The Role of Rac1 in the Epidermis and in the Hair Follicle

Rac1 is a ubiquitously expressed member of the Rho family of small
GTPases, which acts as a molecular switch by shuttling in a highly
regulated manner between an active (GTP-bound) and an inactive
(GDP-bound) state. Different signalling pathways, which involve
integrins, growth factor receptors, cadherins as well as other Rho
GTPases, can induce Rac1 activation. Only in the GTP bound form,
Rac1 can associate with different effector molecules to initiate
cellular responses. Initially described as an important regulator
of the actin cytoskeleton, Rac1 was later found to be also involved
in the modulation of other processes such as cell adhesion,
proliferation, survival, differentiation and migration. In
epithelial cells, Rac1 was shown to regulate the formation and
maintenance of cadherin dependent cell cell contacts, which are
essential for the establishment of the polarized cell morphology.
Before this project was initiated, almost all knowledge about the
function of Rac1 was based on in vitro studies. As constitutive
deletion of the murine rac1 gene leads to early embryonic
lethality, mice allowing for a conditional inactivation of the rac1
gene were generated in this study to enable the analysis of the
function of Rac1 in selected tissues. To investigate the role of
Rac1 in the epidermis and hair follicles and to determine its
function in the establishment and maintenance of cell cell contacts
between epithelial cells in vivo, mice with a
keratinocyte-restricted ablation of the rac1 gene were generated
and analyzed. The results obtained in this study showed that the
absence of Rac1 in the murine epidermis leads to a progressive hair
loss but surprisingly has no effect on the maintenance of the
epidermis. The hair loss is caused by the inability of hair
follicle keratinocytes to maintain their differentiation state,
which leads to the phagocytic removal of the non permanent parts of
the hair follicles by infiltrating macrophages. In contrast,
differentiation and proliferation of epidermal keratinocytes as
well as the formation and maintenance of cell-cell and cell-matrix
contacts, and the deposition of the basement membrane in the
epidermis are not affected by the loss of Rac1. Biochemical
analysis of epidermal lysates demonstrated that the absence of
epidermal defects in vivo is not a result of compensatory
upregulation of closely related members of the Rho family of
GTPases, further indicating that the function of Rac1 in epithelial
cells in vivo is limited. Also, the analysis of the formation of
the embryoid bodies from Rac1-deficient embryonic stem cells showed
that the presence of Rac1 is not required for the establishment of
cell cell contacts during differentiation of the polarized
primitive ectoderm and for the formation of epithelial sheets,
supporting the conclusion that the function of Rac1 in the
regulation of cell cell adhesion between epithelial cells is
dispensable. However, the re epithelialization after wounding was
impaired in the mutant epidermis, demonstrating that Rac1 plays an
important role in pathological conditions. The delayed wound
closure in the absence of Rac1 is caused by impaired cell migration
and proliferation of neo epidermal keratinocytes. Another
interesting finding of this study was the observation that, in
contrast to the steady state in vivo situation, isolated Rac1
deficent primary keratinocytes display severe defects in cell
culture, which lead to their detachment from the matrix. While the
initial adhesion is only mildly affected by the lack of Rac1,
mutant keratinocytes are unable to spread, show an impaired
organization of the actin cytoskeleton and fail to form mature
focal adhesions. The differences between in vivo and in vitro
effects resulting from the inactivation of the rac1 gene indicate
that the function of Rac1 in epithelial cells depends on the
complexity of the cellular system and emphasize the importance of
performing in vivo studies to fully understand its role. Taken
together, the data presented in this study show that Rac1 plays an
important role in the maintenance of hair follicles and during
epidermal wound healing, but that it is not essential for the
homeostasis of the epidermis in physiological conditions and for
the formation and maintenance of cell cell contacts between
epithelial cells in vivo.