Wnt/β-catenin signaling in midbrain dopaminergic neuron specification and neurogenesis

Loss of midbrain dopaminergic （mDA） neurons underlies the motor symptoms of Parkinson＇s disease. Towards cell replacement, studies have focused on mechanisms underlying embryonic mDA production, as a rational basis for deriving mDA neurons from stem cells. We will review studies of [3-catenin, an obligate component of the Wnt cascade that is critical to mDA specification and neuro- genesis, mDA neurons have a unique origin--the midbrain fLoor plate （FP）. Unlike the hindbrain and spinal cord FP, the midbrain FP is highly neurogenic and Wnt/β-catenin signaling is critical to this difference in neurogenic potential. In β-catenin loss-of-function experiments, the midbrain FP resembles the hindbrain FP, and key mDA progenitor genes such as Otx2, Lmxlo, MsxJ, and Ngn2 are downregulated whereas Shh is maintained. Accordingly, the neurogenic capacity of the midbrain FP is diminished, resulting in fewer mDA neurons. Conversely, in β-catenin gain-of.function experiments, the hindbrain FP expresses key mDA progenitor genes, and is highly neurogenic. Interestingly, when excessive β-catenin is supplied to the midbrain FP, less mDA neurons are produced sug- gesting that the dosage ofWntJ β-catenin signaling is critical. These studies of β-catenin have facilitated new protocols to derive mDA neurons from stem cells.     

nhe regulation and deregulation of Wnt signaling by PARK genes in health and disease

Wingless/Int （Wnt） signaling pathways are signal transduction mechanisms that have been widely studied In the field of embryogen- esis. Recent work has established a critical role for these pathways in brain development, especially of midbrain dopaminergic neu- rones, However, the fundamental importance of Wnt signaling for the normal function of mature neurones in the adult central nervous system has also lately been demonstrated by an increasing number of studies. Parkinson＇s disease （PD） is the second most prevalent neurodegenerative disease worldwide and is currently incurable. This debilitating disease is characterized by the progres- sive loss of a subset of midbrain dopaminergic neurones in the substontla nigm leadingto typical extrapyramidal motor symptoms. The aetiology of PD is poorly understood but work performed over the Last two decades has identified a growing number of genetic defects that underlie this condition. Herewe review a growing body of data connecting genes implicated in PD--most notablythe PARKgenes-- with Wnt signaling. These observations provide clues to the normal function of these proteins in healthy neurones and suggest that deregulated Wnt signaling might be a frequent pathomechanlsm leading to PD. These observations have implications for the patho- genesis and treatment of neurodegenerative diseases in general.     

Wnt signaling control of bone cell apoptosis

Wnts are a large family of growth factors that mediate essential biological processes like embryogenesis, morpho- genesis and organogenesis. These proteins also play a role in oncogenesis, and they regulate apoptosis in many tissues. Wnts bind to a membrane receptor complex comprised of a frizzled （FZD） G-protein-coupled receptor and a low-density lipoprotein （LDL） receptor-related protein （LRP）. The formation of this ligand-receptor complex initiates a number of signaling cascades that include the canonical/beta-catenin pathway as well as several noncanonical pathways. In recent years, canonical Wnt signaling has been reported to play a significant role in the control of bone formation. Clinical studies have found that mutations in LRP-5 are associated with reduced bone mineral density （BMD） and fractures. Investigations of knockout and transgenic mouse models of Wnt pathway components have shown that canonical Wnt signaling modulates most aspects ofosteoblast physiology including proliferation, differentiation, function and apoptosis. Transgenic mice expressing a gain of function mutant of LRP-5 in bone, or mice lacking the Wnt antagonist secreted frizzled-related protein-l, exhibit elevated BMD and suppressed osteoblast apoptosis. In addition, preclinical studies with pharmacologic compounds such as those that inhibit glycogen synthase kinase-3β support the importance of the canonical Wnt pathway in modulation of bone formation and osteoblast apoptosis.     

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells （HSCs） are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal （in which the ability to function as HSCs is retained） and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the ＂niche＂, focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.     

Dynamic expression of Wnt signaling molecules and stem-cell marker CK15 in the growth cycle of rat whisker hair follicles

Objective To investigate the distribution and dynamic changes of both Wnt signaling molecules and CK15 throughoutthe three phases of the follicular cycle,and to explore the relationship between Wnt/β-catenin signaling and CK15 in rat whisker hair follicle(HF)growth cycles.Methods Hematoxylin-Eosin(HE)and immunofluorescence stains were used to characterize the expression patterns,including sites and levels of some representative proteins of both canonical and non-canonical Wnt signaling molecules,as well as HF epithelial stem cell marker CK15.Results The expression patterns of bothβ-catenin and Wnt5a were correlated with that of CK15.CK15 was only expressed in anagen.In catagen,β-catenin showed a massive depletion while Wnt5a noticeably increased.In telogen,high level expression ofβ-catenin and low level of Wnt5a were detected.Wnt10b and TCF3 were detected during the entire HF growth cycle.Conclusion These results suggest that Wnt5a is associated with the transition of anagen-catagen phase,accompanied by broad deletion ofβ-catenin and loss of CK15.WntlOb is important for the maintenance of HF activity and is related to the telogenanagen transition.     

