Susan M. Smith, Ph.D.

Professor of Nutritional Sciences; Ph.D., 1987

Emphasis Groups:
Biochemical & Molecular Nutrition
Human Nutrition

Principal Research Interests:
Interactions between dietary agents and prenatal development, roles of vitamin A action, fetal alcohol syndrome, cardiac and neurodevelopmental teratogens

Research Summary:
Our laboratory is interested in how agents in the diet, such as nutrients or toxicants, affect birth defect risk in the offspring. Much of our work focuses on two such agents, vitamin A and alcohol. These are not as disparate as they might seem; alcoholics are frequently vitamin A deficient, and both vitamin A and alcohol are metabolized by the same enzymes, creating opportunities for cross-talk. Periodically, we investigate environmental toxins, such as dioxin and trichloroethylene. Our overall goal is to identify maternal intakes of these agents that affect the risk for birth defects in her children.

Retinoids are the active hormone forms of vitamin A, and when bound to their nuclear receptors, they act as transcriptional regulators of gene expression within their target tissues. In the adult animal, retinoic acid (RA) is a critical mediators of genes that govern diverse cellular activities such as adhesion, differentiation, and proliferation in most tissues and organs of the body. Retinoids have similar functions in the embryo, wherein they also serve to inform the positional identity of many embryonic structures, including the face, heart, limb, and nervous system, through their control of transcriptional factors including those of the homeotic selector class, and growth factors such as TGFs, FGFs, and BMPs. Imbalances in retinoids, through either supplement overuse or deficiency, dysregulates these signals and causes specific birth defects in the above structures.

Perhaps the most common target of gestational retinoid inadequacy is the developing heart; congenital heart malformations affect 0.7/100 live births and often go undiagnosed until adulthood. Our lab has used both avian and mammalian models to investigate retinoid roles in heart development. In this work, we have found that retinoids affect the left-right laterality of the early heart tube (Smith et al., 1997); application of RA to the heart field randomizes the looping process. This is accompanied by changes in sonic hedgehog and cNR1 distribution, and extracellular matrix proteins that govern differential cell adhesion. RA deficiency impairs nervous system, ocular and craniofacial development (Dickman et al. 1997), as well as limb bud outgrowth (Power). Heart development is also impaired, with a thin-walled heart that closely resembles the RXRα-null heart with respect to morphology and gene expression profile (Ruiz-Lozano et al., 1998). We have shown that Retinol Binding Protein is secreted by the early heart (Barron) and that null mutation in RBP causes a similar reduction of trabeculation, and elevations of cardiomyocyte proliferation and fibronectin expression (Wendler et al. 2003). Microarray analysis of embryos revealed a set of genes dysregulated by RA deficiency (Flentke et al. 2004).

More recently, we have become interested in the roles for retinoids in adult cardiac function. This work utilizes the RBP null mouse and represents a new action for retinoids in the adult. This collaboration with J. Lough at the Medical College of Wisconsin (http://www.mcw.edu/display/router.asp?DocID=17123) finds that subclinical retinoid deficiency causes significant cardiac enlargement by 8 wks of age. This does not represent hypertrophy because myocyte diameter is normal, and because the gene expression profile is inconsistent with hypertrophy (e.g. lower levels of bMHC, aSkA). Indeed RBP-null or retinoid-deficient hearts are resistant to hypertrophy induced by pressure overload. A manuscript describing this work is in preparation, and a patent has been filed regarding the possible clinical import of these findings.

The most common teratogen in the human diet is ethanol, and the other half of our lab investigates the mechanisms that underlie Fetal Alcohol Syndrome (FAS) and Alcohol-Related Neurodevelopmental Disorders (ARND). These are the most common known causes of mental retardation, exceeding better known disorders such as Down’s Syndrome and Cerebral Palsy. We study the molecular mechanisms by which pharmacologically relevant alcohol exposures (0.05-0.3% BAC) selectively activate the apoptotic machinery within specific neuronal and craniofacial populations, cells known as the Neural Crest (Smith & Kragtorp, 2006). Using a chick embryo model, we find that that these cells are only sensitive to alcohol-induced apoptosis prior to their migration (C&S 1995a,b), and that the apoptosis pathway converges on endogenous apoptosis pathways (C&S 1998). The embryo’s genetic background affects its sensitivity to ethanol-induced apoptosis (Debelak) and craniofacial malformations (Su et al. 2001), and to heart defects (Cavieres).

