Heat Shock Proteins (HSP):
editPhysiological stressors such as heat, oxidative stress, and ATP depletion induce the expression of Heat Shock Protein genes.[1] Expression of the HSP genes is regulated by Heat Shock Transcription Factor Protein (HSF1)[1]. When the HSF1 pathway is activated, monomers of HSF1 oligomerize as homotrimers, go to the nucleus, and bind to the promoter region all DNA which contain the stress-inducible HSP genes[2]. HSP transcripts are translated to functional Heat Shock Proteins.
HSP and Neurodengereative Diseases:
editHSP facilitate the proper folding of functional proteins and prevent the aggregation of unfolded or mis-folded proteins[3]. In many neurodegenerative diseases, changes in the expression of HSP is said to be the primary cause of rapid onset of the disease[4]. However, induced over-expression of the same HSP has shown to slow down neurodegeneration[4]
HSP and Alzheimers' Disease:
editProminent indications of Alzheimers' Disease include neurofibrillary tangles (NFT) which are formed by aggregates of the tau protein; or amyloid plaques (neuritic), formed by aggregates of the beta amyloid (aβ) proteins[5].
Tau proteins become hyper-phosphorylated due to minimal supervision from HSP and thus form toxic, insoluble NFT aggregates[6].This causes microtubules in neurons to become unstable and promotes further neurodegenerative damage[6]. Recent studies reveal that increasing the expression of HSP, specifically HSP 90 enhances the degradation and solublization of tau aggregates and promotes refolding back their native confirmation[5][6].
Aβ proteins are fragments of amyloid precursor proteins (APP) and from aggregates of hard, insoluble plaques, also known as neuritic plaques (NP)[4]. These neuritic plaques form spontaneously and are stable over time. They interfere with neural activity which increases neurodegeneration[4]. An HSP 40/70/90 complex inhibits the formation of aβ plaques and slows their rate of aggregation to decrease formation of neuritic plaques[5].
Further studies are being conducted which investigate the role of HSP 90 as a therapeutic target[5][7].
HSP and Parkinsons' Disease:
editParkinson's Disease results from a loss of dopaminergic neurons in substantia nigra[3]. Loss of dopamine neurons causes α-synuclein proteins to accumulate in the brain in the form of Lewy bodies[3]. HSP 70 is known to inhibit the accumulation of α-synuclein and decrease protein toxicity in the cell[8][9]. HSP 70 also works to prevent apoptosis and protects against early cell death, specifically neuronal cell death[10].
HSP 70 is a potential therapy target to reduce the progression of Parkinson's Disease[7][10].
HSP and Huntington's Disease
editHuntington's disease is a result of an autosomal dominant mutation due to a large number (>40) of trinucleotide repeats (CAG)[3]. The Huntingtin gene codes for the mutant huntingtin protein, htt. This mutant protein forms ubiquitinated neuronal intranuclear inclusions (NII), a form of neural protein aggregates[11].
These NIIs recruit several proteins which accumulate in the cytoplasm of the cell[11]. HSP40 and HSP70 are also recruited to the area to attempt to refold the mutant protein[12]. However, because the htt protein is very long and forms big aggregates, HSP 40 and HSP 70 are unable to exert their normal protective functions due to the vast amount of aggregates. Also, they cannot compete with the fast rate of aggregation[12]. This results in the overwhelming accumulation of misfolded mutant htt proteins over the course of the disease. This accumulation causes in neuronal degeneration in the striatum and the cortex which interferes with coordination and motor skills, which worsen over time[3].
Over-expression of HSP 40 and HSP 70 is a therapy mechanism that is being investigated to treat the symptoms of Huntington's Disease[12].
HSP and Autophaghy
editThe HSP chaperone system and Autophagy work together to maintain functional proteins within cells under extreme conditions (excessive stress, starvation). In a process known as chaperone-mediated autophagy[13], HSP target proteins in the cytoplasm which expose a a signature motif. Once the HSP recognize the motif, the protein is bound to a lysosomal associated membrane protein (LAMP2) which causes the protein to unfold and translocate into the lysosome to be degraded[13]. Microautophagy and Macroautophagy are another way for cells to degrade dysfunctional proteins, but are unassociated with HSP. Therefore, in certain conditions, cells prioritize HSP mechanisms over autophagy to completely inhibit other modes of cellular autophagy.[13]
Cardiovascular:
editHSP maintain and stabalize cardiac myocytes to ensure that they are functional and operate at their full potential[1]. In instances where cardiac myocytes suffer an injury (ischemia), HSP are expressed at an elevated rate. [14] For example, damage caused by an increase in reactive oxygen species (ROS), which are a result of cardiac ischemia[14] have detrimental effects on the cell and its cellular components. Antioxidants are a form of protective molecules that cells use in response to high levels of oxidative stress caused by ROS. These antioxidants neutralize the ROS to minimize their damage in the cell while HSP work to repair the damage caused by the ROS in the cells.[15]
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- ^ a b c d Adachi, Hiroaki; Katsuno, Masahisa; Waza, Masahiro; Minamiyama, Makoto; Tanaka, Fumiaki; Sobue, Gen (2009-01-01). "Heat shock proteins in neurodegenerative diseases: Pathogenic roles and therapeutic implications". International Journal of Hyperthermia. 25 (8): 647–654. doi:10.3109/02656730903315823. ISSN 0265-6736.
- ^ a b c d Ou, Jiang-Rong; Tan, Meng-Shan; Xie, An-Mu; Yu, Jin-Tai; Tan, Lan (2014-10-13). "Heat Shock Protein 90 in Alzheimer's Disease". BioMed Research International. 2014: 1–7. doi:10.1155/2014/796869. ISSN 2314-6133. PMC 4211323. PMID 25374890.
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- ^ a b c Hansson, Oskar; Nylandsted, Jesper; Castilho, Roger F.; Leist, Marcel; Jäättelä, Marja; Brundin, Patrik (2003-04-25). "Overexpression of heat shock protein 70 in R6/2 Huntington's disease mice has only modest effects on disease progression". Brain Research. 970 (1–2): 47–57. doi:10.1016/S0006-8993(02)04275-0.
- ^ a b c Dokladny, Karol; Myers, Orrin B; Moseley, Pope L. "Heat shock response and autophagy—cooperation and control". Autophagy. 11 (2): 200–213. doi:10.1080/15548627.2015.1009776. PMC 4502786. PMID 25714619.
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