DOI: 10.1369/jhc.3A6213.2004 Volume 52 (8): 1031-1036, 2004 Copyright ©The Histochemical Society, Inc. Differential Expression of Apolipoprotein D in Human Astroglial and Oligodendroglial Cells
Department of Morphology and Cell Biology, Faculty of Biology and Medicine, University of Oviedo, Asturias, Spain Correspondence to: Dr. Jorge Tolivia, Departamento Morfología y Biología Celular, Facultad de Biología y Medicina, Universidad de Oviedo, Julián Clavería s/n, Oviedo 33006, Spain. E-mail: jtolivia{at}correo.uniovi.es
Apolipoprotein D (Apo D) is a secreted lipocalin in the nervous system that may be related to processes of reinnervation and regeneration. Under normal conditions, Apo D is present in the central nervous system in oligodendrocytes, astrocytes, and some scattered neurons. To elucidate the regional and cellular distribution of Apo D in normal human brain, we performed double immunohistochemistry for glial fibrillary acidic protein (GFAP) and Apo D in samples of postmortem human cerebral and cerebellar cortices. Most of the GFAP-positive cells in the gray matter had features of protoplasmic astrocytes and were mainly Apo D-positive. Apo D staining was mostly confined to the cell soma and proximal processes, whereas GFAP extended to a rich and extensive array of processes. The fibrous astrocytes in the white matter were immunoreactive for GFAP but not for Apo D. In the white matter, Apo D was mainly detected in oligodendrocytes and extracellularly in the neuropil. The results of the present study support a specific behavior for each astrocyte type. These findings suggest that Apo D expression may be cell-specific, depending on the particular tissue physiology at the time of examination. (J Histochem Cytochem 52:10311036, 2004)
Key Words: apolipoprotein D GFAP astrocytes protoplasmic fibrous human
ASTROCYTES are among the most abundant cells in brain and are involved in multiple brain functions, participating in neuron development, synaptic activity, and homeostatic control of the extracellular environment. They belong to the family of glia and, as their name suggests, they are star-like cells with many cytoplasmic processes. Some of these processes surround and separate neurons and others have expansions that end over blood vessels, ependyma, and central nervous system (CNS) surfaces. Astrocytes have been classically classified into different types according to their morphology, and recently on the basis of their biochemical characteristics (Privat and Rataboul 1986
Apo D is a member of the lipocalin superfamily and is a component of high-density plasma lipoprotein (HDL). It is also present in breast cyst fluid, lachrymal secretions, apocrine axillary secretions, cerebrospinal fluid (CSF), and urine (Rassart et al. 2000
The synthesis and secretion of Apo D by primary astrocyte cultures of mouse brain have been described. Moreover, Apo D is upregulated by progesterone and oxysterol in these cultured astrocytes (Patel et al. 1995 Because of the important role of astrocytes in the repair and maintenance of neuronal homeostasis and structure, our aim is to study the location and distribution of Apo D in human brain glial cells by immunohistochemical (IHC) techniques to elucidate the significance of Apo D expression in the different astrocyte types of the human nervous system.
Human tissues were provided by the Pathologic Anatomy Department of The General Hospital of Asturias. Twenty subjects between 30 and 58 years old without neurological or psychological disorders were used in this study. Pieces of human brain from different areas (frontal, parietal, and occipital cortices, striatum, hippocampus, and cerebellar cortex) were obtained at autopsy, and fixed by immersion in 10% buffered formalin. After fixation, pieces were dehydrated, cleared in butyl acetate, and embedded in paraffin. Sections 10 µm thick were obtained and attached to gelatin-covered slides.
