DRP1-mediated mitochondrial fission is essential to maintain cristae morphology and bioenergetics

Abstract

Mitochondria and peroxisomes are both dynamic signaling organelles that constantly undergo fission. While mitochondrial fission is known to coordinate cellular metabolism, proliferation, and apoptosis, the physiological relevance of peroxisome dynamics and the implications for cell fate are not fully understood. DRP1 (dynamin-related protein 1) is an essential GTPase that executes both mitochondrial and peroxisomal fission. Patients with de novo heterozygous missense mutations in the gene that encodes DRP1, DNM1L, present with encephalopathy due to defective mitochondrial and peroxisomal fission (EMPF1). EMPF1 is a devastating neurodevelopmental disease with no effective treatment. To interrogate the mechanisms by which DRP1 mutations cause cellular dysfunction, we utilized human-derived fibroblasts from patients with mutations in DRP1 who present with EMPF1. As expected, patient cells display elongated mitochondrial morphology and lack of fission. Patient cells display a lower coupling efficiency of the electron transport chain, increased proton leak, and upregulation of glycolysis. In addition to these metabolic abnormalities, mitochondrial hyperfusion results in aberrant cristae structure and hyperpolarized mitochondrial membrane potential, both of which are tightly linked to the changes in metabolism. Peroxisome structure is also severely elongated in patient cells and results in a potential functional compensation of fatty acid oxidation. Understanding the mechanism by which DRP1 mutations cause these metabolic changes will give insight into the role of mitochondrial dynamics in cristae maintenance and the metabolic capacity of the cell, as well as the disease mechanism underlying EMPF1.

Fission Impossible (?)—New Insights into Disorders of Peroxisome Dynamics

“…Phenotypes consistent with peroxisomal dysfunction, such as hypomyelination, developmental abnormalities and nystagmus, are also sometimes present in patients with DRP1 mutations [ 94 , 95 ]. Analysis of cells derived from these patients normally show major perturbations to the mitochondrial network reflecting a fission defect, including elongation of mitochondria and formation of a hyperfused network [ 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 ], or swollen ‘balloon-like’ mitochondria [ 102 , 103 , 104 ], with abnormal cristae structure [ 102 , 105 ]. Where peroxisomal morphology has been investigated, an elongated phenotype is often seen [ 96 , 100 , 101 , 103 , 104 ], which may be accompanied by constriction [ 95 ] ( Figure 3 ); however, in other patients, peroxisomes are morphologically normal [ 106 , 107 ].…”

“…Notably, in the unusual example of siblings with compound heterozygous frame shifts leading to no detectable DRP1 expression, the mitochondrial morphology in the brain was cell-type specific, with hippocampal and Purkinje neurons showing giant mitochondria, while the mitochondria in glia and non-neuronal cells appeared normal [ 102 ]. Therefore, it is possible that differences observed in peroxisome morphology between patients (and even between different studies on the same patient cells [ 105 , 107 ]), may also arise from differences in the cell type/cellular environment rather than the specific mutation.…”

“…Interestingly, despite the observed morphological changes, the metabolic functions of mitochondria and peroxisomes are only mildly affected (if at all) in patients with DRP1 mutations. Lactate levels may be elevated in some cases [ 95 , 96 , 107 ], presumably as a result of compromised mitochondrial respiration [ 98 , 105 , 106 ]; however, this feature is not shared by all patients [ 99 , 103 ]. Unlike patients with peroxisome biogenesis disorders (PBDs), those with DRP1 mutations do not typically show an increase in serum VLCFA levels [ 103 , 106 , 108 ], nor do their fibroblasts show defects in peroxisomal β-oxidation [ 96 ], indicating that the elongated peroxisomes can still perform their usual metabolic functions.…”

“… (p.E379K): No notable alterations to peroxisomes or mitochondria [ 195 ] Refractory epilepsy with prolonged survival c.1085G>A (p.G362D) De novo heterozygous Missense mutation in middle domain Hyperfusion of the mitochondrial network [ 97 ] Postnatal microcephaly, developmental delay and pain insensitivity c.1084G>A (p.G362S) De novo heterozygous Missense mutation in middle domain Decreased respiratory chain complex IV activity. Impaired fission with elongated mitochondrial structure [ 98 ] Epileptic encephalopathy c.1207C>T (p.R403C) De novo heterozygous Missense mutation in middle domain (Expression in mouse embryonic fibroblasts): Dominant-negative effect, with reduced DRP1 oligomerization and mitochondrial fission activity [ 196 ] Mild developmental delay and intellectual disability, paroxysmal dystonia, acute status epilepticus, progressive global cerebral atrophy Normal mitochondrial and peroxisomal metabolism, elongated mitochondria [ 99 ] Hyperfused elongated mitochondria with abnormal cristae, reduced efficiency of oxidative phosphorylation, increased mitochondrial membrane potential, elongated peroxisomes [ 105 ] Encephalopathy in infancy c.1217T>C (p.L406S) De novo heterozygous Missense mutation in middle domain Elongation of peroxisomes and mitochondria [ 100 ] Hypotonia and absent respiratory effort. Demyelination and reduction of the number of axons in the sural nerve …”

“…Increased lactate levels [ 95 ] Hypotonia, developmental delay, seizures (p.C431Y), optic atrophy (p.G32A) c.1292G>A (p.C431Y) or c.95G>C (p.G32A) De novo heterozygous Missense mutation in middle domain (p.C431Y) or GTPase domain (p.G32A) (p.G32A): Dominant negative effect with elongated, highly connected mitochondrial network. No alterations to peroxisomes [ 107 ] (p.G32A): Hyperfused elongated mitochondria with abnormal cristae, reduced efficiency of oxidative phosphorylation, increased mitochondrial membrane potential, elongated peroxisomes [ 105 ] Hypotonia, developmental delay, abnormal movement c.305C>T (p.T115M) homozygous Missense mutation in GTPase domain Elongated mitochondria, reduced mtDNA content, reduced mitochondrial respiration, impaired cell growth, reduced DRP1 oligomerization Normal peroxisome morphology and VLCFA levels [ 106 ] Static encephalopathy, developmental delay, seizures, nystagmus c.2072A>G, (p.Y691C) De novo heterozygous Missense mutation in GTPase effector domain Normal peroxisomal metabolism (Expression in Drosophila DRP1-deficient cells): Dominant negative effect with enlarged peroxisomes showing abnormal distribution, increased mitochondrial connectivity and abnormal distribution [ 108 ] Severe, early-onset epileptic encephalopathy, developmental delay, progressive cerebral atrophy c.668G>T (p.G223V) or c.1109T>G (p.F370C) De novo heterozygous Missense mutation in GTPase domain (p.G223V) or middle domain (p.F370C) (p.G223V): mixed population of hyperfused and swollen/rod-shaped mitochondria (p.F370C): elongated mitochondria Both: uneven mitochondrial distribution, normal cellular respiration, mixed population of spherical and elongated peroxisomes [ 103 ] ...…”