Mutant SPART causes defects in mitochondrial protein import and bioenergetics reversed by Coenzyme Q

Pathogenic variants in SPART cause Troyer syndrome, characterized by lower extremity spasticity and weakness, short stature and cognitive impairment, and a severe mitochondrial impairment. Herein, we report the identification of a role of Spartin in nuclear-encoded mitochondrial proteins. SPART biallelic missense variants were detected in a 5-year-old boy with short stature, developmental delay and muscle weakness with impaired walking distance. Patient-derived fibroblasts showed an altered mitochondrial network, decreased mitochondrial respiration, increased mitochondrial reactive oxygen species and altered Ca²⁺ versus control cells. We investigated the mitochondrial import of nuclear-encoded proteins in these fibroblasts and in another cell model carrying a SPART loss-of-function mutation. In both cell models the mitochondrial import was impaired, leading to a significant decrease in different proteins, including two key enzymes involved in CoQ10 (CoQ) synthesis, COQ7 and COQ9, with a severe reduction in CoQ content, versus control cells. CoQ supplementation restored cellular ATP levels to the same extent shown by the re-expression of wild-type SPART, suggesting CoQ treatment as a promising therapeutic approach for patients carrying mutations in SPART.

[Study of the Role and Mechanism of Asprosin/Spartin Pathway in Cardiac Microvascular Endothelial Injury Induced by Diabete Mellitus]

OBJECTIVE

To detect the effects and mechanism of asprosin (Asp) and spartin on the injury of mice cardiac microvascular endothelial cells (CMECs) induced by high glucose.

METHODS

The cultured CMECs were divided into 2 groups, one group is normal group (5.5 mmol/L glucose in the medium) and another is HG group (30 mmol/L glucose in the medium). Real-time PCR (qRT-PCR) and Western blot were respectively used to detect the mRNA level of spastic paraplegia 20 (SPG20) and protein expression of spartin in CMECs. Upregulation or downregulation of the expression of spartin was achieved via transfection with adenovirus (Ad) or small interfering RNA (siRNA) respectively. CMECs with downregulation of spartin expression were firstly treated with anti-oxidant N-acetylcysteine (NAC) or Asp respectively for 48 h, and then were interfered with 30 mmol/L glucose for 24 h afterward. The apoptosis of cell was detected by flow cytometry. Nitric oxide (NO) production was detected by NO probe and ELISA kit. The intracellular reactive oxygen species (ROS) levels were tested by DHE staining and ELISA kit. Type 2 diabetic model mice were established and then divided into T2DM group and T2DM+Asp group. After the model mice were established successfully (random blood glucose was more than 16.7 mmol/L), Asp (1 μg/g) was intraperitoneally injected once a day. After 2 weeks, mice echocardiography was performed to test cardiac diastolic function. The integrity of the microvascular endothelium was observed by scanning electron microscopy.

RESULTS

Compared with the normal group, the mRNA level of SPG20 and protein expression of spartin in mice CMECs of HG group were significantly reduced (P < 0.05). Under the condition of high glucose, Ad transfection induced significant decrease of the intracellular ROS level and the apoptosis level of the CMECs (P < 0.05), while NO increased after Ad transfection. In contrast, siRNA intervention resulted in opposite effect. In addition, the antioxidant NAC partly reversed the above changes caused by downregulating spartin. Asp upregulated the level of SPG20 mRNA and spartin protein expression in CMECs, reduced ROS production, reduced apoptosis and increased NO production. However, intervention effects of Asp, such as decreasing of ROS production, inhibiting apoptosis of CMECs and increasing of NO production, were partly reversed in spartin downregulated cells. In vivo, we found that Asp can improve cardiac function and increase the integrity and smoothness of cardiac microvascular endothelium in type 2 diabetic mice.

CONCLUSION

Asp can inhibit oxidative stress in mice CMECs through upregulating spartin signaling pathway, thereby alleviating the damage of microvascular endothelium in diabetic heart.

