Foe or friend? Janus-faces of the neurovascular unit in the formation of brain metastases

The Role of Microglia in Brain Metastases: Mechanisms and Strategies

Brain metastases and related complications are one of the major fatal factors in cancer. Patients with breast cancer, lung cancer, and melanoma are at a high risk of developing brain metastases. However, the mechanisms underlying the brain metastatic cascade remain poorly understood. Microglia, one of the major resident macrophages in the brain parenchyma, are involved in multiple processes associated with brain metastasis, including inflammation, angiogenesis, and immune modulation. They also closely interact with metastatic cancer cells, astrocytes, and other immune cells. Current therapeutic approaches against metastatic brain cancers, including small-molecule drugs, antibody-coupled drugs (ADCs), and immune-checkpoint inhibitors (ICIs), have compromised efficacy owing to the impermeability of the blood-brain barrier (BBB) and complex brain microenvironment. Targeting microglia is one of the strategies for treating metastatic brain cancer. In this review, we summarize the multifaceted roles of microglia in brain metastases and highlight them as potential targets for future therapeutic interventions.

Inflammasome activation in peritumoral astrocytes is a key player in breast cancer brain metastasis development

Inflammasomes, primarily responsible for the activation of IL-1β, have emerged as critical regulators of the tumor microenvironment. By using in vivo and in vitro brain metastasis models, as well as human samples to study the role of the NLRP3 inflammasome in triple-negative breast cancer (TNBC) brain metastases, we found NLRP3 inflammasome components and IL-1β to be highly and specifically expressed in peritumoral astrocytes. Soluble factors from TNBC cells induced upregulation and activation of NLRP3 and IL-1β in astrocytes, while astrocyte-derived mediators augmented the proliferation of metastatic cells. In addition, inhibition of NLRP3 inflammasome activity using MCC950 or dampening the downstream effect of IL-1β prevented the proliferation increase in cancer cells. In vivo, MCC950 reduced IL-1β expression in peritumoral astrocytes, as well as the levels of inflammasome components and active IL-1β. Most importantly, significantly retarded growth of brain metastatic tumors was observed in mice treated with MCC950. Overall, astrocytes contribute to TNBC progression in the brain through activation of the NLRP3 inflammasome and consequent IL-1β release. We conclude that pharmacological targeting of inflammasomes may become a novel strategy in controlling brain metastatic diseases.

Insights into the Molecular Mechanisms Mediating Extravasation in Brain Metastasis of Breast Cancer, Melanoma, and Lung Cancer

Brain metastasis is an incurable end-stage of systemic cancer associated with poor prognosis, and its incidence is increasing. Brain metastasis occurs through a multi-step cascade where cancer cells spread from the primary tumor site to the brain. The extravasation of tumor cells through the blood-brain barrier (BBB) is a critical step in brain metastasis. During extravasation, circulating cancer cells roll along the brain endothelium (BE), adhere to it, then induce alterations in the endothelial barrier to transmigrate through the BBB and enter the brain. Rolling and adhesion are generally mediated by selectins and adhesion molecules induced by inflammatory mediators, while alterations in the endothelial barrier are mediated by proteolytic enzymes, including matrix metalloproteinase, and the transmigration step mediated by factors, including chemokines. However, the molecular mechanisms mediating extravasation are not yet fully understood. A better understanding of these mechanisms is essential as it may serve as the basis for the development of therapeutic strategies for the prevention or treatment of brain metastases. In this review, we summarize the molecular events that occur during the extravasation of cancer cells through the blood-brain barrier in three types of cancer most likely to develop brain metastasis: breast cancer, melanoma, and lung cancer. Common molecular mechanisms driving extravasation in these different tumors are discussed.

