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Svyatoslav Yefimov
Svyatoslav Yefimov

Mature Contacts


Chronic mitochondrial stress associates with major neurodegenerative diseases. Recovering stressed mitochondria constitutes a critical step of mitochondrial quality control and thus energy maintenance in early stages of neurodegeneration. Here, we reveal Mul1-Mfn2 pathway that maintains neuronal mitochondrial integrity under stress conditions. Mul1 deficiency increases Mfn2 activity that triggers the first phasic mitochondrial hyperfusion and also acts as an ER-Mito tethering antagonist. Reduced ER-Mito coupling leads to increased cytoplasmic Ca2+ load that activates calcineurin and induces the second phasic Drp1-dependent mitochondrial fragmentation and mitophagy. Overexpressing Mfn2, but not Mfn1, mimics Mul1-deficient phenotypes, while expressing PTPIP51, an ER-Mito anchoring protein, suppresses Parkin-mediated mitophagy. Thus, by regulating mitochondrial morphology and ER-Mito contacts, Mul1-Mfn2 pathway plays an early checkpoint role in maintaining mitochondrial integrity. Our study provides new mechanistic insights into neuronal mitochondrial maintenance under stress conditions, which is relevant to several major neurodegenerative diseases associated with mitochondrial dysfunction and altered ER-Mito interplay.




mature contacts


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Our previous study in mature cortical neurons revealed that mitophagy is observed in a small proportion of neurons following mitochondrial depolarization and that Parkin translocation to depolarized mitochondria occurs much more slowly than in non-neuronal cells5,6. Consistently, mitophagy is observed in a small proportion of axonal mitochondria following acute depolarization7. Parkin or PINK1 knock-out mice showed subtle changes in mitochondrial morphology and neuronal degeneration8,9,10. In parkin and pink mutant flies, the density and integrity of axonal mitochondria in motor neurons is comparable to WT controls11,12. These findings argue for intrinsic neuronal mechanisms that can maintain or recover mitochondrial integrity, rather than rapid elimination of dysfunctional mitochondria through Parkin-mediated mitophagy. To support this assumption, several fundamental questions remain to be addressed: (1) Do distinct mechanisms are in place in neurons to act as a checkpoint for recovery vs. rapid degradation of chronically stressed mitochondria? (2) Is mitophagy the second resort for neuronal mitochondrial quality control after recovery mechanisms have failed? (3) If this is the case, does mitochondrial ubiquitin ligase 1 (Mul1) play an early role in maintaining neuronal mitochondrial integrity? Addressing these questions is relevant to major neurodegenerative diseases associated with chronic mitochondrial stress.


Mul1, also referred to as mitochondrial-anchored protein ligase (MAPL)13, mitochondrial ubiquitin ligase activator of NF-κB (MULAN)14, or growth inhibition and death E3 ligase (GIDE)15, is a multifunctional mitochondrial membrane protein. In non-neuronal cell lines, Mul1 acts as an E3 ubiquitin ligase that binds, ubiquitinates, and degrades Mfn216 and as a small ubiquitin-like modifier (SUMO) E3 ligase that SUMOylates dynamin-related protein-1 (Drp1) to enhance its stability on mitochondria17,18. Mfn2 is known to control mitochondrial fusion and regulate the interplay between the endoplasmic reticulum and mitochondria (ER-Mito)19,20,21 in mouse embryonic fibroblasts (MEFs) and Hela cells. ER-Mito contacts are tethered by multiple linker proteins with diverse distribution and interactions between the two organelles22. Among these linker proteins is tyrosine phosphatase interacting protein 51 (PTPIP51), which enhances ER-Mito interaction23,24. ER-Mito contacts maintain lipid and energy metabolism, as well as Ca2+ transfer from the ER to mitochondria that is essential for mitochondrial bioenergetics and integrity25,26. The ER extends from the soma into dendrites and axons27, thus allowing for distal ER-Mito signaling28. Disrupted ER-Mito contacts have been implicated in several major neurodegenerative diseases23,29,30.


