Expression analysis of mammalian mitochondrial ribosomal protein genes

Abstract

Mitochondrial ribosomal proteins (MRPs) are essential components for the structural and functional integrity of the mitoribosome complex. Throughout evolution, the mammalian mitoribosome has acquired new Mrp genes to compensate for loss of ribosomal RNA. More than 80 MRPs have been identified in mammals. Here we document expression pattern of 79 Mrp genes during mouse development and adult tissues and find that these genes are consistently expressed throughout early embryogenesis with little stage or tissue specificity. Further investigation of the amino acid sequence reveals that this group of proteins has little to no protein similarity. Recent work has shown that the majority of Mrp genes are essential resulting in early embryonic lethality, suggesting no functional redundancy among the group. Taken together, these results indicate that the Mrp genes are not a gene family descended from a single ancestral gene, and that each MRP has unique and essential role in the mitoribosome complex. The lack of functional redundancy is surprising given the importance of the mitoribosome for cellular and organismal viability. Further, these data suggest that genomic variants in Mrp genes may be causative for early pregnancy loss and should be evaluated as clinically.

Keywords: MRP; Mitochondria; Mitoribosome complex; Mrpl1; Mrpl10; Mrpl11; Mrpl12; Mrpl13; Mrpl14; Mrpl15; Mrpl16; Mrpl17; Mrpl18; Mrpl19; Mrpl2; Mrpl20; Mrpl21; Mrpl22; Mrpl23; Mrpl24; Mrpl27; Mrpl28; Mrpl3; Mrpl30; Mrpl32; Mrpl33; Mrpl34; Mrpl35; Mrpl36; Mrpl37; Mrpl38; Mrpl39; Mrpl4; Mrpl40; Mrpl41; Mrpl42; Mrpl43; Mrpl44; Mrpl45; Mrpl46; Mrpl47; Mrpl48; Mrpl49; Mrpl50; Mrpl51; Mrpl52; Mrpl53; Mrpl54; Mrpl55; Mrpl56; Mrpl57; Mrpl58; Mrpl59; Mrpl9; Mrps10; Mrps11; Mrps12; Mrps14; Mrps15; Mrps16; Mrps17; Mrps18a; Mrps18b; Mrps18c; Mrps2; Mrps21; Mrps22; Mrps23; Mrps24; Mrps25; Mrps26; Mrps27; Mrps28; Mrps29; Mrps30; Mrps31; Mrps33; Mrps34; Mrps35; Mrps36; Mrps37; Mrps38; Mrps39; Mrps5; Mrps6; Mrps7; Mrps9.

Figure 1.

79 Mrp genes are widely expressed throughout early embryo development and in adult mouse tissues. (A-B) RT-PCR gene expression patterns of the 50 Mrpl (A) and 29 Mrps (B) in MII oocytes, E0.5 zygotes, blastocysts and gastrulation stage embryos (E6.5-E8.5), as well as kidney, liver, heart, brain and testes. Gapdh was used as control. “oo” denotes oocytes, “zy” denotes zygotes, “bl” denotes blastocysts, “WE” denotes whole embryos, “emb” denotes embryonic portion, “exe” denotes extraembryonic portion, and “ys” denotes yolk sac. “NTC” denotes no template control. “*” denotes null embryos that are arrested at the pre-gastrulation stage (E6.5); “§” denotes null embryos that are likely to result null embryos that are arrested at E6.5 (they cannot be found at E9.5); “¶” denotes null embryos are not found at E12.5; and “^” denotes mouse knockout strains that failed to produce viable null mouse pups at weaning age but were not examined earlier.

Figure 2.

95% lethality in Mrp knockouts. 22 Mrp genes have been deleted thus far in the mouse. 12 Mrpl (Mrpl3, -l12, -l18, -l22, -l23, -l33, -l44, -l47, -l51, -l56, -l58, and -l59) and 10 Mrps (Mrps5, -s12, -s18c, -s21, -s22, -s25, -s29, -s37, -s38, and -s39). Among these Mrp alleles, seven (Mrpl3, -l22, -l44, -l59, -s18c, -s22, and -s25) result in null embryos that are arrested at the pre-gastrulation stage (E6.5); six more (Mrpl12, -l47, -l51, -s5, -s12, and -s38) are likely to result null embryos that are arrested at E6.5 (they cannot be found at E9.5); Mrpl18, -l33 and -s29 null embryos are not found at E12.5; and five strains (Mrpl23, -l58, -s21, -s37, and -s39) failed to produce viable null mouse pups at weaning age but were not examined earlier. Mrpl56 is the only strain that generated viable homozygous knockout animals. Data extracted from IMPC and other studies.