ABSTRACT Astroglial cells are key to maintain nervous system homeostasis. Neurotrophins are known for their pleiotropic effects on neuronal physiology but also exert complex functions to glial cells. Here, we investigated (i) the signaling competence of mouse embryonic and postnatal primary cortical astrocytes exposed to brain-derived neurotrophic factor (BDNF) and, (ii) the role of kinase D-interacting substrate of 220 kDa (Kidins220), a transmembrane scaffold protein that mediates neurotrophin signaling in neurons. We found a shift from a kinase-based response in embryonic cells to a response predominantly relying on intracellular Ca 2+ transients [Ca 2+ ] i within postnatal cultures, associated with a decrease in the synthesis of full-length BDNF receptor TrkB, with Kidins220 contributing to the BDNF-activated kinase and [Ca 2+ ] i pathways. Finally, Kidins220 participates in the homeostatic function of astrocytes by controlling the expression of the ATP-sensitive inward rectifier potassium channel 10 (Kir4.1) and the metabolic balance of embryonic astrocytes. Overall, our data contribute to the understanding of the complex role played by astrocytes within the central nervous system, and identify Kidins220 as a novel actor in the increasing number of pathologies characterized by astrocytic dysfunctions. This article has an associated First Person interview with the first authors of the paper .

ABSTRACT Ubiquitylation of receptor tyrosine kinases (RTKs) regulates both the levels and functions of these receptors. The neurotrophin receptor TrkB (also known as NTRK2), a RTK, is ubiquitylated upon activation by brain-derived neurotrophic factor (BDNF) binding. Although TrkB ubiquitylation has been demonstrated, there is a lack of knowledge regarding the precise repertoire of proteins that regulates TrkB ubiquitylation. Here, we provide mechanistic evidence indicating that ubiquitin carboxyl-terminal hydrolase 8 (USP8) modulates BDNF- and TrkB-dependent neuronal differentiation. USP8 binds to the C-terminus of TrkB using its microtubule-interacting domain (MIT). Immunopurified USP8 deubiquitylates TrkB in vitro , whereas knockdown of USP8 results in enhanced ubiquitylation of TrkB upon BDNF treatment in neurons. As a consequence of USP8 depletion, TrkB levels and its activation are reduced. Moreover, USP8 protein regulates the differentiation and correct BDNF-dependent dendritic formation of hippocampal neurons in vitro and in vivo . We conclude that USP8 positively regulates the levels and activation of TrkB, modulating BDNF-dependent neuronal differentiation. This article has an associated First Person interview with the first author of the paper.

ABSTRACT Peroxisomes are single-membrane organelles present in eukaryotes. The functional importance of peroxisomes in humans is represented by peroxisome-deficient peroxisome biogenesis disorders (PBDs), including Zellweger syndrome. Defects in the genes that encode the 14 peroxins that are required for peroxisomal membrane assembly, matrix protein import and division have been identified in PBDs. A number of recent findings have advanced our understanding of the biology, physiology and consequences of functional defects in peroxisomes. In this Review, we discuss a cooperative cell defense mechanisms against oxidative stress that involves the localization of BAK (also known as BAK1) to peroxisomes, which alters peroxisomal membrane permeability, resulting in the export of catalase, a peroxisomal enzyme. Another important recent finding is the discovery of a nucleoside diphosphate kinase-like protein that has been shown to be essential for how the energy GTP is generated and provided for the fission of peroxisomes. With regard to PBDs, we newly identified a mild mutation, Pex26-F51L that causes only hearing loss. We will also discuss findings from a new PBD model mouse defective in Pex14, which manifested dysregulation of the BDNF–TrkB pathway, an essential signaling pathway in cerebellar morphogenesis. Here, we thus aim to provide a current view of peroxisome biogenesis and the molecular pathogenesis of PBDs.