Regulating Wnt signaling： a strategy to prevent neurodegeneration and induce regeneration

During the past three decades, Wingtess/Int （Wnt）signaling has emerged as an essential regu{ator crucial for neuronal development and maintenance （Inestrosa and Arenas, 201_0）. In addition, Wnt signal- ing was recently shown to be involved in the regula- tion of synaptic function and plasticity, which is critical for learning and memory （Oliva et aL, 2013）. Deregulation of Wnt signaling has been proposed as a key contributor to the pathogenesis of neurode- generative disorders including Alzheimer＇s disease （AD） and Parkinson＇s disease （PD）. This increasing knowledge of the specific roles of Wnt signaling cascades during different stages of life has suggested innovative therapeutic strategies for the treatment of neurodegenerative diseases.     

DVL mutations identified from human neural tube defects and Dandy-Walker malformation obstruct the Wnt signaling pathway

Wnt signaling pathways,including the canonical Wnt/β-catenin pathway,planar cell polarity pathway,and Wnt/Ca~(2+) signaling pathway,play important roles in neural development during embryonic stages.The DVL genes encode the hub proteins for Wnt signaling pathways.The mutations in DVL2 and DVL3 were identified from patients with neural tube defects(NTDs),but their functions in the pathogenesis of human neural diseases remain elusive.Here,we sequenced the coding regions of three DVL genes in 176 stillborn or miscarried fetuses with NTDs or Dandy-Walker malformation(DWM) and 480 adult controls from a Han Chinese population.Four rare mutations were identified:DVL1 p.R558 H,DVL1 p.R606 C,DVL2 p.R633 W,and DVL3 p.R222 Q.To assess the effect of these mutations on NTDs and DWM,various functional analyses such as luciferase reporter assay,stress fiber formation,and in vivo teratogenic assay were performed.The results showed that the DVL2 p.R633 W mutation destabilized DVL2 protein and upregulated activities for all three Wnt signalings(Wnt/β-catenin signaling,Wnt/planar cell polarity signaling,and Wnt/Ca~(2+) signaling) in mammalian cells.In contrast,DVL1 mutants(DVL1 p.R558 H and DVL1 p.R606 C) decreased canonical Wnt/β-catenin signaling but increased the activity of Wnt/Ca~(2+)signaling,and DVL3 p.R222 Q only decreased the activity of Wnt/Ca~(2+) signaling.We also found that only the DVL2 p.R633 W mutant displayed more severe teratogenicity in zebrafish embryos than wild-type DVL2.Our study demonstrates that these four rare DVL mutations,especially DVL2 p.R633 W,may contribute to human neural diseases such as NTDs and DWM by obstructing Wnt signaling pathways.     

Wnt signaling in the nervous system and in Alzheimer＇s disease

Wnts comprise a large family of proteins that have shown to be part of a signaling cascade that regulates several aspects of develop- ment including organogenesis, mid brain development as welt as stem cell proliferation. Wnt signaling pathway plays different roles in the development of neuronal circuits and also in the adult brain, where it regulates synaptic transmission and plasticity. It has been also implicated in various diseases including cancer and neurodegenerative diseases, reflecting its relevance in fundamental biological pro- cesses. This review summarizes the progress about Wnts function in mature nervous system with a focus on Alzheimer＇s disease （AD）. We discuss the prospects of modulating canonical and non-canonical Wnt signaling as a strategy for neuroprotection. This will include the potential of Wnts to： （i） act as potent regulators of hippocampai synapses and impact in learning and memory; （ii） regulate adult neurogenesis; and finally （iii） control AD pathogenesis.     

Anterograde and Retrograde Regulation of Nuclear Genes Encoding Mitochondrial Proteins during Growth,Development, and Stress

Mitochondrial biogenesis and function in plants require the expression of over 1000 nuclear genes encoding mitochondrial proteins （NGEMPs）. The expression of these genes is regulated by tissue-specific, developmental, internal, and external stimuli that result in a dynamic organelle involved in both metabolic and a variety of signaling processes. Although the metabolic and biosynthetic machinery of mitochondria is relatively well understood, the factors that regu- late these processes and the various signaling pathways involved are only beginning to be identified at a molecular level. The molecular components of anterograde （nuclear to mitochondrial） and retrograde （mitochondrial to nuclear） signaling pathways that regulate the expression of NGEMPs interact with chloroplast-, growth-, and stress-signaling pathways in the cell at a variety of levels, with common components involved in transmission and execution of these signals. This positions mitochondria as important hubs for signaling in the cell, not only in direct signaling of mitochondrial function per se, but also in sensing and/or integrating a variety of other internal and external signals. This integrates and optimizes growth with energy metabolism and stress responses, which is required in both photosynthetic and non-photosynthetic cells.     

