The cellular origin of laminin determines its role in blood pressure regulation

Basal lamina changes in neurodegenerative disorders

BACKGROUND

Neurodegenerative disorders are a group of age-associated diseases characterized by progressive degeneration of the structure and function of the CNS. Two key pathological features of these disorders are blood-brain barrier (BBB) breakdown and protein aggregation.

MAIN BODY

The BBB is composed of various cell types and a non-cellular component---the basal lamina (BL). Although how different cells affect the BBB is well studied, the roles of the BL in BBB maintenance and function remain largely unknown. In addition, located in the perivascular space, the BL is also speculated to regulate protein clearance via the meningeal lymphatic/glymphatic system. Recent studies from our laboratory and others have shown that the BL actively regulates BBB integrity and meningeal lymphatic/glymphatic function in both physiological and pathological conditions, suggesting that it may play an important role in the pathogenesis and/or progression of neurodegenerative disorders. In this review, we focus on changes of the BL and its major components during aging and in neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). First, we introduce the vascular and lymphatic systems in the CNS. Next, we discuss the BL and its major components under homeostatic conditions, and summarize their changes during aging and in AD, PD, and ALS in both rodents and humans. The functional significance of these alterations and potential therapeutic targets are also reviewed. Finally, key challenges in the field and future directions are discussed.

CONCLUSIONS

Understanding BL changes and the functional significance of these changes in neurodegenerative disorders will fill the gap of knowledge in the field. Our goal is to provide a clear and concise review of the complex relationship between the BL and neurodegenerative disorders to stimulate new hypotheses and further research in this field.

Cumulative Damage: Cell Death in Posthemorrhagic Hydrocephalus of Prematurity

Globally, approximately 11% of all infants are born preterm, prior to 37 weeks’ gestation. In these high-risk neonates, encephalopathy of prematurity (EoP) is a major cause of both morbidity and mortality, especially for neonates who are born very preterm (<32 weeks gestation). EoP encompasses numerous types of preterm birth-related brain abnormalities and injuries, and can culminate in a diverse array of neurodevelopmental impairments. Of note, posthemorrhagic hydrocephalus of prematurity (PHHP) can be conceptualized as a severe manifestation of EoP. PHHP impacts the immature neonatal brain at a crucial timepoint during neurodevelopment, and can result in permanent, detrimental consequences to not only cerebrospinal fluid (CSF) dynamics, but also to white and gray matter development. In this review, the relevant literature related to the diverse mechanisms of cell death in the setting of PHHP will be thoroughly discussed. Loss of the epithelial cells of the choroid plexus, ependymal cells and their motile cilia, and cellular structures within the glymphatic system are of particular interest. Greater insights into the injuries, initiating targets, and downstream signaling pathways involved in excess cell death shed light on promising areas for therapeutic intervention. This will bolster current efforts to prevent, mitigate, and reverse the consequential brain remodeling that occurs as a result of hydrocephalus and other components of EoP.

miR-34a regulates phenotypic modulation of vascular smooth muscle cells in intracranial aneurysm by targeting CXCR3 and MMP-2

MicroRNAs (miRNAs) dysregulation is tightly related to diseases including tumor, neuro disease and cardiovascular disease. In this study, we investigated the potential biological effects of miR-34a and its target CXCR3 in phenotypic modulation of vascular smooth muscle cells (VSMCs) of intracranial aneurysms (IAs). MiR-34a was found to be down-regulated in IAs patients tested by Real-time PCR and decreased in GEO data. Meanwhile, our study also showed miR-34a inhibited matrix metalloproteinases (MMPs) and migration of VSMCs. Besides, CXCR3 is a direct target of miR-34a identified via luciferase assay. CXCR3 showed inhibitory effect on SM-MHC, SM22 while promoted MMPs expression, cell proliferation and migration in VSMCs. MiR-34a reversed the effect of CXCR3 in VSMCs. In addition, MMP-2 is a competitive endogenous RNA (ceRNA) of CXCR3 sharing common miR-34a target. CXCR3 increased MMP-2 level through competitive endogenous RNA regulation by sponging endogenous miR-34a. In conclusion, miR-34a is down-regulated in IAs while CXCR3 is the direct target of miR-34a that regulates phenotypic modulation of VSMCs. CXCR3 increased MMP-2 level through competitive endogenous RNA regulation by sharing common miR-34a targets.

