Standardized Preclinical In Vitro Blood-Brain Barrier Mouse Assay Validates Endocytosis-Dependent Antibody Transcytosis Using Transferrin-Receptor-Mediated Pathways.

The presence of the blood-brain barrier (BBB) creates a nigh-on impenetrable obstacle for large macromolecular therapeutics that need to be delivered to the brain milieu to treat neurological disorders. To overcome this, one of the strategies used is to bypass the barrier with what is referred to as a "Trojan Horse" strategy, where therapeutics are designed to use endogenous receptor-mediated pathways to piggyback their way through the BBB. Even though in vivo methodologies are commonly used to test the efficacy of BBB-penetrating biologics, comparable in vitro BBB models are in high demand, as they benefit from being an isolated cellular system devoid of physiological factors that can on occasion mask the processes behind BBB transport via transcytosis. We have developed an in vitro BBB model (In-Cell BBB-Trans assay) based on the murine cEND cells that help delineate the ability of modified large bivalent IgG antibodies conjugated to the transferrin receptor binder scFv8D3 to cross an endothelial monolayer grown on porous cell culture inserts (PCIs). Following the administration of bivalent antibodies into the endothelial monolayer, a highly sensitive enzyme-linked immunosorbent assay (ELISA) is used to determine the concentration in the apical (blood) and basolateral (brain) chambers of the PCI system, allowing for the evaluation of apical recycling and basolateral transcytosis, respectively. Our results show that antibodies conjugated to scFv8D3 transcytose at considerably higher levels compared to unconjugated antibodies in the In-Cell BBB-Trans assay. Interestingly, we are able to show that these results mimic in vivo brain uptake studies using identical antibodies. In addition, we are able to transversely section PCI cultured cells, allowing for the identification of receptors and proteins that are likely involved in the transcytosis of the antibodies. Furthermore, studies using the In-Cell BBB-Trans assay revealed that transcytosis of the transferrin-receptor-targeting antibodies is dependent on endocytosis. In conclusion, we have designed a simple, reproducible In-Cell BBB-Trans assay based on murine cells that can be used to rapidly determine the BBB-penetrating capabilities of transferrin-receptor-targeting antibodies. We believe that the In-Cell BBB-Trans assay can be used as a powerful, preclinical screening platform for therapeutic neurological pathologies.

A single-chain fragment constant design enables easy production of a monovalent blood-brain barrier transporter and provides an improved brain uptake at elevated doses.

The interest for developing antibody-driven therapeutic interventions has exponentially grown over the last few decades. Even though there have been promising leaps in the development of efficacious antibody therapies, problems revolving around production and site-directed delivery of these large macromolecules persist. This is especially pertinent when it comes to designing and producing antibodies to penetrate the blood-brain barrier (BBB) to tackle neurodegenerative diseases. One of the most effective approaches to alleviating this problem is to employ a "Trojan Horse" approach, using receptor-mediated transcytosis, such as those governed by the transferrin receptor (TfR)-mediated pathways, to deliver large protein payloads into the brain. Even though this method is effective, ideal limiting factors, related to how the antibody binds to the TfR, need to be elucidated to improve BBB penetrance. With this said, we have designed and produced a single-chain Fc antibody, conjugated to an scFv8D3 TfR binding motif, creating a single-chain monovalent BBB transporter (scFc-scFv8D3). This recombinant protein is easy to produce and purify, demonstrates monovalent binding to the TfR and is structurally stable at physiologically relevant temperatures. Using an in vitro BBB model system, we show a positive correlation between the concentration of administered antibody and transcytosis efficacy, with scFc-scFv8D3 demonstrating significantly higher transcytosis levels compared with scFv8D3-conjugated bivalent antibodies at elevated administered concentrations. Furthermore, in vivo studies recapitulate the in vitro results, with the scFc-scFv8D3 demonstrating an elevated brain uptake at higher therapeutic doses in wild-type mice, comparable with that of the scFv8D3-conjugated bivalent antibody control. In addition, the half-life of the single-chain monovalent BBB transporter is comparable with that of standard IgG antibodies, indicating that the scFc format does not exacerbate physiological degradation. Our results lead us to the conclusion that valency and affinity are important variables to consider when discerning optimal transport across the BBB using TfR-mediated transcytosis pathways. In addition, we believe the single-chain Fc antibody we have described, which can easily be manipulated to accommodate a bispecific target tactic, provides a simple and efficacious approach for delivering therapeutic payloads to the brain milieu.

Introducing or removing heparan sulfate binding sites does not alter brain uptake of the blood-brain barrier shuttle scFv8D3.

