A Survey of Clinical Immersion Experiences in Biomedical Engineering

Immersion in clinical environments is generally believed to be a valuable experiential learning opportunity for students in biomedical engineering, both at the undergraduate and the graduate level. Immersion is believed to foster an understanding of medical culture, clinical operations, interprofessional collaboration, and oftentimes allows students to either identify unmet clinical needs. The National Institutes of Health supports efforts through grants to incorporate these clinical immersion programs into biomedical engineering curricula, and this has potentially facilitated an expansion of these programs across the United States. Unknown is how common clinical immersion experiences are in biomedical engineering programs, in general how these are organized and executed, and their goals. We conducted a survey of biomedical engineering programs to learn how many programs offer clinical immersion experiences, over what timeframe and in what formats, and what is known about their goals and learning outcomes. We present here the results of that survey which includes 52 clinical immersion courses and programs, 14 of which either are or were previously funded by the NIH. Each of these courses or programs engages, on average, about 27 students per year, but range in size from 2 to 160. The duration of the immersion experience likewise varies greatly from 3 to 400 h. The objectives of these programs are mostly to identify problems, develop engineering solutions to problems, or to learn clinical procedures. Despite the impressive breadth of experiences revealed by this survey, we still know relatively little about their impact on student learning, motivation, identity, or career path. Desired outcomes and assessment strategies must be better aligned with the structure of the clinical immersion experiences themselves if we are to determine if they are effective in meeting those outcomes, including those of professional preparation.

S-nitrosylation of cytoskeletal proteins

Nitric oxide has pronounced effects on cellular functions normally associated with the cytoskeleton, including cell motility, shape, contraction, and mitosis. Protein S-nitrosylation, the covalent addition of a NO group to a cysteine sulfur, is a signaling pathway for nitric oxide that acts in parallel to cyclic guanosine monophosphate (cGMP), but is poorly studied compared to the latter. There is growing evidence that S-nitrosylation of cytoskeletal proteins selectively alters their function. We review that evidence, and find that S-nitrosylation of cytoskeletal targets has complementary but distinct effects to cyclic-GMP in motile and contractile cells-promoting cell migration, and biasing muscle contraction toward relaxation. However, the effects of S-nitrosylation on a host of cytoskeletal proteins and functions remains to be explored.

Design and implementation of a student-taught course on research in regenerative medicine

In the Undergraduate School of Engineering and Applied Sciences (SEAS) at the University of Virginia (UVa), there are few opportunities for undergraduate students to teach, let alone develop, an introductory course for their major. As two undergraduate engineering students (D. N. Tavakol and C. J. Broshkevitch), we were among the first students to take advantage of a new initiative at UVa SEAS to offer student-led courses. As part of this new program, we designed a 1000-level, 1-credit, pass-fail course entitled Introduction to Research in Regenerative Medicine. During a student's first year at the University, opportunities to build research skills and gain exposure to topics within the field of the biomedical sciences are relatively rare, so, to fill this gap, we focused our course on teaching primarily freshman undergraduate students how to synthesize and contextualize scientific literature, covering both basic science and clinical applications. At the end of the course, students self-reported increased confidence in reading and discussing scientific papers and review articles. The critical impact of this course lies not only in an early introduction to the popularized field of regenerative medicine, but also encouragement for younger students to participate in research early on and to appreciate the value of interdisciplinary interactions. The teaching model can be extended for implementation of student-taught introductory courses across diverse undergraduate major tracks at an institution.

Direct measurement of cortical force generation and polarization in a living parasite

Apicomplexa is a large phylum of intracellular parasites that are notable for the diseases they cause, including toxoplasmosis, malaria, and cryptosporidiosis. A conserved motile system is critical to their life cycles and drives directional gliding motility between cells, as well as invasion of and egress from host cells. However, our understanding of this system is limited by a lack of measurements of the forces driving parasite motion. We used a laser trap to measure the function of the motility apparatus of living Toxoplasma gondii by adhering a microsphere to the surface of an immobilized parasite. Motion of the microsphere reflected underlying forces exerted by the motile apparatus. We found that force generated at the parasite surface begins with no preferential directionality but becomes directed toward the rear of the cell after a period of time. The transition from nondirectional to directional force generation occurs on spatial intervals consistent with the lateral periodicity of structures associated with the membrane pellicle and is influenced by the kinetics of actin filament polymerization and cytoplasmic calcium. A lysine methyltransferase regulates both the magnitude and polarization of the force. Our work provides a novel means to dissect the motile mechanisms of these pathogens.

