Specialized cell signaling pathways 2 Dr. Tania A. Shakoori

The Hedgehog Pathway

Holoprosencephaly (HPE) Basal Cell Carcinoma Holoprosencephaly (HPE) refers to a range of problems involving the malformation of the brain and upper face along the mid line. This is extremely rare in live births (~1 in 15,000), but is more commonly detected in early pregnancies (1 in 250); however almost all affected fetuses are miscarried. The cause of these deformities is thought to be due to mutations in the SHH gene Holoprosencephaly (HPE) Medulloblastoma

What’s common? All result from errors of Hedgehog Signaling Pathway key regulator of embryonic development 3 Hedgehog homologues DHH (desert HH) IHH (indian HH) SHH Sonic (SHH) is the best studied

It is translated as a ~45kDa precursor and undergoes autocatalytic processing (Process "1" on Figure 5) to produce an ~20kDa N-terminal signaling domain (referred to as SHH-N) and a ~25kDa C-terminal domain with no known signaling role. SHH can signal in an autocrine fashion, affecting the cells in which it is produced. Secretion and consequent paracrine hedgehog signaling require the participation of Dispatched (DISP) protein (Process "2" on Figure 5). When SHH reaches its target cell, it binds to the Patched-1 (PTCH1) receptor (Process "3" on Figure 5). In the absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the pathway (Process "4"). It has been suggested that SMO is regulated by a small molecule, the cellular localization of which is controlled by PTCH.[16] PTCH1 has homology to Niemann-Pick disease, type C1 (NPC1) that is known to transport lipophilicmolecules across a membrane.[17] PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of SMO activity.[18] A current theory suggests that PTCH regulates SMO by removingoxysterols from SMO. PTCH acts like a sterol pump and removes oxysterols that have been created by 7-dehydrocholesterol reductase.[19] Upon binding of a Hh protein or a mutation in the SSD of PTCH, the pump is turned off allowing oxysterols to accumulate around SMO.

Major Steps Hh ligand precursor Hh ligand (multimeric) released with help of ‘dispatched’ (disp) Binds to ‘patched’ (ptch) on receiving cell Releases smoothened (Smoh) from constitutive inhibition Activation of Gli2/3 complex activation of Gli1 gene transcription

Significance It plays a key role in regulating organogenesis, such as in the growth of digits on limbs and organization of the brain. Sonic hedgehog is the best established example of a morphogen as defined by Lewis Wolpert's French flag model—a molecule that diffuses to form a concentration gradient and has different effects on the cells of the developing embryo depending on its concentration. SHH remains important in the adult. It controls cell division of adult stem cells and has been implicated in the development of some cancers.

video Please Copy paste the following link to get to the video: https://drive.google.com/file/d/0B8lwCh5HN0GTeURlOXpuMW9BM2c/view?usp=sharing

Sonic says: True or false Hh is released with help of ptch? Hh attaches to the receiving cell via disp? Transcription is activated in Hh pathway by ‘Gli’ transcription factors? It maybe involved in holoprosencephaly? In Tumor micro environment Hh secretion is most likely to be inhibited?

Wnt Pathway wingless + Int Dr Tania Shakoori Background and etymology[edit] The discovery of Wnt signaling was influenced by research on oncogenic (cancer-causing) retroviruses. In 1982,Roel Nusse and Harold Varmus infected mice with mouse mammary tumor virus in order to mutate mouse genes to see which genes could cause breast tumors when mutated. They identified a new mouse proto-oncogene that they named int1 (integration 1).[2][6] It was determined that int1 has a high degree of conservation across several species, including humans andDrosophila. Its presence in Drosophila melanogaster led researchers to discover in 1987 that the int1 gene inDrosophila was actually the already known and characterized Drosophila gene known as Wingless (Wg).[2] Since previous research by Christiane Nüsslein-Volhard and Eric Wieschaus (which won them the Nobel Prize in Physiology or Medicine in 1995) had already established the function of Wg as a segment polarity gene involved in the formation of the body axis during embryonic development, researchers determined that the mammalian int1 discovered in mice is also involved in embryonic development.[7] Since int1's discovery in 1982, continued research would lead to the discovery of further genes related to int1; however, since all those genes had not been identified in the same manner as int1, it quickly became clear that the int gene nomenclature, or naming system, was no longer adequate. Thus, the int/Wingless family was renamed the Wnt family and int1 became Wnt1. The name Wnt was chosen because it is a combination, orportmanteau, of int and Wg and stands for Wingless-related integration site.[2

Wnt pathways Types canonical Wnt pathway noncanonical planar cell polarity pathway noncanonical Wnt/calcium pathway

The Canonical Pathway Groucho transcition repressor TCF- transcription factor CBP-CEB binding protein –activates transcrition

Role of Wnt ligand The canonical Wnt pathway (or Wnt/β-catenin pathway) is the Wnt pathway that causes an accumulation of β-catenin in the cytoplasm and its eventual translocation into the nucleus to act as a transcriptional coactivator of transcription factors that belong to the TCF/LEF family. Without Wnt signaling, the β-catenin would not accumulate in the cytoplasm since a destruction complex would normally degrade it. This destruction complex includes the following proteins:  (Axin, adenomatosis polyposis coli (APC), protein phosphatase 2A (PP2A), glycogen synthase kinase 3(GSK3) and casein kinase 1α (CK1α). targeting it for ubiquitination proteasomedigested.

With Wnt ligand Wnt binds Fz and  a co receptor (LRP5/6) the destruction complex function becomes disrupted. translocation of protein Axin, and the destruction complex to the plasma membrane.  Axin becomes de-phosphorylated and its stability and levels are decreased. Dsh then becomes activated and it inhibit the activity of the destruction complex. This allows β-catenin to accumulate and localize to the nucleus and subsequently induce a cellular response via gene transduction alongside other transcription factors.

THINK!! What would happen if one of the proteins of the destruction complex was mutated and not functional???

Roles of the 3 pathways The canonical Wnt pathway  gene transcription the noncanonical planar cell polarity pathway regulates the cytoskeleton that is responsible for the shape of the cell the noncanonical Wnt/calcium pathwayregulates calcium inside the cell

significance Carcinogenesis/tumors Other diseases: Embryogenesis breast and prostate cancer, glioblastoma, Other diseases:  type II diabetes, (role in insulin sensitivity!) and others Embryogenesis

Review video Just copy paste the following link to get to the video: https://drive.google.com/file/d/0B8lwCh5HN0GTYm9renh1NzJUdHc/view?usp=sharing