A Prescription Digital Therapeutic to Support Unsupervised Buprenorphine Initiation for Patients With Opioid Use Disorder: Protocol for a Proof-of-Concept Study.

BACKGROUND: Home-based (unsupervised) buprenorphine initiation is considered safe and effective, yet many patients report barriers to successful treatment initiation. Prescription digital therapeutics (PDTs) are software-based disease treatments regulated by the US Food and Drug Administration (FDA). The reSET-O PDT was authorized by the FDA in 2018 and delivers behavioral treatment for individuals receiving buprenorphine for opioid use disorder (OUD). A prototype PDT (PEAR-002b) designed for use with reSET-O was developed to assist in unsupervised buprenorphine initiation. OBJECTIVE: The primary objective of this pilot study is to evaluate the acceptability of PEAR-002b in individuals with OUD who use it to support buprenorphine initiation, their unsupervised buprenorphine initiation success rate, and their medication adherence. METHODS: Ten adults with OUD will be recruited for acceptability and feasibility testing. Outcomes will be assessed using week-1 visit attendance, participant interviews and satisfaction surveys, and urine drug screening (UDS). Three tools will be used in the study: PEAR-002b, reSET-O, and EmbracePlus. PEAR-002b includes a new set of features designed for use with reSET-O. The mechanism of action for the combined PEAR-002b and reSET-O treatment is a program of medication dosing support during week 1 of the initiation phase, cognitive behavioral therapy, and contingency management. During the medication initiation phase, participants are guided through a process to support proper medication use. PEAR-002b advises them when to take their buprenorphine based on provider inputs (eg, starting dose), self-reported substance use, and self-reported withdrawal symptoms. This study also administers the EmbracePlus device, a medical-grade smartwatch, to pilot methods for collecting physiologic data (eg, heart rate and skin conductance) and evaluate the device's potential for use along with PDTs that are designed to improve OUD treatment initiation. Home buprenorphine initiation success will be summarized as the proportion of participants attending the post-buprenorphine initiation visit (week 1) and the proportion of participants who experience buprenorphine initiation-related adverse events (eg, precipitated withdrawal). Acceptability of PEAR-002b will be evaluated based on individual participants' ratings of ease of use, satisfaction, perceived helpfulness, and likelihood of recommending PEAR-002b. Medication adherence will be evaluated by participant self-report data and confirmed by UDS. UDS data will be summarized as the mean of individual participants' proportion of total urine samples testing positive for buprenorphine or norbuprenorphine over the 4-week study. RESULTS: This project was funded in September 2019. As of September 2022, participant enrollment is ongoing. CONCLUSIONS: This is the first study to our knowledge to develop a PDT that assists with unsupervised buprenorphine initiation with the intent to better support patients and prescribers during this early phase of treatment. This pilot study will assess the acceptability and utility of a digital therapeutic to assist individuals with OUD with unsupervised buprenorphine initiation. TRIAL REGISTRATION: ClinicalTrials.gov NCT05412966; https://clinicaltrials.gov/ct2/show/NCT05412966. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): PRR1-10.2196/43122.

Evaluation of real-world outcomes associated with use of a prescription digital therapeutic to treat substance use disorders.

BACKGROUND AND OBJECTIVES: Digital therapeutics can expand the reach and fidelity of behavioral treatment for substance use disorders (SUDs). This analysis evaluated real-world engagement and clinical outcomes in patients diagnosed with SUD who were prescribed reSET®, an FDA-authorized prescription digital therapeutic (PDT). METHODS: Patients were prescribed a 12-week PDT comprising 61 therapy lessons (31 "core" and 30 "keep learning" lessons) and contingency management rewards (positive reinforcement message or monetary gift cards) based on lesson completion and negative urine drug screens. Engagement (defined as any activity in the PDT), retention (any activity in Weeks 9-12), and substance use data were collected automatically by the PDT and analyzed descriptively. Associations between early lesson completion and end-of-treatment outcomes were assessed. RESULTS: Six hundred and fifty-eight patients filled their prescription. Evaluated were 602 patients who were exposed to therapeutic content by completing at least one lesson (median age 37 years, 33% female, 41% male, 26% unreported sex). Median lessons completed was 33 (out of 61 possible), and 52% of patients completed all core modules. Retention in treatment during the last 4 weeks of treatment was 74%, and 62% were abstinent (missing data considered positive). [Correction added on 13 December 2022, after first online publication: In the preceding sentence, the treatment percentage values were revised from 74.6% to 74%.] DISCUSSION AND CONCLUSIONS: Patients with SUD exhibited robust engagement with a PDT, high rates of retention through 12 weeks, and substantial rates of abstinence at end of treatment when the therapeutic was used in a real-world setting. PDT's hold promise as a new way to access effective SUD treatment. SCIENTIFIC SIGNIFICANCE: This study is the first to report real-world PDT engagement and clinical outcomes data from a large, geographically diverse population of patients with SUDs.

