James I. S. MacDonald

The Signaling Adapter FRS-2 Competes with Shc for Binding to the Nerve Growth Factor Receptor TrkA A MODEL FOR DISCRIMINATING PROLIFERATION AND DIFFERENTIATION

We have isolated a human cDNA for the signaling adapter molecule FRS-2/suc1-associated neurotrophic factor target and shown that it is tyrosine-phosphorylated in response to nerve growth factor (NGF) stimulation. Importantly, we demonstrate that the phosphotyrosine binding domain of FRS-2 directly binds the Trk receptors at the same phosphotyrosine residue that binds the signaling adapter Shc, suggesting a model in which competitive binding between FRS-2 and Shc regulates differentiation versusproliferation. Consistent with this model, FRS-2 binds Grb-2, Crk, the SH2 domain containing tyrosine phosphatase SH-PTP-2, the cyclin-dependent kinase substrate p13 suc1 , and the Src homology 3 (SH3) domain of Src, providing a functional link between TrkA, cell cycle, and multiple NGF signaling effectors. Importantly, overexpression of FRS-2 in cells expressing an NGF nonresponsive TrkA receptor mutant reconstitutes the ability of NGF to stop cell cycle progression and to stimulate neuronal differentiation.

Neutralizing intraspinal nerve growth factor with a trkA-IgG fusion protein blocks the development of autonomic dysreflexia in a clip-compression model of spinal cord injury

Increased intraspinal nerve growth factor (NGF) after spinal cord injury (SCI) is detrimental to the autonomic nervous system. Autonomic dysreflexia is a debilitating condition characterized by episodic hypertension, intense headache, and sweating. Experimentally, it is associated with aberrant primary afferent sprouting in the dorsal horn that is nerve growth factor (NGF)-dependent. Therapeutic strategies that neutralize NGF may ameliorate initial apoptotic cellular responses to the injury and aberrant afferent plasticity that occurs weeks after the injury. Subsequently, the development of autonomic disorders may be suppressed. We constructed a protein including the extracellular portion of trkA fused to the Fc portion of human IgG and expressed it using a baculovirus system. Binding of our trkA-IgG fusion protein was specific for NGF with a K(d) = 4.26 x 10(-11) M and blocked NGF-dependent neuritogenesis in PC-12 cells. We hypothesized that binding of NGF in the injured cord by our trkA-IgG fusion protein would diminish autonomic dysreflexia. Severe, high thoracic SCI was induced with clip compression and the rats were treated with intrathecal infusions (4 microg/day) of trkA-IgG or control IgG. At 14 days post-SCI, the magnitude of autonomic dysreflexia was assessed. Colon distension increased mean arterial pressure (MAP) in control rats by 46 +/- 2 from 96 +/- 5 mmHg. In contrast, MAP of rats treated with trkA-IgG increased by only 30 +/- 2 mmHg. Likewise, the MAP response to cutaneous stimulation was also reduced in rats treated with trkA-IgG (20 +/- 1 vs. 29 +/- 2). In contrast, trkA-IgG treatment had no effect on heart rate responses during colon distension or cutaneous stimulation. These results indicate that treatment with trkA-IgG to block NGF suppresses the development of autonomic dysreflexia after a clinically relevant spinal cord injury.

Posttranslational Modifications of the Retinoblastoma Tumor Suppressor Protein as Determinants of Function

The retinoblastoma tumor suppressor protein (pRB) plays an integral role in G1-S checkpoint control and consequently is a frequent target for inactivation in cancer. The RB protein can function as an adaptor, nucleating components such as E2Fs and chromatin regulating enzymes into the same complex. For this reason, pRBs regulation by posttranslational modifications is thought to be critical. pRB is phosphorylated by a number of different kinases such as cyclin dependent kinases (Cdks), p38 MAP kinase, Chk1/2, Abl, and Aurora b. Although phosphorylation of pRB by Cdks has been extensively studied, activities regulated through phosphorylation by other kinases are just starting to be understood. As well as being phosphorylated, pRB is acetylated, methylated, ubiquitylated, and SUMOylated. Acetylation, methylation, and SUMOylation play roles in pRB mediated gene silencing. Ubiquitinylation of pRB promotes its degradation and may be used to regulate apoptosis. Recent proteomic data have revealed that pRB is posttranslationally modified to a much greater extent than previously thought. This new information suggests that many unknown pathways affect pRB regulation. This review focuses on posttranslational modifications of pRB and how they influence its function. The final part of the review summarizes new phosphorylation sites from accumulated proteomic data and discusses the possibilities that might arise from this data.

