New paper shows novel function of the GFRα1 receptor

In this new paper, we show how the GFRα1 receptor regulates Purkinje cell migration independently of GDNF or RET, by limiting the function of NCAM. The paper has just been published in Cell Reports.

During embryonic development of the cerebellum, Purkinje cells (PCs) migrate away from the ventricular zone to form the PC plate. The mechanisms that regulate PC migration are incompletely understood. Here, we report that the neurotrophic receptor GFRα1 is transiently expressed in developing PCs and loss of GFRα1 delays PC migration. Neither GDNF nor RET, the canonical GFRα1 ligand and co-receptor, respectively, contribute to this process. Instead, we found that the neural cell adhesion molecule NCAM is co-expressed and directly interacts with GFRα1 in embryonic PCs. Genetic reduction of NCAM expression enhances wild-type PC migration and restores migration in Gfra1 mutants, indicating that NCAM restricts PC migration in the embryonic cerebellum. In vitro experiments indicated that GFRα1 can function both in cis and trans to counteract NCAM and promote PC migration. Collectively, our studies show that GFRα1 contributes to PC migration by limiting NCAM function.

Read the full paper HERE.

New paper shows how thalamo-cortical axons regulate the radial dispersion of neocortical GABAergic interneurons

In our latest paper, we show how thalamo-cortical axons regulate the radial dispersion of neocortical GABAergic interneurons. The paper has just been published in eLife.

Neocortical GABAergic interneuron migration and thalamo-cortical axon (TCA) pathfinding follow similar trajectories and timing, suggesting they may be interdependent. The mechanisms that regulate the radial dispersion of neocortical interneurons are incompletely understood. In this new study we report that disruption of TCA innervation, or TCA-derived glutamate, affected the laminar distribution of GABAergic interneurons in mouse neocortex, resulting in abnormal accumulation in deep layers of interneurons that failed to switch from tangential to radial orientation. Expression of the KCC2 cotransporter was elevated in interneurons of denervated cortex, and KCC2 deletion restored normal interneuron lamination in the absence of TCAs. Disruption of interneuron NMDA receptors or pharmacological inhibition of calpain also led to increased KCC2 expression and defective radial dispersion of interneurons. Thus, although TCAs are not required to guide the tangential migration of GABAergic interneurons, they provide crucial signals that restrict interneuron KCC2 levels, allowing coordinated neocortical invasion of TCAs and interneurons.

Read the full paper HERE. (Supplemental information 31.6MB)

New paper demonstrates requirement of p75NTR death domain and transmembrane cysteine for neuronal death in the CNS

In our latest paper, we show how dimers of the p75NTR neurotrophin receptor are indipensable for p75NTR-mediated cell death in the central nervous system. The paper has justg been published in the Journal of Neuroscience.

The oligomeric state and activation mechanism that enable p75 NTR to mediate these effects have recently been called into question. In this new study, we have investigated mutant mice lacking the p75NTR death domain (DD) or a highly conserved transmembrane (TM) cysteine residue (Cys 259) implicated in receptor dimerization and activation. Neuronal death induced by proneurotrophins or epileptic seizures was assessed and compared with responses in p75NTR knock-out mice and wild-type animals. Proneurotrophins induced apoptosis of culturedhippocampalandcorticalneuronsfromwild-typemice,butmutantneuronslackingp75NTR,onlythep75NTR DD,orjustCys259 were all equally resistant to proneurotrophin-induced neuronal death. Homo-FRET anisotropy experiments demonstrated that both NGF and proNGF induce conformational changes in p75 NTR that are dependent on the TM cysteine. In vivo, neuronal death induced by pilocarpine-mediated seizures was significantly reduced in the hippocampus and somatosensory, piriform, and entorhinal cortices of all three strains of p75 NTR mutant mice. Interestingly, the levels of protection observed in mice lacking the DD or only Cys 259 were identical to those of p75 NTR knock-out mice even though the Cys 259 mutant differed from the wild-type receptor in only one amino acid residue. We conclude that,bothinvitroandinvivo,neuronaldeathinducedbyp75NTR requirestheDDandTMCys259,supportingthephysiological relevance of DD signaling by disulfide-linked dimers of p75NTR in the CNS.

Read the full paper HERE.

