**3. β-Amyloid chaperones**

The main function of molecular chaperones is to facilitate their protein targets acquire proper functional conformation or fold into correct oligomeric assemblies [38]. Several proteins have been shown to bind misfolded amyloids and inhibit amyloid aggregation or promote refolding of polymorphs. They usually play other physiological roles, yet show tendency to prevent amyloidosis under specific stimuli. We will discuss some of these non-proteolytic chaperones in more detail below. Additionally, albumin binding to polymeric Aβ, α1-antitrypsin, immunoglobulins A and G are endogenous human cerebrospinal fluid (CSF) proteins with capability to control amyloid formation, although the inhibitory activity is lesser by two orders of magnitude compared to plasma [39].

### **3.1 Heat shock proteins**

Heat shock proteins (Hsp) are the most commonly known chaperones in human body, divided into five classes Hsp70, Hsp90, Hsp60, Hsp40, and small Hsps, differing in protein size. Chaperones in Hsp60 and Hsp70 family mediate folding, Hsp33 family hold partially folded proteins till other chaperones or degradation system can act, and Hsp104 promotes solubilization of aggregated proteins [38]. Mutations in αA- or αB-crystallin and other small heat shock proteins have been linked to increased risk of certain amyloidosis conditions [40]. Hsp70 and Hsp90 are capable of utilizing ATP molecules to perform energy costly unfolding of stable misfolded aggregates and then convert them into properly folded conformations. Similarly other members like Hsp110 can act as disaggregases to forcibly solubilize preformed fibrillar aggregates [41]. Hsp70 recognizes KFERQ motif in proteins to mediate degradation by transporting them to lysosomal compartments inside the cell, a typical example of chaperone mediated autophagy [41]. Disaggregation machinery in humans mainly involves proteins from Hsp family namely, Hsp110, Hsp105, Hsp100, and Hsp70/40 co-chaperone cognates [41]. Proteins in Hsp40 co-chaperone family, mainly DnaJB6 and B8, can decrease the aggregation of polyglutamine peptides in HD in addition to modulating Hsp70 activity [41]. Binding of misfolded proteins and interaction with ATPase domain of Hsp70 are achieved via the highly conserved J domain. Anti-aggregation activity of Hsps, especially Hsp70, is at least partially dependent on presence of ATP [42]. ADP-bound forms of Hsp70 is generated as a result of the ATP hydrolysis by its cochaperones. This form has high affinity for the hydrophobic residues in misfolded peptides and holds the unfolded ensemble till it spontaneously achieved proper conformational populations. Small Hsps (12–42 KDa) do not require ATP since they usually function as holdases and assist other proteins in Hsp chaperone complex [41]. Hsp104 is part of proteostasis network which regulates prion assembly in yeast, Hsp110 is its human counterpart which facilitates chaperone function of Hsp70 and Hsp40 [43]. Hsp90 and its co-chaperones interact with tau and a close control on their interaction can cause the oligomers to turn into benign species instead of toxic aggregates [44]. Free Hsp90 binds and releases substrates in its monomeric extended form, and attains a dimeric ATP-bound closed conformation mediate by co-chaperones p23/Sba1, Hsp70/90 organizing protein (HOP), Cdc37 and other proteins [41, 45]. Hsp60 chaperonins are mainly located in mitochondria and form heptameric, double ring complexes which provide isolated environment for protein folding with co-chaperonins Hsp10 forming the lid of this cavity. Unlike Hsp60, the cytosolic chaperonins in neurons such as TCP-1 Ring Complex (TRiC or CCT), do not require co-chaperones for their function and form a double

**195**

as TSEs.

*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease*

ring complex with each ring consisting of eight subunits. Hsp60 can interact with mutant αS in PD brain [41]. B-chain of monomeric or dimeric insulin can bind to IAPP monomers, thus preventing their aggregation [7]. Presence of chaperone proteins such as Hsp27 (HSPB1), αB-crystallin (HSPB5) and Hsp70 (HSPA1A) can decrease cell toxicity, possibly by binding Aβ oligomers and converting them to larger less toxic aggregates, however it not clear whether they bring about any change in secondary or tertiary structure [44]. HSPA6 is induced in neurons post heat shock, unlike other members of this family HSPA1A and HSP8, thus has a unique probable role as a human neuronal chaperone [46]. Hsp70 and Hsp90 have differential roles in case of inflammation and macrophage recruitment. Hsp90 can also stabilize neurotoxic proteins and should be carefully controlled to achieve

