The basement membrane zone (BMZ) in the skin is a critical interface between the epidermis and dermis and is a highly specialized structure that allows for communication between different cell types.1 Basement membranes throughout the body provide tissue-specific functions; however, all share similar attributes. Regardless of site, the BMZ functions as a vehicle for cellular attachment of cells, a matrix for cellular migration, a gatekeeper for cellular and macromolecule transit, and as the moderator of cellular activities ranging from differentiation to apoptosis. Ultrastructural studies have identified distinct structural components in the BMZ, consisting of the hemidesmosome, anchoring filaments, anchoring fibrils, lamina lucida and lamina densa.2 From this visualization, the BMZ of the skin has been partitioned into four subdivisions: Zone 1 — the plasma membrane and hemidesmosome; Zone 2 — the lamina lucida; Zone 3 — the lamina densa; and Zone 4 — the sub-lamina densa /papillary dermis.3 The purpose of this review is to highlight the BMZ structures and their functions, with an emphasis on the pathophysiology of immuno- and mechano-blistering disorders, as these are always a boards and recertification favorite.
Zone 1 — The Hemidesmosome
Simply put, the hemidesmosome serves as the bridge between the basal keratinocytes and extracellular matrix. These are structurally and functionally different from desmosomes, as hemidesmosomes are “anchoring” plaques connecting the epidermis and dermis, interfacing extracellular and intracellular components, rather then just adjacent keratinocytes. (I am not implying desmosomes are lazy!) The best way to appreciate the hemisdesmosome is to understand it as part of a functional complex, such as the hemidesmosome-anchoring filament complex, which allows for the connection and communication between the basal cells and underlying basement membrane. There are multiple, confusing components that work synergistically in this zone that generally can be subdivided into three groups: the keratin intermediate filaments, the internal plaque proteins and the transmembrane plaque proteins. The basal keratinocyte intermediate filaments function as intracellular scaffolding in the scheme of the overall cutaneous cytoskeleton. These serve as a continuation from the continuous network created by desmosomes, allowing for extensive connections to the underlying BMZ and dermis via the hemidesmosomes. The filaments are composed of keratins (K) 5 and 14. Mutations resulting in dysfunctional K5 and K14 result in various disease patterns, depending on whether one or both are defective. Defects in K5 can result in Dowling-Degos disease, characterized by flexural pigmented reticulate macules and pruritic comedonal papules on the back and/or the neck. Defects in K14 can result in dermatopathia pigmentosa reticularis or Naegeli-Franceschetti-Jadassohn, both demonstrating generalized reticulate hyeprpigmentation and distinguished by noncictracial alopecia in the former and by enamel defects in the latter. In the extreme case, when both K5/14 are impacted, the result is epidermolysis bullosa (EB) simplex, which encompasses several inherited clinical subtypes (Dowling Meara/herpetiform, Weber Cockayne, Koebner) characterized by blister formation in response to mechanical trauma. Of note, EB with mottled hyperpigmentation results in a sole mutation in K5. The major internal plaque proteins consist of bullous pemphigoid antigen 1 (BPa1) and plectin. BPAG1 is a member of the plakin (remember Desmoplakin>S) family and as such, plays an important role in promoting adhesion of the intermediate filament cytoskeleton with “adhesion” units, specifically here, the hemidesmosome.4 BPAG1 is a non-collagenous protein localized to the cytoplasmic plaque of HDZ.5 The amino terminus of BPAG1 associates with the cytoplasmic domain of BPAG2 and integrin subunit β4. (Transmembrane proteins will be discussed later.) BPAG1 is antigenic in several auto-immune blistering diseases such as bullous pemphigoid and paraneoplastic pemphigus. Plectin is also a member of the plakin family, and similarly is associated with the cytoplasmic plaque of HDs.6 Plectin binds to keratin intermediate filaments, and, like BPAG1, binds the cytoplasmic tail of integrin subunit β4 and BPAG2.7 Defects or mutations in the gene encoding plectin are responsible for EBS associated with muscular dystrophy.8 Lastly, a less well-known protein tetraspan CD151, a cell surface protein located on the lateral portion of the basal keratinocyte, is found to associate with integrin α6β4. In fact, it is this interaction that was shown to induce the formation of the HD. Therefore, CD151 is considered a “pre-HD” component whose recruitment into HDs is regulated by integrin α6β4.9 Zone 2 — The Lamina Lucida Although the transmembrane proteins insert into the HD, I would prefer to discuss them in the context of their function; these proteins serve as the bridge between the basal keratinocyte and lamina densa/dermis through the lamina lucida. BPAG2 is probably the best described, but equally as important is integrin α6β4. BPAG2 is also known as collagen 17, likely because the extracellular domain of contains interrupted repeats that have Gly-X-Y repeating amino acid sequences. (Does this structure sound familiar?) BPAG2 is a busy protein, with the intracellular amino terminus interacting with the integrin β4 subunit, plectin, and BPAG1, and the extracellular carboxy end inserting in the lamina densa and looping back around out into the lamina lucida (like an inverted hooked walking cane!).10 BPAG2 has two forms,11 one being the full 180 kDa protein, and the second corresponding to the 120 kDa extracellular domain, which undergoes cleavage from basal keratinocytes via proteolysis. Key BPAG connections include: cytoplasmic — BPAG1, integrin subunit β4 and plectin; extracytoplasmic — the first extracellular portion of BPAG2 (its 16th non-collagenous portion) to the integrin subunit α6, and the carboxy terminal interacts with laminin 5. BPAG2 is famous for its role in both mechano- and autoimmune blistering diseases. Junctional EB, the non-Herlitz subtype, results from null mutations in the gene encoding BPAG2 (COL17A1).12 Clinically, patients demonstrate increased skin fragility and tense blister formation, alopecia, dystrophic nails and dental enamel defects. Subepidermal blistering diseases resulting from auto-antibodies targeting BPAG2 include bullous pemphigoid (BP), pemphigoid gestationis (PG), cicatricial pemphigoid (CP), lichen planus pemphigoides and linear IgA bullous dermatosis (LABD).13,14 Auto-antibodies generally target the NC16A domain of BPAG2; however, in patients with CP, the target is more commonly the distal carboxy terminus of BPAG2, explaining the increased tendency for these patients to scar. Integrins are transmembrane receptors that also promote cell-cell and cell-matrix interactions.15 All integrins must exist as αβ-heterodimers, although several of the α-subunits can associate with more than one β-subunit. The key HD integrin consists of an α6β4 dimer. The HD-plaque associated region of integrin subunit β4 directly associates with plectin, while, as mentioned above, its α6 region binds BPAG216 — the NC16A region of BPAG2, to be precise. Distally, laminin 5 serves as the preferred ligand for this integrin, and is required for stabilization. Mutations in the genes encoding either α6 or β4 integrin subunits result in junctional EB associated with pyloric atresia.17 Clinically, patients have generalized subepidermal blistering of the skin, and oral and respiratory epithelia. Antibodies directed against the integrin subunit β4 cause an ocular form of CP.
