Dendritic atoh1a+ cells serve as transient intermediates during zebrafish Merkel cell development and regeneration

Sensory cells often adopt specific morphologies that aid in the detection of external stimuli. Merkel cells encode gentle touch stimuli in vertebrate skin and adopt a reproducible shape characterized by spiky, actin-rich microvilli that emanate from the cell surface. The mechanism by which Merkel cells acquire this stereotyped morphology from basal keratinocyte progenitors is unknown. Here, we establish that dendritic Merkel cells (dMCs) express atonal homolog 1a (atoh1a), extend dynamic filopodial processes, and arise in transient waves during zebrafish skin development and regeneration. We find that dMCs share molecular similarities with both basal keratinocytes and Merkel cells, yet display mesenchymal-like behaviors, including local cell motility and proliferation within the epidermis. Furthermore, dMCs can directly adopt the mature, microvilliated Merkel cell morphology through substantial remodeling of the actin cytoskeleton. Loss of Ectodysplasin A signaling alters the morphology of dMCs and Merkel cells within specific skin regions. Our results show that dMCs represent an intermediate state in the Merkel cell maturation program and identify Ectodysplasin A signaling as a key regulator of Merkel cell morphology.

Dermal appendage-dependent patterning of zebrafish atoh1a+ Merkel cells

Touch system function requires precise interactions between specialized skin cells and somatosensory axons, as exemplified by the vertebrate mechanosensory Merkel cell-neurite complex. Development and patterning of Merkel cells and associated neurites during skin organogenesis remain poorly understood, partly due to the in utero development of mammalian embryos. Here, we discover Merkel cells in the zebrafish epidermis and identify Atonal homolog 1a (Atoh1a) as a marker of zebrafish Merkel cells. We show that zebrafish Merkel cells derive from basal keratinocytes, express neurosecretory and mechanosensory machinery, extend actin-rich microvilli, and complex with somatosensory axons, all hallmarks of mammalian Merkel cells. Merkel cells populate all major adult skin compartments, with region-specific densities and distribution patterns. In vivo photoconversion reveals that Merkel cells undergo steady loss and replenishment during skin homeostasis. Merkel cells develop concomitant with dermal appendages along the trunk and loss of Ectodysplasin signaling, which prevents dermal appendage formation, reduces Merkel cell density by affecting cell differentiation. By contrast, altering dermal appendage morphology changes the distribution, but not density, of Merkel cells. Overall, our studies provide insights into touch system maturation during skin organogenesis and establish zebrafish as an experimentally accessible in vivo model for the study of Merkel cell biology.

Ancestral role of TNF-R pathway in cell differentiation in the basal metazoan Hydra

Tumour necrosis factor receptors (TNF-Rs) and their ligands, tumour necrosis factors, are highly conserved proteins described in all metazoan phyla. They function as inducers of extrinsic apoptotic signalling and facilitate inflammation, differentiation and cell survival. TNF-Rs use distinct adaptor molecules to activate signalling cascades. Fas-associated protein with death domain (FADD) family adaptors often mediate apoptosis, and TNF-R-associated factor (TRAF) family adaptors mediate cell differentiation and inflammation. Most of these pathway components are conserved in cnidarians, and, here, we investigated the Hydra TNF-R. We report that it is related to the ectodysplasin receptor, which is involved in epithelial cell differentiation in mammals. In Hydra, it is localised in epithelial cells with incorporated nematocytes in tentacles and body column, indicating a similar function. Further experiments suggest that it interacts with the Hydra homologue of a TRAF adaptor, but not with FADD proteins. Hydra FADD proteins colocalised with Hydra caspases in death effector filaments and recruited caspases, suggesting that they are part of an apoptotic signalling pathway. Regulating epithelial cell differentiation via TRAF adaptors therefore seems to be an ancient function of TNF-Rs, whereas FADD-caspase interactions may be part of a separate apoptotic pathway.

Fitness maps to a large-effect locus in introduced stickleback populations

Mutations of small effect underlie most adaptation to new environments, but beneficial variants with large fitness effects are expected to contribute under certain conditions. Genes and genomic regions having large effects on phenotypic differences between populations are known from numerous taxa, but fitness effect sizes have rarely been estimated. We mapped fitness over a generation in an F2 intercross between a marine and a lake stickleback population introduced to a freshwater pond. A quantitative trait locus map of the number of surviving offspring per F2 female detected a single, large-effect locus near Ectodysplasin (Eda), a gene having an ancient freshwater allele causing reduced bony armor and other changes. F2 females homozygous for the freshwater allele had twice the number of surviving offspring as homozygotes for the marine allele, producing a large selection coefficient, s = 0.50 ± 0.09 SE. Correspondingly, the frequency of the freshwater allele increased from 0.50 in F2 mothers to 0.58 in surviving offspring. We compare these results to allele frequency changes at the Eda gene in an Alaskan lake population colonized by marine stickleback in the 1980s. The frequency of the freshwater Eda allele rose steadily over multiple generations and reached 95% within 20 y, yielding a similar estimate of selection, s = 0.49 ± 0.05, but a different degree of dominance. These findings are consistent with other studies suggesting strong selection on this gene (and/or linked genes) in fresh water. Selection on ancient genetic variants carried by colonizing ancestors is likely to increase the prevalence of large-effect fitness variants in adaptive evolution.

