Mitochondrial dysfunction in neurodegeneration

Abstract

The mitochondrial respiratory chain (RC) is responsible for providing most of the cellular energy in the form of ATP, and is also one of the main sites of reactive oxygen species (ROS) formation. The RC, consisting of five enzyme complexes in the inner mitochondrial membrane, is unique in its bipartite dependence on both nuclear and mitochondrial DNA (mtDNA). One example of this is mitochondrial transcription factor A (TFAM), a nuclear encoded protein that is imported into mitochondria where it is essential for transcription and maintenance of mtDNA.

We have studied the two aspects of TFAM function, mitochondrial transcription and mtDNA maintenance, in transgenic mice, and also used tissue-specific knockouts of TFAM to impair mitochondrial function in the mouse brain. An increasing amount of evidence suggest that mitochondrial dysfunction is of central importance in the pathogenesis in common neurodegenerative disorders, such as Parkinson's disease (PD), as well as in the normal aging process.

Neurological symptoms are also prominent in a group of genetic disorders collectively called mitochondrial encephalomyopathies, which are caused by defects in the RC. We show here that human TFAM (hTFAM) is a poor activator of mouse mitochondrial in vitro transcription, despite its strong capacity for unspecific DNA binding. PAC-transgenic mice expressing hTFAM had elevated mtDNA copy number but no changes in levels of most mitochondrial transcripts or in RC function. We estimated the molar ratio of TFAM to mtDNA to 1 TFAM molecule per 15-20 bp of mtDNA. Thus TFAM is an abundant protein within mitochondria capable of regulating mtDNA copy number.

The human TFAM transgene was furthermore unable to complement the loss of endogenous mouse TFAM (mTFAM) in homozygous knockout embryos, probably due to the reduced ability of hTFAM to activate transcription. We were however successful in rescuing tissue-specific knockout mice with mTFAM depletion in the heart by crossing them to PACTFAM mice. Interestingly, such rescued hearts displayed signs of altered transcription regulation in mitochondria, probably as a compensatory response to maintain energy production. Mice with tissue-specific knockout of TFAM in forebrain neurons (MILON mice) survived for a surprisingly long time.

At 4 months of age many neurons were severely RC deficient, but the mice still appeared normal. RC deficient neurons in such presymptomatic animals did however display increased sensitivity to excitotoxic stress. MILON mice died from massive synchronized neurodegenerative events in neocortex and hippocampus, at 5-6 months of age, without showing any major up-regulation of oxidative defenses. We next generated mice with a dopamine (DA) neuron specific inactivation of the Tfam gene. These "Parkinson mice" developed progressive symptoms replicating many of the clinical features of PD, such as bradykinesia, rigidity and abnormal gait. Symptoms were transiently reversed by administration of normal P1) medication (L-DOPA).

In more advanced stages (>30 weeks of age) of the disease, similarly to PD, the efficiency of LDOPA treatment declined and animals showed clear signs of dyskinetic movements. The behavioral disturbances correlated with a slow degeneration of the nigro-striatal DA pathway. Primarily DA neurons in substantia nigra pars compacta were lost, but in later stages also the ventral tegmental area neurons succumbed. Many surviving DA neurons displayed abnormal morphology, containing alpha-synuclein and ubiquitin immunoreactive inclusions, similar to Lewy bodies observed in PD.