by Paul D. Rennert, 28 December 2013
A few weeks ago a paper describing the origin of cells responsible for dermal wound healing showed up in Nature, with a beautiful cover picture showing distinct cell lineages in skin. I actually missed the paper when it came out, so thanks to my friend Katherine Turner (ex-GI, ex-Biogen, now at Scholar Rock) for pointing it out.
The abstract is here, the full text requires a subscription. The paper is from Fiona Watt’s group at the Center for Stem Cells and Regenerative Medicine, King’s College, London.
The paper convincingly shows that the dermal layer of the skin is comprised of two distinct fibroblast lineages. Some context: the dermis is the layer of the skin that lies below the epidermis and includes the upper dermis (the dermal papilla) and the hypodermis (lower dermis). The major cells types residing in the dermis are fibroblasts, which along with the extracellular matrix they produce, form the connective tissue. Other prominent cells include macrophages and adipocytes (fat cells). There are also resident dendritic cells, Mast cells, T cells and other immune system cell types. Other structures include the hair follicles, sweat glands, oil glands, blood vessel endothelium, lymphatic vessels, and nerve endings.
Ok, so why is this paper interesting?
What the authors demonstrate is that the upper and lower dermis originate from distinct lineages of fibroblasts. This is shown using lineage tracing techniques and cell transfer assays. The lineage tracing assays required the use of multiple promoter/expression transgenic mice, and is elegantly and robustly done. The data show that late in mouse embryonic development (day 16.5) the dermis becomes fate-restricted: the Lrig1+ fibroblasts become the cells of the upper dermis and Dlk1+ cells give rise to the lower dermis. Here I’ve just picked a few of the markers they used and have grossly simplified the developmental story because I’m more interested in what they did next.
Having established that there are 2 different fibroblast lineages in the mouse dermis, the investigators asked a pretty simple question: what happens when you wound the skin in the adult mouse? Cellular wound-healing models have been widely used as in vitro models of fibroblast migration, ECM production and differentiation into myofibroblasts (so called because they express smooth muscle actin, a marker of myocytes/smooth muscle cells). Wound healing more broadly is a process of local repair and remodeling. “Deranged” or unregulated wound-healing is one model of fibrosis.
Skin or dermal fibrosis is a highly active area of study. Important fibrotic diseases of the skin include systemic sclerosis (scleroderma), amyloidosis, nephrogenic systemic fibrosis, mixed connective tissue disease, wound-healing fibrosis and others. Scleroderma is modeled in mouse by creating chemical insults on the skin, for example with bleomycin. Such models mimic the effect of chronic wounding, thus the connection to wound healing assays. It has been further demonstrated that many of the processes and pathways involved in dermal wound-healing are dysregulated in skin fibrosis, for example ECM production, TGFbeta production, and CTGF activity among many others.
While there is a good deal of consensus as to the mediators of the fibrotic process, the source of the activated cells that drive fibrosis has been controversial. This is a critical question to address, as effective anti-fibrotic therapies should target the pathogenic cell types. Without getting into immune cells, resident macrophages and Mast cells and the like, we can now at least carefully address this question regarding the source of activated fibroblasts. One model proposes that migrating fibrocytes traffic into wounded/fibrotic sites from the circulation. Another model, widely held, proposes that pericytes, which are mesenchymal cells that support blood vessels, somehow migrate away from endothelium and become myofibroblasts, thus supporting wound healing (and by extension contributing to fibrotic diseases). Finally, it has been proposed that myofibroblasts are derived from local (resident) fibroblasts that migrate into the wound and differentiate.
The Watt’s paper shows clearly that the Dlk1+ fibroblasts of the lower dermis migrate into the wound and that a proportion of these cells express smooth muscle actin and are therefore myofibroblasts. These cells produce collagen, the classic ECM protein that characterizes fibrosis, in this case simply deployed to help close the wound. After the wound is closed the epidermal layer is reformed and only then does the upper dermis reform.
These findings allow for the formulation of specific questions regarding the lower dermal fibroblasts:
– what signals activate and then shut down the lower dermal fibroblast to myofibroblast differentiation process?
– are these signals dysregulated in fibrosis?
– do these same cells deploy in response to fibrotic injury (e.g. with bleomycin)?
– do these cells interact with inflammatory cells, thought to contribute to the pathogenic fibrotic response?
– can we identify actionable targets on this cell type to prevent or decrease the fibrotic response?
Importantly this paper encourages us to ask whether similar resident fibroblast or other mesenchymal cell populations contribute to fibrotic disease in other organs such as the kidney, liver, heart and perhaps even lung. If so, the validity of the circulating fibrocyte and differentiating pericyte models might be reevaluated. Notably, linage tracking was used in support of a pericyte model of myofibroblast differentiation in a similar dermal/muscle injury model as detailed in a paper published in 2012 in Nature Medicine (abstract here). I suspect that squaring these studies will take a careful review of the markers used to identify different cell populations and a detailed look at the injury protocols. I’ll update as more info becomes available.
This is a classic example of the beautiful work that comes from an active and competitive research area and we can and look forward to further studies that will illuminate the processes of wound healing and fibrosis. Such work will certainly support and advance successful fibrosis drug development.
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