No Pain, No Gain: Lack of Exercise Obstructs Neurogenesis

Abstract

Bedridden patients develop atrophied muscles, their daily activities greatly reduced, and some display a depressive mood. Patients who are able to receive physical rehabilitation sometimes show surprising clinical improvements, including reduced depression and attenuation of other stress-related behaviors. Regenerative medicine has advanced two major stem cell-based therapies for CNS disorders, namely, transplantation of exogenous stem cells and amplification of endogenous neurogenesis. The latter strategy embraces a natural way of reinnervating the damaged brain and correcting the neurological impairments. In this study, we discussed how immobilization-induced disuse atrophy, using the hindlimb suspension model, affects neurogenesis in rats. The overarching hypothesis is that immobilization suppresses neurogenesis by reducing the circulating growth or trophic factors, such as vascular endothelial growth factor or brain-derived neurotrophic factor. That immobilization alters neurogenesis and stem cell differentiation in the CNS requires characterization of the stem cell microenvironment by examining the trophic and growth factors, as well as stress-related proteins that have been implicated in exercise-induced neurogenesis. Although accumulating evidence has revealed the contribution of “increased ” exercise on neurogenesis, the reverse paradigm involving “lack of exercise, ” which mimics pathological states (e.g., stroke patients are often immobile), remains underexplored. This novel paradigm will enable us to examine the effects on neurogenesis by a nonpermissive stem cell microenvironment likely produced by lack of exercise. BrdU labeling of proliferative cells, biochemical assays of serum, cerebrospinal fluid and brain levels of trophic factors, growth factors, and stress-related proteins are proposed as indices of neurogenesis, while quantitative measurements of spontaneous movements will reveal psychomotor components of immobilization. Studies designed to reveal how in vivo stimulation, or lack thereof, alters the stem cell microenvironment are needed to begin to develop treatment strategies for enhancing neurogenesis in bedridden patients.