Endocytic receptor-mediated control of morphogen signaling

Emerging Role of Sorting Nexin 17 in Human Health and Disease

The distortion of the cellular membrane transport pathway has a profound impact on cell dynamics and can drive serious physiological consequences during the process of cell sorting. SNX17 is a member of the Sorting Nexin (SNX) family and plays a crucial role in protein sorting and transport in the endocytic pathway. SNX17, SNX27, and SNX31 belong to the SNX-FERM subfamily and possess the FERM domain, which can assist in endocytic transport and lysosomal degradation. The binding partners of SNX27 have been discovered to number over 100, and SNX27 has been linked to the development of Alzheimer's disease progression, tumorigenesis, cancer progression, and metastasis. However, the role and potential mechanisms of SNX17 in human health and disease remain poorly understood, and the function of SNX17 has not been fully elucidated. In this review, we summarize the structure and basic functions of SNX protein, focusing on providing current evidence of the role and possible mechanism of SNX17 in human neurodegenerative diseases and cardiovascular diseases.

Low-Density Lipoprotein Receptor LRP-2 regulates GLR-1 glutamate receptors and glutamatergic behavior in C. elegans

We identified the Low-Density Lipoprotein (LDL) Receptor Related Protein-2 (LRP-2) in a RNAi screen for genes that regulate glutamatergic behavior in C. elegans . lrp-2 loss-of-function mutants have defects in glutamatergic mechanosensory nose-touch behavior and suppress increased spontaneous reversals induced by GLR-1(A/T), a constitutively-active form of the AMPA-type glutamate receptor GLR-1. Total and surface levels of GLR-1 are increased throughout the ventral nerve cord of lrp-2 mutants suggesting that LRP-2 promotes glutamatergic signaling by regulating some aspect of GLR-1 trafficking, localization or function.

Balancing WNT signalling in early forebrain development: The role of LRP4 as a modulator of LRP6 function

The specification of the forebrain relies on the precise regulation of WNT/ß-catenin signalling to support neuronal progenitor cell expansion, patterning, and morphogenesis. Imbalances in WNT signalling activity in the early neuroepithelium lead to congenital disorders, such as neural tube defects (NTDs). LDL receptor-related protein (LRP) family members, including the well-studied receptors LRP5 and LRP6, play critical roles in modulating WNT signalling capacity through tightly regulated interactions with their co-receptor Frizzled, WNT ligands, inhibitors and intracellular WNT pathway components. However, little is known about the function of LRP4 as a potential modulator of WNT signalling in the central nervous system. In this study, we investigated the role of LRP4 in the regulation of WNT signalling during early mouse forebrain development. Our results demonstrate that LRP4 can modulate LRP5- and LRP6-mediated WNT signalling in the developing forebrain prior to the onset of neurogenesis at embryonic stage 9.5 and is therefore essential for accurate neural tube morphogenesis. Specifically, LRP4 functions as a genetic modifier for impaired mitotic activity and forebrain hypoplasia, but not for NTDs in LRP6-deficient mutants. In vivo and in vitro data provide evidence that LRP4 is a key player in fine-tuning WNT signalling capacity and mitotic activity of mouse neuronal progenitors and of human retinal pigment epithelial (hTERT RPE-1) cells. Our data demonstrate the crucial roles of LRP4 and LRP6 in regulating WNT signalling and forebrain development and highlight the need to consider the interaction between different signalling pathways to understand the underlying mechanisms of disease. The findings have significant implications for our mechanistic understanding of how LRPs participate in controlling WNT signalling.

Identification of a De Novo LRP1 mutation in a Saudi Family with Tetralogy of Fallot

BACKGROUND

Tetralogy of Fallot (TOF) is a rare, complex congenital heart defect caused by genetic and environmental interactions that results in abnormal heart development during the early stages of pregnancy. Genetic basis of TOF in Saudi populations is not yet studied. Therefore, the objective of this study is to screen for the molecular defects causing TOF in Saudi patients.

