Apoptosis

Apoptotic cell death is fundamental cell process that utilizes the cell death receptor family signaling network. This key pathway is stimulated by ligands which then regulate downstream adaptor molecules, ultimately activating the caspase proteases. The caspase enzymes exist in an inactive precursor state comprised of a prodomain, as well as large and small catalytic subunits. Caspases are activated by a cleavage that occurs adjacent to an aspartate; this cleavage liberates the individual units and allows formation of an a2b2 tetramer.

The D4-GDI protein is a negative regulator of the Ras-related Rho family of small molecule "molecular switch" GTPases. The Rho GTPases modify cell structure and architecture via rapid changes to the actin cytoskeleton and cell membrane. Many of these physiological processes are associated with apoptotic cell death, thus the in vivo removal of D4-GDI inhibitory block is critical for proper induction and progression of apoptosis in cells.

Cytochrome C is a small heme protein within the inner mitochondrial membrane responsible for carrying electrons within the respiratory transport chain.  Additionally, cytochrome c has also been identified as a player in programmed cell death (apoptosis). During the early phases of apoptotic death reactions, cytochrome c translocates from the mitochondria membrane into the cytoplasm and serves to trigger the apoptotic proteolytic cascade by activating caspase 3, through association with protease activating factor-1 (Apaf-1).

Programmed cell death via apoptosis is a key controlled physiological process instigated by the cell death receptor family, their ligands, and the caspase cysteine protease family. All caspases exist in a precursor form that contains a prodomain, and large and small catalytic subunits. A cleavage event adjacent to an aspartate liberates one large and one small subunit, which are now free to associate into an active a2b2 tetramer. Caspases are activated by triggers such as ligand-receptor interactions, growth factor deprivation, and cell function inhibitors.

FOXO1 belongs to the very large Forkhead family of transcription factors which contain a conserved distinct DNA-binding domain known as the Forkhead Box, or FOX. The Forkhead domain is a 100 amino acid long motif capable of binding and bending DNA, and is also known as a “winged helix”. Forkhead family members are involved in a very diverse and wide range of physiological processes from cell cycle, apoptosis, and oxidative-stress resistance. The “O” class of proteins in particular are all regulated by the insulin/PI3K/AKT pathway.

Cell death via apoptosis is a key cellular function triggered by the cell death receptor family and their ligands. This regulated process then transmits downstream signals through adaptor molecules ending with the caspase cysteine proteases. Caspase 11 has a heterotetrameric structure consisting of two anti-parallel heterodimers. Upon activation, it is cleaved by an autocatalytic mechanism to give rise to individual subunits. This post-translational regulation enables rapid activation. Expression levels of caspase 11 are highest in lung and spleen.

CD90 is a 25-35kD glycosylphosphatidylinositol (GPI)-linked glycoprotein receptor of the immunoglobulin (Ig) superfamily. It is found on murine T-cells, thymocytes, neuronal cells, granulocytic lineage-derived cells, hematopoietic stem cells, fibroblasts, neurons, and Kupffer's cells. CD90 is often used as a marker for a variety of stem cells and mature neuronal axon processes.  CD90 appears to play a role in cell-cell and cell-matrix interactions and adhesion during synaptogenesis, nerve regeneration, apoptosis and necrosis, inflammation, fibrosis, and metastasis.

GAPDH is a 146kD tetramer glycolytic pathway metabolic enzyme composed of four 30-40 kDa subunits. It is responsible for reversibly phosphorylating its substrate glyceraldehyde 3-phosphate within the glycolytic pathway.  Apart from its role in glycolysis, GAPDH may have other roles such as transcriptional activation. Due to its housekeeping role, GAPDH is highly expressed in almost all tissues, allowing its use as an internal loading control (traditionally for mRNA expression comparisons, but also in protein studies.

The BAG proteins are a large family of chaperone regulators governing a wide range of cell processes such as proliferation, survival, stress response, tumorigenesis, neuronal differentiation, growth arrest and apoptosis as reviewed in Takayama et al¹. BAG proteins are co-chaperones that interact with several forms of the chaperone heat shock protein 70 (Hsp70) – the association allows them to both positively and negatively regulate Hsp70.

Fas is a member of the tumor necrosis factor (TNF)-receptor superfamily and plays a key role in the physiological regulation of programmed cell death. This receptor contains a death domain which enables the formation of a signaling complex that includes Fas-associated death domain protein (FADD), caspase 8, and caspase 10. The auto-proteolytic processing of these complexed caspases triggers a downstream cascade that leads to membrane-mediated apoptosis.