Zone 3 — Lamina Densa
The lamina densa can be considered the other side of the river, into which transmembrane proteins can drop anchor via a specific protein family, the laminins. Laminins are heterotrimeric glycoproteins comprising 14 subtypes.18 Laminins consist of three subunits that are joined by disulfide bonds. The various laminin isoforms share functional homology in that they all serve as structural, basement membrane proteins as well as ligands for cell surface receptors, for example, integrins. Laminin 5 is probably the most relevant laminin with respect to dermatology/boards. Laminin 5 plays a crucial role in attaching keratinocytes to the epidermal basement membrane,19 localized to the interface of the lamina lucida and the lamina densa. In fact, integrin α6β4 specifically/preferentially binds laminin 5.20 The binding of laminin 5 to the basement membrane creates a complex containing laminin subunits. These provide binding sites for nidogen, another anchoring component of the lamina densa, allowing for stronger adhesion. Furthermore, Laminin 5 alone can bind type VII collagen, thereby directly linking HD anchoring filament complexes with the rooted anchoring fibrils. Laminin 5’s biological functions are clearly illustrated by resulting clinical phenotypes characterized by subepidermal blister formation in both the inherited junctional EB-Herlitz type21 and auto-immune anti-epiligrin (ie, laminin 5) subtype of cicatricial pemphigoid.22 Of note, this subtype of CP is most commonly associated with underlying adenocarcinoma, especially of the lung. Type IV collagen is also localized within basement membranes23 and structurally resembles procollagen (quick recall: Triple helix of collagen characterized by repeating G-X-Y amino acid sequences, X-Y representing proline or hydroxyproline). Unlike the classic collagens (ie, I and III), type IV collagen is not helical throughout, allowing for increased flexibility. Dr. David Woodley3 coined the “hockey stick” analogy with respect to its structure, with the handle corresponding to the largely continuous triple-helical portion of the molecule. This allows for the formation of a structurally sound lattice of type IV collagen, which ultimately forms the matrix within the lamina densa. Defects in or antibodies to type IV collagen are associated with several diseases. Alport syndrome, which is characterized by hematuria, renal failure and sensorineural hearing loss, results from a mutation in the COL4A5 gene. Goodpasture syndrome, a severe autoimmune disease resulting in pulmonary hemorrhage and glomerulonephritis, represents another type IV collagen disease. More recently, fragments of type IV collagen subunits, canstatin and tumastatin, have been recognized for their important biological activities. Canstatin was shown to be a potent inhibitor of angiogeneisis and tumor growth, and can actually induce apoptosis in vascular endothelial cells (CD31+ cells). Tumstatin was also shown to have anti-cancer properties through antiangiogenic activity, specifically in melanoma cell lines. The potential therapeutic implications here are early but promising.24 Nidogen, or enactin, is a linking glycoprotein found in the lamina densa that brings networks of type IV collagen and different forms of laminin together to form one happy anchoring family. 25 Nidogen is structurally often compared to a dumbbell; 23 however, its physiologic function should not be considered so mundane. In addition to complexing type IV collagen with laminins, nidogen, with the help of laminin, can bind to heparan sulfate proteoglycans and the fibulins to ultimately stabilize the multiple components of the BMZ. To date, there are no autoimmune or inherited blistering diseases associated with nidogen. Lastly, perlecan and other heparin sulfate proteoglycans are important macromolecules that coat the lamina densa core proteins and influence permeability due to their anionic (negative) charges. They are found above, within, and just below the lamina densa, and similarly to nidogden, there are no known associations with autoimmune or inherited blistering diseases.
Zone 4 — Sub lamina densa … To be continued
In order to prevent cerebral overload, Zone 4 will be covered in the next addition of From Bench to Bedside: Science for Boards and Wards in which I will review the components of biological activites of the dermis and, of course, the potential pathology resulting in defects or antigenic responses to these structures.