Axonal Transport

Learning about physiology is changing the way we perceive our work as manual therapists. I started last week with the firm intention to study the neurotransmitters and peptides receptors, thinking this would lead to an interesting article on glucocorticoids. After many days of reading, searching, thinking, one think leading to another one, I ended up learning more about axonal transport and neurotrophic factors. Don't worry, the glucocorticoid article is on its way, it is just in my back pocket.

So in this section, we study a bit further the existence of axonal transport, the role of neurotrophic factors and ultimately, how to apply this new knowledge in the practice of manual therapies. Read to the end !!

I used three main references to write this article :

A. Brown (*) Department of Neuroscience, The Ohio State University, Columbus, OH, USA, Springer Science+Business Media New York 2016 333 D.W. Pfaff, N.D. Volkow (eds.), Neuroscience in the 21st Century, DOI 10.1007/978-1-4939-3474-4_14

Paragraphs written in italic letters refer to this article.

UNSW ANAT3231 Course Coordinator Dr Mark Hill, https://cellbiology.med.unsw.edu.au/cellbiology/index.php/2014_Group_4_Project#Fast_Anterograde_Transport

Paragraphs written in bold letters refer to this article.

E. Castrén (*) Neuroscience Center, University of Helsinki, Helsinki, Springer Science+Business Media New York 2016 1843 D.W. Pfaff, N.D. Volkow (eds.), Neuroscience in the 21st Century, DOI 10.1007/978-1-4939-3474-4_55

Underlined paragraphs refer to this article.

Axons

Axons are long slender cylindrical projections of neurons that enable these cells to communicate directly with other cells in the body over long distances, up to a meter or more in large animals. However, most axonal components originate in the nerve cell body and must be shipped out along the axon by mechanisms of intracellular motility. In addition, signals from the axon and its environment must be conveyed back to the nerve cell body to modulate the nature and composition of the outbound traffic. The outward movement from the cell body toward the axon tip is called anterograde transport and the movement in the opposite direction, back toward the cell body, is called retrograde transport. This bidirectional transport, known collectively as axonal transport.

Axons have the capacity to extend for great distances, but they lack necessary local components within the axoplasm for protein synthesis. Therefore, in neurons, the soma is the primary site of protein synthesis. The distance between site of production and target destination can extend a great length and proteins cannot independently travel these distances. Fortunately, mechanisms of axonal transport and motor proteins allow essential materials to travel significant distance.

Axonal Transport

Membranous structures tend to transit along the axon at fast speeds, and therefore travel via fast axonal transport. On the other hand, non-membranous structures travel along the axon at slower speeds, thus are classified slow axonal transport.

Regrograde Axonal Transport

The primary function of fast retrograde transport (from the tip to the soma) is to return molecules destined for degradation to lysosomes within the soma, preventing accumulation of old molecules in the axon which may cause neuronal dysfunction. Additionally, retrograde transport returns exogenous products of endocytosis from distal axon (neurotrophins), ensuring survival and regulation of gene expression. (See a few paragraphs below : neurotrophic factors).

Fast Anterograde Axonal Transport

The material transported by fast anterograde transport (from the soma to the axon) performs many various functions. Membrane and secretory proteins are transported to functionally different areas within the axons, such as presynaptic terminals, axon membrane and nodes of Ranvier. At these various locations, these membrane-associated proteins function to maintain axonal metabolism, and must therefore be continuously and efficiently be transported at fast rates.

Axonal transport of mitochondria

This form of axonal transport is distinct in its overall functions regarding the neuron. A defining characteristic of mitochondria is its ability to sequester factors of apoptosis, help stabilise the concentration of free calcium ions within the cell and produce ATP (adenosine triphosphate : the universal energy molecule). Due to its unique roles, mitochondria are needed across different locations along the axon, therefore resulting in the continuous starts, stops and change in direction during its transit. Mitochondria are not simply delivered to specific sites to fulfil cell needs, but must continually be repositioned to accommodate for the complex requirements of the neuron; particularly in aerobic metabolism, calcium homoeostasis and cell death.

Slow Anterograde Axonal Transport

Slow anterograde transport carry non-membranous structures in including cytoskeleton polymers (neurofilaments, microtubules and intermediate filaments) and other cytosolic protein complexes. This enables motor proteins to transfer newly synthesised proteins from the soma to specific destinations along the lengths of the axon. The continual transportation and delivery of these proteins allow the cytoskeleton to adapt to the dynamic nature of the neuron. Since they are continually changing in length and shape, the polymers of the cytoskeleton are always being renewed due to axonal transport.

Microtubules and microfilaments

One of the breakthroughs made possible by direct imaging of axonal transport has been the discovery that microtubules and microfilaments serve as the tracks along which all cargoes move, and this is a fundamental feature of intracellular traffic in all eukaryotic cells.

