Keratins: Unraveling the coordinated construction of scaffolds in spermatogenic cells

Recent work shows that two groups of keratins are expressed during mammalia n spermatogenesis. One group, belonging to the classic epidermis-type kerat ins, is present in spermatogonia, spermatocytes, and spermatids. A member o f this group, Sak57, a keratin 5 homologue, has been shown to co-align with microtubules and provide a scaffolding shell while also strengthening inte rcellular cytoplasmic bridges conjoining members of spermatogonial and sper matocyte cohorts. The other, keratin 9, is a component of the perinuclear r ing of the manchette, a microtubular structure developed during the elongat ion and condensation of the spermatid nucleus. The second group, the outer dense fiber (Odf) proteins, is expressed preferentially during mammalian sp ermiogenesis. The family of Odf proteins-Odf1, Odf2, and Odf3-includes an e xpanding group of proteins coassembled along the axoneme during the develop ment of the sperm tail. Investigations on the assembly of epidermis-type an d Odf sperm tail-targeted keratins are now focused on a group of chaperone- like Odf-binding molecules, designated Spags. Spags appear to drive Odfs to a precise destination. A daunting task is to determine how members of the family of keratins get the signal to produce linear scaffolds in specific s permatogenic cell populations and transport keratins to microtubule-contain ing structures such as the manchette and axoneme. Mol. Reprod. Dev, 61:1-2, 2002. (C) 2002 Wiley-Liss, Inc.