Suckling, Feeding, and Swallowing: Behaviors, Circuits, and Targets for Neurodevelopmental Pathology

Abstract

All mammals must suckle and swallow at birth, and subsequently chew and swallow solid foods, for optimal growth and health. These initially innate behaviors depend critically upon coordinated development of the mouth, tongue, pharynx, and larynx as well as the cranial nerves that control these structures. Disrupted suckling, feeding, and swallowing from birth onward—perinatal dysphagia—is often associated with several neurodevelopmental disorders that subsequently alter complex behaviors. Apparently, a broad range of neurodevelopmental pathologic mechanisms also target oropharyngeal and cranial nerve differentiation. These aberrant mechanisms, including altered patterning, progenitor specification, and neurite growth, prefigure dysphagia and may then compromise circuits for additional behavioral capacities. Thus, perinatal dysphagia may be an early indicator of disrupted genetic and developmental programs that compromise neural circuits and yield a broad range of behavioral deficits in neurodevelopmental disorders.

Introduction

Newborn mammals must eat to survive. Thus, precise, genetically defined developmental programs must be executed prenatally to ensure effective nursing at birth and subsequently chewing and swallowing solid foods. Infants often encounter potentially life-threatening difficulties suckling, feeding, and swallowing (SFS)—referred to collectively as pediatric dysphagia. Nevertheless, relatively little is known about the integrated development of the biomechanical apparatus and neural circuits that facilitate effective SFS. We consider how fundamental mechanisms for oropharyngeal, hindbrain, and cranial nerve (CN) development establish SFS and how disrupting these processes leads to pediatric dysphagia. Effective SFS at birth requires the coordination of embryonic patterning; progenitor specification; and cellular differentiation of cranial bones, muscles, and nerves (Alexander et al. 2009, Chai & Maxson 2006, Cobourne et al. 2019, Cordes 2001, Ruder & Arber 2019, Yamane 2005). We assess how these mechanisms are disrupted in dysphagia (Kleinert 2017, LaMantia et al. 2016, Robertson et al. 2017). Finally, we summarize studies of a mouse genetic model for pediatric dysphagia and speculate on how SFS development informs our understanding of behavioral deficits and neural circuit dysfunction in neurodevelopmental disorders.

Defining an innate behavior: starting to suckle

The vocabulary that describes suckling parallels that for adult feeding and swallowing (Jean 2001, Matsuo & Palmer 2008, Sasegbon & Hamdy 2017). Three phases define the adult behavior: oral, pharyngeal, and esophageal (Figure 1, left; Table 1). In the oral phase, food is brought into the mouth and chewed and mixed with saliva—essential for solid food—by jaw and tongue movements, yielding a bolus for swallowing. The pharyngeal phase begins as the bolus is pushed back on the tongue. Pharyngeal muscles pull the larynx forward, elevate the hyoid bone, and close the epiglottis, protecting the airway as the bolus traverses the pharynx. In concert, soft palate muscles elevate to close the nasopharynx, preventing aspiration into nasal sinuses. Finally, in the esophageal phase, the esophageal sphincter relaxes to allow the bolus to enter the esophagus and then closes as a peristaltic wave pushes food toward the stomach.

Figure 1.

Mechanics, musculature, and cranial nerve innervation are distinct for (a) feeding and swallowing in adults versus (b) suckling, feeding, and swallowing (SFS) in infants and toddlers. (a, top row) The functional anatomy and mechanics of adult feeding and swallowing include oral, pharyngeal, and esophageal phases. (Middle and bottom rows) Each phase relies on key sets of muscles (middle) innervated by a subset of cranial nerves (bottom). The essential muscles for each phase, and cranial nerves that innervate them, are color coded as indicated in the bottom right panel. (b) In infants, SFS is performed as a continuous behavior, without a need to pause for breathing as in the adult. (Top) The high position of the larynx in the infant (compare location indicated in the top row of panel a with that in the top row of panel b) allows for the epiglottis to latch against the back of the soft palate, creating separate channels for simultaneous nasal breathing and swallowing liquid. (Top right) Position of the epiglottis during suckling/breathing; dotted arrows indicate flow of milk. (Middle and bottom rows) Different positions and configurations of key muscles and cranial nerves for suckling/swallowing are shown (compare to adult feeding/swallowing in panel a).

Table 1.

The contribution of each of five cranial nerves to the motor and sensory control of suckling, feeding, and swallowing