The blood-brain barrier (BBB) greatly limits the delivery of protein-based drugs into the brain and is a major obstacle for the treatment of brain disorders. Targeting the transferrin receptor (TfR) is a strategy for transporting protein-based drugs into the brain, which can be utilized by using TfR-binding BBB transporters, such as the TfR-binding antibody 8D3. In this current study, we investigated if binding to heparan sulfate (HS) contributes to the brain uptake of a single chain fragment variable of 8D3 (scFv8D3). We designed and produced a scFv8D3 mutant, engineered with additional HS binding sites, HS(+)scFv8D3, to assess whether increased HS binding would improve brain uptake. Additionally, a mutant with a reduced number of HS binding sites, HS(-)scFv8D3, was also engineered to see if reducing the HS binding sites could also affect brain uptake. Heparin column chromatography showed that only the HS(+)scFv8D3 mutant bound HS in the experimental conditions. Ex vivo results showed that the brain uptake was unaffected by the introduction or removal of HS binding sites, which indicates that scFv8D3 is not dependent on the HS binding sites for brain uptake. Conversely, introducing HS binding sites to scFv8D3 decreased its renal excretion while removing them had the opposite effect.

A Brain-Targeting Bispecific-Multivalent Antibody Clears Soluble Amyloid-Beta Aggregates in Alzheimer's Disease Mice.

Amyloid-ß (Aß) oligomers and protofibrils are suggested to be the most neurotoxic Aß species in Alzheimer's disease (AD). Hence, antibodies with strong and selective binding to these soluble Aß aggregates are of therapeutic potential. We have recently introduced HexaRmAb158, a multivalent antibody with additional Aß-binding sites in the form of single-chain fragment variables (scFv) on the N-terminal ends of Aß protofibril selective antibody (RmAb158). Due to the additional binding sites and the short distance between them, HexaRmAb158 displayed a slow dissociation from protofibrils and strong binding to oligomers in vitro. In the current study, we aimed at investigating the therapeutic potential of this antibody format in vivo using mouse models of AD. To enhance BBB delivery, the transferrin receptor (TfR) binding moiety (scFv8D3) was added, forming the bispecific-multivalent antibody (HexaRmAb158-scFv8D3). The new antibody displayed a weaker TfR binding compared to the previously developed RmAb158-scFv8D3 and was less efficiently transcytosed in a cell-based BBB model. HexaRmAb158 detected soluble Aß aggregates derived from brains of tg-ArcSwe and AppNL-G-F mice more efficiently compared to RmAb158. When intravenously injected, HexaRmAb158-scFv8D3 was actively transported over the BBB into the brain in vivo. Brain uptake was marginally lower than that of RmAb158-scFv8D3, but significantly higher than observed for conventional IgG antibodies. Both antibody formats displayed similar brain retention (72 h post injection) and equal capacity in clearing soluble Aß aggregates in tg-ArcSwe mice. In conclusion, we demonstrate a bispecific-multivalent antibody format capable of passing the BBB and targeting a wide-range of sizes of soluble Aß aggregates.

Wide-Ranging Effects on the Brain Proteome in a Transgenic Mouse Model of Alzheimer's Disease Following Treatment with a Brain-Targeting Somatostatin Peptide.

Alzheimer's disease is the most common neurodegenerative disorder characterized by the pathological aggregation of amyloid-ß (Aß) peptide. A potential therapeutic intervention in Alzheimer's disease is to enhance Aß degradation by increasing the activity of Aß-degrading enzymes, including neprilysin. The somatostatin (SST) peptide has been identified as an activator of neprilysin. Recently, we demonstrated the ability of a brain-penetrating SST peptide (SST-scFv8D3) to increase neprilysin activity and membrane-bound Aß42 degradation in the hippocampus of mice overexpressing the Aß-precursor protein with the Swedish mutation (APPswe). Using LC-MS, we further evaluated the anti-Alzheimer's disease effects of SST-scFv8D3. Following a triple intravenous injection of SST-scFv8D3, the LC-MS analysis of the brain proteome revealed that the majority of downregulated proteins consisted of mitochondrial proteins regulating fatty acid oxidation, which are otherwise upregulated in APPswe mice compared to wild-type mice. Moreover, treatment with SST-scFv8D3 significantly increased hippocampal levels of synaptic proteins regulating cell membrane trafficking and neuronal development. Finally, hippocampal concentrations of growth-regulated α (KC/GRO) chemokine and degradation of neuropeptide-Y were elevated after SST-scFv8D3 treatment. In summary, our results demonstrate a multifaceted effect profile in regulating mitochondrial function and neurogenesis following treatment with SST-scFv8D3, further suggesting the development of Alzheimer's disease therapies based on SST peptides.

