Publications

Laminin, a major component of the basal lamina in the CNS, is also expressed in oligodendrocytes (OLs). However, the function of OL-derived laminin remains largely unknown. Here, we performed loss-of-function studies using two OL-specific laminin-α5 conditional knockout mouse lines. Both mutants were grossly normal and displayed intact blood-brain barrier (BBB) integrity. In a mouse model of intracerebral hemorrhage (ICH), control mice and both mutants exhibited comparable hematoma size and neurological dysfunction. In addition, similar levels of hemoglobin and IgG leakage were detected in the mutant brains compared to the controls, indicating comparable BBB damage. Consistent with this finding, subsequent studies revealed no differences in tight junction protein (TJP) and caveolin-1 expression among control and knockout mice, suggesting that neither paracellular nor transcellular mechanism was affected in the mutants. Furthermore, compared to the controls, both mutant lines showed comparable oligodendrocyte number, oligodendrocyte proliferation rate, MBP/MAG levels, and SMI-32 expression, highlighting a minimal role of OL-derived laminin-α5 in OL biology. Together, these findings highlight a dispensable role of OL-derived laminin-α5 in both brain homeostasis and ICH pathogenesis.

Live imaging of the brain extracellular matrix (ECM) provides vital insights into changes that occur in neurological disorders. Current techniques such as second or third-harmonic generation offer limited contrast for live imaging of the brain ECM. Here, a new method, pan-ECM via chemical labeling of extracellular proteins, is introduced for live brain ECM imaging. pan-ECM labels all major ECM components in live tissue including the interstitial matrix, basement membrane, and perineuronal nets. pan-ECM enables in vivo observation of the ECM heterogeneity between the glioma core and margin, as well as the assessment of ECM deterioration under stroke condition, without ECM shrinkage from tissue fixation. These findings indicate that the pan-ECM approach is a novel way to image the entire brain ECM in live brain tissue with optical resolution. pan-ECM has the potential to advance the understanding of ECM in brain function and neurological diseases.

[This corrects the article on p. 299 in vol. 8, PMID: 30693249.].

Microglia play a dual role in stroke depending on their pro-inflammatory and anti-inflammatory polarization. A study in PLOS Biology identifies a new mechanism, through which the transcription factor NR4A1 negatively regulates TNF expression in microglia.

Background: Unlike other proteins that exhibit a diffusion pattern after intracerebral injection, laminin displays a vascular pattern. It remains unclear if this unique vascular pattern is caused by laminin-receptor interaction or laminin self-assembly.

Methods: We compared the distribution of various wild-type laminin isoforms in the brain after intracerebral injection. To determine what causes the unique vascular pattern of laminin in the brain, laminin mutants with impaired receptor-binding and/or self-assembly activities and function-blocking antibodies to laminin receptors were used. In addition, the dynamics of laminin distribution and elimination were examined at multiple time points after intracerebral injection.

Results: We found that β2-containing laminins had higher affinity for the vessels compared to β1-containing laminins. In addition, laminin mutants lacking receptor-binding domains but not that lacking self-assembly capability showed substantially reduced vascular pattern. Consistent with this finding, dystroglycan (DAG1) function-blocking antibody significantly reduced the vascular pattern of wild-type laminin-111. Although failed to affect the vascular pattern when used alone, integrin-β1 function-blocking antibody further decreased the vascular pattern when combined with DAG1 antibody. EDTA, which impaired laminini-DAG1 interaction by chelating Ca²⁺, also attenuated the vascular pattern. Immunohistochemistry revealed that laminins were predominantly located in the perivascular space in capillaries and venules/veins but not arterioles/arteries. The time-course study showed that laminin mutants with impaired receptor-engaging activity were more efficiently eliminated from the brain compared to their wild-type counterparts. Concordantly, significantly higher levels of mutant laminins were detected in the cerebral-spinal fluid (CSF).

Conclusions: These findings suggest that intracerebrally injected laminins are enriched in the perivascular space in a receptor (DAG1/integrin)-dependent rather than self-assembly-dependent manner and eliminated from the brain mainly via the perivascular clearance system.

