Possible risk factors for increased suicide following bariatric surgery

OBJECTIVE

There is a growing research literature suggesting that there may be elevated risk of suicide following bariatric surgery. Most of the data reported thus far has been cross-sectional and observational, and very little is known about the possible specific causal variables involved.

DESIGN AND METHODS

The purpose of this report is to review this literature and to review possible risk factors for increased suicidal risk following bariatric surgery, to delineate future research directions.

RESULTS

First a variety of medical, biological, and genetic factors, including the persistence or recurrence of medical comorbidities after bariatric surgery, the disinhibition and impulsivity secondary to changes in the absorption of alcohol, hypoglycemia, as well as pharmacokinetic changes that may affect the absorption of various medications including antidepressant medications are reviewed. Also reviewed are possible mediating factors involving changes in various peptidergic systems such as GLP-1 and Ghrelin. A number of psychosocial issues that might be involved are discussed, including lack of improvement in quality of life after surgery, continued or recurrent physical mobility restrictions, persistence or recurrence of sexual dysfunction and relationship problems, low self-esteem, and a history of child maltreatment. Inadequate weight loss or weight regain are also discussed.

CONCLUSIONS

A number of possible contributing factors have been identified. Possible theoretical models involved and directions for research are suggested.

Evidence for the essential role of myosin subfragment-2 in the ATP-dependent actin-myosin sliding in muscle contraction

The role of myosin subfragment-2 (myosin S-2) in muscle contraction was studied by using an in vitro motility assay system in which the ATP-dependent sliding between myosin-coated polystyrene beads and actin filament arrays (actin cables) of giant algal cells were recorded under constant external loads provided with a centrifuge microscope. With antibody to myosin S-2 below 0.3 mg/ml, the maximum "isometric" force generated by myosin molecules on the bead decreased markedly, but the unloaded bead-sliding velocity along actin cables did not change appreciably, indicating a decrease in the number of myosin molecules interacting with actin cables. The antibody at 0.3-1.5 mg/ml decreased not only the maximum isometric force, but also the unloaded bead-sliding velocity in a dose-dependent manner. With the antibody at 1.5-3 mg/ml, the beads eventually stopped moving to remain attached to actin cables. These beads could be readily detached from actin cables with very small centrifugal forces, indicating very weak actin-myosin linkages. The antibody had no effect on rigor actin-myosin linkages formed before the antibody application. These results are consistent with the view that myosin S-2 plays an essential role in muscle contraction.

Evidence for bidirectional functional communication between myosin subfragments 1 and 2 in skeletal muscle fibers

It has been shown that, in glycerinated skeletal muscle fibers, polyclonal antibody directed against myosin subfragment 2 (S-2) eliminates Ca(2+)-activated isometric force development, while Mg-ATPase activity of the fibers remains unchanged. To further explore possible involvement of myosin S-2 in muscle contraction, we studied the effect of anti-S-2 antibody on rigor linkage formation in the fibers. It was found that the antibody inhibited development of rigor force in a dose- and time-dependent manner, without changing the relation between muscle fiber stiffness and force. If, however, the antibody was applied after development of rigor force, it had no effect on both the stiffness and force. These results strongly suggest bidirectional functional communication between myosin subfragments 1 and 2 in the fibers.

Effect of cross-linking on the contractile behavior of myofibrils

When rabbit psoas myofibrils in rigor are cross-linked with DMS (dimethyl suberimidate) for various periods of time, they contract on activation to a final sarcomere spacing of 1.3-1.5 microns. This behavior is observed out to 100 min cross-linking time (2 mg/ml DMS; 10 degrees C). Over the next 100 min of cross-linking, the sarcomere spacing, following activation and contraction, gradually increases and finally plateaus near its initial (rigor) value. We also determined the unloaded shortening velocity of the cross-linked myofibrils using an inverted microscope equipped with a video camera. Following photo-activation of caged ATP, the fast contracting process observed in control (untreated) myofibrils decreases in rate and magnitude with increasing cross-linking time. When taken together with earlier cross-linking studies, our present results suggest that the suppression of contraction may result from two distinct cross-linking reactions: (1) Cross-linking of myosin rods in the filament core which immobilizes the S-2 subunit and acts to decrease the isometric force (approximately 90% at 100 min). (2) Cross-linking within the S-1 subunit. This latter reaction is believed to account for the continuous decay in the rate and magnitude of the unloaded shortening process.

