Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

The Arabidopsis SAC9 Enzyme defines a cortical population of early endosomes and restricts PI(4,5)P2 to the Plasma Membrane

Membranes lipids, and especially phosphoinositides, are differentially enriched within the eukaryotic endomembrane system. This generates a landmark code by modulating the properties of each membrane. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] specifically accumulates at the plasma membrane in yeast, animal and plant cells, where it regulates a wide range of cellular processes including endocytosis. However, the functional consequences of mispatterning PI(4,5)P2 in plants are unknown. Here, we functionally characterized the phosphoinositide phosphatase SUPPRESSOR OF ACTIN9 (SAC9) in Arabidopsis thaliana (Arabidopsis). We found that SAC9 depletion led to the ectopic localization of PI(4,5)P2 on cortical intracellular compartments, which depends on PI4P and PI(4,5)P2 production at the plasma membrane. SAC9 localizes to a subpopulation of trans-Golgi Network/early endosomes that are spatially restricted to a region close to the cell cortex and that are coated with clathrin. Furthermore, it interacts and colocalizes with the endocytic component Src Homology 3 Domain Protein 2 (SH3P2). In the absence of SAC9, SH3P2 localization is altered and the clathrin mediated endocytosis rate is significantly reduced. Thus, SAC9 is required to maintain efficient endocytic uptake, highlighting the importance of restricting the PI(4,5)P2 pool at the plasma membrane for the proper regulation of endocytosis in plants.

FIG4-Associated Yunis-Varon Syndrome: Identification of a Novel Missense Variant

Yunis-Varon syndrome (YVS; OMIM 216340) is a rare heterogeneous autosomal recessive disorder with easy recognition of characteristic severe neurological and skeletal abnormalities involving skeletal muscles and cartilages. This cleidocranial dysplasia is characterized by bone and tooth disorders; it also affects the cardiovascular system and tissues from ectoderm with very poor outcomes. Rarely, mutations of the <i>FIG4</i> gene, encoding a 50-phosphoinositide phosphatase have been identified as the cause for YVS. We report a neonate born to a consanguineous couple with typical clinical manifestations of YVS. Using whole-exome sequencing, we identified a novel homozygous missense variant (c.968A&#x3e;G; p.Gln323Arg) in the <i>FIG4</i> gene. Thus, our study expands the molecular and genetic spectrum of <i>FIG4</i>-associated mutations. To our knowledge, this is the first reported case of YVS from the Saudi population.

A structure of substrate-bound Synaptojanin1 provides new insights in its mechanism and the effect of disease mutations

Synaptojanin1 (Synj1) is a phosphoinositide phosphatase, important in clathrin uncoating during endocytosis of presynaptic vesicles. It was identified as a potential drug target for Alzheimer’s disease, Down syndrome, and TBC1D24-associated epilepsy, while also loss-of-function mutations in Synj1 are associated with epilepsy and Parkinson’s disease. Despite its involvement in a range of disorders, structural, and detailed mechanistic information regarding the enzyme is lacking. Here, we report the crystal structure of the 5-phosphatase domain of Synj1. Moreover, we also present a structure of this domain bound to the substrate diC8-PI(3,4,5)P3, providing the first image of a 5-phosphatase with a trapped substrate in its active site. Together with an analysis of the contribution of the different inositide phosphate groups to catalysis, these structures provide new insights in the Synj1 mechanism. Finally, we analysed the effect of three clinical missense mutations (Y793C, R800C, Y849C) on catalysis, unveiling the molecular mechanisms underlying Synj1-associated disease.

Thyroid hormone receptor beta inhibits the PI3K-Akt-mTOR signaling axis in anaplastic thyroid cancer via genomic mechanisms

Thyroid cancer is the most common endocrine malignancy, and the global incidence has increased rapidly over the past few decades. Anaplastic thyroid cancer (ATC) is highly aggressive, dedifferentiated, and patients have a median survival of fewer than six months. Oncogenic alterations in ATC include aberrant PI3K signaling through receptor tyrosine kinase (RTK) amplification, loss of phosphoinositide phosphatase expression and function, and Akt amplification. Furthermore, the loss of expression of the tumor suppressor thyroid hormone receptor beta (TRβ) is strongly associated with ATC. TRβ is known to suppress PI3K in follicular thyroid cancer and breast cancer by binding to the PI3K regulatory subunit p85α. However, the role of TRβ in suppressing PI3K signaling in ATC is not completely delineated. Here we report that TRβ indeed suppresses PI3K signaling in ATC through unreported genomic mechanisms including a decrease in RTK expression and increase in phosphoinositide and Akt phosphatase expression. Furthermore, the reintroduction and activation of TRβ in ATC enables an increase in the efficacy of the competitive PI3K inhibitors LY294002 and buparlisib on cell viability, migration, and suppression of PI3K signaling. These findings not only uncover additional tumor suppressor mechanisms of TRβ but shed light into the implication of TRβ status and activation on inhibitor efficacy in ATC tumors.Abstract FigureGraphical abstract

