When the zombies are all in your head

In your brain there are different types of cells. Firstly there are the neurons (the prima donnas that we believe do most of the communication of information). Next there are the microglia cells, which act as the first and main line of active immune defence in the brain. There are also oligodendrocyte, that wrap protective sheets around the branches of the neurons and help them to pass signals. And then there are astrocytes. These are the ‘helper cells’ which maintain a comfortable environment for the neurons and aid them in their task. Recently, researchers in California reported an curious observation in the Parkinsonian brain: some astrocytes have entered an altered ‘zombie’-like state. And this might not be such a good thing. In today’s post, we’ll review the research and discuss what it could mean – if independently replicated – for the Parkinson’s community.

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Zombies. Source: wallpapersbrowse

I don’t understand the current fascination with zombies.

There are books, movies, television shows, video games. All dealing with the popular idea of dead bodies wandering the Earth terrifying people. But why the fascination? Why does this idea have such appeal to a wide portion of the populous?

I just don’t get it.

Even more of a mystery, however, is where the modern idea of the ‘zombie’ actually came from originally.

You see, no one really knows.

Huh? What do you mean?

Some people believe that the word ‘zombie’ is derived from West African languages – ndzumbi means ‘corpse’ in the Mitsogo language of Gabon, and nzambi means the ‘spirit of a dead person’ in the Kongo language. But how did a word from the African continent become embedded in our psyche?

Others associate the idea of a zombie with Haitian slaves in the 1700s who believed that dying would let them return back to lan guinée (African Guinea) in a kind of afterlife. But apparently that freedom did not apply to situations of suicide. Rather, those who took their own life would be condemned to walk the Hispaniola plantations for eternity as an undead slave. Perhaps this was the starting point for the ‘zombie’.

More recently the word ‘zonbi’ (not a typo) appeared in the Louisiana Creole and the Haitian Creole and represented a person who is killed and was then brought to life without speech or free will.

Delightful stuff for the start of a post on Parkinson’s research, huh?

But we’re going somewhere with this.

The basic idea with a zonbi was that Bokors (or Voodoo sorcerers) possessed the ability to kill someone and then resurrect them from the dead using the administration of coup padre (a powder that is given orally). The primary ingredient of coup padre was tetrodoxin (a potent sodium channel blocker, which inhibits the firing of action potentials in neurons by blocking the access of sodium). It is an extremely toxic compound – more poisonous than cyanide – that was extracted from the poisonous “porcupine fish”.

Porcupine fish. Source: Wikipedia

According to the Creole legends, once given the coup padre, the subject would appear to die (their heart rate would slow to a near stop, their breathing would be greatly subdued, and their body temperature would drop significantly). The Bokor would then bring them ‘back to life’ using their voodoo magic, but their memories would be erased and these mindless drones would remain under the Bokor’s power (until said Bokor departed this world). 

‘The Zombies’​​ by Hector Hyppolite, which hangs in the Museum of Haitian Art. Source: TheAtlantic

But this version of the ‘zombie’ idea probably says less about the Haitian Voodoo culture and more about the post-colonial anxieties of slavery (slavery was banned after the Haitian Revolution in 1804 and the end of French occupation).

Wherever the idea of the zombie came from, Hollywood eventually got hold of the idea and the rest is history.

White Zombie (1932). Source: BBC

In 1932, zombies made their first appearance on the silver screen Victor Halperin’s movie ‘White Zombie‘ starring Bela Lugosi (he of Count Dracula fame). In this movie, zombies were depicted in the Haitian-theme of mindless, unthinking henchmen under the spell of a Bokor wonderfully named ‘Murder Legendre’.

Bela Lugosi as ‘Murder Legendre’. Source: Pinterest

Subsequently there has been an incredibly long list of zombie-associated films/TV shows (Click here to see the list). But the modern version of the ‘zombie’ – distinct from that described in Haitian folklore – only really appeared with the release of George A. Romero’s film ‘Night of the Living Dead‘ (and curiously the word zombie was never mentioned in the film). This film was really the start of the current theme of “zombie apocalypse” story lines.