METHODS

A family with non-syndromic TOF was recruited from the Western region of Saudi Arabia. Whole exome sequencing (WES) was performed on the proband and her parents. The identified candidate variant was verified by sanger sequencing. Also, different computational biology tools were used to figure out how candidate variants affect the structure and function of candidate protein involved in TOF.

RESULTS

A novel heterozygous de novo mutation in LRP1 (p. G3311D) gene was identified in the index case. Also, this variant was absent in the in-house exome sequencing data of 80 healthy Saudi individuals. This variant was predicted to be likely pathogenic, as it negatively affects the biophysical chemical properties and stability of the protein. Furthermore, functional biology data from knock out mouse models confirms that molecular defects in LRP1 gene leads to cardiac defects and lethality. This variant was not previously reported in both Arab and global population genetic databases.

CONCLUSION

The findings in this study postulate that the LRP1 variant has a role in TOF pathogenesis and facilitate accurate diagnosis as well as the understanding of underlying molecular mechanisms and pathophysiology of the disease.

Interplay of Low-Density Lipoprotein Receptors, LRPs, and Lipoproteins in Pulmonary Hypertension

The low-density lipoprotein receptor (LDLR) gene family includes LDLR, very LDLR, and LDL receptor-related proteins (LRPs) such as LRP1, LRP1b (aka LRP-DIT), LRP2 (aka megalin), LRP4, and LRP5/6, and LRP8 (aka ApoER2). LDLR family members constitute a class of closely related multifunctional, transmembrane receptors, with diverse functions, from embryonic development to cancer, lipid metabolism, and cardiovascular homeostasis. While LDLR family members have been studied extensively in the systemic circulation in the context of atherosclerosis, their roles in pulmonary arterial hypertension (PAH) are understudied and largely unknown. Endothelial dysfunction, tissue infiltration of monocytes, and proliferation of pulmonary artery smooth muscle cells are hallmarks of PAH, leading to vascular remodeling, obliteration, increased pulmonary vascular resistance, heart failure, and death. LDLR family members are entangled with the aforementioned detrimental processes by controlling many pathways that are dysregulated in PAH; these include lipid metabolism and oxidation, but also platelet-derived growth factor, transforming growth factor β1, Wnt, apolipoprotein E, bone morpohogenetic proteins, and peroxisome proliferator-activated receptor gamma. In this paper, we discuss the current knowledge on LDLR family members in PAH. We also review mechanisms and drugs discovered in biological contexts and diseases other than PAH that are likely very relevant in the hypertensive pulmonary vasculature and the future care of patients with PAH or other chronic, progressive, debilitating cardiovascular diseases.

LRP1B: A Giant Lost in Cancer Translation

Low-density lipoprotein receptor-related protein 1B (LRP1B) is a giant member of the LDLR protein family, which includes several structurally homologous cell surface receptors with a wide range of biological functions from cargo transport to cell signaling. LRP1B is among the most altered genes in human cancer overall. Found frequently inactivated by several genetic and epigenetic mechanisms, it has mostly been regarded as a putative tumor suppressor. Still, limitations in LRP1B studies exist, in particular associated with its huge size. Therefore, LRP1B expression and function in cancer remains to be fully unveiled. This review addresses the current understanding of LRP1B and the studies that shed a light on the LRP1B structure and ligands. It goes further in presenting increasing knowledge brought by technical and methodological advances that allow to better manipulate LRP1B expression in cells and to more thoroughly explore its expression and mutation status. New evidence is pushing towards the increased relevance of LRP1B in cancer as a potential target or translational prognosis and response to therapy biomarker.

