Huntington’s disease

1. Huntington’s disease Also called Huntington Chorea, affects men and women. 4 to 10 cases in 100,000 people (frequency between 0.004% and 0.01%) . Chorea is defined as brief, irregular, unpredictable, purposeless movements that flow from one body part to another without a rhythmic pattern. HD is a Hyperkinetic Disorder. The symptoms in HD are characterized by Excessive motor activity Involuntary movements (dyskinesias) Decreased muscle tone Behavioral and emotional alteration and cognitive decline, which may precede motor abnormalities. Importance of cognitive and neuropsychological test to identify the pre-symptomatic carriers of the disease. HD is a dominant autosomal inherited disease characterized by the expansion of a triplet repeat CAG, that encodes for Glutamine, in the N-terminal domain of the protein.

2. George Huntington (1850-1916)

3. Specific localization of Basal Ganglia in a coronal section

4. HD is caused by selective degeneration of the GABAergic neurons in the striatum, in particular in the caudate

5. Volumetric analysis in Huntington disease (HD) HD Normal

6. Age of onset: HD is thought to be a true dominant disorder, since homozygous carriers of the disease are no more severely affected than heterozygous carriers. In a normal individual, the number of CAG repeats are <36. In HD patients, age of onset when the repeats are 46 ranges between 25 and 52 years old. However, many factors influence the onset of the disease, such as: Mitochondrial dysfunction Cell death by apoptosis Loss of neurotrophic factors Neuron excitotoxicity Length of the CAG repeat and onset of the disease: Inverse correlation, but not when onset is >50 years of age.

7. Deposition of fibrillar proteinacious material in Huntington’s disease Huntington’s disease: a progressive neurodegenerative disease characterized by CAG repeats causing glutamine expansion motifs (polyQ) in the N-terminal region of the protein huntingtin. Onset of the disease inversely correlates with the number of CAG repeates. Huntingtin can aggregate forming intracellular inclusion bodies that contain also proteasome proteins and ubiquitin. Huntingtin aggregates contain b-sheet structures similar to amyloid.

8. Huntingtin In homo sapiens, the gene for huntingtin encodes a large, cytosolic protein, comprised of 3144 aa and with a molecular weight of about 348 kDa. PolyQ P NH2 COOH aa1 aa3144 In normal individuals, PolyQ<36. In HD patients, PolyQ>36

9. Processing of PolyQ, mutant huntingtin Cdk5, Akt NH2 PolyQ P COOH aa1 aa3144 Cleavage by caspase 2, 3, 6 calpain NH2 COOH N-terminal fragment C-terminal fragment N-term htt forms aggregates

10. In HD, the aggregates are present in the affected brain regions, although not necessarily in the degenerating neurons In HD, are the aggregates protecting from neurodegeneration? Are htt oligomers the most toxic species?

11. Huntington’s disease: loss of function of the wild type huntingtin and gain of toxic function of the mutant huntingtin -Heterozygous patients are NOT different in any way from the homozygous patients: in heterozygous patients, the amount of wild-type “good” huntingtin does not make it to “buffer’ the effects of the mutant, PolyQ expanded huntingtin. -Mutant, PolyQ expanded huntingtin can recruit wild type huntingtin into insoluble aggregates to form the inclusions. -Neurons with larger inclusions are less compromised than those ones with diffused mutant htt (which might contain oligomers not aggregated into inclusions).

12. Potential mechanism of cell death in Huntington’s disease

13. Which is the physiological role of wild type huntingtin? • Huntingtin is a molecule required during development, as hungtintin KO mice die at 7.5 postnatal day. Heterozygosity is enough to reach adulthood. • -Huntingtin interacts with several proteins, i.e. cytoskeletal proteins and others that are associated with membrane vesicles, suggesting a role in intracellular trafficking, membrane recycling and retrograde fast axonal transport. • -The presence of Q repeats (CAG) followed by PolyP domain associates with transcriptional regulatory proteins. Inverse correlation between number of CAG repeats and transcriptional activity. After interacting, huntingtin transports transcriptional regulatory proteins in the cytosol and within the nucleus. (BDNF).

14. Physiological roles of wild type huntingtin

15. Physiologic function of wild-type huntingtin: Cell survival, anti-apoptotic function

16. Cell death is prevented by huntingtin wt, but is promoted by huntingtin PolyQ mutant Normal Serum withdrawal Immortalized embrional striatal cells N548Mu N548WT Rigamonti et al., 2000

17. Temperature-dependent apoptotic DNA fragmentation is sustained by huntingtin PolyQ mutant, but is prevented by huntingtin wt Rigamonti et al., 2000

18. Physiologic function of wild-type huntingtin: modulating the expression and the transport of BDNF (Brain Derived Growth Factor) to striatal neurons

19. -Peculiarity of HD is that lethality occurs specifically in striatal neurons, although huntingtin is ubiquitously expressed. -It is known that huntingtin promotes neuronal survival (and is required during development) -BDNF has certainly a role in maintaining the “health” of striatal neurons. In facts, BDNF: a)promotes growth and differentiation of striatal neurons b)protects striatal neurons from excitotoxicity

20. Wild-type huntingtin, but not mutant PolyQ huntingtin, specifically increase the levels of secreted and cellular BDNF in cell culture Zuccato et al., 2002

21. BDNF seems likely to be produced in cortex and hippocampus, and to be subsequently transported by afferents to the striatal neurons What happens in the cortex of HD patients?