Ok, this is all very interesting, but what on earth does any of it have to do with Parkinson’s?

Well, recently researchers at the Buck Institute for Research on Aging in California noticed something interesting in the brains of people with Parkinson’s: certain types of cells appear to enter a ‘zombie-like’ state.

The Buck Institute for Research on Aging. Source: Buckinstitute

The news media outlets jumped on this idea of course (Click here for an example). And while this might sound like an effort to hype up a bit of scientific research, the depiction of the ‘zombie-like’ state is actually not too far-fetched if one considers the Bokor definition of a zonbi. You see, the researchers found that a toxic chemical could cause the cells to enter this state.

This is the research report in question:

Title: Cellular Senescence Is Induced by the Environmental Neurotoxin Paraquat and Contributes to Neuropathology Linked to Parkinson’s Disease
Authors: Chinta SJ, Woods G, Demaria M, Rane A, Zou Y, McQuade A, Rajagopalan S, Limbad C, Madden DT, Campisi J, Andersen JK.
Journal: Cell Rep. 2018 Jan 23;22(4):930-940.
PMID: 29386135                (This article is OPEN ACCESS if you would like to read it)

In this study, the scientists noticed that a particular type of cell in the brain (called astrocytes) exhibited high levels of proteins associated with senescence in the brains of people with Parkinson’s.

What are astrocytes?

Astrocytes (Astro from Greek astron = star and cyte from Greek “kytos” = cavity but also means cell) are star-shaped cells in the brain that play a critical role in maintaining the carefully balanced environment and provide support to the other types of cells. While neurons get all of the attention because they are the cells sending messages to different parts of the brain, astrocytes are very busy working in the background, holding the show together.

Understand that without astrocytes, the wheels on the wagon would come off very quickly.

An astrocyte (green) supporting other cell types. Source: Sciencenewsforstudents

Astrocytes are some of the hardest working cells in the brain. They really regulate a lot of what is happening up there, from modulating synaptic transmission (the passing of signals from neuron to neuron) and soaking up excess neurotransmitters (the chemicals that pass the signal from neuron to neuron, such as dopamine) to responding to injury in the brain (by forming the ‘glial scar’).

For a really good explanation of what astrocytes do in the brain, I recommend this video from the Khan Academy:

One of the amazing features of astrocytes is how they have changed over time with evolution. Astrocytes look very different between mice and men. When you look at an astrocyte in the human brain, it looks like this:

A human astrocyte. Source: Wikipedia

And when you compare human astrocytes with their mouse/rat counterparts, you will note that human astrocytes are dramatically different. Human astrocytes are vastly bigger (in volume), they have many more branches (or processes), and they support 20 times as many synapses (the point on a neuron where a signal is transmitted):

The evolutionary shift in astrocytes. Source: Ctn

And one of the great myths about the brain is that there are more astrocytes in the brain than neurons (based on the idea that lots of servant cells are required to help the neurons to function properly), but this has been some what debunked over the last decade. For a long time it was believed that glial cells (think astrocytes, microglia and oligodendrocytes) outnumbered the neurons 10:1. But several recent reports have indicated that it is likely to be a 50:50 split. But this ratios depends on which area of the brain you look at. Overall, however, of the approximately 170 billion cells in your brain, 86 billion of them are neurons, while the remaining 84 billion are glial cells (Source and click here for an interesting article about this topic).

Given that astrocytes have such a critical role of normal brain functioning, this revised ratio suggests that we need all of the astrocytes we can get as we get older. We don’t really want any of them to be disappearing or becoming senescent.

What does senescent mean? What is senescence?

Cellular senescence is the phenomenon by which normal cells cease to divide. It is generally considered an anti-cancer mechanism that occurs in cells which are capable of division (astrocytes are known to divide when needed to). Think of senescence as a handbrake that is pulled inside of a cell to stop it from having the ability to divide. But the problem is that after this “brake” has been pulled, these senescent cells remain in the body and start to accumulate with age.

