Findings could help develop new treatments for heart failure, stroke, cancer and neurodegeneration.
Researchers at Temple University School of Medicine report that a mitochondrial membrane protein acts as a gatekeeper for catastrophic cell failure. Their study, published in Molecular Cell, identifies spastic paraplegia 7 (SPG7) as an essential component of the mitochondrial permeability transition pore (PTP), a channel whose opening causes loss of mitochondrial energy production and triggers necrotic cell death. The discovery suggests that blocking SPG7 or its interactions could be a promising strategy to prevent tissue damage in conditions marked by mitochondrial dysfunction, including heart attack and stroke.
The work, led by Muniswamy Madesh, PhD, associate professor in the Department of Biochemistry and the Cardiovascular Research Center at Temple University School of Medicine, establishes SPG7 as a central structural and functional element of the PTP. For decades scientists have known that PTP opening leads to loss of mitochondrial membrane potential, shutdown of ATP production and cell death, but the molecular composition of the pore had been uncertain. Until now, cyclophilin D (CypD) was the only protein widely recognized as necessary for PTP function; this study adds SPG7 as a conserved and indispensable partner.
To identify genes that regulate PTP opening, Dr. Madesh’s team performed an RNA interference (RNAi) screen focused on mitochondrial responses to two main stressors: calcium overload and elevated reactive oxygen species (ROS). These stresses commonly occur in disease states and are primary triggers of PTP opening. Starting from a panel of 128 candidate genes, the researchers narrowed the list to 14 likely components and then showed that knockdown of a single gene, SPG7, was sufficient to prevent pore opening and the resulting collapse of mitochondrial function.
The study clarifies how pathological insults affect mitochondria. In many diseases—especially those involving ischemia or hypoxia—mitochondrial calcium and ROS rise, mitochondria swell, and the PTP opens. This opening disrupts normal electron and proton flow across mitochondrial membranes, collapsing the membrane potential (ΔΨm) and halting ATP synthesis. Cells lose energy rapidly and undergo necrotic death, which contributes to tissue damage after heart attacks, strokes and other acute injuries.
Beyond injury, the physiological role of transient PTP openings is still being explored. Dr. Madesh suggests that under normal conditions SPG7-mediated transient pore activity might help mitochondria eliminate toxic metabolites or maintain homeostasis. He plans follow-up studies with knockout animal models to better define SPG7’s normal cellular functions.
Importantly, the researchers provide biochemical evidence that the PTP is a heterooligomeric complex comprising SPG7, CypD and voltage-dependent anion channel (VDAC) proteins at contact sites between the inner and outer mitochondrial membranes. The work shows that disrupting SPG7 or the interaction between SPG7 and CypD prevents ΔΨm depolarization and protects cells from Ca2+- and ROS-induced necrosis, findings that directly point to therapeutic opportunities.
Therapeutic implications
Because PTP opening plays a central role in cell death across many disorders, targeting the pore—especially by inhibiting SPG7 function or blocking its interaction with CypD—could preserve mitochondrial function and reduce cell loss in acute and chronic diseases. The authors note potential relevance for cardiovascular disease, stroke, cancer and neurodegenerative disorders, all of which involve degrees of hypoxia and mitochondrial dysfunction. Ongoing work in the Temple group will test small-molecule SPG7 inhibitors in animal models and evaluate their promise for translation to humans.
The study team included Santhanam Shanmughapriya, Sudarsan Rajan, Nicholas E. Hoffman, Andrew M. Higgins, Dhanendra Tomar, Neeharika Nemani, Kevin J. Hines, Dylan J. Smith, Akito Eguchi, Sandhya Vallem, Farah Shaikh, Maggie Cheung, Nicole J. Leonard, Ryan S. Stolakis, Matthew P. Wolfers, and Neelakshi R. Jog in the Department of Biochemistry and Center for Translational Medicine at Temple University School of Medicine; Jessica Ibetti, J. Kurt Chuprun, Walter J. Koch, and John W. Elrod at the Center for Translational Medicine and Department of Pharmacology; and Steven R. Houser at the Cardiovascular Research Center. The research was supported by NIH grants R01GM109882, R01HL086699, R01HL119306 and 1S10RR027327.
Source: Temple University School of Medicine. Image credit: public domain. Original research: “SPG7 Is an Essential and Conserved Component of the Mitochondrial Permeability Transition Pore,” Molecular Cell, published online September 17, 2015; doi:10.1016/j.molcel.2015.08.009.
Abstract
SPG7 Is an Essential and Conserved Component of the Mitochondrial Permeability Transition Pore
Highlights
• Mitochondrial SPG7 is required for PTP complex formation across multiple cell types.
• SPG7 interacts with cyclophilin D (CypD) and VDAC at inner/outer mitochondrial membrane contact sites.
• The C-terminal region of SPG7 and the cyclosporin A–binding region of CypD are necessary for pore formation.
• Loss or disruption of SPG7 protects mitochondria against Ca2+- and ROS-induced PTP-dependent necrosis.
Summary
Mitochondrial permeability transition is a process in which abrupt PTP opening causes dissipation of the mitochondrial membrane potential (ΔΨm), collapse of ATP production and cell death. Although multiple candidate proteins have been proposed, the core PTP component was previously unknown. Using an RNAi screen, the authors identified a conserved and necessary role for SPG7 in Ca2+- and ROS-induced PTP opening. SPG7 knockdown produced higher mitochondrial Ca2+ retention and preserved ΔΨm during stress, phenocopying cyclophilin D loss. Biochemical analyses indicate that the PTP is a heterooligomeric complex composed of VDAC, SPG7 and CypD. Interfering with SPG7 or disrupting SPG7–CypD binding prevented ΔΨm depolarization and cell death, identifying SPG7 as an inner-membrane protein that is a core PTP component at inner–outer membrane contact sites.