Helper-Dependent Adenoviral Vectors and Their Use for Neuroscience Applications

CaV 2.1 α1 subunit motifs that control presynaptic CaV 2.1 subtype abundance are distinct from CaV 2.1 preference

Presynaptic voltage-gated Ca2+ channel (CaV ) subtype abundance at mammalian synapses regulates synaptic transmission in health and disease. In the mammalian central nervous system (CNS), most presynaptic terminals are CaV 2.1 dominant with a developmental reduction in CaV 2.2 and CaV 2.3 levels, and CaV 2 subtype levels are altered in various diseases. However, the molecular mechanisms controlling presynaptic CaV 2 subtype levels are largely unsolved. Because the CaV 2 α1  subunit cytoplasmic regions contain varying levels of sequence conservation, these regions are proposed to control presynaptic CaV 2 subtype preference and abundance. To investigate the potential role of these regions, we expressed chimeric CaV 2.1 α1  subunits containing swapped motifs with the CaV 2.2 and CaV 2.3 α1  subunit on a CaV 2.1/CaV 2.2 null background at the calyx of Held presynaptic terminals. We found that expression of CaV 2.1 α1  subunit chimeras containing the CaV 2.3 loop II-III region or cytoplasmic C-terminus (CT) resulted in a large reduction of presynaptic Ca2+ currents compared to the CaV 2.1 α1  subunit. However, the Ca2+ current sensitivity to the CaV 2.1 blocker agatoxin-IVA was the same between the chimeras and the CaV 2.1 α1  subunit. Additionally, we found no reduction in presynaptic Ca2+ currents with CaV 2.1/2.2 cytoplasmic CT chimeras. We conclude that the motifs in the CaV 2.1 loop II-III and CT do not individually regulate CaV 2.1 preference, although these motifs control CaV 2.1 levels and the CaV 2.3 CT contains motifs that negatively regulate presynaptic CaV 2.3 levels. We propose that the motifs controlling presynaptic CaV 2.1 preference are distinct from those regulating CaV 2.1 levels and may act synergistically to impact pathways regulating CaV 2.1 preference and abundance. KEY POINTS: Presynaptic CaV 2 subtype abundance regulates neuronal circuit properties, although the mechanisms regulating presynaptic CaV 2 subtype abundance and preference remain enigmatic. The CaV α1  subunit determines subtype and contains multiple motifs implicated in regulating presynaptic subtype abundance and preference. The CaV 2.1 α1  subunit domain II-III loop and cytoplasmic C-terminus are positive regulators of presynaptic CaV 2.1 abundance but do not regulate preference. The CaV 2.3 α1  subunit cytoplasmic C-terminus negatively regulates presynaptic CaV 2 subtype abundance but not preference, whereas the CaV 2.2 α1  subunit cytoplasmic C-terminus is not a key regulator of presynaptic CaV 2 subtype abundance or preference. The CaV 2 α1  subunit motifs determining the presynaptic CaV 2 preference are distinct from abundance.

CaV2.1 α1 subunit motifs that control presynaptic CaV2.1 subtype abundance are distinct from CaV2.1 preference