Contraction characteristics and ATPase activity of skeletal muscle fibers in the presence of antibody to myosin subfragment 2

To investigate the role of the myosin hinge region in muscle contraction, we examined the contraction characteristics and Mg-ATPase activity of glycerinated muscle fibers prepared from rabbit psoas in the presence and absence of polyclonal antibody directed against the subfragment 2 (S-2) region of myosin. The antibody-induced reduction of Ca(2+)-activated isometric force was always accompanied by a parallel decrease of muscle fiber stiffness, so that the stiffness versus force relation remained unchanged by the antibody treatment. Force-velocity relations of the fibers, obtained by applying ramp decreases in force at steady isometric forces, indicated that the antibody had no effect on maximum shortening velocity or on the shape of force-velocity curves. Simultaneous measurements of Mg-ATPase activity and Ca(2+)-activated force showed that Mg-ATPase activity of the fibers remained unchanged despite the antibody-induced reduction of isometric force even to zero. These results indicate that when anti-S-2 antibody attaches to the S-2 region of myosin molecules, their heads still hydrolyze ATP but no longer contribute to both force generation and muscle fiber stiffness.

Contraction of myofibrils in the presence of antibodies to myosin subfragment 2

In a muscle-based version of in vitro motility assays, the unloaded shortening velocity of rabbit skeletal myofibrils has been determined in the presence and absence of affinity-column-purified polyclonal antibodies directed against the subfragment-2 region of myosin. Contraction was initiated by photohydrolysis of caged ATP and the time dependence of shortening was monitored by an inverted microscope equipped with a video camera. Antibody-treated myofibrils undergo unloaded shortening in a fast phase with initial rates and half-times comparable to control (untreated) myofibrils, despite a marked reduction in the isometric force of skinned muscle fibers in the presence of the antibodies. In antibody-treated myofibrils, this process is followed by a much slower phase of contraction, terminating in elongated structures with well-defined sarcomere spacings (approximately 1 micron) in contrast to the supercontracted globular state of control myofibrils. These results suggest that although the unloaded shortening of myofibrils (and in vitro motility of actin filaments over immobilized myosin heads) can be powered by myosin heads, the subfragment-2 region as well as the myosin head contributes to force production in actively contracting muscle.

Thermal stability of myosin rod from various species

The radius of gyration and fraction helix as a function of temperature have been determined for myosin rod from four different species: rabbit, frog, scallop, and antarctic fish. Measurements from sodium dodecyl sulfate gel electrophoresis indicate that all particles have the same molecular weight (approximately 130K). All fragments are nearly 100% alpha-helical at low temperatures (0-5 degrees C). The melting profiles for each are qualitatively similar in shape, but their midpoints are shifted along the temperature axis in the following order: antarctic fish (Tm = 33 degrees C), scallop (Tm = 39 degrees C), frog (Tm = 45 degrees C), and rabbit (Tm = 49 degrees C). Corresponding radius of gyration vs temperature profiles for each species are shifted to lower temperatures (approximately 5-8 degrees C) with respect to the optical rotation melting curves. From plots of radius of gyration vs fraction helix, we find a marked drop in the radius of gyration (from 43 to approximately 34 nm) with less than a 5% decrease in fraction helix for rabbit, frog, and antarctic fish rods, whereas the radius of gyration of scallop rod never exceeds 34 nm. Results indicate hinging of the myosin rod of each species. The thermal stabilities of the myosin rods shift in parallel with the working temperature of their respective muscles.

Rapid helix--coil transitions in the S-2 region of myosin

Temperature-jump studies on the long S-2 fragment (100,000 daltons) isolated from myosin show that this structure can undergo alpha-helix--random coil transitions in a time range approximating the cycle time of a crossbridge. Two relaxation times are observed after temperature jumps of 5 degrees C over the range 35--55 degrees C, one in the submillisecond (tau f) and the other in the millisecond (tau s) time ranges. Both processes exhibit maxima near the midpoint of the helix--coil transition (tm = 45 +/- 2 degrees C) as determined by optical rotation melt experiments. Similar results were observed for the low temperature transition (tm = 45 degrees C) of the myosin rod. Viscosity studies reveal that the S-2 particles has significant flexibility at physiological temperature. Results are considered in terms of the Huxley--Simmons and helix--coil transition models for force generation in muscle.