aging is the most prominent risk factor for the occurrence of neurodegenerative diseases among others , including oxidative stress ( keller et al . , 2005 ;
jain et al . , 2011 ) , telomere length ( harris et al . , 2006 ) , genetic mutations ( anderton et al . , 2002 ) and head injury ( maiese et al . , 2008 ) . in the united states there are over 35 million of people with a mean age of 65 years and even older , that mainly die from age - related diseases ( drago et al . ,
aging increases susceptibility of people to environmental stressors , thereby increasing the chance to develop neurodegenerative conditions , most likely because the self - repair ability is compromised and tissues and/or organs undergo a progressive decline ( musumeci et al . , 2014a ) .
the aging process is associated with a number of structural , biochemical , functional and neurocognitive changes in the brain .
the structural changes include expansion of cerebral ventricles , regional decreases in cerebral volume ( raz et al . , 2005 ) , loss of neural circuits and reduced brain plasticity ( burke and barnes , 2006 ; kolb and gibb , 2011 ) , thinning of the cortex ( shahani et al . , 2006 ) , decrease in both of the grey and the white matter volume ( bartzokis , 2011 ) , changes in neuronal morphology ( sowell et al . , 2003 ) and formation of neurofibrillary tangles ( hedden and gabrieli , 2004 ; neill , 2012 ) . among the age - related biochemical changes
significant decreases in dopamine receptors d1 , d2 , and d3 ( wang et al . , 1998 ; kaasinen et al . , 2000 ) and decreasing levels of different serotonin receptors and their transporters such as 5-hydroxytryptamine transporters ( 5-htts ) ( chang and martin , 2009 ; chang et al . , 2009 ) have been repeatedly reported . among the neuropsychological changes , alterations in orientation ( benton et al . , 1981 ) and memory ( hof and morrison , 2004 ) are the most common ones . moreover ,
many age - related neurodegenerative diseases are characterized by accumulation of disease - specific misfolded proteins in the central nervous system ( cns ) ( van ham et al . , 2009 ) .
these include -amyloid peptides and tau / phosphorylated tau proteins in alzheimer 's disease ( ad ) , -synuclein in parkinson 's disease ( pd ) , superoxide dismutase ( sod ) in amyotrophic lateral sclerosis ( als ) ( durham et al . ,
1997 ) , and mutant huntingtin in huntington 's disease ( hd ) ( scherzinger et al . , 1997 ) .
the association between age and protein misfolding is not clear yet , but it is probably related to alterations of molecular mechanisms triggered by aging cells , such as telomere shortening , cells shrinkage and decline of quality control over protein synthesis mechanisms ( hung et al . , 2010 ; thanan et al . , 2014 )
schematic representation illustrating some of the most common risk factors that contribute to the onset and/or progression of neurodegenerative diseases and the related mechanisms driving the neurodegenerative process .
telomeres are an evolutionarily conserved repetitive nucleotide sequences ( ttaggg ) localized at the end of each chromosome , that are folded into a t loop structure by a protein complex called shelterin ( stewart et al . , 2012 ) . telomeres play four fundamental roles : protecting genetic information from erosion during dna replication ; protecting dna from damage ; serving as a binding site for dna repair proteins ; and providing information about the cell proliferation history ( stewart et al . , 2012 ; musumeci et al . , 2015 ; giunta et al . ,
the telomere length is the sand glass of the cell since it specifies the number of divisions a cell can undergo before it finally dies ; thus , it indicates the cell proliferative potential .
telomere shortening leads to the attainment of the so - called  hayflick limit , which indicates the transition of cells to the state of senescence . following this step ,
cells progressively enter a state of crisis , which is accompanied metabolic disturbances that culminate in massive cell death .
telomerase plays a pivotal role in the pathology of aging and cancer by maintaining genome integrity , controlling cell proliferation , and regulating tissue homeostasis .
telomerase is essentially composed of an rna component , the telomerase rna or terc , which serves as a template for telomeric dna synthesis , and a catalytic subunit , telomerase reverse transcriptase ( tert ) .
the canonical function of tert is the synthesis of telomeric dna repeats , and the maintenance of telomere length .
