Volume 52 (6): 779-788, 2004 Copyright ©The Histochemical Society, Inc. The Vesicular Monoamine Transporter 2 (VMAT2) Is Expressed by Normal and Tumor Cutaneous Mast Cells and Langerhans Cells of the Skin but Is Absent from Langerhans Cell Histiocytosis
Department of Molecular Neuroscience, Institute of Anatomy and Cell Biology, Philipps University, Marburg, Germany (MA,MK-HS,CD,EW); Department of Pathology, University of Kiel, Kiel, Germany (MA,GK); Department of Neurology, University of Bonn, Bonn, Germany (CD); Department of Dermatology, University of Heidelberg, Heidelberg, Germany (WH); and Section on Molecular Neuroscience, Laboratory of Cellular and Molecular Regulation, National Institute of Mental Health, NIH, Bethesda, Maryland (LEE) Correspondence to: Eberhard Weihe, MD, Inst. of Anatomy and Cell Biology, Dept. of Molecular Neuroscience Robert-Koch Str. 8, 35037 Marburg. Germany. E-mail: weihe{at}staff.uni-marburg.de
Monoamine storage in secretory granules is mediated by the vesicular monoamine transporters 1 and 2 (VMAT1 and VMAT2). The aim of our study was to identify monoamine-handling normal and neoplastic inflammatory cells in the skin by their expression of VMAT1 and VMAT2. Normal skin from various parts of the body, as well as 21 cases of cutaneous mastocytosis and 10 cases of cutaneous Langerhans cell histiocytosis were analyzed by immunohistochemistry, radioactive in situ hybridization, and double-fluorescence confocal microscopy. VMAT2-positive cells in the subepidermal layer were identified as mast cells by their expression of tryptase. Neoplastic mast cells in all cases of cutaneous mastocytosis retained their VMAT2 positivity. The intraepidermal VMAT2-expressing cells were identified as Langerhans cells by their CD1a positivity. VMAT2 was absent from Langerhans cell histiocytosis. VMAT2 is an excellent marker for normal and neoplastic mast cells. The expression of VMAT2 demonstrates the capacity of mast cells for monoamine storage and handling. The presence of VMAT2 in epidermal Langerhans cells revealed a previously unrecognized monoamine-handling phenotype of these cells and indicates possible involvement of amine storage and release associated with antigen presentation. Absence of VMAT2 in neoplastic Langerhans cells indicates a loss of monoamine handling capacity of these cells during tumorigenesis. (J Histochem Cytochem 52:779788, 2004)
Key Words: Birbeck granules CD1a dendritic cell histamine mast cell mastocytosis serotonin tryptase vesicular monoamine transporter
MONOAMINES can act as neurotransmitters, hormones, or autocrine and paracrine factors. These small molecules play an important role in the central and peripheral nervous system as well as in the endocrine and immune system. Their function depends on the location of the cells that synthesize, store and release them. All biogenic monoamine-containing cells are characterized by their expression of proteins that enable them to store and secrete monoamines such as plasma membrane transporters for scavenging and recycling monoamines from the extracellular space, and intracellular transporters for monoamine storage.
The uptake of monoamines from the cytoplasm into secretory granules is mediated by vesicular monoamine transporters (VMATs) along an ATPase-generated proton gradient (Schulinger et al. 1995
Extensive studies employing immunohistochemistry (IHC) and in situ hybridization (ISH) have shown that these two transporters are differentially expressed in monoamine-handling cells of the central nervous system, the autonomic nervous system, and in neuroendocrine cells and related neuroendocrine tumors (Weihe et al. 1994
Tissue Collection and Preparation Various samples of human skin were collected from 39 patients during surgery. The patients consisted of 17 men and 22 women with an average age of 61 years and an age range of 4 to 83 years. The expression patterns of VMAT1 and VMAT2 were analyzed in non-tumorous normal human skin in various regions: head (n=3), nose (n=3), lip (n=2), other facial regions (n=5), neck (n=2), back (n=2), thoracic and abdominal skin (n=3), upper and lower extremities (n=9), scrotum and prepuce of penis (n=2), vulva (n=2), and the anal skin region (n=4). Furthermore, multiple skin specimens from various regions were obtained from five rhesus monkeys, as described previously (Rausch et al. 1994 Samples from cases of cutaneous mastocytosis, including patients with urticaria pigmentosa (n=10), diffuse cutaneous mastocytosis (n=8), and solitary mastocytoma (n=3) were examined for their VMAT expression patterns. The patients with cutaneous mastocytosis consisted of nine males and 12 females with an average age of 14 years and an age range of 2 to 69 years. All these patients exhibited the typical clinical signs of cutaneous mastocytosis with its accompanying skin infiltrates. None of these patients exhibited evidence of extracutaneous involvement or leukemia. Furthermore, tissues from 10 children with CD1a-positive Langerhans cell histiocytosis of the skin were examined. These patients consisted of three boys and seven girls, with an average age of 3 years and an age range of 9 months to 5 years. Adrenal gland tissues from two rhesus monkeys and from two patients who had undergone unilateral adrenalectomy, one suffering from pheochromocytoma (m/63) and one from Conn's syndrome (w/70), were used as control tissues.