			Muscle	Motor function			Sensory function
Oral	Chew food (jaw)	V	Temporalis	Jaw closure	Sense food in mouth	V	Touch, pain, and temperature for face, oral cavity, teeth, anterior tongue
		V	Medial pterygoid	Jaw closure
		V	Lateral pterygoid	Jaw opening, protrusion			Proprioreception for jaw, teeth
		V	Masseter	Jaw closure, protrusion
	Close Mouth	VII	Orbicularis oris	Purses lips		VII	Taste for anterior tongue
		VII	Buccinator	Flattens cheeks
	Move food with tongue	XII	Intrinsic (longitudinal, vertical, transverse)	Alter the shape of the tongue		IX	Taste, sensation for posterior tongue
Pharyngeal	Push food into pharynx (tongue)	XII	Extrinsic (genioglossus, hyoglossus, styloglossus)	Alter the position of the tongue in the mouth	Sensation in pharynx	IX	Touch, pain and temperature for upper pharynx
		X	Palatoglossus	Elevates post. tongue during adult swallowing
	Close nasopharynx (palate)	V	Tensor veli palatine	Stiffens soft palate
		X	Levator veli palatine	Elevates soft palate
		X	Palatopharyngeus
		X	Musculus uvulae	Closes nasopharynx
	Open pharynx	IX	Stylopharyngeus	Elevates larynx and pharynx during swallowing
		X	Salpingopharyngeus	Shortens and widens pharynx
		X	Palatopharyngeus	Shortens and widens pharynx
	Elevate hyoid	V	Ant. belly of digastric	Elevates hyoid bone
		V	Mylohyoid	Elevates hyoid, tongue
		Cer1	Geniohyoid	Elevates hyoid bone forward, helps depress mandible
		VII	Post. belly of digastric	Elevates hyoid, depresses mandible
		VII	Stylohyoid	Elevates hyoid and tongue
Esophageal	Peristalsis; open esophagus	X	Pharyngeal constrictors (superior, middle, inferior)	Sphincters, push food into esophagus		X	Touch, pain and temperature for lower pharynx, larynx, and esophagus (including gag reflex)

Each key cranial nerve is color coded, parallel to the color code used in the figures. For each phase of suckling, feeding, and swallowing behavior, component tasks, in appropriate sequence, are listed; the requisite cranial nerve(s) for motor and sensory control are identified, and the innervated muscles (motor), cranial/oropharyngeal cutaneous domain, or muscle domain (sensory) is identified.

In infants, a suck-swallow-breathe cycle is in place at birth and then advances toward a chew-swallow-breathe cycle (Matsuo & Palmer 2015). To suck liquids, infants use the same oropharyngeal structures used by adults for chewing and swallowing (Lau 2015, Medoff-Cooper et al. 2010); however, their underdeveloped state facilitates sucking liquids (Figure 1b). There are two key distinctions: First, in the oral phase, lips, cheeks, and tongue produce a vacuum for sucking movements. Second, the larynx and hyoid lay relatively higher in the neck, closer to the tongue and soft palate, and the epiglottis is horseshoe shaped rather than flat as in adults. Thus, during the pharyngeal phase, the immature epiglottis presses anteriorly against the soft palate and tongue, diverting milk around the trachea into the esophagus (Figure 1b, right). During the esophageal phase, the tongue pulses to draw milk through the mouth, around the epiglottis, to swallow a continual stream. The trachea remains open, allowing breathing through the nose during uninterrupted feeding. Over the first year, the neck grows, the hyoid and larynx lower, the epiglottis flattens, and the soft palate matures, facilitating the transition to eating solid food between four and six months (Laitman et al. 1977, Westhorpe 1987). Additional feeding difficulties often arise during this transition, suggesting that in some infants, ongoing anatomical and circuit differentiation fail to accommodate the dynamics of SFS.

Babies, behavior, and biomechanics: muscular control of suckling and swallowing

A sequence of activation by muscles that attach to and/or move the skull, jaws, hyoid, and laryngeal cartilages (Matsuo & Palmer 2015) (Figure 1; Table 1) is necessary to suckle. This sequence begins with contracting tongue muscles to latch under the nipple, lip muscles to form a tight seal, cheek muscles to constrict the oral cavity, and tongue and palate muscles to produce a sustained vacuum (Tamura et al. 1998). Suprahyoid and lateral pterygoid muscles open the jaws, and masseter, temporalis, and medial pterygoid muscles close them, facilitating tongue movements. Swallowing requires oropharyngeal relaxation and constriction: The soft palate elevates and the uvula expands to prevent milk entering the nasopharynx, while suprahyoid muscles elevate the hyoid, enlarging the oropharyngeal lumen and upper esophagus (Figure 1b; Table 1). This sequence is engaged for ingesting liquids; however, it can be elicited spontaneously or in response to objects like pacifiers—nonnutritive sucking—which also results in homeostatic responses for digestion and appetite regulation in the absence of food intake (Pinelli & Symington 2000). During infancy, biting fingers, toes, or toys also activates these movements (Tamura et al. 1998), reinforcing SFS as an innate and reflex-driven behavior that then transitions to a learned, discriminatory behavior (Hadders-Algra 2018, Koffman et al. 1998) (Figure 1b). To begin chewing solid food, the infant engages rhythmic constriction/relaxation of jaw muscles with less involvement of facial muscles. The tongue pushes the food bolus into the pharynx, and jaw opener/closer muscles used for reflexive biting generate force for chewing as the suckling reflex is lost (Tamura et al. 1998). When infants swallow, jaws open and lips purse tightly; when adults swallow, maxillary and mandibular teeth are held together, the tongue is placed on the hard palate, and lips relax. The infant-to-adult transition relies upon gradual changes in oropharyngeal muscle recruitment over several years that are not associated with weaning (Festila et al. 2014).

