**5. Pathologic consequences of amyloid formation**

As indicated by **Table 1**, many disorders can be associated with amyloid deposition. In many cases, the precise mechanism of pathologic dysfunction is unknown. Nevertheless, several notions are important.

First, as noted, some polypeptide hormones are apparently stored in amyloidlike conformations. These differ from most of the other types in being reversible upon hormone release. Only in rare situations do these protein accumulations persist as amyloid deposits and become associated with disease.

Second, evolution has often minimized protein sequences that are particularly prone to nucleate amyloid fibrils [19]. Many proteins are located in intracellular regions or are associated with chaperones that reduce their likelihood of assuming alternative conformations. Degradation mechanisms including proteasomes can minimize intracellular aggregation.

In so-called "secondary" amyloid disease, the bulk of accumulated fibrils likely interferes with cellular and organ function. For example, remarkably large

**183**

*Pathophysiology of Amyloid Fibril Formation DOI: http://dx.doi.org/10.5772/intechopen.81965*

deposits.

oligomer pathway.

quantities of AA protein were isolated from the kidneys, liver, and spleen in earlier studies [20, 21]. Examining affected tissues by light microscopy clearly shows large interruptions in organ structural integrity due to massive amyloid deposits. Similar accumulations often accompany "primary" (i.e., immunoglobulin-derived) amyloid disease where fragments of immunoglobulin light chains may be found in large

Evidence from other types of amyloid disease indicates that large, microscopically visible deposits of amyloid fibrils may not always be the initial cause of cell/ organ dysfunction. While fibrils are often seen later in study of affected tissues (and, hence, appear as classic "amyloid" by staining), they may be late consequences. Earlier, oligomeric forms may be more disruptive and lead to organ

**Figure 4** (above) presents a simple scheme for oligomer formation. This is likely to occur as a basic pathway in various types of amyloid-related diseases. As described above, destabilization of the TTR tetramer occurs extracellularly and can lead to oligomers that are relatively small. Various TTR mutations can destabilize the tetramer and accelerate this process although even the normal protein also appears susceptible in some situations. This likely is a problem for various intracellular neurotoxic fibrils and their predecessors (e.g., β-protein and α-synuclein) as well [22]. Thus, finding substantial amounts of extracellular Congo Red staining deposits may be a late feature of the basic disorder rather than the primary cause of dysfunction. As noted above, such material is generally quite stable and often resistant to dissociation. Hence, extracellular deposits (or even substantial accumulations within organelles) may be the end point of the toxic

As described above, once formed, amyloid fibrils are intrinsically quite stable due to intra- and intermolecular bonds and relative inaccessibility to proteases. Ideally, disrupting the fibrils themselves would be an appropriate approach to treating at least some amyloid disorders. In this regard, chaotropic molecules (e.g., urea, guanidinium, etc.), often used in the laboratory, are not options for treatment due to their toxicity. Alternative agents, compatible with in vivo use have not yet been identified. Thus, successful intervention(s) for amyloid disorders must either prevent formation of the precursor(s) or stabilize the protein predecessors of oligomers (or their proteolytic cleavage). Several approaches have been introduced, and these

Immunoglobulin light chain overproduction ("primary" amyloidosis) is basically a clonal disorder of plasma cell proliferation/overexpression. Here, treatment generally depends on suppression or elimination of the responsible cell population. This falls into the spectrum of treatment of myeloma and related disorders and

Amyloid A (SAA) disorders ("secondary" amyloidosis) generally reflect overproduction of the SAA precursor. These usually are related to chronic or periodic and recurrent infection/inflammation. The incidence of these has been reduced by successful treatment of conditions such as tuberculosis and osteomyelitis. However, these and other types of infections and inflammation remain prominent in certain parts of the world and can be accompanied by amyloidosis. Genetic disorders such as familial Mediterranean fever with recurrent, self-limited inflammatory episodes can usually be controlled with agents such as colchicine [23], minimizing SAA

dysfunction well before large deposits are detectable by microscopy.

**6. Therapeutic approaches to amyloid pathophysiology**

depend upon the *specific* type of amyloidosis (recall **Table 1**).

depends on oncologic approaches.

synthesis and AA amyloid accumulation.

