RIPK1


Receptor-interacting serine/threonine-protein kinase 1 is an enzyme that in humans is encoded by the RIPK1 gene, which is located on chromosome 6. This protein belongs to the Receptor Interacting Protein kinases family, which consists of 7 members, RIPK1 being the first member of the family.
RIPK1 is known to have function in a variety of cellular pathways related to both cell survival and death. In terms of cell death, RIPK1 plays a role in apoptosis and necroptosis. Some of the cell survival pathways RIPK1 participates in include NF-κB, Akt, and JNK.

Structure

RIPK1 protein is composed of 671 amino acids, and has a molecular weight of about 76 kDa. It contains a serine/threonine kinase domain in the 300 aa N-Terminus, a death domain in the 112 aa C-Terminus, and a central region between the KD and DD called intermediate domain.

Function

Although, RIPK1 has been primarily studied in the context of TNFR signaling, RIPK1 is also activated in response to diverse stimuli.
The kinase domain, while important for necroptotic functions, appears dispensable for pro-survival roles. Kinase activity of RIPK1 is also required for RIPK1-dependent apoptosis in conditions of IAP1/2 depletion, TAK1inhibition/depletion, RIPK3 depletion or MLKL depletion. Also, proteolytic processing of RIPk1, through both caspase-dependent and -independent mechanisms, triggers lethality that is dependent on the generation of one or more specific C-terminal cleavage product of RIPk1 upon stress.

Role in cell survival

It has been shown that cell survival can be regulated through different RIPK1-mediated pathways that ultimately result in the expression of NF-kB, a protein complex known to regulate transcription of DNA and thus, related to survival processes.

Receptor-mediated signalling

The best well-known pathway of NF-kB activation is that mediated by the death receptor TNFR1, which starts as in the necroptosis pathway with the assembly of TRADD, RIPK1, TRAF2 and clAP1 in the lipid rafts of the plasma membrane. In survival signalling, RIPK1 is then polyubiquitinated, allowing NEMO to bind to the IkB kinase or IKK complex. To activate IKK, TAB2 and TAB3 adaptor proteins recruit TAK1 or MEKK3, which phosphorylate the complex. This results in the phosphorylation of the NF-kB inhibitors by the activated IKK complex, which in turn triggers their polyubiquitination and posterior degradation in the 26S proteasome.
As a result, NF-kB can now migrate to the nucleus where it will control DNA transcription by binding itself to the promoters of specific genes. Some of those genes are thought to have anti-apoptotic properties as well as to promote proteasomal degradation of RIPK1, resulting in a self-regulatory cycle.
While being in complex I, RIPK1 has also been proved to play a role in the activation of MAP kinases such as JNK, ERK and p38. In particular, JNK can be found in both cell death and survival pathways, with its role in the cell death process being suppressed by activated NF-kB.
Cell survival signalling can also be mediated by TLR-3 and TLR-4. In here, RIPK1 is recruited to the receptors where it is phosphorylated and polyubiquitinated. This results in the recruit of the IKK complex activating proteins so eventually NF-kB can now too migrate to the nucleus.
RIPK2 is involved in this TLR-mediated signalling, which suggests that there might be a regulation of cell survival or death through the mutual interaction between the two RIPK family members.

Genotoxic stress-mediated activation

Upon DNA damage, RIPK1 mediates another NF-kB activation pathway where two simultaneous and exclusive processes occur. A pro-apoptotic complex is created while RIPK1 also mediates the interaction between PIDD, NEMO and IKK subunits that will eventually result in the IKK complex activation after interaction with ATM kinase. The interaction between RIPK1 and PIDD through their death domains is thought to promote cell survival to neutralize this pro-apoptotic complex.

Others

It has been observed that RIPK1 may also interact with IGF-1R to activate JNK, it may be related to epidermal growth factor receptor signalling and it is largely expressed in glioblastoma cells, suggesting that RIPK1 is indeed involved in cell survival and proliferation processes.

