In marine fish producing pelagic eggs, the acquirement of buoyancy by the e
ggs through the hydration process is a key event of reproduction; moreover,
the yolk proteolysis, which leads to buoyancy, seems to affect the fertili
ty and survival of the spawned eggs. Recently we demonstrated that cathepsi
n D is the aspartic protease responsible for this intraoocytic processing o
f vitellogenin into yolk proteins. In the present study, we isolated, clone
d, and sequenced the cDNA encoding cathepsin D and studied expression of th
e message by Northern blotting and whole-mount in situ hybridization.
The full-length seabream cathepsin D cDNA is 1837 base pairs long, encoding
a protein of 400 amino acids (aa) consisting of a signal peptide of 19 aa,
a prosequence of 44 aa, and a mature peptide of 336 aa. An absolute sequen
ce conservation at the aspartyl residues (+33 and +221) was found, and ther
e are three potential N-glycosylation sites at +70, aa +189, and aa +274. T
he aa sequence of seabream cathepsin D reveals a high degree of sequence si
milarity with cathepsin D mRNAs from other organisms (73% sequence homology
to mouse and rat, 72% to human and trout, 69% to chicken, 66% to pig, and
65% to Xenopus).
The cathepsin D mRNA in floating eggs was present as a single band that was
approximately 1.9 kilobases (kb) in size, while in the sinking eggs there
were several fast-migrating bands (size range 1.3-0.2 kb). Whole-mount hybr
idization was used to investigate transcription of cathepsin D in the devel
oping embryo; during the hatching period, cathepsin D mRNA-positive cells w
ere distributed in a wide region between the trunk and the tail, and in the
ventral region over the yolk ball. The highest levels of cathepsin D enzym
atic activity were found in the sinking eggs and during the hatching period
of embryonic development.
These data suggest that cathepsin D can be considered a possible marker for
egg quality.