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Retinal Muller glia secrete apolipoproteins E and J which are efficiently assembled into lipoprotein particles
J Shanmugaratnam1, E Berg, L Kimerer
1Department of Biochemistry, Boston University School of Medicine, MA 02118, USA.
Abstract:
We have shown that apolipoprotein E (ApoE) is synthesized by Muller cells, the major glial cell of the rabbit retina, and secreted into the vitreous after which it is taken up by retinal ganglion cells and rapidly transported into the optic nerve [Amaratunga et al., J. Biol. Chem. 271 (1996) 5628-5632]. In this report we demonstrate that the ApoE secreted by Muller cells in vivo and in culture is efficiently assembled into lipoprotein particles. Apolipoprotein J (ApoJ) is also synthesized by these cells and assembled into lipoprotein particles. The lipoproteins are triglyceride-rich and contain cholesterol esters and free cholesterol. They are heterogeneous, with densities between 1.006 and 1.18 and diameters between 14 and 45 nm. We discuss the possible role of these lipoproteins in supplying the needs of neurons for lipids, especially long axonal projection neurons such as retinal ganglion cells, which are vulnerable to age-related neurodegenerative diseases including Alzheimer's disease.
Insights
Muller cells in the retina synthesize and secrete apolipoprotein E (ApoE) and apolipoprotein J (ApoJ) as lipid-rich lipoprotein particles. These particles may supply essential lipids to retinal neurons, potentially impacting neurodegenerative diseases.
Area of Science:
- Neuroscience
- Ophthalmology
- Cell Biology
Background:
- Apolipoprotein E (ApoE) is synthesized by Muller cells in the rabbit retina and utilized by retinal ganglion cells.
- Muller cells are the primary glial cells in the retina.
Purpose of the Study:
- To investigate the assembly and characteristics of apolipoproteins synthesized by Muller cells.
- To explore the potential role of these apolipoproteins in neuronal lipid supply.
Main Methods:
- In vivo and in vitro studies of Muller cell cultures.
- Biochemical analysis of secreted lipoprotein particles (density, diameter, lipid content).
Main Results:
- Muller cells secrete ApoE and ApoJ, assembling them into triglyceride-rich lipoprotein particles.
- These lipoproteins contain cholesterol esters and free cholesterol.
- Particles exhibit heterogeneous densities (1.006-1.18 g/mL) and diameters (14-45 nm).
Conclusions:
- Muller cells produce and secrete functional lipoprotein particles containing ApoE and ApoJ.
- These lipoproteins may serve as a crucial source of lipids for retinal neurons, particularly long-projection neurons.
- This finding has implications for understanding lipid metabolism in neurodegenerative diseases like Alzheimer's.