**1. Introduction**

Alzheimer's disease (AD), a long-lasting progressive neurodegeneration, characterized by synaptic dysfunction, an increase in extracellular amyloid plaques, intracellular tauopathy, extensive neuronal loss in several cerebral areas, and enhanced neuroinflammation. Highly disrupted cholinergic transmission is proving to be a featured biochemical sign of disease. Clinical manifestations include progressive loss of ability to encode new memories, impairment of both declarative and non-declarative memory, and finally the invincible decline of overall intellectual capacity. Today, worldwide spreading of Alzheimer's type dementia is one of the major public health challenges confronting this generation.

its enzymes [10]. In addition, recent neuroimaging results strongly indicate that some nondemented individuals can have amyloid-beta aggregates equivalent in concentration to those found in demented patients [11], and also symptoms of AD can emerge regardless of amyloid

Topological Biomarker of Alzheimer's Disease http://dx.doi.org/10.5772/intechopen.76633 171

The evidences, provided by the animal studies, that amyloid clearance produces decreased symptoms of disease in mouse models of AD [13, 14] had turned the drug development in the direction of targeting beta-amyloid in human brains. The deployment of drugs comprised active and passive immunization directly against amyloid-beta accumulation and inhibition of beta- and gamma-secretase APP cleaving enzyme. Regrettably, none of testing drugs have resulted in recovery of functional abilities or alleviating symptoms of the disease in humans. The treatments did not slow down the course of the disease; on the contrary, some drugs even accelerate the progression of the symptoms. The recently emerged evidences confirmed the low correlation of the cerebral distribution of beta-amyloid plaques with neuropathology, decrease of neural function, or cognitive deficits [3]. A possibility that insoluble plaque is not the primary cause of progressive AD pathology is a strong argument for failure of antiamyloid vaccination trials to improve patient outcome, even when cerebral amyloid plaque was removed [15]. Consequently, failure of amyloid-based AD treatments shifts research hypothesis to the soluble amyloid beta oligomers rather than plaques to be the main cause of neuronal degeneration [16]. However, there is still not reliable evidence in human studies that soluble amyloid oligomers are toxic in vivo. Notably, the cause of pathological cascade, resulting in redundant amyloid forms production and ejection, is still poorly understood along with

the issue concerning the role of different amyloid beta forms in AD pathogenesis [17].

Synapse loss generates a loss of dendritic mass [18] and promotes neuron loss [19], a hallmark of AD pathology. It is interesting that amyloid plaque can show up without synapse loss [20]; moreover, synapse loss can occur without associated amyloid deposition [21]. High quantitative correlations of postmortem cytopathology to premortem cognitive impairments suggest that the decrease in density of synapses and decrease in the number of synapses per neuron are more linked to AD symptoms than are the concentration of amyloid plaques, the number of intracellular tau-tangles, or cortical gliosis [22]. These results appoint synapses as the key feature of AD, strongly indicating that the disease pathogenesis could be the outcome of syn-

It has been shown that the early AD symptoms significantly correlate with a specific dysfunction of cholinergic synapses [24]. Interestingly, acetylcholine receptor agonists affect several of AD hallmarks, including cholinergic deficits, cognitive dysfunction, but also tau and amyloid-beta pathological burdens [25]. The electrophysiological measurements of basal synaptic transmission and long-term potentiation in transgenic mouse with mutations analog of human mutations causing AD suggest that change in synaptic function precedes amyloid plaque production. Increased synaptic activity increases amyloid secretion, while decreasing activity inhibits it [26, 27]. In turn, beta-amyloid burden inhibits synapses and alerts synaptic plasticity [28], implying the existence of a feedback loop between synaptic dynamics and associated amyloid production that might serve as a mechanism to prevent synaptic

**1.2. Synaptic function failure hypothesis**

aptic failure [23].

deposition [12].

Extensive AD research, especially in the last 40 years, brought a significant progress in the development of diagnostic approaches and understanding the etiology. However, despite accumulated knowledge, the cause of the disease has not been found, a postmortem histopathological evaluation is still required to confirm the clinical diagnosis, and finally, effective treatment that would at least slow progression of the disease has not yet been found. As a result, if Augusta Deter, the first patient who had been described with the hallmarks of the disease, was alive today, her prognosis would be the same as in 1906.

#### **1.1. Amyloid hypothesis**

Since the first documentation by Alois Alzheimer, spread of abnormal protein filaments (plaque) throughout the brain, the main histopathological sign of Alzheimer's dementia, had a prominent role in a broad spectrum of proposed mechanisms related to disease pathogenesis. Amyloidogenic fragments were described structurally in extracellular plaques, and tau protein was documented as the main component of intracellular neurofibrillary tangles [1, 2].

According to amyloid hypothesis, the neurotoxic forms of amyloid-beta polypeptides, derived from amyloid precursor protein (APP), induce synaptic injuries followed by substantial intracellular damage in form of tauopathy and subsequently produce the pathological presentations of neurodegeneration leading to dementia [3]. There are reliable evidences, provided by the systematic work of Braak and Braak [4, 5], that the pathological progression of extracellular and intracellular deposits of pathological protein forms starts decades before the onset of clinical symptoms.

The genetic mutations identified on chromosome 21 are conferred to trigger AD through abnormal processing of APP, causing the elevated cerebral concentrations of amyloid-beta or increased production of its specific forms [6]. Also, three other different autosomal dominant mutations that might cause AD have been identified on chromosomes 14 and 1, leading to mutations of presenilin 1 and presenilin 2 proteins, respectively, along with late-onset apolipoprotein genotype ἐ4 [7–9].

The discoveries of mutations in amyloid-beta-related proteins have had significant influence in promoting the amyloid theory. However, while these mutations account for the majority of early-onset AD (<65 years; familial AD), the risk genes linked with late-onset AD (> 65 years; 95% of all AD cases) are subtle, with no direct genetic relation to the APP gene or its enzymes [10]. In addition, recent neuroimaging results strongly indicate that some nondemented individuals can have amyloid-beta aggregates equivalent in concentration to those found in demented patients [11], and also symptoms of AD can emerge regardless of amyloid deposition [12].

The evidences, provided by the animal studies, that amyloid clearance produces decreased symptoms of disease in mouse models of AD [13, 14] had turned the drug development in the direction of targeting beta-amyloid in human brains. The deployment of drugs comprised active and passive immunization directly against amyloid-beta accumulation and inhibition of beta- and gamma-secretase APP cleaving enzyme. Regrettably, none of testing drugs have resulted in recovery of functional abilities or alleviating symptoms of the disease in humans. The treatments did not slow down the course of the disease; on the contrary, some drugs even accelerate the progression of the symptoms. The recently emerged evidences confirmed the low correlation of the cerebral distribution of beta-amyloid plaques with neuropathology, decrease of neural function, or cognitive deficits [3]. A possibility that insoluble plaque is not the primary cause of progressive AD pathology is a strong argument for failure of antiamyloid vaccination trials to improve patient outcome, even when cerebral amyloid plaque was removed [15]. Consequently, failure of amyloid-based AD treatments shifts research hypothesis to the soluble amyloid beta oligomers rather than plaques to be the main cause of neuronal degeneration [16]. However, there is still not reliable evidence in human studies that soluble amyloid oligomers are toxic in vivo. Notably, the cause of pathological cascade, resulting in redundant amyloid forms production and ejection, is still poorly understood along with the issue concerning the role of different amyloid beta forms in AD pathogenesis [17].

#### **1.2. Synaptic function failure hypothesis**

**1. Introduction**

170 Biomarker - Indicator of Abnormal Physiological Process

**1.1. Amyloid hypothesis**

the onset of clinical symptoms.

apolipoprotein genotype ἐ4 [7–9].

Alzheimer's disease (AD), a long-lasting progressive neurodegeneration, characterized by synaptic dysfunction, an increase in extracellular amyloid plaques, intracellular tauopathy, extensive neuronal loss in several cerebral areas, and enhanced neuroinflammation. Highly disrupted cholinergic transmission is proving to be a featured biochemical sign of disease. Clinical manifestations include progressive loss of ability to encode new memories, impairment of both declarative and non-declarative memory, and finally the invincible decline of overall intellectual capacity. Today, worldwide spreading of Alzheimer's type dementia is

Extensive AD research, especially in the last 40 years, brought a significant progress in the development of diagnostic approaches and understanding the etiology. However, despite accumulated knowledge, the cause of the disease has not been found, a postmortem histopathological evaluation is still required to confirm the clinical diagnosis, and finally, effective treatment that would at least slow progression of the disease has not yet been found. As a result, if Augusta Deter, the first patient who had been described with the hallmarks of the