however , accumulating evidence indicates that tert may also exert some fundamental functions that are independent of its enzymatic activity ( verdun and karlseder , 2007 ) ( please refer to figure 2 ) .
a reduction in telomerase expression contributes to telomere shortening in mitotic cells , while high levels of the enzyme in mesenchymal stem cells ( mscs ) contribute to their 
the phenomenon of telomere shortening is closely associated with aging itself , but it has been widely demonstrated that cells can also undergo premature aging due to several factors such as oxidative stress , inflammation and infections , which are able to speed up this process and determine age - related dysfunctions ( hung et al . , 2010 ; jenny , 2012 ; kong et al . , 2013 ; kota et al . ,
therefore , given the involvement of these factors ( and in particular of oxidative stress ) in the development of neurodegenerative / age - associated diseases , it becomes of primary importance to also gain more insights on the underlying mechanisms triggered by these stressors , as this could serve to improve current therapeutic strategies based on the use of mscs to treat neurodegenerative conditions .
the telomerase is composed of an rna component , telomerase rna or telomerase rna component ( terc ) , which serves as a template for telomeric dna synthesis , and a catalytic subunit , tert .
tert besides its canonical function in telomere elongation has also a role as a transcriptional modula - tor of the wnt--catenin ( -cat ) signalling pathway .
tert acts as a cofactor in the -cat transcription complex ; in this complex , tert interacts with brg1 , a chromatin remodeling factor , to regulate the wnt/-cat signalling pathway .
tert is not only acti - vated by the wnt/-cat pathway , but -cat could also be directly regulated by tert induction , which results in maintenance of telomere length . in the mitochondria , tert also plays a role in regulating apoptosis in - duced by oxidative damage of mitochondrial dna ( mtdna ) .
cen : centromere . a reason why telomeres are the preferred targets of oxidative insult seems to be primarily related to their dna composition , which tends to be rich in guanine residues ( coluzzi et al . ,
indeed , the high incidence of guanine bases promotes the generation of alterations to dna bases to species called 8-oxoguanine ( 8-oxog ) , which , if not repaired , may lead to single or double strand breaks , mutations or even genomic instability ( grollman et al . , 1993 ) . of interest , genomic instability , oxidative stress and ageing
are not to be considered as independent causative factors in telomere shortening , but need to be considered as interconnected phenomena .
consistent with this theory , convergent data has identified an accelerated wnt/-catenin cascade activation as a common denominator triggered by these insults .
activation of this pathway reduces mscs proliferation potential , hampers telomerase activity and drives a cellular shift of mscs towards a differentiated / senescent phenotype ( as elegantly reviewed by fukada et al . ,
, it is auspicable that strategies aimed at dampening the occurrence of these detrimental events in neurons or to block the wnt/-catenin intracellular pathways could have the potential to significantly impact the senescent process , including premature telomere shortening .
early neuronal cell death is a feature of neurodegenerative disorders and reduced telomere length has been associated with premature cellular senescence .
studies have shown that reduced telomere length in peripheral blood is associated with the incidence of illnesses associated to the aging phenotypes , such as dementia ( thomas et al . , 2008 ) , neurodegenerative disorders such as hd and genetic neurovascular diseases such as ataxia telangiectasia ( at ) ( metcalfe et al . , 1996 ; kota et al . ,
since ltl is reflective of global cellular morbidity and mortality , it has been proven that it could be used as a useful tool to screen neurodegenerative disorders ( sahin and depinho , 2012 ) .
it is worth emphasizing , however , that leukocytes include diverse cell populations that play complementary roles in tissue homeostasis and responses to infections and diseases , and the possibility exists that simply monitoring ltl may lead to misleading results .
indeed , the three major classifications of leukocytes are granulocytes , lymphocytes , and monocytes .
these populations have different telomere lengths and erosion rates as a result of differences in telomerase activity , proliferation history , and telomere trimming ( stewart et al . , 2012 ) .
these differences have to be carefully taken into account when considering a possible study of age - related processes in neurodegenerative disorders .
however , a recent study has consistently showed that the ltl was reduced in individuals suffering from neurodegenerative disorders as described above , suggesting that the phenomenon of telomere shortening could at least be partly implicated or could contribute to the triggering of pathological pathways activated in these diseases ( kota et al . ,
the reduced telomere length has been attributed to oxidative stress , aberrations in mitochondrial homeostasis , deficient dna repair mechanisms , and decreased dna methylation status ( von zglinicki , 2002 ; blasco , 2007 ; gackowski et al . , 2008 ; copped and migliore , 2009 ; van groen , 2010 ; sahin and depinho , 2012 ) .
interestingly , a review of data from literature concerning the potential use of ltl as a biomarker in ad and pd showed to be inconsistent in both cases , since the number of studies reporting no association between ltl and disease states almost overlapped the ones indicating a correlation between ltl shorthening and neurodegeneration ( eitan et al . , 2014 ) .
interestingly enough , a recent study reported an even longer ltl in pd patients , associating short telomeres with reduced risk of pd ( schrks et al . , 2014 ) .
the reason of these inconsistencies could be dependent on the population number used in these studies , as the low number of patients together with the inter - individual variability may often result in significantly reduced statistical power , and purportedly to unreliable results .
it has been shown that variability in ltl in individuals can be induced by different factors such as chronic stress , diet , lifestyle , chronic inflammation state and hormone levels ( liu et al . , 2010 ; broer et al . ,
the interaction between these factors and genotype can also play a role in ltl variability ( takata et al . , 2012 ) .
certainly , further investigation in this field are needed to clarify the precise role and diagnostic or therapetic potential of ltl in neurodegeneration .
the limited regeneration power of the cns represents a major challenge for the development of new therapeutic strategies efficacious to promote its functional repair .
mscs have been proposed as a viable therapeutic tool for degenerative disorders as they possess high proliferative ability and they are able to differentiate into multiple lineages ( mobasheri et al . ,
2014 ; musumeci et al . , 2014c ; tanna and sachan , 2014 ) .
mscs can differentiate into neuron - like cells and determine a paracrine effect by modulating the plasticity of damaged host tissues ; by secreting neurotrophic and survival - promoting growth factors that inhibit apoptosis and promote neurogenesis , glial scar formation , immunomodulation , angiogenesis and neuronal and glial cell survival ; by restoring synaptic transmitter release ; by integrating into existing neural and synaptic networks ; and by re - establishing functional afferent and efferent connections ( siniscalco et al .
in addition , low immunostimulating and high immunosuppressive properties make mscs a suitable source for cellular therapy ( abumaree et al . , 2012 ; kwon et al . , 2014 ) .
another point in favor to mscs employment in therapy is that cells can be transplanted directly without any prior genetic modification or reprogramming , and are able to migrate to the tissue injury sites ( amado et al . , 2005 ) .
mscs have also been proven to be useful for the treatment of pathologies in which tissue damage is caused by oxidative stress and thus in those pathologies linked to stress - induced telomere shortening and premature aging , where mscs are likely to be more resistant to oxidative insult than normal somatic cells ( benameur et al . , 2015 ) .
this feature is particularly important since it makes mscs an interesting and testable model for the treatment of age - related neurodegenerative disorders .
currently , there is a great interest towards the use of mscs in pioneering therapies aimed at treating chronic and progressive neurodegenerative diseases , which are currently incurable and whose attempts to find disease - modifying therapies have failed , such as ad , pd , als and hd .
it has been shown that after transplantation into the brain , mscs promote neuronal growth , decrease apoptosis , reduce the levels of free radicals , stimulate the formation of new synaptic networks from damaged neurons by supporting axonal outgrowth , modulate neuroinflammatory activities and promote proteosomal degradation of ubiquitinated misfolded proteins ( caplan and dennis , 2006 ; mezey , 2007 ; uccelli et al . , 2011 ) .
through paracrine mechanisms , mscs are also able to interact with neighbouring damaged host cells and influence their microenvironment , by sharing proteins , rnas and even mitochondria ( spees et al .
