A truncated Wnt7a retains full biological activity in skeletal muscle

Abstract

Wnt signaling has essential roles during embryonic development and tissue homoeostasis. Wnt proteins are post-translationally modified and the attachment of a palmitate moiety at two conserved residues is believed to be a prerequisite for the secretion and function of Wnt proteins. Here we demonstrate that a mammalian Wnt protein can be fully functional without palmitoylation. We generate a truncated Wnt7a variant, consisting of the C-terminal 137 amino acids lacking the conserved palmitoylation sites and show that it retains full biological activity in skeletal muscle. This includes binding to and signaling through its receptor Fzd7 to stimulate symmetric expansion of satellite stem cells by activating the planar-cell polarity pathway and inducing myofibre hypertrophy by signaling through the AKT/mTOR pathway. Furthermore, this truncated Wnt7a shows enhanced secretion and dispersion compared with the full-length protein. Together, these findings open important new avenues for the development of Wnt7a as a treatment for muscle-wasting diseases and have broad implications for the therapeutic use of Wnts as biologics.

Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms

“…Thus, current evidence strongly suggests that canonical Wnt/β-catenin signaling plays a significant role in mechanically-induced skeletal muscle growth. More recently, evidence has also demonstrated a link between Wnt signaling, muscle hypertrophy and the activation of mTORC1 signaling [218, 219]. Specifically, the overexpression of Wnt7a (but not Wnt 3a or 5a) induced hypertrophy of muscle fibers in vivo, and in myotubes that was independent of myoblast cell proliferation, differentiation and fusion [218].…”

Wnt signaling in bone and muscle

“…Recently, however, one of the authors of this review developed a variant of Wnt7a that is comprised of the final carboxyl-terminal 137 amino acids (Wnt7aCT), and entirely lacks lipidation sites [103]. Wnt7aCT exhibits similar or superior effect on muscle regeneration relative to full length Wnt7a, is readily expressed to high levels, and exhibits improved dispersal in tissue, and bioavailability.…”

Wnt/β-catenin signaling via Axin2 is required for myogenesis and, together with YAP/Taz and Tead1, active in IIa/IIx muscle fibers

“…C2C12 and primary skeletal myoblasts were transiently transfected by nucleofection according to the manufacturer's instructions (Nucleofector, Lonza). Single myofibers were isolated from the extensor digitorum longus muscle by collagenase digestion and cultured as described elsewhere (von Maltzahn et al, 2013). Culture conditions and transfection protocols are provided in the supplementary Materials and Methods.…”

“…In adult muscle, Wnt proteins are expressed upon regeneration (Polesskaya et al, 2003;von Maltzahn et al, 2012). Non-canonical Wnt signaling through Wnt7a regulates satellite cell number and the size of skeletal muscle (Le Grand et al, 2009;von Maltzahn et al, 2013). Generally, non-canonical Wnt signaling is required for skeletal muscle development, while canonical Wnt signaling, especially via Wnt3a, has been demonstrated to lead to increased fibrosis (Brack et al, 2007).…”

“…In contrast to canonical Wnt signaling, non-canonical Wnt signaling does not require the activity of β-catenin. Examples of non-canonical Wnt signaling pathways in skeletal muscle are the planar cell polarity (PCP) and the AKT/mTOR (Bentzinger et al, 2014;Le Grand et al, 2009;von Maltzahn et al, 2013) pathways.…”