgene expression and astrocyte tropism compared with AAV9 and AAVRec2. In a rodent model of SCI, robust transgene expression by AAV5-GFAP/GfaABC₁D-dYFP was observed through 12 mm of spinal cord tissue and expression was largely restricted to astrocytes. Thus, AAV5-GFAP/GfaABC₁D carries the potential as a potential gene therapy vector, particularly for transducing astrocytes in the damaged spinal cord.

Global Connectivity and Function of Descending Spinal Input Revealed by 3D Microscopy and Retrograde Transduction

The brain communicates with the spinal cord through numerous axon tracts that arise from discrete nuclei, transmit distinct functions, and often collateralize to facilitate the coordination of descending commands. This complexity presents a major challenge to interpreting functional outcomes from therapies that target supraspinal connectivity after injury or disease, while the wide distribution of supraspinal nuclei complicates the delivery of therapeutics. Here we harness retrograde viral vectors to overcome these challenges. We demonstrate that injection of AAV2-Retro to the cervical spinal cord of adult female mice results in highly efficient transduction of supraspinal populations throughout the brainstem, midbrain, and cortex. Some supraspinal populations, including corticospinal and rubrospinal neurons, were transduced with >90% efficiency, with robust transgene expression within 3 d of injection. In contrast, propriospinal and raphe spinal neurons showed much lower rates of retrograde transduction. Using tissue clearing and light-sheet microscopy we present detailed visualizations of descending axons tracts and create a mesoscopic projectome for the spinal cord. Moreover, chemogenetic silencing of supraspinal neurons with retrograde vectors resulted in complete and reversible forelimb paralysis, illustrating effective modulation of supraspinal function. Retrograde vectors were also highly efficient when injected after spinal injury, highlighting therapeutic potential. These data provide a global view of supraspinal connectivity and illustrate the potential of retrograde vectors to parse the functional contributions of supraspinal inputs.SIGNIFICANCE STATEMENT The complexity of descending inputs to the spinal cord presents a major challenge in efforts deliver therapeutics to widespread supraspinal systems, and to interpret their functional effects. Here we demonstrate highly effective gene delivery to diverse supraspinal nuclei using a retrograde viral approach and combine it with tissue clearing and 3D microscopy to map the descending projectome from brain to spinal cord. These data highlight newly developed retrograde viruses as therapeutic and research tools, while offering new insights into supraspinal connectivity.

Combining Constitutively Active Rheb Expression and Chondroitinase Promotes Functional Axonal Regeneration after Cervical Spinal Cord Injury

After spinal cord injury (SCI), severed axons in the adult mammalian CNS are unable to mount a robust regenerative response. In addition, the glial scar at the lesion site further restricts the regenerative potential of axons. We hypothesized that a combinatorial approach coincidentally targeting these obstacles would promote axonal regeneration. We combined (1) transplantation of a growth-permissive peripheral nerve graft (PNG) into an incomplete, cervical lesion cavity; (2) transduction of neurons rostral to the SCI site to express constitutively active Rheb (caRheb; a Ras homolog enriched in brain), a GTPase that directly activates the growth-promoting pathway mammalian target of rapamycin (mTOR) via AAV-caRheb injection; and (3) digestion of growth-inhibitory chondroitin sulfate proteoglycans within the glial scar at the distal PNG interface using the bacterial enzyme chondroitinase ABC (ChABC). We found that expressing caRheb in neurons post-SCI results in modestly yet significantly more axons regenerating across a ChABC-treated distal graft interface into caudal spinal cord than either treatment alone. Excitingly, we found that caRheb+ChABC treatment significantly potentiates the formation of synapses in the host spinal cord and improves the animals' ability to use the affected forelimb. Thus, this combination strategy enhances functional axonal regeneration following a cervical SCI.

Overexpression of Sarcoendoplasmic Reticulum Calcium ATPase 2a Promotes Cardiac Sympathetic Neurotransmission via Abnormal Endoplasmic Reticulum and Mitochondria Ca2+ Regulation

Supplemental Digital Content is available in the text.