Melanoma Cells Produce Large Vesicular-Bodies That Cause Rapid Disruption of Brain Endothelial Barrier-Integrity and Disassembly of Junctional Proteins

It is known that many cells produce extracellular vesicles, and this includes a range of different cancer cell types. Here we demonstrate the profound effects of large vesicular-like bodies produced by melanoma cells on the barrier integrity of human brain endothelial cells. These vesicular-bodies have not been fully characterised but range in size from ~500 nm to >10 µm, are surrounded by membrane and are enzymatically active based on cell-tracker incorporation. Their size is consistent with previously reported large oncosomes and apoptotic bodies. We demonstrate that these melanoma-derived vesicular-bodies rapidly affect brain endothelial barrier integrity, measured using ECIS biosensor technology, where the disruption is evident within ~60 min. This disruption involves acquisition of the vesicles through transcellular uptake into the endothelial cells. We also observed extensive actin-rearrangement, actin removal from the paracellular boundary of the endothelial cells and envelopment of the vesicular-bodies by actin. This was concordant with widespread changes in CD144 localisation, which was consistent with the loss of junctional strength. High-resolution confocal imaging revealed proximity of the melanoma vesicular-bodies juxtaposed to the endothelial nucleus, often containing fragmented DNA themselves, raising speculation over this association and potential delivery of nuclear material into the brain endothelial cells. The disruption of the endothelial cells occurs in a manner that is faster and completely distinct to that of invasion by intact melanoma cells. Given the clinical observation of large vesicles in the circulation of melanoma patients by others, we hypothesize their involvement in weakening or priming the brain vasculature for melanoma invasion.

Recapitulated Crosstalk between Cerebral Metastatic Lung Cancer Cells and Brain Perivascular Tumor Microenvironment in a Microfluidic Co-Culture Chip

Non-small cell lung carcinoma (NSCLC), which affects the brain, is fatal and resistant to anti-cancer therapies. Despite innate, distinct characteristics of the brain from other organs, the underlying delicate crosstalk between brain metastatic NSCLC (BM-NSCLC) cells and brain tumor microenvironment (bTME) associated with tumor evolution remains elusive. Here, a novel 3D microfluidic tri-culture platform is proposed for recapitulating positive feedback from BM-NSCLC and astrocytes and brain-specific endothelial cells, two major players in bTME. Advanced imaging and quantitative functional assessment of the 3D tri-culture model enable real-time live imaging of cell viability and separate analyses of genomic/molecular/secretome from each subset. Susceptibility of multiple patient-derived BM-NSCLCs to representative targeted agents is altered and secretion of serpin E1, interleukin-8, and secreted phosphoprotein 1, which are associated with tumor aggressiveness and poor clinical outcome, is increased in tri-culture. Notably, multiple signaling pathways involved in inflammatory responses, nuclear factor kappa-light-chain-enhancer of activated B cells, and cancer metastasis are activated in BM-NSCLC through interaction with two bTME cell types. This novel platform offers a tool to elucidate potential molecular targets and for effective anti-cancer therapy targeting the crosstalk between metastatic cancer cells and adjacent components of bTME.

Drug delivery for neuronopathic lysosomal storage diseases: evolving roles of the blood brain barrier and cerebrospinal fluid

Whereas significant strides have been made in the treatment of lysosomal storage diseases (LSDs), the neuronopathy associated with these diseases remains impervious mainly because of the blood-brain barrier (BBB), which prevents delivery of large molecules to the brain. However, 100 years of research on the BBB since its conceptualization have clarified many of its functional and structural characteristics, spurring recent endeavors to deliver therapeutics across it to treat central nervous system (CNS) disorders, including neuronopathic LSDs. Along with the BBB, the cerebrospinal fluid (CSF) also functions to protect the microenvironment of the CNS, and it is therefore deeply involved in CNS disorders at large. Recent research aimed at developing therapeutics for neuronopathic LSDs has uncovered a number of critical roles played by the CSF that require further clarification. This review summarizes the most up-to-date understanding of the BBB and the CSF acquired during the development of therapeutics for neuronopathic LSDs, and highlights some of the associated challenges that require further research.