Safety and efficacy of a digital therapeutic for substance use disorder: Secondary analysis of data from a NIDA clinical trials network study.

Background: Traditional treatments for substance use disorders (SUDs) rely heavily on face-to-face interactions, which pose substantial limitations for patients. A clinical trial of a digital therapeutic (DT), delivering behavioral therapy demonstrated safety and efficacy in a population including patients with opioid use disorder (OUD) not treated with buprenorphine, which is not a guideline-recommended approach. This study re-analyzed the data excluding patients with OUD to more closely approximate real-world patient populations. Methods: Secondary analysis of patients with substance use disorders related to alcohol, cannabis, cocaine, or other stimulants (n = 399, patients with OUD excluded) from a previously-published randomized controlled trial. Patients received 12-weeks of outpatient treatment-as-usual (TAU; n = 193) or TAU with reduced counseling plus a DT (n = 206) providing computerized cognitive behavioral therapy and contingency management. Primary outcomes were abstinence in weeks 9-12 and retention in treatment. Results: The 399 patients in this analysis (206 in the DT group and 193 in the TAU group) reported substance use disorders related to alcohol, cannabis, cocaine, or other stimulants (e.g., methamphetamines). Demographic and baseline characteristics including age, sex, race, education, and reported primary substance use disorder were balanced between treatment groups. Abstinence was significantly higher in the DT group compared to TAU (40.3 vs. 17.6%; p < 0.001) as was retention in therapy (76.2 vs. 63.2%, p = 0.004). Intergroup adverse event rates were not significantly different (p = 0.68). Conclusions: The results demonstrate that use of a DT safely increased abstinence (reduced substance use) and retention in treatment among patients with substance use disorders related to alcohol, cannabis, cocaine, or other stimulants (including methamphetamines).

Real-world evidence from users of a behavioral digital therapeutic for chronic insomnia.

BACKGROUND: There have been many research trials of various digital therapeutics, but few real world evaluations of their efficacy. This type of data, however, can provide a more rounded understanding of their impact, utility, reach, and adoption. Findings presented here focus on outcome and patient engagement data of SHUTi (Sleep Health Using the Internet), a digital therapeutic delivering Cognitive Behavioral Therapy for insomnia (CBT-I), in a large real-world dataset of adults with insomnia. METHODS: 7216 adults who purchased access to SHUTi between December 2015 and February 2019 are included in the analysis. The Insomnia Severity Index (ISI) was administered at the beginning of each of six treatment Cores of the intervention. Users entered sleep diaries between Cores to track changes in sleep over time and obtain tailored sleep recommendations. Number of Cores completed and sleep diaries entered indicate program usage. RESULTS: Users showed a reduction in mean ISI scores and a corresponding increase in effect size at the start of each subsequent Core (compared to Core 1) (range: d = 0.3-1.9). Effect sizes at the last Core relative to the first were moderate-to-large for diary-derived sleep onset latency and wake after sleep onset. A reduction in number of medicated nights was also found, with those with severe insomnia showing the largest reduction from last-to-first week of treatment (d = 0.3). At the last Core, 61% met criteria for meaningful treatment response (reduction of >7 points on ISI) and 40% met criteria for remission (ISI<8). Engagement was comparable to SHUTi research trials. CONCLUSION: Consistent with controlled trials, real-world data suggest that digital therapeutics can result in relatively high levels of engagement and clinically meaningful sleep improvements.

Patient Engagement With a Game-Based Digital Therapeutic for the Treatment of Opioid Use Disorder: Protocol for a Randomized Controlled Open-Label, Decentralized Trial.

