ARTICLE IN BRIEF
Three new studies unravel new insights about the molecular pathway of PTEN-induced putative kinase 1 and parkin mutations in familial cases of Parkinson's disease.
Three new papers report a surprising bit of cell biology essential to understanding the normal activity of PTEN-induced putative kinase 1 (PINK1) and parkin, two genes that, when mutated, cause Parkinson's disease (PD). The discovery strengthens the case for intervening in the pathway they share to treat some cases of PD. But as the details of this pathway become clearer, it becomes less clear what connection it may share with other causes of PD, including known genes such as alpha-synuclein and leucine-rich repeat kinase 2 (LRRK2), and the larger number of cases of sporadic disease.
“It means this disease is quite heterogeneous,” said Edward Fon, MD, a co-author of one of the studies that appeared in the June 5 issue of Nature. “And it suggests a therapeutic target for PINK1/parkin disease that may be distinct from other pathways implicated in Parkinson's disease.”
Two other research teams published similar findings — one, in the April 28 issue of the Journal of Cell Biology and, the other, in the May 15 issue of The Biochemical Journal.
A ROLE IN MITOCHONDRIAL QUALITY CONTROL
PINK1 is a protein kinase, whose job is to add phosphates to other proteins. One of its major targets is parkin, and evidence has accumulated in recent years that PINK1 phosphorylation contributes to the activation of parkin. Parkin, in turn, is a ubiquitin ligase, whose job is to add the short protein ubiquitin to proteins; ubiquitination is a signal for degradation of those proteins. Mutations in either gene cause an autosomal recessive form of PD.
Researchers believe the PINK1/parkin system is a key part of mitochondrial quality control. Shortly after it is made, PINK1 is imported into the mitochondrion, where it is quickly degraded by an internal protease. That seemingly futile cycle continues as long as the mitochondrion maintains an electrical potential across its membrane. But a damaged mitochondrion cannot maintain a membrane potential, and when that potential is lost, PINK1 remains stuck to the outside of the mitochondrial membrane, outside the reach of the protease.
What happens next is the surprising bit of cell biology. PINK1 phosphorylates parkin — this much was known already. But the new studies show that PINK1 also phosphorylates ubiquitin, the first known example of ubiquitin phosphorylation. And that phosphorylation contributes to activation of parkin.
“No one had predicted that,” said Dr. Fon, the director of the McGill University Parkinson Program and an associate director of clinical and translational research at Montreal Neurological Institute in Canada.
The three research groups came to that realization in slightly different ways, and with slightly different initial hypotheses. But all of them found that both PINK1 and phospho-ubiquitin were needed to fully activate parkin.
Recent work on the atomic structure of parkin suggests why this may be so, according to Helen Walden, PhD, who helped elucidate the structure, and was not involved in the new studies. “The enzyme's activity is inhibited in at least three different ways,” said Dr. Walden, the program leader in the protein phosphorylation and ubiquitylation unit of the Medical Research Council at University of Dundee, Scotland. “When you take that into account, it is not so weird that you would need at least two moieties to activate it,” one perhaps to relax the structure, and the second to further bring it to its fully active conformation.
Once activated, the system quickly ramps up. “The beauty of the system is that it can be very rapidly upregulated,” said Richard Youle, PhD, who led the research published in the Journal of Cell Biology. Dr. Youle, a senior investigator at the Porter Neuroscience Research Center at the National Institute of Neurologic Diseases and Stroke, added: “As a ubiquitin ligase, parkin ubiquitinates lots of proteins on the outer mitochondrial membrane. Those ubiquitins are very likely to become substrates for PINK1 phosphorylation, activating more parkin molecules as they arrive. It is a powerful feed-forward mechanism.”
Once set in motion, parkin's activity leads — through largely unknown mechanisms — to engulfment of the damaged mitochondrion by autophagosomes, which destroy it. Mutations in either PINK1 or parkin are believed to interrupt this pathway, and — again through largely unknown mechanisms — lead to Parkinson's disease.
IMPLICATIONS FOR THERAPY
“While mitochondrial changes have been linked to Parkinson's disease for decades, the discovery of PINK1 and parkin mutations in familial cases placed mitochondrial damage at the heart of what goes wrong in Parkinson's,” said Miratul Muqit, MD, PhD, lead author of the paper in The Biochemistry Journal.
The three new papers “solve a major puzzle of how these two enzymes interact and function to protect neurons from mitochondrial damage,” said Dr. Muqit, a principal investigator in the Wellcome Trust Senior Research Fellowship in Clinical Science and a consultant neurologist at the University of Dundee, Scotland.
“A major impediment to drug discovery in Parkinson's has been a lack of understanding of the key signal transduction pathways mediating neuronal death,” he said. “Importantly, these studies will aid other experts in the Parkinson's field to potentially exploit these findings to develop biomarkers and therapeutic approaches in Parkinson's.”
That potential is clear for drugs that can upregulate this pathway in the face of mutations in PINK1 or parkin. “Understanding that phospho-ubiquitin turns on parkin,” Dr. Fon said, “you can imagine designing small molecules to do the same thing.”
Indeed, there are companies beginning to investigate such mimics to increase parkin activity. While the obvious candidates for such therapy would be PD patients with PINK1 mutations, Dr. Youle suggested that non-PD candidates might include those with diseases such as Leber's hereditary optic neuropathy; mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; or myoclonic epilepsy with ragged-red fibers, each due to mitochondrial mutations, in which accelerating the removal of damaged mitochondria might be therapeutic.
But can such a therapy help other PD patients? That is much less clear, since there is no obvious connection between the cell biology of PD due to PINK1/parkin and that due to other known PD genes. Dr. Muqit points out that a subset of sporadic patients may have this pathway downregulated, and would therefore be candidates for pro-parkin therapies, but this may be a small minority.
These studies highlight not only that PD may be a heterogeneous disease, but that as more is learned about the detailed biology of different forms, the more it becomes clear that — as in the cancer field — truly effective therapies may require splitting the patient population into ever-narrower subsets based on their molecular, rather than clinical, similarities, said Dr. Muqit.
EXPERTS: ON THE MOLECULAR PATHWAY OF PINK1 AND PARKIN
•. Kane LA, Lazarou M, Fogel AI, et al. PINK1 phosphorylates ubiquitin to activate parkin E3 ubiquitin ligase activity. J Cell Biol. 2014; 205:(2):143–153; Epub 2014 Apr 21.
•. Koyano F, Okatsu K, Kosako H, et al. Ubiquitin is phosphorylated by PINK1 to activate parkin. Nature. 2014; 510:(7503):162–166; Epub 2014 Jun 4.
•. Kazlauskaite A, Kondapalli C, Gourlay R, et al. Parkin is activated by PINK1-dependent phosphorylation of ubiquitin at Ser65. Biochem J. 2014; 460:(1):127–139.