KERATINOGENEZA

Este functia specifica prin care epidermul produce zilnic 0,6 – 1 gram keratina. Biosinteza keratinei respecta schema sintezei proteinelor si se desfasoara incepand cu celulele stratului bazal (keratinoblaste), continuandu-se in celelalte straturi ale epidermului. O ipotetica pierdere a acestei functii nu ar fi compatibila cu viata.

In celulele bazale se formeaza prekeratina. Se sintetizeaza initial lanturi polipeptidice care formeaza protofilamente apoi, prin agregare longitudinala formeaza filamente si ulterior tonofilamente (denumite si filamente intermediare de keratina). La nivelul stratului bazal, tonofilamentele sunt subtiri (50 Å), marimea lor marindu-se odata cu avansarea keratinocitelor spre suprafata epidermului. In stratul malpighian, grosimea tonofilamentelor este de 100 Å, iar in stratul cornos de 400 Å. Concomitent are loc transformarea cisteinei din prokeratina in cistina, care prezinta legaturi disulfidice (S-S) stabile, ireversibile.

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In stratul granulor este sintetizata keratohialina, care este liantul fibrelor de keratina. Studii recenta arata ca in structura keratohialinei intra:

- Profilagrina – este o proteina insolubila, neutra, bogata in serina, glicina, glutamina, histidina si arginina. In zona de tranzitie granulos/cornos, sufera proccesul de defosforilare si proteoliza, formanduse filagrina. Filagrina, ca proteina matrice, contribuie la formarea macrofibrelor de keratina.

- Loricrina – este un polipeptid bogat in glicina, serina, cisteina, cu masa moleculara 26 kDa.

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In malpighinianul superior apar keratosomii (corpii Odland), considerati ca fiind cea de-a treia componenta implicata in procesul de keratogeneza.

In urma procesului de keratinizare epidermica, keratinocitele ajunse in stratul cornos au un invelis celular format prin legaturi ireversibile intre numeroase tipuri de proteine (locrina, involucrina, keratolinina, polipeptidul de 195 kDa, cornifina), iar in interior macrofibrile de keratina care ocupa inreaga celula, nucleul si organitele lipsind.

Filamente de keratina in celule epiteliale

Fotografie la microscop cu imunofluorescenta a unor filamente de keratina (verde) pe o lama cu celule epiteliale. Filamentele fiecarei celule sunt conectate indirect cu cele adiacente prin desmozomi. O a doua proteina (albastru) a fost marcata pentru a evidentia membranele celulare.

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Exista mai multe clasificari ale keratinelor:

- Dupa structura spatiala: α-keratina, care este keratina stabila β-keratina, considerata keratina reversibila.

- Dupa localizare: Keratina moale, continuta in epiderm. Are cantitate redusa de sulf si de cistina Keratina tare, prezenta in unghii si in corticala firului de par

- Dupa Moll si colaboratorii, exista: Keratine tip I (acide) ce cuprind K10 – K20, caracterizate prin masa moleculara de 40-

56,5 kDa, codate de o familie de gene de pe cromozomul 17; Keratine de tip II (neutre/bazice) ce cuprind K1 – K9. Acestea au masa moleculara 52

– 67 kDasi sunt codate de o familie de gene de pe cromozomul 12.

Keratinele epiteliale formeaza heterodimeri prin asocierea unei keratine de tip I cu keratina de tip II, cum ar fi K10/K1, K10/K2, K13/K4, K14/K5, K16/K6. In stratul bazal se sintetizeaza predominant K1/K5 care se asambleaza in filamente intermediare de keratina. Aceste keratine scad in stratul spinos si creste sinteza de K1 si K10 care vor forma la randul lor filamente intermediare de keratina. Cele 20 de keratine despre care am vorbit pana acum sunt keratine epiteliale.

In structura parului si unghiilor intra o combinatie de keratine epiteliale si keratine specifice. Keratinele pilare, in numar de 10, denumite generic keratine H, apartin celor doua tipuri (acid – Ha si bazic – Hb). Ele sunt codate de gene localizate pe cromozomul 17q 12-21 si 12q 13.

Factorii reglatori ai keratinogenezei

- Factorii intrinseci: Chalona epidermica este sintetizata de celulele malpighianului superior si are actiune

pe keratinocitele ce poseda receptori specifici. Ea suprima mitozele, actiune ce este potentata de hormonii de stres (adrenalina, catecolamine) si de glucocorticoizi. Chalona epidermica este in concentratie mai mica la bolnavii cu psoriazis si in zonele cu presiune mecanica pe epiderm. S-a derscris un ritm circadian al chalonei epidermice, cu acrofaza matinala.

