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Proto-oncogene protein Wnt-1 protein, short for Wingless-Type MMTV Integration Site Family Member 1, is encoded by the Wnt-1 gene. ^[1] Wnt-1 is part of the Wnt-gene family, together forming a highly conserved secreted signaling pathway that regulate cell-to-cell interaction during embryogenesis. ^[2]

Wnt-1 is a secreted glycoprotein, usually around 350-400 amino acids in length with conserved pattern of 23-25 cysteine residues across species. ^[3]

During embryonic development, Wnt-1 plays a role in generation of cell polarity, body axis formation, ^[4] and development of the central nervous system. ^[5] It also has a role in the regeneration of tissues in adult organisms. ^[6] A mis-expression of Wnt-1 is correlated with tumorogenesis. ^[3]

Wnt signals through cell-surface receptors in the Frizzled and LRP families. ^{[7] [2]} Binding of Wnt protein to cell-surface receptor triggers a signaling cascade involving intracellular signaling protein beta-catenin, which leads to the increased expression of 50+ genes. ^[7]

Wnt-1 protein can be referred to as Int-1 or Wg in literature, varying by the organism studied and the time of research. Int-1 (Integration site-1) was first described by Nusse and Varmus in 1982 as the proto-oncogene activated by mouse mammary tumor virus (MMTV). ^[8] In 1976, the ortholog gene in Drosophila melanogaster was described as Wg (Wingless), from the wingless phenotype of mutants. ^[9] To reduce confusion, Int-1 and Wg were combined and renamed as Wnt-1 around the 1990s. ^[10]

Wnt-1 is required for proper formation of the midbrain and anterior hindbrain in the mouse. ^[5] Mutations of the Wnt-1 gene in embryonic mice results in a wide range of phenotypes: some die at birth lacking an entire cerebellum and parts of the midbrain, while some live through adulthood suffering from ataxia and lacking the anterior half of the cerebellum. ^[5] The malformed parts of the brain in Wnt-1 mutants correspond to not only the region where Wnt-1 would normally be highly expressed, but also the surrounding tissues. ^[11] This suggests Wnt-1 protein’s importance in inducing development of surrounding tissues. ^[12] No explanation for the variability of Wnt-1 mutant phenotype expressed is known, but it may be due to several factors, including genetic background. ^[13]

The role of Wnt-1 protein in body-axis formation was studied in the model organism Xenopus laevis. The injection of Wnt-1 mRNA into early Xenopus embryo results in duplicated dorsal axis. This suggests the role of Wnt-1 as an organizer molecule and embryonic signal inducer for pattern formation. ^[14]

Wnt proteins are involved in a multitude of pathways. In most pathways, Wnt is found to interact directly with transmembrane proteins of the Frizzled family. ^[15] Wnt1 is specifically found to interact with Fzd8. The binding of Wnt to Fzd8 requires a dimerization of two CRD domains of Fzd. ^[16]

In the absence of Wnt, β-catenin is phosphorylated and marked for degradation by ubiquitin mediated proteosomes. ^[17] This phosphorylation is catalyzed by serine/threonine glycogen synthase kinase-3beta (GSK-3β) in complex with casein kinase 1 (CK1), the adenomatous polyposis coli (APC) protein, axin and protein phosphatase 2A (PP2A). ^{[15] [17]}This process of phosphorylation by GSK-3 β occurs at serines-33 and -37, as well as at serine-45. ^[18] The process is continued by β-TrCP in conjunction with ubiquitin ligase in order to mark the protein with ubiquitin for degradation. ^[19]

A change in the cytoplasmic conformation of the proteins occurs when Wnt binds to the Fzd/LRP complex, resulting in the binding of Axin to LRP and the binding of the Dishevelled protein (Dsh) to Fzd. ^[20] Other transmembrane proteins found to interact with Fzd8 and assist in the signal transmission across the membrane are two low density lipoprotein receptor-related proteins, LRP5 and LRP6. ^{[20] [15]}The deactivation of GSK-3β is coupled to the interaction between axin and dishevelled (DIX) domain, thus preventing the formation of the axin dependant phosphorylation complex. ^{[21] [22]} This prevents phosphorylation of β-catenin and allows for its accumulation.

