Plasmalogens regulate retinal connexin 43 expression and Müller glial cells gap junction intercellular communication and migration.

Karadayi R, Mazzocco J, Leclere L, Buteau B, Gregoire S, Belloir C, Koudsi M, Bessard P, Bizeau J, Dubus E, Fenech C, Briand L, Bretillon L, Bron AM, Fioramonti X, and Acar N. (2022) Plasmalogens regulate retinal connexin 43 expression and Müller glial cells gap junction intercellular communication and migration. Frontiers in Cell and Developmental Biology

Plasmalogens are a class of phospholipid that contain a vinyl-ether bond at the sn-1 position on the glycerol backbone causing a more compact structure. Because of this, plasmalogens have roles in cell membrane structure and function, vesicular fusion, and protecting against oxidative stress. Reductions in plasmalogens is associated with many diseases including Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis, while a plasmalogen deficiency caused by the inability to synthesize plasmalogens results in rhizomelic chondrodysplasia punctata (RCDP). RCDP is an ultra-rare genetic disorder that causes a severe form of dwarfism and cognitive deficits characterized by short stature, skeletal abnormalities, recurrent respiratory illnesses, cardiac defects, and a shortened lifespan. A plasmalogen deficiency in mice has been shown to be linked to reduced expression of brain connexin 43 (Cx43). Cx43 is the main connexin type in retinal glial cells and is important for other cellular mechanisms including calcium-based gap junction communication between cells and cell migration. Karakayi et al were interested in determining how a plasmalogen deficiency affects Cx43 expression in the retina and on the function of Müller cells, the main glial cell in the retina involved in water and ion homeostasis, angiogenesis, and inflammation.

To accomplish these studies, 13 week old DAPAT+/+ and DAPAT-/- (the gene that encodes dihydroxyacetone phosphate-acyltransferase, DHAPAT, the enzyme that initiates plasmalogen biosynthesis in the peroxisome) mice were used. To confirm the effects of plasmalogen level on Cx43 gene expression and protein level, the retinas of these mice were tested. Both the Cx43 gene and protein were found to be downregulated in the DAPAT-/- mice compared to the DAPAT+/+ mice by 83% and 89%, respectively. Using immunohistochemistry, DHAPAT expression was detected in several layers of the retina which corresponded to the ganglion cell layer, inner nuclear layer, and the photoreceptors inner segments which are also the location of Müller cell endfeet, soma, and distal processes, respectively. To confirm the plasmalogen activity in Müller cells, lipids were extracted from rat Müller cells and whole mouse retinas and compared. A significantly higher concentration of plasmalogens were found in the Müller cells, indicating that these cells store plasmalogens in the retina and that they may have roles in Müller cell function. As well, Müller cells that had been transfected with siRNA targeting DHAPAT also demonstrated a 56% decrease in plasmalogen levels.

Karadayi et al also wanted to study the relationship between plasmalogen deficiency in Müller cells, Cx43 downregulation, and Müller cell function using a primary Müller cell line. Decreased plasmalogens were associated with a 47% reduction in Cx43 protein expression and a 36% increase in ATP (adenosine triphosphate) mediated calcium response. To determine how the plasmalogen and Cx43 decreases affect the function of Müller cells, gap junction intercellular communication (GJIC) was investigated. Müller cells engage in GJIC with other Müller cells and astrocytes, another type of glial cells in the central nervous system. Calcium imaging was used to determine if a plasmalogen deficiency affects calcium-based communication between these two cell types. Plasmalogen depletion in Müller cells was accompanied by an increase in astrocyte activation time and a decrease in calcium velocity. As well, ATP stimulation of plasmalogen-deficient Müller cells also resulted in increased amplitude activation in astrocytes, suggesting that the over-activation seen in Müller cells is preserved throughout the calcium propagation to astrocytes. These findings indicate that plasmalogens have a role in regulating ATP-stimulated calcium response of retinal Müller cells and calcium signaling to astrocytes.

Karadayi et al were interested in determining the role of plasmalogens on the expression of Cx43, an important protein for gap junction communication, and how its function is affected. Using plasmalogen deficient mouse models, they demonstrated that this depletion causes a downregulation of Cx43 expression and protein production and that Müller cells are a predominant supplier and storage location for plasmalogens in the retina. Their in vitro model showed that plasmalogens have a role in the regulation of the ATP-stimulated calcium response and the calcium wave propagation to astrocytes. These results directly implicate the role of plasmalogens in the function of glial cells in the central nervous system and in the ability of these cells to communicate through calcium signaling. To increase our understanding, the authors suggest that further work is needed to narrow down the exact processes that link plasmalogen levels to Cx43 expression.