6,612 research outputs found
Commissural Axon Kinetics and the Role of Netrin in Early Brain Circuitry Development
No full textAs neurons begin to differentiate, they send out processes called axons to initiate the formation of functional nerve connections. A specialized structure at the end of an axon called the growth cone is believed to possess the impressive navigational and target recognition ability crucial for this process. The goal of the research presented in this thesis was to understand the cellular and molecular mechanisms that shape axon growth and guidance in vivo during early brain development using a multifaceted experimental approach.
Towards this goal we employed the simple, well-characterized neuronal scaffold of the embryonic zebrafish brain in combination with cell labeling techniques and performed studies in three specific areas: (1) the dynamic behaviors of navigating growth cones to obtain information about their cellular interactions with each other and the local environment, (2) the action of specific proteins (netrin and its receptor DCC) known to be involved in axon guidance in order to determine their function in vivo, and (3) the mobility of GFP in growth cones as a way to gain insight into the dynamics of molecular species in these structures as they actively navigate.
Critical to our studies was a stable transgenic gata2::GFP zebrafish line in which we found high level of GFP expression in early forebrain neuronal clusters allowing in vivo timelapse study of the growth cones that pioneer the postoptic commissural (POC) axon tract. Following the development of the POC also allowed us to investigate how early commissural growth cones behave at the midline.
Timelapse analysis of POC axon kinetics revealed important insight into growth cone interactions with each other and their environment and showed that these have behavioral consequences. While it was known that commissural axons slow down while crossing the midline, our data showed that this is only true for the leader axons. Follower axons do not slow down unless the leader axon is ablated. Together this analysis revealed that in addition to specific molecular cues, axon-axon interactions are important for establishing early axon tract.
This characterization of POC axon kinetics and growth cone behavior in turn, provided us with an assay for studying the specific role of netrin, primarily a midline attractant for commissural axons in the spinal cord and its receptor, deleted in colorectal cancer (DCC). Loss- and gain-of-function experiments in combination with timelapse imaging uncovered a novel function for netrin as a positional repellent cue for POC axons.
Finally, prompted by the observed differences in leader and follower POC growth cones, we developed a new experimental approach to assay GFP mobility as a reporter for the diffusion rates of other molecular species inside growth cones in vivo. We found that diffusion rates in actively pioneering growth cones are significantly decreased compared to follower axons suggesting that diffusion rates might be linked to growth cone pathfinding.
Collectively, the findings presented in this thesis constitute a framework that allows for an integrative approach of studying growth cone navigation in vivo. Basic integrative knowledge of this sort is expected to aid the development of medical therapies related to nerve injury and repair.</p
Neuronal deletion of GSK3beta increases microtubule speed in the growth cone and enhances axon regeneration via CRMP-2 and independently of MAP1B and CLASP2
No full textBackgroundIn the adult central nervous system, axonal regeneration is abortive. Regulators of microtubule dynamics have emerged as attractive targets to promote axonal growth following injury as microtubule organization is pivotal for growth cone formation. In this study, we used conditioned neurons with high regenerative capacity to further dissect cytoskeletal mechanisms that might be involved in the gain of intrinsic axon growth capacity.ResultsFollowing a phospho-site broad signaling pathway screen, we found that in conditioned neurons with high regenerative capacity, decreased glycogen synthase kinase 3beta (GSK3beta) activity and increased microtubule growth speed in the growth cone were present. To investigate the importance of GSK3beta regulation during axonal regeneration in vivo, we used three genetic mouse models with high, intermediate or no GSK3beta activity in neurons. Following spinal cord injury, reduced GSK3beta levels or complete neuronal deletion of GSK3beta led to increased growth cone microtubule growth speed and promoted axon regeneration. While several microtubule-interacting proteins are GSK3beta substrates, phospho-mimetic collapsin response mediator protein 2 (T/D-CRMP-2) was sufficient to decrease microtubule growth speed and neurite outgrowth, of conditioned neurons and of GSK3beta-depleted neurons, prevailing over the effect