Design, development, and evaluation of a novel retraction device for gallbladder extraction during laparoscopic cholecystectomy

BACKGROUND

A source of frustration during laparoscopic cholecystectomy involves extraction of the gallbladder through port sites smaller than the gallbladder itself. We describe the development and testing of a novel device for the safe, minimal enlargement of laparoscopic port sites to extract large, stone-filled gallbladders from the abdomen.

METHODS

The study device consists of a handle with a retraction tongue to shield the specimen and a guide for a scalpel to incise the fascia within the incision. Patients enrolled underwent laparoscopic cholecystectomy. Gallbladder extraction was attempted. If standard measures failed, the device was implemented. Extraction time and device utility scores were recorded for each patient. Patients returned 3-4 weeks postoperatively for assessment of pain level, cosmetic effect, and presence of infectious complications.

RESULTS

Twenty (51 %) of 39 patients required the device. Average extraction time for the first eight patients was 120 s. After interim analysis, an improved device was used in 12 patients and average extraction time was 24 s. There were no adverse events. Postoperative pain ratings and incision cosmesis were comparable between patients with and without use of the device.

CONCLUSION

The study device enables safe and rapid extraction of impacted gallbladders through the abdominal wall.

A High-Throughput Technique Reveals the Load- and Site Density-Dependent Kinetics of E-Selectin

The kinetics of bond rupture between receptors and ligand are critically dependent on applied mechanical force. Force spectroscopy of single receptor-ligand pairs to measure kinetics is a laborious and time-consuming process that is generally performed using individual force probes and making one measurement at a time when typically hundreds of measurements are needed. A high-throughput approach is thus desirable. We report here a magnetic bond puller that provides high-throughput measurements of single receptor-ligand bond kinetics. Electromagnets are used to apply pN tensile and compressive forces to receptor-coated magnetic microspheres while monitoring their contact with a ligand-coated surface. Bond lifetimes and the probability of forming a bond are measured via videomicroscopy, and the data are used to determine the load dependent rates of bond rupture and bond formation. The approach is simple, customizable, relatively inexpensive, and can make dozens of kinetic measurements simultaneously. We used the device to investigate how compressive and tensile forces affect the rates of formation and rupture, respectively, of bonds between E-selectin and sialyl Lewisa (sLea), a sugar on P-selectin glycoprotein ligand-1 to which selectins bind. We confirmed earlier findings of a load-dependent rate of bond formation between these two molecules, and that they form a catch-slip bond like other selectin family members. We also make the novel observation of an "ideal" bond in a highly multivalent system of this receptor-ligand pair.

Force spectroscopy reveals multiple "closed states" of the muscle thin filament

Tropomyosin (Tm) plays a critical role in regulating the contraction of striated muscle. The three-state model of activation posits that Tm exists in three positions on the thin filament: "blocked" in the absence of calcium when myosin cannot bind, "closed" when calcium binds troponin and Tm partially covers the myosin binding site, and "open" after myosin binding forces Tm completely off neighboring sites. However, we recently showed that actin filaments decorated with phosphorylated Tm are driven by myosin with greater force than bare actin filaments. This result cannot be explained by simple steric hindrance and suggests that Tm may have additional effects on actin-myosin interactions. We therefore tested the hypothesis that Tm and its phosphorylation state affect the rate at which single actin-myosin bonds form and rupture. Using a laser trap, we measured the time necessary for the first bond to form between actin and rigor heavy meromyosin and the load-dependent durations of those bonds. Measurements were repeated in the presence of subsaturating myosin-S1 to force Tm from the closed to the open state. Maximum bond lifetimes increased in the open state, but only when Tm was phosphorylated. While the frequency with which bonds formed was extremely low in the closed state, when a bond did form it took significantly less time to do so than with bare actin. These data suggest there are at least two closed states of the thin filament, and that Tm provides additional points of contact for myosin.

