homozygous loss - of - function mutations in the gba1 gene cause gaucher disease ( gd ) , the commonest lysosomal storage disorder . this gene encodes glucocerebrosidase ( gcase ) , a lysosomal enzyme responsible for the hydrolysis of the glycolipid substrate glucosylceramide ( glccer ) to ceramide and glucose . reductions in enzymatic activity result in the lysosomal accumulation of glccer , as well as glucosylsphingosine , within reticulo - endoendothelial cells , leading to the systemic sequelae of gd , including organomegaly , bone disease , anaemia and thrombocytopenia . the clinical spectrum of gd has historically been sub - divided into type 1 gd , the so - called non - neuronopathic milder form of the disease , and types ii and iii gd , which are severe acute and chronic neuronopathic forms respectively ( cox , 2010 ) . in recent years heterozygous gba1 mutations have been identified as key genetic risk factors for parkinson 's disease ( pd ) , and increase the risk of developing pd by approximately 20-fold ( sidransky et al . , 2009 ; migdalska - richards and schapira , 2016 ) . the neuropathological hallmark of pd is the presence of proteinacious intraneuronal inclusions , known as lewy bodies , predominantly composed of aggregated -synuclein ( -syn ) , in addition to other proteins such as ubiquitin and p62 ( zatloukal et al . , 2002 ; shults , 2006 ) . -syn is a presynaptic protein of unknown function , which is thought to play a central role in the pathogenesis of pd . moreover , there is mounting evidence that soluble -syn aggregation intermediates ( so - called oligomeric or protofibrillar species ) represent the most toxic form of the protein ( volles and lansbury , 2003 ) . -syn is removed from the cell by both macroautophagy and chaperone - mediated autophagy ( cma ) ( cuervo et al . , 2004 ; watanabe et al . , 2012 ) , and indeed -syn aggregates accumulate in response to the pharmacological inhibition of autophagy in mice ( klucken et al . , 2012 ) . -syn is a well - characterised substrate of cma due to the presence of a cma - specific pentapeptide sequence motif . it is selectively translocated across the lysosomal membrane in a complex with the heat shock cognate protein 70 ( hsc70 ) , a process dependent on lysosomal - associated membrane protein 2a ( lamp-2a ) . furthermore pathogenic forms of -syn inhibit cma through the blockade of receptor - mediated uptake into the lysosome ( cuervo et al . , 2004 ) . macroautophagy is a less selective degradative pathway , which is responsible for the bulk removal of defective organelles and mis - folded cytoplasmic proteins , including -syn , from the cell . it involves the sequestration of the cytosolic contents into double - membrane autophagosomes , which are then delivered to the lysosome to form a single - membrane autophagolysosome . macroautophagy dysregulation is increasingly being recognized as a pathogenic factor in neurodegeneration , including in pd ( ravikumar et al . , 2010 ) . consistent with this , the selective suppression of autophagy , through the neuronal loss of the autophagy genes atg7 or atg5 , results in a number of phenotypes in mice , including locomotor defects , accumulation of polyubiquitinated proteins and neurodegeneration ( hara et al . both loss - of - function and toxic gain - of - function hypothesizes have been put forward to explain the link between gba1 mutations and pd ( kinghorn , 2011 ; migdalska - richards and schapira , 2016 ) . there is growing evidence supporting the role of gcase loss - of - function in pd . the majority of gba1 mutations , including missense , nonsense and frame - shift mutations , insertions or deletions , are associated with reduced lysosomal gcase levels ( montfort et al . , moreover , milder mutations , associated with slightly diminished gcase levels , confer a much lower risk of pd than more severe mutations resulting in severe enzymatic dysfunction ( swan and saunders - pullman , 2013 ) . moreover , gcase protein levels and enzymatic activity are both decreased in the post - mortem brain tissue from patients with both idiopathic and gba1-linked pd ( gegg et al . , 2012 ) . a number of mechanisms linking gcase loss - of - function with neurodegeneration have been demonstrated ( kinghorn , 2011 ; migdalska - richards and schapira , 2016 ) . loss of gcase activity has been shown to lead to glccer accumulation , resulting in stabilisation of -syn oligomers and -syn accumulation . the subsequent increase in -syn may inhibit gcase er - golgi trafficking and lysosomal gcase levels , thus creating a bidirectional pathogenic loop ( mazzulli et al . , 2012 ) . we recently investigated the role of the autophagy - lysosomal ( als ) system in neuronopathic gd using drosophila melanogaster ( kinghorn et al . , 2016 ) . employing homologous recombination techniques we developed a fly model lacking gcase in the brain . gcase - deficient flies displayed reduced lifespan , age - dependent locomotor abnormalities , as well clear evidence of synaptic loss and neurodegeneration . in keeping with the hallmark lysosomal dysfunction seen in gd cells , staining with lysotracker