Mitochondrial Biogenesis Functions Research Articles

Abstract P3180: Cardiomyocyte Pgc-1α Mediates The Adaptive Response To Endurance Exercise Training In Mice

Exercise has myriad systemic benefits in reducing the incidence from and morbidity of cardio-metabolic diseases. In healthy individuals, aerobic exercise promotes physiologic hypertrophy of the heart, which requires increased mitochondrial numbers and function. In skeletal muscle and heart, a key orchestrator of the adaptation to increased energetic needs is the transcriptional regulator PGC-1α. PGC-1α may mediate cardiac adaptation to physiological stress through its function in mitochondrial biogenesis but also through other mechanisms. The role of cardiac PGC-1α to the systemic training response is poorly understood. We generated PGC-1α cardiomyocyte-specific knockout (cKO) mice to study its role in endurance exercise training. Sedentary cKO mice demonstrated normal cardiac function and no significant differences in acute treadmill exercise tolerance. However, cKO mice demonstrated a lack of ability to augment exercise capacity after 6 weeks of wheel running compared to WT (work achieved 32.9 J in WT [39% increase] vs 23.1 J in cKOs [no change vs baseline], p<0.001, unpaired t-test). This was despite comparable voluntary wheel distance run during training (2.93-3.67 km/day for both over 5 weeks, p=0.58). Unexpectedly, after 6 weeks of training, cKO mice developed a dilated cardiomyopathy at rest and exercise-related dysfunction (contractile reserve [ΔFS stress-rest ] +6.0 in WT vs -6.4% in KO, p<0.05, unpaired t-test). They also demonstrated features of cardiac cachexia as measured by 30% and 21% reductions in inguinal adipose and gastrocnemius mass, respectively (P<0.05, unpaired t-test). Gene expression profiling demonstrated activation of pathological ( Nppa , Nppb , Myh7/Myh6 ) and atrophic pathways ( Gdf11 , Gdf15 , Fstl3 ) in the trained cKOs. In vitro, gain of function studies using adenoviral expression in neonatal rat ventricular myocytes demonstrated that PGC-1α overexpression could abrogate pathologic hypertrophy and atrophy gene expression pathways stimulated by phenylephrine. Our preliminary studies support cardiomyocyte PGC-1α as a key mediator of the adaptive response to exercise training and unveil a key role of this transcriptional activator in protecting from exercise-related cardiac dysfunction in mice.

Characterization of a novel transcriptional feedback loop regulating lipid metabolism

The peroxisome proliferator‐activated receptor gamma coactivator 1α (PGC‐1α) is a transcriptional coactivator that plays a key regulatory function in mitochondrial biogenesis and lipid metabolism. It is highly induced upon cold exposure in brown fat, exercise in skeletal muscle and fasting in liver. The function of PGC‐1α has been extensively studied, but not much is known about how PGC‐1α activity is regulated. In order to identify novel PGC‐1α modulators, we characterized novel interacting molecules from nuclear liver extracts via biochemical purification followed by mass‐spectrometry analysis. Among a number of interactors we chose to study ACOX1 (Acyl Coenzyme A Oxidase). ACOX1 is the first and rate‐limiting enzyme of the peroxisomal fatty acid β‐oxidation and its expression is known to be under the control of PPARα, a transcription factor involved in fatty acid metabolism, potently coactivated by PGC‐1α. By in vitro pull‐down assay and by co‐immunoprecipitation in Hek293 cells, we reconfirmed that PGC‐1α interacts with ACOX1. ACOX1 is able to increase the transcriptional activity of the PGC‐1α/PPARα complex in luciferase assays. These results suggest that ACOX1 is able to modulate its own expression by enhancing the activity of its own transcriptional regulator PGC‐1α, creating a positive feedback loop that regulates lipid metabolism.Supported by the Intramural Research Program of NIDDK, NIH.

Nuclear gene OPA1, encoding a mitochondrial dynamin-related protein, is mutated in dominant optic atrophy

Autosomal dominant optic atrophy (ADOA), also referred to as optic atrophy type 1 (OPA 1, MIM 165500) or Kjer’s disease, is the most prevalent hereditary optic neuropathy. It typically presents within the first two decades of life with insidious bilateral and symmetric visual acuity loss, centrocecal scotomas and temporal optic atrophy. The disease is highly variable in expression, with both interfamilial and intrafamilial variability. The fundamental pathology of ADOA is a primary degeneration of the retinal ganglion cells followed by ascending atrophy of the optic nerve. The predominant locus for this disorder has been mapped to a 1.4-cM interval on chromosome 3q28-q29 flanked by markers D3S3669 and D3S35662. In these two independent publications, a nuclear gene, OPA1, is reported that maps within the candidate region and encodes a dynamin-related protein that localizes to mitochondria. In the first publication, using sequence analysis, mutations were identified in seven independent families with ADOA. The mutations included missense and nonsense alterations, deletions and insertions, which all segregated with the disease in these families. In the second paper, four different OPA1 mutations, including frameshift and missense mutations segregated with the disease. OPA1 is widely expressed and is most abundant in the retina. The presence of consensus signal peptide sequences suggests that the product of the gene OPA1 is targeted to mitochondria and may exert its function in mitochondrial biogenesis and stabilization of mitochondrial membrane integrity. Combined with what we already know regarding an underlying mitochondrial pathophysiology in Leber’s hereditary optic neuropathy, this information suggests a role for mitochondria in retinal ganglion cell pathophysiology.—Nancy J. Newman

OPA1, encoding a dynamin-related GTPase is mutated in autosomal dominant optic atrophy linked to chromosome 3q28

