Despite advances in understanding the pathogenesis of diabetic retinopathy (DR), the nature and time course of the molecular changes associated with DR remain incompletely understood. We propose to investigate the mechanism of bone marrow (BM) failure in diabetes and test diverse therapeutic approaches to correct BM and bone marrow progenitor cell (BMPC) dysfunction in order to restore retinal vascular health. Healthy BMPC participate in the process of re-endothelialization and also contribute indirectly to vascular repair by the secretion of paracrine factors. In health, distinct populations of BMPC work in concert to orchestrate vascular repair. The small pool of CD34+ cells has robust reparative function and modulates, via paracrine signaling, the function of the much larger pool of CD14+ cells which, depending on the local milieu, have the potential to also become efficient reparative cells. Freshly isolated BMPC can be expanded in vitro to generate two additional populations of reparative cells: early endothelial precursor cells (eEPC) or late outgrowth endothelial cells (OEC) that may have more therapeutic utility than freshly isolated cells. To date the optimal progenitor cell for vascular repair has not been identified.

In diabetes, BMPC demonstrate deficient reparative function and may even have a deleterious effect on the repair process. We demonstrated that: 1) diabetic CD34+ cells do not repair acellular capillaries in the diabetic retina, whereas CD34+ cells from nondiabetic individuals do (a similar observation was also made using CD14+ cells); 2) compared to nondiabetic CD34+ cells, diabetic CD34+cells have reduced bioavailable nitric oxide (NO) which results in their reduced migratory ability that can be corrected by restoring NO levels to normal; and 3) diabetes is associated with the loss of circadian rhythmicity of BMPC release from the BM into the circulation. Decrease in blood flow through the vasa nervorum, the small arteries that provide blood supply to peripheral nerves, has been implicated in the development of diabetic neuropathy. We postulate that in diabetes reduced NO initiates changes in the vasa nervorum and is a mechanistic contributor to reduced sympathetic neurotransmission which ultimately results in BM dysfunction. Loss of this neurotrophic support leads to changes in the BM microenvironment which contribute to, not only reduced release of these cells, but also to their impaired function. This dysfunction is further exacerbated by elevated reactive oxygen species (ROS) in these cells. Based on our findings that diabetic retinopathy is linked to peripheral neuropathy, we put forth the following hypothesis:

Loss of circadian rhythmicity of release of BMPC from the BM (due to dysregulated NO in the vasa nervorum and subsequent BM neuropathology) and increasing levels of intracellular ROS, increasing endogenous NOS inhibitors and inappropriate secretion of pro-inflammatory cytokines and proangiogenic growth factors compromise the reparative ability of BMPC leading to development of diabetic retinopathy.