BACKGROUND: Prescription digital therapeutics are software-based disease treatments that are regulated by the US Food and Drug Administration; the reSET-O prescription digital therapeutic was authorized in 2018 and delivers behavioral treatment for individuals receiving buprenorphine for opioid use disorder. Although reSET-O improves outcomes for individuals with opioid use disorder, most of the therapeutic content is delivered as narrative text. PEAR-008 is an investigational device based on reSET-O that uses an interactive, game-based platform to deliver similar therapeutic content designed to enhance patient engagement, which may further improve treatment outcomes. OBJECTIVE: We aim to investigate how participants interact with the prescription digital therapeutic's new content delivery format. Secondary objectives include evaluating treatment success, symptoms of co-occurring mental health disorders, recovery capital, and skill development. METHODS: Due to the COVID-19 pandemic, this study was redesigned using a decentralized model because it was not possible to conduct medication initiation and study visits in person, as initially intended. A decentralized, randomized controlled trial design will be utilized to compare patient engagement with PEAR-008 and that with reSET-O using both subjective and objective assessments. The study population will consist of approximately 130 individuals with opioid use disorder (based on Diagnostic and Statistical Manual of Mental Disorders 5 criteria) who have recently started buprenorphine treatment for opioid use disorder. Participants will be virtually recruited and randomly assigned to receive either PEAR-008 or reSET-O. All study sessions will be virtual, and the duration of the study is 12 weeks. The primary outcome measure of engagement is operationalized as the number of active sessions per week with either PEAR-008 or reSET-O. (An active session is any session that contains some active participation in the app, such as navigating to a different screen, engaging with a learning module, or responding to a notification.) Subjective dimensions of engagement will be assessed with participant surveys. The hypothesis is that PEAR-008 will have significantly greater participant engagement than reSET-O. RESULTS: As of February 2021, participant enrollment is ongoing. CONCLUSIONS: This randomized controlled trial will investigate if changing the delivery format and enhancing the content of a prescription digital therapeutic for opioid use disorder will affect how participants use and interact with the prescription digital therapeutic. The study design may serve as a useful model for conducting decentralized studies in this patient population. TRIAL REGISTRATION: ClinicalTrials.gov NCT04542642; https://clinicaltrials.gov/ct2/show/NCT04542642. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): DERR1-10.2196/32759.

Engagement patterns with a digital therapeutic for substance use disorders: Correlations with abstinence outcomes.

INTRODUCTION: Patient engagement may play a key role in the success or failure of treatments for substance use disorder (SUD). This exploratory analysis of data from a large, multisite effectiveness trial (NCT01104805) sought to determine how patient engagement with a digital therapeutic for SUD delivered at clinics was associated with abstinence outcomes. METHODS: The study evaluated engagement for 206 participants enrolled in a treatment program for SUDs related to cocaine, alcohol, cannabis, or other stimulants who were randomized to receive treatment as usual (TAU) or reduced TAU plus the digital Therapeutic Education System (TES) for 12 weeks. Participants were eligible for contingency management incentives for module completion (modules cover Community Reinforcement Approach topic areas) and negative urine drug screens. Analyses examined the association of module completion with end-of-treatment abstinence. RESULTS: Participants completed a mean of 38.8 (range 0-72) TES modules over 12 weeks of treatment. Study completers (n = 157) completed a mean of 45.5 (range 9-72) TES modules, whereas study noncompleters (n = 49) completed a mean of 17.4 (range 0-45) TES modules. The study observed a strong positive correlation between TES engagement (i.e., total number of modules completed) and the probability of abstinence during weeks 9-12 of treatment among 157 study completers (OR = 1.11; 95% CI 1.08-1.14). Each module completed increased the odds of abstinence during weeks 9-12 by approximately 11% for study completers and 9% for the full sample. The study observed a similar, but weaker, association between engagement and abstinence among 49 patients who did not complete the study (OR = 1.02; 95% CI 0.98-1.07). CONCLUSIONS: Greater engagement with a digital therapeutic for patients with SUD (i.e., number of modules completed over time) was strongly associated with the probability of abstinence in the last four weeks of treatment among those who completed the recommended 12-week treatment. TRIALS REGISTRATION: ClinicalTrials.gov Identifier: NCT01104805.

Evaluation of the cost-utility of a prescription digital therapeutic for the treatment of opioid use disorder.