Reduced cardiomyocyte excitation–contraction coupling and downregulation of the SERCA2a (sarcoendoplasmic reticulum calcium ATPase 2a) is associated with heart failure. This has led to viral transgene upregulation of SERCA2a in cardiomyocytes as a treatment. We hypothesized that SERCA2a gene therapy expressed under a similar promiscuous cytomegalovirus promoter could also affect the cardiac sympathetic neural axis and promote sympathoexcitation. Stellate neurons were isolated from 90 to 120 g male, Sprague–Dawley, Wistar Kyoto, and spontaneously hypertensive rats. Neurons were infected with Ad-mCherry or Ad-mCherry-hATP2Aa (SERCA2a). Intracellular Ca²⁺ changes were measured using fura-2AM in response to KCl, caffeine, thapsigargin, and carbonylcyanide-p-trifluoromethoxyphenylhydrazine to mobilize intracellular Ca²⁺ stores. The effect of SERCA2a on neurotransmitter release was measured using [³H]-norepinephrine overflow from 340 to 360 g Sprague–Dawley rat atria in response to right stellate ganglia stimulation. Upregulation of SERCA2a resulted in greater neurotransmitter release in response to stellate stimulation compared with control (empty: 98.7±20.5 cpm, n=7; SERCA: 186.5±28.41 cpm, n=8; P<0.05). In isolated Sprague–Dawley rat stellate neurons, SERCA2a overexpression facilitated greater depolarization-induced Ca²⁺ transients (empty: 0.64±0.03 au, n=57; SERCA: 0.75±0.03 au, n=68; P<0.05), along with increased endoplasmic reticulum and mitochondria Ca²⁺ load. Similar results were observed in Wistar Kyoto and age-matched spontaneously hypertensive rats, despite no further increase in endoplasmic reticulum load being observed in the spontaneously hypertensive rat (spontaneously hypertensive rats: empty, 0.16±0.04 au, n=18; SERCA: 0.17±0.02 au, n=25). In conclusion, SERCA2a upregulation in cardiac sympathetic neurons resulted in increased neurotransmission and increased Ca²⁺ loading into intracellular stores. Whether the increased Ca²⁺ transient and neurotransmission after SERCA2A overexpression contributes to enhanced sympathoexcitation in heart failure patients remains to be determined.

Neurotrophic Factors Used to Treat Spinal Cord Injury

The application of neurotrophic factors as a therapy to improve morphological and behavioral outcomes after experimental spinal cord injury (SCI) has been the focus of many studies. These studies vary markedly in the type of neurotrophic factor that is delivered, the mode of administration, and the location, timing, and duration of the treatment. Generally, the majority of studies have had significant success if neurotrophic factors are applied in or close to the lesion site during the acute or the subacute phase after SCI. Comparatively fewer studies have administered neurotrophic factors in order to directly target the somata of injured neurons. The mode of delivery varies between acute injection of recombinant proteins, subacute or chronic delivery using a variety of strategies including osmotic minipumps, cell-mediated delivery, delivery using polymer release vehicles or supporting bridges of some sort, or the use of gene therapy to modify neurons, glial cells, or precursor/stem cells. In this brief review, we summarize the state of play of many of the therapies using these factors, most of which have been undertaken in rodent models of SCI.

Re-Establishment of Cortical Motor Output Maps and Spontaneous Functional Recovery via Spared Dorsolaterally Projecting Corticospinal Neurons after Dorsal Column Spinal Cord Injury in Adult Mice

