Novel molecular biomarkers at the blood-brain barrier in ALS

Abstract

Recently neuroinflammation has gained a particular focus as a key mechanism of ALS. Several studies in vivo as well as in vitro have nominated immunoglobulin G (IgG) isolated from ALS patients as an active contributor to disease onset and progression. We have shown that ALS IgG affects astroglial Ca(2+) excitability and induces downstream activation of phosphatidylinositol 3-kinase. These studies were hampered by a lack of knowledge of the pathway of entry of immune factors in the CNS. Our MRI data revealed the blood-brain barrier BBB leakage and T cell infiltration into brain parenchyma in ALS G93A rats. Since astrocyte ensheathes blood vessel wall contributing to BBB stability and plays an important role in ALS pathogenesis, we have studied astrocytic membrane proteins water channel aquaporin-4 and the inwardly rectifying potassium channel. In this review, we will summarize data related to BBB disruption with particular emphasis on impaired function of astrocytes in ALS. We will discuss implication of membrane proteins expressed on astrocytic endfeet, aquaporin-4, and inwardly rectifying potassium channel in the pathology of ALS. In addition to ALS-specific IgGs, these membrane proteins are proposed as novel biomarkers of the disease.

Figure 1

The molecular mechanism of ALS pathology in astrocytes. WT astrocyte (left) contains a balanced ratio of Kir4.1 and AQP4 channels in its cell membrane that also forms the endfeet around the endothelial layer of the blood vessel (center). ALS astrocyte (right) shows a misbalance of channel molecules with an abundance of AQP4 over Kir4.1. This causes swelling of endfeet and affects the BBB that becomes leaky for immune factors such as immunoglobulins (IgGs) that are known to cause intracellular calcium spikes (red trace and the pseudocolor image of a Ca²⁺-sensitive dye at the peak of the response in cultured astrocytes) and may start excitotoxic processes in situ. Colored panels in the bottom illustrate immunocytochemistry of AQP4 and GFAP in WT versus ALS astrocytes in the rat brain while the panels with traces illustrate whole-cell Kir currents from WT versus ALS astrocytes in culture and two examples of respective Western blot bands for Kir protein.