Direct regulation of striated muscle myosins by nitric oxide and endogenous nitrosothiols

BACKGROUND

Nitric oxide (NO) has long been recognized to affect muscle contraction, both through activation of guanylyl cyclase and through modification of cysteines in proteins to yield S-nitrosothiols. While NO affects the contractile apparatus directly, the identities of the target myofibrillar proteins remain unknown. Here we report that nitrogen oxides directly regulate striated muscle myosins.

PRINCIPAL FINDINGS

Exposure of skeletal and cardiac myosins to physiological concentrations of nitrogen oxides, including the endogenous nitrosothiol S-nitroso-L-cysteine, reduced the velocity of actin filaments over myosin in a dose-dependent and oxygen-dependent manner, caused a doubling of force as measured in a laser trap transducer, and caused S-nitrosylation of cysteines in the myosin heavy chain. These biomechanical effects were not observed in response to S-nitroso-D-cysteine, demonstrating specificity for the naturally occurring isomer. Both myosin heavy chain isoforms in rats and cardiac myosin heavy chain from human were S-nitrosylated in vivo.

SIGNIFICANCE

These data show that nitrosylation signaling acts as a molecular "gear shift" for myosin--an altogether novel mechanism by which striated muscle and cellular biomechanics may be regulated.

The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit

Tropomyosin (Tm) is one of the major phosphoproteins comprising the thin filament of muscle. However, the specific role of Tm phosphorylation in modulating the mechanics of actomyosin interaction has not been determined. Here we show that Tm phosphorylation is necessary for long-range cooperative activation of myosin binding. We used a novel optical trapping assay to measure the isometric stall force of an ensemble of myosin molecules moving actin filaments reconstituted with either natively phosphorylated or dephosphorylated Tm. The data show that the thin filament is cooperatively activated by myosin across regulatory units when Tm is phosphorylated. When Tm is dephosphorylated, this "long-range" cooperative activation is lost and the filament behaves identically to bare actin filaments. However, these effects are not due to dissociation of dephosphorylated Tm from the reconstituted thin filament. The data suggest that end-to-end interactions of adjacent Tm molecules are strengthened when Tm is phosphorylated, and that phosphorylation is thus essential for long range cooperative activation along the thin filament.

Peroxynitrite inhibits myofibrillar protein function in an in vitro assay of motility

We determined the effects of peroxynitrite (ONOO-) on cardiac myosin, actin, and thin filaments in order to more clearly understand the impact of this reactive compound in ischemia/reperfusion injury and heart failure. Actin filaments, native thin filaments, and alpha-cardiac myosin from rat hearts were exposed to ONOO- in the presence of 2 mM bicarbonate. Filament velocities over myosin, calcium sensitivity, and relative force generated by myosin were assessed in an in vitro motility assay in the absence of reducing agents. ONOO- concentrations > or =10 microM significantly reduced the velocities of thin filaments or bare actin filaments over alpha-cardiac myosin when any of these proteins were exposed individually. These functional deficits were linearly related to the degree of tyrosine nitration, with myosin being the most sensitive. However, at 10 microM ONOO- the calcium sensitivity of thin filaments remained unchanged. Cotreatment of myosin and thin filaments, analogous to the in vivo situation, resulted in a significantly greater functional deficit. The load supported by myosin after ONOO- exposure was estimated using mixtures experiments to be increased threefold. These data suggest that nitration of myofibrillar proteins can contribute to cardiac contractile dysfunction in pathologic states in which ONOO- is liberated.

