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Crisp proteins appear to play multiple roles in the life history of sperm. One of these roles is to act as a sperm chemoattractant. Allurin, a 21 kDa Crisp protein rapidly released from the egg jelly of at least two frogs, X. laevis and X. tropicalis, elicits directed motility in both homospecific and heterospecific sperm. In X. tropicalis, allurin is coded for by the newly documented Crisp A gene. Recently, the observation that allurin can also elicit chemotaxis in mouse sperm raises the question of whether allurin-like proteins might act as sperm chemoattractants in mammals. Although an allurin gene has yet to be documented in mammals, Crisp proteins truncated post-translationally appear to exist in both the male and female reproductive tract of mammals.
Fig. 4. Secretion of allurin in the frog oviduct and its application to the egg. (A) The X.
laevis oviduct consists of tubular jelly secreting glands capped by arches of epithelial cells
that face the oviduct lumen. Below the arches, cells that synthesize and secrete jelly
glycoconjugates are filled with large, strongly basophilic granules. The cells of each tubular
gland fan out from a central duct that empties into the lumen at the crevices between the
epithelial arches. Bar, 25 μm. (B) Each arch contains at its center a capillary loop filled with
blood cells. Covering the capillary is a single layer of ciliated epithelial cells and interspersed
among these cells are occasional secretory cells that have clusters of lightly stained granules
(asterisks). These are thought to be the allurin producing cells. Bar, 10 μm. From Chandler
and Roberson, 2008 with copyright permission from Jones and Bartlett Publishers (C)
Electron micrograph of cilia emerging at the apical plasma membrane of arch epithelial cells.
Bar, 1 μm. (D) Immunocytochemical localization of allurin at the ultrastructural level. Arrows
point to colloidal gold beads marking the presence of allurin on the extracellular surface of
the cilia. Bar, 0.4 μm. (E) Immunocytochemical localization of allurin as it is applied to an egg
passing down the oviduct. The secondary antibody signal (red) is superimposed on a
transmitted light image of the egg and oviductal epithelium. Heavy deposits of allurin on the
ciliary border are mixed with jelly and �brushed� onto the egg. Bar, 10 μm. (F) In the freshly
spawned egg, allurin has become concentrated in the outmost (J3) jelly layer poised to be
released into the medium. Bar, 100 μm. From Xiang et al., Dev. Biol. 275:343-355 (2004) with
copyright permission from Elsevier Science.
Fig. 5. Sperm responses to allurin. (A) Release of
allurin from the egg into the surrounding medium
occurs rapidly with a half time of 4 minutes (open
circles). Release appears to involve both diffusion
and convection. Based on computer modeling, diffusion
alone is slower (solid triangles; half time 20
minutes), intermittent convection faster (solid squares,
half time 10 minutes) and continuous mixing best
approximating allurin release (solid circles, half time 4
minutes). From Xiang et al., Mol. Reprod. Dev. 70:344-
60 (2005) with copyright permission from John Wiley
and Sons. (B) Binding of Oregon Green-conjugated
allurin to the midpiece and head of X. laevis sperm
(left). Very little binding is observed on the flagellum.
A phase contrast image of the same sperm is shown
on the right. Bar, 5 μm. (C,D) Trajectories of X. laevis
sperm surrounding a capillary filled with concentrated
egg water. Sperm turning toward the pipette
(solid lines) outnumber those turning away (dashed
lines) as shown quantitatively in panel (D). From Al-
Anzi and Chandler, Dev. Biol. 198:366-375. (1998) with copyright permission from Elsevier Science. (E) Allurin-mediated chemotaxis by X. laevis sperm
as measured in a two-chamber assay. Both solubilized egg jelly and egg water elicit ~5 fold greater sperm passage across a porous membrane than
do controls. Purified X. laevis allurin stimulates sperm passage to a similar extent while jelly depleted of allurin exhibits reduced activity.
crisp3 (cysteine-rich secretory protein 3) gene expression in sections of Xenopus laevis adult oviduct, NF stage 66, as assayed by immunohistochemistry. Red immunofluorescence is superimposed on bright field image.
