ArticleslgStudy

biology

Tropic cues involved in growth cone guidance

Tropic cues involved in growth cone guidance is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Tropic cues involved in growth cone guidance rather than just read about it. In short: The growth cone is a highly dynamic structure of the developing neuron, changing directionality in response to different secreted and contact-dependent guidance cues; it navigates through the developing nervous system in search of its target. The migration of the growth cone is mediated through the interaction of numerous trophic and tropic factors; netrins, slits, ephrins and semaphorins are four well-studied tropi…

Tropic cues involved in growth cone guidance — main illustration
Tropic cues involved in growth cone guidance — illustration

Key takeaways

  • Tropic cues involved in growth cone guidance belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Tropic cues involved in growth cone guidance to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Tropic cues involved in growth cone guidance from memory before moving on to harder problems.

Reference excerpt

The growth cone is a highly dynamic structure of the developing neuron, changing directionality in response to different secreted and contact-dependent guidance cues; it navigates through the developing nervous system in search of its target. The migration of the growth cone is mediated through the interaction of numerous trophic and tropic factors; netrins, slits, ephrins and semaphorins are four well-studied tropic cues (Fig.1). The growth cone is capable of modifying its sensitivity to these guidance molecules as it migrates to its target; this sensitivity regulation is an important theme seen throughout development.

Netrins

Netrins are diffusible chemoattractive molecules that guide commissural axons across the midline; they are secreted by floor plate cells at ventral midline of the spinal cord. Netrins establish a gradient to direct commissural axons at a distance; Netrin-2 is expressed broadly in the ventral two thirds of the spinal cord, but not in the floor plate. Mice with netrin-1 loss-of-function exhibit severe disruption in commissural axon migration; this experiment established the importance of Netrin-1 in guidance decisions. Netrin-1 gradient in Xenopus laevis ganglion cell can induce turning of retinal growth cones in vitro to steer axons out of the retina. Netrin (unc-6, Caenorhabditis elegans homologue) and its corresponding receptor DCC (Deleted in Colorectal Cancer) were initially identified as an attractive interaction. DCC, expressed by commissural axons, binds to netrin with high affinity; inhibiting netrin/DCC signaling interferes with the attractive turning of retinal growth cones. Netrin-1 has also been shown to act as a chemorepellent in vivo for trochlear motor axons that migrate dorsally away from the floor plate. In netrin-1 deficient mice, trochlear axon projections are normal, suggesting the existence of other redundant guidance cues working in tandem with netrin-1 to repel trochlear axons. Studies in C. elegans revealed a possible mechanism for netrin acting as a chemorepulsive agent (Fig.2). Unc-5, a transmembrane protein, is required for dorsal migration of axons in nematodes; it was determined that unc-5 acts as a repulsive receptor for Netrin (unc-6). The switch between attractive and repulsive netrin signaling can be mediated by misexpression of unc-5 in commissural axons. Netrin-1/DCC binding induces DCC homodimerization leading to an attractive response; on the other hand, the chemorepellent response is triggered via netrin-1 binding to unc-5/DCC heterodimers. Netrin repulsion can also be mediated by changes in cyclic nucleotide levels; netrin-1 induces a repulsive response when cAMP signaling is inhibited. Cis interactions of netrin/DCC (attractive) and Slit/Robo (repulsive) in commissural axons silence both signaling cues; this illustrates how multiple tropic cues interact to guide the commissural axons to their targets.

Slits

Repulsive cues play an important role in guiding growth cones to their appropriate target; roundabout (Robo) receptors and their ligand, Slit, are a well-studied example of repulsive guidance. Robo receptors were initially identified in Drosophila melanogaster using a forward genetic screen to search for molecules involved in midline crossing at the floor plate. Robo/Slit loss-of-function mutations result in axons crossing the midline multiple times, whereas gain-of-function results in little to no midline crossing; consequently this interaction was determined to be important in preventing non-commissural axons from crossing the midline and commissural axons from recrossing. How the commissural axon regulates its response to Slit repulsion at the midline has been extensively studied in both Drosophila and vertebrates; in these two models the growth cone's response to Slit has been shown to be regulated through receptor trafficking and alternative splicing, respectively (Fig.3). Receptor trafficking is used extensively throughout growth cone migration; in Drosophila prior to crossing the midline these neurons express commissureless (comm), a protein involved in Robo receptor trafficking. Comm prevents Robo from reaching the cell membrane by targeting the receptor for the endosomal pathway; this allows the growth cone of the commissural axon to cross the midline by preventing Robo/Slit repulsive interactions. Comm expression turns off after the growth cone has crossed the midline; this permits Robo/Slit repulsion and prevents the growth cone from crossing the midline again. Vertebrates, on the other hand, do not possess a comm homolog; instead they facilitate midline crossing through alternative splicing of Robo3 (aka.Rig-1). Robo3 has two isoforms, 3.1 and 3.2, and these isoforms interact with Robo1 and Robo2 (Robo1/2) through cis interactions at the leading edge of the growth cone. Before crossing the midline Robo3.1 inhibits Slit/Robo repulsive signaling, allowing the commissural axon to cross; after crossing the midline Robo 3.1 is replaced by Robo3.2 to facilitate the repulsive Slit/Robo signaling through cis interactions with Robo1/2. Slit/Robo signaling is seen throughout the developing nervous system and is demonstrative of the importance of repulsive cues in growth cone migration; the aforementioned regulation of these repulsive barriers determines the path of the commissural axon.