Motor cortical plasticity contributes to spontaneous recovery after incomplete spinal cord injury (SCI), but the pathways underlying this remain poorly understood. We performed optogenetic mapping of motor cortex in channelrhodopsin-2 expressing mice to assess the capacity of the cortex to re-establish motor output longitudinally after a C3/C4 dorsal column SCI that bilaterally ablated the dorsal corticospinal tract (CST) containing ∼96% of corticospinal fibers but spared ∼3% of CST fibers that project via the dorsolateral funiculus. Optogenetic mapping revealed extensive early deficits, but eventual reestablishment of motor cortical output maps to the limbs at the same latency as preoperatively by 4 weeks after injury. Analysis of skilled locomotion on the horizontal ladder revealed early deficits followed by partial spontaneous recovery by 6 weeks after injury. To dissociate between the contributions of injured dorsal projecting versus spared dorsolateral projecting corticospinal neurons, we established a transient silencing approach to inactivate spared dorsolaterally projecting corticospinal neurons specifically by injecting adeno-associated virus (AAV)-expressing Cre-dependent DREADD (designer receptor exclusively activated by designer drug) receptor hM4Di in sensorimotor cortex and AAV-expressing Cre in C7/C8 dorsolateral funiculus. Transient silencing uninjured dorsolaterally projecting corticospinal neurons via activation of the inhibitory DREADD receptor hM4Di abrogated spontaneous recovery and resulted in a greater change in skilled locomotion than in control uninjured mice using the same silencing approach. These data demonstrate the pivotal role of a minor dorsolateral corticospinal pathway in mediating spontaneous recovery after SCI and support a focus on spared corticospinal neurons as a target for therapy.

SIGNIFICANCE STATEMENT

Spontaneous recovery can occur after incomplete spinal cord injury (SCI), but the pathways underlying this remain poorly understood. We performed optogenetic mapping of motor cortex after a cervical SCI that interrupts most corticospinal transmission but results in partial recovery on a horizontal ladder task of sensorimotor function. We demonstrate that the motor cortex can reestablish output to the limbs longitudinally. To dissociate the roles of injured and uninjured corticospinal neurons in mediating recovery, we transiently silenced the minor dorsolateral corticospinal pathway spared by our injury. This abrogated spontaneous recovery and resulted in a greater change in skilled locomotion than in uninjured mice using the same approach. Therefore, uninjured corticospinal neurons substantiate remarkable motor cortical plasticity and partial recovery after SCI.

The role of the PI3K/Akt/mTOR pathway in glial scar formation following spinal cord injury

Several studies suggest that glial scars pose as physical and chemical barriers that limit neurite regeneration after spinal cord injury (SCI). Evidences suggest that the activation of the PI3K/Akt/mTOR signaling pathway is involved in glial scar formation. Therefore, inhibition of the PI3K/Akt/mTOR pathway may beneficially attenuate glial scar formation after SCI. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates the PI3K/Akt/mTOR pathway. Therefore, we hypothesized that the overexpression of PTEN in the spinal cord will have beneficial effects after SCI. In the present study, we intrathecally injected a recombinant adenovirus carrying the pten gene (Ad-PTEN) to cause overexpression of PTEN in rats with contusion injured spinal cords. The results suggest overexpression of PTEN in spinal cord attenuated glial scar formation and led to improved locomotor function after SCI. Overexpression of PTEN following SCI attenuated gliosis, affected chondroitin sulfate proteoglycan expression, and improved axon regeneration into the lesion site. Furthermore, we suggest that the activation of the PI3K/Akt/mTOR pathway in astrocytes at 3 days after SCI may be involved in glial scar formation. Because delayed treatment with Ad-PTEN enhanced motor function recovery more significantly than immediate treatment with Ad-PTEN after SCI, the results suggest that the best strategy to attenuate glial scar formation could be to introduce 3 days after SCI. This study's findings thus have positive implications for patients who are unable to receive immediate medical attention after SCI.

Controlling AAV Tropism in the Nervous System with Natural and Engineered Capsids

More than one hundred naturally occurring variants of adeno-associated virus (AAV) have been identified, and this library has been further expanded by an array of techniques for modification of the viral capsid. AAV capsid variants possess unique antigenic profiles and demonstrate distinct cellular tropisms driven by differences in receptor binding. AAV capsids can be chemically modified to alter tropism, can be produced as hybrid vectors that combine the properties of multiple serotypes, and can carry peptide insertions that introduce novel receptor-binding activity. Furthermore, directed evolution of shuffled genome libraries can identify engineered variants with unique properties, and rational modification of the viral capsid can alter tropism, reduce blockage by neutralizing antibodies, or enhance transduction efficiency. This large number of AAV variants and engineered capsids provides a varied toolkit for gene delivery to the CNS and retina, with specialized vectors available for many applications, but selecting a capsid variant from the array of available vectors can be difficult. This chapter describes the unique properties of a range of AAV variants and engineered capsids, and provides a guide for selecting the appropriate vector for specific applications in the CNS and retina.