Atg7 Silencing Inhibits Laminin-5 Expression to Suppress Tube Formation by Brain Endothelial Cells

Cerebral angiogenesis is a key event during brain development and recovery from brain injury. We previously demonstrated that Atg7 knockout impaired angiogenesis in the mouse brain. However, the role of Atg7 in angiogenesis is not completely understood. In this study, we used human brain microvascular endothelial cells (HBMECs) to investigate the mechanism of Atg7-regulated cerebral angiogenesis. We found that Atg7 depletion specifically diminished the expression of the β3 and γ2 chains of laminin-5, a major component of the extracellular matrix. In contrast, autophagy inhibitors did not affect laminin-5 expression, suggesting that Atg7-regulated laminin-5 expression is autophagy-independent. We also found that Atg7-regulated laminin-5 expression occurred at the transcriptional level through NF-κB signaling. Exogenous laminin-5 or the NF-κB agonist betulinic acid effectively rescued tube formation by Atg7-deficient HBMECs. Taken together, our study identified a novel mechanism by which Atg7 regulates laminin-5 expression via NF-κB to modulate tube formation by brain endothelial cells during cerebral angiogenesis. Anat Rec, 302:2255-2260, 2019. © 2019 American Association for Anatomy.

Pericytic Laminin Maintains Blood-Brain Barrier Integrity in an Age-Dependent Manner

Brain pericytes synthesize and deposit laminin at the blood-brain barrier (BBB). The function of pericyte-derived laminin in BBB maintenance remains largely unknown. In a previous study, we generated pericytic laminin conditional knockout (PKO) mice, which developed BBB breakdown and hydrocephalus in a mixed genetic background. However, since hydrocephalus itself can compromise BBB integrity, it remains unclear whether BBB disruption in these mutants is due to loss of pericytic laminin or secondary to hydrocephalus. Here, we report that, in C57Bl6 dominant background, the PKO mice fail to show hydrocephalus, have a normal lifespan, and develop BBB breakdown in an age-dependent manner. Further mechanistic studies demonstrate that abnormal paracellular transport, enhanced transcytosis, decreased pericyte coverage, and diminished AQP4 level are responsible for BBB disruption in PKO mice. These results suggest that pericyte-derived laminin plays an indispensable and age-dependent role in the maintenance of BBB integrity under homeostatic conditions.

Laminins and their receptors in the CNS

Laminin, an extracellular matrix protein, is widely expressed in the central nervous system (CNS). By interacting with integrin and non-integrin receptors, laminin exerts a large variety of important functions in the CNS in both physiological and pathological conditions. Due to the existence of many laminin isoforms and their differential expression in various cell types in the CNS, the exact functions of each individual laminin molecule in CNS development and homeostasis remain largely unclear. In this review, we first briefly introduce the structure and biochemistry of laminins and their receptors. Next, the dynamic expression of laminins and their receptors in the CNS during both development and in adulthood is summarized in a cell-type-specific manner, which allows appreciation of their functional redundancy/compensation. Furthermore, we discuss the biological functions of laminins and their receptors in CNS development, blood-brain barrier (BBB) maintenance, neurodegeneration, stroke, and neuroinflammation. Last, key challenges and potential future research directions are summarized and discussed. Our goals are to provide a synthetic review to stimulate future studies and promote the formation of new ideas/hypotheses and new lines of research in this field.

Basement membrane and stroke

Located at the interface of the circulation system and the CNS, the basement membrane (BM) is well positioned to regulate blood-brain barrier (BBB) integrity. Given the important roles of BBB in the development and progression of various neurological disorders, the BM has been hypothesized to contribute to the pathogenesis of these diseases. After stroke, a cerebrovascular disease caused by rupture (hemorrhagic) or occlusion (ischemic) of cerebral blood vessels, the BM undergoes constant remodeling to modulate disease progression. Although an association between BM dissolution and stroke is observed, how each individual BM component changes after stroke and how these components contribute to stroke pathogenesis are mostly unclear. In this review, I first briefly introduce the composition of the BM in the brain. Next, the functions of the BM and its major components in BBB maintenance under homeostatic conditions are summarized. Furthermore, the roles of the BM and its major components in the pathogenesis of hemorrhagic and ischemic stroke are discussed. Last, unsolved questions and potential future directions are described. This review aims to provide a comprehensive reference for future studies, stimulate the formation of new ideas, and promote the generation of new genetic tools in the field of BM/stroke research.