of decreased levels of phosphorylated microtubule-associated protein 1B (MAP1B) and through a mechanism unrelated to decreased levels of phosphorylated cytoplasmic linker associated protein 2 (CLASP2). In addition, phospho-resistant T/A-CRMP-2 counteracted the inhibitory myelin effect on neurite growth, further supporting the GSK3beta-CRMP-2 relevance during axon regeneration.ConclusionsOur work shows that increased microtubule growth speed in the growth cone is present in conditions of increased axonal growth, and is achieved following inactivation of the GSK3beta-CRMP-2 pathway, enhancing axon regeneration through the glial scar. In this context, our results support that a precise control of microtubule dynamics, specifically in the growth cone, is required to optimize axon regrowth
Development of the early axon scaffold in the rostral brain of the small spotted cat shark (<i>Scyliorhinus canicula</i>) embryo
No full textInternational audienceThe cat shark is increasingly used as a model for Chondrichthyes, an evolutionarily important sister group of the bony vertebrates that include teleosts and tetrapods. In the bony vertebrates, the first axon tracts form a highly conserved early axon scaffold. The corresponding structure has not been well characterised in cat shark and will prove a useful model for comparative studies. Using pan-neural markers, the early axon scaffold of the cat shark, Scyliorhinus canicula, was analysed. Like in other vertebrates, the medial longitudinal fascicle was the first axon tract to form from a small cluster of neurones in the ventral brain. Subsequently, additional neuronal clusters and axon tracts emerged which formed an array of longitudinal, transversal, and commissural axons tracts in the Scyliorhinus canicula embryonic brain. The first structures to appear after the medial longitudinal fascicle were the tract of the postoptic commissure, the dorsoventral diencephalic tract, and the descending tract of the mesencephalic nucleus of the trigeminal nerve. These results confirm that the early axon scaffold in the embryonic brain is highly conserved through vertebrate evolution
Inflammation contributes to axon reflex vasodilatation evoked by iontophoresis of an alpha-1 adrenoceptor agonist
No full textIontophoresis of aradrenoceptor agonists in the human forearm evoke axon reflex vasodilatation, possibly due to an accumulation of inflammatory agents at the site of iontophoresis. To investigate this possibility, skin sites in the forearm of healthy participants were treated with an anti-inflammatory gel containing ibuprofen 5% before the iontophoresis of the aradrenoceptor agonist phenylephrine (350 mu A for 3 min). Red cell flux was measured with laser Doppler flowmetry at the site of iontophoresis and 8 mm away in the region of axon reflex vasodilatation. In additional experiments, skin sites were treated with the vasodilator sodium nitroprusside (to counteract vasoconstriction and disperse inflammatory mediators produced during the iontophoresis of phenylephrine); local anaesthetic agent (to determine whether the axon reflex to phenylephrine was neurally-mediated); or the alpha(2)-adrenoceptor agonist clonidine (to investigate the specificity of the adrenergic axon reflex). Phenylephrine evoked marked vasodilatation 8 mm from the site of iontophoresis whereas clonidine and saline-control did not (mean flux increase+/-S.E. 485+/-132% for phenylephrine; 44+/-24% for clonidine; 39+/-19% for saline-control; p<0.05 for phenylephrine versus control). Axon reflex vasodilatation to phenylephrine was unaffected by variations in blood flow at the site of phenylephrine iontophoresis, but was reduced by ibuprofen pretreatment and abolished by local anaesthetic pretreatment. These findings suggest that prostaglandin synthesis at the site of iontophoresis contributes to but does not account entirely for axon reflex vasodilatation to phenylephrine. Alpha-1 adrenoceptor mediation of axon reflexes could play a role in aberrant sensory-sympathetic coupling in neuro-inflammatory diseases
Repeated cycles of electrical stimulation decrease vasoconstriction and axon-reflex vasodilation to noradrenaline in the human forearm
No full textTo investigate whether desensitization to the vasomotor effects of noradrenaline is a specific effect of electrical stimulation. Three sites on the forearm of 10 healthy volunteers were stimulated with 0.2 mA direct current for 2 min twice daily for 10 days. Noradrenaline and histamine were then displaced from ring-shaped iontophoresis chambers into two of the pretreated sites and two untreated sites on the contralateral forearm. Axon-reflex vasodilation was measured from the centre of the ring described by the iontophoresis chamber with a laser Doppler flowmeter. One or two days later, noradrenaline and vasopressin were introduced into pretreated and untreated sites by iontophoresis, and vasoconstriction at sites of administration