Apoptosis in Alzheimer's disease: insight into the signaling pathways and therapeutic avenues

Alzheimer's disease (AD) is characterized by the accumulation of hyperphosphorylated tau and amyloid-beta (A beta) protein resulting in synaptic loss and apoptosis. A beta and tau deposition trigger apoptotic pathways that result in neuronal death. Apoptosis is considered to be responsible for manifestations associated with AD under pathological conditions. It regulates via extrinsic and intrinsic pathways. It activates various proteins including Bcl-2 family proteins like Bax, Bad, Bid, Bcl-XS, Bcl-XL and caspases comprising of initiator, effector and inflammatory caspases carried out through a cascade of events that finally lead to cell disintegration. The apoptotic elements interact with trophic factors, signaling molecules including Ras-ERK, JNK, GSK-3 beta, BDNF/TrkB/CREB and PI3K/AKT/mTOR. Ras-ERK signaling is involved in the progression of cell cycle and apoptosis. JNK pathway is also upregulated in AD which results in decreased expression of anti-apoptotic proteins. JAK-STAT triggers caspase-3 mediated apoptosis leading to neurodegeneration. The imbalance between autophagy and apoptosis is regulated by PI3K/Akt/mTOR pathway. GSK-3 beta is involved in the stimulation of pro-apoptotic factors resulting in dysregulation of apoptosis. Drugs like filgrastim, epigallocatechin gallate, curcumin, nicergoline and minocycline are under development which target these pathways and modulate the disease condition. This study sheds light on apoptotic pathways that are cardinal for neuronal survival and perform crucial role in the occurrence of AD along with the trends in therapeutics targeting apoptosis induced AD. To develop prospective treatments for AD, it is desirable to elucidate potential targets including restoration apoptotic balance, regulation of caspases, Bcl-2 and other crucial proteins involved in apoptosis mediated AD.