Mitochondrial gene expression in male germ cells of the mouse

Inheritance of the mitochondrial genome is known to be exclusively maternal. To determine whether the loss of paternal mitochondria could be due to a deficiency of RNA in the spermatozoal mitochondria, the expression of mitochondrial genes was studied in testicular cells at various stages of spermatogenesis and in epididymal spermatozoa. The presence of mitochondrial transcripts was examined by Northern blot analysis using probes for the following mitochondrially encoded genes: 12 S and 16 S ribosomal RNAs and a group of mRNAs including cytochrome oxidase subunits I and II (COI-COII), cytochrome b (cyt b), adenosine triphosphatase (ATPase) subunits 6 and 8, and subunit 1 of the respiratory chain NADH dehydrogenase (ND1). Comparison of total testicular RNA preparations from prepuberal (6, 8, 12, 16, 18, 20, 22, and 30 days old) and sexually mature (45 days old) mice revealed no major qualitative or quantitative differences in the levels of the mitochondrial transcripts described above. Similar results were observed from enriched preparations of type A and B spermatogonia and interstitial cells obtained from the testes of 8-day-old mice. Transcripts for COI-COII, ATPase 6, or ND1 were reduced in amount in the enriched preparations of pachytene spermatocytes, round spermatids, and residual bodies when compared to the amount in total testis or liver RNA. Transcripts of all the mitochondrial genes analyzed were present in RNA preparations isolated from sperm midpiece tails obtained after sonication of epididymal spermatozoa. These studies demonstrate that (a) during testicular development the levels of mitochondrial RNA in total testicular extracts show no major qualitative and quantitative differences; (b) the mitochondrial transcripts in enriched populations of type A and type B spermatogonia are not different from those obtained from total testes extracts; (c) mitochondrial transcript levels gradually decrease in enriched preparations of pachytene spermatocytes, round spermatids, and residual bodies; and (d) the mitochondrial rRNAs and mRNAs encoded by several mitochondrial genes can be isolated from sperm midpiece tails.