The endoplasmic reticulum Ca²⁺‐ATPase SERCA2b is upregulated in activated microglia and its inhibition causes opposite effects on migration and phagocytosis

Abstract

Activation of microglia is an early immune response to damage in the brain. Although a key role for Ca2+ as trigger of microglial activation has been considered, little is known about the molecular scenario for regulating Ca2+ homeostasis in these cells. Taking into account the importance of the endoplasmic reticulum as a cellular Ca2+ store, the sarco(endo)plasmic reticulum Ca2+‐ATPase (SERCA2b) is an interesting target to modulate intracellular Ca2+ dynamics. We found upregulation of SERCA2b in activated microglia of human brain with Alzheimer's disease and we further studied the participation of SERCA2b in microglial functions by using the BV2 murine microglial cell line and primary microglia isolated from mouse brain. To trigger microglia activation, we used the bacterial lipopolysaccharide (LPS), which is known to induce an increase of cytosolic Ca2+. Our results showed an upregulated expression of SERCA2b in LPS‐induced activated microglia likely associated to an attempt to restore the increased cytosolic Ca2+ concentration. We analyzed SERCA2b contribution in microglial migration by using the specific SERCA inhibitor thapsigargin in scratch assays. Microglial migration was strongly stimulated with thapsigargin, even more than with LPS‐induction, but delayed in time. However, phagocytic capacity of microglia was blocked in the presence of the SERCA inhibitor, indicating the importance of a tight control of cytosolic Ca2+ in these processes. All together, these results provide for the first time compelling evidence for SERCA2b as a major player regulating microglial functions, affecting migration and phagocytosis in an opposite manner.

Upregulation of the secretory pathway Ca2+/Mn2+‐ATPase isoform 1 in LPS‐stimulated microglia and its involvement in Mn2+‐induced Golgi fragmentation

“…Scale bar in (c): 20 μm. SPCA1, secretory pathway Ca2+/Mn2+-transport ATPase isoform 1. transporters of the ER (SERCA2b) (Morales-Ropero et al, 2021) or Golgi (SPCA1, this work). We failed to detect SPCA2 with specific antibodies (data not shown), which is in line with gene expression databases highlighting an almost complete absence of Atp2c2 transcripts in mouse microglia (Saunders et al, 2018), suggesting that SPCA1 represents the primary isoform in microglia.…”

“…Primary cultures of mouse brain microglia were prepared from newborn (1 or 2-days old) C57BL/6 mice as previously described (Neubrand et al, 2018) with modifications (Morales-Ropero et al, 2021). Animals were obtained from the Animal Facility Service of the Scientific Instrument Centre of the University of Granada (UGR), and experiments were performed with the approval from the UGR Ethical Committee.…”

“…The luminal Ca 2+ concentration in these organelles is critical to perform their intrinsic functions, but also serves as Ca 2+ stores that participate in Ca 2+ signaling processes across the cell. In this sense, we recently reported the upregulation of the ER Ca 2+ -ATPase (SERCA2b) after lipopolysaccharide (LPS)-stimulation of microglia in culture and in ameboid microglia in brains of Alzheimer's disease patients (Morales-Ropero et al, 2021). Inhibition of SERCA2b in microglia caused opposite effects on migration and phagocytosis, highlighting the crucial role of these Ca 2+ -transporters and the ER Ca 2+ store in mechanisms that modulate microglia in health and disease.…”

The Meeting of Micropeptides with Major Ca2+ Pumps in Inner Membranes—Consideration of a New Player, SERCA1b

“…SERCA2a is the most expressed isoform after SERCA1a in rodent, it is confined to slow skeletal muscle fibres and serves as the main SR calcium pump in the heart. SERCA2b is found in virtually every cell, but its ubiquitous expression is reconciled with occasional induction i.e., in glial cells when it fulfils special functions [ 14 ]. SERCA2b has the highest affinity to Ca 2+ among these pumps.…”

Microglia and Microglia-Like Cells: Similar but Different

“…For example, the entry of microglial cells into the developing quail retina is related to extracellular ATP/ADP or UTP/UDP (Martin-Estebane et al, 2017). In this sense, intracellular Ca 2+ waves, which are elicited by ATP and propagate from cell to cell, affect the migratory response of microglial cells (Sieger et al, 2012;Morales-Ropero et al, 2021), and might be involved in the colonization of the developing nervous tissue.…”