Treatment of Neuronopathic Mucopolysaccharidoses with Blood-Brain Barrier-Crossing Enzymes: Clinical Application of Receptor-Mediated Transcytosis

Enzyme replacement therapy (ERT) has paved the way for treating the somatic symptoms of lysosomal storage diseases (LSDs), but the inability of intravenously administered enzymes to cross the blood-brain barrier (BBB) has left the central nervous system (CNS)-related symptoms of LSDs largely impervious to the therapeutic benefits of ERT, although ERT via intrathecal and intracerebroventricular routes can be used for some neuronopathic LSDs (in particular, mucopolysaccharidoses). However, the considerable practical issues involved make these routes unsuitable for long-term treatment. Efforts have been made to modify enzymes (e.g., by fusing them with antibodies against innate receptors on the cerebrovascular endothelium) so that they can cross the BBB via receptor-mediated transcytosis (RMT) and address neuronopathy in the CNS. This review summarizes the various scientific and technological challenges of applying RMT to the development of safe and effective enzyme therapeutics for neuronopathic mucopolysaccharidoses; it then discusses the translational and methodological issues surrounding preclinical and clinical evaluation to establish RMT-applied ERT.

Microphysiological systems to study tumor-stroma interactions in brain cancer

Brain tumors still lack effective treatments, and the mechanisms of tumor progression and therapeutic resistance are unclear. Multiple parameters affect cancer prognosis (e.g., type and grade, age, location, size, and genetic mutations) and election of suitable treatments is based on preclinical models and clinical data. However, most candidate drugs fail in human trials due to inefficacy. Cell lines and tissue culture plates do not provide physiologically relevant environments, and animal models are not able to adequately mimic characteristics of disease in humans. Therefore, increasing technological advances are focusing on in vitro and computational modeling to increase the throughput and predicting capabilities of preclinical systems. The extensive use of these therapeutic agents requires a more profound understanding of the tumor-stroma interactions, including neural tissue, extracellular matrix, blood-brain barrier, astrocytes and microglia. Microphysiological brain tumor models offer physiologically relevant vascularized 'minitumors' that can help deciphering disease mechanisms, accelerating the drug discovery and predicting patient's response to anticancer treatments. This article reviews progress in tumor-on-a-chip platforms that are designed to comprehend the particular roles of stromal cells in the brain tumor microenvironment.

Drug delivery and targeting to brain tumors: considerations for crossing the blood-brain barrier

Introduction: The blood-brain barrier (BBB) selectively impedes the transportation of drug molecules into the brain, which makes the drug delivery and targeting of brain tumors very challenging.Areas covered: Having surveyed the recent literature, comprehensive insights are given into the impacts of the BBB on the advanced drug delivery and targeting modalities for brain tumors.Expert opinion: Brain capillary endothelial cells form the BBB in association with astrocytes, pericytes, neurons, and extracellular matrix. Coop of these forms the complex setting of neurovascular unite. The BBB maintains the brain homeostasis by restrictive controlling of the blood circulating nutrients/substances trafficking. Despite substantial progress on therapy of brain tumors, there is no impeccable strategy to safely deliver chemotherapeutics into the brain. Various strategies have been applied to deliver chemotherapeutics into the brain (e.g. BBB opening, direct delivery by infusion, injection, microdialysis, and implants, and smart nanosystems), which hold different pros and cons. Of note, smart nanoscale multifunctional nanomedicines can serve as targeting, imaging, and treatment modality for brain tumors. Given that aggressive brain tumors (e.g. gliomas) are often unresponsive to any treatments, an in-depth understanding of the molecular/cellular complexity of brain tumors might help the development of smart and effective treatment modalities.

Picturing Breast Cancer Brain Metastasis Development to Unravel Molecular Players and Cellular Crosstalk

Simple Summary

Breast cancer is a devastating disorder affecting millions of women worldwide. With improved therapeutics for the primary tumor, the appearance of metastasis has been increasing. Breast cancer frequently metastasizes to the brain, constituting a major hurdle without cure and with a poor survival. It is imperative to better understand the mechanisms involved in malignant cell transposition of the brain microvasculature and parenchymal colonization by deciphering the alterations occurring in the tumor and microvascular cells, as well as the occurrence of intercellular communication during the process. We aimed to profile the process of the formation of breast cancer brain metastasis and the timeline of events governing it. We used a specific mouse model of the disease to perform extensive microscopic analyses. We identified phenotypic changes and the activation of relevant molecular players in tumorigenesis, together with vascular alterations, and the occurrence of crosstalk. Our findings unravel putative therapeutic targets to tackle breast cancer brain metastasis.