ABSTRACT The secretory protein brain-derived neurotrophic factor (BDNF) is assumed to be a key factor for the induction of synaptic plasticity processes in neurons. However, the molecular mechanisms for activity-dependent release of the protein largely remain elusive. Here, we demonstrate the relevance of the priming factor CAPS1 (also known as CADPS) for the maturation and exocytosis of BDNF-containing secretory granules, as well as for neurotransmitter release from synaptic vesicles. Using live-cell imaging and RNA silencing methods, we show that CAPS1 has a previously unrecognized function in regulating the intragranular pH of BDNF-containing secretory granules. Furthermore, our results demonstrate that acute single-cell knockdown of CAPS1 with unaltered expression in neighboring neurons leads to a strong reduction in the number of fusion-competent secretory granules and to a significant decrease of released BDNF following exocytosis in dendrites of CAPS1-deficient neurons. In addition, our results show a reduction in synaptic vesicle turnover after CAPS1 knockdown without affecting the density of active boutons in hippocampal neurons. Thus, our results reveal new functions of endogenous CAPS1 in the BDNF secretory granule life cycle, thereby representing a new mechanism of neuronal plasticity.

ABSTRACT The p75 neurotrophin receptor (p75 NTR ; also known as NGFR) can mediate neuronal apoptosis in disease or following trauma, and facilitate survival through interactions with Trk receptors . Here we tested the ability of a p75 NTR -derived trophic cell-permeable peptide, c29, to inhibit p75 NTR -mediated motor neuron death. Acute c29 application to axotomized motor neuron axons decreased cell death, and systemic c29 treatment of SOD1 G93A mice, a common model of amyotrophic lateral sclerosis, resulted in increased spinal motor neuron survival mid-disease as well as delayed disease onset. Coincident with this, c29 treatment of these mice reduced the production of p75 NTR cleavage products. Although c29 treatment inhibited mature- and pro-nerve-growth-factor-induced death of cultured motor neurons, and these ligands induced the cleavage of p75 NTR in motor-neuron-like NSC-34 cells, there was no direct effect of c29 on p75 NTR cleavage. Rather, c29 promoted motor neuron survival in vitro by enhancing the activation of TrkB-dependent signaling pathways, provided that low levels of brain-derived neurotrophic factor (BDNF) were present, an effect that was replicated in vivo in SOD1 G93A mice. We conclude that the c29 peptide facilitates BDNF-dependent survival of motor neurons in vitro and in vivo .

ABSTRACT Brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival, neurite outgrowth and synaptic plasticity by activating the receptor tropomyosin receptor kinase B (TrkB, also known as NTRK2). TrkB has been shown to undergo recycling after BDNF stimulation. We have previously reported that full-length TrkB (TrkB-FL) are recycled through a Rab11-dependent pathway upon BDNF stimuli, which is important for the translocation of TrkB-FL into dendritic spines and for the maintenance of prolonged BDNF downstream signaling during long-term potentiation (LTP). However, the identity of the motor protein that mediates the local transfer of recycled TrkB-FL back to the plasma membrane remains unclear. Here, we report that the F-actin-based motor protein myosin Va (Myo5a) mediates the postendocytic recycling of TrkB-FL. Blocking the interaction between Rab11 and Myo5a by use of a TAT-tagged peptide consisting of amino acids 55–66 of the Myo5a ExonE domain weakened the association between TrkB-FL and Myo5a and thus impaired TrkB-FL recycling and BDNF-induced TrkB-FL translocation into dendritic spines. Finally, inhibiting Myo5a-mediated TrkB-FL recycling led to a significant reduction in prolonged BDNF downstream signaling. Taken together, these results show that Myo5a mediates BDNF-dependent TrkB-FL recycling and contributes to BDNF-induced TrkB spine translocation and prolonged downstream signaling.

Summary Brain-derived neurotrophic factor (BDNF) is known to be a crucial regulator of neuronal survival and synaptic plasticity in the mammalian brain. Furthermore, BDNF positively influences differentiation of embryonic neural precursors, as well as that of neural stem cells from adult neurogenic niches. To study the impact of cell-released BDNF on neural differentiation of embryonic stem cells (ESCs), which represent an attractive source for cell transplantation studies, we have generated mouse ESC clones overexpressing BDNF–GFP by use of knock-in technology. After neural differentiation in vitro , we observed that ESC clones overexpressing BDNF–GFP gave rise to an increased number of neurons as compared to control ESCs. Neurons derived from BDNF–GFP-expressing ESCs harbored a more complex dendritic morphology and differentiated into the GABAergic lineage more than controls. Moreover, we show that ESC-derived neurons released BDNF–GFP in an activity-dependent manner and displayed similar electrophysiological properties as cortical neurons. Thus, our study describes the generation of ESCs stably overexpressing BDNF–GFP, which are ideally suited to investigate the ameliorating effects of BDNF in cell transplantation studies of various neuropathological conditions.

