Molecular cloning of a novel alternatively spliced nuclear protein

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 5: intercellular junctions and contacts between germs cells and Sertoli cells and their regulatory interactions, testicular cholesterol, and genes/proteins associated with more than one germ cell generation

In the testis, cell adhesion and junctional molecules permit specific interactions and intracellular communication between germ and Sertoli cells and apposed Sertoli cells. Among the many adhesion family of proteins, NCAM, nectin and nectin-like, catenins, and cadherens will be discussed, along with gap junctions between germ and Sertoli cells and the many members of the connexin family. The blood-testis barrier separates the haploid spermatids from blood borne elements. In the barrier, the intercellular junctions consist of many proteins such as occludin, tricellulin, and claudins. Changes in the expression of cell adhesion molecules are also an essential part of the mechanism that allows germ cells to move from the basal compartment of the seminiferous tubule to the adluminal compartment thus crossing the blood-testis barrier and well-defined proteins have been shown to assist in this process. Several structural components show interactions between germ cells to Sertoli cells such as the ectoplasmic specialization which are more closely related to Sertoli cells and tubulobulbar complexes that are processes of elongating spermatids embedded into Sertoli cells. Germ cells also modify several Sertoli functions and this also appears to be the case for residual bodies. Cholesterol plays a significant role during spermatogenesis and is essential for germ cell development. Lastly, we list genes/proteins that are expressed not only in any one specific generation of germ cells but across more than one generation.

Identification of a novel estrogen receptor-alpha variant and its upstream splicing regulator

Alternative splicing of precursor mRNA is a fundamental mechanism to generate multiple proteins from a single gene. Although constitutive and alternative mRNA splicing is temporally and spatially regulated, deregulation of mRNA splicing could cause development, progression, and metastasis of tumors. Through yeast two-hybrid screening of a human breast cDNA library using estrogen receptor-alpha (ERalpha) as bait, we identified a novel nuclear receptor box containing full-length protein, nuclear protein E3-3 (NPE3-3). Our results revealed that NPE3-3 associates with not only ERalpha but also with splicing factors, serine/arginine-rich protein (SRp)-30c, SRp40, and splicing factor SC-35, suggesting that NPE3-3 is likely to be involved in regulation of mRNA splicing. Accordingly, transient expression of NPE3-3 in cells resulted in expected splicing of the CD44 control minigene. We also discovered that NPE3-3-overexpressing clones produced a novel, previously unrecognized, alternatively spliced variant of ERalpha (termed ERalphaV), which had a molecular size of 37 kDa composed of only exons 1, 2, 7, and 8. ERalphaV was expressed and sequestered in the cytoplasm in MCF-7 cells stably overexpressing NPE3-3, suggesting its involvement in nongenomic hormone signaling. NPE3-3 clones exhibited up-regulation of ERK1/2 signaling, cyclin D1, and cathepsin D and enhanced tumor cell proliferation, migration, and tumorigenicity. Moreover, direct expression of the ERalphaV in breast cancer cells stimulated ERK1/2 up-regulation and cyclin D1 expression. We found that ERalphaV physically interacted with MAPK kinase (MEK)-1/2, and thus, an ERalphaV and MEK1/2 complex could lead to the activation of the ERK1/2 pathway. Interestingly, NPE3-3 was up-regulated in human breast tumors. These findings revealed a role for NPE3-3 in alternative splicing and suggest that ERalpha is a physiological target of NPE3-3, leading to a constitutive nongenomic signaling pathway in breast cancer cells.

The interaction and colocalization of Sam68 with the splicing-associated factor YT521-B in nuclear dots is regulated by the Src family kinase p59(fyn)

Alternative pre-mRNA splicing patterns can change an extracellular stimulus, but the signaling pathways leading to these changes are still poorly characterized. Here, we describe a tyrosine-phosphorylated nuclear protein, YT521-B, and show that it interacts with the nuclear transcriptosomal component scaffold attachment factor B, and the 68-kDa Src substrate associated during mitosis, Sam68. Northern blot analysis demonstrated ubiquitous expression, but detailed RNA in situ analysis revealed cell type specificity in the brain. YT521-B protein is localized in the nucleoplasm and concentrated in 5-20 large nuclear dots. Deletion analysis demonstrated that the formation of these dots depends on the presence of the amino-terminal glutamic acid-rich domain and the carboxyl-terminal glutamic acid/arginine-rich region. We show that the latter comprises an important protein-protein interaction domain. The Src family kinase p59(fyn)-mediated tyrosine phosphorylation of Sam68 negatively regulates its association with YT521-B, and overexpression of p59(fyn) dissolves nuclear dots containing YT521-B. In vivo splicing assays demonstrated that YT521-B modulates alternative splice site selection in a concentration-dependent manner. Together, our data indicate that YT521-B and Sam68 may be part of a signal transduction pathway that influences splice site selection.