Axonal Branching Research Articles

Embryonic medial ganglionic eminence cells survive and integrate into the inferior colliculus of adult mice

Acoustic overexposure can lead to decreased inhibition in auditory centers, including the inferior colliculus (IC), and has been implicated in the development of central auditory pathologies. While systemic drugs that increase GABAergic transmission have been shown to provide symptomatic relief, their side effect profiles impose an upper-limit on the dose and duration of use. A treatment that locally increases inhibition in auditory nuclei could mitigate these side effects. One such approach could be transplantation of inhibitory precursor neurons derived from the medial ganglionic eminence (MGE). The present study investigated whether transplanted MGE cells can survive and integrate into the IC of non-noise exposed and noise exposed mice. MGE cells were harvested on embryonic days 12-14 and injected bilaterally into the IC of adult mice, with or without previous noise exposure. At one-week post transplantation, MGE cells possessed small, elongated soma and bipolar processes, characteristic of migrating cells. By 5 weeks, MGE cells exhibited a more mature morphology, with multiple branching processes and axons with boutons that stain positive for the vesicular GABA transporter (VGAT). The MGE survival rate after 14 weeks post transplantation was 1.7% in non-noise exposed subjects. MGE survival rate was not significantly affected by noise exposure (1.2%). In both groups the vast majority of transplanted MGE cells (>97%) expressed the vesicular GABA transporter. Furthermore, electronmicroscopic analysis indicated that transplanted MGE cells formed synapses with and received synaptic endings from host IC neurons. Acoustic stimulation lead to a significant increase in the percentage of endogenous inhibitory cells that express c-fos but had no effect on the percentage of c-fos expressing transplanted MGE cells. MGE cells were observed in the IC up to 22 weeks post transplantation, the longest time point investigated, suggesting long term survival and integration. These data provide the first evidence that transplantation of MGE cells is viable in the IC and provides a new strategy to explore treatment options for central hearing dysfunction following noise exposure.

Zebrafish Larvae Rapidly Recover from Locomotor Effects and Neuromuscular Alterations Induced by Cholinergic Insecticides.

Owing to the importance of acetylcholine as a neurotransmitter, many insecticides target the cholinergic system. Across phyla, cholinergic signaling is essential for many neuro-developmental processes including axonal pathfinding and synaptogenesis. Consequently, early-life exposure to such insecticides can disturb these processes, resulting in an impaired nervous system. One test frequently used to assess developmental neurotoxicity is the zebrafish light-dark transition test, which measures larval locomotion as a response to light changes. However, it is only poorly understood which structural alterations cause insecticide-induced locomotion defects and how persistent these alterations are. Therefore, this study aimed to link locomotion defects with effects on neuromuscular structures, including motorneurons, synapses, and muscles, and to investigate the longevity of the effects. The cholinergic insecticides diazinon and dimethoate (organophosphates), methomyl and pirimicarb (carbamates), and imidacloprid and thiacloprid (neonicotinoids) were used to induce hypoactivity. Our analyses revealed that some insecticides did not alter any of the structures assessed, while others affected axon branching (methomyl, imidacloprid) or muscle integrity (methomyl, thiacloprid). The majority of effects, even structural, were reversible within 24 to 72 h. Overall, we find that both neurodevelopmental and non-neurodevelopmental effects of different longevity can account for the reduced locomotion. These findings provide unprecedented insights into the underpinnings of insecticide-induced hypoactivity.

Central projections of cercal giant interneurons in the adult field cricket, Gryllus bimaculatus.