, 2006 ; olson et al . , 2012 ) . as a proof - of - concept , mazzini et al
. demonstrated that mscs can decrease motor neuron cell death through paracrine actions when implanted into the cns of als patients ( mazzini et al .
recently , the paracrine properties of bone marrow - derived mscs ( bm - mscs ) have been also shown in rat model of ad , suggesting their potential therapeutic role in this disease ( salem et al . , 2014 ) .
the potential efficacy of human mscs ( hmscs ) has been also confirmed recently , as treatment succeeded to ameliorate some behavioral defects observed in a rodent model of hd , hence demonstrating that xenologous transplantation of hmscs could be considered a potentially successful approach to counteract neurodegeneration caused by hd , and perhaps other cns disorders ( hosseini et al . , 2014 ) .
mscs can be readily isolated from various tissues , show high plasticity and are capable to differentiate into many functional cell types ( woodbury et al . , 2000 ;
numerous studies have shown that bm - mscs can differentiate into cells that display neuronal or even dopaminergic characteristics both in vitro and in vivo ( ni et al . , 2010 ; zeng et al . , 2011 )
a recent study reported that mouse bm - mscs provided neuroprotection by secreting a key factor , prosaposin , a molecule capable of rescuing mature neurons from apoptotic death .
the secretome of bm - mscs showed to reduce toxin - induced cell death in cultures of rat pheochromocytoma cells , human rencell cortical neurons , and rat cortical primary neurons ( li et al . , 2010 ) .
unfortunately , the medical procedure to obtain bm - mscs from the bone marrow is invasive and definitely painful to patients . therefore , efforts have been made to find more practical alternatives .
indeed , recently other mscs sources have gained clinical interest for use in regenerative medicine ; and adipose tissue represents one of these sources with a broad spectrum of benefits .
human adipose tissue represents a readily available autologous source of mscs ( ghasemi and razavi , 2014 ) .
human adipose tissue - derived mscs ( hat - mscs ) retain morphological , phenotypic and functional characteristics resembling those of bm - mscs ( zuk et al . , 2002 ) , are stable over long term culture , expand efficiently in vitro and possess multi - lineage differentiation potential ( zuk et al .
, 2001 ; musumeci et al . , 2011 ; choudhery et al . , 2013 ;
latest observations suggest that transplantation of hat - mscs into the brains of elderly mice improved both locomotor activity and cognitive functions .
transplanted cells rapidly differentiated into neurons and in part , into astrocytes , and produced choline acetyltransferase proteins , restoring acetylcholine levels in thebrain .
moreover , transplantation of hat - mscs restored neuronal integrity by stimulating the release of neurotrophic factorsby neighbouring cells ( park et al . , 2013 ) .
in this regard , an aspect to be considered in mscs therapies is that it is now well - recognized that many pleiotrophic molecules endowed with neuroprotective potential , including some neuropeptides produced locally by resident glial cells or neurons ( i.e. , pituitary adenylyl cyclase activating polypeptide and/or vasoactive intestinal peptide ) , when stimulated by neighbouring cells ( i.e. , implanted mscs ) may prevent cognitive decline caused by aging ( pirger et al . , 2014 ) , facilitate nerve recovery after injury both in the cns ( reviewed by waschek , 2013 ) and the periphery ( tamas et al . ,
2012 ) , stimulate remielination processes and glial regenerative support to neurons ( castorina et al . , 2014 , 2015 ) and are even capable to prevent retinal damage and mantain retinal barrier properties ( giunta et al . , 2012 ; scuderi et al . ,
2013 ) or impede oxidative insults ( castorina et al . , 2012 ) , a broad spectrum of physiopathological events that , at different degrees , are negatively impacted by senescence .
even more interesting , combinatorial administration of these molecules with mscs has been suggested to support spinal cord recovery after damage ( fang et al . , 2010 ) , inferring on the mutual reciprocity between the two , especially desirable to complement the existing gaps determined by the single therapeutic employment of mscs in aged patients affected by neurodegenerative disorders .
another source of mscs that has captured minor scientific interest is represented by dental pulp stem cells ( dpscs ) .
dpscs have also been recognized as capable to differentiate into a variety of cell lineages ( zhang et al . , 2006
; huang et al . , 2009 ) , but more studies are required to better define their potential .
other sources of stem cells are that obtained from human - exfoliated deciduous teeth ( shed ) , which have been shown to contain multipotent stem cells ( miura et al . , 2003 ) .