Dysfunction of Wnt signaling and synaptic ：lisassembly in neurodegenerative diseases

The molecular mechanisms that regulate synapse formation have been well documented. However, little is known about the factors that modulate synaptic stability. Synapse loss is an early and invariant feature of neurodegenerative diseases including Alzheimer＇s lAD） and Parkinson＇s disease. Notably, in AD the extent of synapse loss correlates with the severity of the disease. Hence, understanding the molecular mechanisms that underlie synaptic maintenance is crucial to reveal potential targets that will allow the development of ther- apies to protect synapses. Writs play a central role in the formation and function of neuronal circuits. Moreover, Wnt signaling compo- nents are expressed in the adult brain suggesting their role in synaptic maintenance in the adult. Indeed, blockade of Wnts with the Wnt antagonist Dickkopf-1 （Dkkl） causes synapse disassembly in mature hippocampal cells. Dkkl is elevated in brain biopsies from AD patients and animal models. Consistent with these findings, Amyloid-β （Aβ） oUgomers induce the rapid expression of Dkkl. Importantly, Dkkl neutralizing antibodies protect synapses against Aβ toxicity, indicating that Dkkl is required for Aβ-mediated synapse loss. In this review, we discuss the role of Wnt signaling in synapse maintenance in the adult brain, particularly in relation to synaptic loss in neurodegenerative diseases.     

Targeting stem cell signaling pathways for drug discovery: advances in the Notch and Wnt pathways

Signaling pathways transduce extracellular stimuli into cells through molecular cascades to regulate cellular functions.In stem cells,a small number of pathways,notably those of TGF-?/BMP,Hedgehog,Notch,and Wnt,are responsible for the regulation of pluripotency and differentiation.During embryonic development,these pathways govern cell fate specifications as well as the formation of tissues and organs.In adulthood,their normal functions are important for tissue homeostasis and regeneration,whereas aberrations result in diseases,such as cancer and degenerative disorders.In complex biological systems,stem cell signaling pathways work in concert as a network and exhibit crosstalk,such as the negative crosstalk between Wnt and Notch.Over the past decade,genetic and genomic studies have identified a number of potential drug targets that are involved in stem cell signaling pathways.Indeed,discovery of new targets and drugs for these pathways has become one of the most active areas in both the research community and pharmaceutical industry.Remarkable progress has been made and several promising drug candidates have entered into clinical trials.This review focuses on recent advances in the discovery of novel drugs which target the Notch and Wnt pathways.     

Preventing aging with stem cell rejuvenation: Feasible or infeasible?

Characterized by dysfunction of tissues, organs, organ systems and the whole organism, aging results fromthe reduced function of effective stem cell populations. Recent advances in aging research have demonstrated that old tissue stem cells can be rejuvenated for the purpose of maintaining the old-organ function by youthful re-calibration of the environment where stem cells reside. Biochemical cues regulating tissue stem cell function include molecular signaling pathways that interact between stem cells themselves and their niches. Historically, plasma fractions have been shown to contain factors capable of controlling age phenotypes; subsequently, signaling pathways involved in the aging process have been identified. Consequently, modulation of signaling pathways such as Notch/Delta, Wnt, transforming growth factor-β, JAK/STAT, mammalian target of rapamycin and p38 mitogen-activated protein kinase has demonstrated potential to rejuvenate stem cell function leading to organismic rejuvenation. Several synthetic agents and natural sources, such as phytochemicals and flavonoids, have been proposed to rejuvenate old stem cells by targeting these pathways. However, several concerns still remain to achieve effective organismic rejuvenation in clinical settings, such as possible carcinogenic actions; thus, further research is still required.     

Emerging roles of transforming growth factor β signaling in wet age-related macular degeneration

Age-related macular degeneration (AMD) is one of the major causes of irreversible blindness among aging populations in developed countries and can be classified as dry or wet according to its progression.Wet AMD,which is characterized by angiogenesis on the choroidal membrane,is uncommonly seen but more severe.Controlling or completely inhibiting the factors that contribute to the progression of events that lead to angiogenesis may be an effective strategy for treating wet AMD.Emerging evidence has shown that transforming growth factor-β(TGF-β) signaling plays a significant role in the progression of wet AMD.In this review,we described the roles of and changes in TGF-β signaling in the development of AMD and discussed the mechanisms of the TGF-β superfamily in choroidal neovascularization (CNV) and wet AMD,including the modulation of angiogenesis-related factors,inflammation,vascular fibrosis,and immune responses,as well as cross-talk with other signaling pathways.These remarkable findings indicate that TGF-β signaling is a potential target for wet AMD treatment.