Integrin-linked Kinase Modulation of Melanocytic Lineage Cells
Abstract
Melanocytes are specialized melanin-producing cells found in the skin, inner ear and heart. Melanocyte abnormalities cause many human disorders, including pigmentation defects, deafness, and melanoma. A better understanding of melanocyte biology is essential to address those disorders. Integrin-linked kinase (ILK) is a ubiquitous scaffold protein, essential for epidermal development. Importantly, the mechanisms by which ILK modulates the development and functions of melanocytic lineage cells remains unknown. To help address this void, I have developed cell-based models, as well as a reporter mouse model that allows tamoxifen-inducible Ilk gene inactivation specifically in melanocytic cells.
I observed that inactivation of Ilk in first-wave melanoblasts significantly reduces their ability to form long pseudopods, migrate and proliferate. As a result, they do not populate the developing epidermis and hair follicles, normally. In post-natal melanocytes, ILK is required for proliferation and formation of cell extensions. Rac1 activation in response to growth factors, as well as Rac1-associated morphological changes in response to collagen-1 are defective in the absence of ILK. Exogenous expression of constitutive active Rac1 restores, to some degree, the formation of cell extensions. Thus, Rac1 functions downstream from ILK to regulate melanocyte morphology.
Melanocytes synthesize melanin in vesicles termed melanosomes. The latter travel along microtubule tracks to the tip of dendrites, where they are then transferred to neighbouring keratinocytes. I have examined the role of ILK in melanosome trafficking and melanin transfer to keratinocytes. In the absence of ILK, melanosome trafficking is abnormal, but is partially restored following stabilization of microtubules. Thus, it is likely that the alterations in melanosome movement are due to microtubule destabilization in the absence of ILK. Furthermore, I have shown that dendrite formation and melanin transfer in response to keratinocyte-secreted factors is severely reduced in the absence of ILK. Inhibition of GSK-3 partially restored dendricity, suggesting that ILK may be required to modulate GSK-3 activity and promote dendrite formation. In summary, my data suggest key roles for ILK in establishment of the melanocyte lineage, and for postnatal melanocyte functions.