Background: The opioid epidemic continues to generate a significant mental and physical health burden on patients, and claims the life of almost 150 Americans daily. Making matters worse, an increase in relapses and/or opioid-related deaths has been reported in more than 40 U.S. states since the start of the COVID-19 pandemic. Opioid use disorder (OUD) is one of the single most expensive disorders in the United States, generating average medical costs of $60B from just 2 million Americans diagnosed with the disorder. In commercial use since 2019, reSET-O is a non-drug, prescription digital therapeutic (PDT) that delivers evidence-based neurobehavioral treatment for OUD and helps overcome the barriers associated with access to care, stigma, and social distancing. Although shown to be cost effective and efficacious in clinical trials and real-world evidence studies, respectively, information on its value for money from a health utilities and cost per quality-adjusted life-year is needed to inform policy discussions.Objectives: To evaluate the impact of reSET-O on health utilities and assess its overall cost per quality-adjusted life year (QALY) gained vs. treatment-as-usual (TAU).Methods: Decision analytic model comparing reSET-O plus TAU to TAU alone (i.e. buprenorphine, face-to-face counseling, and contingency management) over 12 weeks. Clinical effectiveness data (abstinence and health utility) were obtained from a clinical trial, and resource utilization and cost data were adapted from a recent claims data analysis to reflect less frequent face-to-face counseling with the therapeutic.Results: The addition of reSET-O to TAU decreases total health care costs by -$131 and resulted in post-treatment utility values within population norms, with a corresponding gain of 0.003 QALYs. reSET-O when used adjunctively to TAU was economically dominant (less costly, more effective) vs. TAU alone.Conclusion: reSET-O is an economically-dominant adjunctive treatment for OUD and is associated with an overall reduction in total incremental cost vs TAU.

Real-world evidence for a prescription digital therapeutic to treat opioid use disorder.

OBJECTIVES: To evaluate patient engagement and usage of a prescription digital therapeutic (PDT) and associated outcomes of opioid use and treatment retention in a large real-world dataset of patients with opioid use disorder (OUD) treated with buprenorphine medication for opioid use disorder (MOUD). PDTs are software-based disease treatments evaluated for safety and effectiveness in randomized clinical trials (RCTs), and authorized by the U.S. Food and Drug Administration (FDA) to treat disease with approved directions for use (label). METHODS: A real-world observational evaluation of an all-comer population of patients who redeemed a 12-week prescription for the reSET-O PDT. Engagement and therapeutic use data were collected and analysed on a population level. Substance use was evaluated as a composite of self-reports recorded with reSET-O and urine drug screens (UDS). RESULTS: Data from 3144 individuals with OUD were evaluated. 45.5% were between ages 30 and 39 years. 80% completed at least 8 of the 67 possible therapeutic modules, 66% completed half of all modules, and 49% completed all modules. Abstinence during the last 4 weeks of treatment was calculated with two imputation methodologies: 66% abstinent using "missing data excluded (patients with no data as positive)", and 91% abstinent with "missing data removed (patients with no data excluded)". 91% of patients met the responder definition of ≥80% of self-report or UDS negative. 74.2% of patients were retained through the last 4 weeks of treatment. Subgroup analysis of patients using reSET-O appropriately (4 or more modules per week for the first 4 weeks) showed 88.1% abstinence using "missing data excluded (patients with no data as positive)", and retention at weeks 9-12 of 85.8%. CONCLUSIONS: Results demonstrate that reSET-O is readily and broadly used by patients with OUD and that high real-world engagement with the therapeutic is positively associated with abstinence and retention in treatment. ReSET-O is a potentially valuable adjunct to buprenorphine MOUD therapy for patients with OUD.

Safety and efficacy of a prescription digital therapeutic as an adjunct to buprenorphine for treatment of opioid use disorder.

OBJECTIVES: To evaluate the safety and efficacy of a digital therapeutic in treatment-seeking individuals with opioid use disorder (OUD) in an analysis of randomized clinical trial (RCT) data (ClinicalTrials.gov identifier: NCT00929253). METHODS: Secondary analysis of an RCT including 170 adults meeting DSM-IV criteria for OUD. Participants were randomized to 12-weeks of treatment-as-usual (TAU) or TAU plus a digital therapeutic providing 67 digital, interactive educational modules based on the Community Reinforcement Approach. TAU consisted of buprenorphine maintenance therapy, 30 min biweekly clinician interaction, and abstinence-based contingency management. Primary endpoints were treatment retention and abstinence (negative urine drug screen) during weeks 9-12 of treatment. Safety was assessed by evaluating adverse events. RESULTS: Participants randomized to TAU plus a digital therapeutic had significantly greater odds of opioid abstinence during weeks 9-12 compared to TAU: 77.3 versus 62.1%, respectively (p=.02), OR 2.08, 95% CI 1.10-3.95. The risk of patients leaving treatment was significantly lower in the digital therapeutic group (HR 0.49, 95% CI 0.26-0.92). No significant difference was observed in the rate of adverse events between groups (p=.42). CONCLUSIONS: A prescription digital therapeutic (PDT) in combination with buprenorphine therapy improves clinically significant patient outcomes including abstinence from illicit opioids and retention in treatment compared with treatment as usual.

