Spermatogenesis generates functional sperm cells from initially undifferentiated germ cells. This involves the proliferation of spermatogonia, meiosis of spermatocytes and the differentiation of spermatids into spermatozoa. It is a complex developmental program in which myriad events take place to ensure that the germ cells reach their proper stages of development at the appropriate times. Normal spermatogenesis requires a well-regulated balance of several processes, including cell proliferation, differentiation and apoptosis.
Apoptosis is a key phenomenon during spermatogenesis. For instance, an early, massive wave of germ cell apoptosis occurs at puberty. This event takes place during postnatal weeks 2 to 4 in mice, with a peak after 3 weeks [1–3]. It is estimated that 75% of spermatogenic cells undergo apoptosis during development [4, 5], ensuring the maintenance of a critical ratio between maturing germ cells and Sertoli cells [2, 6]. Sporadic apoptosis also occurs, primarily in spermatogonia and spermatocytes , eliminating defective germ cells with mutated DNA .
The Rhomboid family comprises polytopic membrane proteins, which may be the most widely-conserved membrane proteins identified to date . They share conserved biochemical properties in all biological kingdoms. Rhomboid proteases, which have been well studied in Drosophila, appear to regulate EGF receptor signalling pathways, thereby controlling growth and development [9, 10]. In addition, some yeast Rhomboid proteases have been found to play important roles in mitochondrial membrane remodelling , while some parasite proteases containing a Rhomboid domain are important for invasiveness .
Vertebrate rhomboid genes have been grouped into three classes: (1) active cellular rhomboids, including RHBDL1, RHBDL2, RHBDL4 and RHBDD1 ; (2) inactive cellular rhomboids, including RHBDL5 and RHBDL6; (3) a mitochondrial rhomboid, PARL . However, the physiological functions of these proteases remain to be clarified. Recently, RHBDL2 was found to cleave thrombomodulin and ephrin B3 [15, 16]. An exciting finding in this field was the discovery of PARL (Presenilin-associated rhomboid-like), the function of which appears to be associated with apoptosis [17, 18]. Drosophila Rhomboid 7 is believed to be associated with spermatogenesis, as male mutants tend to be afflicted with familial sterility .
mRHBDD1 is the first Rhomboid serine protease known to be involved in mammalian spermatogenesis. We found that RHBDD1, which contains a Rhomboid domain, is highly expressed in testes and is involved in regulating apoptosis . In the present study, the roles of mRHBDD1 in germ cell apoptosis and mouse spermatogenesis were investigated. We found that it is involved in regulating apoptosis in a mouse spermatagonia cell line, GC-1, and is essential for their capacity to differentiate into spermatids in mouse seminiferous tubules.