Wilhelm bernhard workshop on the cell nucleus

ROLE OF THE NUCLEOLUS IN HUMAN REPRODUCTION

Kittur N.¹, Bazett-Jones D.P² and Meier U.T. ¹

¹Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York, USA; ²The Hospital For Sick Cbildren, University of Toronto, Ontario, Canada
The ultrastructurally well-defined nucleolar channel system (NCS) consists of intranuclear stacks of membrane cisternae embedded in an electron dense matrix. It is specific to the buman endometrial epithelium where it appears transiently post-ovulation, coincident with the height of receptivity. Molecularly, the NCS is uncharacterized. By overexpression of the nucleolar chaperone Noppl40 we induced intranuclear membrane systems (R-rings) in tissue culture cells that are indistinguishable from the NCS. Using histo-chemical labeling for the endoplasmic reticulum (ER) marker enzyme glucose-6-phosphatase, we show that the NCS, like the R-rings, consists of bona fide ER. Not only is this the first labeling of the NCS but it also suggests molecular identity of NCS and R-rings. Noppl40 localizes to the R-rings and apparently the NCS implicating this protein as a key player in NCS/R-ring formation Indeed, we provide evidence that NCS/ring biogenesis is initiated by a calcium-mediated Nopp140-membrane interaction. Thus, Noppl40 binds calcium and complexed calcium is enriched between the membrane stacks of the NCS/R-rings as determined by electron spectroscopic imaging. Moreover, calcium chelators delay R-ring formation. A direct role for the NCS in human reproduction is supported by our observation that the NCS occurs in and adjacent to cells exhibiting pinopodes, apical membrane protrusions thought to be involved in blastocyst attachment. Specifically, we speculate that the NCS affects the localization and/or expression of integral membrane proteins (e.g., by sequestering them within the nucleus) and thereby locally creating an epithelial cell surface that is favorable for blastocyst attachment. Indeed, CLIMP 63 and p58/ERGIC53, two integral membrane proteins, are misrouted to the R-rings. More importantly, expression of the plasma membrane marker, caveolin-1, is reduced on the surface of R-ring positive cells. Therefore, in the special case of human endometrium, the omnipresent nucleolus or some of its components may be crucial for reproduction.