Recently, we have elucidated the mechanism by which alcohol activates apoptosis signals within the neural crest (Cartwright et al 1997). Ethanol causes a rapid and selective elevation of intracellular calcium concentrations within the premigratory neural crest (Kragtorp et al. 2003). This calcium transient requires the activity of phospholipase Cβ and production of PIP2; this is sensitive to inhibition by pertussis toxin and likely originates from a G protein-coupled receptor linked to Gαi2/3 and Gβγ (Garic-Stankovic et al. 2005). Interestingly, GPCRs are common targets in mediating alcohol’s addictive and reward properties. It is now clear that alcohol does not affect cell function through effects on membrane fluidity. Rather, as in other models, we find that ethanol’s stimulation of Ca release is saturable, implicating a protein target. N-Alcohols through C-5 can mobilize Ca in neural crest and longer alcohols abruptly lose activity, suggesting the alcohol binding pocket in the hypothesized GPCR target cannot accommodate alcohols larger than pentanol (Garic-Stankovic et al. 2006). Our current research has shown that CaMKII is a critical target activated by calcium, and its inhibition prevents the subsequent apoptosis. We are also investigating the targets of CaMKII activity, and the identity of the GPCR on neural crest that is activated by alcohol. Finally, we have elucidated several of the signals that mediate endogenous apoptosis of neural crest, to establish how alcohol co-opts these endogenous signals in effecting its teratogenicity.

Our lab maintains an interest in the actions of cardiac teratogens. In the past we have investigated the basis for dioxin (TCDD) cardiac teratogenicity, work that originated with and continues with Dr. Mary Walker at the University of New Mexico (Thackaberry et al. 2002, 2003). We also studied the cardioteratogenesis of retinoic acid (Dickman, Smith et al. 1997) and ethanol (Cavieres). Our recent investigations address the putative cardiac teratogenicity of the common groundwater contaminant Trichloroethylene (TCE). In another collaboration with John Lough at the Medical College of Wisconsin (http://www.mcw.edu/display/router.asp?DocID=17123), we find that the embryonic chick heart is sensitive to TCE during the period of cushion development (Drake EHP 2006) but not during the earlier heart specification period (Drake ToxSci 2006). TCE exposure enhances cushion cell proliferation, reduces cardiac function as measured by Doppler imaging, and increases embryo mortality. In this work we also established a useful model for the study of cardiac teratogens during the early gestational period (Drake et al BDRA 2006).

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Representative Publications

Drake VJ, Koprowski SL, Hu N, Lough JW, Smith SM. 2006. Trichloroethylene exposure during cardiac valvuloseptal morphogenesis alters cushion formation and cardiac hemodynamics in the avian embryo. Env. Health Perpect. 114:842-847. [Abstract]

Garic-Stankovic A, Hernandez M, Flentke GR, Smith SM. 2006. Structural constraints for alcohol-stimulated Ca2+ release in neural crest, and dual agonist/antagonist properties of n-octanol. Alcohol Clin Exp Res 30:552-559. [Abstract]

Garic-Stankovic A, Hernandez MA, Flentke GR, Debelak-Kragtorp KA, Armant DR, Smith SM. 2005. Ethanol triggers neural crest apoptosis thru the selective activation of a pertussis toxin-sensitive G-protein and a phospholipase Cβ-dependent Ca2+ transient. Alcohol Clin Exp Res 29:1237-1246. [Abstract]

Kilburn BA, Chiang PJ, Wang J, Flentke GR, Smith SM, Armant DR. 2006. Rapid induction of apoptosis in gastrulating mouse embryos by ethanol and its prevention by HB-EGF. Alcohol Clin Exp Res 30:127-134. [Abstract]

Flentke GR, Baker MW, Docterman KE, Power S, Lough J, Smith SM. 2004. Microarray analysis of retinoid-dependent gene activity during rat embryogenesis: increased collagen fibril production in a model of retinoid insufficiency. Dev Dyn 229:886-898. [Abstract]

Wendler CC, Schmoldt A, Flentke GR, Case LC, Quadro L, Blaner WS, Lough J, Smith SM. 2003. Increased fibronectin deposition in embryonic hearts of retinol-binding protein-null mice. Circ Res 92:920-928. [Abstract]

Debelak-Kragtorp KA, Armant DR, Smith SM. 2003. Ethanol-induced cephalic apoptosis requires phospholipase C-dependent intracellular calcium signaling. Alcohol Clin Exp Res 27:515-523. [Abstract]

Dickman, E.D., Thaller, C., Smith, S.M. (1997) Temporally regulated retinoic acid depletion produces specific neural crest, ocular, and nervous system defects. Development 124:3111-3121. [Abstract]

Smith, S.M. (1998) Alcohol-induced cell death in the embryo. Alc. Health Res. World 21:287-297.

Cartwright, M.M., Tessmer, L.L., Smith, S.M. (1998) Ethanol-induced neural crest apoptosis is coincident with their endogenous death but is mechanistically distinct. Alcohol Clin. Exp. Res. 22:142-149. [Abstract]

Ruiz-Lozano, P., Smith, S.M., Perkins, G., Kubalak, S.W., Boss, G.R., Sucov, H.M., Evans, R.M., Chien, K.R . (1998) Energy deprivation and a deficiency in downstream metabolic target genes during the onset of embryonic heart failure in RXRa-l-embryos. Development 125:533-544. [Abstract]