Molecular chaperones are capable of binding and folding intracellular soluble amyloid aggregates, however solubilization of amyloid plaques is out of scope for these proteins. While these chaperones may not be able to completely reverse neurodegenerative symptoms, they play crucial anti-apoptotic functions through protein folding and degradation of unfolded or misfolded proteins. There are fundamental mechanisms underlying a chaperone function, namely, unfolded protein response (UPR), protein compartmentalization, heat shock response, chaperone-mediated autophagy and lysosome system, ER associated degradation (ERAD) and ubiquitin-proteasome system (UPS) [46]. Extracellular chaperones mediate proper protein folding and refolding by providing isolated environment or through intermolecular interactions. If proper folding cannot be achieved, they may play a role in mediating intracellular proteasomal degradation or microglial digestion of unfolded protein fragments. Cytosolic chaperone systems like Hsp70 and Hsp40, comprising heat shock cognate 70 (Hsc70), and their interacting proteins like C-terminus of Hsc70-interacting protein (CHIP) can mediate targeting of misfolded proteins to proteasome machinery [38]. Intracellular pathways for degradation of misfolded proteins include the UPS, 26S proteasome and lysosomemediated phagocytosis [9]. Hsp70 recruits misfolded proteins, especially αS, for degradation by directing them to proteasomes or autophagy-lysosomal pathway [44]. UPS comprises of a cascade of enzymes E1 (Ub activating enzyme), E2 (Ub conjugating enzyme) and E3 (Ub ligase) facilitating binding of ubiquitin (Ub) to target proteins. Ligase enzymes in this cascade can employ chaperone proteins such as Hsc70 for recognition of exposed hydrophobic regions misfolded peptides and together mediate proteolysis of misfolded intracellular proteins. E4 enzyme from UPS can recruit additional Ub molecules to the protein substrate which can be either directed for degradation or other protein interactions based on their topology [41]. 26S proteasome particles degrades polyUb-conjugated proteins with the help of its two 19S regulatory particles responsible for recognition and de-ubiquitination and a 20S core particle through which proteolysis of these substrates into short peptides is mediated [41]. ERAD withholds misfolded proteins from continuing with on-going cargo to golgi apparatus for further downstream processes. UPR is initiated by migration of transcription factors to the nucleus causing upregulation of ER chaperones-encoding gene expression in response to increasing in unfolded proteins, such as Aβ aggregates in neuronal ER, which require assistance of chaperones present in the ER [47, 48]. Reduction in UPS proteolytic activity is linked with disease pathology in AD, PD, ALS, HD as well

*DOI: http://dx.doi.org/10.5772/intechopen.84238*

required therapeutic outcomes.

**3.2 Intracellular degradation machinery**

*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease DOI: http://dx.doi.org/10.5772/intechopen.84238*

ring complex with each ring consisting of eight subunits. Hsp60 can interact with mutant αS in PD brain [41]. B-chain of monomeric or dimeric insulin can bind to IAPP monomers, thus preventing their aggregation [7]. Presence of chaperone proteins such as Hsp27 (HSPB1), αB-crystallin (HSPB5) and Hsp70 (HSPA1A) can decrease cell toxicity, possibly by binding Aβ oligomers and converting them to larger less toxic aggregates, however it not clear whether they bring about any change in secondary or tertiary structure [44]. HSPA6 is induced in neurons post heat shock, unlike other members of this family HSPA1A and HSP8, thus has a unique probable role as a human neuronal chaperone [46]. Hsp70 and Hsp90 have differential roles in case of inflammation and macrophage recruitment. Hsp90 can also stabilize neurotoxic proteins and should be carefully controlled to achieve required therapeutic outcomes.

### **3.2 Intracellular degradation machinery**

*Amyloid Diseases*

**3. β-Amyloid chaperones**

**3.1 Heat shock proteins**

two orders of magnitude compared to plasma [39].

