Leptomeningeal anastomoses: Mechanisms of pial collateral remodeling in ischemic stroke

Collateral blood vessels in stroke and ischemic disease: Formation, physiology, rarefaction, remodeling

Collateral blood vessels are unique, naturally occurring endogenous bypass vessels that provide alternative pathways for oxygen delivery in obstructive arterial conditions and diseases. Surprisingly however, the capacity of the collateral circulation to provide protection varies greatly among individuals, resulting in a significant fraction having poor collateral circulation in their tissues. We recently reviewed evidence that the presence of naturally-occurring polymorphisms in genes that determine the number and diameter of collaterals that form during development (ie, genetic background), is a major contributor to this variation. The purpose of this review is to summarize current understanding of the other determinants of collateral blood flow, drawing on both animal and human studies. These include the level of smooth muscle tone in collaterals, hemodynamic forces, how collaterals form during development (collaterogenesis), de novo formation of additional new collaterals during adulthood, loss of collaterals with aging and cardiovascular risk factor presence (rarefaction), and collateral remodeling (structural lumen enlargement). We also review emerging evidence that collaterals not only provide protection in ischemic conditions but may also serve a physiological function in healthy individuals. Primary focus is on studies conducted in brain, however relevant findings in other tissues are also reviewed, as are questions for future investigation.

Tie2-Dependent Mechanisms Promote Leptomeningeal Collateral Remodeling and Reperfusion Following Stroke

Leptomeningeal collaterals are distal pial arterial anastomotic vessels that provide an alternative route for redistributing cerebral blood flow following arterial obstruction, thereby limiting tissue damage. However, the regulatory mechanisms and strategies to enhance this adaptive response remain under investigation. This study explored the pharmacological effects of Tie2 receptor activation, using the peptide agonist Vasculotide, following permanent middle cerebral artery occlusion (pMCAO). Vasculotide improved collateral growth and remodeling, which correlated with reduced infarct volume, enhanced blood flow, and functional recovery within 24hrs post-pMCAO. In contrast, collateral growth was attenuated in Tie2 and EphA4/Tie2 double knockdown mice, while the loss of EphA4 increased Tie2 and Ang-1 expression and mimicked the positive effects of Vasculotide following stroke. Furthermore, bulk RNA sequencing of meningeal tissue identified key transcriptomic changes, including alterations in AJ-associated transcripts, such as Krt5 , Krt14 , and Col17a1 , in the ipsilateral meninges of both endothelial cell-specific EphA4 knockout and Vasculotide-treated mice. Krt5 expression was found upregulated on meningeal arterial vascular network in injured KO mice, highlighting a potential new mediator of meningeal vascular remodeling. These findings illustrate that EphA4 and Tie2 play opposing roles in collateral remodeling, including the regulation of Krt5. Modulating their activity could potentially enhance the collateral response to stroke.

Genetic determinants of insufficiency of the collateral circulation

It has been estimated that approximately two million neurons, sixteen billion synapses and twelve kilometers of axons are lost each minute following anterior large-vessel stroke. The level of collateral blood flow has become recognized as a primary determinant of the pace of this loss and an important factor in clinical decision-making. Many of the topics in this review cover recent developments that have not been reviewed elsewhere. These include that: the number and diameter of collaterals and collateral blood flow vary greatly in the brain and other tissues of healthy individuals; a large percentage of individuals are deficient in collaterals; the underlying mechanism arises primarily from naturally occurring polymorphisms in genes/genetic loci within the pathway that drives collateral formation during development; evidence indicates collateral abundance does not exhibit sexual dimorphism; and that collaterals-besides their function as endogenous bypass vessels-may have a physiological role in optimizing oxygen delivery. Animal and human studies in brain and other tissues, where available, are reviewed. Details of many of the studies are provided so that the strength of the findings and conclusions can be assessed without consulting the original literature. Key questions that remain unanswered and strategies to address them are also discussed.

Determinants of Leptomeningeal Collateral Status in Acute Ischemic Stroke: A Systematic Review and Meta-Analysis of Observational Studies

BACKGROUND

Leptomeningeal collateral status is a major determinant of outcomes in patients with acute ischemic stroke; however, the factors that determine collateral status are not well understood. We conducted a comprehensive systematic review and meta-analysis to identify determinants associated with collateral status in patients with anterior circulation infarction.

METHODS AND RESULTS

The PubMed, EMBASE, Web of Science, and Cochrane Central Register of Controlled Trials databases were searched for studies that reported the determinants of leptomeningeal collateral status in acute ischemic stroke between January 2000 and June 2023. A random-effects meta-analysis model was used to pool the determinants of leptomeningeal collateral status. Eighty-one studies with 17 366 patients met the inclusion criteria. We analyzed 31 potential risk factors, and the results indicated that worse leptomeningeal collateral status was significantly associated with older age (weighted mean difference, 1.22 [95% CI, 0.69 to 1.76]), male sex (odds ratio [OR], 1.12 [95% CI, 1.02 to 1.23]), hypertension (OR, 1.27 [95% CI, 1.15 to 1.40]), diabetes (OR, 1.21 [95% CI, 1.10 to 1.33]), atrial fibrillation (OR, 1.26 [95% CI, 1.09 to 1.46]), cardioembolic stroke (OR, 1.27 [95% CI, 1.04 to 1.55]), internal carotid artery occlusion (OR, 1.84 [95% CI, 1.50 to 2.25]), and higher admission blood glucose (weighted mean difference, 8.74 [95% CI, 2.52 to 18.51]).