Endocytic Protein Defects in the Neural Crest Cell Lineage and Its Pathway Are Associated with Congenital Heart Defects

Endocytic trafficking is an under-appreciated pathway in cardiac development. Several genes related to endocytic trafficking have been uncovered in a mutagenic ENU screen, in which mutations led to congenital heart defects (CHDs). In this article, we review the relationship between these genes (including LRP1 and LRP2) and cardiac neural crest cells (CNCCs) during cardiac development. Mice with an ENU-induced Lrp1 mutation exhibit a spectrum of CHDs. Conditional deletion using a floxed Lrp1 allele with different Cre drivers showed that targeting neural crest cells with Wnt1-Cre expression replicated the full cardiac phenotypes of the ENU-induced Lrp1 mutation. In addition, LRP1 function in CNCCs is required for normal OFT lengthening and survival/expansion of the cushion mesenchyme, with other cell lineages along the NCC migratory path playing an additional role. Mice with an ENU-induced and targeted Lrp2 mutation demonstrated the cardiac phenotype of common arterial trunk (CAT). Although there is no impact on CNCCs in Lrp2 mutants, the loss of LRP2 results in the depletion of sonic hedgehog (SHH)-dependent cells in the second heart field. SHH is known to be crucial for CNCC survival and proliferation, which suggests LRP2 has a non-autonomous role in CNCCs. In this article, other endocytic trafficking proteins that are associated with CHDs that may play roles in the NCC pathway during development, such as AP1B1, AP2B1, FUZ, MYH10, and HECTD1, are reviewed.

Blockage of undesirable endocytosis of recombinant human growth/differentiation factor-5 in Chinese hamster ovary cell cultures requires heparin analogs with specific chain lengths

Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis lowers the yield of recombinant human bone morphogenetic proteins (rhBMPs), such as rhBMP-2 and rhBMP-4, from Chinese hamster ovary (CHO) cell cultures. Exogenous recombinant human growth/differentiation factor-5 (rhGDF-5), a member of the BMP family, bound to cell surface HSPGs and was actively internalized into CHO cells. Knockdown of heparan sulfate (HS) synthesis enzymes in CHO cells revealed that the chain length and N-sulfation of HS affected the binding of rhGDF-5 to HSPGs and subsequent rhGDF-5 internalization. To increase product yield by minimizing rhGDF-5 internalization in recombinant CHO (rCHO) cell cultures, heparin, and dextran sulfate (DS) of various polysaccharide chain lengths, which are structural analogs of HS, were examined for blockage of rhGDF-5 internalization. Heparin fragments of four monosaccharides (MW of 1.2 kDa) and DS (MW of 15 kDa) did not inhibit rhGDF-5 internalization whereas unfractionated heparin and DS of 200 kDa could significantly inhibit it. Compared to the control cultures, supplementation with unfractionated heparin or DS of 200 kDa at 1 g L^-1 resulted in more than a 10-fold increase in the maximum rhGDF-5 concentration. Taken together, the supplementation of structural HS analogs improved rhGDF-5 production in rCHO cell cultures by inhibiting rhGDF-5 internalization. GRAPHICAL ABSTRACT AND LAY SUMMARY: Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis lowers the yield of rhGDF-5 from CHO cell cultures. In this study, the authors found that the length and N-sulfation of HS chain determine the binding of rhGDF-5 to HSPGs and subsequent rhGDF-5 internalization. Based on this finding, the authors successfully used heparin analogs with specific chain lengths to enhance the rhGDF-5 yield by blocking rhGDF-5 internalization.

Rab7-Mediated Endocytosis Establishes Patterning of Wnt Activity through Inactivation of Dkk Antagonism

Endocytosis has been proposed to modulate cell signaling activities. However, the role of endocytosis in embryogenesis, which requires coordination of multiple signaling inputs, has remained less understood. We previously showed that mouse embryos lacking a small guanosine triphosphate (GTP)-binding protein Rab7 implicated in endocytic flow are defective in gastrulation. Here, we investigate how subcellular defects associated with Rab7 deficiency are related to the observed developmental defects. Rab7-deficient embryos fail to organize mesodermal tissues due to defects in Wnt-β-catenin signaling. Visceral endoderm (VE)-specific ablation of Rab7 results in patterning defects similar to systemic Rab7 deletion. Rab7 mutants accumulate the Wnt antagonist Dkk1 in the extracellular space and in intracellular compartments throughout the VE epithelium. These data indicate that Rab7-dependent endocytosis regulates the concentration and availability of extracellular Dkk1, thereby relieving the epiblast of antagonism. This intercellular mechanism therefore organizes distinct spatiotemporal patterns of canonical Wnt activity during the peri-gastrulation stages of embryonic development.

