Expression of neurofilament proteins by horizontal cells in the rabbit retina varies with retinal location

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Abstract
Classical neurofibrillar staining methods and immunocytochemistry with antibodies to the light, medium and heavy chain subunits of the neurofilament triplet have been used forin situ andin vitro investigation of the organization of neurofilaments in A- and B-type horizontal cells of the adult rabbit retina. Surprisingly, their expression and organization within a cell is dependent on its location along the dorso-ventral axis of the retina. A-type horizontal cells in superior retina consistently stained with a wide variety of neurofibrillar methods to reveal neurofibrillar bundles, which immunocytochemistry showed to contain all three neurofilament subunits. A-type horizontal cells in inferior retina were uniformly refractory to neurofibrillar staining, although they expressed all three subunits. However, there was less of the light and medium subunits; the organization of the filaments into bundles (neurofibrils) is minimal. B-type horizontal cells could not be stained with any neurofibrillar method and were not recognizable byin situ immunocytochemistry. However, B-type cells could be seen to express all three subunitsin vitro, but the expression of the light and medium subunits was weak. There was only a slight difference between B-type cells taken from superior and inferior retina. Combined with the results of recent transfection studies, these findings suggest that the amount of the light neurofilament subunit present in a horizontal cell determines its content of neurofibrillar bundles, and that rabbit horizontal cells may contain more neurofilament protein, particularly of the heavy subunit, than is used for neurofilament formation. The results also show that determination of the neurofilament protein content of a population of nerve cells can be critically dependent on examination of single isolated cells. Isolated rabbit horizontal cells provide a promising system for studies of the mechanical, molecular and biochemical properties of neurofilaments.