Lipocalins in bioscience: the first family gathering

Expression patterns and changes of the LCN2 gene in the testes of induced cryptorchidism and busulfan-treated mice

Lipocalin-2 (LCN2) was known to play various roles in different type cells; however, little was known about the effect of LCN2 in male fertility. In this study, we aimed to explore the expression pattern of LCN2 with increasing age in mice, and to obtain insight into the role of LCN2 in mice testes by induced cryptorchidism and busulfan-treated infertility. In situ hybridization showed that LCN2 was localized primarily in Leydig cells, but was absent in Sertoli and germ cells. Its expression in testes exhibited an age-related increase from day 1 to 8 months, then reduced by the twelth month. The mRNA and protein levels of LCN2 in the testes of both infertile models increased as measured by real-time PCR and western blotting, respectively. LCN2 mRNA and protein levels were higher (p<0.05) in busulfan treated mice than that of cryptorchidism. These observations have shown that LCN2 is developmentally regulated and highly expressed in the Leydig cells of mouse testes.

Structure of a heterogeneous, glycosylated, lipid-bound, in vivo-grown protein crystal at atomic resolution from the viviparous cockroach Diploptera punctata

Macromolecular crystals for X-ray diffraction studies are typically grown in vitro from pure and homogeneous samples; however, there are examples of protein crystals that have been identified in vivo. Recent developments in micro-crystallography techniques and the advent of X-ray free-electron lasers have allowed the determination of several protein structures from crystals grown in cellulo. Here, an atomic resolution (1.2 Å) crystal structure is reported of heterogeneous milk proteins grown inside a living organism in their functional niche. These in vivo-grown crystals were isolated from the midgut of an embryo within the only known viviparous cockroach, Diploptera punctata. The milk proteins crystallized in space group P1, and a structure was determined by anomalous dispersion from the native S atoms. The data revealed glycosylated proteins that adopt a lipocalin fold, bind lipids and organize to form a tightly packed crystalline lattice. A single crystal is estimated to contain more than three times the energy of an equivalent mass of dairy milk. This unique storage form of nourishment for developing embryos allows access to a constant supply of complete nutrients. Notably, the crystalline cockroach-milk proteins are highly heterogeneous with respect to amino-acid sequence, glycosylation and bound fatty-acid composition. These data present a unique example of protein heterogeneity within a single in vivo-grown crystal of a natural protein in its native environment at atomic resolution.

Signaling by retinol and its serum binding protein

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP) which, in turn, associates with transthyretin (TTR) to form a retinol-RBP-TTR ternary complex. At some tissues, retinol-bound (holo-) RBP is recognized by a membrane protein termed STRA6, which transports retinol from extracellular RBP into cells and, concomitantly, activates a JAK2/STAT3/5 signaling cascade that culminates in induction of STAT target genes. STRA6-mediated retinol transport and cell signaling are critically inter-dependent, and they both require the presence of cellular retinol-binding protein 1 (CRBP1), an intracellular retinol acceptor, as well as a retinol-metabolizing enzyme such as lecithin:retinol acyltransferase (LRAT). STRA6 thus functions as a "cytokine signaling transporter" which couples vitamin A homeostasis and metabolism to cell signaling, thereby regulating gene transcription. Recent studies provided molecular level insights into the mode of action of this unique protein.

New concepts and aids to facilitate crystallization

Novel tools and technologies are required to obtain structural information of difficult to crystallize complex biological systems such as membrane proteins, multiprotein assemblies, transient conformational states and intrinsically disordered proteins. One promising approach is to select a high affinity and specificity-binding partner (crystallization chaperone), form a complex with the protein of interest and crystallize the complex. Often the chaperone reduces the conformational freedom of the target protein and additionally facilitates the formation of well-ordered crystals. This review provides an update on the recent successes in chaperone-assisted crystallography. We also stress the importance of synergistic approaches involving protein engineering, crystallization chaperones and crystallization additives. Recent examples demonstrate that investment in such approaches can be key to success.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

The adult boar testicular and epididymal transcriptomes

Background

Mammalians gamete production takes place in the testis but when they exit this organ, although spermatozoa have acquired a specialized and distinct morphology, they are immotile and infertile. It is only after their travel in the epididymis that sperm gain their motility and fertility. Epididymis is a crescent shaped organ adjacent to the testis that can be divided in three gross morphological regions, head (caput), body (corpus) and tail (cauda). It contains a long and unique convoluted tubule connected to the testis via the efferent ducts and finished by joining the vas deferens in its caudal part.

Results

In this study, the testis, the efferent ducts (vas efferens, VE), nine distinct successive epididymal segments and the deferent duct (vas deferens, VD) of four adult boars of known fertility were isolated and their mRNA extracted. The gene expression of each of these samples was analyzed using a pig generic 9 K nylon microarray (AGENAE program; GEO accession number: GPL3729) spotted with 8931 clones derived from normalized cDNA banks from different pig tissues including testis and epididymis. Differentially expressed transcripts were obtained with moderated t-tests and F-tests and two data clustering algorithms based either on partitioning around medoid (top down PAM) or hierarchical clustering (bottom up HCL) were combined for class discovery and gene expression analysis. Tissue clustering defined seven transcriptomic units: testis, vas efferens and five epididymal transcriptomic units. Meanwhile transcripts formed only four clusters related to the tissues. We have then used a specific statistical method to sort out genes specifically over-expressed (markers) in testis, VE or in each of the five transcriptomic units of the epididymis (including VD). The specific regional expression of some of these genes was further validated by PCR and Q-PCR. We also searched for specific pathways and functions using available gene ontology information.

Conclusion

This study described for the first time the complete transcriptomes of the testis, the epididymis, the vas efferens and the vas deferens on the same species. It described new genes or genes not yet reported over-expressed in these boar tissues, as well as new control mechanisms. It emphasizes and fulfilled the gap between studies done in rodents and human, and provides tools that will be useful for further studies on the biochemical processes responsible for the formation and maintain of the epididymal regionalization and the development of a fertile spermatozoa.

Production of recombinant human alpha1-microglobulin and mutant forms involved in chromophore formation

Alpha(1)-Microglobulin, a 26 kDa lipocalin present in plasma and tissues, carries a set of unknown chromophores, bound to C34, K92, K118 and K130, which cause its charge and size heterogeneity. In man, the protein is found in two forms, full length and lacking the C-terminal tetrapeptide LIPR (t-alpha(1)-microglobulin), both which are heme-binding and the latter with heme-degrading properties. We report cloning and overexpression of full length alpha(1)-microglobulin (wt protein), t-alpha(1)-microglobulin (wtdeltaLIPR) and the mutants C34S, K(92,118,130)T and C34S/K(92,118,130)T, the latter subsequently abbreviated as K(3)T and C34S/K(3)T, in Escherichia coli. After purification and refolding from inclusion bodies, all proteins were correctly folded as determined by far-UV circular dichroism and radioimmunoassay. As revealed by gel filtration, recombinant alpha(1)-microglobulins had lower tendencies to form dimers than human plasma or urine analogues. All alpha(1)-microglobulin forms displayed higher amounts of the chromophore than bovine serum albumin but significantly lower than the human urine or plasma counterparts. Differences in the absorbance and fluorescence profiles are consistent with a model where the chromophore is formed by a series of reactions with heme or other chromophore precursors and where C34 is essential for binding of the ligand, K92, K118 and K130 are involved in transformation into the chromophore and LIPR inhibits the latter reaction.