Cellular Signal Transduction Lab

Cellular signaling is a biological system that cannot be separated from all lives for their development, differentiation, and maintenance. Each signaling pathway is based on kinase cascades composed of numerous proteins, and they affect biological phenomena of cells and the whole organism by regulating their target gene expression. The studies on cellular signaling has been conducted with aims to understand mechanisms for the development of organisms and find ways for the cure and prevention of various diseases.

Among different cellular signaling pathways, we focus on Wnt and Hippo signaling pathways in our laboratory. Wnt and Hippo signaling pathways have been discovered as important factors in developmental stages. However, since their connection with diverse diseases, especially cancer and neuronal diseases, now the Wnt and Hippo signalings are recognized as essential fields for curing the illnesses. The results we have in our lab are expected to give a deep understanding of the pathways, as well as information on the cross-talk between the Wnt and Hippo signalings. We also expect the research we are performing will provide better understanding of basic science, while discovering therapeutic strategies for various diseases.

Contents of Research

Identification of noble signal transduction of Wnt signaling by TFEB

Wnt signaling has been studied as a proto-oncogene that causes cancer initially, and since its role in Drosophila wing development was found, the study on the Wnt signaling has been expanded to broader fields. As Wnt signaling pathway has a critical role in regulating many biological phenomena, the research on the pathway has an importance as a basic study, and also provides therapeutic targets for various diseases. A lot of potential therapeutic targets have been discovered so far. However, since the targets do not have clinical significance yet, more targets should be discovered based on the studies of new biological regulatory mechanisms. To achieve our goals, our team has been conducting different projects funded by national research funds as shown below.

It is well known that β-catenin is a key mediator of Wnt signaling for the regulation of target gene expression. However, we found that transcription factor EB (TFEB) is required for the expression of about 27% of genes that are increased by the treatment of Wnt3a. TFEB is a well-known master regulator of lysosomal biogenesis and autophagy. Under nutrient deprivation condition, TFEB migrates into nucleus and stimulate expression of lysosomal genes. We found that the target genes whose expressions are regulated by Wnt/TFEB are different from the genes that are involved in lysosomal biogenesis. More data for the regulation of TFEB nuclear localization and activation of target genes will be discussed. If our data are really true, the terminology “Wnt/β-catenin signaling” should be changed to “Wnt/β-catenin-TFEB signaling”. Currently, we are trying to confirm whether our biochemical findings have physiological relevance using in vivo mice model system. Since many Wnt/TFEB target genes encode proteins for ion channels, we hypothesize that the neuronal degeneration caused by mis-regulation of Wnt signaling may be due to lower or higher expression of ion channels.

2. Studying cross-talks with Wnt and Hippo signaling and providing therapeutic targets

Controlled cell growth and proliferation are essential for tissue homeostasis and development. Wnt and Hippo signaling are well known as positive and negative regulators of cell proliferation, respectively. The regulation of Hippo signaling by the Wnt pathway has been shown, but how and which components of Wnt signaling are involved in the activation of Hippo signaling are unknown.

Merlin has been known to be the causative gene in Neurofibromatosis type II(NFII), though through what mechanism cancer is induced remains to be discovered. Merlin was also found to be an upstream regulator in Hippo signaling pathway. Our team found that Merlin binds with Wnt signaling co-receptor LRP6, leading to the inhibition of Wnt pathway and found that the key molecule of Wnt signaling β-catenin level being upregulated in the tissues of NFII patients, not the key molecule of Hippo signaling (YAP). We focus on enlightening the cross-talk between Wnt and Hippo signaling for the discovery of potential therapeutic targets of such diseases.

In recent years, we discovered that the Wnt co-receptor LRP6 controls Hippo signaling in reaction to the metabolic status of a cell, and are conducting relevant experiments. We found that a reduction in the level of low-density lipoprotein receptor-related protein 6 (LRP6) during nutrient starvation induces phosphorylation and cytoplasmic localization of YAP, inhibiting YAP-dependent transcription. Phosphorylation of YAP via loss of LRP6 is mediated by large tumor suppressor kinases 1/2 (LATS1/2) and Merlin. We found that O-GlcNAcylation of LRP6 was reduced and the overall amount of LRP6 was decreased via endocytosis-mediated lysosomal degradation during nutrient starvation. Merlin binds to LRP6; when LRP6 is less O-GlcNAcylated, Merlin dissociates from it and becomes capable of interacting with LATS1 to induce phosphorylation of YAP. Our data suggest that LRP6 has unexpected roles as a nutrient sensor and Hippo signaling regulator. Currently we are trying to identify mechanisms for the regulation of O-GlcNAcylation of LRP6 during starvation and other stress conditions.

