Tal Burstyn-Cohen

Reduced IGF-1 signaling delays age-associated proteotoxicity in mice.

The insulin/insulin growth factor (IGF) signaling (IIS) pathway is a key regulator of aging of worms, flies, mice, and likely humans. Delayed aging by IIS reduction protects the nematode C. elegans from toxicity associated with the aggregation of the Alzheimers disease-linked human peptide, Abeta. We reduced IGF signaling in Alzheimers model mice and discovered that these animals are protected from Alzheimers-like disease symptoms, including reduced behavioral impairment, neuroinflammation, and neuronal loss. This protection is correlated with the hyperaggregation of Abeta leading to tightly packed, ordered plaques, suggesting that one aspect of the protection conferred by reduced IGF signaling is the sequestration of soluble Abeta oligomers into dense aggregates of lower toxicity. These findings indicate that the IGF signaling-regulated mechanism that protects from Abeta toxicity is conserved from worms to mammals and point to the modulation of this signaling pathway as a promising strategy for the development of Alzheimers disease therapy.The insulin/insulin growth factor (IGF) signaling (IIS) pathway is a key regulator of aging of worms, flies, mice, and likely humans. Delayed aging by IIS reduction protects the nematode C. elegans from toxicity associated with the aggregation of the Alzheimers disease-linked human peptide, Aβ. We reduced IGF signaling in Alzheimers model mice and discovered that these animals are protected from Alzheimers-like disease symptoms, including reduced behavioral impairment, neuroinflammation, and neuronal loss. This protection is correlated with the hyperaggregation of Aβ leading to tightly packed, ordered plaques, suggesting that one aspect of the protection conferred by reduced IGF signaling is the sequestration of soluble Aβ oligomers into dense aggregates of lower toxicity. These findings indicate that the IGF signaling-regulated mechanism that protects from Aβ toxicity is conserved from worms to mammals and point to the modulation of this signaling pathway as a promising strategy for the development of Alzheimers disease therapy.

TAM receptor function in the retinal pigment epithelium.

The TAM receptor tyrosine kinase Mer is expressed by cells of the retinal pigment epithelium (RPE), and genetic studies have demonstrated that Mer is essential for RPE function. RPE cells that lack Mer exhibit a severely compromised ability to phagocytose the distal ends of photoreceptor (PR) outer segments, which leads to the complete postnatal degeneration of photoreceptors and to blindness. Although in vitro experiments have implicated Gas6 as the critical TAM ligand for this process, we find that Gas6 mutant mice have a histologically intact retina with no photoreceptor degeneration. We further find that, in addition to Mer, RPE cells also express another TAM receptor--Tyro 3--and that both of these receptors are instead activated independently by the Gas6-related ligand Protein S. This protein is also expressed by RPE cells. Finally, we demonstrate that loss of Mer function is accompanied by a substantial down-regulation in Tyro 3 as well. These observations indicate that both Mer and Tyro 3 act in mouse RPE cells and suggest that their biologically relevant ligand in these cells is Protein S.

Canonical Wnt activity regulates trunk neural crest delamination linking BMP/noggin signaling with G1/S transition.

Delamination of premigratory neural crest cells depends on a balance between BMP/noggin and on successful G1/S transition. Here, we report that BMP regulates G1/S transition and consequent crest delamination through canonical Wnt signaling. Noggin overexpression inhibits G1/S transition and blocking G1/S abrogates BMP-induced delamination; moreover, transcription of Wnt1 is stimulated by BMP and by the developing somites, which concomitantly inhibit noggin production. Interfering with β-catenin and LEF/TCF inhibits G1/S transition, neural crest delamination and transcription of various BMP-dependent genes, which include Cad6B, Pax3 and Msx1, but not that of Slug, Sox9 or FoxD3. Hence, we propose that developing somites inhibit noggin transcription in the dorsal tube, resulting in activation of BMP and consequent Wnt1 production. Canonical Wnt signaling in turn stimulates G1/S transition and generation of neural crest cell motility independently of its proposed role in earlier neural crest specification.

F-Spondin Is Required for Accurate Pathfinding of Commissural Axons at the Floor Plate

The commissural axons project toward and across the floor plate. They then turn into the longitudinal axis, extending along the contralateral side of the floor plate. F-spondin, a protein produced and secreted by the floor plate, promotes adhesion and neurite extension of commissural neurons in vitro. Injection of purified F-spondin protein into the lumen of the spinal cord of chicken embryos in ovo resulted in longitudinal turning of commissural axons before reaching the floor plate, whereas neutralizing antibody (Ab) injections caused lateral turning at the contralateral floor plate boundary. These combined in vitro and in vivo results suggest that F-spondin is required to prevent the lateral drifting of the commissural axons after having crossed the floor plate.

