Cerebral adrenoleukodystrophy: a demyelinating disease that leaves the door wide open: Figure 1

Abstract

This scientific commentary refers to ‘Brain endothelial dysfunction in cerebral adrenoleukodystrophy’ by Musolino et al. (doi: 10.1093/awv250) Cerebral adrenoleukodystrophy (ALD) can be distinguished from other inherited diseases of white matter (known collectively as ‘leukodystrophies’) by the presence of severe neuroinflammation and blood–brain barrier disruption, evidenced in brain MRI by the leakage of gadolinium-diethylenetriamine pentaacetic acid (Gd-DPTA) at the periphery of demyelinating lesions on T1-weighted sequences. In contrast to multiple sclerosis, blood–brain barrier breakdown occurs downstream of the demyelinating process. Moreover, it is always followed by very rapid progression of demyelinating lesions, likely resulting from myelin destruction by inflammatory cells. Mechanisms by which loss of ABCD1 function in brain endothelial cells, and also in astrocytes, microglia and pericytes, could disrupt the blood–brain barrier in cerebral ALD and enhance neuroinflammation. Loss of ABCD1 function (a fatty acid transporter localized to peroxisomes) in brain endothelial cells increases the expression of ICAM-1 and VCAM, facilitating the adhesion of monocytes at the cell surface. It also displaces CLDN5 (Claudin-5) and ZO-1 from the cell surface to the cytosol, resulting in increased blood-brain barrier (BBB) permeability. Synthesis of MMP9 contributes to complete disruption of the blood–brain barrier, enhancing the entry of monocytes and T and B lymphocytes into the white matter. Loss of ABCD1 function in pericytes could give rise to abnormal crosstalk between pericytes and endothelial cells, mediated by bidirectional TGF-β–TGF-β receptor signalling. ABCD1-deficiency in astrocytes might impair sonic hedgehog (SHH), Tie2 and AT1 signalling, and thus the expression and localization of CLDN5 and occludin. TGF-β1 produced by astrocytes could downregulate the expression of CLDN5 through transcriptional inhibition of MYC. Lastly, ABCD1-deficient microglia release IL-1β, which downregulates ZO-1 expression, which could in turn abolish the protective effect of astrocytes mediated by SHH signalling. In this issue of Brain, Musolino et al. (2015) unravel for the first time some of the molecular mechanisms that underlie blood–brain barrier breakdown in ALD. Using ALD brain tissue, and human brain microvascular cells (HBMECs) in which the ABCD1 (ALD) gene was silenced via siRNA, they demonstrate an upregulation of intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion protein 1 (VCAM1) expression that reflected endothelial activation and that facilitated monocyte adhesion. They also show an increase in the expression of transforming growth factor β1 (TGF-β1), and in the number of vessels, probably due to production of vascular endothelial growth factor (VEGF) by astrocytes and/or microglia. Furthermore, they reveal an increase in the expression of matrix metallopeptidase 9 (MMP9), which contributed to blood–brain barrier disruption, and in turn resulted in leakage of fibrinogen (an exclusively intravascular protein) into the perivascular space, recruitment of monocytes/macrophages and the activation of microglia around vessels in active demyelinating areas. This correlated with the displacement of two members of the endothelial tight junction complex, claudin 5 (CLDN5) and ZO-1, from the membrane to the cytoplasm of endothelial cells: a clear indication that the blood–brain barrier had been breached, boosting the ingress of proinflammatory lymphocytes and monocytes into the white matter. Importantly, ABCD1 silencing in HBMECs also displaced CLDN5 to the endothelial cell cytoplasm, as in vivo. Studies in HBMECs provided two key additional pieces of information: (i) the silencing of ABCD1 resulted in transcriptional inhibition of CLDN5 and MYC (also known as c-MYC); and (ii) deleterious effects in ABCD1-silenced HBMECs were seen before any increase in very long chain fatty acids (VLCFAs) could be observed. ABCD1 encodes one half of an ATP-binding transporter that homodimerizes within the membranes of peroxisomes and which is thought to import CoA derivatives of VLCFAs (fatty acids with >22 carbon atoms) into these organelles, wherein they are degraded by β-oxidation. All mutations of ABCD1 result in loss of ABCD1 transporter function and all patients with ALD accumulate VLCFAs in their plasma, cells and tissues. The mechanism by which CLDN5 gene expression is downregulated in ABCD1-silenced HBMECs is puzzling. Using a PCR-array screen of only 84 genes involved in neuroinflammation, Musolino and colleagues found that MYC gene expression was markedly decreased. Western blot confirmed that c-MYC protein expression was also reduced. Silencing cMYC in HBMECs changed the expression of CLDN5 and ICAM1 in a similar manner to that seen in ABCD1-silenced HBMECs. In macrophages, inhibition of MYC blocks the expression of MMP9 and TGF-β1 (which are upregulated in ALD brain tissue and ABCD1-silenced HBMECs) bu