Cell proliferation, survival and differentiation are commonly known to be regulated by stereotyped developmental programs and physiological feedback mechanisms. However, far less is understood how extrinsic sensory stimuli, through local innervation by the peripheral nervous system, modulate the signaling and responses of cells and tissues in the developmental adaptation and homeostasis of animal tissues. We address this question using a simple Drosophila melanogaster model of niche support by the PNS, focusing on the hematopoietic pockets (HPs) in the body wall of the optically transparent larva. In this system, blood cells (hemocytes) reside in direct physical contact with segmentally repeated sensory PNS clusters, functionally rely on the PNS for their localization and trophic survival, and are induced to proliferate in these microenvironments. We identified PNS neuron-produced Activin as a key regulator of hemocyte adhesion, localization and number, demonstrating that factors from the PNS determine hemocyte signaling and biological responses (Makhijani et al. in prep.). Examining the role of neuron excitation in the HPs, we find that transient silencing of PNS neuronal activity through inducible genetic systems or acetylcholine antagonists results in the rapid scattering or dispersal of resident hemocytes;conversely, PNS stimulants such as the irritant chemicals AITC (wasabi), menthol (mint), or blue light induce recruitment of blood cells to HPs and cause a rise in blood cell numbers over time. Exposure of intact living larvae to these noxious stimuli causes a rapid increase in intracellular calcium (Ca2+), both in PNS neurons and, subsequently, in hemocytes, consistent with an activation of Trp (Transient receptor potential) channels. We hypothesize that activation of the PNS by noxious stimuli regulates blood cell responses through the release or presentation of molecular factors;PNS signals promote hemocyte localization to HPs and facilitate exposure to inductive signals from the microenvironment, resulting in an adaptation of the animal's blood cell pool. The objective of the proposed research is to (1) Determine which aspects of blood cell development are regulated by PNS activity;(2) Identify an inducible mechanism how blood cells are activated by the PNS, focusing on Act and acetylcholine as candidate inducible signals;(3) Dissect the mechanistic sequence by which the irritant AITC (wasabi) regulates blood cell responses. This research is innovative, because we established this simple Drosophila model to study the role of the PNS in the support of a target tissue during development, and now propose to utilize the system to understand the signaling events and biological responses that are triggered in target cells following induction by extrinsic sensory stimuli. This work is significan because it is expected to reveal general principles of how the PNS and its afferent inputs regulate signaling in the homeostasis and developmental adaptation of animal tissues. Interruptions in this communication, such as in peripheral neuropathies, may be the cause of developmental defects and organ degeneration.
The proposed research is relevant to NIH's mission, because it will address fundamental principles how sensory inputs from the environment regulate the signaling of target cells in the homeostasis and developmental adaptation of animal tissues. This will lay the foundation to understand how interruptions in this communication, such as in peripheral neuropathies or neural dysfunction, may be the cause of developmental defects, organ degeneration and other niche-induced diseases. As a consequence, novel clinical therapies and preventive strategies may emerge that intervene at the level of the PNS or its interface with tissue microenvironments.