The p75 neurotrophin receptor (p75NTR) potentiates Trk signaling, but the underlying mechanisms remain uncertain. Here, we examine the relationship between p75NTR cleavage and Trk signaling. We found that, in PC12 cells, nerve growth factor (NGF) induces rapid and robust α-secretase- and γ-secretase-dependent cleavage of p75NTR, releasing the resulting intracellular domain into the cytosol. Brain-derived neurotrophic factor similarly induces p75NTR cleavage in primary cerebellar granule neurons. p75NTR cleavage occurs by means of Trk-dependent activation of MEK-Erk signaling and induction of α-secretase activity, and is independent of ligand binding to p75NTR. Neurons and PC12 cells lacking p75NTR display defects in neurotrophin-dependent Akt activation. Normal Akt activation is rescued using full-length p75NTR or the p75 intracellular domain, but not cleavage-resistant p75NTR. We then demonstrate that NGF-dependent growth arrest of PC12 cells requires p75NTR cleavage and generation of the intracellular domain. We conclude that generation of the soluble p75NTR intracellular domain by Trk-induced cleavage plays a fundamental role in Trk-dependent signaling events.

The Trk family of receptor tyrosine kinases and the p75 receptor (p75 NTR ) mediate the effects of neurotrophins on neuronal survival, differentiation and synaptic plasticity. The neurotrophin BDNF and its cognate receptor tyrosine kinase, TrkB.FL, are highly expressed in neurons of the central nervous system. At later stages in postnatal development the truncated TrkB splice variants (TrkB.T1, TrkB.T2) become abundant. However, the signalling and function of these truncated receptors remained largely elusive. We show that overexpression of TrkB.T1 in hippocampal neurons induces the formation of dendritic filopodia, which are known precursors of synaptic spines. The induction of filopodia by TrkB.T1 occurs independently of neurotrophin binding and of kinase activity of endogenous TrkB.FL. Coexpression of a p75 NTR lacking an intracellular domain inhibits the TrkB.T1-induced effect in a dominant negative manner. Steric hindrance of extracellular p75 NTR interactions with a specific antibody, or absence of p75 NTR with an intact extracellular domain also inhibit this TrkB.T1-induced effect. We thus propose a novel signalling pathway initiated by neurotrophin-independent extracellular or intramembrane interaction of TrkB.T1 with the p75 NTR receptor, which modulates dendritic growth via p75 NTR signalling cascades.

Exocytotic incorporation of plasmalemmal precursor vesicles (PPVs) into the cell surface is necessary for neurite extension and is known to occur mainly at the growth cone. This report examines whether this is a regulated event controlled by growth factors. The Golgi complex and nascent PPVs of hippocampal neurons in culture were pulse-labeled with fluorescent ceramide. We studied the dynamics of labeled PPVs upon arrival at the axonal growth cone. In controls and cultures stimulated with brain-derived neurotrophic factor (BDNF), PPV clusters persisted in growth cones with a half-life( t 1/2 ) of >14 minutes. Upon challenge with IGF-1,however, fluorescent elements cleared from the growth cones with a t 1/2 of only 6 minutes. Plasmalemmal expansion was measured directly as externalization of membrane glycoconjugates in resealed growth cone particles (GCPs) isolated from fetal forebrain. These assays demonstrated that membrane expansion could be stimulated by IGF-1 in a dose-dependent manner but not by BDNF, even though intact, functional BDNF receptor was present on GCPs. Because both BDNF and IGF-1 are known to enhance neurite growth, but BDNF did not stimulate membrane expansion at the growth cone, we studied the effect of BDNF on the IGF-1 receptor. BDNF was found to cause the translocation of the growth-cone-specific IGF-1 receptor subunitβ gc to the distal axon, in a KIF2-dependent manner. We conclude that IGF-1 stimulates axonal assembly at the growth cone, and that this occurs via regulated exocytosis of PPVs. This mechanism is affected by BDNF only indirectly, by regulation of the β gc level at the growth cone.