Pathophysiological significance of calcium signaling at Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs)

Calcium acts as a critical determinant of mitochondria-nuclear networking driven retrograde signaling

Mitochondria are robust signaling organelle that regulate a variety of cellular functions. One of the key mechanisms that drive mitochondrial signaling is inter-organelle crosstalk. Mitochondria communicates with other organelles primarily via exchange of calcium (Ca2+), reactive oxygen species (ROS) and lipids across organelle membranes. Mitochondria has its own genome but a majority of mitochondrial proteins are encoded by nuclear genome. Therefore, several mitochondrial functions are controlled by nucleus via anterograde signaling. However, the role of mitochondria in driving expression of genes encoded by nuclear genome has recently gained attention. Recent studies from independent groups have demonstrated a critical role for mitochondrial Ca2+signaling in stimulating nuclear gene expression. These studies report that inhibition of mitochondrial Ca2+uptake through silencing of Mitochondrial Ca2+Uniporter (MCU) leads to Ca2+oscillations in the cytosol. The rise in cytosolic Ca2+ results in activation of Ca2+ sensitive transcription factors such as NFATs and NF-κB. These transcription factors consequently induce expression of their target genes in the nuclear genome. It is important to highlight that these groups used different cell types and elegantly presented a phenomenon that is conserved across various systems. Notably, mitochondrial Ca2+ signaling mediated transcriptional regulation controls diverse cellular functions ranging from B-cell activation, melanogenesis and aging associated inflammation. Future studies on this signaling module would result in better understanding of this axis in human pathophysiology and could lead to development of novel therapeutic strategies.

Mitochondrial calcium uptake orchestrates vertebrate pigmentation via transcriptional regulation of keratin filaments

Mitochondria regulate several physiological functions through mitochondrial Ca2+ dynamics. However, role of mitochondrial Ca2+ signaling in melanosome biology remains unknown. Here, we show that pigmentation requires mitochondrial Ca2+ uptake. In vitro gain and loss of function studies demonstrate that mitochondrial Ca2+ uniporter (MCU) is crucial for melanogenesis while MCU rheostat, MCUb negatively control melanogenesis. Zebrafish, MCU+/- and MCUb-/- mice models show that MCU complex drives pigmentation in vivo. Mechanistically, MCU silencing activates transcription factor NFAT2 to induce expression of keratin (5, 7, and 8) filaments. Interestingly, keratin5 in turn augments mitochondrial Ca2+ uptake and potentiates melanogenesis by regulating melanosome biogenesis and maturation. Hence this signaling module acts as a negative feedback loop that fine-tunes both mitochondrial Ca2+ signaling and pigmentation. Notably, mitoxantrone, an FDA approved drug that inhibits MCU, reduces pigmentation thereby highlighting therapeutic potential of targeting mitochondrial Ca2+ uptake for clinical management of pigmentary disorders. Taken together, we reveal an MCU-NFAT2-Keratin5 driven signaling axis that acts as a critical determinant of mitochondrial Ca2+ uptake and pigmentation. Given the vital role of mitochondrial Ca2+ signaling and keratin filaments in cellular physiology, this feedback loop could be operational in a variety of other patho-physiological processes.

Exploring the intricacies of calcium dysregulation in ischemic stroke: Insights into neuronal cell death and therapeutic strategies

Calcium ion (Ca2+) dysregulation is one of the main causes of neuronal cell death and brain damage after cerebral ischemia. During ischemic stroke, the ability of neurons to maintain Ca2+ homeostasis is compromised. Ca2+ regulates various functions of the nervous system, including neuronal activity and adenosine triphosphate (ATP) production. Disruptions in Ca2+ homeostasis can trigger a cascade of events, including activation of the unfolded protein response (UPR) pathway, which is associated with endoplasmic reticulum (ER) stress and mitochondrial dysfunction. This response occurs when the cell is unable to manage protein folding within the ER due to various stressors, such as a high influx of Ca2+. Consequently, the UPR is initiated to restore ER function and alleviate stress, but prolonged activation can lead to mitochondrial dysfunction and, ultimately, cell death. Hence, precise regulation of Ca2+ within the cell is mandatory. The ER and mitochondria are two such organelles that maintain intracellular Ca2+ homeostasis through various calcium-operating channels, including ryanodine receptors (RyRs), inositol trisphosphate receptors (IP3Rs), sarco/endoplasmic reticulum calcium ATPases (SERCAs), the mitochondrial Na+/Ca2+ exchanger (NCLX), the mitochondrial calcium uniporter (MCU) and voltage-dependent anion channels (VDACs). These channels utilize Ca2+ sequestering and release mechanisms to maintain intracellular Ca2+ homeostasis and ensure proper cellular function and survival. The present review critically evaluates the significance of Ca2+ and its physiological role in cerebral ischemia. We have compiled recent findings on calcium's role and emerging treatment strategies, particularly targeting mitochondria and the endoplasmic reticulum, to address Ca2+ overload in cerebral ischemia.