Basement membrane and blood-brain barrier

The blood–brain barrier (BBB) is a highly complex and dynamic structure, mainly composed of brain microvascular endothelial cells, pericytes, astrocytes and the basement membrane (BM). The vast majority of BBB research focuses on its cellular constituents. Its non-cellular component, the BM, on the other hand, is largely understudied due to its intrinsic complexity and the lack of research tools. In this review, we focus on the role of the BM in BBB integrity. We first briefly introduce the biochemical composition and structure of the BM. Next, the biological functions of major components of the BM in BBB formation and maintenance are discussed. Our goal is to provide a concise overview on how the BM contributes to BBB integrity.

Laminin: loss-of-function studies

Laminin, one of the most widely expressed extracellular matrix proteins, exerts many important functions in multiple organs/systems and at various developmental stages. Although its critical roles in embryonic development have been demonstrated, laminin's functions at later stages remain largely unknown, mainly due to its intrinsic complexity and lack of research tools (most laminin mutants are embryonic lethal). With the advance of genetic and molecular techniques, many new laminin mutants have been generated recently. These new mutants usually have a longer lifespan and show previously unidentified phenotypes. Not only do these studies suggest novel functions of laminin, but also they provide invaluable animal models that allow investigation of laminin's functions at late stages. Here, I first briefly introduce the nomenclature, structure, and biochemistry of laminin in general. Next, all the loss-of-function mutants/models for each laminin chain are discussed and their phenotypes compared. I hope to provide a comprehensive review on laminin functions and its loss-of-function models, which could serve as a reference for future research in this understudied field.

The role of pericytic laminin in blood brain barrier integrity maintenance

Laminin, a major component of the basement membrane, plays an important role in blood brain barrier regulation. At the neurovascular unit, brain endothelial cells, astrocytes, and pericytes synthesize and deposit different laminin isoforms into the basement membrane. It has been shown that laminin α4 (endothelial laminin) regulates vascular integrity at embryonic/neonatal stage, while astrocytic laminin maintains vascular integrity in adulthood. Here, we investigate the function of pericyte-derived laminin in vascular integrity. Using a conditional knockout mouse line, we report that loss of pericytic laminin leads to hydrocephalus and BBB breakdown in a small percentage (10.7%) of the mutants. Interestingly, BBB disruption always goes hand-in-hand with hydrocephalus in these mutants, and neither symptom is observed in the rest 89.3% of the mutants. Further mechanistic studies show that reduced tight junction proteins, diminished AQP4 expression, and decreased pericyte coverage are responsible for the BBB disruption. Together, these data suggest that pericyte-derived laminin is involved in the maintenance of BBB integrity and regulation of ventricular size/development.

Laminin regulates PDGFRβ(+) cell stemness and muscle development

Muscle-resident PDGFRβ⁺ cells, which include pericytes and PW1⁺ interstitial cells (PICs), play a dual role in muscular dystrophy. They can either undergo myogenesis to promote muscle regeneration or differentiate into adipocytes and other cells to compromise regeneration. How the differentiation and fate determination of PDGFRβ⁺ cells are regulated, however, remains unclear. Here, by utilizing a conditional knockout mouse line, we report that PDGFRβ⁺ cell-derived laminin inhibits their proliferation and adipogenesis, but is indispensable for their myogenesis. In addition, we show that laminin alone is able to partially reverse the muscle dystrophic phenotype in these mice at the molecular, structural and functional levels. Further RNAseq analysis reveals that laminin regulates PDGFRβ⁺ cell differentiation/fate determination via gpihbp1. These data support a critical role of laminin in the regulation of PDGFRβ⁺ cell stemness, identify an innovative target for future drug development and may provide an effective treatment for muscular dystrophy.

Muscle PDGFRβ⁺ cells are interstitial stem/progenitor cells with myogenic potential. Here, Yao et al. show that PDGFRβ⁺ cell-derived laminin actively regulates their proliferation, differentiation and fate determination.