Abstract

With breast cancer (BC) therapy improvements, the appearance of brain metastases has been increasing, representing a life-threatening condition. Brain metastasis formation involves BC cell (BCC) extravasation across the blood–brain barrier (BBB) and brain colonization by unclear mechanisms. We aimed to disclose the actors involved in BC brain metastasis formation, focusing on BCCs’ phenotype, growth factor expression, and signaling pathway activation, correlating with BBB alterations and intercellular communication. Hippocampi of female mice inoculated with 4T1 BCCs were examined over time by hematoxylin-eosin, immunohistochemistry and immunofluorescence. Well-established metastases were observed at seven days, increasing thereafter. BCCs entering brain parenchyma presented mesenchymal, migratory, and proliferative features; however, with time, they increasingly expressed epithelial markers, reflecting a mesenchymal–epithelial transition. BCCs also expressed platelet-derived growth factor-B, β₄ integrin, and focal adhesion kinase, suggesting autocrine and/or paracrine regulation with adhesion signaling activation, while balance between Rac1 and RhoA was associated with the motility status. Intercellular communication via gap junctions was clear among BCCs, and between BCCs and endothelial cells. Thrombin accumulation, junctional protein impairment, and vesicular proteins increase reflect BBB alterations related with extravasation. Expression of plasmalemma vesicle-associated protein was increased in BCCs, along with augmented vascularization, whereas pericyte contraction indicated mural cells’ activation. Our results provide further understanding of BC brain metastasis formation, disclosing potential therapeutic targets.

Circulating MicroRNAs and Blood-Brain-Barrier Function in Breast Cancer Metastasis

Brain metastases are a major cause of death in breast cancer patients. A key event in the metastatic progression of breast cancer in the brain is the migration of cancer cells across the blood-brain barrier (BBB). The BBB is a natural barrier with specialized functions that protect the brain from harmful substances, including anti-tumor drugs. Extracellular vesicles (EVs) sequestered by cells are mediators of cell-cell communication. EVs carry cellular components, including microRNAs that affect the cellular processes of target cells. Here, we summarize the knowledge about microRNAs known to play a significant role in breast cancer and/or in the BBB function. In addition, we describe previously established in vitro BBB models, which are a useful tool for studying molecular mechanisms involved in the formation of brain metastases.

Site-specific metastasis: A cooperation between cancer cells and the metastatic microenvironment

The conclusion derived from the information provided in this review is that disseminating tumor cells (DTC) collaborate with the microenvironment of a future metastatic organ site in the establishment of organ‐specific metastasis. We review the basic principles of site‐specific metastasis and the contribution of the cross talk between DTC and the microenvironment of metastatic sites (metastatic microenvironment [MME]) to the establishment of the organ‐specific premetastatic niche; the targeted migration of DTC to the endothelium of the future organ‐specific metastasis; the transmigration of DTC to this site and the seeding and colonization of DTC in their future MME. We also discuss the role played by DTC‐MME interactions on tumor dormancy and on the differential response of tumor cells residing in different MMEs to antitumor therapy. Finally, we summarize some studies dealing with the effects of the MME on a unique site‐specific metastasis—brain metastasis.

miRNAs in Health and Disease: A Focus on the Breast Cancer Metastatic Cascade towards the Brain

MicroRNAs (miRNAs) are small non-coding RNAs that mainly act by binding to target genes to regulate their expression. Due to the multitude of genes regulated by miRNAs they have been subject of extensive research in the past few years. This state-of-the-art review summarizes the current knowledge about miRNAs and illustrates their role as powerful regulators of physiological processes. Moreover, it highlights their aberrant expression in disease, including specific cancer types and the differential hosting-metastases preferences that influence several steps of tumorigenesis. Considering the incidence of breast cancer and that the metastatic disease is presently the major cause of death in women, emphasis is put in the role of miRNAs in breast cancer and in the regulation of the different steps of the metastatic cascade. Furthermore, we depict their involvement in the cascade of events underlying breast cancer brain metastasis formation and development. Collectively, this review shall contribute to a better understanding of the uniqueness of the biologic roles of miRNAs in these processes, to the awareness of miRNAs as new and reliable biomarkers and/or of therapeutic targets, which can change the landscape of a poor prognosis and low survival rates condition of advanced breast cancer patients.