The main function of molecular chaperones is to facilitate their protein targets acquire proper functional conformation or fold into correct oligomeric assemblies [38]. Several proteins have been shown to bind misfolded amyloids and inhibit amyloid aggregation or promote refolding of polymorphs. They usually play other physiological roles, yet show tendency to prevent amyloidosis under specific stimuli. We will discuss some of these non-proteolytic chaperones in more detail below. Additionally, albumin binding to polymeric Aβ, α1-antitrypsin, immunoglobulins A and G are endogenous human cerebrospinal fluid (CSF) proteins with capability to control amyloid formation, although the inhibitory activity is lesser by

Heat shock proteins (Hsp) are the most commonly known chaperones in human body, divided into five classes Hsp70, Hsp90, Hsp60, Hsp40, and small Hsps, differing in protein size. Chaperones in Hsp60 and Hsp70 family mediate folding, Hsp33 family hold partially folded proteins till other chaperones or degradation system can act, and Hsp104 promotes solubilization of aggregated proteins [38]. Mutations in αA- or αB-crystallin and other small heat shock proteins have been linked to increased risk of certain amyloidosis conditions [40]. Hsp70 and Hsp90 are capable of utilizing ATP molecules to perform energy costly unfolding of stable misfolded aggregates and then convert them into properly folded conformations. Similarly other members like Hsp110 can act as disaggregases to forcibly solubilize preformed fibrillar aggregates [41]. Hsp70 recognizes KFERQ motif in proteins to mediate degradation by transporting them to lysosomal compartments inside the cell, a typical example of chaperone mediated autophagy [41]. Disaggregation machinery in humans mainly involves proteins from Hsp family namely, Hsp110, Hsp105, Hsp100, and Hsp70/40 co-chaperone cognates [41]. Proteins in Hsp40 co-chaperone family, mainly DnaJB6 and B8, can decrease the aggregation of polyglutamine peptides in HD in addition to modulating Hsp70 activity [41]. Binding of misfolded proteins and interaction with ATPase domain of Hsp70 are achieved via the highly conserved J domain. Anti-aggregation activity of Hsps, especially Hsp70, is at least partially dependent on presence of ATP [42]. ADP-bound forms of Hsp70 is generated as a result of the ATP hydrolysis by its cochaperones. This form has high affinity for the hydrophobic residues in misfolded peptides and holds the unfolded ensemble till it spontaneously achieved proper conformational populations. Small Hsps (12–42 KDa) do not require ATP since they usually function as holdases and assist other proteins in Hsp chaperone complex [41]. Hsp104 is part of proteostasis network which regulates prion assembly in yeast, Hsp110 is its human counterpart which facilitates chaperone function of Hsp70 and Hsp40 [43]. Hsp90 and its co-chaperones interact with tau and a close control on their interaction can cause the oligomers to turn into benign species instead of toxic aggregates [44]. Free Hsp90 binds and releases substrates in its monomeric extended form, and attains a dimeric ATP-bound closed conformation mediate by co-chaperones p23/Sba1, Hsp70/90 organizing protein (HOP), Cdc37 and other proteins [41, 45]. Hsp60 chaperonins are mainly located in mitochondria and form heptameric, double ring complexes which provide isolated environment for protein folding with co-chaperonins Hsp10 forming the lid of this cavity. Unlike Hsp60, the cytosolic chaperonins in neurons such as TCP-1 Ring Complex (TRiC or CCT), do not require co-chaperones for their function and form a double

**194**

Molecular chaperones are capable of binding and folding intracellular soluble amyloid aggregates, however solubilization of amyloid plaques is out of scope for these proteins. While these chaperones may not be able to completely reverse neurodegenerative symptoms, they play crucial anti-apoptotic functions through protein folding and degradation of unfolded or misfolded proteins. There are fundamental mechanisms underlying a chaperone function, namely, unfolded protein response (UPR), protein compartmentalization, heat shock response, chaperone-mediated autophagy and lysosome system, ER associated degradation (ERAD) and ubiquitin-proteasome system (UPS) [46]. Extracellular chaperones mediate proper protein folding and refolding by providing isolated environment or through intermolecular interactions. If proper folding cannot be achieved, they may play a role in mediating intracellular proteasomal degradation or microglial digestion of unfolded protein fragments. Cytosolic chaperone systems like Hsp70 and Hsp40, comprising heat shock cognate 70 (Hsc70), and their interacting proteins like C-terminus of Hsc70-interacting protein (CHIP) can mediate targeting of misfolded proteins to proteasome machinery [38]. Intracellular pathways for degradation of misfolded proteins include the UPS, 26S proteasome and lysosomemediated phagocytosis [9]. Hsp70 recruits misfolded proteins, especially αS, for degradation by directing them to proteasomes or autophagy-lysosomal pathway [44]. UPS comprises of a cascade of enzymes E1 (Ub activating enzyme), E2 (Ub conjugating enzyme) and E3 (Ub ligase) facilitating binding of ubiquitin (Ub) to target proteins. Ligase enzymes in this cascade can employ chaperone proteins such as Hsc70 for recognition of exposed hydrophobic regions misfolded peptides and together mediate proteolysis of misfolded intracellular proteins. E4 enzyme from UPS can recruit additional Ub molecules to the protein substrate which can be either directed for degradation or other protein interactions based on their topology [41]. 26S proteasome particles degrades polyUb-conjugated proteins with the help of its two 19S regulatory particles responsible for recognition and de-ubiquitination and a 20S core particle through which proteolysis of these substrates into short peptides is mediated [41]. ERAD withholds misfolded proteins from continuing with on-going cargo to golgi apparatus for further downstream processes. UPR is initiated by migration of transcription factors to the nucleus causing upregulation of ER chaperones-encoding gene expression in response to increasing in unfolded proteins, such as Aβ aggregates in neuronal ER, which require assistance of chaperones present in the ER [47, 48]. Reduction in UPS proteolytic activity is linked with disease pathology in AD, PD, ALS, HD as well as TSEs.