#### *Pathophysiology of Amyloid Fibril Formation DOI: http://dx.doi.org/10.5772/intechopen.81965*

*Amyloid Diseases*

species (step F) [15, 16].

*Scheme for nucleation of TTR amyloid oligomers.*

**Figure 4.**

tion, and tissue toxicity [18].

**5. Pathologic consequences of amyloid formation**

persist as amyloid deposits and become associated with disease.

Nevertheless, several notions are important.

minimize intracellular aggregation.

largely acellular, region where amyloid fibrils become the predominant structured

A contrasting situation can occur in situations where the fibril precursor is not only soluble but also intrinsically capable of β structure formation without cleavage. **Figure 4** shows events for transthyretin (TTR). TTR is a 127 amino acid protein circulating in the blood as a stable tetramer that binds thyroid hormone and retin A (hence, its name). The TTR monomer itself contains prominent β-sheet domains. If the tetramer dissociates, the free monomers can misfold into various forms. Among these, some can associate as oligomers which then can be extended into fibrils. TTR amyloid fibrils are particularly prone to cause dysfunction in nerves and the heart. Interestingly, over 100 amino acid substitutions (i*.*e., mutations) have been identified in TTR [17]. Mutations differentially affect tetramer stability—some increase it, while others reduce it. Among the latter are several that are associated with inherited amyloid diseases, and the Val30Met and Val122Ile mutations have been particularly well-studied (affecting nerves and/or the heart). Kinetic and other evidence implicates the oligomer form(s) as directly involved in organ dysfunction. Amyloid fibrils become detectable by microscopy as the disease progresses. One proposed therapeutic strategy involves developing small molecules that stabilize the circulating tetramer, hence reducing (or eliminating) dissociation, oligomer forma-

As indicated by **Table 1**, many disorders can be associated with amyloid deposition. In many cases, the precise mechanism of pathologic dysfunction is unknown.

First, as noted, some polypeptide hormones are apparently stored in amyloidlike conformations. These differ from most of the other types in being reversible upon hormone release. Only in rare situations do these protein accumulations

Second, evolution has often minimized protein sequences that are particularly prone to nucleate amyloid fibrils [19]. Many proteins are located in intracellular regions or are associated with chaperones that reduce their likelihood of assuming alternative conformations. Degradation mechanisms including proteasomes can

In so-called "secondary" amyloid disease, the bulk of accumulated fibrils likely interferes with cellular and organ function. For example, remarkably large

**182**

quantities of AA protein were isolated from the kidneys, liver, and spleen in earlier studies [20, 21]. Examining affected tissues by light microscopy clearly shows large interruptions in organ structural integrity due to massive amyloid deposits. Similar accumulations often accompany "primary" (i.e., immunoglobulin-derived) amyloid disease where fragments of immunoglobulin light chains may be found in large deposits.

Evidence from other types of amyloid disease indicates that large, microscopically visible deposits of amyloid fibrils may not always be the initial cause of cell/ organ dysfunction. While fibrils are often seen later in study of affected tissues (and, hence, appear as classic "amyloid" by staining), they may be late consequences. Earlier, oligomeric forms may be more disruptive and lead to organ dysfunction well before large deposits are detectable by microscopy.

**Figure 4** (above) presents a simple scheme for oligomer formation. This is likely to occur as a basic pathway in various types of amyloid-related diseases. As described above, destabilization of the TTR tetramer occurs extracellularly and can lead to oligomers that are relatively small. Various TTR mutations can destabilize the tetramer and accelerate this process although even the normal protein also appears susceptible in some situations. This likely is a problem for various intracellular neurotoxic fibrils and their predecessors (e.g., β-protein and α-synuclein) as well [22]. Thus, finding substantial amounts of extracellular Congo Red staining deposits may be a late feature of the basic disorder rather than the primary cause of dysfunction. As noted above, such material is generally quite stable and often resistant to dissociation. Hence, extracellular deposits (or even substantial accumulations within organelles) may be the end point of the toxic oligomer pathway.
