Biochem post midterm- autism and PD
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Pyruvate dehydrogenase control
- Can be phosphorylated on its serine to inhibit activity (an ex. of a post-translational modification)
- -dephosphorylated by protein phosphatases
Function of GTP
- Important for cell signalling as its used by GPCR in its signalling cascade
- Can be converted to ATP
How can you measure the ETCs rate?
- 14C labelled pyruvate and measure 14 CO2 to estimate overall capacity of energy generation
- spectrophotometry and radiochemical assays can also be used
- can also measure rate of synthesis of ATP
- oxygen consumption rate
- assays can be used for every complex
What is mitochondrial myopathy
- genetic defects in mitochondrial structure and function leading to defective aerobic energy transduction and resulting in :
- exercise intolerance
- lactic acidosis
- Official diagnosis requires less than 30% functionality of mitochondria
What is a good indication of muscle mitochondrial disorder?
High lactic acid: pyruvate ratio
Stats on mitochondrial dysfunction
39% of people with mitochondrial dysfunction have reduced ATP production (of these people, 2/3rds showed reduced activity of oxidative phosphorylation enzymes)
Mitochondrial disease occurs in 1/5000 people
- only transferred maternally
- small, circular
- 16,000 bp codes for around 15 mitochondrial proteins (15/1300 total mitochondrial proteins)
Stats on Autism spectrum
- 1/100 children= prevalence (increased incidence/prevalence last few decades, could be better detection or a biological reason)
- 4% of people with ASD have definite mitochondrial disease (an underestimate as there's overlap with probable and possible mitochondrial disease)
What is fragile X syndrome
Most frequent single gene known to cause autism
expression of gene CGG repeats (normally about 50X but 200-300 X in FXS)
CGG repeats get methylated, leading to gene silencing and a protein important for neural development isn't transcribed.
To receive a diagnosis of autism, you must:
- Have a social disorder (communication problem, difficulty learning/understanding language)
- repititive or stereotyped behaviour
Calcium regulation and mitochondrial disease
- calcium usually bound to proteins or sequested in organelles
- free calcium in cells is in nanomolar range
- muscarinic acetylcholine receptors will activate calcium release that functions as a signalling moecule (thus Ca2+ usually tightly controlled, but not in mitochondrial diseases)
- Lots of proteins are calcium dependent: protein Kinase A, cAMP, proteases, phosphatases
Physiological consequences of Mitochondrial Dysfunction
- Increased ROS
- Reduced GABA interneuron activity (also associated with schizophrenia)
- Abnormal calcium regulation
- Reduced synaptic plasticity
What is increased in terms of mitochondrial DNA in autistic children? What is decreased?
mtDNA replication and mtDNA deletions.
low NADH oxidase activity, low succinate oxidase activity (this leads to lactate:pyruvate ratio of 6, compared to 12 for controls and implies NAD:NADH ratio of 1500:1, as opposed to 750:1 for controls)
Increased H2O2 production in lymphocytic mitochondria (Complex 1 and complex 2)
What isthe hallmark of Parkinson's?
- degeneration of DA neurons in nigrostiatal pathway
- a-synuclein aggregation causing Lewy bodies
- Dopamine deficiency
- Treated with L-Dopa
What are some cellular responses to damage?
- DNA repair
- mitochondrial unfolded protein response
What are protein chaperones?
- Proteins that help other proteins fold
- if under energy stresor mutation in chaperone, protein undergoes unfolded protein response (tags the unfolded protein for degradation
What is ubuiquitin?
Tags unfolded proteins on their lysine for degradation (ubiquitination)
What is fusion? What is fission?
- Fusion-stimulated by energy demand and stress(2 small mt to one big mt)
- Facillitates exchange of internal components, may help remove dysfunctional mitochondria with reduced mitochondrial membrane potential through autophagy-lysosomal pathway called mitophagy
Fission: generates new organelles and facillitates quality control( 1 big mt to 2 small mt)
What is autophagy?
bulk lysosomal degradation pathway essential for turnover of long-lived, misfolded, or aggregated proteins as well as damaged or excess organelles
loss of autophagy related genes results in neurodegeneration and accumulation of abnormal proteins
How is a-synuclein different in PD?
a-synuclein is overexpressed in PD, leads to imapired autophagy (leading to further accumulation)
a-synuclein degraded by chaperone mediated autophagy
What are some toxins used to emulate PD models?