The structures of neurons, such as dendrites and axonal projections, are closely related to their response properties and their specific functions in neural circuits. Identified neurons, having genetically determined morphological features and pre- and postsynaptic partners, play significant roles in specific behaviors. Giant interneurons (GIs) are identified in the terminal abdominal ganglion of the cricket as mechanosensory projection neurons and are sensitive to airflow stimulation of the cerci. GIs are classified into ventral GIs (vGIs) or dorsal GIs (dGIs) depending on the location of their axons running within the connective nerve cord. Based on their response properties to airflow, vGIs are presumed to be involved in triggering the wind-elicited escape response, whereas dGIs are thought to be airflow direction-encoding neurons. The previous findings regarding airflow sensitivity point to possible differences in the morphology of the central projections that may correspond to their neural functions. However, the detailed morphologies of the GIs in the cephalic and thoracic ganglia of adult crickets remain unclear. In this study, we stained six GIs, namely, GI 8-1 (medial giant interneuron, MGI), 9-1 (lateral giant interneuron, LGI), 9-2, 9-3, 10-2, and 10-3, using intracellular iontophoretic or pressure injection of dyes. Staining revealed remarkable differences in the axonal branching patterns between vGIs and dGIs. The dGIs were further divided into subgroups based on the profiles of their axon collaterals and projection sites in the brain. The anatomical differences between the GIs' central projections seemed to be related to their information encodement and behavioral functions.

Palisade Endings in Extraocular Muscle Exhibit Clear Motor Features but their Exact Function is still not known

ObjectiveProprioception from extraocular muscles (EOMs) is important for proper visually‐guided behavior. Surprisingly, the classical proprioceptor pair (muscle spindles and Golgi tendon organs) is absent in the EOMs of most mammals and instead, palisade endings have been found in virtually all species analyzed so far. Due to their widespread distribution it is believed that palisade endings are candidates for EOM proprioception. We have analyzed the molecular profile and the central connection of palisade endings.MethodsWhole mount of cat EOMs were triple labeled with neuronal markers, including markers for cholinergic axons, exocytosis proteins, and acetylcholine receptors. In anterograde tracing experiments, neuronal tracer was injected into the EOM motor nuclei.ResultsPalisade endings were formed by nerve fibers that, coming from the muscle, extended into the tendon where they made a u‐shaped turn to approach the muscle tendon junction. Recurrent nerves divided into axonal branches and terminal varicosities to form palisade endings at the tip of single muscle fibers. Molecular analyses showed that palisade endings had a cholinergic phenotype because they expressed choline acetyl transferase, the synthesizing enzyme of acetylcholine. Additionally, palisade ending expressed SNAP25, synaptobrevin, synaptotagmin, complexin, and syntaxin, all of which are key proteins involved in neurotransmitter release. Acetylcholine esterase, the degrading enzyme of acetylcholine was absent in palisade ending and no receptors for cholinergic transmission were associated with palisade endings as demonstrated by the absence of alpha‐bungarotoxin signals. Following neuronal tracer injection in the EOM motor nuclei, palisade endings were labelled.ConclusionOur findings show that palisade endings are cholinergic, have an exocytosis machinery for neurotransmitter (acetylcholine) release and originate from the EOM motor nuclei. However, no receptors for cholinergic transmission are associated with palisade endings suggesting that palisade ending belong to an effector system that is different from that in other skeletal muscles.

Investigating the Role of the Locus Coeruleus in Periaqueductal Gray-mediated Antinociception

Activity in the ventrolateral periaqueductal gray (vlPAG) has been shown to be antinociceptive and is considered to be a major component in the expression of opioid drug-induced analgesia. Canonically, opioids disinhibit vlPAG activity, which modulates neuronal responses in the rostroventral medulla (RVM) and ultimately gates spinal outflow of nociceptive information via serotonergic and opioidergic mechanisms in the spinal dorsal horn. Fundamental pharmacological studies, however, indicate that spinal noradrenergic signaling is consistently implicated in opioid antinociception, suggesting that there is a gap in our knowledge of how the vlPAG may control the release of antinociceptive neuromodulators in the dorsal horn through different output regions. Although the locus coeruleus (LC) has been shown to modulate nociception, its role in and recruitment by the canonical descending circuitry is unclear. Using genetically-encoded and anatomically-targeted viral tools, immunohistochemistry, and acute pain behavior assays in conjunction with intrathecal pharmacology in mice, we show that activity in the vlPAG is necessary for the expression of systemic morphine analgesia at low doses, and that both systemic morphine-induced and PAG-driven antinociception require opioidergic and noradrenergic signaling in the spinal cord. Additionally, we find that vlPAG projection neurons send branching axons to both the RVM and LC, which may account for the LC activation we observed in both opioid-induced and PAG-driven antinociception. These findings reveal a previously underappreciated architecture of the descending pain modulatory circuitry and advance our understanding of the circuit mechanisms by which morphine produces analgesia. Grant support from NIDA R00 DA034648, Rita Allen Foundation (non-profit), Klingenstein-Simons Foundation (non-profit).

