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Dyskinesias are involuntary muscle movements associated with long-term use of levodopa therapy (use of levodopa is not a certainty for developing dyskinesias, but there is an association). A better understanding of the underlying biology of dyskinesias is required in order to alleviate this condition for those affected by it.
Recently researchers have reported that an imbalance between dopamine levels (associated with levodopa treatment) and a protein called sonic hedgehog could be partly underlying the development of dyskinesias.
In today’s post, we will explore what sonic hedgehog does in the body, provide an overview of dyskinesias, review the new research, and discuss the implications of the research.
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The humble fly (Drosophila). Source: Ecolab
No one should ever be allowed to say that fly geneticists don’t have a sense of humour.
When it comes to the naming of genes, these guys are the best!
A gene is a section of DNA that provides the instructions for making a particular protein, and each gene has been given a name. Some names are just boring – such as leucine-rich repeat kinase 2 (or LRRK2) – while other names are rather amusing. Especially the fly genes.
For example, there is one fly gene called “indy”, which stands for I‘m Not Dead Yet. Flies with genetic variation in this gene have longer than average lifespans (Click here to read more about this):
Another amusingly named gene is “Cheap Date”. Flies with a genetic mutation in this gene are very susceptible to alcohol (Click here to read more about this):
There is also “Ken and Barbie” – genetic variations in this gene result in a lack of external genitalia (Click here to read more about this).
The fly research community have a lot of really great names for genes: “lunatic fringe”, “headcase” and “mothers against decapentaplegia (MAD)”.
But one of the most popular gene names in all of biology is a gene called “Sonic Hedgehog”
What is Sonic Hedghog?
First discovered in flies, the Hedgehog signaling pathway controls a wide range of developmental processes and is implicated in a variety of cancers.
In flies, there is only one hedgehog proteins, but in humans there is a family of three (sonic, desert, and indian). The name ‘hedgehog’ comes from the original discovery of the gene in flies (Drosophila) – flies that carry a mutated form of the hedgehog gene will grow up and develop into something that is said to resemble a hedgehog (another example of fly genetists humour).
This extreme effect on development points towards a major role for hedgehog proteins in the development of organisms though, and in mammals, sonic hedgehog is a major player.
How does sonic hedgehog function?
Sonic hedgehog is a protein that is released by cells, and it functions by binding to a receptor called ‘Patched’. A receptor is like a light switch, which is waiting for the right protein to come along and turn it on. Patched is sitting on the outer surface of a cell waiting for sonic to come along and activate it.
In the absence of sonic hedgehog, Patched inhibits a transmembrane protein called ‘Smoothened’ which allows the ‘Suppressor of Fused’ (SUFU) to block any transcriptional activity – the transcribing of DNA into RNA – of particular sonic hedgehog-associated genes. When sonic hedgehog does bind to Patched it blocks ‘Smoothened’, which results in transcriptional activity of those sonic hedgehog-associated genes.
Sonic hedgehog signaling. Source: Cincancerres
In the image above, in panel B there are a number of drugs that are named as inhibitors of this pathway (all of them acting as inhibitors, noting the role Sonic Hedgehog plays in several cancers).
Is sonic hedgehog named after the 1990s video game?
The story behind the name is that a British research scientist named, Robert Riddle (who was working in Professor Cliff Tabin’s lab where sonic hedgehog was first discovered – one of three labs that co-discovered it) proposed the name from a comic book that his daughter brought over from the U.K. At the time the videogame was not available in the U.S.
The evolution of Sonic the hedgehog (& computer games). Source: Youtube
The name was not universally popular though, and some members the Human Genome Organization’s Nomenclature Committee (yes, such an organisation does exist – sounds like a hoot!) put sonic on their top 10 list of gene names that they wanted to change.
A big argument ensued, but silliness won the day and the name stuck.
It’s a great name. Why did they want to change it?
Basically, clinicians were having a hard time explaining to patients that their conditions were associated with genes that had silly names.
I mean, imagine your doctor telling you that you have a very serious medical condition which is associated with an error in your ‘lunatic fringe’ gene!
Or even better: your ‘headcase’ gene!
And it actually got even more silly: A potential inhibitor of the sonic hedgehog signaling was being developed for cancer, and the developers called it “Robotnikinin” – a salute to Sonic the Hedgehog’s nemesis, Dr. Ivo “Eggman” Robotnik.