A retinoblastoma allele that is mutated at its common E2F interaction site inhibits cell proliferation in gene targeted mice

ABSTRACT The retinoblastoma protein (pRB) is best known for regulating cell proliferation through E2F transcription factors. In this report, we investigate the properties of a targeted mutation that disrupts pRB interactions with the transactivation domain of E2Fs. Mice that carry this mutation endogenously (Rb1ΔG) are defective for pRB-dependent repression of E2F target genes. Except for an accelerated entry into S phase in response to serum stimulation, cell cycle regulation in Rb1ΔG/ΔG mouse embryonic fibroblasts (MEFs) strongly resembles that of the wild type. In a serum deprivation-induced cell cycle exit, Rb1ΔG/ΔG MEFs display a magnitude of E2F target gene derepression similar to that of Rb1−/− cells, even though Rb1ΔG/ΔG cells exit the cell cycle normally. Interestingly, cell cycle arrest in Rb1ΔG/ΔG MEFs is responsive to p16 expression and gamma irradiation, indicating that alternate mechanisms can be activated in G1 to arrest proliferation. Some Rb1ΔG/ΔG mice die neonatally with a muscle degeneration phenotype, while the others live a normal life span with no evidence of spontaneous tumor formation. Most tissues appear histologically normal while being accompanied by derepression of pRB-regulated E2F targets. This suggests that non-E2F-, pRB-dependent pathways may have a more relevant role in proliferative control than previously identified.

Loss of the mammalian DREAM complex deregulates chondrocyte proliferation.

ABSTRACT Mammalian DREAM is a conserved protein complex that functions in cellular quiescence. DREAM contains an E2F, a retinoblastoma (RB)-family protein, and the MuvB core (LIN9, LIN37, LIN52, LIN54, and RBBP4). In mammals, MuvB can alternatively bind to BMYB to form a complex that promotes mitotic gene expression. Because BMYB-MuvB is essential for proliferation, loss-of-function approaches to study MuvB have generated limited insight into DREAM function. Here, we report a gene-targeted mouse model that is uniquely deficient for DREAM complex assembly. We have targeted p107 (Rbl1) to prevent MuvB binding and combined it with deficiency for p130 (Rbl2). Our data demonstrate that cells from these mice preferentially assemble BMYB-MuvB complexes and fail to repress transcription. DREAM-deficient mice show defects in endochondral bone formation and die shortly after birth. Micro-computed tomography and histology demonstrate that in the absence of DREAM, chondrocytes fail to arrest proliferation. Since DREAM requires DYRK1A (dual-specificity tyrosine phosphorylation-regulated protein kinase 1A) phosphorylation of LIN52 for assembly, we utilized an embryonic bone culture system and pharmacologic inhibition of (DYRK) kinase to demonstrate a similar defect in endochondral bone growth. This reveals that assembly of mammalian DREAM is required to induce cell cycle exit in chondrocytes.

HDAF transgenic pig livers are protected from hyperacute rejection during ex vivo perfusion with human blood.

The aim of this study was to determine if human decay‐accelerating factor (hDAF) protects against hyperacute rejection in an ex vivo liver perfusion system using human blood. Pig livers were perfused ex vivo via the portal vein for an average of 5–6 h using a membrane oxygenator. Three groups were studied. Group I: Wild‐type pig livers were alloperfused with fresh pig blood (n=5). Group II: Wild‐type pig livers were xenoperfused with fresh human blood (n=5). Group III: hDAF transgenic pig livers were xenoperfused with fresh human blood (n=5). The graft ischemic time, ratio of perfusate volume to liver weight, flow rate, and perfusate hematocrit were similar in each group. The hDAF livers perfused with human blood (Group III) had a lower ALT level, less protein and albumin losses, lower bilirubin levels in the perfusate, less weight gain, and greater bile production than the wild‐type livers perfused with human blood. Histology showed classic features of hyperacute rejection in Group II, including massive hemorrhage, severe vasculitits, fibrin and C5b‐9 deposition, and endothelial damage within 1 h of perfusion, whereas liver histology studies in Groups I and III were near normal. IgG and IgM deposits were seen in the xenoperfused livers. Electron microscopy (EM) and immuno‐EM showed loss of endothelial cells, trapping of white blood cells and platelets, and diffuse fibrin deposits in Group II only. hDAF pig livers perfused with human blood showed superior function and histology when compared with wild‐type pig livers. These data suggest that (1) hyperacute rejection may contribute to the inconsistent results using wild‐type pig livers for extracorporeal liver support and (2) genetically modified pigs that express hDAF may provide a better donor source than wild‐type pigs for extracorporeal liver support.

