Does mitochondrial dysfunction of hair follicle epithelial stem cells play a role in the pathobiology of lichen planopilaris?

Lichen planopilaris (LPP) is a difficult to treat condition leading to permanent hair loss and scarring. Pathobiologically, LPP shows T- cell mediated interferon-γ driven loss of keratin 15 (K15) positive hair follicle (HF) epithelial stem cells (eHFSCs) due to immune privilege (IP) collapse and pathological epithelial-mesenchymal transition (EMT). This exhausts the eHFSC pool; impacting the HF's capacity to regenerate and cycle and ultimately resulting in HF destruction, fibrosis and scarring.

Dear Editor, Lichen planopilaris (LPP) is a difficult-to-treat condition leading to permanent hair loss and scarring. 1 Pathobiologically, LPP shows T-cell-mediated interferon-c-driven loss of keratin 15-positive hair follicle (HF) epithelial stem cells (eHFSCs) due to immune privilege (IP) collapse and pathological epithelial-to-mesenchymal transition (EMT). 1,2 This exhausts the eHFSC pool, impacting the HF's capacity to regenerate and cycle, ultimately resulting in HF destruction, fibrosis and scarring. 1 Defective or insufficient peroxisome proliferator-activated receptor (PPAR)-c signalling in eHFSCs has been implicated in LPP pathogenesis, 3 which can be partly rescued by PPAR-c agonists. 2 However, currently available systemic PPAR-c agonists also stimulate PPAR-a and therefore have potential for considerable adverse effects, highlighting a need to identify more effective treatment strategies that target central elements of LPP pathobiology.
In the current pilot study we explored the possibility that mitochondrial dysfunction in eHFSCs may represent one such target in LPP pathobiology. Apart from the increasing appreciation of keratinocyte mitochondrial function in HF biology, 4 this hypothesis was encouraged by our recent finding that PPAR-c stimulation profoundly upregulates mitochondrial activity in organ-cultured human scalp HFs. 5 Therefore, we investigated whether eHFSCs in lesional human LPP HFs show indications of mitochondrial dysfunction.
To investigate mitochondrial function in LPP we first searched for ultrastructural mitochondrial abnormalities in outer-rootsheath keratinocytes below the level of the sebaceous gland, including the bulge region, by transmission electron microscopy. Compared with healthy HFs, which had small cylindrical mitochondria, both nonlesional and lesional LPP HFs (five patients) showed mitochondria that had undergone swelling, being rounded and enlarged or swollen (Figure 1a). Notably, in nonlesional HFs, most mitochondria retained their inner-membrane cristae, suggesting they are still at least partially functional.
However, the cristae were found to be completely degenerated in lesional HFs, which is a characteristic ultrastructural sign of severe mitochondrial damage and/or mitochondrial leakage.
Mitochondrial transcription factor A (TFAM) is critical for mitochondrial DNA transcription and genome replication 6 and is upregulated by PPAR-c in human HFs. 5 Therefore, we next investigated TFAM protein expression by quantitative and standardized immunohistomorphometry of keratin 15-positive cells in lesional and nonlesional LPP, and healthy HFs. TFAM immunoreactivity was significantly lower in lesional HFs than in healthy or nonlesional HFs in the bulge (P = 0Á003), with nonlesional HFs showing a slight but nonsignificant decrease compared with controls ( Figure 1b). As TFAM is essential for mitochondrial genome replication, transcription and packaging, 6 defective or insufficient TFAM, even in nonlesional HFs, suggests that these HFs have a constitutive problem in TFAM expression, which might contribute to the ultrastructural mitochondrial abnormalities observed (Figure 1a). Preliminary evidence suggests that HFs may attempt to compensate by upregulating MT-CO1 and VDAC1, but this requires further investigation (unpublished work by R.P.).
Next, to probe how the controlled induction of EMT and IP collapse affected bulge mitochondrial function, we utilized a 'cocktail' to promote EMT and IP collapse in the bulge of healthy human scalp HFs, thereby imitating LPP pathogenesis ex vivo. 1,2 Vimentin and E-cadherin expression was used to verify EMT induction in treated HFs (not shown).
A significant decrease in the expression of TFAM in the bulge of healthy anagen scalp HFs was observed following 3 days of cocktail treatment (Figure 1c). Moreover, the respiratory rate of cocktail-treated HFs was drastically reduced compared with vehicle-treated control HFs, as assessed by O 2 consumption assay (Figure 1d). These data suggest that a proinflammatory signalling milieu sufficient to induce bulge IP collapse and EMT also promotes mitochondrial dysfunction in human eHFSCs.
Together, our gene and protein expression, ultrastructural and energy metabolism data highlight a functionally important role of eHFSC mitochondrial dysfunction in LPP development. This not only introduces an important new principle into LPP pathobiology, but further encourages systematic exploration of novel mitochondrial stimulatory agents that target eHFSCs in LPP management. 2 Going forward, future research following up this pilot study firstly needs to elucidate whether mitochondrial defects are secondary to LPP-associated HF inflammation or represent a constitutive abnormality that predisposes to eHFSC damage and LPP development; this could in part be investigated by ultrastructural analysis of mitochondria after EMT induction. Secondly it should be examined how bulge mitochondrial dysfunction is acquired, how it progresses and whether it is reversible therapeutically. On this basis, it would be interesting to investigate PPAR-c coactivator a, whose expression is increased upon mitochondrial dysfunction. 7 Thirdly, it would be useful to investigate whether and how mitochondrial dysfunction contributes to bulge IP collapse and/or pathological EMT. Finally, we need to understand whether this dysfunction is linked to LPP-associated abnormalities in PPAR-c-mediated signalling and whether PPAR-c-specific agonists may be therapeutic in stimulating HF epithelial mitochondrial function, given the recognized impact of the latter on mitochondrial HF physiology. 5,8