In vivo imaging of endothelial cell adhesion molecule expression after radiosurgery in an animal model of arteriovenous malformation

Key biomarkers in cerebral arteriovenous malformations: Updated review

The formation of vascular networks consisting of arteries, capillaries, and veins is vital in embryogenesis. It is also crucial in adulthood for the formation of a functional vasculature. Cerebral arteriovenous malformations (CAVMs) are linked with a remarkable risk of intracerebral hemorrhage because arterial blood is directly shunted into the veins before the arterial blood pressure is dissipated. The underlying mechanisms responsible for arteriovenous malformation (AVM) growth, progression, and rupture are not fully known, yet the critical role of inflammation in AVM pathogenesis has been noted. The proinflammatory cytokines are upregulated in CAVM, which stimulates overexpression of cell adhesion molecules in endothelial cells (ECs), leading to improved leukocyte recruitment. It is well-known that metalloproteinase-9 secretion by leukocytes disrupts CAVM walls resulting in rupture. Moreover, inflammation alters the angioarchitecture of CAVMs by upregulating angiogenic factors impacting the apoptosis, migration, and proliferation of ECs. A better understanding of the molecular signature of CAVM might allow us to identify biomarkers predicting this complication, acting as a goal for further investigations that may be potentially targeted in gene therapy. The present review is focused on the numerous studies conducted on the molecular signature of CAVM and the associated hemorrhage. The association of numerous molecular signatures with a higher risk of CAVM rupture is shown through inducing proinflammatory mediators, as well as growth factors signaling, Ras-mitogen-activated protein kinase-extracellular signal-regulated kinase, and NOTCH pathways, which are accompanied by cellular level inflammation and endothelial alterations resulting in vascular wall instability. According to the studies, it is assumed that matrix metalloproteinase, interleukin-6, and vascular endothelial growth factor are the biomarkers most associated with CAVM and the rate of hemorrhage, as well as diagnostic methods, with respect to enhancing the patient-specific risk estimation and improving treatment choices.

Externalization of Mitochondrial PDCE2 on Irradiated Endothelium as a Target for Radiation-Guided Drug Delivery and Precision Thrombosis of Pathological Vasculature

Endothelial cells are highly sensitive to ionizing radiation, and exposure leads to multiple adaptive changes. Remarkably, part of this response is the translocation of normally intracellular proteins to the cell surface. It is unclear whether this ectopic expression has a protective or deleterious function, but, regardless, these surface-exposed proteins may provide unique discriminatory targets for radiation-guided drug delivery to vascular malformations or tumor vasculature. We investigated the ability of an antibody–thrombin conjugate targeting mitochondrial PDCE2 (E2 subunit of pyruvate dehydrogenase) to induce precision thrombosis on irradiated endothelial cells in a parallel-plate flow system. Click-chemistry was used to create antibody–thrombin conjugates targeting PDCE2 as the vascular targeting agent (VTA). VTAs were injected into the parallel-plate flow system with whole human blood circulating over irradiated cells. The efficacy and specificity of fibrin-thrombus formation was assessed relative to non-irradiated controls. The PDCE2-targeting VTA dose-dependently increased thrombus formation: minimal thrombosis was induced in response to 5 Gy radiation; doses of 15 and 25 Gy induced significant thrombosis with equivalent efficacy. Negligible VTA binding or thrombosis was demonstrated in the absence of radiation or with non-targeted thrombin. PDCE2 represents a unique discriminatory target for radiation-guided drug delivery and precision thrombosis in pathological vasculature.

Acute and endothelial-specific Robo4 deletion affect hematopoietic stem cell trafficking independent of VCAM1

Hematopoietic stem cell (HSC) trafficking is regulated by a number of complex mechanisms. Among them are the transmembrane protein Robo4 and the vascular cell adhesion molecule, VCAM1. Endothelial VCAM1 is a well-known regulator of hematopoietic cell trafficking, and our previous studies revealed that germline deletion of Robo4 led to impaired HSC trafficking, with an increase in vascular endothelial cell (VEC) numbers and downregulation of VCAM1 protein on sinusoidal VECs. Here, we utilized two Robo4 conditional deletion models in parallel with Robo4 germline knockout mice (R4KO) to evaluate the effects of acute and endothelial cell-specific Robo4 deletion on HSC trafficking. Strikingly similar to the R4KO, the acute deletion of Robo4 resulted in altered HSC distribution between the bone marrow and blood compartments, despite normal numbers of VECs and wild-type levels of VCAM1 cell surface protein on sinusoidal VECs. Additionally, consistent with the R4KO mice, acute loss of Robo4 in the host perturbed long-term engraftment of donor wild-type HSCs and improved HSC mobilization to the peripheral blood. These data demonstrate the significant role that endothelial Robo4 plays in directional HSC trafficking, independent of alterations in VEC numbers and VCAM1 expression.

