Keratins: unraveling the coordinated construction of scaffolds in spermatogenic cells

Recent work shows that two groups of keratins are expressed during mammalian spermatogenesis. One group, belonging to the classic epidermis-type keratins, is present in spermatogonia, spermatocytes, and spermatids. A member of this group, Sak57, a keratin 5 homologue, has been shown to co-align with microtubules and provide a scaffolding shell while also strengthening intercellular cytoplasmic bridges conjoining members of spermatogonial and spermatocyte cohorts. The other, keratin 9, is a component of the perinuclear ring of the manchette, a microtubular structure developed during the elongation and condensation of the spermatid nucleus. The second group, the outer dense fiber (Odf) proteins, is expressed preferentially during mammalian spermiogenesis. The family of Odf proteins-Odf1, Odf2, and Odf3-includes an expanding group of proteins co-assembled along the axoneme during the development of the sperm tail. Investigations on the assembly of epidermis-type and 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 spermatogenic cell populations and transport keratins to microtubule-containing structures such as the manchette and axoneme.