Enhancement of L-selectin, but not P-selectin, bond formation frequency by convective flow

L-selectin-mediated leukocyte rolling has been proposed to require a high rate of bond formation compared to that of P-selectin to compensate for its much higher off-rate. To test this hypothesis, a microbead system was utilized to measure relative L-selectin and P-selectin bond formation rates on their common ligand P-selectin glycoprotein ligand-1 (PSGL-1) under shear flow. Using video microscopy, we tracked selectin-coated microbeads to detect the formation frequency of adhesive tether bonds. From velocity distributions of noninteracting and interacting microbeads, we observed that tether bond formation rates for P-selectin on PSGL-1 decreased with increasing wall shear stress, from 0.14 +/- 0.04 bonds/microm at 0.2 dyn/cm(2) to 0.014 +/- 0.003 bonds/microm at 1.0 dyn/cm(2). In contrast, L-selectin tether bond formation increased from 0.017 +/- 0.005 bonds/microm at 0.2 dyn/cm(2) to 0.031 +/- 0.005 bonds/microm at 1.0 dyn/cm(2). L-selectin tether bond formation rates appeared to be enhanced by convective transport, whereas P-selectin rates were inhibited. The transition force for the L-selectin catch-slip transition of 44 pN/bond agreed well with theoretical models (Pereverzev et al. 2005. Biophys. J. 89:1446-1454). Despite catch bond behavior, hydrodymanic shear thresholding was not detected with L-selectin beads rolling on PSGL-1. We speculate that shear flow generated compressive forces may enhance L-selectin bond formation relative to that of P-selectin and that L-selectin bonds with PSGL-1 may be tuned for the compressive forces characteristic of leukocyte-leukocyte collisions during secondary capture on the blood vessel wall. This is the first report, to our knowledge, comparing L-selectin and P-selectin bond formation frequencies in shear flow.

Alterations to myofibrillar protein function in nonischemic regions of the heart early after myocardial infarction

Remote-zone left ventricular dysfunction (LVD) contributes to global reductions in contractile function after localized myocardial infarction (MI). However, the molecular mechanisms underlying this form of LVD are not clear. This study tested the hypothesis that myofibrillar protein function is directly affected in remote-zone LVD early after MI. Cardiac myosin and native thin filaments were purified from mouse myocardium taken from both the nonnecrotic zone adjacent to and the nonischemic zone remote from an infarct induced by 1 h of coronary occlusion followed by 24 h of reperfusion. Thin filament velocities were measured using the in vitro motility assay. Results showed that overall function was significantly reduced in samples from both the adjacent (43 +/- 12% of control, n = 7) and remote (53 +/- 8% of control, n = 13) zones when compared with control proteins (P < 0.05). Myosin from the remote zone propelled control thin filaments at reduced velocities similar to those measured above. In contrast, the Ca(2+) sensitivity of remote-zone thin filaments over control myosin was unchanged from control thin filaments (half-maximal at pCa 6.32 +/- 0.08 and 6.27 +/- 0.06, respectively) but showed a 20% increase in velocity at saturating Ca(2+) that parallels an increase in tropomyosin phosphorylation. Myosin dysfunction may be related to oxidation of cysteines in the myosin heavy chains or carbonylation of myosin binding protein-C. We hypothesize that phosphorylation of tropomyosin may serve a compensatory role, augmenting contraction during periods of oxidative stress when myosin function is compromised.

Mechanics of actomyosin bonds in different nucleotide states are tuned to muscle contraction

Muscle contraction and many other cell movements are driven by cyclic interactions between actin filaments and the motor enzyme myosin. Conformational changes in the actin-myosin binding interface occur in concert with the binding of ATP, binding to actin, and loss of hydrolytic by-products, but the effects of these conformational changes on the strength of the actomyosin bond are unknown. The force-dependent kinetics of the actomyosin bond may be particularly important at high loads, where myosin may detach from actin before achieving its full power stroke. Here we show that over a physiological range of rapidly applied loads, actomyosin behaves as a "catch" bond, characterized by increasing lifetimes with increasing loads up to a maximum at approximately 6 pN. Surprisingly, we found that the myosin-ADP bond is possessed of longer lifetimes under load than rigor bonds, although the load at which bond lifetime is maximal remains unchanged. We also found that actomyosin bond lifetime is ultimately dependent not only on load, but loading history as well. These data suggest a complex relationship between the rate of actomyosin dissociation and muscle force and shortening velocity. The 6-pN load for maximum bond lifetime is near the force generated by a single myosin molecule during isometric contraction. This raises the possibility that all catch bonds between load-bearing molecules are "mechanokinetically" tuned to their physiological environment.