It was once thought that senescent cells simply ceased to have major functional roles after the brake has been pulled. More recent research has changed this idea, however, and it is now believed that senescent cells are still very much active, but functionally altered.

And not altered in a good way (think ‘zombie-like’).

Several years ago now, some researchers at the Mayo Clinic College of Medicine (Rochester, Minnesota) reported something interesting about senescent cells: they shorten the life span of mice.

Title: Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders.
Authors: Baker DJ, Wijshake T, Tchkonia T, LeBrasseur NK, Childs BG, van de Sluis B, Kirkland JL, van Deursen JM.
Journal: Nature. 2011 Nov 2;479(7372):232-6.
PMID: 22048312                (This article is OPEN ACCESS if you would like to read it)

In this study, the scientists genetically engineered a mouse that would get rid of all senescent cells from all of the organs in the body (throughout life). When they compared these mice to normal mice, the investigators found that life-long removal of cells producing high levels of p16^Ink4a delayed the onset of many age-related complications. Regular elimination of senescent cells from mice resulted in the animals remained youthful longer, based on measures of their mobility, muscle mass, and fat storage.

Subsequent research suggests that some of these senescent cells secrete molecules that could profoundly affect neighbouring cells (think pro-inflammatory molecules), and there are many researchers now speculating as to whether this may also contribute to age-related conditions, such as Alzheimer’s and Parkinson’s (Click here to read a good review on cellular senescence).

Previous research has suggested that the number of astrocytes with high levels of senescence proteins actually increase in the human brain during the normal ageing process (Click here to read more about this). This phenomenon has also been observed in the brains people with Alzheimer’s disease:

Title: Astrocyte senescence as a component of Alzheimer’s disease.
Authors: Bhat, R., Crowe, E.P., Bitto, A., Moh, M., Katsetos, C.D., Garcia, F.U., Johnson, F.B., Trojanowski, J.Q., Sell, C., Torres, C.
Journal: PLoS ONE, 2012; 7: e45069
PMID: 22984612                  (This article is OPEN ACCESS if you would like to read it)

In this study, the researchers looked at levels of two proteins associated with senescence (metalloproteinase-1 (or MMP-1) and p16^INK4a) in astrocytes in fetal, middle aged (35 to 50 years) and elderly (78 to 90 years) brain tissue samples. Compared with the fetal tissue samples, the investigators observed a significant increase in the number of astrocytes with high levels of these two proteins. And this number increased when the researchers looked at elderly brains. In addition, samples from people who passed away with Alzheimer’s harboured a significantly greater level of p16^INK4a-in astrocytes compared with samples from normal-healthy adult control subjects of similar ages.

The Buck Institute researchers behind the research report we are reviewing today found very a similar result in samples of postmortem brain from people who passed away with Parkinson’s, thus supporting a possible role for senescent cell burden in neurodegenerative conditions.

They found elevated levels of the senescence proteins in the substantia nigra (the region of the brain where the dopamine-producing neurons reside). This was particularly evident when they looked at a protein called lamin B1 (a marker of senescence). Astrocytes in the Parkinsonian brain were found to be lacking lamin B1 while no significant difference was detected in aged-matched control tissues. You can see an example of this result in the image below: blue staining indicates the nucleus of cells, red staining denotes astrocytes, and green staining labels lamin B1. Note the lack of green staining where the red astrocyte should be in bottom right panel (from the Parkinson’s sample; indicated by a red arrow) even though the nucleus of the neighbouring cell still has lamin B1 protein (green;indicated by a yellow arrow).

Source: CellReports

Next the Buck Institute researchers sought to determine if chemicals associated with increased risk of Parkinson’s could also induce cellular senescence in human astrocytes. They selected the herbicide paraquat for this analysis, and they found that after exposing astrocytes grown in cell culture to paraquat, there was a robust increase in levels of protein associated with senescence. They also found that low but sustained doses of paraquat increased the percentage of senescent astrocytes, suggesting that even low exposures to such a chemical over time could have an impact. 

