Neurobiology, Stem Cell Biology, and Immunology: An Emerging Triad for Understanding Tissue Homeostasis and Repair

Abstract

The peripheral nervous system (PNS) endows organisms with the remarkable ability to sense and respond to our dynamic world. Emerging evidence shows the PNS also participates in tissue homeostasis and repair by integrating local changes with organismal and environmental changes. Here, we provide an in-depth summary of findings delineating the diverse roles of peripheral nerves in modulating stem cell behaviors and immune responses under steady state conditions and in response to injury and duress, with a specific focus on the skin and in the hematopoietic system. These examples showcase how elucidating neuro-stem cell and neuro-immune cell interactions provides a conceptual framework that connects tissue biology and local immunity with systemic bodily changes to meet varying demands. They also demonstrate how changes in these interactions can manifest in stress, aging, cancer, and inflammation, as well as how these findings can be harnessed to guide the development of new therapeutics.

Introduction

Stem cells and immune cells are central players in tissue maintenance and repair in diverse organ systems. Stem cells self-renew and generate differentiated progeny essential for the maintenance and repair of tissues. Immune cells survey peripheral tissues, identify pathogens and damage, and mount appropriate attack or repair responses. However, tissue production and immune responses are not only localized phenomena: they often need to be coordinated with and tailored to an organism’s physiological state and changing external environment.

Because the nervous system has a footprint in every organ, and because of its specialized role in sensing environmental changes, integrating diverse information across organ systems, and modulating responses systemically—all with rapid speed—it is uniquely poised to initiate and integrate local tissue changes in response to both systemic and environmental cues. Indeed, recent findings have begun to elucidate how the nervous system modulates somatic stem cell behavior or immune responses in various contexts across development, homeostasis, aging, and disease. These discoveries are fueled by technical advances in (1) mouse genetic tools that enable targeting of specific populations of stem cells, immune cells, or neurons (Barker et al 2007, Beronja et al 2010, Cavanaugh et al 2011, Chan et al 2017, Clausen et al 1999, Ferron & Vacher 2005, Ito et al 2005, Li et al 2011, Mattiuz et al 2018, Stirling et al 2005); (2) single-cell sequencing approaches that delineate molecular changes and identify markers within heterogeneous cell populations (Baryawno et al 2019, Chiu et al 2014, Grover et al 2016, Joost et al 2020, Ma et al 2020, Stickels et al 2021, Tanay & Regev 2017, Tikhonova et al 2019); and (3) chemogenetic and optogenetic tools that modulate neuronal activities (Alexander et al 2009, Armbruster et al 2007, Boyden et al 2005, Farrell et al 2013, Stachniak et al 2014). Neuro-stem cell and neuro-immune cell interactions are particularly abundant at barrier sites—such as the skin and the gastrointestinal tract—as these locations are exposed to frequent insults where rapid tissue production and immune modulations are vital (Jacobson et al 2021, Trier et al 2019). These interactions are also found in the bone marrow, the host site for hematopoietic stem cells (HSCs), which sit at the top of the hierarchy that produces diverse immune cell types.

Here, we discuss recent advances that showcase how the nervous system interacts with stem cells and with immune cells. We focus our discussions on the skin, as an example of a barrier site, and on the bone marrow, as an example of a non-barrier site. Comparison of these two seemly divergent systems reveals shared principles as well as interesting differences. Collectively, these advances demonstrate how neural modulation of stem cells and immune cells is crucial for organisms to adapt to dynamic changes both locally and systemically. They also delineate experimental strategies for future investigation of these cross-system interactions and lay the foundation for development of therapeutics that embrace a more holistic and systemic approach that treats the root cause of diseases rather than only alleviates symptoms that manifest locally.

The Anatomy and Function of the Peripheral Nervous System

The peripheral nervous system (PNS) consists of two main branches, the somatic nervous system and the autonomic nervous system. Together, they transmit peripheral signals to and from the central nervous system (CNS, which contains the brain and the spinal cord). The somatic nervous system consists of sensory neurons that enable a diverse array of sensations and motor neurons that control voluntary movement (Akinrodoye & Lui 2022). The autonomic nervous system contains three branches—the sympathetic, parasympathetic, and enteric nervous system—that connect the CNS to all peripheral organs to regulate bodily functions and physiological responses (Karemaker 2017). For the purposes of this review, we will focus on the sensory nervous system, sympathetic nervous system, and parasympathetic nervous system due to their emerging roles in regulating stem cells and immune cells.

Sensory nervous system

The sensory nervous system relays incredibly diverse cues from all the peripheral organs and the external environment to the CNS (Figure 1A). Most sensory neuron cell bodies reside in dorsal root ganglia (DRG) close to the spinal cord. Morphologically, most neurons have a single, long projection, called an axon, through which they deliver information to targets, and many branch-like short projections stemming from the cell body, called dendrites, through which they receive information. However, sensory neurons are uniquely pseudouniploar and lack dendrites. Instead, sensory axons are bifurcated—a single axon split into two branches. One branch innervates targeted peripheral tissues to receive sensory information that is then relayed to the other branch, which delivers the information to the spinal cord. Sensory neurons are specifically attuned to detecting either innocuous or nociceptive thermal, mechanical, or chemical stimuli, with the vast majority of the DRG sensory neurons dedicated to responding to harmful stimulants (Han et al 2013, Liu et al 2007, Luiz et al 2019).

Figure 1. The peripheral nervous system (PNS).

The PNS is composed of two major arms, the afferent and efferent. The afferent arm collects information from peripheral organs, relays it to the central nervous system (CNS), and then the efferent arm sends signals from the CNS back to the periphery to modulate tissue behavior and function. A. Sensory neurons, located in the dorsal root ganglia juxtaposed to the spine, innervate diverse peripheral tissues and represent the “afferent” arm that brings peripheral information to the CNS. Sensory nerves display unique innervation patterns in different tissues. In the skin, sensory nerves form lanceolate endings that wrap around the hair follicle to detect touch. Dense sensory innervation of the skin with free nerve endings in the epidermis and dermis predominantly sense painful stimuli. There are also free ending nerves for thermal sensations, with innocuous sensing cold neurons located near the epidermis and heat sensing neurons deeper in the dermis. Sensory neurons have a unique morphology with a bifurcated axon that extends one branch to peripheral tissues to detect different sensations that are then relayed to the other axonal branch that extends to the CNS. Information integrated in the CNS is relayed out through the sympathetic nervous system. B. Sympathetic neurons innervate virtually every organ in the body to regulate diverse physiological processes and represent the “efferent” arm. The cell bodies of preganglionic sympathetic neurons are located in the spinal cord and they transmit signals from the CNS to postganglionic neurons. The cell bodies of postganglionic sympathetic neurons are located in sympathetic ganglia that form a chain near the spine. Postganglionic neurons relay the signal from presynaptic neurons to regulate innervated tissues in the periphery.