P417. In Vivo Evidence of Microstructural Hypo-Connectivity of Brain White Matter in 22q11.2 Deletion Syndrome

22q11.2 Deletion Syndrome, or 22q11.2DS, is a genetic syndrome associated with high rates of schizophrenia, autism, and attention deficit hyperactivity disorder, in addition to widespread structural and functional abnormalities throughout the brain. Experimental animal models have identified neuronal connectivity deficits, e.g., decreased axonal length and complexity of axonal branching, as a primary mechanism underlying atypical brain development in 22q11.2DS. However, it is still unclear whether deficits in axonal morphology can also be observed in people with 22q11.2DS.

Selective axonal transport through branch junctions is directed by growth cone signaling and mediated by KIF1/kinesin-3 motors.

SUMMARYDevelopment and function of nerve cells rely on the orchestration of microtubule-based transport from the cell body into distal axonal terminals. Neurons often have highly elaborate branches innervating multiple targets, but how protein or membrane cargos navigate through branch junctions to specific branch targets is unknown. Here, we demonstrate that anterograde transport of membrane vesicles through axonal branch junctions is highly selective, which is influenced by branch length and more strongly by growth cone motility. Using an optogenetic tool, we demonstrate that signaling from the growth cone can rapidly direct transport through branch junctions. We further demonstrate that such transport selectivity is differentially regulated for different vesicles and mediated by the KIF1/kinesin-3 family motors. We propose that this transport regulation through branch junctions could broadly impact neuronal development, function, and regeneration.

An automated method for precise axon reconstruction from recordings of high-density micro-electrode arrays

Objective: Neurons communicate with each other by sending action potentials (APs) through their axons. The velocity of axonal signal propagation describes how fast electrical APs can travel. This velocity can be affected in a human brain by several pathologies, including multiple sclerosis, traumatic brain injury and channelopathies. High-density microelectrode arrays (HD-MEAs) provide unprecedented spatio-temporal resolution to extracellularly record neural electrical activity. The high density of the recording electrodes enables to image the activity of individual neurons down to subcellular resolution, which includes the propagation of axonal signals. However, axon reconstruction, to date, mainly relies on manual approaches to select the electrodes and channels that seemingly record the signals along a specific axon, while an automated approach to track multiple axonal branches in extracellular action-potential recordings is still missing. Approach: In this article, we propose a fully automated approach to reconstruct axons from extracellular electrical-potential landscapes, so-called ‘electrical footprints’ of neurons. After an initial electrode and channel selection, the proposed method first constructs a graph based on the voltage signal amplitudes and latencies. Then, the graph is interrogated to extract possible axonal branches. Finally, the axonal branches are pruned, and axonal action-potential propagation velocities are computed. Main results: We first validate our method using simulated data from detailed reconstructions of neurons, showing that our approach is capable of accurately reconstructing axonal branches. We then apply the reconstruction algorithm to experimental recordings of HD-MEAs and show that it can be used to determine axonal morphologies and signal-propagation velocities at high throughput. Significance: We introduce a fully automated method to reconstruct axonal branches and estimate axonal action-potential propagation velocities using HD-MEA recordings. Our method yields highly reliable and reproducible velocity estimations, which constitute an important electrophysiological feature of neuronal preparations.

