p73 is a member of the p53 family , although p73 ( and p63 ) are more closely related to the ancestral form of the protein than p53 . these three mammalian genes encode transcription factors that play key roles as regulators of proliferation , differentiation , cell death , stem cell renewal , and cell fate commitment [ 24 ] . all p53 family genes contain the same modular domain structure , including an amino - terminal transactivation domain ( ta ) , a dna - binding domain and a carboxy - terminal oligomerization domain . the p73 gene contains 15 exons and transcription can be initiated from two n - terminal promoters ( fig .  1 ) . transcription from the most upstream atg ( promoter 1 ) generates the transactivating ( ta ) isoforms , while the second promoter , situated within intron 3 , yields amino - terminal truncated proteins ( n isoforms ) . in general , ta and n isoforms display distinct biological activities . the ta isoforms induce cell cycle arrest and apoptosis , and are therefore candidate tumour suppressors [ 5 , 6 ] while the truncated n isoforms are ( generally ) pro - survival and favour oncogenic transformation . in addition , both tap73 and np73 transcripts undergo alternative c - terminal splicing , generating , in theory , up to seven different variants of each in normal cells , although not all these have been detected at the protein level [ 810 ] . a genomic organisation of p73 and representation of different splicing variants that give rise to the isoforms of p73 . the p1 promoter generates the ta isoforms , while the p2 promoter produces the n isoforms . b schematic representation of the domains encoded by the different isoforms of p73 . on top ta transactivation domain , dbd dna - binding domain , od oligomerization domain , sam sam domain , tid transactivation inhibitory domain p73 gene structure . a genomic organisation of p73 and representation of different splicing variants that give rise to the isoforms of p73 . the p1 promoter generates the ta isoforms , while the p2 promoter produces the n isoforms . b schematic representation of the domains encoded by the different isoforms of p73 . on top , there are indicated the aminoacids included in each domain . ta transactivation domain , dbd dna - binding domain , od oligomerization domain , sam sam domain , tid transactivation inhibitory domain it is , however , now generally accepted that n isoforms largely act as dominant negative inhibitors of the activity of ta isoforms , both by competing for consensus elements in promoter dna and by dimerisation [ 7 , 11 ] . however , this generalisation must be qualified since the longest np73 isoform contains a second c - terminal ta domain and can transactivate a set of genes distinct from that recognised by tap73 . moreover , the  isoforms of both ta and np73 contain a c - terminal transactivation inhibitory domain , which can compromise the transcriptional activity mediated by the n - terminal ta domain as a result of intramolecular interactions . thus , to regard the output of p73 expression as the resultant of the ratio between ta and n isoform expression may be an oversimplification . the human trp73 gene was identified in 1997 when it was localised within chromosome 1p36 , a region frequently deleted in tumours such as neuroblastoma and other late - stage human cancers [ 12 , 13 ] . although genes other than p73 within the 1p36 region , such as chd5 , may also contribute to these phenotypes , these data suggest that p73 may have a role in neural development as well as in cancer . this developmental role for p73 is substantiated by the phenotype of total p73 knockout mice . unlike the tumour - susceptible p53 null mice , total p73 knockout mice do not develop tumours but show developmental defects in the central nervous system ( table  1 ) , including congenital hydrocephalus and hippocampal dysgenesis , with abnormalities in the ca1ca3 pyramidal cell layers and the dentate gyrus . in keeping with these anatomical abnormalities , the total p73 knockout mice also have defects in both embryonal and adult neurogenesis , suggesting that p73 isoforms may be survival factors for neural stem cells . p73 mice also have a reduction in cortical thickness as a consequence of loss of mature cortical neurons . table 1neuronal defects in p73 transgenic micep73 ( yang et al . , pozniak et al . ) tap73 ( tomasini et al . ) dnp73 ( wilhelm et al . ) anatomical defectcortexhypoplasianoneage - dependence hypocellularity and cortex thinninghippocampusdysgenesisabsence lower blade of dentate gyrusnoneventricularthird ventricle enlarged and hydrocephalusnoneenlarged ventricleothersloss of cajal - retzius neurons and reelin defective . , wetzel et al . ) ( personal communication)(personal communication)youngagedyoungagedyoungagedburrowingntntreducedreducedntnormalclaspingpositivereducedpositivepositiventpositiveopen fieldreducedreducedreducedreducedntreducedbarnes mazereducedreducedntntntnthind griploosereducedntntntntlight  dark boxpositiventntntntntyoung mice : 24  months old . aged mice : 1517  months oldnt not testedthe behavioural experiments were performed in p73 mice neuronal defects in