Since the first documentation by Alois Alzheimer, spread of abnormal protein filaments (plaque) throughout the brain, the main histopathological sign of Alzheimer's dementia, had a prominent role in a broad spectrum of proposed mechanisms related to disease pathogenesis. Amyloidogenic fragments were described structurally in extracellular plaques, and tau protein was documented as the main component of intracellular neurofibrillary tangles [1, 2]. According to amyloid hypothesis, the neurotoxic forms of amyloid-beta polypeptides, derived from amyloid precursor protein (APP), induce synaptic injuries followed by substantial intracellular damage in form of tauopathy and subsequently produce the pathological presentations of neurodegeneration leading to dementia [3]. There are reliable evidences, provided by the systematic work of Braak and Braak [4, 5], that the pathological progression of extracellular and intracellular deposits of pathological protein forms starts decades before

The genetic mutations identified on chromosome 21 are conferred to trigger AD through abnormal processing of APP, causing the elevated cerebral concentrations of amyloid-beta or increased production of its specific forms [6]. Also, three other different autosomal dominant mutations that might cause AD have been identified on chromosomes 14 and 1, leading to mutations of presenilin 1 and presenilin 2 proteins, respectively, along with late-onset

The discoveries of mutations in amyloid-beta-related proteins have had significant influence in promoting the amyloid theory. However, while these mutations account for the majority of early-onset AD (<65 years; familial AD), the risk genes linked with late-onset AD (> 65 years; 95% of all AD cases) are subtle, with no direct genetic relation to the APP gene or

one of the major public health challenges confronting this generation.

disease, was alive today, her prognosis would be the same as in 1906.

Synapse loss generates a loss of dendritic mass [18] and promotes neuron loss [19], a hallmark of AD pathology. It is interesting that amyloid plaque can show up without synapse loss [20]; moreover, synapse loss can occur without associated amyloid deposition [21]. High quantitative correlations of postmortem cytopathology to premortem cognitive impairments suggest that the decrease in density of synapses and decrease in the number of synapses per neuron are more linked to AD symptoms than are the concentration of amyloid plaques, the number of intracellular tau-tangles, or cortical gliosis [22]. These results appoint synapses as the key feature of AD, strongly indicating that the disease pathogenesis could be the outcome of synaptic failure [23].

It has been shown that the early AD symptoms significantly correlate with a specific dysfunction of cholinergic synapses [24]. Interestingly, acetylcholine receptor agonists affect several of AD hallmarks, including cholinergic deficits, cognitive dysfunction, but also tau and amyloid-beta pathological burdens [25]. The electrophysiological measurements of basal synaptic transmission and long-term potentiation in transgenic mouse with mutations analog of human mutations causing AD suggest that change in synaptic function precedes amyloid plaque production. Increased synaptic activity increases amyloid secretion, while decreasing activity inhibits it [26, 27]. In turn, beta-amyloid burden inhibits synapses and alerts synaptic plasticity [28], implying the existence of a feedback loop between synaptic dynamics and associated amyloid production that might serve as a mechanism to prevent synaptic hyperactivation and excitotoxicity [26]. In addition, recent studies demonstrate an important physiological role of cortical amyloid secretion, showing that low concentrations of betaamyloid peptides increase long-term potentiation, needed for successful memory formation [29, 30]. There are also evidences that synaptic dysfunction occurs before changes in synaptic morphology or the number of synapses per neuron [31, 32]. These breakthroughs in the field provide strong evidences that amyloid homeostasis is controlled by the synaptic functions and emphasize prominent amyloid involvement in healthy memory coding.

widespread neurodegeneration, during the preclinical stage of disease, when neural injuries may still be reversible. Consequently, it is crucial to detect very early, possibly reversible, pathological changes related to AD in cognitively intact individuals, before the occurrence of

Topological Biomarker of Alzheimer's Disease http://dx.doi.org/10.5772/intechopen.76633 173

A wealth of evidence suggests that the amyloid hypothesis of AD etiology is insufficient to explain all pathological changes associated with AD and their temporal evolution. Considering the wide spectra of data in AD research, it cannot be overlooked their common link: synaptic dysfunction and degradation as a very early characteristic of the disease. Unlike neuron death, changes in synaptic functions are very likely still reversible. These subtle neurofunctional alternations are likely detectable by functional neuroimaging techniques. The neuroimaging has the capability to provide an assessment of the altered neurophysiology, before anatomical abnormalities and divergent neuropathology of the later disease stages. Focusing the synaptic level, at the phase when the neuron cell potentially is still healthy, provides an excellent opportunity to intervene at a reversible stage of the disease when neural networks are vulnerable, but not lost. With regard to focal neural activation during the early phases of processing external inputs, studying the early sensory responses enable unique insight into the synchronized synaptic activity of functionally related neural substrates, which are on the larger scale recognized as sensory networks. The first manifestations of declining synaptic function could involve desynchronization of synaptic transmission, which may cause the "virtual" absence of activation when it is measured extracranially. These alterations in topology of sensory networks could be even associated with specific patterns of attention, memory,

the first symptoms, i.e., to reveal the reliable preclinical biomarker of the disease.

or behavioral disorder that could indicate the preclinical stage of disease.

