Sympathetic signaling as a candidate mechanism underlying bone loss after spinal cord injury

Project Summary Spinal cord injury (SCI) is a devastating, life-altering event that affects nearly 27 million people globally. Along with the loss of motor and sensory function, there are also underestimated consequences of SCI, including autonomic dysreflexia, sexual dysfunction, and osteoporosis, that compromise quality of life. Around 80% of people living with SCI will receive a diagnosis of osteoporosis or osteopenia, with loss of up to 50% of sublesional bone within the first 2 years of injury. Traditionally, bone loss after SCI has been attributed to disuse, however, the rate of SCI-induced bone loss is much more rapid than other disuse-related forms like osteoporosis from spaceflight or bedrest. Moreover, pharmacological and rehabilitation therapies that are effective for disuse osteoporosis lack efficacy in the SCI population. Our rodent SCI model also supports the rationale that SCI-induced osteoporosis cannot be explained by disuse alone, as our rats recover weight- supported stepping by 14 days post-SCI and still lose bone. In the proposed studies, I hypothesize that a reduction in local sympathetic signaling drives bone loss after SCI. This is be studied in the following 2 aims: Aim 1: Determine SCI-level dependent effects of increasing and decreasing norepinephrine (NE) on bone loss. SCI will damage sympathetic preganglionic neurons which innervate peripheral organs in a level- dependent manner. I will test the sufficiency and necessity of sympathetic signaling for SCI-induced bone loss by locally administering NE and 6-hydroxydopamine, respectively. Aim 2: Determine SCI level-dependent effects of sympathetic signaling on hematopoiesis. Changes in the hematopoietic response have been shown as early as 1-day post-SCI. Considering that the cells of the bone marrow niche represent a heterogenous population contributing to multiple processes like bone metabolism and the immune response, I will employ two single-cell applications – flow cytometry and single-cell RNA sequencing (scRNA-seq) – to probe temporal hematopoietic changes after SCI. Preliminary data for this proposal suggest that bone loss persists at T11 SCI, for at least 6 months post injury, and that increasing local sympathetic signaling has a protective effect against bone loss, supporting the premise of Aim 1. Interestingly, however, a higher level of injury at T6 shows bone volume similar to shams and no effect of NE treatment. This proposal will introduce a chemical denervating agent to test if local sympathetic signaling is necessary for this form of bone loss. I will also investigate the bone marrow to further investigate not only how SCI changes the bone marrow niche, but also the effects of the presence and absence of sympathetic signaling. The training plan's focus on probing effects of SCI on bone and the use of advanced techniques, like scRNA-seq, will set me on a path to achieve my goal of becoming an independent researcher in spinal cord injury.