Nephronectin promotes breast cancer brain metastatic colonization via its integrin-binding domains

This study demonstrates a role for the extracellular matrix protein nephronectin (NPNT) in promoting experimental breast cancer brain metastasis, possibly through enhanced binding to- and migration through brain endothelial cells. With the introduction of more targeted breast cancer treatments, a prolonged survival has resulted during the last decade. Consequently, an increased number of patients develop metastasis in the brain, a challenging organ to treat. We recently reported that NPNT was highly expressed in primary breast cancer and associated with unfavourable prognosis. The current study addresses our hypothesis that NPNT promotes brain metastases through its integrin-binding motifs. SAGE-sequencing revealed that NPNT was significantly up-regulated in human breast cancer tissue compared to pair-matched normal breast tissue. Human brain metastatic breast cancers expressed both NPNT and its receptor, integrin α8β1. Using an open access repository; BreastMark, we found a correlation between high NPNT mRNA levels and poor prognosis for patients with the luminal B subtype. The 66cl4 mouse cell line was used for expression of wild-type and mutant NPNT, which is unable to bind α8β1. Using an in vivo model of brain metastatic colonization, 66cl4-NPNT cells showed an increased ability to form metastatic lesions compared to cells with mutant NPNT, possibly through reduced endothelial adhesion and transmigration.

Pericyte-secreted IGF2 promotes breast cancer brain metastasis formation

Brain metastases are life-threatening complications of triple-negative breast cancer, melanoma, and a few other tumor types. Poor outcome of cerebral secondary tumors largely depends on the microenvironment formed by cells of the neurovascular unit, among which pericytes are the least characterized. By using in vivo and in vitro techniques and human samples, here we show that pericytes play crucial role in the development of metastatic brain tumors by directly influencing key steps of the development of the disease. Brain pericytes had a prompt chemoattractant effect on breast cancer cells and established direct contacts with them. By secreting high amounts of extracellular matrix proteins, pericytes enhanced adhesion of both melanoma and triple-negative cancer cells, which might be particularly important in the exclusive perivascular growth of these tumor cells. In addition, pericytes secreted insulin-like growth factor 2 (IGF2), which had a very significant pro-proliferative effect on mammary carcinoma, but not on melanoma cells. By inhibiting IGF2 signaling using silencing or picropodophyllin (PPP), we could block the proliferation-increasing effect of pericytes on breast cancer cells. Administration of PPP (a blood-brain barrier-permeable substance) significantly decreased the size of brain tumors in mice inoculated with triple-negative breast cancer cells. Taken together, our results indicate that brain pericytes have significant pro-metastatic features, especially in breast cancer. Our study underlines the importance of targeting pericytes and the IGF axis as potential strategies in brain metastatic diseases.

Supportive roles of brain macrophages in CNS metastases and assessment of new approaches targeting their functions

Metastases to the central nervous system (CNS) occur frequently in adults and their frequency increases with the prolonged survival of cancer patients. Patients with CNS metastases have short survival, and modern therapeutics, while effective for extra-cranial cancers, do not reduce metastatic burden. Tumor cells attract and reprogram stromal cells, including tumor-associated macrophages that support cancer growth by promoting tissue remodeling, invasion, immunosuppression and metastasis. Specific roles of brain resident and infiltrating macrophages in creating a pre-metastatic niche for CNS invading cancer cells are less known. There are populations of CNS resident innate immune cells such as: parenchymal microglia and non-parenchymal, CNS border-associated macrophages that colonize CNS in early development and sustain its homeostasis. In this study we summarize available data on potential roles of different brain macrophages in most common brain metastases. We hypothesize that metastatic cancer cells exploit CNS macrophages and their cytoprotective mechanisms to create a pre-metastatic niche and facilitate metastatic growth. We assess current pharmacological strategies to manipulate functions of brain macrophages and hypothesize on their potential use in a therapy of CNS metastases. We conclude that the current data strongly support a notion that microglia, as well as non-parenchymal macrophages and peripheral infiltrating macrophages, are involved in multiple stages of CNS metastases. Understanding their contribution will lead to development of new therapeutic strategies.