A distributed cranial nerve network controls suckling, feeding, and swallowing

SFS musculoskeletal activation and modulation require an equally dynamic neural circuit. The essential SFS neural circuit is defined by 5 of the 12 CNs: CNs V, VII, IX, X, and XII (Figure 1; Table 1). CNs V and VII mediate the oral phase; CNs IX, X, and XII regulate the pharyngeal and esophageal phases. Peripheral trigeminal ganglion sensory neurons (CNgVs) detect and relay somatosensation from the lower face and anterior oropharynx to brainstem trigeminal nuclei. The mesencephalic trigeminal nucleus relays mechanosensory information from jaw-closer muscles and teeth. CN V motor neurons innervate jaw closer and some oropharyngeal muscles. The facial nerve, CN VII, has several roles: Motor neurons innervate lip, cheek, hyoid elevator, and jaw-opening muscles; preganglionic parasympathetic fibers innervate salivary glands; and geniculate ganglion sensory neurons relay taste from the tongue to the solitary nucleus. The remaining three CNs are responsible for motor control of the pharyngeal and esophageal phases (Figure 1; Table 1). CNs IX (glossopharyngeal) and X (vagus) include nucleus ambiguus motor neuron axons that innervate laryngeal muscles and the palatoglossus, an extrinsic tongue muscle. CN XII (hypoglossal) innervates the remaining tongue muscles. Together, peripheral sensory and brainstem motor neurons that contribute to these five CNs, relevant brainstem motor or sensory relay nuclei, and their interconnections constitute the primary SFS neural circuit. Subsequent SFS refinement occurs as chemosensory (Al Ain et al. 2013, Coureaud et al. 2006, Logan et al. 2012, Loos et al. 2019), hypothalamic (Zimmer et al. 2019), and cortical inputs to CN networks mature (Muscatelli & Bouret 2018).

Ready at birth: evidence for a distinct developmental program for suckling, feeding, and swallowing

Antecedents of SFS emerge before birth. Sucking and swallowing movements are detected by ultrasound by the end of the first trimester (Delaney & Arvedson 2008, Festila et al. 2014, Hadders-Algra 2018, McCain 2003, Miller et al. 2003, Reissland et al. 2012). Muscle differentiation begins with the tongue—around 30 days. Hypoglossal (CN XII) nerve rootlets emerge from the brainstem around 28 days and reach nascent tongue muscles by 37 days (O'Rahilly & Muller 1984). Axons from the nucleus ambiguus (CNs IX and X) arrive as early as 30 days (Brown 1990). All CNs exit the brainstem by gestational week 6, as palatine and pharyngeal muscles—subsequently innervated by CNs V, IX, and X—appear (Domenech-Ratto 1977, Muller & O'Rahilly 2011). In most mammals, including those born at relatively early (altricial) stages, SFS muscles and nerves are established at similar stages (Chandrasekhar 2004, Schmidt et al. 2013). Nevertheless, additional development is required. In preterm infants, suckling is not fully effective until 32–34 gestational weeks (Delaney & Arvedson 2008), a clinical challenge for their care. The early emergence of the integrated biomechanic/neural SFS system and behavior suggests a new significance for cranial and hindbrain developmental mechanisms (Figure 2): They define a dedicated developmental program based upon a distinct genetic architecture that ensures SFS functional integrity from birth onward.

Figure 2.

The integrated developmental program for suckling, feeding, and swallowing (SFS) reflects the regulation of pharyngeal arch and hindbrain patterning, progenitor specification, and initial cellular differentiation, controlled by an extensive gene network. (a) Anterior-posterior (A-P) organization of key muscles and essential cranial motor nerves used in SFS arises based upon hindbrain/pharyngeal arch patterning. The left column illustrates muscles derived from each pharyngeal arch or from the posterior (somitic) mesoderm. The muscles and cranial nerves providing motor innervation are color coded as in Figure 1. (b) The A-P axes of the pharyngeal arches and hindbrain reflect metameric divisions (rhombomeres, r1–r7). Secreted signals [e.g., Fgf, retinoic acid (RA)] define this organization as well as a nested mosaic of gene expression, including homeobox (Hox) transcription factors. The dorsal-ventral (D-V) axis, which establishes sensory/motor domains, is defined primarily by the secreted signals Bmp (dorsal) and Shh (ventral). (c) A-P organization of the sensory cranial nerves for SFS. The correspondence between the sensory component of each cranial nerve and its craniofacial field is indicated by the same color code used in Figure 1. (d) A mixture of neural crest (Wnt1:Cre-recombined, green)- and placode (Six1 protein, red)-derived progenitors generate sensory neurons in the cranial ganglia (left). These progenitors, as well as those for cranial motor neurons, generate the rudimentary embryonic cranial nerves, labeled by beta-3 Tubulin (Tubb3, blue, right). (e) A-P- and D-V-hindbrain patterning is reflected in localized expression of specific markers, including the dorsal marker Pax3 (red, left) and the r4-specific protein Hoxb1 (red, right). (f) Mutations in multiple genes, including many homeobox transcription factors, disrupt the formation of specific cranial nerves (for a more complete description of the relationship between genes and each cranial nerve, see Supplemental Table 1).