The Vitamin D Receptor Regulates Tissue Resident Macrophage Response to Injury.

Ligand-dependent actions of the vitamin D receptor (VDR) play a pleiotropic role in the regulation of innate and adaptive immunity. The liganded VDR is required for recruitment of macrophages during the inflammatory phase of cutaneous wound healing. Although the number of macrophages in the granulation tissue 2 days after wounding is markedly reduced in VDR knockout (KO) compared with wild-type mice, VDR ablation does not alter macrophage polarization. Parabiosis studies demonstrate that circulatory chimerism with wild-type mice is unable to rescue the macrophage defect in the wounds of VDR KO mice and reveal that wound macrophages are of local origin, regardless of VDR status. Wound cytokine analyses demonstrated a decrease in macrophage colony-stimulating factor (M-CSF) protein levels in VDR KO mice. Consistent with this, induction of M-CSF gene expression by TGFß and 1,25-dihydroxyvitamin D was impaired in dermal fibroblasts isolated from VDR KO mice. Because M-CSF is important for macrophage self-renewal, studies were performed to evaluate the response of tissue resident macrophages to this cytokine. A decrease in M-CSF induced proliferation and cyclin D1 expression was observed in peritoneal resident macrophages isolated from VDR KO mice, suggesting an intrinsic macrophage abnormality. Consistent with this, wound-healing assays in mice with macrophage-specific VDR ablation demonstrate that a normal wound microenvironment cannot compensate for the absence of the VDR in macrophages and thus confirm a critical role for the macrophage VDR in the inflammatory response to injury.

The vitamin D receptor is required for activation of cWnt and hedgehog signaling in keratinocytes.

Alopecia (hair loss) in vitamin D receptor (VDR)-null mice is due to absence of ligand-independent actions of the VDR that are required for initiation of postmorphogenic hair cycles. Investigations were undertaken to determine whether the VDR is required for the induction of signaling pathways that play an important role in this process. The induction of cWnt and hedgehog target genes that characterizes early anagen was found to be dramatically attenuated in VDR(-/-) mice, relative to wild-type (WT) mice. To determine whether this reflects impaired responsiveness to cWnt ligands, in vitro studies were performed in primary keratinocytes. These studies demonstrated impaired induction of cWnt target genes in response to Wnt3a in VDR(-/-) keratinocytes, relative to wild-type keratinocytes. Chromatin immunoprecipitation analyses revealed that the VDR was recruited to the regulatory regions of cWnt and hedgehog target genes in WT keratinocytes but not in VDR(-/-) or Lef1(-/-) keratinocytes. Lef1 was enriched on these same regulatory regions in WT keratinocytes but not in VDR(-/-) keratinocytes. In vivo studies were performed to determine whether activation of the hedgehog pathway could bypass the defect in cWnt signaling observed in the absence of the unliganded VDR. In WT, but not VDR(-/-), mice, hedgehog agonist treatment resulted in an induction of cWnt and hedgehog target genes and the generation of mature anagen hair follicles. Thus, these studies demonstrate that the unliganded VDR interacts with regulatory regions in the cWnt and hedgehog target genes and is required for the induction of these pathways during the postnatal hair cycle.

Ligand-dependent actions of the vitamin D receptor are required for activation of TGF-ß signaling during the inflammatory response to cutaneous injury.

