Targeting Autophagy to Treat Neurodegenerative Diseases Flashcards
Targeting Autophagy in NDDs
For most NDDs the available evidence favors a strategy of enhancing the efficacy of autophagy
• Treatment with rapamycin provided a proof of principle
Limitations in targeting autophagy in NDDs
- Strategy might not work is cargo recognition by the autophagosome is impaired (if cargo isn’t recognized –> increased autophagy isn’t helpful)
- Timing is very important
- Inducing autophagy if autophagy flux is inhibited may have serious detrimental effects (more autophagosome that cannot be cleared = worsens the issues)
Enhancing Protein Clearance by Activating Autophagy through
- mTOR inhibitors
- AMPK activators
- Inositol lowering agents
mTOR inhibitors
Rapamycin
Kinase inhibitors
AMPK activators
- Metformin
- Trehalose
Inositol Lowering agent
lithium
Rapamycin (Sirolimus)
Rapamycin: Antibiotic with antifungal and
immunosuppressive properties
How rapamycin binds
- Rapamycin works by an allosteric mechanism: Does not bind to the kinase domain (activation loop)
- Rapamycin binds to the cytoplasmic receptor FKBP12
- FKBP12-rapamycin complex binds to mTOR
- FKBP12-rapamycin complex destabilizes mTORC1
T/F: Rapamycin does NOT impair kinase activity directly
TRUE
binds to the FKP12 receptor rather than the activation loop
Kinase Inhibitors of mTOR
- Selective ATP-competitive small molecule directly inhibit the kinase domain of mTOR
- gets into activation loop and directly inhibits kinase activity
Kinase inhibitors vs. Rapamycin
Autophagy induction by ATP-competitive
inhibitors is higher than by rapamycin
- but rapamycin has been widely used in humans (safe for human use)
Rapamycin facilitates autophagy through
– induction of autophagy initiation (ULK complex)
– enhancement of lysosomal biogenesis (enhances TFEB translocation to nucleus)
mTOR inhibitors is complicated because
- Use of mTOR inhibitors is complicated due to mTOR effects independent of autophagy
non-autophagy effects of mTOR
- mTOR regulates translation and cell growth, and whole-body metabolism)
- treatment with rapamycin impairs wound healing and has immunosuppressive effects
(prevent its use in infectious diseases and immunosuppressed individuals) - Treatment not ideal for chronic diseases
Rapamycin in AD
• In AD rapamycin is an effective inhibitor of neurodegeneration in mouse models of AD – improves AD-linked cognitive deficits – reduces Aβ accumulation – Attenuates tau hyper-phosphorylation
Success of any autophagy induction intervention may depend on the…
TIMING
– Should be performed very very early, before Ab accumulation
Rapamycin use in HD: results
Studies on cellular models gave promising results indicating a possibility to enhance degradation of aggregated proteins
In animal experiments rapamycin caused:
– a decrease in levels of mHTT aggregates
– amelioration of neurodegeneration
– Improved animal behavior
Timing for rapamycin use
- Rapamycin is effective at early stages of protein aggregation
- When aggregates become highly stable they seem to be too large to be degraded by autophagy
Downsides of rapamycin use in HD
- Autophagy activators may have a limited beneficial effect if cargo recognition is defective
- Despite these encouraging results clinical trials with the use of rapamycin for treatment of HD have not been started yet
Metformin: drug type
- Prototypical activator of AMPK
- Widely used anti-diabetic agent
Metformin & autophagy
INDUCES autophagy
• Some of the effects of Metformin in diabetes may be mediated via autophagy
• Worth considering use if autophagy upregulation is beneficial
• Mediates multiple AMPK-unrelated effect
Metformin in NDDs
- Metformin abrogated α-synuclein toxicity in primary cultures of cortical neurons
- Metformin reduced neuronal loss in a neurochemical lesion model of PD in mice
- In metformin-treated neurons, hyperphosphorylated tau and Aβ were reduced
Trehalose: what is it
is a a disaccharide present in bacteria, yeast, fungi and plants but not in vertebrates
How trehalose works
- It affects mTOR-independent regulation of autophagy
- It also affects mTOR-dependent regulation of autophagy