Building the biomechanics for suckling, feeding, and swallowing

The integrated SFS developmental program initially engages all three germ layers—ectoderm, mesoderm, and endoderm—to generate bone, cartilage, skeletal and smooth muscle, glandular tissue, epidermis, ciliated epithelia, neurons, and glia. The oral cavity, trachea, and esophagus derive from the endodermal foregut tube, with some mesodermal/ectodermal contributions (Billmyre et al. 2015, Jacobs et al. 2012, Lewis & Tam 2006, Nowotschin et al. 2019, Ziermann et al. 2018). The SFS musculoskeletal scaffold develops via coordination of pharyngeal and anterior neural tube/hindbrain/neural crest patterning (Rinon et al. 2007) (Figure 2). The vertebrate cranial skeleton mostly derives from the hindbrain neural crest (Kuratani 2018). Oropharyngeal and esophageal skeletal muscles derive from the mesoderm of the pharyngeal arches, with six evaginations (five in humans and mice) on either side of the endoderm-derived pharynx (Kaplan et al. 2015, Ziermann et al. 2018) (Figure 2). Each arch includes mesodermal mesenchyme covered by outer ectodermal and inner endodermal epithelia. Hindbrain-derived neural crest cells in the arches contribute to cranial cartilage and tendons. Cranial paraxial mesoderm generates the myogenic precursors of key SFS muscles (Figure 2). The anterior-posterior (A-P) coordination of foregut, musculoskeletal, and hindbrain differentiation, based upon A-P position of the pharyngeal arches, is essential for the biomechanical and neural circuit differentiation that ensure effective SFS at birth.

Building neural circuits for suckling, feeding, and swallowing

SFS stereotypy and the imperative for integrated behavior at birth raise a central neurobiological question: What is the specific developmental program that ensures functionally competent neural circuits for neonatal SFS? We suggest a developmental program divided into two phases. The first phase starts before the neural tube closes and depends critically on genetic constraints that impose an A-P pattern on the entire head, including the hindbrain, in all vertebrates (Diogo et al. 2015, Parker et al. 2016, Schmidt et al. 2013) (Figure 2). Hindbrain metameric organization (rhombomeres) emerges at this time, embryonic day (E)8.0 in the mouse, and constrains the development of cranial bones and muscles as well as the sensory and motor neurons that constitute the SFS primary circuit. The cranial sensory ganglia—CNgV, VII, IX, and X—coalesce as a mosaic of rhombomere-specified neural crest and cranial placode cells (Breau & Schneider-Maunoury 2015, Fode et al. 1998, Karpinski et al. 2016, Steventon et al. 2014) (Figure 2). They begin to extend axons into the undifferentiated pharyngeal periphery and hindbrain between E9.0 and E9.5. Their dual origin determines cranial sensory neuron functional identity—placode cells differentiate, mostly, as mechanoreceptors and neural crest cells generate primarily nociceptors (Klein et al. 1994, Smeyne et al. 1994).

In parallel, hindbrain motor neurons acquire excitable properties during this period. CN V and VII motor neurons are physiologically active by E9.5, based upon optically recorded Ca⁺⁺ transients (Gust et al. 2003). By E10.5, CN V, VII, IX, and X motor neurons—key populations for SFS—fire autonomously (Abadie et al. 2000), and by E12.5, there is rhythmic bilateral firing across the entire hindbrain ensemble. Apparently, the early signaling and transcriptional regulation that underlie hindbrain axial organization and rhombomere specification are critical for initial SFS-related motor circuit development. The identities of rhombomeres r2, r3, r4, r6, and r7—the sources of CN V, VII, IX, and XII sensory and motor neurons—must be established for optimal SFS circuit development (Figure 2). CNs from r4 and r5 contribute to other aspects of SFS (e.g., taste, salivation; see Table 1) but are not as crucial for the SFS behavioral sequence. Thus, patterning hindbrain rhombomeric axes emerges as a crucial contributor to establishing the fundamental behavior.

This second phase, when neurons differentiate and synapses are made, ultimately facilitates experience- and activity-dependent circuit maturation as feeding behaviors mature. At the outset, axon growth and dendritic differentiation and synaptogenesis from, to, and within the brainstem accelerate, influenced by trophic interactions that depend upon the hindbrain or oropharyngeal target differentiation (Buchman & Davies 1993, Huang et al. 1999, Lindsay 1996, Mikaels et al. 2000, Vogel & Davies 1991). These interactions also refine mechanoreceptive versus nociceptive identities of cranial sensory neurons. Neurotrophic ligands characterize distinct targets, and cognate receptors are expressed by subclasses of mechanoreceptors or nociceptors (Ernsberger 2009, Snider & Silos-Santiago 1996, Wright & Snider 1995). In addition, central pattern-generator circuits emerge in the trigeminal, facial, ambiguus, and hypoglossal motor nuclei to regulate SFS (Dellow & Lund 1971, Morquette et al. 2012, Nakamura et al. 2004). Presumably, this process reflects the acquisition of burst-related intrinsic properties (Cifra et al. 2009), additional interconnections between nuclei, and maturation of local interneuron networks (Bourque & Kolta 2001, Kolta 1997) to generate the rhythmicity and force necessary to chew solid food. Subsequently, forebrain inputs influence hindbrain CN circuits to refine modes of food intake and preferences and to define homeostatic versus hedonic feeding (Muscatelli & Bouret 2018, Rossi & Stuber 2018). These circuit changes may also contribute to adaptive responses to varying food characteristics (e.g., hardness, viscosity, chemosensory aesthetics) that operate over the remainder of life (Ashiga et al. 2019, Woda et al. 2006)