22. Levels of BDNF mRNA are reduced in cortex of HD patient Zuccato et al., 2002

23. Pathogenic mechanisms of mutant PolyQ huntingtin: Htt processing and formation of aggregates

24. Processing of PolyQ, mutant huntingtin Cdk5, Akt NH2 PolyQ P COOH aa1 aa3144 Cleavage by caspase 2, 3, 6 calpain NH2 COOH N-terminal fragment C-terminal fragment N-term fragment forms aggregates

25. Different translocation of PolyQ, mutant huntingtin N-terminal fragment NH2 N-terminal fragment Capabilities to form aggregates Soluble form activates caspase-1 positive feedback in regulating apoptosis Glu vesicle cytosol nucleus Inhibits Glu Uptake

26. How does the CAG, PolyQ expanded motif cause HD? -It induces mitochondrial dysfunction, that leads to mitochondrial disruption of Ca++ homeostasis: trigger of cytochrome c and caspase-dependent apoptosis. -It avoids certain post-translation modifications (like SUMOylation and palmitoylation) to occur on the protein, with consequences on the protein’s functionality. -The N-terminal fragment translocates to the nucleus where it causes: a) reduction of the synthesis of neurotrophic factors (like BDNF Brain-Derived Neurotrophic Factor), that are essential for the neuronal growth and life (REST/NRSF). b) formation of intranuclear aggregates that recruit also transcription factors (CREB binding protein CBP), inducing transcriptional deregulation.

27. Pathogenic mechanisms of mutant PolyQ huntingtin: Mitochondrial dysfunction

28. Membrane Potential: Difference in the voltage between the two sides of a membrane, caused by an influx and efflux of ions to and from a cell or neuron and an organelle. In the mitochondrion, the membrane potential is caused in particular by exchange of Ca++ ions between the mitochondrion and the cytosol. PTPs: Permeability Transition Pores Ca++ Ca++ PTP

29. Membrane potential and calcium levels are reduced in HD mitochondria Lymphoblasts Panov et al., 2002

30. In HD patients, mitochondrial depolarization occurs at lower Ca++ levels PTP compromised? Physiologically, the potential drops when the PTP open Juvenile HD patient: 65 repeats Adult HD patient: 46 repeats Panov et al., 2002

31. The tendency of the PTP to be open directly correlates with the number of PolyQ repeats in huntingtin mutant molecule

32. Huntingtin aggregates are visible on the mitochondrial membrane of PolyQ mutant huntingtin transgenic mice, but not of wild-type huntingtin transgenic mice Huntingtin PolyQ mutant transgenic mice Huntingtin wild-type transgenic mice Panov et al., 2002

33. Evidences that Calcium homeostasis is altered 1-in mitochondria of HD patients 2- in mitochondria of transgenic mice over-expressing mutant htt, but not in mice over-expressing wild type htt 3-administration of synthetic PolyQ repeats to human wild-type mitochondria in vitro causes alteration of calcium homeostasis Altered PTP and mitochondrial calcium homeostasis EARLY EVENT in HD

34. Mutant PolyQ huntingtin disrupts Calcium homeostasis, probably forming aggregates on the mitochondrial membrane that cause abnormal opening of the Permeability Transition Pores. Can mutant htt cause mitochondrial dysfunction also using different mechanisms?

35. Serum deprivation increases mitochondrial fragmentation in cells over-expressing mutant htt HeLa cells Serum deprivation to induce oxidative stress

36. Mutant htt sensitizes cells to mitochondrial abnormalities following oxidative stress

37. The only over-expression of toxic mutant htt causes mitochondrial fragmentation and dysfunction

38. Mutant htt affecting mitochondrial dynamics?

39. Mitochondrial fusion seems to occur only in cells over-expressing wt htt, but not mutant htt

40. Abolishing fission (Drp1 mutant) or inducing fusion (Mfn2 expression) RESCUES the mitochondrial fragmentation phenotype Cellular death by apoptosis caused by mutant htt is rescued too

41. Mutant htt PREDISPOSES cells to toxic stimuli leading to increased mitochondrial fission and therefore dysfunction A role for unbalanced fusion/fission in mitochondrial dysfunction in HD?

43. Mutant htt causes mitochondrial fragmentation in fibroblasts of HD patients

44. PolyQ expansion associated with HD impairs mitochondrial movement

45. PolyQ expansion associated with HD affects mitochondrial size … WT YAC128

46. …and structure: loss of cristae’s density and presence of constrictions normal YAC128 “sagittal” Side view of outer membrane “coronal”

47. Mutant HTT associates with DRP1 in HD Colocalization is also found in neurons Human Brains Mitochondrial lysates from mouse brain

48. Mutant HTT has greater affinity than wt HTT for DRP1: effects on DRP1 GTPase acrivity DRP1 forms oligomers in presence of GTP (is a GTPase), even thicker in presence of mutant htt. DRP1 + nucleotides +httQ ext1-53 DRP1 + nucleotides DRP1 No nucleotides

49. Inactive Drp1 mutant rescues the effects of polyQ htt on mitochondrial fragmentation and cell death (and also transport) Same effects achieved also by over-expression on Mfn2

50. Re-established mitochondrial fission and fusion balance improves the toxic effects of polyQ htt on mitochondrial motility, fragmentation and on cell death.