Dr. Ivo Robotnik. Source: Wikipedia
Imagine your experimental treatment being named after a Disney character.
Or one of the Simpsons.
One can certainly appreciate where the clinicians were coming from.
Got it. So what does sonic hedgehog do?
Sonic hedgehog plays an important role in cell growth, cell specialization, and the normal shaping (patterning) of the body.
This video explains some of the pathways involving sonic hedgehog:
Important for our discussion here is that sonic hedgehog is critical for the development of dopamine neurons.
Remind me again, what are dopamine neurons?
Dopamine is a chemical that the brain uses to help pass messages/signals between cells (also known as a neurotransmitter). The bulk of the dopamine generated by your brain is produced by a population of dopamine producing neurons in a region of the midbrain called the substantia nigra.
The loss of these dopamine neurons is one of the cardinal features of the Parkinsonian brain. The substantia nigra (or ‘substance dark’) region is visible (with the human eye) on postmortem sections of brain due to the production of a molecule called neuromelanin in the dopamine neurons. And as you can see in the image below, the Parkinsonian brain has less dark pigmented cells in the substantia nigra region of the midbrain.
The dark pigmented dopamine neurons in the substantia nigra are reduced in the Parkinsonian brain (right). Source:Memorangapp
The loss of dopamine in the brain is associated with the appearance of the motor features of Parkinson’s. By the time an individual is diagnosed with the condition, they have lost approximately 40-60% of the dopamine neurons in the substantia nigra region.
Ok, and this protein called sonic hedgehog is important for the development of these dopamine neurons?
Back in 1995, researchers first reported that sonic hedgehog was required for the generation of dopamine neurons:
Title: Induction of midbrain dopaminergic neurons by Sonic hedgehog.
Authors: Hynes M, Porter JA, Chiang C, Chang D, Tessier-Lavigne M, Beachy PA, Rosenthal A.
Journal: Neuron. 1995 Jul;15(1):35-44.
PMID: 7619528 (This report is OPEN ACCESS if you would like to read it)
In this study, the researchers found that inhibiting sonic hedgehog signalling blocked the development of dopamine neurons. They also found that when sonic hedgehog protein was added to the solution cells were growing in, it was able to induce the production of dopamine neurons. This finding was hugely exciting for the scientists as they were interested in the idea of growing dopamine neurons in cell culture for the purpose of cell replacement therapy for Parkinson’s (Click here to read more about this). They knew that any factor that increases the development of dopamine neurons would be very useful.
And these results were independently replicated shortly after this report was published (Click here to read more about the second report).
These results naturally lead to questions regarding how sonic might be involved in Parkinson’s.
RECAP #1: Sonic hedgehog is a protein released by cells that plays a key role in cell growth and specialization, as well as the normal shaping of the body .
Sonic is also critical for the development of dopamine neurons.
So how might sonic hedgehog be involved with Parkinson’s?
The substantia nigra (where the dopamine neurons reside) is located near the base of the brain, but the dopamine neurons project their branches (or axons) to the other areas of the brain, including an area called the putamen, and this is where they release most of their actual dopamine (the chemical which helps us to move properly).
The projections of the substantia nigra dopamine neurons & location of the putamen. Source: MyBrainNotes
In addition to dopamine, these neurons also produce and release sonic hedgehog (Click here to read more about this) and many of the putamen-located target cells of these dopamine neurons have the sonic hedgehog receptor Patched and its downstream effector Smoothened on their outer surface and respond to its stimulation.
This fact got some researchers wondering what could be happening in the case of Parkinson’s, where we use administration of levodopa tablets to create higher levels of dopamine in the brain, but this treatment is occurring in a situation where there is less sonic hedgehog (due to the absence of the dopamine neurons).
What could be the impact of the missing sonic hedgehog in that scenario?
And very recently, scientists conducted some experiments to address this, and they published the results of those experiments in this report:
Title: Dopaminergic co-transmission with sonic hedgehog inhibits abnormal involuntary movements in models of Parkinson’s disease and L-Dopa induced dyskinesia.
Authors: Malave L, Zuelke DR, Uribe-Cano S, Starikov L, Rebholz H, Friedman E, Qin C, Li Q, Bezard E, Kottmann AH.
Journal: Commun Biol. 2021 Sep 22;4(1):1071.