Immuno-imaging of ICAM-1 in tumours by SPECT

PURPOSE

Molecular imaging of cancer cells' reaction to radiation damage can provide a non-invasive measure of tumour response to treatment. The cell surface glycoprotein ICAM-1 (CD54) was identified as a potential radiation response marker. SPECT imaging using an 111In-radiolabelled anti-ICAM-1 antibody was explored.

METHODS

PSN-1 cells were irradiated (10 Gy), and protein expression changes were investigated using an antibody array on cell lysates 24 h later. Results were confirmed by western blot, flow cytometry and immunofluorescence. We confirmed the affinity of an 111In-labelled anti-ICAM-1 antibody in vitro, and in vivo, in PSN-1-xenograft bearing mice. The xenografts were irradiated (0 or 10 Gy), and [111In]In-anti-ICAM-1 SPECT/CT images were acquired 24, 48 and 72 h after intravenous administration.

RESULTS

ICAM-1 was identified as a potential marker of radiation treatment using an antibody array in PSN-1 cell lysates following irradiation, showing a significant increase in ICAM-1 signal compared to non-irradiated cells. Western blot and immunohistochemistry confirmed this upregulation, with an up to 20-fold increase in ICAM-1 signal. Radiolabelled anti-ICAM-1 bound to ICAM-1 expressing cells with good affinity (Kd = 24.0 ± 4.0 nM). [111In]In-anti-ICAM-1 uptake in tumours at 72 h post injection was approximately 3-fold higher than non-specific isotype-matched [111In]In-mIgG2a control (19.3 ± 2.5%ID/g versus 6.3 ± 2.2%ID/g, P = 0.0002). However, ICAM1 levels, and [111In]In-anti-ICAM-1 uptake in tumours was no different after irradiation (uptake 9.2%ID/g versus 14.8%ID/g). Western blots of the xenograft lysates showed no significant differences, confirming these results.

CONCLUSION

Imaging of ICAM-1 is feasible in mouse models of pancreatic cancer. Although ICAM-1 is upregulated post-irradiation in in vitro models of pancreatic cancer, it shows little change in expression in an in vivo mouse xenograft model.

Targeting of externalized αB-crystallin on irradiated endothelial cells with pro-thrombotic vascular targeting agents: Potential applications for brain arteriovenous malformations

BACKGROUND

Vascular targeting uses molecular markers on the surface of diseased vasculature for ligand-directed drug delivery to induce vessel occlusion or destruction. In the absence of discriminatory markers, such as in brain arteriovenous malformations (AVMs), stereotactic radiosurgery may be used to prime molecular changes on the endothelial surface. This study explored αB-crystallin (CRYAB) as a radiation induced target and pre-tested the specificity and efficacy of a CRYAB-targeting coaguligand for in vitro thrombus induction.

METHODS

A parallel-plate flow system was established to circulate human whole blood over a layer of human brain endothelial cells. A conjugate of anti-CRYAB antibody and thrombin was injected into the circuit to compare binding and thrombus formation on cells with or without prior radiation treatment (0-25 Gy).

RESULTS

Radiation increased CRYAB expression and surface exposure in human brain endothelial cells. In the parallel-plate flow system, the targeted anti-CRYAB-thrombin conjugate increased thrombus formation on the surface of irradiated cells relative to non-irradiated cells and to a non-targeting IgG-thrombin conjugate. Fibrin deposition and accumulation of fibrinogen degradation products increased significantly at radiation doses at or above 15 Gy with conjugate concentrations of 1.25 and 2.5 μg/mL.

CONCLUSIONS

CRYAB exposure can be detected at the surface of human brain endothelial cells in response to irradiation. Pro-thrombotic CRYAB-targeting conjugates can bind under high flow conditions and in the presence of whole blood induce stable thrombus formation with high specificity and efficacy on irradiated surfaces. CRYAB provides a novel radiation marker for potential vascular targeting in irradiated brain AVMs.

