Differential expression of VEGF isoforms and receptors in the phrenic motor nucleus of end‐stage SOD‐1G93A mutant rats

Abstract

Amyotrophic lateral sclerosis (ALS) is a devastating motor neuron disease causing paralysis and death from respiratory failure. Despite major loss of phrenic motor neurons (PMN) In a rodent model of familial ALS (SOD1G93A rats), breathing capacity is maintained until late in disease progression (Nichols, et al., 2013). Thus, compensatory respiratory plasticity preserves breathing capacity despite progressive respiratory motor neuron cell death. Vascular endothelial growth factor (VEGF), often regarded as an angiogenic factor, also plays a pivotal role in neuroprotection and neuroplasticity. VEGF‐A and VEGF receptor 2 (VEGFR‐2) are expressed in PMN of naïve rats, and intrathecal VEGF‐A injections induce long‐lasting phrenic motor facilitation (Dale‐Nagle et al., 2011). Little is known concerning in the expression of different VEGF isoforms or their corresponding receptors in PMN, or changes in their expression with motor neuron disease. Since VEGF may play a key role in compensating for respiratory motor neuron loss, we investigated expression of multiple VEGF isoforms (A, B, C, D, E) and receptor subtypes (VEGFR‐1/Flt‐1, VEGFR‐2/KDR/Flk‐1, VEGFR‐3) in the phrenic motor nucleus of end‐stage SOD1G93A rats (20% decrease from peak body mass). We utilized immunohistochemistry to label different VEGF isoforms and receptors within neurons (NeuN positive), astrocytes (GFAP positive) and microglia (OX‐42 positive). Presumptive phrenic motor neurons expressed VEGF‐A, with minimal or no expression of VEGF‐B, ‐C, ‐D, and ‐E. These neurons expressed all VEGF receptors (1, 2 and 3). On the other hand, reactive astrocytes expressed all VEGF isoforms and VEGF receptors 1 and 2, but not VEGFR‐3. Interestingly, activated microglia expressed only VEGF‐B, ‐C and ‐D, but not VEGF‐A or ‐E; microglia expressed VEGFR‐1 and VEGFR‐2, but not VEGFR‐3. In conclusion, multiple VEGF isoforms and receptors are differentially expressed among different cells types of the phrenic motor nucleus of SOD‐1 mutant rats. Increased VEGF expression in glial cells neighboring motor neurons may confer neuroprotection, or help surviving motor neurons compensate for the loss of their neighbors during diseases progression. However, knowledge of the specific role(s) played by different VEGF isoforms/receptors in ALS is lacking.Support or Funding InformationSupported by McKnight Brain Institute