Subthalamic deep brain stimulation of an anatomically detailed model of the human hyperdirect pathway.

The motor hyperdirect pathway (HDP) is considered a key target in the treatment of Parkinson's disease with subthalamic deep brain stimulation (DBS). This hypothesis is partially derived from the association of HDP activation with evoked potentials (EPs) generated in the motor cortex and subthalamic nucleus (STN) after a DBS pulse. However, the biophysical details of how and when DBS-induced action potentials (APs) in HDP neurons reach their terminations in the cortex or STN remain unclear. Therefore, we used an anatomically detailed representation of the motor HDP, as well as the internal capsule (IC), in a model of human subthalamic DBS to explore AP activation and transmission in the HDP and IC. Our results show that small diameter HDP axons exhibited AP initiation in their subthalamic terminal arbor, which resulted in relatively long transmission latencies to cortex (∼3.5-8 ms). Alternatively, large diameter HDP axons were most likely to be directly activated in the capsular region, which resulted in short transmission times to the cortex (∼1-3 ms). However, those large diameter HDP antidromic APs would be indistinguishable from any other IC axons that were also activated by the stimulus. Conversely, DBS-induced APs in both small and large diameter HDP axons reached their synaptic boutons in the STN with similar timings, but both spanned a wide temporal range (∼0.5-5 ms). We also found that using anodic or bipolar stimulation helped to bias activation of the HDP over the IC. These computational results provide useful information for linking HDP activation with EP recordings in clinical experiments.NEW & NOTEWORTHY We used biophysical models to study pathway recruitment and conduction latencies of the hyperdirect pathway (HDP) in response to subthalamic deep brain stimulation (DBS). The model system allowed us to assess the influence of increased anatomical realism on pathway activity and the possibility of identifying HDP activity in evoked potentials (EPs) recorded in either the subthalamic nucleus (STN) or cortex. The model predicts that HDP activation is accentuated by complex axonal branching in the STN.

Neuromodulatory control of inhibitory network arborization in the developing postnatal neocortex.

Interregional neuronal communication is pivotal to instructing and adjusting cortical circuit assembly. Subcortical neuromodulatory systems project long-range axons to the cortex and affect cortical processing. However, their roles and signaling mechanisms in cortical wiring remain poorly understood. Here, we explored whether and how the cholinergic system regulates inhibitory axonal ramification of neocortical chandelier cells (ChCs), which control spike generation by innervating axon initial segments of pyramidal neurons. We found that acetylcholine (ACh) signaling through nicotinic ACh receptors (nAChRs) and downstream T-type voltage-dependent calcium (Ca2+) channels cell-autonomously controls axonal arborization in developing ChCs through regulating filopodia initiation. This signaling axis shapes the basal Ca2+ level range in varicosities where filopodia originate. Furthermore, the normal development of ChC axonal arbors requires proper levels of activity in subcortical cholinergic neurons. Thus, the cholinergic system regulates inhibitory network arborization in the developing neocortex and may tune cortical circuit properties depending on early-life experiences.

In situ cryo-electron tomography reveals local cellular machineries for axon branch development

Neurons are highly polarized cells forming an intricate network of dendrites and axons. They are shaped by the dynamic reorganization of cytoskeleton components and cellular organelles. Axon branching allows the formation of new paths and increases circuit complexity. However, our understanding of branch formation is sparse due to the lack of direct in-depth observations. Using in situ cellular cryo-electron tomography on primary mouse neurons, we directly visualized the remodeling of organelles and cytoskeleton structures at axon branches. Strikingly, branched areas functioned as hotspots concentrating organelles to support dynamic activities. Unaligned actin filaments assembled at the base of premature branches accompanied by filopodia-like protrusions. Microtubules and ER comigrated into preformed branches to support outgrowth together with accumulating compact, ∼500-nm mitochondria and locally clustered ribosomes. We obtained a roadmap of events supporting the hypothesis of local protein synthesis selectively taking place at axon branches, allowing them to serve as unique control hubs for axon development and downstream neural network formation.