Sensory neurons display a high degree of heterogeneity, and classification of both innocuous and nociceptive sensory neurons depends on cell body size, axon diameter, degree of myelination, and conduction velocity (Abraira & Ginty 2013). These neurons can be further divided into an array of subtypes based on the expression of different receptors, ion channels, signaling factors, cytoskeletal proteins, and neuropeptides (Braz et al 2005, Chiu et al 2014, Dong et al 2001, Hjerling-Leffler et al 2007, Li et al 2016, Usoskin et al 2015, Zylka et al 2005). Unique markers for sensory subtypes have enabled the generation of genetic tools to study the function of sensory populations and to visualize their complex innervation morphologies in different tissues. Sensory neurons form specific innervation patterns that are tailored to their various roles in individual tissues (Table 1). For example, sensory neurons in the skin form a dense network of free nerve endings within the dermis and epidermis (Figure 1A) (Vrontou et al 2013, Wu et al 2012, Zylka et al 2005). This web-like morphology may uniquely position them to survey environmental changes such as temperature fluctuations to regulate physiological responses. While sensory perceptions—in particular pain sensations—are important for detecting tissue damage, emerging data show that sensory neurons actively participate in tissue homeostasis and repair via modulation of stem cell behaviors and immune responses.

Table 1.

Select examples of the functions of the peripheral nervous system in tissue regulation

	Site of action	Action	References
Sensory	hair follicle	gentle touch detection by hair deflection	Biemesderfer 1978; Li 2011, Vrontou et al. 2013
	dermis & epidermis	temperature sensing	Story et al 2003, Bautista et al 2007, Lv & Liu 2007, Dhaka et al 2008, Basbaum et al 2009
		injury or environmental hazards	Koizumi et al 2004, Lumpkin & Caterina 2007
	taste bud	regeneration	Lu et al 2018
	bone marrow	stem cell activity and differentiation	Gao et al 2021
	mineralized bone	sensing tissue damage	Mach 2002
Sympathetic	secretory cells	hormonal secretion	Karemaker 2017, Lin et al 2021
	smooth muscle	control of involuntary movements	Karemaker 2017
	hair follicle	piloerection (goosebumps)	Furlan et al 2016, Zhao et al 2017, Shwartz et al 2020
	adipose	thermal regulation	Sipe et al 2017, Zeng et al 2019
	bone marrow	trafficking and differentiation of HSCs	Méndez-Ferrer et al 2008, Méndez-Ferrer et al 2010, Maryanovich et al 2018, García-García et al 2019
	skin	activation of HFSCs	Shwartz et al 2020
		depletion of MeSC	Zhang et al 2020
	intestine	promotes tissue protective programs in immune response	Gabanyi et al 2016, Matheis et al 2020
Parasympathetic	gut	regulating immune cell landscape and response	de Jonge et al 2005, Meregnani et al 2011, Matteoli et al 2014
	liver	immune response	Borovikova et al 2000
	bone marrow	HSC trafficking	García et al 2013, García-García et al 2019
	salivary gland	development and regeneration	Knox et al 2010, Knox et al 2013
	prostate	cancer formation and metastasis	Magnon et al 2013

Sympathetic nervous system

Sensory neurons represent the “afferent” arm of the PNS—they sense stimuli in peripheral tissues and relay this information to the CNS where the information is integrated and interpreted. A reflexive response is then delivered from the CNS back to the periphery through the sympathetic nervous system (the “efferent” arm) to regulate organ function. This circuit enables an immediate response in innervated organs to adapt to internal or external changes. For example, cold detection by sensory neurons in the skin is transmitted to the hypothalamus region of the brain, which can then relay a response through cutaneous innervating sympathetic neurons to trigger goosebumps for thermal regulation (Bautista et al 2007, Dhaka et al 2008, Furlan et al 2016, Shwartz et al 2020, Story et al 2003, Zhao et al 2017).

The ganglia of sympathetic neurons form a bilateral chain located ventro-laterally to the spinal cord. The cell bodies of preganglionic sympathetic neurons lie in the spinal cord and extend their axons to synapse with the cell bodies of peripheral postganglionic neurons in the sympathetic ganglia (Figure 1B). Postganglionic sympathetic neurons extend their axons out from the sympathetic ganglia to innervate and regulate virtually every organ in the body (Figure 1B). Sympathetic neurons are constantly active under steady-state to regulate physiological homeostasis—including heart rate, respiration, blood glucose levels, and digestion (Patel et al 2015, Scott-Solomon et al 2021, Zeng et al 2019). The level of basal sympathetic activity also follows a circadian pattern, with different levels of basal activity associated with rest and wakefulness (Biaggioni 2008, Somers et al 1993). Under duress, rapid elevations in sympathetic activity mobilize the “fight-or-flight” responses—including elevations in blood pressure and heart rate, and increases in blood sugar—to enable an organism to immediately react to threats (Lin et al 2021, Scott-Solomon et al 2021, Sipe et al 2017).

Being an “efferent arm”, postganglionic sympathetic neuronal axons innervate and transmit neuronal signals to regulate diverse target tissues in the periphery. Sympathetic axons secrete a neurotransmitter, norepinephrine, which binds to adrenergic receptors expressed by innervated tissues to regulate physiological responses (Table 1). Recent studies have begun to elucidate diverse roles for the sympathetic nervous system in regulating somatic stem cells and immune cells. These studies show that beyond its known roles in regulating physiology, the sympathetic nervous system also regulates tissue formation, regeneration, and repair. In fact, because sympathetic neurons are essential for modulating body physiology in response to environmental changes, they are uniquely poised to couple tissue renewal and immunity with systemic demands and changes in the external environment for optimal outcomes.