the importance of shed is that they are derived from a tissue similar to the umbilical cord .
notably , both kinds of dscs can be induced to differentiate into neuron - like cells and be transplanted in brain injury and/or neurodegenerative disease animal models to conduct neuroregeneration studies ( sakai et al . , 2012 ;
based on these findings , it is plausible to believe that the extracted teeth , considered a common waste product from dental extraction procedures , could be employed in the future to exploit in tissue engineering strategies as a promising substitute of bm - mscs .
a major issue that has significantly limited the use of mscs - based therapy is the low yielding of viable mscs from donor tissue .
in fact , in order to harvest sufficient mscs to procure some clinical benefits cells need to replicate several times in vitro .
unfortunately , a number of studies have demonstrated that mscs from various animals undergo spontaneous transformation when cultured for long terms , posing a limit to this approach .
indeed , transformed mscs show some of the features of senescent cells , with a progressive shortening of telomers and consequently , cell death ( ahmadbeigi et al . ,
2011 ; ren et al . , 2011 ; he et al . , 2014 ) .
such an aging process occurring in mscs appears to be tissue - specific and has been shown to be regulated by evolutionarily conserved signaling pathways .
more recently , a signaling pathway that has shown to be tightly associated to age - related cellular changes is the wnt/-catenin signaling cascade ( decarolis et al . , 2008 ; hiyama et al . , 2010 ; stevens et al . ,
wnt/-catenin signaling plays a functional role as a key regulator of self - renewal and differentiation properties in mscs .
( 2014 ) found that activation of the wnt/-catenin pathway delays the progression of cellular senescence as shown by the decrease in senescence effectors p53 and phospho - retinoblastoma ( prb ) , lowered senescence - associated -galactosidase ( sa--gal ) activity , and increased telomerase activity .
in contrast , suppression of the wnt pathway promoted senescence in mscs ( jeoung et al . , 2014 ) .
2012 ) also showed that wnt/-catenin pathway is connected and regulates tert expression through the interaction with kruppel - like factor 4 ( klf4 ) , a core component of the pluripotency transcriptional network ( a schematic representation is depicted in figure 2 ) .
unfortunately , to date , the mechanism through which the wnt/-catenin signaling pathway regulates age - related neurogenic differentiation in mscs still remains unclear and needs further investigations .
it has been assumed that aging is presumably linked to diminished organ repair capacity due to reduced functionality of mscs .
for this reason , it should be taken into account that the effectiveness of mscs - based therapies are highly influenced by donor age .
it was observed that progressively aging murine bm - mscs exhibit a decline in mscs number , proliferation , differentiation , angiogenic and wound healing properties , along with enhanced apoptotic and senescent features ( kretlow et al . , 2008 ; choudhery et al .
, 2012a , b ) . in a study using adipose tissue - derived mesenchymal stem cells ( at - mscs ) from both young and old donors
, it was observed that both were able to form colonies , but at - msc from younger donors produce more colonies containing larger numbers of cells and increased proliferative rate than those obtained from older donors ( alt et al . , 2012 ) .
moreover , at - mscs obtained from aged donors displayed increased senescent features , as indicated by the greater expression levels of p16 and p21 genes , which have been indicated as markers of senescence ( stolzing et al . , 2008 ) .
in the latter study , the expression of sa--gal was measured and it was also found at higher levels in aged at - mscs cultures , while sod activity was decreased ( stolzing et al . , 2008 ) .
it was further identified that mscs from elderly donors became more granular and developed a more flat and larger morphology at passages 56 , indicating the appearance of typical morphological signs of replicative senescence ( khan et al . ,
recently , in a study conducted on hbm - mscs from young and old donors used to differentiate and promote neurite outgrowth from dorsal root ganglia neurons ( drgn ) , brohlin and coworkers observed that treatment of hbm - mscs with growth factors induced protein expression of the glial cell marker s100 in cultures from young but not old donors .