Because of their length and organisation, it is generally assumed that microtubules are the tracks for long-range axial movements in axons, whereas microfilaments are the tracks for short-range movements. However, it is important to note that even though microtubules can be very long, they do not extend for the entire length of the axon. Thus, the overlap of microtubules along axons is critical to establishing an uninterrupted highway from cell body to axon tip; gaps in this overlapping array obviously cannot occur because axonal transport is a lifeline for axons. Any interruption in the continuity of the overlapping microtubule array in axons would have profound and devastating consequences for the nerve cell.

Retrograde transport of neurotrophins (retrograde signalling)

Neurotrophic factors support survival, process outgrowth, and phenotypic differentiation of neurons during development. Many neurotrophic factors are also expressed in adult brain, where they regulate neuronal connectivity and network plasticity. Neurotrophic factors are typically released from the target cells or postsynaptic neurons, taken up by the presynaptic axon terminals and retrogradely transported to the soma. Access of the axon terminal to neurotrophic factors selects those that have established an optimal connection with the target cell, whereas neurons or processes with an insufficient access or transport of neurotrophic factors degenerate. Therefore, the primary function of neurotrophic factors is to mediate information to the neuronal soma about the quality of the connection between a neuron and its target.

Individual liver cells can metabolise chemicals, and individual red blood cells can carry oxygen, but individual neurons are not useful alone; their function is to mediate information between individual cells. To achieve this, neurons extend extremely long processes to contact distant target cells or other neurons. The maintenance of these lengthy processes and the transportation of nutrients and metabolic products up and down the axon are energetically extremely costly and are also an Achilles’ heel for several disease processes. It is therefore vital for a neuronal cell body to be informed that the distant axon terminal is engaged in a functional connection with an active target cell. If this connection is lost or becomes inactive, the neuron loses its reason for existence and begins to search for a new contact, or degenerates and eventually dies. Neurotrophic factors are critical molecules that mediate information to a neuron about the status and well-being of its contact with its target cell.

Let’s see how we can summarise this :

The soma of the neuron, or the cell body, produces most axonal components which must be transported in the axon (and dendrites), through fast or slow anterograde axonal transport. Also, signals from the axon and its environment must be conveyed back to the neuron cell body through retrograde axonal transport which also functions as a pathway for the recycling or degradation of membranous organelles and their macromolecular components.

So, what is so interesting for us in manual therapy?

Well, it is in the discovery of axonal transport that we will find something for us :

In 1948, Paul Weiss and Helen Hiscoe used a surgical technique to apply a gentle but gradual constriction to regenerating axons in vivo. The axons gradually swelled proximal to the constriction (the side closer to the cell body) over several weeks due to the accumulation of anterogradely transporter materials. When the constriction was released, the bolus of accumulated materials appeared to propagate distally along the axon (way from the cell body). As Anthony Brown says in his article, this study remains a landmark in the fields of axonal transport.

Today, the scientists have three ways to study axonal transport. Let's see the first one, the accumulation technique, which consists, as its name suggests, of blocking movement locally along an axon or nerve and observe what cargo structures accumulate and the rate at which they do so. The most common strategy is to ligate a nerve in vivo using surgical thread. And this is the most important for us : anterogradely moving materials accumulate on the proximal side of the ligation and become depleted on the distal side, whereas retrogradely moving materials accumulate on the distal side and become depleted on the proximal side. This technique is nonselective (everything that moves accumulates) and scientists need techniques yielding to more molecular and kinetic information. But for us, this is great.

It means that all the material necessary for membrane repair, synaptic efficiency, axonal metabolism, cytoskeleton repair, and the energetic supply from the mitochondria will be missing on the distal side of the axon. And that material due for recycling will also accumulate and not reach the cell body, but more : that the neurotrophic factors will not reach the cell body, depriving the it of this essential signal from the target cell. The whole neuron will suffer from compression, anywhere on its length. Accumulation also means swelling. It is clearly expressed and explained in the research from Weiss and Hiscoe, less clear in the description of the accumulation technique. But when matter accumulates and that there is no way out (no lymphatic drainage either in or around a neuron), then it swells.

And what do we feel on the skin ? What if the « blumps » described by Diane Jacobs (DNM) or the « grains of rice » described by Raymond Branly (Niromathé) were areas of accumulation of axonal material on each side of a compression ? Yes, I know, we cannot « feel » one neuron, it is too small. But if one neuron is compressed at an aponeurotic ring for example, I bet you that it is not alone and that many other neurons are also compressed, if not all (more or less). And let us not forget that when there is a nerve, there are blood vessels and that if the nerve is compressed, so will the vessels, hence the increased risk of epineural edema. Then the sum of accumulations starts to mean something that our really sensitive fingers could really start to feel.

Remember : it is vital for a neuronal cell body to be informed that the distant axon terminal is engaged in a functional connection with an active target cell. If this connection is lost or becomes inactive, the neuron loses its reason for existence and begins to search for a new contact, or degenerates and eventually dies.

I know it is a very long explanation for a very short ending. But what an ending !

Or is it conceptual hallucination ?