### **3.3 Non-proteolytic amyloid chaperones**

Aging is a major risk factor for many neurodegenerative diseases. The UPS system is known to get deregulated with increase in neuronal age and therefore offers less resistance to pathological protein aggregates. Proteins such as Human serum albumin show competitive binding towards Aβ oligomers, with its different domains binding to many oligomeric molecules [12]. It binds almost 90% of plasma Aβ peptides potentially through dual binding mechanism involving Aβ(1-40) monomers and protofibrils likely utilizing residues involved in fibril formation.

Here we suggest non-proteolytic endogenous proteins which show potential as AD therapeutics directed against Aβ, namely, Lipocalin-type prostaglandin D synthase (L-PGDS, also known as β-trace), apolipoprotein E (ApoE), α2 macroglobulin (α2M), haptoglobin and clusterin as alternatives to intracellular Aβ degradation machinery. These proteins have been sporadically studied for their holdase or transporter activity, yet their Aβ chaperone function is still unexplored. L-PGDS is the second most abundant protein in human CSF after albumin [49]. It exhibits dual functions, as a lipophilic ligand transporter in cells and as isomerase in arachidonic acid pathway to convert prostaglandin H2 to prostaglandin D2 [50]. L-PGDS plays protective roles in different neurological diseases including genetic demyelinating disease, brain injuries and multiple sclerosis [51]. It plays protective role against cerebral ischemia as well [52]. It promotes recruitment of astrocytes and glial cells to the source of injury [53]. Early stress stimulus can upregulate L-PGDS suggesting its unknown, yet novel stress protection mechanism. L-PGDS deficient mice showed additional neuronal apoptosis strongly indicating its important protective function in neurons and surrounding oligodendrocytes [51]. L-PGDS binds to various Aβ peptides including fibrils and is colocalized in Aβ plaques [54]. Furthermore, Aβ is physiologically secreted to CSF under normal conditions where L-PGDS is abundant. L-PGDS ratio in CSF:serum is already identified as early biomarker for detection of potential damage to blood–brain barrier and quantifying its complex with transthyretin in CSF is also suggested to be a possible diagnostic marker [55]. L-PGDS colocalizes with amyloid plaques and mediates inhibition of aggregates through cysteine residue [54]. Based on previous studies and our own findings, we posit that secreted L-PGDS binds to monomeric and prefibrillar Aβ and inhibits amyloid aggregation in synergy with its ability to break down mature fibrils.

α2M is also an extracellular glycoprotein showing potential chaperone properties. It can prevent proteases from hydrolyzing proteins and mediates Aβ clearance through formation of α2M/protease complexes [56]. It protects cells from apoptosis through receptor interactions. α2M selectively binds proteins in non-native conformations and in process prevents their aggregation [56]. α2M decreases with age and its clearance mechanisms for Aβ get impaired [57]. A2M gene polymorphisms are associated with sporadic AD in some populations and α2M is also present in amyloid plaques [58]. ApoE isoforms E2, E3 and E4 can delay amyloid aggregation through differential activity as extracellular chaperones [59]. The suggested role of apoE4 as pathological chaperone is partly due to its role in delaying fibril formation from Aβ monomers leading to increase in toxic oligomeric species [60]. Isoform E3 and E2 show binding affinity to oligomeric Aβ as well, thus decreasing their neurotoxic effects [61]. Clusterin, or apolipoprotein J, is also extracellularly secreted and moonlights as a chaperone upon cellular stress stimulus [62]. Very similar to sHsps in its chaperone activity, clusterin preferentially interacts with off-pathway aggregates which are highly toxic and prone to precipitation [63]. Haptoglobin is very similar to α2M as an extracellular chaperone glycoprotein, though is activated in acidic

