Oscar Marcelo Lazo

Brain derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) play crucial roles in neuronal development, synaptic transmission, and neuroplasticity. Deficits in BDNF/TrkB signalling and trafficking have been identified in several neurodegenerative diseases, including Alzheimer’s disease (AD). Individuals with Down syndrome (DS) are at an increased risk of developing AD compared to the general population. Basal forebrain neurons (BFNs) are among the first to degenerate in AD and DS, but the mechanisms underlying their vulnerability remain unclear. Using BFNs derived from the Dp1Tyb mouse model of DS, we investigated neurotrophic signalling and trafficking deficits in AD-DS. We found enlarged early endosomes and elevated levels of active Rab5, a GTPase critical for early endosome formation, in Dp1Tyb BFNs. These abnormalities were associated with impaired transport of internalised TrkB from axon terminals to the soma. Using microfluidic devices, we demonstrated that axonal BDNF stimulation enhanced signalling endosome dynamics in wild-type but not Dp1Tyb BFNs, which is likely due to impaired axonal ERK1/2 signalling. Our findings establish a link between Rab5 hyperactivation, endosomal dysfunction, and impaired ERK1/2 signalling, highlighting the interplay between trafficking and neurotrophic signalling, and underscore the importance of targeting endolysosomal and signalling pathways to mitigate neuronal dysfunction in AD-DS.

Brain derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) play crucial roles in neuronal development, synaptic transmission, and neuroplasticity. Deficits in BDNF/TrkB signalling and trafficking have been identified in several neurodegenerative diseases, including Alzheimer’s disease (AD). Individuals with Down syndrome (DS) are at an increased risk of developing AD compared to the general population. Basal forebrain neurons (BFNs) are among the first to degenerate in AD and DS, but the mechanisms underlying their vulnerability remain unclear. Using BFNs derived from the Dp1Tyb mouse model of DS, we investigated neurotrophic signalling and trafficking deficits in AD-DS. We found enlarged early endosomes and elevated levels of active Rab5, a GTPase critical for early endosome formation, in Dp1Tyb BFNs. These abnormalities were associated with impaired transport of internalised TrkB from axon terminals to the soma. Using microfluidic devices, we demonstrated that axonal BDNF stimulation enhanced signalling endosome dynamics in wild-type but not Dp1Tyb BFNs, which is likely due to impaired axonal ERK1/2 signalling. Our findings establish a link between Rab5 hyperactivation, endosomal dysfunction, and impaired ERK1/2 signalling, highlighting the interplay between trafficking and neurotrophic signalling, and underscore the importance of targeting endolysosomal and signalling pathways to mitigate neuronal dysfunction in AD-DS.

Neurons process real-time information from axon terminals to coordinate gene expression, growth, and plasticity. Inputs from distal axons are encoded as a stream of endocytic organelles, termed signalling endosomes, targeted to the soma. Formation of these organelles depends on target-derived molecules, such as brain-derived neurotrophic factor (BDNF), which is recognised by TrkB receptors on the plasma membrane, endocytosed, and transported to the cell body along the microtubules network. Notwithstanding its physiological and neuropathological importance, the mechanism controlling the sorting of TrkB to signalling endosomes is currently unknown. In this work, we use primary mouse neurons to uncover the small GTPase Rab10 as critical for TrkB sorting and propagation of BDNF signalling from axon terminals to the soma. Our data demonstrate that Rab10 defines a novel membrane compartment that is rapidly mobilised towards the axon terminal upon BDNF stimulation, enabling the axon to fine-tune retrograde signalling depending on BDNF availability at the synapse. These results help clarifying the neuroprotective phenotype recently associated to Rab10 polymorphisms in Alzheimer’s disease and provide a new therapeutic target to halt neurodegeneration.

Pathological tau aggregates propagate across functionally connected neuronal networks in human neurodegenerative pathologies, such as Alzheimer’s disease. However, the mechanism underlying this process is poorly understood. Several studies have showed that tau release is dependent on neuronal activity and that pathological tau is found in the extracellular space in free form, as well as in the lumen of extracellular vesicles. We recently showed that metabotropic glutamate receptor activity and the SNAP25 integrity modulate the release of pathological tau from human and mouse synaptosomes. Here, we have leveraged botulinum neurotoxins (BoNTs), which impair neurotransmitter release by cleaving specific synaptic SNARE proteins, to dissect molecular mechanisms related to tau release at synapses. In particular, we have tested the effect of botulinum neurotoxin A (BoNT/A) on the synaptic release of tau in primary mouse neurons. Hippocampal neurons were grown in microfluidic chambers and transduced with lentiviruses expressing human tau (hTau). We found that neuronal stimulation significantly increases the release of mutant hTau, whereas wild-type hTau is unaffected. Importantly, BoNT/A blocks mutant hTau release, indicating that this process is modulated by SNAP25 in intact neurons. These results suggest that BoNTs are potent tools to study the spreading of pathological proteins in neurodegenerative diseases and will play a central role in identifying novel molecular targets for the development of therapeutic interventions to treat tauopathies.

Neurons process real-time information from axon terminals to coordinate gene expression, growth and plasticity. Input from distal axons is encoded as a stream of endocytic organelles, termed signalling endosomes, targeted to the soma. Formation of these organelles depends on target-derived molecules, such as brain-derived neurotrophic factor (BDNF), which is recognised by TrkB receptors on the plasma membrane, endocytosed and transported to the cell body along the microtubules network. Notwithstanding its neuropathological importance, the mechanism controlling the sorting of TrkB to signalling endosomes is currently unknown. In this work, we uncover the small GTPase Rab10 as critical for TrkB sorting and propagation of BDNF signalling from axon terminals to the soma. Our data demonstrate that Rab10 defines a novel class of axonal organelles, rapidly mobilised towards the axon terminal upon BDNF stimulation, enabling the axon to fine-tune retrograde signalling depending on BDNF availability at the synapse. These results help clarifying the neuroprotective phenotype recently associated to Rab10 polymorphisms and provide a new therapeutic target to halt neurodegeneration.