A biomarker in medicine is conceptualized as a measurable detector of a physiological, anatomic, or biochemical alternation that can distinct normal biologic processes from pathological. Biomarker should be able to provide reliable diagnosis, follow the development of disease, and measure responses to a therapeutic intervention. The development of a new biomarker for any clinical condition is based on the ability to accurately detect specific pathophysiology against the gold or reference standard. This creates problem specifically to AD because there is no in vivo gold standard. Currently, the final diagnosis of AD requires both the presence of amnestic symptoms and postmortem histopathological confirmation. Premortem diagnosis of AD is only "probable" or "possible" based on symptoms characteristic of Alzheimer's type

The growing body of evidences evolves AD research field toward the concept of disease pathogenesis as a continuum of long-term phases in which clinical symptomatology and underlying pathophysiological process have different temporal development rates [45]. The existing findings indicate that the onset of the first symptoms marked the already irreversible stage of the disease [44]. Therefore, only detection of preclinical phase, before the occurrence of the first

**1.4. Conclusion**

**2. Biomarkers of AD**

of dementia and neuroimaging findings.

Cerebral dysfunction found in non-demented elderly individuals with amyloid plaques before any memory disturbances [33] points at impairment in neural function as a very early pathophysiological sign of AD. Interestingly, functional changes may be driven in both directions, increased [34, 35] or decreased [36, 37] neuronal excitability, usually depending on the disease stage and a specific brain area and its function. The increased synaptic activity, found in early stages of symptomatic AD, might be an adaptive response driving neuroprotection [38]. On the other hand, the decreased neural network excitability could be induced by activation of gamma-aminobutyric acid (GABA) receptors, which decrease glutamate excitatory transmission, implying that in vivo glutamate-mediated neuronal excitability is controlled by interactions between inhibitory systems [39]. The co-transmission of acetylcholine and GABA, first found within the cholinergic system only, recently was demonstrated as a common feature of nearly all cholinergic forebrain neurons [40, 41]. These results appoint GABA as a fast neurotransmitter utilized throughout the forebrain cholinergic system and emphasized acetylcholine-GABA co-release as a major modulation factor of cortical functions transmitted by cholinergic neurons. Moreover, there is evidence that GABA receptor agonists defend neurons in culture from the toxicity of beta amyloids and of different glutamate receptor agonists [42]. In conclusion, given their major role in both sensory processing and cognition, and high susceptibility to AD pathology, the ability of the cholinergic neurons to co-release GABA could explain the failure of specific synaptic inhibitory processes found in AD that may trigger the cascade of events resulting in characteristic neuropathology.

#### **1.3. Dynamic model of AD pathogenesis**

The presence of cerebral amyloid aggregation in cognitively normal individuals, the lack of systematic correlation between amyloid plaque deposition and cognition, insufficiently explained influence of soluble amyloid oligomeric *in vivo*, the bias of preclinical disease models toward the amyloid hypothesis, and finally failures of clinical trials with anti-amyloid drugs are the strong arguments for urgent need to revise present model of AD to include alternative possibilities that could account for all the research results associated to AD.

Failures of clinical trials of anti-amyloid drugs in symptomatic AD patients (mild cognitive impairment (MCI) and moderate Alzheimer's disease patients) in addition to reliable evidence that the hallmarks of disease could be found in the brain decades before symptom onset set up a view that the onset of clinical symptoms is a sign of irreversible neural damage. Consequently, the concept of AD pathogenesis is evolving toward a view of the disease as a long-term continuum, which differs only by symptom appearance; that is, a non-symptomatic (preclinical) AD phase and an irreversible symptomatic AD phase [43, 44]. This dynamic concept of the disease emphasizes the neurobiological advantage of early intervention before of widespread neurodegeneration, during the preclinical stage of disease, when neural injuries may still be reversible. Consequently, it is crucial to detect very early, possibly reversible, pathological changes related to AD in cognitively intact individuals, before the occurrence of the first symptoms, i.e., to reveal the reliable preclinical biomarker of the disease.

