Impaired blood–brain/spinal cord barrier in ALS patients

Abstract

Vascular pathology, including blood–brain/spinal cord barrier (BBB/BSCB) alterations, has recently been recognized as a key factor possibly aggravating motor neuron damage, identifying a neurovascular disease signature for ALS. However, BBB/BSCB competence in sporadic ALS (SALS) is still undetermined. In this study, BBB/BSCB integrity in postmortem gray and white matter of medulla and spinal cord tissue from SALS patients and controls was investigated. Major findings include (1) endothelial cell damage and pericyte degeneration, (2) severe intra- and extracellular edema, (3) reduced CD31 and CD105 expressions in endothelium, (4) significant accumulation of perivascular collagen IV, and fibrin deposits (5) significantly increased microvascular density in lumbar spinal cord, (6) IgG microvascular leakage, (7) reduced tight junction and adhesion protein expressions. Microvascular barrier abnormalities determined in gray and white matter of the medulla, cervical, and lumbar spinal cord of SALS patients are novel findings. Pervasive barrier damage discovered in ALS may have implications for disease pathogenesis and progression, as well as for uncovering novel therapeutic targets.

Introduction

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by progressive motor neuron degeneration in the brain and spinal cord leading to muscle atrophy, paralysis and death. Sporadic ALS (SALS) dominates, with only 5–10% of cases genetically linked (FALS); 20% of FALS cases show mutations in the Cu/Zn superoxide dismutase (SOD1) gene (Rosen et al., 1993). Clinical presentation and pathology of SALS and FALS are similar. Although numerous hypotheses have been proposed regarding ALS etiopathology (Bruijn et al., 2004, Cleveland and Rothstein, 2001, Rothstein, 2009, Strong et al., 2005), the causes of motor neuron degeneration and pathogenic mechanisms are still uncertain. Existing therapies for ALS are largely palliative.

Determining putative mechanism(s) of motor neuron degeneration in ALS is challenging due to the diffuse motor neuron death and the complexity of disease manifestation. Vascular impairment has only recently been recognized as a key factor in ALS, identifying a neurovascular disease signature (Garbuzova-Davis et al., 2011). Evidence of compromised blood–brain barrier (BBB) and/or blood–spinal cord barrier (BSCB) integrity was recently identified in ALS patients and in an animal model of ALS. Structural and functional BBB/BSCB impairments were demonstrated in an animal model of ALS at early stage disease and worsened with disease progression (Garbuzova-Davis et al., 2007a, Garbuzova-Davis et al., 2007b, Nicaise et al., 2009a, Nicaise et al., 2009b, Zhong et al., 2008). Additionally, vascular leakage, decreased capillary length and blood flow, microhemorrhages, reduced expression of basement membrane components and tight junction proteins have been shown in the spinal cords of SOD1 transgenic animals. Importantly, BSCB breakdown was found in SOD1 mutant mice and rats prior to motor neuron degeneration and neurovascular inflammation (Miyazaki et al., 2011, Nicaise et al., 2009a, Zhong et al., 2008), suggesting vascular alteration as an early ALS pathological event. Evidence of BBB/BSCB impairment has also been observed in postmortem tissue from ALS patients. Loss of endothelium integrity as shown by significant reductions of occludin and ZO-1 mRNA was recently observed in spinal cords from ALS patients (Henkel et al., 2009). Decreased perivascular occludin and collagen IV, as well as astrocyte end-feet dissociated from the endothelium were also seen in postmortem ALS spinal cord tissue (Miyazaki et al., 2011). These results strengthen the likelihood that barrier disruption contributes to disease progression and knowledge of this damage may lead to novel therapeutic targets.

Thus, impairment of the BBB/BSCB, preceding CNS entry of blood-borne toxins could be a key early factor in ALS pathogenesis, accelerating motor neuron death. The majority of findings on microvascular pathology in ALS, including BBB/BSCB alterations, have been established in mutant SOD1 rodent models, identifying barrier damage during disease development which might similarly occur in FALS patients carrying the SOD1 mutation. However, BBB/BSCB competence, as part of the tightly integrated neurovascular unit, is still largely a mystery in sporadic ALS cases.

The aim of this study was to determine integrity of the BBB/BSCB in postmortem tissues from patients with the sporadic form of ALS. A specific focus was analyzing barrier competence in the gray and white matter of the brainstem and spinal cord, CNS structures known to experience motor neuron pathology.