Merkel Cell Development is Independent of L 1 Adhesion Molecule

D. Magnani, S. Kief, J.M. Brandner, U. Bartsch, M. Schachner, I. Moll

Department of Dermatology and Venerology, University Hospital Hamburg­Eppendorf, Hamburg, Germany

Summary

Merkel cells express epithelial and neuronal markers. In the skin of snout and footpad of L1 knock-out and wild-type mice, similar densities of Merkel cells were observed by immunohistochemistry. This result indicates that Merkel cells are independent ofL1 presence.

Introduction

L1 adhesion molecule (Ll) is a 200--220 kDa type I membrane glycoprotein of the immunoglobulin superfamily expressed in a variety of neural and epithelial cells. It supports homophilic as well as integrin-mediated cell binding. Besides cell adhesion, it is involved in cell migration, and cell growth especially of neu­ronal cells. It has been shown to deliver signals for the growth and survival of neu­ronal cells.

The neuroendocrine cells of mammalian skin, the Merkel cells (MC) are sup­posed to be epithelially derived, but they show both epithelial and neuronal mark­ers. They express cytokeratins (nos. 8, 18, 19, 20), desmosomal proteins and chromogranin A, neuron-specific enolase and various neuropeptides. Because of the importance of L1 for both epithelial and neuronal cells, we were interested in the existence ofMC in L1 knock-out mice in comparison to wild-type mice.

Material and Methods

The skin of the snout and footpad of L1 knock-out and wild-type mice were frozen immediately after preparation and 6-j.tm-thick cryostat sections were made. MC were localized by using antibodies against cytokeratin 18 and 20 (Fig. 1 ), as

K. I. Baumann et al. (eds.), The Merkel Cell© Springer-Verlag Berlin Heidelberg 2003

122 Magnani et al.

B

('

Fig. 1. Indirect immunostaining on an acetone-fixed cryosection of a murine foot pad using monoclonal antibodies against CK18 (A, B) and CK20 (C, D) comparing wild-type mice (A, C) and Ll knock-out mice (B, D). Arrows MC. Note the presence of MC in both wild-type and Ll knock-out skin

described by Moll et al. (1996), counted and estimated per mm2 of frozen section area. Primary antibodies were kindly provided by W.W. Franke and coworkers, German Cancer Research Center, Heidelberg. Tissues of Ll knock-out mice were kindly provided by M. Schachner and coworkers, ZMNH, Hamburg ( cf. Dahme et al. 1997).

Results and Discussion

We could identify MC in knock-out and wild-type mice with identical density in the foot pad (Fig. 2, left columns). The higher number of MC in the snout of wild-type mice compared to knock-out mice was not significant (Fig. 2, right col­umns).

The very similar numbers of MC in the skin of Ll knock-out and of wild-type mice strongly argue for a regular fetal development ofMC in the absence ofLI.

Ll provided by nerves may not be a prerequisite for their development. This is in accordance with former data which have shown the presence of MC in murine epidermis clearly before cutaneous nerves are present (Pasche et al. 1990).

The same is true for human epidermis; MC are detectable around fetal week 8 without any cutaneous nerves in their neighbourhood (Moll et al. 1996). However, it still remains to be clarified whether Ll is important for homeostasis and func­tion of MC and their synapses in later life. This shall be elucidated in future ex­periments.

Merkel Cell Development is Independent ofLl Adhesion Molecule 123

N

E 35 E

~ 30 "ii 25 u "ii 20 .:.: ...

15 Q)

::!: 10 -0 ... 5 Q)

..c 0 E :::1 pad snout z

Fig. 2. Number of Merkel cells/mm2 in snout and foot pad of Ll knock-out mice in comparison to wild-type mice. MC were localized by antibodies against CK20, counted and

estimated per mm2. We identified MC in both wild-type and knock-out mice. The numbers of MC in foot pads of wild-type and knock-out mice were similar. The higher number of

MC/mm2 in the snout of wild-type in comparison toLl knock-out mice was not significant

References

Dahme M, Bartsch U, Martini R, Anliker B, Schachner M, Mantei N (1997) Disruption of the mouse Ll gene leads to malformations of the nervous system. Nat Genet 17:346-349

Moll T, Paus R, Moll R (1996) Merkel cells in mouse skin: intermediate filament pattern, localization, and hair cycle-dependent density. J Invest Dermatol I 06:281- 286

Pasche F, Merot Y, Carraux P, Saurat J-H (1990) Relationship between Merkel cells and nerve endings during embryogenesis in the mouse epidermis. J Invest Dermatol 95:247- 251