CDR3 Variants of the TXB2 Shuttle with Increased TfR1 Association Rate and Enhanced Brain Penetration.

Since the delivery of biologic drugs to the brain is greatly hampered by the existence of the blood-brain barrier (BBB), brain shuttles are being developed to enhance therapeutic efficacy. As we have previously shown, efficient and selective brain delivery was achieved with TXB2, a cross-species reactive, anti-TfR1 VNAR antibody. To further explore the limits of brain penetration, we conducted restricted randomization of the CDR3 loop, followed by phage display to identify improved TXB2 variants. The variants were screened for brain penetration in mice using a 25 nmol/kg (1.875 mg/kg) dose and a single 18 h timepoint. A higher kinetic association rate to TfR1 correlated with improved brain penetration in vivo. The most potent variant, TXB4, showed a 3.6-fold improvement over TXB2, which had on average 14-fold higher brain levels when compared to an isotype control. Like TXB2, TXB4 retained brain specificity with parenchymal penetration and no accumulation in other organs. When fused with a neurotensin (NT) payload, it led to a rapid drop in body temperature upon transport across the BBB. We also showed that fusion of TXB4 to four therapeutic antibodies (anti-CD20, anti-EGFRvIII, anti-PD-L1 and anti-BACE1) improved their brain exposure between 14- to 30-fold. In summary, we enhanced the potency of parental TXB2 brain shuttle and gained a critical mechanistic understanding of brain delivery mediated by the VNAR anti-TfR1 antibody.

A Single Domain Shark Antibody Targeting the Transferrin Receptor 1 Delivers a TrkB Agonist Antibody to the Brain and Provides Full Neuroprotection in a Mouse Model of Parkinson's Disease.

Single domain shark antibodies that bind to the transferrin receptor 1 (TfR1) on brain endothelial cells have been used to shuttle antibodies and other cargos across the blood brain barrier (BBB) to the brain. For these studies the TXB4 brain shuttle was fused to a TrkB neurotrophin receptor agonist antibody. The TXB4-TrkB fusion retained potent agonist activity at its cognate receptor and after systemic administration showed a 12-fold increase in brain levels over the unmodified antibody. Only the TXB4-TrkB antibody fusion was detected within the brain and localized to TrkB positive cells in the cortex and tyrosine hydroxylase (TH) positive dopaminergic neurons in the substantia nigra pars compacta (SNc), where it was associated with activated ERK1/2 signaling. When tested in the 6-hydroxydopamine (6-OHDA) mouse model of Parkinson's disease (PD), TXB4-TrkB, but not the unmodified antibody, completely prevented the 6-OHDA induced death of TH positive neurons in the SNc. In conclusion, the fusion of the TXB4 brain shuttle allows a TrkB agonist antibody to reach neuroprotective concentrations in the brain parenchyma following systemic administration.

Single domain shark VNAR antibodies neutralize SARS-CoV-2 infection in vitro.

Single domain shark variable domain of new antigen receptor (VNAR) antibodies can offer a viable alternative to conventional Ig-based monoclonal antibodies in treating COVID-19 disease during the current pandemic. Here we report the identification of neutralizing single domain VNAR antibodies selected against the severe acute respiratory syndrome coronavirus 2 spike protein derived from the Wuhan variant using phage display. We identified 56 unique binding clones that exhibited high affinity and specificity to the spike protein. Of those, 10 showed an ability to block both the spike protein receptor binding domain from the Wuhan variant and the N501Y mutant from interacting with recombinant angiotensin-converting enzyme 2 (ACE2) receptor in vitro. In addition, three antibody clones retained in vitro blocking activity when the E484K spike protein mutant was used. The inhibitory property of the VNAR antibodies was further confirmed for all 10 antibody clones using ACE2 expressing cells with spike protein from the Wuhan variant. The viral neutralizing potential of the VNAR clones was also confirmed for the 10 antibodies tested using live Wuhan variant virus in in vitro cell infectivity assays. Single domain VNAR antibodies, due to their low complexity, small size, unique epitope recognition, and formatting flexibility, should be a useful adjunct to existing antibody approaches to treat COVID-19.

Blood-brain barrier transport using a high affinity, brain-selective VNAR antibody targeting transferrin receptor 1.

