ubiquitin and human pa tho genesis

8/3/2019 Ubiquitin and Human Pa Tho Genesis

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UBIQUITIN AND HUMAN

PATHOGENESIS

Presenter : Nikunj Raninga


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Due to many important roles of ubiquitin, itsobvious to have disease when there are aberrationsin system.

Inactivation of a major enzyme such as E1 is lethal.

Mutations in enzymes or in recognition motifs insubstrates that do not affect vital pathways or thataffect the involved process only partially may result

in a broad array of phenotypes.

Aquired changes in UPS also lead to variouspathologies.


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The pathological states associated with theubiquitin system can be classified into

(a) those that result from loss of function mutation in

a ubiquitin system enzyme or in the recognition motifin the target substrate that lead to stabilization ofcertain proteins, and

(b) those that result from gain of function - abnormal

or accelerated degradation of the protein target Studies has been acquired by inactivating gene of

particular pathway enzyme of ubiquitin pathway


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Better understanding of the processes andidentification of the components involved in thedegradation of key regulatory proteins will lead

to the development of mechanism-based drugsthat will target specifically only the involvedproteins.

Many diseases are related to aberrations in the

ubiquitin system like malignancies, syndromerelated to membrane proteins, neurologicaldisorders, etc.


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MALIGNANCIES

Specific cancers can result from stabilization ofoncoproteins/growth promoting factors, ordestabilization of tumor suppressors.

Oncoproteins, if not properly removed from the cell,can induce malignant transformation.

Ubiquitin targets N-myc, c-Myc, c-Fos, c-Jun, Src,and the adenovirus E1A proteins.

Destabilization of tumor suppressor proteins such asp53 and p27 has also been implicated in thepathogenesis of malignancies.


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1. p53

It was noted that the level of the tumor suppressorprotein p53 is extremely low in uterine cervical carcinomatumors caused by high-risk strains of the HumanPapillomavirus (HPV)

Studies shown that the suppressor is targeted forubiquitin-mediated degradation by the HPV oncoproteinE6 coded by high risk strains of the virus

Low risk strains that encode slightly different E6 proteins(such as E611) do not transform cells and do not target

p53 for degradation .

Degradation is mediated by the HECT domain E3 enzymeE6-Associated Protein, E6-AP, where E6 serves as anancillary protein that allows recognition of p53


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E6 bring E3 and substrate to close proximity forligation

In normal conditions also, p53 is targated but by Mdm2

ligase but p53 gets stabilized when there isDNAdamage

In certain tumors it was found that Mdm2 isoverexpressed, often as the result of gene amplification.

It is possible that removal of p53 in these cells plays arole in their transformation


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2. p27 Apart from p53, signalling molecules like p27

which regulates (arrests) cell cycle

Its decreased level has been reported in breastcancers and others

Detailed mechanistic analysis revealed that thelow level of p27 correlates directly with increasedlevel of Skp2, the protein involved in p27ubiquitination, and overexpression of Skp2 inexperimental animals is oncogenic


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Other protein pathogenesis associated withmalignancies

b-Catenin

c-Jun

pVHL

c-Cbl and Hakai


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MEMBRANE PROTEINS DISORDERS

1. Liddles syndrome :

Aberration in the regulation of the kidney EpithelialNa+ Channel (ENaC)

Channel-composed of three homologous subunits a, hand g, is a short-lived protein complex that is degradedby the ubiquitin system and is recognized via a proline-rich (PY) motif by NEDD4, a member of the HECT

domain family of E3 Binding of NEDD4 is mediated via a WW domain in the

ligase


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A mutation in the PY motif leads to stabilization of

the channel complex & it does not associate anymorewith the ligase and so there is accumulation of itssubunits

Result- excessive reabsorption of Na+ and H2O, and,

consequently, a severe form of early-onsethypertension

An interesting finding relates to the regulation of thedegradation pathway of ENaC

NEDD4

-2 is phosphorylated by the aldosterone-inducedSgk1 kinase on specific Ser residues.

This phosphorylation weakens the interaction betweenNedd4-2 and ENaC, leading to elevated ENaC cell surfaceexpression.


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NEURODEGENERATIVE DISEASES

Accumulation of ubiquitin conjugates and/or inclusionbodies associated with ubiquitin, proteasome, andcertain disease-characteristic proteins has beenreported in a broad array of chronic neurodegenerative

diseases, such as the neurofibrillary tangles ofAlzheimers disease (AD), brainstem Lewy bodies(LBs)the neuropathological hallmark in Parkinsonsdisease (PD), LBs in LB dementia

Also found in nuclear inclusions in CAG repeatexpansion (polyglutamine extension) disorders such asHuntingtons disease (HD), Spinoccerebellar Ataxias(SCAs), and Spinobulbar Muscular Atrophy (KennedysSyndrome)


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In these diseases, direct link between ubiquitin anddisease is not established

Even though there may be defect related to thesesyndrome, this may be secondary effect and notprimary

In some cases there is accumulation of inclusionbodies(in Lewis syndrom) which is unsuccessfullytried to be removed by ubiquitin proteasomal system

Initial hypothesis-inclusion bodies are generatedbecause of the inherent tendency of the abnormalproteins to associate with one another and aggregate

Now -the process maybe more complex and invol-

ves active cellular machineries


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Emerging concept The sequestration of theaggregated proteins from the cytosol andnucleoplasm and their concentration in definedinclusion bodies separates them from sensitivecellular machineries, such as the transcriptionalapparatus, and is therefore protective

