Final Report Summary - MITOSCAFFOLD (Mitochondrial membrane organization by protein scaffolds and lipid dynamics)
Biological membranes are complex structures made of membrane lipids and proteins that control essential functions of the cell. Their importance is highlighted by mitochondria, dynamic organelles that are bound by double membranes with essential roles in cellular metabolism and signaling. Disturbance of mitochondrial function is linked to aging and both muscular and neurological diseases in humans. However, we are only beginning to understand the mechanisms which ensure the correct lipid composition and guide the organization of proteins and lipids within these membranes.
The MITOSCAFFOLD project studied the mechanisms that ensure the correct spatial organization of mitochondrial membranes and their functionality. We focused on membrane scaffold proteins, which we hypothesized to form membrane domains that contain defined proteins and lipids, and on lipid transport mechanisms responsible for ensuring the correct lipid composition of mitochondrial membranes.
Our studies revealed general principles of how phospholipids are transported within mitochondria. We identified a new group of lipid transfer proteins (LTPs) called the Ups1/PRELI family of proteins, which transport phospholipids between mitochondrial membranes. These LTPs form dimers with Mdm35/TRIAP1, which bind specific lipid types depending on their protein composition. In collaboration with Dr. Steven Matthews (Imperial College London), we were able to determine the first three-dimensional structure of one member of this novel LTP family. Moreover, our studies showed that regulated lipid trafficking between both mitochondrial membranes can define the phospholipid accumulation and functionality of these membranes.
Cardiolipin, a phospholipid that only occurs in mitochondria, is essential to protect cells against programmed cell death and to ensure mitochondrial function during aging. We have identified the complete set of enzymes involved in the synthesis of cardiolipin within mitochondria. Our studies on human LTPs showed an important function of intramitochondrial lipid transport and cardiolipin levels for cell survival and protection from programmed cell death. Lower cardiolipin levels alone facilitated the release of cell death-inducing proteins from mitochondria. In order to examine the role of members of the Ups/PRELI protein family in vivo, we generated conditional knockout mice allowing the tissue-specific ablation of specific mitochondrial LTPs. Our initial experiments already established an essential role of intramitochondrial lipid trafficking for embryonic development and will now be extended to specific tissues.
It is becoming increasingly clear that a specific membrane organization is essential to maintain mitochondrial function. The second part of the MITOSCAFFOLD project studied ring-shaped prohibitin protein complexes that function as membrane scaffolds in the mitochondrial inner membrane. Using quantitative mass spectroscopy, we identified the proteins that are found in membrane areas defined by prohibitin complexes. Among these proteins, we identified DNAJC19; defects in this protein cause dilated cardiomyopathy with ataxia (DCMA), a heritable heart disease. We could show that DNAJC19-prohibitin complexes regulate the cardiolipin metabolism. Notably, the clinical symptoms of DCMA patients resemble those of Barth syndrome patients, which have mutations in a gene controlling cardiolipin metabolism. Thus, we demonstrated similarities in the disease mechanisms of two cardiomyopathies and linked both diseases to disturbed cardiolipin metabolism. Barth syndrome and DCMA thus belong to a growing family of heart diseases which are caused by changes in the composition of mitochondrial membranes.
Another focus of the project was to study the physiological role of membrane domains defined by prohibitin ring complexes using tissue-specific, conditional gene inactivation in the mouse. We found that the loss of prohibitins affects mitochondrial functions and cell survival in the brain, heart, liver, kidney and skin, underlining the importance of mitochondrial membrane structure. In the absence of Phb2, death of neurons in the brain was observed, triggered by stress-induced changes in protein turnover by the protease OMA1. Further studies revealed activation of OMA1 by various stress and pathological conditions, identifying OMA1 as a central regulator of cell survival. Surprisingly, we detected in Phb2-deficient mouse brains tau aggregates, characteristic of many neurodegenerative diseases such as Alzheimer’s disease. This is the first demonstration that tau aggregation can be caused by mitochondrial dysfunction. Our observation is of broad interest for neurodegenerative disorders where tau aggregates are formed or that are linked to disturbed mitochondrial function and dynamics.
The MITOSCAFFOLD project studied the mechanisms that ensure the correct spatial organization of mitochondrial membranes and their functionality. We focused on membrane scaffold proteins, which we hypothesized to form membrane domains that contain defined proteins and lipids, and on lipid transport mechanisms responsible for ensuring the correct lipid composition of mitochondrial membranes.
Our studies revealed general principles of how phospholipids are transported within mitochondria. We identified a new group of lipid transfer proteins (LTPs) called the Ups1/PRELI family of proteins, which transport phospholipids between mitochondrial membranes. These LTPs form dimers with Mdm35/TRIAP1, which bind specific lipid types depending on their protein composition. In collaboration with Dr. Steven Matthews (Imperial College London), we were able to determine the first three-dimensional structure of one member of this novel LTP family. Moreover, our studies showed that regulated lipid trafficking between both mitochondrial membranes can define the phospholipid accumulation and functionality of these membranes.
Cardiolipin, a phospholipid that only occurs in mitochondria, is essential to protect cells against programmed cell death and to ensure mitochondrial function during aging. We have identified the complete set of enzymes involved in the synthesis of cardiolipin within mitochondria. Our studies on human LTPs showed an important function of intramitochondrial lipid transport and cardiolipin levels for cell survival and protection from programmed cell death. Lower cardiolipin levels alone facilitated the release of cell death-inducing proteins from mitochondria. In order to examine the role of members of the Ups/PRELI protein family in vivo, we generated conditional knockout mice allowing the tissue-specific ablation of specific mitochondrial LTPs. Our initial experiments already established an essential role of intramitochondrial lipid trafficking for embryonic development and will now be extended to specific tissues.
It is becoming increasingly clear that a specific membrane organization is essential to maintain mitochondrial function. The second part of the MITOSCAFFOLD project studied ring-shaped prohibitin protein complexes that function as membrane scaffolds in the mitochondrial inner membrane. Using quantitative mass spectroscopy, we identified the proteins that are found in membrane areas defined by prohibitin complexes. Among these proteins, we identified DNAJC19; defects in this protein cause dilated cardiomyopathy with ataxia (DCMA), a heritable heart disease. We could show that DNAJC19-prohibitin complexes regulate the cardiolipin metabolism. Notably, the clinical symptoms of DCMA patients resemble those of Barth syndrome patients, which have mutations in a gene controlling cardiolipin metabolism. Thus, we demonstrated similarities in the disease mechanisms of two cardiomyopathies and linked both diseases to disturbed cardiolipin metabolism. Barth syndrome and DCMA thus belong to a growing family of heart diseases which are caused by changes in the composition of mitochondrial membranes.
Another focus of the project was to study the physiological role of membrane domains defined by prohibitin ring complexes using tissue-specific, conditional gene inactivation in the mouse. We found that the loss of prohibitins affects mitochondrial functions and cell survival in the brain, heart, liver, kidney and skin, underlining the importance of mitochondrial membrane structure. In the absence of Phb2, death of neurons in the brain was observed, triggered by stress-induced changes in protein turnover by the protease OMA1. Further studies revealed activation of OMA1 by various stress and pathological conditions, identifying OMA1 as a central regulator of cell survival. Surprisingly, we detected in Phb2-deficient mouse brains tau aggregates, characteristic of many neurodegenerative diseases such as Alzheimer’s disease. This is the first demonstration that tau aggregation can be caused by mitochondrial dysfunction. Our observation is of broad interest for neurodegenerative disorders where tau aggregates are formed or that are linked to disturbed mitochondrial function and dynamics.