p73 transgenic mice young mice : 24  months old . aged mice : 1517  months old the behavioural experiments were performed in p73 mice this neurological phenotype has been attributed primarily to the loss of np73 isoforms , since these are expressed in post - mitotic neurons and act as survival factors . thus , ngf withdrawal or overexpression of p53 in sympathetic cervical ganglion cells leads to a reduction of endogenous np73 levels and apoptosis which is prevented by overexpression of np73 . recently , the generation of np73 isoform - specific ko mice has confirmed the prosurvival role of np73 in differentiated mature neurons . indeed , neuronal density in the motor cortex of np73 mice is significantly reduced after 10  months of age and progresses with evidence of neurodegeneration , although there were no striking hippocampal abnormalities . in particular , it has been observed that the number of vomeronasal neurons and cajal - rezius cells was profoundly reduced , and that the choroid plexuses were atrophic . however , the severity of the neurological defects observed in the np73-specific knockout is not as dramatic as that seen in the total p73 mice , suggesting that tap73 may also contribute to the development of the cns . while cortical thickness in selective tap73 null mice is normal , they show hippocampal dysgenesis with loss of the lower blade of the dentate gyrus similar to that seen in total p73 mice at p14 , before the complicating effect of ventricular enlargement further distorts the hippocampal architecture in the total knockout . this anatomical phenotype is reflected in the reduction in neurogenesis in the subgranular zone of the dentate gyrus ( fig . indeed , tap73 has been shown to regulate the negative bhlh hey2 which is known to sustain maintenance of neural precursors . thus , the isoform - selective knockout studies suggest that tap73 and np73 contribute to cns development in ways that are only partially overlapping . the dentate gyrus from day  7 after birth ( p7 ) of normal ( p73 ) and knockout ( p73 ) mice was stained with antibodies to glial fibrillary acidic protein ( gfap ) and nestin . knockout mice show nearly half of gfap / nestin cells , indicating a very reduced stemness potential in these mice . scale bars 50  m reduction of the putative stem cell in the dentate gyrus from p73 mice . the dentate gyrus from day  7 after birth ( p7 ) of normal ( p73 ) and knockout ( p73 ) mice was stained with antibodies to glial fibrillary acidic protein ( gfap ) and nestin . knockout mice show nearly half of gfap / nestin cells , indicating a very reduced stemness potential in these mice . both embryonic and adult neural stem cells ( nsc ) are primary precursors that have the ability to differentiate into different cell types ( neurons , astrocytes and oligodendrocytes ) while retaining the capacity to produce identical nsc progeny ( self - renewal ) , and p53 and np63 have already been implicated in nsc biology [ 25 , 26 ] . recently , four independent groups [ 16 , 23 , 27 , 28 ] have demonstrated that p73 is also a positive regulator of embryonic and adult nsc and some of this in vivo evidence has been discussed above . neurospheres derived from p73 mice are smaller , with a reduced number of cells in s phase and an increase in the senescent population . at the molecular level , it has been shown that nsc from p73 mice have transcriptional dysregulation of sox-2 , sox-3 , nanog , notch - i , notch-2 , hes-5 , jag2 and deltex , which are all components of signalling pathways involved in the regulation of proliferation and/or self - renewal , although further studies are required to address how p73 physiologically regulates these factors . tap73 is the predominant isoform expressed in embryonic nsc , and endogenous expression of tap73 increases during differentiation of nsc . further studies have shown that smaller numbers of neurons can be derived from p73 nsc which do not fully differentiate , with defects in arborization of the dendritic tree and in physical connectivity . this is reflected in the anatomy of hippocampal neurons in the dentate gyrus of p73 mice in vivo , which show a reduced number of branches when compared to normal mice ( fig . 3hippocampal neuron morphology is altered in the dentate gyrus of knockout ( p73 ) mice . golgi staining of dentate gyrus from normal ( p73 ) and p73 mice ( age=18  days after birth ) . scale bars 100  m ( top panel ) and 50  m ( low panel ) hippocampal neuron morphology is altered in the dentate gyrus of knockout ( p73 ) mice . golgi staining of dentate gyrus from normal ( p73 ) and p73 mice ( age=18  days after birth ) . scale bars 100  m ( top panel ) and 50  m ( low panel ) p73 has also been implicated in oligodendrocyte as well as neuronal differentiation , and oligodendrocytic differentiation from p73 nsc is also impaired , with lower numbers and poorer quality than those derived from wild type nsc . however , dissociated p73 nsc retain the ability to differentiate into neurons , astrocytes and oligodendrocytes , indicating that loss of p73 does not affect the multipotency of nsc . another , though perhaps less definitive model