For IHC analysis the human tissues were fixed in either BouinHollande fixative, 10% formalin, or 4% formaldehyde/PBS for 2448 hr and then embedded in paraffin. The monkey tissues were perfused with 4% formaldehyde/PBS before postfixation in BouinHollande for 2448 hr, as described previously (Rausch et al. 1994 After dehydration in a graded series of 2-propanol solution, the tissues were embedded in Paraplast Plus (Merck; Darmstadt, Germany). Adjacent sections (3 µm or 7 µm thick) were cut and deparaffinized. Antigen retrieval to increase the sensitivity of immunodetection was performed by heating the sections at 9295C for 15 min in 0.01 M citrate buffer (pH 6) according to the DAKO protocol (Hamburg, Germany). Nonspecific binding sites were blocked with 5% bovine serum albumin (BSA; Serva, Heidelberg, Germany) in PBS (pH 7.45), followed by an avidinbiotin blocking step (avidinbiotin blocking kit; Boehringer, Ingelheim, Germany). For ISH, human non-neoplastic skin tissues from six patients and three patients with cutaneous mastocytosis were immediately frozen on dry ice and stored at 80C. Cryosections (1416 µm thick) were placed on presilanized glass slides, fixed in 4% phosphate-buffered formaldehyde for 60 min, followed by three 10-min washes in 50 mM PBS (pH 7.4). The slides were then briefly rinsed in distilled water, incubated in 0.1 M triethanolamine (pH 8.0) for 1 min and for 10 min in the same solution containing 0.25% v/v acetic anhydride under rapid stirring. They were then quickly rinsed in 2 x SSC, dehydrated in 50% and 70% ethanol, and air-dried.
Immunohistochemistry
Tissue sections were incubated with the primary antibodies overnight at 18C (diluted as shown in Table 1) and further incubated for 2 hr at 37C. The sections were then washed in distilled water and in 50 mM PBS and incubated with species-specific biotinylated secondary antibodies (Dianova; Hamburg, Germany) for 45 min at 37C, washed several times, and incubated for 30 min with the ABC reagents (Vectastatin Elite ABC kit; Boehringer, Ingelheim, Germany). Immunoreactions were visualized with 3'3-diaminobenzidine (DAB; Sigma, Deisenhofen, Germany) enhanced by the addition of 0.08% ammonium nickel sulfate (Fluka; Buchs, Switzerland), resulting in dark blue staining. No binding was detected in the absence of the primary antibody. The specificity of the IHC staining was observed by preabsorbing the antisera with 25 µmol of the C-terminal human VMAT1 and VMAT2 peptide, respectively.
Radioactive ISH To each section, hybridization buffer (3 x SSC, 50 mM NaPO4, pH 7.4, 1 x Denhardt's solution, 0.25 mg/ml yeast tRNA, 10% dextran sulfate, 50% formamide, 10 mM dithiothreitol) was applied. The hybridization mix contained 50,000 dpm/ml of 35S-labeled RNA probes. The sections were coverslipped and incubated in moist chambers at 60C for 16 hr. The coverslips were then removed in 2 x SSC and the sections were subjected to the following posthybridization steps: RNase treatment (20 µg/ml RNase A and 1 U/ml RNase T1 in 10 mM Tris, pH 8.0, 0.5 M NaCl, 1 mM EDTA) for 60 min at 37C and successive washes in decreasing salt concentrations (2 x, 1 x, 0.5 x, and 0.2 x SSC) for 10 min each, followed by incubation in 0.2 x SSC at 60C for 60 min. For autoradiography, slides were dipped in NTB-2 nuclear emulsion (Eastman Kodak; Rochester, NY) and developed after 3 weeks of exposure time. Developed sections were stained with hematoxylin and eosin, analyzed, and photographed in darkfield and brightfield modus with an AX 70 microscope (Olympus; Hamburg, Germany).
Confocal Laser Scanning Microscopy
Ethics
Expression of VMAT2 in Cutaneous Mast Cells and Cutaneous Mastocytosis In all regions of the human and monkey skin, VMAT2 but not VMAT1 immunoreactivity was present in a considerable subpopulation of connective tissue immune cells and in varicose nerve fibers innervating the blood vessels (Figure 1) . The VMAT2-positive cells were usually found in the dermis, with some enrichment around the superficial vascular plexus. They measured 815 µm in diameter, were round, oval, or fusiform, and possessed the morphological features of mast cells. Their number varied in the skin of different parts of the body, being most abundant in the scrotum and the prepuce of penis (data not shown). This pattern was seen in tissues fixed either in BouinHollande or in 4% (para)formaldehyde with or without antigen retrieval.
