Maintenance of respiratory chain function in mouse hearts with severely impaired mtDNA transcription

Abstract

The basal mitochondrial transcription machinery is essential for biogenesis of the respiratory chain and consists of mitochondrial RNA polymerase, mitochondrial transcription factor A (TFAM) and mitochondrial transcription factor B2. This triad of proteins is sufficient and necessary for mtDNA transcription initiation. Abolished mtDNA transcription caused by tissue-specific knockout of TFAM in the mouse heart leads to early onset of a severe mitochondrial cardiomyopathy with lethality within the first post-natal weeks. Here, we describe a mouse model expressing human TFAM instead of the endogenous mouse TFAM in heart. These rescue mice have severe reduction in mtDNA transcription initiation, but, surprisingly, are healthy at the age of 52 weeks with near-normal steady-state levels of transcripts. In addition, we demonstrate that heavy-strand mtDNA transcription normally terminates at the termination-associated sequence in the control region. This termination is abolished in rescue animals resulting in heavy (H)-strand transcription of the entire control region. In conclusion, we demonstrate here the existence of an unexpected mtDNA transcript stabilization mechanism that almost completely compensates for the severely reduced transcription initiation in rescue hearts. Future elucidation of the underlying molecular mechanism may provide a novel pathway to treat mitochondrial dysfunction in human pathology.

The core human mitochondrial transcription initiation complex

“…The direct test of this first concept is not possible because knocking-out mtTFA results in loss of mtDNA and hence mitochondrial transcription would not be possible. However, in mice in which mouse mtTFA has been replaced by human, which has significantly reduced transcription activation capacity on mouse mtDNA promoters in vitro, some transcription still occurs 28 that could be due to mtTFA-independent initiation by mouse POLRMT and mtTFB2 in cells (i.e., the basal twocomponent machinery). In addition, Goto et al showed that reducing Drosophila mtTFA levels to less than five percent of normal in Kc137 cells did not significantly affect transcription rates or steady-state levels of mitochondrial RNAs.…”

Dietary isoflavone daidzein promotes Tfam expression that increases mitochondrial biogenesis in C2C12 muscle cells

“…5 experiments, cardiac muscle-specific ablation of Tfam results in severely impaired mtDNA transcription and a respiratory chain deficiency in the heart [9]. Thus, Tfam might play a critical role in mitochondrial biogenesis.…”

Mitochondrial transcription in mammalian cells

“…This is particularly puzzling considering that precise spacing between TFAM-binding sites and transcription initiation sites appears to be important for efficient transcription initiation (59). Also, functional interchangeability of human and mouse TFAMs in vitro and in vivo (60, 61) and similar footprints of mouse and human TFAMs on both human and mouse LSPs (62), together with the lack of considerable sequence similarity in upstream regions of human and mouse promoters (63), collectively suggest that correct TFAM positioning upstream of the transcription start site may be sequence-independent. Finally, some studies failed to detect substantial differences in TFAM binding to specific and non-specific sequences as reported by dissociation constants (64) or by genome-scale mapping of TFAM-mtDNA interactions (65).…”