Phosphatidylinositol phosphates directly bind to neurofilament light chain (NF-L) for the regulation of NF-L self assembly

Phosphatidylinositol phosphates (PtdInsPs) are ubiquitous membrane phospholipids that play diverse roles in cell growth and differentiation. To clarify the regulation mechanism acting on neurofilament light chain (NF-L) self assembly, we examined the effects of various PtdInsPs on this process. We found that PtdInsPs, including PI(4,5)P2, directly bind to the positively charged Arg54 of murine NF-L, and this binding promotes NF-L self assembly in vitro. Mutant NF-L (R53A/R54A) proteins lacking binding affinity to PtdInsPs did not have the same effect, but the mutant NF-L proteins showed greater self assembly than the wild-type in the absence of any PtdInsP. These results collectively suggest that Arg54 plays a pivotal role in NF-L self assembly by binding with PtdInsPs.

Pleckstrin homology domain of phospholipase C-gamma1 directly binds to 68-kDa neurofilament light chain

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) has two pleckstrin homology (PH) domains: an amino-terminal domain (PH1) and a split PH domain (PH2). Here, we show that overlay assay of bovine brain tubulin pool with glutathione-S-transferase (GST)-PLC-gamma1 PH domain fusion proteins, followed by matrix-assisted laser-desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), identified 68-kDa neurofilament light chain (NF-L) as a binding protein of amino-terminal PH domain of PLC-gamma1. NF-L is known as a component of neuronal intermediate filaments, which are responsible for supporting the structure of myelinated axons in neuron. PLC-gamma1 and NF-L colocalized in the neurite in PC12 cells upon nerve growth factor stimulation. In vitro binding assay and immunoprecipitation analysis also showed a specific interaction of both proteins in differentiated PC12 cells. The phosphatidylinositol 4, 5-bisphosphate [PI(4,5)P2] hydrolyzing activity of PLC-gamma1 was slightly decreased in the presence of purified NF-L in vitro, suggesting that NF-L inhibits PLC-gamma1. Our results suggest that PLC-gamma1-associated NF-L sequesters the phospholipid from the PH domain of PLC-gamma1.

Mutational Analysis of the Neurofilament Light Chain (NEFL) Gene in Patients with Charcot-Marie-Tooth Disease

BACKGROUND: Charcot-Marie-Tooth (CMT) disease is the most common form of inherited motor and sensory neuropathy. Neurofilament light chain polypeptide (NEFL) is one of the most abundant cytoskeletal components of the neuron. The NEFL gene encoding the neurofilament light chain plays an important role in the axonal structure that includes an extensive fibrous network in the cytoplasm of the neuron. Mutations in the NEFL gene are also present in CMT2E, CMT type 1 and Dejerine-Sottas syndrome. However, there have been no reports to investigate the NEFL genes in Korean CMT patients. Therefore, we investigated to find the clinical characteristics in patients with the NEFL gene mutation. METHODS: We examined mutations of the NEFL gene in 125 Korean CMT families. Mutations were confirmed by the sequencing of both strands. Nerve conduction studies were carried out on CMT patients having each mutation. RESULTS: Three pathogenic mutations were found in 3 families, and 2 polymorphisms in 2 families. Two mutations (Leu334Pro, Pro22Arg) were determined too novel, and those were not detected in 105 healthy controls. A de novo missense mutation was found in a CMT family with the NEFL mutation. The frequency of the NEFL mutation was 2.4%, which was similar in Europeans, and lower than those found in Japanese. Pro22Arg and Glu397Lys mutations showed demyelinating neuropathy and Leu334pro mutation showed axonal neuropathy. CONCLUSIONS: We found NEFL mutations in patients with sporadic or dominantly inherited CMT. NEFL mutations should be considered in the evaluation of CMT or related neuropathies with various clinical features.