A neural progenitor mitotic wave is required for asynchronous axon outgrowth and morphology.

Spatiotemporal mechanisms generating neural diversity are fundamental for understanding neural processes. Here, we investigated how neural diversity arises from neurons coming from identical progenitors. In the dorsal thorax of Drosophila, rows of mechanosensory organs originate from the division of sensory organ progenitor (SOPs). We show that in each row of the notum, an anteromedial located central SOP divides first, then neighbouring SOPs divide, and so on. This centrifugal wave of mitoses depends on cell-cell inhibitory interactions mediated by SOP cytoplasmic protrusions and Scabrous, a secreted protein interacting with the Delta/Notch complex. Furthermore, when this mitotic wave was reduced, axonal growth was more synchronous, axonal terminals had a complex branching pattern and fly behaviour was impaired. We show that the temporal order of progenitor divisions influences the birth order of sensory neurons, axon branching and impact on grooming behaviour. These data support the idea that developmental timing controls axon wiring neural diversity.

Baccharin from Brazilian green propolis induces neurotrophic signaling pathways in PC12 cells: potential for axonal and synaptic regeneration.

Neurodegenerative diseases are characterized by progressive loss of the structure and function of specific neuronal populations, and have been associated with reduced neurotrophic support. Neurotrophins, like NGF (nerve growth factor), are endogenous proteins that induce neuritogenesis and modulate axonal growth, branching, and synapsis; however, their therapeutic application is limited mainly by low stability, short half-life, and inability to cross the blood-brain barrier (BBB). Small neurotrophic molecules that have suitable pharmacokinetics and are able to cross the BBB are potential candidates for neuroprotection. Baccharin is a bioactive small molecule isolated from Brazilian green propolis. In the present study, we investigated the neurotrophic and neuroprotective potential of baccharin in the PC12 cell neuronal model. We used pharmacological inhibitors (K252a, LY294002, and U0126), and ELISA (phospho-trkA, phospho-Akt, and phospho-MEK) to investigate the involvement of trkA receptor, PI3k/Akt pathway, and MAPK/Erk pathway, respectively. Additionally, we evaluated the expression of axonal (GAP-43) and synaptic (synapsin I) proteins by western blot. The results showed that baccharin induces neuritogenesis in NGF-deprived PC12 cells, through activation of trkA receptor and the downstream signaling cascades (PI3K/Akt and MAPK/ERK), which is the same neurotrophic pathway activated by NGF in PC12 cells and neurons. Baccharin also induced the expression of GAP-43 and synapsin I, which mediate axonal and synaptic plasticity, respectively. Additionally, in silico predictions of baccharin showed favorable physicochemical properties, pharmacokinetics, drug-likeness, and medicinal chemistry friendliness. Altogether, these findings suggest that baccharin is a promising neurotrophic agent whose therapeutic application in neurodegeneration should be further investigated.

Dendritic Domain-Specific Sampling of Long-Range Axons Shapes Feedforward and Feedback Connectivity of L5 Neurons.

Feedforward and feedback pathways interact in specific dendritic domains to enable cognitive functions such as predictive processing and learning. Based on axonal projections, hierarchically lower areas are thought to form synapses primarily on dendrites in middle cortical layers, while higher-order areas are posited to target dendrites in layer 1 and in deep layers. However, the extent to which functional synapses form in regions of axo-dendritic overlap has not been extensively studied. Here, we use viral tracing in the secondary visual cortex of male mice to map brain-wide inputs to thick-tufted layer 5 pyramidal neurons. Furthermore, we provide a comprehensive map of input locations through subcellular optogenetic circuit mapping. We show that input pathways target distinct dendritic domains with far greater specificity than appears from their axonal branching, often deviating substantially from the canonical patterns. Common assumptions regarding the dendrite-level interaction of feedforward and feedback inputs may thus need revisiting.