**197**

*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease*

**species**

amyloids and oligomers

amyloids and oligomers

Higher order aggregates

fibrils

Prefibrillar oligomers

misfolded proteins

Nascent polypeptides

oligomeric species

Misfolded protein aggregated

Monomers, fibrils

plaques/ diffusible aggregates

species

Holdase Nascent

Holdase Amyloid

**Chromosomal and cellular location**

6p21; intracellular

14q32; intracellular

19p13; intracellular

11q23; intracellular, membrane

13q12; intracellular

7q11; intracellular

11q24; intracellular

2q33; intracellular

16p13; intracellular

12p13; extracellular **Brain expression levels (HPA) [66]** **Refs.**

68]

70]

66.3 TPM [42, 67,

1021.5 TPM [69,

110.5 TPM [71]

1888.7 TPM [72]

109.4 TPM [73]

98.7 TPM [74]

1119.0 TPM [75]

141.6 TPM [76,

137.7 TPM [78]

172.0 TPM [80]

9q34; secreted 1224.7 TPM [54]

TPM

8p21; secreted 10875.4

77]

[79]

environment and interacts with prefibrillar amyloids to prevent further aggregation [64]. These endogenous proteins and many others may play very important yet

Ig antibodies such as aducanumab, targeting specific oligomeric forms of Aβ have already reached clinical trials, showing promising therapeutic effects focusing on delaying cognitive decline. However recent withdrawals of some of these antibodies like bapineuzumab and solanezumab, have forced scientists to look for more robust options [65]. Enzymes such as insulin degrading enzymes, neprilysin, cathepsin B are capable of degrading amyloidogenic peptides. Enzymes belonging to peptidyl-prolyl cis/trans isomerase (PPIase) family, e.g., CypB, can convert

undiscovered role to maintain proteostasis in physiological environment.

*DOI: http://dx.doi.org/10.5772/intechopen.84238*

Hsp70 (DnaK)

Hsp40 (DnaJ)

Hsc70 (HSPA8)

STUB1 (CHIP)

L-PGDS (β-trace)

Clusterin (ApoJ)

**Table 2.**

αB crystallin (Hsp27)

**Chaperone Function Amyloid** 

Hsp90 Holdase Nascent

Hsp110 Disaggregase;

Holdase; disaggregase in ternary complex

holdase

Hsp28 (sHsp) Holdase Unfolded or

Chaperone mediate autophagy

Ubiquitinmediated protein degradation

Aggregation inhibitor

α2-Macroglobulin Inhibitor Prefibrillar

*Mode of function of endogenous amyloid chaperones.*

Stabilizer Senile

Hsp60 Chaperonin Early


*Neuroprotective Function of Non-Proteolytic Amyloid-β Chaperones in Alzheimer's Disease DOI: http://dx.doi.org/10.5772/intechopen.84238*

