Aging from a Biological Perspective
Senescence is the biological term for aging, or deterioration of cellular function. This refers to the phenomenon that cells can only undergo a certain number of cycles before complications arise. These complications lead to aging and eventually the death of an organism. Hydra, however, do not exhibit this quality, making them a perfect model organism for developmental biology studies . Senescence has been a hot topic lately due to the relation of immune responses to the aging of cells .
Why Study Injury-Induced Immune Response?
The inmate immune system of an organism directs the transition from injury to regeneration. As an organism ages this immune response typically becomes less effective at restoring function of damaged cells. Hydras in contradticiton, exhibit many immune abilities that lead to full regeneration of lost limbs that are not affected by age, giving this animal characteristics of immortality . Examining the immune response of hydras during regeneration is important to establishing a link between the signaling pathways that lead to this phenomenon. Given that certain progenitor cells play a huge role in the regeneration of hydra these questions may just be answered from a developmental biology perspective.
Answering the abundance of questions that revolve around the immune response of hydra regeneration could provide insight as to how to repair damaged or amputated tissues in issues of human injury. This regeneration has been compared to hepatocyte regeneration in the human liver .
Hydra exhibit the ability to discriminate between their own cells and cells of another organism- even if that organism is another hydra. This is called allorecognition and plays an important role in the immune response of regeneration .
Immediately Following Injury:
Initiated by the generation of reactive oxygen species (ROS) at the site of damage, an immune signaling cascade leads to the wound healing followed by regeneration of the missing limb.
Due to the large amount of ROS produced at the damage site, apoptosis is observed in neighboring cells due to MAPK activation. This ROS production is also believed to drive a stress response via Nrf activation.
Immediately the ectodermal and endodermal cells (Figure below) close the wound over the span of two hours by stretching over the space .
These epithelial cells are not affected by ROS, and conversely have phagocytic activities that devour cell debris to clear the area for regeneration.
NFKB activation is a key immune response that is exhibited in most organisms; therefore, it was necessary to see which upregulated genes post injury impact this pathway. It has been shown that immune pathways such as this are not modified on a transcriptional level in hydra, but instead undergo PTM following injury .
Tumor Necrosis Factor Receptor (TNFR, shown above) is a redox-dependent apoptosis regulator that likely plays a role in the hydra immune response because it plays a similar role in mammalian cells. This factor is upregulated about 60 min post ablation in hydra which leads to enhancement of NFkB signaling and a reduction of apoptosis-inducing JNK signaling.
Neural precursor cell expressed developmentally down regulated (NEDD8) is a factor that is transferred to Cullin by Ubc12 (in schematic above) to activate Cullin ring ligase-1 (CRL1). The activation of this complex leads to ubiquination of phosphorylated IkB which leads to its degradation. NF-kB is then free to move into the nucleus to activate transcription, as shown in the schematic above. A heat map shows that Ubc12 is upregulated within the first two hours post ablation (as annotated in Figure 2A below ).
Heat shock proteins (HSPs) are immune stress proteins that assist the regeneration process. They are often referred to as “cytoprotective proteins” These are upregulated in 4 successive waves immediately following bisection.
- Allow organisms to resist stresses such as injury or heat exposure.
- Facilitate protein folding that is ATP-dependent.
- CRYAB1, CRYAB2, and HSP16.2 are all upregulated early and encode small HSPs that prevent premature apoptosis from non-specific aggregation
- HSP90a1 assists in folding of several evolutionarily conserved client proteins including transcription factors such as MAPK, TBK JNK etc. This indirectly affects the NF-kB pathway .
Activator Protein 1 (AP1) is formed from heterodimerization of Jun and Fral1 that are upregulated within the first hour of regeneration (Figure 5 below). Genes encoding antimicrobial peptides (AMPs) are upregulated which points to NF-kB activation by AP1.
Redox factor 1 (Ref-1) is a bZIP transcription factor that controls the redox regulation of a cysteine residue that is present in the Hydra DNA binding domain.
- Leads to tighter DNA binding and therefore an up regulation of certain genes
- Ref-1 is activated by ROS generation at the wound site and then initiates antioxidant response that is mediated by Jun, Fral1, ATF1/CREB and Nrf.
Other Immune Genes Involved:
TNFR, TRAF2l, TRAF5l, IRF2-BP, SIK2
Salt Inducible Kinase 2 (SIK2) regulates TLR (Toll-Like Receptor) signaling in Hydra. It also affects CRTC phosphorylation.
Interferon Regulatory Factor (IRF2-BP) is a co-repressor of IRF2 that targets Jun Dimerization Protein 2 (JDP2) for degradation in order to allow the AP1 complex to form.
TNF Receptor Associated Factors (TRAF) activates ASK1 (apoptosis signal regulating kinase 1) and upregulates NF-kB and downgrades JNK activity with decreases in ROS production .
To summarize, the immune response in hydra provides cues as to how the human immunity might be manipulated to repair extensive tissue damage in a shorter period of time. Though it is not likely that humans could ever regenerate to the capacity that these invertebrates can, it still holds promise for potential treatments. Reducing senescence in cells that already regenerate, such as liver cells, via immune modification has a great potential for future research .
The Wenger et al. paper, “Injury-induced immune responses in Hydra,” was a major source for this webpage. Though the paper was very thorough in exploring the immune pathways that are triggered when a hydra undergoes regeneration, some data was left very incomplete. It seemed that Figure 5 was far too busy and difficult to process without including many pathways that were discussed in the paper. It seems that perhaps making several schematics that coincide with the data in the paper would be a helpful solution to this problem. A figure of this nature was created for this webpage and can be viewed above. Figure 5 is pictured below for reference.
1. Boehm, A.-M., et al. (2013). “Stem cells and aging from a quasi-immortal point of view.” BioEssays 35(11): 994-1003.
2. Galliot, B. (2012). “Hydra, a fruitful model system for 270 years.” Int. J. Dev. Biol. 56(6/7/8): 411-423.
3. Lasry, A. and Y. Ben-Neriah (2015). “Senescence-associated inflammatory responses: aging and cancer perspectives.” Trends Immunol 36(4): 217-228.
4. Wenger, Y., et al. (2014). “Injury-induced immune responses in Hydra.” Semin. Immunol. 26(4): 277-294.