Genetic variations in a region of our DNA called PARKIN is associated with an increased risk of developing Parkinson’s – particularly young-onset PD (diagnosed before the age of 40yrs).
This area of DNA provides the instructions for making a protein (also referred to as PARKIN), which plays a number of important roles inside of cells.
Recently, a South Korean biotech company called Cellivery has published research on an experimental therapeutic agent that easily penetrates both the brain and cells within, delivering PARKIN protein to the cells that need it.
In today’s post, we will discuss what PARKIN does, review the new research report, and explore what could happen next.
Here on the SoPD we often talk about research regarding the prominent Parkinson’s associated proteins, think of alpha synuclein, LRRK2 and GBA. And they are of interest as there is a great deal of activity now at the clinical level exploring agents targetting these proteins.
But there are a number of interesting therapeutics being developed that are exploring some of the other Parkinson’s-associated proteins.
A good example was published this week:
Title: Intracellular delivery of Parkin rescues neurons from accumulation of damaged mitochondria and pathological α-synuclein
Authors: Chung E, Choi Y, Park J, Nah W, Park J, Jung Y, Lee J, Lee H, Park S, Hwang S, Kim S, Lee J, Min D, Jo J, Kang S, Jung M, Lee PH, Ruley HE & Jo D
Journal: Science Advances, 29 Apr 2020:6, 18, eaba1193
In this study, South Korean researchers demonstrated that a brain penetrating compound (including the PARKIN protein) can rescue numerous models of Parkinson’s.
Hang on a second: What is PARKIN?
Sometimes referred to as PARK2 (as it was the second gene to be associated with Parkinson’s), PARKIN protein functions as an E3 ubiquitin ligase.
A ligase is an enzyme that initiates the joining of two molecules. It forms a new chemical bond between them.
Ubiquitin is a marvellous little protein that – as the name suggests – is ‘ubiquitous’ in all cells, and it affects all aspects of cell biology. It works its magic by being bound to proteins through a process known as ubiquitination.
The structure of ubiquitin. Source: Wikipedia
Ubiquitination – which can change the way a protein functions – is a 3 steps process with specific groups of enzymes performing each step. Those 3 steps are:
- Activation with ubiquitin-activating enzymes (conducted by E1 enzymes)
- Conjugation with ubiquitin-conjugating enzymes (conducted by E2 enzymes)
- Ligation with ubiquitin ligases (conducted by E3 enzymes)
PARKIN is an enzyme involved with that last step (“E3”).
But it is important to understand that ubiquitin can affect all kinds of biological processes, including:
- Apoptosis (cell death)
- Cell division and multiplication
- Degeneration of neurons and muscular cells
- DNA transcription and repair
- Immune and inflammatory response
- Stress response pathway
- Viral infection
How is PARKIN associated with Parkinson’s?
Tiny variations in the region of DNA that provide the instructions for making the PARKIN protein are some of the most common genetic risk factors for individuals with young-onset Parkinson’s (or YOPD). These are cases diagnosed at some point under 40 years of age.
There isn’t one single PARKIN genetic variation associated with Parkinson’s. Rather, there are currently 10 common mutations in the PARKIN gene that can give rise to early-onset Parkinson’s. And it should be noted that PARKIN has also been associated with different types of cancer – there are 13 cancer-related genetic variants (PARKIN is recognised as a tumor suppressor – click here to read more about this).
Importantly, only two of the Parkinson’s related variants are associated with an increased risk of cancer (they are R24P and R275W – red+black arrow heads in the image below).
Comparing PARK2 Cancer and PD associated mutations. Source: Nature
Thus it is important to know exactly where your mutation is, if in fact you have one. And if you have the R24P and R275W mutation, it doesn’t necessarily mean that you are definitely going to develop a particulartype of cancer. It just means that you are slightly more predisposed to it than someone without that PARKIN genetic variation (and please remember that a lot of genetic variations are required for a cancer to start!).
It is good to be aware of this association though and to have regular check ups.
What does PARKIN-associated Parkinson’s look like?
Individuals diagnosed with PARKIN-associated YOPD are usually diagnosed around 30 years of age (but this can vary greatly). The disease is slower in progression than other forms of PD. The condition often presents itself clinically in the form of tremor, bradykinesia, and lower-limb dystonia (sustained muscle contractions). The affected person will have a preserved sense of smell and cognitive deficits are rare. Interestingly, at the postmortem stage, there are few reported cases with lewy bodies (one of the fundamental hallmarks of the Parkinsonian brain).
For those interested in learning more about PARKIN-associated YOPD, click here for a thorough review of the condition.
RECAP #1: Tiny variations in the DNA that provides the instructions for making a protein called PARKIN are some of the most common genetic risk factors for young onset Parkinson’s (diagnosed before 40 years of age).
PARKIN has many functions inside of cells, but it is primarily associated with attaching another protein – called ubiquitin – to other proteins.
Ok so what does the new study report?