Receptor-mediated yolk uptake is required for oskar mRNA localization and cortical anchorage of germ plasm components in the Drosophila oocyte

The Drosophila germ plasm is responsible for germ cell formation. Its assembly begins with localization of oskar mRNA to the posterior pole of the oocyte. The oskar translation produces 2 isoforms with distinct functions: short Oskar recruits germ plasm components, whereas long Oskar remodels actin to anchor the components to the cortex. The mechanism by which long Oskar anchors them remains elusive. Here, we report that Yolkless, which facilitates uptake of nutrient yolk proteins into the oocyte, is a key cofactor for long Oskar. Loss of Yolkless or depletion of yolk proteins disrupts the microtubule alignment and oskar mRNA localization at the posterior pole of the oocyte, whereas microtubule-dependent localization of bicoid mRNA to the anterior and gurken mRNA to the anterior-dorsal corner remains intact. Furthermore, these mutant oocytes do not properly respond to long Oskar, causing defects in the actin remodeling and germ plasm anchoring. Thus, the yolk uptake is not merely the process for nutrient incorporation, but also crucial for oskar mRNA localization and cortical anchorage of germ plasm components in the oocyte.

A study of the fruit fly Drosophila reveals that receptor-mediated yolk uptake is not merely a nutrient storage process for future embryogenesis, but is also required for localization of Oskar mRNA and cortical anchorage of germ plasm components in the oocyte during oogenesis.

Deletion of LRP1 From Astrocytes Modifies Neuronal Network Activity in an in vitro Model of the Tripartite Synapse

The low-density lipoprotein receptor-related protein 1 (LRP1) is a transmembrane receptor that binds over 40 potential ligands and is involved in processes such as cell differentiation, proliferation, and survival. LRP1 is ubiquitously expressed in the organism and enriched among others in blood vessels, liver, and the central nervous system (CNS). There, it is strongly expressed by neurons, microglia, immature oligodendrocytes, and astrocytes. The constitutive LRP1 knockout leads to embryonic lethality. Therefore, previous studies focused on conditional LRP1-knockout strategies and revealed that the deletion of LRP1 causes an increased differentiation of neural stem and precursor cells into astrocytes. Furthermore, astrocytic LRP1 is necessary for the degradation of Aβ and the reduced accumulation of amyloid plaques in Alzheimer’s disease. Although the role of LRP1 in neurons has intensely been investigated, the function of LRP1 with regard to the differentiation and maturation of astrocytes and their functionality is still unknown. To address this question, we generated an inducible conditional transgenic mouse model, where LRP1 is specifically deleted from GLAST-positive astrocyte precursor cells. The recombination with resulting knockout events was visualized by the simultaneous expression of the fluorescent reporter tdTomato. We observed a significantly increased number of GLT-1 expressing astrocytes in LRP1-depleted astrocytic cultures in comparison to control astrocytes. Furthermore, we investigated the influence of astrocytic LRP1 on neuronal activity and synaptogenesis using the co-culture of hippocampal neurons with control or LRP1-depleted astrocytes. These analyses revealed that the LRP1-deficient astrocytes caused a decreased number of single action potentials as well as a negatively influenced neuronal network activity. Moreover, the proportion of pre- and postsynaptic structures was significantly altered in neurons co-cultured with LPR1-depleted astrocytes. However, the number of structural synapses was not affected. Additionally, the supernatant of hippocampal neurons co-cultured with LRP1-deficient astrocytes showed an altered set of cytokines in comparison to the control condition, which potentially contributed to the altered neuronal transmission and synaptogenesis. Our results suggest astrocytic LRP1 as a modulator of synaptic transmission and synaptogenesis by altering the expression of the glutamate transporter on the cell surface on astrocytes and the release of cytokines in vitro.