Mitochondria-associated endoplasmic reticulum membrane as a mediator of vanadium-induced endoplasmic reticulum quality control in duck brains

Vanadium (V) plays a crucial role in normal cells, but excess V causes multi-organ toxicity, including neurotoxicity. Mitochondria-associated endoplasmic reticulum membrane (MAM) is a dynamic structure between endoplasmic reticulum (ER) and mitochondria that mediates ER quality control (ERQC). To explore the effects of excess V on MAM and ERQC in the brain, 72 ducks were randomly divided into two groups: the control group (basal diet) and the V group (30 mg V/kg basal diet). On days 22 and 44, brain tissues were collected for histomorphological observation and determination of trace element contents. In addition, the mRNA and protein levels of MAM and ERQC-related factors in the brain were analyzed. Results show that excessive V causes the imbalance of trace elements, the integrity disruption of MAM, rupture of ER and autophagosomes formation. Moreover, it inhibits IP3R and VDAC1 co-localization, down-regulates the expression levels of MAM-related factors, but up-regulates the expression levels of ERQC and autophagy related factors. Together, results indicate that V exposure causes disruption of MAM and activates ERQC, which is further causing autophagy.

Mitochondrial calcium signaling mediated transcriptional regulation of keratin filaments is a critical determinant of melanogenesis

Mitochondria are versatile organelles that regulate several physiological functions. Many mitochondria-controlled processes are driven by mitochondrial Ca2+ signaling. However, role of mitochondrial Ca2+ signaling in melanosome biology remains unknown. Here, we show that pigmentation requires mitochondrial Ca2+ uptake. In vitro gain and loss of function studies demonstrated that Mitochondrial Ca2+ Uniporter (MCU) is crucial for melanogenesis while the MCU rheostats, MCUb and MICU1 negatively control melanogenesis. Zebrafish and mouse models showed that MCU plays a vital role in pigmentation in vivo. Mechanistically, MCU controls activation of transcription factor NFAT2 to induce expression of three keratins (keratin 5, 7 and 8), which we report as positive regulators of melanogenesis. Interestingly, keratin 5 in turn modulates mitochondrial Ca2+ uptake thereby this signaling module acts as a negative feedback loop that fine-tunes both mitochondrial Ca2+ signaling and melanogenesis. Mitoxantrone, an FDA approved drug that inhibits MCU, decreases physiological melanogenesis. Collectively, our data demonstrates a critical role for mitochondrial Ca2+ signaling in vertebrate pigmentation and reveal the therapeutic potential of targeting MCU for clinical management of pigmentary disorders. Given the centrality of mitochondrial Ca2+ signaling and keratin filaments in cellular physiology, this feedback loop may be functional in a variety of other pathophysiological conditions.

Inhibition of ferroptosis through regulating neuronal calcium homeostasis: An emerging therapeutic target for Alzheimer's disease

Alzheimer's disease (AD), a chronic and progressive neurodegenerative disease, generates a serious threat to the health of the elderly. The AD brain is microscopically characterized by amyloid plaques and neurofibrillary tangles. There are still no effective therapeutic drugs to restrain the progression of AD though much attention has been paid to exploit AD treatments. Ferroptosis, a type of programmed cell death, has been reported to promote the pathological occurrence and development of AD, and inhibition of neuronal ferroptosis can effectively improve the cognitive impairment of AD. Studies have shown that calcium (Ca²⁺) dyshomeostasis is closely related to the pathology of AD, and can drive the occurrence of ferroptosis through several pathways, such as interacting with iron, and regulating the crosstalk between endoplasmic reticulum (ER) and mitochondria. This paper mainly reviews the roles of ferroptosis and Ca²⁺ in the pathology of AD, and highlights that restraining ferroptosis through maintaining the homeostasis of Ca²⁺ may be an innovative target for the treatment of AD.