The function of fibroblasts in intracerebral hemorrhage (ICH) remains elusive. By targeting Col1α1, a fibroblast-specific marker, we generate mice with ablated Col1α1⁺ fibroblasts. These mutants show exacerbated blood-brain barrier (BBB) damage, enlarged injury volume, and worse neurological function, highlighting a beneficial role of Col1α1⁺ fibroblasts in ICH. Echoing these findings, fibroblasts significantly decrease endothelial permeability in an in vitro ICH model. Next, we demonstrate that fibroblasts promote BBB integrity in ICH mainly via up-regulating tight junction proteins without affecting transcytosis-associated proteins, indicating a paracellular rather than transcellular mechanism. A subsequent mechanistic study reveals that the BBB-protective effect of fibroblasts is partially mediated by TIMP metallopeptidase inhibitor 2 (TIMP2). Furthermore, we find that exogenous TIMP2 attenuates BBB disruption in these mutants after ICH. These results suggest that Col1α1⁺ fibroblasts repair BBB damage in ICH via the paracellular pathway in a TIMP2-dependent manner, and that Col1α1⁺ fibroblasts and TIMP2 may be targeted in ICH treatment.

Laminin, a major component of the basal lamina (BL), is a heterotrimeric protein with many isoforms. In the CNS, laminin is expressed by almost all cell types, yet different cells synthesize distinct laminin isoforms. By binding to its receptors, laminin exerts a wide variety of important functions. However, due to the reciprocal and cell-specific expression of laminin in different cells at the neurovascular unit, its functions in blood-brain barrier (BBB) maintenance and BBB repair after injury are not fully understood. In this review, we focus on the expression and functions of laminin and its receptors in the neurovascular unit under both physiological and pathological conditions. We first briefly introduce the structures of laminin and its receptors. Next, the expression and functions of laminin and its receptors in the CNS are summarized in a cell-specific manner. Finally, we identify the knowledge gap in the field and discuss key questions that need to be answered in the future. Our goal is to provide a comprehensive overview on cell-specific expression of laminin and its receptors in the CNS and their functions on BBB integrity.

Pericytes are multipotent perivascular cells that play important roles in CNS injury. However, controversial findings exist on how pericytes change and whether they differentiated into microglia-like cells after ischemic stroke. This discrepancy is mainly due to the lack of pericyte-specific markers: the “pericyte” population identified in previous studies contained vascular smooth muscle cells (vSMCs) and/or fibroblasts. Therefore, it remains unclear which cell type differentiates into microglia-like cells after stroke. In this study, lineage-tracing technique was used to mark α-smooth muscle actin (SMA)^{low/undetectable} pericytes, vSMCs, and fibroblasts, and their fates were analyzed after ischemic stroke. We found that SMA^{low/undetectable} pericytes and fibroblasts but not vSMCs substantially proliferated at the subacute phase after injury, and that SMA^{low/undetectable} pericyte but not vSMCs or fibroblasts differentiated into Iba1⁺ cells after ischemic stroke. Further imaging flow cytometry analysis revealed that SMA^{low/undetectable} pericytes differentiated into both microglia and macrophages at day 7 after stroke. These results demonstrate that SMA^{low/undetectable} pericytes rather than vSMCs or fibroblasts differentiate into both microglia-like and macrophage-like cells after stroke, suggesting that these pericytes may be targeted in the treatment of ischemic stroke.

Pericytes are a heterogenous population that plays multiple important roles in both physiological and pathological conditions. Although many markers and transgenic mouse lines have been used to identify pericytes, these tools all have limitations. For example, many of them are not pericyte-specific and none of them are able to distinguish different subtypes of pericyte. Here, we summarize commonly used pericyte markers and transgenic mouse lines, compare their unique features and limitations, and discuss key points to consider when using these tools or interpreting data generated by using them. Identifying/developing pericyte-specific and subtype-specific markers/tools will fill the gap of knowledge and substantially move the field forward.

Adenylyl cyclases (ADCYs), by generating second messenger cAMP, play important roles in various cellular processes. Their expression, regulation and functions in the CNS, however, remain largely unknown. In this review, we first introduce the classification and structure of ADCYs, followed by a discussion of the regulation of mammalian ADCYs (ADCY1-10). Next, the expression and function of each mammalian ADCY isoform are summarized in a region/cell-specific manner. Furthermore, the effects of GPCR-ADCY signaling on blood–brain barrier (BBB) integrity are reviewed. Last, current challenges and future directions are discussed. We aim to provide a succinct review on ADCYs to foster new research in the future.