Dysphagia: a first hit of neurodevelopmental behavioral pathology

The high frequency of perinatal dysphagia—up to 25% in otherwise typical infants, and as high as 85% in those with developmental disorders—suggests that the genetic network that specifies the SFS developmental program is vulnerable to mutation and environmental disruption. These disruptions may contribute to broader circuit pathology and indicate risk for additional neurodevelopmental impairment (Berlin et al. 2011, Nicholls & Bryant-Waugh 2009). Dysphagia is a frequent complication in cerebral palsy, in which motor control is globally altered (Asgarshirazi et al. 2017). Perinatal dysphagia also coincides with craniofacial anomalies in a broad range of neurodevelopmental disorders (Compton & Walker 2009, Solzak et al. 2013, Tripi et al. 2019) (Table 2). Accordingly, dysphagia may indicate a risk for deficits not diagnosed until an infant matures and complex behaviors emerge (Berlin et al. 2011). Some infants with significant morphogenetic anomalies are dysphagic at birth and subsequently diagnosed with intellectual disability (ID) or autism spectrum disorder (ASD). In other children already diagnosed with ASD, SFS difficulties are recognized later due to diminished growth, aspiration-based infections, or altered food preferences (Betalli et al. 2013, Field et al. 2003, Osugo et al. 2017, Twachtman-Reilly et al. 2008). In addition, children diagnosed with attention deficit hyperactivity disorder (ADHD) (Beck et al. 2005, Celletti et al. 2015) and fetal alcohol syndrome (Amos-Kroohs et al. 2016, Shen et al. 2013, Werts et al. 2014) have feeding and swallowing complications. Dysphagia in congenital heart disease (CHD) may reflect altered neural as well as pharyngeal arch differentiation (Table 2). In CHD, hindbrain motor and sensory relay neurons as well as the neural crest can be compromised (Calmont et al. 2018, Plein et al. 2015). Moreover, CHD, due to either hypoxia/ischemia or parallel disruption of cardiovascular and neural development, results in a higher frequency of complex behavioral deficits (Cohen & Earing 2018, Snookes et al. 2010). Thus, dysphagia, pharyngeal and neural crest anomalies, and complex behavioral deficits are shared by multiple neurodevelopmental disorders.

Table 2.

Clinically and genetically defined developmental disorders with increased incidence of pediatric dysphagia

Disorder	Diagnosis of associated genetic defect	Craniofacial anomalies	Cardiovascular anomalies	Dysphagia onset	Reference(s)
Developmental/neurodevelopmental disorders
Premature birth	ND	ND	Variable	Early	NA
Cerebral palsy	Rare CNVs and monogenic mutations	Moderately frequent	Moderately frequent	Early	Asgarshirazi et al. 2017, Fahey et al. 2017, Pharoah 2007, Self et al. 2012
Attention deficit/ hyperactivity disorder	Rare CNVs and monogenic mutations	ND	ND	Variable, later	Beck et al. 2005, Celletti et al. 2015
Autistic spectrum disorder	Rare CNVs and monogenic mutations	Moderately frequent	Rare	Variable, later	Timonen-Soivio et al. 2015, Twachtman-Reilly et al. 2008
Intellectual disability	Multiple CNVs and rare monogenic mutations	Frequent	NA	Variable, early	Berlin et al. 2011
Fetal alcohol syndrome	NA	Frequent	Moderately frequent	Variable	Keyte & Hutson 2012, Sant'Anna & Tosello 2006
Congenital heart disease	Multiple CNVs and rare monogenic mutations	Variable	100%	Early onset/correlated with heart repair surgery	Indramohan et al. 2017, Pereira et al. 2015
Genetic syndromes
22q11.2 deletion syndrome	Heterozygous deletion 1.5 to 3 MB hChr 22q11.2	Frequent, including some cleft lip/palate	Frequent	Early	NA
Down syndrome	Duplication, hChr21, variable size	Frequent	Moderately frequent	Variable, early	Stanley et al. 2019, Versacci et al. 2018
Rett syndrome	Monogenic MECP2, X-linked, male lethality, females affected	Variable	Rare	Variable, later	Isaacs et al. 2003, Mezzedimi et al. 2017
Noonan syndrome	Monogenic autosomal dominant; PTPN11, SOS1, RAF1, RIT1	Frequent	Frequent	Frequent, early	Roberts et al. 2013, Shah et al. 1999
CHARGE syndrome	CHD7	Frequent	Frequent	Early	Bergman et al. 2011, Dobbelsteyn et al 2008
Kabuki syndrome	KMT2D, KMD6A	Frequent	Frequent	Early	Adam et al. 2019
Troyer syndrome	SPG20	Variable	Not reported	Variable	Baple & Crosby 2004
Christianson syndrome	NHE6/SLC9A6	Variable	Not reported	Variable	Morrow & Pescosolido 2018

Developmental/neurodevelopmental disorders for which diagnosis is primarily clinical and etiologies are diverse appear first. Genetic syndromes associated with specific copy number, variants, or mutations in a limited number of single genes appear second. For both groups, pediatric dysphagia is frequently associated with a much broader phenotypic spectrum, including craniofacial anomalies and cardiovascular malformations. Abbreviations: CNV,copy number variant ; NA, not applicable; ND, no data.