Occlusion of Animal Model Arteriovenous Malformations Using Vascular Targeting

Brain arteriovenous malformations (AVMs) are a significant cause of intracerebral hemorrhage in children and young adults. Currently, one third of patients have no viable treatment options. Vascular targeting agents (VTAs) are being designed to deliver pro-thrombotic molecules to the abnormal AVM vessels for rapid occlusion and cure. This study assessed the efficacy of a pro-thrombotic VTA targeting phosphatidylserine (PS) in a radiation-primed AVM animal model. The model AVM was surgically created in rats by anastomosis of the left external jugular vein to the adjacent common carotid artery. After 6 weeks, the AVM was irradiated (20 Gy) using gamma knife surgery (GKS). A PS-targeting VTA was created by conjugation of annexin V with human thrombin and administered intravenously 3 weeks post-GKS or sham. Unconjugated thrombin was used as a non-targeting control. AVM thrombosis and occlusion was monitored 3 weeks later by angiography and histology. Preliminary experiments established a safe dose of active thrombin for systemic administration. Subsequently, a single dose of annexin V-thrombin conjugate (0.77 mg/kg) resulted in angiographic AVM occlusion in sham (75%) and irradiated (63%) animals, while non-targeted thrombin did not. Lowering the conjugate dose (0.38 mg/kg) decreased angiographic AVM occlusion in sham (13%) relative to irradiated (80%) animals (p = 0.03) as did delivery of two consecutive doses of 0.38 mg/kg, 2 days apart (sham (0%); irradiated (78%); p = 0.003). These findings demonstrate efficacy of the PS-targeting VTA and the feasibility of a vascular targeting approach for occlusion of high-flow AVMs. Targeting specificity can be enhanced by radiation-sensitization and VTA dose modification.

Radiation-Stimulated Translocation of CD166 and CRYAB to the Endothelial Surface Provides Potential Vascular Targets on Irradiated Brain Arteriovenous Malformations

Vascular targeting with pro-thrombotic antibody-conjugates is a promising biological treatment for brain arteriovenous malformations (bAVMs). However, targeted drug delivery relies on the identification of unique or overexpressed markers on the surface of a target cell. In the absence of inherent biological markers, stereotactic radiosurgery may be used to prime induction of site-specific and targetable molecular changes on the endothelial surface. To investigate lumen-accessible, endothelial targets induced by radiation, we combined Gamma knife surgery in an AVM animal model with in vivo biotin-labeling and comparative proteomics. Two proteins, αB-crystallin (CRYAB)-a small heat shock protein that normally acts as an intracellular chaperone to misfolded proteins-and activated leukocyte cell adhesion molecule CD166, were further validated for endothelial surface expression after irradiation. Immunostaining of endothelial cells in vitro and rat AVM tissue ex vivo confirmed de novo induction of CRYAB following irradiation (20 Gy). Western analysis demonstrated that CRYAB accumulated intracellularly as a 20 kDa monomer, but, at the cell surface, a novel 65 kDa protein was observed, suggesting radiation stimulates translocation of an atypical CRYAB isoform. In contrast, CD166 had relatively high expression in non-irradiated cells, localized predominantly to the lateral surfaces. Radiation increased CD166 surface exposure by inducing translocation from intercellular junctions to the apical surface without significantly altering total protein levels. These findings reinforce the dynamic molecular changes induced by radiation exposure, particularly at the cell surface, and support further investigation of radiation as a priming mechanism and these molecules as putative targets for focused drug delivery in irradiated tissue.

Sensitization to endothelial cell antigens: Unraveling the cause or effect paradox

Anti-endothelial cell antibodies (AECAs) have been correlated with increased acute and chronic rejection across all organ types and early graft dysfunction in kidney and heart transplantation. Nevertheless, the lack of appropriate tools and clear criteria for defining injurious versus non-injurious AECAs prohibits their routine inclusion in clinical risk assessments and diagnostic algorithms for antibody mediated injury. Clinical characterization of AECAs is complicated due to the wide range of polymorphic and non-polymorphic antigens expressed across different vascular tissues and the diverse array of specificities observed between individuals. This complexity is also reflected in the broad spectrum of reported injury phenotypes. AECAs detected at time of allograft dysfunction may represent biomarkers of past vascular injury or active contributors to a current rejection process. New tools within the fields of proteomics, genomics, bioinformatics, and imaging are currently being validated and hold great promise for unraveling the AECA paradox.