Cell polarity plays a key role in the development of the central nervous system (CNS). Interestingly, disruption of cell polarity is seen in many cancers. ASPP2 is a haplo-insufficient tumor suppressor and an activator of the p53 family. In this study, we show that ASPP2 controls the polarity and proliferation of neural progenitors in vivo, leading to the formation of neuroblastic rosettes that resemble primitive neuroepithelial tumors. Consistent with its role in cell polarity, ASPP2 influences interkinetic nuclear migration and lamination during CNS development. Mechanistically, ASPP2 maintains the integrity of tight/adherens junctions. ASPP2 binds Par-3 and controls its apical/junctional localization without affecting its expression or Par-3/aPKC lambda binding. The junctional localization of ASPP2 and Par-3 is interdependent, suggesting that they are prime targets for each other. These results identify ASPP2 as a regulator of Par-3, which plays a key role in controlling cell proliferation, polarity, and tissue organization during CNS development.

NPM/ALK is an oncogenic fusion protein expressed in ϳ50% of anaplastic large cell lymphoma cases. It derives from the t(2; 5)(p23;q35) chromosomal translocation that fuses the catalytic domain of the tyrosine kinase, anaplastic lymphoma kinase (ALK), with the dimerization domain of the ubiquitously expressed nucleophosmin (NPM) protein. Dimerization of the ALK kinase domain leads to its autophosphorylation and constitutive activation. Activated NPM/ALK stimulates downstream survival and proliferation signaling pathways leading to malignant transformation. Herein, we investigated the molecular mechanisms of autoactivation of the catalytic domain of ALK. Because kinases are typically regulated by autophosphorylation of their activation loops, we systematically mutated (Tyr 3 Phe) three potential autophosphorylation sites contained in the "YXXXYY" motif of the ALK activation loop, and determined the effect of these mutations on the catalytic activity and biological function of NPM/ALK. We observed that mutation of both the second and third tyrosine residues (YFF mutant) did not affect the kinase activity or transforming ability of NPM/ALK. In contrast, mutation of the first and second (FFY), first and third (FYF), or all three (FFF) tyrosine residues impaired both kinase activity and transforming ability of NPM/ALK. Furthermore, a DFF mutant, in which the aspartic residue introduces a negative charge similar to a phosphorylated tyrosine, possessed catalytic activity similar to the YFF mutant. Together, our findings indicate that phosphorylation of the first tyrosine of the YXXXYY motif is necessary for the autoactivation of the ALK kinase domain and the transforming activity of NPM/ALK. The anaplastic lymphoma kinase (ALK)2 is a receptor tyrosine kinase that belongs to the insulin receptor (IR) kinase subfamily of receptor tyrosine kinases (1). ALK is normally expressed in both the central and peripheral nervous systems during embryogenesis (2, 3). Expression of ALK is down-regulated before birth and in the adult ALK expression is restricted to specific cells in the central and peripheral nervous system (4). However, ALK is also aberrantly expressed in cancer as a result of chromosomal rearrangements involving the alk gene located on the 2p23 chromosome. ALK fusion proteins have been detected in anaplastic large cell lymphoma, diffuse large B-cell lymphoma, and in inflammatory myofibroblastic tumors (5-7).The most commonly expressed ALK fusion protein in anaplastic large cell lymphoma is NPM/ALK, which derives from the t(2;5)(p23;q35) chromosomal translocation that fuses the kinase domain of ALK to the dimerization domain of nucleophosmin (NPM) (5). Dimerization of NPM/ALK stimulates autophosphorylation and activation of the ALK kinase domain (8, 9) and, subsequently, phosphorylation of other tyrosine residues within NPM/ALK that are important for the activation of oncogenic signaling pathways. Phosphorylated tyrosine residues act as docking sites for SH2 and PTB domains contained within signaling molecules or adap...

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