In effect, the herbicide paraquat could be acting like the Haitian Bokor’s ‘coup padre’ which apparently brought on the zonbi state in individuals.

Their next experiment was an interesting one: the investigators grew dopamine neurons in a cell culture solution that had been previously used to support senescent astrocytes and they found a significant reduction in the viability of dopamine neurons (compared to a control group of dopamine neurons that they grew in normal solution). This suggested that the senescent astrocytes were releasing certain compounds into their surrounding environment that could be having a negative impact (even after the solution is transferred to a different set of cells).

Next, the investigators used genetically engineered mouse in which senescent cells are automatically killed off (similar to the study mentioned further above). They began exposing these mice (and some normal control mice) to a continuous low dose of paraquat, which increased levels of senescence-associated protein p16^INK4a in the substantia nigra (the region where the dopamine neurons live).

A lab mouse. Source: USNews

When they looked at dopamine neurons in the genetically engineered mice, the investigators found that the removal of senescent cells reduced the levels of dopamine cell loss associated with paraquat treatment (compared to normal mice given the same toxin). Thus, the researchers concluded that “therapies that target senescent cells may constitute a strategy for treatment of sporadic Parkinson’s, for which environmental exposure is a major risk factor”.

Interesting idea huh?

Yeah, very interesting. So is anyone trying to take drugs to the clinic for eliminating senescent cells?

Good question.

Buried deep in the notes of this ‘zombie cell’ research report is the acknowledgement that one of the researchers in the study Prof Judith Campisi is one of the founders of a small biotech company called Unity biotechnology which is developing “senolytics” therapies for age-related conditions.

 Prof Judith Campisi. Source: BuckInstitute

And this company seems to be very serious about moving forward with treatments that selectively target senescent cells (the management team of Unity biotechnology have collectively taken 91 therapeutic candidates into human clinical trials and they are responsible for the creation of 13 FDA-approved medicines – like I said, serious) and they have some serious money backing them (Amazon boss Jeff Bezos’ venture fund Bezos Expeditions and PayPal co-founder Peter Thiel’s Founders Fund are investors – again: serious). Currently the company has pre-clinical programs for inflammatory joint diseases and ophthalmology, but they are certainly exploring other conditions, Parkinson’s is hopefully one of them in the wake of this current research report.

And Unity biotechnology are not alone in this area of targeting cellular senescence. Oisin biotechnology are another company designing at such therapies.

And there are certainly many different avenues to explore with regards to therapeutic options for senescent cell-based complications. For example, pro-senescence therapies could be considered for cancer (forcing cancer cells to stop dividing):

Options for senescence cell therapies. Source: Researchgate

To be fair, this is rather blue sky research (with clinical applications still some ways off in the future), but it is a very interesting idea and major biotech firms are certainly having a look at it.

What does it all mean?

Researchers in California has reported that there is an increase in the number of senescent astrocytes in the brains of people with Parkinson’s. They have also reported that exposing human astrocytes to the herbicide paraquat can cause the cells to flip from a healthy state into a state of senescence. Significantly, they report that lowering these levels of senescence reduced the neurodegeneration typically observed in a model of Parkinson’s, suggesting that senescent cells could be contributing to the development of neurodegenerative conditions like Parkinson’s.

While I was initially disturbed by the use of the term ‘zombie’ with regards to the cells in this study, I have come around to appreciating that the word is rather appropriate in the more classical version of a Haitian zonbi. I am also very intrigued to see if independent research groups can replicate these findings. If so, it would represent a very different way of looking at neurodegenerative conditions, and could help to why things may be going wrong in Parkinson’s for specific groups of vulnerable cells, like dopamine neurons.

I thought this research report was interesting.

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The banner for today’s post was sourced from Octobergallery