Enzymes involved in the biosynthesis of sphingolipids with very-long-chain PUFA concur with male germ cell differentiation

Abstract

The sphingolipids sphingomyelin (SM) and ceramide (Cer), of male rodent germ cells (spermatocytes, spermatids) and spermatozoa contain very-long-chain (C28-C32) polyenoic fatty acids (VLCPUFA) in nonhydroxy (n-V) and 2-hydroxy forms (h-V) not present in Sertoli cells. During postnatal development, SM and Cer species with n-V appear in testes concomitantly with pachytene spermatocytes and those with h-V with spermatids. Both are able to biosynthesize their own sphingolipids, the former more actively than the latter. The biosynthesis of n-V requires elongation of PUFA (20:4n-6, 22:5n-6) by elongation of very-long-chain fatty acid (Elovl) proteins, while that of h-V requires a fatty acid 2-hydroxylase (Fa2h). Elovl5 and Elovl2, coding for enzymes responsible for PUFA biosynthesis, and Elovl4, in turn responsible for the formation of PUFA longer than C26, are actively expressed in germ cells. The Elovl4 protein is exclusively expressed in germ cells in a stage-dependent manner, spermatocytes displaying the highest Elovl4 protein levels and enzymatic activity. Along with high proportion of h-V Cer and h-V SM species, the Fa2h protein is mainly concentrated in late spermatids, in the step of spermiogenesis in which they elongate and their heads change shape. Consistently, spermatocytes express the highest levels of ceramide synthase 3, required for the N-acylation of sphingosine with a VLCPUFA, while spermatids express the highest levels of SM synthase 2. Irrespective of the maturation stage, the small raftlike domains of the germ cell plasma membrane contain species of SM and Cer with saturated fatty acids, while those containing n-V and h-V abound in the large non-raft areas. These species, with unique physico-chemical properties, could play a structural role facilitating the germ cell shape changes associated with the progress of spermatogenesis. If released from such species, free n-V and h-V could be involved in germ cell differentiation as potential precursors of uncommon elovanoid-like bioactive derivatives.