Cathepsin L inhibitor I blocks mitotic chromosomes decondensation during cleavage cell cycles of sea urchin embryos

We have previously reported that sperm histones (SpH) degradation after fertilization is catalyzed by a cystein–protease(SpH- protease). Its inhibition blocks the degradation of SpH in vivo and also aborts sea urchin development at the initial embryonic cell cycles. It remains unknown if this effect is a consequence of the persistence of SpH on zygotic chromatin, or if this proteaseis involved per-se in the progression of the embryonic cell cycles. To discriminate among these two options we have inhibited this proteaseat a time when male chromatin remodelingwas completed and the embryos were engaged in the second cell cycle of the cleavage divisions. The role of this enzyme in cell cycle was initially analyzed by immuno-inhibiting its SpH degrading activity in one of the two blastomeresafter the initial cleavage division, while the other blastomerewas used as a control. We found that in the blastomereinjected with the anti-SpH- proteaseantibodies the cytokinesis was arrested, the chromatin failed to decondense after mitosisand BrdU incorporation into DNA was blocked. Since the N-terminal sequence and the SpH proteasewas homologous to the cathepsin L(Cat L) family of proteases, we subsequently investigated if the deleterious effect of the inhibition of this proteaseis related to its Cat L activity. In this context we analyzed the effect of Cat L inhibitor I (Z–Phe-Phe–CH2F) on embryonic development. We found that the addition of 100 uM of this inhibitor to the embryos harvested at the time of the initial cleavage division (80 min p.i.) mimics perfectly the effects of the immuno-inhibition of this enzyme obtained by microinjecting the anti-SpH– proteaseantibodies. Taken together these results indicate that the activity of this proteaseis required for embryonic cell cycle progression. Interestingly, we observed that when this proteasewas inhibited the chromatin decondensation after mitosiswas abolished indicating that the inhibition of this enzyme affects chromosomes decondensation after mitosis. J. Cell. Physiol. 216: 790–795, 2008, © 2008 Wiley-Liss, Inc.