Presynaptic voltage-gated Ca2+ channels (CaV) subtype abundance at mammalian synapses regulates synaptic transmission in health and disease. In the mammalian central nervous system, most presynaptic terminals are CaV2.1 dominant with a developmental reduction in CaV2.2 and CaV2.3 levels, and CaV2 subtype levels are altered in various diseases. However, the molecular mechanisms controlling presynaptic CaV2 subtype levels are largely unsolved. Since the CaV2 α1 subunit cytoplasmic regions contain varying levels of sequence conservation, these regions are proposed to control presynaptic CaV2 subtype preference and abundance. To investigate the potential role of these regions, we expressed chimeric CaV2.1 α1subunits containing swapped motifs with the CaV2.2 and CaV2.3 α1 subunit on a CaV2.1/CaV2.2 null background at the calyx of Held presynaptic terminal. We found that expression of CaV2.1 α1 subunit chimeras containing the CaV2.3 loop II-III region or cytoplasmic C-terminus (CT) resulted in a large reduction of presynaptic Ca2+ currents compared to the CaV2.1 α1 subunit. However, the Ca2+ current sensitivity to the CaV2.1 blocker Agatoxin-IVA, was the same between the chimeras and the CaV2.1 α1 subunit. Additionally, we found no reduction in presynaptic Ca2+ currents with CaV2.1/2.2 cytoplasmic CT chimeras. We conclude that the motifs in the CaV2.1 loop II-III and CT do not individually regulate CaV2.1 preference, but these motifs control CaV2.1 levels and the CaV2.3 CT contains motifs that negatively regulate presynaptic CaV2.3 levels. We propose that the motifs controlling presynaptic CaV2.1 preference are distinct from those regulating CaV2.1 levels and may act synergistically to impact pathways regulating CaV2.1 preference and abundance.

Stereotactic Delivery of Helper-dependent Adenoviral Viral Vectors at Distinct Developmental Time Points to Perform Age-dependent Molecular Manipulations of the Mouse Calyx of Held

Synapses are specialized structures that enable neuronal communication, which is essential for brain function and development. Alterations in synaptic proteins have been linked to various neurological and neuropsychiatric disorders. Therefore, manipulating synaptic proteins in vivo can provide insight into the molecular mechanisms underlying these disorders and aid in developing new therapeutic strategies. Previous methods such as constitutive knock-out animals are limited by developmental compensation and off-target effects. The current approach outlines procedures for age-dependent molecular manipulations in mice using helper-dependent adenovirus viral vectors (HdAd) at distinct developmental time points. Using stereotactic injection of HdAds in both newborn and juvenile mice, we demonstrate the versatility of this method to express Cre recombinase in globular bushy cells of juvenile Rac1fl/fl mice to ablate presynaptic Rac1 and study its role in synaptic transmission. Separately, we overexpress CaV2 α1 subunits at two distinct developmental time points to elucidate the mechanisms that determine presynaptic CaV2 channel abundance and preference. This method presents a reliable, cost-effective, and minimally invasive approach for controlling gene expression in specific regions of the mouse brain and will be a powerful tool to decipher brain function in health and disease. Key features Virus-mediated genetic perturbation in neonatal and young adult mice. Stereotaxic injection allows targeting of brain structures at different developmental stages to study the impact of genetic perturbation throughout the development.

Development and evaluation of helper dependent adenoviral vectors for inner ear gene delivery

Viral vector gene therapy is an attractive strategy to treat hearing loss. Since hearing loss is due to a variety of pathogenic signaling cascades in distinct cells, viral vectors that can express large or multiple genes in a cell-type specific manner are needed. Helper-dependent adenoviral vectors (HdAd) are safe viral vectors with a large packaging capacity (-36 kb). Despite the potential of HdAd, its use in the inner ear is largely unexplored. Therefore, to evaluate the utility of HdAd for inner ear gene therapy, we created two HdAd vectors that use distinct cellular receptors for transduction: HdAd Serotype Type 5 (HdAd5), the Coxsackie-Adenovirus Receptor (CAR) and a chimeric HdAd 5/35, the human CD46+ receptor (hCD46). We delivered these vectors through the round window (RW) or scala media in CBA/J, C57Bl6/J and hCD46 transgenic mice. Immunostaining in conjunction with confocal microscopy of cochlear sections revealed that multiple cell types were transduced using HdAd5 and HdAd 5/35 in all mouse models. Delivery of HdAd5 via RW in the C57Bl/6 J or CBA/J cochlea resulted in transduced mesenchymal cells of the peri‑lymphatic lining and modiolar region while scala media delivery resulted in transduction of supporting cells and inner hair cells. Hd5/35 transduction was CD46 dependent and RW delivery of HdAd5/35 in the hCD46 mouse model resulted in a similar transduction pattern as HdAd5 in the peri‑lymphatic lining and modiolar region in the cochlea. Our data indicate that HdAd vectors are promising vectors for use in inner ear gene therapy to treat some causes of hearing loss.