Dysphagia is also a frequent complication in genetic neurodevelopmental syndromes, including Down (Stanley et al. 2019), Rett (Mezzedimi et al. 2017), Christianson (Morrow & Pescosolido 2018), Troyer (Baple & Crosby 2004), Kabuki (Adam et al. 2019), Noonan (Shah et al. 1999), and CHARGE syndromes (Dobbelsteyn et al. 2008) (Table 2). Often, the genes mutated in these syndromes are expressed widely or ubiquitously in the developing brain. This suggests that broader gene networks associated with each syndrome may first contribute to hindbrain development underlying early optimal SFS and then compromise additional circuit development. DiGeorge or 22q11.2 deletion syndrome (22q11DS) also disrupts both craniofacial and neural development as well as cardiac, limb, and digit differentiation (McDonald-McGinn et al. 2015). The coincidence of pharyngeal and neural phenotypes suggests that children with 22q11DS may have a higher frequency of feeding and swallowing difficulties. This is indeed the case; at least 85% are diagnosed with dysphagia early in life (Eicher et al. 2000). In addition, many 22q11-deleted genes are expressed first in the neural crest or in the developing hindbrain early and then in multiple brain regions, including the cerebral cortex, hippocampus, and basal ganglia (Maynard et al. 2003, 2008; Meechan et al. 2007; Motahari et al. 2019). Thus, dysphagia in 22q11DS may reflect the diminished dosage of key genes in critical brain regions at essential times in their development.

Dysphagia also arises after a lifetime of normal feeding and swallowing (Rommel & Hamdy 2016) due to stroke, brain injury, or neurodegenerative diseases, including amyotrophic lateral sclerosis, muscular dystrophy, multiple sclerosis, and Parkinson’s and Alzheimer’s diseases (Aghaz et al. 2018, Audag et al. 2019, Chouinard 2000, Polychronis et al. 2019). Adult dysphagia is primarily a disorder of the pharyngeal and esophageal phases of swallowing due to diminished cortical control of CN function as well as cranial motor neuron dysfunction or degeneration (Gonzalez-Fernandez et al. 2015). Apparently, additional forebrain/midbrain control enables adult SFS, distinguishing it from the neonatal behavior. The association of dysphagia with adult neurodegenerative disorders raises two issues for individuals with neurodevelopmental disorders: First, perinatal dysphagia, even if resolved, may establish greater risk for later neurodegenerative changes. Second, subclinical disruptions in oropharyngeal or hindbrain/CN circuit development may enhance dysphagia risk later in life.

Interrupted programs: divergent oropharyngeal differentiation in dysphagia

Craniofacial and oropharyngeal developmental mechanisms are likely pathogenic targets for perinatal dysphagia. Disrupted cranial neural crest patterning and differentiation, leading to cleft lip/palate and micrognathia (Baudon et al. 2009, Breik et al. 2016, Miller 2011), compromise the SFS oral phase. The pharyngeal phase relies on optimal tongue development. Tongue morphogenesis engages a neural crest scaffold for cranial mesodermal progenitors that generate muscles of the tongue, influenced by axial signals (i.e., Shh, Bmps, Fgfs) and downstream transcription factors, including Dlx, Pax, and MyoD family members (Parada & Chai 2015, Parada et al. 2012). Human aglossia, often in the context of oromandibular-limb hypogenesis (Milam et al. 2014) or situs inversus (Amor & Craig 2001), likely reflects disruption of similar signals and effectors. Failed trachea/esophagus separation, a fairly common occurrence (1/3,500 births) (Torfs et al. 1995), often accompanies CHD or genetic syndromes (Billmyre et al. 2015), complicating the esophageal phase. In animal models, tracheal/esophageal malformations reflect altered foregut/notochord interactions via Bmp, Wnt, and Fgf signaling and altered transcription factor expression, including Nkx2.1 and Sox2 (Que 2015, Que et al. 2006). Thus, craniofacial and oropharyngeal anomalies associated with dysphagia often arise due to disrupted axial signals and downstream patterning of related transcription factors.