Cdon mutation and fetal alcohol converge on Nodal signaling in a mouse model of holoprosencephaly

Holoprosencephaly (HPE), a defect in midline patterning of the forebrain and midface, arises ~1 in 250 conceptions. It is associated with predisposing mutations in the Nodal and Hedgehog (HH) pathways, with penetrance and expressivity graded by genetic and environmental modifiers, via poorly understood mechanisms. CDON is a multifunctional co-receptor, including for the HH pathway. In mice, Cdon mutation synergizes with fetal alcohol exposure, producing HPE phenotypes closely resembling those seen in humans. We report here that, unexpectedly, Nodal signaling is a major point of synergistic interaction between Cdon mutation and fetal alcohol. Window-of-sensitivity, genetic, and in vitro findings are consistent with a model whereby brief exposure of Cdon mutant embryos to ethanol during gastrulation transiently and partially inhibits Nodal pathway activity, with consequent effects on midline patterning. These results illuminate mechanisms of gene-environment interaction in a multifactorial model of a common birth defect.

Improving the production of recombinant human bone morphogenetic protein-4 in Chinese hamster ovary cell cultures by inhibition of undesirable endocytosis

Endocytic regulation serves a critical role in modulating the extracellular level of signaling molecules, such as bone morphogenetic proteins (BMPs). Unfortunately, endocytosis may result in poor yields of recombinant human BMP-4 (rhBMP-4) from Chinese hamster ovary (CHO) cell cultures. When rhBMP-4 was incubated with CHO cells, rhBMP-4 was actively internalized into cells. Cell surface bound heparan sulfate proteoglycans (HSPGs) served as the major receptors for rhBMP-4 internalization. Removal of cell surface heparan sulfate (HS) by heparinases or reduction of HSPG synthesis by knockdown of xylosyltransferase2 (xylt2) in CHO cells decreased internalization of rhBMP-4. In addition, treatment with endocytosis inhibitors (chlorpromazine, genistein, and dynasore) identified a clathrin- and dynamin-dependent endocytic pathway as the major route for rhBMP-4 internalization. To enhance product yield by minimizing rhBMP-4 internalization in recombinant CHO (rCHO) cell cultures, we have tested various strategies to reduce HSPG synthesis (knockdown of xylt2 and sodium chlorate treatment) or inhibit the binding of rhBMP-4 to cell-surface-bound HSPGs (supplementation with heparin or dextran sulfate [DS]). Among these approaches, DS, which is a linear anionic sulfated polysaccharide with similarity to HS chains, was the most effective in enhancing rhBMP-4 production in rCHO cell cultures. Compared with the control cultures, DS addition to the culture medium (1.0 g/L) resulted in 1.4-fold and 2.3-fold increases in maximum rhBMP-4 concentration in batch and fed-batch cultures, respectively. Taken together, the addition of DS, an effective competitor of HSPGs, improved rhBMP-4 production in rCHO cell cultures through blockage of rhBMP-4 internalization.

Multiple modes of Lrp4 function in modulation of Wnt/β-catenin signaling during tooth development

During development and homeostasis, precise control of Wnt/β-catenin signaling is in part achieved by secreted and membrane proteins that negatively control activity of the Wnt co-receptors Lrp5 and Lrp6. Lrp4 is related to Lrp5/6 and is implicated in modulation of Wnt/β-catenin signaling, presumably through its ability to bind to the Wise (Sostdc1)/sclerostin (Sost) family of Wnt antagonists. To gain insights into the molecular mechanisms of Lrp4 function in modulating Wnt signaling, we performed an array of genetic analyses in murine tooth development, where Lrp4 and Wise play important roles. We provide genetic evidence that Lrp4 mediates the Wnt inhibitory function of Wise and also modulates Wnt/β-catenin signaling independently of Wise. Chimeric receptor analyses raise the possibility that the Lrp4 extracellular domain interacts with Wnt ligands, as well as the Wnt antagonists. Diverse modes of Lrp4 function are supported by severe tooth phenotypes of mice carrying a human mutation known to abolish Lrp4 binding to Sost. Our data suggest a model whereby Lrp4 modulates Wnt/β-catenin signaling via interaction with Wnt ligands and antagonists in a context-dependent manner.