VikAD, a Vika site-specific recombinase-based system for efficient and scalable helper-dependent adenovirus production

Recombinant viral vectors have become integral tools for basic in vivo research applications. Helper-dependent adenoviral (HdAd) vectors have a large packaging capacity of ∼36 kb of DNA that mediate long-term transgene expression in vitro and in vivo. The large carrying capacity of HdAd enables basic research or clinical applications requiring the delivery of large genes or multiple transgenes, which cannot be packaged into other widely used viral vectors. Currently, common HdAd production systems use an Ad helper virus (HV) with a packaging signal (Ψ) that is flanked by either loxP or FRT sites, which is excised in producer cells expressing Cre or Flp recombinases to prevent HV packaging. However, these production systems prevent the use of HdAd vectors for genetic strategies that rely on Cre or Flp recombination for cell-type-specific expression. To overcome these limitations, we developed the VikAD production system, which is based on producer cells expressing the Vika recombinase and an HV that contains a Ψ flanked by vox sites. The availability of this production system will greatly expand the utility and flexibility of HdAd vectors for use in research applications to monitor and manipulate cellular activity with increased specificity.

Advanced imaging and labelling methods to decipher brain cell organization and function

The brain is arguably the most complex organ. The branched and extended morphology of nerve cells, their subcellular complexity, the multiplicity of brain cell types as well as their intricate connectivity and the scattering properties of brain tissue present formidable challenges to the understanding of brain function. Neuroscientists have often been at the forefront of technological and methodological developments to overcome these hurdles to visualize, quantify and modify cell and network properties. Over the last few decades, the development of advanced imaging methods has revolutionized our approach to explore the brain. Super-resolution microscopy and tissue imaging approaches have recently exploded. These instrumentation-based innovations have occurred in parallel with the development of new molecular approaches to label protein targets, to evolve new biosensors and to target them to appropriate cell types or subcellular compartments. We review the latest developments for labelling and functionalizing proteins with small localization and functionalized reporters. We present how these molecular tools are combined with the development of a wide variety of imaging methods that break either the diffraction barrier or the tissue penetration depth limits. We put these developments in perspective to emphasize how they will enable step changes in our understanding of the brain.

Presynaptic development is controlled by the core active zone proteins CAST/ELKS

KEY POINTS

CAST/ELKS are positive regulators of presynaptic growth and are suppressors of active zone expansion at the developing mouse calyx of Held. CAST/ELKS regulate all three Ca_V 2 subtype channel levels in the presynaptic terminal and not just Ca_V 2.1. The half-life of ELKS is on the timescale of days and not weeks. Synaptic transmission was not impacted by the loss of CAST/ELKS. CAST/ELKS are involved in pathways regulating morphological properties of presynaptic terminals during an early stage of circuit maturation.

ABSTRACT

Many presynaptic active zone (AZ) proteins have multiple regulatory roles that vary during distinct stages of neuronal circuit development. The CAST/ELKS protein family are evolutionarily conserved presynaptic AZ molecules that regulate presynaptic calcium channels, synaptic transmission and plasticity in the mammalian CNS. However, how these proteins regulate synapse development and presynaptic function in a developing neuronal circuit in its native environment is unclear. To unravel the roles of CAST/ELKS in glutamatergic synapse development and in presynaptic function, we used CAST knockout (KO) and ELKS conditional KO (CKO) mice to examine how their loss during the early stages of circuit maturation impacted the calyx of Held presynaptic terminal development and function. Morphological analysis from confocal z-stacks revealed that combined deletion of CAST/ELKS resulted in a reduction in the surface area and volume of the calyx. Analysis of AZ ultrastructure showed that AZ size was increased in the absence of CAST/ELKS. Patch clamp recordings demonstrated a reduction of all presynaptic Ca_V 2 channel subtype currents that correlated with a loss in presynaptic Ca_V 2 channel numbers. However, these changes did not impair synaptic transmission and plasticity and synaptic vesicle release kinetics. We conclude that CAST/ELKS proteins are positive regulators of presynaptic growth and are suppressors of AZ expansion and Ca_V 2 subtype currents and levels during calyx of Held development. We propose that CAST/ELKS are involved in pathways regulating presynaptic morphological properties and Ca_V 2 channel subtypes and suggest there is developmental compensation to preserve synaptic transmission during early stages of neuronal circuit maturation.

