Dynamin-related protein 1 is required for normal mitochondrial bioenergetic and synaptic function in CA1 hippocampal neurons

Abstract

Disrupting particular mitochondrial fission and fusion proteins leads to the death of specific neuronal populations; however, the normal functions of mitochondrial fission in neurons are poorly understood, especially in vivo, which limits the understanding of mitochondrial changes in disease. Altered activity of the central mitochondrial fission protein dynamin-related protein 1 (Drp1) may contribute to the pathophysiology of several neurologic diseases. To study Drp1 in a neuronal population affected by Alzheimer's disease (AD), stroke, and seizure disorders, we postnatally deleted Drp1 from CA1 and other forebrain neurons in mice (CamKII-Cre, Drp1lox/lox (Drp1cKO)). Although most CA1 neurons survived for more than 1 year, their synaptic transmission was impaired, and Drp1cKO mice had impaired memory. In Drp1cKO cell bodies, we observed marked mitochondrial swelling but no change in the number of mitochondria in individual synaptic terminals. Using ATP FRET sensors, we found that cultured neurons lacking Drp1 (Drp1KO) could not maintain normal levels of mitochondrial-derived ATP when energy consumption was increased by neural activity. These deficits occurred specifically at the nerve terminal, but not the cell body, and were sufficient to impair synaptic vesicle cycling. Although Drp1KO increased the distance between axonal mitochondria, mitochondrial-derived ATP still decreased similarly in Drp1KO boutons with and without mitochondria. This indicates that mitochondrial-derived ATP is rapidly dispersed in Drp1KO axons, and that the deficits in axonal bioenergetics and function are not caused by regional energy gradients. Instead, loss of Drp1 compromises the intrinsic bioenergetic function of axonal mitochondria, thus revealing a mechanism by which disrupting mitochondrial dynamics can cause dysfunction of axons.

Mitochondrial dynamics in neuronal injury, development and plasticity

“…In support of an importance of Drp1 for synapse development, brain-specific Drp1-KO mice die shortly after birth due to impaired forebrain development and synapse formation that likely is a result of mitochondrial aggregation, their impaired transport and defective mitophagy in neurons (Ishihara et al, 2009;Wakabayashi et al, 2009). Two recent studies that use mice with postnatal Drp1 deletion in forebrain neurons support these findings; the mice exhibited impaired mitochondrial ATP production, hippocampal atrophy, defects in synaptic transmission, as well as deficits in learning and memory (Shields et al, 2015;Oettinghaus et al, 2016).…”

“…More recently, hippocampal neurons cultured from Drp1 KO mice were shown to have ATP deficits specifically at nerve terminals, resulting in impaired synaptic vesicle cycling. Whereas Drp1 KO neurons had fewer axonal mitochondria, the deficits in ATP were similar among the synaptic boutons with or without mitochondria, indicating that an intrinsic bioenergetic deficit, rather than mislocalization of mitochondria, accounts for impaired neurotransmission in the Drp1 KO mice (Shields et al, 2015). Although rapid diffusion of ATP and spatiotemporal energy buffers, such as the phosphocreatine shuttle (Andres et al, 2008;Linton et al, 2010), can obviate the need for local ATP production by mitochondria, Ca 2+ cycling and the production of ROS, and other short-lived metabolites might, nevertheless, depend to a greater extent on the precise localization of the organelle.…”

DNM1L-related mitochondrial fission defect presenting as refractory epilepsy

“…5,6 In neurons, deficiency of Drp1 prevents mitochondria from trafficking correctly to the axon and synaptic bouton, with ensuing abnormalities of synaptic vesicle trafficking. [7][8][9] Altered mitochondrial fission, fusion, or distribution are seen in several neurodegenerative conditions, including Charcot-Marie-Tooth disease type 2A, Parkinson's disease, and autosomal dominant optic atrophy, indicating a specific structural requirement for mitochondrial fission in neurons. 2 In 2007, Waterham et al 10 reported a de novo heterozygous DNM1L pathogenic variant in a female neonate with a lethal encephalopathy characterized by cerebral dysgenesis, seizures, lactic acidosis, elevated very long chain fatty acids, and abnormal mitochondrial and peroxisomal elongation.…”

Lessons Learned from Transgenic Mouse Models for the Therapeutic Use of Drp1 Inhibitors

“…Greater resilience to genetic ablation of Drp1 was found in hippocampal neurons of 2-month-old mice which did not display signs of neurodegeneration for up to 3 months [37]. In another model, deleting Drp1 in hippocampal neurons of newborn mice rendered neurons viable for up to one year [38]. However, hippocampal Drp1 ablation in these later two studies was not without effects: both studies reported that short-term memory and synaptic short-term potentiation was decreased, accompanied by hippocampal atrophy [37,38].…”