The vitamin D receptor (VDR) has both 1,25-dihydroxyvitamin D-dependent and -independent actions in the epidermis. Ligand-dependent actions of the VDR have been shown to promote keratinocyte differentiation and to regulate formation of the epidermal barrier. In contrast, the actions of the VDR that regulate postmorphogenic hair cycling do not require 1,25-dihydroxyvitamin D. The VDR also has immunomodulatory actions that are dependent on its ligand, 1,25-dihydroxyvitamin D. To determine whether the ligand-dependent or -independent actions of the VDR regulate the inflammatory response to cutaneous injury, studies were performed in control, VDR knockout, and vitamin D-deficient mice. These investigations demonstrate that absence of receptor or ligand impairs the dermal response to cutaneous injury. Although neutrophil recruitment is not affected, the absence of VDR signaling leads to defects in macrophage recruitment and granulation tissue formation. Studies performed to identify the molecular basis for this phenotype demonstrate that absence of the VDR, or its ligand, impairs TGF-ß signaling in the dermis, characterized by decreased expression of monocyte chemotactic protein-1 and reduced phosphorylation of phosphorylated Smad-3 as well as attenuated phosphorylated Smad-3 phosphorylation in response to TGF-ß in primary dermal fibroblasts lacking the VDR. Thus, these data demonstrate that the liganded VDR interacts with the TGF-ß signaling pathway to promote the normal inflammatory response to cutaneous injury.

Lymphoid enhancer-binding factor-1 (LEF1) interacts with the DNA-binding domain of the vitamin D receptor.

Ligand-independent actions of the vitamin D receptor (VDR) are required for normal post-morphogenic hair cycles; however, the molecular mechanisms by which the VDR exerts these actions are not clear. Previous studies demonstrated impaired regulation of the canonical Wnt signaling pathway in primary keratinocytes lacking the VDR. To identify the key effector of canonical Wnt signaling that interacts with the VDR, GST pulldown studies were performed. A novel interaction between the VDR and LEF1 (lymphoid enhancer-binding factor-1) that is independent of ß-catenin was identified. This interaction is dependent upon sequences within the N-terminal region of the VDR, a domain required for VDR-DNA interactions and normal hair cycling in mice. Mutation of specific residues within the N-terminal region of the VDR not only abrogated interactions between the VDR and LEF1 but also impaired the ability of the VDR to enhance Wnt signaling in vdr(-/-) primary keratinocytes. Thus, this study demonstrates a novel interaction between the VDR and LEF1 that is mediated by the DNA-binding domain of the VDR and that is required for normal canonical Wnt signaling in keratinocytes.

The vitamin D receptor, the skin and stem cells.

The active metabolite of vitamin D, 1,25-dihydroxyvitamin D, has been shown to have pro-differentiation and antiproliferative effects on keratinocytes that are mediated by interactions with its nuclear receptor. Other cutaneous actions of the vitamin D receptor have been brought to light by the cutaneous phenotype of humans and mice with non-functional vitamin D receptors. Although mice lacking functional vitamin D receptors develop a normal first coat of hair, they exhibit impaired cyclic regeneration of hair follicles that leads to the development of alopecia. Normal hair cycling involves reciprocal interactions between the dermal papilla and the epidermal keratinocyte. Studies in mice with targeted ablation of the vitamin D receptor demonstrate that the abnormality in the hair cycle is due to a defect in the keratinocyte component of the hair follicle. Furthermore, expression of mutant vitamin D receptor transgenes in the keratinocytes of vitamin D receptor knockout mice demonstrates that the effects of the receptor that maintain hair follicle homeostasis are ligand-independent. Absence of a functional vitamin D receptor leads to impaired function of keratinocyte stem cells, both in vivo and in vitro. This is manifested by impaired cyclic regeneration of the hair follicle, a decrease in bulge keratinocyte stem cells with ageing and an abnormality in lineage progression of these cells, leading to their preferential differentiation into sebocytes.

Acute phosphate restriction leads to impaired fracture healing and resistance to BMP-2.

Hypophosphatemia leads to rickets and osteomalacia, the latter of which results in decreased biomechanical integrity of bones, accompanied by poor fracture healing. Impaired phosphate-dependent apoptosis of hypertrophic chondrocytes is the molecular basis for rickets. However, the underlying pathophysiology of impaired fracture healing has not been characterized previously. To address the role of phosphate in fracture repair, mice were placed on a phosphate-restricted diet 2 days prior to or 3 days after induction of a mid-diaphyseal femoral fracture to assess the effects of phosphate deficiency on the initial recruitment of mesenchymal stem cells and their subsequent differentiation. Histologic and micro-computed tomographic (microCT) analyses demonstrated that both phosphate restriction models dramatically impaired fracture healing primarily owing to a defect in differentiation along the chondrogenic lineage. Based on Sox9 and Sox5 mRNA levels, neither the initial recruitment of cells to the callus nor their lineage commitment was effected by hypophosphatemia. However, differentiation of these cells was impaired in association with impaired bone morphogenetic protein (BMP) signaling. In vivo ectopic bone-formation assays and in vitro investigations in ST2 stromal cells confirmed that phosphate restriction leads to BMP-2 resistance. Marrow ablation studies demonstrate that hypophosphatemia has different effects on injury-induced intramembranous bone formation compared with endochondral bone formation. Thus phosphate plays an important role in the skeleton that extends beyond mineralized matrix formation and growth plate maturation and is critical for endochondral bone repair.

