Tài liệu Báo cáo khoa học: A possible role of mitochondria in the apoptotic-like programmed nuclear

A possible role of mitochondria in the apoptotic-like

programmed nuclear death of Tetrahymena thermophila

Takashi Kobayashi and Hiroshi Endoh

Division of Life Science, Graduate School of Natural Science and Technology, Kanazawa University, Japan

Mitochondria are known to play a major role in apop￾tosis or programmed cell death (reviewed in [1,2]).

Multiple cell death-associated factors have been identi￾fied in mitochondria. These factors may be divided into

three categories based on their functions: cyto￾chrome c, Smac ⁄ DIABLO, and Omi ⁄ HtrA2, all of

which are involved in caspase activation [3–7], while

apoptosis-inducing factor (AIF) and endonuclease G

(EndoG) are direct effectors of nuclear condensation

and DNA degradation [8,9]. The pro- and antiapoptotic

members of the Bcl-2 family proteins regulate loss of

mitochondrial inner membrane potential, which results

in the release of these apoptogenic factors [1,10]. The

involvement of mitochondria in apoptosis is common

among metazoans and plants [11]. Homologues of the

aforementioned mitochondrial apoptosis factors have

been identified even in protistans, such as the cellular

slime moulds and kinetoplastids [12,13]. Taking these

discoveries into consideration, the crucial role played

by mitochondria in apoptosis appears to have an early

evolutionary origin.

The ciliated protozoan Tetrahymena thermophila

undergoes a unique process during conjugation, i.e.

programmed nuclear degradation. Unicellular Tetra￾hymena has two morphologically and functionally dif￾ferent nuclei within the same cytoplasm. One is the

germinal micronucleus and the other is the somatic

macronucleus. These nuclei both originate from a ferti￾lized micronucleus (synkaryon) during conjugation

[14,15]. As the new macronuclei differentiate from the

synkaryon via two postzygotic nuclear divisions, the

parental macronucleus begins to degenerate, in a

Keywords

nuclear apoptosis; autophagosome;

endonuclease; mitochondria; Tetrahymena

Correspondence

T. Kobayashi, Institute for Molecular

Science of Medicine, Aichi Medical

University, Yazako, Nagakute, Aichi

480-1195, Japan

Fax: +81 561 63 3532

Tel: +81 561 62 3311 (ext. 2087)

E-mail: [email protected]

(Received 13 April 2005, revised 19 July

2005, accepted 24 August 2005)

doi:10.1111/j.1742-4658.2005.04936.x

The ciliated protozoan Tetrahymena has a unique apoptosis-like process,

which is called programmed nuclear death (PND). During conjugation, the

new germinal micro- and somatic macro-nuclei differentiate from a zygotic

fertilized nucleus, whereas the old parental macronucleus degenerates,

ensuring that only the new macronucleus is responsible for expression of

the progeny genotype. As is the case with apoptosis, this process encompas￾ses chromatin cleavage into high-molecular mass DNA, oligonucleosomal

DNA laddering, and complete degradation of the nuclear DNA, with the

ultimate outcome of nuclear resorption. Caspase-8- and caspase-9-like

activities are involved in the final resorption process of PND. In this report,

we show evidence for mitochondrial association with PND. Mitochondria

and the degenerating macronucleus were colocalized in autophagosome

using two dyes for the detection of mitochondria. In addition, an endo￾nuclease with similarities to mammalian endonuclease G was detected in

the isolated mitochondria. When the macronuclei were incubated with iso￾lated mitochondria in a cell-free system, DNA fragments of 150–400 bp

were generated, but no DNA ladder appeared. Taking account of the pre￾sent observations and the timing of autophagosome formation, we conclude

that mitochondria might be involved in Tetrahymena PND, probably with

the process of oligonucleosomal laddering.

Abbreviations

AIF, apoptosis-inducing factor; DAPI, 4,6-diamino-2-phenylindole; DePsipher, 5,5¢,6,6¢-tetrachloro-1,1¢,3,3¢-

tetraethylbenzimidazolylcarbocyanine iodide; EndoG, endonuclease G.

5378 FEBS Journal 272 (2005) 5378–5387 ª 2005 FEBS