In this new study, the researchers present an experimental therapeutic agent called “improved cell-permeable Parkin protein” (iCP-Parkin). It is basically the Parkin protein with a few additional attachments that allows it to pass easily through cell membranes.
The compound has been developed by a South Korean biotech company called Cellivery Therapeutics.
The company has developed “Therapeuticmolecule Systemic Delivery Technology” (or TSDT) to enable better delivery of molecules, not only to the brain but also into cells. The brain is surrounded by a protective membrane that limits a lot of chemicals and drugs from entering the brain. TSDT allows for easier access. Once inside the brain, TSDT also allows molecules to easily access cells as well.
This video explains TSDT:
In their study, the Cellivery researchers wanted to explore whether their iCP-PARKIN could be used as a means of ‘PARKIN replacement’ in PARKIN-associated Parkinson’s.
They firstly demonstrated that iCP-PARKIN was detectable in the entire body (including the brain) at 3 hours after injection into normal mice (with maximum levels being detected in the brain at 2 hours).
Next they demonstrated that iCP-PARKIN can reduce alpha synuclein-associated toxicity in cell culture models of Parkinson’s.
What is alpha synuclein?
Alpha synuclein is a protein that accumulates in certain neurons in Parkinson’s.
By itself, the alpha synuclein protein is considered a monomer, or a single molecule that will bind to other molecules to form an oligomer (a collection of a certain number of monomers in a specific structure). In Parkinson’s, alpha-synuclein also aggregates to form what are called ‘fibrils’.
Microscopic images of Alpha Synuclein (AS) monomers, oligomers and fibrils. Source: Brain
Oligomer versions of alpha-synuclein are believed to have a key role in the pathology of Parkinson’s. They can lead to the generation of fibrils and may even cause damage by themselves.
The researchers found that iCP-PARKIN treatment significantly reduced oligomeric alpha synuclein levels (by more than 80%) in cells that had been treated with neurotoxins. It should also be noted that treating cells with just PARKIN protein (a control) had no effect – due to the protein not entering the cells.
How was iCP-PARKIN reducing alpha synuclein levels?
The researchers found that if they chemically blocked the waste disposal systems of the cell – namely, the proteasome and the autophagy processes – they would block the alpha synuclein reducing effects of iCP-PARKIN. Thus, they proposed that iCP-PARKIN was having its alpha synuclein effect via these two systems.
Next, the investigators reported that iCP-PARKIN treatment (30mg/kg) rescued a neurotoxin-induced mouse model of Parkinson’s (6-OHDA lesion of the medial forebrain bundle) when 3 days of treatment began 4 days post administration of the toxin. This is a delayed treatment approach to a very severe model of PD – the fact that they saw any effect here is interesting. The delayed treatment approach is suppose to replicate what is happening in the human situation, where motor features are appearing before treatment begins.
The researchers repeated this feat in two additional rodent models of Parkinson’s:
- A 6-OHDA lesion in the mouse striatum, with treatment (3x per week for 4 weeks) starting 3 weeks post toxin (again a delayed treatment model), and
- A MPTP mouse model (different lesion regimes), with treatment (1x per day for 5 days) starting after the toxin was delivered
In both models, the iCP-PARKIN treatment improved the motor complications and survival of the dopamine neurons (compared to controls).
And as if that body of work wasn’t impressive, the investigators continued their study by testing iCP-PARKIN in a more “disease relevant” model of PD (intracranially delivery of AAV-viral alpha synuclein in mice – these mice were injected in the brain with a virus that produces high levels of alpha synuclein). Mice that are injected with the virus develop motor symptoms at approximately 5 weeks post viral delivery and exhibit 80% dopamine neuron loss by 8 weeks.
The investigators treated the mice with iCP-Parkin (30 mg/kg, 3x per week for 4 weeks starting at 8 weeks post virus delivery) and reported not only improvements in motor function, but also better survival of the dopamine neurons and reduced alpha synuclein levels.
The researchers concluded that their new therapuetic agent has “the desired activity, specificity, and bioavailability” required for a condition like Parkinson’s, and that their results were consistent across multiple models (using different mechanisms of pathology).
Wait a minute. How do they explain the survival of dopamine neurons after “80% dopamine neuron loss”?
They suggest that a ‘self-protection’ mechanism might be at play in which the dopamine neurons turn off a lot of the dopamine-related activity while they deal with the toxin/stress affecting them (we have recently explored this kind of idea in a previous SoPD post on dormant-but-not-dead dopamine neurons – click here to read that post). The researchers propose that during Parkinson’s progression the ‘loss of dopamine neuron activity’ may not initially mean dopamine neuronal death, and if this is the case then there is the possibility of a window of opportunity for some kind of reversal. This is what the authors of the study propose.
RECAP #2: Researchers from Cellivery Therapeutics have developed a version of the PARKIN protein – called iCP-PARKIN – that can enter the brain, access cells and function like normal PARKIN protein.
