Zeh 2013 Thesis

Zeh RM (2013) Neurological and molecular biological characterisation of the mutant mouse line Tom40. Thesis Technische Universität München 243pp.

» tum 1100739 Open Access

Zeh RM (2013) Thesis Technische Universität Muenchen

Abstract: The mitochondrial import machinery consists of several translocases. By gene trap mutagenesis, a knockout mouse model for the translocase of the outer mitochondrial membrane (Tom) 40 was created, the protein which forms the general import pore of the outer membrane. The mutation is homozygous lethal. Reduced expression of Tom40 leads to mild cardiac dysfunction and causes a slowly progressing movement disorder, combined with neurological impairments and neurodegeneration. Mitochondrial morphology is altered widely in mutants and there is a strong age-dependant worsening of mitochondrial function. These mutants can serve as a model for age-associated motor disorders.

Labels: MiParea: Respiration, mtDNA;mt-genetics, Genetic knockout;overexpression 

Organism: Rat  Tissue;cell: Heart, Nervous system  Preparation: Isolated mitochondria 

Coupling state: OXPHOS, ET  Pathway: N, S, ROX  HRR: Oxygraph-2k 

The vast majority of mitochondrial proteins is encoded by nuclear genes and then imported into the organelle. The TOM complex mediates the import of all proteins of mitochondria into the intermembrane space and also the insertion of proteins into the outer membrane. TOM40 comprises the main component of the TOM complex as it forms the general import pore. Recent reports showed that in human Alzheimer’s disease (AD) brains amyloid precursor protein (APP) accumulates in the TOM40 import channel and thereby inhibits the entry of various nuclear-encoded proteins. Moreover, a variant in the TOM40 gene has been associated with age of onset in AD. The Tom40 mutant mouse line was generated by gene trap mutagenesis using a non-retroviral pT1ßgeo vector. Homozygous Tom40-/- mice are not viable, embryos die before E3.5. Heterozygous Tom40+/- mice with a 50% reduction of expressed Tom40 mRNA showed normal development but a reduced life span with a 30% higher mortality after two years. Systemically analysis of mice at different time points during their life time revealed a mild cardiac dysfunction already early in young age and slowly progressing neurological impairments in mutant mice. Young heterozygous Tom40 mutants showed a subtle heart phenotype in the electrocardiogram (ECG) analysis. A decreased P-wave duration and prolonged Q-T and S-T intervals in the mutants indicate conduction impairments. Electron microscopic images of heart tissue illustrate several alterations concerning structure and arrangement of mutant mitochondria. Despite this alteration, the composition and function of the respiratory chain in the inner mitochondrial membrane was not affected at all in young mutant mice, indicating that the animals were able to compensate the defect to a great extent. With aging, this compensation seemed to fail more and more. Evaluation of basic neurological functions showed no differences in locomotor activity and muscle force but a slight sensorimotor impairment in motor coordination and balance tasks. Motoric nerve conduction velocity (NCV) in the sciatic nerve of mutants was markedly reduced compared to wildtypes, suggesting that there is a peripheral neuropathy already very early in development. There were very subtle alterations in the enzyme activity of mutant brain mitochondria, while the abundance of the respiratory chain complex (RCC) subunits still was normal. However, respirometric analyses of isolated mitochondria revealed a strong genotype-dependant worsening of mitochondrial function in mutants, both in heart and in brain. The same effect could be shown for the motor and the xi i electrophysiological phenotypes. Irrespective of genome-wide association studies (GWAS) which stated a possible association to AD, there was no evidence for the development of AD specific phenotypes like a memory loss. Tyrosin-hydroxylase (TH) positive neurons in the substantia nigra of aged mutants revealed a reduced number compared to age-matched wildtypes. Since alterations in the abundance of TH-positive neurons and the consequent impairment of the dopaminergic system in the brain are a common phenotype observed in patients suffering of Parkinson’s disease (PD), the Tom40 mouse model may serve as model for the development of neurodegenerative diseases like Parkinsonism. Systemic reduction of the main import protein Tom 40 is sufficient to cause cardiac symptoms and progressive neurological impairment.