was measured in the heated forearm. The pretreatment blocked vasoconstriction to noradrenaline [median increase in flow 1%, interquartile range (IR) -41 to 52%; median decrease at the untreated site 53%, IR. -70 to -10%; P 0.05], but did not block vasoconstriction to vasopressin (median decrease 42% at the untreated site and 45% at the pretreated site). Axon-reflex vasodilation to noradrenaline was diminished at the pretreated site (median increase in flow 33%, IR 2–321%; untreated site 247%, IR 31–1087%; P 0.05). However, axon-reflex vasodilation to histamine did not differ significantly between the pretreated site (median increase 1085%) and the untreated site (median increase 1345%). The conditioning pretreatment appears to evoke a specific decrease in responsiveness to noradrenaline. Repeated cycles of electrical stimulation may downregulate neural and vascular responses to noradrenaline by repetitively activating cutaneous sympathetic nerve fibres
Electrical activity suppresses intrinsic growth competence in adult primary sensory neurons
No full textThe ability of neurons to regenerate in the adult mammalian central nervous system (CNS) is often poor, leading to persistent deficits after injury. Failure of axon regeneration in the CNS has been attributed to the presence of an extrinsic inhibitory environment and to an intrinsic limitation to support growth. Remarkably, in adult primary sensory neurons of the dorsal root ganglia (DRG), a peripheral lesion primes neurons to grow and to override the inhibitory environment. Under this condition not only their peripheral axons regrow, but also their injured central axons coursing in the spinal cord regenerate. However, the nature of the signal that is sensed by the cell upon peripheral lesion to initiate the regenerative response is poorly understood.
This study started from the hypothesis that electrical silencing caused by peripheral deafferentiation is an important signal to trigger axon regrowth in adult DRG neurons. I first examined the effect of electrical activity on axon growth of cultured DRG neurons. I found that either chronic depolarization or electrical field stimulation strongly inhibits axon outgrowth in cultured DRG neurons. The inhibitory effect depends on Ca2+ influx through L-type voltage-gated calcium channels and involves transcriptional changes. Consistently, after a peripheral lesion, L-type current is diminished and the L-type pore-forming subunit Cav1.2 is downregulated. To determine whether the lack of L-type channels is sufficient to promote axon growth, mice lacking the pore-forming subunit of L-type channel, Cav1.2, in the nervous system were generated. Neurons isolated from adult Cav1.2 knockout (KO) mice grew more extensively than those from their control littermates.
Taken together, these data provide evidence that electrical activity is a limiting factor for axon growth in adult DRG neurons and that releasing this “brake” is sufficient to induce axon growth. My results further suggest that electrical silencing might promote axon regeneration in vivo. Consequently, I have attempted to apply this knowledge to a model of spinal cord injury. However, these in vivo experiments have been so far hampered by technical limitations. Further endeavors are currently in progress
Neuropilin 1 and 2 control cranial gangliogenesis and axon guidance through neural crest cells
No full textNeuropilin (NRP) receptors and their class 3 semaphorin (SEMA3) ligands play well-established roles in axon guidance, with loss of NRP1, NRP2, SEMA3A or SEMA3F causing defasciculation and errors in growth cone guidance of peripherally projecting nerves. Here we report that loss of NRP1 or NRP2 also impairs sensory neuron positioning in the mouse head, and that this defect is a consequence of inappropriate cranial neural crest cell migration. Specifically, neural crest cells move into the normally crest-free territory between the trigeminal and hyoid neural crest streams and recruit sensory neurons from the otic placode; these ectopic neurons then extend axons between the trigeminal and facioacoustic ganglia. Moreover, we found that NRP1 and NRP2 cooperate to guide cranial neural crest cells and position sensory neurons; thus, in the absence of SEMA3/NRP signalling, the segmentation of the cranial nervous system is lost. We conclude that neuropilins play multiple roles in the sensory nervous system by directing cranial neural crest cells, positioning sensory neurons and organising their axonal projections
Microstructural parameter estimation in vivo using diffusion MRI and structured prior information.
No full textDiffusion MRI has recently been used with detailed models to probe tissue microstructure. Much of this work has been performed ex vivo with powerful scanner hardware, to gain sensitivity to parameters such as axon radius. By contrast, performing microstructure imaging on clinical scanners is extremely challenging
- …