SARM1 Suppresses Axon Branching Through Attenuation of Axonal Cytoskeletal Dynamics.

Axon branching is a fundamental aspect of neuronal morphogenesis, neuronal circuit formation, and response of the nervous system to injury. Sterile alpha and TIR motif containing 1 (SARM1) was initially identified as promoting Wallerian degeneration of axons. We now report a novel function of SARM1 in postnatal sensory neurons; the suppression of axon branching. Axon collateral branches develop from axonal filopodia precursors through the coordination of the actin and microtubule cytoskeleton. In vitro analysis revealed that cultured P0-2 dorsal root ganglion sensory neurons from a SARM1 knockout (KO) mouse exhibit increased numbers of collateral branches and axonal filopodia relative to wild-type neurons. In SARM1 KO mice, cutaneous sensory endings exhibit increased branching in the skin in vivo with normal density of innervation. Transient axonal actin patches serve as cytoskeletal platforms from which axonal filopodia emerge. Live imaging analysis of axonal actin dynamics showed that SARM1 KO neurons exhibit increased rates of axonal actin patch formation and increased probability that individual patches will give rise to a filopodium before dissipating. SARM1 KO axons contain elevated levels of drebrin and cortactin, two actin regulatory proteins that are positive regulators of actin patches, filopodia formation, and branching. Live imaging of microtubule plus tip dynamics revealed an increase in the rate of formation and velocity of polymerizing tips along the axons of SARM1 KO neurons. Stationary mitochondria define sites along the axon where branches may arise, and the axons of SARM1 KO sensory neurons exhibit an increase in stationary mitochondria. These data reveal SARM1 to be a negative regulator of axonal cytoskeletal dynamics and collateral branching.

The History of Neuroscience 5: Discovery of the Neuron and Its Connections

Thanks to the invention of the microscope equipped with an achromatic lens, research on the histology of the nervous system flourished during the 19th century. Jan Evangelista Purkinje, together with Gabriel Valentin, first depicted individual nerve cells. Robert Remak, one of the most remarkable microscopists of the 19th century, made the pivotal discovery that the cell body (globule) and fibers arising from the globule formed a single unit. In addition to improvements in microscopes, a new method of visualizing nervous tissue was developed—namely, the silver staining method introduced by Camillo Golgi. Golgi could identify the types and delicate branches of axons. However, he believed that axons always directly merged with other axons, which was the basis of the reticular or nerve net theory. Santiago Ramón y Cajal, the founder of modern neuroscience, finally established the cornerstone of the neuron doctrine, according to which the nerve cell is the basic unit of the nervous system and information flows from unit to unit without direct fusion. This structure is called the synapse. Key words: Achromatic lens; Histology; Silver staining; Neuron doctrine; Synapses.

Modified working heart brainstem preparation for studying spike propagation through the dorsal root ganglion

The dorsal root ganglion (DRG) contains cell bodies of peripheral somatosensory nerves. These nerves generate and transmit to the CNS signals related to physical changes in the body's immediate environment or state of internal organs. DRG neurons are pseudounipolar with a short stem axon branching into peripheral and central (dorsal root; DR) aspects. Peripheral branches innervate the end organs (e.g. skin, muscle etc.), while DRs synapse in the spinal cord. On the way from peripheral nerve endings to synaptic terminals, spikes pass through branching points (t-junctions) in the ganglia.

