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While the mechanisms by which chemical signals control cell fate have been well studied, the impact of mechanical inputs on cell fate decisions is not well understood. Here, using the well-defined system of keratinocyte differentiation in the skin, we examine whether and how direct force transmission to the nucleus regulates epidermal cell fate. Using a molecular biosensor, we find that tension on the nucleus through linker of nucleoskeleton and cytoskeleton (LINC) complexes requires integrin engagement in undifferentiated epidermal stem cells and is released during differentiation concomitant with decreased tension on A-type lamins. LINC complex ablation in mice reveals that LINC complexes are required to repress epidermal differentiation in vivo and in vitro and influence accessibility of epidermal differentiation genes, suggesting that force transduction from engaged integrins to the nucleus plays a role in maintaining keratinocyte progenitors. This work reveals a direct mechanotransduction pathway capable of relaying adhesion-specific signals to regulate cell fate.

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Cutaneous leishmaniasis (CL) is a common disease affecting millions in endemic areas worldwide. We present a case of lupoid leishmaniasis, a rare variant of CL, which clinically mimicked sarcoidosis and/or granulomatous rosacea for 10 years until ultimate diagnosis. An 82-year-old U.S. citizen with an extensive travel history presented with a 10-year history of facial plaques on the cheeks and was previously diagnosed and treated as sarcoidosis. Multiple biopsies (previously and at presentation) revealed tuberculoid granulomas with negative special stains for microorganisms and negative sterile tissue cultures for acid-fast bacilli, bacteria, and fungal organisms. A diagnosis of granulomatous rosacea was rendered and multiple medical therapies were attempted, none with sustained improvement. Repeat biopsy of a new lesion revealed intracellular organisms consistent with leishmaniasis, which was confirmed by polymerase chain reaction (PCR). Lupoid leishmaniasis is a rare presentation of CL including facial plaques that can mimic granulomatous diseases affecting the face including sarcoidosis and granulomatous rosacea. CL can sometimes be challenging to diagnose through standard histopathologic examination; immunohistochemistry for CD1a can be used to augment tissue-based examination and PCR should be sent early in cases with sufficient concern.

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Goltz syndrome (focal dermal hypoplasia) is an X-linked dominant disorder that is classically associated with yellowish papules representing fat herniation (superficial adipocytes). We report a series of three cases, with clinicopathologic correlation of biopsies from Blaschkoid streaks. A range of histopathologic features, including some underreported findings (increased papillary dermal blood vessels, decreased thickness of the dermis, and adipocytes high in the dermis), are reproducible and can strongly point to the correct diagnosis of Goltz syndrome.

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Linear lichen planus (LP) is a rare dermatologic disease in which lichenoid lesions conform to a blaschkolinear distribution, most commonly on the extremities. Linear discoid lupus erythematosus (DLE) is a cutaneous manifestation of lupus that also conforms to Blaschko's lines. Blaschkolinear disorders have been shown to result from somatic mosaicism, most recently in nevus sebaceus, epidermal nevi, and syringocystadenoma papilliferum. In linear LP and DLE, presentation of papules along Blaschko's lines suggests that these disorders can result from keratinocytic genetic mosaicism. Their onset later in life suggests that a secondary trigger is necessary to drive inflammatory reactions to these linear lesions. To date causative mutations have not be identified. We report 2 cases of linear lichen planus and 1 case of linear discoid lupus erythematosus, all histologically confirmed. Both LP patients have experienced episodic regression and recurrence of their lesions in precisely the same distribution, with moderate symptomatic benefit from topical steroids and non-steroidal anti-inflammatories. The DLE patient showed gradual response to hydroxychloroquine over the course of 14 months. These cases highlight linear inflammatory diseases which represent localized variants of disorders for which there are few efficacious therapies. Their linear presentations suggest that they result from somatic mosaicism and genetic investigation of such disorders may reveal relevant therapeutic targets.

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The linker of nucleoskeleton and cytoskeleton (LINC) complex allows cells to actively control nuclear position by coupling the nucleus to the cytoplasmic cytoskeleton. Nuclear position responds to the formation of intercellular adhesions through coordination with the cytoskeleton, but it is not known whether this response impacts adhesion function. In this paper, we demonstrate that the LINC complex component SUN2 contributes to the mechanical integrity of intercellular adhesions between mammalian epidermal keratinocytes. Mice deficient for Sun2 exhibited irregular hair follicle intercellular adhesions, defective follicle structure, and alopecia. Primary mouse keratinocytes lacking Sun2 displayed aberrant nuclear position in response to adhesion formation, altered desmosome distribution, and mechanically defective adhesions. This dysfunction appeared rooted in a failure of Sun2-null cells to reorganize their microtubule network to support coordinated intercellular adhesion. Together, these results suggest that cross talk between the nucleus, cytoskeleton, and intercellular adhesions is important for epidermal tissue integrity.

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Plakophilin 2 (PKP2), an armadillo family member closely related to p120 catenin (p120ctn), is a constituent of the intercellular adhesive junction, the desmosome. We previously showed that PKP2 loss prevents the incorporation of desmosome precursors enriched in the plaque protein desmoplakin (DP) into newly forming desmosomes, in part by disrupting PKC-dependent regulation of DP assembly competence. On the basis of the observation that DP incorporation into junctions is cytochalasin D-sensitive, here we ask whether PKP2 may also contribute to actin-dependent regulation of desmosome assembly. We demonstrate that PKP2 knockdown impairs cortical actin remodeling after cadherin ligation, without affecting p120ctn expression or localization. Our data suggest that these defects result from the failure of activated RhoA to localize at intercellular interfaces after cell-cell contact and an elevation of cellular RhoA, stress fibers, and other indicators of contractile signaling in squamous cell lines and atrial cardiomyocytes. Consistent with these observations, RhoA activation accelerated DP redistribution to desmosomes during the first hour of junction assembly, whereas sustained RhoA activity compromised desmosome plaque maturation. Together with our previous findings, these data suggest that PKP2 may functionally link RhoA- and PKC-dependent pathways to drive actin reorganization and regulate DP-IF interactions required for normal desmosome assembly.

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Armadillo family proteins known as plakophilins have been characterized as structural components of desmosomes that stabilize and strengthen adhesion by enhancing attachments with the intermediate filament cytoskeleton. However, plakophilins and their close relatives are emerging as versatile scaffolds for multiple signaling and metabolic processes that not only facilitate junction dynamics but also more globally regulate diverse cellular activities. While perturbation of plakophilin functions contribute to inherited diseases and cancer pathogenesis, the functional significance of the multiple PKP isoforms and the mechanisms by which their behaviors are regulated remain to be elucidated.

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Plakophilins (PKPs) are armadillo family members related to the classical cadherin-associated protein p120(ctn). PKPs localize to the cytoplasmic plaque of intercellular junctions and participate in linking the intermediate filament (IF)-binding protein desmoplakin (DP) to desmosomal cadherins. In response to cell-cell contact, PKP2 associates with DP in plaque precursors that form in the cytoplasm and translocate to nascent desmosomes. Here, we provide evidence that PKP2 governs DP assembly dynamics by scaffolding a DP-PKP2-protein kinase C alpha (PKC alpha) complex, which is disrupted by PKP2 knockdown. The behavior of a phosphorylation-deficient DP mutant that associates more tightly with IF is mimicked by PKP2 and PKC alpha knockdown and PKC pharmacological inhibition, all of which impair junction assembly. PKP2 knockdown is accompanied by increased phosphorylation of PKC substrates, raising the possibility that global alterations in PKC signaling may contribute to pathogenesis of congenital defects caused by PKP2 deficiency.

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