Neuropilin-1 promotes Hedgehog signaling through a novel cytoplasmic motif

Hedgehog (HH) signaling critically regulates embryonic and postnatal development as well as adult tissue homeostasis, and its perturbation can lead to developmental disorders, birth defects, and cancers. Neuropilins (NRPs), which have well-defined roles in Semaphorin and VEGF signaling, positively regulate HH pathway function, although their mechanism of action in HH signaling remains unclear. Here, using luciferase-based reporter assays, we provide evidence that NRP1 regulates HH signaling specifically at the level of GLI transcriptional activator function. Moreover, we show that NRP1 localization to the primary cilium, a key platform for HH signal transduction, does not correlate with HH signal promotion. Rather, a structure-function analysis suggests that the NRP1 cytoplasmic and transmembrane domains are necessary and sufficient to regulate HH pathway activity. Furthermore, we identify a previously uncharacterized, 12-amino acid region within the NRP1 cytoplasmic domain that mediates HH signal promotion. Overall, our results provide mechanistic insight into NRP1 function within and potentially beyond the HH signaling pathway. These insights have implications for the development of novel modulators of HH-driven developmental disorders and diseases.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Membrane dynamics in mammalian embryogenesis: Implication in signal regulation

Eukaryotes have evolved an array of membrane compartments constituting secretory and endocytic pathways that allow the flow of materials. Both pathways perform important regulatory roles. The secretory pathway is essential for the production of extracellular, secreted signal molecules, but its function is not restricted to a mere route connecting intra- and extracellular compartments. Post-translational modifications also play an integral function in the secretory pathway and are implicated in developmental regulation. The endocytic pathway serves as a platform for relaying signals from the extracellular stimuli to intracellular mediators, and then ultimately inducing signal termination. Here, we discuss recent studies showing that dysfunction in membrane dynamics causes patterning defects in embryogenesis and tissue morphogenesis in mammals.

LRP2, an auxiliary receptor that controls sonic hedgehog signaling in development and disease

To fulfill their multiple roles in organ development and adult tissue homeostasis, hedgehog (HH) morphogens act through their receptor Patched (PTCH) on target cells. However, HH actions also require HH binding proteins, auxiliary cell surface receptors that agonize or antagonize morphogen signaling in a context-dependent manner. Here, we discuss recent findings on the LDL receptor-related protein 2 (LRP2), an exemplary HH binding protein that modulates sonic hedgehog activities in stem and progenitor cell niches in embryonic and adult tissues. LRP2 functions are crucial for developmental processes in a number of tissues, including the brain, the eye, and the heart, and defects in this receptor pathway are the cause of devastating congenital diseases in humans. Developmental Dynamics 245:569-579, 2016. © 2016 Wiley Periodicals, Inc.

Low Density Lipoprotein Receptor Related Proteins as Regulators of Neural Stem and Progenitor Cell Function

The central nervous system (CNS) is a highly organised structure. Many signalling systems work in concert to ensure that neural stem cells are appropriately directed to generate progenitor cells, which in turn mature into functional cell types including projection neurons, interneurons, astrocytes, and oligodendrocytes. Herein we explore the role of the low density lipoprotein (LDL) receptor family, in particular family members LRP1 and LRP2, in regulating the behaviour of neural stem and progenitor cells during development and adulthood. The ability of LRP1 and LRP2 to bind a diverse and extensive range of ligands, regulate ligand endocytosis, recruit nonreceptor tyrosine kinases for direct signal transduction and signal in conjunction with other receptors, enables them to modulate many crucial neural cell functions.

New genetic regulators question relevance of abundant yolk protein production in C. elegans

Vitellogenesis or maternal yolk formation is considered critical to the reproduction of egg-laying animals. In invertebrates, however, most of its regulatory genes are still unknown. Via a combined mapping and whole-genome sequencing strategy, we performed a forward genetic screen to isolate novel regulators of yolk production in the nematode model system Caenorhabditis elegans. In addition to isolating new alleles of rab-35, rab-10 and M04F3.2, we identified five mutant alleles corresponding to three novel regulatory genes potently suppressing the expression of a GFP-based yolk reporter. We confirmed that mutations in vrp-1, ceh-60 and lrp-2 disrupt endogenous yolk protein synthesis at the transcriptional and translational level. In contrast to current beliefs, our discovered set of mutants with strongly reduced yolk proteins did not show serious reproduction defects. This raises questions as to whether yolk proteins per se are needed for ultimate reproductive success.