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Factorul de crestere al epidermului (Epidermal Growth Factor) stimuleaza epidermopoieza, diferentierea celulara si keratinogeneza;

TGF α (Transforming Growth Factor α) si KGF (Keratinocyte Growth Factor) stimuleaza epidermopoieza.

TGF β1 si TGF β2 suprima proliferarea epidermica iar in anumite conditii pot induce diferentierea epidermica

Interferonul γ suprima proliferarea epidermica Prostaglandinele F si poliaminele stimuleaza epidermopoieza si keratinogeneza

- Factori extrinseci: Factorul hormonal: hormonii androgeni stimuleaza epidermopoieza, iar hormonii

tiroidieni o deprima. Factorul nervos: la hemiparetici si polinevritici epidermopoieza este redusa in zona

interesata; Factorul mecanic creste epidermopoieza si keratinizarea Factorul medicamentos – vitamina A, precum si acizii grasi hidroxilati incetinesc

epidermopoieza. De asemenea, vitamina D este un inhibitor puternic al proliferarii epidermice.

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Major cellular functions of cytoplasmic intermediate filaments.

(a) Mechanical support. The epidermis is a good example to illustrate this function that is shared by all major types of intermediate filament (see Table 1). Keratin intermediate filaments are abundant in keratinocytes, ranging from > 10% of total proteins in progenitor basal cells to > 70% in late-differentiating cells. Changes in filament colour reflect differential expression and composition of keratins in basal, early- and late-differentiating cells (differentiation proceeds with the arrow). Keratin filament networks extend throughout the entire cytoplasm in individual keratinocytes and are integrated between cells by attachment at desmosome cell–cell junctions (red dots) and between basal cells and the basal lamina by attachment at hemidesmosomes (yellow dots). This organization maximizes the mechanical support provided by keratin filaments. (b) Cytoarchitecture. In motor neurons, the radial growth of axonal processes requires their interaction with neurofilaments (light blue) to exhibit correct stoichiometry between the light (NF-L), medium (NF-M) and heavy (NF-H) subunits (see Table 1). The large C-terminal tail domains of NF-H (red) and NF-M (orange) subunits are hyperphosphorylated and project away from the filament core, thereby determining interfilament spacing and axonal calibre. The many neurofilaments interact with the less frequent microtubules (dark blue) and with subcortical actin filaments (dark green) through cytoskeletal linker proteins, such as plectin and BPAG1 (yellow). Adapted from reference 32. A cytoarchitecture role has also been shown for nuclear lamins and other cytoplasmic intermediate filaments, such as desmin and keratin. (c) Cell migration. In circulation, lymphocytes resist haemodynamic and mechanical stresses, owing in part to their vimentin intermediate filament network (see Table 1), which is organized in a cage configuration at the cytoplasmic periphery (left cell). After chemokine-induced chemotaxis, for example at sites of active inflammation, vimentin intermediate filaments are rapidly move to the perinuclear region at the cell uropod. This is made possible partly through the site-specific

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phosphorylation of vimentin subunits (depicted by a change in filament colour in the cell, right), and correlates with a softening of the viscoelastic properties of the cytoplasm, presumably to allow the pliability needed during extravasation. The same general principles underlie the ability of epithelial cells to migrate into a wound site after injury. (d) Signal modulation. Cytoplasmic intermediate filaments can bind and modulate the activity of signalling proteins, thereby influencing the flow of extracellular signals to relevant terminal effectors inside the cell. Two possible mechanisms are shown. Left, interactions of intermediate filaments with cell surface receptors, such as Fas, modulate their density and function; right, regulated interactions between intermediate filaments and an adaptor protein, such as TRADD (pink), near the cell surface limits the availability of this adaptor to a ligand-bound receptor that is poised to transmit a signal to the cell. Regulation of either type of interaction, conveyed here by a local change in filament colour (blue or red), can be mediated by dynamic post-translation modifications, association with other proteins or local differences in the composition of intermediate-filament subunits. Both of these mechanisms may participate in regulating the response of epithelial cells to pro-apoptotic signals and other signalling events.

A (neutral-basic) B (acidic) Occurrence

keratin 1, keratin 2 keratin 9, keratin 10 stratum corneum, keratinocytes

keratin 3 keratin 12 cornea

keratin 4 keratin 13 stratified epithelium

keratin 5 keratin 14, keratin 15 stratified epithelium

keratin 6 keratin 16, keratin 17 squamous epithelium

keratin 7 keratin 19 ductal epithelia

keratin 8 keratin 18, keratin 20 simple epithelium

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