Institute Affiliations


  • Member, Maternal & Child Health Research Institute (MCHRI)

Professional Education


  • Fellowship, Columbia University Medical Center - Morgan Stanley Children's Hospital, Pediatric Cardiology (2020)
  • Residency, Loma Linda University Children's Hospital, Pediatrics (2017)
  • Doctor of Medicine, Oregon Health & Science University, Medicine (2014)
  • Bachelor of Arts, Stanford University, Human Biology (2007)

Stanford Advisors


All Publications


  • Single cell RNA sequencing approaches to cardiac development and congenital heart disease. Seminars in cell & developmental biology Samad, T., Wu, S. M. 2021

    Abstract

    The development of single cell RNA sequencing technologies has accelerated the ability of scientists to understand healthy and disease states of the cardiovascular system. Congenital heart defects occur in approximately 40,000 births each year and 1 out of 4 children are born with critical congenital heart disease requiring surgical interventions and a lifetime of monitoring. An understanding of how the normal heart develops and how each cell contributes to normal and pathological anatomy is an important goal in pediatric cardiovascular research. Single cell sequencing has provided the tools to increase the ability to discover rare cell types and novel genes involved in normal cardiac development. Knowledge of gene expression of single cells within cardiac tissue has contributed to the understanding of how each cell type contributes to the anatomic structures of the heart. In this review, we summarize how single cell RNA sequencing has been utilized to understand cardiac developmental processes and congenital heart disease. We discuss the advantages and disadvantages of whole cell versus single nuclei RNA sequencing and describe the approaches to analyze the interactomes, transcriptomes, and differentiation trajectory from single cell data. We summarize the currently available single cell RNA sequencing technologies and technical aspects of performing single cell analysis and how to overcome common obstacles. We also review data from the recently published human and mouse fetal heart atlases and advancements that have occurred within the field due to the application of these single cell tools. Finally we highlight the potential for single cell technologies to uncover novel mechanisms of disease pathogenesis by leveraging findings from genome wide association studies.

    View details for DOI 10.1016/j.semcdb.2021.04.023

    View details for PubMedID 34006454

  • The PDZ motif of the α1C subunit is not required for surface trafficking and adrenergic modulation of CaV1.2 channel in the heart. The Journal of biological chemistry Yang, L., Katchman, A., Weinberg, R. L., Abrams, J., Samad, T., Wan, E., Pitt, G. S., Marx, S. O. 2015; 290 (4): 2166-74

    Abstract

    Voltage-gated Ca(2+) channels play a key role in initiating muscle excitation-contraction coupling, neurotransmitter release, gene expression, and hormone secretion. The association of CaV1.2 with a supramolecular complex impacts trafficking, localization, turnover, and, most importantly, multifaceted regulation of its function in the heart. Several studies hint at an important role for the C terminus of the α1C subunit as a hub for multidimensional regulation of CaV1.2 channel trafficking and function. Recent studies have demonstrated an important role for the four-residue PDZ binding motif at the C terminus of α1C in interacting with scaffold proteins containing PDZ domains, in the subcellular localization of CaV1.2 in neurons, and in the efficient signaling to cAMP-response element-binding protein in neurons. However, the role of the α1C PDZ ligand domain in the heart is not known. To determine whether the α1C PDZ motif is critical for CaV1.2 trafficking and function in cardiomyocytes, we generated transgenic mice with inducible expression of an N-terminal FLAG epitope-tagged dihydropyridine-resistant α1C with the PDZ motif deleted (ΔPDZ). These mice were crossed with α-myosin heavy chain reverse transcriptional transactivator transgenic mice, and the double-transgenic mice were fed doxycycline. The ΔPDZ channels expressed, trafficked to the membrane, and supported robust excitation-contraction coupling in the presence of nisoldipine, a dihydropyridine Ca(2+) channel blocker, providing functional evidence that they appropriately target to dyads. The ΔPDZ Ca(2+) channels were appropriately regulated by isoproterenol and forskolin. These data indicate that the α1C PDZ motif is not required for surface trafficking, localization to the dyad, or adrenergic stimulation of CaV1.2 in adult cardiomyocytes.

