The neuroprotective and neurotrophic actions of EPO have made it a useful molecule to investigate in clinical studies of neuropsychiatric disorders. As an FDA approved biologic drug that is widely prescribed to treat anemia, the safety profile is well documented. However, the potential for adverse hematological effects with chronic dosing is a major limitation for its use as a CNS drug. CEPO is devoid of erythropoietic activity and helps address this key limitation. Our results also show that CEPO had no effect on platelet counts, whereas EPO sharply elevated it. EPO’s effect on reticulocyte number (9x higher than control) is rather striking. As with other hematological parameters, CEPO’s effect on reticulocyte number was comparable to controls. The apparent lack of hematopoietic cascade activation raises interesting questions regarding CEPO’s mechanisms of action in mediating behavioral effects which are comparable to EPO (6, 16). Gene expression studies have shown that EPO (16) and CEPO (11) share an overlap in the neurotrophic factors that they induce in the hippocampus, such as BDNF, VGF and neuritin. These three neurotrophic molecules have been previously shown to independently produce antidepressant-like and cognitive enhancing effects in rodent models (17–22).
Our comparative analysis of EPO and CEPO-induced protein expression profiles provides additional insight into their potential mechanisms of action. Both ligands elevated the expression of neurotrophic and neurogenic proteins. Interestingly, more classes of trophic factor molecules were induced by EPO than CEPO. Trophic factors such as Transforming growth factor beta (TGF b), Myotrophin and Neudesin were elevated only by EPO. This suggests that CEPO has a more limited trophic role in comparison to EPO which is known to be pleiotrophic. It is likely that this is due to differential activation of intracellular signal transduction cascades by EPO and CEPO.
Both molecules comparably induced MAPK and Akt in the rat hippocampus. These cascades could be involved in their behavioral effects as these kinase pathways have been strongly implicated in antidepressant-like activity (23, 24). The elevation of Growth factor receptor-bound 2 (Grb2), a crucial adaptor molecule that links growth factor receptors to intracellular signaling, provides additional support for the overlap in trophic factor signaling pathways induced by CEPO and EPO. It is tempting to speculate that CEPO (6) recapitulates EPO’s antidepressant (2, 3) effects by virtue of activating trophic signaling pathways but is non-erythropoietic because it does not induce the canonical Jak-STAT hematopoietic cascade. While the results from this study indicate an overlap in trophic pathways, we did not find evidence indicating selective activation of the hematopoietic pathway by EPO. Previous work that carefully examined the differences in hematopoietic signaling molecules induced by wildtype EPO and a non-erythropoietic mutant EPO, reported differences that were subtle and dynamic (25). Our studies, conducted at a single timepoint, were likely unable to capture these dynamic changes.
Bioinformatics pathway analysis revealed the enrichment of CEPO-induced proteins with functions related to neurogenesis, synaptic plasticity, neurotransmitter transport, synaptic vesicle priming, LTP and dendritic spine development. Upregulated proteins included Nenf, Chgb, Cttn, Camk1, Eif3a, Rplp2 and Psmc3. A neurotrophic molecule, Neudesin neurotrophic factor (Nenf), regulates hippocampal neurogenesis. Nenf knockout mice display a decrease in cell proliferation and newborn neurons in the sub granular zone (SGZ) of the hippocampus (26). A secretory protein present in synaptic vesicles, Chromogranin b (Chgb), promotes neurotransmitter release and differentiation of hippocampal neuronal precursor cells (27, 28). An F-actin binding protein, Cortactin (Cttn), is present in dendritic spines in the hippocampus. During the synaptic activity, it causes changes in spine shape and size by interacting with actin filaments and supporting the induction of LTP. Additionally, Cttn interacts with PSD-95, causing an increase in spine density and facilitates LTP and synaptic plasticity (29, 30). CEPO induced Cortactin specifically in the molecular layer of the dentate gyrus, which could indicate that CEPO’s actions prominently involve the hippocampus. Long term memory formation occurs due to an increase in synaptic strength and is facilitated by new protein synthesis. The key components of protein synthesis are elongation factors such as Eif3a that aid in the protein synthesis initiation step and ribosomal subunits (Rplp2) involved in protein translation (31–33).
The upregulation of ubiquitination and proteasomal proteins such as Psmc3 by both EPO and CEPO can seem counterintuitive. However, it is useful to note that synaptic plasticity causes rapid protein synthesis as well as protein degradation. Ubiquitination and proteasomal pathways play important roles in controlled protein degradation. Interestingly, inhibiting these degradative pathways can cause a reduction in potentiation and LTP formation (34, 35). Overall, the EPO and CEPO-induced protein expression profiles provide mechanistic insight into their behavioral actions, particularly the cognitive effects that have been reported in preclinical and clinical studies. Our study was focused on global protein expression changes that are essentially downstream from receptor activation and did not capture alterations that are transient and dynamic. In future studies aimed at understanding CEPO’s lack of hematopoietic effects it will be useful to focus on posttranslational modifications that regulate signal transduction. It is widely thought that CEPO signals via a betacommon receptor and EPO receptor heteromer rather than the EPO receptor dimer employed by EPO. Further studies are needed to understand this important ligand-receptor interaction and how it affects cellular signaling. The possibility of additional receptors and adaptor molecules should also be considered. A global phospho-proteome approach shortly after receptor activation has the potential to shed light on differential signaling pathway activation by EPO and CEPO.