Natural Killers at the Neuro-Immune Axis!

Much has been said about the role Natural Killer (NK) cells play in positively and negatively influencing events in tissues and cells. Summarized facts about the healthy state of NK cells in humans and animals explain how innate immune cells, including NK cells differ from adaptive immune cells. One significant feature of NK cells is that they can act independently of MHC antigen presentations and that makes them tantalizing, but enormously challenging for scientists seeking to embrace their  influence over cells and their killing capabilities.

The variety and combination of inhibitory and activating receptors differentially expressed by as many as 30,000 human NK cell subsets makes heterogeneity difficult to relate across different conditions, organs and tissue types. Notwithstanding, positive rates of overall patient survival resoundingly corelated to the presence of as few as one NK cell infiltrating a tumor in a microscopic field.  

Innate immune cells including NK and macrophages have also been directly tied to conditions of neurological pain and more specifically to afferent and efferent fibers that signal through the vagus nerve. In these models at the immune-neurological interface similarities exists and both organs must interact for proper function. 

In each of these organs communication is mediated by direct cellular contact eg. synapse formation and via soluble mediators like cytokines or neurotransmitters that also communicate bi-directionally between cells of each system. The nervous and immune systems can influence each other’s activity because immune cells express neurotransmitter receptors, and neurons express cytokine receptors. Immune cells can synthesize and release neurotransmitters themselves, thus using neurotransmitter-mediated pathways via autocrine and paracrine mechanisms. This may indicate that NK cells extend nerve end signaling further into tissues and at a cellular level. 

A recent paradigm in physiology describes the existence of neuro-immune cell units, at an organ-tissue level and identifies the enormous complexity inherent in this globally unifying approach that also connects neuro-immune-gut, at least in Parkinson's disease.

Parkinson’s is a brain disorder where certain nerve cells slowly die and symptoms worsen. The risk of developing the condition increases with age, but in certain patients the illness is caused by defects in two proteins, PINK1 and parkin. NK cells are capable of homing to the central nervous system in neurological disorders that exhibit exacerbated inflammation and inhibit hyperactivated microglia. Recently, a study demonstrated that NK cells scavenge alpha-synuclein aggregates and systemic depletion of NK cells results in exacerbated neuropathology in a mouse model of alpha-synucleinopathy, making NK cells highly relevant in Parkinson’s disease.

We recently described a mechanism by which the sentinel state of NK cells is impaired and suggested the senescent phenotype, induced by age related mitophagy could be the primary cause. Increase in mitophagy (mitochondrial autophagy) is age-dependent and abrogated by PINK1 or parkin deficiency suggesting, in Parkinson's disease compromised mitophagy is associated with neurological degeneration. Further  PINK1 and Parkin, which are regulated by p53 specifically repress mitochondrial antigen presentation of both MHC classes. Therefore, excessive PINK1 or parkin increases rates of NK cell mitophagy and repress the presentation of mitochondrial antigens for MHC classes at the axis of this neuro-immune related disease.

The healthy state of NK cells at the axis of neuro-immune systems may indeed have more far reaching implications for the future of human diseases and therapies.

 

Synapses By p53 And CD40L in Reproduction and Immunity

Cell membranes constitute a diverse range of lipid molecules each attached to a varying, odd or even length hydrocarbon chain (a tail) that, collectively pack together to form a membrane. Packing is a dynamic that generally occurs according to surrounding pressure, concentration, hydrophobic conditions and motion. The mix of molecules and their hydrocarbon chains in each membrane play a crucial role in determining functions of complex organisms in cells.

Two complex membrane bound organisms of eukaryotic cells are mitochondria - primary provider of ATP energy powering reactions of the cell and endoplasmic reticulum (ER) - protein folding organelle surrounding the nucleus. The mitochondria comprise a double membrane containing electron transport chains - sets of four membrane bound proteins which pump protons between inner and outer membranes to maintain optimal inner mitochondrial membrane pressure through which oxygen is metabolized into water by phosphorylation of ADP to ATP molecules, which are the basic energy unit of the cell.

ER is a convoluted extension of the nucleus membrane into which translated amino acids are transported and where they fold before being released and packaged in the golgi apparatus and cytoplasm. The process of translation, folding and transport requires significant energy as such mitochondria and ER are closely associated. Recently and for the first time C18 ceramide transportation between ER and outer mitochondrial membrane was described as a cellular stress response mechanism.

Another important membrane lipid C16-ceramide was found to tightly bind within the p53 DNA-binding domain. This interaction was highly selective toward the C16 ceramide acyl chain length with its C10 atom being proximal to Ser240 and Ser241. This binding stabilized p53 and disrupted its complex with E3 ligase MDM2 leading to the p53 accumulation, nuclear translocation and activation of downstream targets. The p53-MDM2 axis has been extensively covered in previous articles describing allorecognition, reproduction, immunity and auto-regulation. Ser241 was the only residue that interacted with all three p53 DNA sequences (p21, puma and a non-specific DNA system) persistently, indicating that Ser241 is a [response element] sequence-independent H-bond donor/acceptor for DNA.

It was also determined that Folate stress induces apoptosis via p53-dependent de novo Ceramide synthesis and up-regulation of Ceramide synthase 6 [C16], which is a transcriptional target of p53. In particular, Folate metabolism affects ovarian function, implantation, embryogenesis and the entire process of pregnancy. We observed that folate withdrawal leads to CerS6 up-regulation and C16-ceramide accumulation in a p53-dependent manner as a pro-apoptotic cue.

It has been demonstrated that clustering of the CD40 receptor depends on reciprocal clustering of the CD40 ligand, which is mediated by an association with p53, a translocation of acid sphingomyelinase (ASM) to the cell membrane, activation of the ASM (enzyme for ceramide), and a formation of ceramide. Ceramide appears to modify preexisting sphingolipid-rich membrane microdomains to fuse and form ceramide-enriched signaling platforms that serve to cluster CD40 ligand. Genetic deficiency of p53 or ASM or disruption of [C16] ceramide-enriched membrane domains prevents clustering of CD40 ligand. If the ligand is membrane-bound, the contact site between clustered ligands and receptors forms an immune synapse.

Finally, immune activation during the implantation phase causes preeclampsia-like symptoms via the CD40–CD40 ligand pathway in pregnant mice. The CD40 ligand (CD40L) is expressed by T cells and has a critical role in immune system regulation. Interventions targeting CD40L interactions following embryo implantation represent an approach to preventing preeclampsia (PE).

Here we have demonstrated a relationship between p53, C16 ceramide in reproduction and immunity via CD40 receptor-ligand in membrane bound concentrations of cells, particularly in respect of immunological synapse formation and blastocyst implantation. This further supports the notion that immunity and reproduction share common innate origins linked by p53.