Role in cell death

Necroptosis

is a programmed form of necrosis which starts with the assembly of the TNF ligand to its membrane receptor, the TNFR. Once activated, the intracellular domain of TNFR starts the recruitment of the adaptor TNFR-1-associated death domain protein TRADD, which recruits RIPK1 and two ubiquitin ligases: TRAF2 and clAP1. This complex is called the TNFR-1 complex I.
Complex-I is then modified by the IAPs and the LUBAC, which generate linear ubiquitin linkages. The ubiquitination of complex-I leads to the activation of NF-κB, which in turn activates the expression of FLICE-like inhibitory protein FLIP. FLIP then binds to caspase-8, forming a caspase-8 FLIP heterodimer in the cytosol that disrupts the activity of caspase-8 and prevents caspase-8 mediated apoptosis from taking place.
The assembly of complex II-b then starts in the cytosol. This new complex contains the caspase-8 FLIP heterodimer as well as RIPK1 and RIPK3. Caspase inhibition within this complex allows RIPK1 and RIPK3 to autotransphosphorylate each other, forming another complex called the necrosome. The necrosome starts recruiting MLKL, which is phosphorylated by RIPK3 and immediately translocates to lipid rafts inside the plasma membrane. This leads to the formation of pores in the membrane, allowing the sodium influx to increase -and consequently the osmotic pressure-, which eventually causes cell membrane rupture.

Apoptosis

The apoptotic extrinsic pathway starts with the formation of the TNFR-1 complex-I, which contains TRADD, RIPK1, and two ubiquitin ligases:TRAF2 and clAP1.
Unlike the necroptotic pathway, this pathway doesn’t include the inhibition of caspase-8. Thus, in absence of NF-κB function, FLIP is not produced, and therefore active caspase-8 assembles with FADD, RIPK1 and RIPK3 in the cytosol, forming what is known as complex IIa.
Caspase-8 activates Bid, a protein that binds to the mitochondrial membrane, allowing the release of intermembrane mitochondrial molecules such as cytochrome c. Cytochrome c then assembles with Apaf 1 and ATP molecules, forming a complex called apoptosome. The activation of caspase 3 and 9 by the apoptosome starts a proteolitic cascade that eventually leads to the degradation of organelles and proteins, and the fragmentation of the DNA, inducing apoptotic cell death.

Neurodegenerative diseases

Alzheimer's disease

Patients with Alzheimer's disease, a neurodegenerative disease characterized by a cognitive deterioration and a behavioural disorder, experience a chronic brain inflammation which leads to the atrophy of several brain regions.
A sign of this inflammation is an increased number of microglia, a type of glial cells located in the brain and the spinal cord. RIPK1 is known to appear in larger quantities in brains from those affected with AD. This enzyme regulates not only necroptosis, but cell inflammation as well, and as a result it is involved in the regulation of microglial functions, specially those associated with the appearance and development of neurodegenerative diseases such as AD.

Amyotrophic Lateral Sclerosis

is characterized by the degeneration of motor neurons which leads to the progressive loss of mobility. Consequently, patients are unable to do any physical activity due to the atrophy of their muscles.
The optineurin gene and its mutation are known to be involved in ALS. When the organism loses OPTN, the dysmyelination of axons and its degeneration start. The degeneration of the axons is produced by several components from the Central Nervous System including RIPK1 and another enzyme from the Receptor Interacting Protein kinases family, RIPK3, as well as other proteins such as MLKL.
Once RIPK1, RIPK3 and MLKL have contributed to the dysmyelination and the consequent degeneration of axons, the nerve impulse can't to go from one neuron to another due to the lack of myelin, which leads to the consequent mobility problems as the nerve impulse does not arrive to its final destination.

Autoinflamatory disease

An autoinflammatory disease characterised by recurrent fevers and lymphadenopathy has been associated with mutations in this gene.

Interactions

RIPK1 has been shown to interact with: