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Reelin expression in brain endothelial cells: an electron microscopy study.

Perez-Costas E, Fenton EY, Caruncho HJ - BMC Neurosci (2015)

Bottom Line: As expected, at the electron microscope reelin labeling was observed in neurons of the cortex, where most of the labeling was associated with the rough endoplasmic reticulum.Importantly, reelin was also observed in some endothelial cells located in small capillaries, which confirmed the findings obtained at the light microscope.In addition, some scarce labeling was observed in the nuclear membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychology, University of Alabama at Birmingham, College of Arts and Sciences, Campbell Hall 415, 1720 2nd Avenue South, Birmingham, Alabama, 35294, USA. epcostas@uab.edu.

ABSTRACT

Background: Reelin expression and function have been extensively studied in the brain, although its expression has been also reported in other tissues including blood. This raises the possibility that reelin might be able to cross the blood-brain barrier, which could be functionally relevant. Up-to-date no studies have been conducted to assess if reelin is present in the blood-brain barrier, which is mainly constituted by tightly packed endothelial cells. In this report we assessed the expression of reelin in brain capillaries using immunocytochemistry and electron microscopy.

Results: At the light microscope, reelin immunolabeling appeared in specific endothelial cells in brain areas that presented abundant diffuse labeling for this protein (e.g., layer I of the cortex, or the stratum lacunosum moleculare of the hippocampus), while it was mostly absent from capillaries in other brain areas (e.g., deeper cortical layers, or the CA1 layer of the hippocampus). As expected, at the electron microscope reelin labeling was observed in neurons of the cortex, where most of the labeling was associated with the rough endoplasmic reticulum. Importantly, reelin was also observed in some endothelial cells located in small capillaries, which confirmed the findings obtained at the light microscope. In these cells, reelin labeling was located primarily in caveolae (i.e., vesicles of transcytosis), and associated with the plasma membrane of the luminal side of endothelial cells. In addition, some scarce labeling was observed in the nuclear membrane.

Conclusions: The presence of reelin immunolabeling in brain endothelial cells, and particularly in caveolar vesicles within these cells, suggests that reelin and/or reelin peptides may be able to cross the blood-brain barrier, which could have important physiological, pathological, and therapeutic implications.

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Reelin labeling at the light microscope. A) Reelin immunolabeling in superficial and mid-layers of the cortex. As expected, reelin-labeled neurons are present across different layers of the cortex, and diffuse labeling is mostly present in the superficial layers. B) High magnification of the area indicated with a dashed-line box in A. This image shows a reelin-immunolabeled blood capillary in layer I of the cortex (empty black arrows). C) Reelin labeling in deeper layers of the cortex. Scattered reelin-labeled neurons in these deeper layers present moderate labeling compared to superficial layers. D) High magnification image of the area indicated with a box in C. The capillary is negative for reelin labeling (empty white arrows). E) Reelin labeling in the lateral part of the rostral hippocampus. Note the row of lightly reelin-labeled neurons in the CA1, as well as the diffuse reelin immunolabeling in the stratum lacunosum moleculare (slm). F) High magnification image of an area neighboring the slm, corresponding to the dashed-line area indicated in E. Note the intense reelin labeling surrounding this capillary (empty black arrow). In addition there is a strongly labeled neuron in close proximity (black arrowhead). G) Reelin labeling in the medial part of the rostral hippocampus. The corpus callosum (cc) appears dorsally bordering the hippocampus. H) High magnification image of the dashed-line box area in G. Note the unlabeled capillary (empty white arrow) located in close proximity to a strongly labeled neuron (black arrowhead). cc: corpus callosum; CA1: Cornu Ammonis layer I; slm: stratum lacunosum moleculare. Scale bars: 10 microns in A, C, G, E; 100 microns in B, D, F, H.
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Fig1: Reelin labeling at the light microscope. A) Reelin immunolabeling in superficial and mid-layers of the cortex. As expected, reelin-labeled neurons are present across different layers of the cortex, and diffuse labeling is mostly present in the superficial layers. B) High magnification of the area indicated with a dashed-line box in A. This image shows a reelin-immunolabeled blood capillary in layer I of the cortex (empty black arrows). C) Reelin labeling in deeper layers of the cortex. Scattered reelin-labeled neurons in these deeper layers present moderate labeling compared to superficial layers. D) High magnification image of the area indicated with a box in C. The capillary is negative for reelin labeling (empty white arrows). E) Reelin labeling in the lateral part of the rostral hippocampus. Note the row of lightly reelin-labeled neurons in the CA1, as well as the diffuse reelin immunolabeling in the stratum lacunosum moleculare (slm). F) High magnification image of an area neighboring the slm, corresponding to the dashed-line area indicated in E. Note the intense reelin labeling surrounding this capillary (empty black arrow). In addition there is a strongly labeled neuron in close proximity (black arrowhead). G) Reelin labeling in the medial part of the rostral hippocampus. The corpus callosum (cc) appears dorsally bordering the hippocampus. H) High magnification image of the dashed-line box area in G. Note the unlabeled capillary (empty white arrow) located in close proximity to a strongly labeled neuron (black arrowhead). cc: corpus callosum; CA1: Cornu Ammonis layer I; slm: stratum lacunosum moleculare. Scale bars: 10 microns in A, C, G, E; 100 microns in B, D, F, H.

Mentions: In this study we focused on reelin immunolabeling in the adult rat cortex and hippocampus. As shown in previous studies [6-9,13,34,35], at the light microscope the most intense reelin labeling was observed in layer I of the cortex and in the stratum lacunosum moleculare of the hippocampus, where neuronal and diffuse labeling were observed (Figure 1). Interestingly, at higher magnification reelin labeling also appeared in some (but not all) capillaries within areas containing strong diffuse immunolabeling, such as layer I of the cortex (Figures 1A-B) and the stratum lacunosum moleculare of the hippocampus (Figures 1E-F). On the other hand, reelin-labeled capillaries were not observed in areas lacking diffuse immunostaining, such as deeper layers of the cortex, and the CA1 area of the hippocampus (Figures 1C-D, G-H). The fact that reelin immunostaining was only observed in some (but not all) capillaries within areas presenting strong diffuse labeling argues in favor of its specificity. In addition, this also suggests that a possible secretion and/or transport of reelin by endothelial cells may take place primarily in areas of heavy diffuse reelin staining (i.e., brain areas where reelin tends to accumulate in the extracellular matrix) [see reference 32].Figure 1


Reelin expression in brain endothelial cells: an electron microscopy study.

Perez-Costas E, Fenton EY, Caruncho HJ - BMC Neurosci (2015)