Ras Guanine Nucleotide Releasing Factor 1 (RasGrf1) Enhancement of Trk Receptor-Mediated Neurite Outgrowth Requires Activation of Both H-Ras and Rac

We previously demonstrated that the guanine nucleotide exchange factor, RasGrf1, binds nerve growth factor (NGF)-activated TrkA and facilitates neurotrophin-induced neurite outgrowth in PC12 cells. RasGrf1 can activate both Ras and Rac, via intrinsic Cdc25 and DH domains, respectively, suggesting that the activation of both could contribute to this process. Previous studies have assayed constitutive neurite outgrowth following RasGrf1 over-expression in PC12 cells, in either the absence or presence of ectopic H-Ras, and have suggested an essential role for either Ras or Rac depending on the presence of H-Ras over-expression. In contrast, in this study, we have addressed the mechanism of how RasGrf1 facilitates neurite outgrowth in response to the neurotrophins, NGF and BDNF. Using Ras/Rac activation assays and site-directed RasGrf1 mutants, we find that both Ras and Rac are essential to neurotrophin-induced neurite outgrowth. Moreover, we find that H-Ras over-expression rescues the loss of neurite outgrowth observed with a Rac minus mutant and that RasGrf1 differentially stimulates NGF-dependent activation of Rac or Ras, depending on cell type. Collectively, these studies clarify the mechanism of how RasGrf1 expression facilitates neurotrophin-induced neurite outgrowth. Moreover, they suggest that H-Ras over-expression should be used with caution to measure phenotypic responses.

Nesca, a novel neuronal adapter protein, links the molecular motor kinesin with the pre-synaptic membrane protein, syntaxin-1, in hippocampal neurons.

J. Neurochem. (2012) 121, 861–880.

A systematic analysis of negative growth control implicates the DREAM complex in cancer cell dormancy.

Epithelial ovarian cancer (EOC) generates multicellular aggregates called spheroids that detach from the primary tumor and disseminate through ascites. Spheroids possess a number of characteristics of tumor dormancy including withdrawal from the cell cycle and resistance to chemotherapeutics. This report systematically analyzes the effects of RNAi depletion of 21 genes that are known to contribute to negative regulation of the cell cycle in 10 ovarian cancer cell lines. Interestingly, spheroid cell viability was compromised by loss of some cyclin-dependent kinase inhibitors such as p57Kip2, as well as Dyrk1A, Lin52, and E2F5 in most cell lines tested. Many genes essential for EOC spheroid viability are pertinent to the mammalian DREAM repressor complex. Mechanistically, the data demonstrate that DREAM is assembled upon the induction of spheroid formation, which is dependent upon Dyrk1A. Loss of Dyrk1A results in retention of the b-Myb–MuvB complex, elevated expression of DREAM target genes, and increased DNA synthesis that is coincident with cell death. Inhibition of Dyrk1A activity using pharmacologic agents Harmine and INDY compromises viability of spheroids and blocks DREAM assembly. In addition, INDY treatment improves the response to carboplatin, suggesting this is a therapeutic target for EOC treatment. Implications: Loss of negative growth control mechanisms in cancer dormancy lead to cell death and not proliferation, suggesting they are an attractive therapeutic approach. Mol Cancer Res; 15(4); 371–81. ©2016 AACR.

Unravelling the Mechanism of TrkA-Induced Cell Death by Macropinocytosis in Medulloblastoma Daoy Cells

ABSTRACT Macropinocytosis is a normal cellular process by which cells internalize extracellular fluids and nutrients from their environment and is one strategy that Ras-transformed pancreatic cancer cells use to increase uptake of amino acids to meet the needs of rapid growth. Paradoxically, in non-Ras transformed medulloblastoma brain tumors, we have shown that expression and activation of the receptor tyrosine kinase TrkA overactivates macropinocytosis, resulting in the catastrophic disintegration of the cell membrane and in tumor cell death. The molecular basis of this uncontrolled form of macropinocytosis has not been previously understood. Here, we demonstrate that the overactivation of macropinocytosis is caused by the simultaneous activation of two TrkA-mediated pathways: (i) inhibition of RhoB via phosphorylation at Ser185 by casein kinase 1, which relieves actin stress fibers, and (ii) FRS2-scaffolded Src and H-Ras activation of RhoA, which stimulate actin reorganization and the formation of lamellipodia. Since catastrophic macropinocytosis results in brain tumor cell death, improved understanding of the mechanisms involved will facilitate future efforts to reprogram tumors, even those resistant to apoptosis, to die.