Radioactive ISH revealed a corresponding expression pattern for VMAT2 mRNA in cells of the dermal layer. VMAT2-positive cells were identified as mast cells by poststaining with Giemsa and by double immunofluorescence, which revealed full co-positivity of VMAT2 and the mast cell marker tryptase in the granules of mast cells (Figure 2) . All other (tryptase-negative) immune cells of the connective tissue were also negative for VMAT2, including lymphocytes and macrophages.
Tumor mast cells in patients with urticaria pigmentosa, solitary mastocytoma and diffuse cutaneous mastocytosis were fully co-positive for VMAT2, tryptase, and HDC (Figure 3) . The specificity of the IHC staining was demonstrated by preabsorbing the VMAT2 antisera with 25 µmol of the recognizing C-terminal human VMAT2 peptide (Figure 3). In contrast, VMAT1 was absent from tumor mast cells in all cases of cutaneous mastocytosis (Figure 3).
Expression of VMAT2 in Cutaneous Langerhans Cells but Absence of VMATs in Langerhans Cell Histiocytosis A considerable subpopulation of epidermal cells situated in the epidermis above the basal layer and within the pilosebaceous epithelium of human skin showed gene and protein expression of VMAT2 but not of VMAT1, as demonstrated by IHC and radioactive ISH (Figures 4 and 5) . The VMAT2-immunoreactive cells exhibited dendritic features, with thin irregularly shaped cell processes (Figure 4). On the basis of their CD1a co-positivity, the VMAT2-positive cells were identified as epidermal Langerhans cells (Figure 6) . In the epidermal layer, VMAT2 immunoreactivity was seen only in CD1a-positive cells. All other cell types of the epidermal layer were negative for VMAT2 and for VMAT1. High-resolution confocal laser scanning microscopy demonstrated three subcellular staining patterns of VMAT2 and CD1a. The cell surface was exclusively VMAT2-/CD1a+, some organelles in close association with the cellular surface were VMAT2+/CD1a+, and the majority of cytoplasmic granules were VMAT2+/CD1a (Figure 6). In contrast to the normal skin, VMAT2 was absent from all tumor CD1a-positive cells in Langerhans cell histiocytosis (Figure 7) .
Monoamines are stored in and secreted from immune cells. Storage of monoamines in secretory granules is mediated by either VMAT1 or VMAT2. Although VMAT2 was originally cloned from a rat basophilic leukemia cell line (Erickson et al. 1992
Mast cells, which derive from CD34-positive bone marrow stem cells, produce and liberate a variety of biologically active agents, especially monoamines such as histamine. They appear to participate in a variety of inflammatory dermatoses and in some tumors (Atkins and Clark 1987
Recently, VMAT1 and VMAT2 were found to be excellent markers for the diagnosis of certain monoamine-handling endocrine tumors, such as tumors deriving from histamine-containing enterochromaffin-like cells and serotonin-containing enterochromaffin cells of the gastroenteropancreatic system (Eissele et al. 1999
Surprisingly, VMAT2 but not VMAT1 was also expressed in a subpopulation of cells in the epidermal layer. According to their CD1a phenotype, these cells were identified as epidermal Langerhans cells (Klareskog et al. 1977
The selective expression of VMAT2, which is responsible for the uptake and accumulation of biogenic amines to make them available for exocytotic release, adds a new important feature to the functional spectrum of this particular cell type. VMAT2 expression in epidermal Langerhans cells suggests that these cells constitutively accumulate, store, and release monoamines. The physiological action of monoamines such as catecholamines on skin immune function is well documented (Maestroni 2000
Analysis of the subcellular distribution of VMAT2 revealed a granular cytoplasmic pattern that was partially CD1a-positive. In contrast, the cell surface was exclusively CD1a-positive but VMAT2-negative. It has been shown that CD1a molecules traffic through the early recycling endosomal pathway (Salamero et al. 2001
Based on the expression of VMAT2 in cutaneous Langerhans cells, we anticipated that VMAT2 might also serve as a marker for the neoplastic counterparts of these cells in the skin. However, VMAT2 was absent from all cases of Langerhans cell histiocytosis examined. It has been demonstrated that Langerhans cell histiocytosis is a true neoplastic disease with clonal proliferation of theses cells (Willman et al. 1994
In summary, the identification of VMAT2 in mast cells and Langerhans cells of the skin provides evidence of a molecular mechanism for monoamine storage and handling in these cells and provides a basis for imaging, as well as promoting and interfering with monoamine loading of these cells based on the known transport properties of human VMAT2 (Erickson et al. 1996
We wish to thank Maike Pacena, Anja Paulus, Elke Rodenberg-Frank, Petra Sack, and Marion Zibuschka for their excellent technical assistance, and Heidemarie Schneider and Klaus Schoenheiz for photographic documentation. We are indebted to Katherine Dege for critically reading the manuscript. We thank Ivo Leuschner from the Institute for Paediatric Pathology, University of Kiel, for the generous gift of histiocytosis tissue specimens. The grant support of the Volkswagen-Stiftung (to EW, LEE) and of the DFG (to EW and MK-HS) is gratefully acknowledged.
Received for publication January 26, 2004; accepted February 4, 2004
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