Autosomal dominant optic atrophy (ADOA), also referred to as optic atrophy type 1 (OPA 1, MIM 165500) or Kjer’s disease, is the most prevalent hereditary optic neuropathy. It typically presents within the first two decades of life with insidious bilateral and symmetric visual acuity loss, centrocecal scotomas and temporal optic atrophy. The disease is highly variable in expression, with both interfamilial and intrafamilial variability. The fundamental pathology of ADOA is a primary degeneration of the retinal ganglion cells followed by ascending atrophy of the optic nerve. The predominant locus for this disorder has been mapped to a 1.4-cM interval on chromosome 3q28-q29 flanked by markers D3S3669 and D3S35662. In these two independent publications, a nuclear gene, OPA1, is reported that maps within the candidate region and encodes a dynamin-related protein that localizes to mitochondria. In the first publication, using sequence analysis, mutations were identified in seven independent families with ADOA. The mutations included missense and nonsense alterations, deletions and insertions, which all segregated with the disease in these families. In the second paper, four different OPA1 mutations, including frameshift and missense mutations segregated with the disease. OPA1 is widely expressed and is most abundant in the retina. The presence of consensus signal peptide sequences suggests that the product of the gene OPA1 is targeted to mitochondria and may exert its function in mitochondrial biogenesis and stabilization of mitochondrial membrane integrity. Combined with what we already know regarding an underlying mitochondrial pathophysiology in Leber’s hereditary optic neuropathy, this information suggests a role for mitochondria in retinal ganglion cell pathophysiology.—Nancy J. Newman

OPA1, encoding a dynamin-related GTPase, is mutated in autosomal dominant optic atrophy linked to chromosome 3q28.

Autosomal dominant optic atrophy (ADOA) is the most prevalent hereditary optic neuropathy resulting in progressive loss of visual acuity, centrocoecal scotoma and bilateral temporal atrophy of the optic nerve with an onset within the first two decades of life. The predominant locus for this disorder (OPA1; MIM 165500) has been mapped to a 1.4-cM interval on chromosome 3q28-q29 flanked by markers D3S3669 and D3S3562 (ref. 3). We established a PAC contig covering the entire OPA1 candidate region of approximately 1 Mb and a sequence skimming approach allowed us to identify a gene encoding a polypeptide of 960 amino acids with homology to dynamin-related GTPases. The gene comprises 28 coding exons and spans more than 40 kb of genomic sequence. Upon sequence analysis, we identified mutations in seven independent families with ADOA. The mutations include missense and nonsense alterations, deletions and insertions, which all segregate with the disease in these families. Because most mutations probably represent null alleles, dominant inheritance of the disease may result from haploinsufficiency of OPA1. OPA1 is widely expressed and is most abundant in the retina. The presence of consensus signal peptide sequences suggests that the product of the gene OPA1 is targeted to mitochondria and may exert its function in mitochondrial biogenesis and stabilization of mitochondrial membrane integrity.

A mutation in a novel yeast proteasomal gene, RPN11/MPR1, produces a cell cycle arrest, overreplication of nuclear and mitochondrial DNA, and an altered mitochondrial morphology.

We report here the functional characterization of an essential Saccharomyces cerevisiae gene, MPR1, coding for a regulatory proteasomal subunit for which the name Rpn11p has been proposed. For this study we made use of the mpr1-1 mutation that causes the following pleiotropic defects. At 24 degreesC growth is delayed on glucose and impaired on glycerol, whereas no growth is seen at 36 degreesC on either carbon source. Microscopic observation of cells growing on glucose at 24 degreesC shows that most of them bear a large bud, whereas mitochondrial morphology is profoundly altered. A shift to the nonpermissive temperature produces aberrant elongated cell morphologies, whereas the nucleus fails to divide. Flow cytometry profiles after the shift to the nonpermissive temperature indicate overreplication of both nuclear and mitochondrial DNA. Consistently with the identification of Mpr1p with a proteasomal subunit, the mutation is complemented by the human POH1 proteasomal gene. Moreover, the mpr1-1 mutant grown to stationary phase accumulates ubiquitinated proteins. Localization of the Rpn11p/Mpr1p protein has been studied by green fluorescent protein fusion, and the fusion protein has been found to be mainly associated to cytoplasmic structures. For the first time, a proteasomal mutation has also revealed an associated mitochondrial phenotype. We actually showed, by the use of [rho degrees] cells derived from the mutant, that the increase in DNA content per cell is due in part to an increase in the amount of mitochondrial DNA. Moreover, microscopy of mpr1-1 cells grown on glucose showed that multiple punctate mitochondrial structures were present in place of the tubular network found in the wild-type strain. These data strongly suggest that mpr1-1 is a valuable tool with which to study the possible roles of proteasomal function in mitochondrial biogenesis.

A path from mitochondria to the yeast nucleus.

We have identified a path in yeast, from mitochondria to the nucleus, which may have a regulatory function in mitochondrial biogenesis. This path is evident as an elevated expression of a number of nuclear DNA sequences in response to specific defects in the mitochondrial genome, including the absence of mitochondrial DNA in rho 0 petites. Among those nuclear sequences preferentially expressed in certain respiratory-deficient cells are stable poly(A)+ transcripts derived from the so-called non-transcribed spacer region of the nuclear ribosomal DNA repeat, where they are most abundant in the rho 0 petite. Although the function of these unusual RNAs is unclear, the observations may reflect the presence of a mitochondrial homeostatic control system in yeast, which we suggest could function to adjust the mass of mitochondria and mitochondrial DNA in the cell in response to inequities in organelle apportionment during cell budding.