New review article published: Biology of GDNF and its receptors — Relevance for disorders of the central nervous system

A targeted effort to identify novel neurotrophic factors for midbrain dopaminergic neurons resulted in the isolation of GDNF (glial cell line-derived neurotrophic factor) from the supernatant of a rat glial cell line in 1993. Over two decades and 1200 papers later, the GDNF ligand family and their different receptor systems are now recognized as one of the major neurotrophic networks in the nervous system, important for the devel- opment, maintenance and function of a variety of neurons and glial cells. The many ways in which the four mem- bers of the GDNF ligand family can signal and function allow these factors to take part in the control of multiple types of processes, from neuronal survival to axon guidance and synapse formation in the developing nervous system, to synaptic function and regenerative responses in the adult. In this review, recently published in Neurobiology Of Disease, basic aspects of GDNF signaling mechanisms and receptor systems are first summarized followed by a review of current knowledge on the physiology of GDNF activities in the central nervous system, with an eye to its relevance for neurodegenerative and neuropsychiatric diseases. Read the full paper HERE.

New paper describes first structures of protein complexes of the p75NTR death domain

Our latest paper describes new NMR structures of the death domain in complex with downstream interactions RhoGDI and RIP2 as well as the death domain dimer. These are the first structural insights into p75NTR signaling and reveal many surprises for the death domain superfamily. The paper is now available online at eLife

Death domains (DDs) mediate assembly of oligomeric complexes for activation of downstream signaling pathways through incompletely understood mechanisms. We report structures of complexes formed by the DD of p75 neurotrophin receptor (p75NTR) with RhoGDI, for activation of the RhoA pathway, with caspase recruitment domain (CARD) of RIP2 kinase, for activation of the NF-kB pathway, and with itself, revealing how DD dimerization controls access of intracellular effectors to the receptor. RIP2 CARD and RhoGDI bind to p75NTR DD at partially overlapping epitopes with over 100-fold difference in affinity, revealing the mechanism by which RIP2 recruitment displaces RhoGDI upon ligand binding. The p75NTR DD forms non-covalent, low-affinity symmetric dimers in solution. The dimer interface overlaps with RIP2 CARD but not RhoGDI binding sites, supporting a model of receptor activation triggered by separation of DDs. These structures reveal how competitive protein-protein interactions orchestrate the hierarchical activation of downstream pathways in non-catalytic receptors.

Mechanism for neuron-type-specific signaling by the p75NTR death receptor unravelled

In our latest paper, we show that the p75 neurotrophin receptor p75NTR can signal very differently in diferent types of neurons. Using pharmacological and genetic techniques, we demonstrate that this is partly controlled by differential proteolytic cleavage of the receptor in different cell types. The new work has appeared online in the Journal of Cell Science

Signaling by the p75 neurotrophin receptor (p75NTR) is often referred to as cell-context dependent, but neuron-type specific signaling by p75NTR has not been systematically investigated. Here, we report that p75NTR signals very differently in hippocampal neurons (HCNs) and cerebellar granule neurons (CGNs), and present evidence indicating that this is partly controlled by differential proteolytic cleavage. NGF induced caspase-3 activity and cell death in HCNs but not in CGNs, while it stimulated NFκB activity in CGNs but not in HCNs. HCNs and CGNs displayed different patterns of p75NTRproteolytic cleavage. While the p75NTR carboxy terminal fragment (CTF) was more abundant than the intracellular domain (ICD) in HCNs, CGNs exhibited fully processed ICD with very little CTF. Pharmacological or genetic blockade of p75NTR cleavage by gamma-secretase abolished NGF-induced upregulation of NFκB activity and enabled induction of CGN death, phenocopying the functional profile of HCNs. Thus, the activities of multifunctional receptors, such as p75NTR, can be tuned into narrower activity profiles by cell-type-specific differences in intracellular processes, such as proteolytic cleavage, leading to very different biological outcomes. Read the full article HERE.

New discovery explains how ALK7 receptor regulates fat accumulation in obesity

In our latest paper, we report that the sensitivity of fat cells to signals that increase the breakdown of fat is linked to the receptor ALK7. The discovery, which is published today in eLife, suggests that ALK7 is an interesting target for future strategies to treat obesity.

The ALK7 receptor is predominantly found in fat cells and tissues involved in controlling the metabolism. Intriguingly, mice with a mutation in ALK7 accumulate less fat than mice with a functional version of the protein. Until now, it has not been known why.

We created mice whose fat cells lack ALK7, but whose other cells all produce ALK7 as normal. We found that fat cells lacking the ALK7 receptor are more sensitive to adrenaline and noradrenaline signals, a finding that can explain why they accumulate less fat even though the mice were on a high-fat diet. Adrenaline and noradrenaline are central players in metabolism. These hormones trigger the burst of energy and increase in heart rate and blood pressure that are needed for the “fight-or-flight” response. The hormones normally stimulate the breakdown of fat, but when nutrients are plentiful, fat cells become resistant to this signal and instead store fat. This mechanism evolved to facilitate energy storage during times of abundant food supply, enhancing survival upon starvation. In the industrialized world where food is constantly accessible, this resistance can cause an unhealthy increase in body fat and result in obesity.

