Exam 3: Biology

Passage In 1906, the Nobel Prize in Physiology or Medicine was awarded to two biologists for their groundbreaking work in the field of neuroanatomy.  Despite using the same technique to visualize neurons, the two biologists formed opposing theories regarding nervous system organization.Hypothesis 1:  Reticular TheoryBiologist 1 developed a neural staining technique that was carried out by fixing brain tissue in potassium dichromate (K₂Cr₂O₇) before treating it with silver nitrate (AgNO₃).  A silver chromate (Ag₂CrO₄) precipitate deposited within the neurons, making them appear black.  The silver impregnation method was revolutionary because of two features.  First, only a random subset of neurons within a given tissue sample (~1% of the total cell population) was subject to silver impregnation.  Second, the impregnated cells were stained in their entirety, allowing for visualization of the complete neuron.  Biologist 1 concluded that his findings supported the idea that the nervous system is a single reticulum of fused neural processes that transmit information via cytoplasmic continuity.Hypothesis 2:  Neuron DoctrineBiologist 2 modified the silver impregnation method by first subjecting the brain tissue to two rounds of staining ("double impregnation") rather than one.  Second, where Biologist 1 studied the brain tissue of older animals with myelinated nerve fibers, Biologist 2 employed the silver stain on brain tissue from younger animals, which made it easier for the silver to impregnate the neural processes due to relative lack of myelination.  Biologist 2 reasoned that the nervous system is not a single web of interconnected fibers but rather is made up of discrete neural units that communicate via close, but not continuous, contact.The neuron doctrine was later proven correct with the advent of electron microscopy, which allowed scientists to visualize the synapse (the point of communication between two distinct neurons).  Further investigation allowed researchers to ultimately discover that there are two types of synapses:  electrical and chemical. -Compared to the axons investigated by Biologist 2, the axons studied by Biologist 1 most likely had:

A gene regulating tail length in bobtail cats has a genetic variation at a single locus, where organisms can have an allele that produces long tails (L) or an allele that produces short tails (l).  A male cat homozygous for short tail length (ll) was mated with a female cat homozygous for long tail length (LL).  The tail lengths of the parents and offspring were recorded below. A scientist concludes that the alleles regulating tail length in these cats exhibit incomplete dominance.  Do the data support this conclusion?

Passage A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications. -Glucocorticoid deficiency is known to impair healing of superficial wounds, memory formation, and other physiologic phenomena.  This deficiency most likely leads to these wide-ranging effects because:

Passage Beta-thalassemia is a blood disorder caused by reduced production of the β-globin subunit of hemoglobin.  The erythrocytes in patients with beta-thalassemia are abnormally small, lack sufficient functional hemoglobin, and are decreased in number.  These individuals require lifelong blood transfusions to correct their erythrocyte shortage.  Each unit of blood transfused contains many milligrams of iron and, consequently, blood iron levels increase due to repeated transfusions.  The excess iron is then continuously deposited in the organs, leading to organ damage.Transferrin (Tf) is a protein that binds free iron in the blood and plays a key role in erythropoiesis, which requires uptake of iron from the blood in a process facilitated by Tf and its cell surface receptor (TfR).  Iron-bound Tf, known as holotransferrin (holoTf), binds TfR to form a complex that is brought into the developing erythrocyte in an endosome.  Acidification of the endosome triggers a conformational change in holoTf, which allows the release of free iron into the cell and the recycling of the remaining complex components to the cell surface.  Non-iron-bound Tf is referred to as apotransferrin (apoTf), and due to its iron-binding capability, researchers believe that apoTf injections may help prevent deposition of excess iron in the organs of patients with beta-thalassemia.To investigate the effect of apoTf administration in a mouse model of beta-thalassemia, mice homozygous for an inactivating deletion in the murine β-globin gene (Hbb^th−3/Hbb^th−3) were given blood transfusions followed by apoTf injection.  Measurements of non-transferrin-bound iron in the blood (NTBI), plasma holoTf, and urinary holoTf were taken immediately and at 15 days post-injection.  Results revealed that NTBI decreased over the tested period.  Experimenters hypothesized that apoTf mediated certain regulatory changes that reduced NTBI in Hbb^th−3/Hbb^th−3 mice by increasing urinary holoTf excretion. -In cells expressing TfR, decreased endocytic vessel pH during normal iron metabolism would trigger the recycling of:

Passage Adeno-associated virus 2 (AAV2) is a nonpathogenic, nonenveloped DNA virus that requires helper viruses such as adenovirus (AV) or herpes simplex virus (HSV) to replicate efficiently.  Because it can transduce human cells, including quiescent (nondividing) cells, without causing disease, AAV2 is attractive as a gene therapy vector.  In clinical trials, a modified AAV2 virus has successfully treated the retinal disease Leber's congenital amaurosis as well as the blood clotting disorder hemophilia B.  AAV2 transduces cells via a multistep process, shown in Figure 1. Figure 1  Schematic showing the mechanism of transduction by AAV2Studies indicate that unlike AV and HSV, AAV2 inhibits pathways other than the endosomal pathway, such as trafficking to the Golgi, increasing the percentage of invading virus particles that arrive at the nucleus. Adapted from Xiao PJ, Samulski RJ. Cytoplasmic trafficking, endosomal escape, and perinuclear accumulation of adeno-associated virus type 2 particles are facilitated by microtubule network. J Virol. 2012;86(19):10462-73. -AAV2 entry into cells could best be blocked by a drug that inhibits which of the following mechanisms?

Passage Mitochondria are thought to have been independent bacterial organisms that were engulfed and integrated into eukaryotic cells approximately two billion years ago.  Most of the mitochondrial genome was lost or transferred into the large central genome of the eukaryotic nucleus, leaving only a residual genome within each mitochondrion.Because multiple mitochondria are found in the cell, mitochondrial DNA (mtDNA) mutations result in a condition known as heteroplasmy, or the intracellular mixture of wild-type mtDNA and mutant mtDNA.  Just as the cellular ratio of wild-type to mutant mtDNA varies, the overall mtDNA content within a tissue or organ is also highly variable.  For this reason, disease-causing mutant mtDNA manifests phenotypically only when the majority of mitochondria within a given tissue express the deleterious allele.  In addition, the variability in mtDNA content makes it possible for individuals with the same mitochondrial mutation to display drastically different clinical symptoms.To investigate how heteroplasmy influences disease manifestation, researchers analyzed an A to G substitution at nucleotide 3243 in the tRNA^leu gene of mtDNA.  This mutation has been associated with hereditary hearing loss, diabetes mellitus, and a syndrome characterized by mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes.Family members who were confirmed to have the A3243G mutation were tested using an audiometry examination to assess hearing impairment, as shown in Table 1.  The subjects carried no other genetic mutations known to cause hearing loss.Table 1  Audiometry Results From Family Members with the A3243G Mutation The blood samples of these subjects were collected, and the 3243 region of the tRNA^leu gene was amplified via standard PCR.  The PCR products were subsequently treated with the restriction enzyme ApaI and visualized on a PAGE gel.  The normal undigested PCR product is 161 bp in length, but the PCR product amplified from the mutant A3243G is digested into two fragments by ApaI. Figure 1  PAGE visualization of PCR products following ApaI digestion Adapted from Hadjivasiliou Z, Pomiankowski A, Seymour RM, Lane N. Selection for mitonuclear co-adaptation could favour the evolution of two sexes. Proc Biol Sci. 2012;279(1734):1865-72. -Which of the following cell types would be LEAST sensitive to mitochondrial dysfunction due to mtDNA mutation?

Passage Skin grafting is a procedure in which healthy skin from a different region of the body is transplanted onto an area of absent or damaged skin.  Skin grafts are used for partial-thickness burns, which damage the epidermis and dermis, and for full-thickness burns, which damage the epidermis, dermis, and hypodermis.  Superficial burns injure only epidermal tissue and heal without skin grafts.  Skin grafting has greatly improved the long-term survival of burn victims.  However, because grafted skin cannot effectively dissipate heat, burn survivors are at an increased risk of hyperthermia (abnormally high body temperature), a condition that can lead to heat-related medical emergencies such as heat stroke.  The greater the grafted body surface area (BSA), the greater the risk for complications related to thermoregulation.Heat acclimation (HA) is the process by which the body increases heat tolerance through adaptive changes in the sympathetic control of skin heat dissipation.  It can be induced in healthy individuals by repeated exercise at elevated ambient temperature and humidity.  A group of researchers tested whether skin graft recipients could also undergo HA.  Skin graft recipients who were otherwise healthy and had at least 15% grafted BSA were recruited for the study, as were controls with no history of receiving grafts.  The skin graft recipients were split into two groups, those with 15%-40% grafted BSA and those with >40% grafted BSA.  For a period of seven days, subjects participated daily in 90 min of exercise at 50% maximal oxygen consumption in a room where the temperature was maintained at 40°C.  Exercise modality, fluid intake, and frequency and duration of rest were strictly monitored.On the days immediately preceding and following the HA regimen, the heat tolerance of participants was tested with a 90-min exercise regimen at a fixed heat production rate (Figure 1).  Tolerance testing was halted before 90 min for subjects whose internal body temperature exceeded 40°C.  The change in internal temperature of participants was calculated by subtracting their internal temperature before exercise from their internal temperature at exercise cessation (either once temperature reached 40°C or at the 90-min endpoint). Figure 1  HA in controls and in skin graft recipients Adapted from Schlader ZJ, Ganio MS, Pearson J, et al. J Appl Physiol. 2015;119(1):69-76. -What physiological change occurs in the skin of control subjects during HA exercise?