Most dysphagic infants and children, including those with neurodevelopmental syndromes, do not have these extreme defects. Infants and children with 22q11DS have an increased incidence most likely due to subtler, but functionally significant, oropharyngeal dysmorphology. In 22q11DS, the palatal velum, superior pharyngeal, and levator palatine muscles are altered (Filip et al. 2018, Huang & Shapiro 2000, Kollara et al. 2019, Vantrappen et al. 2001, Zim et al. 2003; but see Widdershoven et al. 2011). One of the most-studied 22q11 candidate genes for pharyngeal and cardiovascular anomalies is the T-box transcription factor Tbx1, a regulator of pharyngeal arch mesoderm/endoderm differentiation (Dastjerdi et al. 2007, Grifone et al. 2008, Kelly et al. 2004, Scambler 2010). In Tbx1^−/− mouse embryos, which die by mid- to late gestation, first and second pharyngeal arch–derived cranial muscles are hypoplastic or lost, the mandible is diminished, soft palate and submucosal clefts are seen, and cranial bones are dysmorphic or absent. In both Tbx1^−/− and Tbx1^+/− embryos, fourth pharyngeal arch–derived muscles that elevate the soft palate are compromised (Jerome & Papaioannou 2001, Kong et al. 2014, Lindsay et al. 2001, Merscher et al. 2001), and CNs X and IX, which innervate these structures, are dysmorphic (Calmont et al. 2018, Karpinski et al. 2014, Okubo & Takada 2015). Accordingly, disrupted transcription factor activity in the pharyngeal mesoderm and endoderm compromises oropharyngeal development in 22q11DS. Apparently, mutations associated with neurodevelopmental syndromes target the earliest phases of the SFS developmental program, prefiguring perinatal dysphagia.

Even mice have dysphagia: animal models of disrupted suckling, feeding, and swallowing

This account of typical SFS and perinatal dysphagia raises key questions: What is the genetic architecture that underlies the SFS developmental program, and how can disruption lead to perinatal dysphagia? Clinical history indicates that key events happen before the mid–third trimester in humans: Premature infants cannot successfully suckle until 32–34 gestational weeks (McCain 2003). Insights from embryological and genetic analyses of craniofacial and hindbrain development in fish, frogs, chicks, and mice indicate that critical events likely occur very early in embryogenesis (see Figure 2); however, phenotypic embryos rarely survive to birth and thus are not models for altered SFS behavior (Figure 2; Supplemental Table 1). The mutant genes include transcription factors, adhesion molecules, protein scaffolds, secreted and cell surface ligand receptors, and other signaling intermediates. Existing dysphagia animal models—mostly focused on adults—rely upon surgical or vascular lesions in animals without prior history of feeding or swallowing difficulties. Parallel analyses in newborn animals of adequate size to perform selective lesions, particularly of the superior laryngeal nerve, confirm the essential contributions of CNs to perinatal SFS (Ding et al. 2013, Gould et al. 2017). Nevertheless, this approach creates pathology in newborns that would otherwise suckle normally rather than characterizing pathogenesis that leads to disrupted SFS at birth. The challenge, therefore, is to define an animal model for perinatal dysphagia that exhibits behavioral disruption without experimental lesions and then analyze oropharyngeal, hindbrain, and CN development prior to birth with sufficient resolution to determine when key mechanisms diverge, leading to pathology.

Novel mouse genetic models of some of the human neurodevelopmental syndromes described above may include dysphagia in their phenotypic spectrum, providing a new approach to understanding the typical and pathogenic mechanisms for SFS. We approached this problem in mouse models of 22q11DS (Meechan et al. 2015), based upon the association of 22q11DS with craniofacial and brain anomalies (McDonald-McGinn et al. 2015) and, more essentially, a high frequency of perinatal dysphagia (Eicher et al. 2000, LaMantia et al. 2016). A valid animal model of perinatal dysphagia should share key characteristics with dysphagic infants, including those with 22q11DS: (a) failure to gain weight compared to controls, (b) acute or chronic milk or food aspiration, (c) craniofacial anomalies, and (d) altered CN function. The genomically accurate LgDel mouse model of 22q11DS (Meechan et al. 2015, Merscher et al. 2001, Motahari et al. 2019) meets these criteria (Karpinski et al. 2014, Wang et al. 2017) (Figure 3a-d). Accordingly, the LgDel mouse provides an opportunity to assess when and how genetic and developmental divergence in a neurodevelopmental disorder prefigure perinatal dysphagia.

Figure 3.