Potential of targeting host cell calcium dynamics to curtail SARS-CoV-2 infection and COVID-19 pathogenesis

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection and associated coronavirus disease 2019 (COVID-19) has severely impacted human well-being. Although vaccination programs have helped in reducing the severity of the disease, drug regimens for clinical management of COVID-19 are not well recognized yet. It is therefore important to identify and characterize the molecular pathways that could be therapeutically targeted to halt SARS-CoV-2 infection and COVID-19 pathogenesis. SARS-CoV-2 hijacks host cell molecular machinery for its entry, replication and egress. Interestingly, SARS-CoV-2 interacts with host cell Calcium (Ca²⁺) handling proteins and perturbs Ca²⁺ homeostasis. We here systematically review the literature that demonstrates a critical role of host cell Ca²⁺ dynamics in regulating SARS-CoV-2 infection and COVID-19 pathogenesis. Further, we discuss recent studies, which have reported that SARS-CoV-2 acts on several organelle-specific Ca²⁺ transport mechanisms. Moreover, we deliberate upon the possibility of curtailing SARS-CoV-2 infection by targeting host cell Ca²⁺ handling machinery. Importantly, we delve into the clinical trials that are examining the efficacy of FDA-approved small molecules acting on Ca²⁺ handling machinery for the management of COVID-19. Although an important role of host cell Ca²⁺ signaling in driving SARS-CoV-2 infection has emerged, the underlying molecular mechanisms remain poorly understood. In future, it would be important to investigate in detail the signaling cascades that connect perturbed Ca²⁺ dynamics to SARS-CoV-2 infection.

Natural Polyphenols as SERCA Activators: Role in the Endoplasmic Reticulum Stress-Related Diseases

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a key protein responsible for transporting Ca²⁺ ions from the cytosol into the lumen of the sarco/endoplasmic reticulum (SR/ER), thus maintaining Ca²⁺ homeostasis within cells. Accumulating evidence suggests that impaired SERCA function is associated with disruption of intracellular Ca²⁺ homeostasis and induction of ER stress, leading to different chronic pathological conditions. Therefore, appropriate strategies to control Ca²⁺ homeostasis via modulation of either SERCA pump activity/expression or relevant signaling pathways may represent a useful approach to combat pathological states associated with ER stress. Natural dietary polyphenolic compounds, such as resveratrol, gingerol, ellagic acid, luteolin, or green tea polyphenols, with a number of health-promoting properties, have been described either to increase SERCA activity/expression directly or to affect Ca²⁺ signaling pathways. In this review, potential Ca²⁺-mediated effects of the most studied polyphenols on SERCA pumps or related Ca²⁺ signaling pathways are summarized, and relevant mechanisms of their action on Ca²⁺ regulation with respect to various ER stress-related states are depicted. All data were collected using scientific search tools (i.e., Science Direct, PubMed, Scopus, and Google Scholar).

Mitofusin-2 Negatively Regulates Melanogenesis by Modulating Mitochondrial ROS Generation

Inter-organellar communication is emerging as one of the most crucial regulators of cellular physiology. One of the key regulators of inter-organellar communication is Mitofusin-2 (MFN2). MFN2 is also involved in mediating mitochondrial fusion–fission dynamics. Further, it facilitates mitochondrial crosstalk with the endoplasmic reticulum, lysosomes and melanosomes, which are lysosome-related organelles specialized in melanin synthesis within melanocytes. However, the role of MFN2 in regulating melanocyte-specific cellular function, i.e., melanogenesis, remains poorly understood. Here, using a B16 mouse melanoma cell line and primary human melanocytes, we report that MFN2 negatively regulates melanogenesis. Both the transient and stable knockdown of MFN2 leads to enhanced melanogenesis, which is associated with an increase in the number of mature (stage III and IV) melanosomes and the augmented expression of key melanogenic enzymes. Further, the ectopic expression of MFN2 in MFN2-silenced cells leads to the complete rescue of the phenotype at the cellular and molecular levels. Mechanistically, MFN2-silencing elevates mitochondrial reactive-oxygen-species (ROS) levels which in turn increases melanogenesis. ROS quenching with the antioxidant N-acetyl cysteine (NAC) reverses the MFN2-knockdown-mediated increase in melanogenesis. Moreover, MFN2 expression is significantly lower in the darkly pigmented primary human melanocytes in comparison to lightly pigmented melanocytes, highlighting a potential contribution of lower MFN2 levels to higher physiological pigmentation. Taken together, our work establishes MFN2 as a novel negative regulator of melanogenesis.