Automated Live-Cell Imaging of Synapses in Rat and Human Neuronal Cultures

Synapse loss and dendritic damage correlate with cognitive decline in many neurodegenerative diseases, underlie neurodevelopmental disorders, and are associated with environmental and drug-induced CNS toxicities. However, screening assays designed to measure loss of synaptic connections between live cells are lacking. Here, we describe the design and validation of automated synaptic imaging assay (ASIA), an efficient approach to label, image, and analyze synapses between live neurons. Using viral transduction to express fluorescent proteins that label synapses and an automated computer-controlled microscope, we developed a method to identify agents that regulate synapse number. ASIA is compatible with both confocal and wide-field microscopy; wide-field image acquisition is faster but requires a deconvolution step in the analysis. Both types of images feed into batch processing analysis software that can be run on ImageJ, CellProfiler, and MetaMorph platforms. Primary analysis endpoints are the number of structural synapses and cell viability. Thus, overt cell death is differentiated from subtle changes in synapse density, an important distinction when studying neurodegenerative processes. In rat hippocampal cultures treated for 24 h with 100 μM 2-bromopalmitic acid (2-BP), a compound that prevents clustering of postsynaptic density 95 (PSD95), ASIA reliably detected loss of postsynaptic density 95-enhanced green fluorescent protein (PSD95-eGFP)-labeled synapses in the absence of cell death. In contrast, treatment with 100 μM glutamate produced synapse loss and significant cell death, determined from morphological changes in a binary image created from co-expressed mCherry. Treatment with 3 mM lithium for 24 h significantly increased the number of fluorescent puncta, showing that ASIA also detects synaptogenesis. Proof of concept studies show that cell-specific promoters enable the selective study of inhibitory or principal neurons and that alternative reporter constructs enable quantification of GABAergic or glutamatergic synapses. ASIA can also be used to study synapse loss between human induced pluripotent stem cell (iPSC)-derived cortical neurons. Significant synapse loss in the absence of cell death was detected in the iPSC-derived neuronal cultures treated with either 100 μM 2-BP or 100 μM glutamate for 24 h, while 300 μM glutamate produced synapse loss and cell death. ASIA shows promise for identifying agents that evoke synaptic toxicities and screening for compounds that prevent or reverse synapse loss.

Helper-Dependent Adenovirus Transduces the Human and Rat Retina but Elicits an Inflammatory Reaction When Delivered Subretinally in Rats