Our study also reveals unique mitochondrial phenotypes in Mul1 mutant neurons: transient hyperfusion as an early response to maintain bioenergetic capacity. Prolonged stress impairs ER-Mito interplay and disturbs mitochondrial functions and Ca2+ homeostasis. Elevated cytoplasmic Ca2+ load activates Drp1 through calcineurin, thus triggering mitochondrial fragmentation. We provided six lines of evidence to support this mechanistic model: (1) mitochondrial association of Drp1 is increased at DIV13, a time point just before fragmentation occurs; (2) mitochondria are locked at the hyperfusion state when expressing catalytically inactive mutant Drp1K38A; (3) cytoplasmic Ca2+ load is elevated in Mul1-deficient neurons; (4) Drp1 is activated through the Ca2+/calcineurin when ER-Mito contacts are impaired; (5) expressing ER-Mito anchoring protein PTPIP51 rescues mitochondrial fragmentation. We further confirmed this model by applying two blockers of calcineurin activity, which arrest mitochondria in the hyperfusion state in Mul1-depleted neurons. Thus, our study reveals a new mechanistic pathway that links impaired ER-Mito contacts to Drp1-dependent mitochondrial fragmentation, a process necessary for Parkin-mediated mitophagy.


Several critical roles of mitochondria, including lipid and energy metabolism and Ca2+ homeostasis, rely on their interplay with the ER. Altered ER-Mito coupling has been reported in the pathogenesis of several major neurodegenerative diseases23,70. Although affected neurons in AD, PD, ALS/FTD, and HSP display various pathological changes, they all share common features: chronic mitochondrial dysfunction, altered ER-Mito interactions with impaired Ca2+ homeostasis and energy metabolism, and mitochondrial fragmentation23. Our study in mature cortical neurons reveals that sustaining ER-Mito contacts is critical to maintaining mitochondrial integrity, suppressing mitochondrial fragmentation and mitophagy, thus providing a mechanistic explanation for the seemingly disparate features of these neurodegenerative diseases.


The distribution of the neuropeptides substance P (SP), vasoactive intestinal polypeptide (VIP) and calcitonin gene related peptide (CGRP) was studied immunohistochemically in psoriatic skin during the Koebner response (6 h, 2 days, 7 days, 14 days, 21 days), and in mature psoriatic plaques, of 37 psoriatic patients. The morphological association of sensory nerves, SP and VIP with papillary mast cells was also monitored. The nerves containing SP, VIP or CGRP were very scanty in control skin, and in non-lesional and Koebner-negative psoriatic skin. The first psoriatic lesions were seen 7 days after tape stripping the symptomless psoriatic skin. SP- and VIP-containing nerves were slightly increased in Koebner-positive specimens, but the increase was very prominent in dermal papillae of mature psoriatic plaques. In the plaques, nerve-mast cell contacts were significantly increased (p


Descending systems have a crucial role in the selection of motor output patterns by influencing the activity of interneuronal networks in the spinal cord. Commissural interneurons that project to the contralateral grey matter are key components of such networks as they coordinate left-right motor activity of fore and hind-limbs. The aim of this study was to determine if corticospinal (CST) and reticulospinal (RST) neurons make significant numbers of axonal contacts with cervical commissural interneurons. Two classes of commissural neurons were analysed: 1) local commissural interneurons (LCINs) in segments C4-5; 2) long descending propriospinal neurons (LDPNs) projecting from C4 to the rostral lumbar cord. Commissural interneurons were labelled with Fluorogold and CST and RST axons were labelled by injecting the b subunit of cholera toxin in the forelimb area of the primary somatosensory cortex or the medial longitudinal fasciculus respectively. The results show that LCINs and LDPNs receive few contacts from CST terminals but large numbers of contacts are formed by RST terminals. Use of vesicular glutamate and vesicular GABA transporters revealed that both types of cell received about 80% excitatory and 20% inhibitory RST contacts. Therefore the CST appears to have a minimal influence on LCINs and LDPNs but the RST has a powerful influence. This suggests that left-right activity in the rat spinal cord is not influenced directly via CST systems but is strongly controlled by the RST pathway. Many RST neurons have monosynaptic input from corticobulbar pathways therefore this pathway may provide an indirect route from the cortex to commissural systems. The cortico-reticulospinal-commissural system may also contribute to functional recovery following damage to the CST as it has the capacity to deliver information from the cortex to the spinal cord in the absence of direct CST input. 041b061a72


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