cGMP via PKG Activates 26S Proteasomes and Enhances Degradation of Proteins, Including Ones that Cause Neurodegenerative Diseases
This week we profile a recent publication in PNAS from Dr. Jordan VerPlank
and the laboratory of Dr. Alfred Goldberg (pictured) at Harvard Medical School.
Can you provide a brief overview of your lab’s current research focus?
Our laboratory in the Department of Cell Biology at Harvard Medical School has long been a center for the study of mechanisms and regulation of protein degradation, especially by the Ubiquitin Proteasome System (UPS). It has been generally assumed that rates of degradation by the UPS are regulated solely at the ubiquitination step, and consequently major drug-development efforts are underway at many biotech companies to influence protein ubiquitination by approaches like ProTacs. However, recent studies primarily from our lab have demonstrated that the activity of the 26S proteasome, the molecular machine that degrades ubiquitinated proteins, is also tightly regulated. Furthermore, cells have mechanisms to activate the proteasome, some of which are critical in determining the cell’s capacity to eliminate misfolded, potentially toxic proteins, including several that cause common major neurodegenerative diseases.
Recently, we have focused on an important new mechanism that cells utilize to enhance proteasome function and intracellular protein degradation – proteasome phosphorylation (1). Several years ago, we demonstrated that pharmacological agents that raise cyclic AMP (cAMP) increase rapidly the 26S proteasome’s ability to hydrolyze substrates via phosphorylation by the cyclic AMP-dependent protein kinase (PKA). cAMP has long been known to mediate the actions of many hormones (e.g., epinephrine and glucagon) and we showed that activation of the proteasome occurs under various physiological conditions that raise cAMP (e.g., fasting and exercise) (2). These pharmacological agents and hormones via PKA also selectively increase the degradation by the UPS of misfolded mutant and regulatory proteins, without affecting the bulk of cell proteins. It is now clear that many neurodegenerative diseases (e.g., tauopathies like Alzheimer’s disease or frontotemporal dementia or prion diseases like “mad cow”) are associated with an accumulation of protein aggregates which in experimental models impair proteasome function and protein homeostasis. In cultured cells (2) and in mouse (3) and zebrafish (4) models of tauopathies, treatment with a drug that raises cAMP by blocking its breakdown by phosphodiesterase 4 increased proteasome activity and reduced the levels of the disease-associated mutant tau and the resulting pathologies. Thus, enhancing proteasome activity and stimulating the clearance of mutant proteins by increasing cAMP appears to be a promising new strategy to combat neurodegenerative diseases.
Because of the therapeutic potential of cAMP in activating protein degradation by the UPS, we explored other signaling pathways that could also induce phosphorylation and activation of the 26S proteasome. These studies have uncovered a new role for cyclic GMP (cGMP) in controlling cellular protein degradation by the UPS (4). Aside from their biological interest, these findings are also of clear therapeutic promise because there are many widely-used, FDA-approved drugs that elevate cGMP in cells and are well tolerated, even when used chronically.
What is the significance of the findings in this publication?
In this article, we demonstrate that agents that raise cGMP and activate the cGMP-dependent protein kinase (PKG), including the widely-used phosphodiesterase 5 inhibitors sildenafil (Viagra) and tadalafil (Cialis), stimulate proteasome activities and protein breakdown by the ubiquitin proteasome system, without affecting autophagy. Like cAMP, cGMP enhances the degradation of misfolded and regulatory proteins, but cGMP also stimulates the breakdown of the bulk of cell proteins, which is unaffected by cAMP. We also found in cells and cell extracts that raising cGMP enhances within minutes the conjugation of ubiquitin to cytosolic proteins (4). Furthermore, we showed that sildenafil reduced the levels of the mutant tau and polyQ-expanded huntingtin by increasing their degradation in zebrafish larvae models of tauopathies and Huntington’s disease. Also, treatment of the zebrafish larvae expressing mutant tau also decreased the associated neuronal death and morphological abnormalities. Thus, agents that raise cGMP and activate PKG are an exciting new strategy to prevent the progression of neurodegenerative diseases in which the accumulation of misfolded proteins impairs proteasomal function.
What are the next steps for this research?
Enhancing ubiquitination, proteasome activities, and protein degradation are newly-appreciated actions of cGMP and PKG. Like most new findings, these observations raise many more questions to be investigated. One open question is how activating protein degradation is linked to cGMP’s physiological effects (e.g. vasodilation caused by nitric oxide signaling). We also do not know at the molecular level how phosphorylation alters proteasome structure to increase its activity. Another key issue is whether the activation of the proteasome, of ubiquitination, or both, are necessary for the therapeutic effects we observed in these disease models. We also are exploring further how these findings may lead to improvements in the treatment of different neurodegenerative diseases, and we already have promising results in other animal disease models.
This work was funded by:
We are very grateful for research grants from the National Institute of General Medical Sciences (NIGMS), Cure Alzheimer’s Fund, Muscular Dystrophy Association, and Project ALS, and for a postdoctoral fellowship from the NIGMS. Our collaborators in this work, Prof. David Rubinsztein, Sylwia Tyrkalska, and Angeleen Fleming (University of Cambridge) were supported by the UK Dementia Research Institute, The Tau Consortium, Alzheimer’s Research UK, and The Roger de Spoelberch Foundation.
- Lokireddy, S, et. al. PNAS 2015 Dec 29; 112(52): E716-85. Doi 10.1073. PNAS 1522332112.
- VerPlank, JS, et. al. PNAS. 2019 Mar, 116 (10) 4228-4237. Doi: 10.1073/pnas.1809254116.
- Myeku, N, et. al., Nature Medicine. 2016 Jan;22(1): 46-53. Doi:10.1038/nm.4011.
- VerPlank, JS, et. al. PNAS. 2020;202003277. doi:10.1073/pnas.2003277117.