The identification of >100 genes causing inherited retinal degeneration and the promising results of recent gene augmentation trials have led to an increase in the number of studies investigating the preclinical efficacy of viral-mediated gene transfer. Despite success using adeno-associated viruses, many disease-causing genes, such as ABCA4 or USH2A, are too large to fit into these vectors. One option for large gene delivery is the family of integration-deficient helper-dependent adenoviruses (HDAds), which efficiently transduce postmitotic neurons. However, HDAds have been shown in other organ systems to elicit an immune response, and the immunogenicity of HDAds in the retina has not been characterized. In this study, HDAd serotype 5 (HDAd5) was found to successfully transduce rod and cone photoreceptors in ex vivo human retinal organ cultures. The ocular inflammatory response to subretinal injection of the HDAd5 was evaluated using a rat model. Subretinal injection of HDAd5 carrying cytomegalovirus promoter-driven enhanced green fluorescent protein (HDAd5-CMVp-eGFP) elicited a robust inflammatory response by 3 days postinjection. This reaction included vitreous infiltration of ionized calcium-binding adapter molecule 1 (Iba1)-positive monocytes and increased expression of the proinflammatory protein, intercellular adhesion molecule 1 (ICAM-1). By 7 days postinjection, most Iba1-positive infiltrates migrated into the neural retina and ICAM-1 expression was significantly increased compared with buffer-injected control eyes. At 14 days postinjection, Iba1-positive cells persisted in the retinas of HDAd5-injected eyes, and there was thinning of the outer nuclear layer. Subretinal injection of an empty HDAd5 virus was used to confirm that the inflammatory response was in response to the HDAd5 vector and not due to eGFP-induced overexpression cytotoxicity. Subretinal injection of lower doses of HDAd5 dampened the inflammatory response, but also eGFP expression. Despite their larger carrying capacity, further work is needed to elucidate the inflammatory pathways involved and to identify an immunomodulation paradigm sufficient for safe and effective transfer of large genes to the retina using HDAd5.

Presynaptic Mitochondria Volume and Abundance Increase during Development of a High-Fidelity Synapse

The calyx of Held, a large glutamatergic presynaptic terminal in the auditory brainstem undergoes developmental changes to support the high action-potential firing rates required for auditory information encoding. In addition, calyx terminals are morphologically diverse, which impacts vesicle release properties and synaptic plasticity. Mitochondria influence synaptic plasticity through calcium buffering and are crucial for providing the energy required for synaptic transmission. Therefore, it has been postulated that mitochondrial levels increase during development and contribute to the morphological-functional diversity in the mature calyx. However, the developmental profile of mitochondrial volumes and subsynaptic distribution at the calyx of Held remains unclear. To provide insight on this, we developed a helper-dependent adenoviral vector that expresses the genetically encoded peroxidase marker for mitochondria, mito-APEX2, at the mouse calyx of Held. We developed protocols to detect labeled mitochondria for use with serial block face scanning electron microscopy to carry out semiautomated segmentation of mitochondria, high-throughput whole-terminal reconstruction, and presynaptic ultrastructure in mice of either sex. Subsequently, we measured mitochondrial volumes and subsynaptic distributions at the immature postnatal day (P)7 and the mature (P21) calyx. We found an increase of mitochondria volumes in terminals and axons from P7 to P21 but did not observe differences between stalk and swelling subcompartments in the mature calyx. Based on these findings, we propose that mitochondrial volumes and synaptic localization developmentally increase to support high firing rates required in the initial stages of auditory information processing.SIGNIFICANCE STATEMENT Elucidating the developmental processes of auditory brainstem presynaptic terminals is critical to understanding auditory information encoding. Additionally, morphological-functional diversity at these terminals is proposed to enhance coding capacity. Mitochondria provide energy for synaptic transmission and can buffer calcium, impacting synaptic plasticity; however, their developmental profile to ultimately support the energetic demands of synapses following the onset of hearing remains unknown. Therefore, we created a helper-dependent adenoviral vector with the mitochondria-targeting peroxidase mito-APEX2 and expressed it at the mouse calyx of Held. Volumetric reconstructions of serial block face electron microscopy data of immature and mature labeled calyces reveal that mitochondrial volumes are increased to support high firing rates upon maturity.

