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Possible Function of Caudal Nuclear Pocket: Degradation of Nucleoproteins by Ubiquitin–Proteasome System in Rat Spermatids and Human Sperm

Celina M. Haraguchi, Tadashi Mabuchi, Shuji Hirata, Tomoko Shoda, Toshinobu Tokumoto, Kazuhiko Hoshi and Sadaki Yokota

Biology Laboratory (CMH,SY), Department of Biochemistry (TM), and Department of Gynecology and Obstetrics (SH,TS,KH), Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi, Japan, and Faculty of Science, Department of Biology and Geosciences, Shizuoka University, Shizuoka, Japan (TT)

Correspondence to: Dr. Sadaki Yokota, Biology Laboratory, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Chuo-shi, Yamanashi 409-3898, Japan. E-mail: syokota{at}yamanashi.ac.jp

Many temporarily functioning proteins are generated during the replacement of nucleoproteins in the nuclei of late spermatids and seem to be degraded in the nucleus. This study was designed to clarify the involvement of the ubiquitin–proteasome degradation system in the nucleus of rat developing spermatids. Thus, we studied the nuclear distribution of polyubiquitinated proteins (pUP) and proteasome in spermiogenic cells and sperm using postembedding immunoelectron microscopy. We divided the nuclear area of late spermatids into two regions: (1) a dense area composed of condensed chromatin and (2) a nuclear pocket in the neck region. The latter was located in the caudal nuclear region and was surrounded by redundant nuclear envelope. We demonstrated the presence of pUP in the dense area and nuclear pocket, proteasome in the nuclear pocket, and clear spots in the dense area of rat spermatids. Using quantitative analysis of immunogold labeling, we found that fluctuation of pUP and proteasome levels in late spermatogenesis was mostly synchronized with disappearance of histones and transitional proteins reported previously. In the nuclei of human sperm, pUP was detected in the dense area, whereas proteasome was in the nuclear vacuoles and clear spots. These results strongly suggest that pUP occur in the dense nuclear area of developing spermatids and that the ubiquitin–proteasome system is more actively operational in the nuclear pocket than dense area. Thus, the nuclear pocket might be the degradation site for temporarily functioning proteins generating during condensation of chromatin in late spermatids. (J Histochem Cytochem 55:585–595, 2007)

Key Words: nucleoproteins • polyubiquitin • proteasomes • spermatogenesis • sperm • degradation sites • nuclear pocket • nuclear vacuoles

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