Passage The lumen of the human gut is lined by a monolayer of epithelial cells that acts as a selectively permeable barrier, preventing the passage of harmful intraluminal foreign antigens, flora, and toxins into the circulation while allowing digestion and absorption of essential dietary nutrients along with the transfer of electrolytes and water.Proteins in the tight junctions of intestinal epithelial cells maintain barrier integrity, but barrier dysfunction occurs when these cells are damaged in the setting of infection, burns, shock, or hypoxia (low oxygen levels).  The transcription factor HIF-1, a heterodimer composed of the macromolecules HIF-1α and HIF-1β, regulates the adaptive cellular response to hypoxia and the consequent expression of tight junction proteins.Researchers assessed the concentration of HIF-1 heterodimer components in human intestinal Caco-2 cells subjected to hypoxia/reoxygenation (H/R) in vitro.  Caco-2 cells, a colon-derived cell line, were cultured under specific conditions to mimic the functional and morphological phenotype of wild-type enterocytes lining the small intestine.  These cells were prepared and grown as a monolayer on a collagen-coated membrane.Next, the monolayer was cultured in hypoxic conditions and then exposed to atmospheric oxygen levels (normoxia) for 30, 60, and 120 minutes.  Protein levels were quantified using direct enzyme-linked immunosorbent assay (ELISA), in which an antibody linked to a reporter enzyme was utilized to bind and detect expression of the target molecule (analyte) in a sample.  When the colorless substrate of the reporter enzyme was added, the enzyme generated a visible colored product that could be quantified based on color intensity. Figure 1  Concentration of (A) HIF-1α and (B) HIF-1β in Caco-2 cells subjected to H/R (Note:  0 minutes = cells cultured in hypoxic conditions)Emodin, an anthraquinone compound that prevents hypoxia-induced epithelial cell disruption, was used in conjunction with a chemical HIF-1α inhibitor (HIF-1α-I) to treat Caco-2 cells in a separate experiment.  Transepithelial electrical resistance (TEER) was assessed as a measure of barrier function, with higher TEER values indicating a more intact epithelial cell barrier.  HIF-1α-I was found to block emodin's protective effect on epithelial barrier integrity. Adapted from Lei Q, Qiang F, Chao D, et al. Int J Mol Med. 2014;34(6):1629-39. -Developmental protocols for Caco-2 monolayers involve the use of cells with high proliferation potential that can differentiate in a synchronized manner to form homogenous monolayers.  This proliferation of Caco-2 cells is primarily achieved through:

Passage The sex chromosomes X and Y are involved in mammalian sex determination.  Organisms with two wild-type (WT) X chromosomes develop as females (XX), but those with a combination of WT X and Y chromosomes develop as males (XY) (Table 1).  Both sexes also inherit a set of autosomes (nonsex chromosomes).  Humans inherit 22 pairs of autosomes (44 total).  In contrast, rabbits inherit 21 pairs of autosomes (42 total).Table 1  Chromosome Types Inherited by a Male and Female Rabbit (Note: The Y chromosome has fewer alleles than the X chromosome.) Specifically, the SRY gene, located on the mammalian Y chromosome, is crucial for male sex determination and induction of testis development.  Studies in mammals have shown that regardless of an organism's combination of sex chromosomes, fetuses will always develop as female in the absence of the SRY protein.  In addition, one of the X chromosomes becomes inactivated in WT,XX females to prevent them from having double the amount of X-chromosome gene products as WT,XY males.Sp1 is a transcription factor with demonstrated regulatory control of SRY gene expression, and XY individuals with mutations affecting the Sp1 binding site on the SRY gene have been shown to develop female sex characteristics.  Unlike SRY, the gene for Sp1 is located on an autosome.Researchers knocked out (KO) Sp1 binding sites on the SRY gene in rabbit embryos (Sp1 KO,XY rabbits) and observed their sexual development following implantation in surrogate mothers.  The relationship between the SRY gene and sexual development can be explained by the following experimental observations: Most Sp1 KO,XY rabbits had female genitalia and displayed normal mating behaviors. Sp1 KO,XY rabbits had significantly smaller ovaries and fewer follicles than WT,XX rabbits. No significant differences were observed in the cervix and uterus between Sp1 KO,XY and WT,XX rabbits. There was significantly reduced SRY expression in Sp1 KO,XY rabbits when compared to WT,XY males. WT,XY male rabbits had significantly higher testosterone levels when compared to Sp1 KO,XY rabbits. Sp1 KO,XY rabbits were infertile when mated with WT,XY males. Sp1 KO,XY rabbits could become pregnant via embryo transfer. Song Y et al. Mutation of the Sp1 binding site in the 5' flanking region of SRY causes sex reversal in rabbits. Oncotarget. 2017; 8(24):38176-83. -The number of each type of sex chromosome in a group of 5 male (WT,XY) and 5 female (WT,XX) rabbits is shown below. In the combined population of 15 X chromosomes and 5 Y chromosomes, which of the following alleles, if any, would be most susceptible to loss due to random chance alone?

Passage The Namibian cheetah (Acinonyx jubatus jubatus), the world's fastest land-dwelling animal, hunts across the African grasslands using high-speed sprints.  Cheetahs possess unique anatomical features specialized for speed, including an aerodynamic skull, elongated legs, and an enlarged heart.  These physiological adaptations have occurred over time and allow cheetahs to achieve speeds greater than 100 km/h.  However, due to their small body and slender limbs, cheetahs (50-64 kg) are limited with regard to the size of the animals they can successfully hunt and therefore typically prey on slightly smaller species that inhabit open grasslands.It is believed that around 12,000 years ago, Namibian cheetahs experienced an environmental catastrophe that drastically reduced their population.  The small number of surviving cheetahs began mating with one another, which led to severe inbreeding depression.  Over time, the offspring of subsequent generations exhibited decreased fitness, reduced fecundity (number of offspring), and sustained a loss of major histocompatibility complex (MHC) allele diversity.  To study if low MHC diversity correlates with low fitness in cheetahs, scientists analyzed the genetic diversity and disease prevalence in a group of free-ranging and captive male cheetahs.Experiment 1MHC allele genotyping and analysis was performed in wild (free-ranging) and captive male cheetahs (Figure 1).  Because MHC diversity is strongly correlated with genetic diversity, MHC diversity is used as a marker for an organism's overall genetic variability. Figure 1  Evaluation of heterozygosity in (A) wild and (B) captive male cheetahs born between 1976 and 2007 (Note:  Each data point represents a single male cheetah.)Experiment 2Disease prevalence was evaluated in wild and captive male cheetahs by quantitative analysis of symptoms relating gastrointestinal, liver, and kidney disease.  The percentages of cheetahs affected by each disease are shown in Figure 2. Figure 2  Evaluation of disease prevalence in wild and captive male cheetahsExperiment 3Fecal cortisol levels were noninvasively evaluated in wild and captive cheetahs over a period of 6 months (Figure 3). Figure 3  Average fecal cortisol concentrations of wild and captive male cheetahs Adapted from Castro-prieto A, Wachter B, Sommer S. Cheetah paradigm revisited: MHC diversity in the world's largest free-ranging population. Mol Biol Evol. 2011;28(4):1455-68. -If wild Namibian cheetahs continue breeding as described in the passage, what is most likely to occur to their population size in the long term?