"Shrink wrapping" lectures: teaching cell and molecular biology within the context of human pathologies

Students are most motivated and learn best when they are immersed in an environment that causes them to realize why they should learn. Perhaps nowhere is this truer than when teaching the biological sciences to engineers. Transitioning from a traditionally mathematics-based to a traditionally knowledge-based pedagogical style can challenge student learning and engagement. To address this, human pathologies were used as a problem-based context for teaching knowledge-based cell biological mechanisms. Lectures were divided into four modules. First, a disease was presented from clinical, economic, and etiological standpoints. Second, fundamental concepts of cell and molecular biology were taught that were directly relevant to that disease. Finally, we discussed the cellular and molecular basis of the disease based on these fundamental concepts, together with current clinical approaches to the disease. The basic science is thus presented within a "shrink wrap" of disease application. Evaluation of this contextual technique suggests that it is very useful in improving undergraduate student focus and motivation, and offers many advantages to the instructor as well.

The tail of myosin reduces actin filament velocity in the in vitro motility assay

It has been observed that heavy meromyosin (HMM) propels actin filaments to higher velocities than native myosin in the in vitro motility assay, yet the reason for this difference has remained unexplained. Since the major difference between these two proteins is the presence of the tail in native myosin, we tested the hypothesis that unknown interactions between actin and the tail (LMM) slow motility in native myosin. Chymotryptic HMM and LMM were mixed in a range of molar ratios (0-5 LMM/HMM) and compared to native rat skeletal myosin in the in vitro motility assay at 30 degrees C. Increasing proportions of LMM to HMM slowed actin filament velocities, becoming equivalent to native myosin at a ratio of 3 LMM/HMM. NH4+ -ATPase assays demonstrated that HMM concentrations on the surface were constant and independent of LMM concentration, arguing against a simple displacement mechanism. Relationships between velocity and the number of available heads suggested that the duty cycle of HMM was not altered by the presence of LMM. HMM prepared with a lower chymotrypsin concentration and with very short digestion times moved actin at the same high velocity. The difference between velocities of actin filament propelled by HMM and HMM/LMM decreased with increasing ionic strength, suggesting that ionic bonds between myosin tail and actin filaments may play a role in slowing filament velocity. These data suggest the high velocities of actin filaments over HMM result from the absence of drag generated by the myosin tail, and not from proteolytic nicking of the motor domain.

Creating multiple time-shared laser traps with simultaneous displacement detection using digital signal processing hardware

We present a design for implementing multiple laser traps for single-molecule studies through time-sharing using commercially available digital signal processing hardware in a computer running a standard multitasking operating system. This design enables four to six independent laser traps with a visitation frequency of 10,000s(-1)trap(-1) and a timing jitter of +/-0.5 micros to be created. The design also achieves nanometer-resolution detection of displacement in all of the traps simultaneously via back focal-plane interferometry and only a single quadrant photodiode detector. Practical design considerations and limitations together with the use of fiberlasers in laser traps are discussed. Using this device, the mechanokinetics of multiple molecular motors or adhesion proteins may be measured simultaneously. We present the example biological application of two kinesin-coated beads in separate traps moving on different portions of a microtubule.