#### **Table 2.**

*Amyloid Diseases*

**3.3 Non-proteolytic amyloid chaperones**

Aging is a major risk factor for many neurodegenerative diseases. The UPS system is known to get deregulated with increase in neuronal age and therefore offers less resistance to pathological protein aggregates. Proteins such as Human serum albumin show competitive binding towards Aβ oligomers, with its different domains binding to many oligomeric molecules [12]. It binds almost 90% of plasma Aβ peptides potentially through dual binding mechanism involving Aβ(1-40) monomers and protofibrils likely utilizing residues involved in fibril formation. Here we suggest non-proteolytic endogenous proteins which show potential as AD therapeutics directed against Aβ, namely, Lipocalin-type prostaglandin D synthase (L-PGDS, also known as β-trace), apolipoprotein E (ApoE), α2 macroglobulin (α2M), haptoglobin and clusterin as alternatives to intracellular Aβ degradation machinery. These proteins have been sporadically studied for their holdase or transporter activity, yet their Aβ chaperone function is still unexplored. L-PGDS is the second most abundant protein in human CSF after albumin [49]. It exhibits dual functions, as a lipophilic ligand transporter in cells and as isomerase in arachidonic acid pathway to convert prostaglandin H2 to prostaglandin D2 [50]. L-PGDS plays protective roles in different neurological diseases including genetic demyelinating disease, brain injuries and multiple sclerosis [51]. It plays protective role against cerebral ischemia as well [52]. It promotes recruitment of astrocytes and glial cells to the source of injury [53]. Early stress stimulus can upregulate L-PGDS suggesting its unknown, yet novel stress protection mechanism. L-PGDS deficient mice showed additional neuronal apoptosis strongly indicating its important protective function in neurons and surrounding oligodendrocytes [51]. L-PGDS binds to various Aβ peptides including fibrils and is colocalized in Aβ plaques [54]. Furthermore, Aβ is physiologically secreted to CSF under normal conditions where L-PGDS is abundant. L-PGDS ratio in CSF:serum is already identified as early biomarker for detection of potential damage to blood–brain barrier and quantifying its complex with transthyretin in CSF is also suggested to be a possible diagnostic marker [55]. L-PGDS colocalizes with amyloid plaques and mediates inhibition of aggregates through cysteine residue [54]. Based on previous studies and our own findings, we posit that secreted L-PGDS binds to monomeric and prefibrillar Aβ and inhibits amyloid aggregation in synergy with its ability to break down mature

α2M is also an extracellular glycoprotein showing potential chaperone properties. It can prevent proteases from hydrolyzing proteins and mediates Aβ clearance through formation of α2M/protease complexes [56]. It protects cells from apoptosis through receptor interactions. α2M selectively binds proteins in non-native conformations and in process prevents their aggregation [56]. α2M decreases with age and its clearance mechanisms for Aβ get impaired [57]. A2M gene polymorphisms are associated with sporadic AD in some populations and α2M is also present in amyloid plaques [58]. ApoE isoforms E2, E3 and E4 can delay amyloid aggregation through differential activity as extracellular chaperones [59]. The suggested role of apoE4 as pathological chaperone is partly due to its role in delaying fibril formation from Aβ monomers leading to increase in toxic oligomeric species [60]. Isoform E3 and E2 show binding affinity to oligomeric Aβ as well, thus decreasing their neurotoxic effects [61]. Clusterin, or apolipoprotein J, is also extracellularly secreted and

moonlights as a chaperone upon cellular stress stimulus [62]. Very similar to sHsps in its chaperone activity, clusterin preferentially interacts with off-pathway aggregates which are highly toxic and prone to precipitation [63]. Haptoglobin is very similar to α2M as an extracellular chaperone glycoprotein, though is activated in acidic

**196**

fibrils.

*Mode of function of endogenous amyloid chaperones.*

environment and interacts with prefibrillar amyloids to prevent further aggregation [64]. These endogenous proteins and many others may play very important yet undiscovered role to maintain proteostasis in physiological environment.

Ig antibodies such as aducanumab, targeting specific oligomeric forms of Aβ have already reached clinical trials, showing promising therapeutic effects focusing on delaying cognitive decline. However recent withdrawals of some of these antibodies like bapineuzumab and solanezumab, have forced scientists to look for more robust options [65]. Enzymes such as insulin degrading enzymes, neprilysin, cathepsin B are capable of degrading amyloidogenic peptides. Enzymes belonging to peptidyl-prolyl cis/trans isomerase (PPIase) family, e.g., CypB, can convert

proline containing peptides from cis to trans conformational isomers thereby facilitating their folding process [47]. Some inherently amyloidogenic proteins such as PrP, hTTR and BRICHOS-domain containing proteins can paradoxically also inhibit aggregation of other amyloids. BRICHOS-domain containing protein ITM2B (Bri2) is a CNS membrane protein which shows chaperone functions for amyloids by binding to tyrosine and other charged residues [43]. Tetrameric form of Transthyretin (TTR) can also prevent aggregation of amyloid oligomers [44]. In recent years, some small molecules have also been put forward as possible inhibitors of amyloid aggregation—such as polyphenols, e.g., epigallocatechin gallate (EGCG), curcumin, resveratrol, etc. [7, 12]. Understanding inhibition mechanisms of non-proteolytic endogenous chaperone proteins and other molecules is very crucial to develop therapies targeting amyloid aggregates in AD (**Table 2**).