In the nucleus without β-catenin, two transcription factors, T-cell factor (TCF) and Lymphoid enhancer factor (LEF) in conjugation with the protein Groucho, act as inhibitors of Wnt controlled genes. ^[17] Once build up of β-catenin occurs, it acts to from a complex with TCF/LEF. The complex then acts to induce the same genes that TCF/LEF inhibited in the absence of β-catenin, specifically those relating to cell growth and differentiation. ^[17]

The two major non-canonical pathways are the planar cell polarity (PCP) pathway and the Wnt/Ca2+ pathway. Both of these pathways are also reliant on the Frizzled transmembrane protein. ^[17] It is suggested that the activation of these alternate pathways is dependent on the specific Fzd related membrane proteins present, such as LRP5 or LRP6. The Wnt/Ca2+ pathway is reliant on a reaction between dishevelled protein and Fzd, causing the activation of a G-protein coupled signal transduction cascade.The ultimate release of calcium from this mechanism activates calcium/calmodium-dependent protein kinase II (CaMKII). ^[17] In developmental processes, the release of such kinases results in a decrease in the effect of the canonical beta catenin pathway, and plays a role in cell migration and axis development. Planar Cell Polarity (PCP) is a pathway involving many of the same proteins such as Fzd and Dishevelled , but primarily plays a role in altering cell morphology. ^[23] This pathway involves the activation of Rho- and Rac-GTPases at the membrane, followed by the activation of different Myosins and Kinases which alter cell structure. ^{[15] [23]}

Wnt-1 mis-expression are of clinical significance since mutations are associated chronic diseases.

An overexpression of Wnt-1 is correlated with tumorogenesis. ^[3] Wnt-1 was first identified in 1982 by Nesse and Varmus as a proto-oncogene activated by integration of mouse mammary tumor virus (MMTV). ^[8] Today, the mis-expression of Wnt-1 is correlated with diseases such as oral squamous cell carcinoma (OSCC), ^[24] breast tumours in human breast cancer ^[25] (19), non-small cell lung cancer (NSCLC), ^[26] and gastric cancer. ^[27]

The up-regulation of Wnt proteins can enhance the process of osteocyte proliferation. ^[28] Injecting mice with purified Wnt3a protein resulted in proliferation of liposomes at the sites of bone damage. This not only suggests an additional area of therapeutic research using Wnt pathways, but also the potential positive effects of up-regulated Wnt protein expressions in tissue/organ regeneration. Wnt-1 over-expression can suppress the lymphangiogenesis and potentiates the delay of metastasis in melanoma. ^[29] In addition, Wnt-1 expression has shown to aid the restoration of cardiac functions by up-regulating the formation of new cardiac fibroblast cells, along the Wnt-1/β-catenin pathway in the cases of acute cardiac ischemia. ^[30]

Inhibition studies of the Wnt-gene family were long focused due to their cancer-causing properties when over-expressed. Over-expression of Wnt-1 has demonstrated a strong causational relationship to the formation of mammary gland tumor, ^[25] a deeper understanding of the specific Wnt-1 pathway in relevant organs will potentially bring a more effective treatment therapy for breast cancer.

The highly conserved nature of Wnt gene throughout evolution creates an opportunity for research to take place in multiple model organisms, including worms, flies, and mice. ^{[31] [2]} The human Wnt-1 protein sequence is 99% identical to that of the mouse homologue. ^[32] Wnt-1 is usually around 350-400 amino acids in length with conserved pattern of 23-25 cysteine residues across species. ^[3]

Evolutionary biologists speculate the early amplification and divergence of the Wnt family were the cause for increasing complexity of animal body plans. Wnt-1 has been found conserved throughout the entire animal kingdom, often expressed in regionalized patterns. ^[33]