Parasympathetic nervous system

As a counterbalance to the sympathetic nervous system, the parasympathetic nervous system innervates many of the same targets as the sympathetic nervous system to decrease arousal after a threat has subsided and to promote “rest-and-digest” under steady state. Parasympathetic ganglia are located in close proximity to or embedded in the tissues they regulate, which they achieve through secretion of the neurotransmitter acetylcholine (Karemaker 2017). The counterbalancing natures of the sympathetic and parasympathetic nervous system is crucial for creating an equilibrium that maintains organismal physiology. Although much remains to be learned, several interesting findings have begun to elucidate new roles of parasympathetic nerves in tissue homeostasis and cancer (Table 1).

Neuro-stem cell interactions in the skin

The skin contains some of the most highly regenerative tissues in mammals that are maintained by distinct populations of somatic stem cells. The skin is also highly innervated with sympathetic nerves and a wide array of sensory nerves, endowing organisms with the remarkable ability to sense and respond to diverse external stimuli. Recent findings have revealed that the PNS plays a crucial role in regulating skin stem cell behaviors in tissue homeostasis, injury repair, and in response to stressful stimuli. Although these findings represent perhaps only the tip of the iceberg for the potentially vast complicity of nerve-stem cell interactions in the skin, they provide conceptual frameworks for understanding how neurobiology and stem cell biology can be integrated at the surface of an organism where external stimuli and insults are frequent.

Sympathetic nerves and hair follicle stem cells—bridging the gap between stem cells and the environment

Tissue production and renewal is not only important for maintaining organ function long-term but it is also essential for tailoring tissue growth to changing environmental demands. Interactions between sympathetic nerve endings and hair follicle stem cells (HFSCs) exemplify how the nervous system can contribute to both homeostatic maintenance and “on-demand” growth via direct regulation of somatic stem cells.

Hair is a key trait shared by all mammals. The hair follicle—the epithelial tissue hosting each hair—undergoes cycles of growth (anagen), regression (catagen), and rest (telogen). HFSCs reside in the bulge and a small cluster below the bulge, known as the hair germ. The telogen hair follicle contains only the bulge and the hair germ at the base of the hair follicle (Figure 2A). Entry into anagen depends on the activation of HFSCs, which proliferate transiently to regenerate a transit-amplifying population that produces 7 morphologically and molecularly distinct differentiated cell types including the hair shaft that forms the hair (Hsu & Fuchs 2021, Zhang & Hsu 2017). The hair follicle grows deep down into the dermis in anagen, while the newly formed hair grows upwards and protrudes out from the skin surface (Figure 2A). Anagen is the only phase in which the hair can grow longer.

Figure 2. Sympathetic neurons regulate two distinct populations of stem cells residing in the hair follicle under steady state, cold, and stress.

A. The hair follicle contains two stem cell populations of distinct origin and function. Hair follicle stem cells (HFSCs) are of epithelial origin and are located in the bulge and hair germ. Melanocyte stem cells (MeSCs) arise from the neural crest and are intermingled with HFSCs in the bulge and hair germ. HFSCs and MeSCs are both quiescent during the telogen (resting) phase of the hair cycle. At the onset of anagen (growing) phase, both stem cell populations become activated to proliferate—HFSCs produce transit-amplifying cells (TACs, also known as matrix) that regenerate a new hair and all other differentiated cells of the hair follicle. MeSCs make differentiated melanocytes that produce pigments that color the hair. B. Basal sympathetic nerve activity primes HFSCs for activation via the neurotransmitter norepinephrine (NE). Norepinephrine binds to ADRB2 receptors on the surface of HFSCs to downregulate expression of a quiescence promoting secreted protein, FGF18. Following cold exposure, elevated sympathetic nerve activity not only induces goosebumps (piloerection), but also accelerates HFSC activation and promotes new hair growth. C. Following strong acute stress, hyperactivated sympathetic nerves secrete an excess of norepinephrine around the bulge, which binds to ADRB2 on MeSCs. Activated ADRB2 signaling in MeSCs causes them to hyperproliferate, differentiate, and be permanently depleted from the bulge. Loss of MeSCs leads to the production of an unpigmented hair in the next hair cycle.

Sympathetic neurons are known to innervate arrector pili muscles, a bundle of smooth muscle cells connected to HFSCs in each hair follicle (Botchkarev et al 1999, Fujiwara et al 2011). Cold temperature induces an elevation in the activity of the sympathetic nervous system and causes the contraction of innervated arrector pili muscles, pulling the hair to stand erect—a reaction known as “piloerection” or “goosebumps”. However, piloerection is only a rapid, first-line defense towards protection against cold. Using the arrector pili muscle as a highway, sympathetic nerves also innervate HFSCs. In mice, elevated sympathetic nerve activity in cold temperature leads to precocious activation of HFSCs to regenerate a new hair coat (Figure 2B), a mechanism that may allow animals to grow a thicker hair coat for insulation in the winter to meet seasonal demands (Shwartz et al 2020). Besides temperature changes, activation of the sympathetic nerve activity via strong blue light stimulation also leads to hair growth, underscoring another example of how tissue production can be triggered by external environmental stimuli via the sympathetic nervous system (Fan et al 2018).

Besides its role in stimulating HFSC activation through elevated neural activity, basal activity of the sympathetic nervous system also modulates HFSCs during normal tissue turnover. Ablation of cutaneous sympathetic nerves leads to a substantial delay in anagen entry, suggesting that low levels of sympathetic nerve activity under steady-state conditions prime HFSCs for activation. In fact, sympathetic neurons form synapse-like connections with HFSCs and modulate HFSCs directly via norepinephrine, similar to how sympathetic neurons regulate their conventional targets such as smooth muscles or glands. Norepinephrine binds to the β2-adrenergic receptor (ADRB2) on HFSCs and down-regulates the expression of a secreted signaling protein, FGF18, which normally enforces HFSC quiescence (Figure 2B) (Shwartz et al 2020). In this sense, a fast-acting, short-range neurotransmitter signal relays to a slow-acting, broad-range secreted protein signal to combine the best of both worlds—sympathetic neurons transmit rapid signals reflective of body physiology state or external environment changes, while FGF18 levels around HFSCs can further fine-tune HFSC activity for a much longer duration than the initial neuronal impulse.