- rotenone (pesticide)
- paraquat (pesticide)
all show specificity for toxicity in DA neurons
How does MPTP exert its toxicity?
- Selective inhibitor of mitochondrial complex 1, leading to ROS
- MPTP gets oxidized to MPP+, which mimics DA and utilizes the DA transporter and selectively harms dopamine neurons
What are Lewy bodies?
- Seen in PD brain tissue (but not in autosomal recessive PD)
- proteinaceous, intracelular inclusions containing ubiquitin and a-synuclein among other components deposited in nigrostriatal pathway
- Mutation of parkin is the most common cause of recessive PD
- mediates clearance of abnormal mitochondria through autophagy
- enhances mt proliferation by increasing Tfam expression
- protein kinase
- mutation that can cause recessive PD
- involved in regulating mitochondrial morphology and maintenance
- usually very short half-life
- upon reduction of mitochondrial membrane potential, pink1 stabilizes on outer mitochondrial membrane where it will accumulate
- accumulation of PINK1 induces translocation of Parkin from cytosol to mitochondria, leading to Parkin dependent ubiquitination-> degradation of mitochondrial proteins, and subsequent activation of autophagy machinery
Neurodegenerative disorders of the brain
- late-onset, progressive, age=dependent brain disorders characterize by impairment of cognitive function
- linked to energy deficiency and increased ROS
Bioenergetic deficits in PD
- Activity of complex 1 is diminished in ideopathic PD (not restricted to DA neurons, but just seems to affect DA neurons more)
- Altered regulation of complex 1 activity by transcription factors
- Reduced taining of TCA cycle rate limiting step enzyme: alpha-ketoglutarate dehydrogenase
What are cybrids? What is a rho-o cell line?
Nuclear DNA of normal cell + mt DNA of cell of interest
Rho-o cell line is a cell devoid of mtDNA (so only nuclear DNA), can combine this with cytoplast of mtDNA to create a cybrid cell
Cybrids of PD show:
reduced SIRT1 phosphorylation, reduced PGC-1alevels,reduced cellular respiration, increased NF-kB activation
What is miro?
protein that anchors mitochondria to motor proteins.
Overexpression of Miro enhances clearance of defective mitochondria and reduces defects in mitochondrial dynamics
Pink and parkin quarantine damaged mitochondria by phosphorylating miro, not allowing mitochondrial movement.
What is DJ-1
Plays a protective role by inhibiting ROS generation
What does enhanced ROS cause in PD
decreased mt proliferation, decreased ubiquitin activity, increased mitochondria membrane permeability transition,enhanced BAX mediated Cyt-C release
what does PARIS do?
ZINC-finger folding motif (usually associated with DNA bidingin)
Parkin-interacting substrate; represses expression of PGC-1a by binding to PGC-1a promoter, leading to selective loss of DA neurons.
Resveratrol activates SIRT 1 (which is thought to disrupt epigenetic silencing), and which actiavtes PGC-1a
Resveratrol blocks MPTP mediated cell death and inhibits mitochondrial dysfunction
Fusion and fission as it relates to damaged mt
Fusion: health mitochondria will fuse with unhealthy mitochondria to maintain cell homeostasis (fusion also occurs constantly in dividing cells)
Fission: damaged portion of mitochondria divides off (and is tagged by PINK1). Helps expand mitochondrial genome
Parkin and its relation to innate immunity
- Parkin is also found in macrophages
- Parkin causes a macrophage to degrade itself (autophagosome) after bacterial ingestion
- PARK2 knockout mice= increased infection and death from bacteria
PD patients have increased:
Catalase, SOD, GSH, and mtDNA content
probably to fend off oxidative stress.
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