PMID: 34552196 (This report is OPEN ACCESS if you would like to read it)
In this study, the researchers wanted to know if levodopa-induced dyskinesias emerge in part as a result of an imbalance between dopamine and sonic hedgehog signaling.
Wait, what are dykinesias?
Dyskinesias (from Greek: dys – abnormal; and kinēsis – motion, movement) are a category of movement disorders that are characterised by involuntary muscle movements. And they are certainly not specific to Parkinson’s.
But in the case of Parkinson’s, dyskinesias have generally been believed to be associated with long-term use of levodopa (also known as Sinemet or Madopar – levodopa with carbidopa and levodopa with benserazide, respectively).
Sinemet is levodopa. Source: Drugs
NOTE: Long-term use of levodopa is not a certainty for developing dyskinesias, but there is an association. It will differ from person to person.
As for how they develop, there is a lot of debate over this topic, but there are some basic details that researchers generally tend to agree on.
Before being diagnosed and beginning a course of levodopa, the locomotion parts of the brain in a person with Parkinson’s gradually becomes more and more inhibited. This increasing level of inhibition results in the slowness and difficulty in initiating movement that characterises this condition.
A person with Parkinson’s may want to move, but they can’t – they are inhibited. In effect, they are akinetic (from Greek: a-, not, without; and kinēsis – motion).
Drawing of an akinetic individual with Parkinson’s, by Sir William Richard Gowers. Source: Wikipedia
Once inside the brain, levodopa is quickly converted into dopamine. It is changed into dopamine by an enzyme called DOPA decarboxylase, and this change rapidly increases the levels of dopamine in the brain, allowing the locomotion parts of the brain to function more normally.
The chemical conversion of L-DOPA to dopamine. Source: Nootrobox
In understanding this process, it is important to appreciate that when an levodopa tablet is consumed and levodopa enters the brain, there is a rapid increase in the levels of dopamine. This ‘spike’ in the supply of dopamine will last for the next few hours, before the dopamine is eventually used up.
As the effects of the levodopa tablet wear off, another tablet will be required. This use of multiple levodopa pills across the day gives rise to a wave-like shape to the dopamine levels in the brain over the course of the day (see the figure below). The first pill in the morning will quickly lift the levels of dopamine enough that the individual will no longer feel akinetic. This will allow them to be able to function with normal controlled movement for several hours before the levodopa begins to wear off. As the levodopa wears off, the dopamine levels in the brain drop back towards levels that will leave the person feeling akinetic and at this point another levodopa tablet is required.
After several years of levodopa use, many people with Parkinson’s will experience a weaker response to each tablet. They will also find that they have more time during which they will be unable to move (exhibiting akinesia). This is simply the result of the slow progression of Parkinson’s – levodopa treats the motor features of the condition but only hides/masks the fact that the disease is still progressing.
To combat this shorter response time, the dose of levodopa is usually increased. This will result in increasing levels of dopamine in the brain (as illustrated by the higher wave form over time in the image below). Gradually it will take more levodopa medication-induced dopamine to lift the individual out of the akinetic state.
This increasing of levodopa dosage, however, results in too much dopamine being present in the brain at times. And this situation is often associated with the gradual development of abnormal involuntary movements that appear when the levels of levodopa induced dopamine are the highest.
These are the involuntary muscle movements that we refer to as dyskinesias.
RECAP #2: Sonic hedgehog is produced by dopamine neurons and released on to target cells, which can be stimulated by it. In Parkinson’s, this stimulation is missing – given the loss of the dopamine neurons that release sonic.
Researchers have been curious about the role of sonic hedgehog in the movement issues associated with Parkinson’s, particularly the development of dyskinesias – involuntary movements associated with long-term use of levodopa.
Ok, so the researchers were interested in what effect sonic hedgehog could be having in terms of dyskinesias. What did they find in their experiments?
First the researchers investigated whether enhancement of sonic hedgehog signaling during levodopa treatment could stop or reduce the development of dyskinesia in different preclinical models. They tested this idea in the classical neurotoxin (6-OHDA) rodent model, then in a genetic mouse model (the aphakia mice, which lose dopamine neurons due to the absence of the Pitx3 gene), and also in a neurotoxin (MPTP) primate model.
