Bi-allelic variants of FILIP1 cause congenital myopathy, dysmorphism and neurological defects

Restoration of Shal/KV4 proteostasis and motor function in a Drosophila model of spinocerebellar ataxia type 19/22

Loss-of-function mutations in the human KCND3 gene encoding KV4.3 K+ channels are linked to the autosomal dominant neurodegenerative disease spinocerebellar ataxia type 19/22 (SCA19/22). Previous biophysical and biochemical analyses in vitro support the notion that the autosomal dominant inheritance pattern of SCA19/22 is associated with the dominant-negative effects of disease-causing KV4.3 mutants on proteostasis of their wild-type (WT) counterpart. Herein we aimed to explore whether the disease-causing mutants might perturb protein expression of endogenous KV4.3 channel in human cells, as well as contributing to in vivo pathomechanisms underlying motor impairments and neurodegeneration in an animal model of SCA19/22. Substantial reduction in human KV4.3 protein level was validated in skin fibroblasts derived from heterozygous SCA19/22 patients. Genetic knockdown of endogenous Shal, the fly ortholog of human KV4.3, in Drosophila led to locomotor impairment, ommatidia degeneration, and reduced brain cortex thickness, all of which was effectively ameliorated by transgenic expression of human KV4.3, but not KV1.1 K+ channel. Transgenic expression of SCA19/22-causing human KV4.3 mutants resulted in notable disruption of endogenous Shal proteostasis, locomotor function, and ommatidia morphology in Drosophila. Enhanced expression of the Drosophila molecular chaperones HSC70 and HSP83 in our fly model of SCA19/22 corrected Shal protein deficit, locomotor dysfunction, and neurodegeneration. Overexpression of Hsp90β also upregulated endogenous human KV4.3 protein level in patient-derived skin fibroblasts. Our findings highlight Drosophila as a suitable animal model for studying KV4.3 channelopathy in vivo, and accentuate a critical role of defective KV4.3 proteostasis in the pathogenesis of motor dysfunction and neurodegeneration in SCA19/22.

Deletion of filamin A-interacting protein (FILIP) results in a weak grip strength and abnormal responses to nociceptive stimulation

Filamin A-interacting protein (FILIP in mice, FILIP1 in humans) was first identified as a protein that negatively controls neuronal migration in rodents, and was subsequently demonstrated to be pivotal for the development of the neocortex. In the previous study, we generated FILIP knockout mice to investigate the in vivo functions of FILIP in cortical development. Since FILIP mRNA is widely expressed in the body, we systematically examined FILIP-knockout mice to determine the functions of FILIP throughout the body. Our results showed that FILIP-knockout mice exhibited weak grip strength and sensory abnormalities. Interestingly, we also found that FILIP was expressed in a subset of neurons in the dorsal root ganglion (DRG). Recent research has reported that FILIP1 mutations lead to severe neurological and musculoskeletal abnormalities, resulting in the proposal of a new disease entity, termed FILIP1opathy. It is expected that our FILIP-knockout mice could be used as a model for the pathological investigation of FILIP1opathy.

Protein phosphatase-1 regulates the binding of filamin C to FILIP1 in cultured skeletal muscle cells under mechanical stress

The actin-binding protein filamin c (FLNc) is a key mediator in the response of skeletal muscle cells to mechanical stress. In addition to its function as a structural scaffold, FLNc acts as a signaling adaptor which is phosphorylated at S2234 in its mechanosensitive domain 20 (d20) through AKT. Here, we discovered a strong dephosphorylation of FLNc-pS2234 in cultured skeletal myotubes under acute mechanical stress, despite high AKT activity. We found that all three protein phosphatase 1 (PP1) isoforms are part of the FLNc d18-21 interactome. Enzymatic assays demonstrate that PP1 efficiently dephosphorylates FLNc-pS2234 and in vitro and in cells upon PP1 activation using specific modulators. FLNc-pS2234 dephosphorylation promotes the interaction with FILIP1, a mediator for filamin degradation. Altogether, we present a model in which dephosphorylation of FLNc d20 by the dominant action of PP1c prevails over AKT activity to promote the binding of the filamin degradation-inducing factor FILIP1 during acute mechanical stress.

Revealing myopathy spectrum: integrating transcriptional and clinical features of human skeletal muscles with varying health conditions

Myopathy refers to a large group of heterogeneous, rare muscle diseases. Bulk RNA-sequencing has been utilized for the diagnosis and research of these diseases for many years. However, the existing valuable sequencing data often lack integration and clinical interpretation. In this study, we integrated bulk RNA-sequencing data from 1221 human skeletal muscles (292 with myopathies, 929 controls) from both databases and our local samples. By applying a method similar to single-cell analysis, we revealed a general spectrum of muscle diseases, ranging from healthy to mild disease, moderate muscle wasting, and severe muscle disease. This spectrum was further partly validated in three specific myopathies (97 muscles) through clinical features including trinucleotide repeat expansion, magnetic resonance imaging fat fraction, pathology, and clinical severity scores. This spectrum helped us identify 234 genuinely healthy muscles as unprecedented controls, providing a new perspective for deciphering the hallmark genes and pathways among different myopathies. The newly identified featured genes of general myopathy, inclusion body myositis, and titinopathy were highly expressed in our local muscles, as validated by quantitative polymerase chain reaction.