Electron Microscopic Radioautographic Study on Protein Synthesis in Mitochondria of Binucleate Hepatocytes of Aging Mice

Tetsuji Nagata

doi:10.1100/tsw.2007.140

Abstract

In order to study the aging changes of intramitochondrial protein synthesis in mouse hepatocytes, 10 groups of aging mice, each consisting of three individuals, total 30, from fetal day 19 to postnatal year 2, were injected with 3H-leucine, a protein precursor, sacrificed 1 h later, and the liver tissues processed for electron microscopic radioautography. On electron microscopic radioautograms obtained from each animal, the numbers of mitochondria, the numbers of labeled mitochondria, and the mitochondrial labeling index labeled with 3H-leucine that showed protein synthesis in each hepatocyte, both mononucleate and binucleate cells, were counted and the averages in respective aging groups were compared. From the results, it was demonstrated that the numbers of mitochondria, the numbers of labeled mitochondria, and the labeling indices of intramitochondrial protein syntheses in both mononucleate and binucleate hepatocytes of mice at various ages increased due to development of animals. The numbers of mitochondria, the numbers of labeled mitochondria, and the labeling indices of intramitochondrial protein synthesis in binucleate hepatocytes were more than those of mononucleate hepatocytes at the same aging stages.

Highlights

Intramitochondrial nucleic acid syntheses, both DNA and RNA, in mammalian and avian cells were first demonstrated morphologically by the present author by means of electron microscopic radioautography with accurate localization in primary cultured cells of the livers and kidneys of mice and chickens in vitro[1,2], and in some other established cell lines, such as HeLa cells[3,4,5,6] or mitochondrial fractions prepared from in vivo cells[7,8,9]
With regard to the macromolecular synthesis in various cells in various organs of experimental animals as observed by light and electron microscopic radioautography, it is well known that the silver grains demonstrate DNA, RNA, and protein syntheses due to the radiolabeled precursors, such as 3H-thymidine, 3H-uridine, and 3H-leucine, respectively[1,2,3,22,43,44,48]
The present results obtained from the livers of aging mice revealed that the incorporation of 3H-leucine, indicating protein synthesis, resulted in silver grain localization over the nuclei and cell bodies of almost all binucleate hepatocytes from newborn postnatal days 1, 3, and 14 to adult and senescent stages at postnatal months 1, 2, 6, 12, and 24, which showed the localization of newly synthesized proteins in the nuclei and the cytoplasmic cell organelles including mitochondria

Summary

Introduction

Intramitochondrial nucleic acid syntheses, both DNA and RNA, in mammalian and avian cells were first demonstrated morphologically by the present author by means of electron microscopic radioautography with accurate localization in primary cultured cells of the livers and kidneys of mice and chickens in vitro[1,2], and in some other established cell lines, such as HeLa cells[3,4,5,6] or mitochondrial fractions prepared from in vivo cells[7,8,9]. This paper deals with the relationship between the protein synthesis and aging in hepatocytes of mice in vivo at various ages by means of electron microscopic radioautography as a part of serial studies on special cytochemistry[47] and radioautographology[48]

Methods

Results

Conclusion