A novel mutation in SPART gene causes a severe neurodevelopmental delay due to mitochondrial dysfunction with complex I impairments and altered pyruvate metabolism

Loss-of-function mutations in the SPART gene cause Troyer syndrome, a recessive form of spastic paraplegia resulting in muscle weakness, short stature, and cognitive defects. SPART encodes for Spartin, a protein linked to endosomal trafficking and mitochondrial membrane potential maintenance. Here, we identified with whole exome sequencing (WES) a novel frameshift mutation in the SPART gene in 2 brothers presenting an uncharacterized developmental delay and short stature. Functional characterization in an SH-SY5Y cell model shows that this mutation is associated with increased neurite outgrowth. These cells also show a marked decrease in mitochondrial complex I (NADH dehydrogenase) activity, coupled to decreased ATP synthesis and defective mitochondrial membrane potential. The cells also presented an increase in reactive oxygen species, extracellular pyruvate, and NADH levels, consistent with impaired complex I activity. In concordance with a severe mitochondrial failure, Spartin loss also led to an altered intracellular Ca²⁺ homeostasis that was restored after transient expression of wild-type Spartin. Our data provide for the first time a thorough assessment of Spartin loss effects, including impaired complex I activity coupled to increased extracellular pyruvate. In summary, through a WES study we assign a diagnosis of Troyer syndrome to otherwise undiagnosed patients, and by functional characterization we show that the novel mutation in SPART leads to a profound bioenergetic imbalance.-Diquigiovanni, C., Bergamini, C., Diaz, R., Liparulo, I., Bianco, F., Masin, L., Baldassarro, V. A., Rizzardi, N., Tranchina, A., Buscherini, F., Wischmeijer, A., Pippucci, T., Scarano, E., Cordelli, D. M., Fato, R., Seri, M., Paracchini, S., Bonora, E. A novel mutation in SPART gene causes a severe neurodevelopmental delay due to mitochondrial dysfunction with complex I impairments and altered pyruvate metabolism.

Novel SPG20 mutation in an extended family with Troyer syndrome

Troyer Syndrome (TRS) is a rare autosomal recessive complicated spastic paraplegia disorder characterized by various neurological and musculoskeletal manifestations. Pathogenicity stems from mutations in SPG20 which encodes Spartin, a multifunctional protein that is thought to be essential for neuron viability. Here we report on the clinical and molecular characterization of TRS in five patients from an extended consanguineous family in the United Arab Emirates. Molecular analysis involved Whole Exome Sequencing and Sanger sequencing for identification and confirmation of the causative variant respectively. In silico tools including CADD and Polyphen-2 were used to assess pathogenicity of the variant. The clinical description of these patients included spastic paraparesis, motor and cognitive delay, gait abnormalities, musculoskeletal features, as well as white matter abnormalities and emotional liability. Molecular analysis revealed a novel homozygous missense mutation in SPG20 (c.1324G > C; p.Ala442Pro) occurring at an evolutionarily conserved residue in the Plant-Related Senescence domain of Spartin. The mutation segregated with the clinical phenotype in all patients. In silico algorithms predict the mutation to be disease causing, and the variant had not been previously reported in public or ethnic specific variant repositories.

Distinct mechanisms of recognizing endosomal sorting complex required for transport III (ESCRT-III) protein IST1 by different microtubule interacting and trafficking (MIT) domains

The endosomal sorting complex required for transport (ESCRT) machinery is responsible for membrane remodeling in a number of biological processes including multivesicular body biogenesis, cytokinesis, and enveloped virus budding. In mammalian cells, efficient abscission during cytokinesis requires proper function of the ESCRT-III protein IST1, which binds to the microtubule interacting and trafficking (MIT) domains of VPS4, LIP5, and Spartin via its C-terminal MIT-interacting motif (MIM). Here, we studied the molecular interactions between IST1 and the three MIT domain-containing proteins to understand the structural basis that governs pairwise MIT-MIM interaction. Crystal structures of the three molecular complexes revealed that IST1 binds to the MIT domains of VPS4, LIP5, and Spartin using two different mechanisms (MIM1 mode versus MIM3 mode). Structural comparison revealed that structural features in both MIT and MIM contribute to determine the specific binding mechanism. Within the IST1 MIM sequence, two phenylalanine residues were shown to be important in discriminating MIM1 versus MIM3 binding. These observations enabled us to deduce a preliminary binding code, which we applied to provide CHMP2A, a protein that normally only binds the MIT domain in the MIM1 mode, the additional ability to bind the MIT domain of Spartin in the MIM3 mode.