Miratul Muqit, MD, PhD, in 2004, together with Patrick Sleiman, identified and characterized the first mutations in the PINK1 gene in families with inherited Parkinson’s disease. In 2008 he joined the MRC Protein Phosphorylation Unit in Dundee as a Wellcome Trust Intermediate Fellow to initiate a program of research into the biochemistry of the PINK1 kinase under the mentorship of Dario Alessi. Away from the bench he manages patients with Parkinson’s disease in the clinic.
“My laboratory is utilizing state of the art biochemical, proteomic, transgenic and structural methodologies to address the next major questions for understanding the PINK1 signaling pathway. We wish to understand in more detail how PINK1 activity is regulated by mitochondrial depolarization and identify novel regulatory molecules of PINK1 activity. We are also very interested in determining the mechanism of how Serine 65 phosphorylation leads to activation of the Parkin E3 ligase activity and identifying key substrates of Parkin whose ubiquitylation is critically dependent on Serine 65 phosphorylation.” (Source)
The following has been paraphrased from an interview with Prof. Miratul Muqit on April 26th, 2018.
(Click here for the full audio version)
What is meant when a cell biologist uses the word ‘pathway’?
They are referring to a biological readout that is controlled at a molecular level by various bundles of proteins. These are organized and coordinated in a way that leads to a response that alters the behavior of a cell. Such pathways are regulated at multiple levels, and in disease context, the pathway could be perturbed at any one of these levels.
How well defined is the PINK1 pathway?
What is clear is that the protein encoded by the gene, which is a particular class of enzyme known as a kinase, is involved in helping cells cope with damage to mitochondria. We also know pretty well that people who have mutations in this gene have disturbances in this pathway.
(Click here for a description of PINK1 and its role in Parkinson’s disease, from the Science of Parkinson’s)
How big of a disturbance is needed in a pathway to perturb homeostasis (the ability of the cell to main equilibrium or stability)?
In Parkinson’s, homeostasis has been well studied in the context of protein folding and misfolding. There is a certain balance that needs to be maintained in order for the cell to properly make proteins and then break them down after they have been used. In the context of mitochondrial function, homeostasis refers to the cells ability to clean up the harmful by-products that result from mitochondria’s primary role as the energy producers in the cell. Homeostasis is also a measure of cell trafficking, which is the movement of different parts within a cell, and has to be very well maintained for the overall health of the cell.
There have been approximately 20 genes linked to familial Parkinson’s disease. It is incredible that if you have one point mutation in one of those genes, which is just a single letter change in the three billion letters of your DNA, that alone can perturb homeostasis and lead to disease. That tells us that subtle changes in the protein can have massive effects on the cellular level. It also tells us that studying these changes can give us clues about the mechanisms that lead to disease.
How well do we need to understand these mechanisms before we can design drugs that purposely intervene?
If you look at the current pipeline of drugs entering the clinic, it is dominated by small molecules targeting the signal transduction pathways to treat cancers. They are almost all designer drugs based on the fundamental knowledge of the basic mechanisms. Most of that knowledge is founded on genetic discoveries from 30 years ago. Knowing the genes is important in understanding the biological pathways linked to disease, but it is not enough, it usually takes years of research trying to understand the molecular and atomic level interactions before we can get a really useful drug.
However, there are a few examples where a gene encodes for a specific enzyme where a mutation in it results in the up-regulation of enzyme activity. In those you know pretty clearly how the mutation changes its function. These genetic discoveries quickly stimulate drug discovery without having to look at all of the downstream effects. This applies to a lot of cancer mechanisms, as well as people with Parkinson’s who carry LRRK2 mutations. But, in most cases you need a very detailed mechanistic understanding to know if your drugs are working and whether they might be effective in clinical trials.
Would you be able to identify a PINK1 carrier just based on their symptoms?
I could have a reasonable guess. One of the features is that patients tend to be very young, usually under 45. They also often develop dystonia (abnormal muscle contractions), as well as certain psychiatric features. These are quite similar to carriers of another mutation called parkin, so I could probably tell that someone is either a PINK1 or parkin carrier.
Could you briefly explain autophagy and how integral PINK1 is to it?
Autophagy mainly involves the disposal and recycling of damaged proteins within cells. There are many different forms of autophagy, PINK1 has been linked to a form called mitophagy that disposes of damaged mitochondria. There are several steps in the pathway, first the damage is sensed by the PINK1 protein, leading to tagging of the mitochondria with ubiquitin chains, that triggers the development of a new membrane which forms around the damaged mitochondria, and then it gets sent to the lysosome to be broken down into parts.
What role is PINK1 playing in helping us understand selective vulnerability (why certain cells die and not others) in PD?
At the moment we don’t have a good explanation for selective vulnerability. We do know that cell loss in PD goes beyond just dopamine cells and seems to spread to different areas of the brain. The fact that patients with PINK1 tend not to get dementia, like others forms of PD do, suggests that their disease doesn’t spread to the cortical regions of the brain.
What I think will happen in the future is that the genes associated with PD, which are now studied in isolation (Parkin and PINK1 being the exception), will be studied together. I think there are common pathways that different genes feed into, and that once we identify the nodes that connect them it will help explain selective vulnerability.
What role is PINK1 thought to play in idiopathic (no-known cause) PD?
Because there are some distinct clinical features of PINK1-PD, some think there might not be a link to idiopathic PD. Parkinson’s is also classified by the presence of Lewy bodies in post-mortem brains. We know from the few autopsies done on PINK1-PD carriers, that some had Lewy bodies and some didn’t, suggesting again that it may not be linked to idiopathic PD. Finally, genome wide association studies indicate that there are 40 subtle gene polymorphisms that can increase your risk of developing sporadic PD, but PINK1 isn’t implicated.
So you might think that PINK1 has nothing to do with idiopathic PD, but I think the present age of molecular research is teaching us that pathology isn’t that black and white. I think the idiopathic disease forms are driven by disturbances in some of the same pathways as those of mutation carriers, and that there is a subtle disturbance along the same pathways causing up-regulation or down-regulation of important activity. That is what we are trying to establish now using very sensitive antibody markers to monitor the activity of the PINK1 pathway. We are taking tissue of people with idiopathic forms of the disease and seeing if their PINK1 activity levels are higher or lower than tissue from people without the disease. Until that is done we can’t really answer this question, but I think we are going to discover that PINK1 is involved in cases of idiopathic PD where the person’s disease also develops slowly and they share symptoms with PINK1 patients.
When do you believe we might we see PINK1 based therapies make it to trial?
I think we may see PINK1 based therapies linked in the next 5-10 years. There is exciting work currently to develop small molecule drugs that can activate PINK1 or its target Parkin and several PINK1 activators have been reported by the Shokat lab. There is also fast moving work targeting a class of enzyme known as deubiquitinases (DUBs). DUBs are involved in reversal of PINK1 function and so inhibiting DUBs would be predicted to be beneficial. Currently a DUB known as USP30 is attracting the most interest amongst drug companies.