What floats our boat Part II - Brain Manganese and alpha-Synuclein
Manganese is another essential element required by mammals, participating as a cofactor for various metabolic enzymes. Similar to iron, manganese deficiency or overload in the human body can result in a variety of diseases. In particular, dysregulation in manganese levels have been implicated in neurological diseases such as manganism, hypermanganesemia and Parkinson’s. In patients with these disorders, hyperintensities are quantified in MRI scans due to the effect of paramagnetic Mn(II); this Mn accumulation is localized to specific brain regions e.g. the midbrain where the dopaminergic neurons associated with Parkinson’s disease reside. There is not much known as to how manganese infiltrates into the brain parenchyma. Our objective is to understand the mechanisms by which manganese is delivered to the brain via the blood-brain barrier (BBB) from the systemic circulation (see previous page for illustration on brain vasculature).

While many have considered that the same proteins responsible for iron trafficking also transport manganese, we have observed that this is not always the case. For instance, ferroportin  (Fpn), which is the only known mammalian iron exporter, was assumed to be the predominant manganese exporter. We investigated the role of Fpn in manganese export at the BBB by treating human brain microvascular endothelial cells (hBMVEC) with the peptide hormone hepcidin, which induces Fpn internalization and degradation by the cell by directly binding to Fpn, and inducing conformational its ubiquitination. As shown in the figure to the right, hepcidin treatment ablates iron efflux in BMVEC but has no effect on manganese efflux, suggesting that Fpn is a primary iron, but not manganese efflux transporter. Clearly, the manganese transport pathway is discernably different than the mechanisms that regulate iron trafficking. Our objective is to identify the specific proteins involved in manganese transport at the BBB.

 
We believe that BMVEC tightly regulate manganese efflux into the brain. The image shown to the right, illustrates the trajectory of manganese flux in a our transwell model of the BBB (see previous page for illustration of BMVEC in grown in transwell inserts). Most of the manganese secreted by BMVEC is directed back into the capillary lumen (apical chamber), rather than entering into the brain (basal chamber). What is modulating this specific pattern of manganese flux at the BBB? Is this trajectory sensitive to systemic signals, such as those associated with infection, inflammation or sepsis? These are questions we seek to answer. One possible modulator of Mn-trafficking is alpha-synuclein. The physiologic function of alpha-synuclein remains to be delineated; alpha-synuclein may have a role in synaptic vesicle formation at presynaptic terminals and the release of dopamine from neurons. Recently, alpha-synuclein has been proposed to loosely bind to manganese and other transition metals and thereby prevent neurotoxicity. We have found that alpha-synuclein is endogenously expressed by BMVEC and its expression and secretion is induced when cells are exposed to with manganese. Out premise is that  alpha-synuclein plays a key role in managing manganese homeostasis in the neurovascular unit: we propose that it 1) modulates the efflux of Mn(II) from the capillary endothelial cells and 2) suppresses its neurotoxicity in the brain's abluminal space. Our studies on the function of alpha-synuclein range from cell biologi functional assays like those illustrated here to biophysical quantification of the metal binding characteristics of recombinant forms of this protein.