Transfer across the blood-brain barrier (BBB) remains a significant hurdle for the development of biopharmaceuticals with therapeutic effects within the central nervous system. We established a functional selection method to identify high affinity single domain antibodies to the transferrin receptor 1 (TfR1) with efficient biotherapeutic delivery across the BBB. A synthetic phage display library based on the variable domain of new antigen receptor (VNAR) was used for in vitro selection against recombinant human TfR1 ectodomain (rh-TfR1-ECD) followed by in vivo selection in mouse for brain parenchyma penetrating antibodies. TXB2 VNAR was identified as a high affinity, species cross-reactive VNAR antibody against TfR1-ECD that does not compete with transferrin or ferritin for receptor binding. IV dosing of TXB2 when fused to human Fc domain (TXB2-hFc) at 25 nmol/kg (1.875 mg/kg) in mice resulted in rapid binding to brain capillaries with subsequent transport into the brain parenchyma and specific uptake into TfR1-positive neurons. Likewise, IV dosing of TXB2-hFc fused with neurotensin (TXB2-hFc-NT) at 25 nmol/kg resulted in a rapid and reversible pharmacological response as measured by body temperature reduction. TXB2-hFc did not elicit any acute adverse reactions, bind, or deplete circulating reticulocytes or reduce BBB-expressed endogenous TfR1 in mice. There was no evidence of target-mediated clearance or accumulation in peripheral organs except lung. In conclusion, TXB2 is a high affinity, species cross-reactive, and brain-selective VNAR antibody to TfR1 that rapidly crosses the BBB and exhibits a favorable pharmacokinetic and safety profile and can be readily adapted to carry a wide variety of biotherapeutics from blood to brain.

Mathematical model of early Reelin-induced Src family kinase-mediated signaling.

Reelin is a large glycoprotein with a dual role in the mammalian brain. It regulates the positioning and differentiation of postmitotic neurons during brain development and modulates neurotransmission and memory formation in the adult brain. Alterations in the Reelin signaling pathway have been described in different psychiatric disorders. Reelin mainly signals by binding to the lipoprotein receptors Vldlr and ApoER2, which induces tyrosine phosphorylation of the adaptor protein Dab1 mediated by Src family kinases (SFKs). In turn, phosphorylated Dab1 activates downstream signaling cascades, including PI3-kinase-dependent signaling. In this work, a mechanistic model based on ordinary differential equations was built to model early dynamics of the Reelin-mediated signaling cascade. Mechanistic models are frequently used to disentangle the highly complex mechanisms underlying cellular processes and obtain new biological insights. The model was calibrated on time-resolved data and a dose-response measurement of protein concentrations measured in cortical neurons treated with Reelin. It focusses on the interplay between Dab1 and SFKs with a special emphasis on the tyrosine phosphorylation of Dab1, and their role for the regulation of Reelin-induced signaling. Model selection was performed on different model structures and a comprehensive mechanistic model of the early Reelin signaling cascade is provided in this work. It emphasizes the importance of Reelin-induced lipoprotein receptor clustering for SFK-mediated Dab1 trans-phosphorylation and does not require co-receptors to describe the measured data. The model is freely available within the open-source framework Data2Dynamics (www.data2dynamics.org). It can be used to generate predictions that can be validated experimentally, and provides a platform for model extensions both to downstream targets such as transcription factors and interactions with other transmembrane proteins and neuronal signaling pathways.

Intersectin 1 is a component of the Reelin pathway to regulate neuronal migration and synaptic plasticity in the hippocampus.

Brain development and function depend on the directed and coordinated migration of neurons from proliferative zones to their final position. The secreted glycoprotein Reelin is an important factor directing neuronal migration. Loss of Reelin function results in the severe developmental disorder lissencephaly and is associated with neurological diseases in humans. Reelin signals via the lipoprotein receptors very low density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2), but the exact mechanism by which these receptors control cellular function is poorly understood. We report that loss of the signaling scaffold intersectin 1 (ITSN1) in mice leads to defective neuronal migration and ablates Reelin stimulation of hippocampal long-term potentiation (LTP). Knockout (KO) mice lacking ITSN1 suffer from dispersion of pyramidal neurons and malformation of the radial glial scaffold, akin to the hippocampal lamination defects observed in VLDLR or ApoER2 mutants. ITSN1 genetically interacts with Reelin receptors, as evidenced by the prominent neuronal migration and radial glial defects in hippocampus and cortex seen in double-KO mice lacking ITSN1 and ApoER2. These defects were similar to, albeit less severe than, those observed in Reelin-deficient or VLDLR/ ApoER2 double-KO mice. Molecularly, ITSN1 associates with the VLDLR and its downstream signaling adaptor Dab1 to facilitate Reelin signaling. Collectively, these data identify ITSN1 as a component of Reelin signaling that acts predominantly by facilitating the VLDLR-Dab1 axis to direct neuronal migration in the cortex and hippocampus and to augment synaptic plasticity.

Contact between the ß1 and ß2 Segments of α-Synuclein that Inhibits Amyloid Formation.

