ROLE OF THE PHOSPHATE EXPORTER XPR1 IN VASCULAR CALCIFICATION – CALCIPHOS

Phosphate is one of the key minerals in the body that participates in the synthesis of nucleic acids and membranes, bone and tooth mineralization, energy production (ATP) and signal transduction. Phosphate is present in blood, cerebrospinal fluid (CSF) and cells, and its concentration is tightly regulated by sodium/phosphate transporters present at the plasma membrane of all cell types including renal, intestinal and choroid plexus epithelia, and vascular smooth muscle cells (VSMC). Defect in phosphate transport may have severe clinical consequences such as bone demineralization, and arterial and vascular calcifications that can lead to cardiac and cerebral complications.

Primary familial brain calcification (PFBC) is a rare disorder characterized by calcium phosphate deposits in microvessel walls of the brain. PFBC is associated with diverse neuropsychiatric expression and is inherited as an autosomal trait. We recently discovered that the retroviral receptor XPR1 had phosphate export activity (Giovannini et al., Cell Reports 2013) and that mutations in the XPR1 gene were present in up to 5% of PFBC patients (Legati et al., Nature Genetics 2015). XPR1 is the second gene, after SLC20A2/PiT2, which encodes a phosphate transporter and the mutations of which cause PFBC. We think that XPR1 is a key player of cellular phosphate homeostasis and metabolism and that its dysfunction leads to phosphate regulation disorders like calcification. The CALCIPHOS proposal involves two complementary teams of expertise and stems on our recent discoveries to study the XPR1 function and to evaluate its role in phosphate metabolic disorders.

Objective 1. To study the role of XPR1 in cellular phosphate homeostasis and metabolism. In particular, we aim:

(i) to characterize the N-terminal SPX domain of XPR1 for which we have yet unpublished evidence that it acts as a regulator of phosphate export, and in which all the mutations associated with PFBC have been found. Newly identified SPX-interacting factors obtained from a genetic screen will be evaluated for this regulation.
(ii) to establish a molecular link between PiT2 (phosphate importer) and XPR1 (phosphate exporter) which are both associated to the same PFBC disease.
(iii) to study the role of XPR1 and PiT2 in directional apico-basal phosphate transport in a choroid plexus cell model. This role is supported by the elevated phosphate levels observed in CSF of PFBC mice.

Objective 2. To study the role of XPR1 in vascular calcification. In particular, we aim:

(iv) to understand mechanisms by which XPR1 dysfunction leads to calcification. We will use a model of vascular smooth muscle cells (VSMC) in which calcification can be induced. Parental cells will be compared to cells presenting XPR1 haploinsufficiency or expressing only XPR1 variants with PFBC mutations that have been identified in patients.
(v) to measure the appearance of cerebral calcifications and the increase of phosphate concentration in CSF of heterozygous mice presenting XPR1 haploinsufficiency.

Objective 3. To establish and explore a cohort of PFBC patients. In particular, we aim:

(vi) to search for novel mutations in the XPR1 and SLC20A2/PiT2 genes of patients with PFBC, and to use patient samples for functional studies on phosphate transport.

This integrated study will identify new key actors of phosphate metabolism and evaluate precisely the role of XPR1 in phosphate-associated dysfunctions and diseases.