Passage Skin grafting is a procedure in which healthy skin from a different region of the body is transplanted onto an area of absent or damaged skin.  Skin grafts are used for partial-thickness burns, which damage the epidermis and dermis, and for full-thickness burns, which damage the epidermis, dermis, and hypodermis.  Superficial burns injure only epidermal tissue and heal without skin grafts.  Skin grafting has greatly improved the long-term survival of burn victims.  However, because grafted skin cannot effectively dissipate heat, burn survivors are at an increased risk of hyperthermia (abnormally high body temperature), a condition that can lead to heat-related medical emergencies such as heat stroke.  The greater the grafted body surface area (BSA), the greater the risk for complications related to thermoregulation.Heat acclimation (HA) is the process by which the body increases heat tolerance through adaptive changes in the sympathetic control of skin heat dissipation.  It can be induced in healthy individuals by repeated exercise at elevated ambient temperature and humidity.  A group of researchers tested whether skin graft recipients could also undergo HA.  Skin graft recipients who were otherwise healthy and had at least 15% grafted BSA were recruited for the study, as were controls with no history of receiving grafts.  The skin graft recipients were split into two groups, those with 15%-40% grafted BSA and those with >40% grafted BSA.  For a period of seven days, subjects participated daily in 90 min of exercise at 50% maximal oxygen consumption in a room where the temperature was maintained at 40°C.  Exercise modality, fluid intake, and frequency and duration of rest were strictly monitored.On the days immediately preceding and following the HA regimen, the heat tolerance of participants was tested with a 90-min exercise regimen at a fixed heat production rate (Figure 1).  Tolerance testing was halted before 90 min for subjects whose internal body temperature exceeded 40°C.  The change in internal temperature of participants was calculated by subtracting their internal temperature before exercise from their internal temperature at exercise cessation (either once temperature reached 40°C or at the 90-min endpoint). Figure 1  HA in controls and in skin graft recipients Adapted from Schlader ZJ, Ganio MS, Pearson J, et al. J Appl Physiol. 2015;119(1):69-76. -Which table likely illustrates the change in average internal body temperature that occurred during heat tolerance testing pre-HA and post-HA in grafted and control subjects?  Assume all participants had an average internal body temperature of 37°C before heat-tolerance testing began.

Passage Each time a eukaryotic cell replicates its DNA, the telomeres at the ends of each chromosome become slightly shorter.  When telomeres reach a critically short length, the cells enter the senescent state, in which cell division ceases (Figure 1).  In humans, conditions such as hypertension, liver disease, and Alzheimer disease have been associated with the decreased telomere length and resulting senescence that occur as part of the normal aging process. Figure 1Cells that must be able to divide indefinitely (stem cells and germ cells) express an enzyme known as telomerase.  Functional telomerase consists of a protein subunit called telomerase reverse transcriptase (TERT) and a noncoding RNA subunit called telomerase RNA (TR).  Using a portion of TR as a template, telomerase extends telomeres by continuously adding the sequence 5′-TTAGGG-3′ to the ends of chromosomes.  The complement strand is then filled in by DNA polymerase.  Telomere extension allows these cells to avoid senescence.In healthy somatic cells, however, TERT transcription is generally downregulated by the absence of the oncogene c-Myc and the presence of the tumor suppressor protein WT1.  When telomerase is suppressed, the resulting critical telomere shortening has been hypothesized to prevent uncontrolled cell proliferation in adult tissues.  In cancerous somatic cells, on the other hand, increased telomerase activity is often associated with tumorigenesis.  WT1 is frequently absent and c-Myc is frequently overexpressed in these cells, leading to increased TERT expression.In addition to transcriptional control, TERT can be regulated at the post-transcriptional level.  The TERT gene consists of 16 exons and 15 introns that can be spliced into more than 20 different isoforms in humans.  Only one isoform (the active isoform) is known to extend telomeres.  However, the i2 isoform, in which intron 2 is partially included in the mature mRNA (Figure 2), has been hypothesized to downregulate residual TERT activity in somatic cells. Figure 2 Adapted from Cifuentes-rojas C, Shippen DE. Telomerase regulation. Mutat Res. 2012;730(1-2):20-7. -Some eukaryotic organisms have multiple TERT genes with high sequence similarity that are expressed at different times in the organism's life cycle.  These genes could have arisen by:

In humans, meiosis and mitosis are most similar with respect to:

Passage Alzheimer disease (AD) is a progressive condition affecting memory and cognition that can be pathologically characterized by the presence of neurofibrillary tangles (NFTs).  NFTs, aggregates of hyperphosphorylated tau proteins that have dissociated from microtubules, are believed to contribute to neuronal degradation and subsequent AD symptoms.  In general, ribosomes initiate translation of tau mRNA by binding the 7-methylguanosine (m7Gpp) cap present at the end of the 5′ untranslated region (UTR) of the mRNA molecule.  However, researchers believe that tau protein translation initiation can occur without recognition of the m7Gpp cap.  This alternate mechanism, referred to as cap-independent translation, indicates that ribosomes are recruited by internal ribosome entry sites (IRES) generally located in the 5′ UTR of the mRNA sequence.To determine the initiation mechanism of tau mRNA translation, neuroblastoma cells of AD mice were transfected with two different DNA constructs (Figure 1).  Construct 1 contained the 5′ UTR of tau mRNA isoforms (v1 and v2) extracted from cells displaying NFTs.  Construct 2 included the 5′ UTR of the β-globulin gene, which normally codes for a plasma protein.  The luciferase reporter genes, Renillla and Photinus, were also included in the two constructs. /_ /_ Figure 1  DNA constructs containing β-globulin or tau gene used to transfect neuroblastoma cellsLuciferase gene expression from the constructs was monitored in the absence and presence of a synthetic m7Gpp cap analog (Figure 2).  Translation of Renillla luciferase mRNA depended on ribosomal binding to the m7Gpp cap of the mRNA, but translation of Photinus luciferase required ribosome recruitment by an IRES in the 5′ UTR of the adjacent gene. Figure 2  Luciferase activity in neuroblastoma cells exposed to m7Gpp cap analogIn a second study, researchers used siRNA to target initiation factor eIF4E, a peptide translated in a cap-dependent manner that facilitates the binding of ribosomes to the m7Gpp cap.  Expression of factor eIF4E, elongation factor eEF2K, and tau v2 protein was assessed in siRNA-treated cells (Figure 3). Figure 3  Western blot of eIF4E, eEF2K, and tau v2 extracted from non-AD cells treated with eIF4E siRNA Adapted from Veo BL, Krushel LA. Translation initiation of the human tau mRNA through an internal ribosomal entry site. J Alzheimers Dis. 2009; 16(2):271-5. -Type 1 diabetes mellitus (T1DM) differs from T2DM in that affected patients stop producing insulin at a young age.  Which of the following is most likely impaired in T1DM?Exocrine function of pancreatic beta cellsEndocrine function of pancreatic beta cellsExocrine function of pancreatic alpha cellsEndocrine function of pancreatic alpha cells

Passage Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common heterogeneous disorders, with a reported incidence of 1 in 1,000 people.  The primary pathology is the development of numerous fluid-filled renal cysts that ultimately expand and severely impair kidney function.  By age 60, approximately 50% of patients with ADPKD progress to end-stage renal disease (ESRD) and kidney failure.  Clinical complications include high blood pressure, urinary tract infections, and cysts in other organs such as the liver and pancreas.Approximately 85% of ADPKD cases are caused by mutations in the PKD1 gene, which codes for the protein polycystin-1 (PC1).  The remaining 15% of cases are caused by mutations in the PKD2 gene, which codes for the protein polycystin-2 (PC2).  The PC2 protein functions as a mechanically activated calcium (Ca²⁺) channel in ciliated renal epithelial cells.  PC1 and PC2 interact via their C-terminal cytoplasmic tails to form a heteromer that modulates intracellular calcium homeostasis.  Disruption of calcium signaling as a result of mutations in either PKD1 or PKD2 is associated with increased cell growth and cyst formation in ADPKD patients.To identify individuals with mutations in PKD1 and PKD2, researchers analyzed phenotypic and genetic data from patients with established renal disease, as shown in Table 1.Table 1  Comparison of Renal Disease Onset and Symptom Severity in Various Patient Groups Next, researchers examined how phenotypic disease severity was affected by both the position of the mutations in PKD1 (5′ or 3′ end) and the type of mutation in PKD1 or PKD2.  In Figure 1, disease severity is given as the cumulative probability of renal survival as a function of age. Figure 1  Probability of maintaining healthy kidney physiology as a function of age, analyzed by mutation type and position Adapted from Cornec-le gall E, Audrézet MP, Chen JM, et al. J Am Soc Nephrol. 2013;24(6):1006-13. -Based on the results presented in Figure 1, which of the following statements about patients with ADPKD are true?Truncating mutations result in more severe ADPKD symptoms than nontruncating mutations.The closer the mutation is to the 5′ end of the gene, the more severe the disease phenotype.PKD1 mutations result in more severe disease pathology than PKD2 mutations.