Conversion of the intrinsically disordered protein α-synuclein (α-syn) into amyloid aggregates is a key process in Parkinson's disease. The sequence region 35-59 contains ß-strand segments ß1 and ß2 of α-syn amyloid fibril models and most disease-related mutations. ß1 and ß2 frequently engage in transient interactions in monomeric α-syn. The consequences of ß1-ß2 contacts are evaluated by disulfide engineering, biophysical techniques, and cell viability assays. The double-cysteine mutant α-synCC, with a disulfide linking ß1 and ß2, is aggregation-incompetent and inhibits aggregation and toxicity of wild-type α-syn. We show that α-syn delays the aggregation of amyloid-ß peptide and islet amyloid polypeptide involved in Alzheimer's disease and type 2 diabetes, an effect enhanced in the α-synCC mutant. Tertiary interactions in the ß1-ß2 region of α-syn interfere with the nucleation of amyloid formation, suggesting promotion of such interactions as a potential therapeutic approach.

A ß-hairpin-binding protein for three different disease-related amyloidogenic proteins.

Amyloidogenic proteins share a propensity to convert to the ß-structure-rich amyloid state that is associated with the progression of several protein-misfolding disorders. Here we show that a single engineered ß-hairpin-binding protein, the ß-wrapin AS10, binds monomers of three different amyloidogenic proteins, that is, amyloid-ß peptide, α-synuclein, and islet amyloid polypeptide, with sub-micromolar affinity. AS10 binding inhibits the aggregation and toxicity of all three proteins. The results demonstrate common conformational preferences and related binding sites in a subset of the amyloidogenic proteins. These commonalities enable the generation of multispecific monomer-binding agents.

Impact of subunit linkages in an engineered homodimeric binding protein to α-synuclein.

Aggregation of the protein α-synuclein (α-syn) has been implicated in Parkinson's disease and other neurodegenerative disorders, collectively referred to as synucleinopathies. The ß-wrapin AS69 is a small engineered binding protein to α-syn that stabilizes a ß-hairpin conformation of monomeric α-syn and inhibits α-syn aggregation at substoichiometric concentrations. AS69 is a homodimer whose subunits are linked via a disulfide bridge between their single cysteine residues, Cys-28. Here we show that expression of a functional dimer as a single polypeptide chain is achievable by head-to-tail linkage of AS69 subunits. Choice of a suitable linker is essential for construction of head-to-tail dimers that exhibit undiminished α-syn affinity compared with the solely disulfide-linked dimer. We characterize AS69-GS3, a head-to-tail dimer with a glycine-serine-rich linker, under oxidized and reduced conditions in order to evaluate the impact of the Cys28-disulfide bond on structure, stability and α-syn binding. Formation of the disulfide bond causes compaction of AS69-GS3, increases its thermostability, and is a prerequisite for high-affinity binding to α-syn. Comparison of AS69-GS3 and AS69 demonstrates that head-to-tail linkage promotes α-syn binding by affording accelerated disulfide bond formation.

Sequestration of a ß-hairpin for control of α-synuclein aggregation.

The misfolding and aggregation of the protein α-synuclein (α-syn), which results in the formation of amyloid fibrils, is involved in the pathogenesis of Parkinson's disease and other synucleinopathies. The emergence of amyloid toxicity is associated with the formation of partially folded aggregation intermediates. Here, we engineered a class of binding proteins termed ß-wrapins (ß-wrap proteins) with affinity for α-synuclein (α-syn). The NMR structure of an α-syn:ß-wrapin complex reveals a ß-hairpin of α-syn comprising the sequence region α-syn(37-54). The ß-wrapin inhibits α-syn aggregation and toxicity at substoichiometric concentrations, demonstrating that it interferes with the nucleation of aggregation.

Functional cross-talk between ras and rho pathways: a Ras-specific GTPase-activating protein (p120RasGAP) competitively inhibits the RhoGAP activity of deleted in liver cancer (DLC) tumor suppressor by masking the catalytic arginine finger.

The three deleted in liver cancer genes (DLC1-3) encode Rho-specific GTPase-activating proteins (RhoGAPs). Their expression is frequently silenced in a variety of cancers. The RhoGAP activity, which is required for full DLC-dependent tumor suppressor activity, can be inhibited by the Src homology 3 (SH3) domain of a Ras-specific GAP (p120RasGAP). Here, we comprehensively investigated the molecular mechanism underlying cross-talk between two distinct regulators of small GTP-binding proteins using structural and biochemical methods. We demonstrate that only the SH3 domain of p120 selectively inhibits the RhoGAP activity of all three DLC isoforms as compared with a large set of other representative SH3 or RhoGAP proteins. Structural and mutational analyses provide new insights into a putative interaction mode of the p120 SH3 domain with the DLC1 RhoGAP domain that is atypical and does not follow the classical PXXP-directed interaction. Hence, p120 associates with the DLC1 RhoGAP domain by targeting the catalytic arginine finger and thus by competitively and very potently inhibiting RhoGAP activity. The novel findings of this study shed light on the molecular mechanisms underlying the DLC inhibitory effects of p120 and suggest a functional cross-talk between Ras and Rho proteins at the level of regulatory proteins.