Sympathetic nerves and melanocyte stem cells—a stressful connection

The hair follicle also hosts another stem cell population—melanocyte stem cells (MeSCs), which are crucial for making pigments. MeSCs are derived from the neural crest and are intertwined with HFSCs in the bulge and hair germ. Like HFSCs, MeSCs are quiescent throughout most of the hair cycle and only become activated transiently during early anagen (Chang et al 2013). Concurrent activation of HFSCs and MeSCs ensures the regeneration of a pigmented hair in anagen—HFSCs produce the hair follicle and the hair, while MeSCs produce differentiated melanocytes that color the hair (Figure 2A) (Lu et al 2020, Rabbani et al 2011). In the absence of MeSCs, the newly formed hair in anagen becomes unpigmented, a process commonly referred to as hair graying.

We are all familiar with the maxim that stress turns hair gray or have heard anecdotes of someone’s hair turning gray following a highly stressful event. Recently, it has been shown that sympathetic neurons drive stress-induced hair graying. Under strong acute stress in mice, the sympathetic nervous system becomes highly activated to trigger the “fight or flight” response. Excess norepinephrine secreted from the sympathetic axon terminals near the hair follicle binds to the ADRB2 receptor on MeSCs and triggers their permanent depletion from the hair follicle within days (Figure 2C) (Zhang et al 2020). Norepinephrine triggers a similar response in human MeSCs in cultured hair follicle explants (Rachmin et al 2021). This rapid depletion of MeSCs demonstrates how an acutely stressful event can have detrimental and irreversible consequences on tissue regeneration by causing the rapid loss of a somatic stem cell population through overactivation of the sympathetic nervous system. Stress is often associated with damage to multiple organ systems (Choi et al 2021, Shukla et al 2021, Sivamani et al 2009, Song et al 2018, Yirmiya et al 2006), but how a psychological state influences tissue changes at the cellular and molecular levels remains unclear. It will be interesting to investigate if other stress-related tissue changes also involve changes in stem cell populations linked to the PNS.

In addition to being associated with stress, hair graying also occurs with aging. Natural aging is marked by a decline in tissue homeostasis and regeneration, which in part is attributed to changes in somatic stem cells. Interestingly, patients who underwent partial sympathectomy (removal of sympathetic innervation) developed fewer unpigmented hairs on the side that no longer had sympathetic innervation with age (Lerner 1966, Ortonne et al 1982). Therefore, it is possible that the sympathetic nervous system contributes to the age-related gradual loss of MeSCs and that hyperactivation of the sympathetic nervous system under acute stress accelerates this process. Aged skin also shows many deficits in regeneration, including a decline in hair follicle regeneration and a delay in wound healing, with some of these defects also seen under stress (Choi et al 2021, Ge et al 2020, Keyes et al 2016, Keyes et al 2013). Further work investigating the similarities and differences between stress response and the aging process will provide new insights into how the PNS contributes to the tissue decline seen with both processes and identify potential ways to halt or reverse these changes.

The role of sensory neurons during wound repair

Normal tissue turnover utilizes discrete stem cell populations for compartment-specific regeneration. Following injury, individual stem cell populations often lose their regionally restricted behavior and exhibit a high degree of plasticity for tissue repair. This plasticity is seen in the skin epithelium, which contains the stratified epidermis, the hair follicle, and the isthmus region connecting the hair follicle with the epidermis. Under steady state, the epidermis is maintained by epidermal stem cells (EpSCs) located at the basal layer of the epidermis. Upon injury, in addition to EpSCs, cells from the hair follicle and the isthmus region become activated and migrate to the epidermis to participate in the repair process (Figure 3A) (Brownell et al 2011, Huang et al 2021a, Ito et al 2005). Interestingly, when sensory nerves are ablated, the contribution of hair follicle cells to the recovered epidermis after wounding is reduced (Brownell et al 2011), suggesting a role for sensory nerves in mobilizing hair follicle cells for wound repair.

Figure 3. Sensory neurons mobilize stem cells in the hair follicle and epidermis for wound repair.

A. Sensory neurons wrap around hair follicles to regulate the migration of hair follicle cells (HFC) from the upper bulge/isthmus toward the epidermis upon injury (indicated by the star). The absence of sensory nerves reduces the contribution of HFCs to epidermal repair. B. Sensory neurons innervate the epidermis in regions containing LGR6⁽⁺⁾ epidermal stem cells (EpSCs). Following injury, sensory innervation drives the rapid mobilization of LGR6⁽⁺⁾ EpSCs to begin re-epithelization. Upon denervation, LGR6⁽⁺⁾ EpSCs lose their rapid activation status and contribute equally with LGR6⁽⁻⁾ EpSCs to wound repair. C. The touch dome is a tiny structure composed of basal epidermal cells and specialized mechano-sensing Merkel cells. Merkel cells depend on sensory nerve derived Sonic hedgehog (SHH) for maintenance. Constitutive SHH signaling induces basal cell carcinoma (BCC) formation that is dependent on signals from sensory nerves for development.

Sensory nerves also innervate a subset of EpSCs marked by LGR6 (Leucine Rich Repeat Containing G Protein-Coupled Receptor 6). LGR6⁽⁺⁾ and LGR6⁽⁻⁾ EpSCs behave similarly during normal epidermal turnover. However, upon wounding, LGR6⁽⁺⁾ EpSCs proliferate faster than the LGR6⁽⁻⁾ EpSCs and are the early responders for re-epithelialization (Figure 3B). Surgical ablation of sensory innervation delays wound closure at the early phase due to a decrease in LGR6⁽⁺⁾ EpSC proliferation, underscoring the importance of sensory innervation in mobilizing EpSCs in the early stage of wound repair.