In all three cases, high dose levodopa treatment led to the development of dykinesias, and in all three cases, parallel treatment with a sonic hedgehog stimulator reduced the intensity of the dyskinesias (as measured by the abnormal involuntary movement (or AIM) scale scores). Interestingly, when the investigators treated the dyskinetic animals with a sonic hedgehog inhibitor (called cyclopamine) the treatment made the dyskinesias significantly worse (increasing the AIM scores):
The researchers also found that the dose of sonic hedgehog stimulation needed to reduce the levodopa-induced dyskinesias by 50% in each of the studies scaled with the dose of levodopa used to induce dyskinesia (in other words, the more levodopa required, the more sonic hedgehog stimulation needed).
In addition, that severity of the dyskinesias could be repeatedly reduced (or reinstated) by dosing the same animals sequentially with (or without) sonic hedgehog stimulation in addition to levodopa.
Next the researchers investigated what was happening in the target cells in the putamen. The target cells that they focused on were the cholinergic neurons.
What are cholinergic interneurons?
Cholinergic neurons make up only about 1–2% of all the cells in the putamen, but they are very large cells which send out dense networks of branches throughout the region, allowing them to communicate with lots of other cells. Cholinergic interneurons produce a neurotransmitter called acetylcholine.
Cholinergic neurons (green) in the putamen. Source: Neuro-marseille
The researchers found that reducing Smoothened activity in cholinergic neurons promoted the appearance of levodopa-induced dyskinesias. And in the reverse experiment, selectively increasing Smoothened activity in cholinergic neurons was sufficient to render mice resistant to levodopa-induced dyskinesias.
Interesting. So what are the implications for this research?
No, wait. We haven’t got to the really intriguing part of this study.
In a further experiment, the investigators artificially depleted sonic hedgehog in dopamine neurons in normal mice, and guess what happened?
Did they develop dyskinesias???
In the absence of any Parkinson’s modelling or levodopa treatment, the researchers found that reducing sonic hedgehog in dopamine neurons in normal mice produced dyskinesia-like involuntary movements.
The scientists concluded that their data provides compelling evidence that sonic hedgehog released by dopamine neurons can reduce dyskinesias in models of Parkinson’s.
It also suggests that the development of levodopa-induced dyskinesias in Parkinson’s could be resulting from an imbalance between dopamine and sonic hedgehog release.
The results (if they can be independently replicated) provide a strong rationale for exploiting modulators of Smoothened signaling in cholinergic neurons for the development of anti-levodopa-induced dyskinesias medication.
Amazing. Where can I get some sonic hedgehog stimulators?
So this is where I rain on the parade and say that ‘more research is required‘.
Up near the top of this post, I presented this image:
Sonic hedgehog signaling. Source: Cincancerres
In panel B of the image, there are a number of sonic hedgehog and smoothened inhibitors mentioned (such as Robotnikinin) which have been developed for cancer therapy. To date, very few sonic hedgehog stimulators have been developed because all of the emphasis has been focused on inhibitors for stopping cancer.
The one sonic hedgehog pathway stimulator that has been developed, but it has only been used as an investigational tool. It has never been clinically tested as there are serious concerns about off-target side effects.
There are concerns that pharmacologically activating a major regulator of cellular function and patterning like sonic hedgehog could have a lot of unintended consequences (side effects) in various areas of the body. A sonic hedgehog pathway stimulator would need to be targeted to the organ of interest (in our case: the brain).
The researchers who published the results that have been reviewed in today’s post are now preclinically exploring novel approaches to modulate the sonic hedgehog pathway stimulation in the Parkinson’s brain. They are also investigating whether sonic hedgehog pathway-based biomarkers could possibly be used to identify different stages of PD or even individuals at risk of developing dyskinesias (Source).
Lots of interesting potential from this research – watch this space.
So what does it all mean?
Sonic hedgehog is not just a character from a video game, but also a major regulator of cellular activity. As a protein, sonic is absolutely critical to the development of dopamine neurons, which has led many to ask if this powerful agent could be having a role in Parkinson’s.
Recently, scientists have presented data indicating that an imbalance of dopamine and sonic hedgehog in the levodopa-treated Parkinson’s brain could be influential in the development of levodopa-induced dyskinesias (debilitating involuntary movements).
The study suggests that activation of the sonic hedgehog pathway could help to prevent the development of dyskinesias and reduce their severity. The issue going forward will be identifying methods of stimulating sonic in specific regions of the brain, without causing off-target side effects elsewhere in the body.
It will be interesting to watch how this line of research develops.
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