    View details for DOI 10.1074/jbc.M114.602508

    View details for PubMedID 25505241

    View details for PubMedCentralID PMC4303668

  • β-adrenergic regulation of the L-type Ca2+ channel does not require phosphorylation of α1C Ser1700. Circulation research Yang, L., Katchman, A., Samad, T., Morrow, J., Weinberg, R., Marx, S. O. 2013; 113 (7): 871-80

    Abstract

    Sympathetic nervous system triggered activation of protein kinase A, which phosphorylates several targets within cardiomyocytes, augments inotropy, chronotropy, and lusitropy. An important target of β-adrenergic stimulation is the sarcolemmal L-type Ca(2+) channel, CaV1.2, which plays a key role in cardiac excitation-contraction coupling. The molecular mechanisms of β-adrenergic regulation of CaV1.2 in cardiomyocytes, however, are incompletely known. Recently, it has been postulated that proteolytic cleavage at Ala(1800) and protein kinase A phosphorylation of Ser(1700) are required for β-adrenergic modulation of CaV1.2.To assess the role of Ala(1800) in the cleavage of α1C and the role of Ser(1700) and Thr(1704) in mediating the adrenergic regulation of CaV1.2 in the heart.Using a transgenic approach that enables selective and inducible expression in mice of FLAG-epitope-tagged, dihydropyridine-resistant CaV1.2 channels harboring mutations at key regulatory sites, we show that adrenergic regulation of CaV1.2 current and fractional shortening of cardiomyocytes do not require phosphorylation of either Ser(1700) or Thr(1704) of the α1C subunit. The presence of Ala(1800) and the (1798)NNAN(1801) motif in α1C is not required for proteolytic cleavage of the α1C C-terminus, and deletion of these residues did not perturb adrenergic modulation of CaV1.2 current.These results show that protein kinase A phosphorylation of α1C Ser(1700) does not have a major role in the sympathetic stimulation of Ca(2+) current and contraction in the adult murine heart. Moreover, this new transgenic approach enables functional and reproducible screening of α1C mutants in freshly isolated adult cardiomyocytes in a reliable, timely, cost-effective manner.

    View details for DOI 10.1161/CIRCRESAHA.113.301926

    View details for PubMedID 23825359

    View details for PubMedCentralID PMC3864014

  • Stem cell antigen-1 in skeletal muscle function. PLoS currents Bernstein, H. S., Samad, T., Cholsiripunlert, S., Khalifian, S., Gong, W., Ritner, C., Aurigui, J., Ling, V., Wilschut, K. J., Bennett, S., Hoffman, J., Oishi, P. 2013; 5

    Abstract

    Stem cell antigen-1 (Sca-1) is a member of the Ly-6 multigene family encoding highly homologous, glycosyl-phosphatidylinositol-anchored membrane proteins. Sca-1 is expressed on muscle-derived stem cells and myogenic precursors recruited to sites of muscle injury. We previously reported that inhibition of Sca-1 expression stimulated myoblast proliferation in vitro and regulated the tempo of muscle repair in vivo. Despite its function in myoblast expansion during muscle repair, a role for Sca-1 in normal, post-natal muscle has not been thoroughly investigated. We systematically compared Sca-1-/- (KO) and Sca-1+/+ (WT) mice and hindlimb muscles to elucidate the tissue, contractile, and functional effects of Sca-1 in young and aging animals. Comparison of muscle volume, fibrosis, myofiber cross-sectional area, and Pax7+ myoblast number showed little differences between ages or genotypes. Exercise protocols, however, demonstrated decreased stamina in KO versus WT mice, with young KO mice achieving results similar to aging WT animals. In addition, KO mice did not improve with practice, while WT animals demonstrated conditioning over time. Surprisingly, myomechanical analysis of isolated muscles showed that KO young muscle generated more force and experienced less fatigue. However, KO muscle also demonstrated incomplete relaxation with fatigue. These findings suggest that Sca-1 is necessary for muscle conditioning with exercise, and that deficient conditioning in Sca-1 KO animals becomes more pronounced with age.

    View details for DOI 10.1371/currents.md.411a8332d61e22725e6937b97e6d0ef8

    View details for PubMedID 24042315

    View details for PubMedCentralID PMC3770837