however , exogenous administration of growth factors enhanced the levels of brain - derived neurotrophic factor ( bdnf ) and of vascular endothelial growth factor ( vegf ) transcripts in both donor cell groups and partly recovered stemness properties of mscs from elderly , supporting the hypothesis stated above . finally , in the same study it was demonstrated that mscs co - cultured with drgn significantly enhanced total neurite length only when obtained from young but not old donors .
moreover , mscs from young donors maintained their proliferation rate while those from the old ones showed increased population doubling times ( brohlin et al . , 2012 ) .
these observations suggest that mscs isolated from either young or old donors may benefit of a combinatorial approach to retain , at least in part , their regenerating properties on neurons .
nevertheless , to date mscs from young donors are still to be considered the first choice mscs source to use for cns repair ( figure 3 ) .
the fact that mscs can be conveniently obtained from different accessible tissues ( such as bone marrow , blood , adipose and dental tissue ) and demonstrate neuroprotective effects , immunomodulatory properties and self - migratory activity , makes them an attractive therapeutic tool for potential application in neurodegenerative disorders . however , there are some critical points that still need to be clarified before msc - based therapy can be adopted in clinical practice .
these include the reduced stemness properties of mscs isolated from elderly or caused by long - term expansion in vitro , which could result in reduced efficacy for regenerative cellular therapy .
the complex pathways involved in neurodegenerative disorders , should be evaluated with care , in the attempt to extend the current understanding of the pathogenesis of these diseases and identifying targets for intervention . to be suitable for use inneuroregenerative therapy , the mechanisms that govern the self renewal capacity of mscs
, it is proposed that scientific effort should focus more on finding the appropriate microenvironment ( culture conditions ) that more likely will allow to yield sufficient number of functional mscs .
as previously discussed , amolecular mechanism worthy of attention could be represented by the wnt/-catenin signaling pathway , whose involvement in triggering the shift of mscs towards a senescent phenotype appears to be clear .
these findings , together with the evidences obtained with combinatorial approaches using neuroprotective agents , support the idea that trophic molecules , including some neuropeptides , may elicit a regulatory function on the wnt signaling cascade , which in turn , could be the key element in controlling mscs senescence ( jeoung et al . , 2014 ) .
alternatively , another critical mechanism to target could be telomere regulation , but this strategy has already reach general consensus , since studies on the mechanisms controlling telomere status and regulation in these cells have progressively gained importance in the last years .
in fact , strategies to prevent telomere loss or to increase telomere length of mscs may prevent or delay degeneration and hence the onset of symptoms in neurodegenerative disorders , improving the results of mscs - based therapetic approaches .
finally , a further and reasonable method to expand mscs validity in therapy could be represented by banking younger adipose tissue for later use .
preservation of mscs at a younger age , when their biological utility is maximal , could provide a usable source of functional mscs with full regenerative potential for future applications in regenerative medicine .
 aging is the most prominent risk factor contributing to the development of neurodegenerative disorders . in the united states , 
 over 35 million of elderly people suffer from age - related diseases . 
 aging impairs the self - repair ability of neuronal cells , which undergo progressive deterioration . 
 once initiated , this process hampers the already limited regenerative power of the central nervous system , making the search for new therapeutic strategies particularly difficult in elderly affected patients . 
 so far , mesenchymal stem cells have proven to be a viable option to ameliorate certain aspects of neurodegeneration , as they possess high proliferative rate and differentiate in vitro into multiple lineages . however , accumulating data have demonstrated that during long - term culture , mesenchymal stem cells undergo spontaneous transformation 
 . transformed mesenchymal stem cells show typical features of senescence , including the progressive shortening of telomers , which results in cell loss and , as a consequence , hampered regenerative potential . 
 these evidences , in line with those observed in mesenchymal stem cells isolated from old donors , suggest that senescence may represent a limit to mesenchymal stem cells exploitation in therapy , prompting scholars to either find alternative sources of pluripotent cells or to arrest the age - related transformation . in the present review , 
 we summarize findings from recent literature , and critically discuss some of the major hurdles encountered in the search of appropriate sources of mesenchymal stem cells , as well as benefits arising from their use in neurodegenerative diseases . 
 finally , we provide some insights that may aid in the development of strategies to arrest or , at least , delay the aging of mesenchymal stem cells to improve their therapeutic potential . 