The Annulus of the Mouse Sperm Tail Is Required to Establish a Membrane Diffusion Barrier That Is Engaged During the Late Steps of Spermiogenesis1

Abstract

The annulus is a higher order septin cytoskeletal structure located between the midpiece and principal piece regions of the sperm tail. The annulus has been hypothesized to generate the diffusion barrier that exists between these two membrane domains. We tested this premise directly on septin 4 knockout mice, whose sperm are viable but lack an annulus, by following the diffusing membrane protein basigin. Basigin is normally confined to the principal piece domain on testicular and caput sperm, but undergoes relocation into the midpiece during sperm epididymal transit. On Sept4(-/-) sperm, domain confinement was lost, and basigin localized over the entire plasma membrane. Both immunofluorescence and immunoblotting further revealed reduced levels of basigin expression on sperm from the knockout. Testicular immunohistochemistry showed similar basigin expression and tail targeting in wild-type (WT) and Sept4(-/-) tubules until step 15 of spermatid development, at which point basigin was redistributed throughout the plasma membrane of Sept4(-/-) spermatids. The basigin outside of the tail was subsequently lost around the time of sperm release into the lumen. The redistribution in the knockout coincides with the time in WT sperm when the annulus completes its migration from the neck down to the midpiece-principal piece junction. We posit that basigin may not diffuse freely until after the annulus arrives at the midpiece-principal piece junction to restrict lateral movement. These results are the strongest evidence to date of a mammalian septin structure establishing a membrane diffusion barrier.

Relationship between absence of annulus and asthenozoospermia in Iranian men

“…The annulus is an electron-dense septinbased structure, which could function as a kind of physical barrier to lateral diffusion between the midpiece and principal piece regions of the sperm tail [2]. Thus, anomalous annuli that are secondarily disorganized by other genetic or nonhereditary etiologies could also accompany impaired motility [1].…”

Ultrastructure of Spermatozoa from Infertility Patients

“…The mid-piece and principal piece of the tail are separated by a ring structure known as the annulus (Figure 5e), which presumably performs a barrier function to prevent molecular diffusion between the two pieces [82].…”

Septins in kidney: A territory little explored

“…Septins are essential for forming this diffusion barrier Kwitny, Klaus, & Hunnicutt, 2010). Septins also form a ring at the base of dendritic spines in neurons, which is essential for dendrite branching and morphology (Xie et al, 2007).…”