Citrin mediated metabolic rewiring in response to altered basal subcellular Ca2+ homeostasis

In contrast to long-term metabolic reprogramming, metabolic rewiring represents an instant and reversible cellular adaptation to physiological or pathological stress. Ca2+ signals of distinct spatio-temporal patterns control a plethora of signaling processes and can determine basal cellular metabolic setting, however, Ca2+ signals that define metabolic rewiring have not been conclusively identified and characterized. Here, we reveal the existence of a basal Ca2+ flux originating from extracellular space and delivered to mitochondria by Ca2+ leakage from inositol triphosphate receptors in mitochondria-associated membranes. This Ca2+ flux primes mitochondrial metabolism by maintaining glycolysis and keeping mitochondria energized for ATP production. We identified citrin, a well-defined Ca2+-binding component of malate-aspartate shuttle in the mitochondrial intermembrane space, as predominant target of this basal Ca2+ regulation. Our data emphasize that any manipulation of this ubiquitous Ca2+ system has the potency to initiate metabolic rewiring as an instant and reversible cellular adaptation to physiological or pathological stress.

Dysregulation of host cell calcium signaling during viral infections: Emerging paradigm with high clinical relevance

Viral infections are one of the leading causes of human illness. Viruses take over host cell signaling cascades for their replication and infection. Calcium (Ca2+) is a versatile and ubiquitous second messenger that modulates plethora of cellular functions. In last two decades, a critical role of host cell Ca2+ signaling in modulating viral infections has emerged. Furthermore, recent literature clearly implicates a vital role for the organellar Ca2+ dynamics (influx and efflux across organelles) in regulating virus entry, replication and severity of the infection. Therefore, it is not surprising that a number of viral infections including current SARS-CoV-2 driven COVID-19 pandemic are associated with dysregulated Ca2+ homeostasis. The focus of this review is to first discuss the role of host cell Ca2+ signaling in viral entry, replication and egress. We further deliberate on emerging literature demonstrating hijacking of the host cell Ca2+ dynamics by viruses. In particular, a variety of viruses including SARS-CoV-2 modulate lysosomal and cytosolic Ca2+ signaling for host cell entry and replication. Moreover, we delve into the recent studies, which have demonstrated the potential of several FDA-approved drugs targeting Ca2+ handling machinery in inhibiting viral infections. Importantly, we discuss the prospective of targeting intracellular Ca2+ signaling for better management and treatment of viral pathogenesis including COVID-19. Finally, we highlight the key outstanding questions in the field that demand critical and timely attention.

Molecular machinery regulating mitochondrial calcium levels: The nuts and bolts of mitochondrial calcium dynamics

Mitochondria play vital role in regulating the cellular energetics and metabolism. Further, it is a signaling hub for cell survival and apoptotic pathways. One of the key determinants that calibrate both cellular energetics and survival functions is mitochondrial calcium (Ca2+) dynamics. Mitochondrial Ca2+ regulates three Ca2+-sensitive dehydrogenase enzymes involved in tricarboxylic acid cycle (TCA) cycle thereby directly controlling ATP synthesis. On the other hand, excessive Ca2+ concentration within the mitochondrial matrix elevates mitochondrial reactive oxygen species (mROS) levels and causes mitochondrial membrane depolarization. This leads to opening of the mitochondrial permeability transition pore (mPTP) and release of cytochrome c into cytosol eventually triggering apoptosis. Therefore, it is critical for cell to maintain mitochondrial Ca2+ concentration. Since cells can neither synthesize nor metabolize Ca2+, it is the dynamic interplay of Ca2+ handling proteins involved in mitochondrial Ca2+ influx and efflux that take the center stage. In this review we would discuss the key molecular machinery regulating mitochondrial Ca2+ concentration. We would focus on the channel complex involved in bringing Ca2+ into mitochondrial matrix i.e. Mitochondrial Ca2+ Uniporter (MCU) and its key regulators Mitochondrial Ca2+ Uptake proteins (MICU1, 2 and 3), MCU regulatory subunit b (MCUb), Essential MCU Regulator (EMRE) and Mitochondrial Ca2+ Uniporter Regulator 1 (MCUR1). Further, we would deliberate on major mitochondrial Ca2+ efflux proteins i.e. Mitochondrial Na+/Ca2+/Li+ exchanger (NCLX) and Leucine zipper EF hand-containing transmembrane1 (Letm1). Moreover, we would highlight the physiological functions of these proteins and discuss their relevance in human pathophysiology. Finally, we would highlight key outstanding questions in the field.