Chromatin remodeling during spermiogenesis: molecular components involved in nuclear import of transition protein 2 (TP2) and role of acetylation in modulating TP2 function

Abstract

Spermatogenesis is a process in which spermatogonial stem cells undergo a series of biochemical and morphological changes resulting in the production of highly differentiated haploid cells called spermatozoa. The entire process of spermatogenesis can be divided into three phases: 1) stem cell renewal and differentiation, 2) meiosis and 3) spermiogenesis. In mammals, spermatogenesis is characterised by a unique chromatin remodeling process, in which somatic histones are sequentially replaced by testis specific variants, followed by the replacement of both somatic and testis specific histones with a class of basic proteins, transition proteins (TP1, TP2 and TP4). There are three types of spermatogonia or stem cells, the A type, Intype (intermediate) and the B-type, which are located in the periphery of seminiferous tubules. A-type cells that are undifferentiated (Courot et al., 1970; Clermont, 1977) undergo four successive divisions giving rise to cell types A1 to A4 in rat and mouse. The A4 cells undergo several rounds of division producing A0 or A1 and In-type cells. The In-type cells then undergo further division and differentiation giving rise to B-type spermatogonia. There is a progressive decrease in the size of the nucleus, followed by the appearance of dark, spherical structures with coarse granules attached to the nuclear membrane as the cells undergo differentiation. After successful mitotic division, the B-type spermatogonia produce preleptotene primary spermatocytes (Courot et al., 1970). Most of the meiotic DNA and nuclear protein synthesis takes place at this stage of preleptotene primary spermatocyte. After this synthesis phase the cells enter the prophase of the first meiotic division.