Expressing Constitutively Active Rheb in Adult Neurons after a Complete Spinal Cord Injury Enhances Axonal Regeneration beyond a Chondroitinase-Treated Glial Scar

After a spinal cord injury (SCI), CNS axons fail to regenerate, resulting in permanent deficits. This is due to: (1) the presence of inhibitory molecules, e.g., chondroitin sulfate proteoglycans (CSPG), in the glial scar at the lesion; and (2) the diminished growth capacity of adult neurons. We sought to determine whether expressing a constitutively active form of the GTPase Rheb (caRheb) in adult neurons after a complete SCI in rats improves intrinsic growth potential to result in axon regeneration out of a growth-supportive peripheral nerve grafted (PNG) into the SCI cavity. We also hypothesized that treating the glial scar with chondroitinase ABC (ChABC), which digests CSPG, would further allow caRheb-transduced neurons to extend axons across the distal graft interface. We found that targeting this pathway at a clinically relevant post-SCI time point improves both sprouting and regeneration of axons. CaRheb increased the number of axons, but not the number of neurons, that projected into the PNG, indicative of augmented sprouting. We also saw that caRheb enhanced sprouting far rostral to the injury. CaRheb not only increased growth rostral and into the graft, it also resulted in significantly more regrowth of axons across a ChABC-treated scar into caudal spinal cord. CaRheb(+) neurons had higher levels of growth-associated-43, suggestive of a newly identified mechanism for mTOR-mediated enhancement of regeneration. Thus, we demonstrate for the first time that simultaneously addressing intrinsic and scar-associated, extrinsic impediments to regeneration results in significant regrowth beyond an extremely challenging, complete SCI site.

SIGNIFICANCE STATEMENT

After spinal cord injury (SCI), CNS axons fail to regenerate, resulting in permanent deficits. This is due to the diminished growth capacity of adult neurons and the presence of inhibitory molecules in the scar at the lesion. We sought to simultaneously counter both of these obstacles to achieve more robust regeneration after complete SCI. We transduced neurons postinjury to express a constitutively active Rheb to enhance their intrinsic growth potential, transplanted a growth supporting peripheral nerve graft into the lesion cavity, and enzymatically modulated the inhibitory glial scar distal to the graft. We demonstrate, for the first time, that simultaneously addressing neuron-related, intrinsic deficits in axon regrowth and extrinsic, scar-associated impediments to regeneration results in significant regeneration after SCI.

Lentiviral vectors as tools to understand central nervous system biology in mammalian model organisms

Lentiviruses have been extensively used as gene delivery vectors since the mid-1990s. Usually derived from the human immunodeficiency virus genome, they mediate efficient gene transfer to non-dividing cells, including neurons and glia in the adult mammalian brain. In addition, integration of the recombinant lentiviral construct into the host genome provides permanent expression, including the progeny of dividing neural precursors. In this review, we describe targeted vectors with modified envelope glycoproteins and expression of transgenes under the regulation of cell-selective and inducible promoters. This technology has broad utility to address fundamental questions in neuroscience and we outline how this has been used in rodents and primates. Combining viral tract tracing with immunohistochemistry and confocal or electron microscopy, lentiviral vectors provide a tool to selectively label and trace specific neuronal populations at gross or ultrastructural levels. Additionally, new generation optogenetic technologies can be readily utilized to analyze neuronal circuit and gene functions in the mature mammalian brain. Examples of these applications, limitations of current systems and prospects for future developments to enhance neuroscience knowledge will be reviewed. Finally, we will discuss how these vectors may be translated from gene therapy trials into the clinical setting.

Neurotrophic factors for spinal cord repair: Which, where, how and when to apply, and for what period of time?

