Posttranslational modification of mitochondrial transcription factor A in impaired mitochondria biogenesis: Implications in diabetic retinopathy and metabolic memory phenomenon

Abstract

Mitochondrial transcription factor A (TFAM) is one of the key regulators of the transcription of mtDNA. In diabetes, despite increase in gene transcripts of TFAM, its protein levels in the mitochondria are decreased and mitochondria copy numbers become subnormal. The aim of this study is to investigate the mechanism(s) responsible for decreased mitochondrial TFAM in diabetes. Using retinal endothelial cells, we have investigated the effect of overexpression of cytosolic chaperone, Hsp70, and TFAM on glucose-induced decrease in mitochondrial TFAM levels, and the transcription of mtDNA-encoded genes, NADH dehydrogenase subunit 6 (ND6) and cytochrome b (Cytb). To investigate the role of posttranslational modifications in subnormal mitochondrial TFAM, ubiquitination of TFAM was accessed, and the results were confirmed in the retina from streptozotocin-induced diabetic rats. While overexpression of Hsp70 failed to prevent glucose-induced decrease in mitochondrial TFAM and transcripts of ND6 and Cytb, overexpression of TFAM ameliorated decrease in its mitochondrial protein levels and transcriptional activity. TFAM was ubiquitinated by high glucose, and PYR-41, an inhibitor of ubiquitination, prevented TFAM ubiquitination and restored the transcriptional activity. Similarly, TFAM was ubiquitinated in the retina from diabetic rats, and it continued to be modified after reinstitution of normal glycemia. Our results clearly imply that the ubiquitination of TFAM impedes its transport to the mitochondria resulting in subnormal mtDNA transcription and mitochondria dysfunction, and inhibition of ubiquitination restores mitochondrial homeostasis. Reversal of hyperglycemia does not provide any benefit to TFAM ubiquitination. Thus, strategies targeting posttranslational modification could provide an avenue to preserve mitochondrial homeostasis, and inhibit the development/progression of diabetic retinopathy.

Pancreas: Do All Roads Lead to Mitochondria?

“…7 Santos, et al have shown that ubiquitination of TFAM in diabetes prevents its transport to the mitochondria, which disrupts mitochondrial homeostasis indicating the strong relation with diabetes and mitochondrial dysfunction. 7 Kearns-Sayre Syndrome (KSS) is a rare cause of diabetes in children. KSS has a group of diseases, known as the classic triad, which includes retinitis pigmentosa, progressive external ophthalmoplegia, and cardiac conduction problems.…”

“…7 Also, Transcription Factor A Mitochondrial (TFAM) is a key regulator of the transcription of mt-DNA. 7 Santos, et al have shown that ubiquitination of TFAM in diabetes prevents its transport to the mitochondria, which disrupts mitochondrial homeostasis indicating the strong relation with diabetes and mitochondrial dysfunction. 7 Kearns-Sayre Syndrome (KSS) is a rare cause of diabetes in children.…”

Mitochondrial Transcription Factor A's role in heart failure.

“…The HSP60-70 complex is Londependent, and, importantly, TFAM binds to the HSP70 component. The interaction of TFAM bound to HSP-70 at the nucleus allows for transportation of TFAM into the mitochondria (Santos, Mishra, and Kowluru 2014). Lon can be personified as TFAM's personal trainer, keeping TFAM fit for mitochondrial biogenesis.…”

“…This regulates Ca 2+ , leading to decreased protease activity and apoptotic signaling. Santos describes TFAM's binding to HSP60, but at a lower affinity when compared to HSP70 (Santos, Mishra, and Kowluru 2014). TFAM transport may be inhibited in high ROS conditions such as cardiomyopathies.…”

“…TFAM packages single copies of mtDNA, creating a mitochondrial nucleoid (Kukat et al 2015;Wang et al 1999). TFAM is transported from the nucleus into the mitochondria by chaperone HSP70 (Santos, Mishra, and Kowluru 2014).…”