| The link between histopathological hallmarks
of Alzheimer’s disease (AD), i.e. amyloid plaques, and
neurofibrillary tangles, and AD-associated cognitive
impairment, has long been established. However, the
introduction of interactions between amyloid-beta (Aβ)
as well as hyperphosphorylated tau, and the cholinergic
system to the territory of descriptive neuropathology
has drastically changed this field by adding the theory of
synaptic neurotransmission to the toxic pas de deux in
AD. Accumulating data show that a multitarget approach
involving all amyloid, tau, and cholinergic hypotheses
could better explain the evolution of events happening
in AD. Various species of both Aβ and tau could be
traced in cholinergic neurons of the basal forebrain system
early in the course of the disease. These molecules
induce degeneration in the neurons of this system. Reciprocally,
aberrant cholinergic system modulation promotes
changes in amyloid precursor protein (APP) metabolism
and tau phosphorylation, resulting in neurotoxicity, neuroinflammation,
and neuronal death. Altogether, these
changes may better correlate with the clinical findings and
cognitive impairment detected in AD patients. Failure of
several of Aβ- and tau-related therapies further highlights
the need for special attention to molecules that target all
of these mentioned pathologic changes. Another noteworthy
fact here is that none of the popular hypotheses of AD
such as amyloidopathy or tauopathy seem to be responsible
for the changes observed in AD alone. Thus, the main culprit should be sought higher in the stream somewhere
in APP metabolism or Wnt signaling in the cholinergic
system of the basal forebrain. Future studies should target
these pathological events. |