The molecular mechanics of P- and L-selectin lectin domains binding to PSGL-1

A laser trap was used to compare the load-dependent binding kinetics between truncated P- and L-selectin to their natural ligand, P-selectin glycoprotein ligand-1 (PSGL-1) over the predicted physiological range of loading rates. Human PSGL-1 was covalently coupled to polystyrene beads. Chimeric selectins were adsorbed to nitrocellulose-coated glass beads on a coverslip. A PSGL-1 bead was held in a laser trap and touched to a vertical surface of the glass bead, allowing a bond to form between selectin and ligand. The surface was moved away from the microsphere, applying load at a constant rate until bond rupture. Rupture force for both selectins increased with loading rate, but significant differences in rupture force between P- and L-selectin were observed only above 460 pN/s. These data are best represented as two energy barriers to unbinding, with the transition from the low to high loading rate regime at 260-290 pN/s. The data also allow the first estimate of a two-dimensional specific on-rate for binding of these two selectins to their natural ligand (1.7 microm2/s). These data suggest that P- and L-selectin lectin domains have very similar kinetics under physiological conditions.

Teaching peer review and the process of scientific writing

Many undergraduate and graduate students understand neither the process of scientific writing nor the significance of peer review. In response, some instructors have created writing assignments that teach or mimic parts of the scientific publishing process. However, none fully reproduced peer review and revision of papers together with the writing and publishing process from research to final, accepted draft. In addition, most have been instituted at the graduate rather than undergraduate level. We present a detailed method for teaching undergraduate students the full scientific publishing process, including anonymous peer review, during the process of writing a "term paper." The result is a review article in the format for submission to a major scientific journal. This method has been implemented in the course Cell and Molecular Biology for Engineers at the University of Virginia. Use of this method resulted in improved grades, much higher quality in the final manuscript, greater objectivity in grading, and improved understanding of the importance of peer review.

Quantitative comparison of algorithms for tracking single fluorescent particles

Single particle tracking has seen numerous applications in biophysics, ranging from the diffusion of proteins in cell membranes to the movement of molecular motors. A plethora of computer algorithms have been developed to monitor the sub-pixel displacement of fluorescent objects between successive video frames, and some have been claimed to have "nanometer" resolution. To date, there has been no rigorous comparison of these algorithms under realistic conditions. In this paper, we quantitatively compare specific implementations of four commonly used tracking algorithms: cross-correlation, sum-absolute difference, centroid, and direct Gaussian fit. Images of fluorescent objects ranging in size from point sources to 5 microm were computer generated with known sub-pixel displacements. Realistic noise was added and the above four algorithms were compared for accuracy and precision. We found that cross-correlation is the most accurate algorithm for large particles. However, for point sources, direct Gaussian fit to the intensity distribution is the superior algorithm in terms of both accuracy and precision, and is the most robust at low signal-to-noise. Most significantly, all four algorithms fail as the signal-to-noise ratio approaches 4. We judge direct Gaussian fit to be the best algorithm when tracking single fluorophores, where the signal-to-noise is frequently near 4.

The light chain binding domain of expressed smooth muscle heavy meromyosin acts as a mechanical lever

Structural data led to the proposal that the molecular motor myosin moves actin by a swinging of the light chain binding domain, or "neck." To test the hypothesis that the neck functions as a mechanical lever, smooth muscle heavy meromyosin (HMM) mutants were expressed with shorter or longer necks by either deleting or adding light chain binding sites. The mutant HMMs were characterized kinetically and mechanically, with emphasis on measurements of unitary displacements and forces in the laser trap assay. Two shorter necked constructs had smaller unitary step sizes and moved actin more slowly than WT HMM in the motility assay. A longer necked construct that contained an additional essential light chain binding site exhibited a 1.4-fold increase in the unitary step size compared with its control. Kinetic changes were also observed with several of the constructs. The mutant lacking a neck produced force at a somewhat reduced level, while the force exerted by the giraffe construct was higher than control. The single molecule displacement and force data support the hypothesis that the neck functions as a rigid lever, with the fulcrum for movement and force located at a point within the motor domain.