Peripheral neuropathy (loss of peripheral nerves) and chronic wounds are two common morbidities experienced by diabetic patients and cancer patients undergoing chemotherapy (Kanat et al 2017, McNees & Meneses 2007, Volmer-Thole & Lobmann 2016). The discoveries that peripheral nerves may play a role in wound healing suggest that these two morbidities may be more related than previously anticipated. Delineating the specific types of peripheral neurons and the signals they send to modulate the healing process may inform the development of new treatments for chronic wounds.

Sensory nerves modulate cancer development and progression—the touch dome as a cancer hotspot

The touch dome is a tiny, self-contained epidermal compartment critical for perceiving gentle touch. A touch dome is composed of basal epidermal cells, Merkel cells (specialized mechanoreceptor derived from touch dome epidermal cells), and innervations from Aβ sensory neurons (Figure 3C) (a type of low-threshold mechanoreceptor or LTMR) (Iggo & Muir 1969, Jenkins et al 2019, Li et al 2011, Morrison et al 2009, Nguyen et al 2018, Van Keymeulen et al 2009). Denervation leads to changes in the touch dome morphology and loss of Merkel cells over time, suggesting that neuronal signals are important for Merkel cell maintenance (English et al 1983, Xiao et al 2015). Sonic Hedgehog (SHH) was identified as a sensory nerve-derived signal that partially contributes to touch dome homeostasis and Merkel cell formation (Perdigoto et al 2016, Xiao et al 2016, Xiao et al 2015). Interestingly, regions of the epidermis containing touch domes are often “hot spots” for basal cell carcinoma (BCC) compared to the rest of the epidermis. BCC is the most prevalent cancer in the world and is caused by mutations that lead to a constitutive activation of the Hedgehog pathway (Figure 3C). Loss of sensory nerves attenuates touch dome-derived BCCs, suggesting a role for sensory nerves, and potentially nerve-derived SHH, in modulating BCC formation (Peterson et al 2015).

Many solid tumors are highly innervated (Magnon et al 2013, Rabben et al 2016, Seifert & Spitznas 2002). Interestingly, solid tumors with denser peripheral innervation are often associated with poorer prognosis (Hayakawa et al 2017, Kamiya et al 2019, Madeo et al 2018, Pan et al 2021, Venkatesh et al 2017). Although much remains to be learned about the role of peripheral nerves in cancer, several studies have shown improved prognosis in breast cancer mouse models and human melanoma patients following treatment with b-blockers (drugs that dampen sympathetic neuron signaling) (De Giorgi et al 2018, Glasner et al 2010). Future studies exploring the dependence of tumor formation and progression on peripheral nerves could uncover how neuromodulation influences cancer and identify potential new therapies.

Peripheral nerve regulation of HSCs and the bone marrow

HSCs produce and maintain all blood cell types (Figure 4) (Haas et al 2018, Olson et al 2020, Wang & Wagers 2011). HSCs reside primarily in the bone marrow, but they also rhythmically egress from the bone marrow and circulate in the blood stream under steady state. Bone marrow is a highly vascularized tissue filled with diverse mesenchymal cells that regulate HSC self-renewal and egress. It is also densely innervated by peripheral autonomic and sensory neurons. Studies have revealed a significant contribution of this dense innervation network to the regulation of HSCs and their progeny.

Figure 4. The hematopoietic stem cell (HSC) lineage.

HSCs give rise to multipotent progenitors that further produce myeloid or lymphoid progeny downstream. The myeloid progenitor gives rise to all monocytes and granulocytes as well as red blood cells and megakaryocytes that produce platelets. Macrophages are monocytes that have migrated from the bloodstream into tissues. The lymphoid progenitor produces T-cells, B-cells, and natural killer (NK) cells.

Sympathetic neurons and HSCs—a daily rhythm

Many stem cells display rhythmic behaviors that may contribute to their maintenance and functionality in tissue renewal. One of the most well described oscillatory behaviors is the daily egress of HSCs from the bone marrow. The sympathetic nervous system regulates the proliferation and differentiation of HSCs as well as the migration of HSCs and their progeny in and out of the bone marrow (Katayama et al 2006, Lucas et al 2008, Méndez-Ferrer et al 2008, Scheiermann et al 2012). Sympathetic neurons were first shown to be required for HSC egress from the bone marrow under G-CSF (Granulocyte colony-stimulating factor) stimulation—a cytokine used to mobilize the release of HSCs from the bone marrow for transplantation purposes (Katayama et al 2006). Emerging evidence now suggests basal autonomic activity is an essential regulator of the circadian release of HSCs into the bloodstream. The number of circulating HSCs and progenitors oscillates throughout the day, peaking during the early resting phase and reaching a low during active wakefulness (Lucas et al 2008). Cyclic egress of HSCs from the bone marrow is mediated by the rhythmic release of norepinephrine from the sympathetic nerve terminals innervating the bone marrow (Figure 5). This rhythmic pattern is abolished in chemically sympathectomized mice or mice lacking the β3-adrenergic receptor (Adrb3) (Méndez-Ferrer et al 2008), suggesting a key role for sympathetic nerves in orchestrating rhythmic stem cell behavior according to the circadian clock. Instead of regulating HSCs directly, sympathetic nerves suppress the expression of Chemokine (C-X-C Motif) Ligand 12 (CXCL12) by the mesenchymal stromal cells, which normally promotes HSC egress from the bone marrow (Méndez-Ferrer et al 2010, Méndez-Ferrer et al 2008). A recent study further suggests the parasympathetic nervous system modulates the activity of the sympathetic nervous system to reduce the egress of HSCs and leukocytes from the bone marrow during wakefulness (Figure 5) (García-García et al 2019), which exemplifies the counterbalancing nature of the two branches of autonomic nervous system in regulating stem cell behaviors. Harnessing this rhythmic nature may lead to significantly higher yields of HSCs clinically if the blood is harvested during the peak of HSC release. It may also inform optimal treatment regimens for leukemia or inflammatory diseases.

Figure 5. Autonomic nerves regulate hematopoietic stem cell (HSC) trafficking in the bone marrow.

