Myotubularin-Related (MTMR) Phospholipid Phosphatase Proteins in the Peripheral Nervous System

Abstract

Myotubularin-related proteins (MTMRs) constitute a broad family of ubiquitously expressed phosphatases with 14 members in humans, of which eight are catalytically active phosphatases, while six are catalytically inactive. Active MTMRs possess 3-phosphatase activity toward both PtdIns3P and PtdIns(3, 5)P 2 poliphosphoinositides (PPIn), suggesting an involvement in intracellular trafficking and membrane homeostasis. Among MTMRs, catalytically active MTMR2 and inactive MTMR13 have a nonredundant function in nerve. Loss of either MTMR2 or MTMR13 causes Charcot-Marie-Tooth type 4B1 and B2 neuropathy, respectively, characterized by demyelination and redundant loops of myelin known as myelin outfoldings. In Mtmr2-null mouse nerves, these aberrant foldings occur at 3-4 weeks after birth, a time when myelination is established, and Schwann cells are still elongating to reach the final internodal length. Moreover, Mtmr2-specific ablation in Schwann cells is both sufficient and necessary to provoke CMT4B1 with myelin outfoldings. MTMR2 phospholipid phosphatase might regulate intracellular trafficking events and membrane homeostasis in Schwann cells during postnatal nerve development. In this review, we will discuss recent findings on the MTMR family with a major focus on MTMR2 and MTMR13 and their putative role in Schwann cell biology.