revealed numerous abnormally enlarged lysosomes in the brains of flies lacking gcase . this abnormal lysosomal pathology was associated with the accumulation of glccer , similar to that seen in neuronopathic gd brains ( conradi et al . , 1984 ) glucocerebrosidase ( gcase ) deficiency results in autophagy - lysosomal system ( als ) dysfunction . under normal physiological conditions gcase normal lysosomal function is required for the autophagic clearance of defective cellular organelles and mis - folded proteins . mutations in the gba1 gene result in gcase loss - of - function and accumulation of its substrates . this leads to lysosomal dysfunction and subsequent defects in both macroautophagy and chaperone - mediated autophagy ( cma ) , with the consequent accumulation of -synuclein ( -syn ) . als dysfunction in a fly model of neuronal gcase deficiency was associated with the accumulation of abnormal giant mitochondria ( representative giant mitochondria in gba1 knockout ( gba1b ) and healthy mitochondria in wild type control fly brains are shown with yellow asterisks , shown under identical magnification ) , accumulation of autophagy substrates ( p62 and polyubiquitinated proteins ) , in addition to neurodegeneration and synaptic loss . these neuropathological defects resulted in a number of neurotoxic phenotypes , including reduced lifespan , locomotor abnormalities and decreased resistance to oxidative stress . likely as a compensatory response to the autophagy block , mammalian target of rapamycin ( mtor)-complex 1 ( mtorc1 ) activity was decreased and mitf / tfeb gene expression was up - regulated in the brains of gcase - deficient flies . the mtor inhibitor rapamycin was able to functionally rescue the lifespan , locomotor and oxidative stress phenotypes of the gcase - deficient flies , highlighting the potential therapeutic benefits of rapamycin and other inhibitors of mtorc1 in gaucher disease ( gd ) and parkinson 's disease ( pd ) ( kinghorn et al . , 2016 ) . normal lysosomal functioning is required for the formation of autolysosomes and thus the autophagic removal of misfolded proteins and defective organelles from the cell ( ravikumar et al . , 2010 ) . during macroautophagy lc3-i is converted into its lipidated form , lc3-ii , which is recruited to autophagosomal membranes ( ravikumar et al . , 2010 ) . we therefore probed macroautophagy function by measuring the levels of the fly homologue of lc3 , atg8 , by western blot analysis . this demonstrated that both atg8(lc3)-i and atg8(lc3)-ii are increased in the gcase - deficient fly brain , in addition to the ratio of atg - ii / atg - i , suggestive of increased autophagosome number and a block in autophagy flux . this mirrors the increased lc3-ii levels seen in the substantia nigra of pd patients ( dehay et al . , 2010 ) . in addition , we demonstrated increased accumulation in gcase - deficient fly brains of autophagy substrates , namely polyubiquinated proteins and ref(2)p , the drosophila homologue of p62 . both ubiquitin and p62 are abundant constituents of protein inclusions associated with neurodegenerative diseases , including lbs in pd ( zatloukal et al . . further interrogation of autophagy at the regulatory level in fly brains revealed a decrease in the phosphorylation of p70s6 kinase , a substrate of the energy sensing mammalian target of rapamycin ( mtor)-complex 1 ( mtorc1 ) . mtorc1 plays a central role in macroautophagy by initiating the formation and elongation of the autophagosomal membrane ( ravikumar et al . , 2010 ) . thus the decrease in mtorc1 activity seen in gcase - deficient fly brains likely represents a compensatory response to the autophagy block and is similar to that seen in fibroblasts derived from pd patients harbouring gba1 mutations ( magalhaes et al . , , abnormal giant mitochondria were observed in the brains of gcase - deficient flies , a reflection of the inability of the cells to clear aged and defective mitochondria by autophagy ( the process of mitophagy ) . mitf ( microphthalmia transcription factor ) is the fly homologue of mammalian tfeb ( transcription factor eb ) , the master regulator of lysosomal biogenesis . it has many conserved functions in the fly including regulation of the lysosomal - autophagy pathway and lipid metabolism ( bouche et al . , 2016 ) . gcase deficiency in the fly was also associated with mitf gene up - regulation , likely in response to the lysosomal - autophagy block . lastly treatment of gcase - deficient flies with the mtor inhibitor rapamycin resulted in a significant rescue of the lifespan and locomotor phenotypes , as well as resistance to oxidative stress ( figure 1 ) . als dysfunction is increasingly being implicated in the pathogenesis of gd and pd , and has been identified in several gd and gcase - deficient pd models . autophagy defects , in association with inflammasone activation , were observed in gd macrophages ( aflaki et al . , 2014 ) . pharmacological inhibition of gcase in mice , through the chronic systemic treatment with conduritol--epoxide , lead to the accumulation of insoluble -syn aggregates and neurodegeneration , in addition