Passage Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes.  The transition from a hypoxic (low O₂) to an oxic atmosphere (21% O₂) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation.  This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome.  On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, there is a possibility of gene transfer from eukaryotes to prokaryotes.  This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi.  The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS).  ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups, but can accumulate during times of biochemical and environmental stress.  Superoxide (O₂⁻) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H₂O₂) and O₂ gas. Adapted from Bannister JV, Parker MW. The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proc Natl Acad Sci USA. 1985;82(1):149-52. -In anaerobic environments, P. leiognathi can produce energy by using an inorganic final electron acceptor other than oxygen in the electron transport chain.  Under anaerobic conditions, which of the following is NOT active in ponyfish cells but active in P. leiognathi?

Passage Endometriosis is a chronic, treatable disease of the female reproductive tract associated with infertility and pelvic pain in women of reproductive age.  The endometrium, a layer of epithelial tissue that grows and sheds during the menstrual cycle, normally makes up the interior lining of the uterus.  In individuals with endometriosis, endometrial tissue is also found ectopically (outside the uterine cavity).The mechanism by which endometrial tissue successfully implants outside the uterine cavity is thought to be dependent on signaling molecules that promote the formation of blood vessels locally.  One of these signaling molecules is vascular endothelial growth factor (VEGF), a protein that is normally synthesized and secreted by various tissues in response to reduced oxygen levels.  Because VEGF is thought to be implicated in ectopic endometrial tissue growth, researchers hypothesize that the abdominal and pelvic structures of women with endometriosis would contain higher levels of VEGF compared to women without endometriosis.In an experiment, women ages 25-40 were divided into groups based on endometriosis disease status.  Individuals on any type of medication were excluded from the study.  Using minimally invasive surgery, samples of peritoneal fluid were taken from healthy subjects (n = 19) and from subjects diagnosed with endometriosis (n = 30).  An enzyme-linked immunosorbent assay (ELISA) was used to characterize the concentration of VEGF protein present within these samples.In a separate procedure that took place five days after peritoneal fluid collection, uterine endometrial samples were obtained from all subjects and visualized under a microscope to determine the menstrual cycle phase of each participant.  Figure 1 shows the level of VEGF in the abdominal fluid categorized by disease status and menstrual cycle phase. Figure 1  VEGF concentration in sampled peritoneal fluid (Note:  * = p < 0.05; Proliferative phase = stage immediately preceding ovulation; Secretory phase = stage immediately following ovulation) -Infertility in patients with endometriosis can result from ectopic endometrial tissue deposits in all of the following structures EXCEPT:

Passage In 1906, the Nobel Prize in Physiology or Medicine was awarded to two biologists for their groundbreaking work in the field of neuroanatomy.  Despite using the same technique to visualize neurons, the two biologists formed opposing theories regarding nervous system organization.Hypothesis 1:  Reticular TheoryBiologist 1 developed a neural staining technique that was carried out by fixing brain tissue in potassium dichromate (K₂Cr₂O₇) before treating it with silver nitrate (AgNO₃).  A silver chromate (Ag₂CrO₄) precipitate deposited within the neurons, making them appear black.  The silver impregnation method was revolutionary because of two features.  First, only a random subset of neurons within a given tissue sample (~1% of the total cell population) was subject to silver impregnation.  Second, the impregnated cells were stained in their entirety, allowing for visualization of the complete neuron.  Biologist 1 concluded that his findings supported the idea that the nervous system is a single reticulum of fused neural processes that transmit information via cytoplasmic continuity.Hypothesis 2:  Neuron DoctrineBiologist 2 modified the silver impregnation method by first subjecting the brain tissue to two rounds of staining ("double impregnation") rather than one.  Second, where Biologist 1 studied the brain tissue of older animals with myelinated nerve fibers, Biologist 2 employed the silver stain on brain tissue from younger animals, which made it easier for the silver to impregnate the neural processes due to relative lack of myelination.  Biologist 2 reasoned that the nervous system is not a single web of interconnected fibers but rather is made up of discrete neural units that communicate via close, but not continuous, contact.The neuron doctrine was later proven correct with the advent of electron microscopy, which allowed scientists to visualize the synapse (the point of communication between two distinct neurons).  Further investigation allowed researchers to ultimately discover that there are two types of synapses:  electrical and chemical. -Which of the following synaptic events is most likely to contribute to the generation of an action potential in a postsynaptic neuron?

