Unique carboxyl-terminal sequences of wild type and alternatively spliced variant forms of transforming growth factor-alpha precursors mediate specific interactions with ErbB4 and ErbB2

Nolz1 expression is required in dopaminergic axon guidance and striatal innervation

Midbrain dopaminergic (DA) axons make long longitudinal projections towards the striatum. Despite the importance of DA striatal innervation, processes involved in establishment of DA axonal connectivity remain largely unknown. Here we demonstrate a striatal-specific requirement of transcriptional regulator Nolz1 in establishing DA circuitry formation. DA projections are misguided and fail to innervate the striatum in both constitutive and striatal-specific Nolz1 mutant embryos. The lack of striatal Nolz1 expression results in nigral to pallidal lineage conversion of striatal projection neuron subtypes. This lineage switch alters the composition of secreted factors influencing DA axonal tract formation and renders the striatum non-permissive for dopaminergic and other forebrain tracts. Furthermore, transcriptomic analysis of wild-type and Nolz1^−/− mutant striatal tissue led to the identification of several secreted factors that underlie the observed guidance defects and proteins that promote DA axonal outgrowth. Together, our data demonstrate the involvement of the striatum in orchestrating dopaminergic circuitry formation.

The mechanisms regulating midbrain dopaminergic innervation during development are unclear. Here, the authors showed that Nolz1 is required for axonal guidance of dopaminergic neurons during embryonic development of the mouse brain.

Proteolytic processing of TGFα redirects its mitogenic activity: the membrane-anchored form is autocrine, the secreted form is paracrine

Wild-type transforming growth factor alpha (TGFalpha) expression in lactotrope cells in the pituitary gland led to lactotrope-specific pituitary hyperplasia and adenomata. To indicate whether the EGF receptor is involved in this TGFalpha-mediated phenotype, we bred TGFalpha mice with mice expressing the cytoplasmic truncated-EGF receptor (EGFR-tr), which is dominant-negative in other models. These bitransgenic mice developed pituitary pathology despite expression of the dominant-negative receptor. To further characterize this observation, we generated two lineages of transgenic mice that overexpress mutant forms of TGFalpha: a processed soluble form (s TGFalpha) and a cytoplasmic-deleted form (TGFalphaDeltaC). While sTGFalpha expression in lactotrope cells failed to induce autocrine lactotrope hyperplasia, the pituitary became very enlarged due to proliferation of neighboring interstitial cells. In contrast, the TGFalphaDeltaC mice did not develop a phenotype, although the mRNA and protein were present in the pituitary and this form of TGFalpha was confirmed to be biologically active and targeted properly to the plasma membrane of cultured CHO cells. The results suggest that the cytoplasmic domain of TGFalpha is required for autocrine parenchymal tumor formation in the pituitary gland. This signal cannot be inhibited by the EGFR-tr. Conversely, the released form of TGFalpha appears to have primarily paracrine activity.

Dynamic assembly of the urokinase-type plasminogen activator signaling receptor complex determines the mitogenic activity of urokinase-type plasminogen activator

The urokinase-type plasminogen activator (uPA) receptor (uPAR) functions in concert with co-receptors, including integrins, FPR-like receptor-1/lipoxin A4 receptor, and the epidermal growth factor receptor (EGFR), to initiate cell signaling. uPAR co-receptors may be dynamically organized into a multiprotein signaling receptor complex. In Chinese hamster ovary-K1 (CHO-K1) cells, uPA-binding to uPAR activates ERK/MAP kinase, even though these cells do not express the EGFR; however, when CHO-K1 cells are transfected to express the EGFR, ERK activation becomes EGFR-dependent. In this study, we demonstrate that ERK activation in response to uPA follows equivalent biphasic kinetics in EGFR-expressing and -deficient CHO-K1 cells. In both cell types, the response is pertussis toxin-sensitive; however, uPA promotes cell proliferation exclusively in the EGFR-expressing cells. uPA-induced mitogenic activity requires activation of both STAT5b and ERK. STAT5b was tyrosine-phosphorylated, in response to uPA, only in EGFR-expressing cells. uPA-induced cell proliferation was blocked by dominant-negative MEK1, dominant-negative STAT5b, and by expression of an EGFR that is mutated at Tyr-845, which is essential for STAT5b activation. In two cell culture models of uPA-stimulated breast cancer growth, MDA-MB 468 cells treated with uPA and MCF-7 cells treated with uPA-plasminogen activator inhibitor-1 complex, proliferation was completely inhibited when EGFR expression or activity was blocked. We conclude that expression and assembly of uPAR co-receptors in a specific cell type determines the response to uPA. The EGFR selectively cooperates with uPAR to mediate mitogenesis.

MUC1 membrane trafficking is modulated by multiple interactions

MUC1 is a mucin-like transmembrane protein found on the apical surface of many epithelia. Because aberrant intracellular localization of MUC1 in tumor cells correlates with an aggressive tumor and a poor prognosis for the patient, experiments were designed to characterize the features that modulate MUC1 membrane trafficking. By following [(35)S]Met/Cys-labeled MUC1 in glycosylation-defective Chinese hamster ovary cells, we found previously that truncation of O-glycans on MUC1 inhibited its surface expression and stimulated its internalization by clathrin-mediated endocytosis. To identify signals for MUC1 internalization that are independent of its glycosylation state, the ectodomain of MUC1 was replaced with that of Tac, and chimera endocytosis was measured by the same protocol. Endocytosis of the chimera was significantly faster than for MUC1, indicating that features of the highly extended ectodomain inhibit MUC1 internalization. Analysis of truncation mutants and tyrosine mutants showed that Tyr(20) and Tyr(60) were both required for efficient endocytosis. Mutation of Tyr(20) significantly blocked coimmunoprecipitation of the chimera with AP-2, indicating that Y(20)HPM is recognized as a YXXphi motif by the mu2 subunit. The tyrosine-phosphorylated Y(60)TNP was previously identified as an SH2 site for Grb2 binding, and we found that mutation of Tyr(60) blocked coimmunoprecipitation of the chimera with Grb2. This is the first indication that Grb2 plays a significant role in the endocytosis of MUC1.

Proteomics methods for probing molecular mechanisms in signal transduction1

mRNA splicing and various posttranslational modifications to proteins result in a larger number of proteins than genes. Assessing the dynamic nature of this proteome is the challenge of modern proteomics. Recent advances in high throughput methods greatly facilitate the analysis of proteins involved in signal transduction, their production, posttranslational modifications and interactions. Highly reproducible two dimensional polyacrylamide gel electrophoresis (2D-PAGE) methods, coupled with matrix assisted laser desorption-time of flight-mass spectrometry (MALDI-TOF-MS) allow rapid separation and identification of proteins. These methods, alone or in conjunction with other techniques such as immunoprecipitation, allow identification of various critical posttranslational modifications, such as phosphorylation. High throughput identification of important protein-protein interactions is accomplished by yeast two hybrid approaches. In vitro and in vivo pulldown assays, coupled with MALDI-TOF-MS, provide an important alternative to two hybrid approaches. Emerging advances in production of protein-based arrays promise to further increase throughput of proteomics-based approaches to signal transduction.

ErbB-4: a receptor tyrosine kinase

ErbB-4 is a receptor tyrosine kinase that is activated by the binding of specific growth factors to its ectodomain. In addition to the initiation of signal transduction pathways that direct cell responses, such as proliferation or differentiation, this receptor is subject to ligand-dependent trafficking events. The signal transduction events are controlled by ligand-dependent activation of the receptor tyrosine kinase activity, which results in receptor autophosphorylation and the tyrosine phosphorylation of other cellular proteins. The trafficking events include migration into and out of membrane microdomains, entry into internalization pathways and endocytosis, plus proteolytic fragmentation.

Stimulation of mitogenic pathways through kinase-impaired mutants of the epidermal growth factor receptor

Two residues have been shown to be critical for the kinase activity of the receptor for epidermal growth factor (EGF): lysine-721, which functions in the binding of ATP by correctly positioning the gamma-phosphate for phosphoryl transfer, and aspartate-813, which functions as the catalytic base of the kinase. Mutation of either of these two residues has been shown to disrupt kinase activity of the receptor. However, studies performed in different laboratories had suggested that while EGF receptors mutated at lysine-721 are unable to stimulate significant increases of [(3)H]thymidine incorporation into DNA in response to EGF treatment, cells expressing EGF receptors mutated at aspartate-813 do stimulate significant incorporation of [(3)H]thymidine into DNA in response to EGF. In the present study, EGF receptors mutated at lysine-721 or aspartate-813 (K721R and D813A, respectively), as well as wild-type EGF receptors, were expressed in the same cellular background, Chinese hamster ovary cells, and side-by-side experiments were performed to investigate possible signaling-related differences. Our results indicate that while there are measurable differences in the abilities of the two mutant receptors to stimulate [(3)H]thymidine incorporation between 20 and 24 h after addition of EGF, these differences cannot be correlated with significant differences in EGF-stimulated tyrosine phosphorylation of mutant EGF receptor and endogenous ErbB2, the extent of receptor internalization, EGF-stimulated ion uptake, stimulation of SHC activity, or receptor association with Grb2. Flow cytometric data suggest that populations of cells expressing either kinase-impaired mutant EGF receptor progress similarly into S phase in response to addition of EGF. These observations suggest that D813A and K721R retain similar ability to stimulate mitogenic signaling events through transactivation of ErbB2 with only subtle temporal differences, and they emphasize the importance of expressing mutant receptors in an identical cellular context to make valid comparisons of functions.