Dendritic arborization of neurons is regulated by brain-derived neurotrophic factor (BDNF) together with its receptor TrkB. Endocytosis is required for dendritic branching and regulates TrkB signaling, but how post-endocytic trafficking determine the neuronal response to BDNF is not well understood. The monomeric GTPase Rab11 regulates the dynamics of recycling endosomes and local delivery of receptors to specific dendritic compartments. Our aim was to study whether Rab11-dependent trafficking of TrkB in dendrites regulates BDNF-induced dendritic branching in rat hippocampal neurons. We report that TrkB in dendrites is a cargo for Rab11 endosomes and both Rab11 and its effector MyoVb are required for BDNF/TrkB-induced dendritic branching. In turn, BDNF induces accumulation of Rab11-positive endosomes and GTP-bound Rab11 in dendrites. Moreover, the expression of a constitutively active mutant of Rab11 is sufficient to increase dendritic branching by increasing TrkB localization in dendrites and enhancing sensitization to endogenous BDNF. We propose that Rab11-dependent dendritic recycling provides a mechanism to retain TrkB in dendrites and increase local signaling to regulate arborization.

Brain-derived neurotrophic factor (BDNF) and its receptors TrkB and p75 regulate dendritic and axonal growth during development and maintenance of the mature nervous system; however, the cellular and molecular mechanisms underlying this process are not fully understood. In recent years, several advances have shed new light on the processes behind the regulation of BDNF-mediated structural plasticity including control of neuronal transcription, local translation of proteins, and regulation of cytoskeleton and membrane dynamics. In this review, we summarize recent advances in the field of BDNF signaling in neurons to induce neuronal growth. V C 2016 Wiley Periodicals, Inc.Key Words: BDNF; dendritic growth; signaling; cytoskeleton dynamics; membrane trafficking Introduction T he functionality of the nervous system (NS) depends on the extent of connectivity achieved by neurons. Although there is wide range of neuronal morphologies, neurons are specialized to form part of a given neuronal circuit and thus their morphology must adjust to this task [Gao, 2007]. In this regard, neurons are highly polarized cells composed of two main domains: the somato-dendritic compartment where neurons receive and integrate information from several axonal inputs and axons that are responsible for transmitting the action potential and conveying the information to the next relay in the network. A close relationship between the appropriate development of dendrites and axons and the functionality of the NS has been described. For example, alterations in dendrites, axonal inputs and dendritic spines are linked to neurodevelopmental and neuropathological conditions. Thus, the study of the mechanisms implicated in the regulation of neuronal morphology during development and maintenance of the adult NS is a focus of intense research [Armstrong et al., 1998;Wood et al., 2004;Bronfman et al., 2007;Dickstein et al., 2010;Cabeza et al., 2012;Eiland and McEwen, 2012;Kulkarni and Firestein, 2012].Neuronal morphology is regulated by both intrinsic and extrinsic factors, whose actions are overlapping. The intrinsic factors are described as the action of the genetic program of neurons that leads to a basic pattern of branching. In addition to this, there is an extensive list of molecules grouped as extrinsic factors that shape axonal and dendritic morphology throughout development and in response to sensory experience. These include bone morphogenetic family proteins (BMPs), semaphorins, Reelin, neurotrophins, and neuronal activity, among others [Jan and Jan, 2010]. The mechanisms underlying the effects of external cues to induce dendritic and axonal branching have only begun to be understood, and several steps including stimulation of intracellular signaling pathways to activate specific transcription factors, local synthesis of proteins and cellular membrane addition and turnover are involved. An interesting point of convergence is the effect of these external signals on the stability and dynamics of the cytoskeleton and the activity of molecular mo...

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Studies in recent years have established that the principal effects in cardiac cell therapy are associated with paracrine/autocrine factors. We combined several complementary techniques to define human cardiac progenitor cell (CPC) secretome constituted by 914 proteins/genes; 51% of these are associated with the exosomal compartment. To define the set of proteins specifically or highly differentially secreted by CPC, we compared human mesenchymal stem cells and dermal fibroblasts; the study defined a group of growth factors, cytokines and chemokines expressed at high to medium levels by CPC. Among them, IL-1, GROa (CXCL1), CXCL6 (GCP2) and IL-8 are examples whose expression was confirmed by most techniques used. ELISA showed that CXCL6 is significantly overexpressed in CPC conditioned medium (CM) (18- to 26-fold) and western blot confirmed expression of its receptors CXCR1 and CXCR2. Addition of anti-CXCL6 completely abolished migration in CPC-CM compared with anti-CXCR2, which promoted partial inhibition, and anti-CXCR1, which was inefficient. Anti-CXCL6 also significantly inhibited CPC CM angiogenic activity. In vivo evaluation also supported a relevant role for angiogenesis. Altogether, these results suggest a notable angiogenic potential in CPC-CM and identify CXCL6 as an important paracrine factor for CPC that signals mainly through CXCR2.

Neurotrophin receptors use endosomal pathways for signaling in neurons. However, how neurotrophins regulate the endosomal system for proper signaling is unknown. Rabs are monomeric GTPases that act as molecular switches to regulate membrane trafficking by binding a wide range of effectors. Among the Rab GTPases, Rab5 is the key GTPase regulating early endosomes and is the first sorting organelle of endocytosed receptors. The objective of our work was to study the regulation of Rab5-positive endosomes by BDNF at different levels, including dynamic, activity and protein levels in hippocampal neurons. Short-term treatment with BDNF increased the colocalization of TrkB in dendrites and cell bodies, increasing the vesiculation of Rab5-positive endosomes. Consistently, BDNF increased the number and mobility of Rab5 endosomes in dendrites. Cell body fluorescence recovery after photobleaching of Rab-EGFP-expressing neurons suggested increased movement of Rab5 endosomes from dendrites to cell bodies. These results correlated with the BDNF-induced activation of Rab5 in dendrites, followed by increased activation of Rab5 in cell bodies. Long-term treatment of hippocampal neurons with BDNF increased the protein levels of Rab5 and Rab11 in an mTOR-dependent manner. While BDNF regulation of Rab5a levels occurred at both the transcriptional and translational levels, Rab11a levels were regulated at the translational level at the time points analyzed. Finally, expression of a dominant-negative mutant of Rab5 reduced the basal arborization of nontreated neurons, and although BDNF was partially able to rescue the effect of Rab5DN at the level of primary dendrites, BDNF-induced dendritic branching was largely reduced. Our findings indicate that BDNF regulates the Rab5-Rab11 endosomal system at different levels and that these processes are likely required for BDNF-induced dendritic branching.