Dynamic Regulation of Mitochondrial [Ca2+] in Hippocampal Neurons

Abstract

While neuronal mitochondria have been studied extensively in their role in health and disease, the rules that govern calcium regulation in mitochondria remain somewhat vague. In the present study using cultured rat hippocampal neurons transfected with the mtRCaMP mitochondrial calcium sensor, we investigated the effects of cytosolic calcium surges on the dynamics of mitochondrial calcium ([Ca2+]m). Cytosolic calcium ([Ca2+]c) was measured using the high affinity sensor Fluo-2. We recorded two types of calcium events: local and global ones. Local events were limited to a small, 2–5 µm section of the dendrite, presumably caused by local synaptic activity, while global events were associated with network bursts and extended throughout the imaged dendrite. In both cases, cytosolic surges were followed by a delayed rise in [Ca2+]m. In global events, the rise lasted longer and was observed in all mitochondrial clusters. At the end of the descending part of the global event, [Ca2+]m was still high. Global events were accompanied by short and rather high [Ca2+]m surges which we called spikelets, and were present until the complete decay of the cytosolic event. In the case of local events, selective short-term responses were limited to the part of the mitochondrial cluster that was located directly in the center of [Ca2+]c activity, and faded quickly, while responses in the neighboring regions were rarely observed. Caffeine (which recruits ryanodine receptors to supply calcium to the mitochondria), and carbonyl cyanide m-chlorophenyl hydrazine (CCCP, a mitochondrial uncoupler) could affect [Ca2+]m in both global and local events. We constructed a computational model to simulate the fundamental role of mitochondria in restricting calcium signals within a narrow range under synapses, preventing diffusion into adjacent regions of the dendrite. Our results indicate that local cytoplasmic and mitochondrial calcium concentrations are highly correlated. This reflects a key role of signaling pathways that connect the postsynaptic membrane to local mitochondrial clusters.

Cultured Rat Hippocampal Neurons Exposed to the Mitochondrial Uncoupler Carbonyl Cyanide Chlorophenylhydrazone Undergo a Rapid, Presenilin-Dependent Change in Neuronal Properties

“…The ER and MT are linked to allow a shift of excess Ca 2+ from one compartment to the other. The regulation of calcium efflux into and out of mitochondria includes several molecular mechanisms that have been studied extensively in recent years, by us and others [ 34 , 35 , 36 ]. Among them is the Na/Ca exchanger that is controlled by NCLX to allow for an influx of calcium into the MT [ 37 ].…”

“…Alternatively, it modulates and enhances calcium leakage through ryanodine receptors [ 8 , 14 ]. Since the loading of the postsynaptic segment of the ER with calcium in each specific compartment depends on the activity of the entire synaptic machinery [ 34 , 38 ], it therefore can be assumed that PS1 is highly involved in the control of Ca 2+ homeostasis inside the local mitochondria. In particular, in the present article, we demonstrate that mPS1 suppresses Ca 2+ fluctuations in local postsynaptic mitochondria and reduces their potential under CCCP.…”

“…We have recently suggested that synaptic mitochondria are capable of limiting the diffusion of Ca 2+ inside the dendritic space to 2–5 μm. This mechanism may provide the specificity of the second messenger effects, which regulate the metabolic activity of MTs or even their compartments within a synaptic cluster [ 34 ].…”

Differential Role of Active Compounds in Mitophagy and Related Neurodegenerative Diseases

“…In the dendritic spine, disruption of the mitochondria can lead to a decrease in the ability to maintain the function of the spine. This violation may be caused both by a decrease in energy supply and by a deficiency in the local Ca 2+ signaling [ 33 ].…”

Perturbation of 3D nuclear architecture, epigenomic aging and dysregulation, and cannabinoid synaptopathy reconfigures conceptualization of cannabinoid pathophysiology: part 2—Metabolome, immunome, synaptome

“…Activities at the presynaptic terminal are generally heavily energy dependent. Terminal depolarization by an action potential causes calcium to flow into the terminal and the exocytosis of neurotransmitters from presynaptic vesicles ( 294 , 335 ). Restoration of ionic gradients, synaptic cargo transport, synapse assembly and maintenance, pumping out presynaptic calcium, and synaptic vesicle refiling and recycling all require ATP.…”