CONCLUSIONS

Hypertension and diabetes could be modifiable risk factors associated with leptomeningeal collateral status. Older age and male sex could be nonmodified risk factors. Further high-quality therapeutic studies focusing on controlling risk factors are needed to support our findings.

Evolucollateral dynamics in stroke: Evolutionary pathophysiology, remodelling and emerging therapeutic strategies

Leptomeningeal collaterals (LMCs) are crucial in mitigating the impact of acute ischemic stroke (AIS) by providing alternate blood flow routes when primary arteries are obstructed. This article explores the evolutionary pathophysiology of LMCs, highlighting their critical function in stroke and the genetic and molecular mechanisms governing their development and remodelling. We address the translational challenges of applying animal model findings to human clinical scenarios, emphasizing the need for further research to validate emerging therapies-such as pharmacological agents, gene therapy and mechanical interventions-in clinical settings, aimed at enhancing collateral perfusion. Computational modelling emerges as a promising method for integrating experimental data, which requires precise parameterization and empirical validation. We introduce the 'Evolucollateral Dynamics' hypothesis, proposing a novel framework that incorporates evolutionary biology principles into therapeutic strategies, offering new perspectives on enhancing collateral circulation. This hypothesis emphasizes the role of genetic predispositions and environmental influences on collateral circulation, which may impact therapeutic strategies and optimize treatment outcomes. Future research must incorporate human clinical data to create robust treatment protocols, thereby maximizing the therapeutic potential of LMCs and improving outcomes for stroke patients.

Integrated Strategies for Targeting Arteriogenesis and Angiogenesis After Stroke

The interdependence between arteriogenesis and angiogenesis is crucial for enhancing perfusion by synchronously improving leptomeningeal collaterals (LMCs) and microvascular networks after stroke. However, current approaches often focus on promoting arteriogenesis and angiogenesis separately, neglecting the potential synergistic benefits of targeting both processes simultaneously. Therefore, it is imperative to consider both arteriogenesis and angiogenesis as integral and complementary strategies for post-stroke revascularization. To gain a deeper understanding of their relationships after stroke and to facilitate the development of targeted revascularization strategies, we compared them based on their timescale, space, and pathophysiology. The temporal differences in the occurrence of arteriogenesis and angiogenesis allow them to restore blood flow at different stages after stroke. The spatial differences in the effects of arteriogenesis and angiogenesis enable them to specifically target the ischemic penumbra and core infarct region. Additionally, the endothelial cell, as the primary effector cell in their pathophysiological processes, is promising target for enhancing both. Therefore, we provide an overview of key signals that regulate endothelium-mediated arteriogenesis and angiogenesis. Finally, we summarize current therapeutic strategies that involve these signals to promote both processes after stroke, with the aim of inspiring future therapeutic advances in revascularization.

Inter-Rater Reliability of Collateral Status Assessment Based on CT Angiography: A Retrospective Study of Middle Cerebral Artery Ischaemic Stroke

The importance of assessing the collateral status (CS) in patients with ischaemic stroke (IS) has repeatedly been emphasised in clinical guidelines. Various publications offer qualitative or semiquantitative scales with gradations corresponding to the different extents of the collaterals, visualised mostly on the basis of CTA images. However, information on their inter-rater reliability is limited. Therefore, the aim of this study is to investigate the inter-rater reliability of the scales for collateral assessment. CTA images of 158 patients in the acute period of IS were used in the study. The assessment of CS was performed by two experts using three methodologies: the modified Tan scale, the Miteff scale, and the Rosenthal scale. Cohen's kappa, weighted kappa and Krippendorff's alpha were used as reliability measures. For the modified Tan scale and the Miteff and Rosenthal scales, the weighted kappa values were 0.72, 0.49 and 0.59, respectively. Although the best measure of consistency was found for the modified Tan scale, no statistically significant differences were revealed among the scales. The impact of the CS on the degree of neurological deficit at discharge was shown for the modified Tan and Rosenthal scales. In conclusion, the analysis showed a moderate inter-rater reliability of the three scales, but was not able to distinguish the best one among them.

Remodeling of the Neurovascular Unit Following Cerebral Ischemia and Hemorrhage

Formulated as a group effort of the stroke community, the transforming concept of the neurovascular unit (NVU) depicts the structural and functional relationship between brain cells and the vascular structure. Composed of both neural and vascular elements, the NVU forms the blood-brain barrier that regulates cerebral blood flow to meet the oxygen demand of the brain in normal physiology and maintain brain homeostasis. Conversely, the dysregulation and dysfunction of the NVU is an essential pathological feature that underlies neurological disorders spanning from chronic neurodegeneration to acute cerebrovascular events such as ischemic stroke and cerebral hemorrhage, which were the focus of this review. We also discussed how common vascular risk factors of stroke predispose the NVU to pathological changes. We synthesized existing literature and first provided an overview of the basic structure and function of NVU, followed by knowledge of how these components remodel in response to ischemic stroke and brain hemorrhage. A greater understanding of the NVU dysfunction and remodeling will enable the design of targeted therapies and provide a valuable foundation for relevant research in this area.