Ash1L ameliorates psoriasis via limiting neuronal activity-dependent release of miR-let-7b

BACKGROUND AND PURPOSE

Psoriasis is a common autoimmune skin disease that significantly diminishes patients' quality of life. Interactions between primary afferents of the somatosensory system and the cutaneous immune system mediate the pathogenesis of psoriasis. This study aims to elucidate the molecular mechanisms of how primary sensory neurons regulate psoriasis formation.

EXPERIMENTAL APPROACH

Skin and total RNA were extracted from wild-type (WT) and ASH1-like histone lysine methyltransferase (Ash1l+/- ) mice in both naive and imiquimod (IMQ)-induced psoriasis models. Immunohistochemistry, quantitative real-time polymerase chain reaction (qRT-PCR) and fluorescence-activated cell sorting (FACS) were then performed. Microfluidic chamber coculture was used to investigate the interaction between somatosensory neurons and bone marrow dendritic cells (BMDCs) ex vivo. Whole-cell patch clamp recordings were used to evaluate neuronal excitability after Ash1L haploinsufficiency in primary sensory neurons.

KEY RESULTS

The haploinsufficiency of ASH1L, a histone methyltransferase, in primary sensory neurons causes both neurite hyperinnervation and increased neuronal excitability, which promote miR-let-7b release from primary afferents in the skin in a neuronal activity-dependent manner. With a 'GUUGUGU' core sequence, miR-let-7b functions as an endogenous ligand of toll-like receptor 7 (TLR7) and stimulates the activation of dermal dendritic cells (DCs) and interleukin (IL)-23/IL-17 axis, ultimately exacerbating the symptoms of psoriasis. Thus, by limiting miR-let-7b release from primary afferents, ASH1L prevents dermal DC activation and ameliorates psoriasis.

CONCLUSION AND IMPLICATIONS

Somatosensory neuron ASH1L modulates the cutaneous immune system by limiting neuronal activity-dependent release of miR-let-7b, which can directly activate dermal DCs via TLR7 and ultimately lead to aggravated psoriatic lesion.

Structural Insights into Stimulation of Ash1L's H3K36 Methyltransferase Activity through Mrg15 Binding

The evolutionarily conserved Trithorax group protein Ash1 is a SET domain histone methyltransferase that mono- and dimethylates lysine 36 of histone H3 (H3K36). Ash1 forms a complex with Mrg15 and Nurf55, and the binding of Mrg15 greatly stimulates the catalytic activity of Ash1, yet the underlying molecular mechanisms remain unknown. Here we report the crystal structure of the tandem Mrg15-interacting and SET domains of human Ash1L in complex with Mrg15. Ash1L interacts with Mrg15 principally via a segment located N-terminal to the catalytic SET domain. Surprisingly, an autoinhibitory loop in the post-SET region of Ash1L is destabilized on Mrg15 binding despite no direct contact. Dynamics of the autoinhibitory loop can be attributed to subtle structural changes of the S-adenosylmethionine (SAM) binding pocket induced by Mrg15 binding, implicating a mechanism of conformational coupling between SAM and substrate binding sites. The findings broaden the understanding of regulation of H3K36 methyltransferases.

Roles of H3K36-specific histone methyltransferases in transcription: antagonizing silencing and safeguarding transcription fidelity

Histone H3K36 methylation is well-known for its role in active transcription. In Saccharomyces cerevisiae, H3K36 methylation is mediated solely by SET2 during transcription elongation. In metazoans, multiple H3K36-specific methyltransferases exist and contribute to distinct biochemical activities and subsequent functions. In this review, we focus on the H3K36-specific histone methyltransferases in metazoans, and discuss their enzymatic activity regulation and their roles in antagonizing Polycomb silencing and safeguarding transcription fidelity.