A variety of neurotrophic factors have been used in attempts to improve morphological and behavioural outcomes after experimental spinal cord injury (SCI). Here we review many of these factors, their cellular targets, and their therapeutic impact on spinal cord repair in different, primarily rodent, models of SCI. A majority of studies report favourable outcomes but results are by no means consistent, thus a major aim of this review is to consider how best to apply neurotrophic factors after SCI to optimize their therapeutic potential. In addition to which factors are chosen, many variables need be considered when delivering trophic support, including where and when to apply a given factor or factors, how such factors are administered, at what dose, and for how long. Overall, the majority of studies have applied neurotrophic support in or close to the spinal cord lesion site, in the acute or sub-acute phase (0-14 days post-injury). Far fewer chronic SCI studies have been undertaken. In addition, comparatively fewer studies have administered neurotrophic factors directly to the cell bodies of injured neurons; yet in other instructive rodent models of CNS injury, for example optic nerve crush or transection, therapies are targeted directly at the injured neurons themselves, the retinal ganglion cells. The mode of delivery of neurotrophic factors is also an important variable, whether delivered by acute injection of recombinant proteins, sub-acute or chronic delivery using osmotic minipumps, cell-mediated delivery, delivery using polymer release vehicles or supporting bridges of some sort, or the use of gene therapy to modify neurons, glial cells or precursor/stem cells. Neurotrophic factors are often used in combination with cell or tissue grafts and/or other pharmacotherapeutic agents. Finally, the dose and time-course of delivery of trophic support should ideally be tailored to suit specific biological requirements, whether they relate to neuronal survival, axonal sparing/sprouting, or the long-distance regeneration of axons ending in a different mode of growth associated with terminal arborization and renewed synaptogenesis. This article is part of a Special Issue entitled SI: Spinal cord injury.

Gene therapy strategies for the treatment of spinal cord injury

Spinal cord injury is a complex pathology often resulting in functional impairment and paralysis. Gene therapy has emerged as a possible solution to the problems of limited neural tissue regeneration through the administration of factors promoting axonal growth, while also offering long-term local delivery of therapeutic molecules at the injury site. Of note, gene therapy is our response to the requirements of neural and glial cells following spinal cord injury, providing, in a time-dependent manner, growth substances for axonal regeneration and eliminating axonal growth inhibitors. Herein, we explore different gene therapy strategies, including targeting gene expression to modulate the presence of neurotrophic growth or survival factors and increase neural tissue plasticity. Special attention is given to describing advances in viral and non-viral gene delivery systems, as well as the available routes of gene delivery. Finally, we discuss the future of combinatorial gene therapies and give consideration to the implementation of gene therapy in humans.

Widespread and efficient transduction of spinal cord and brain following neonatal AAV injection and potential disease modifying effect in ALS mice

The architecture of the spinal cord makes efficient delivery of recombinant adeno-associated virus (rAAV) vectors throughout the neuraxis challenging. We describe a paradigm in which small amounts of virus delivered intraspinally to newborn mice result in robust rAAV-mediated transgene expression in the spinal cord. We compared the efficacy of rAAV2/1, 2/5, 2/8, and 2/9 encoding EGFP delivered to the hindlimb muscle (IM), cisterna magna (ICM), or lumbar spinal cord (IS) of neonatal pups. IS injection of all four capsids resulted in robust transduction of the spinal cord with rAAV2/5, 2/8, and 2/9 vectors appearing to be transported to brain. ICM injection resulted in widespread expression of EGFP in the brain, and upper spinal cord. IM injection resulted in robust muscle expression, with only rAAV2/8 and 2/9 transducing spinal motor and sensory neurons. As proof of concept, we use the IS paradigm to express murine Interleukin (IL)-10 in the spinal cord of the SOD1-G93A transgenic mouse model of amyotrophic lateral sclerosis. We show that expression of IL-10 in the spinal axis of SOD1-G93A mice altered the immune milieu and significantly prolonged survival. These data establish an efficient paradigm for somatic transgene delivery of therapeutic biologics to the spinal cord of mice.