Deletion of the epidermis derived laminin γ1 chain leads to defects in the regulation of late hair morphogenesis.

Laminins are the most abundant non-collagenous basement membrane (BM) components, composed of an α, ß and γ chain. The laminin γ1 chain, encoded by LAMC1, is the most abundant γ chain. The main laminin isoforms in the dermo-epidermal junction (DEJ) are laminin-332, laminin-511 and laminin-211, the latter being restricted to the lower part of hair follicles (HFs). Complete deletion of LAMC1 results in lethality around embryonic day 5.5. To study the function of laminin γ1 containing isoforms in skin development and maturation after birth, we generated mice lacking LAMC1 expression in basal keratinocytes (LAMC1EKO) using the keratin 14 (K14) Cre/loxP system. This deletion resulted in loss of keratinocyte derived laminin-511 and in deposition of fibroblast derived laminin-211 throughout the whole DEJ. The DEJ in areas between hemidesmosomes was thickened, whereas hemidesmosome morphology was normal. Most strikingly, LAMC1EKO mice showed delayed HF morphogenesis accompanied by reduced proliferation of hair matrix cells and impaired differentiation of hair shafts (HS). However, this deletion did not interfere with early HF development, since placode numbers and embryonic hair germ formation were not affected. Microarray analysis of skin revealed down regulation of mainly different hair keratins. This is due to reduced expression of transcription factors such as HoxC13, FoxN1, FoxQ1 and Msx2, known to regulate expression of hair keratins. While the role of laminin-511 in signaling during early hair germ formation and elongation phase has been described, we here demonstrate that epidermal laminin-511 is also a key regulator for later hair development and HS differentiation.

Basement membrane deposition of nidogen 1 but not nidogen 2 requires the nidogen binding module of the laminin gamma1 chain.

The nidogen-laminin interaction is proposed to play a key role in basement membrane (BM) assembly. However, though there are similarities, the phenotypes in mice lacking nidogen 1 and 2 (nidogen double null) differ to those of mice lacking the nidogen binding module (γ1III4) of the laminin γ1 chain. This indicates different cell- and tissue-specific functions for nidogens and their interaction with laminin and poses the question of whether the phenotypes in nidogen double null mice are caused by the loss of the laminin-nidogen interaction or rather by other unknown nidogen functions. To investigate this, we analyzed BMs, in particular those in the skin of mice lacking the nidogen binding module. In contrast to nidogen double null mice, all skin BMs in γ1III4-deficient mice appeared normal. Furthermore, although nidogen 1 deposition was strongly reduced, nidogen 2 appeared unchanged. Mice with additional deletion of the laminin γ3 chain, which contains a γ1-like nidogen binding module, showed a further reduction of nidogen 1 in the dermoepidermal BM; however, this again did not affect nidogen 2. This demonstrates that in vivo only nidogen 1 deposition is critically dependent on the nidogen binding modules of the laminin γ1 and γ3 chains, whereas nidogen 2 is independently recruited either by binding to an alternative site on laminin or to other BM proteins.