A. Sympathetic activity regulates the circadian egress of HSCs and progenitors. Norepinephrine (NE) released from sympathetic terminals binds to the adrenergic receptor (ADRB3) on mesenchymal stromal cells (MSCs) to downregulate the expression of CXCL12 and permit HSC release into circulation during rest. B. Parasympathetic signaling from the CNS regulates circadian retention of HSCs and progenitors in the bone marrow. Acetylcholine (ACh) signals released from central parasympathetic neurons represses sympathetic activity to promote homing and retention of HSCs during wakefulness.

Sympathetic neurons in stress, chemotherapy, and aging

Interestingly, the same sympathetic nerve-stromal axis that regulates the rhythmic HSC behavior is further activated under stress. When mice are subjected to chronic stress, elevated sympathetic nerve activity causes enhanced norepinephrine release, which in turn reduces CXCL12 levels in the stroma. These changes lead to the activation of the most quiescent HSC populations and the release of both HSCs and leukocytes (in particular neutrophils and inflammatory monocytes) into the bloodstream (Figure 6A) (Heidt et al 2014). In fact, the same responses were detected in medical residents during their stressful shift hours at a hospital intensive care unit (Heidt et al 2014). Increased leukocytes in the bloodstream can be advantageous for fighting infections. However, the increased output of neutrophils and inflammatory monocytes also promotes plaque formation that can cause atherosclerosis, myocardial infarction, and stroke (Heidt et al 2014). Additionally, increased monocyte production and release in response to chronic stress is associated with prolonged exacerbations in neuropsychiatric disorders, including anxiety (McKim et al 2016, Powell et al 2013). Going forward, it will be important to investigate how long-term elevations in autonomic activity affect HSC populations and progeny and the downstream implications of these effects on disease development and progression.

Figure 6. Alterations in peripheral nerve activity can dysregulate HSCs to contribute to disease, but they may also have the potential to be harnessed for therapies.

A. Chronic stress elevates sympathetic activity causing aberrant mobilization of HSCs. Excessive norepinephrine (NE) increases the release of HSCs and leukocytes from the bone marrow leading to higher circulating levels in the blood. B. Sympathectomy by chemotherapeutic agents like cisplatin impair HSC engraftment following transplantation. Pretreatment with neuroprotective agents 4-methylcatechol or glial derived neurotrophic factor (GDNF) prevent cisplatin-induced loss and improve HSC engraftment. C. Reduction or loss of sympathetic innervation in the bone marrow during aging alters HSCs towards a myeloid-biased fate. Activation of ADRB3 in the niche with an agonist partially rescues the age-related HSC phenotype. D. Nociceptive sensory neurons directly regulate HSCs to promote their egress. Capsaicin can activate nociceptive neurons to increase release of the neuropeptide calcitonin gene-related peptide (CGRP). CGRP binds to receptor activity modifying protein 1 (RAMP1) expressed by HSCs to promote their release from the bone marrow.

Sympathetic neuropathy following therapeutic drug treatments can also have pathological consequences. For example, many chemotherapeutic agents like cisplatin damage sympathetic innervation (Adams et al 2015, Lucas et al 2013). Loss of sympathetic innervation causes significant alterations to the bone marrow microenvironment that can impair HSC engraftment following transplantation (Figure 6B). Prevention of sympathetic nerve damage following cisplatin treatment improves HSC recovery (Lucas et al 2013). Cisplatin and other platinum-based chemotherapeutic compounds are known neurotoxins that can lead to severe peripheral neuropathy beyond their impact on sympathetic neurons (Argyriou et al 2012). Future studies could explore how loss of peripheral innervation may contribute to other long-term side effects seen with chemotherapy and if preventing neuronal death mitigates them.

With age, HSCs display a decline in regeneration potential and a shift to a myeloid-biased fate. Coinciding with these HSC changes, the bone marrow also displays a significant loss in sympathetic innervation. This age-related HSC dysfunction can be partially rescued with an ADRB3 agonist, suggesting that decreases in norepinephrine signaling through loss of sympathetic innervation in the bone marrow influence the aging process of HSCs (Figure 6C) (Maryanovich et al 2018). Collectively, these findings suggest that sympathetic nerve degeneration contributes to several HSC pathologies. It will be interesting to explore if changes in peripheral nerve innervation is a common theme among different organ systems during aging, and if these nerve alterations contribute to age-related stem cell declines.

Sensory nerves and HSCs— spicing things up

The bone marrow is also innervated with sensory nerves, in particular nociceptive neurons. Sensory nerves innervating the bone marrow work in concert with the sympathetic nerves to regulate HSCs. Dual ablation of sympathetic neurons and nociceptive neurons leads to a significant HSC expansion in the bone marrow and biases hematopoiesis towards a myeloid fate, a phenotype not seen with the ablation of sympathetic neurons or nociceptive neurons alone. By contrast, chemical sympathectomy alone specifically leads to lower lymphopoiesis without affecting myeloid lineages (Gao et al 2021). These results suggest that different peripheral neurons may preferentially regulate different subsets of HSC populations.

Additionally, similar to sympathetic neurons, nociceptive neurons also regulate G-CSF induced HSC egress from the bone marrow. However, in contrast to the sympathetic nerves, which regulate the egress process indirectly via their influence on the stroma, sensory neurons directly regulate HSCs via secretion of the neuropeptide calcitonin gene-related peptide (CGRP) (Figure 6D). Interestingly, feeding mice with food containing capsaicin (an active component of chili peppers), which activates nociceptive neurons via binding to receptor activity modifying protein 1 (RAMP1), can enhance G-CSF-induced HSC mobilization (Gao et al 2021). Together, these findings suggest multiple nerve-based strategies that could be exploited to promote HSC egress for clinical uses.

Autonomic imbalance: how stress hurts the bone

Besides regulating HSCs residing in the bone marrow, the nervous system also has an interesting role in regulating the bone tissue itself. Sympathetic activity supports bone resorption by promoting the differentiation of progenitors to osteoclasts while parasympathetic activity inhibits osteoclasts through activation of apoptotic pathways for bone accrual (Bajayo et al 2012, Elefteriou et al 2005). Patients with neuropsychiatric disorders, neurodegenerative diseases, and epilepsy, all of which dysregulate autonomic activity, commonly have low bone mass and osteoporosis (Brady et al 2019, Melton et al 1994, Moen et al 2011, Yirmiya et al 2006, Zhou et al 2011, Zhou et al 2014). Chronic depression is also associated with a decrease in bone mass that may be caused by stress-induced elevation in sympathetic activity (Yirmiya et al 2006). By contrast, traumatic brain injury that decreases sympathetic activity is associated with an increase in bone formation (Tam et al 2008). Future work to uncover the mechanisms by which sympathetic neurons regulate these different changes in the bone will clarify how stress impacts bones.