Recent findings suggests that aggregated proteinscan inhibit the ubiquitin system


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Some neurodegenerative diseasesinvolving aberrations in UPS :

Parkinsons disease

Alzheimers disease

Angelman syndrome


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IMMUNE AND INFLAMMATORY

RESPONSE

Peptide epitopes on CTLs on class-I MHC moleculesare generated by limited processing of antigenprocessing by Ub-proteasomal pathway

Gamma IFN stimulates ag presentation & leads toinduction & exchange of 3 proteasomal subunitsand result in alterations in cleavage sitepreferances

This change in cleavage generates peptides thatterminate with hydrophobic and basic amino acidresidues and are presented on MHC-1

Ub system degrades them in non discreminatemanner -both self and nonself(e.g. viral) protein


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Among them self antigen doesnt create immune response

But alteration in cleavage site presentation ofdifferently cleaved protein self protein recognized asforeign Generation of immune response Autoimmunediseases

Immune disorders are also caused by other way likedeficiency in activation of immune system keytranscription factor like NF-kB by Uq via two stepactivation mechanism

It leads to the transcription of various cytokines, adhesionmolecules, inflammatory responses and stress proteins &immune system receptors.

Mutation related to these proteins can cause variousserious disorders like incontinentia pigmenti,hypohidrotic/anhidrotic extodermal dysplasia, but also, asexpected, immune deficiency


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Some viruses have evolved the mechanism for theirproteins to be escaped from ubiquitin degradationthus not presented on MHC No immune response

EBV encodes for protein termed EBMA-1 which isnot cleaved by proteasome and its amount can bedetected lifetime of certain patients like BerkitsLymphoma

Its common characteristics is protein encoded ishaving Gly-Ala repeats


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Mechanism of Cytomegalivirus is also interesting

It encodes resident ER protein US2 & US11 whichreduces MHC-1 expression on cell surface


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MUSCLE WASTING

Muscle degeneration that follows long-termimmobilization, denervation, and severe catabolicstates, such as occurs in sepsis and cancer-inducedcachexia, leads to activation of the ubiquitin pathwayand induction of many of its enzymatic components

In turn results in massive degradation of muscleproteins

Interestingly, it appears that N-end rule pathwayplays an important role in stress-induced muscle

proteolysis

Actin and myosin-major muscle proteins aredegraded like this but they are not having NTRwhich follows this pathway, so they are converted in

it.


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An important development in this area is thediscovery of two, degeneration-induced ubiquitinligases, Murf1 (Muscle RING Finger) and Murf2

But substrates of induced E3s have not beenidentified and also signalling mechanism related tohypertrophy and atrophy is not identified


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DISEASES ASSOCIATED WITH

ANIMAL MODELS The utilization of animal models in which single genes

along the ubiquitinproteasome pathway aremanipulated can be used as powerful tools to study the

effects of such manipulations on the development andpathophysiology of the mature organism

They can also be used to study disease models andthe activity of novel therapeutic modalities

Studies on D.malanogaster found the pathologiesobserved in mammals in a variety ofneurodegenerative disorders


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E.g. mutations in the D. melanogasters Bendless genethat codes for an E2 enzyme result in impairment information of neuronal networks, due most probably to adefect in neuronal guidance

Inactivation of the flys Ariadne- 1 that codes for aRING finger E3 results, in most cases, in embryoniclethality. The few survivors demonstrate a severedefect in the development of the central nervous system


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DRUG DEVELOPMENT FOR

TARGETING ABERRANT ACTIVITY OFTHE UBIQUITIN SYSTEM

Because of the central role the ubiquitin system playsin such a broad array of basic cellular processes,

development of drugs that modulate the activity of thesystem may be difficult

Also ub enzymes are not specific to particular process,so its targeting is difficult

Recent studies strongly suggests that proteasomeinhibitors may indeed be beneficial in certainpathologies, such as in cancer, asthma, brain infarct,and autoimmune encephalomyelitis


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A completely different approach to drug developmentcan be the development of small molecules that bindand inhibit specific E3s. For example, specific phospho-peptide derivatives that span the phosphorylation

targeting domains in different substrates can serve asbaits to the respective E3s

A double-edged sword.

In the case of p27 and InB-alpha, where

phosphorylation destabilizes negative regulators,inhibition of the E3 can control dysregulated cellcycle and decrease untoward activity of the immunesystem


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Also, the similarity between the phosphorylation sitesof InB-alpha and h-catenin may also lead tostabilization of h-catenin, which is a transcriptionalactivator, and its excessive transcriptional activity can

result in malignant transformation of benign cells

A better approach-development of small moleculesthat are substrate-specific and bind, preferably, tospecific substrates or to their ancillary proteins

rather then to an E3


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When accelerated degradation of a tumor suppressorresults in sensitization of cells to malignanttransformation, selective inhibition of the recognitionmachinery can potentially reverse the malignantphenotype.

Peptides that bind specifically to HPV-E6, and preventits association with p53, may interfere with p53targeting. Such peptides were able to induce p53 inHPV-transformed cells with subsequent reversal ofcertain malignant characteristics or induction ofapoptosis

So there are many possibilities related to variousdiseaeses and ubiquitin proteasome system as itstarget for cure


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ubiquitin and human pa tho genesis

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