implicating p73 in neurogenesis is the terminal differentiation of neuroblastoma cells induced by retinoic acid ( ra ) and which is associated with an increased expression of tap73 . in addition , ectopic expression of tap73 itself induces terminal neuronal differentiation . during ra treatment , tap73 isoforms regulate the n - cam promoter , while ectopic expression of tap73 led to a down - regulation of n - myc and an increased expression of prb , mimicking the ra effect on these two genes , which are crucial for neuroblastoma survival [ 32 , 33 ] . in conclusion , while recent work indicates a clear requirement for p73 , and particularly tap73 in the maintenance of stemness via a yet not fully identified mechanism both in vivo and in vitro [ 16 , 23 , 27 , 28 ] , other data suggest an involvement of p73 in neuronal   and oligodendrocyte   differentiation . these multiple biological activities in the nervous system may reflect the molecular complexity of the 14 protein isoforms of p73 , their interactions with each other as well as their interplay with other p53/p63 family members , which are themselves expressed as multiple isoforms . alternatively , since p73 appears to regulate between 1,000 and 2,000 genes , including 100200 transcription factors , several apparently divergent pathways could be simultaneously activated . during the last 15  years , it has emerged that the p53 family has an important role in several neurodegenerative diseases . of special interest to us , and on which we will focus , is their role in the most ( and increasingly ) prevalent form of neurodegeneration , alzheimer s disease ( ad ) . ad exists as both familial and sporadic forms . the familial early onset ( eoad ) form , accounting for only a small percentage of cases , is inherited via mutations in either the -amyloid precursor protein ( app ) or one of the two presenilins ( ps ) , ps1 and ps2 . in contrast , the greatest genetic risk for the sporadic , late onset ( load ) form is the 4 allele of apolipoprotein e ( apoe ) . recently , genome - wide association studies have identified three additional risk genes , albeit with much weaker effects than apoe ; apoj / clu , picalm and cr1 [ 35 , 36 ] both sporadic and familial forms of the disease are characterised by two brain lesions : senile plaques extracellular deposits of the -amyloid ( a ) peptide ; and neurofibrillary tangles ( nfts)intracellular aggregates of paired helical filaments ( phf ) composed of hyperphosphorylated forms of the microtubule associated protein , tau . a is derived by proteolytic processing from the app , with the presenilins being a necessary component of the -secretase complex responsible for the final stage of this process . the phosphorylation state of tau regulates its ability to bind microtubules ; in its hyperphosphorylated state tau is unbound and eventually polymerises into paired helical filaments , which then aggregate into nfts . it is emerging that it is not these insoluble aggregates of tau , but rather some soluble form that is the toxic species responsible for neuronal dysfunction and eventually neuronal death . that mutations in app give rise to increased a production ( as do mutations in the presenilins , as subsequently observed ) , and as senile plaque appearance precedes that of nfts , the amyloid cascade hypothesis was formulated . this hypothesis , which has not yet been refuted , holds that increased levels of a lead to the hyperphosphorylation of tau which then leads to neuronal dysfunction and eventually neurodegeneration . attempts to model this cascade of events in mice has for the most part failed , as even when mutant transgenic forms of both app and ps1 are introduced , despite high levels of a production leading to extensive plaque formation , the aggregation of tau is not observed . it is only when app , ps1 and human tau are concomitantly over expressed in triple transgenic mice that both plaques and tangles are observed . the first indication that the p53 family may play a role in ad came in 1996 with the demonstration that intracellular a upregulates p53 in the brains of transgenic mice overexpressing just the a fragment of app . that same year , the kosik group examined neurons from a p53 mouse , observing that p53 has a role in neuronal differentiation and in tau phosphorylation . in 2001 use of pifithrin- , an inhibitor of p53-dependent gene transcription was shown to protect neurons against a-induced apoptosis . the p53 protein was also found to be upregulated in the brain of ad sufferers , a finding which we later confirmed . in 2004 , caricasole et al .   