Reelin labeling at the light microscope. A) Reelin immunolabeling in superficial and mid-layers of the cortex. As expected, reelin-labeled neurons are present across different layers of the cortex, and diffuse labeling is mostly present in the superficial layers. B) High magnification of the area indicated with a dashed-line box in A. This image shows a reelin-immunolabeled blood capillary in layer I of the cortex (empty black arrows). C) Reelin labeling in deeper layers of the cortex. Scattered reelin-labeled neurons in these deeper layers present moderate labeling compared to superficial layers. D) High magnification image of the area indicated with a box in C. The capillary is negative for reelin labeling (empty white arrows). E) Reelin labeling in the lateral part of the rostral hippocampus. Note the row of lightly reelin-labeled neurons in the CA1, as well as the diffuse reelin immunolabeling in the stratum lacunosum moleculare (slm). F) High magnification image of an area neighboring the slm, corresponding to the dashed-line area indicated in E. Note the intense reelin labeling surrounding this capillary (empty black arrow). In addition there is a strongly labeled neuron in close proximity (black arrowhead). G) Reelin labeling in the medial part of the rostral hippocampus. The corpus callosum (cc) appears dorsally bordering the hippocampus. H) High magnification image of the dashed-line box area in G. Note the unlabeled capillary (empty white arrow) located in close proximity to a strongly labeled neuron (black arrowhead). cc: corpus callosum; CA1: Cornu Ammonis layer I; slm: stratum lacunosum moleculare. Scale bars: 10 microns in A, C, G, E; 100 microns in B, D, F, H.
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Fig1: Reelin labeling at the light microscope. A) Reelin immunolabeling in superficial and mid-layers of the cortex. As expected, reelin-labeled neurons are present across different layers of the cortex, and diffuse labeling is mostly present in the superficial layers. B) High magnification of the area indicated with a dashed-line box in A. This image shows a reelin-immunolabeled blood capillary in layer I of the cortex (empty black arrows). C) Reelin labeling in deeper layers of the cortex. Scattered reelin-labeled neurons in these deeper layers present moderate labeling compared to superficial layers. D) High magnification image of the area indicated with a box in C. The capillary is negative for reelin labeling (empty white arrows). E) Reelin labeling in the lateral part of the rostral hippocampus. Note the row of lightly reelin-labeled neurons in the CA1, as well as the diffuse reelin immunolabeling in the stratum lacunosum moleculare (slm). F) High magnification image of an area neighboring the slm, corresponding to the dashed-line area indicated in E. Note the intense reelin labeling surrounding this capillary (empty black arrow). In addition there is a strongly labeled neuron in close proximity (black arrowhead). G) Reelin labeling in the medial part of the rostral hippocampus. The corpus callosum (cc) appears dorsally bordering the hippocampus. H) High magnification image of the dashed-line box area in G. Note the unlabeled capillary (empty white arrow) located in close proximity to a strongly labeled neuron (black arrowhead). cc: corpus callosum; CA1: Cornu Ammonis layer I; slm: stratum lacunosum moleculare. Scale bars: 10 microns in A, C, G, E; 100 microns in B, D, F, H.
Mentions: In this study we focused on reelin immunolabeling in the adult rat cortex and hippocampus. As shown in previous studies [6-9,13,34,35], at the light microscope the most intense reelin labeling was observed in layer I of the cortex and in the stratum lacunosum moleculare of the hippocampus, where neuronal and diffuse labeling were observed (Figure 1). Interestingly, at higher magnification reelin labeling also appeared in some (but not all) capillaries within areas containing strong diffuse immunolabeling, such as layer I of the cortex (Figures 1A-B) and the stratum lacunosum moleculare of the hippocampus (Figures 1E-F). On the other hand, reelin-labeled capillaries were not observed in areas lacking diffuse immunostaining, such as deeper layers of the cortex, and the CA1 area of the hippocampus (Figures 1C-D, G-H). The fact that reelin immunostaining was only observed in some (but not all) capillaries within areas presenting strong diffuse labeling argues in favor of its specificity. In addition, this also suggests that a possible secretion and/or transport of reelin by endothelial cells may take place primarily in areas of heavy diffuse reelin staining (i.e., brain areas where reelin tends to accumulate in the extracellular matrix) [see reference 32].Figure 1

Bottom Line: As expected, at the electron microscope reelin labeling was observed in neurons of the cortex, where most of the labeling was associated with the rough endoplasmic reticulum.Importantly, reelin was also observed in some endothelial cells located in small capillaries, which confirmed the findings obtained at the light microscope.In addition, some scarce labeling was observed in the nuclear membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychology, University of Alabama at Birmingham, College of Arts and Sciences, Campbell Hall 415, 1720 2nd Avenue South, Birmingham, Alabama, 35294, USA. epcostas@uab.edu.

ABSTRACT

Background: Reelin expression and function have been extensively studied in the brain, although its expression has been also reported in other tissues including blood. This raises the possibility that reelin might be able to cross the blood-brain barrier, which could be functionally relevant. Up-to-date no studies have been conducted to assess if reelin is present in the blood-brain barrier, which is mainly constituted by tightly packed endothelial cells. In this report we assessed the expression of reelin in brain capillaries using immunocytochemistry and electron microscopy.

Results: At the light microscope, reelin immunolabeling appeared in specific endothelial cells in brain areas that presented abundant diffuse labeling for this protein (e.g., layer I of the cortex, or the stratum lacunosum moleculare of the hippocampus), while it was mostly absent from capillaries in other brain areas (e.g., deeper cortical layers, or the CA1 layer of the hippocampus). As expected, at the electron microscope reelin labeling was observed in neurons of the cortex, where most of the labeling was associated with the rough endoplasmic reticulum. Importantly, reelin was also observed in some endothelial cells located in small capillaries, which confirmed the findings obtained at the light microscope. In these cells, reelin labeling was located primarily in caveolae (i.e., vesicles of transcytosis), and associated with the plasma membrane of the luminal side of endothelial cells. In addition, some scarce labeling was observed in the nuclear membrane.

Conclusions: The presence of reelin immunolabeling in brain endothelial cells, and particularly in caveolar vesicles within these cells, suggests that reelin and/or reelin peptides may be able to cross the blood-brain barrier, which could have important physiological, pathological, and therapeutic implications.

Show MeSH
Related in: MedlinePlus