Mitochondrial Calcium Uniporter (MCU) deficiency reveals an alternate path for Ca2+ uptake in photoreceptor mitochondria

Abstract

Rods and cones use intracellular Ca2+ to regulate many functions, including phototransduction and neurotransmission. The Mitochondrial Calcium Uniporter (MCU) complex is thought to be the primary pathway for Ca2+ entry into mitochondria in eukaryotes. We investigate the hypothesis that mitochondrial Ca2+ uptake via MCU influences phototransduction and energy metabolism in photoreceptors using a mcu-/- zebrafish and a rod photoreceptor-specific Mcu-/- mouse. Using genetically encoded Ca2+ sensors to directly examine Ca2+ uptake in zebrafish cone mitochondria, we found that loss of MCU reduces but does not eliminate mitochondrial Ca2+ uptake. Loss of MCU does not lead to photoreceptor degeneration, mildly affects mitochondrial metabolism, and does not alter physiological responses to light, even in the absence of the Na+/Ca2+, K+ exchanger. Our results reveal that MCU is dispensable for vertebrate photoreceptor function, consistent with its low expression and the presence of an alternative pathway for Ca2+ uptake into photoreceptor mitochondria.

Metabolism in the Zebrafish Retina

“…Additionally, mitochondria in the cell body take up Ca 2+ from the cytosol via the mitochondrial Ca 2+ uniporter (MCU), driving ATP production through OXPHOS by stimulating dehydrogenases in the TCA cycle [ 97 ]. Knockouts of MCU in zebrafish and mice showed mild alterations in metabolism, while phototransduction and neurotransmission remained unperturbed [ 98 ]. Interestingly, the absence of MCU did not completely halt Ca 2+ uptake into the mitochondria in these KOs, suggestive of an alternative route for its uptake [ 98 ].…”

“…Knockouts of MCU in zebrafish and mice showed mild alterations in metabolism, while phototransduction and neurotransmission remained unperturbed [ 98 ]. Interestingly, the absence of MCU did not completely halt Ca 2+ uptake into the mitochondria in these KOs, suggestive of an alternative route for its uptake [ 98 ]. This is a possible adaptation to maximise Ca 2+ homeostasis and mitochondrial metabolism, and thereby sustain key photoreceptor functions.…”

Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans

“…This enables faster regulation of Ca 2+ , however, the affinity of Na + /Ca 2+ exchangers to Ca 2+ is rather low and they are not able to drive Ca 2+ to extremely low levels. In some cases, plasma membrane ATPases (PMCAs) (Weeraratne et al, 2006 ; Castillo et al, 2007 ) as well as intracellular stores (ER and mitochondria) (Molnar et al, 2012 ; Bisbach et al, 2020 ; Hutto et al, 2020 ; Liu et al, 2020 ) can also contribute to cytosolic Ca 2+ homeostasis in sensory cilia or microvilli, but the contribution of these mechanisms to the overall Ca 2+ homeostasis and transduction is not well-understood.…”

“…Their role in vertebrate rod and cone photoreceptors was recently reviewed in Vinberg et al ( 2018 ). Rod photoreceptor outer segments rely almost exclusively on NCKX1 for Ca 2+ extrusion (Reilander et al, 1992 ; Vinberg et al, 2018 ) although some other mechanism(s) may also have a small contribution to Ca 2+ homeostasis in the mouse rod outer segment (Vinberg et al, 2015 ; Bisbach et al, 2020 ). Cones were originally thought to rely exclusively on NCKX2, however, a recent study showed that vertebrate cones (including primate cones) also express NCKX4 in their outer segments and that NCKX4 is important for the fast termination and light adaptation of mouse cones (Vinberg et al, 2017 ).…”

Dynamic Control of Mitochondrial Ca2+ Levels as a Survival Strategy of Cancer Cells

“…The mitochondrial uniporter complex (MCUC) further consists of the dominant-negative pore-forming subunit MCUb (Raffaello et al, 2013 ) and the scaffold factor MCU regulator 1 (MCUR1) (Tomar et al, 2016 ). However, mitochondria can sequester Ca 2+ even in the absence of MCU, thus, indicating alternative pathways that directly or indirectly affect mitochondrial Ca 2+ uptake (Bisbach et al, 2020 ). The Ca 2+ /H + exchanger leucine-zipper and EF-hand containing transmembrane protein 1 (LETM1) modulates mitochondrial Ca 2+ homeostasis (Jiang et al, 2009 ), metabolic signaling (Doonan et al, 2014 ) and cristae organization (Nakamura et al, 2020 ), while it does not mediate mitochondrial Ca 2+ extrusion (De Marchi U. et al, 2014 ).…”