mTORC1 activates PASK-Wdr5 signaling to epigenetically connect the nutrient status with myogenesis

Abstract

SummaryIn the tissue microenvironment, stem cell functions are modulated by extrinsic signaling cues such as peptide hormones and dietary nutrients. These signaling cues maintain the balance between self-renewal and differentiation of its resident stem cells. The mechanistic Target of Rapamycin Complex 1 (mTORC1) is implicated to play an important role in regulating this balance, although its downstream effectors in stem cells have been elusive. We have recently shown that the PASK protein kinase phosphorylates Wdr5 to stimulate muscle stem cell differentiation by epigenetically activating the Myogenin promoter. Here, we show that the PASK-Wdr5 signaling pathway is a nutrient-sensitive downstream target of mTORC1 in muscle stem cells. We show that phosphorylation of PASK, and in turn of Wdr5, by mTORC1 is required for the activation of Myogenin transcription, exit from the self-renewal and induction of the myogenesis program. Thus, mTOR connects the diverse extrinsic signaling cues to a central epigenetic process to regulate the muscle stem cell fate between self-renewal and differentiation.

Nutrient Signaling-Dependent Quaternary Structure Remodeling Drives the Catalytic Activation of metazoan PASK

“…The remaining portion of the human PASK sequence (aa 403-998), approximately 600 amino acids, is considered unstructured but functionally significant. This assertion is based on our recent studies, which identified critical regulatory elements within this unstructured region that regulate PASK subcellular distribution, interactions with the substrate Wdr5, and catalytic activation upon mTORC1-mediated phosphorylation [10][11][12]. Because these biochemical signals primarily target sequences outside the functional domains of PAS-A (PDB-ID: 1LL8) and serine/threonine kinase (PDB-ID: 3DLS), we aimed to understand how these regulatory inputs in the unstructured region of PASK affect the known structural elements that control PASK functions.…”

“…We previously reported the catalytic activation of PASK via the mTOR complex 1 (mTORC1)-mediated phosphorylation of residues S640 and T642 within the PAS 1-737 region and S949, T953, and S957 within the C-terminal Kinase 841-1323 region of PASK juxtaposed with the PAC motif (Figure 6F, green circles) [11]. Since these phosphorylation motifs are outside of well-defined domain boundaries, it remains unclear how mTORC1-mediated phosphorylation could stimulate PASK catalytic activity.…”

“…PASK is the only known mammalian protein kinase that combines a nutrient-sensing PAS domain with a serine/threonine-kinase domain. PASK is vital for stem cell differentiation and regulation of organismal metabolism [7][8][9][10][11][12]. We previously elucidated the crystal structure of the kinase domain of PASK and revealed its activation loop-phosphorylation independent mode of substrate catalysis [13].…”

“…Additionally, the solution structure of the PAS-A domain of PASK has been determined, revealing its metabolite-binding capacity [14]. Although PASK exhibits modest activity under unstimulated conditions, nutrient signaling enhances PASK catalytic activity via mechanistic Target of Rapamycin (mTOR) complex 1 (mTORC1)-mediated multisite phosphorylation [11]. Notably, mTORC1 phosphorylates the unstructured region between the PAS-A and kinase domains.…”