Two heads of myosin are better than one for generating force and motion

Several classes of the myosin superfamily are distinguished by their "double-headed" structure, where each head is a molecular motor capable of hydrolyzing ATP and interacting with actin to generate force and motion. The functional significance of this dimeric structure, however, has eluded investigators since its discovery in the late 1960s. Using an optical-trap transducer, we have measured the unitary displacement and force produced by double-headed and single-headed smooth- and skeletal-muscle myosins. Single-headed myosin produces approximately half the displacement and force (approximately 6 nm; 0.7 pN) of double-headed myosin (approximately 10 nm; 1.4 pN) during a unitary interaction with actin. These data suggest that muscle myosins require both heads to generate maximal force and motion.

The molecular mechanics of smooth muscle myosin

Smooth muscle cells are capable of generating forces comparable to those of skeletal muscle cells but with far less myosin, the molecular motor that powers muscle contraction. This unique capability may be inherent to the myosin molecule. We have directly characterized the molecular mechanics of smooth muscle myosin using new technologies developed to measure the forces generated by these proteins. The data help explain the differences in force and velocity in whole smooth and skeletal muscles.

Smooth muscle and skeletal muscle myosins produce similar unitary forces and displacements in the laser trap

Purified smooth muscle myosin in the in vitro motility assay propels actin filaments at 1/10 the velocity, yet produces 3-4 times more force than skeletal muscle myosin. At the level of a single myosin molecule, these differences in force and actin filament velocity may be reflected in the size and duration of single motion and force-generating events, or in the kinetics of the cross-bridge cycle. Specifically, an increase in either unitary force or duty cycle may explain the enhanced force-generating capacity of smooth muscle myosin. Similarly, an increase in attached time or decrease in unitary displacement may explain the reduced actin filament velocity of smooth muscle myosin. To discriminate between these possibilities, we used a laser trap to measure unitary forces and displacements from single smooth and skeletal muscle myosin molecules. We analyzed our data using mean-variance analysis, which does not rely on scoring individual events by eye, and emphasizes periods in the data with constant properties. Both myosins demonstrated multiple but similar event populations with discrete peaks at approximately +11 and -11 nm in displacement, and 1.5 and 3.5 pN in force. Mean attached times for smooth muscle myosin were longer than for skeletal-muscle myosin. These results explain much of the difference in actin filament velocity between these myosins, and suggest that an increased duty cycle is responsible for the enhanced force-generating capacity of smooth over skeletal-muscle myosin.

Actin filament mechanics in the laser trap

Numerous biological processes, including muscular contraction, depend upon the mechanical properties of actin filaments. One such property is resistance to bending (flexural rigidity, EI). To estimate EI, we attached the ends of fluorescently labelled actin filaments to two microsphere 'handles' captured in independent laser traps. The positions of the traps were manipulated to apply a range of tensions (0-8 pN) to the filaments via the microsphere handles. With increasing filament tension, the displacement of the microspheres was inconsistent with a microsphere-filament system that is rigid. We maintain that this inconsistency is due to the microspheres rotating in the trap and the filaments bending near either attachments to accommodate this rotation. Fitting the experimental data to a simple model of this phenomena, we estimate actin's EI to be approximately 15 x 10(3) pNnm2, a value within the range of previously reported results, albeit using a novel method. These results both: support the idea that actin filaments are more compliant than historically assumed; and, indicate that without appropriately pretensioning the actin filament in similar laser traps, measurements of unitary molecular events (e.g. myosin displacement) may be significantly underestimated.