Ectodysplasin-A signaling is a key integrator in the lacrimal gland-cornea feedback loop

A lack of ectodysplasin-A (Eda) signaling leads to dry eye symptoms, which have so far only been associated with altered Meibomian glands. Here, we used loss-of-function (Eda ^-/-) mutant mice to unravel the impact of Eda signaling on lacrimal gland formation, maturation and subsequent physiological function. Our study demonstrates that Eda activity is dispensable during lacrimal gland embryonic development. However, using a transcriptomic approach, we show that the Eda pathway is necessary for proper cell terminal differentiation in lacrimal gland epithelium and correlated with modified expression of secreted factors commonly found in the tear film. Finally, we discovered that lacrimal glands present a bilateral reduction of Eda signaling activity in response to unilateral corneal injury. This observation hints towards a role for the Eda pathway in controlling the switch from basal to reflex tears, to support corneal wound healing. Collectively, our data suggest a crucial implication of Eda signaling in the cornea-lacrimal gland feedback loop, both in physiological and pathophysiological conditions. Our findings demonstrate that Eda downstream targets could help alleviate dry eye symptoms.

Ongoing niche differentiation under high gene flow in a polymorphic brackish water threespine stickleback (Gasterosteus aculeatus) population

BACKGROUND

Marine threespine sticklebacks colonized and adapted to brackish and freshwater environments since the last Pleistocene glacial. Throughout the Holarctic, three lateral plate morphs are observed; the low, partial and completely plated morph. We test if the three plate morphs in the brackish water Lake Engervann, Norway, differ in body size, trophic morphology (gill raker number and length), niche (stable isotopes; δ15N, δ13C, and parasites (Theristina gasterostei, Trematoda spp.)), genetic structure (microsatellites) and the lateral-plate encoding Stn382 (Ectodysplasin) gene. We examine differences temporally (autumn 2006/spring 2007) and spatially (upper/lower sections of the lake - reflecting low versus high salinity).

RESULTS

All morphs belonged to one gene pool. The complete morph was larger than the low plated, with the partial morph intermediate. The number of lateral plates ranged 8-71, with means of 64.2 for complete, 40.3 for partial, and 14.9 for low plated morph. Stickleback δ15N was higher in the lower lake section, while δ13C was higher in the upper section. Stickleback isotopic values were greater in autumn. The low plated morph had larger variances in δ15N and δ13C than the other morphs. Sticklebacks in the upper section had more T. gasterostei than in the lower section which had more Trematoda spp. Sticklebacks had less T. gasterostei, but more Trematoda spp. in autumn than spring. Sticklebacks with few and short rakers had more T. gasterostei, while sticklebacks with longer rakers had more Trematoda. spp. Stickleback with higher δ15N values had more T. gasterostei, while sticklebacks with higher δ15N and δ13C values had more Trematoda spp. The low plated morph had fewer Trematoda spp. than other morphs.

CONCLUSIONS

Trait-ecology associations may imply that the three lateral plate morphs in the brackish water lagoon of Lake Engervann are experiencing ongoing divergent selection for niche and migratory life history strategies under high gene flow. As such, the brackish water zone may generally act as a generator of genomic diversity to be selected upon in the different environments where threespine sticklebacks can live.

Ectodysplasin A (EDA) - EDA receptor signalling and its pharmacological modulation

The TNF family ligand ectodysplasin A (EDA) regulates the induction, morphogenesis and/or maintenance of skin-derived structures such as teeth, hair, sweat glands and several other glands. Deficiencies in the EDA - EDA receptor (EDAR) signalling pathway cause hypohidrotic ectodermal dysplasia (HED). This syndrome is characterized by the absence or malformation of several skin-derived appendages resulting in hypotrychosis, hypodontia, heat-intolerance, dry skin and dry eyes, susceptibility to airways infections and crusting of various secretions. The EDA-EDAR system is an important effector of canonical Wnt signalling in developing skin appendages. It functions by stimulating NF-κB-mediated transcription of effectors or inhibitors of the Wnt, Sonic hedgehog (SHH), fibroblast growth factor (FGF) and transforming growth factor beta (TGFβ) pathways that regulate interactions within or between epithelial and mesenchymal cells and tissues. In animal models of Eda-deficiency, soluble EDAR agonists can precisely correct clinically relevant symptoms with low side effects even at high agonist doses, indicating that efficient negative feedback signals occur in treated tissues. Hijacking of the placental antibody transport system can help deliver active molecules to developing foetuses in a timely manner. EDAR agonists may serve to treat certain forms of ectodermal dysplasia.