Disrupted suckling, feeding, and swallowing (SFS) parallels pediatric dysphagia in human infants in the LgDel mouse model of 22q11.2 deletion syndrome and reflects early divergence of the SFS developmental program. (a) Similar to human infants, LgDel mice show a failure to gain weight and aspirate milk into the nasopharynx, as illustrated by a swallowing test using fluorescently labeled milk. Evidence of aspiration can be seen in the nasal sinuses and lungs (bottom right), where residual milk can be immunohistochemically detected, as can neutrophils, suggesting a resulting infection. (b) LgDel mice have craniofacial anomalies, including delayed palate closure. This can be seen grossly as a gap in the developing palate at embryonic day (E)14.5 (top panels, compare black arrows) and histologically as a failure of the LgDel palatal shelves (black arrows) to elevate to a horizontal position as in the wild type (WT). (c) The excitable properties of cranial motor neurons are altered. Motor neurons in the hypoglossal nucleus [cranial nerve (CN) XII] have been labeled using a choline acetyl transferase (ChAT):Cre driver and an eGFP reporter. Patch-clamp electrophysiological recordings of WT and LgDel hypoglossal motor neurons demonstrate that the amplitude and duration of action potential after-hyperpolarization (AP/AHP) is diminished in the LgDel. The amplitude, but not frequency, of spontaneous excitatory postsynaptic currents (EPSCs) is diminished in the LgDel. The frequency of inhibitory postsynaptic currents (IPSCs) and GABAergic miniature inhibitory postsynaptic currents (mIPSCs) is diminished in the LgDel. (d) CNs in a whole-mount E10.5 mouse embryo, labeled immunofluorescently for beta-III tubulin (Tubb3). The image is a composite of a three-dimensional confocal z stack through the entire embryo, taken at a final magnification of 200x. The colors reflect a depth code, illustrating more superficial CNs in red and those deeper in yellow, green, and blue. The inset shows tight fascicles of axons in the maxillary (Mx, top inset) and mandibular (Md, bottom inset) of CN V extending into the as yet undifferentiated pharyngeal arches. (e) WT CNs are patterned normally. In parallel, anterior-posterior (A-P) rhombomere patterning, demonstrated by posterior expression of Cyp26b1, is distinct. (f) In the LgDel embryo, A-P patterning is disrupted due to an apparent increase in the retinoic acid (RA) signaling gradient. Cyp26b1, whose expression is regulated by RA, expands into anterior rhombomeres (r2, r3, r4). CN V growth and fasciculation is impaired; the ophthalmic (Op), maxillary (Mx), and mandibular (Md) branches are hypomorphic; CN VII fails to bifurcate; and CNs IX and X are frequently fused. (g) Reduction of RA signaling levels by heterozygous deletion of Raldh2 restores Cyp26b1 expression to the WT pattern and rescues the CN V/VII (black arrow) but not the CN IX/X (black arrowhead) phenotypes. Whole-embryo image in panel d provided by Zahra Motahari and Anastas Popratiloff; elements of panels a, b, and e–g adapted from Karpinski et al. (2014) under a Creative Commons Attribution (CC-BY) 4.0 License; and elements of panel c adapted with permission from Wang et al. (2017).

Our work on LgDel mice offers two key insights into the SFS developmental program as a target for dysphagia pathology (Figure 3). First, a highly specified sequence of differentiation ensures that SFS is online at birth. Second, disrupting early steps in this sequence prefigures dysphagia. In typically developing mouse embryos, hindbrain and craniofacial gene–expression patterns and levels are distinct and quantitatively precise (Karpinski et al. 2014, Meechan et al. 2007) (Figure 3), as are early coalescence, lineage, proliferation, identities, and differentiation of cranial sensory ganglia precursors and neurons (Karpinski et al. 2016) (Figure 3). Initial CN projections are stereotyped, with limited variation (Karpinski et al. 2014, 2016), and matched by fasciculated projections of small subsets of sensory and motor axons into as yet undifferentiated targets. By the end of the first postnatal week, motor neurons in the hypoglossal nucleus that innervate a majority of the muscles of the tongue (CN XII) have mature firing patterns as well as excitatory and inhibitory synaptic responses (Wang et al. 2017). The precision of these molecular and cellular features declines in the LgDel, resulting in early and consistent, but survivable, divergence in SFS circuit differentiation, growth of individual CN axons, and physiological properties of hypoglossal motor neurons. A singular change precedes many of these differences: A-P hindbrain patterning via retinoic acid (RA) signaling that normally defines the posterior axis (r6, r7) shifts anterior rhombomeres toward a more posterior identity (Figure 3). When rescued genetically by diminishing RA levels (Karpinski et al. 2014, Maynard et al. 2013), CN differentiation in LgDel embryos returns to the wild-type state (Figure 3). Thus, in LgDel, disrupting the earliest stages of the developmental program for optimal neonatal SFS, particularly the initial step of A-P hindbrain patterning, prefigures perinatal dysphagia.

Suckling, feeding, and swallowing: a key to neurodevelopmental pathology

Simple innate behaviors like SFS are often considered separately from complex cognitive behaviors. We suggest that disruptions of developmental programs for neural circuits that enable simple behaviors, especially those essential at birth like SFS, provide insight into complex circuits and behavioral dysfunction. Loss, gain, or polymorphic function of genes that regulate circuit development for simple behaviors may fulfill similar roles for circuits that mediate complex behaviors. In addition, common circuit anomalies may be shared by simple and complex behaviors—disrupted synaptic transmission, excitatory/inhibitory balance, or neuromodulation—even if these anomalies occur via distinct pathological processes. Indeed, mutations and polymorphisms associated with clinically defined neurodevelopmental disorders like ID, ASD, and ADHD overlap with genes implicated in hindbrain and CN development (Demontis et al. 2019, Krishnan et al. 2016, Willsey et al. 2018). Anomalies in another innate behavior that relies critically upon a different set of hindbrain CNs (eye movements) are also correlated with neurodevelopmental disorders (Oystreck et al. 2011, Whitman & Engle 2017). Mutated genes associated with these disorders are essential for hindbrain patterning (Tischfield et al. 2005) or neuronal differentiation (Tischfield et al. 2010, Yamada et al. 2003). The genetic architecture of craniofacial, oropharyngeal, CN, and hindbrain patterning may provide a blueprint for circuit differentiation underlying several innate, but nevertheless sophisticated, behaviors. These behaviors emerge early, depend less on learning, and are critical for survival—like SFS for nutrition or eye movements for vigilance. A similar fundamental plan, relying on patterning, downstream transcriptional regulation, and subsequent cell-cell signaling, may also establish dedicated circuits for cognitive behaviors and be targeted by the same pathologies that alter SFS.