Mitochondrial DNA in Mammalian Reproduction
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Reviews of Reproduction (1998) 3, 172–182 Mitochondrial DNA in mammalian reproduction Jim Cummins Division of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, Western Australia 6150, Australia Mitochondrial DNA (mtDNA) forms a semi-autonomous asexually reproducing genome in eukaryotic organisms. It plays an essential role in the life cycle through the control of energy production, by the inherently dangerous process of oxidative phosphorylation. The asym- metric nature of its inheritance – almost exclusively through the female – imposes different evolutionary constraints on males and females, and may lie at the heart of anisogamy. This review examines the implications of recent findings on the biology of mtDNA for reproduction and inheritance in mammals. Although the existence of mitochondria has been known since encodes for tryptophan in mammalian mitochondria). The in- the last century, mitochondrial DNA (mtDNA) has been studied heritance of mitochondria through the female lineage remains most extensively in the past two decades. Mitochondria have one of the central enigmas of reproductive biology. a profound role to play in mammalian tissue bioenergetics, in Mitochondria have certain tissue-specific configurations that growth, in ageing and in apoptosis, and yet they descend from presumably reflect local energetic requirements (Fawcett, 1981). an asexually reproducing independent life form. It has been New techniques for visualizing mitochondria in whole cells, hypothesized that tensions between the evolutionary ‘interests’ such as the incorporation of green fluorescent protein coupled of the eukaryotic host and its subservient organelles have led to with confocal microscopy, reveal that mitochondrial form can asymmetrical inheritance, so that mitochondria derive pre- not only reflect pathological states but may also be extremely dominantly from the female in most organisms (Hurst, 1992; diverse even within the same cell (Kanazawa et al., 1997). The Hurst et al., 1996). This has consequences that are only just be- basic design is a double membrane surrounding an inner mito- coming apparent. There is considerable interest in the potential chondrial matrix that contains one or more circular mtDNA role of mitochondria and cytoplasmic inheritance on growth molecules. The outer mitochondrial membrane is thought to and performance factors such as muscle development and milk represent the original invaginated host plasma membrane, production in domestic animals. As might be expected, the while the inner mitochondrial membrane represents the bac- maternally inherited mitochondrial genome has significant terial wall and contains the site of oxidative phosphorylation non-Mendelian effects on steroidogenesis and on respiratory- (OXPHOS) on its inner surface. The matrix also contains ribo- dependent functions such as growth, oxygen consumption and somes for local protein synthesis. lean:fat ratios, but not on anaerobic metabolism. Smith and The two membranes differ profoundly in composition. The Alcivar (1993) have reviewed this topic recently and compre- lipid:protein ratio of the outer membrane is about 50:50 and hensively; therefore, the focus of this article will be the role of it is permeable to molecules with molecular weights of up to mtDNA in the life cycle. 10 000 (Lodish et al., 1995). The inner membrane is relatively Mitochondria are semi-autonomous organelles found in all impermeable and is about 80% protein and is thrown up into eukaryotic cells (except mature red blood cells and some pro- infoldings – crystae – the sites of OXPHOS enzymes. Here the tozoans). It is generally accepted that mitochondria originated oxidation of metabolites generates ATP through a series of in ancestral eukaryotic cells through endosymbiosis of free- integral membrane multi-subunit protein complexes which living bacteria capable of metabolizing oxygen – a suggestion couple electron transport to ATP synthesis. These complexes first made over a century ago and noted by Ozawa in his review are unique in that they consist of proteins encoded by two (Ozawa, 1997a). Our ancestral eukaryotes thus exploited the separate yet cooperating genomes, that of the nucleus and that capacity of mitochondria to metabolise oxygen. This allowed of the mitochondrion (Poyton and McEwen, 1996; Shadel and them to flourish despite the increasing concentrations of this Clayton, 1997) There are separate translocase systems in the highly reactive and potentially poisonous element in the en- inner and outer membranes that coordinate the recognition, vironment. While most of the mitochondrial genes have moved import and assortment of essential proteins from the cytosol to the nucleus, mitochondria retain their bacterial facility for (Neupert, 1997). multiplying by simple fission – and even fusing – indepen- dently of the host cell cycle. Excess mitochondria are removed Mitochondrial DNA by autophagic lysosomal activity. These population control measures act in response to the energetic demands of different Most cells in the body contain between 103 and 104 copies of tissues. The mitochondrial genome also has idiosyncrasies in mtDNA. There are much higher copy numbers (about 105) in RNA processing and in its genetic code that differ from those of mature oocytes. This may be in preparation for the energetic nuclear DNA (for example UGA is normally a stop codon, but demands of embryogenesis (Pikó and Matsumoto, 1976) but © 1998 Journals of Reproduction and Fertility 1359-6004/98 $12.50 Downloaded from Bioscientifica.com at 09/24/2021 01:14:10PM via free access Mitochondrial DNA in the life cycle 173 Table 1. Fate of light and heavy chain transcripts from mitochondrial DNA Displaced heavy strand (D-loop) Light chain transcripts Heavy chain transcripts Heavy strand Nascent heavy origin strand 8 tRNAs 14 tRNAs 1 mRNA 12 mRNAS Heavy strand RNA primers for heavy chain replication 2 tRNAs promoter Transcription Modified from Shadel and Clayton, 1997 factor binding Light strand sites promoter an alternative explanation is that replication does not occur during early embryogenesis and that high copy numbers are needed to give a sufficient reservoir (see below). The DNA Light strand origin exists mainly as a circular molecule of approximately 16.6 kb, (stem-loop) encoding 13 proteins that are transcribed and translated in the mitochondrion (Table 1). These are essential subunits of the electron transport complexes on the inner mitochondrial mem- brane. The mitochondrial genome also encodes the RNA mol- ecules that are necessary for the translation of these proteins (Table 1) (Lodish et al., 1995; Shadel and Clayton, 1997). mtDNA structure Fig. 1. This diagram summarizes the major features of mtDNA re- ferred to in the text. (See Shadel and Clayton, 1997.) The separate strands of the mtDNA molecule differ in buoy- ant density; the heavier ‘H-strand’ has a higher G + T content than the light ‘L-strand’. Transcription occurs simultaneously and in opposite directions and many genes overlap. By con- Box 1 Major mitochondrial import proteins vention mtDNA is depicted as a circle (Fig. 1), but alternatives, such as dimer loops and catenated (chain-linked) circles, are Mitochondrial DNA and RNA polymerases known (Clayton, 1982). In some single-celled organisms (many Transcription, translation and transcription termination factors RNA processing enzymes pathogenic to mammals) aberrant linear mtDNA molecules Mitochondrial ribosomal proteins with telomere-like endings are also found (Nosek et al., 1998). Aminoacyl-tRNA synthetases There is a specialised, somewhat unstable and hypervariable region called the D-(displacement) loop, where there is a triplex DNA structure at the site of origin of the H strand (Fig. 1). This structure is formed by a short nascent H-strand that remains closely associated with the parental molecule. This region is crit- Neupert, 1997). Besides structural components of the mitochon- ical for the initiation of transcription and translation (see below). dria, these imports involve factors that regulate and specifically The mammalian mitochondrial genome is extremely com- recognize mtDNA and regulate gene expression (Box 1). pressed with no introns. Some genes even overlap. This con- trasts strikingly with, for example, yeast (five times larger at Nuclear–mitochondrial interactions 78 000 bp) or plants, in which there are multiple recombining molecules with enormous size variation even within a family. Any alterations that arise in the components of the mtDNA For example, sizes vary from 330 000 bp in watermelons to or RNA that recognize or bind to nuclear-encoded regulatory 2.5 + 106 bp in muskmelon (Lodish et al., 1995). Most of the 100 elements must be balanced by compensatory mutations in the or more genes controlling the synthesis of mammalian mito- nuclear genes, as the mitochondrial genome mutates much chondrial proteins have moved to the nucleus over the course more rapidly than the nuclear genome (see below). This mu- of evolution. tuality is thought to drive species specificity in nuclear– mitochondrial interaction (Kenyon and Moraes, 1997; Wallace, 1997), but surprisingly little is known about the coordination of Import–export mechanisms expression between the nuclear and mitochondrial genomes. Proteins are assembled on cytoplasmic ribosomes and trans- However, it is clearly a sensitive system. Nagao et al. (1998) ported into the mitochondrion through protein-lined channels, found decreased physical performance and growth rates in with the aid of cytoplasmic