CaV2.1 α1 Subunit Expression Regulates Presynaptic CaV2.1 Abundance and Synaptic Strength at a Central Synapse

The abundance of presynaptic Ca_V2 voltage-gated Ca²⁺ channels (Ca_V2) at mammalian active zones (AZs) regulates the efficacy of synaptic transmission. It is proposed that presynaptic Ca_V2 levels are saturated in AZs due to a finite number of slots that set Ca_V2 subtype abundance and that Ca_V2.1 cannot compete for Ca_V2.2 slots. However, at most AZs, Ca_V2.1 levels are highest and Ca_V2.2 levels are developmentally reduced. To investigate Ca_V2.1 saturation states and preference in AZs, we overexpressed the Ca_V2.1 and Ca_V2.2 α₁ subunits at the calyx of Held at immature and mature developmental stages. We found that AZs prefer Ca_V2.1 to Ca_V2.2. Remarkably, Ca_V2.1 α₁ subunit overexpression drove increased Ca_V2.1 currents and channel numbers and increased synaptic strength at both developmental stages examined. Therefore, we propose that Ca_V2.1 levels in the AZ are not saturated and that synaptic strength can be modulated by increasing Ca_V2.1 levels to regulate neuronal circuit output. VIDEO ABSTRACT.

Synaptotagmin-7 controls the size of the reserve and resting pools of synaptic vesicles in hippocampal neurons

Continuous neurotransmitter release is subjected to synaptic vesicle availability, which in turn depends on vesicle recycling and the traffic of vesicles between pools. We studied the role of Synaptotagmin-7 (Syt-7) in synaptic vesicle accessibility for release in hippocampal neurons in culture. Synaptic boutons from Syt-7 knockout (KO) mice displayed normal basal secretion with no alteration in the RRP size or the probability of release. However, stronger stimuli revealed an increase in the size of the reserve and resting vesicle pools in Syt-7 KO boutons compared with WT. These data suggest that Syt-7 plays a significant role in the vesicle pool homeostasis and, consequently, in the availability of vesicles for synaptic transmission during strong stimulation, probably, by facilitating advancing synaptic vesicles to the readily releasable pool.

Orphan receptor GPR158 controls stress-induced depression

Stress can be a motivational force for decisive action and adapting to novel environment; whereas, exposure to chronic stress contributes to the development of depression and anxiety. However, the molecular mechanisms underlying stress-responsive behaviors are not fully understood. Here, we identified the orphan receptor GPR158 as a novel regulator operating in the prefrontal cortex (PFC) that links chronic stress to depression. GPR158 is highly upregulated in the PFC of human subjects with major depressive disorder. Exposure of mice to chronic stress also increased GPR158 protein levels in the PFC in a glucocorticoid-dependent manner. Viral overexpression of GPR158 in the PFC induced depressive-like behaviors. In contrast GPR158 ablation, led to a prominent antidepressant-like phenotype and stress resiliency. We found that GPR158 exerts its effects via modulating synaptic strength altering AMPA receptor activity. Taken together, our findings identify a new player in mood regulation and introduce a pharmacological target for managing depression.

A novel region in the CaV2.1 α1 subunit C-terminus regulates fast synaptic vesicle fusion and vesicle docking at the mammalian presynaptic active zone

In central nervous system (CNS) synapses, action potential-evoked neurotransmitter release is principally mediated by Ca_V2.1 calcium channels (Ca_V2.1) and is highly dependent on the physical distance between Ca_V2.1 and synaptic vesicles (coupling). Although various active zone proteins are proposed to control coupling and abundance of Ca_V2.1 through direct interactions with the Ca_V2.1 α1 subunit C-terminus at the active zone, the role of these interaction partners is controversial. To define the intrinsic motifs that regulate coupling, we expressed mutant Ca_V2.1 α₁ subunits on a Ca_V2.1 null background at the calyx of Held presynaptic terminal. Our results identified a region that directly controlled fast synaptic vesicle release and vesicle docking at the active zone independent of Ca_V2.1 abundance. In addition, proposed individual direct interactions with active zone proteins are insufficient for Ca_V2.1 abundance and coupling. Therefore, our work advances our molecular understanding of Ca_V2.1 regulation of neurotransmitter release in mammalian CNS synapses.