Vitamin D and human health: lessons from vitamin D receptor null mice.

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

The LIM protein LIMD1 influences osteoblast differentiation and function.

The balance between bone resorption and bone formation involves the coordinated activities of osteoblasts and osteoclasts. Communication between these two cell types is essential for maintenance of normal bone homeostasis; however, the mechanisms regulating this cross talk are not completely understood. Many factors that mediate differentiation and function of both osteoblasts and osteoclasts have been identified. The LIM protein Limd1 has been implicated in the regulation of stress osteoclastogenesis through an interaction with the p62/sequestosome protein. Here we show that Limd1 also influences osteoblast progenitor numbers, differentiation, and function. Limd1(-/-) calvarial osteoblasts display increased mineralization and accelerated differentiation. While no significant differences in osteoblast number or function were detected in vivo, bone marrow stromal cells isolated from Limd1(-/-) mice contain significantly more osteoblast progenitors compared to wild type controls when cultured ex vivo. Furthermore, we observed a significant increase in nuclear beta-catenin staining in differentiating Limd1(-/-) calvarial osteoblasts suggesting that Limd1 is a negative regulator of canonical Wnt signaling in osteoblasts. These results demonstrate that Limd1 influences not only stress osteoclastogenesis but also osteoblast function and osteoblast progenitor commitment. Together, these data identify Limd1 as a novel regulator of both bone osetoclast and bone osteoblast development and function.

The LIM protein, Limd1, regulates AP-1 activation through an interaction with Traf6 to influence osteoclast development.

Increasingly a number of proteins important in the regulation of bone osteoclast development have been shown primarily influence osteoclastogenesis under conditions of physiologic or pathologic stress. Why basal osteoclastogenesis is normal and how these proteins regulate stress osteoclastogenic responses, as opposed to basal osteoclastogenesis, is unclear. LIM proteins of the Ajuba/Zyxin family localize to cellular sites of cell adhesion where they contribute to the regulation of cell adhesion and migration, translocate into the nucleus where they can affect cell fate, but are also found in the cytoplasm where their function is largely unknown. We show that one member of this LIM protein family, Limd1, is uniquely up-regulated during osteoclast differentiation and interacts with Traf6, a critical cytosolic regulator of RANK-L-regulated osteoclast development. Limd1 positively affects the capacity of Traf6 to activate AP-1, and Limd1(-/-) osteoclast precursor cells are defective in the activation of AP-1 and thus induction of NFAT2. Limd1(-/-) mice, although having normal basal bone osteoclast numbers and bone density, are resistant to physiological and pathologic osteoclastogenic stimuli. These results implicate Limd1 as a potentially important regulator of osteoclast development under conditions of stress.

Canine PKD1 is a single-copy gene: genomic organization and comparative analysis.

Most cases of autosomal dominant polycystic kidney disease are caused by mutations in the gene PKD1, encoding polycystin-1. To gain insight into the role of polycystin-1 in tubulogenesis and cystogenesis using the well-characterized canine kidney epithelial cell line MDCK, we have now cloned and characterized the exon/intron structure of the canine gene PKD1. FISH analysis showed that the dog genome lacks the multiple PKD1 homologs present in human. Intron 21 of dog PKD1 lacked the polypyrimidine tract characteristic of the human gene, whereas pyrimidine-rich elements were identified in canine intron 30. Canine polycystin-1 showed a higher degree of homology with the human counterpart and lower homology with mouse and rat. A striking degree of conservation (97% identity) was determined for the leucine-rich repeat domain between dog and human. Also, the homology analysis indicated that 4 of 16 Ig-like repeats (IgIII, IgVII, IgX, and IgXV) are likely to be functionally significant. This is particularly important in light of our recent findings demonstrating that Iglike domains form strong homophilic interactions and can mediate cell-cell adhesion. These data enable detailed analysis of the role of polycystin-1 in cystogenesis and tubulogenesis using the canine MDCK cell line.