Sections were deparaffinized in xylene and rehydrated, and a double immunostaining for Apo D protein and GFAP was carried out according to the following protocol. The sections were treated sequentially with Triton X-100 (0.1%, 5 min) at room temperature (RT), washed in distilled water, treated with H2O2 (3%, 5 min) in a wet chamber at RT, washed in distilled water, and treated with PBS for 2 min. Nonspecific binding was blocked by incubation with bovine serum. Incubation with a specific antibody against Apo D (1:2000 dilution) was carried out overnight at 4C (provided by Dr. C. López-Otín, Departamento de Bioquímica y Biología Molecular, Universidad de Oviedo; see López-Boado et al. 1994
All specimens of normal human brain used in the present work exhibited a similar pattern of IHC staining for Apo D, and the labeling was more intense in white than in gray matter in all areas sampled. As expected, strong immunoreactivity for Apo D was also detected in fibroblasts and perivascular cells of the choroid plexus and in fibroblasts of the pial layer. Moreover, Apo D staining was also found in perivascular but not in endothelial cells of blood vessels. Inconstant and slight staining of scattered neurons was also observed. Scattered Apo D-positive glial cells were observed in gray matter opposite to the many Apo D-positive glial cells found in white matter. To detect astrocytes, double IHC for GFAP and Apo D was achieved. GFAP-positive astrocytes were observed in all layers of the cerebral and cerebellar cortices, but mainly in white matter. No IHC differences between sampled areas of cerebral cortex were observed. Positive staining for GFAP was found in the cytoplasm and processes of scattered astrocytes of the gray matter in cerebral cortex (Figure 1A). Some of these cells showed immunoreactivity for Apo D mainly concentrated in cytoplasmic granules (Figure 1B). These double-positive cells were mainly observed in molecular and pyramidal layers of the cerebral cortex. Apo D-positive astrocytes were most frequently observed in association with blood vessels, and their morphology resembled that of protoplasmic astrocytes that were highly comparable to those described with the Golgi technique (Figure 1B). On the other hand, the white matter appeared more stained for GFAP and Apo D than the gray matter. However, co-localization of both antibodies in astroglial cells there was not observed (Figures 1C and 1D). Strong positive immunostaining for Apo D was observed in other glial cells that were round in shape with an eccentric nucleus and few straight processes, resembling oligodendrocytes (Figure 1D). Significant immunoreactivity for Apo D was found in neuropil surrounding the white matter (Figures 1C and 1D).
The three layers of the cerebellum presented a similar pattern of double immunostaining (Figure 1D). Astrocytes immunoreactive for GFAP, similar in morphology to those observed in human cerebrocortical white matter, were sparsely found throughout the cerebellar white matter. These fibrous astrocytes have no reactivity to Apo D (Figure 1F). As in cerebral white matter, the Apo D immunostaining was found mainly in oligodendrocytes (Figure 1F). GFAP-positive astrocytes were regularly distributed throughout the granular layer of the cerebellar cortex. Only a few of these cells showed a slight co-localization of both proteins. Most of the astroglial cells were Apo D-immunonegative (Figure 1G). Sparsely distributed Apo D-immunoreactive Purkinje cells or granular cells were seen in all sampled human cerebellar cortices (Figure 1G). The other subpopulation of GFAP-positive astrocytes in the human cerebellum was identified as Bergman glia based on the position of their cell bodies, near the granular layer/Purkinje cell interface, and the extension of their feather-like processes across the width of the molecular layer to contact the pia mater. Like other astrocytes in the cerebellum, no immunoreactivity for Apo D was observed in Bergman glia (Figure 1H). Finally, the molecular layer, subjacent to the glia limitans of the cerebellum, displayed immunoreactivity for GFAP and Apo D (Figure 1H). Co-localization of both proteins was not found in this layer. GFAP staining was present only in Bergman processes, and only a few sparse neurons were slightly positive for Apo D (Figure 1H).
The present study showed that Apo D is present in glial cells of cerebellar and cerebral cortices of normal young human brains. These results are consistent with previous IHC studies revealing that Apo D was present in mammalian neuroglial cells, astrocytes, and oligodendrocytes (Boyles et al. 1990b
In human cerebral and cerebellar gray matter, we have found few GFAP-positive astrocytes, and most of them co-localized Apo D. Moreover, protoplasmic and fibrous astrocytes show clear differences in the expression of Apo D. Protoplasmic astrocytes of the gray matter show co-localization of GFAP and Apo D, especially when they are activated, whereas fibrous astrocytes do not exhibit Apo D staining. In cerebellar cortex, specialized protoplasmic astrocytes, i.e., the Bergman glia, are rarely immunopositive for Apo D. These glial cells behaved in the same way as fibrous astrocytes of the white matter. The different behavior of protoplasmic vs fibrous astrocytes, with regard to Apo D synthesis, suggests that astrocytes can be arranged in different compartments with different roles in brain homeostasis. Glial activation results in increments in the size and number of activated cells and in the expression of a new panel of genes (Topp et al. 1989 The functional role of Apo D in the CNS is poorly known. Apo D is a member of the lipocalin family and is considered a "multiligand-multifunctional" protein, and it has been suggested that Apo D could achieve diverse functions in different tissues and organs. Particularly in the CNS, the role and the molecular mechanisms that control Apo D expression would be different in the various types of neural cells. Our results show a heterogeneous expression of Apo D in astrocytes, which support a specific behavior for each astrocyte type. The Apo D in protoplasmic astrocytes probably acts as a lipid transporter protein in neurites promoting responses.
Supported by grants from Fondo de Investigación Sanitaria Española (PI020324, Red C03/06).
Received for publication December 5, 2003; accepted March 23, 2004
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