## **1.4. Conclusion**

hyperactivation and excitotoxicity [26]. In addition, recent studies demonstrate an important physiological role of cortical amyloid secretion, showing that low concentrations of betaamyloid peptides increase long-term potentiation, needed for successful memory formation [29, 30]. There are also evidences that synaptic dysfunction occurs before changes in synaptic morphology or the number of synapses per neuron [31, 32]. These breakthroughs in the field provide strong evidences that amyloid homeostasis is controlled by the synaptic functions

Cerebral dysfunction found in non-demented elderly individuals with amyloid plaques before any memory disturbances [33] points at impairment in neural function as a very early pathophysiological sign of AD. Interestingly, functional changes may be driven in both directions, increased [34, 35] or decreased [36, 37] neuronal excitability, usually depending on the disease stage and a specific brain area and its function. The increased synaptic activity, found in early stages of symptomatic AD, might be an adaptive response driving neuroprotection [38]. On the other hand, the decreased neural network excitability could be induced by activation of gamma-aminobutyric acid (GABA) receptors, which decrease glutamate excitatory transmission, implying that in vivo glutamate-mediated neuronal excitability is controlled by interactions between inhibitory systems [39]. The co-transmission of acetylcholine and GABA, first found within the cholinergic system only, recently was demonstrated as a common feature of nearly all cholinergic forebrain neurons [40, 41]. These results appoint GABA as a fast neurotransmitter utilized throughout the forebrain cholinergic system and emphasized acetylcholine-GABA co-release as a major modulation factor of cortical functions transmitted by cholinergic neurons. Moreover, there is evidence that GABA receptor agonists defend neurons in culture from the toxicity of beta amyloids and of different glutamate receptor agonists [42]. In conclusion, given their major role in both sensory processing and cognition, and high susceptibility to AD pathology, the ability of the cholinergic neurons to co-release GABA could explain the failure of specific synaptic inhibitory processes found in AD that may trig-

The presence of cerebral amyloid aggregation in cognitively normal individuals, the lack of systematic correlation between amyloid plaque deposition and cognition, insufficiently explained influence of soluble amyloid oligomeric *in vivo*, the bias of preclinical disease models toward the amyloid hypothesis, and finally failures of clinical trials with anti-amyloid drugs are the strong arguments for urgent need to revise present model of AD to include alternative possibilities that could account for all the research results associated to AD.

Failures of clinical trials of anti-amyloid drugs in symptomatic AD patients (mild cognitive impairment (MCI) and moderate Alzheimer's disease patients) in addition to reliable evidence that the hallmarks of disease could be found in the brain decades before symptom onset set up a view that the onset of clinical symptoms is a sign of irreversible neural damage. Consequently, the concept of AD pathogenesis is evolving toward a view of the disease as a long-term continuum, which differs only by symptom appearance; that is, a non-symptomatic (preclinical) AD phase and an irreversible symptomatic AD phase [43, 44]. This dynamic concept of the disease emphasizes the neurobiological advantage of early intervention before of

and emphasize prominent amyloid involvement in healthy memory coding.

ger the cascade of events resulting in characteristic neuropathology.

**1.3. Dynamic model of AD pathogenesis**

172 Biomarker - Indicator of Abnormal Physiological Process

A wealth of evidence suggests that the amyloid hypothesis of AD etiology is insufficient to explain all pathological changes associated with AD and their temporal evolution. Considering the wide spectra of data in AD research, it cannot be overlooked their common link: synaptic dysfunction and degradation as a very early characteristic of the disease. Unlike neuron death, changes in synaptic functions are very likely still reversible. These subtle neurofunctional alternations are likely detectable by functional neuroimaging techniques. The neuroimaging has the capability to provide an assessment of the altered neurophysiology, before anatomical abnormalities and divergent neuropathology of the later disease stages. Focusing the synaptic level, at the phase when the neuron cell potentially is still healthy, provides an excellent opportunity to intervene at a reversible stage of the disease when neural networks are vulnerable, but not lost. With regard to focal neural activation during the early phases of processing external inputs, studying the early sensory responses enable unique insight into the synchronized synaptic activity of functionally related neural substrates, which are on the larger scale recognized as sensory networks. The first manifestations of declining synaptic function could involve desynchronization of synaptic transmission, which may cause the "virtual" absence of activation when it is measured extracranially. These alterations in topology of sensory networks could be even associated with specific patterns of attention, memory, or behavioral disorder that could indicate the preclinical stage of disease.