Ephrins

… excerpt ends here. Continue reading the full article.

Illustrations

Tropic cues involved in growth cone guidance: Fig.1: Various tropic cues interact to guide the growth cone; the initial role of these cues was identified as either repulsive (red) or attractive (green).
Fig.1: Various tropic cues interact to guide the growth cone; the initial role of these cues was identified as either repulsive (red) or attractive (green).
Tropic cues involved in growth cone guidance: Fig.2: Netrin signaling has multiple roles in guidance.
Fig.2: Netrin signaling has multiple roles in guidance.
Tropic cues involved in growth cone guidance: Fig.3: Drosophila and vertebrates use different mechanisms to regulate their sensitivity to Slit mediated repulsion at the midline. Drosophila regulate their sensitivity through Comm mediated endosomal trafficking of Robo (top panel); vertebrates use alternatively spliced Robo3 isoforms to regulate Slit signaling (bottom panel).
Fig.3: Drosophila and vertebrates use different mechanisms to regulate their sensitivity to Slit mediated repulsion at the midline. Drosophila regulate their sensitivity through Comm mediated endosomal trafficking of Robo (top panel); vertebrates use alternatively spliced Robo3 isoforms to regulate Slit signaling (bottom panel).
Tropic cues involved in growth cone guidance: Fig.4: Ephrin/Eph bidirectional signaling; forward signaling (ligand to receptor) and reverse signaling (receptor to ligand).
Fig.4: Ephrin/Eph bidirectional signaling; forward signaling (ligand to receptor) and reverse signaling (receptor to ligand).
Tropic cues involved in growth cone guidance: Fig.5: Semaphorins can exist either in a soluble form or membrane bound form. Both are utilized in axonal repulsion, but recent studies demonstrate that Semaphorins can also mediate attraction. In addition, soluble Sema3A binds to Neuropilin-1, which lacks an intracellular domain and thus co-signals with Plexin A. Sema4A in its membrane form binds to Plexin B1 for its signaling pathway. Together, these mammalian Semaphorins can mediate axonal guidance and target selection.
Fig.5: Semaphorins can exist either in a soluble form or membrane bound form. Both are utilized in axonal repulsion, but recent studies demonstrate that Semaphorins can also mediate attraction. In addition, soluble Sema3A binds to Neuropilin-1, which lacks an intracellular domain and thus co-signals with Plexin A. Sema4A in its membrane form binds to Plexin B1 for its signaling pathway. Together, these mammalian Semaphorins can mediate axonal guidance and target selection.

Worked examples

Example 1 — a first encounter with Tropic cues involved in growth cone guidance

Start with the simplest possible case. Write down what Tropic cues involved in growth cone guidance claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Tropic cues involved in growth cone guidance before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Tropic cues involved in growth cone guidance ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Tropic cues involved in growth cone guidance

In research
Tropic cues involved in growth cone guidance appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Tropic cues involved in growth cone guidance in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Tropic cues involved in growth cone guidance is common in secondary-school and first-year university syllabi. It links to neighbouring topics Developmental neuroscience, so understanding it makes those chapters shorter.
In everyday life
Look for Tropic cues involved in growth cone guidance outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Tropic cues involved in growth cone guidance” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Tropic cues involved in growth cone guidance in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Tropic cues involved in growth cone guidance means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Tropic cues involved in growth cone guidance out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tropic cues involved in growth cone guidance in simple terms?

The growth cone is a highly dynamic structure of the developing neuron, changing directionality in response to different secreted and contact-dependent guidance cues; it navigates through the developing nervous system in search of its target. The migration of the growth cone is mediated through the…

Why does Tropic cues involved in growth cone guidance matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Tropic cues involved in growth cone guidance?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Tropic cues involved in growth cone guidance.

Tags

  • Developmental neuroscience

Keep exploring