Downregulation of circulating miR 802-5p and miR 194-5p and upregulation of brain MEF2C along breast cancer brain metastasization

Breast cancer brain metastases (BCBMs) have been underinvestigated despite their high incidence and poor outcome. MicroRNAs (miRNAs), and particularly circulating miRNAs, regulate multiple cellular functions, and their deregulation has been reported in different types of cancer and metastasis. However, their signature in plasma along brain metastasis development and their relevant targets remain undetermined. Here, we used a mouse model of BCBM and next‐generation sequencing (NGS) to establish the alterations in circulating miRNAs during brain metastasis formation and development. We further performed bioinformatics analysis to identify their targets with relevance in the metastatic process. We additionally analyzed human resected brain metastasis samples of breast cancer patients for target expression validation. Breast cancer cells were injected in the carotid artery of mice to preferentially induce metastasis in the brain, and samples were collected at different timepoints (5 h, 3, 7, and 10 days) to follow metastasis development in the brain and in peripheral organs. Metastases were detected from 7 days onwards, mainly in the brain. NGS revealed a deregulation of circulating miRNA profile during BCBM progression, rising from 18% at 3 days to 30% at 10 days following malignant cells’ injection. Work was focused on those altered prior to metastasis detection, among which were miR‐802‐5p and miR‐194‐5p, whose downregulation was validated by qPCR. Using targetscan and diana tools, the transcription factor myocyte enhancer factor 2C (MEF2C) was identified as a target for both miRNAs, and its expression was increasingly observed in malignant cells along brain metastasis development. Its upregulation was also observed in peritumoral astrocytes pointing to a role of MEF2C in the crosstalk between tumor cells and astrocytes. MEF2C expression was also observed in human BCBM, validating the observation in mouse. Collectively, downregulation of circulating miR‐802‐5p and miR‐194‐5p appears as a precocious event in BCBM and MEF2C emerges as a new player in brain metastasis development.

Hypoxia-Induced MicroRNA-212/132 Alter Blood-Brain Barrier Integrity Through Inhibition of Tight Junction-Associated Proteins in Human and Mouse Brain Microvascular Endothelial Cells

Blood-brain barrier (BBB) integrity is one of the important elements of central nervous system (CNS) homeostasis. MicroRNAs (miRs) have been demonstrated to play a role in many CNS disorders such as stroke and traumatic brain injury. MiR-212/132 are highly expressed in the CNS but their role at the BBB has not been characterized yet. Thus, we analyzed the expression of miR-212/132 in hypoxic mouse and human brain microvascular endothelial cells (BMEC) as well as in posttraumatic mouse and human brain tissue and serum exosomes. MiR-212/132 expression was detected in brain capillaries by in situ hybridization and was increased up to ten times in hypoxic BMEC. Over-expression of pre-miR-212/132 in BMEC decreased barrier properties and reduced migration of BMEC in the wound healing assay. We identified and validated tight junction proteins claudin-1 (Cldn1), junctional adhesion molecule 3 (Jam3), and tight junction-associated protein 1 (Tjap1) as potential miR-212/132 targets. Over-expression of miRs led to a decrease in mRNA and protein expression of Cldn1, Jam3, and Tjap1, which could be rescued by a respective anti-miR. In conclusion, our study identifies miR-212/132 as critical players at the hypoxic BBB. In addition, we propose three new direct miR-212/132 targets to be involved in miR-212/132-mediated effects on BBB properties.