3. The discovery of novel components and their function in Hippo Signaling Pathway

Hippo pathway is a signaling pathway that controls organ sizes and cellular proliferation rate through a kinase cascade with various core components. When the pathway is inactivated, the transcriptional co-activator YAP stays dephosphorylated and localizes to the nucleus, thereby binding to the transcription factor TEAD and leads to the expression of cellular proliferation-related genes. On the contrary, when Hippo pathway is activated, kinases called MST and LATS are activated, phosphorylating and sequestering YAP in the cytoplasm through its binding with 14-3-3. No proliferation-related gene expressed in this case. It has been shown that YAP levels are enriched in many cancers, hence enhanced cellular proliferation. Therefore, we believe proper regulation of Hippo pathway is critical in inhibiting cancer formation and proliferation, and we expect to provide more effective therapeutic strategies in cancers caused by abnormal regulation of the pathway.

In our lab, we aim to discover noble components of Hippo pathway and apply them to cancer therapy as novel therapeutic targets. Recently, we found a kinase called NLK(Nemo-like kinase) that phosphorylates S128 residue of YAP, thereby preventing S127 phosphorylation, which leads to YAP sequestration in the nucleus. We also discovered deubiquitinase YOD1, that stabilizes ITCH, an E3 ligase that ubiquitinates LATS, a core kinase of Hippo pathway. The stabilization of ITCH by YOD1 increases LATS degradation, leaving YAP unphosphorylated, and eventually increases cellular proliferation and liver size.

YAP ultimately increases target gene expression, therefore, controlling its nuclear localization is essential for the regulation of Hippo signaling. We have identified MAML1/2 as critical regulators for YAP/TAZ nuclear retention and transcription activities. Clinical analysis with specimens of a human cancer patient and a public cancer database reveals pathological association between MAML expression and YAP signature. Our findings provide mechanistic insights of YAP/TAZ-dependent growth control pathway and tumorigenesis. Currently, we are trying to identify mechanisms how transcriptional activation and chromatin remodeling by MAML1/2 are modulated by using whole genome analysis.

4. Verifying Mest/Peg1 loss-of-imprinting as a causative mechanism of Alzheimer’s disease

South Korea is a rapidly aging society along with the increase of elderly population, and the rate of occurrence rate of patients with Alzheimer’s disease is increasing as well. If Alzheimer’s disease can be detected and diagnosed in early stages, it is possible to delay its progression significantly. So for the early detection, studies on the discovery of biomarkers of Alzheimer’s disease has been conducted through decades in developed countries. To this day, the biomarkers for Alzheimer’s disease have revolved and developed around the amyloid and Tau hypothesis, and their relevant regulators. However, all novel medications for Alzheimer’s disease developed based on these approaches have failed. Therefore, it has been suggested that it is necessary for the old paradigm based on neuron virulence/defense/death-regulatory factors, to be dismantled.

Mammals are inherited with a pair of alleles from their parents, and in most cases, it does not matter from which parent they received them. However, in genomic imprinting, from which parent one had received the allele determines what gene is expressed or not. According to the research of Cui et al. (2003), through aging, the imprinting that has been regulating the expression of IGF is lost (loss of imprinting), thereby increasing IGF expression and ultimately leading to colon cancer. Our team has found that an imprinted gene Mest/Peg1 regulates the modification of a Wnt signaling co-receptor LRP6 and inhibits Wnt signaling (Jung et al. 2011). Mutation of LRP6 gene was found to be related with late-onset of Alzheimer’s disease, and it was proposed that it would act synergistically with the carrier status of APOE ε4 gene, causing Wnt signaling pathway abnormalities and associated neural degeneration (De Ferrari et al. 2007).

From the former research mentioned above, we are trying to elucidate whether the misregulation of Mest/Peg1 imprinting is involved in neuro-degeneration and the pathogenesis of Alzheimer’s disease. We incorporate genome editing techniques using the CRISPR-Cas9 system to prove our hypothesis and develop animal models for dementia research. Currently, we are setting up collaboration with clinical doctors who maintain cohorts of Alzheimer’s disease patients to translate our findings to clinical sides.