Introduction 1 2 3 2 1-40 1-42 4 6 7 8 9 10 11 13 Chaperones can be defined as proteins that: (1) have a role in the intracellular handling of misfolded proteins, (2) induce conformational changes of proteins, (3) act as transporter of proteins. “Professional” chaperones, such as the heat shock protein family (Hsp), are defined as proteins that have a specific function in facilitating normal folding of proteins and intracellular handling of misfolded proteins. Members of the Hsp family recognize misfolded proteins and transport them to the proteasome for degradation. Therefore, this protein family acts as the first line of defense against toxicity induced by misfolded proteins such as Aβ and tau. In contrast to professional chaperones, “amateur” chaperones can be defined as proteins that bind to other proteins and induce conformational changes or, alternatively, serve as transporter proteins. Examples of putative amateur chaperones are apolipoprotein E (ApoE), heparan sulfate proteoglycans (HSPGs), and complement factors such as C1q. They have, in contrast to the professional chaperones, primarly an extracellular function. In this paper, we will review the role of both amateur and professional chaperones in the pathogenesis of AD. Aβ-Binding Proteins in Extracellular Interaction with Aβ Apolipoproteins The apolipoprotein family consists of proteins that conjugate with lipids to form different classes of lipoprotein particles. In human brain, several members of this protein family are expressed, such as apolipoprotein E (ApoE), apolipoprotein J (ApoJ), and apolipoprotein D (apoD). 14 18 19 20 21 24 25 26 27 28 29 30 29 18 31 33 31 32 34 35 36 37 38 39 40 36 25 41 42 43 45 5 6 46 47 50 51 52 53 54 56 57 58 59 In conclusion, ApoE and ApoJ can be regarded as amateur chaperones that regulate Aβ aggregation in vitro. By accelerating the Aβ aggregation process towards mature fibril formation, (human) ApoE prevents formation of toxic Aβ intermediates such as oligomers and protofibrils, and thus, may have a protective function towards development of AD. Moreover, ApoE protects against the development of AD by suppressing the inflammatory reactions associated with AD lesions. Besides its role in inducing conformational changes in Aβ, ApoE facilitates Aβ clearance from brain by serving as a transporter molecule of Aβ, which will be discussed in paragraph 4. Heparan Sulfate Proteoglycans N 60 61 60 62 63 65 65 67 68 69 69 72 73 70 74 76 77 78 79 80 81 82 83 84 85 86 61 Complement Factors 46 87 89 90 91 87 92 93 94 95 96 97 97 98 100 12 90 101 102 103 104 Although none of the complement factors directly regulate conformational changes of Aβ, complement activation as a whole plays a role in the Aβ aggregation in vivo. Therefore, complement factors might act as amateur chaperones, although their exact role in Aβ aggregation remains to be elucidated. Professional Chaperones 105 105 106 107 1 Fig. 1 Hsp sHsps  Small Heat Shock Proteins 108 109 110 111 114 115 116 111 117 119 120 121 116 117 122 113 123 126 127 128 Miscellaneous Proteins 1 2 129 132 133 135 134 135 136 137 138 139 140 141 142 104 143 144 145 144 146 147 148 149 150 151 152 153 154 155 156 In summary, several proteins are associated with Aβ aggregates in the AD brain and contribute to the aggregation of Aβ and should, therefore, be considered as amateur chaperones. In addition, they might play a role in triggering inflammation. Aβ-Binding Proteins and Intracellular Interactions with Aβ 157 159 160 161 162 160 163 164 165 166 167 132 168 171 172 173 174 1 Table 1 Summary of the expression of chaperones in AD brains and their interaction and effects on Aβ and tau   SP/CAA NFT Direct interaction Effects on Aβ or tau in general Apolipoproteins  ApoE + + Aβ/tau ↑ Fibrillar Aβ /↓ hyperph. Tau  ApoJ + ? Aβ ↓ Aβ aggregation HSPGs  Perlecan ± ± Aβ HSPGs:  Agrin + − Aβ ↓ Proteolytic breakdown Aβ  Glypican 1 + − ? ↑ Non-fibrillar → fibrillar Aβ  Syndecan 1–3 + − ? ↑ Phosphorylation tau  Collagen XVIII + − ?   GAGs + + Aβ/tau  Complement factors  Hageman Factor + + ? Aβ activates complement in AD  C1q + + Aβ C3 ↓ Aβ deposition  C3/C4 + + Aβ   C5-9 + + ?  Heat shock proteins  Hsp90 + ? Tau ↓ Tau aggregation  Hsp70 + ? Aβ/tau ↓ Tau aggregation Small Hsps  αB-crystallin − − Aβ ↓ Aβ fibril formation  Hsp27 − ± Aβ/tau ↓ Aβ fibril formation  Hsp20 + − Aβ ↓ Aβ fibril formation  HspB2/B3 + + – No effect  HspB8 + − Aβ ↓ Aβ fibril formation Acute phase proteins 1 + − Aβ ↑ Aβ fibrillization 2 + − Aβ   serum amyloid P + + Aβ ↑ Aβ fibrillization Miscellaneous compounds  tPA − − Aβ ↓ Aβ fibril formation  Gelsolin − − Aβ ↑ Aβ fibrillization Expression of chaperones in a specific lesion is illustrated as follows: present (+), by conflicting reports (±), absence (−), and unknown (?); ↓ = inhibition or down-regulation, ↑ = induction or up-regulation SP CAA NFT HSPGs Aβ Hsp Apo SAP tPA GAGs LDLR LRP-1 BBB Aβ-Binding Proteins and Aβ Clearance Aβ-binding proteins, amateur chaperones, play a role in the clearance of Aβ from brain by functioning as a transporter molecule. Two major pathways govern Aβ clearance. By the first pathway, Aβ is removed from brain to blood via active transport across the BBB. This active transport is performed by specialized transporters, so-called “Aβ-receptors”, expressed by endothelial cells. Second, Aβ is removed from brain via phagocytosis by both microglial cells and astrocytes. In both pathways, interaction of Aβ with cell surface Aβ-receptors is crucial; therefore, the expression levels of Aβ-binding proteins might contribute to Aβ clearance by regulating its binding with Aβ receptors. 4 175 10 176 177 178 179 179 7 139 180 182 183 184 184 181 184 185 187 132 188 189 40 146 Concluding Remarks 2 Fig. 2 Aβ 1 2  184 Overexpression of professional chaperones, such as the Hsps, to prevent aggregation of misfolded proteins will have to be evaluated carefully, as they also interact with other chaperones and are dependent on this interaction to fulfill some of their functions. This strategy may therefore result in instability of the cell-stress mechanism, which may cause the system to collapse. A solution may be found in the overexpression of several chaperones, which may be required to achieve an impact on the progression of the disease. 121 In conclusion, studying the role of chaperones, both professional and amateur, in the pathophysiology of AD will provide us with a better understanding of the mechanisms underlying the formation and accumulation of toxic aggregates in AD, which, eventually, will lead to the design of more effective therapeutic strategies.