Genome-Wide Identification and Characterization of SAC Domain-Containing Protein Family in Cotton

Background: Phosphoinositides(PIs) are important regulators of a diverse range of cellular functions. The Suppressor of Actin (SAC) domain-containing proteins are a class of phosphoinositide phosphatase involved in the synthesis of PIs. Though the cellular functions of SAC domain-containing proteins have been characterized in yeast, information of SAC genes in cotton is largely undefined.Results: In the present study, 12,12and24 putative SAC genes were identified in the G. ramondii, G. arboreum and G. hirsutum respectively. Detailed gene information, including genomic organization, structural feature, conserved domain and phylogenetic relationship of the genes were systematically characterized. All SAC family members in cotton were divided into three clades, Group I, Group II and Group III, based on their sequence similarities and phylogenetic relationship. The SAC domains consist of seven highly conserved motifs that are believed to be important for the phosphoinositide phosphatase activities from yeast to animal. Expression analysis of GhSAC from Group II and Group III shared similar moderate pattern in different tissues and insensitive to different abiotic stresses. Different members in Group I showed different expressional profiles. Four genes (GhSAC2.1A/GhSAC2.1D and GhSAC4.2A/GhSAC4.2D) from Group I predominantly expressed in anther, pistil and petal. The results suggested the functional divergence among different groups and members of SAC in cotton.Conclusions: Systematical analysis of the SAC gene family in cotton provided a solid foundation for further investigation of the biological functions of SAC genes.

Roles for a lipid phosphatase in the activation of its opposing lipid kinase

The phosphoinositide phosphatase Fig4 is predicted to turn over the signaling lipid PI3,5P2. It is shown that a major role of Fig4 is to elevate PI3,5P2 via dynamic regulation of the protein complex that activates its opposing lipid kinase, Fab1.

Absence of Sac2/INPP5F enhances the phenotype of a Parkinson’s disease mutation of synaptojanin 1

Numerous genes whose mutations cause, or increase the risk of, Parkinson’s disease (PD) have been identified. An inactivating mutation (R258Q) in the Sac inositol phosphatase domain of synaptojanin 1 (SJ1/PARK20), a phosphoinositide phosphatase implicated in synaptic vesicle recycling, results in PD. The gene encoding Sac2/INPP5F, another Sac domain-containing protein, is located within a PD risk locus identified by genome-wide association studies. Knock-In mice carrying the SJ1 patient mutation (SJ1RQKI) exhibit PD features, while Sac2 knockout mice (Sac2KO) do not have obvious neurologic defects. We report a “synthetic” effect of the SJ1 mutation and the KO of Sac2 in mice. Most mice with both mutations died perinatally. The occasional survivors had stunted growth, died within 3 wk, and showed abnormalities of striatal dopaminergic nerve terminals at an earlier stage than SJ1RQKI mice. The abnormal accumulation of endocytic factors observed at synapses of cultured SJ1RQKI neurons was more severe in double-mutant neurons. Our results suggest that SJ1 and Sac2 have partially overlapping functions and are consistent with a potential role of Sac2 as a PD risk gene.

Synj1 haploinsufficiency causes dopamine neuron vulnerability and alpha-synuclein accumulation in mice

Synaptojanin1 (synj1) is a phosphoinositide phosphatase with dual SAC1 and 5′-phosphatase enzymatic activities in regulating phospholipid signaling. The brain-enriched isoform has been shown to participate in synaptic vesicle (SV) recycling. More recently, recessive human mutations were identified in the two phosphatase domains of SYNJ1, including R258Q, R459P and R839C, which are linked to rare forms of early-onset Parkinsonism. We now demonstrate that Synj1 heterozygous deletion (Synj1+/−), which is associated with an impaired 5′-phosphatase activity, also leads to Parkinson’s disease (PD)-like pathologies in mice. We report that male Synj1+/− mice display age-dependent motor function abnormalities as well as alpha-synuclein accumulation, impaired autophagy and dopaminergic terminal degeneration. Synj1+/− mice contain elevated 5′-phosphatase substrate, PI(4,5)P2, particularly in the midbrain neurons. Moreover, pharmacological elevation of membrane PI(4,5)P2 in cultured neurons impairs SV endocytosis, specifically in midbrain neurons, and further exacerbates SV trafficking defects in Synj1+/− midbrain neurons. We demonstrate down-regulation of SYNJ1 transcript in a subset of sporadic PD brains, implicating a potential role of Synj1 deficiency in the decline of dopaminergic function during aging.