Neuro-immune interactions

Similar to the nervous system, the immune system is highly specialized for sensing and responding to the environment. HSCs sit at the top of the hierarchy to produce and maintain an array of cell types that constitute the immune system (Figure 4). Emerging evidence has revealed various interactions between the immune system and the nervous system during development, upon injury, and in disease (Chen et al 2021, Chu et al 2020, Godinho-Silva et al 2019, Ordovas-Montanes et al 2015, Udit et al 2022, Veiga-Fernandes & Mucida 2016). Here, we focus on how the PNS regulates immune cell trafficking from the bone marrow and the lymph nodes to the peripheral tissues and how these neuro-immune interactions impact skin immunity.

Neural regulation of immune cell trafficking

Under steady state, low levels of leukocytes (white blood cells)—including granulocytes (neutrophils, eosinophils, and basophiles), monocytes, and lymphocytes (T and B cells)—constantly circulate between the bone marrow, lymph nodes, and peripheral organs. This circulation is part of the immunosurveillance that maximizes encounters between pathogens and immune cells (Massberg et al 2007). The number of leukocytes in peripheral tissues varies throughout the day, coinciding with rhythmic oscillations in sympathetic nerve activity (Brown & Dougherty 1956, Halberg et al 1953, Scheiermann et al 2012). Inflammation—the body’s response to infection or injury—often involves substantial leukocyte infiltration to the affected sites. Leukocyte recruitment still exhibits strong oscillations even under inflammation, and different numbers of leukocytes can directly impact the outcome of inflammatory diseases (Scheiermann et al 2012). Ablation of sympathetic innervation in peripheral tissues abolishes the circadian fluctuation of immune cells in the periphery, in particular macrophages and neutrophils, suggesting that sympathetic activity regulates leukocyte retention (Figure 7A) (Scheiermann et al 2012). ADRB2 signaling is also important for regulating lymphocyte egress from lymph nodes to peripheral tissues (Figure 7B) (Nakai et al 2014). The rhythmic pattern of leukocyte recruitment allows more leukocytes to be available at peripheral tissues during daily active periods, when injuries and infections are more likely to occur.

Figure 7. Sympathetic nerves regulate immune cell trafficking.

A. Sympathetic nerves regulate the circadian recruitment of leukocytes to peripheral tissues. (Top) Leukocytes circulate in the blood when sympathetic activity is low during rest. (Bottom) During wakefulness, sympathetic nerves release norepinephrine (NE) that binds and signals through ADRB2 and ADRB3 (β2- and β3-adrenergic receptors) in vascular endothelial cells to increase the expression of extracellular proteins that tether circulating leukocytes for retention and infiltration into peripheral tissues. B. Sympathetic nerves also directly regulate the trafficking of lymphocytes. Activation of ADRB2 receptors on T-cells and B-cells prevents their egress from lymph nodes.

Interestingly, the neuroanatomy of acupuncture—an ancient practice involving stimulating specific regions of somatic tissues to modulate distant organs or whole-body physiology—has gradually begun to reveal the significance of the neuro-immune axis in tissue repair (Ma 2020). Although many open questions remain about the mechanisms of acupuncture, it has been shown to modulate inflammatory responses in part via the sympathetic nervous system and parasympathetic nervous system (Liu et al 2021, Liu et al 2020). The neuroanatomical basis for acupuncture provides a compelling demonstration of how the interconnectivity of neurocircuitry can be leveraged to achieve desired modulations at a systemic level or at a distal organ far from the stimulation site.

Besides the autonomic nervous system, nociceptive sensory neurons also innervate the lymph node and may modulate the trafficking of leukocytes both directly and indirectly via endothelial cells or stroma in the lymph node (Huang et al 2021b). These findings suggest potentially diverse roles for peripheral nerves in regulating the migratory behavior of immune cells and their retention in different tissues.

Neuro-immune interactions in the skin

As the first line of defense against environmental insults and hazards, the skin functions as a crucial barrier against infection. Immune cells in the skin must remain vigilant, and they are poised to sense and respond to intruding pathogens, including bacteria, fungi, viruses, and parasites. Emerging evidence has suggested that nociceptive sensory neurons both transmit painful sensations and orchestrate immune responses by directly sensing pathogens. For example, in Candida albicans skin infections—a type of fungal infection—fungal derived β-glucan binds to the dectin-1 receptor expressed by nociceptive neurons (Figure 8A). The activation of nociceptive neurons leads to pain sensation and triggers the release of the neuropeptide CGRP, which through a relaying cassette via dermal dendritic cells, eventually drives IL-17 production in γδ T cells (Cohen et al 2019, Maruyama et al 2018). IL-17 is critical for eliciting protective immunity against C. albicans. Therefore, patients who are defective in IL-17 responses (such as AIDS patients) are particularly susceptible to C. albicans infection (Kashem et al 2015). Optogenetic activation of cutaneous nociceptive sensory neurons in the absence of tissue damage elicits IL-17 production in γδ T cells and primes the skin to clear C. albicans infections faster (Cohen et al 2019), demonstrating the importance of nociceptive neurons in mounting immune responses against pathogens.

Figure 8. Sensory nerves modulate immune function in the skin.

A. Skin infection with C. albicans induces inflammation through sensory nerves. Pathogen-secreted β-glucan binds to Dectin-1 receptors on sensory nerves. CGRP released by activated sensory nerves at the site of infection elevates expression of IL-23 by dendritic cells. IL-23 induces IL-17 production in γδ T cells, thereby activating immune responses for pathogen clearance. Activated sensory neurons also signal to immune cells at adjacent sites, priming them in case the pathogen spreads. B. Subpopulations of sensory nerves regulate tissue protective responses in immune cells. Formyl peptides and α-hemolysin from S. aureus activate NAV1.8⁺ and TRPV1⁺ sensory neurons. Activated sensory neurons release CGRP to downregulate expression of inflammatory signals like TNF-α in macrophages, preventing monocyte influx. UV induced skin damage activates a population of C-low threshold mechanoreceptor (C-LTMR) sensory neurons that release TAFA4 to downregulate expression of inflammatory genes and increase expression of genes involved in tissue protection in macrophages.

