Short peptides from leucyl-tRNA synthetase rescue disease-causing mitochondrial tRNA point mutations.

Mutations in mitochondrial (mt) genes coding for mt-tRNAs are responsible for a range of syndromes, for which no effective treatment is available. We recently showed that the carboxy-terminal domain (Cterm) of human mt-leucyl tRNA synthetase rescues the pathologic phenotype associated either with the m.3243A>G mutation in mt-tRNA(Leu(UUR)) or with mutations in the mt-tRNA(Ile), both of which are aminoacylated by Class I mt-aminoacyl-tRNA synthetases (mt-aaRSs). Here we show, by using the human transmitochondrial cybrid model, that the Cterm is also able to improve the phenotype caused by the m.8344A>G mutation in mt-tRNA(Lys), aminoacylated by a Class II aaRS. Importantly, we demonstrate that the same rescuing ability is retained by two Cterm-derived short peptides, ß30_31 and ß32_33, which are effective towards both the m.8344A>G and the m.3243A>G mutations. Furthermore, we provide in vitro evidence that these peptides bind with high affinity wild-type and mutant human mt-tRNA(Leu(UUR)) and mt-tRNA(Lys), and stabilize mutant mt-tRNA(Leu(UUR)). In conclusion, we demonstrate that small Cterm-derived peptides can be effective tools to rescue cellular defects caused by mutations in a wide range of mt-tRNAs.

Impaired mitochondrial biogenesis is a common feature to myocardial hypertrophy and end-stage ischemic heart failure.

Mitochondrial (mt) DNA depletion and oxidative mtDNA damage have been implicated in the process of pathological cardiac remodeling. Whether these features are present in the early phase of maladaptive cardiac remodeling, that is, during compensated cardiac hypertrophy, is still unknown. We compared the morphologic and molecular features of mt biogenesis and markers of oxidative stress in human heart from adult subjects with compensated hypertrophic cardiomyopathy and heart failure. We have shown that mtDNA depletion is a constant feature of both conditions. A quantitative loss of mtDNA content was associated with significant down-regulation of selected modulators of mt biogenesis and decreased expression of proteins involved in mtDNA maintenance. Interestingly, mtDNA depletion characterized also the end-stage phase of cardiomyopathies due to a primary mtDNA defect. Oxidative stress damage was detected only in failing myocardium.

Defective PITRM1 mitochondrial peptidase is associated with Aß amyloidotic neurodegeneration.

Mitochondrial dysfunction and altered proteostasis are central features of neurodegenerative diseases. The pitrilysin metallopeptidase 1 (PITRM1) is a mitochondrial matrix enzyme, which digests oligopeptides, including the mitochondrial targeting sequences that are cleaved from proteins imported across the inner mitochondrial membrane and the mitochondrial fraction of amyloid beta (Aß). We identified two siblings carrying a homozygous PITRM1 missense mutation (c.548G>A, p.Arg183Gln) associated with an autosomal recessive, slowly progressive syndrome characterised by mental retardation, spinocerebellar ataxia, cognitive decline and psychosis. The pathogenicity of the mutation was tested in vitro, in mutant fibroblasts and skeletal muscle, and in a yeast model. A Pitrm1(+/-) heterozygous mouse showed progressive ataxia associated with brain degenerative lesions, including accumulation of Aß-positive amyloid deposits. Our results show that PITRM1 is responsible for significant Aß degradation and that impairment of its activity results in Aß accumulation, thus providing a mechanistic demonstration of the mitochondrial involvement in amyloidotic neurodegeneration.

Targeting estrogen receptor ß as preventive therapeutic strategy for Leber's hereditary optic neuropathy.

Leber's hereditary optic neuropathy (LHON) is a maternally inherited blinding disease characterized by degeneration of retinal ganglion cells (RGCs) and consequent optic nerve atrophy. Peculiar features of LHON are incomplete penetrance and gender bias, with a marked male prevalence. Based on the different hormonal metabolism between genders, we proposed that estrogens play a protective role in females and showed that these hormones ameliorate mitochondrial dysfunction in LHON through the estrogen receptors (ERs). We also showed that ERß localize to the mitochondria of RGCs. Thus, targeting ERß may become a therapeutic strategy for LHON specifically aimed at avoiding or delaying the onset of disease in mutation carriers. Here, we tested the effects of ERß targeting on LHON mitochondrial defective metabolism by treating LHON cybrid cells carrying the m.11778G>A mutation with a combination of natural estrogen-like compounds that bind ERß with high selectivity. We demonstrated that these molecules improve cell viability by reducing apoptosis, inducing mitochondrial biogenesis and strongly reducing the levels of reactive oxygen species in LHON cells. These effects were abolished in cells with ERß knockdown by silencing receptor expression or by using specific receptor antagonists. Our observations support the hypothesis that estrogen-like molecules may be useful in LHON prophylactic therapy. This is particularly important for lifelong disease prevention in unaffected LHON mutation carriers. Current strategies attempting to combat degeneration of RGCs during the acute phase of LHON have not been very effective. Implementing a different and preemptive approach with a low risk profile may be very helpful.

Homozygous NOTCH3 null mutation and impaired NOTCH3 signaling in recessive early-onset arteriopathy and cavitating leukoencephalopathy.

Notch signaling is essential for vascular physiology. Neomorphic heterozygous mutations in NOTCH3, one of the four human NOTCH receptors, cause cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Hypomorphic heterozygous alleles have been occasionally described in association with a spectrum of cerebrovascular phenotypes overlapping CADASIL, but their pathogenic potential is unclear. We describe a patient with childhood-onset arteriopathy, cavitating leukoencephalopathy with cerebral white matter abnormalities presented as diffuse cavitations, multiple lacunar infarctions and disseminated microbleeds. We identified a novel homozygous c.C2898A (p.C966*) null mutation in NOTCH3 abolishing NOTCH3 expression and causing NOTCH3 signaling impairment. NOTCH3 targets acting in the regulation of arterial tone (KCNA5) or expressed in the vasculature (CDH6) were downregulated. Patient's vessels were characterized by smooth muscle degeneration as in CADASIL, but without deposition of granular osmiophilic material (GOM), the CADASIL hallmark. The heterozygous parents displayed similar but less dramatic trends in decrease in the expression of NOTCH3 and its targets, as well as in vessel degeneration. This study suggests a functional link between NOTCH3 deficiency and pathogenesis of vascular leukoencephalopathies.