LRP2 Acts as SHH Clearance Receptor to Protect the Retinal Margin from Mitogenic Stimuli

During forebrain development, LRP2 promotes morphogen signaling as an auxiliary SHH receptor. However, in the developing retina, LRP2 assumes the opposing function, mediating endocytic clearance of SHH and antagonizing morphogen action. LRP2-mediated clearance prevents spread of SHH activity from the central retina into the retinal margin to protect quiescent progenitor cells in this niche from mitogenic stimuli. Loss of LRP2 in mice increases the sensitivity of the retinal margin for SHH, causing expansion of the retinal progenitor cell pool and hyperproliferation of this tissue. Our findings document the ability of LRP2 to act, in a context-dependent manner, as activator or inhibitor of the SHH pathway. Our current findings uncovered LRP2 activity as the molecular mechanism imposing quiescence of the retinal margin in the mammalian eye and suggest SHH-induced proliferation of the retinal margin as cause of the large eye phenotype observed in mouse models and patients with LRP2 defects.

Altered TGF-β signaling in fetal fibroblasts: what is known about the underlying mechanisms?

Scarless wound healing is a unique and intrinsic capacity of the fetal skin that is not fully understood. Further insight into the underlying mechanisms of fetal wound healing may lead to new therapeutic approaches promoting adult scarless wound healing. Differences between fetal and adult wound healing are found in the extracellular matrix, the inflammatory reaction and the levels of growth factors present in the wound. This review focuses specifically on transforming growth factor β (TGF-β), as this growth factor is prominently involved in wound healing and fibroblast-to-myofibroblast differentiation. Although fetal fibroblasts do respond to TGF-β, they lack a proliferative and a contractile response and display short-lived myofibroblast differentiation, autocrine response, and collagen up-regulation in comparison with adult fibroblasts. Curiously, prolonged TGF-β activation is associated with fibrosis, and therefore, this short-lived response in fetal fibroblasts might contribute to scarless healing. This review gives an overview of the current knowledge on TGF-β signaling and the intracellular TGF-β signaling pathway in fetal fibroblasts. Furthermore, this review also describes the various components that regulate the cellular TGF-β response and hypothesizes about the possible roles these components might play in the altered response of fetal fibroblasts to TGF-β.

P53 mutations in triple negative breast cancer upregulate endosomal recycling of epidermal growth factor receptor (EGFR) increasing its oncogenic potency

There is no available targeted therapy for triple-negative or its more aggressive subtype, basal-like breast cancer. Multiple therapeutic strategies based on translational knowledge have not improved the treatment options for triple negative patients. As understanding of molecular pathways that drive tumor development is rapidly increasing, it is imperative to adapt our treatment strategies to perturbations in molecular pathways driving the malignant process. Basal-like breast cancers over-express EGFR (without mutations or EGFR gene amplifications) and have p53 mutations. While EGFR drives the malignant behavior in triple negative breast cancer (TNBC), anti-EGFR therapies have fallen short of the expected results in clinical trials. Here we bring evidence that the less than optimal results of the anti-EGFR therapies may be explained in part by the increased potency of the EGFR signaling due to increased endosomal recycling. The functional connection between EGFR and endosomal trafficking in TNBC is mutant p53 found in the most aggressive forms of TNBC. Mutant p53 acquires oncogenic functions and binds p63 protein, a member of p53 family with tumor suppressor activities. In the absence of functional p63 there is an upregulation of endosomal recycling EGFR and integrin to the membrane with increased proinvasive abilities of cancer cells. Blocking endosomal trafficking combined with anti-EGFR treatments may result in better clinical outcomes in TNBC.