Another example in which neurons can directly sense pathogens is in Staphylococcus aureus infection—one of the most common and deadly bacterial infections associated with wounds and surgery. S. aureus-derived N-formylated peptides and the pore-forming toxin α-hemolysin induce action potentials and calcium influx in NAV1.8⁺ and TRPV1⁺ nociceptive neurons to cause painful sensations at the site of infection. Activation of NAV1.8⁺ and TRPV1⁺ nociceptive neurons by these bacterial components leads to the release of CGRP in the skin and inhibits tumor necrosis factor (TNF)-α production in macrophages (Figure 8B). Reduction in TNF-α suppresses monocyte influx into the skin and reduces draining lymph node hypertrophy (Chiu et al 2013).

Besides nociceptive neurons, touch-sensing neurons also play a role in modulating skin immunity. After UV exposure, C-LTMR sensory nerves secrete a neuropeptide, TAFA4, which downregulates inflammation and prevents fibrosis after UV-induced tissue damage by modulating dermal macrophages (Figure 8B) (Hoeffel et al 2021). Although inhibition of immune cells during infection seems counter-intuitive, it is essential to prevent detrimental consequences associated with an overactive immune system. Together, these studies suggest that sensory neurons modulate the immune response for pathogen clearance while preventing an overactivation that could damage the skin.

Some bacteria have evolved to hijack this neuro-immune circuitry. In the case of Streptococcus pyogenes infection—another common pathogen causing skin infections—the bacteria secrete a pore-forming toxin to activate TRPV1⁺ nociceptive sensory neurons to induce pain. CGRP secreted from these neurons then suppresses neutrophil recruitment and prevents neutrophils from killing S. pyogenes. Blocking the neuro-immune communication with Botulinum neurotoxin A, which prevents CGRP release, or injection of a CGRP antagonist significantly improves the outcome of S. pyogenes infection (Pinho-Ribeiro et al 2018).

Neuro-immune interactions also contribute to immune-related diseases. Psoriasis is an immune-related disorder that accelerates epidermal proliferation and causes raised, scaly skin patches. It is one of the most prevalent immune disorders, affecting more than 3% of the adult population in the US alone (Armstrong et al 2021). Patients with psoriasis often report sensations of itch and discomfort within skin lesions. Clinical studies suggest that damage to cutaneous nerves can ameliorate inflammation and skin changes in psoriasis patients, indicating a role for peripheral nerves in modulating the severity of this disease (Farber et al 1990). In mice, topical imiquimod (IMQ) application induces γδT17-cell-mediated inflammation that resembles psoriasis. Chemical ablation of sensory neurons prevents IMQ-induced inflammation, whereas ablation of sympathetic neurons only reduces swelling but does not prevent immune cell activation and infiltration, suggesting that different peripheral nerves regulate different aspects of the immune response. Sensory nerves mediate inflammatory responses and orchestrate skin immune cell behaviors by acutely regulating IL-23 production from dermal dendritic cells (Riol-Blanco et al 2014).

Collectively, these examples demonstrate the vital role of peripheral nerves in regulating immune cells during host defense, inflammation, and injury. Given the unique ability of the nervous system to transmit rapid responses, it is particularly suited to act as the first responder to initiate the slower immune cell cascade that will then have a longer lasting impact. This relaying mechanism is reminiscent of how the nervous system modulates hair growth in response to cold by relaying its effect to a longer lasting signal. The wide-spread nature of peripheral innervation may also serve as a convenient route to prime surrounding tissues or influence distant areas, like the bone marrow, that are far away from the infection site. This interconnection could then coordinate tissue-specific immune cells with HSC behaviors to expedite pathogen clearance and potentially help create a memory of the infection.

Conclusions

Neurobiology, stem cell biology, and immunology have traditionally been studied as three largely separate fields. In this review, we discuss studies that break these boundaries and merge these fields. Several common themes have emerged from these synergistic studies. First, tissue regeneration and repair do not operate in isolation in individual organs. Instead, local tissue changes and immune responses are tightly integrated with bodily and environmental changes by the PNS. Second, neurons communicate with stem cells and immune cells either directly or indirectly via neurotransmitters (such as norepinephrine) and neuropeptides (such as CGRP). Although these molecules act over short distances and have short half-lives, their effect is often sustained and amplified by inducing the expression of secreted proteins (such as cytokines) in target cells. In this sense, neurons are often the activators that initiate downstream responses. Third, changes in neuronal activity and innervation density can be the root cause for a wide variety of conditions and diseases that may seem unrelated to the nervous system, including stress-related tissue deterioration and immune changes, defects in wound repair, aging, and cancer.

These findings suggest a number of avenues for future explorations. Some of these studies may focus on further delineating initial observations. For example, although neuronal inputs are critical modulators of many tissue processes, the specific class of neurons involved as well as the cellular and molecular mechanisms by which neurons alter tissue biology and immune changes remain to be elucidated in most cases. Other studies may continue to integrate neurobiology into our understanding of tissue homeostasis. For example, neurons can have tonic firing (steady action potential firing at a low background level that is constant or sustained over a long duration) and phasic firing (one or a few short bursts of action potential that is substantially above background activity). Can the amplitude and duration of neuronal activity lead to different outcomes in tissue changes, and, if so, what are the mechanisms by which these types of activity are interpreted? Finally, these studies suggest new opportunities for therapeutic interventions, and future studies could aim to further develop these ideas. For example, one could explore how internal organs may be modulated by stimulating innervations at a more superficial level, such as the skin, and thereby take advantage of the elaborate wiring of the nervous system. While current medical interventions often take an organ specific approach to treat symptoms manifested at a specific location, a better understanding of neuro-stem cell and neuro-immune interactions offers tremendous opportunities to achieve a more holistic approach for both treating and preventing diseases in the future.