Bisphenol a affects neurodevelopmental gene expression, cognitive function, and neuromuscular synaptic morphology in Drosophila melanogaster

Bisphenol A (BPA) is an environmentally prevalent endocrine disrupting chemical that can impact human health and may be an environmental risk factor for neurodevelopmental disorders. BPA has been associated with behavioral impairment in children and a variety of neurodevelopmental phenotypes in model organisms. We used Drosophila melanogaster to explore the consequences of developmental BPA exposure on gene expression, cognitive function, and synapse development. Our transcriptome analysis indicated neurodevelopmentally relevant genes were predominantly downregulated by BPA. Among the misregulated genes were those with roles in learning, memory, and synapse development, as well as orthologs of human genes associated with neurodevelopmental and neuropsychiatric disorders. To examine how gene expression data corresponded to behavioral and cellular phenotypes, we first used a predator-response behavioral paradigm and found that BPA disrupts visual perception. Further analysis using conditioned courtship suppression showed that BPA impairs associative learning. Finally, we examined synapse morphology within the larval neuromuscular junction and found that BPA significantly increased the number of axonal branches. Given that our findings align with studies of BPA in mammalian model organisms, this data indicates that BPA impairs neurodevelopmental pathways that are functionally conserved from invertebrates to mammals. Further, because Drosophila do not possess classic estrogen receptors or estrogen, this research suggests that BPA can impact neurodevelopment by molecular mechanisms distinct from its role as an estrogen mimic.

Microcephaly gene Cenpj regulates axonal growth in cortical neurons through microtubule destabilization.

Neocortex development comprises of a complex series of time- and space-specific processes to generate the typical interconnected six-layered architecture of adult mammals. Axon growth is required for the proper establishment of cortical circuits. Malformations in axonal growth and pathfinding might lead to severe neuropathologies, such as corpus callosum dysgenesis. Cenpj, a microcephaly gene, encodes a scaffold protein that regulates centrosome biogenesis and microtubule stabilization. During corticogenesis, Cenpj regulates progenitor division and neuronal migration. Since microtubule stabilization is crucial for axon extension, we investigated the role of Cenpj in axon growth during cortical development in a mouse model. Through loss- and gain-of-function assays ex vivo and in utero, we quantified callosal axonal length, branching, and growth cone size compared to controls. We observed that silencing Cenpj results in an increased axonal length. Ex vivo, we assessed the number of branches, the area of growth cones and the stability of microtubules. In silenced Cenpj axons, there were more branches, larger growth cone area, and more stable microtubules. Rescue experiments confirmed that neurons present axonal length comparable to controls. Here we propose that Cenpj regulates axon growth by destabilizing microtubules during cortical development. Finally, our findings suggest that Cenpj might be a novel target for axonal regeneration.

MtIF3 is locally translated in axons and regulates mitochondrial translation for axonal growth

BackgroundThe establishment and maintenance of functional neural connections relies on appropriate distribution and localization of mitochondria in neurites, as these organelles provide essential energy and metabolites. In particular, mitochondria are transported to axons and support local energy production to maintain energy-demanding neuronal processes including axon branching, growth, and regeneration. Additionally, local protein synthesis is required for structural and functional changes in axons, with nuclear-encoded mitochondrial mRNAs having been found localized in axons. However, it remains unclear whether these mRNAs are locally translated and whether the potential translated mitochondrial proteins are involved in the regulation of mitochondrial functions in axons. Here, we aim to further understand the purpose of such compartmentalization by focusing on the role of mitochondrial initiation factor 3 (mtIF3), whose nuclear-encoded transcripts have been shown to be present in axonal growth cones.ResultsWe demonstrate that brain-derived neurotrophic factor (BDNF) induces local translation of mtIF3 mRNA in axonal growth cones. Subsequently, mtIF3 protein is translocated into axonal mitochondria and promotes mitochondrial translation as assessed by our newly developed bimolecular fluorescence complementation sensor for the assembly of mitochondrial ribosomes. We further show that BDNF-induced axonal growth requires mtIF3-dependent mitochondrial translation in distal axons.ConclusionWe describe a previously unknown function of mitochondrial initiation factor 3 (mtIF3) in axonal protein synthesis and development. These findings provide insight into the way neurons adaptively control mitochondrial physiology and axonal development via local mtIF3 translation.