Structural basis for the differential interaction of Scribble PDZ domains with the guanine nucleotide exchange factor β-PIX

Scribble is a highly conserved protein regulator of cell polarity that has been demonstrated to function as a tumor suppressor or, conversely, as an oncogene in a context-dependent manner, and it also controls many physiological processes ranging from immunity to memory. Scribble consists of a leucine-rich repeat domain and four PDZ domains, with the latter being responsible for most of Scribble's complex formation with other proteins. Given the similarities of the Scribble PDZ domain sequences in their binding grooves, it is common for these domains to show overlapping preferences for the same ligand. Yet, Scribble PDZ domains can still exhibit unique binding profiles toward other ligands. This raises the fundamental question as to how these PDZ domains discriminate ligands and exert specificities in Scribble complex formation. To better understand how Scribble PDZ domains direct cell polarity signaling, we investigated here their interactions with the well-characterized Scribble binding partner β-PIX, a guanine nucleotide exchange factor. We report the interaction profiles of all isolated Scribble PDZ domains with a β-PIX peptide. We show that Scribble PDZ1 and PDZ3 are the major interactors with β-PIX and reveal a distinct binding hierarchy in the interactions between the individual Scribble PDZ domains and β-PIX. Furthermore, using crystal structures of PDZ1 and PDZ3 bound to β-PIX, we define the structural basis for Scribble's ability to specifically engage β-PIX via its PDZ domains and provide a mechanistic platform for understanding Scribble-β-PIX-coordinated cellular functions such as directional cell migration.

A Chemical Screening Approach to Identify Novel Key Mediators of Erythroid Enucleation

Erythroid enucleation is critical for terminal differentiation of red blood cells, and involves extrusion of the nucleus by orthochromatic erythroblasts to produce reticulocytes. Due to the difficulty of synchronizing erythroblasts, the molecular mechanisms underlying the enucleation process remain poorly understood. To elucidate the cellular program governing enucleation, we utilized a novel chemical screening approach whereby orthochromatic cells primed for enucleation were enriched ex vivo and subjected to a functional drug screen using a 324 compound library consisting of structurally diverse, medicinally active and cell permeable drugs. Using this approach, we have confirmed the role of HDACs, proteasomal regulators and MAPK in erythroid enucleation and introduce a new role for Cyclin-dependent kinases, in particular CDK9, in this process. Importantly, we demonstrate that when coupled with imaging analysis, this approach provides a powerful means to identify and characterize rate limiting steps involved in the erythroid enucleation process.

The polarity protein Scrib mediates epidermal development and exerts a tumor suppressive function during skin carcinogenesis

BACKGROUND

The establishment and maintenance of polarity is vital for embryonic development and loss of polarity is a frequent characteristic of epithelial cancers, however the underlying molecular mechanisms remain unclear. Here, we identify a novel role for the polarity protein Scrib as a mediator of epidermal permeability barrier acquisition, skeletal morphogenesis, and as a potent tumor suppressor in cutaneous carcinogenesis.

METHODS

To explore the role of Scrib during epidermal development, we compared the permeability of toluidine blue dye in wild-type, Scrib heterozygous and Scrib KO embryonic epidermis at E16.5, E17.5 and E18.5. Mouse embryos were stained with alcian blue and alizarin red for skeletal analysis. To establish whether Scrib plays a tumor suppressive role during skin tumorigenesis and/or progression, we evaluated an autochthonous mouse model of skin carcinogenesis in the context of Scrib loss. We utilised Cre-LoxP technology to conditionally deplete Scrib in adult epidermis, since Scrib KO embryos are neonatal lethal.

RESULTS

We establish that Scrib perturbs keratinocyte maturation during embryonic development, causing impaired epidermal barrier formation, and that Scrib is required for skeletal morphogenesis in mice. Analysis of conditional transgenic mice deficient for Scrib specifically within the epidermis revealed no skin pathologies, indicating that Scrib is dispensable for normal adult epidermal homeostasis. Nevertheless, bi-allelic loss of Scrib significantly enhanced tumor multiplicity and progression in an autochthonous model of epidermal carcinogenesis in vivo, demonstrating Scrib is an epidermal tumor suppressor. Mechanistically, we show that apoptosis is the critical effector of Scrib tumor suppressor activity during skin carcinogenesis and provide new insight into the function of polarity proteins during DNA damage repair.

CONCLUSIONS

For the first time, we provide genetic evidence of a unique link between skin carcinogenesis and loss of the epithelial polarity regulator Scrib, emphasizing that Scrib exerts a wide-spread tumor suppressive function in epithelia.

Dissecting the role of polarity regulators in cancer through the use of mouse models

Loss of cell polarity and tissue architecture is a hallmark of aggressive epithelial cancers. In addition to serving as an initial barrier to tumorigenesis, evidence in the literature has pointed towards a highly conserved role for many polarity regulators during tumor formation and progression. Here, we review recent developments in the field that have been driven by genetically engineered mouse models that establish the tumor suppressive and context dependent oncogenic function of cell polarity regulators in vivo. These studies emphasize the complexity of the polarity network during cancer formation and progression, and reveal the need to interpret polarity protein function in a cell-type and tissue specific manner. They also highlight how aberrant polarity signaling could provide a novel route for therapeutic intervention to improve our management of malignancies in the clinic.

Differential coding potential of ADAM22 mRNAs

ADAM22 is one of three catalytically inactive ADAM family members highly expressed in the brain. Preliminary functional studies suggest possible roles in epilepsy and myelination. We report an additional eight new splice variants of human ADAM22. Analysis of the altered splicing patterns of ADAM22 mRNAs in glioma allows us to suggest alternate splicing patterns in normal brain compared to glioma may represent differential use of exon 32. We also report diversity in the 5' leader sequences of ADAM22 mRNAs as a consequence of alternate transcriptional initiation sites. ADAM22 has an additional transcriptional initiation element producing transcripts lacking the exon 1 sequence including the signal peptide. Variable transcriptional initiation in exon 1 produces a range of ADAM22 5' leader sequence lengths, all of which are significantly longer than those described in NCBI reference sequences. Longer 5' leader sequences contain a second upstream AUG codon which acts to inhibit ADAM22 translation.

Efficient ADAM22 surface expression is mediated by phosphorylation-dependent interaction with 14-3-3 protein family members

ADAM22 is one of three catalytically inactive ADAM family members highly expressed in the brain. ADAM22 has numerous splice variants, all with considerable cytoplasmic tails of up to 148 amino acids. ADAM22 can act to inhibit cell proliferation, however, it has been suggested that it also acts as an adhesion protein. We identified three 14-3-3 protein members by a yeast two-hybrid screen and show by co-immunoprecipitation that the cytoplasmic domain of ADAM22 can interact with all six 14-3-3 proteins expressed in the brain. In addition, we show that 14-3-3 proteins interact preferentially with the serine phosphorylated precursor form of ADAM22. ADAM22 has two 14-3-3 protein binding consensus motifs; the first binding site, spanning residues 831-834, was shown to be the most crucial for 14-3-3 binding to occur. The interaction between ADAM22 and 14-3-3 proteins is dependent on phosphorylation of ADAM22, but not of 14-3-3 proteins. ADAM22 point mutants lacking functional 14-3-3 protein binding motifs could no longer accumulate efficiently at the cell surface. Deletion of both 14-3-3 binding sites and newly identified ER retention motifs restored localization of ADAM22 at the cell surface. These results reveal a role for 14-3-3 proteins in targeting ADAM22 to the membrane by masking ER retention signals.