to an increase in the cma - associated protein lamp-2a and the macroautophagy marker lc3-ii ( rocha et al . gcase sirna knockdown in cell culture also inhibited macroautophagy flux with a rise in lc3-ii and p62 , and gba1 knockout in mouse embryonic fibroblasts lead to a compromise in cma . furthermore the autophagy defects were associated with loss of autophagic lysosomal reformation and maturation of endosomes . they also observed a decrease in functional acidified lysosomes within gcase - deficient cells ( magalhaes et al . , 2016 ) . enlargement of the lysosomal compartment , as observed in gcase - deficient fly brains , was also seen in gba1 mutant pd induced pluripotent stem cell ( ipsc)-derived dopamine neurons , in association with autophagy defects and accumulation of intracellular p62 ( fernandes et al . , 2016 ) . it has been suggested that both gd patients and gba1 mutation carriers display a similar risk of developing pd ( schapira , 2015 ) . consistent with this , in vitro work has shown that the degree of autophagy inhibition and -syn accumulation is comparable in ipsc - derived neurones from gd patients and gba1 mutation carriers with pd ( schndorf et al . , 2014 ) . moreover , autophagic lysosomal reformation defects in gcase - deficient cellular models were also similar in heterozygote and homozygote mutant gba1 models ( magalhaes et al . , 2016 ) . these observations are in keeping , at least in part , by the complex interplay between the loss and toxic gain - of - functions of gcase activity , the ageing cellular degradative machinery , including the als , and additional genetic risk factors predisposing to pd . work in our gcase knockout fly model , as well as in other cellular and mouse models , has demonstrated that loss of gcase activity is sufficient to cause neurodegeneration , even in the absence of -syn , when gcase levels are sufficiently low ( kinghorn et al 2016 ) . as already mentioned , -syn can further potentiate gcase deficiency through the generation of a bidirectional feedback loop , leading to lysosomal dysfunction and the generation of toxic oligomeric -syn species ( mazzulli et al . , in addition to the effects of gcase loss - of - function , the cellular sequelae of aging undoubtedly play a role . . during ageing gcase activity also declines in the substantia nigra and putamen of healthy controls , and is comparable to the enzymatic activity in pd patients with gba1 mutations ( rocha et al . , 2015a ) . this suggests that an age - dependent reduction in gcase activity may lower the threshold for developing pd , and contribute to the lysosomal dysfunction that occurs in ageing and pd . therefore moderate reductions in gcase activity in gba1 mutation carriers , in addition to the age - related decline in the als , and other cellular perturbations secondary to additional genetic and environmental risk factors , may be sufficient to promote -syn accumulation and neurodegeneration . further studies are required to better characterize the relationship between specific pathogenic gba1 mutations , their corresponding gcase activity and als function , in addition to their dose effect and clinical pd phenotypes . such studies will undoubtedly aid our understanding of the variable penetrance of gba1 mutations , in both the heterozygous and homozygous state , in causing pd . the beneficial effects of rapamycin , demonstrated in our gcase knockout fly model , suggest that the inhibition of mtorc1 may represent an additional therapeutic strategy in the treatment of gd and pd , as well as other neurodegenerative disorders , in which there is significant als dysfunction . in support of this , rapamycin increased the clearance of exogenously expressed human wild - type and mutant -syn in an inducible cell model ( webb et al . , 2003 ) as well as endogenous -syn in gba1-knockdown primary cortical neurons ( du et al . , 2015 ) . rapamycin has also been shown to mediate neuroprotective effects in other neurodegenerative diseases , including in a tdp-43-expressing amyotrophic lateral sclerosis mouse model ( bov et al . , 2011 ; wang et al . , however , inhibition of mtor by rapamycin is not invariably protective in all gba1 mutant models , and indeed rapamycin was found to trigger cell death in gd ipsc - derived neuronal cells harboring a gba1 mutation ( awad et al . , 2015 ) . furthermore chronic rapamycin treatment aggravated the pathology and muscle weakness in a mouse model of vcp ( valosin containing protein)-associated myopathy , despite a defect in mtor signaling ( ching and weihl , 2013 ) . thus the effects of inhibiting mtorc1 activity appear to vary depending on disease pathology , the model organism , and also likely the timing of the therapy . further studies in mammalian models are now required to verify the benefits of rapamycin , and other inhibitors of mtor , in ameliorating the neuropathology in gd and pd . it is likely that the clinical success of pharmacological mtor inhibitors will rely on the precise titration of mtor activity , in order to stimulate autophagy appropriately in the face of als dysfunction , minimalizing unwanted side effects .