showed that a activates the expression of the p53 target gene encoding the soluble wnt antagonist , dickkopf-1 ( dkk1 ) , and that knock - down of dkk1 in primary neurons almost completely blocked a-induced tau phosphorylation , implicating the p53 family in the  amyloid cascade  pathway . in the same year , it was also reported   that the p73 protein exhibits an altered subcellular distribution in ad brain . in hippocampal pyramidal neurons of control subjects , p73 immunoreactivity was predominately cytoplasmic , while in ad samples increased levels of p73 were found in the nuclei of pyramidal neurons and in dystrophic neurites . we employed a simple cell model to show that transcriptionally active forms of p53 are able to induce tau phosphorylation at specifc phosphoepitopes , particularly the at8/tau-1 ( s199 , s202 and s205 ) and phf-1 ( s396 and s404 ) sites . we found that this also holds true for p73 , with the ta forms increasing tau phosphorylation , while the n forms do not . in this system , we also found that transcriptionally active forms of p63 activate tau phosphorylation , while np63 forms do not ( unpublished observations ) . in 2008 , the miller / kaplan group   reported that in brains of aged ( 1618  months ) heterozygous p73 mice , there were substantial increases in tau phosphorylation levels together with filamentous aggregates of hyperphosphorylated tau with similarities to nfts . when they crossed these haploinsufficent p73 mice with mice harbouring a double mutant form of app ( the tgcrnd8 mouse ) , tau phosphorylation and tau filament formation occurred as early as 1.5  months . this finding is especially remarkable given that a-based mouse models of ad do not manifest overt tau pathology . it also lends yet further support to the amyloid cascade hypothesis , showing that a lies upstream of tau pathology and also that the cascade of events that a must set into motion , leading to the hyperphosphorylation of tau and subsequent neurodegeneration , involves the p53 family , and in particular p73 . the miller / kaplan group went on to show   that in primary cortical neurons generated from p73 ko mice the activity of c - jun n - terminal kinase ( jnk ) increased as the number of wt p73 alleles decreased , and that jnk inhibition decreased tau phosphorylation in these neurons . they propose that , since np73 can bind and inhibit jnk , which the ta forms of p73 can not , it is the loss of the n forms of p73 that brings about tau phosphorylation and neurodegeneration . the same group has previously demonstrated that np73 plays a neuroprotective role in the cns [ 17 , 50 ] and also claim that n forms of p73 predominate , at least in the postnatal mouse brain . the loss of one copy of the entire p73 gene results in a reduction in np73 isoforms leading to an increase in jnk activity . in support of this claim , there is an extensive literature implicating jnk in ad pathology , e.g. [ 5153 ] . our group has shown that jnk directly targets tau   and also plays a role in the regulation of app processing . inhibiting jnk has also been shown to protect against a toxicity in an app / ps1 brain slice model of ad . there exists one other knock - out mouse with a phenotype that includes neurodegeneration with accompanying aggregates of hyperphosphorylated tau filaments . the deleted gene is prolylisomerase-1 , which is associated with a production and aggregation through its binding with phospho - thr688 of app , and tau phosphorylation through interaction with phospho - thr231 of tau . of interest , pin1 a has been shown to activate c - abl and increase p73 levels , while the swedish mutant form of app increases expression from both the ta and n promoters of the p73 gene , but results in an overall increase in the ta forms . it appears then that p73 plays a key role in ad and , in particular , that the balance between the ta and n forms is crucial . however , from the above , and other observations such as the fact that p53 transcriptional activity is regulated by the presenilins and that many of the components of the -secretase complex are themselves regulated by p53 , it would seem unlikely that p73 acts alone . further investigations into p73 interactions with other p53 family members , in particular p53 itself , within nervous tissues will undoubtedly aid our understanding of its roles in both normal adult brain and in the pathogenic processes underlying alzheimer s disease . from these studies , it emerges that p73 is the p53 family member with a fundamental role in central nervous system development and maintenance , and even in its degeneration . indeed , as shown in fig .  4 , the p73 gene plays a role throughout the neurogenesis process , from neural stem cells to mature postmitotic neurons . although perhaps simplistic at this stage , these new findings suggest that tap73 is the isoform essential for neuronal differentiation and maintenance of neural stem cells . however , the phenotype observed in isoform specific knock - out mice is milder than that in the full knock - out mice , indicating that , as in cancer , it is the interaction between p73 isoforms , which ultimately determines the phenotype . functional neurons are generated from neural stem cells and then after maturation , integrated in neuronal circuits . np73 plays a major role both as a survival mechanism as well as yet unknown pathways . question marks indicate that molecular mechanism has not been fully investigated yet role of p73 in neurogenesis . functional neurons are generated from neural stem cells and then after maturation , integrated in neuronal circuits . np73 plays a major role both as a survival mechanism as well as yet unknown pathways .