Loss of the mitochondrial protease activity of Omi causes mitochondrial dysfunction, neurodegeneration with parkinsonian features and premature death in
mnd2 (motor neuron degeneration 2) mice. However, the detailed mechanisms underlying this pathology remain largely unknown. Here, we report that Omi participates in the process of mitochondrial biogenesis, which has been linked to several neurodegenerative diseases. The mitochondrial biogenesis is deficit in
mnd2 mice, evidenced by severe decreases of mitochondrial components, mitochondrial DNA and mitochondrial density. Omi cleaves glycogen synthase kinase 3
β (GSK3
β), a kinase promoting PPARγ coactivator-1
α (PGC-1
α) degradation, to regulate PGC-1
α, a factor important for the mitochondrial biogenesis. In
mnd2 mice, GSK3
β abundance is increased and PGC-1
α abundance is decreased significantly. Inhibition of GSK3
β by SB216763 or overexpression of PGC-1
α can restore mitochondrial biogenesis in
mnd2 mice or
Omi-knockdown N2a cells. Furthermore, there is a significant improvement of the movement ability of
mnd2 mice after SB216763 treatment. Thus, our study identified Omi as a novel regulator of mitochondrial biogenesis, involving in Omi protease-deficient-induced neurodegeneration.Mitochondria have a vital role in neuronal death and survival.
1 As critical cellular organelles, mitochondria have highly dynamic properties, including mitochondrial fission, fusion, transport, biogenesis and degradation. The changes of those properties affect mitochondrial functions, leading to the occurrence of diseases.
2, 3 Growing lines of evidence suggest that the mitochondrial dysfunction is involved in aging and neurodegenerative diseases, such as Alzheimer''s disease (AD), Huntington''s disease (HD) and Parkinson''s disease (PD).
4, 5 Similar to other neurodegenerative diseases, PD is a progressive neurological disorder, which is characterized by the development of cytoplasmic aggregates known as Lewy bodies and degeneration of dopaminergic (DA) neurons in the substantia nigra of midbrain and other brain regions.
6 In PD, dysfunction of mitochondria has been documented to be associated with disease pathogenesis in PD brains and both genetic- and toxin-induced PD animal models. In PD brains, mutations in mitochondrial DNA (mtDNA) occur more frequently than those in age-matched control; and mutations in the nuclear-encoded mtDNA polymerase-
γ gene, which impair mtDNA replication and result in multiple mtDNA deletions, cause PD-like symptoms.
5 Meanwhile, several PD-associated gene products, including
α-synuclein, parkin, DJ-1, PINK1 (PTEN-induced putative kinase 1), leucine-rich repeat kinase 2, ubiquitin carboxy-terminal hydrolase L1 and Omi, have been identified to be associated with PD, and lead to mitochondrial dysfunction with changes in mitochondrial morphology, biogenesis and mitophagy
in vivo and
in vitro.
5, 7, 8, 9 Besides, mitochondrial toxins, such as MPTP (1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and rotenone that inhibit complex I of the mitochondrial respiratory chain, cause clinically parkinsonian phenotype.
10, 11The serine protease Omi (also known as HtrA2) belongs to the high-temperature requirement factor A (HtrA) family, and was originally identified as a mammalian homolog of the
Escherichia coli heat-shock-induced serine protease HtrA/DegP and DegS.
12 Omi is mainly localized in mitochondria, although a fraction of it is also found in nucleus.
13 Omi is released from the mitochondria into the cytosol to mediate cell death by caspase-dependent or -independent pathways in response to apoptotic stimuli.
14, 15 However, the notion that Omi is an apoptosis inducer in the central nervous system was challenged by studies of Omi-overexpressing or -deficient mice.
Omi-overexpressing mice show normal development without any sign of apoptotic cell death.
16 On the other hand,
mnd2 (motor neuron degeneration 2) mice that harbor protease-deficient Omi S276C mutants, and
Omi-knockout mice both suffer from progressive neurodegeneration, especially in striatum, and motor abnormalities similar to PD. Both mice fail to gain weight and die before postnatal day 40 due to neurodegeneration with progressive mitochondrial damage.
17, 18, 19 Besides, mutations in the
Omi gene have also been identified in PD patients.
20, 21 Previous studies have shown that Omi has a vital role in the mitochondrial integrity, and the loss of protease activity leads to mitochondrial dysfunction, such as abnormal mitochondrial morphology and increased mtDNA mutation and deletions, increased susceptibility of mitochondrial membrane permeabilization, decreased mitochondrial membrane potential, and reduced mitochondrial density in
mnd2 mice and
Omi-knockout mice.
17, 18, 22 Omi has been found to act downstream of PINK1, but parallel to parkin, in a mitochondrial stress sensing pathway to sense the different stresses, which may be defective in PD.
23 These findings suggest that the primary function of Omi is involved in neuroprotection, especially in the maintenance of mitochondrial homeostasis.
23, 24In this article, we identified that Omi cleaves glycogen synthase kinase 3
β (GSK3
β) to regulate PPARγ coactivator-1
α (PGC-1
α) abundance and to ensure mitochondrial biogenesis.
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