We then investigated if it is possible to prevent obesity by blocking ALK7. At present, there are no known ALK7 inhibitors, but we solved this by generating mice with a special mutation in ALK7 which renders it sensitive to inhibition by a chemical substance. This made it possible for us to block the receptor at any time in an otherwise normal adult animal.

– Using this approach, we could get these mice to be leaner on a high fat diet simply by administration of the chemical. This suggests that acute inhibition of the ALK7 receptor can prevent obesity in adult animals, says Tingqing Guo, first author of the study.

We have also showed that the ALK7 receptor works in a similar way in human fat cells as it does in mice.

– Overall, these results suggest that blockade of the ALK7 receptor could represent a novel strategy to combat human obesity, says Carlos Ibanez, principal investigator of the study.

The work was supported by grants from the European Research Council, Swedish Research Council, Strategic Research Program in Diabetes of Karolinska Institutet, Swedish Cancer Society, Knut and Alice Wallenberg Foundation, the National University of Singapore and the National Medical Research Council of Singapore. eLife is a peer-reviewed open-access scientific journal established at the end of 2012 by Nobel Prize Winner Randy Schekman, with support from the Howard Hughes Medical Institute, Max Planck Society and Wellcome Trust.

The paper is freely accessible and can be found HERE.

New paper reveals differential regulation of pancreatic insulin secretion by Smad proteins and activin ligands

Diabetologia has now published online our latest paper describing differential actions of activins A and B and Smad proteins 2 and 3 on the regulation of insulin secretion by pancreatic beta cells (Wu et al., 2013).

Glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells is regulated by paracrine factors whose identity and mechanisms of action are incompletely understood. Activins are expressed in pancreatic islets and have been implicated in the regulation of GSIS. Activins A and B signal through a common set of intracellular components, but it is unclear whether they display similar or distinct functions in glucose homeostasis. Glucose homeostatic responses were examined in mice lacking activin B and in pancreatic islets derived from these mutants. The ability of activins A and B to regulate downstream signalling, ATP production and GSIS in islets and in beta-cells was compared. Mice lacking activin-B displayed elevated serum insulin levels and glucose-stimulated insulin release. Injection of a soluble activin B antagonist phenocopied these changes in wild type mice. Isolated pancreatic islets from mutant mice showed enhanced GSIS which could be rescued by exogenous activin B. Activin B negatively regulated GSIS and ATP production in wild type islets, while activin-A displayed opposite effects. The downstream mediator Smad3 responded preferentially to activin B in pancreatic islets and beta-cells, while Smad2 showed preference for activin A, indicating distinct signalling effects of the two activins. In line with this, overexpression of Smad3, but not Smad2, decreased GSIS in pancreatic islets. These results reveal a tug-of-war between activin ligands in the regulation of insulin secretion by beta-cells and suggest that manipulation of activin signalling could be a useful strategy for the control of glucose homeostasis in diabetes and metabolic disease.

Read the full article HERE.

New paper provides insights into the logic of neurotrophin signaling through the p75 neurotrophin receptor

Cell Reports publishes today our latest paper describing a structure-function map of the death domain of the p75 neurotrophin receptor (Charalampopoulos et al. 2012)

Structural determinants underlying signaling specificity in the tumor necrosis factor receptor superfamily (TNFRSF) are poorly characterized and it is unclear whether different signaling outputs can be genetically dissociated. The p75 neurotrophin receptor (p75NTR), also known as TNFRSF16, is a key regulator of trophic and injury responses in the nervous system. In this paper, we describe a genetic approach to dissect p75NTR signaling and decipher its underlying logic. Structural determinants important for regulation of cell death, NF-kB and RhoA pathways were identified in the p75NTR death domain. Pro-apoptotic and pro-survival pathways mapped onto non-overlapping epitopes, demonstrating that different signaling outputs can be genetically separated in p75NTR. Dissociation of JNK and caspase-3 activities indicated that JNK is necessary but not sufficient for p75NTR-mediated cell death. RIP2 recruitment and RhoGDI release were mechanistically linked, indicating that competition for DD binding underlies cross-talk between NF-kB and RhoA pathways in p75NTR signaling. These results provide new insights into the logic of p75NTR signaling and pave the way for a genetic dissection of p75NTR function and physiology.

Read the full paper HERE.

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Death Domain Signaling by Disulfide-Linked Dimers of the p75 Neurotrophin Receptor Mediates Neuronal Death in the CNS

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