The mechanics of arteriole-tissue interaction

Arterioles are embedded in the extensive connective tissue matrix of the interstitium. Mechanical interactions with the interstitium may affect the length-tension characteristics of arterioles, and thus affect their reactivity. However, no studies have adequately characterized the coupling between arterioles and the interstitium or investigated how the interstitium might change the physiological expression of arterioles. Therefore, the goal of this project was to investigate the mechanical interactions between arterioles and the interstitium and then to predict the physiological consequences of these interactions. We measured in situ the mechanical coupling of arterioles to the interstitium, the mechanical properties of the interstitium, and the structure of the interstitium in the hamster cheek pouch. We demonstrated that there are mechanical interactions between arterioles and the interstitium that are mediated both through direct connections and through the movement of extracellular fluid through the connective tissue network. We also found that the elastic modulus of the interstitium increases in the vicinity of the arteriole. Finally, both the mechanical coupling of arterioles to the interstitium and the mechanical properties of the interstitium are explained by the structure of the connective tissue matrix. The arterioles appear to be connected to adjacent fibroblasts and fibrocytes by collagen fibrils. These cells are in turn connected to the fiber matrix of the interstitium. Furthermore, the presence of these cells may explain the mechanical heterogeneity of the interstitium. We propose that the physiological role of the interstitium surrounding arterioles is to protect arterioles from stretching and deformation of the tissue while allowing these vessels to constrict freely.

Locomotive forces produced by single leukocytes in vivo and in vitro

We report here the first time-resolved measurements of the forces produced during the migration of single leukocytes in vivo and in vitro. Pulmonary macrophages from hamsters and mice, in vitro, and Nembutal (pentobarbital sodium)-anesthetized hamster neutrophils, in vivo, generated maximum locomotive forces ranging from 1.9 to 10.7 nN or tenths of microdynes. Force production was periodic and correlated with the length of the leading lamellipod but not with generalized cell ruffling. Although the extension of the leading lamella is critical to locomotive force generation, these direct measurements suggest that lamellar extension may not arise from the same contractile processes driving forward motion of the cell mass. Indeed, cell ruffling, lamellar extension, and locomotive force generation may be differentially controlled and have different origins. This technique may be extended to test numerous hypotheses of how these and other nonmuscle cells crawl.

Smooth muscle myosin: a high force-generating molecular motor

Smooth muscle generates as much force per cross sectional area of muscle as skeletal muscle with only one-fifth the myosin content. Although this apparent difference could be explained at the tissue or cellular level, it is possible that at the molecular level smooth muscle cross-bridges generate greater average force than skeletal muscle cross-bridges. To test this hypothesis, we used an in vitro motility assay (VanBuren et al., 1994) in which either chicken thiophosphorylated gizzard smooth or pectoralis skeletal muscle monomeric myosin is adhered to a nitrocellulose surface. A fluorescently labeled actin filament, attached to an ultracompliant (50-200 nm/pN) glass microneedle, is brought in contact with the myosin surface. Isometric force, being generated by myosin cross-bridges pulling on the attached actin filament, is calculated from the extent to which the calibrated microneedle is deflected. By measuring the density of myosin adhered to the surface, we estimated the number of myosin cross-bridges that are able to interact with a length of actin filament in contact with the myosin surface. In a direct comparison between smooth and skeletal muscle myosin, the average force per cross-bridge was 0.8 and 0.2 pN, respectively. Surprisingly, smooth muscle myosin generates approximately 4 times greater average force per cross-bridge head than skeletal muscle myosin. Because average isometric force is the product of the cross-bridge unitary force and duty cycle, we are presently using a laser optical trap in an attempt to measure unitary events from single myosin molecules. This approach should allow us to determine whether an increase in unitary force, duty cycle, or both contribute to smooth muscle myosin's enhanced force-generating capacity compared with skeletal muscle myosin.

A novel remote-sensing isometric force transducer for micromechanics studies

We have developed an innovative transducer for measuring force with femtonewton-to-micronewton resolution in biological systems. A magnetic microsphere is attached to the specimen being studied and is positioned between two electromagnets. Video microscopy and edge detection are used to monitor small movements of the microsphere that occur when the specimen generates force. An automatic control system adjusts the current through the electromagnets to keep the microsphere stationary. Measured force is a linear function of this current. This transducer is unique in its combination of sensitivity and isometric properties and its ability to measure force without direct connections to the specimen. That is, the transducer is "remote sensing" and can measure force through intervening membrane or tissue. The transducer is isometric at steady state to the limit at which displacement of the microsphere can be resolved, which can be as low as 19 nm. The completed system is being used to study the mechanics of interstitial connective tissue but may also be used to study molecular generation of force.