F-Spondin, Expressed in Somite Regions Avoided by Neural Crest Cells, Mediates Inhibition of Distinct Somite Domains to Neural Crest Migration

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial-half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

TAM receptors and the clearance of apoptotic cells.

The Tyro3, Axl, and Mer (TAM) receptor tyrosine kinases and their ligands Gas6 and Protein S are required for the optimal phagocytosis of apoptotic cells in the mature immune, nervous, and reproductive systems. Genetic analyses in mice, rats, and humans reveal that this receptor‐ligand system plays an especially important role in the phagocytosis that is triggered by the “eat‐me” signal phosphatidylserine. Deficiencies in TAM signaling lead to human retinal dystrophies and may contribute to lupus and other human autoimmune diseases. The TAM system appears to interact and cooperate with several other phagocytic networks, including scavenger receptor and integrin‐based systems, and may serve as a signaling hub that integrates these systems.

Association between the Cell Cycle and Neural Crest Delamination through Specific Regulation of G1/S Transition

Delamination of premigratory neural crest cells from the dorsal neural tube depends both upon environmental signals and cell-intrinsic mechanisms and is a prerequisite for cells to engage in migration. Here we show that avian neural crest cells synchronously emigrate from the neural tube in the S phase of the cell cycle. Furthermore, specific inhibition of the transition from G1 to S both in ovo and in explants blocks delamination, whereas arrest at the S or G2 phases has no immediate effect. Thus, the events taking place during G1 that control the transition from G1 to S are necessary for the epithelial to mesenchymal conversion of crest precursors.

Lack of Protein S in mice causes embryonic lethal coagulopathy and vascular dysgenesis

Protein S (ProS) is a blood anticoagulant encoded by the Pros1 gene, and ProS deficiencies are associated with venous thrombosis, stroke, and autoimmunity. These associations notwithstanding, the relative risk that reduced ProS expression confers in different disease settings has been difficult to assess without an animal model. We have now described a mouse model of ProS deficiency and shown that all Pros1-/- mice die in utero,from a fulminant coagulopathy and associated hemorrhages. Although ProS is known to act as a cofactor for activated Protein C (aPC), plasma from Pros1+/- heterozygous mice exhibited accelerated thrombin generation independent of aPC, and Pros1 mutants displayed defects in vessel development and function not seen in mice lacking protein C. Similar vascular defects appeared in mice in which Pros1 was conditionally deleted in vascular smooth muscle cells. Mutants in which Pros1 was deleted specifically in hepatocytes, which are thought to be the major source of ProS in the blood, were viable as adults and displayed less-severe coagulopathy without vascular dysgenesis. Finally, analysis of mutants in which Pros1 was deleted in endothelial cells indicated that these cells make a substantial contribution to circulating ProS. These results demonstrate that ProS is a pleiotropic anticoagulant with aPC-independent activities and highlight new roles for ProS in vascular development and homeostasis.

Accumulation of F-Spondin in Injured Peripheral Nerve Promotes the Outgrowth of Sensory Axons

F-spondin, an extracellular matrix protein, is present in peripheral nerve during embryonic development, but its amount diminishes by birth. Axotomy of adult rat sciatic nerve, however, causes a massive upregulation of both F-spondin mRNA and protein distal to the lesion. F-spondin in the distal stump of axotomized nerve promotes neurite outgrowth of sensory neurons, as revealed by protein neutralization with F-spondin-specific antibodies. Thus, F-spondin is likely to play a role in promoting axonal regeneration after nerve injury.

F-spondin is a contact-repellent molecule for embryonic motor neurons.

The floor plate plays a key role in patterning axonal trajectory in the embryonic spinal cord by providing both long-range and local guidance cues that promote or inhibit axonal growth toward and across the ventral midline of the spinal cord, thus acting as an intermediate target for a number of crossing (commissural) and noncrossing (motor) axons. F-spondin, a secreted adhesion molecule expressed in the embryonic floor plate and the caudal somite of birds, plays a dual role in patterning the nervous system. It promotes adhesion and outgrowth of commissural axons and inhibits adhesion of neural crest cells. In the current study, we demonstrate that outgrowth of embryonic motor axons also is inhibited by F-spondin protein in a contact-repulsion fashion. Three independent lines of evidence support our hypothesis: substrate-attached F-spondin inhibits outgrowth of dissociated motor neurons in an outgrowth assay; F-spondin elicits acute growth cone collapse when applied to cultured motor neurons; and challenging ventral spinal cord explants with aggregates of HEK 293 cells expressing F-spondin, causes contact-repulsion of motor neurites. Structural–functional studies demonstrate that the processed carboxyl-half protein that contains the thrombospondin type 1 repeats is more prominent in inhibiting outgrowth, suggesting that the processing of F-spondin is important for enhancing its inhibitory activity.