Why Is There No PiT1/SLC20A1 Pathogenic Variants Yet Linked to Primary Familial Brain Calcification?

Astrocytes modulate brain phosphate homeostasis via polarized distribution of phosphate uptake transporter PiT2 and exporter XPR1

Aberrant inorganic phosphate (Pi) homeostasis causes brain calcification and aggravates neurodegeneration, but the underlying mechanism remains unclear. Here, we found that primary familial brain calcification (PFBC)-associated Pi transporter genes Pit2 and Xpr1 were highly expressed in astrocytes, with importer PiT2 distributed over the entire astrocyte processes and exporter XPR1 localized to astrocyte end-feet on blood vessels. This polarized PiT2 and XPR1 distribution endowed astrocyte with Pi transport capacity competent for brain Pi homeostasis, which was disrupted in mice with astrocyte-specific knockout (KO) of either Pit2 or Xpr1. Moreover, we found that Pi uptake by PiT2, and its facilitation by PFBC-associated galactosidase MYORG, were required for the high Pi transport capacity of astrocytes. Finally, brain calcification was suppressed by astrocyte-specific PiT2 re-expression in Pit2-KO mice. Thus, astrocyte-mediated Pi transport is pivotal for brain Pi homeostasis, and elevating astrocytic Pi transporter function represents a potential therapeutic strategy for reducing brain calcification.

Basal ganglia calcification in hypoparathyroidism and pseudohypoparathyroidism: local and systemic metabolic mechanisms

BACKGROUND

Hypoparathyroidism and pseudohypoparathyroidism are rare disorders of mineral metabolism which may be associated with soft tissue calcification in the basal ganglia in the brain, and occasionally the skin and other tissues. The basal ganglia are the most common sites of calcification in the central nervous system in these disorders, and were first associated with this manifestation in a report from the Mayo Clinic in 1939. The reasons why the basal ganglia are a common site of soft tissue calcification in these rare disorders has been a matter of investigation for many years.

FINDINGS

Due to recent increased understanding of phosphate transport and new insights gained from mRNA expression in the basal ganglia, the pathophysiology of basal ganglia calcification (BGC) is now clearer. There is evidence that the absence of parathyroid hormone in hypoparathyroidism may play a direct role, but this is clearly not the case in pseudohypoparathyroidism, which is associated with increased parathyroid hormone levels. Maintaining the calcium/phosphorus ratio as close to normal as possible, and maintaining normal serum phosphate levels, may help mitigate the progression of BGC. There is no evidence of regression of BGC with conventional treatment, and long-term data with adjunctive or replacement therapy with parathyroid hormone or its analogues are not yet available.

PURPOSE OF THE REVIEW

This review will focus on the pathophysiology of BGC in hypoparathyroidism and pseudohypoparathyroidism, and review the proposed pathophysiologic mechanisms, as well as the clinical implications of BGC on patient quality of life.