Trehalose: mTOR-independent regulation of autophagy
– induces TFEB translocation to the nucleus:
• Enhances expression of autophagy-related genes (Beclin1, p62, LC3, etc)
– activates AMPK which activates the ULK complex
Trehalose: mTOR-dependent regulation of autophagy
– activates AMPK, which inhibits mTOR
Mettformin in human us: retrospective studies
Retrospective studies with Metformin use for more than 2 years:
– significant reduction in neurodegenerative disease
– neuroprotective
– promotes neurogenesis
Metformin in human observational studies
Multiple observational studies reported varying results on the value of metformin for preventing AD in DMT2 patients
– Three studies reported a decreased risk for cognitive impairment or dementia
– One meta-analysis suggested a trend for reduced risk of dementia with metformin use in diabetics
– Three other studies reported an increased risk for impaired cognitive performance, dementia, or AD
– One study reported that longer metformin use was associated with an increasing risk for dementia
MIXED results
Regulation of AMPK by Trehalose
- Trehalose inhibits a family of glucose transporters (SLC2A) generating a starvation-like state
- trehalose competes with sugar at the transporters –> prevent sugar entry –> decrease ATP –> AMPK activated
AMPK & autophagy
- Low energy levels (high AMP) stimulates autophagy through AMPK
Inhibiting AMPK effects
fructose/glucose entry increases ATP –> inhibits AMPK –> activates mTOR –> no autophagy
Trehalose–regulation of AMPK
Low energy levels (high AMP) stimulates autophagy through AMPK –> mimicked by trehalose (by competing with sugars for entry and decreasing ATP)–> AMPK phosphorylates and activates of ULK1 + inhibits mTOR –> autophagosome biogenesis
Trehalose in Models of NDDs
Trehalose enhances the removal of misfolded proteins in several neurodegenerative models
Trehalose in cellular vs. animal models
- Trehalose proved effective in cellular models of PD, HD and AD –> In cultured cells trehalose decreases cytosolic aggregates of mHtt, a- synuclein and p- tau
- Trehalose was also effective in mouse models of HD, AD and tauopathies–> In animal models it cleared aggregates, reduced neurodegeneration and ameliorated motor and cognitive performance
Trehalose for Human Therapy: issues
approved for human use BUT issues with availability
- Uncertainty if trehalose crosses BBB or enter neurons –> requires transporter not expressed in neurons
- Trehalose cannot be delivered orally in humans– because it is degraded by trehalase, an enzyme present in the gastrointestinal tract
Trehalose use in humans: how to make it work
Nanolipid-trehalose conjugated have been developed
– effective autophagy inducers
– overcome the poor pharmacokinetics of this sugar
Lithium: as a therapeutic agent
- Lithium inhibits inositol monophosphatase
- Lithium also activates AMPK
- Lithium has effects independent of autophagy
T/F: lithium Lithium induces autophagy through mTOR
FALSE
Lithium induces autophagy independent of mTOR
How lithium works
- Lithium prevents inositol recycling, which leads to depletion of cellular inositol and inhibition of the phosphoinositol cycle
- Inositol and IP3 are negative regulators of
autophagosome production
THEREFORE lithium increases autophagy by depleting cellular inositol
Lithium in Models of NDDs
Lithium enhances the cellular clearance of aggregate-prone forms of huntingtin, α-synuclein, tau and SOD1 in cellular models
Lithium efficacy against NDDs
Lithium has neuroprotective effects in most NDDS, similar to rapamycin
– It ameliorates motor function in a mouse model of tauopathy
– It slows disease progression in HD models
lithium as a combination therapy
Effects are additive to rapamycin, metformin and other autophagy enhancers
Human use of lithium
–> Lithium used off-label in experimental therapy in 3 HD patients–> each patient showed different results
– some neurological parameters were improved, but no changes in chorea
could be or viceversa
– Problem: all these patients received also other drugs
Other clinical trials with HD patients also did not give conclusive results: extremely different responses of various persons