Response of the neurovascular unit to brain metastatic breast cancer cells

Therapeutic resistance of cerebral secondary tumours largely depends on unique aspects linked to the neurovascular unit, especially cerebral endothelial cells and astrocytes. By using advanced microscopy techniques, here we explored novel mechanisms related to the neurovascular unit during extravasation and proliferation of triple negative breast cancer cells in the brain. Metastatic mammary carcinoma cells arrested and elongated within one hour in cerebral microvessels, but their number decreased by almost 80% in the first two days. Interestingly, malignant cells induced vasoconstriction and development of intraluminal endothelial plugs, which isolated invading cells from the circulation. During diapedesis - which usually took place on day four and five after inoculation of the tumour cells - continuity of cerebral endothelial tight junctions remained intact, indicating migration of cancer cells through the transcellular pathway. In addition, metastatic cells induced formation of multiluminal vessels and claudin-5-positive endothelial blebs. However, even severe endothelial blebbing could be reversed and the vessel morphology was restored shortly after the tumour cells completed transendothelial migration. Similar to neuro-inflammatory leukocytes, tumour cells migrated not only through the endothelial layer, but through the glia limitans perivascularis as well. Nevertheless, along with the growth of metastatic lesions by co-option of pre-existing capillaries, astrocytes and astrocyte end-feet were gradually expelled from the vessels to the border of the tumour. Taken together, we identified previously unknown mechanisms involved in the reaction of brain resident cells to invading breast cancer cells. Our results contribute to a better understanding of the complex cross-talk between tumour cells and host cells in the brain, which is essential for the identification of new therapeutic targets in this devastating disease.

Paracellular and transcellular migration of metastatic cells through the cerebral endothelium

Breast cancer and melanoma are among the most frequent cancer types leading to brain metastases. Despite the unquestionable clinical significance, important aspects of the development of secondary tumours of the central nervous system are largely uncharacterized, including extravasation of metastatic cells through the blood-brain barrier. By using transmission electron microscopy, here we followed interactions of cancer cells and brain endothelial cells during the adhesion, intercalation/incorporation and transendothelial migration steps. We observed that brain endothelial cells were actively involved in the initial phases of the extravasation by extending filopodia-like membrane protrusions towards the tumour cells. Melanoma cells tended to intercalate between endothelial cells and to transmigrate by utilizing the paracellular route. On the other hand, breast cancer cells were frequently incorporated into the endothelium and were able to migrate through the transcellular way from the apical to the basolateral side of brain endothelial cells. When co-culturing melanoma cells with cerebral endothelial cells, we observed N-cadherin enrichment at melanoma-melanoma and melanoma-endothelial cell borders. However, for breast cancer cells N-cadherin proved to be dispensable for the transendothelial migration both in vitro and in vivo. Our results indicate that breast cancer cells are more effective in the transcellular type of migration than melanoma cells.

Conflicting Roles of Connexin43 in Tumor Invasion and Growth in the Central Nervous System

The tumor microenvironment is known to have increased levels of cytokines and metabolites, such as glutamate, due to their release from the surrounding cells. A normal cell around the tumor that responds to the inflammatory environment is likely to be subsequently altered. We discuss how these abnormalities will support tumor survival via the actions of gap junctions (GJs) and hemichannels (HCs) which are composed of hexamer of connexin43 (Cx43) protein. In particular, we discuss how GJ intercellular communication (GJIC) in glioma cells, the primary brain tumor, is a regulatory factor and its attenuation leads to tumor invasion. In contrast, the astrocytes, which are normal cells around the glioma, are “hijacked” by tumor cells, either by receiving the transmission of malignant substances from the cancer cells via GJIC, or perhaps via astrocytic HC activity through the paracrine signaling which enable the delivery of these substances to the distal astrocytes. This astrocytic signaling would promote tumor expansion in the brain. In addition, brain metastasis from peripheral tissues has also been known to be facilitated by GJs formed between cerebral vascular endothelial cells and cancer cells. Astrocytes and microglia are generally thought to eliminate cancer cells at the blood–brain barrier. In contrast, some reports suggest they facilitate tumor progression as tumor cells take advantage of the normal functions of astrocytes that support the survival of the neurons by exchanging nutrients and metabolites. In summary, GJIC is essential for the normal physiological function of growth and allowing the diffusion of physiological substances. Therefore, whether GJIC is cancer promoting or suppressing may be dependent on what permeates through GJs, when it is active, and to which cells. The nature of GJs, which has been ambiguous in brain tumor progression, needs to be revisited and understood together with new findings on Cx proteins and HC activities.