ADAR-related activation of adenosine-to-inosine RNA editing during regeneration.

Urodele amphibians possess an amazing regenerative capacity that requires the activation of cellular plasticity in differentiated cells and progenitor/stem cells. Many aspects of regeneration in Urodele amphibians recapitulate development, making it unlikely that gene regulatory pathways which are essential for development are mutually exclusive from those necessary for regeneration. One such post-transcriptional gene regulatory pathway, which has been previously shown to be essential for functional metazoan development, is RNA editing. RNA editing catalyses discrete nucleotide changes in RNA transcripts, creating a molecular diversity that could create an enticing connection to the activated cellular plasticity found in newts during regeneration. To assess whether RNA editing occurs during regeneration, we demonstrated that GABRA3 and ADAR2 mRNA transcripts are edited in uninjured and regenerating tissues. Full open-reading frame sequences for ADAR1 and ADAR2, two enzymes responsible for adenosine-to-inosine RNA editing, were cloned from newt brain cDNA and exhibited a strong resemblance to ADAR (adenosine deaminase, RNA-specific) enzymes discovered in mammals. We demonstrated that ADAR1 and ADAR2 mRNA expression levels are differentially expressed during different phases of regeneration in multiple tissues, whereas protein expression levels remain unaltered. In addition, we have characterized a fascinating nucleocytoplasmic shuttling of ADAR1 in a variety of different cell types during regeneration, which could provide a mechanism for controlling RNA editing, without altering translational output of the editing enzyme. The link between RNA editing and regeneration provides further insights into how lower organisms, such as the newt, can activate essential molecular pathways via the discrete alteration of RNA sequences.

Plasticity and recovery of skeletal muscle satellite cells during limb regeneration.

Salamander limb regeneration depends on local progenitors whose progeny are recruited to the new limb. We previously identified a Pax7(+) cell population in skeletal muscle whose progeny have the potential to contribute to the regenerating limb. However, the plasticity of individual Pax7(+) cells, as well as their recovery within the new limb, was unclear. Here, we show that Pax7(+) cells remain present after multiple rounds of limb amputation/regeneration. Pax7(+) cells are found exclusively within skeletal muscle in the regenerating limb and proliferate where the myofibers are growing. Pax7 is rapidly down-regulated in the blastema, and analyses of clonal derivatives show that Pax7(+) cell progeny are not restricted to skeletal muscle during limb regeneration. Our data suggest that the newt regeneration blastema is not entirely a composite of lineage-restricted progenitors. The results demonstrate that except for a transient and subsequently blunted increase, skeletal muscle satellite cells constitute a stable pool of reserve cells for multiple limb regeneration events.-Morrison, J. I., Borg, P., Simon, A. Plasticity and recovery of skeletal muscle satellite cells during limb regeneration.

Targeted gene delivery to differentiated skeletal muscle: a tool to study dedifferentiation.

Cellular dedifferentiation is required for functional regeneration in salamanders. Dedifferentiating multinucleate skeletal muscle gives rise to mononucleate cells during limb regeneration. Efficient methods and tools must be developed in order to understand the molecular cues underlying dedifferentiation. Here we describe a non-viral method to express extra-chromosomal DNA exclusively in terminally differentiated muscle without the need for cell purification steps. After cytoplasmic injection of various expression vectors into myotubes or myofibres, we detect long-lasting mRNA and protein expression in up to 70% of the injected cells. The combination of the transfection protocol with live imaging allows a time- and cost-effective screen of candidate genes in terminally differentiated muscle cells of both amphibian and mammalian origin.

Salamander limb regeneration involves the activation of a multipotent skeletal muscle satellite cell population.

In contrast to mammals, salamanders can regenerate complex structures after injury, including entire limbs. A central question is whether the generation of progenitor cells during limb regeneration and mammalian tissue repair occur via separate or overlapping mechanisms. Limb regeneration depends on the formation of a blastema, from which the new appendage develops. Dedifferentiation of stump tissues, such as skeletal muscle, precedes blastema formation, but it was not known whether dedifferentiation involves stem cell activation. We describe a multipotent Pax7+ satellite cell population located within the skeletal muscle of the salamander limb. We demonstrate that skeletal muscle dedifferentiation involves satellite cell activation and that these cells can contribute to new limb tissues. Activation of salamander satellite cells occurs in an analogous manner to how the mammalian myofiber mobilizes stem cells during skeletal muscle tissue repair. Thus, limb regeneration and mammalian tissue repair share common cellular and molecular programs. Our findings also identify satellite cells as potential targets in promoting mammalian blastema formation.