Across multiple preclinical models of Parkinson’s, the investigators observed beneficial effects from iCP-PARKIN treatment.
Is this the first time this molecule has been tested?
Yes, but the Cellivery researcher have previously published data on an earlier version of iCP-PARKIN:
Title: Cell-permeable parkin proteins suppress Parkinson disease-associated phenotypes in cultured cells and animals
Authors: Duong T, Kim J, Ruley HE, Jo D
Journal: PLoS One. 2014 Jul 14;9(7):e102517
PMID: 25019626 (This report is OPEN ACCESS if you would like to read it)
In this study, the researchers tested ‘CP-PARKIN’ (note the missing ‘i’ for improved) in models of Parkinson’s. This was the first generation of the agent and it suffered from low solubility (meaning it didn’t dissolves in water very well) which therefore made it unsuitable for clinical development. The CP-PARKIN compound still demonstrated beneficial effects in both cell culture and mouse models of Parkinson’s, but the results were not as robust as those reported with the new iCP-PARKIN agent.
And have other researchers seen similar neuroprotective effects with PARKIN in models of Parkinson’s?
So the iCP-PARKIN and CP-PARKIN compounds haven’t been independently tested (as far as I’m aware, and I’m happy to be corrected on this – it would be nice to see independent replication using these agents), but yes, other research groups have seen neuroprotective properties associated with introducing PARKIN protein into models of Parkinson’s.
The first report was published in 2004 (just a few years after PARKIN was discovered to be associated with PD):
Title: Lentiviral vector delivery of parkin prevents dopaminergic degeneration in an alpha-synuclein rat model of Parkinson’s disease.
Authors: Lo Bianco C, Schneider BL, Bauer M, Sajadi A, Brice A, Iwatsubo T, Aebischer P.
Journal: Proc Natl Acad Sci U S A. 2004 Dec 14;101(50):17510-5
PMID: 15576511 (This report is OPEN ACCESS if you would like to read it)
In this study, the researchers rescued a rodent alpha-synuclein model of Parkinson’s by introducing PARKIN (using viruses to produce high levels of the protein). And this finding was independently reproduced in a neurotoxin model of PD several years later:
Title: Parkin-mediated protection of dopaminergic neurons in a chronic MPTP-minipump mouse model of Parkinson disease.
Authors: Yasuda T, Hayakawa H, Nihira T, Ren YR, Nakata Y, Nagai M, Hattori N, Miyake K, Takada M, Shimada T, Mizuno Y, Mochizuki H.
Journal: J Neuropathol Exp Neurol. 2011 Aug;70(8):686-97.
PMID: 21760537 (This report is OPEN ACCESS if you would like to read it)
So there is a growing body of evidence supporting PARKIN as a potential neuroprotective agent for Parkinson’s.
# # #
RECAP #3: iCP-PARKIN is a new version of a similar PARKIN-based compound (called CP-PARKIN), which also demonstrated beneficial effects in models of Parkinson’s.
Other research groups have previously published reports of PARKIN exhibiting neuroprotective properties in models of Parkinson’s.
# # #
Interesting. So what is happening next?
This is not immediately clear.
Cellivery Therapeutics has conducted all of the iCP-PARKIN research discussed above, but they have provided very little in further information regarding the development of this agent. The assumption is that they are keen to take it into clinical testing.
The studies presented in the new research report were supported by a grant from the Michael J Fox Foundation (Click here to read more about this). But as interesting as the data is, there are still a few hurdles to clear before iCP-PARKIN will be ready for clinical testing.
The company will need to conduct long-term toxicology studies to determine if the agent is safe and tolerable for long term use. They will also need to identify the dose range that can be safely used in any future clinical trial.
We will hopefully learn more about this later this year, but the company has already partnered up with another South Korean biotech firm called Ildong Pharmaceutical to further develop iCP-Parkin for the clinic (Click here to read more about this).
So what does it all mean?
Tiny variations in specific regions of DNA have been associated with an increased risk of developing Parkinson’s. These genetic errors can result in the production of a faulty protein that does not do its job properly (for example, the GCase protein in the case of GBA-associated PD – click here to read more about this). An obvious therapeutic approach for these types of Parkinson’s is to simply replace the faulty protein with a correct version.
The South Korean biotech company Cellivery is attempting to do this with the Parkinson’s associated protein PARKIN. Their drug – a PARKIN-based agent – has demonstrated some impressive results in multiple models of Parkinson’s and it will be interesting to see what happens next with this experimental approach.
And given that their agent has beneficial effects in models of non-genetic forms of PD, the company must be looking at evaluating iCP-PARKIN in more that simply PARKIN-associated PD. In addition, as we discussed above, the company is probably not blind to the fact that if iCP-PARKIN is successful in PD, it could potentially be repurposed for oncological indications.
But there is still a long way to go before we get to that point, so until then we will be keeping an eye out for any news relating to PARKIN or iCP-PARKIN developments here.
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The banner for today’s post was sourced (and adapted) from Cellivery.