Passage Viruses are known as obligate intracellular parasites because they cannot reproduce independently (outside of a host cell).  Viral genomes (RNA or DNA) are enclosed by a protein shell known as the capsid.  Some viruses, known as enveloped viruses, have an additional phospholipid bilayer that fully envelops the capsid.  Viruses that lack this outermost phospholipid covering are labeled non-enveloped.  The entry of either enveloped or nonenveloped viruses into a host cell enables the production of viral proteins and the subsequent assembly of viral progeny.Immune cells (B and T cells) protect host organisms against viral infections by recognizing viral particles (antigens) as foreign.  In response to the recognition of foreign particles, antigen-presenting cells display cleaved antigens (also known as antigen fragments) on their surface, which enable the activation of other immune cells that stimulate an immune response, including the production of neutralizing antibodies.An example of an infectious disease is viral hepatitis (inflammation of the liver), which can be caused by any one of five distinct hepatitis viruses (A-E) that infect hepatocytes (liver cells).  HAV, HCV, and HEV contain positive-sense, single-stranded RNA (+ssRNA) as genetic material.  In contrast, HBV has a partially double-stranded circular DNA genome.  HDV is unique because it has a negative-sense, single-stranded RNA (−ssRNA) genome.Researchers hypothesized that synthetic antibodies could be used to treat viral hepatitis in patients with compromised immune systems.  Fluorescent antibodies, designed to bind the capsid proteins of the five hepatitis viruses, were added to separate wells containing each type of virus.  The fluorescence assay that was performed detected only bound antibodies in media with HAV or HEV. Adapted from Shin EC, Sung PS, Park SH. Immune responses and immunopathology in acute and chronic viral hepatitis. Nat Rev Immunol. 2016;16(8):509-23. -Assume bacteria are infected with a vector containing HBV genetic material.  This vector has been shown to introduce its genomic content into bacterial cells without fully gaining entry into the bacterial cytoplasm.  Based on this information, the vector is most likely a:

Passage The Namibian cheetah (Acinonyx jubatus jubatus), the world's fastest land-dwelling animal, hunts across the African grasslands using high-speed sprints.  Cheetahs possess unique anatomical features specialized for speed, including an aerodynamic skull, elongated legs, and an enlarged heart.  These physiological adaptations have occurred over time and allow cheetahs to achieve speeds greater than 100 km/h.  However, due to their small body and slender limbs, cheetahs (50-64 kg) are limited with regard to the size of the animals they can successfully hunt and therefore typically prey on slightly smaller species that inhabit open grasslands.It is believed that around 12,000 years ago, Namibian cheetahs experienced an environmental catastrophe that drastically reduced their population.  The small number of surviving cheetahs began mating with one another, which led to severe inbreeding depression.  Over time, the offspring of subsequent generations exhibited decreased fitness, reduced fecundity (number of offspring), and sustained a loss of major histocompatibility complex (MHC) allele diversity.  To study if low MHC diversity correlates with low fitness in cheetahs, scientists analyzed the genetic diversity and disease prevalence in a group of free-ranging and captive male cheetahs.Experiment 1MHC allele genotyping and analysis was performed in wild (free-ranging) and captive male cheetahs (Figure 1).  Because MHC diversity is strongly correlated with genetic diversity, MHC diversity is used as a marker for an organism's overall genetic variability. Figure 1  Evaluation of heterozygosity in (A) wild and (B) captive male cheetahs born between 1976 and 2007 (Note:  Each data point represents a single male cheetah.)Experiment 2Disease prevalence was evaluated in wild and captive male cheetahs by quantitative analysis of symptoms relating gastrointestinal, liver, and kidney disease.  The percentages of cheetahs affected by each disease are shown in Figure 2. Figure 2  Evaluation of disease prevalence in wild and captive male cheetahsExperiment 3Fecal cortisol levels were noninvasively evaluated in wild and captive cheetahs over a period of 6 months (Figure 3). Figure 3  Average fecal cortisol concentrations of wild and captive male cheetahs Adapted from Castro-prieto A, Wachter B, Sommer S. Cheetah paradigm revisited: MHC diversity in the world's largest free-ranging population. Mol Biol Evol. 2011;28(4):1455-68. -The cheetahs that survived the environmental catastrophe that occurred 12,000 years ago are at greater risk of extinction because of: