טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKaufman Gili
SubjectThe Molecular Basis of Dermatophyte Pathogenicity
DepartmentDepartment of Medicine
Supervisors Dr. Israela Berdicevsky
Professor Benjamin Horwitz


Abstract

Dermatophytes are pathogenic fungi that infect human skin, nails and hair and cause a disease called dermatophytosis. The dermatophyte colonizes and infects the outermost layer of the skin tissue (stratum corneum) and causes skin lesions. Skin infections due to dematophytosis are distributed worldwide and T. mentagrophytes is one of the most widespread species belonging to the group that cause a skin disease.

Dermatophytes are adapted to infect keratinized tissues. Infection of the skin tissue includes: adhesion, invasion and secretion of enzymes that degrade skin components.

Little is known about the initial contact of the dermatophyte and the subsequent events that take place before and after the active lesion develops. A novel ex vivo model for the study of adherence and invasion of dermatophytes to the stratum corneum was developed in our laboratory. The initial stages of growth on the host can be studied by scanning and transmission electron microscopy (SEM and TEM).

Adherence, germination (24 h) and proliferation (3 d) were observed.

Dermatophyte infection on the skin model was followed on the skin model by a genetically-encoded vital marker (GFP). The skin was inoculated with a transgenic isolate of T. mentagrophytes expressing the GFP and followed by Confocal Scanning Laser Microscopy (CSLM).  Invasion of the tissue showed hyphal branching and growth in multiple directions.

The proteolytic profile of the proteases secreted from T. mentagrophytes was assayed by substrate gel and proteolytic activity analysis. Two serine proteases of similar molecular weight were secreted during growth on the epidermal matrix components keratin and elastin. We have characterized the expression of an aminopeptidase gene that was called Tri m 4. This T. mentagrophytes gene is a homolog of Trichophyton rubrum Tri r 4. Tri m 4 is closely related to Tri r 4 (almost 94% identity at the protein level). Tri m 4 resembles other protease-encoding genes thought to be virulence factors, for example DPP V of Aspergillus fumigatus. The Tri m 4 protein was detected immunochemically both in fungal extracts and in the culture medium. Expression of the Tri m 4 gene was induced several fold when T. mentagrophytes was grown on keratin and elastin. In order to identify additional genes encoding putative virulence factors, differential cDNA screening was performed. By this method, a fungal thioredoxin and a cellulase were identified and both genes were found to be strongly induced by skin extracellular matrix proteins and a model for their function is proposed.

Dermatophytes, which are well adapted infectious agents, seem to use their mechanical and biochemical capabilities to invade the skin tissue effectively.