Brain-derived neurotrophic factor (BDNF) is a key regulator of the morphology and connectivity of central neurons. We have previously shown that BDNF/TrkB signaling regulates the activity and mobility of the GTPases Rab5 and Rab11, which in turn determine the postendocytic sorting of signaling TrkB receptors. Moreover, decreased Rab5 or Rab11 activity inhibits BDNF-induced dendritic branching. Whether Rab5 or Rab11 activity is important for local events only or for regulating nuclear signaling and gene expression is unknown. Here, we investigated, in rat hippocampal neuronal cultures derived from embryos of unknown sex, whether BDNF-induced signaling cascades are altered when early and recycling endosomes are disrupted by the expression of dominant-negative mutants of Rab5 and Rab11. The activity of both Rab5 and Rab11 was required for sustained activity of Erk1/2 and nuclear CREB phosphorylation, and increased transcription of a BDNF-dependent program of gene expression containing CRE binding sites, which includes activity-regulated genes such asArc,Dusp1,c-fos,Egr1, andEgr2, and growth and survival genes such asAtf3andGem. Based on our results, we propose that early and recycling endosomes provide a platform for the integration of neurotrophic signaling from the plasma membrane to the nucleus in neurons, and that this mechanism is likely to regulate neuronal plasticity and survival.SIGNIFICANCE STATEMENTBDNF is a neurotrophic factor that regulates plastic changes in the brain, including dendritic growth. The cellular and molecular mechanisms underlying this process are not completely understood. Our results uncover the cellular requirements that central neurons possess to integrate the plasma membrane into nuclear signaling in neurons. Our results indicate that the endosomal pathway is required for the signaling cascade initiated by BDNF and its receptors at the plasma membrane to modulate BDNF-dependent gene expression and neuronal dendritic growth mediated by the CREB transcription factor. CREB is a key transcription factor regulating circuit development and learning and memory.

Brain-derived neurotrophic factor (BDNF) is a key regulator of the morphology and connectivity of central neurons. We have previously shown that BDNF/TrkB signaling regulates the activity and mobility of the GTPases Rab5 and Rab11, which in turn determine the post-endocytic sorting of signaling TrkB receptors. Moreover, altered Rab5 or Rab11 activity inhibits BDNF-induced dendritic branching. Whether Rab5 or Rab11 activity is important for local events only, or also for regulating nuclear signaling and gene expression, is unknown. Here, we investigated whether BDNF-induced signaling cascades were altered when early and recycling endosomes were disrupted by the expression of dominant negative mutants of Rab5 and Rab11. The activities of both Rab5 and Rab11 were required for sustained activity of Erk1/2 and nuclear CREB phosphorylation and for increased transcription of BDNF-dependent genes containing CRE-binding sites that include activity-regulated genes such as Arc , Dusp1 , c-fos and Egr1 and growth and survival genes such as Atf3 and Nf1 . Based on our results, we propose that the early and recycling endosomes provide a platform for the integration of neurotrophic signaling from the plasma membrane to the nucleus in neurons and that this mechanism likely regulates neuronal plasticity and neuronal survival. Significance Statement: BDNF is a soluble neurotrophic factor that regulates plastic changes in the brain, including dendritic growth, by binding to its plasma membrane receptor TrkB. BDNF/TrkB activates signaling cascades leading to activation of CREB, a key transcription factor regulating circuit development and learning and memory. Our results uncover the cellular mechanisms that central neurons use to integrate the signaling of plasma membrane receptors with nuclear transcriptional responses. We found that the endosomal pathway is required for the signaling cascade initiated by BDNF and its receptors in the plasma membranes to modulate BDNF-dependent gene expression and neuronal dendritic growth mediated by the CREB transcription factor in the nucleus.

Neurons are highly polarised cells that critically depend on long‐range, bidirectional transport between the cell body and synapse for their function. This continual and highly coordinated trafficking process, which takes place via the axon, has fascinated researchers since the early twentieth century. Ramon y Cajal first proposed the existence of axonal trafficking of biological material after observing that dissociation of the axon from the cell body led to neuronal degeneration. Since these first indirect observations, the field has come a long way in its understanding of this fundamental process. However, these advances in our knowledge have been aided by breakthroughs in other scientific disciplines, as well as the parallel development of novel tools, techniques and model systems. In this review, we summarise the evolution of tools used to study axonal transport and discuss how their deployment has refined our understanding of this process. We also highlight innovative tools currently being developed and how their addition to the available axonal transport toolkit might help to address key outstanding questions.

Neurons are highly complex and polarised cells that must overcome a series of logistic challenges to maintain homeostasis across their morphological domains. A very clear example is the propagation of neurotrophic signalling from distal axons, where target‐released neurotrophins bind to their receptors and initiate signalling, towards the cell body, where nuclear and cytosolic responses are integrated. The mechanisms of propagation of neurotrophic signalling have been extensively studied and, eventually, the model of a ‘signalling endosome’, transporting activated receptors and associated complexes, has emerged. Nevertheless, the exact nature of this organelle remains elusive. In this Review, we examine the evidence for the retrograde transport of neurotrophins and their receptors in endosomes, outline some of their diverse physiological and pathological roles, and discuss the main interactors, morphological features and trafficking destinations of a highly flexible endosomal signalling organelle with multiple molecular signatures.